Depositional History and Lateral Variability of a Microbial Carbonate, Three Mile Canyon and Evacuation Creek, Eastern Uinta Basin, Utah
Michael Swierenga, J. Frederick Sarg, and Kati Tänavsuu-Milkeviciene
Colorado School of Mines
Marginal lacustrine carbonates of the Eocene Green River Formation form important reservoir rocks in the Uinta basin, yet are often underrepresented in studies. This study seeks to describe and interpret a microbial carbonate unit along a continuous three mile long outcrop in Three Mile Canyon, on the eastern edge of the Uinta basin, near the Utah-Colorado border. This canyon is 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 outcrop runs obliquely to the paleo-shoreline, giving insight into marginal-lacustrine depositional variability. The objectives of this study are to 1) identify and fully describe the facies of a thick microbialite unit in this study area, 2) measure and describe porosity, 3) describe the lateral variability of microbialite facies along the outcrop, 4) determine depositional history, 5) place this unit into the larger, existing stratigraphic framework developed for the Uinta basin, and 6) describe the diagenetic history of this microbialite deposit. To this end, rock samples and measured sections were taken at intervals along outcrop where lateral changes were observed. Samples were prepared for thin section and stable isotope analysis. Littoral carbonate facies include oolitic grainstone to packstone, intraclast rudstone, and skeletal grainstone. Microbialite facies include stromatolite, thrombolite, and dendrolite. These facies form an upward deepening cycle, transitioning from coated grains or intraclast rudestone to agglutinated microbialite, capped by fine-grained stromatolite. A sandstone unit underlies the microbial carbonate layer, indicating that deltaic sands provided a stable growth platform for microbial accumulations. Microbialite thickness is greatest where it directly overlays sandstone bodies, and large thrombolite to stromatolite heads are dominant. The microbialite layer persists laterally for kilometers beyond sandstone pinch-out, but quickly thins, transitioning from large-scale (several meter) laterally-linked stromatolite and thrombolite heads, to thin (cm-scale) planar laminations with smaller isolated microbial mounds. Graded pisoids and ooids commonly form the early core of mound, and fill between mounds. Porosity observed in outcrop is primarily interparticle and fenestral.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013