Depositional Environments and Sequence Stratigraphy of the Lower Cretaceous Dakota Sandstone in the Ridgway Area, Southwestern Colorado

Abstract

The Dakota Sandstone records the initial transgression of the Cretaceous western interior seaway across southwestern Colorado. This transgression does not show a simple pattern. The sequence stratigraphic analysis of the Dakota Sandstone indicates that the initial part of this transgression occurred in multiple steps. This study concentrates on eleven sections in the Ridgway, Colorado area. Facies analysis identified five depositional facies: delta plain, delta front, radial bifurcating channel (distributary-channel), lower shoreface and fluvial channel. Deltaic facies are the predominant facies and show interaction between fluvial and marine processes. Higher wave influence is present in the upper part of the formation, seen by the presence of hummocky cross stratification in a lower shoreface setting. This increase in wave energy suggests a change in shoreline configuration. At the base of the formation, deltaic sediment accumulated in an embayed coastline, protected from wave energy. Near the top of the formation, the shoreline became straighter with higher wave influence. At the top of the formation, the shoreline returned to an embayed configuration. Sequence stratigraphic interpretation of the Dakota Sandstone reveals the presence of eleven parasequences and three depositional sequences, with two incised valleys. Sequences are bounded by erosional surfaces or their correlative interfluve expressions. Sequence one starts in the underlying Burro Canyon Formation, continues into the Dakota Sandstone, and includes seven parasequences represented by delta plain, delta front and distributary channel facies. These parasequences show a retrogradational followed by an aggradational stacking pattern. Sequence two contains three parasequences and includes an incised valley-fill at the base overlain by lower shoreface and delta-front facies. The staking pattern is defined as aggradational within this sequence. The lower part of sequence three occurs within the Dakota Sandstone; the remainder of the sequence occurs in the overlying Mancos Shale. The part of sequence three within the Dakota Sandstone is represented entirely by fluvial deposits within an incised valley. The sequence stratigraphic interpretation of the Dakota Sandstone in southwestern Colorado reveals the complexities related to the initial transgression of the Cretaceous Western Interior Seaway. These complexities are seen via the presence of different sequences, as opposed to a single parasequence set characterized by a retrogradational stacking pattern.