High-Frequency Sequence Stratigraphy of Nearshore Strata: Corcoran Member, Mount Garfield Formation, Book Cliffs, Colorado, U.S.A.

Mustapha Zater and Diane L. Kamola
University of Kansas, Lawrence, KS

Shallow-marine strata within the Corcoran Member of the Upper Cretaceous Mount Garfield Formation were investigated to develop a high resolution sequence stratigraphic framework for this member. This investigation consists mainly of outcrop analysis from the eastern Book Cliffs, and limited amount of subsurface data, taken from a series of wells drilled directly north and east of the field area. Sedimentological analysis of nine sections was completed. Strata, as well as significant surfaces, were followed canyon-by-canyon to understand the along-dip variability of the various facies.

Strata of the Corcoran Member are interpreted to comprise three high-frequency sequences. Sequences are bounded by surfaces characterized by erosional truncation, a basinward shift in facies and a rapid increase in grain size. Outcrop expression of the first sequence consists of two progradationally stacked parasequences, comprised of wave-dominated shoreface successions, and interpreted as deposits of the highstand system tract. The second sequence consists of a thin incised valley fill succession, overlain by a wave-dominated shoreface succession, which is truncated by a sequence boundary. Directly overlying the sequence boundary is a 7-8 m succession of nonmarine and marginal marine strata. Extensive lateral variability is seen in this succession, which includes the following: carbonaceous shale and poorly developed, thinly bedded coals; interstratified coal, siltstone, and sandstone; and upward fining sandstones with tidal indicators. This final succession has a strong aggradational element and is interpreted as the transgressive systems tract of the third sequence. The third sequence straddles the lithostratigraphic member boundary between the Corcoran and overlying Cozette Member.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005