Microbialite Investigations of the Douglas Creek Member of the Green River Formation (Eocene), Evacuation Creek Area, Uinta Basin, Utah, U.S.A.

Abstract

Three transgressive, microbialite-bearing carbonate units were studied in detail in a 37 km² area near the confluence of Evacuation and Missouri Creeks, eastern Uinta Basin, Utah. Each unit is capped by siliciclastics interpreted as prograding deltas. The microbialite intervals were investigated from a macrostructural, mesostructural and microstructural perspective using outcrop descriptions, petrology, mineralogy and O and C stable isotopes.

The lowest microbial carbonate unit is the most laterally continuous. The vertical succession consists of ostracodal grainstones with rip-up clasts, ooidal to pisoidal coarsening upward grainstones and microbialite biostrome facies. All are dominated by dolomite (Avg=80%). The biostromes contain beds of layered (undulatory to pseudocolumnar), minicolumnar and closely spaced domical stromatolites. The middle unit has lateral facies variations ranging from well-developed large bioherms (central area), to smaller bioherms and ostracodal-ooidal grainstones to the northwest, to kerogen-rich mudstones to the southeast. The unit is dominated by dolomite (Avg=70%) with ≤10% quartz grains, and begins with a debris layer that is the substrate for the overlying bioherms. The bioherms are composed of domical stromatolites (≤80 cm) followed by 50–70 cm shrubby branching minicolumnar stromatolites, then 10–30 cm columnar stromatolites and culminates with 0–50 cm of microbial boundstone. The upper unit is composed of undulatory, pseudocolumnar to columnar layered stromatolites and microbial boundstones that were deposited on top of paleohighs developed by the older, larger bioherms. Ostracodal-ooidal grainstones and kerogenous mudstones were deposited in surrounding paleolows. Dolomite (Avg=83%) dominates this unit, with some calcite and ankerite.

The lowest unit was deposited on a shallow ramp with little siliciclastic input. The middle unit was superimposed on the lowstand delta unit that ended the first cycle. The middle unit’s higher siliciclastic content and the δ¹⁸O signal indicate that the delta was active during transgression. The uppermost unit shows no fluvial influence and was deposited in slightly deeper water as indicated by δ¹⁸O signal. The vertical stacking pattern of facies for each microbialite-bearing carbonate unit (transgressive part of a cycle) is distinctive and each unit’s microbialite assemblage is unique.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018