This six-day Lacustrine Basin Exploration field seminar has been developed on a number of classic, world-famous field localities in Utah and Wyoming. Localities in the vicinity of Salt Lake City, UT will be used to illustrate key limnologic, geomorphic, and stratigraphic features of pluvial lake Bonneville (Gilbert deltas and other shoreline features) and modern Great Salt Lake (playa-lake environments). These “actualistic” observations will help provide context for understanding the deposits of Eocene Lake Gosiute in Wyoming, where recent radioisotopic work provides a chronostratigrahic framework of unprecedented resolution. There we will focus on basin margin to basin center transects of the Bridger and Washakie basins, based on excellent exposures of fine-grained lacustrine carbonate mudstone facies and alluvial to deltaic sandstone facies. Participants will build two basin-scale cross sections of Lake Gosiute strata by recording their own guided outcrop observations on the chronostratigraphic framework provided.
At the outcrop scale, we will examine the heterogeneous reservoir architecture of alluvial, deltaic, and lake-marginal carbonate deposits associated with overfilled, balanced-fill, and underfilled lacustrine basins. These deposits range in style from classic Gilbert deltas to more “dryland” fluvial facies dominated by upper flow regime deposition, to microbiliate carbonate strata. The latter are spectacularly exposed near LaBarge, Wyoming, and represent one of the best available analogs to recently discovered Brazilian reservoirs.
Laterally equivalent mudstone facies are similarly heterogeneous, ranging from laminated oil shale with abundant fish fossils to pedogenically modified playa facies associated with nonmarine evaporites. Distinctive biological marker compounds (biomarkers) are associated with each of these facies, and can be used to help determine paleoenvironmental setting. Mudstone mineralogy ranges from clay-rich to mostly carbonate, implying widely different fracture properties.
In addition to field studies, several classroom lectures will be used to illustrate the main course concepts and to provide geologic background information on the field areas. Because fine-grained rocks can appear rather different in core than in outcrop, we will also conduct a half-day core workshop based on representative examples of the major facies associations. We will spend some time developing criteria for subsurface recognition using wire-line logs, seismic, and organic geochemistry.