Sheeted Clastic Dikes in the Megaflood Region

Sheeted Clastic Dikes in the Megaflood Region, WA-OR-ID-MT

Skye W. Cooley

Mission Valley, MT

Abstract

Clastic dikes are sediment-filled fractures found worldwide in deformed sediments from the Precambrian to the Pleistocene. Most are soft sediment deformation features and the products of liquefaction caused by seismic shaking. The dikes described in this article are different. They are wedge-shaped, vertically sheeted, and sourced from above - characteristics that starkly contrast with most liquefaction dikes. For this study, I investigated unconsolidated sediments exposed along the route of Pleistocene megafloods between Priest River, ID and The Dalles, OR. I measured the widths of thousands of clastic dikes at 285 exposures (as of Nov 2024). The dikes are Pleistocene in age, occur entirely within Ice Age floodways, and formed by hydraulic fracture concurrent with floodwater loading. They are flood injectites, not seismites. This study is one of the largest ever conducted on sedimentary dikes.

Keywords: clastic dike, sand dike, sand injectite, hydraulic fracture, Channeled Scablands, Missoula floods, megafloods, Columbia Basin, Cordilleran Ice Sheet, Touchet Beds

Previous Work on the Clastic Dikes

Clastic dikes in Missoula flood deposits are noted in classic papers on Channeled Scablands geology (Bretz, 1928-29 unpublished field notes; Baker, 1973; Waitt, 1985; Smith, 1993; Atwater, 1986). Reports containing detailed descriptions of the features are few (Jenkins, 1925; Lupher, 1944; Black, 1979; Woodward-Clyde Associates, 1981) and those containing measurements are rare (Alwin and Scott, 1970; Cooley and others, 1996; Neill and others, 1997; Ward and others, 2006). A few articles containing otherwise detailed stratigraphic descriptions, photographs, and sketches ignore the dikes entirely (Waitt, 1985; Smith, 1988a,b; Lindsey and others, 1996; Benito and O'Connor, 2003). Numerous authors speculate on the dikes' origin, but few provide data to support their assertions (Flint, 1938; Newcomb, 1962; Bingham and Grolier, 1966; Jones and Deacon, 1966; Beaulieu, 1974; Carson and others, 1978; Shaw and others, 1999; Pritchard and Cebula, 2016). Eighty years of reporting on dike-riddled deposits at the U.S. Department of Energy's Hanford Site has provided no clarity on their origin. Some Hanford authors appear to have neatly tailored titles to avoid being found in keyword searches (i.e., Newman and others, 2002). The voluminous Hanford gray literature lacks measurement data and a regional perspective (i.e., Bjornstad, 1980; Bjornstad and others, 1990, 2001; Bjornstad and Teel, 1993; Fecht and others, 1999; Bjornstad, 2006, Bjornstad and Lanigan, 2007). It is rife with speculation, much of it contradictory, and best ignored. See Footnotes 1 and 13.

Proposed Origins

Four origins for the dikes have been proposed: earthquakes (Jenkins, 1925), ground ice (Alwin and Scott, 1970; Black, 1979), desiccation (Lupher, 1944), and floodwater loading (Brown and Brown, 1962; Baker, 1973). A dubious fifth, “multigenetic” (Black, 1979; Fecht and others, 1999), suggests the dikes formed by a combination of these processes. Cooley (2015) provides a concise summary of the arguments for and against each hypothesis. A detailed treatment is provided here.

This Study

I investigated unconsolidated sediments, partially-lithified sediments, and flood-scoured bedrock exposed along the path of the Ice Age floods between Priest River, ID and The Dalles, OR. Field work totaling more than 250 days was conducted between 1995 and 2024. Clastic dikes with vertically sheeted fills were identified in 285 of 531 exposures. Dozens of never-reported locations were discovered, described, and mapped. Sheeted clastic dikes with identical characteristics (size, shape, fill, age) were found only within the margins of Pleistocene floodways. All appear to have formed by the same mechanism during the Pleistocene, and not before or since. This summary provides the most current information and supersedes previous reports (Cooley, 1996, 2008, 2014, 2015, 2020; Cooley and others, 1996). The photos and figures included here are mine unless otherwise credited.

The megaflood region. Pleistocene megafloods and associated slackwater lakes covered >30,000 km2 of Washington, Oregon, and Idaho. The margins of the floodway can be approximated by clipping a DEM to the 400m elevation contour.

Clastic dikes in the Channeled Scablands. I have documented thousands of clastic dikes in hundreds of exposures throughout the Channeled Scablands including a few sites well north of the former Cordilleran Ice Sheet margin. Sheeted dikes occur within the floodway (blue area), are all but absent in unconsolidated sediments outside the floodway, and are largest and most abundant in slackwater lake deposits that contain abundant silt. A notable gap in diking appears on the map as an east-west swath from Moses Coulee to the eastern Cheney-Palouse, where only gravel and bare rock occur. Rhythmite deposition is for the most part controlled by Wallula Gap (elevation limited), thus most dikes occur south of Quincy Basin. Purple squares are townships with one or more exposures containing dikes (n = 132). Each square equals ~36 square miles.

Typical sheeted clastic dike. A vertically-sheeted clastic dike intrudes silty-sandy late Pleistocene megaflood rhythmites (Touchet Beds) at Smith Hollow Rd in the Tucannon Valley, WA. Wall-parallel sheeting is the product of crack-and-fill cycles accompanied by leakoff and skin wall formation. Sheeting in Touchet-type dikes is not flow banding commonly observed in sandstone dikes in other regions.

Dike-sill-dike geometry. Dike follows a least-resistance pathway through a stack of Touchet Beds at Lewiston, ID. The dike tends to cut vertically across silty, low-permeability layers (tan) and follow bedding in the coarser, higher-permeability sands (gray). The orientation of fill bands flips between vertical in the dike segments and horizontal in the sill segments. Geometry of the entire dike, mostly out-of-plane, is likely a branching structure with blade-like pinchouts both laterally and on bottom (i.e., KGD hydraulic fracture model). Fractures controlled fluid flow in dike portions while porosity controlled in sill portions. Resistance was nearly equal.

Bedded fills. Most fill bands (sheets) are subhorizontally bedded, consistent with top-down infilling and a component of lateral flow along strike. Bedding in sandy fills is concave-up to planar with angles that range from flat to greater than 35 degrees. Less common are bands filled with unstratified silt. The fill material was always a slurry when it entered fractures.

Sheeted fills. This dike contains more than 40 sheets (fill bands).

More than a Touchet Beds story. The dikes intrude a variety of substrates - surficial deposits, partially-lithified sediments, and bedrock. Here, a sheeted silty dike intrudes colluvium composed of angular, locally-derived basaltic clasts. The host material was swept downstream from a scree slope on the flank of the Alder Ridge anticline by a Missoula flood. Near Alderdale, Columbia Gorge, WA.

Huge dikes in slackwater lake basins. Very large dikes with true widths exceeding a meter often contain >100 fill bands. The largest dikes are found where rhythmite stacks are thickest and slackwater lakes were deepest. Foster Wells Rd north of Pasco, WA.

Jenkins was first. This 1923 photo shows field geologist Olaf P. Jenkins standing next to a large dike exposed in a gravel pit near Touchet, WA. The caption reads, "Clastic dikes in Touchet Beds and dust dune between Touchet and Walla Walla". This dike is sourced in light colored slackwater sediments that overlie the dark, laminated sand. Source: Washington Geological Survey Archives (c. 1923, No. 00604)

Publications on the dikes. 107 articles, abstracts, field guides, agency gray lit, and consultant reports have been published on the clastic dikes in Eastern Washington since 1925. Most simply mention the dikes in passing. Few contain measurements of any kind. Many of the Hanford articles derive from a single infiltration experiment conducted in the 2000's on one large dike located off Army Loop Road.

Touchet Beds in Burlingame Canyon. Many geologists have visited this classic locality near Gardena, WA. About 40 Touchet Beds are exposed here (Waitt, 1980, 1985). Rhythmically-bedded flood deposits like these are widely distributed across Columbia Basin to an elevation of approximately 366m, corresponding with the highstand level of Lake Lewis. They all contain dikes. Thick, well-exposed stacks of rhythmites like those at Burlingame Canyon are rare. Most slackwater sections are thinner with greater sedimentologic variability, reflecting the site-specific flood/backflood/ponding conditions. Some Touchet-equivalent beds are found well above 366m elevation (i.e., Lacrosse, WA) and well below 366m (i.e., Cecil, OR). Slackwater at those locations was not controlled by Wallula Gap, but by other bedrock constrictions. Photo source: Washington Geological Survey Archives (1978, No. 3455). See Footnote 5.

Study sites. Locations with sheeted clastic dikes are shown as black circles (n > 285). White circles denote locations where no dikes were observed (n > 225). Stars mark abundant soft sediment deformation (non-dike features). Star symbols are offset from outcrop circles for clarity. A dashed line approximates the Cordilleran Ice Sheet margin at LGM. Glacial Lake Columbia is shown at its 600 m elevation shoreline. Sediments observed at all locations (488 shown on map) consist primarily of unconsolidated Pleistocene-age flood and non-flood silts, sands, and gravels (Touchet Beds, Pasco Gravels, Palouse Loess) that commonly overlie Neogene sediments (Ellensburg Fm, Ringold Fm) and bedrock (mostly basalt). A number of thick Holocene alluvial sections were surveyed as well. Light gray areas show the extent of ephemeral slackwater lakes Lewis, Condon, Latah, Upper Columbia, and Foster that formed behind bedrock or ice constrictions (A,B,C,D,E). Dikes are common in the southern part of the Channeled Scablands, where silt-sand rhythmites are common and exposures are more numerous. Bar gravels and Palouse Loess contain few dikes and poorly preserve deformation in general. Varved silt-clay beds in the basins of glacial Lakes Missoula and Columbia contain small, non-dike deformation structures. An anomalous black dot between Wenatchee and Yakima is a single dike at the Ellensburg Rodeo Grounds. It is Miocene age, unsheeted, and not flood related. Dozens more sites have been logged since the drafting of this map in 2020.

Deformation style varies along the flood route. Various types of soft sediment deformation structures occur in the Channeled Scabland (light blue area). Wedge-shaped sheeted dikes are common in slackwater deposits, while t-shaped mud squirts are more common to the north where varved lacustrine sediments dominate. The particular type of deformation feature present at any given location appears governed by grain size, sediment thickness, water depth, lake residence time, and a few other factors that affect material response. Dikes in different flood facies take different shapes (i.e., slackwater rhythmites vs. bar gravels). Local flow dynamics may affect dike size and abundance (i.e., high energy channel vs. deep slackwater lake). The dashed blue line follows Atwater's longitudinal profile of the Columbia River between Coeur d'Alene and Wallula Gap (Atwater, 1987, Figure 2). Each map symbol represents one or more outcrops.

Typical Clastic Dikes vs. Sheeted Injectites

Clastic dikes are sediment-filled fractures found worldwide in deformed sediments from the Precambrian to the Holocene. Most sand dikes are the products of liquefaction and fluid escape triggered by seismic shaking. Strong shaking elevates pore fluid pressures in wet, unconsolidated sediment layers at depth, causing sandy material to mobilize and vent to the surface, often forming sand blows (Obermeier, 1998). Therefore, most earthquake-generated clastic dikes are upward-pinching structures, sourced from below, that contain massive sandy fills.

The clastic dikes described here are different. They are slender, vertically-sheeted, wedge-shaped structures that were filled from the top. They are sediment-filled hydraulic fractures propagated downward into sedimentary and bedrock substrates during periods when energetic glacial floods and deep, slow-draining slackwater lakes inundated the landscape. Their geometry is consistent with hydraulic fractures described in the literature for sand injectites in submarine turbidite fans (Jolly and Lonergan, 2002; Hurst et al., 2011 Appendix A; Cobain and others, 2016) and certain subglacial settings (von Brunn and Talbot, 1986; Broster, 1991; Larsen and Mangerud, 1992; Dreimanis and Rappol, 1997; LeHeron and Etienne, 2005). Lessons from the well-exposed, human-scale, terrestrial sand dikes described here may inform industry work on sand injectites in offshore petroleum basins.

Reinjection concept. Dike growth of composite dikes by LeHeron and Etienne (2005).

Dikes and flute casts. (A) A typical clastic dike in slackwater rhythmites (Touchet Beds) exhibits the characteristic vertical sheeting composed of darker fill bands (or sheets) separated by light-colored silt skin partitions. This example contains ~12 sheets and is filled with silty, sandy sediment closely resembling the host material. Umatilla Basin at Cecil, OR (Slackwater Lake Condon). (B) A typical dike in gravelly deposits is truncated at its top by a second floodbed. Its fill is crudely sheeted and lacks silt skins. Dikes in coarse-grained deposits tend to have lower length-to-width ratio than dikes in fine-grained sediments. Umatilla Basin at Willow Creek, OR (Slackwater Lake Condon). (C) Examples of flute casts that ornament the faces of silt skins. Upward-pointing noses are clear directional indicators. Sediment entered the fractures from the top. Quarters for scale. Walla Walla Valley, WA (Slackwater Lake Lewis). (D) A sheeted dike, filled with a mix of silty, sandy flood sediment (Late Pleistocene) and quartzite-rich gravel from the Ellensburg Fm eroded from local exposures by floodwaters, intrudes micaceous, oxidized fluvial sandstones (Miocene) at Snipes Mountain. The dike contains ~10 sheets. Hoe is 28 cm long. Emerald Road at Granger, WA (Slackwater Lake Lewis).

Size and Shape - The wedge-shaped dikes are found in more than a dozen geologic units. The largest examples contain >100 fill bands, exceed 2 m in width, and penetrate to depths >50 m. Most are <15 cm wide and contain fewer than a dozen sheets. Dikes in silt-sand rhythmites are long and slender (H >> W). Dikes in coarse sands and gravel are few in number, crudely sheeted, and stubby. Dikes that penetrate bedrock (Columbia River Basalt) are slender and resemble those in rhythmites. In most cases, sediments filled tensile fractures (Mode I joints), but en echelon geometries are not uncommon (shear). In three dimensions, the dikes resemble wing-shaped hydraulic fractures with curved fronts (i.e., PKN fracture model of Belin and Carey, 1997).

Fills - Sedimentology of dike fills reflects the local geology. Dike fills typically contain a mix of basaltic sediment (local bedrock) and micaceous silty-sandy sediment (older rhythmites and Palouse loess). At sites along the margins of the Columbia River Basalt province, dike fills contain non-basaltic clasts. At Snipes Mountain, WA (Yakima Valley), dikes are filled by both quartzite gravel from deposits on the top of the ridge and Touchet Bed sediment from a stack of rhythmites inset into the side of the ridge. Dikes at West Foster Creek (upper Columbia Valley) contain Miocene gruss shed from granitic highlands that poke up through the basalt. Dikes near Walla Walla Valley contain Touchet Bed sediment and basaltic colluvium in locations beneath cliffs (Jenkins, 1925; Cooley, 2015). In the Palouse Hills, composite dikes filled are filled with muddy, cemented sediment and unconsolidated sandy-silty sediment injected later (Cooley and others, 1996; Spencer and Jaffee, 2002; Bader and others, 2016).

Sheeting and Growth - The dikes are conspicuously sheeted structures that grew in pulses crack-and-fill cycling. Vertical sheeting records increments of widening and lengthening. Hyashi (1966) distinguished sheeted sedimentary dikes from single-fill dikes and categorized them as "compound" or "composite" based on the particular way they grew. Compound dikes are formed during single diking events. Composite dikes form by reinjection during multiple events separated in time. In both types, new fractures opened into and alongside older sheets. Strong grain size contrasts between adjacent sheets are evidence of a variable and changing sediment source during single diking events and over longer periods and multiple diking events.

Growth in stages. Sheeted dikes grow sheet by sheet as illustrated above. New sheets may intrude alongside older ones (B parallels A) or split older ones (C2 splits B). The example dike, from Touchet River Road, contains 7 sheets formed in 4 widening stages (crack and fill pulses). Growth stages: A = 1 sheet, B = 1 sheet, C = 3 sheets, D = 2 sheets.

Stacked fills between skin walls. I measured the thickness of 20 coherent packages of sediment stacked between vertical skin walls in a few random dikes that were well exposed and easy to reach at road level. Each "stack" constitutes a small portion of the sediment within a single sheet. Stacks are separated from one another by subhorizontal silt skin partitions that form breaks between infill pulses and often correspond with abrupt changes in the bedding angle of stratified fills. Most fills are less than a meter tall, but taller stacks are not uncommon. Some exceeded 2-3 meters at the study site (Touchet River Rd), where large dikes are numerous.

Per descendum. Sheeted dikes cut downward through sandy Missoula Flood deposits at Latah Creek near the Qualchan Golf Course west of Spokane, WA. Small branches mimic the form of the larger dike.

Arris and aperture. The dikes are sediment-filled fractures with 3D shapes resembling an axe blade. The fracture has width (aperture), volume, and a curving, irregular cutting edge (arris). In cross section, the 2D shape will appear differently depending on where along its length it intersects the outcrop (a plane). The example above shows a dike that tapers both vertically and horizontally. Its 2D cross section appears to taper downward in outcrop Plane C, upward in outcrop Plane B, and both upward and downward in outcrop Plane A. The geometry of Touchet-type dikes is consistent with the PKN model for fluid-driven fractures.

