Clastic Dikes: Formed by Lateral Spreading?
Lateral spreads. A network of lateral spread fractures in pavement formed by an earthquake. Peru 1970. USGS photo.
Lateral Spreading: A proposed origin for sheeted clastic dikes in Eastern Washington
Earthquake-triggered lateral spreading is invoked to explain the many thousands of sheeted Pleistocene clastic dikes in south-central Washington (Cooley et al., 1996; Neill et al., 1997; Pogue, 1998). The process is not spelled out in any article, but I imagine the explanation would go something like this: Shaking triggers lateral translation of and minor shearing between of bedrock blocks along unseen sliding surfaces, creating a series of narrow sub-vertical cracks (gouge zones <1m wide). Spreading that forms vertical joints in the basalt bedrock is ostensibly transferred into sandy sediments that fill the adjacent basin creating the dikes. Gouge-filled joints and sheeted clastic dikes are, by this reasoning, analogous structures in different materials. But didn't the megafloods also strip away basalt and cause basalt cliffs to topple?
What is Lateral Spreading?
The U.S. Geological Survey's Fact Sheet 2004-3072 (USGS, 2004) tells us that lateral spreading is a specific type of low-angle landsliding that involves liquefaction. Lateral spreads,
...occur on very gentle slopes or flat terrain...the dominant mode of movement is lateral extension accompanied by...tensile fractures. The failure is caused by liquefaction...usually triggered by rapid ground motion, such as that experienced during an earthquake...Lateral spreading in fine-grained materials on shallow slopes is usually progressive. The failure starts suddenly in a small area and spreads rapidly.
According to Varnes (1978), lateral spreading involves,
Movements may involve fracturing and extension of coherent material...owing to liquefaction or plastic flow of subjacent material. The coherent upper units may subside, translate, rotate, or disintegrate, or they may liquefy and flow.
Types of Landslides. Ten different types of slides, flows, topples, avalanches, and creep according to USGS (2004). Tensile cracks appear in nine of the ten diagrams. The fractures are strongly parallel and oriented perpendicular to the direction of slip.
Unique or Gradational?
The ten styles of mass wasting are not themselves end members, but share certain aspects in common; one type commonly grades into another. For example, "block slides", "rock falls", and "topples" largely describe the same thing, except the size of the blocks varies and maybe the dip of the beds. Likewise, "debris flows" and "debris avalanches" vary in the amount of sediment, water, and organic material they contain. Debris flows and debris avalanches often occur together. Whether you call a slope failure a "lateral spread" or a "creep" can come down the rate of its movement downslope. Slow movement is typically called creep. Fast, a spread. Many landslides involve more than one process, thus are "complex" according to the Fact Sheet.
Lateral Spreads: The geological stem cell
Lateral spreads are the geological equivalent of a stem cell in biology. They grow with time and assume one form or another if permitted to develop. Hear me out.
In most landslides, tension gashes (ground fissures) appear first, often at the location of the soon-to-be head scarp. Down-slope translation (extension) causes the fractures to form in sediment, rock, or soil. The fractures warn us of things to come. With time and continued movement, they will grow and merge at depth to form a basal sliding surface. Tensile cracks (not unique to any slide type) will evolve into a scarp from which the slide mass will separate and become something we name - a topple, a slide, a flow, a creep. Geologists wait until mass wasting develops for awhile before we give the thing a name because the name identifies the dominant process responsible. Luckily, landslides generally proceed quickly, so we rarely have to wait long.
Before and after. Lateral spreading extends the upper unit (stratified sand and silt). Tensile fractures separate coherent, stratified blocks. How well this model fits dike-bearing strata in Eastern Washington (i.e., Touchet Beds) is unclear. Some basic questions must be asked: Did liquefaction occur at a low level in the Touchet Beds? Can we see the base of the translated blocks? Are sheeted fills in the dikes consistent with liquefaction below and brittle cracking above as shown in the diagram? What is the sedimentary record of strong shaking in the region over the past 2 million years? Did lateral spreading occur in the same way everywhere the dikes are found - that is, does it explain identical dikes that occur throughout >30,000 km2 region? Where else on Earth do we find analogous structures: vertically-sheeted, wedge-shaped sand-silt dikes formed by earthquake-triggered lateral spreading?
What Are We Really Saying?
What are we actually saying when we say, "these clastic dikes formed by lateral spreading" (i.e., Pogue, 1998)? First, that liquefaction was involved. Second, that sliding occurred on a liquefied basal plane (a thin body of fluidized sediment). Third, that all Touchet-type clastic dikes with their distinctive sheeted fills and distributed throughout a vast region are products of a single mechanism, the dominant mechanism, lateral spreading.
A few field-testable questions (and answers) come to mind:
Q: Is there evidence of intense or widespread liquefaction in the Touchet Beds of Eastern Washington, northeastern Oregon, and west-central Idaho?
