Abstract

The Paleogene sedimentary record of the Green River Basin, Wyoming was largely controlled by the regressions and transgressions of Paleolake Gosiute, a large shallow lake that occupied the center of this intermontane basin. Fluctuations in lake depth, occurring in response to both climatic and tectonic controls, resulted in extensive inundation or exposure of the lowlands adjacent to the lake. In the southwestern Green River Basin, lacustrine deposits of the Green River Formation intertongue extensively with the predominantly fluvial strata of the Wasatch and Bridger Formations. These fluvial units reflect periods of lacustrine lowstand during which the lake contracted to the center of the basin. In intermontane basins, stratigraphic base level is not directly affected by changing sea level. Fluctuations in lake level due to variations in climate and local tectonics therefore play a dominant role in the stratigraphic evolution of the basin.

Paleomagnetic and biostratigraphic data obtained from the Little Muddy Creek area provide a chronostratigraphic framework to facilitate reconstruction of the sequence stratigraphic history in the study area and provide a means for assessing temporal equivalency between outcrop exposures in different parts of the Green River Basin. Early Eocene strata within the Little Muddy Creek area are characterized by four fluvial-lacustrine sequences separated by erosional unconformities (lacustrine lowstand surfaces of erosion). The first unconformity occurs in the latest Wasatchian (Lostcabinian, base of GPTS chron C23r). The medial unconformity occurs in the earliest Bridgerian (early Gardnerbuttean, middle chron C23r). The upper unconformity occurs in the medial early Bridgerian (late Gardnerbuttean, upper chron C23r).

Three types of regionally significant stratal surfaces occur within the study interval. Sequence bounding unconformities are characterized by abrupt upward changes in lithology, from lacustrine carbonates to amalgamated fluvio-deltaic sandsheets. Initial transgressive surfaces are characterized by a shift from sandstone zones to heterolithic, mud-dominated, alluvial floodplain deposits. Maximum lacustrine flooding surfaces mark the point of maximum lake advance on the margins of the basin. Individual sequences have a predictable geometry, akin to that observed in marine successions, reflecting lake level variations and concomitant fluctuations in fluvial base level and available accommodation space.