STORM DEPOSITION AND SEQUENCE STRATIGRAPHY OF A CRETACEOUS NEAR-SHORE MUDSTONE UNIT – THE SKULL CREEK SHALE FORMATION, COLORADO, USA

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 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 for the overlying well-explored J sandstones.

In this study, the Skull Creek Formation is subdivided into three facies: a sandy portion characterized by sandy gutter casts lined with shale at their bases, combined flow ripples and HSC sandstones, a siltstone-dominated part with low-angle combined flow ripples, and a silt-rich and in places laminated mudstone with lenticular siltstone laminae. All facies are bioturbated throughout.

The unit is interpreted as an offshore environment subdivided into three depositional facies belts: a proximal, sand-rich part received abundant 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 both high- and low-energy deposition. Filling of the gutter casts in the proximal facies, however, was most likely the result of two consecutive storms, the shale linings reflecting waning energy after the scouring and prior to their infill with sand.

Based on its internal stacking patterns it seems likely that the Skull Creek Formation reflects an initial transgression, a weakly developed regression reflected e.g. the gutter casts, and a subsequent slight transgression before a final shallowing.