Pleistocene dikes in older sandstone. Energetic flooding at The Dalles incised deep gullies into fluvial sandstone of the Chenoweth Fm (Dalles Group) and filled them with gravel. This example funnels down to become a parallel-sided dike that continues for several meters below grade. Flood-deposited boulders are scattered about the undulating bench above the roadcut. Chenoweth Creek Valley, Columbia Gorge, OR.

Truncation and reinjection. Two episodes of injection are preserved in this composite dike. The first cuts Bed A. The second cuts Bed B and Bed A. The younger dike merges with the older, following the path it established. The dike is truncated twice at bedding contacts. This geometry is impossible with upward fluid escape (i.e., liquefaction).

En echelon. Dike is offset by a set of small, bedding-parallel faults (shear), the result of slumping long after the dike formed. R1-R4 are Touchet Bed rhythmites. Last Chance Rd near Lowden, WA.

Polygonal Networks - Burned areas and bladed cutslopes expose polygonal dike networks in plan view (i.e., Silver and Pogue, 2002). Horizontal exposures reveal how dike fills coalesced and how fill bands intertwined as fractures lengthened. Well-developed polygonal networks containing large dikes tend to occur near the centers of low elevation basins inundated repeatedly by glacial outburst floods. Sheet counts are highest where many floods gathered - primarily near Wallula Gap through which all floods flowed.

Silt Skins - Thin silt partitions (silt skin walls) separate vertical sheets of sediment (fills bands) inside the dikes and sills. Silt skins form when pore water migrates out of the saturated fill, through the fracture wall, and into the surrounding material. Interior skin walls form by dewatering of new fills into adjacent fills. Fines are screened at the fracture wall and accumulate in continuous layers (1–10 mm thick filter cake), sealing the fracture. Crack sealing begins almost immediately after the fill enters the fracture and progresses quickly. Skin walls in the dikes are similar to those formed in poured-concrete slurry walls used in heavy construction and building foundations.

Flute Casts - Upward-pointing flute casts ornament the interior faces of skin walls and unambiguously indicate infill from the top. Exterior walls indicate the host sediment was well-drained, ice-free, and above the water table (vadose zone) during infill. Exterior walls formed as pore water moved out of the fill into surrounding host material (leakoff). Dikes that penetrate impermeable bedrock lack outer skin walls, but contain interior partitions.

Halos and Lumpkins - "Leakoff halos" are slightly discolored zones that extend a few centimeters beyond the dike wall. They indicate a slight modification of the host material caused by leakoff. Leakoff halos contain slightly more fines and some are weakly cemented. "Lumpkins" are bulbous forms on the exterior walls of some dikes. They are convexities formed by leakoff.

Rip-ups - Fragments of older fills, skin walls, and host material are a component of most fills. New fractures slightly crosscut older material, shattering it and incorporating the broken bits into the fill.

Stress Orientation During Formation - The dikes are dominantly vertical to nearly vertical structures that cut across bedding at high angles. Sills and horizontal spurs are less numerous and commonly pinchout within a few meters. Dike shape is consistent with a maximum principle stress oriented vertically. Most dikes cut cleanly across bedding contacts without offsetting them. Because the dikes form polygons when viewed from above (i.e., random strike orientations), we can infer both the intermediate and minimum principle stresses (horizontal) were roughly equal during injection (i.e., O2 = O3). Equal horizontal stresses can also be inferred from the twisting of dikes commonly observed in large outcrops where entire dikes are exposed. Dikes change strike along their length due to a lack of a dominant horizontal stress. The dikes fill tensile fractures (Type I fracture mode), but en echelon forms (i.e., evidence of shear) are routinely found. Expression of shear in the dikes, inferred from the arrangement of disconnected segments exposed in cross section, appears to be an opening mode secondary to jointing and might only emerge when fracture propagation comes to a halt, near pinchouts. No tests to determine whether dike spacing (joint spacing) decreased through time with repeated flood-load cycles have been conducted, though field evidence suggests dike numbers and dike widths have increased over time at many locations. However, since younger dikes commonly follow the paths of older ones, it is unclear by what amount spacing may have decreased during the Pleistocene.

Dikes and Faults - Most dikes cut cleanly through host sediments and do not offset bedding contacts. Most exposures examined for this study were not appreciably deformed or faulted, but some were. Faulting does not appear to be a primary control on diking. Normal faults with small offsets in scabland flood deposits were observed at certain locations. Many are listric (slumps) with displacements from a few centimeters to a few meters, formed sometime after most if not all floodbeds were deposited. The dikes both crosscut and intrude along normal faults similar to how they follow older dikes and bedding. Small thrust faults are less common in scabland deposits and having personally observed fewer than half a dozen examples, I am hesitant to opine on how dikes behave near them. Shear deformation in flood deposits is not a dominant mode, yet slip along bedding planes and within beds does occur. En echelon dikes and "stairstepping" dikes offset by numerous small, low-angle faults are occasionally found. Larger mapped structures such asYakima Fold Belt faults that offset Quaternary deposits are not associated with dense networks of clastic dikes. For this study, I surveyed many tens of kilometers of tilted sediments along mapped Quaternary faults, finding far fewer dikes than in undeformed sections located at considerable distances from them. Likewise, subsidiary normal faults associated with the collapsed crests of thrust cored anticlines (i.e., hanging wall grabens) are not associated with large or numerous dikes. In general, the number, size, and orientation of dikes does not appear related to bedrock controls or tectonic stresses except in a few specific locations.

Fluted walls. Flute casts with upward-pointing noses decorate the interior silt walls of Touchet-type clastic dikes. Flutes are directional indicators. Sediment entered from the top. Identical fluting was observed in thousands of dikes throughout the study area. Quarter for scale.

Diking and leakoff. A fluid-driven fracture (hydraulic fracture) propagates by the rapid advance of the crack tip through the host material. Tip advance is jumpy and dike growth incremental. In fracked wells, fluid pressure is created at the wellbore. In the dikes, fluid pressure is created at the ground surface during overland megaflooding. In both, fracture opening is accompanied by simultaneous infilling by sediment slurry (proppant slurry) and leakoff beings immediately. Repeated cracking, filling, and dewatering creates the conspicuous vertically sheeted fills clastic in Touchet-type dikes. Figure modified from Phillips and others (2013).

Leak-off halo. Fining and cementation is apparent just beyond the margin of some dikes. This halo is evidence of leak-off, the diffusion of pore water and fines out of the fill during injection and for a short time after. While fines migrate during diking, cementation is diagenetic and occurs later. Hwy 24 near crest of Yakima Ridge.

Leak-off lumpkins. The surface of a dike's outer silt skin wall is often covered with bulbous structures formed as the fill dewaters. They can look very much like tiny load casts. "Lumpkins" are consistent with outward diffusion of pore water from the wet fill into the drier surrounding sediment. Walla Walla Valley.

Lumpkins aplenty. Leak-off creates bulbous forms on the outer walls of some dikes. Hwy 397 west of Finley, WA.

Lumpkin-o-rama. Bulbous forms on thin silt walls are easily damaged despite a soft brush. White Bluffs, WA.

Distribution - Sheeted dikes are not isolated features; they are widely distributed throughout the region inundated by Ice Age floods (>30,000 km2). Great distances separate outcrops containing dikes with identical characteristics. For example, sites near Kettle Falls, WA and Salem, OR are separated by more than 500 km. The dikes do not occur above the local elevation of maximum flooding (~366m in south-central Washington and higher to the north), nor in unconsolidated sediments beyond the margins of Ice Age floodways. No dikes are known in Palouse Loess outside of flood coulees. Identical dikes occur in close proximity to mapped Quaternary faults (i.e., Wallula fault zone) and at >150 km distances from them. They are most abundant, well-formed, and best-exposed in thick sections of silty flood rhythmites in backwater valleys.

Age - Field relationships constrain the timing of dike injection to between ~1.8 Ma to ~14 ka, the period of ice sheet growth and scabland flooding in Eastern Washington (Easterbrook, 1994; Baker and others, 2016; Waitt and others, 2016). Most dikes formed late, during Late Wisconsin Missoula flooding (18–14 ka). A few cemented dikes are found in ancient flood deposits. The dikes penetrate all formations exposed to flooding, including pre-Wisconsin scabland deposits (“ancient” flood gravels, silt diamicts, paleosols), calcrete-capped Pliocene-Pleistocene alluvium ("Cold Creek unit"), Pliocene Ringold Fm sediments, Pliocene Chenoweth Fm sediments, Miocene-Pliocene Ellensburg/Latah Fm sediments, and Miocene Columbia River Basalt flows. Rip-ups and clasts from these formations are routinely found in sediments that host the dikes. The dikes cut the Mount St. Helens Set S tephra (16 ka), but not the Mazama Ash (6.8 ka). No dikes were found in Holocene deposits during this study and I am aware of no other studies reporting sheeted dikes in Holocene alluvium east of the Cascade divide. The preserved record indicates sheeted dikes did not form in significant numbers prior to the Pleistocene or since.

Slender, sheeted dikes are Touchet-type dikes. Missoula flood rhythmites (R1 through R7), each deposited by a separate flood, are intruded by a Touchet-type clastic dike. The dike originates at the base of R7 and descends through several underlying beds. It does not crosscut the entire the section, rather it formed midway through the Missoula flood period (18-14 ka). Other dikes descend from older and younger beds in the stack. The dike cuts a clean path through the host sediment and does not follow a rubbly zone between toppled blocks. Bedding contacts are not offset or tilted by the dike. No low angle sliding surface is present in the outcrop, which is floored in basalt bedrock. Both branches taper downward to a point. The sediment that fills the dike was not supplied by a liquefied layer at depth (a bed somewhere below R1); it does not feed a sand blow (i.e., Obermeier, 1998). The source of the dike is clear. It begins at the base of R7, widening from a small sag. Bedding in R7 grades smoothly into the top of the dike. Flute casts on the interior faces of the dike's walls provide clear evidence of downward infilling. The dike exhibits the characteristics typical of sheeted Touchet-type dikes found throughout the floodway region. Burlingame Canyon in Walla Walla Valley near Gardena, WA.

Lessons from Warden. I discovered this excellent exposure of ~10 Touchet Bed rhythmites a few years ago near Warden, WA. It contains many features and important relationships in one place not commonly seen in more popular exposures near Walla Walla or TriCities. Sags, load casts, contorted bedding, and small wedge-shaped dikes occur at nearly every bedding contact. A large dike is truncated by a prominent erosional surface with wetland soil features above (longer hiatus). A conspicuous 2"-thick gray layer just above the truncation surface is probably reworked volcanic ash; I've found it in other outcrops nearby. If you interpret the dikes as seismites, then you must ignore the obvious relationship between the dikes, other loading features, and bedding contacts (floodwater loading events). Deformed zones near the top of the exposure are ancient wetland deposits that were overridden by an energetic flood carrying gravel.

Injection during flooding. My conceptual model for sheeted clastic dikes in the megaflood region developed from relationships observed in the field. Downward injection occurred only during Pleistocene overland flood events and associated periods when slackwater lakes were present. Flood loads fractured the relatively dry, brittle substrate allowing sediment circulating at the base of the flood (or soupy lake bottom) to immediately fill the fractures, forming dikes.

Flood injectites vs. sand blows. (A) The sketch illustrates differences between clastic dikes formed by liquefaction (sand blows and fluid escape structures) and those formed by floodwater loading and hydrofracture (flood injectites). Liquefaction dikes propagate upward and are sourced in wet, sandy beds deposited sometime in the past and remobilized by strong shaking. Liquefaction often produces feeder dikes that vent to the surface as sand blows (volcanic edifices of sand). Flood injectites are sediment-filled filled hydrofractures that propagate downward from the surface. The fractures are immediately filled with sediment sourced in circulating bottom currents of glacial floods. Liquefaction dikes in A cut younger strata and are filled with older sediment. Injection dikes in B cut older strata and are filled with younger sediment. (B) My dike-fill generations concept sketch explains the formation of sheeted clastic dikes in aggrading flood sediments (each bed = one flood). The four geometries represent the range of forms found in the study area: a). Unsheeted - Single-fill, b). Sheeted - Multi-fill Compound, c). Sheeted - Single-fill Composite, d). Sheeted - Multi-fill Composite. Compound = Multiple fill bands injected during a single event. Composite = Multiple fill bands injected during two or more events separated in time.

Dike abundance, shape, and grainsize. A correlation exists between sediment porosity (millidarcy, mD), permeability (percent, %), and dike shape. (A) The tightness of the host material, a function of grainsize, determines whether pore fluid pressures will build or disperse, and whether slender or stubby dikes will form. Silty mixtures are tight and tend to respond to stress by fracturing (pore fluids move in fractures), while sandy-gravelly mixtures respond by matrix flow (pore fluids flush through interconnected pores). (B) Diking can occur in all phases of flooding, but dike fills are sourced at or very near the surface. Coarser fills correspond with the initial flood rush (coarse sand and gravel dikes), while finer fills correspond with the slackwater phase (silt-sand dikes). If a flood carries only finer material (no gravel), then all dikes will contain silty fills (i.e., northern Walla Valley, Skyrocket Hills, and loess islands in the Marengo-Benge area). Dikes with coarse fills were triggered by floods. Dikes with fine fills were triggered by lake loads.Dikes with coarse fills were triggered by floods. Dikes with fine fills were triggered by lake loads. (C) Numerous slender, sheeted dikes correspond with high silt content source beds, slackwater settings, prolonged loading (deep lake), and better preservation (protected valley settings). Sparse, crudely-sheeted, stubby dikes form in coarse sand, laminated sand, and gravelly bar deposits that lack silt.

Vadose sandwich? Thick piles of unconsolidated sediment (rhythmites) raised the ground surface, but not the water table. A dry vadose zone sandwiched and sealed between the base of the flood and the water table may have created a wet-over-dry-over-wet situation. Brittle fractures may have initiated at the top of the dry interval (ground surface), which might have been frost-hardened. Mere speculation at this point. Do hydraulic fractures require brittle sediment or is wet just the same? Absent an obvious mud seal or other impermeable cap, the mind stretches for alternative ways to confine fluid pressure and invert the pressure gradient. It happened, but its difficult to explain how.

Discrete deformation. Dikes fill tensile, Type I fractures. Shear is rarely involved. Deformation associated with diking does not extend beyond the dike wall. Material surrounding the dikes is extended slightly to accommodate the fill, but otherwise tends to remain undeformed. Clear bedding contacts and delicate bedforms in the host sediments continue across each dike in the photo. Dikes in the photo are filled with gray Touchet Bed sediment and intrude oxidized, quartzite-bearing, fluvial sandstones of the Miocene Ellensburg Fm. Snipes Mountain near Granger, WA.

Clean crosscuts. A sand-filled dike cuts cleanly across silt-sand rhythmites at Starbuck, WA.

Younger dikes in older deposits. A sheeted dike descends from Touchet Beds through a thick stack of cemented loess, calcrete, and weathered fanglomerate near Finley, WA.

Pleistocene dikes in Miocene basalt. Sheeted sand-silt dikes, sourced from above, intrude Columbia River Basalt where exposed to Ice Age floods. The dikes exploit joints in the bedrock. A.) Weaver Pit near Gardena, Walla Walla Valley, B.) Hwy 12 near Alpowa Creek, Lewiston Basin, C.) Hwy 14 near Alderdale, Umatilla Basin.

Gravel infill from above. Unconsolidated flood gravel fills a clastic dike in older tuffaceous sandstone of the Chenoweth Fm at The Dalles, OR.

Pleistocene dikes in Ellensburg sandstone. Silt-sand dike cuts a crossbedded fluvial sandstone of the Ellensburg Fm at West Foster Creek near Bridgeport, WA.

Sheeted fills in fractured basalt. Sheeted sand dike fed from above cuts Columbia River Basalt at Prosser, WA. An old fracture in the rock appears to have been widened further by incremental injections of pressurized sediment.

Early to Middle Pleistocene dikes cut Pliocene Ringold Fm. Downward-tapering, partially-cemented dikes are truncated by ancient flood deposits above Ringold Road at White Bluffs, WA.

Gray dike in red fan gravel. Gray dike sourced in unconsolidated flood-laid sediment cuts older, reddened fanglomerate shed from the north flank of the Saddle Mountain anticline. Smyrna Bench, WA.

Liquefaction in Eastern Washington

Liquefaction features are rare in Pleistocene flood deposits, Ellensburg Fm, Ringold Fm, and Holocene alluvium. In the few instances where liquefaction in Pleistocene sediments has been reported, the features are small, irregular bodies of debatable origin. Two examples from the literature include,

Foundation Sciences (1980) reported possible liquefaction features at Finley Quarry near Pasco, WA (Wallula Fault Zone). The blobby forms are small, unconnected to a source bed, and partially trend with bedding. A USGS geologist confirmed the features were evidence of shaking-induced liquefaction (Sherrod and others, 2016). A seismic origin is, however, disputed by Coppersmith and others (2014) and me.
USGS reported liquefaction features in Holocene loess in a trench opened near Wallula Gap ("SUK" trench of Angster and others, 2020). The surface lineament they targeted for trenching has since been revealed to be an old ranch road, not a fault scarp. Remnants of old pavement are clear in trench walls. No fault was discovered in the SUK trench and Touchet Beds beneath the "liquefied" loess remain undeformed. Read my critique of the SUK trench work HERE.

