top of page

Lind Coulee Fault near O'Sullivan Dam - Potholes Reservoir, WA

Any excusion into Lind Coulee will include some adventure. This outing involves recent faulting and interesting Plio-Pleistocene stratigraphy exposed in shoreline bluffs along Lower Crab Creek (lower Lind Coulee). As for the geography, Crab Creek, which flows west through Lind Coulee, empties to Potholes Reservoir forming its southeastern arm. Potholes Reservoir (Moses Lake) began filling in 1949 with the construction of O'Sullivan Dam across the spectacular Drumheller Channels scabland (USFWS Columbia National Wildlife Refuge). While the geography can be a bit confusing, its an easy place to access. The hiking is moderate, mostly off-trail and along the shoreline. Brush and loose footing will be encountered. Total roundtrip hiking distance is less than two miles no matter where you park and turn around points are everwhere. Geology here includes Miocene Roza basalt, Pliocene Ringold sediments, Pleistocene loess beds young and old, several paleosols (Plio-Pleist), an old(?) slug of flood gravel, late Wisconsin Missoula flood deposits, and Holocene dune sand.

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. Either way, your goal is the south-side shoreline bluffs. Low water level is essential; some amount of shoreline must be present. From the bridge, follow an informal footpath west along the top of the bluff through cheatgrass and sage to outcrops exposing the Lind Coulee Fault, an eastern extension of the Frenchman Hills structure. Also, explore the other outcrops east of the bridge (alluvium, flood deposits, loess, dune sand) and along Hwy 262 (spheroidal basalt). The intrepid geologist might even hike the full shoreline east to the Warden Outfall public parking area (off map to east); its a rougher go and the outcrops are not as good. Gaiters, boots, long pants, and bug spray recommended. I might also take a set of clothes to change into afterward.

View looking east along shoreline bluff toward Rd M SE bridge. 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 swim out and splash around if you step on it. I saw great horned owl, blue heron, pelican, and white egret when I last visited.

Tha dam geology. The 1954 report on construction of O'Sullivan Dam contains an interesting observation about the Lind Coulee Fault,

The drilling operations revealed that, although the basalt flows were severely faulted and sharply folded along the Lind Coulee flexure, they were elsewhere essentially flat-lying. Along the northern edge of the Lind Coulee flexure zone, the basalt flows were so severely crushed as to create a vertical (underground) barrier which effectually blocked the regional southward movement of ground water through the interflow zones. As a result artesian groundwater conditions existed north of and up to the very northern edge of the flexure, whereas normal groundwater levels existed in the central and southern portions of the flexure and in the undisturbed basalt flows to the south…The entire site lies downstream from the tightly sheared section of the Lind Coulee flexure.

Grolier and Bingham's 1971 map. Site A at upper left is the location of West's trenches.

Lind Coulee Fault. One splay of the Lind Coulee Fault, a south-dipping thrust, places Miocene Roza basalt (middle Miocene Wanapum) over younger sediments. Grolier and Bingham first mapped the area, publishing draft and final reports (1971, 1978 Figs. 14, 23) that identified the fault and loosely described the local stratigraphy. In the photo above, rubbly Roza is shoved over their "brown tuffaceous sand" (1978, p.44-46). Nearby, the fault places Roza over young, weakly-cemented Palouse loess. The so-called brown sand is not a helpful name in today's parlance. Grolier and Bingham did not clearly specify its age or whether it is part of the Pliocene Ringold Fm or stands alone as Pleistocene alluvium of ancestral Crab Creek. Unit descriptions in the older literature is problematic with respect to non-flood, non-loess sediments; we use different unit names today. Alluvium-loess-paleosol strata exposed in shoreline bluffs may correlate with sediments Bryan (1927) suspected were the "basal part of the Palouse loess" or with the "red sandstone and terra cotta silt" of Calkins (1905). A messy red sandstone layer often capped by variegated white-green paleosols is common in the area (Watt Ln slide, Scooteney Rd, White Bluffs Overlook, etc.). Culver (1937) also observed the alluvium or something similar, but his description is a bit vague. West and Shaffer (1988) call it "a poorly understood sequence of Pleistocene paleosols and loessial sediments". The following quote by Grolier and Bingham belies their lack of confidence regarding the stratigraphic position of the deposits,

"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"

I suspect the odd ball alluvial section exposed here at Lind Coulee is a bit compressed. Parts appear to correlate with paleosol intervals in the upper Ringold at White Bluffs Overlook and elsewhere in Pasco Basin. These sediments were deposited near the eastern margin of Pliocene Lake Ringold, trace west to match the 2-3 soil-forming intervals that separate sections of laminated lacustrine siltstones nearer the lake's center. Pre-glacial alluvium-loess-paleosols at Lind Coulee (wetland setting with low-relief uplands) correlate with 2-3 shoalings of Lake Ringold about 3-4 Ma. And other portions of the Lind Coulee section appear to be time-equivalent with deeper water facies in the Ringold.

Stratigraphy in shoreline outcrops. 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. 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" might not be calcrete; its a cemented, burrowed Ringold-like mudstone with soil features. The burrows are often larger diameter than cicada, but otherwise have the same appearance.

