Abstract: Late Cretaceous Paleogeography, Sediment Accumulation, and Coal Distribution, the Western Interior of Middle North America

Laura N. Robinson Roberts, Mark A. Kirschbaum

A synthesis of Late Cretaceous paleogeography and sediment accumulation of the Western Interior, from Mexico to southwestern Canada, provides insight into the relative importance of sea-level variation and tectonism as factors controlling the distribution of Late Cretaceous coal. Results include a sequence of paleogeographic maps of six biostratigraphically constrained intervals that depict depositional environments and emphasize the area distribution of peatforming environments. Isopach maps of strata for each time interval were also constructed. Information from these maps, and a knowledge of sea-level variation during the Late Cretaceous, allowed comparison of areas of thick coal with areas affected by varying sediment accumulation rates and by varying sea level.

Thick peat accumulated during the early Cenomanian (Dakota Formation coal) on low-gradient alluvial plains where sediment accumulation rate was relatively slow, well inland from the rising Cretaceous Interior seaway. Thicker, more extensive peats formed from the latest Cenomanian to the middle Maastrichtian within a foreland basin east of the Sevier orogenic front. The majority of peat accumulated on coastal plains in the more actively subsiding parts of the foreland basin. This time interval is characterized by fluctuating shorelines resulting from the interaction of sediment supply, basin subsidence, and eustasy. During the late Maastrichtian, when Laramide deformation predominated, less significant amounts of peat accumulated (Lance Formation coal), either confined to intermontane asins, where sediment accumulation rate was high, or scattered over short-lived coastal plains that trailed the final retreat of the seaway from the Western Interior.

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