Abstract: Episodic Regression in Offshore-Lower Shoreface Facies, Skull Creek Shale, Lower Cretaceous, Northern Colorado

John P. Graham, Frank Ethridge

The Lower Cretaceous, Skull Creek Shale and Ft. Collins member of the Muddy Sandstone at Horsetooth Reservoir form a progradational parasequence within a highstand systems tract punctuated by two heretofore unrecognized regressive pulses. The marine flooding surface juxtaposes marine shales of the Skull Creek over near-shore sandstones and paleosols of the Plainview Formation. The upper boundary of the parasequence is a sequence boundary.

Gutter casts at the base of storm-generated, hummocky cross-stratification mark the first abrupt shallowing within the Skull Creek. Upper offshore to lower shoreface, thinly bedded sandstones and siltstones rapidly prograde over offshore shales. The gutter casts represent the distal expression of a forced regression on a shoreline prograding from the west. In addition, a local synsedimentary slump at Soldier Canyon Dam contains overturned hummocky sandstones and siltstones translated only a short distance down depositional dip. Movement may have occurred when a pressure gradient developed at the sediment-seawater interface with rapid sea level lowering.

Stratigraphic relationships suggest that irregularities in an offshore geometry were subdued with the progradation of shoreface facies in the upper Skull Creek. The abrupt introduction of Cruziana ichnofacies and increased sand/shale ratio signals a second progradational pulse over an established offshore facies. Following this pulse, the oscillations cease and the lower to middle shoreface of the Skull Creek becomes transitional into the upper shoreface and foreshore of the Ft. Collins member before the parasequence is truncated by the sequence boundary.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994