The Paleobiogeography of the Late Cretaceous, Western Interior Basin Tepee Butte Mounds (Hydrocarbon Seeps) of North America and Possible Tectonic Factors Controlling Their Distribution

Within the Late Cretaceous, Western Interior Basin of North America hundreds of "tepee buttes" mounds have been mapped. Comparisons of the paleobiogeography of the tepee buttes to subsurface structures, basinal subsidence patterns, and shoreline position, indicate an association between mound formation, western shoreline migration and changes in basin tectonics. Initiation of tepee butte formation within the basin is concurrent with changes in the tectonics of North American and Farallon Plates, and the subsequent shift eastward in the region of maximum subsidence and sediment deposition within the basin. Mound distribution is basinward of the loci of maximum subsidence, delineating the forebulge region of the basin.

The tepee buttes occur as localized fields of widely spaced, often linearly arranged, conical shaped, anomalous carbonate bodies within the basinal Pierre Shale. Carbonate production is related to oxidation of hydrocarbon emissions at the sea floor by chemosynthetic bacteria. The geographic distribution of the tepee buttes is constrained to roughly between 101° 30' W and 105 °30' W longitude ranging from the northern Black Hills into southern Colorado. Four discernable intervals of seep activity are identified over a time span of 10 million years from late Middle Campanian through the Early Maastrichtian, 78.7 Ma through 69.1 Ma.

Intervals of mound formation are tied to migration of the western shoreline, with mound formation starting at maximum transgressive phases, and ending at maximum regressive phases. Shoreline migration alters sediment loading across the basin affecting the position and degree of flexure of the forebulge region. Such flexural changes are postulated to alter the intensity of seep emissions, and the opening and closing of the fluid-conduits that supply the hydrocarbon-seeps, thus affecting carbonate production. Detailed mapping of the age distribution of individual mounds across more localized areas has the potential to record the movement of the forebulge across the basin.