"Considerable work awaits sedimentologists on and adjacent to the Columbia Plateau before we will be cognizant of all aspects of the tectonic, volcanic, and climatic influences on regional sedimentation during the Neogene." - Gary Smith and others (1989)
Where is the Neogene record of seismic shaking in eastern Washington?
The Yakima Fold Belt (YFB) in central Washington is a set of west trending, fault-cored folds located in the back-arc of the Cascadia Subduction Zone. The have been rising for at least the past 10 million years. The largest folds in the in the ~14,000 km2 province (i.e., Saddle Mountains anticline) have accommodated >1 km of shortening over this time (Staisch et al., 2017). Shortening on other structures appear roughly similar (Kelsey et al., 2017). Despite their topographic prominence and proximity to major infrastructure (hydroelectric dams, nuclear sites), we know little about the seismic history prior to 2 Ma.
Was rise of the Yakima Folds over the past 10 million years accompanied by large earthquakes?
Upper Ringold sediments at the White Bluffs contain small-scale soft deformation structures consistent with local sedimentation. No seismites (continuously deformed beds) have been reported to date. The Ringold is a Miocene-Pliocene formation (8.0-3.4 Ma). The Neogene consists of the Miocene and Pliocene (23.0-2.6 Ma).
Aeromagnetic mapping reveals the crustal connection between Eastern and Western Washington beneath the Cascade Mountains (Blakely et al., 2011). The Yakima Fold Belt is located at lower right, the Cascades at center, and Puget Sound at upper left.
Soft sediment deformation structures (clastic dikes, sand blows, liquefaction/fluid escape structures, continuously-contorted beds, etc.) in sediments near active mountain belts are commonly used as indicators of past seismic activity. To date, widespread soft sediment deformation attributable to seismic shaking has not been identified in the Miocene-Pliocene Ringold Fm (8.0-3.4 Ma) (Merriam and Buwalda, 1917; Newcomb, 1958; Hays and Schuster, 1983; Lindsey, 1996; DOE, 2002; Williams et al., 2002; DOE, 2010; Staisch et al., 2017). Late Neogene sediments with characteristics favorable to both form and preserve soft sediment deformation structures are abundant within Yakima Fold Belt province, though surprisingly few are known. Snipes Mountain near Granger, WA appears to be one of few places such evidence may exist, but no paleoseismic investigation of the Snipes Mountain area has been published to date.
Geodetic station data for the fold belt describe the current situation as "a slow strain environment". Reidel (1984) found deformation rates were different in the geologic past, with the period from 17 to about 10.5 Ma being a distinctly more active period of fold deformation than the period from 10.5 Ma to the present. He argued 80% of the total strain occurred from 17–10.5 Ma. Staisch et al. (2018) reported deformation rate increased during the Pleistocene (<2.6 Ma). This finding contradicts previous interpretations and notions that "the entire fold belt has continued to develop in a pattern similar to that of the Pliocene and Miocene" (Reidel et al., 1994).
Mapped surface ruptures in the fold belt are all young features (late Quaternary) (Campbell and Bentley, 1981; West et al., 1996; Blakely et al., 2011). Data from trenched faults in Pasco Basin (Bennett et al., 2016), scarps visible in lidar images (Kelsey et al., 2017), and dates on young strath terraces in Yakima Canyon (Bender et al., 2015) also demonstrate late Quaternary fault movements, but provide little information on older (Neogene) uplift other than measurements of structural relief and steady-state uplift rate estimates for a few of the folds.
Uplift of the Cascade Range and rise of the Yakima Folds, with exception of the north-trending Naneum structure, occurred after Grande Ronde Basalt inundated a low-relief landscape and cooled around 15.6 Ma. Kelsey et al. (2017) places the onset of YFB tectonism at 12-7 Ma based on dating of a Quaternary strath terrace at the Manastash anticline, "the anticline started to contract a few to many millions of years after emplacement of the Grande Ronde Basalt".
If the fold belt's 12 fault-cored folds are capable of producing earthquakes with magnitudes >M 6.5 as researchers have claimed, most recently USGS (Staisch et al., 2018), there should be ample sedimentary evidence of strong shaking preserved in >100m of Ringold Fm section exposed at the White Bluffs. The Ringold sediments there represent >4 million years of near-continuous sedimentary filling of the Pasco Basin syncline. Ringold sedimentation was coeval with and post-dated YFB uplift; Ringold strata are tilted and folded in places, flat-lying in others.
Other large, well-known exposures of thick sedimentary sections, including Saddle Mountains, Taunton, Kittitas Valley, and Yakima Valley should likewise show evidence of earthquake disturbance at recurrence intervals reported for YFB faults. Abundant borehole information from the Hanford Site (>7500 wells) and Quincy Basin should, too. Sedimentary interbeds in the Columbia River Basalt (Ellensburg Fm/Latah Fm), the Thorp Gravels (4.9 to 2.9 Ma), and Ellensburg-equivalent basin fill deposits in the Dalles-Umatilla syncline (Newcomb, 1966; Madin and McClaughry, 2019) should contain similar evidence.