At the broader scale, liquefaction has not been identified in any trenched fault in Eastern Washington.

Ahtanum Ridge-Burbank trench near Yakima, WA (Bennett and others, 2016).
Toppenish Ridge trench (Repasky and others, 1998; Campbell and others, 1995; Campbell and Bentley, 1981).
Wenas Valley trench (Sherrod and others, 2013).
Saddle Mountains trenches at Smyrna Bench (Bingham and others, 1970 Plates 4,5,6).
Buroker trench southeast of Walla Walla (Farooqui and Thoms, 1980).
Lind Coulee trenches at O'Sullivan Dam-lower Lind Coulee (GEI/West & Shaffer, 1988).
Gable Mountain trenches at the Hanford Site (Bingham and others, 1970; Golder Associates/PSPL, 1982).
Spencer Canyon trench near Entiat, WA. The trenched scarp is believed to have formed during the magnitude 7 1872 Chelan quake (Sherrod and others, 2015).
Wallula-SUK Trench (Angster and others, 2020).
Kittitas Valley trench (Huddleston, 2022; Dr. Walter Szeliga, personal and written communications, 2023).
Gate Creek trench near Mt. Hood (Bennett and others, 2021; Madin and others, 2021).

Paleoseismic trenches in the Yakima Fold Belt. Trench locations across surface scarps and mapped Quaternary faults. The Spencer Canyon site is located ~60km north of the Kittitas Valley site. Paleoseismic trenching has revealed no connection between sheeted clastic dikes and movement of Yakima Fold Belt faults. Basemap by Lidke and others (2003). Box area on map is ~44,000 km2 (17,000 square miles).

Liquefied sediment. Deformation pictured here occurred during a Missoula flood, that is, during sedimentation. The swirls and flames are not products of sediments first laid down flat, then remobilized sometime later by seismic shaking. All the clues are here to interpret the history correctly. White Bluffs, WA.

Deformation during deposition. Flame structures in the light-colored mud formed during a flood, which deposited a thick bed of gray sand. A dense, sand-choked current moved left to right over the soupy, unconsolidated bed, dragged some of the bottom sediment upward and into the flow. The silty-clayey sediment holds together, forming spectacular flames. Very common near TriCities. No earthquake required.

T-shaped mudsquirts. Rapid deposition of a sand bed on top of soupy lake bottom mud with consolidated varved beds below. The deformation shown here was triggered by rapid deposition and loading, not by shaking. The gray sand was dumped by a Missoula flood, temporarily disrupting quiet water deposition in Glacial Lake Columbia. West shore of Sanpoil Valley, WA.

Flood and repeat. Three flood rhythmites exhibiting identical bedform progression. Syn-depositional deformation. Ringold Rd, White Bluffs, WA.

In Depth: Gable Mountain Seismic Trenches The Gable Mountain trenches at the Hanford Site are instructive. Golder Associates/Puget Sound Power and Light (Bingham and others, 1970 Plates 8, 9; Golder Associates/PSPL, 1982) opened several trenches across two thrust faults. The South Fault displaced Miocene basalt and the Rattlesnake Ridge sedimentary interbed that separates Pomona and Elephant Mountain flows ~50'. The overlying Hanford Fm sediments (Missoula flood deposits) were not displaced. The Central Fault displaced the Rattlesnake interbed by 182' and the Hanford Fm by only 0.2' (Reidel and others, 1992, p. 43-44). No liquefaction was found in any trench. A single clastic dike, sourced from above and filled with flood-laid gravel, descends into brecciated Elephant Mountain basalt (Trench log GT-2 in Reidel and others, 1992, Figure 39, p. 45). No other dikes were noted in the 92'-long trench. Philip S. Justus, a geologist for the Nuclear Regulatory Commission, clearly linked the dikes to flooding, not faulting, in a report to management outlined below (Justus, 1980),

Flood deposits on Gable Mountain bear a close resemblance to typical Missoula flood deposits.
Two distinct cycles of [Ice Age flood] deposition are present on the north side of Gable Mtn; possibly three on south side.
Clastic dikes on Gable Mountain are similar in lithology and fabric to those found elsewhere in the Pasco Basin.
Clastic dikes associated with each overlying [flood] cycle are found in Trenches CD-8, G-2, and G-3.
The youngest clastic dikes originate from the base of the coarse upper unit of flood deposits which is bounded at the top by St. Helens S ash as found in Trenches CD-4 and G-1.
Clastic dikes in Trenches CD-5 and G-3 are displaced by shearing on the fault plane (CD-5) and in the hanging wall (G-3).
In Trench G-3 displacements in the flood deposits appear to post date the youngest clastic dike.
Shears, possibly associated with the thrust fault, appear to cross and slightly displace clastic dikes in the footwall in an area of Trench CD-6.
Clastic dikes along fault plane in Trench CD-6 have slickensides surfaces with strikes parallel to the dip of the fault.
Oriented slickensides in clastic dikes parallel to slickensides in gouge on fault breccia (Trench CD-5).
Wherever fine-grained material is present along fault plane, slickensides are present.

Key takeaways from the Gable Mountain trenches are a.) clastic dikes sourced in unconsolidated surficial material (flood deposits) that intrude the faulted basalt below are typical Touchet-type dikes, b.) dikes are not large or numerous along the Gable Mountain Fault scarp, c.) liquefaction was not observed in the trenches, and d.) crosscutting relationships, slickensides, and offset dikes suggest most of the faulting occurred prior to Late Wisconsin flooding of the Hanford Plain and only a small amount of movement on Gable Mountain faults post-dates flooding and diking ~12 ka.

In Depth: Lind Coulee Fault Seismic Trenches

Paleoseismic trenches opened across the Lind Coulee Fault by Michael West/GEI exposed a few clastic dikes in sheared basalt. The Lind Coulee Fault is an eastern extension of the Frenchman Hills thrust. The fault, which places Roza basalt over Pleistocene loess, is well-exposed in shoreline bluffs along the south shore of O'Sullivan reservoir west of the Rd M SE bridge.

Lind Coulee West Trench Site. Trenches at Lind Coulee were opened in the 1980s as part of a seismic safety study of O'Sullivan Dam. The U.S. Bureau of Reclamation operates the dam which spans the head of Drumheller Channels near MarDon Resort. The dam impounds Potholes Lake. Lower Crab Creek, empties from the west through Lind Coulee. Hwy 262 crosses the dam.

Findings in the Lind Coulee West Trench are similar to those at Gable Mountain, but took the team of investigators more time and a considerable amount of back and forth to arrive at a correct interpretation (Grolier and Bingham, 1971, 1978; Galster/USBOR memo, 1987, "Area No. 2"; Levfevre and MCConnell memo, 1987; West and Shaffer, 1988; Shaffer and West, 1989; Reidel and Campbell, 1989, "Stop 21-A", Figure 14; Geomatrix Consultants Inc., 1990, "East Fault Exposure"; Reidel and Fecht, 1994; Schuster and others, 1997; Lidke and Haller, 2016).

Pleistocene dikes intruding fault gouge initially got investigators excited,

The Lind Coulee trench initially presented strong circumstantial evidence for fault displacement of [basalt, a very old reverse-magnetized loess, and a younger less cemented loess]. The evidence for displacement was magnified by the protruding knob of brecciated basalt [seven meters from west end of NE-trending trench], the apparent overturned contact with flood deposlts on the north side of the knob, flood sands injected along a shear plane in the fault zone and discontinuity of the petrocalcic horizon and infiltration of loess north of Station 7 [near west end of trench]. The geometry of flood deposits overlying the paleosol on the footwall block was also suggestive of colluvial wedge geometry.

Upon further investigation, West and Shaffer modified their interpretation,

In spite of this body of circumstantial evidence, we could find no evidence of shearing, tectonic displacement or colluviation characteristic of surface fault rupture. The [flood-deposited] sands along the shear plane appear to have been injected hydraulically along the plane rather than dragged along it.

They ultimately conclude dike injection post-dated faulting,

The lacustrine silt [that separates “intermediate” flood deposits from “youngest” flood deposits] could be traced as a continuous, uninterrupted horizon across the main fault zone, indicating with 100% certainty that the fault had not moved since deposition of the silt layer. Careful excavation of Unit 5B [“intermediate”] flood deposits disclosed no evidence of shearing or tectonic colluviation. These deposits were in intimate contact with the eroded basalt surface on the hanging wall and exhibited an open-work fabric that we attribute to high energy flood deposition.

Busacca and McDonald (Appendix V) conclude the flood deposits exposed in the trench are not related to the most recent episodes of flooding (about 12 to 16 Ka) but are older…based on soil development and stratigraphic position that the age of flood deposits in the Lind Coulee West area is 40 to 50 Ka. The last surface fault displacement, therefore, occurred before 40 to 50 Ka.

The apparent injection of flood sands along a shear plane in the fault zone is more difficult to explain. We are of the opinion that the sand was injected hydraulically from the top down. The sands filling the shear however do not appear to be continuous with flood deposits mapped as Unit 5 [“intermediate” flood deposits].

Similar injection of flood sands along shear planes was noted in fault trenches excavated on Gable Mountain (DOE/Westinghouse, 1987b).

The authors invoke interpretations by Woodward-Clyde Consultants (1981), namely the few dikes formed by,

…either hydraulic injection associated with catastrophic flooding or hydraulic injection resulting from fault movement and liquefaction offer reasonable interpretations for the origin of clastic dikes including the feature in the Lind Coulee West trench.

They dutifully entertain an alternative origin for the dikes (liquefaction), though no evidence was found,

Another possibility is that the sands were injected from below and are part of an older flood deposit preserved deeper in the footwall. The exposures in both cross-cut trenches suggest older flood deposits are indeed involved in faulting and could be preserved at depth in the footwall and locally along shear planes…

Lind Coulee Fault at O'Sullivan Reservoir. The Lind Coulee Fault is a south-dipping thrust that places Miocene basalt (Wanapum Roza flow) over younger sediments. There are several splays. Grolier and Bingham first identified the fault (Grolier and Bingham, 1971; 1978 Figures 14, 23). West and Shaffer trenched it in the 1980s. Easily accessible exposures remain. The photo and sketches above show Roza basalt shoved over alluvial Ringold Fm sediments and at 2-3 generations of loess. The fault shatters the Roza basalt. A thin white gouge zone is observable in places. Gouge is 10-20cm wide and associated with boudin-like lenses of deformed dark and light brown mudstone, rock flour, or broken basalt. Beneath the gouge is a sliver of brown mudstone (hanging wall) and cemented loess. Faint bedding in the loess confirms its vertical to overturned tilt beneath portions of the fault. The shattered footwall Roza is weathered above the fault and takes on a greenish-yellow hue. The rubbly zone grades upward to competent basalt then to spheroidally weathered basalt at the flow top. The boulder-sized spheroidal forms are also exposed along nearby Hwy 262. My own investigations of the Lind Coulee Fault Trench site and all nearby bluffs have yielded no evidence of liquefaction. Lind Coulee Fault is part of the larger Frenchman Hills thrust, known to have Quaternary movement (Reidel and Fecht, 1994; Schuster and others, 1997; Lidke and Haller, 2016). USBOR memos recount some details of West's trenching work from the perspective of the client (Lefevre and O'Connell, 1987; Galster, 1987). Much of the work on the Lind Coulee Fault was done pre-Internet, which makes written reports difficult to find. Relevant articles include Grolier and Bingham (1971, 1978), Galster/USBOR memo (1987, "Area No. 2"); Levfevre and MCConnell memo (1987), West and Shaffer (1988), Shaffer and West (1989), Reidel and Campbell (1989, "Stop 21-A", Figure 14), Geomatrix Consultants Inc. (1990, "East Fault Exposure"), Reidel and Fecht (1994), Schuster and others (1997), Lidke and Haller (2016). A big thanks to Brian Sherrod for sending me a scanned version of West and Shaffer (1988, Vol. 2).

In Depth: Willamette Valley, Lower Columbia Gorge, and Coastal Areas

The Missoula floods backflooded the Willamette Valley dozens of times, depositing gravels and silt-sand rhythmites across the valley floor from Portland to Eugene (170 km south of the Columbia River). Hundreds of ice-rafted erratics are mapped throughout the Willamette Valley (Allison, 1935; Minervini and others, 2003) and nearby Columbia Gorge (Bretz, 1919). In general, Pleistocene deposits in the valley contain few dikes, though the exposure is limited due to dense development and vegetation.

PhD student Jerry L. Glenn (1965) found a few sheeted dikes in Missoula flood rhythmites (Willamette Silt) at his River Bend and Irish Bend sites near Corvallis. An Ira Allison photo shows a clastic dike cutting rhythmites near St. Paul, OR (1978, Figure 14). Dikes were exposed in highway excavations at Portland (Ian Madin, 2014 written communication and photos) and in dirt walls in the basement of the Capital Building at Salem during seismic refitting (Ray Wells, written communication and photos).

Thurber and Obermeier (1996) reported finding 16 clastic dikes at 7 sites along the lower Calapooia River, a tributary to the Willamette River. The largest features measured 5 m long x 10 cm wide. They attributed the dikes to liquefaction triggered by a Holocene earthquake. The Calapooia report contains no photos of dike fills or relationships between the dikes and the sediments they intrude, which makes independent evaluation difficult. Consultant John Sims (2002) reviewed Thurber and Obermeier's report, finding their data set too small to support the interpretation,

The limited area surveyed by [Thurber and Obermeier] in the Willamette Valley does not allow for a high level of confidence in determining if the features result from large subduction events or local intracrustal events. The age of the structures is somewhat in doubt as few radiocarbon dates are available for the host deposits and Thurber and Obermeier (1996) do not report any radiocarbon dates as part of their study. They also do not mention any evidence for liquefaction in post Pleistocene deposits of which there are many in the banks of the Willamette River and its tributaries. Thus, with incomplete coverage and lack of dating of paleoliquefaction features, the question of source zones is moot. Earthquake source determination can only be addressed with broader coverage of liquefaction features and better age data to constrain timing of events and to allow regional correlations of liquefaction features. In addition, we need a more complete picture of the size distribution of similar-aged features for the purposes of evaluating the magnitudes of prehistoric earthquakes.

River Bend section. Glenn (1965, Figures 3 and 15) reported finding a few clastic dikes in Touchet Bed-equivalent flood rhythmites (Willamette Silt) along the Willamette River. Note the lack of deformed bedding. Outcrop photos taken prior to dense urban and residential development of the Portland area as well as serviceable unit descriptions are found in Bretz (1925, 1928), Allison (1932, 1933, 1936, 1953, 1978), Piper (1942), Treasher (1942), Lowry and Baldwin (1952), Baldwin and others (1955), Allison and Felts (1956), Wells and Peck (1961), Trimble (1957, 1963), Balster and Parsons (1969), Hampton, (1972), Robert (1984), McDowell (1991), Yeats and others (1996), and McDowell and Roberts (1987).

Obermeier and Dickenson (2000) describe "relict liquefaction features" in low shoreline bluffs of sandy islands in the Columbia River between Astoria, OR (Marsh Island) and Kalama, WA (Bonneville Dam) and in cutbanks of 10 rivers east of the Cascade divide (Hood River). The thickest dikes measured 30 cm. The thickest sills 5 cm. The authors attributed the dikes to lateral spreading, hydraulic fracturing, and surface oscillations (ground shattering and warping) triggered by earthquakes. Similar investigations by USGS and DOGAMI were conducted in the Columbia gorge (Obermeier, 1993; Peterson and Madin, 1997; Atwater, 1994) and contain some of the same information as earlier reports.

Atwater (1994) and Takada and Atwater (2004 + Appendix A Supplement) describe sandy riverbank sediments in the lower Columbia River gorge deformed by the 1700 AD Cascadia earthquake. Their Holocene dikes of fluidized sand were generally narrow and outnumbered by sills that "mostly follow and locally invade the undersides of mud beds. The mud beds probably impeded diffuse upward flow of water expelled from liquefied sand. Trapped beneath mud beds, this water flowed laterally, destroyed bedding by entraining (fluidizing) sand, and locally scoured the overlying mud."

Peterson and Madin (1997) and Peterson and others (2014) describe sand dikes and sills (unsheeted fluid escape structures) intruding Holocene overbank muds at sites along the lower Willamette River and coastal areas of Washington and Oregon. They interpreted the dikes as features triggered by seismicity at the Cascadia margin, possibly the 1700 AD event. A field guide was prepared for the Friends of the Pleistocene (Peterson and others, 1993).

The dikes and sills described by Atwater (1994), Thurber and Obermeier (1996), Obermeier and Dickenson (2000), Sims (2002), Takada and Atwater (2004), and Peterson and others (2014) are liquefaction/fluidization features not wedge-shaped, sheeted injection features.

Holocene liquefaction dikes in the Columbia gorge. Caption for Figure 13b in Atwater (1994) reads, "Dikes with raised edges at upper Wallace Island [near Longview, WA]...The dikes transect mud beds that extend parallel to shoreline." This is the same dike pictured in Peterson and Madin (1997, Fig. 11b).

