Liquefaction at Finley Quarry? A critique of Sherrod et al. (2016)
"Our current understanding of the Western Cordillera was built from equal parts science and personality. The Geology and the Geologist cannot be separated."
Opening comment from Dr. Art Snoke on the first day of his graduate course
on Cordilleran Tectonics, University of Wyoming, 1998
"The Columbia Plateau...is tectonically active...it is foolhardy to pretend that we can predict what will happen in the Pasco Basin in the next 10,000 years. "
Synopsis of a presentation given by P.R. Hooper to the Pacific Northwest Metals and Minerals Conference by R.G. Raney, USDOI Bureau of Mines, 1987
Two reports by two different teams of geologists describe the same outcrop, sediments, and features at Finley Quarry, WA (Coppersmith et al., 2014; Sherrod et al., 2016). I visited this site several times in years prior to those teams' visits, as have numerous geologists in decades past (Jones and Deacon, 1966; Jahns, 1967; Brown, 1968; Bingham et al., 1970; Rockwell, 1979; Farooqui, 1977; Farooqui and Thoms, 1980; Foundation Sciences, 1980). When the Sherrod/USGS article was published, I was a bit surprised by some of their descriptions and interpretations. I consulted my own field notes and photographs. I found a few apparent discrepancies and emailed my questions to the lead author. Receiving no response from him, I contacted his senior colleague at U.S. Geological Survey (Department of Interior) to ask if he might be willing to forward my questions. He did, but again, no response. My reply to the Sherrod et al. (2016) article is below.
Oblique aerial photo of Finley Quarry from Google Earth.
My field sketch of the basic geology at Finley Quarry. The area of concern lies at the far left.
A "clastic dike" shown in Figure 7.15 of Coppersmith et al. (2014) caught my attention. There are several conspicuous breccia-filled fractures in the high wall of the basalt quarry (fault gouge), so my first thought was that might be describing those. But the clastic dike was not one of those. It was clearly enclosed by sediment and labeled a "liquefaction" feature.
The fault gouge was determined to be middle Pleistocene, dated by U-series method (Paces, 2014):
"Carbonate‐rich gouge present along the fault exposed at Finley quarry shows complex layering and cementation paralleling the attitude of the fault...ages range from 700 +270/‐160 ka to 224 ±36 ka...These ages are consistent with repeated faulting over a several hundred thousand year time frame in the middle Pleistocene. If younger fracturing and cementation events are present (for instance, the event that formed the colluvial fault wedge dated at 17 ka), they were not sampled in this block of gouge."
I believe this feature is misinterpreted by Sherrod et al. (2016) as a Holocene liquefaction feature created by seismic shaking along the Wallula Fault Zone (Olympic-Wallowa Lineament). While other information at the site, documented by these two teams and by others in the past, supports faulting and seismicity, the clastic dike could be interpreted differently.
A simpler explanation is that the dike is Pleistocene in age (not Holocene). Thousands of such dikes intrude the Touchet Beds in the area. The quarry lies below the maximum level of Missoula floods (Lake Lewis shoreline = 380m elevation) in which the Touchet Beds were deposited.
I reinterpret their "liquefaction dike/sill (E4)" to be older than the modern soil (Units, 17, 18 in Fig. 7.15), thus does not cross-cut it. Rather, like many other clastic dikes in the region, the feature predates the modern soil (Holocene "Humic-stained L1 loess"). The top of the dike is overprinted (truncated) by the modern soil. The tops of many clastic dikes in the vicinity similarly appear to fade upward over a distance of a few centimeters into the base of the modern soil profile. Pedogenic processes have obliterated the top of the dike.
If the dike is Pleistocene in age, there is a good chance it is of non-seismic origin. Liquefaction features are not common in Eastern Washington (no reports to date) despite abundant wet, unconsolidated sediments with grainsize distributions that make them susceptible to seismic shaking located in close proximity to faults known to be active since the Neogene.
No source bed identified and no taper direction provided by Sherrod et al. The dike-sill feature could just as easily be a typical downward-injected feature, like thousands of others in the Touchet Beds, locally disrupted by mass wasting at the site.
I believe the Sherrod team was overenthusiastic in attributing the "clastic dike" to earthquake-caused liquefaction. Localized liquefaction features, given their rare occurrence in the region, should be clearly distinguished from the more familiar and widespread set of sheeted silt-, sand-, gravel-filled dikes associated with megaflood deposits throughout the region (Jenkins, 1925; Cooley, 2020).
Figure 7.15 in Coppersmith et al. (2014) shows the outcrop at Finley Quarry examined by the Sherrod/USGS team and the Coppersmith/Quaternary Studies Team. The dike is circled in red.
Geologists from Foundation Sciences, Inc. drew this sketch of the Finley Quarry exposure some 36 years prior to arrival of the Sherrod/USGS team when the surficial strata was still present (Foundation Sciences, 1980, Figure 4). Their caption reads, "Sketch of Finley Quarry Fault. Note unfaulted post-flood colluvium and loess overlying older faulted materials.". Sherrod should have compared features clear in this sketch to those in his own.