Coarse, basaltic "fanglomerates" (boulder gravels) that mantle the flanks of YFB ridges, interpreted as proximal alluvial fan deposits shed into adjacent synclinal valleys during uplift, also warrant examination. If uplift was continuous since the Miocene, the coarse gravels should be abundant throughout not only the Pleistocene-Holocene section, but the entire Pliocene and latter part of the Miocene section as well. Are the gravels wedge-shaped fans or broad blankets? Is reconstruction of the paleoslope the gravels were deposited upon on possible? Are some gravels confused with cataclysmic outburst flood deposits?
In short, the sedimentary record preserved in synclinal basins of central Washington should reflect 10 million years of continued uplift. Does it?
Shaded relief map of the top of the Ringold Fm (~11,500 square km area). Map by Kennedy/Jenks Consultants for Franklin Conservation District (Triangle Associates, 2003, p. 10).
Patterns of fault displacement come in 5 flavors: Episodic-quiescent, Episodic-active, Decelerating, Constant, Accelerating. Which style describes Yakima Fold Belt over its lifetime? Have uplift rates and seismic recurrence increased or decreased over time? How do Holocene and Pleistocene recurrence data for these faults compare to the Neogene record? Figure by McAlpin and Nelson (Figure 1.4, p. 10) in McAlpin (2009).
A well-established relationship exists between earthquake magnitude and the distribution of liquefaction structures in the region surrounding the epicenter (Ambraseys, 1988; Galli, 2000). The study authors suggest an M 6.5 to 7.0 quake may be expected to produce surface ruptures and subsurface soft sediment deformation structures out to a distance of ~100km. The yellow circle shows the predicted damage halo for a quake with its epicenter placed near the center of the Yakima Fold Belt. The circle encompasses portions of the eastern slope of the Cascades, Pasco Basin, Umatilla Basin, southern Okanogan, and western Palouse Slope.
Data compiled from earthquake-affected regions around the world reveal a fairly robust relationship between magnitude and liquefaction (figure redrawn from Galli, 2000). Based on the various curves, the damage halo for an M 6.5-7.0 quake would extend outward 25-125 km from the epicenter. A reasonable search area for liquefaction features in Eastern Washington could reasonably be constructed from a set of 75 km-radius circles placed on the centroids of all mapped YFB faults.
If the faults that core the Yakima Folds are capable of generating M 6.5 to 7.0 magnitude earthquakes, then "secondary sedimentary evidence" of strong shaking should be present somewhere in central Washington (Ringold Fm, Ellensburg Fm, Pleistocene flood deposits, etc.). The figure highlights one example from the Miocene-Pliocene Ringold Fm. Figure modified from McAlpin and Nelson (Figure 1.6, p. 16).
A simplified cross section through a portion of the Yakima Fold Belt centered on the Pasco Basin. Researchers suspect a basal decollement exists beneath the YFB (Campbell and Bentley, 1981; Miner, 2002), though cross sections like the one above (Reidel et al., 1989) rarely depict deeper parts of the thrust belt.
Foreland systems have been described by DeCelles (2012). They consists of several parts, including the orogenic wedge (wedge top), foredeep, and forebulge. If we consider the Yakima Fold Belt to be a foreland system (albeit a small one), the fault cored-ridges are the orogenic wedge, the Pasco Basin the foredeep, and the western Palouse Slope the forebulge. In such systems, denudation atop forebulge high advances cratonward with the orogenic wedge, creating an unconformity (Crampton and Allen, 1995). Forebulge steepening and relative drift toward the thrust wedge (seemingly backward, increasing erosion there) appears possible, based on lithospheric flexure models.
Linear elastic lithosphere model for forebulge formation, migration, and erosion (modified from Crampton and Allen, 1995). As the thrust wedge advances east, the crest of the forebulge shifts west. The amount of erosion during thrust wedge growth occurs at the forebulge unconformity (post-Ringold unconformity). By this logic, more Ringold sediment should have been removed from what today is the northeastern Pasco Basin-western Palouse Slope. A stronger uplift signal (wave-like passage of the forebulge) should be present in the sedimentary record there. Other foreland flexure models migrate the forebulge cratonward with the orogenic wedge and depict no reverse migration of the forebulge crest. The erosion amount would be similar, but distributed over a longer baseline. I have not previously heard or read any discussions of a forebulge or forebulge unconformity with respect to the YFB.
According to DeCelles (2012),
The standard [foreland] stratigraphic succession consists of a several km-thick upward-coarsening sequence marked in its lower part by a zone of intense stratigraphic condensation [condensed stratigraphic section] or a major disconformity (owing to passage of the forebulge), and in its upper part by coarse-grained proximal facies with growth structures (the wedge-top depozone). Foredeep deposits always reside between the forebulge disconformity/condensation zone and wedge-top deposits, and backbulge deposits may be present in the lowermost part of the succession. Wedge-top deposits are vulnerable to erosion because of their high structural elevation, and preservation of backbulge and forebulge deposits depends in part on tectonic setting.
Can the Yakima Fold Belt be considered a thin-skinned foreland system with parts described by DeCelles (2012), a basal decollement, and duplex-style thickening at depth?
Do strata in and around the Yakima Fold Belt fit the standard stratigraphic succession?
Are the calcrete pedocomplex (developed atop the post-Ringold unconformity) and the basaltic boulder gravel "fanglomerates" in northeastern Pasco Basin, the foredeep and wedge-top portions of a standard upward-coarsening foreland succession constructed during uplift of the Yakima Fold Belt? That is, are these deposits represent the proximal wedge-top over foredeep deposits over the forebulge unconformity?