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Eagle Ford - Colorado Connection: Cenomanian to Coniacian in Southwestern North America

Thomas Ewing
Frontera Exploration Consultants

Preliminary reconstructions of early Late Cretaceous paleogeography suggest a complex series of events for organic mudrock deposition in southwestern North America. In middle Cenomanian time, shelf carbonate deposition in Texas suddenly ceased with formation of a major hardground and/or exposure surface. About the same time, Dakota clastic shorelines were being transgressed in New Mexico. In late Cenomanian and earliest Turonian time, marine transgression reached a peak. Clastics from the emerging Cordillera were trapped far westward. The deepwater Ojinaga trough connected northward to the Rio Salado tongue of the Mancos and ultimately to the Graneros and Greenhorn section to the north, and bounded the Eagle Ford shelf platform on the west. In middle Turonian time, a rapid shoreline progradation in New Mexico formed the Tres Hermanos regression, a zone of remarkably stable character and thickness suggesting a forced regression. At the same time, the Lozier Canyon section records a significant unconformity, perhaps on the flanks of the San Marcos Arch. The San Vicente section contains a few clastic grains but major clastic progradation was probably stopped by the Ojinaga trough. In the late Turonian, renewed transgression resulted in the D-Cross member of the Mancos, possibly time-correlative with the Langtry member of the Eagle Ford. However, a major deltaic headland developed in southern New Mexico causing progradation of Gallup shorelines and deposition of the thick Crevasse Canyon Formation. This headland sealed the north end of the Ojinaga trough and possibly inhibited the circulation of anoxic waters in and out of the Western Interior Basin. This disruption of circulation may be partly responsible for gradual lithologic changes from Eagle Ford to Austin (oxygenated facies) in Texas, and a partial separation of Austin from Niobrara facies during Coniacian-Santonian time. Additional work is needed on key outcrop sections in west Texas and New Mexico to confirm this conjecture.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013