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.