Depositional Environments and Sequence Stratigraphy of the Lower Cretaceous Dakota Sandstone, SW Colorado

Serradji, hayet¹, Diane Kamola² (1) Kansas University, lawrence, KS (2) University of Kansas, Lawrence, KS

Facies and sequence stratigraphic analysis of the Lower Cretaceous Dakota Sandstone was completed in SW Colorado. Delta-front sandstones, channel-fill sandstone, and deltaplain and/or floodplain siltstone are the dominant facies. Delta-front sandstones are upward-coarsening intervals, with abundant planar-to-current-rippled beds indicating river dominance. Individual deltas can be traced for up to 10 km and show facies changes from distributary channel to proximal delta to distal delta-front settings. The facies belts are usually over 3 km width. Arenicolites, found throughout the deltaic intervals, is interpreted to indicate stressed, possibly brackish-water conditions. Eight parasequences are present. Parasequence boundaries correspond to a sudden increase of accommodation seen by the vertical stacking of the various depositional facies. In places, a thin (cm-thick) coal horizon, which can be traced up to 5km, defines the parasequence boundaries. Parasequences at the base of the studied interval usually contain floodplain/deltaplain or fluvial deposits. Paresequence toward the top of the studied interval contain deltaic deposits and record the sudden input of coarser material to the study area. Progressively greater wave influence is observed in the deltas that are stratigraphically higher in the section, seen through the presence of thick HCS and (10 to 15 cm) wave-ripple beds towards the top of the formation. This vertical stacking of parasequences (greater marine influence up section) is interpreted to reflect a gradual landward movement of the shoreline across the study area while the Dakota accumulated. Based on this, a retrogradational parasequence stacking pattern is inferred.