Calcrete Field Trip - Corfu Landslide Overlook

Directions - From Othello, drive Hwy 26 west to the White Bluffs Area gate. Do not turn. Instead, continue west on the highway 2.8 miles farther. Turn R (north) onto the gravel road (Corfu Rd) and locate the sign "Saddle Mountains Overlook 5 miles". Drive north up the south flank of Saddle Mountains. At 4.9 miles cross the canal, after which potholes and washboard increase. At 7.1 miles reach the saddle crest and 4-way intersection (565m elevation). Turn R (east) at the sign to "Saddle Mts Overlook 1 Mile". Continue up the easy incline to concrete slabs and road's end. Park and walk to the head scarp of the Corfu Landslide Complex.

Saddle Crest 4-way Intersection 46.793399, 119.470546

Red Tank 46.802443, 119.465334

Return route and other stops - Return to the 4-way intersection at the saddle crest. Choose one of three options: a.) Stay L to return to Hwy 26. b.) Stay R for Red Tank Hike. The north half of Corfu Road is a rough, high-clearance route that descends through dusty, loose switchbacks to Crab Creek Valley. c.) The middle route leads to Wahatis Peak summit and radio towers. Gravel road that roughens near the top.

Aerial view of the Corfu Landslide Complex. Star is the overlook location. The Old Milwaukee railway cuts a path through Ringold Fm sediments and slide debris. The Taunton Substation is just off the left side of the photo along the tracks. Base image was brazenly stolen from Baker et al. (2016). I added some annotations to assuage my guilt. View looking south.

Green areas are benches cut into landslide deposits by glacial outburst floods moving west down Crab Creek Coulee. Pink area is Corfu Landslide released from the north flank of the Saddle Mountains. Pink slide debris appears to truncate the green benches (overlap area), which would indicate sliding predated and postdated flooding. Lidar hillshade base image from Washington Geological Survey.

Western portion of Taunton Bench and eastern portion of Corfu Landslide.

Corfu Landslide Complex - The shear cliff below your feet is the head scarp of the Corfu Landslide Complex. The complex consists of more than two dozen distinct slides covering some 20 km2 and a volume of ~1km3 (Lewis, 1985). Sliding occurred during the Late Pleistocene probably along a weak, clay-rich interbed within the basalt. Slide debris includes boulder-sized clasts of broken basalt, Ringold, and loess. Sliding may have initiated when the slope was undercut by the Missoula floods. Failure of thick, saturated loess (Hart et al., 2012) and ground vibrations generated by cataclysmic floods might also have played a role. Outburst floods post-date some lobes of the slide complex; flood-beveled benches in slide debris are visible at low elevations. Higher benches are truncated by the slide. Failures of mid-level benches creates a blocky pattern on lidar imagery and hints at more recent instability. Crab Creek has completely reworked slide debris spilled onto the valley floor. Both MSH Set S and Mazama ash are found in loess overlying slide deposits.

Silcrete on basalt. A short hike downhill from the overlook leads to a thin silcrete layer exposed at the cliff edge. The 10-40cm thick bright white material lies unconformably atop weathered Pomona Member of the Saddle Mountain Basalt (10.5 Ma). Parent material may be a thin skiff of loess. The white layers is visible intermittently along the rim of the head scarp. Its age is not known. Did it form a.) At the base of the Ringold, since stripped from the ridge (late Miocene), b.) Atop the eroded fold crest standing above the Ringold lake shoreline (late Pliocene), or c.) In exposed bedrock long after the fold had emerged and the Ringold basin drained (Pleistocene)? Was the basalt flatlying when the silcrete formed (e.g., silcrete predates the anticline)?

Silcrete nearby - Similar silcrete overprints basalt beneath a Miocene sedimentary interbed exposed along the high crest of Saddle Mountains several kilometers west of here.