Others would seem to agree. Curious comments that appear in a table near the end of the Coppersmith et al. article (Coppersmith et al., 2014, Table 7.2) suggest there were disagreements about the stratigraphy observed at Finley Quarry, including the clastic dike-sill feature,
The postulated liquefaction feature (dike and sill d) that appears to post-date the deposition of [~11,000 year old Glacier Peak G] tephra is the strongest evidence for a strong shaking earthquake in the general vicinity of Finley quarry (which may or may not be on the RAW-Rattles fault source). The best expressed part of the mapped feature cross cuts Unit 13. The USGS log interprets Unit 13 as L1 loess. However, this unit shares characteristics similar to massive to poorly bedded slackwater deposits observed elsewhere (e.g., at the MCBONEs locality) (i.e., it contains scattered clasts and irregular blocks of pre-flood ice-rafted or ripped up material). Although the mapped extent of the feature up into the modern soil suggests a post-flooding event, additional work or review is needed to preclude a flood-related liquefaction phenomenon.
I disagree with some of the interpretations made at Finley Quarry by the Sherrod/USGS team, mostly because their description of the liquefaction feature, a key element in their young, strong seismicity narrative, is so abbreviated and their field sketch crude. Its a pattern I've found in previous work by Sherrod. In his first few years with USGS, he recalculated the 1936 Stateline/Milton-Freewater Earthquake magnitude (no change made) and located a scarp using lidar which he called the 1872 Chelan Earthquake epicenter (epicenter relocated several miles south and quake renamed it the North Cascades Earthquake). Both recalculation and renaming projects were wastes of time. Sherrod tends to inflate the seismic hazard of the Inland Pacific Northwest and his own image in the press.
Questions about the geology at Finley Quarry:
1.) Is the "liquefaction dike-sill" in Unit 19 something different than the vertically-sheeted clastic dikes ubiquitous in the Touchet Beds?
If the same, the feature would properly be called a Pleistocene clastic dike deformed by Holocene mass wasting associated with the local hillslope or other injection process. Unit 19, "gravelly silt to fine-medium well-bedded sand interbedded with silt laminae", appears to describe Missoula flood rhythmites. If not, are similar dike-sill features present in other Eastern Washington locations?
2.) Did you examine nearby outcrops?
Huge roadcuts that beautifully expose the same bedrock and surficial strata occur along strike just west of Finley Quarry along Hwy 397. Dozens of clastic dikes intrude Pleistocene sediment in the vicinity. The Hwy 397 cuts are hundreds of meters long and twenty meters high. All are hikeable. A representative suite of features and relationships are fully developed here. The Finley Quarry site is a highly disturbed, manmade site and a far cry from anything resembling a type section for surficial units.
3.) Was a source bed for the liquefaction dike-sill feature identified?
To date, no clastic dikes of Pleistocene age in Eastern Washington have been shown to ascend from a buried (liquefied) source bed. I am unaware of any reports describing Holocene liquefaction dikes in the Cascade Range or east of the Cascades. Perhaps recent trenching by USGS has uncovered some in in Yakima Fold Belt (i.e., Bennett's work).
4.) Could your trench Units 16 and 13 be diamicts similar to those described by Spencer & Jaffee (2002) and Bader et al. (2016) at locations in the northern Walla Walla Valley?
These authors interpret massive to crudely bedded, silty deposits that contain anomalous pebbles to ancient floods sweeping loess-covered hillslopes clean. In the western WWV, eastern Pasco Basin, and southern Palouse Slope (areas located just north of Finley Quarry), these silty beds are relatively common. They are associated with Palouse loess below 380m elevation and contain calcic paleosols, mammal fossils, and several tephras. Bedding is very difficult to resolve in water-lain silt. The presence of pebbles suspended in the otherwise massive matrix is the clue to a water-lain origin, thus diamict. If so, these would not be Touchet Beds, as indicated on p. 1652 and in Fig. 12, but older flood deposits or reworked loess (probably pre-late Wisconsin).
An undeformed clastic dike in the Touchet Beds of south-central Washington State.
Bingham et al., 1970 , Geologic investigation of faulting in the Hanford region, Washington with a section on the occurrence of microearthquakes , U.S. Atomic Energy Commission, Division of Reactor Development and Technology Report #90-27, 126 pgs.
Camp et al., 2017, Field-Trip Guide to the vents, dikes, stratigraphy, and structure of the Columbia River Basalt Group, eastern Oregon and southeastern Washington, USGS/DOI Scientific Investigations Report 2017–5022–N, p. 65-66
Cooley, S.W., 2020, Sheeted clastic dikes in the megaflood region, MT-WA-OR-ID, Northwest Geology (Tobacco Root Geological Society), v. 49
Coppersmith et al., 2014, Appendix E: Structural analysis and Quaternary investigations in support of the Hanford PSHA in Hanford Sitewide Probabilistic Seismic Hazard Analysis, Pacific Northwest National Lab Report #23361
Foundation Sciences, Inc., 1980, Geologic reconnaissance of parts of the Walla Walla and Pullman, Washington, and Pendleton, Oregon 1 degree x 2 degree AMS quadrangles , a consultant's report to U.S. Army Corps of Engineers-Seattle District, Contract DACW67-80-C-0125, 144 pgs.
Jenkins, O.P., 1925, Clastic dikes of eastern Washington and their geologic significance, American Journal of Science, 5th Series, v. 10
Paces, J.B., 2014, 230Th/U ages supporting Hanford site-wide Probabilistic Seismic Hazard Analysis, AMEC/PNNL/USDOE Office of Environmental Management/USGS Report, 28 pgs.
Sherrod et al., 2016, Active Faulting on the Wallula Fault within the Olympic-Wallowa Lineament, Washington State, USA, Geological Society of America Bulletin v. 128