Storm Deposition and Sequence Stratigraphy of a Cretaceous Near-Shore Mudstone Unit — The Skull Creek Shale Formation, Colorado, USA

Abstract

The Skull Creek Formation is a fine-grained siliciclastic unit in the Cretaceous Dakota Group along the Front Range in the western US. Well-known for its storm-generated structures, the unit is sandwiched between two sandstone packages, the Plainview and underlying Lytle Formations below, and the overlying J sandstone. Current models interpret the Skull Creek Formation as late transgressive to early highstand sediments deposited in an offshore environment with a maximum flooding surface in its lower part. A detailed sedimentology of this unit, however, is still lacking despite its importance as a potential hydrocarbon source rock for the overlying well-explored J sandstones. In this study, the Skull Creek Formation is subdivided into three facies associations: a sandy portion characterized by sandy gutter casts lined with shale at their bases, combined flow ripples and HCS sandstones, a siltstone-dominated part with low-angle combined flow ripples, and a silt-rich and in places laminated mudstone with lenticular siltstone laminae. The level of bioturbation varies throughout the section and is generally better developed the coarser the grain size. The Skull Creek Formation is interpreted as an offshore environment subdivided into three depositional facies belts that are equivalent to the three facies associations: a proximal, sand-rich part that received abundant sediment input from nearshore settings during storms; an intermediate portion consisting of siltstones deposited above storm wave base but beyond the reach of sand-rich currents from the proximal shelf; and a distal environment showing relatively high-energy deposition during storms, with intermittent mudstone sedimentation during fair-weather. Filling of the gutter casts in the proximal facies, however, was most likely the result of two consecutive storms, the shale linings reflect waning energy after the scouring and prior to their infill with sand. Based on its stacking patterns the Skull Creek Formation reflects an initial transgression expressed in the fining-upward in the lower part of the unit. A weakly developed regression is reflected in an overall coarsening-upward and the presence of sand-rich gutter casts and HCS sandstones. Following a slight transgression the succession shallows until the overlying J sandstones reflected in a gradual increase in sand content.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015