Control of Lake Type on Physical and Geochemical Nature of Depositional Sequence Development: Examples from the Green River Formation, Wyoming
K. M. Bohacs, B. Horsfield, D. J. Curry, and A. R. Carroll
Lake depositional sequences often differ significantly from marine sequences. Although there is a wide variety of lake types, their geological record appear to be stacks of two end-member-type packages deposited by different processes of shoreline regression: progradation and desiccation. These two types of parasequences and the sequences and sequence sets they form are recorded in distinctly different lithologies, stratal stacking, and organic and inorganic geochemistries.
The Eocene Green River Formation records a wide range of lake conditions. The Luman Tongue was deposited under fresh, oxic waters in an open hydrologic system; It is composed of asymmetric shoaling-upward stratal packages 2.5 to 9 meters thick, analogous to marine parasequences: they formed by shoreline progradation, from profundal shales to coals. The Laney Shale Member accumulated under alkaline to saline waters in an intermittently closed basin. The lake system deposited carbonate-rich parasequences 1 to 5 meters thick whose shoaling upward is due mostly to lake desiccation with minor shoreline progradation.
Both members had a relatively uniform primary input of organic matter (mostly alginite), but significant differences in preservation. The Laney Shale contains total-organic-carbon concentrations and hydrogen indices 2 to 3 times greater than the Luman Tongue and 5 to 10 times greatc concentrations of thermally labile aliphatic, mono-, and di-aromatic carbon. The geochemistry varic systematically within each unit at the parasequence, sequence, and sequence-set scale. Maximum organic concentration in both parasequence types tends to occur a short distance above the basal flooding surface, after the initial transgression, when the lake was reaching maximum extent. Within each Luman sequence, the concentration of labile aliphatic carbon increases to a maximum in the middle of the sequence around the surface of maximum flooding and then decreases as the lake shallowed. In contrast, the maximum organic content in the Laney sequences occurs in the early highstand (above the maximum flooding surface). At the sequence-set scale, the regressive a- transgressive phases contain significantly different organic facies. In the regressive sequence sets the Laney Shale, TOC contents are higher, but labile aliphatic carbon concentrations are lower indicating enhanced productivity but decreased preservation. Transgressive sequence sets had significantly higher isotope ratios of both organic and inorganic carbon, possibly due to slow sediment accumulation and increased CO2 recycling.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California