Changes in Eocene Sediment Supply to the Northern Gulf Coast and Implications for Petroleum Systems

Abstract

It has long been established that within the Cenozoic, the Middle Eocene was a time of anomalously low sediment supply to the northern Gulf Coast. To better understand this, we present detrital zircon (DZ) U/Pb age spectra for 12 samples (n=1620) from Middle Eocene Claiborne Group strata and 5 samples (n=657) from Late Eocene Jackson Group strata. In both cases, samples were collected from outcropping strata extending from the Alabama-Mississippi border region to central Texas. Work on additional samples is underway. In general, DZ data from the Claiborne and Jackson Group are similar to data from the Upper Paleocene-Lower Eocene Wilcox Group and define at least five sediment input axes (paleo-Appalachian-derived, Mississippi, Arkansas, Brazos-Red, and Colorado rivers). This finding supports recent interpretations that both eastern and western North American source areas fed into the Mississippi Embayment continuously during the Paleogene. Although Claiborne Group DZ age populations broadly mirror overlying and underlying strata, the Claiborne Group consistently exhibits a much higher proportion of DZs that must ultimately derive from igneous rocks in the eastern United States. Consequently, we interpret relatively low sediment transfer from western cordilleran sources to a large part of the northern Gulf Coast in the Middle Eocene, perhaps due to diminished erosion or upstream trapping of sediment. Jackson Group DZ data indicate an increased proportion of western cordilleran sediment on the Texas coastal plain by the Late Eocene. Diminished sediment supply from western sources corresponded to the deposition of several intervals of fine-grained sediment in the Claiborne and eastern Jackson Group. The fine-grained strata are expressed in seismic reflection data as reflective intervals across the paleo-shelf. Based on continuity of a reflective seismic facies, a basal, eastern Claiborne Group fine-grained interval extends downdip into lightly explored regions. Similar to the Wilcox Group, sparse core data hint that the basal, shelfal Claiborne Group is consistently enriched in organic carbon (up to a few wt. %), and downdip continuity of a fine-grained stratal package suggests the presence of downdip, Claiborne organic carbon rich strata in lightly explored coastal parishes/counties. Reduced sand supply appears to have resulted in Claiborne Group reservoir potential different from the Wilcox Group, which may require a different exploration strategy.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016