--> Decreased Channel Dimensions and Sediment Flux Through the Paleocene Raton-Wilcox Rivers: Implications for Wilcox Shelf Margin

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Decreased Channel Dimensions and Sediment Flux Through the Paleocene Raton-Wilcox Rivers: Implications for Wilcox Shelf Margin

Abstract

The Paleocene-Eocene Wilcox Group of onshore South Texas reaches a thickness of 1300 m, and contains a linked facies succession passing from coastal plain to shoreline to shelf and out to deepwater slope deposits. This spectrum of facies associations across Texas were the very deposits giving rise to the original concepts of ‘depositional systems’ by Fisher and McGowen (1967). A re-visit of some 500 wells and 1500 feet of cores through the Wilcox succession in South Texas shows that the fundamental organization of the linked facies tracts was one of some 24 transgressive(T) to regressive(R) units, thickening from av. 25 m on inner shelf to av. 75 m on outer shelf. Cores show that the transgressive part of units or sequences formed mainly from mixed energy, tidally-influenced estuaries, whereas the regressive part represents mainly river- and wave-dominated deltas. These fundamental sequences reflect the fact that the Wilcox shelf-margin prism was constructed by the repeated T-R transits of shorelines across the widening shelf, driven ultimately by northerly rivers. As successive T-R sequences accumulated, the shelf break of the clinoform system migrated up to 8 km basinward per sequence. To further evaluate the feeder river systems that drove the Wilcox shoreline, the Paleocene, Raton and Poison Canyon formations of the Laramide Raton Basin were examined, as these represent coeval fluvial systems that flowed southwards across New Mexico and Texas. The fluvial channels of the older Raton Formation are up to 6 times wider and up to 5 times deeper than the younger Poison Canyon channels. In addition, the former were more interconnected and sometimes sheet-like in their amalgamation, compared to the more isolate, lower net-to-gross channels of the Poison Canyon Formation. It is likely that the decreased sediment discharge through time in the Raton Basin reflects the change from coarser-grained Lower Wilcox to finer-grained Middle Wilcox on the GOM shelf-margin prism, and is also consistent with the recorded decrease in rates of shelf-margin progradation from Lower Wilcox (20–30 km/My) to Middle and Upper Wilcox (4–8 km/My). Although the large Raton channels and Wilcox distributary channels indicate Paleocene high sediment discharges to the coeval Gulf coast, the decreasing sediment discharge in late Paleocene and Eocene suggests lower Laramide relief generation and possibly northward diversion of some of the Laramide rivers.