Coastal liquefaction. Caption for Figure 2 in Peterson and Madin (1997) reads, "Drawing of subsurface fluidization features including clastic dikes and sills and flames. Internal structures include intruded contacts with host deposit and disoriented mud blocks in sandy matrix. Fluidization features such as clastic sills are often enhanced under thin capping deposits of mud overlying thick source beds of sand."

In Depth: Toppenish Ridge

Two large gravel pits at Toppenish Ridge near Granger, WA expose conglomerates of the Miocene Ellensburg Fm. The active Toppenish Ridge Fault is mapped between the two pits. At the Lower Pit (225-250 m elevation) several large, sheeted dikes sourced in Touchet Beds cut downward through flat-lying conglomerate with sandy lenses. The Touchet Beds are inset into the older sediment. At the Upper Pit (265-295 m elevation), located <200m from the fault, the conglomerate dips steeply south (>50 deg). Very few dikes were found in the tilted beds.

Trenching elsewhere along Toppenish Ridge was done in the 1980s by Newell Campbell and Ted Repasky, but I've not been able to obtain copies of the following reports.

Campbell, N.P., Ring, T., Repasky, T., 1995, Final report, 1994 NEHRP grant earthquake hazard study of the vicinity of Toppenish Basin, south-central Washington: Technical report to USGS (Contract 1434-94-G-249), 9 pgs.

Repasky, T.R., Campbell, N.P., Busacca, A.J., 1998, Earthquake hazard study in the vicinity of Toppenish Basin, south-central Washington: Technical report to USGS (Contract 1434-HQ-97-GR-03013), 27 pgs., 7 plates

Toppenish Ridge exposures near Granger, WA. In the Yakima Valley, Ellensburg sediments, deposited by the ancestral Columbia River and smaller streams draining the Cascades, interfinger with and overlie the Columbia River Basalts. Green areas of the map are Miocene Ellensburg Fm sediments. Light brown areas are basalt flows. Gray areas are late Pleistocene Touchet Beds. Yellow is recent alluvium of the Yakima River. Sheeted dikes and Touchet Beds occur in the Lower Pit (minimally deformed strata), but not the Upper Pit (steeply tilted strata). Landowner is the Yakama Indian Reservation.

Upper Pit at Toppenish Ridge. Steeply-dipping, partially lithified fluvial sediments contain almost no clastic dikes and no liquefaction features despite abundant sand lenses. Sheeted dikes are present in a thin section of slumped or tilted(?) Touchet rhythmites at the top of the exposure (~295m elevation). The Toppenish Ridge Fault, an active structure, is mapped less than 200 m away (Schuster and others, 1994; Lidke and others, 2003; online USGS Quaternary Fold and Fault Database for the United States).

Lower Pit at Toppenish Ridge. Touchet Beds are the unambiguous source for dikes off Tule Rd.

Lower Pit at Toppenish Ridge. Flat-lying Ellensburg Fm sediments are cut by a number of sheeted dikes sourced in the overlying Touchet Beds. Diking at this lower elevation site appears related to flooding, not faulting or seismicity. The dikes do not rise from a liquefied source bed. They post-date deposition of the Ellensburg and most, if not all, of the tilting.

Pliocene-age Dikes in the Ringold Formation

Sheeted clastic dikes are rare in Miocene and Pliocene sediments, even where deformed. The Ringold Fm, mostly floodplain alluvium and alluvial fan deposits, does, however, contain a few unsheeted clastic dikes.

A sparse set of thin, short, mud-filled dikes is found in certain fine-grained beds in the Ringold Fm (9.5-3.4 Ma) at Smyrna Bench, White Bluffs, and Othello. The vertical dikes are rarely thicker than a notebook or extend for more than meter. They are sourced and contained within the upper Ringold, often a hard white claystone bed and a gray, tuffaceous sand that thickens toward the Columbia River. These dikes do not appear in many exposures and have not been studied in detail (Fecht and others, 1999).

I interpret this set of dikes as incidental features commonly found in sandy sedimentary deposits near fault zones worldwide. They do not appear to form polygonal networks or feed sand blows. Their fills are unsheeted and otherwise bear little resemblance to Touchet-type dikes.

Small dikes in Ringold sediments at Saddle Mountains. A few small white dikes cut an oxidized alluvial fan gravel atop Elephant Mountain basalt. Dikes in this same layer occur at Smyrna Bench.

Small dikes in the Ringold Fm. Small, single-fill dikes cut a white claystone at Othello Canal. I examine these dikes in a YouTube video Pliocene Clastic Dikes at Othello Canal.

Columbia River Road. Mineralized dikes cut tens of meters through Ringold sediments at White Bluffs. White mineral is calcite and gypsum from irrigation water.

Small dikes in eastern Yakima Valley. A thin clastic dike, truncated at its top, cuts fluvial-lacustrine strata at Houghton Rd north of Sunnyside, WA. These quiet water sediments closely resemble those in pits off Emerald Rd at Snipes Mountain (mapped as Miocene Ellensburg or undifferentiated Miocene) and at many locations in Pasco Basin (mapped as Pliocene Ringold). The contact between Ellensburg and Ringold is not well defined in western Pasco Basin.

Do Clastic Dikes Indicate Paleoseismicity?

Clastic dikes are commonly observed in earthquake-prone regions of the world and highlighted in post-quake damage assessments. Methods for measuring and mapping liquefaction dikes and related features have been developed by USGS, state geological surveys, and consultants to some extent (Gohn and others, 1984; Atwater, 1994; Obermeier, 1996, 2009; Peterson and Madin, 1998; McCalpin, 2009; Holtzer and others, 2011). Maps of dike width can help define the spatial extent of deformation caused by shaking. The value of such maps largely depends on the size of the dataset. A small number of measurements or measurements collected within a small area (i.e., a trench) have low value because they lack statistical power and may not reflect the actual pattern of damage. Borradaile (1984) highlights inappropriate analyses of dike measurement data and other mistakes that can mislead policy makers.

Misinterpretation of deformation features, dike taper direction, and other field relationships can also be a problem, especially for inexperienced staff or where exposure is poor. In the absence of abundant outcrops or unfamiliar geology, investigators should be especially aware of their biases. The assumption that all clastic dikes form by liquefaction and are triggered by earthquakes has led some to incorrectly interpret features formed by aseismic processes as seismites.

Seismic hazard in Columbia Basin vs. New Madrid. Earthquake hazard probability based on the 2018 USGS model (fault-slip rates, frequency, magnitude). Red-orange indicates high probability for damaging quakes. Green-blue indicates low probability. Note the stark difference between the Columbia Basin (green-yellow) the New Madrid Fault Zone (dark red-red-orange). Dikes in the Columbia Basin are wedge-shaped and filled from above. Dikes in the New Madrid Seismic Zone are feeder conduits to sand blows. Columbia Basin dikes occur entirely within the Ice Age floodway. New Madrid dikes occur in floodplain deposits of the lower Mississippi River and valleys of large tributaries.

Modest historical shaking east of the Cascades. Map of earthquake epicenters recorded between 1970-2015 (Brocher and others, 2017, Fig. 2). East of the Cascade divide, quakes are mostly small magnitude, shallow, and locally clustered (i.e., Entiat and certain Yakima Fold Belt faults). Plenty of epicenter dots fall nowhere near mapped faults and belong to no clusters (Gomberg and others, 2012). The distribution of dense networks of clastic dikes shows little spatial overlap with locations of recent epicenters. YFTB = Yakima Fold Thrust Belt, GRZ = Goat Rocks zone, SHZ = St. Helens zone, UL = Umtanum lineation, WRZ = Western Rainier zone. Quaternary faults are heavy gray lines.

Double Bluff on Whidbey Island, WA. An example from Puget Sound for comparison. Deformed clay-rich units (orange-tan) are intebedded with thick sands (brown-gray). Though the grainsizes are not too different from many exposures in the megaflood region, the setting and the deformation features are. This is an estuarine outwash plain and the features are t-shaped mud squrts, not sheeted, wedge-shaped dikes (or sand blows). Depositional hiatuses and low angle unconformities are seen here and there in the stack; its not one big sediment dump. There is rhythmicity here to; sand-clay-sand-clay. The entire section is deformed, though deformation changes from top to bottom and appears partitioned according to the grainsize and strength characteristics of the different layers. Dike-like flame structures rise from each clay-rich bed and intrude the overlying sand. Each band of clay flames scales with the thickness of its source bed. Bedding in the formerly flat-lying sands now swirls sympathetically with the margins of the clay flames and mud squirts. All of the layers here were remobilized and deformed long after they were deposited. According to local geologists, the deformation was caused by mass wasting triggered by seismic shaking. We can see the effects of mass wasting and loading, but have to infer a seismic trigger; no age data links a specific quake to this deformation. Curious to know where the locals believe the shoreline bluff was during that shaking event? Hasn't this erodible cliff face retreated a considerable distance since glacial times? To my eye, the deformation occurred in buried strata, hundreds of meters back from the steep escarpment that today is being eroded by waves and tides. Was there even a bluff here at that time? Where was sea level? If no bluff, then is mass wasting relevant? What evidence places this vertical cut and its suite of deformation features at the water's edge during glacial times? Is the deformation attributable to a single event? Why couldn't rapidly-deposited slugs of sand from a few subglacial floods spilled into a muddy trough have produced everything we see here?

In 2017, an international conference was convened to coordinate proper reporting on seismites in sedimentary sequences. The conference emphasized the need for caution (Feng, 2017). It seems “seismite” (Seilacher, 1969; Montenat and others, 2007; Van Loon, 2014) has for some time been assigned too liberally to features of nonseismic or ambiguous origin, making reexamination of "classic" seismite localities necessary. Clear-eyed geoscientists who participated reattributed many features formerly identified as seismites to nonseismic processes, most commonly to rapid sedimentation and loading (Moretti and Van Loon, 2014; Shanmugam, 2016 and references therein). The following quotes capture the feelings of some participants:

“Nonseismic events can create structures that are virtually indistinguishable from seismically-deformed sediments, or seismites. Therefore, paleoseismologists must correlate candidate seismites over regions and rule out nontectonic origins before concluding that an earthquake occurred.”

– L.B. Grant

“A great progress has been made in researches [sic] of soft-sediment deformation structures (SSDs) and seismites in China. However, the research thought was not open-minded. About the origin of SSDs, it was almost with one viewpoint, i.e., almost all papers published in journals of China considered the beds with SSDs as seismites. It is not a good phenomenon.”

– Z-Z. Feng

“At present, there are no criteria to distinguish...soft-sediment deformation structures formed by earthquakes from SSDs formed by the other 20 triggering mechanisms...the current practice of interpreting all SSDs as “seismites” is a sign of intellectual indolence.”

– G. Shanmugam

Sheeted dikes in the Channeled Scablands and Palouse Hills. Sheeted dikes originate in Pleistocene deposits, including both “ancient” flood deposits (>35 ka, pre-Late Wisconsin) and younger Missoula Flood deposits (<35 ka, Late Wisconsin). The vertically sheeted, wedge-shaped structures number in the tens of thousands (a conservative estimate), are visually distinctive, and occur only within the boundaries of the Ice Age floodways. They penetrate to many meters depth, including basalt bedrock. Miocen basalts and Neogene basin fill sediments, where swept by floods, contain dikes.

Maximum Width Method is Inappropriate for Sheeted Dikes (Compound and Composite)

Liquefaction-hazard maps prepared in the wake of damaging earthquakes are based on observations and point data collected in the field, specifically the locations of surface ruptures, water spouts, sand boils, and widths of clastic dikes. Liquefaction dikes feed sand blows. It is common practice to record the width of the widest feeder dike at a number of sites and contour the data. This is the “maximum width method”. It is based on the notion that seismic shaking is most intense near an epicenter and drops off with distance away as energy attenuates. Larger and more numerous dikes should occur near the epicenter where ground acceleration is greater, shaking more intense, and pore pressures higher in wet sediments. Relationships between liquefaction and shaking intensity are established (Ambraseys, 1991; Galli, 2000; Zhong and others, 2022).

An often-cited liquefaction mapping study involving dikes was conducted in the New Madrid Seismic Zone. USGS geologist Steve Obermeier, seeking to quantify earlier observations (Fuller, 1912; Boyd and Schumm, 1995), measured the widths of sand blow feeder dikes formed by shaking (Obermeier, 1998; Obermeier and others, 2005). Magnitude 7.2–8.2 quakes with Modified Mercalli Intensities >VIII struck the region in 1811-1812, forming the dikes, toppling unreinforced structures, disrupting transportation networks, and changing local hydrology. Two earlier seismic events are now recognized as well. Wet sediment was vented over hundreds of square kilometers., much of which is still visible on aerial photos. Obermeier used dike width measurements to approximate the location of the paleoepicenter, demonstrating the utility of liquefaction features in paleoseismic reconstruction (McCalpin, 2009).

Sand blows in the New Madrid Seismic Zone. Obermeier (1998) used the maximum widths of sand blow feeder dikes to approximate the location of a paleoepicenter and delineate the region affected by liquefaction. Black circles correspond to his 3 maximum width categories (15 cm, 15-50 cm, >50 cm). Dashed ovals are the interpreted damage halos associated with the 19th century quakes.

The USGS's 'maximum width method' for sand blow feeder dikes is not appropriate for sheeted injection dikes because it assumes single-fill structures and a single earthquake. It uses dike width (fracture aperture) to predict the strength of shaking. However, in sheeted dikes (compound and composite structures), width increases incrementally by multiple subparallel fractures with different apertures. Widening of composite dikes reflects more than one triggering event, where diking events may be separated by decades to millennia. The widths of single-fill dikes and sheeted dikes are not comparable. The failure mode is different (i.e., hydraulic fracture and injection vs. fluidized escape). An appropriate metric, one that provides an apples-to-apples comparison of fracture apertures, is the width of the widest feeder dike at a site (single-fill dikes at New Madrid) versus the width of the widest individual sheet in any dike at a site (sheeted dikes in Columbia Basin).

Columbia Basin Crust vs. New Madrid Crust The tectonic settings of the two regions, composition of the crust beneath each, and the potential for faults to generate strong shaking are not comparable. The New Madrid is an ancient failed rift in crystalline basement. Seismicity >M 7.0 is generated by deep, steep faults in strong crust. The Columbia Basin, by contrast, is a young back-arc flood basalt province resting atop extended Tertiary crust capable of <M 7.0 quakes. At New Madrid, Holocene floodplain deposits liquefied during shaking and vented sand upward. In Columbia Basin, dikes were injected downward into various substrates during Ice Age megaflood events.

Bedrock geology or floodway processes? Sheeted clastic dikes riddle sediments in Pasco Basin, Yakima Fold Belt, Palouse, and Willamette Valley, but are not found in sediments overlying thinner basalts of the Blue Mountains or Idaho-Nevada Graben. Basaltic bedrock does not appear to be a control on diking. Map modified from Tolan and others (2009, Figure 1).

Missing Holocene Deformation

Thick, unconsolidated alluvium in dozens of valleys across Eastern Washington lack clastic dikes. The absence of dikes in wet, fine grained Holocene floodplains suggests that a.) faults of the region do not currently generate large enough earthquakes to produce dikes, but did so in the past, b.) Pleistocene dikes are not seismites, but products of another process, or c.) the recurrence interval for large earthquakes is much longer than 15,000 years.

Undeformed Holocene alluvium. Thick sections of Holocene floodplain alluvium (>4m) like this along Dry Creek near Walla Walla show no evidence of strong, pre- or post-Mazama shaking. The bright white ash is conspicuous in many roadcuts, railcuts, and cutbanks in the Walla Walla Valley and elsewhere. If present, convolute bedding, soft sediment deformation features, and faults would have long ago been identified by local geologists, farmers, and soil scientists given the strong visual contrast between the ash and darker alluvium. Photo location is the intersection of Harvey Shaw Rd and Dague Rd ~8 km north of Walla Walla, WA. Mapped Quaternary faults in the vicinity include the Wallula Fault Zone (21 km away), Hite Fault (33 km away), Kooskooskie Fault (23 km away), and Promontory Point Fault (6 km away). Photographed in June 2021.

Thick alluvium. No soft sediment deformation or dikes has been noted in the floodplain of Union Flat Creek near Dusty, WA.

Undeformed alluvium. Thick alluvial fills along Willow Creek near LaCrosse, WA are undeformed.

Undeformed late Pleistocene deposits and Holocene alluvium. A mix of sandy Ice Age flood deposits, reworked colluvium, and varved lake beds capped by younger alluvium containing Mazama Ash is exposed along Latah Creek west of Spokane, WA. Lake beds are especially prone to landsliding along the creek, but sections more than 2km upstream of the Hatch Rd bridge remain largely undeformed and without dikes. Dikes are found mostly downstream of Hatch Rd. I've seen nothing in the upper reaches of Latah/Hangman Valley resembling liquefaction. Local folds (centimeter to meter scale) are occasionally encountered - rollups formed where coarse bedload gravels overrode finer grained sediments (high energy backflood flows). Photo is a cutbank below Hangman Valley Rd northwest of Hangman Valley Golf Course.

