Abstract: Detailed Analysis of the Turonian to Campanian Part of the Global Cycle Chart: Evidence from the Western Interior of the United States

Keith W. Shanley, Peter J. McCabe

At least five sequence boundaries, recognized on the basis of basinward shifts in facies and the development of incised valleys, are recognized within Turonian to Campanian strata of the Four Corners area--they are dated approximately at 90.5, 90.0, 88.8, 88.5 and 85 ma. However, because of regional variations in subsidence rates, the expression of the sequences varies greatly across the area. In areas of relatively low tectonic subsidence such as the San Juan basin, Black Mesa, and Kaiparowits Plateau, the 90.5, 90.0, 88.8, an 88.5 ma sequence boundaries are characterized by a pronounced basinward shift in facies and the development of incised valleys. In the Henry Mountains and Wasatch Plateau, faster rates of tectonic subsidence resulted in a more subtle basinward shift in facies a the 90.5 and 90.0 ma sequence boundaries. The higher rates of subsidence precluded subaerial exposure and a basinward shift from developing at 88.8 and 88.5 ma and instead resulted in retrogradational parasequence stacking patterns. The Kaiparowits Plateau and Black Mesa are located significantly updip of the San Juan basin and Wasatch Plateau, and in this region the 90.0 and 88.8 ma sequence boundaries are merged forming a composite sequence boundary representing a prolonged period of subaerial erosion. Much of the Gallup and Ferron sandstones that lie between the 90.0 and 88.8 sequence boundaries in the San Juan basin, Henry Mountains, and Wasatch Plateau are temporally equivalent to this erosion surface.

The global cycle chart can be broadly applied to the Turonian to Santonian strata of the Four Corners region, however, the lateral changes in parasequence stacking patterns and the expression of sequence boundaries also reflect changes in the rates of tectonic subsidence. Furthermore, careful consideration of position within the basin suggests that the 90.0 ma sequence boundary--one of the more dramatic base-level falls on the global chart--is a composite boundary of at least two sequence boundaries.

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