STRATAL ARCHITECTURE OF AN EARLY EOCENE FLUVIAL LACUSTRINE DEPOSITIONAL SYSTEM

Lacustrine deposits of the Green River Formation intertongue extensively with predominantly fluvial strata of the Wasatch and Bridger Formations within the Green River Basin (GRB) of Wyoming. These fluvial units reflect periods of lacustrine lowstand during which Paleolake Gosiute contracted to the center of the basin. In intermontane basins of the Rocky Mountain interior stratigraphic base level is not directly affected by sea level change. In the Green River Basin, fluctuations in lake level due to variations in climate and local tectonics resulted in extensive inundation or exposure of the lowlands adjacent to the lake and thus played a dominant role in the stratigraphic evolution of the basin.

Early Eocene strata in the southwestern Green River Basin (Little Muddy Creek area) are characterized by four fluvial-lacustrine sequences separated by erosional unconformities (lacustrine lowstand surfaces of erosion). The first of these unconformities 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 early Bridgerian (late Gardnerbuttean, upper chron C23r). Paleomagnetic and biostratigraphic data have allowed for accurate correlation of significant stratal surfaces between the study area in the west and South Pass on the eastern side of the basin.

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 sheet sands. 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.