Shaking Intensity-Liquefaction Distance Relationships Fail

Shallow, intraplate faults in the study area are believed capable of producing magnitude >6.5 earthquakes and MMI VII–VIII shaking (Lidke and others, 2003). However, sheeted clastic dikes sourced in Pleistocene sediments are found at distances far beyond the limits for soft sediment deformation established by Galli (2000) and Zhong and others (2022).

An epicenter at Wallula Gap (Wallula Fault Zone) is located >285 km from large dikes near Kettle Falls, WA.
An epicenter at Burbank, WA (Umtanum–Gable Mountain Fault) is >260 km from large dikes in Lewiston Basin, ID.
An epicenter on the Hite Fault is >265 km from large dikes near Granger, WA in the western Yakima Valley.
An epicenter near Arlington, OR (Arlington–Shutler Fault Zone) is >230 km from dikes in the central Willamette Valley, OR.
An epicenter at Smyrna, WA (Saddle Mountains Fault) is 225 km from large dikes at Kettle Falls, 205 km from Tammany Creek, ID, 135 km from Cecil, OR, and 120 km from Bridgeport, WA.
An epicenter at Wyeth, OR (Mount Hood Fault Zone) is 250 km from large dikes at Warden, WA, 375 km from Lewiston, ID, and 395 km from Latah Creek, WA.
An epicenter at Spencer Canyon near Entiat, WA is >210 km from large dikes at Touchet, WA.

Magnitude–distance curves. Liquefaction outer-distance limits compiled from studies on several continents (Galli (2000; Qiao and others, 2017); Zhong and others, 2022). A robust relationship exists between earthquake magnitude and the radial distance away from an epicenter liquefaction features will form. The limit of liquefaction from an M 6.5 earthquake is ~75 km. For an M 7.5 quake, the limit approaches 150 km. Many large dikes documented in this study are located at distances >150 km from mapped Quaternary faults.

Distances from an assumed epicenter. Radial distances from outcrops I visited containing clastic dikes (circles) measured from an assumed epicenter at Wallula Gap (Wallula Fault Zone). Most dikes occur within 150 km of the assumed epicenter, but many occur at distances far beyond distance limits of liquefaction established by Galli (2000). Black bars represent the boundaries of subbasins along the Ice Age floodway. Subbasin count, at bottom of figure, is a proxy for outcrop availability. Outcrops are more numerous near Wallula Gap, where several rivers converge, thus more dikes were found there. Dikes are, in general, largest and most abundant in exposures immediately upstream and downstream of Wallula Gap, though very large dikes are found in distant exposures. Subbasins: CC = Crab Creek Valley, GT = Gorge Tributary valleys downstream of Wallula Gap to The Dalles, LB = Lewiston Basin, OK = Okanogan Valley, PB = Pasco Basin, RP = Rathdrum Prairie, QB = Quincy Basin, SR = Snake River Valley, TV = Tucannon River Valley, UB = Umatilla Basin, UC = Upper Columbia River Valley, WC = Willow Creek Valley, WW = Walla Walla Valley, WV = Willamette Valley, YV = Yakima Valley.

Same dikes hundreds of kilometers apart. One of several very large Touchet-type clastic dikes in the Upper Columbia River gorge, some 285 km north of Wallula Gap. Colville River mouth south of Kettle Falls, WA.

Weak Evidence of Strong Shaking East of the Cascades

If the dikes in the Channeled Scablands are the products of seismic shaking, then one or more of the Yakima Fold Belt structures would be the likely trigger. However, the dikes are distributed over too large an area for a single fault to be the culprit. If movement on the Saddle Mountains Fault, for example, triggered diking, then we should expect repeated diking in the same area according to its recurrence record. Since the Saddle Mountains have been rising for at least the past 15 million years, dikes should be abundant in Miocene, Pliocene, Pleistocene, and Holocene strata. Likewise, Neogene sediments cut by other YFB faults such as those at Toppenish Ridge, Lind Coulee, Kittitas Valley, and Naches Valley should host dikes and SSDs. Dozens of published measured sections through Ellensburg, Latah, and Ringold Fm should contain evidence of liquefaction, but do not. If hazard studies are correct, a radial pattern of strong shaking by YFB faults should be obvious in sedimentary sections, but it isn't.

Fault zone investigations have likewise failed to reveal a pattern of strong shaking in Eastern Washington. The often-referenced Stateline earthquake of 1936 that struck the Walla Walla Valley was a sub-magnitude 6.0 event that formed no sheeted dikes and caused no damage to speak of beyond its immediate epicenter, the tiny community of Umapine, OR. The Hite Fault, located in the Blue Mountains southeast of Walla Walla, appears to be inactive. I am aware of no reports of liquefaction or other seismites associated with the Hite Fault. Widespread liquefaction was not reported following the 1872 North Cascades earthquake (~M 7) and its many aftershocks (Milne, 1956; Sherrod and others, 2015; Brocher and others, 2018) despite vast quantities of silty-sandy glaciofluvial and glaciolacustrine sediments in terraces along the nearby Columbia, Wenatchee, Methow, Okanogan, and Sanpoil Rivers. It is entirely possible that the YFB ridges rose one M 5.9 quake at a time.

Accounts of the 1872 North Cascades quake, the largest on record for Eastern Washington, came mainly from local newspapermen, whose job it was to amplify the spectacle and sell newspapers. News reports should be taken with a grain of salt, especially those from 150 years ago. The quake, centered at Entiat, caused water spouts, ground cracks, landslides, and collapsed cabin roofs (Washington Standard Newspaper 11 Jan 1873; Coombs and others, 1976; Brocher and others, 2018, Appendix B), but historical context is needed. Wenatchee in 1872 was a frontier town. Residents - all 100 of them - occupied a community that would not be platted for another 20 years. Chelan County didn't exist at that time. The light bulb and the telephone had not yet been invented. Ulysses S. Grant was President. Washington, Idaho, Colorado, Wyoming, Utah, New Mexico, and Arizona were not yet states. Just 6 rudimentary seismographs monitored ground motions for the entire PNW region, including parts of British Columbia, until the mid-1960s. Newspapers ain't science.

Saddle Mountains Fault. Excellent exposures of sandy interbeds and tilted basalt flows (Elephant Mountain) are found along the Saddle Mountains front (Lower Crab Creek). None contain sheeted clastic dikes. The thin, light-colored fractures in the photo at right are not liquefaction features, but bleached shear bands, which are structures common in deformed sandstones worldwide.

Saddle Mountains crest. I've worked methodically along the entire crest of the Saddle Mountains, Smryna Bench, and Taunton Bench, examining Pliocene and Pleistocene sediments preserved there (Cooley, 2023). I found no sheeted clastic dikes above ~360 m elevation. In fact, surprisingly little evidence of strong shaking is found in numerous tilted sections along the 85 km-long fault.

Minimally deformed interbeds. Ebinghaus and others (2012) examined Miocene-age sedimentary interbeds (Ellensburg Fm) at 14 sites in Pasco and Quincy Basins near the Saddle Mountains and Frenchman Hills Faults. Minor soft sediment deformation - flame structures and load casts - were noted at 3 sites. At his Wagon Road 1 and 2 (Moses Coulee) and Mabton (Yakima Valley) sites deformation occurred at contacts between mudstones and overlying sands. Flame structures and load casts are commonly found where sand is repeatedly spilled through openings in levees onto off-channel muds. No clastic dikes were reported. Ebinghaus' findings are consistent with my own observations at dozens of exposures in the region and those of others (i.e., Hays and Schuster, 1983; Smith, 1988a,b).

Other evidence deserves consideration. Strong seismic shaking has not been recognized in a.) hundreds of borehole cores logged at the Hanford Site, b.) in dozens of measured sections at White Bluffs published by several geologists, c.) in cores from alpine lakes in the Cascades and Okanogan Highlands, d.) in ODP cores off WA and OR, e.) in thick Ellensburg/Thorp/Latah Fm sections in Kittitas, Yakima, and Naches Valleys, f.) in Neogene sediments in the Dalles-Umatilla syncline, and g.) in thick and thin sedimentary interbeds in the CRBs across the region. A strong paleoseismic signal remains largely unrecognized despite more than a century of geological investigation by USGS, USBOR, Washington Geological Survey, various mapping crews, university researchers, graduate students, and others.

Strong shaking produced by a Puget Sound fault or by the Cascadia Subduction Zone are far-fetched explanations for the presence of dikes in Eastern Washington. Shaking generated west of the Cascade divide would attenuate long before reaching the Columbia Basin (Peterson and others, 2011; Wood and others, 2014). No evidence of megathrust shaking at 1700 AD (Atwater and others, 2005) is recognized in Eastern Washington. Likewise, no shaking effects are known from the 1918 Vancouver Island M 7.2, 1946 Vancouver Island M 7.5, 1949 Olympia M 6.7, or 2001 Nisqually M 6.8 quakes.

To date, no study has established an association between Yakima Fold Belt seismicity and sheeted clastic dikes. Instead, an aseismic driver - large overland floods - appears to control where, when, and how the dikes formed (see Footnote 6).

Lost near Lyons Ferry. About 15 Touchet Beds overlie a thick bar gravel in the Snake River canyon near Lyons Ferry. John Whitmer photo (WGS Archive No. 03144).

Sheeted Dikes Without Earthquakes

Examples of sheeted, per descendum clastic dikes that closely resemble those in Columbia Basin are reported by others around the world. In all cases, overloading, rapid sedimentation, and hydrofracture were involved. Sheeted, wedge-shaped dikes intrude muddy deposits beneath tidewater glaciers in Sweden (Von Brunn and Talbot, 1986; Jolly and Lonergan, 2002; Le Heron and Etienne, 2005; Phillips and others, 2013), New England (Kruger, 1938), and in sediments of alpine glacial lakes (Sutherland and others, 2022). Sheeted dikes intrude lahar deposits on the side of an Aleutian volcano in Alaska (Herriott and others, 2014) and beneath ash flows in the Lake Atitlan caldera, Guatemala (Brocard and Moran-Ical, 2014). Winglike sand intrusions formed by hydrofracture during deposition propagate downward, upward, and laterally in deep water clastic systems (Jenkins, 1930; Duranti and Hurst, 2004; Huuse and others, 2007; Monnier and others, 2015; Cobain and others, 2015, 2016). Wedge-shaped sand dikes descend from the base of debris flow deposits into underlying sandstone at Black Dragon Canyon in the San Rafael Swell, UT (Author's field notes). Braccini and others (2008) was first to recognize the dikes in Eastern Washington as injectites comparable to those formed offshore.

Mount Spurr, Alaska. Sheeted dikes with characteristics identical observed to dikes in the Touchet Beds were discovered by Herriott (2014) in sandy lahar deposits on the side of an Aleutian volcano. Rapid deposition, surface loading, wet over dry sediments, and hydraulic fracturing were all involved. Image courtesy of Herriott. The red arrows are his and point to silt skins.

Voss, Norway. Descending "laminated dikes" at Voss, Norway are identical to dikes in the Touchet Beds of Washington State. Wall-parallel laminations formed by "a repetitive process operating during formation of these types of dikes" (Mangerud and Skreden, 1972; Mangerud and others, 1981; Larsen and Mangerud, 1992). Similar downward-injected dikes formed in glacial settings are reported in Scandinavia, British Columbia, Quebec, Ontario, and New England (Kruger, 1938; Dionne and Shilts, 1974; Amark, 1985; Boulton and Caban, 1995; Brunn and Talbot, 1986; Broster and Clague, 1987; Dreimanis and Rappol, 1996; Rijsdijk and others, 1999; ).

Hat Creek, British Columbia. Gravel dike from above penetrates underlying outwash sand in British Columbia (Broster and Clague, 1987).

Dikes in southwest BC. Sheeted gravel-sand dikes filled from above in British Columbia (Broster, 1991, Fig. 9b).

San Rafael Swell. Clastic dikes injected downward beneath an overriding debris flow in Black Dragon Canyon, UT.

Patagonia. Sheeted dike in varved glaciolacustrine sediments in the northern Patagonian Andes (Perucca and Bastias, 2008, Fig.11). Pocket knife in shadow.

Sweden. Top-loading by a grounding glacier and hydraulic fracture conspire to force wedge-shaped till-filled dikes into a muddy substrate. Figure by von Brunn and Talbot (1986, Fig. 16).

Polish coal mine. A sheeted dike nearly a meter wide with characteristics identical to those in the Touchet Beds descends from unconsolidated overburden into bedrock rock, following extensional fractures developed in the crest of a fold (Haluszczak and others, 2007, Fig. 6e).

Vocontian Basin, France. 3D representation of an injectite network formed offshore in a turbidite fan setting (Monnier and others, 2025 Fig. 8). Dikes of remobilized sand at scales approaching the resolution of modern seismic imagery connect larger and thicker sills. Though the stress gradient in the deep-water setting is opposite that associated with terrestrial floods, many similarities between the two systems exist, including hydraulic fracture and downward injection (Gottis, 1953; Parize, 1988; Huang, 1988; Jolly and Lonergan, 2002; Rowe and others, 2002; Parize and Fries, 2003; LeHeron and Etienne, 2005; Scholz, 2009, 2010; Jonk, 2010; Kane, 2010; Beyer and Griffith, 2016).

South Africa. Kilometer scale fluid migration and injectite growth in the Karoo Basin, South Africa. Downward and lateral injections propagate when fluid pressure in the sand body exceeds the confining strength of the seal. Figure redrawn from Cobain and others (2026).

Idaho vs. Utah. On the left is a sketch of a Touchet-type clastic dike at Lewiston, ID that I measured in several places (sheeting not shown). On the right is a cartoon of a typical deep sea sand injectite from a slideshow by Dr. Lansing Taylor, formerly of the University of Utah's Energy & Geosciences Institute. I flipped the injectite image upside down. Fractures follow the most efficient pathway. In these two examples, pathways alternate between horizontal and vertical in response to changes in grainsize.

Branching at bedding contacts. Branching geometry in a dike at Snipes Mountain, WA. The dike is sourced from above in Missoula flood rhythmites (late Pleistocene Touchet Beds) that unconformably overlie a quartzite-bearing gravel (Miocene Snipes Mountain conglomerate). The dike pinches as it descends through the stack, branching in opposite directions and thinning where it encounters a strong grainsize contrast at the contact. Both sheeted sills pinchout within a few meters. Both dike and sill segments show clear intrusive relationships.

Rubbly Injectites at Indian Creek, WA

In November 2017, I discovered and measured several breccia-filled dikes that cut varved Glacial Lake Columbia beds along lower Indian Creek Rd (Hawk Creek) east of Lincoln, WA. The unsheeted dikes formed in the tributary valley just off the energetic floodway probably in response to slumping of blocks of varved sediment. Several large (>10 m), coherent, rotated blocks were present in nearby outcrops. Fills contain broken, stratified clasts ripped-up from the host material. The dikes intrude the lower portion of the >20 m-thick section with at least 24 rhythmites composed of varved intervals (lacustrine) and sand beds (flood).

Pleistocene injectites exposed in the northernmost portion of the Ice Age floodway.

Rubbly injectite crosscuts bedding at a low angle.

Rubbly, unsheeted fills contain stratified rip-up clasts - chunks liberated from the surrounding material.

Field Work Matters

The origin of clastic dikes in sedimentary sequences can be ambiguous. Earthquakes, though often involved, are not required. In fact, clastic dikes are reported in many settings where active seismicity played no role whatsoever (Shanmugam, 2016). Lessons learned from coastal California or the Wabash Valley do not apply universally. Only when anchored by evidence gathered at the outcrop will an investigation into the origin of clastic dikes tilt toward a correct interpretation. Office-generated theories and probability models serve society best when they are rooted in and remain subordinate to field observations.

Dikes are threshold features that, if interpreted one way, may prompt policy makers to brand a landscape hazardous and unfit for occupation and/or future development. Interpreted another way, the same dikes become Ice Age relicts of little importance to anyone other than academics and megaflood enthusiasts.

Careful field work that involves a significant number of observations, measurements, descriptions, samples and a study area scaled to the geological phenomenon under investigation should be de rigueur. Overuse of "seismite", shoddy field documentation, and the application of methods poorly suited to the region are unacceptable practices.

Project planning is the responsibility of the Field Geologist. The subdiscipline Paleoseismology will hopefully remain a field-based discipline going forward, one focused on determining the timing and effects of prehistoric earthquakes, not getting one's name in the newspaper (or on NPR). Data gathered in the course of a paleoseismic investigation (fault slip rates, event dates, and shaking effects) are critical inputs to building codes, hazard planning documents, and land use policies. Data from the field informs and often drives policymaking, which affects the lives of real people. Unlike journal articles and tables of recurrence probabilities, maps constructed from field measurements are easily understood by all audiences. They are uniquely influential and tend to find their way into land use policy documents, which persist for decades.

Dike geometries in outcrop. (A) Twin-tapering forms that do not look like typical dikes. They are axe blade-shaped fractures propagating laterally and emerging from the face of the outcrop. A trick of geometry in the third dimension (see Arris and Aperture figure earlier in article). (B) Three dikes that lack a taper direction are truncated at their tops by erosional surfaces (bedding contacts). (C) Buried sediment remobilized in response to shaking vents sand upward to a higher stratigraphic position (sill) or to the surface via a feeder dike (sand blow). May be sourced from above or below. (D) Upward and downward tapering dikes. Local shearing may have offset a single dike, causing it to appear as two with opposite tapers. A trick of limited exposure. Excavate features or keep looking to find more conclusive relationships. (E) Downward tapering dike-sill geometry with upward-curving intersections are sourced from above. (F) Upward tapering dike-sill geometry with upward (tree branch-like curving intersections are sourced from below.

