Sequence-Stratigraphic Analysis of the Rollins and the Cozzette Sandstone Members, the Upper Cretaceous Mount Garfield Formation of the Piceance Basin, Colorado.

Abstract

Sequence-stratigraphic study of the Cozzette and the Rollins Sandstone members, of the Mt. Garfield Formation of the Mesaverde Group, in the southern part of the Piceance basin (western Colorado), utilizes mainly well-log data along with limited outcrop data. Outcrop description of the Rollins Sandstone Member indicates a depositional succession that changes from complex marginal marine deposits at the base to marine wave-dominated shoreface successions at the top. The lower marginal-marine deposits are interpreted to occur within multiple incised-valley fills that nest and form a main stratigraphic element landward, particularly within the uppermost part of the Cozzette Sandstone Member. Incised-valley fills thin basinward. Sequence-stratigraphic interpretation of the subsurface data provides a stratigraphic history similar to that interpreted from the outcrop exposures across a regional realm. The subsurface analysis of the study interval distinguishes 5 depositional sequences that change in thickness throughout the study area and are listed as follows: CZ1, CZ2, CZ3, R1 and R2. The depositional sequence R2 is the youngest incomplete sequence within the study interval. Each depositional sequence is composed of incised-valley fills at the base and highstand deposits with marine shoreface at the top. The incomplete depositional sequence R2 is represented by incised-valley fills alone. The vertical chronostratigraphic architecture of the sequence set (CZ1, CZ2, CZ3, R1) show a regional change in stacking pattern from retrogradational (CZ1, CZ2, and CZ3) to progradational (R1). The turnaround from retrogradational to progradational stacking is probably the stratigraphic limit between the Cozzette and the Rollins Sandstone members; its stratigraphic expression is probably gradational and complex in a landward direction. Incised valleys are superimposed landward, probably along axes between raised mires, and exhibit highly variable log patterns that reflect complex marginal-marine deposits.