Sedimentation and pedogenesis of the Lower Cretaceous Hensel Formation, central Texas
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The Lower Cretaceous Hensel Formation in central Texas consists of predominantly terrigenous sediments deposited during the marine transgression of the Glen Rose and Fredericksburg Group carbonates. Hensel outcrops south of the Llano Uplift are composed of coarse-grained, lenticular, framework deposits and finer grained, extensive, matrix deposits. Framework facies include cross-stratified conglomerate, conglomeratic sandstone, and sandstone. These facies represent small fluvial systems whose channel-fill textures and morphologies changed in response to lowered channel gradients and decreased sediment supply. Matrix facies include calcretized (calichified) red mudstone and thin, sheet sandstone, interpreted as paleosols and overbank deposits. The pervasive development of calcrete, desiccation cracks, and pseudo-anticlines within the Hensel paleosols, as well as the overall depositional style of the unit imply an arid to semi-arid, seasonal climate. Paleopedogenesis resulted in the formation of authigenic kaolin and illite, intense red-orange coloration, and various types of calcrete. Paleocalcrete is most abundant within the mudstone facies where it assumes a nodular habit, commonly found as rhizoconcretions or coalesced into honeycomb structures. Calcretized, thin-bedded sandstone and mudstone buckled into pseudo-anticlines in one locality. Microscopic soil features in all matrix calcretes include orthic and disorthic glaebules of low-Mg calcite, crystallaria, pedodes, circumgranular cracks, and joint planes. Calcian-ferroan dolomite with limpid overgrowths and fibrous calcite are confined to the "tepee" calcrete. Paleocalcrete accumulated in the framework sandstone facies as undulating hardpans and vertically oriented rhizoliths. Microscopic textures in these calcretes are similar to those in the nodules and calcretized sandstones of the matrix facies. Detrital grains are widely dispersed and silica is but slightly replaced by calcium carbonate. This evidence suggests that displacive crystallization of calcitic glaebules and fibers was the most significant precipitation mechanism during calcretization in both the mudstone and sandstone facies. Carbon and oxygen isotopic values from each representative calcrete confirm a pedogenic origin for the authigenic calcite. "Heavier" isotopic values for the pseudo-anticline dolomite denote an enrichment of meteoric groundwaters by extreme evaporation. These conditions would have existed near a continental playa, an environmental interpretation considered appropriate for such a setting.