NEW PHYSICAL STRATIGRAPHY FOR THE LOWER LANEY MEMBER (GREEN RIVER FORMATION, WYOMING): IMPLICATIONS FOR ANCIENT LAKE CLASSIFICATION

The lower Laney Member of the Green River Formation (Eocene) of Wyoming is regarded by Carroll and Bohacs (1999) as an example of a “balanced fill” lake-type. A new physical stratigraphy for this interval, based on laterally persistent ash layers, implies that their classification is inappropriate. Taking additional lacustrine examples into account (e.g., Triassic of the eastern US), we suggest that in the absence of the key element of time, the Carroll and Bohacs scheme fails to encompass the full range of lacustrine facies variability.

Shallow water facies of the Laney Member, such as stromatolitic and oolitic carbonates and micrite with desiccation cracks, are present at 10 to 15 horizons along the western and eastern sides of the Rock Spring Uplift, an anticline that today separates the Bridger and Washakie basins. However, these facies are not laterally persistent as previously assumed. Following consecutive time markers towards the depocenter of the Bridger basin, the shallow water facies change into microlaminated, black, gray-weathering oil shale, with abundant fish fossils. Consequently, shallow water facies are present at only 3 levels within the lower Laney Member in the central part of the basin.

The lateral facies distribution suggests a deep lake environment in which shallow water deposition was confined to the shores, and not a playa lake with laterally persistent shallow water facies. Furthermore, the localization of shallow water indicators along the Rock Spring Uplift implies that this was an island at the time, separating two distinct depocenters. The low frequency and limited vertical extent of shallow depositional environments at the basin center imply a stable lake level, with only minor lake level fluctuations recorded along the margins.

The new facies scheme for the lower Laney is hard to place in any of the three lake-types of Carroll and Bohacs because parts of this interval can be classified in any of the categories, depending on their chosen length. A similar problem is expected for any lacustrine deposits containing a hierarchy of lithological variability, whether cyclical or not. For this reason, the underfilled - balanced fill - overfilled classification for lacustrine sequences is of limited utility.