Depositional history and lateral variability of microbial carbonates, Three Mile Canyon and Evacuation Creek, Eastern Uinta Basin, Utah

Abstract

The Eocene Green River Formation was deposited in several Laramide foreland basins in the Rocky Mountain region. The formation is well known for its abundance of high-grade oil shale. In the Uinta basin, Green River hydrocarbons are being produced from large fields such as Altamont-Bluebell, Monument Butte, and Red Wash. Pre-salt discoveries in offshore Brazil and Angola have also helped bring recent focus to microbial carbonates as hydrocarbon reservoirs. Lacustrine systems are extremely variable, being controlled both by climate and tectonics. Continuous and well-exposed outcrops are invaluable for characterizing these important reservoirs. This study describes and interprets a continuous, three mile carbonate outcrop within the R5 section of the Green River Formation in Three Mile Canyon, Utah, on the eastern edge of Eocene Lake Uinta. This canyon is currently a tributary of Evacuation Creek, an area well known for its excellent exposures of the Green River Formation. The units exposed in Three Mile Canyon are marginal lacustrine deposits of shale, deltaic sandstone, and littoral to sublittoral carbonates. The study outcrop follows an obliquely basinward transect through shore to nearshore facies. The carbonate unit geometries display a lateral transition from large-scale (m-scale) laterally linked stromatolite and thrombolite heads, to thin (cm-scale) planar laminations with smaller isolated microbial mounds. Moving basinward toward Evacuation Creek, the unit pinches out into low grade oil shales. The carbonate consists of two facies associations: (1) microbial and (2) marginal non-microbial lacustrine carbonates. The microbialites are comprised of stromatolite, thrombolite, and dendrolite fabrics. Non-microbial carbonates occur in association with these, and consist of five lithofacies that record changing energy conditions associated with water depth. Facies transitions appear to describe two overall deepening-upward cycles, with localized shallowing sequences. Thin section analysis reveals that the carbonates have undergone a complex diagenetic history that began syndepositionally and continued through burial, including micritization, dissolution, neomorphism, dolomitization, mechanical and chemical compaction, calcite cementation, dolomite cementation, and dedolomitization. Significant porosity has been created and preserved through these processes.