A: The firm answer is no.
Q: Is a basal shear surface present in Touchet Beds?
A: Again, no.
Q: Does lateral spreading (liquefaction required) explain the presence of identical sheeted dikes in both the unconsolidated Touchet Beds and in bedrock of various lithologies and ages in the region?
A: Not well.
Three alternatives. The three scenarios above assume a near-level ground surface, which comports with known locations of very large clastic dike (basin centers). A free face cliff or bluff is needed in order for spreading to occur, specifically because valley floors have so little slope and without one blocks of sediment have nowhere to go. Scenario A.) Channel incision and creation of a free face removes support and facilitates lateral spreading. Scenario B.) Seismic shaking, liquefaction in an in situ sandy layer at depth results in mobilization of in situ sediment and venting of a sand slurry to the surface (sand blows). Scenario C.) A large vertical load imposed by a megaflood increased pore fluid pressures in the substrate below (sediment or rock) initiating hydraulic fracture. Fractures immediately fill with sediment sourced in circulating, sediment-laden currents at the base of the flood. Fractures are propped by the sandy fill, thus become clastic dikes. Repeated flooding creates composite clastic dikes (merged dikes of different ages).
Burlingame. Is this evidence of lateral spreading or sand blow activity? R1 through R7 are flood rhythmites deposited by separate floods. The dike begins at the base of R7 and cuts downward through the section. It does not crosscut the entire the section, rather it formed during the cycle of Missoula flooding (18-14 ka). The dike cuts its own path and does not follow a rubble zone between laterally-translated blocks. Bedding contacts are not offset or tilted across the dike. The branched dike tapers downward to a point on both of it legs. It was not fed from a liquefied sandy layer below; the source of the dikes is clear. Flute casts on silt skins ornament the interior faces of the dike's walls, providing clear evidence of downward infilling. This example at Burlingame is a typical Touchet-type dike. It is not a sand blow (i.e., Obermeier, 1998).
Lewiston. Is this form consistent with lateral spreading? There is no bedding-parallel fault offset here. The structure exhibits dike-sill-dike geometry, its original form when it was intruded. This geometry results from fluid-driven fracture and sediment injection (hydraulic fracture). The dike-sill exploits the most efficient pathway available to it, diking across less-permeable silty layers, paralleling high-permeability sandy ones. Can you visualize the form this dike must take in 3D? Hellsgate Recreation Area. More info on this exposure see my post: https://www.skyecooley.com/single-post/dike-sill-dike-relationships-in-a-fluid-driven-sediment-filled-fracture-clastic-dike
Landslides Are Local
By their very nature, landslides comprise a relatively small portion of most landscapes. Geologists get excited about really big landslides because they are really big. If big slides were the norm, nobody would bat an eye at the way Anchorage slipped into the sea in 1964. No one would care about the Heart Mountain Detachment - and certainly not carry on decades-long arguments about its origin. And Dr. David Malone doesn't have a job.
Lateral spreading over the area in which Touchet-type clastic dikes are found would constitute one of Earth's largest landslide complexes. The trigger for such a slide would require an enormous amount of energy. The dikes show clear evidence of repeated injection, so that trigger must also repeat many times - but only during the Pleistocene and only within Ice Age floodways. No Touchet-type dikes originate in Tertiary or Holocene deposits. The sheeted dikes do not occur beyond the margins of scabland routes followed by Ice Age megafloods (Cooley, 2020).
Spreading is Common to Multiple Landslide Types
The USGS Fact Sheet clearly depicts tensile fractures in nine of ten landslide types. Lateral spreading is not unique to one landslide type.
Mass Wasting Disrupts Bedding
Slides and flows destroy depositional layering. Liquefaction is effective at homogenizing bedded sandy materials. In fact, sandy layers that have been liquefied often appear structureless; some geologists call them "homogenites". If the dikes are products of lateral spreading, delicate bedding and fine scale laminae in dike fills and in the unconsolidated sediments that host them would be disrupted. Rubbly zones of between translated blocks would be numerous, especially where the dikes are numerous. Disrupted zones would form a halo around at least some dikes, as tensile fracturing must precede diking.
Flat-lying and mostly undisturbed. Touchet Beds at Burlingame Canyon near Lowden in the Walla Walla Valley are intruded by several large, slender dikes. The dikes cut cleanly through bedding, descending and pinching as they go. Deformation at Burlingame, other than the dikes, is meager. There are very few faults and no evidence of liquefaction. No sand blow edifices have ever been reported here or elsewhere in the Touchet Beds. If the dikes are taken to be lateral spread structures, the total amount of lateral extension is minor, equal to the cumulative width of the dike fills. Since the dikes are not oriented parallel to one another and the spread direction is unclear (no free face apparent), the total amount of spreading amount would actually be some fraction of cumulative width (not true width). It would seem strange to interpret the Burlingame exposure as "a set of coherent, shifted, stratified blocks separated by vertical clastic dikes occupying tensile fractures". Where is the liquefied sliding surface required by lateral spreading as defined by USGS and Varnes? Photo taken in 1978 by an unnamed photographer, Washington Geological Survey archive.
