Lacustrine Sedimentology, Stratigraphy and Stable Isotope Geochemistry of the Tipton Member of the Green River Formation

Abstract

The Tipton Member of the Green River Formation occupies much of the Greater Green River Basin (GGRB) of Wyoming and Colorado. Long hypothesized to record a single shift from open to partly closed hydrology, new detailed stratigraphy and stable isotope geochemistry indicates that its strata record open, then partly closed, then open, then partly closed hydrology, which are each recorded by distinct transitions in facies associations, geochemistry, carbonate mineralogy, and organic content. Intervals of open hydrology occur coincident with the progradation of deltaic sandstones that are absent during the partly closed intervals, suggesting that environmental transitions were controlled by avulsions of the Idaho River. The first of these transitions occurs at the contact between the Scheggs bed and overlying Rife bed, and is thought to reflect the initial impoundment of Lake Gosiute. The Scheggs bed ranges from 23.5 to 36.5 m, and is characterized by fluvial-lacustrine lithofacies, calcitic mineralogy, an average Fischer Assay content of 7.6 gal./ton, and low δ18O and δ13C values (25.3‰ and 0.7‰, respectively). These deposits transition over a five meter interval to the overlying 2–15 m-thick lower Rife bed. The lower Rife bed is characterized by fluctuating profundal lithofacies, dolomitic mineralogy, an average Fischer Assay content of 17.6 gal./ton, and high δ18O and δ13C values (29.3 and 5.3‰). The lower Rife bed transitions up-section over a two meter interval into fluvial-lacustrine lithofacies of the 2.5–20 m thick middle Rife bed, which exhibits calcitic mineralogy, an average Fischer Assay content of 9.7 gal./ton, and low δ18O and δ13C values (23.0 and 1.9‰). The third and final transition, from the middle Rife bed to the upper Rife bed, occurs gradationally over 6 m of section. The 6.5–22 m-thick upper Rife bed is characterized by fluctuating profundal deposits, dolomitic mineralogy, an average Fischer Assay content of 19.2 gal./ton, and high δ18O and δ13C values (29.8‰ and 8.5‰, respectfully). We interpret this succession of abrupt changes in lithofacies and isotope geochemistry within the Tipton Member to reflect the diversion, recapture, and ultimate diversion of a major source(s) of water and sediment into the basin.