Facies, stratigraphy, and mineralogy of the carbonate marker and D marker units, lower Green River Formation, Uinta Basin, Utah

Abstract

This study documents and correlates the Carbonate Marker Unit and the D Marker Unit, two regionally extensive carbonate beds within the Lower Green River Formation in the Uinta basin. Both units were measured in a series of accessible canyons along the Roan Cliffs on the southern margin of the basin. This was accomplished by incorporating measured outcrop, petrographic analysis, X-ray fluorescence, X-ray diffraction, QEMSCAN analysis, and stable isotopic analysis. Four facies associations were identified that correlated to unique depositional environments within the lake. These include delta, mudflat, shoal complex, and sublittoral deposits. Facies associations were incorporated into a lacustrine depositional model to track transgressive and regressive periods in the lake. Five transgressive-regressive cycles were identified within the Carbonate Marker Unit and two in the D Marker Unit. Cycles were observed in two distinct styles characterized by either rapid lake level fluctuation or by slow lake level fluctuation. Some basal cycles in the Carbonate Marker Unit onlap onto the unit below as a result of the dip component between the three canyons in this study. The D Marker Unit, however, is a consistent thickness across all three sections as the result of a rapid transgression which flooded all three sections simultaneously. Lake Uinta during the lower Eocene Green River Formation is interpreted to be a closed-lake system with water inputs controlled by monsoon-style seasonality and whose salinity varies considerably. Two periods of freshwater are identified within the Carbonate Marker Unit, characterized by the resurgence of mollusc taxa, broad swamps which hosted terrigenous plant life sufficient to produce coals, and nutrient driven ostracod blooms. The upper freshwater period in the Carbonate Marker Unit also corresponds with fish remains found in the formation. Lake Uinta during Carbonate Marker and D Marker times is interpreted to be a stratified lake with a well-developed chemocline. Beneath the chemocline waters were dense, highly saline, reducing, and anoxic to dysoxic. As a result, precipitation of analcime cement, ferroan dolomite, and the severe alteration or dissolution of clay species is common in the sublittoral and profundal zones.