DIFFERENTIAL PRESERVATION AS A PREDICTIVE CHRONOSTRATIGRAPHIC TOOL: AN OUTCROP BASED EXAMPLE FROM THE (EOCENE) WILKINS PEAK MEMBER, WYOMING

Geomorphic processes are transformed into stratigraphic volumes through differential preservation, and the physical relationships that define high and low preservation provide a powerful predictive tool for stratigraphic correlation. Geomorphic elements like point bars produce dramatically different stratigraphic bodies as a function of how much of the point bar is preserved. Patterns of differential preservation are the result of changes in the flux of sediment moving across ancient landscapes. High preservation in one area reduces the net sediment available down-dip. This fundamental relationship provides a physical basis for prediction of sedimentary and stratigraphic attributes up-dip and down-dip of any location in a profile.

Low stratigraphic preservation is characterized by decreased bedform diversity, better sorted and generally coarser grains, numerous surfaces of reworking/bypass, and greater amalgamation. High preservation stratigraphic elements are dominated by greater diversities of bedforms and grain-sizes, and show relatively less reworking or amalgamation. Low preservation suites are indicative of decreased storage at a given locality, and therefore predict greater down-dip export of sediment and the presence of coeval high preservation elements in more distal locations. Conversely, increased preservation up-dip reduces the amount of material available for translation down-dip. These are predictable relationships that are expressed at the outcrop scale.

The clastic phases of the Wilkins Peak Member of the Eocene Green River Formation provide an ideal natural laboratory for testing the predictive power of differential preservation. Detailed analysis of well-channelized fluvial strata in a portion of the basin provides a predictive framework based on changing preservation. This prediction is tested by comparing the Wilkins Peak to cores of coeval down-dip deltaic sediments of Lake Gosiute as well as up-dip alluvial/fluvial outcrops of the equivalent Cathedral Bluffs Member. Recognizing the role of preservation in building the stratigraphic record allows for the construction of high resolution chronostratigraphic correlations that relate architectural evolution within a basin to fundamental patterns of changing storage, erosion, and bypass within a basin.