Know your SSD. Many soft sediment deformation features are distinctive, but many others can look alike. This is because ductile material is involved, more than one process may be at work, and features at an early stage of development may morph into very different shapes over time. Careful observation is usually the key to sorting things out correctly.

Key Characteristics of Clastic Dikes Assessable in the Field

Three key physical characteristics of clastic dikes - vertical sheeting, taper direction, and truncation at bedding contacts - are readily assessable in the field.

Sheeted Fills - Sheeting is the result of repeated fracturing and sediment injection. Dikes in the study area grew in staccato fashion by filling of newly opened fractures by new pulses of sediment. Sheeting in dikes found elsewhere likely records a similar pattern of reinjection along preexisting weaknesses. New sheets erode older ones; rip-ups derived from adjacent bands should be present in abundance. Small packages of parallel sheets, typically up to six or so in the case of study area dikes, commonly crosscut older packages and reveal variations in the way fractures propagate. If new sheets tap progressively younger source beds, small collapses should be present above fractures opening below. At some of my study sites, multiple collapses appear in successive rhythmites, each initiating a dike. Brief periods of dike widening alternate with longer periods of inactivity (hiatus), a pattern that tracks with cyclic filling and spilling by proglacial lakes.
Taper Direction - Taper direction is strongly tied to dike origin as it often reveals how fractures propagated, the mode of fracture, and the orientation of the fluid pressure gradient. In many cases, taper direction reveals the source of dike fills. Upward-tapering dikes rise from a buried sediment source layer and formed by upward escape of fluidized sediment under a normal pressure gradient (higher deeper, lower shallower). During shaking, pore fluid pressure rises. If the pressure exceeds the confining stress, then wet sediment is expelled upward, down the pressure gradient. Downward-tapering dikes indicate injection from the surface and a temporarily inverted pressure gradient. Fluid driven fractures propagate downward and are filled by sediment circulating at the surface. Upward-tapering dikes are almost always the result of seismic shaking. Downward-tapering dikes (per descendum) form in response to rapid loading and hydraulic fracture. Fractures tend to follow efficient pathways within the substrate.
Truncations - The tops of downward-tapering dikes are truncated at bedding contacts or unconformities. Dikes truncated tops constrain the timing of injection. A set of dikes truncated by depositional or erosional contacts is evidence of a repetitive trigger and repeated diking over time.

Flood counts and the development of vertical sheeting. Stacks of rhythmites (Touchet Beds) deposited by Pleistocene megafloods accumulated to different thicknesses in different parts of the Channeled Scabland. Rhythmite counts vary depending on location. The most complete rhythmite sections occur in slackwater basins repeatedly filled by Lake Lewis, Lake Condon, and Lake Allison. Lake Lewis filled the Walla Walla Valley (Waitt, 1980; 1985), Lewiston Basin (Bretz, 1929 field notes; Webster and others 1982), and Tucannon Valley (Smith, 1993). Lake Condon filled the Umatilla Valley (Benito and O'Connor, 2003), Willow Creek Valley (Cooley, 2015), and Sixmile Valley. Lake Allison filled the Willamette Valley (Glenn, 1965). Glacial Lake Columbia filled the Sanpoil Valley (Atwater, 1986), Upper Columbia Valley (Kiver and Stradling, 1982; Hanson and Clague, 2012), Latah Creek Valley (Rigby, 1982; Kiver and Stradling, 1982; Waitt, 1983; Meyer, 1999), and Foster Coulee (Russell, 1893). Rhythmites also occur in the Glacial Priest Lake basin (Walker, 1967; Breckenridge, 1989). The rhythmite count at a site approximates the flood count. All floods flowed through Wallula Gap and slackwater lakes that ponded there were the deepest anywhere in the region (>200m). Consequently, the largest dikes occur in full rhythmite sections in the southern Pasco Basin, eastern Umatilla Basin, and western Walla Walla Valley. Fill band counts (sheet counts) record repeated flood-loading, substrate failure, and sediment injection. Sheet counts (injections) are a multiple of rhythmite counts (flood counts), though a one-sheet-per-flood isn't the pattern. The sheet count data suggest that up to about 10 sheets may form in a given dike during a single flood. Local conditions seem to play a role (flow regime, water depth, valley configuration, grainsize, slackwater lake residence time, etc.). Very large composite dikes widen by the addition of new fractures and new sediment injections, so their widest portions occur near the base of rhythmite stacks (lower in the section) rather than near their tops (higher in the section). Dikes can appear to taper upward because newer fills that tapped successively younger flood beds intruded alongside older fills.

Truncated dikes at multiple levels. Sheeted dikes intrude more than a dozen geologic units exposed along the Ice Age floodway. Here, I've revisited key sites reported by others and redrawn their stratigraphy to include dikes. The sketches above are representative of studies published to date and show diking was recurrent with flooding. (A) Walla Walla Valley sites from Spencer and Jaffee (2002). (B) Lind Coulee site from Daugherty (1956). (C) Moxee Mammoth site from Lillquist and others (2005). (D) Hanford's FMEF site from Bjornstad and others (1990). (E) Rulo site from Bader and others (2016). A = Alluvium, C = Colluvium, CRB = Columbia River Basalt, DIA = Silt diamict, EG = Exotic-clast bearing gravel, FG = Fanglomerate/Alluvial fan gravel, L = Loess, P = Paleosol, S = Sandy, SCR = Silt-clay rhythmites, TB = Touchet Beds/Hanford Fm.

Warden Canal. The dikes do not cut entirely through stacks of rhythmites. Several beds overlie this truncated dike near Warden, WA.

Silt-sealed Cracks and Hydrofracture

Sand-propped hydraulic fractures are used by the petroleum industry to stimulate oil and gas reservoirs, a procedure known as "fracking". Hydrofractures are induced by shutting in a portion of the well bore, adding a proppant slurry (sand + water + chemicals), and using pumps to jack up the fluid pressure. When fluid pressure exceeds the formation's resistance, the rock surrounding the wellbore fails and fluid-driven fractures begin to propagate outward. The pressurized proppant slurry immediately fills the expanding fractures and holds them slightly open, permitting hydrocarbons to flow back to the well. Fractures propagated beyond the well bore exponentially increase a well's effective surface area and open new pathways into the reservoir.

Unlike a shut-in wellbore, however, sediments that host the dikes are typically unconsolidated and sandy with no low-permeability layers that might act as a seal. Yet clastic dikes abound. Two key factors explain the formation of the dikes: high strain rate and the formation of silt skin walls.

High strain rate - When loaded by a catastrophic flood, pressure in the shallow subsurface built so rapidly that hydraulic fracturing was induced. The normally loose (ductile) material failed in the brittle mode at the high strain rate. Pressure rose above that required for fracture and exceeded the sediment's capacity to dissipate pressurized fluid through its pore network. Rapid loading alone appears adequate to initiate fracture in a low tensile strength material with no true seal such as the Touchet Beds.

Silt skins - Once fractures began to form and fill, silt skin walls began to build. The sealing effect of the low-permeability skins delayed leakoff and facilitated further fracture. New fractures as well as natural flaws (cracks, soil macropores, burrows, etc.) provided low-resistance routes for new fractures to follow. Fractures immediately filled with sediment, the injected slurry (a natural proppant) sourced from within the overriding flood. Dewatering (leakoff), integral to the formation of the skin wall, begins the moment sediment enters a fracture. The skin-sealed crack begins to behave as a pressure vessel almost immediately. Pressure inside of the sealed fracture (pore fluid pressure, Pf) rises until it exceeds the confining strength of the material (Pf > 03) and the crack tip advances or, if leakoff loss exceeds Pf, the fracture closes. The fracture propagates in the 01–02 plane (vertical), widening against 03. As the fluid pressure equilibrates to the confining pressure (Pf = 03), the fracture tip halts, filling ceases, the crack closes down on its proppant, and pressure begins to build again if the load is still present. Each time Pf exceeds 03, the tip jumps forward or a new fracture initiates nearby. With each increment of widening, fluid pressure drops (volume increase = pressure decrease), but soon rebuilds. This loading-driven crack-fill-seal cycling created the dikes’ vertically sheeted fabric.

Sheeted infill illustrated. Fluid pressure-driven crack and fill (crack volume cycling) is shown at the scale of a dike (nearfield scale) during a flood loading event. Time steps 1 through 12 in the pressure-time curve correspond with crack tip locations. During overloading, dike growth corresponds with pressure-volume cycling where fluid pressure remains between the minimum and maximum principal stress values. Silt-sealed fractures become sheeted dikes in my study area and in other geologic settings where silt is present and similar overloading has occurred (lahar, grounding glacier, debris flow, etc.). Diking seems to have occurred twice during a flood-load event. The first is the initial onrush of the overland flood (or backflood). The second occurs once a slackwater lake has formed (sustained load). Gravelly or sandy dikes are likely produced by the overland flood, while silty-sandy dikes result from the lake.While hydraulic fracture is well understood, the development of wall-parallel laminae (sheeting) in clastic dikes by a combination of rapid loading, hydraulic fracture, and silt wall seals, as I've illustrated here, has not previously been described in detail. Figure 9 in LeHeron and Etienne (2005) is the closest I've seen. My illustrations are not copied from anyone; I created them to clarify my thoughts and convey my argument to others.

Sheeting forms pulse by pulse. Cyclic fluid-driven fracture results in the growth of dikes with vertically-sheeted fills during flood events (a single hydrofracture event). The cross sections correspond to the gray shaded portion of the pressure-time curve in the figure above. During hydraulic fracture, new fractures open, propagate, and fill. Here I show 4 pulses corresponding to 4 episodes of adjacent diking (2a, 3a, 3b-c, 4b, 4c) and nonadjacent diking (1a, 2b, 4a). Incremental growth of dikes involves the cycling of fluid pressure, the repeated opening of new fractures, and near simultaneous infilling by sediment carried by a flood. Evidence of repeated flooding (stacks of rhythmites) and repeated fracture injection (sheeted fills) is a pattern observed throughout the Channeled Scablands.

Near field and far field fracture. I modified the stress-strain curve that describes hydraulic fracture to illustrate my concept of sheeted diking during floods. The curves relate floodwater loads imposed over a broad area (far field flood load) to cyclic pressure pulses that occur at the local scale (near field injections). The fracking/leak-off test framework captures most of the important elements. Leak-off begins when a fracture opens (not at closure) and continues after the fracture closes, but that point, somewhat secondary, is not well captured in this diagram.

Relevant equations. This article provides limited discussion on the set of equations relevant to fluid-driven fracture and diking. The equations above are basic elements of undergraduate-level courses in Fracture Mechanics. Quality YouTube channels teaching this stuff include Nicholas Espinoza, Scott Ramsay, Taylor Sparks, and others. Elastic Pressure (Pe) describes properties of the fractured material. Specifically, the stress perpendicular to a crack required to keep it open, where h is crack width, L is crack length, G is the shear modulus, and v is Poisson's ratio. Source Pressure (Pr) is fluid pressure of the source, considered here to be the pressure of the sediment-water slurry at base of a megaflood or slackwater lake, measured at the opening (top) of the crack. Source Pressure is assumed to be constant during diking. E is Young's modulus of the host sediment or rock, delta p is the density difference between host and the injected fill, g is gravity, Q is the volumetric flux of material injected into the crack, and u is the average injection velocity. Viscous Pressure Drop (Pv) is the pressure change along its length from crack opening to crack tip, where n is the slurry viscosity. Fracture Pressure (Pf) is the pressure required to propagate the crack tip forward, where Kc is the critical fracture toughness. Hydrostatic Pressure (buoyancy) is ignored for near-surface sedimentary dikes at atmospheric temperatures.

Deep sea analogs? A 2m-wide sand injectite intrudes pillow basalts erupted off Angola (Hurst and Cartwright, 2007, Fig. 4). Deep sea injectites are larger than terrestrial Touchet-type dikes and the orientation of crustal stresses is different, yet important similarities exist.

Evaluating Proposed Origins

In this section, I evaluate seven proposed origins based on my observations and the literature.

(A) Desiccation hypothesis - Little evidence supports a desiccation origin. Dike geometry, distribution, size, sedimentology, and internal characteristics are fundamentally at odds with an origin involving the passive infilling of meters-deep, open-standing cracks. The dikes are not filled mudcracks.

(B) Ground ice hypothesis - Permafrost is soil that remains below 0 degC for at least two years. Ice wedges are common to permafrost lowlands in northern North America, Europe, and Asia. Ice wedges grow by annual freeze-thaw cycling where ground cracks open and fill with ice and sediment. Fossil ice wedge casts reported in glacial outwash near the margins of Late Wisconsin glaciers (Horber, 1949; Dylik, 1966; Burbidge and others, 1988; Stone and Ashley, 1992; Demoulin, 1996). Cold-formed wedges commonly contain vertically-laminated fills (sheeting) and coalesce to form polygonal networks (Lachenbruch, 1962; Romanovskiy, 1973). Ice wedge growth at middle latitudes, while relatively common during the Pleistocene, is rare today.

A few geologists have interpreted the clastic dikes near Richland, WA as fossil ice wedge casts based on polygonal networks, vertically-laminated fills, and age (Alwin and Scott, 1970, Lupher, 1944, and Black, 1979). While the dikes bear some resemblance to fossil wedges in mid-latitude France (Antoine and others, 2005), The Netherlands (Van Huissteden and others, 2000), Poland (Zoller and others, 2022), Germany (Grube, 2012), Niger (Denis and others, 2010), Patagonia (Perucca and Bastias, 2008), and certain high-latitude sites (Van Vliet-Lanoe, 2005), other cold-climate features are all but absent. Additional corroborating evidence is needed to prove the past presence of frozen ground in Eastern Washington.

No permafrost. Compiled climate-proxy information indicates permafrost never formed in Columbia Basin during Late Wisconsin glacials and interglacials.

Periglacial features are not widely recognized in the nearby Blue Mountains or Cascade Mountains. Hints of frost-cracked ground are found in buried soil profiles of the Palouse and Umatilla Plateau. Rock glaciers lingering in cold hollows east of the Cascade divide (Lillquist and Weidenaar, 2021) lie at elevations significantly higher than all sites where clastic dikes are found. Frost shattering of Columbia River Basalt, exposed over thousands of square kilometers, does not appear to have been unusually intense or comparable to the modern Arctic (Pidwirny, 2006). Pleistocene cirque elevations in the Rocky Mountains (Pierce, 2003, Fig. 1) project well above the crests of Yakima Fold Belt ridges. Relict ground ice features are few in the Columbia Basin (French, 2018). No mention of soil wedges, frost stirring, or gelifluction is made in NRCS Soil Surveys for the Colville Indian Reservation (NRCS, 2002), Okanogan County (NRCS, 2010), Chelan County (USDA, 1975), Douglas County (NRCS, 2008), Grant County (USDA, 1984), or Lincoln County (USDA, 1981). Stacked Pleistocene paleosols in the Palouse and Channeled Scabland contain abundant evidence of soil life. Phytoliths, rodent burrows, and cicada burrows are incompatible with deep, prolonged freezing. Backfilled burrows that riddle the Touchet Beds attest to rapid recolonization after each Ice Age flood event. Mammoth that once roamed south-central Washington were nourished by steppe-grassland forage, not tundra plants (Fry, 1969; Last and Barton, 2014). Pollen samples from lake bottom cores indicate both cold-tolerant plant species and conifers occupied the landscape throughout the Lake Wisconsin (Blinnikov and others, 2002; Whitlock and Brunelle, 2006).

Small frost wedges in varved beds of Glacial Lake Missoula (Chambers, 1984; Chambers and Currey, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021) have not be found in similar varved beds of Glacial Lake Columbia (Lake Roosevelt, Lake Rufus Woods, Banks Lake). Fossil soil wedges such as those in Idaho's Lemhi Range (Butler, 1984; D.R. Butler written communication), at the Owl Cave-Wasden Site on the Snake River Plain (Dort, 1968; Butler, 1969), in terrace gravels near Lewistown, MT (Schafer, 1949), in prairie soils near Browning, MT (unpublished field notes by the author) and Laramie, WY (Grasso, 1979; Mears, 1981, 1987; Nissen and Mears, 1990; Munn and Spackman, 1991; Dillon and Sorenson, 2007) apparently never formed west of the Rocky Mountains.

Ice wedges in the literature. Hundreds of studies have been published on fossil ice wedges, ice wedge casts, and soil wedges formed in permafrost (Lachenbruch, 1962; Pewe, 1973; Romanovskij, 1973; Mears, 1987; Yershov, 1998; Bockheim, 2002; Murton, 2020). Several lines of climate-proxy evidence indicate the Columbia Basin was never glaciated and free of permafrost, even during the very coldest parts of the Pleistocene. Eastern Washington's clastic dikes are not fossil ice wedge casts, despite their laminated fills, tendency to form polygons, and speculation by authors (Lupher, 1944; Alwin, 1970; Black, 1979). At its coldest, the Pleistocene Columbia Basin was "tundra-like" (Cooley, 2008) and perhaps best described as a "cold steppe" (Spencer and Knapp, 2010, p. 50) with widespread sagebrush and pockets of pine forest refugia supporting species commonly "found in alpine and sub-alpine valleys in the [present-day] Cascade Mountains of Washington...cool-to-cold, moist, open-park conditions...consistent with the presence of continental ice to the north". While the use of periglacial terminology may persist among certain paleoecology groups (i.e., O'Geen and Busacca, 2001), Eastern Washington was never tundra and always contained trees.

