SUBSURFACE TO OUTCROP CORRELATION OF FLUVIAL ARCHITECTURE OF THE LANCE FORMATION, WASHAKIE BASIN, SOUTHERN WYOMING

The sequence stratigraphy and paleohydraulic analysis of the fluvial Lance Formation in the Washakie Basin, Southern Wyoming is using the correlation of subsurface data to outcrop. Subsurface and outcrop correlations evaluate the connection of key surfaces between Lance formation alluvial environments and Fox Hills shallow marine deposits and the calculation of Lance dimensions and paleocurrent. Previous studies have constrained the Fox Hills sandstone (shallow water deposits) and Lewis shale (deep water deposits) within genetic sequences, bounded by maximum flooding surfaces (MFS), and identified the paleodrainage divide and total sediment yield for the fluvial Lance formation. A correlation of subsurface to outcrop of the fluvial Lance provides detailed Lance characterization, with beneficial academic and economic implications.

Correlation of subsurface to outcrop includes an extensive dataset of over 1000 wells with gamma-ray curves in the depositional system and 20 measured sections from the outcrops in the eastern part of the basin. Sixteen MFS determined by previous studies divide the system based on genetic sequences and Carvajal and Steel (2012) provides the estimates of sediment supply, 4-16 x 10⁶ ton/y, and yield, 200-2000 ton/km²/y, for the Lance rivers. Correlation of flooding surfaces through the Lance formation is focused between MFS 10-13 and will allow descriptive analysis of the evolution of sandstone and shale patterns and comparison to outcrop. Paleohydraulic calculations for the Lance Fm. are based on the methodology of Bridge and Tye (2000) and can be used as analogs for subsurface data. Using cross-strata thickness (dune height) from outcrop, the paleo-depth, range of channel belt width, proportion, and connectedness of the Lance river system has been analyzed.

A series of paleogeographic maps based on subsurface data illustrate multiple, roughly north to south oriented rivers of the Lance formation with significant channel size changes to the south (basinward). The equivalent shoreline was likely fed by multiple southward flowing rivers. The river paleo-depth analysis based on outcrop data indicates channels with typical depths between 10-20 m. Comparison of fluvial paleogeography, channel size, and facies patterns between successive Lance Fm. sequences will be shown.