Periods of Rapid Environmental Change Identified by High-Resolution Analysis of an Oil Shale/Stromatolite/Oil Shale Succession, Laney Member, Eocene Green River Formation, Wyoming, U.S.A.

Abstract

A 45 cm thick oil shale/stromatolite/oil shale succession was analyzed at mm-scale intervals using mineralogy (XRD), textures in slabs and thin sections, and O and C stable isotope composition to determine the environmental factors controlling depositional trends. Five distinct units were identified based on mineralogy and texture (from bottom to top): 1. oil shale laminasets (3 cm) rich in calcite, kerogen, and silicates; 2. dolomitic, kerogen-poor, silicate-poor laminasets (5.5 cm) with two interbedded thin tuffs; 3. domical, ~15 cm wide dolomitic, silicate-poor stromatolite (19.5 cm); 4. columnar, silicate-poor, stromatolite (5 cm) with dolomite comprising 60% of total carbonate; 5. calcite-rich, kerogen-rich, silicate-rich oil shale laminasets (12 cm). Transitions between units span intervals of 1.5 cm or less with no evidence of subaerial exposure. Apart from the unit 2 to unit 3 transition, significant ∂¹⁸O isotopic fluctuations coincide with unit boundaries.

Isotopic, mineralogic, and petrologic data suggest that this succession represents five periods of distinct (but relatively stable) lake environments punctuated by four, short-duration, climatic events. The increase in dolomite and decrease in siliciclastics and kerogen found through units 1 and 2 indicate lake regression, with a positive excursion in ∂¹⁸O signaling a sharp increase in lake salinity between the units. Lake level then stabilized as the first stages of transgression began (unit 3), creating an environment conducive to microbialite growth. The relatively uniform stromatolite morphology and mineralogy in unit 3 probably reflect stable chemical conditions. Distinct changes in stromatolite morphology and mineralogy, along with a negative shift in ∂¹⁸O, signal an influx of fresh water at the base of unit 4. However, continuous stromatolite growth and the scarcity of siliciclastics through unit 4 suggest that the effect of fresh-water inflow on lake level was minor. The full effects of lake transgression are apparent in unit 5, as stromatolite growth ceases and kerogen-, calcite-, silicate-rich, more negative ∂¹⁸O oil shale returns.

The application of this high-resolution approach will aid in better understanding the paleoenvironmental conditions affecting the rates of deposition and growth of microbialites during dynamic periods of transgressions/regressions of Eocene Lake Gosiute.

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