Calcrete Field Trip - Stokrose Gravel
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 (stokrose.com). 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.
An example of basalt pillows surrounded by palagonite. Frenchman Hills, WA.
A chunk of palagonite in the boulder gravel.
Boulders of fragmental, volcaniclastic sediment in gravel.
A variation on the sandstone found among basalt boulders.
Large boulders of volcaniclastic material (cemented sandstone) contain "pods" of basalt and felsic volcanic clasts, many with angular shapes. Crossbedding and other bedforms in other examples indicate water transport. The sandy, light-colored matrix is very firmly cemented and might be baked, perhaps the basalt part of the pillow basalt-palagonite complex that felt heat, but that was not in direct contact with lava. This rock unit is unfamiliar to me and may be unique to this part of Lind Coulee. Appears to be a local interbed deposited between flows, possibly correlative with Vantage sandstone or sediments exposed in the Frenchman Hills.
Pods of clasts and similarly-sized voids (vugs?) surrounded by hard sandstone. Did the pods form during deposition, prior to overtopping by the lava flow?
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 don't mind sand.
Three to four flood rhythmites overlie >5m of foreset 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 seen what.
Lithologies in the 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.
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).