14 Green River Formation, Laney Member, Eocene, Wyoming, USA—A Balanced-Fill Lake System with Microbial Carbonate and Oil Shale, an Analog for Part of the South Atlantic Pre-Salt

Abstract

ABSTRACT The Green River Formation illustrates the expression of sequence-stratigraphic surfaces and units in lacustrine and alluvial strata. These settings are distinctly different from those of most of the mudstone units considered in this book. Our study shows how applying the sequence-stratigraphic method and approach from first principles in continental settings can provide insights into the accumulation of mudstones enriched in organic matter and biogenic material. These settings also have substantial hydrocarbon source, reservoir, and seal potential. Indeed, lacustrine settings host many of the largest oil discoveries of this century. This setting offers an opportunity to examine the expression of parasequences, depositional sequences, and key surfaces in a setting that is significantly different from the more commonly studied marine settings. Although lakes may seem completely different from oceans, they have enough similarities with oceans that their differences tell us much about what is really essential about sequence stratigraphy—and what is an accident of the depositional setting. The sequence-stratigraphic approach of recognizing a hierarchy of rock packages bounded by various surfaces works very well in lake strata. In studying lacustrine strata, we recognize the same types of sequence-stratigraphic surfaces as in marine settings along with similar stratal stacking patterns. The expressions of parasequences and sequences differ between marine and lacustrine settings, however, because of significant differences among the dynamics and responses of these systems. Despite these differences, we see that the sequence-stratigraphic approach works well for lakes. Separate models, however, are needed for each of three lake-basin types to summarize the lacustrine sequence expression—just as shallow-marine carbonate sequences look different from shallow-marine siliciclastic sequences and require separate models. Contrasts among lake and marine systems make it inappropriate to directly apply one unmodified marine sequence-stratigraphic model to all lake systems. Indeed, one lacustrine model is not applicable to all lake-basin types.