Geochemical Reconstruction of Milankovitch-Driven Lacustrine Conditions: Green River Formation, Wyoming

Stan Teerman¹, Al Fischer², and Alessandro Grippo^2
1 ChevronTexaco, Houston, TX
² University of Southern California, Los Angeles, CA

In the spectrum of lacustrine facies in the Green River Formation, Wyoming, the Tipton Scheggs and Rife Beds represent freshwater, overfilled and brackish/saline, balanced-fill lake systems, respectively. Time-series analysis of Fischer assay oil-yields from the paleo-geographic lake center of the Tipton Member defines the full suite of Milaknovitch cyclicity. This continuous, well-defined cyclicity documents that in addition to tectonic modification and regional paleo-climatic variations, the limnology of paleo-lake Gosiute was controlled by orbital-driven changes in insolation.

The lithologically uniform 6 to 9' precessional cycles consist of an organic-rich base with a reduced but variable organic content in the remainder of the interval. The cycle base contains an algal dominated hydrogen-rich kerogen that is sometimes depleted in 13C. Above the cycle base, small to moderate differences in the kerogen and bitumen reflect a greater proportion of bacterial input and subtle increase in degradation.

These precessional cycles record a perennial, two-stage cyclical lake with repetitive trends in precipitation and evaporation. The cycle base was deposited during the rainy precessional phase when an expanded, nutrient-rich lake enhanced productivity and elevated stratification. During the dry precessional phase, lower lake levels, and reduced eutrophication and stratification occurred. The orbital-derived climatic signal dictated lacustrine conditions that continuously imprinted sediments. Yet, the evolving Tipton lake systems simultaneously modified, magnified or suppressed lacustrine processes, resulting in different cyclical expressions. Identification of cyclical geochemical patterns and their lacustrine reconstruction define mechanisms by which orbital signals were transferred through different paleo-climatic conditions and lake systems into the sedimentary record.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005