Lind Coulee Fault at O'Sullivan Reservoir

Field trip map. If you have a Discover Pass, park at the WDFW fishing/hunting area and the hike short distance east to shoreline bluff exposures. If not, park at the informal pullout near the south abutment of the bridge on Road M SE or in the larger public lot with a pit toilet north of bridge. Follow an informal footpath west along top of bluff through cheatgrass and sage to outcrops exposing the fault. Explore the other outcrops east of the bridge and the roadcut along Hwy 262. Gaiters, boots, long pants, and bug spray recommended. I also take a set of clothes to change into afterward.

View looking east from near WDFW parking area. The shoreline makes for easy walking during low water periods. There's a low beaver lodge in the gully near the powerline crossing. They'll splash around and swim out if you step on it. I saw great horned owl, blue heron, pelican, and white egret when I last visited.

Lind Coulee Fault. The Lind Coulee Fault, a south-dipping thrust, places Miocene basalt (Wanapum Roza) over younger sediments. Mapping geologists, Grolier and Bingham identify it in their draft and final reports (1971, 1978 Figs. 14, 23). In the photo above, rubbly Roza is shoved over the "brown tuffaceous sand" of Grolier and Bingham (1978, p.44-46). Nearby, the fault places the Roza over Palouse loess. The so-called brown sand is an unusual unit that is not often well exposed. Confusion exists over its age. It may correlate with sediments Bryan (1927) suspected were the "basal part of the Palouse loess". Grolier and Bingham suggest it is found with the "red sandstone and terra cotta silt" of Calkins (1905). They also indicate it interfingers with laminated lacustrine siltstones of the Savage Island Member of the Ringold Fm. I have not observed that relationship myself, but have seen a messy red sandstone layer associated with variegated paleosols in the area (Watt Ln slide, Scooteney Rd, White Bluffs Overlook, etc.). The following quote suggests they were not confident of the relationship between this particular alluvium and the Ringold,

"In this report, where the field identification of rocks was based solely on megascopic characteristics, a layer-by-layer correlation of the brown tuffaceous sand underlying the uplands with the sand intercalated in the buff laminated clay at or near the White Bluffs was not possible"

Culver (1937) also observed the alluvium or something similar, but seems a bit vague and might mistake it for the Washtucna Soil or another older loess unit. At Lind Coulee Wasteway, some 4m of the uncemented to weakly cemented water-laid alluvium is exposed. Its more than just sand. It consists mainly of dark brown overbank muds with lenses of gravelly sand and paleosols. A cobby sand bed lies at the base of the unit, beveling the top of a prominent white to green to dark brown paleosol developed in very fine grained mud. The age of the alluvium is not known. If it is Pliocene, it is remarkably uncemented; hardness appears mostly a function of clay content, not lithification. If it is Pleistocene (early to middle Pleistocene?), then it may be a rare remnant, possibly from the ancestral Crab Creek floodplain.

Stratigraphy in outcrops. As you walk west from the bridge, the stratigraphy changes from one exposure to the next. This is how it looked to me. The lower "calcrete" is not calcrete; its a cemented, burrowed Ringold mudstone.

Close up of fault. Fault zone stratigraphy moving left to right: Rubbly Roza basalt, a thin white gouge zone along fault, a sliver of brown mudstone in footwall underlain by cemented buff-colored loess with light band of caliche (steeply dipping) and disturbed uncemented loess. Shattered Roza is brecciated along the fault and takes on a greenish-yellow hue. It grades upward to competent basalt that is spheroidally weathered in places.

Variation on gouge. Elsewhere along the fault, the gouge zone (10-20cm wide) includes boudin-like lenses of deformed dark and light brown mudstone, rock flour, and broken basalt. Fault puts basalt over brown alluvium here.

Basalt breccia. The fault also expresses as a zone of breccia. In fact, this may be a small shear zone that parallels the main fault. Fault puts basalt over basalt here, if this actually is the main fault. Note the vesicular flow top below is not present above.

Footwall boulders. In September 2021, I discovered rounded basalt boulders embedded in white-green mudstone beneath the fault plane. The mudstone is distinctive paleosol that lies at the base of the brown alluvium, well exposed nearby. The boulders struck me as unusual. A colluvial deposit? An old megaflood deposit? Something else? Previous geologists surely would have mentioned these boulders in earlier reports, if seen.

Corestone supply and transport. Here's what I think is going on. The boulders below the fault are actually corestones caught up in the wide gouge zone. They were liberated from footwall Roza by faulting, but are not far-traveled. The sketch shows the same weathered zone in the hanging wall Roza. Ball bearing action on the Lind Coulee Fault? Maybe. Sounds pretty cool, anyway.

