STRATIGRAPHIC EVOLUTION AND ARCHITECTURE OF ALLUVIAL PALEOVALLEYS IN THE LOWER CRETACEOUS BURRO CANYON FORMATION, SOUTHWEST PICEANCE BASIN, COLORADO

The Lower Cretaceous Burro Canyon Formation (BCF) in the southwest Piceance Basin contains a variety of fluvial systems that are displayed in numerous high-quality, three-dimensional outcrops along the Gunnison River in Delta and Mesa Counties, CO. Exposures were studied at seven representative locations along a transect of ~80 kilometers using traditional sedimentological techniques (e.g., measured sections, paleocurrents) and by UAS photogrammetry.

In the study areas, the lower BCF (15-27 m thick) is comprised of sandstone-rich (high net-to-gross ratio), amalgamated channel complexes, overlain by an aggregational interval (20-36 m thick), characterized by non- to semi-amalgamated channel complexes (lower net-to-gross ratio). Paleocurrent data from both intervals show a sediment transport direction to the northeast.

Channel complexes in the lower interval contain vertically and laterally stacked channel-fill elements with medium- to large-scale cross-stratification and numerous truncated contacts. The channel-fill elements have an apparent-width range of 40 to 420 m and a thickness range of 1.5 to 18 m. Mudrock interbeds are rare. Sediment deposition was dominated by low-sinuosity, braided rivers.

In contrast, the fluvial complexes in the upper interval have isolated channel-fill elements that are interbedded with floodplain mudrock. The apparent-width values for the channel-fill elements range from 60 to 300 m and thickness ranges from 6 to18 m. Lateral-accretion stratification in the sandstone bodies is common. Deposition was by higher sinuosity rivers.

The characteristics and spatial distribution of architectural elements in the depositional sequence, as well as lateral changes in thickness and net-to-gross ratios, suggest that the BCF was deposited within a set of small incised-valleys. These systems formed in a complex dynamic between incision, deposition, and sediment supply.