Contact of Burro Canyon Formation with Dakota Sandstone, Slick Rock District, Colorado, and Correlation of Burro Canyon Formation

Abstract

Weathering of shales in the upper part of the Burro Canyon formation and the lower part of the overlying Dakota sandstone has made recognition of the disconformity between the formations difficult in the Disappointment basin area of the Slick Rock district, San Miguel and Dolores counties, Colorado. In lieu of the basal conglomerate of the Dakota sandstone, which marks the contact between the formations in the surrounding region, the presence of abundant carbonaceous material in shales of the Dakota sandstone differentiates them from the green shales in the Burro Canyon formation. The contact of the two formations, where clearly exposed in Disappointment basin, is conformable, sharp, and not gradational. Correlation of the Burro Canyon formation of western Colorado with the Cedar Mountain formation of central and eastern Utah is substantiated by the discovery in the Burro Canyon formation of two pelecypods, Protelliptio douglassi and Unio farri, with the conifer, Frenelopsis varians. The pelecypods also occur in the Kootenai-Cloverly fauna of Montana and Wyoming. The Kootenai-Cloverly fauna contains the pelecypod Eupera onestae which is found in the Cedar Mountain formation. The conifer also occurs in the Trinity group of Texas. The Trinity group contains the charophyte Clavator harrisi which is found in the Cedar Mountain formation.