The Wilcox Group in east-central Louisiana consists of a variable sequence of fine-grained sand, mud, and lignite units with a composite thickness of about 3,000 feet. Studies of sand-body geometry, lithologic composition, and facies relationships indicate that the Wilcox Group consists dominantly of deltaic plain deposits (the Holly Springs and an overlying, unnamed delta system) which filled the Mississippi trough during late Paleocene and early Eocene times. Depositional history of these deposits is divided into four phases: (1) a basal progradational phase, characterized by thick bar-finger and upper deltaic plain sequences indicative of delta construction onto a deep and muddy shelf; (2) a thick transgressive deltaic phase including several shoal-water delta lobes with many distributaries separated by destructional phase units; (3) an upper deltaic phase characterized by small shoal-water delta lobes; and (4) a fluvial-transgressive phase consisting of a massive sand unit of coalescing fluvial deposits capped by a veneer of glauconitic, transgressive sands. Deltaic deposits of the lower part of the Wilcox Group closely resemble corresponding facies of the Recent Mississippi River delta system. The deltaic mass of the upper part of the Wilcox Group differs from both in several significant lithologic and geometric parameters, including: (1) an increase in carbonate accumulation; (2) a decrease in thickness and width of the channel sand and related facies; (3) a decrease in sand percentage; and (4) a decrease in the amount of lignite. A shift in paleodrainage from south to southeast accompanies these changes. / text

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. / text