Named as the upper member (of 3 members) of Wood River Formation of the newly named Sun Valley Group for Wilson Creek, a tributary of Trail Creek southeast of Sun Valley, Blaine Co, ID, Snake River basin. Name was proposed earlier (Thomasson, 1959) in an unpublished thesis). Rocks of this member were assigned to units 7 and 8 by Hall and others (1974). Type section is the upper part of measured section of Hall and others at Seamans Creek, sec 21, T3N, R19E, north of Bellevue, Blaine Co. Some exposures of member east of Bellevue and Hailey; good exposures in alpine ridges of high Boulder Mountains northeast of Ketchum. Gradationally overlies Eagle Creek Member (new) of Wood River Formation. Unconformably beneath Eocene Challis Volcanic Group. Consists of more than 800 m of thin-bedded micritic siltstone, silty micrite, micritic mudstone, allochem silty limestone with subordinate medium-bedded micritic sandstone and sandy micrite. Correlates with upper member of Dollarhide Formation of Sun Valley Group and with members 3 and 4 of Grand Prize Formation of Sun Valley. Stratigraphic charts and figures. Fossils--fusulinids and scaphopods from Boulder Mountain, Blaine Co, yield a Wolfcampian to Leonardian age. May be as young as Guadalupian.

Source: GNU records (USGS DDS-6; Denver GNULEX).