Fossils from the Middle Jurassic Wanakah formation near Delta in western Colorado

Abstract

The Middle Jurassic Wanakah Formation averages about 30 m in thickness in Colorado. Fossils are sparse and include fish, ostracodes, and trace fossils. A thin (0.03-0.45 m) fossil bed near the middle of the formation extends for some 48 km along the northeast flank of the Uncompahgre Plateau near Delta. The fossil bed at one locality contains one pelecypod identified possibly as Modiolus cf. M. subimbricatus (Meek), as well as other specimens too poorly preserved for identification. Previously, Mytilus was found in the same fossil bed at another locality by C.N. Holmes. The Wanakah Formation is primarily of terrestrial origin, but the fossil bed represents a shallow-marine incursion.

Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

“…Paleosol features such as root traces, blocky peds, disrupted bedding, and carbonate concretions suggest subaerial exposure of the sediments and periods of nondeposition where soils could develop in a terrestrial setting. Tabular sandstones topped with microbialites (wavy laminated mi- Most depositional environment interpretations of the Wanakah Formation suggest a lacustrine origin, particularly those done on the Todilto Limestone to the south of the study area (Peterson, 1988a(Peterson, , 1994Turner and Peterson, 1992;Lucas, 1992, 1994;Kirkland et al, 1995;Benan et al, 2000;O'Sullivan, 2004;O'Sullivan et al, 2006;Lucas et al, 2006). In the study area, the lack of marine fossils lends support for a lacustrine interpretation, and the overall restricted biodiversity (limited to trace fossils and microbialites) suggests hypersaline water chemistry.…”

“…2; Gilluly, 1929;Bernier and Chan, 2006). The J-5 unconformity is typically extended into western Colorado and placed in between the Wanakah Formation and the Tidwell Member (O'Sullivan, 2004;O'Sullivan et al, 2006) because the Summerville Formation and the Wanakah Formation are interpreted as coeval (Peterson, 1994). However, this interpreted extension of the J-5 unconformity into Colorado is controversial (O'Sullivan and Pipiringos, 1983;O'Sullivan, 2004).…”

“…Controversies exist over Wanakah Formation depositional environmental interpretations (marine or lacustrine) and whether the unit is even present in western Colorado (Tanner, 1970;Adler, 1974;Ridgley and Goldhaber, 1983;Peterson, 1988aPeterson, , 1994Kirkland et al, 1995;Turner and Peterson, 1992;Lucas, 1992, 1994;O'Sullivan, 2004;O'Sullivan et al, 2006;Lucas et al, 2006). The Wanakah Formation is lithologically variable and contains several dissimilar members.…”

“…The Wanakah Formation is lithologically variable and contains several dissimilar members. For example, in Ouray, Colo-rado, the type section includes an 18-m-thick basal limestone member (The Pony Express Limestone) that is not present north of the western Black Canyon of the Gunnison (O'Sullivan, 1992O'Sullivan et al, 2006). In the Black Canyon of the Gunnison, the Wanakah Formation contains 3-7-m-thick gypsum lenses, also not present at any other exposure (O'Sullivan, 1992(O'Sullivan, , 2004O'Sullivan et al, 2006).…”