Lind Coulee East Fault. Shear zone stratigraphy moving left to right: Rubbly Roza basalt, a thin white gouge zone along fault, a sliver of brown mudstone in footwall is overturned. Below, but stratigraphically above, is cemented buff-colored loess with light band of caliche and disturbed uncemented loess (steeply dipping to overturned). West and Shaffer (1988, p. 80) note this same relationship in their western trench, "...the fault has transported slivers of footwall material to overturned positions within the fault zone". 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.

Photo from GEI/West and Shaffer (1988) showing geologists at the same location in photo above. They trenched nearby.

Paleoseismic trenches. Locations of paleoseismic trenches dug by GEI/West and Shaffer (1988) for the Bureau of Reclamation. Outcrop photos in this post are taken at various locations along the shoreline bluff that extends out of frame to the left (east). The Rd M SE bridge is to the east. View to SE. Locations of trenches, roads, etc. were estimated from historic photos in the 1988 report using a modern Google Earth photo base.

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 deformed mudstone (gouge) is a distinctive paleosol that lies at the base of the brown alluvium. 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 they had seen them. There is no mention of sub-fault boulders in the literature. Michael West told me he had not uncovered them either.

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

Round boulders in situ nearby. Spheroidal weathering marks the top of th 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 and above Lind Coulee Fault.

Corestones. Corestones of weathered Roza basalt formed in place.

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 columnar 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 Drumheller Channels area - they may not be very far-traveled at all. Some came pre-rounded from the factory. View looking north from Hwy 262.

Cross section. It's not this simple, but here's my interpretation of the general geometry of the Lind Coulee Fault. The "calcrete" above the Roza is not calcrete; its a cemented Ringold mudstone. Trenching by Michael West/GEI (West & Shaffer/GEI, 1988; Shaffer and West, 1989) shows a clearer cross section, though I think their unit descriptions are too generalized. 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 referenced 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 are larger than typical cicada) 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. There are several erosional surfaces here and quite a bit of time represented, given the amount of soil development. The cemented, light green paleosol at the bottom looks very much like a Ringold unit. This distinctive unit may be the same that crops out near Othello (McCammon Rd bridge, etc.). If you look close, you'll see lenses of gravel in it.

Slug of 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. Silty and not far traveled. Swept from dusty hillslopes it would seem. Stream gravels typically contain abundant rounded clasts and are winnowed of their fines. This deposit appears to represent a short period of energetic flow within a shallow channel sculpting the base of a low basalt cliff and the toe of its colluvial apron (basalt talus and loess). The channel did not flow long enough to winnow out the fines. It appears choked by angular material and finer matix material - sediment contributed from the hillslope. A flash flood confined to Lind Coulee?. A few kilometers west, the slug of gravel appears in the same shoreline, but there its thicker and more interesting. The deposit is also traceable east of Rd M SE bridge for ~1.5 km. The simplest explanation involves a flash flood down ancestral Crab Creek, not megaflooding. But who believes that? Approximate GPS: 46.987, -119.2112.

The slug grows. Body of angular gravel thickens to >1m in shoreline exposures at WDFW fishing access area located ~1 km to the west. Same gray mudstone and white layer below slug. GPS: 46.9859, -119.2247.

Swaley brown alluvium. A swaley brown alluvium overlies the distinct cracked paleosol that grades east, where it becomes green and white. The alluvium maintains its stratigraphic position and its brown color. This clear, sharp contact runs all along the bluff exposure. I think the truncated paleosol is Pliocene and the brown swaley stuff Pleistocene and local to Lind Coulee. Its more interesting if its Pleistocene because little mention of Pleistocene alluvium is made in the Ccablands literature. It all got swept away by megafloods, didn't it? I've observed little in the Ringold that looks like this brown stuff. The Savage Island paleosol intervals (lake shoaling periods) never contains thick alluvium of this character.

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.

Other than tilting and brecciation immediately within the fault zone (a few meters wide) there is no evidence of strong shaking preserved in unconsolidated sediments at Lind Coulee. I have found no clastic dikes, no features resembling seismites, nor other soft sediment deformation consistent with liquefaction. West and Shaffer (1988) report finding "injection of flood sands along a shear plane in the fault zone" in a portion of their western trench. They admit the features are "difficult to explain", but note similar features at Gable Mountain (DOE/John Bingham trenches) and smartly conclude,

We are of the opinon that either hydraulic injection asscociated with catastrophic flooding or hydraulic injection resulting from fault movement and liquefaction offer reasonable interpretations for the origin of clastic dikes including the feature in Lind Coulee West trench.

My own work over the past 20 years (Cooley, 2020 and an updated article on this site titled Sheeted Clastic Dikes in the Megaflood Region) has shown there is abundant and widespread evidence for the former, little to none for the latter.


USBOR (1954), Grolier and Bingham (1971, 1978), Galster/USBOR memo (1987, "Area No. 2"); Levfevre and McConnell memo (1987), West and Shaffer/GEI (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).

Links to Other Stops:

Lind Coulee Fault at O'Sullivan Reservoir

Bình luận

Last 50 Posts
All Posts by Month
    bottom of page