Provenance of the south Texas Paleocene-Eocene Wilcox Group, western Gulf of Mexico basin : insights from sandstone modal compositions and detrital zircon geochronology
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Sandstone modal compositions and detrital zircon U-Pb analysis of the Paleocene-Eocene Wilcox Group of the southern Gulf Coast of Texas indicate long-distance sediment transport primarily from volcanic and basement sources to the west, northwest and southwest. The Wilcox Group of south Texas represents the earliest series of major post-Cretaceous pulses of sand deposition along the western margin of the Gulf of Mexico (GoM). Laramide basement uplifts have long been held to be the provenance of the Wilcox Group, implying that initiation of basement uplifts was the driving factor for this transition from carbonate sedimentation to clastic deposition. To determine the provenance of the Wilcox Group and test this conventional hypothesis, 40 thin sections were point-counted using the Gazzi-Dickinson method to determine sandstone composition and 10 detrital zircon samples were analyzed by LA-ICP-MS to determine U-Pb age spectra for each of the sampled areas. Modal data for sand grain populations suggest mixed sources including basement rocks, magmatic arc rocks and subordinate sedimentary rocks for the Wilcox Group. Zircon age spectra for these sandstones reveal a complex grain assemblage derived from older sediments and crystalline rocks ranging in age from Archean to Cenozoic. Sediment was primarily derived from Laramide uplifted crystalline blocks of the central and southern Rocky Mountains, the Cordilleran arc of western North America, and arc related extrusive and intrusive igneous rock of northern Mexico. Comparisons of Upper and Lower Wilcox zircon age spectra show that more arc related material was deposited in the Lower Wilcox, whereas more basement material was deposited in the Upper Wilcox.