SPRING DEPOSITS IN THE WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION: STEED CANYON, LINCOLN COUNTY, WYOMING

An isolated cluster of 1 m thick carbonate spring mounds occurs in the Wilkins Peak Member of the Green River Formation near the town of La Barge, Wyoming. Each mound is 1-2 m in diameter, circular in plan view and contains a single centrally located, vertically directed spring vent up to 20 cm in diameter. Internal mound fabrics consist of layered carbonate (travertine), irregular ooids and brecciated carbonate fragments. Sand tufa spires up to 3 m thick occur in sandstone 15 m below the spring mounds and are considered strong evidence of vertical groundwater flow directly toward the overlying carbonate spring mounds. These spring deposits are assigned to the Wilkins Peak Member of the Green River Formation because they occur with caddisfly-dominated microbial-carbonate mounds that have been correlated with the Wilkins Peak “layered tuff”. Furthermore, the Steed Canyon spring deposits occur with 20 m of overlying Wilkins Peak silver “oil shale”. Intermound deposits are chiefly oolitic micrite. One significant intermound oolite deposit contains abundant molds of avian bones (likely Presbyornis).

The co-occurrence of spring deposits and caddisfly-dominated microbial-carbonate mounds indicates that the spring deposits were formed subaqueously (since caddisfly larvae are fully aquatic). It is not known whether the birds were killed by the springs or if the bird bones were accidentally washed into the spring area by another mechanism.