Round boulders in situ nearby. Spheroidal weathering is common in basalt, especially the Roza flow. All of the corestones in the photo are attached to their footings. This hanging wall exposure is located just a few meters west of (above) the fault.

Corestones. Corestones of weathered Roza basalt

Boulders made while you wait. Spheroidally-weathered Roza basalt is exposed in a nearby roadcut along Hwy 262 across from the gravel pit (see map). Chemical weathering has sculpted the blocky basalt into these smooth, rounded forms. Some of these corestones have been liberated from their rocky footings by vigorous Ice Age floods and are deposited in great piles downstream. Be careful when interpreting the roundedness of basalt boulders in the Columbia National Wildlife Refuge/Drumheller Channels area - they may not be very far-traveled at all. Some came pre-rounded from the factory. View to the north.

Cross section. It's not this simple, but here's my interpretation of the general fault geometry at Lind Coulee Wasteway. The "calcrete" above the Roza is not calcrete; its a cemented Ringold mudstone. I've not seen the Michael West/GEI reports written for U.S. Bureau of Reclamation (West & Shaffer/GEI, 1988; Shaffer and West, 1989), which may contain a cross section and/or better info. Lind Coulee Fault is part of the larger Frenchman Hills structure, known to have Quaternary movement (Reidel and Fecht, 1994; Schuster et al., 1997; Lidke and Haller, 2016). USBOR memos refenced below recount some details of West's trenching project from the perspective of the client (Lefevre and O'Connell, 1987; Galster, 1987). A review of the investigation is included in a later report written by West (Geomatrix Consultants Inc., 1990).

Calcrete ledge. Below the brown alluvium is an 80cm-thick light-colored ledge with concrete-like hardness. It is densely burrowed (1" diameter tubes) and appears to be ancient loess or reworked loess thoroughly cemented. The flat ledge crops out along the shoreline and is exposed during low water periods (August or later). Basalt is underneath. The pictured exposure is on the south shoreline <100m east of the bridge, just north of the informal parking area (see map). View to the south.

Ledge tilts south. The light-colored, cemented ledge is tilted to the right (south). The fault is exposed in outcrops in the background. Though weeds hide it from view, the tilted ledge is continuous for tens of meters and holds the same dip. Tilted by faulting or just a slump block? View to the east.

Alluvial stratigraphy. My interpretation of a portion of the alluvial section ("brown tuffaceous sand") at Lind Coulee Wasteway. Looks like an aggrading stream and floodplain, deposition just beyond the margin of a larger lake basin to the west and south. The cemented, light green paleosol at the bottom looks very much like a Ringold unit. This distinctive paleosol, or one identical to it, crops out in the Savage Island section around the Othello area. If you look close, you'll see lenses of gravel in it.

Angular gravel. Zooming in on the angular gravel. The white-green paleosol is unconformably overlain by a tan sandy-gravelly unit that pinches laterally. Shown here at its thickest, the gravelly bed contains subangular to subrounded clasts, mostly basaltic, but some mudstone rip ups are there too. The muddy matrix is curious. Not far traveled. Stream gravels typically contain abundant rounded clasts. This may represent a shallow channel moving along a the base of a basalt cliff and its colluvial apron (talus and loess). The channel, choked by angular material and sediment contributed from the hillslope during a flash flood or high water storm event, became a mud flow. The simplest explanation involves local channel and hillslope processes, not megaflooding.

Swaley brown alluvium. The swaley brown alluvium overlies the distinct cracked paleosol that grades east, becoming green and white. It maintains the same stratigraphic position as the brown and hold this clear, sharp contact all along the exposure. I think this is a Pliocene paleosol with Pleistocene brown alluvium above. Its more fun if its Pleistocene. I've observed little in the Ringold that looks like this brown stuff. The Savage Island paleosol intervals never contain alluvium this thick.

Mazama ash. Mazama ash with backfilled rodent and insect burrows occurs in young alluvium and loess at the top of the section. The gully wash-thickened ash deposit is not well exposed in shoreline bluffs west of the bridge. This outcrop is just north of the informal parking area, east of the bridge. View to southwest.

Sedimentary record of seismicity. There is no evidence of strong shaking preserved in unconsolidated sediments at the site. I found no clastic dikes or other soft sediment deformation structures in the alluvium that lies in direct contact with the Lind Coulee Fault.


Grolier and Bingham (1971, 978), 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", Fig. 14), Geomatrix Consultants Inc. (1990, "East Fault Exposure"), Reidel and Fecht (1994), Schuster et al. (1997), Lidke and Haller (2016)

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