Lateral Spreads Produce Tilted Blocks
Landslides often produce jumbled, chaotic deposits. Deposits produced by lateral spreads are decidedly less jumbled, consisting of slightly-tilted, but coherent blocks. After more than a century of investigation, no such pattern has been found in any outcrop (Jenkins, 1925; Black, 1979; Cooley, 2020).
Landslides Are Messy
If lateral spreading occurred at a regional scale, diking would be just one effect, one type of evidence left behind. A whole suite of other mass wasting structures and deposits would accompany the dikes. However, the Touchet Beds are neither jumbled nor chaotic. Some warping (open folds) and some faulting (meter-scale listric normal faults) are observed in certain locations (Cooley et al., 1996), but overall the Touchet Beds remain largely undeformed.
Pristine preservation. Fine laminae and delicate primary sedimentary structures in the fills of clastic dikes are nearly always perfectly preserved. Back-filled rodent burrows (originally round), common in Touchet Beds, have not all been flattened by compaction. Adjacent dikes are not sympathetically folded nor are their margins uniformly crenulated. Gentle, open folds and some saggin in slackwater rhythmite sections is common, but wholesale homogenization and loss of original bedding has never been observed by the author or reported by others. My photos from Pine Creek, OR.
Where's the Free Face?
The dikes form polygonal networks near the center of the Pasco Basin, visible in aerial photos. Numerous reports discuss this same location on the Hanford plain off Army Loop Road (Newcomb, 1962; Grolier and Bingham, 1978; Lillie et al., 1978; Black, 1979; Silver and Pogue, 1997; Fecht et al., 1999; Johnson et al., 1999; Murray et al., 2003; Gee and Ward, 2006). The site is today a low-relief valley bottom and was the same during the Pleistocene, when the dikes formed. The Hanford plain accumulated coarse, sandy sediment delivered by all Ice Age floods that drained through Wallula Gap. Incised channels are absent in the remnant Pleistocene topography. Borehole studies make no mention of buried valley fills or incised channels beneath the Hanford plain. If lateral spreads translated blocks toward a free face, where was the free face in the center of the Pasco Basin?
Dike polygons. A polygonal network of clastic dikes appears in aerial photos of the Army Loop Road onthe Hanford Site, Pasco Basin, WA. Vegetation was burned away by wildfire several years ago, revealing the dikes. Google Earth photo.
What Does the Literature Say?
Textbooks always list lateral spreading as one of several common responses of wet sediment to seismic shaking. Photographs of small, single-fill sand dikes ascending from a liquefied source bed are common in chapters on soft sediment deformation. Journal articles, however, contain remarkably few examples of sheeted clastic dikes. The number of articles describing sheeted clastic dikes filling lateral spread fractures from the top is minuscule. I've reviewed >300 articles on clastic dikes published since 1830s. To my knowledge, there is not a single article that describes sheeted, wedge-shaped dikes formed by earthquake-triggered lateral spreading (i.e., no analog in the known geological record). Several articles do, however, describe such dikes where the ground surface was overridden by glaciers, lahars, and debris flows.
Where Is Clear Evidence of Lateral Spreading in Eastern Washington?
Lateral spreading coincident with diking is clear along the banks of the Upper Columbia River (Lake Roosevelt) north of the Spokane River. The varved beds exposed in high shoreline bluffs near Hunters and Inchelium were deposited on the bottom of Glacial Lake Columbia during the late Pleistocene. Large, wedge-shaped, single-fill dikes (unsheeted) descend for 10-20m through the section, terminating just above a highly-strained zone in the same material. Textures in the basal shear zone resemble those in high grade metamorphic rocks like mylonite. The dikes, up to 60cm wide at their tops, appear to meet the USGS/Varnes criteria for lateral spreading.
How it works up north. Lateral spreading at Hunters and Inchelium went something like this. To learn more about the dike at Hunters, see my other post: https://www.skyecooley.com/single-post/2019/04/06/Clastic-Dikes-in-Lake-Roosevelt-Bluffs
Dikes at Hunters. Dikes descend many meters through shoreline bluffs along Lake Roosevelt.
Single-fill structures. Dikes at Hunters, WA are not vertically-sheeted like those in the Touchet Beds. These formed near a free face that laterally spread and partially toppled into an open channel. They contain sandy-gravelly material supplied by glacial outwash that overlies the lake beds and armors the terrace surface above. Cobbles up to grapefruit size are contained in dike fills.
