Stevensite, Oolite, and Microbialites in the Eocene Green River Formation, Sanpete Valley, Uinta Basin, Utah
Buchheim, Paul; Awramik, Stanley M.
Stevensite has been reported as a clay mineral in lacustrine carbonate successions commonly associated with hydrocarbon reservoirs of the pre-salt Aptian South Atlantic conjugate basins of Brazil, and is problematic in that it compromises reservoir porosity. Stevensite is an authigenic clay mineral, a Mg smectite, indicative of lacustrine saline-alkaline depositional environments. It has been reported from the Eocene Green River Formation in both the Uinta (Utah) and Green River (Wyoming) Basins. In the Uinta Basin (Sanpete Valley), it occurs in the same section with calcareous claystone, shale, tuff, oolite, and microbialites. Stevensite is variously described as forming or composing "pellets" or ooids and has been referred to as "coffee-ground beds". Grains are 1-2 mm in size, irregularly shaped, and commonly concentrically laminated. However, they are absent from the carbonate oolite facies in the upper half of the section.
Although various studies have noted and described the presence of stevensite, none have provided details of its depositional environment. Our preliminary study of successions in the Sanpete Valley, Utah, provides insights that contribute to both a better understanding of stevensite deposition as well as the oolites and associated microbialites in the succession. The succession represents an overall shallowing upward sequence, with profundal claystones and shales dominating the lower half of the section and littoral oolite and microbialites the upper half of the section. The microbialites are nearly always associated with oolite and best developed in meter-thick, oolite beds that are ripple-cross laminated. Stevensite occurs in the lower half, in beds 1-25 cm thick, associated with shales and claystones. The exact origin of the stevensite grains is still the subject of further study. However, the facies association of the stevensite with shales and claystones suggests quiet water, profundal depositional conditions. This is contrary to the high-energy conditions suggested by the "oolite" interpretation for stevensite grains.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013