Mima mounds in Columbia Basin are not diagnostic of past periglacial conditions. Silt mounds are common to glaciated and unglaciated landscapes blanketed by loess worldwide (Busacca and others, 2004). In North America, silt mounds are found from central Mexico to the Arctic and some mound fields in Washington clearly date to the Holocene. Mima mounds indicate abundant wind, dust, and some aridity. Little else.

Though the southern limit of the Cordilleran Ice Sheet is well defined across northern Washington (Porter and others, 1983; Atwater 1986; Cheney, 2016), a corresponding periglacial zone remains loosely delineated. Murton (2020) depicts a conspicuously narrow permafrost zone south of the Okanogan Lobe, implying the southern limit of periglaciation extended only a short distance south of Upper Grand Coulee. Periglacial features are abundant in a 200 km-wide swath south of the continental Laurentide Ice Sheet (Pewe, 1983; Clark and Ciolkosz, 1988). No such swath exists south of the maritime Cordilleran Ice Sheet (Orme, 2002; French and Millar, 2013; French, 2017).

Ice wedges active and relict. Left: Ice wedge penetrating thick silt at the USACE Permafrost Tunnel Research Facility near Fairbanks, AK (www.erdc.usace.army.mil/CRREL/Permafrost-Tunnel-Research-Facility). Right: A fossil ice wedge cast (sand wedge) penetrating sandy alluvium in northern Europe. Photo by Richter/Freiberg Instruments.

Wedges in Glacial Lake Missoula beds. Small wedges descend from numerous horizons in Glacial Lake Missoula "varve" beds exposed in the Clark Fork River Valley, MT. Specific sites include Rail Line (A,B), Jocko River (C), Crow Dam, and Garden Gulch sections (Chambers, 1984; Chambers and Currey, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021). These structures are though to have formed during lowstand periods (shoaling, subaerial exposure) and resemble wedges commonly found in the Arctic. A combination of desiccation and shallow ice wedging seem to be at play. Similar wedges are not known in varved beds in northern Washington. Photo B by Michelle Hanson. Photos A and C are mine.

Soil wedges in Montana. These wedges formed 465 km east of Grand Coulee Dam in treeless prairie soils. Periglacial wedges like these help define the former ground ice region south of the Laurentide Ice Sheet (Murton 2020, French 2017). Similar wedges are not known in Eastern Washington. Roadcut is along Hwy 89 between the Two Medicine River and Badger Creek south of Browning, MT.

(C) Lateral spreading hypothesis - Lateral spreading can form wedge-shaped cracks by a combination of liquefaction and extension. Surface cracks open as a block of material slides sideways along a low-angle slip plane. The figure below shows three scenarios where wedge-shaped cracks may form in gentle terrain underlain by a thick body of sediment. The 'free face', created by channel incision, is the key; space is needed to accommodate spreading.

Numerous aspects of lateral spreading are inconsistent with dike geometry, dike fill characteristics, and dike distribution. According to the model, tension fractures form near shoreline bluffs (within 100m?), which predicts dikes will be more numerous at topographic breaks. However, dense networks of clastic dikes correspond with broad valleys and coulee bottoms. Concave valley floors are under weak compression, not extension. Incised channels necessary to accommodate spreading are simply not found in the Touchet Beds or other formations that host the dikes. Slopes of benches underlain by Touchet Beds are too flat to translate blocks of sandy sediment sideways. Slide blocks in un-channeled valley fills have little reason to form and, if formed, have nowhere to go. The dikes are not associated with rubbly landslide deposits, high-relief erosional surfaces, or incised channels.

Lateral spreading falls short. Liquefaction does, too. The diagrams above show why. (A) Channel incision removes support and creates a free face that can facilitate lateral spreading in shoreline bluffs. Block sliding due to gravity. (B) Seismic shaking that triggers liquefaction in a wet, sandy layer at depth can produce clastic dikes (water escape structures). Sediment is vented to the surface, forming sand blows. Cone-shaped sheets and "volcanic" sand edifices are formed. (C) A large vertical load imposed by a megaflood (overland flood, backflood, slackwater lake) increases pore fluid pressures in the underlying sediments and trigger hydraulic fracture. Fractures immediately fill with sediment sourced from the surface. Hydraulic fractures initiated at the ground surface propagate downwards. Fractures are propped by the sandy fill to become clastic dikes. Internal sheeting develops during single events (pressure-volume cycling) and over time as new dikes form during subsequent flood events. New dikes merge with older ones. Repeated flooding creates both compound and composite dikes.

Free face extension and shearing at depth. Lateral spreading can create wedge-shaped dikes with massive fills in certain locations. A set of wedge-shaped gravel dikes near Hunters, WA formed when unstable shoreline bluffs composed of varved glacial lake sediments capped by outwash gravel began to slip and topple into the Columbia River. Gravity, accommodation by the free face, and a subhorizontal slide plane at depth acted together. Read more about the Hunters, WA dikes HERE.

(D) Rebound following slackwater lake drainage - Ice Age floods imposed enormous loads on the crust. In the southern Pasco Basin, the depth of Lake Lewis depth exceeded 200m. Each flood imposed its load very rapidly (hours to days). Each slackwater lake load persisted longer (weeks). While we can assume the crust was depressed a bit during flooding and rebounded as the water drained away, we don't know the of depression depression, or the rate of depression and recovery, or whether the effects of loading and unloading are preserved in the geologic record. No one has studied crustal loading effects.

Belly up to the bar. Wedge-shaped fractures result from up-bending caused by unloading of floodwater by drainage.

(E) Seismic shaking and liquefaction hypothesis - Fault movements independent of floods or in tandem with floods are capable of triggering earthquakes and forming clastic dikes. Whether scabland floods played any role in triggering strong earthquakes is unknown. If the dikes are the products of seismicity, then diking by liquefaction and lateral spreading would the likely mechanism. However, the dikes discussed here do not resemble liquefaction dikes formed elsewhere and described thoroughly in the literature. The nearest set of liquefaction dikes is found in Holocene alluvium of the lower Columbia River Gorge and thought to result from shaking by the 1700 Cascadia earthquake (Dickenson, 1997; Obermeier and Dickenson, 1997; Atwater and others, 2005, 2015). Dikes of the Columbia Gorge set differ in size and character with those in the Columbia Basin set. The latter do not resemble feeder conduits of sand blows like those found following the 1964 Alaska quake (McCulloch and Bonilla, 1970) or those at New Madrid (Fuller, 1912; Obermeier, 1989). Atwater (2000) suspects the mechanical properties of sediments in the Gorge (sand islands and river banks) vary in ways not anticipated, which might explain their spotty appearance; discontinuous liquefaction occurred in visually-similar deposits. Sheeted dikes in the Channeled Scabland formed only during the Pleistocene, are not associated with sand blows, and cannot be linked to any particular earthquake.

(F) Flood-generated vibration hypothesis - Ice Age floods would have produced a tremendous rumble as they coursed through the countryside. The cataclysm must have terrified humans and animals who witnessed their passage. In addition to a roar, overland floods may have induced a vibratory resonance in certain rocks and sediments. The dikes may be somehow related to resonant deformation, though the mechanism remains elusive. No clear analog has yet come to light, though research into seismicity generated by large, sediment-laden floods is underway at Université de Grenoble-Alpes, France as of 2023 (Kristen Cook, Florent Gimbert, Alain Recking).

(G) Hydraulic fracture triggered by floodwater loading hypothesis - This is my preferred origin, the evidence for is presented in this article.

Numerous non-seismic triggers. Clastic dikes often form in response to rapid sedimentation and overloading. Dikes are widely documented in both seismically-active and low-seismicity areas and in a variety of geologic environments and deposits. While earthquakes commonly trigger liquefaction and can a produce clastic dikes, they are most commonly unsheeted, upward-tapering, and small. Dike morphology indicates whether liquefaction was involved. Evaluate taper direction, internal sheeting, and the connection to source bed to distinguish each type, its trigger, its origin. Not all clastic dikes are seismites. Figure modified from Shanmugam and others (2016, Figure 16).

Dike-sill-dike. Fluid-driven fracture in unconsolidated granular material is central to the formation of sheeted clastic dikes in the Touchet Beds. An understanding of how hydraulic fractures initiate and propagate is necessary. This can be taught at the undergraduate level. Read Bons and others (2022) and half a dozen articles on sand injectites. Students will get it. Or avoid the physics and keep floundering about with liquefaction and earthquakes.

Different Features, Same Floodway

The same floods produced different types of deformation features depending on the grainsize and rheology of the sediment they encountered. In silt-sand rhythmites near Walla Walla, Lewiston, and Zillah, expect to find sheeted, wedge-shaped dikes in the hundreds. In varved beds of the upper Columbia River, expect abundant t-shaped mud squirts, a few rubbly injectites, and other features associated with mass wasting. In eddy bars near Umatilla and Washtucna, expect a few stubby, gravel-filled dikes with crude vertical sheeting. In gravel-free silt rhythmites in backwater valley of the Palouse near the upper limit of flooding, a few thin dikes will appear here and there. Where basalt is exposed at low elevation along energetic canyons (Snake and Columbia gorges), a few sheeted dikes will fill (and widen) fractures in the bedrock. Coarse, laminated pebble-sands like those at at Qualchan, the mouth of Rock Creek, and the big quarry north of Corfu will be devoid of soft sediment deformation structures. Same floods, same forces, different substrates, different features.

Conclusions

This article summarizes my work on sheeted clastic dikes in the Channeled Scablands of WA, OR, ID, and MT. This inland region was repeatedly swept by colossal glacial floods during the Pleistocene. Thousands of sheeted clastic dikes documented here occur exclusively within the margins of the floodway, are identical at all locations (size, shape, sedimentology, age), and formed by the same mechanism: hydraulic fracture. Megafloods and deep, slow-draining slackwater lakes imposed enormous loads on sedimentary and bedrock substrates, opening wedge-shaped fractures that rapidly grew and filled with sediment sourced in circulating bottom currents. Vertical sheeting reflects crack-and-fill cycling during each flood event (compound dikes). New dikes followed the paths of older dikes (composite dikes). Flutes on the interior surfaces of skin walls indicate infilling from above. Dike width distributions roughly scale with rhythmite counts (flood events). The largest dikes occur near basin centers, where floodwaters were deepest and rhythmite stacks thickest. Unlike most clastic dikes in the literature, the features described here did not form by liquefaction, were not triggered by earthquakes, and are not feeder conduits to sand blows. They are flood injectites, not seismites. This study confirms a hydrofracture origin proposed by Pogue (1998).

This article expands on one published in Northwest Geology v. 49 in August 2020. Northwest Geology is published annually by the Tobacco Root Geological Society in conjunction with the TRGS field conference. TRGS is a Montana-based group of geoscience professionals. I update this online version from time to time as new information becomes available. Online version was first posted here 15 Sept 2020.

LAST UPDATED: 20 Dec 2024

Footnote 1 Bruce Bjornstad, a retired career Quaternary geologist at Hanford, has, more than any of his PNNL colleagues, mentioned the clastic dikes in his writing, beginning with his university work in the late 1970s and continuing with his recent guidebooks on scabland geology. The primary focus of his professional career was the hydrogeological behaviour of megaflood deposits at the Hanford Nuclear Site and, to a lesser extent, sediments of the Ringold Formation. Though an author of numerous agency reports, Bjornstad has never published original work on clastic dikes. He is listed as a coauthor on Fecht and others (1999) - a mystifying publication. I believe it was compiled from Fecht's notes. The list of quotes below reveals Bjornstad's drifting opinion on the origin of the dikes through time. What accounts for these changes in opinion remains unclear. Based on their 40 years of writing, I consider Bjornstad and his Hanford colleagues to be casual observers of the dikes. They failed to seriously address the dike problem during their time.

a.) Bjornstad (1980)

"The assemblage of sedimentary structures within the Touchet Beds comparable to turbidites ...suggest periodic, rapid, subaqueous deposition of successive rhythmites by turbidity-like currents created by flood surges during a single flood. Additional evidence suggesting that flood surges rather than separate floods were responsible for rhythmite formation [includes]...the possible association relating clastic dikes with soft sediment deformation."

b.) Bjornstad (1990)

"These dikes are thought to represent dewatering structures that developed during compaction and settling of cataclysmic flood deposits during or soon after floodwaters drained from the Pasco Basin (Bergeron and others, 1987)." "Most clastic dikes, ubiquitous in flood deposits throughout the Pasco Basin, appear to have formed through forcible injection during waning stages of flooding (Black, 1979; WCC, 1981) during this time."

c.) Bjornstad and Teel (1993)

"In the Pasco Basin, clastic dikes are believed to be dewatering structures associated with lake draining following cataclysmic floods." d.) Bjornstad and others (2001)

"The dikes signify soft-sediment deformation during or soon after flooding, perhaps associated with flood-induced seismicity (Cooley and others, 1996; Fecht and others, 1999)."

e.) Bjornstad (2006)

"Clastic dikes formed during or soon after Ice Age flooding, perhaps because of ground shaking during earthquakes...If earthquakes occurred more frequently, we might expect to see more dikes in sequences of flood beds with truncations atop flood beds. But this is not the case..."

f.) Bjornstad and Lanigan (2007)

"Clastic dikes may be the result of ground shaking, which caused the wet sediments to liquefy and flow along paths of weakness down into or up along vertical earthquake-generated cracks in the flood deposits."

Footnote 2

I have not yet submitted this, or some version of it, manuscript to an academic journal. A reasonably complete version was published in Northwest Geology v. 49 published by the Tobacco Root Geological Society (Cooley, 2020). That manuscript was reviewed and approved by Mike Stickney, Director of Earthquake Studies Office at Montana Bureau of Mines and Geology and Jeff Lonn, Research Geologist also at Montana Bureau of Mines and Geology. Supporting TRGS's excellent annual field conference is far more important to me than whatever prestige I might gain through publication in a traditional journal. Thousands have read this article because it is offered free online. A few dozen might find it if it were tucked behind a journal's paywall. Paying exorbitant page rates so that others might publish my work seems a bit anachronistic, given the tools available today. When the project is complete (I consider everything here draft information), I will likely submit a much leaner version emphasizing the measurement data to Northwest Science Journal for review. Or maybe Whitman College or the Washington Geologic Survey would be interested.

Footnote 3 In our work as Whitman College geology students (Cooley, 1996; Cooley and others, 1996) and follow on studies (Niell and others, 1997; Pogue, 1998), we imprecisely stated that the dikes penetrate from top to bottom through the entire stack of rhythmites, thus were late-flooding and/or post-flooding features formed by lateral spreads triggered seismic shaking. While many dikes do cut from top to bottom through the stack, their internal structure - their vertical sheeting - preserves a more nuanced history of incremental growth coincident with flooding. Vertical sheets of sediment that comprise large dikes (sheets = dikelets = fill bands), are often truncated at their tops by depositional contacts between rhythmites and surfaces within rhythmites that correspond to abrupt changes in flow regime (i.e., upvalley flow, slackwater, downvalley drainage). The dikes do not descend from the top of the rhythmite stack.

As geologists, we think of a "clastic dike" as a single structure, but sheeted dikes are actually compound structures (multiple parts) and many are composite structures (new parts added over time). While the "dike" may crosscut an exposure, each sheet (or packages of sheets) traverses only a portion of it. The dikes grew as single fills and sheet packages during single events, and by the addition of new sheets/packages through time, each sourced from a different rhythmite. Dike growth occurred in tandem with Ice Age flood cycles, which punctuated the Pleistocene. As students, we regularly observed truncated sheets (and entire dikes) and were somewhat puzzled by them. We routinely commented to one another about them, photographed them, and sketched various truncation relationships in our field books. We did not, however, fully recognize, much less emphasize the fundamental importance sheet/package truncation plays in dike growth. The dikes are composite structures that grew wider and deeper by reinjection during dozens of catastrophic glacial outburst flood events. Truncated sheets indicate the dikes are not single-event structures injected at the tail end of Missoula flooding (crosscutting features that post-date deposition of all or most floodbeds), as our clumsy early interpretations suggest. Rather, they are long-lived structures that grew in pulses during floods over time. Many large dikes grew by repeated sheet injection over thousands of years. Their growth recurrence interval is the flood interval.

College try. My undergraduate thesis suggested slope stability controlled dike distribution in the Walla Walla Valley (Cooley, 1996). The working model, developed from readings and observations at ~30 outcrops, invoked earthquake-triggered lateral spreading to explain higher dike counts along the sloping valley sides. Listric normal faults and dikes were thought to be closely linked (i.e., the dikes are sedimented-filled tensile fractures). I recall struggling to reconcile my own field observations with diking mechanisms proposed by others: slumping during flooding (Baker, 1973) and post-flood lateral spreading (McCalpin, 1996). Neither seemed to fully explain the relationships in outcrops. This sketch approximates a flawed understanding of the factors that control diking I held at that time.

