| Abstract: | Late Cretaceous, Western Interior basin cold-seep mounds or tepee buttes,
reported hydrocarbon emission (methane) sites, are anomalous carbonate bodies
within basinal shales. Stratigraphic sleuthing has revealed the cold-seep mounds
to be narrowly restricted in their geographic, stratigraphic, and age
distributions. The geographic distribution ranges from the northern Black Hills
southward to the Texas-Mexican border, restricted to roughly between 101°30" W
and 105°30" W longitude. The age distribution of cold-seep formation consist of
five discrete intervals during a time span of approximately 14 million years,
basal Campanian through the Early Maastrichtian.
The oldest cold-seeps are reported from the upper Ojinaga Formation, West Texas, within basal Campanian Gulf Coast biozone, Submortoniceras tequesquitense. Later cold-seep mounds, the Tepee Buttes, are reported from the central interior of the basin (Black Hills to Colorado/New Mexico border) within the Pierre Shale. These occurrences are distributed within four intervals from the Middle Campanian through Early Maastrichtian within the Western Interior basin ammonite biozones of: 1) Baculites perplexus and B. gregoryensis, 2) B. scotti through Didymoceras cheyennense, 3) B. reesidei through B. eliasi, and 4) B. grandis and possibly B. clinolobatus. Comparisons in the geographic and temporal distribution of the interior cold-seep mounds to subsurface structures, basinal subsidence patterns, and strandline position, suggest an association between cold-seep formation and changes in basin tectonics, western strandline migration, and the possible delineation of the forebulge region. |