Deposition and diagenesis of the lower cretaceous Antlers Sandstone on the Young Ranch, Nolan County, Texas
Wood, Leonard Wayne
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The Lower Cretaceous Antlers Sandstone on the Young Ranch is the time equivalent to the Twin Mountains, Glen Rose, and Paluxy Formations of the Cretaceous Trinity Group. The Antlers represents the initial deposition of sediment onto the Wichita Paleoplain unconformity of North and West-Central Texas. The Antlers on the Young Ranch is interpreted as a bed-load channel deposit consisting of sandstone and gravelly sandstone. On the Young Ranch, the Antlers Sandstone is a discontinuous sandstone that is only present where post-depositional cementation by quartz and microquartz has occurred, or where the sandstone is overlain by uneroded limestones of the Walnut Formation. The silicification is similar to sllcretes (quartz cemented soils) found in other Mesozoic and Cenozoic sediments. Petrographic analysis of the Antlers silcrete on the Young Ranch reveals large amounts of syntaxial overgrowths on detrital quartz grains. Microquartz cement is also observed, although it is not present in the large volumes like the quartz overgrowths. Syntaxial quartz overgrowths were observed when using the SEM, as well as the small euhedral and subhedral crystals characteristic of microquartz. The reconstruction of the diagenetic history of the Antlers Sandstone on the Young Ranch indicates that the Antlers most resembles a groundwater silcrete. The maximum depth of burial that the Antlers could have undergone (< 160 ft.) excludes deep burial (>3 km), therefore silicification must have formed under shallow burial conditions (diagenetic quartzarenite or silcrete). Moreover, the multicolored nature of the Antlers in outcrop indicates shallow silicification (silcrete). The predominance of quartz overgrowths (--70%) compared to mircoquartz cement (--30%) indicates a groundwater silcrete as opposed to a pedogenic silcrete. The absence of numerous soil features also supports the interpretation of the Antlers being primarily a groundwater silcrete. The silicification of the Antlers sandstone probably occurred penecontemporaneous with Antlers deposition, but could have occurred during post-Edwards exposure. Fluctuations in the water table on the Young Ranch are the likely mechanism for silicification, which transported silica liberated from intraformational sources such as dissolutloned chert and polycrystalline quartz grains. This accumulation of silica from internal sources aids in the Interpretation of the Antlers Sandstone on the Young Ranch being classified as a paleosilcrete horizon.