Control of Lake Type on Physical and Geochemical Nature of Depositional Sequence Development: Examples from the Green River Formation, Wyoming: ABSTRACT

Abstract

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. This 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 Lunian Tongue and 5 to 10 times greater concentrationsmore » of thermally labile aliphatic, mono-, and di-aromatic carbon. The geochemistry varies 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.« less