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ABSTRACT: Development of Laramide-Style Subsidence Trends During Late Cretaceous Foreland Basin Deposition, Northwest Colorado

Ronald C. Johnson

Isopach maps of Upper Cretaceous rocks in the Laramide Piceance basin of northwest Colorado have revealed that Laramide-style subsidence trends developed long before the end of Cretaceous foreland basin deposition in this area. Foreland basin deposits in northwest Colorado can be subdivided into three major units: (1) the Cenomanian through lower upper Campanian marine Mancos Shale, (2) the upper Campanian marginal marine Iles Formation, and (3) the upper Campanian through Maestrichtian largely nonmarine Williams Fork Formation. Isopach maps of the Mancos Shale show west- and northwest-thickening trends similar to regional isopach trends of foreland basin deposits. In contrast, isopach maps of the Iles interval reveal a change to east- and northeast-thickening trends iden ical to those found in the Laramide Piceance basin. However, the Iles interval continues to thicken over the White River uplift, which forms the eastern boundary of the Laramide Piceance basin. This shift occurred near the end of Baculites asperiformis faunal zone (78 Ma), or considerably before the White River uplift began to rise in Paleocene time. Shoreline trends for the Iles interval were not noticeably influenced by these variations in subsidence; they typically run northeast-southwest, nearly perpendicular to isopachs. Thickening trends of the Williams Fork interval are similar to those of the Iles. An unconformity occurs at the top of the Williams Fork, however, and it was previously thought that early Laramide uplift and beveling increased toward the west, producing this thicken ng trend. Significant beveling may have occurred, but the westward thinning of the Williams Fork may also be depositionally controlled.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990