Footnote 4

The term injection has no directional implication. Injected material may have moved upward, downward, or sideways. Injection describes fracture-filling where wet or slurried material is mobilized and moves into fractured sediment or rock. The usage of injection and injectite in this article is consistent with the relevant geoscience literature (sediments and structure of petroleum reservoirs), not general textbooks on sedimentology and stratigraphy. For example, injection wells move water from the surface to the subsurface. Hydraulic injection involves the lateral propagation of fractures and proppant from the well bore into the formation. Fluidized injection is commonly used to describe both upward-pinching clastic dikes and dikes that were filled from the top.

Footnote 5 Burlingame Canyon is on private land is not accessible to the public without permission from landowners.

Footnote 6 In the course of my investigation of calcrete-bearing sediments of Plio-Pleistocene age near Othello, WA, I have not observed soft sediment deformation consistent with strong, recurrent shaking in the two dozen sections I have described. I recently correlated 28 detailed stratigraphic columns from White Bluffs by Kevin Lindsey (Lindsey and others, 1996 Appendix A), finding no evidence of repeated, widespread shaking. Soft sediment deformation in Pleistocene sediments is plainly syn-depositional. Local deformation and some small, unsheeted fluidization structures in Ringold sediments are found along Saddle Mountains' frontal thrust. Trenching by Michael West and others decades ago documented young faulting higher on the mountain. Steve Reidel never mentioned seismites in reports on Yakima Fold Belt uplifts or in map unit descriptions accompanying his geologic maps. Smith (1988a,b) and Ebinghaus and others (2012) studied CRB interbeds (Ellensburg Fm), finding no evidence of strong shaking. About 10 paleoseismic trenches opened across YFB faults and logged by USGS and others contain no liquefaction features.

Footnote 7

A package of fill bands injected during a single flood. Three fill bands comprise a composite clastic dike in the idealized example above. Each band formed at a slightly different time during a megaflood event. Each fracture opening corresponds with a slightly different flow regime, taps a slightly different stratigraphic level within a rhythmite as it forms, and accesses sediment of a different grainsize. Substitute different flood beds for stratigraphic position within a single bed to explain reinjected dikes in the region. At a larger scale, dike fills reflect the caliber of the sediment available to them. Grainsize in flood deposits is primarily determined by the local flow regime - high-energy channel, backflooded valley, slackwater lake. Since the configuration of most valleys and bedrock water gaps were not radically changed by flood erosion, successive floods produced more or less the same flow regimes and deposited the same grainsizes in the same places over and over. For example, the protected Touchet Valley received mostly medium to fine sand and silt. Dikes there are filled with the same. The Starbuck area, situated close to high-velocity coulees, received more gravelly sand. Dikes there are filled with coarser material. A page from one of my field books.

Footnote 8

I update this online document from time to time as new information becomes available. This seems a modern way to do things. I prefer to provide updates as they come rather than wait for some journal to publish something formatted to their liking.

Footnote 9

If my work informs yours, you should cite this web-based article or the original print article (Cooley, 2020). What is presented here is new work and original work. It is entirely my own. Please include the date you accessed it in your citation.

Cooley, S.W., date accessed, Sheeted clastic dikes in the megaflood region, WA-OR-ID-MT, www.skyecooley.com/single-post/2020/09/15/Sheeted-Clastic-Dikes-in-the-Megaflood-Region

Cooley, S.W., 2020, Sheeted clastic dikes in the megaflood region, WA-OR-ID-MT in Lonn, J; English, A.; McDonald, K.; Hargrave, P. (editors), Northwest Geology: Journal of the Tobacco Root Geological Society, 45th Annual Field Conference - Geology of the Bitterroot Region and Other Papers v. 49, p. 1-17

Footnote 10

As recently as 1996 and publication of the Final EIS for tank wast remediation (USDOE/WADOE, 1996), a fundamental lack of understanding on the part of geologists regarding the dikes' origin, geometry at depth, connection to stratigraphic units, and possibly their role in conducting fluids in the subsurface was recognized by geohydrologists at Hanford. Nevertheless, they forward an interpretation,

As with the genesis of [the] clastic dikes, little is known about their hydraulic characteristics...the inferred hydraulic nature of the dikes is that of potentially a minor barrier to flow perpendicular to the dike. The clay content and lack of sand stringer continuity suggest that clastic dikes do not function as preferential flow paths for vertical flow.

Concerns over the potential for dikes to serve as "preferential" or "fast" conduits for liquid waste leaked to subsurface aquifers have been raised by a number of Hanford researchers (Cushing, 1994; Finfrock, 1994; Fayer and Ritter, 1999; Faybishenko and others, 2000; Fendorf and Jardine, 2003; Fayer and others, 2010) was partially tested using an infiltration field experiment on a single, large dike in Touchet Beds at the Hanford Site (Murray and others, 2003, 2007; Ward and Gee, 2003; Ward and others, 2006). This single experiment at the "Army Loop Clastic Dike Site" generated several other reports with various titles and authors.

A lithified clastic dike penetrating bedrock at the Rocky Mountain Arsenal in Colorado raised similar concerns decades prior to the Murray report (Miller and others, 1979). Liquid waste introduced deliberately to the suburface was also a problem at RMA. Characterization of clastic dikes as lateral barriers to migrating fluids is an idea developed at RMA and later adopted by Hanford authors, though few geological similarities between dikes in Washington and Colorado exist,

A vertical, tabular body embedded in fine-grained, tuffaceous, calcareous sandstones, siltstones, and shales has been reported at the Rocky Mountain Arsenal. The tabular body has the mineralogy and structure of an igneous dike, but it may also be a clastic dike of sedimentary origin. The possibility that the body may be an igneous dike is of concern to the staff geologists at the Arsenal because it could extend to great depth and be impermeable enough to provide a barrier to laterally migrating ground water or, conversely, it could be relatively permeable and provide an aquifer.

The figure below shows the results from an infiltration test where water-borne dye tracer was introduced to the ground surface via driplines and allowed to spread by gravity into in a.) a large, vertical, sheeted clastic dike, b.) a large, sheeted, subhorizontal sill joined to the dike, and c.) non-dike Touchet Bed sediment surrounding both. Red and yellow areas indicate high reflectance values (high moisture content). Odd how reflectance values and moisture values in the chart below disagree. The figure is redrawn from Murray and others (2001, Fig. 7.33, 7.34). I've reframed their nearly unintelligible artwork, thus presented their study's key finding clearly. Note the sill at left is as wet or wetter (containing and conducting as much or more dye tracer) as the vertical dike at center. The report did not address the intersecting sill.

The Murray study, a follow-on study similar to one conducted by Sisson and Lu (1981), has been regularly referenced in subsequent reports, despite it's obvious design flaws and limited value to geological community or waste remediation managers at Hanford. The study failed to consider flow in the sill and failed to test flow in a multi-dike network, the typical configuration in Columbia Basin. Reading through the project's various interim and final reports leaves one with the impression that it was a start-and-stop affair involving many enthusiastic lab scientists doing something (outdoors!) for the first time. Ten collaborators are listed.

If Hanford geologists actually wanted to find out how clastic dikes conduct fluids leaked from the surface, they would have repeated this experiment at several locations, evaluating flow behavior in multiple configurations of clastic dike-sill networks (i.e., flow in typical dike networks). The Murray team studied one dike at one location. Hardly the rigorous investigation of which our National Labs are capable. But Murray gave it the old College Try, didn't he?

Clastic dike exposed in the Army Loop Road excavation. Grid on lowest tier is 2m wide x 1m high. Blue tent covers infiltration dripline equipment. Photo: Murray team.

Clastic dike and sill are much finer grained than host sediment. Photo: Murray and others, 2003, Fig. 10.

Infiltration in dike, sill, and host sediment after 3 days. Murray and others, 2003, Fig. 16.

Infrared image (top) and a composite photograph of the experiment pit face. Black vertical dike is on the left in both images. Sill is the long, black, horizontal feature extending from the right side of the dike. It looks like bedding. Photo: Murray and others, 2003, Fig. 12.

Clastic dike polygonal network along the Army Loop Road near the infiltration study site. Google Earth photo.

Footnote 11

J Harlan Bretz did not pay much attention to clastic dikes in the Channeled Scabland, but a few mentions do pop up in his copious field notes (see Bretz's newly found notebooks archived at nickzentner.com). On August 7, 1928 at "Gardena Cliffs" just south of Touchet, WA he writes, "Clastic dikes are prominent, some of them wedging out halfway down the cliff, some continuing to the bottom of the section...Wedges of the fine sand at angles of 40 degrees from horizontal penetrate or appear to penetrate up into the gravel for three or four feet" (1928 typed field notes, p. 27). On August 10, 1928 in the Walla Walla Valley, he reports, "Clastic dike of marvellous development abundant in these sections. But tho they contain the coarse black sand, they are not responsible for the pockets and lenses [ice rafted berg mounds containing non-basaltic clasts]. One clastic dike has seams of sand separated by seams of clay in the prevailing mode but seams of good brown sand" (1928 typed field notes, p. 34-35). On July 29, 1928 at Willow Creek, WA he finds, "fingers of the black sand extend from the gravel into the silt." (1928 typed field notes, p. 8). Dikes are again observed near Clarkson and Asotin in his 1928 field notes (p. 43). Bretz also describes aseismic deformation in the Touchet Beds in his typed field notes and in one article (August 14, 1923; Bretz, 1928b, Fig. 9 photo above; July 15, 1929).

The caption for the 1928 photo above reads,

Contorted bedding in silt and fine sand, Walla Walla Valley. Etched into relief by the wind.

In the body of the article, he notes,

Marked irregularities in stratification of sandy phases occur. One such structure is illustrated in Figure 9. No satisfactory explanation for this is at hand.

In 1923, Bretz writes,

Along Sunset Hiway, 4-5 miles north of Wenatchee, is a cut showing a seasonably banded silt about 200 ft above the altitude of Wenatchee. 30-foot section. Silt is pale buff in color. Some portions a very fine sand, somewhat more gray or brown colored. The seasonal banding is far from perfect thruout...In most summer bands, is a delicate foreset or current bedding, showing very gentle southward drift of the material along the bottom. This doesn't seem right for a water body which recorded varve clays. And there are several other unorthodox things too. Portions of the deposit which do not show varve clay stratification are composed of fine sand to silt, not of clay. Current bedding is prominent in this, much more so than in the summer varves. Here and there in the section are worn pebbles and cobble not of basalt or the local granite. These undoubtedly show the presence of floating ice. In certain layer of the finer silt are most peculiar contortions of the laminae, tho the strat immediately above or below are not affected. Some of these contortions were sketched. In each sketch, one prominent continuous lamina has been shown. The others are squeezed or thickened to conform. In [sketch 3] the little triangular areas show fragmentation and porosity. These plications or contortions have a strike. They are rolls! And the strike is essentially parallel to the wall of the valley here. Other evidence, in the existence of gentle flexures in the strata themselves, seems to point to settling of the material down a subaqueous slope toward the deeper parts of the valley as the cause for the distortion.

Bretz's only mention of clastic dikes arrives on July 15, 1929 in typed field notes on Tammany Bar at Lewiston, ID. This is four years after publication of the first article on Touchet Bed clastic dikes by O.P. Jenkins, who visited this same exposure. At Lewiston, Touchet Beds ("silts") overlie a boulder gravel bar left by the Bonneville flood.

The silt rests directly on the rough bouldery surface of the gravel deposit, some boulders a foot or two in diameter setting up on top of the gravel project for their full diameter into the silt...The silt is well sorted, no scattered particle of basalt, etc., rather irregular bedding. Lenses of black coarse sand occur irregularly, the contact of sand on silt being so irregular that it seems like a cross section of miniature mountain topography. To add to the puzzle, clastic dikes can be traced up in the deposit, thickening with increasing height and some of them abruptly ending at various levels. This silt looks more like a normal stream alluvium, the cracks developing at successive intervals while the accumulation was going on per saltum, thus some of the cracks being filled and covered over before other cracks were formed. Yet there is no such sand on top of the silt today from which the the major number of the cracks (coming to the top) could be filled. Such sand deposit may once have been present, and now gone by erosion.

Footnote 12

Future Work. My field data collection effort has spanned many years, yet could be repeated if the outcrops remain. I have taken copius field notes and built an organized archive of information on the outcrops I have visited. If a future geologist is interested in revisiting sites at which I ahve collected measurements, they would complete the task much quicker since a map now exists to guide them. Locating and accessing dozens of new (previously unreported) outcrops throughout this large study area took hundreds of days in the field. If you are a student or geologist interested in conducting your own project on clastic dikes, please feel free to contact me. Happy to help.

Footnote 13

Newman and others titled their 2002 article "High frequency electromagnetic impedance imaging for vadose zone and groundwater characterization". The first sentence of the Executive Summary reads,

Executive Summary - A geophysical experiment is described for characterizing the clastic dike systems, which are ubiquitous within the vadose zone at the Hanford Nuclear Reservation.

The Abstract contrasts markedly with the Executive Summary. It also removes the words "clastic" or "dike". The author replaced "clastic dike" with the word "heterogeneity". Who does that?

Abstract - Accurate description of transport pathways on the gross scale, the location of contamination, and characterization of heterogeneity within the vadose zone, are now realized as vital for proper treatment, confinement and stabilization of subsurface contamination at Department of Energy (DOE) waste sites. Electromagnetic (EM) methods are ideal for these tasks since they are directly sensitive to the amount of fluid present in porous media, as well as fluid composition. At many DOE sites it is necessary to employ lower frequency (<1 MHz) or diffusive electromagnetic fields because of the inability of ground penetrating radar (GPR) to penetrate to sufficient depths. The high frequency impedance method, which operated in the diffusive frequency range (10 Hz to 1 MHz), as well as the low end of the spectrum employed by GPR (1MHz-10 MHz), is an ideal technique to delineate and map the aforementioned targets. The method has clearly shown the potential to provide needed information on variations in subsurface saturation due to local storage tanks and perched water zones, as well as mapping geological structures related to the subsurface hydrological properties and heterogeneity within the vadose zone. Although it exhibits certain advantages over other EM methods, the impedance method comes with a set of assumptions and practices that can limit its potential. The first is the desire to locate receivers in the far-field of the transmitter which allows the use of magnetotelluric (MT) inversion codes to interpret the data. Unfortunately, one does not precisely know when one is in the far-field of the transmitter, because this depends on the geology we wish to image. The second limiting factor is the scarcity of complete 2D and 3D inversion schemes necessary to properly invert the data. While approximate 2D schemes are now emerging, rigorous 2D and 3D inversion codes are needed to bound the range of applicability of the approximate methods. We propose to address these problems in the following manner: (1) implement full non-linear 2D/3D inverse solutions that incorporate source coordinates and polarization characteristics, (2) use these solutions to study improvements in image resolution that can be obtained by making measurements in the near- and mid-field regimes using multiple source fields, (3) collect data at the Hanford Reservation with recently developed earth impedance measurement systems, and (4) interpret the field data with the newly developed inversion capability, as well as with additional and independent information such as well logs from boreholes. The benefit of this research to the DOE would be a combined measurement/interpretation package for non-invasive, high-resolution characterization of larger transport pathways, certain types of contamination, and heterogeneity within the vadose zone at the Hanford reservation, as well as other DOE facilities.

Dikes in Patagonia. The caption reads, "Another detail of the clastic dike, which reveals the exceptional apophyseal body, the only dike like this in this group of clastic viens in Tierra del Fuego. ar = Tertiary sandstone, ae = Tertiary claystone." Source: Borrello, A.V. (1962), Sorbre los diques clastico de Tierra del Fuego (About the clastic dikes of Tierra del Fuego), Universidad Nacional de la Plata, Facultad de Ciencias Naturales y Museo, Revista del Museo de la Plata, Tomo V, Geologia No. 32, p. 155-191

*Footnote 14*

In 2016, while living in Alaska, I received a phone call from Steve Obermeier. I'd not corresponded with the nearly-retired geologist prior to that call. He wanted to discuss clastic dikes in Washington and offer me a co-authorship on an article in prep. I was excited, but wary. Excited to be called out of the blue by someone at USGS, but wary because I was familiar with Obermeier's deep work on sand blows at New Madrid as well as his superficial understanding of the clastic dikes in Eastern Washington. We spoke for about 15 minutes and I asked if he would send me the manuscript to review. Instead, he sent two articles from some engineering journal. A follow up email and the manuscript eventually arrived. At the top of the document was this note from an internal reviewer at USGS, Gregory Gohn. Obermeier and coauthors clearly intended to publish a weak article that contained no new data and several figures that looked suspiciously like my own. I would have been excluded, but for Gohn's suggestion to contact the only person actively working on the dikes in question. Forehead slap. I was Kevin Pogue's student on the first clastic dike thesis completed at Whitman College (Cooley, 1996; Cooley et al., 1996). Odd that my former advisor didn't call me and Steve did. A few days later, I wrote Obermeier to decline co-authorship and any further involvement. Their paper was never published. I guess peer review works. Karma, too.

References

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Sheeted Clastic Dikes in the Megaflood Region

Sheeted Clastic Dikes in the Megaflood Region, WA-OR-ID-MT

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