Glenn R. Sharman
Postdoctoral Fellow, Bureau of Economic Geology, University of Texas at Austin
Continental-scale drainages host the worlds largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics, climate) on landscape evolution, yet deciphering between these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled rate of sediment delivery to the northern GOM during Paleocene-Eocene time. Sedimentation rate increased two orders of magnitude, reaching approximately three times the Cenozoic average, during deposition of the Lower Wilcox Group, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this dramatic increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide hinterland (~910,000 km^2) that included the headwaters of the California and Idaho rivers. Capture of this California River drainage may have occurred in the vicinity of the Hanna basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta that accumulated within the Houston embayment during Lower Wilcox time. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide syn-orogenic units, including enrichment in detritus derived from the Cordilleran Arc and basement terranes of western North America relative to older and younger units in the Houston embayment. Subsequent decline in sedimentation rate by an order of magnitude to the GOM can be partly attributed to well-documented drainage closure (~780,000 km^2) that accompanied lake formation in the Laramide hinterland (ca. 53-51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.