Basal shear zone. Mylonite-like shear fabric occurs in a bedding-parallel one near the base of the exposure, below where dikes pinch out. The deformed material is unconsolidated silt and clay.
Shear zone in context. Flat-lying varve sets occur above the shear zone and beneath the swirled zone below it. The dark-colored lake-bottom clays represent background sedimentaton (varves) that alternates with light-colored silts. Silts are the coarsest grain size here and likely represent the plume of suspended sediment and some portion of the associated bottom-hugging density flow. Flood currents moved up the valley out of the main body of Glacial Lake Columbia. Hunters is the distal, northern limit of Missoula backflooding in the Columbia Valley (Hanson and Clague, 2016, Fig. 1). The Hunters area would be a great place for an MS student with a good kayak interested in subaqueous deformation structures to base out of. The place is peaceful and beautiful. Good campsites abound. The local tavern attracts a cast of colorful characters. Discoveries would be made.
Lateral spreading is a type of landsliding commonly observed in tide flats, recent alluvial fills, or deltaic deposits subjected to shaking by strong earthquakes. The desire to attribute sheeted clastic dikes in Missoula Flood rhythmites to lateral spreading is understandable. Extension certainly did occur with diking, but spreading wasn't the dominant mechanism. Field evidence provides little support for the hypothesis; I've changed my mind since my undergraduate thesis work in 1995 (Cooley, 1996) The dikesnumber in the hundreds of thousands and 99.9% of them have identical characteristics. They were not produced by repeated mass wasting of any sort. Different types of slides, slumps, and flows would have occurred with spreads, each leaving behind its own evidence (deformation structures and deposits) different from that left by lateral spreads. Fine laminae and delicate primary structures are pristinely preserved in both dikes and rhythmites today, more than 10,000 years later. Diking occurred along discrete fractures; dikes did not passively fill tensile surface fractures nor follow rubble zones between slide blocks. An explanation consistent with dike morphology, distribution, and age is detailed in Cooley (2020) and elsewhere on this website. Flood-load triggered, fluid-driven fractures were opened and rapidly filled by sediments sourced in circulating sediment-laden currents of glacial outburst floods. Flooding, loading, fracturing, and diking were coincident. Touchet-type dikes are flood injectites (naturally-propped hydraulic fractures), not liquefaction structures (soft sediment deformation features).
Eastern Washington's clastic dike problem has suffered from too much speculation and too little data, especially from college faculty. Articles and abstracts published by local geologists to date are nearly devoid of any actual measurements. I've attempted to change that by measuring thousands of dikes at >500 localities (Cooley, 2020). While certainly inadequate, mine is a unique and substantial dataset representative of the entire region the dikes are found (>30,000 km2)
Those who favor a lateral spread origin either need to redefine the term, create a new term for their purposes, or explain the absence of a liquefied sliding surface at classic localities such as Touchet Valley and Lewiston Basin. Data supporting a lateral spread origin has not been presented to date. Got data? I'd be happy to hear you out. Do something new, something quantitative (science), then give me a call. Observing the same outcrops year after year and professing to "just know" how the dikes formed (faith) - or worse, calling them "multigenetic" structures (ignorance) - is no longer acceptable. We're all learning here, but some of us have solved a few things. Recognize it. Keep current. Embrace relevant phenomena not addressed by your own education or by traditional Geology curricula (i.e., fluid-driven fracture).
Your comments are always welcome: firstname.lastname@example.org
Cooley, S.W., 2020, Sheeted clastic dikes in the megaflood region of Washington, Oregon, Idaho, and Montana, Northwest Geology, v. 49, p. 1-17, https://www.skyecooley.com/single-post/2020/09/15/sheeted-clastic-dikes-in-the-megaflood-region
Cooley, S.W., 1996, Timing and emplacement of clastic dikes in the Touchet Beds of south-central Washington, BA thesis, Whitman College, 37 pgs.
Cooley, S.W.; Pidduck, B.K.; Pogue, K.R., 1996, Mechanism and timing of emplacement of clastic dikes in the Touchet Beds of the Walla Walla Valley, GSA Cordilleran Section Abstracts with Programs, v. 28, p. 57
Hanson, M.A., Clague, J.J., 2016, Record of glacial Lake Missoula floods in glacial Lake Columbia, Washington, Quaternary Science Reviews, v. 133, p. 62-76
Obermeier, S.F., 1998, Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA, Engineering Geology, v. 50, p. 227-254
Pogue, K., 1998, Earthquake-generated(?) structures in Missoula Flood slackwater sediments (Touchet Beds) of southeastern Washington [abstract], Geological Society of America Annual Meeting Abstracts with Programs, Session 174, Abstract T43
U.S. Geological Survey, 2004, Fact Sheet 2004-3072, Types of landslides, https://pubs.usgs.gov/fs/2004/3072