EVALUATION OF CONTROLS ON CHANNEL CLUSTERING IN THE LANCE FORMATION, BIGHORN BASIN, WYOMING

Extrinsic factors (relative sea level, climate, and tectonics) have long been considered the prime control on the distribution of channel bodies in areas influenced by base-level changes. However, the importance of intrinsic processes on the construction of the stratigraphic record is gaining appreciation. Recent studies have found that stratigraphic organization may occur within fluvial systems on basin-filling time scales (10⁵-10⁶ years). Sand-dominated intervals in the fluvial Lance Formation (Maestrichtian) have been interpreted as incised-valley fills formed during sea-level lowstand. However, closely spaced sand bodies in the Ferris Formation (Lance equivalent) are interpreted as aggradational in origin, and have been compared to autogenic avulsion stratigraphy produced in experimental basins. This study evaluates the Lance Formation in the Bighorn Basin, Wyoming in an effort to determine whether these sand-dominated intervals are truly incised-valley fills resulting from sea-level changes, or if they were generated by another process. In order to distinguish between these different controls on architecture we have established the following criteria for recognition of incised valley fills: 1) Recognition of a discrete container against which channel fill deposits are emplaced; 2) Evidence of valley incision that is more than ~5x average river paleoflow depth; 3) Paleosol development along extant interfluves; 4) Highly amalgamated channel-belt deposits within the container that rapidly become less amalgamated above the container bounds; 5) Evidence of time separation between valley wall incision and subsequent infilling.

Exposures of the Lance Formation in the study area are along two outcrop belts located down paleoflow of each other along the two margins of the basin. As such, comparison between exposures allows us to evaluate how alluvial architecture changes from upstream to downstream during Lance deposition. We tentatively interpret our results to indicate that avulsion clusters dominate upstream, but that relative sea level change may play a role in alluvial architecture in the downstream part of the system. Comparison of the size and geometry of channel groupings may provide a means for determining the relative importance of base level controls for all of the observed architecture.