Active fault segments. The length of the Saddle Mountains between Foster Creek near Badger Gap to the Sagehill Rd-Radar Rd intersection is 85 km using a sinuous (generous) path. Lengths from 90-115km are reported by others (Reidel, 1984; Geomatrix, 1990; Staisch et al., 2017). Quaternary fault activity is known for the Smyrna Bench segment (15.5km) and Saddle Gap segment (27.5km), a total of ~50km or about 1/3 of the total length of the Saddle Mountains Fault. West and Shaffer/GEI (1988) reported no evidence of Quaternary faulting for the Sentinel Gap segment and judged it inactive. The Corfu Landslide Complex is located between Smyrna Bench and Taunton Substation.

No soft sediment deformation - There is a distinct lack of soft sediment deformation in this 3m-high section despite its proximity to the Saddle Mountains Fault and a major league landslide. Evidence of shaking should be here. A thin interval of mildly contorted bedding occurs in places, but its quite unremarkable.

Fishy people - The nearby Taunton Bench site is an important fossil locality. Taunton is stratigraphically the highest of three Blancan faunas in the Ringold and appears to span the Plio-Pleistocene boundary (2.8–3.0 Ma). The three local faunas are the White Bluffs (~4.5 Ma), the Blufftop (~3.7 Ma), and the Taunton (2.8-3.0 Ma). Blufftop equates to our field stop at White Bluffs Overlook. Fish fossils excavated at Taunton indicate that the eastern Pasco Basin was connected to the Pacific via the ancestral Columbia-Salmon-Clearwater River through the Dalles-Umatilla syncline (Columbia Trans-arc Low) in the late Miocene, but isolated from the Pacific Ocean in the Pliocene, likely by waterfalls and water temperature (Smith et al., 2000).

Thick loess on Smyrna Bench - When we think of thick loess in eastern Washington, the Palouse comes to mind. Palouse loess was derived from outburst flood deposits, piled to a depth of 75m in places. Repeated episodes of watery southbound transport alternated with northbound recycling by wind. The oldest Palouse loess dates to 1.15 Ma (Sweeney et al. 2017). Smyrna Bench also hosts impressively thick loess, which farmers take advantage of with the help of center pivot irrigation. 3-4m of loess were exposed in a paleoseismic trench excavated by M.W. West/GEI off Road 17 SW near the base of Smyrna Bench (Section 36, T16N, R26E). Holocene loess 2m thick is exposed in roadcuts of Corfu Road below the mythical Red Tank. Dust accumulation continues today with Pasco storms thickening Palouse soils and Palouse storms thickening Okanogan soils.

Taunton Powerhouse site.

Taunton Powerhouse site.

Taunton Powerhouse site.

Taunton Powerhouse is located on the Old Milwaukee Line.

Taunton redrawn. Compiled stratigraphic, fossil, and paleomagnetic information for the Taunton site. Much of the literature on the Ringold Fm suffers from the use of colloquial terminology, informal units, loose location information, and a paucity of maps showing where actual work was done (i.e., measured sections). A complicating factor is Hanford. Ringold stratigraphy was first worked out for the Hanford Site using exposures outside is fences. If you want to see Hanford's geology, you have to leave Hanford. Even with a neatly compiled figure like this, its difficult to figure out where the study was done. Its been an 80 year struggle to figure out who did what where.

Refs: Sweeney et al. (2017), Baker et al. (2016, Stop 10, p. 42-43), King et al. (2016), McDonald et al. (2012), Hart et al. (2012), Sweeney et al., (2007), Sweeney et al. (2004), Bjornstad (2006), Richardson et al. (1997), West et al. (1996), Busacca and McDonald (1994), Busacca (1989), McDonald and Busacca (1988), Galster (1987), Lewis (1985), Reidel (1984), Bingham (1970).

Links to Other Stops:

Calcrete Field Trip Overview

Stokrose Gravel Pit

Warden Canal

Hendricks Road

Red Tank Hike

Corfu Landslide Overlook

Offramp Stop

Herman Railcut

Coyan Road

Liesle Road

Booker Road at Canal

Lemaster-Hatton Intersection

Lind Coulee Fault at O'Sullivan Reservoir

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