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Calcrete Field Trip 2021 - Stokrose Gravel Pit

Stokrose Gravel is a large, active pit and gravel stockpile located in Lind Coulee 7 miles east of Warden, WA off Lind-Warden Road (Road 8 SE) near intersection with Johnson Rd. Mining occurs entirely in flood gravels. The pit is owned by the Clausen family ( I obtained written permission from Aaron Golladay to visit the pit with Northwest Geological Society. Thank you, Aaron.

An excellent exposure of west-dipping foresets in Missoula flood boulder gravels along the north wall.

The largest flood-transported clast in the pit measured 3m x 4m x 2.5m.

Colorful boulders of amorphous silica (opal) derive from a pillow basalt-palagonite complex (Miocene age) quarried by floodwaters and incorporated into the Pleistocene deposit. Columbia River Basalt lavas flowing into a lake, river, and/or wet sediment chill quickly and commonly form pillow basalts and palagonite with pockets of opal and basaltic glass. Opal is amorphous silica. Palagonite is a loose, often friable rusty orange colored mixture of clay and weathered basaltic glass. Basalt pillows are round, nested blobs with glassy rinds with palagonite between the pillows.

A classic example of basalt pillows surrounded by palagonite from a location on the south flank of Frenchman Hills, WA. Molten lava invaded a body of wet sediment.

A liberated chunk of palagonite in the flood-deposited boulder gravel at the Stokrose Pit

Boulders of fragmental, volcaniclastic sediment in flood gravel.

A variation on the Miocene sandstone found among basalt boulders.

Large boulders of volcaniclastic material (cemented sandstone) contain blobby pods of basaltic and felsic volcanic clasts, many with angular shapes. Crossbedding and other bedforms in other examples indicate water transport. The light-colored sandy matrix is very firmly cemented and slightly baked. These boulder appear to be have begun life as sandy alluvium of a stream that developed between lava eruptions in the Lind Coulee area during the Miocene. Lind Coulee may have been a persistent drainage valley that reestablished itself more or less the same place between lava eruptions; a stream valley also scoured by Ice Age floods. The stream was perched atop basalt bedrock, but carried clasts derived from a felsic volcanic source. At some point, new basaltic lava filled the valley. The boulders preserve part of the flow bottom complex, or pillow basalt-palagonite complex, in which a suite of features can be found, including basalt pillows, glassy hyaloclasite, breccia, peperites, and spiracles. Flow-bottoms are often a few meters thick and quite colorful. The wet alluvium was apparently thick enough to escape complete baking by the overriding lava. Some of the large, crossbedded sandstone boulders in the pit show no evidence of contact baking with the lava, while others do. The distinctive characteristics of this alluvial unit - a sedimentary interbed in the CRBs - is unfamiliar to me. It is local to lower Lind Coulee, but almost certainly correlates to one of the named interbeds.

Pods of clasts and similarly-sized voids (large vesicles?) surrounded by hard sandstone. Did the pods form as lava flowed over top?

Cobbly flood gravel grades upward into a silt-sand diamict, then into sandy silt as the flow energy decreased. West wall exposure. Bedforms are not well preserved in silt, which can be confusing. Silt transported by water can appear very much like silt transported by wind. One key to differentiating between the two is the presence/absence of "floating" clasts larger than sand size. Only water can move those.

Cemented cicada burrows overprint sandy flood sediment that contains floating pebble- to cobble-sized clasts. East wall.

Sandy sediment with pebbles later colonized by cicada. Cicada are not exclusive to loess; they'll take sand if they have to.

Three to four flood rhythmites overlie >5m of foreset, flood-laid boulder gravel. Color change from gray (rhythmite base) to brown (rhythmite top) corresponds with a change from silty to sandy sediment. Best exposure is along the rim of the west wall.

Stratigraphy on west wall near SW corner of pit. Tan diamicts are the upper portions of flood rhythmites (slow flow and slackwater). No indication of upvalley flow was observed; all foresets dip west (down Lind Coulee). Unclear if "slackwater" period in floodbeds this far north are controlled by ponding at Wallula Gap. Backwater might be forming as overland flows join a filled mainstem Columbia Valley. At least 3 flood beds are exposed here, though the upper portion of the exposure may contain a few very thin beds that are not each distinct.

Hasty sketch of rhythmite stratigraphy shown in photo above. Three flood beds are preserved here, possibly more. West wall (not south).

Pebbles floating in silty deposits are associated here with gravels, a clear flood origin. Silt diamicts associated with erosional surfaces that truncate paleosols and clastic dikes, but not associated with gravels are found elsewhere in the region, typically in areas away from high energy flood coulees. Such diamicts also appear to be flood deposits, but remain somewhat controversial. Depends who you talk to and who's familiar with what.

Rhythmite stratigraphy.

Lithologies in the boulder gravel include Columbia River Basalt, red vesicular basalt, palagonite, calcrete rip-ups, cemented volcaniclastic sandstone, thin-bedded "quartzite" (lower right in photo), and minor granite. The "quartzite" is probably from the cemented sandstone unit, not Montana.

Boulder spoil piles remain after the more valuable gravel is removed. Boulders to 1.5m are common. Flood energy down Lind Coulee was high.

Basalt dike in a basalt boulder.

The Lind Coulee Archaeological Site (45GR97) is located 7km west of the Stokrose Gravel Pit. The site was discovered in 1947 and excavated by Washington State University in the 1950s and again in the 1970s (Drucker 1948; Daugherty, 1956; Irwin and Moody, 1978; Kirk and Dougherty, 2007). Excavated materials are now in museums and archives.

Links to Other Stops:

Lind Coulee Fault at O'Sullivan Reservoir


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