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AAPG Southwest Section Annual Convention

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New Applications of Detailed Geologic Mapping in the Glass Mountains

Abstract

The Glass Mountains expose the shelf-to-basin transition on the SE margin of the Delaware basin. Ongoing mapping-based studies in two small areas within the Glass Mountains are producing significant results. A detrital zircon geochronology study (reference on poster) that included samples from both map areas indicates a southern, Coahuila terrane, source for Pennsylvanian and Permian sands. Lower slopes of Leonard Mountain previously mapped as Carboniferous Tesnus Formation expose folded Pennsylvanian Haymond Formation below an angular unconformity and overlying Pennsylvanian Gaptank Formation. Our two ~40 km2 field areas are in the eastern and central Glass Mountains, 27 km NW and 15 km N of Marathon, Texas. Field areas expose different facies of approximately the same section: Pennsylvanian Haymond and Gaptank Formations, Wolfcampian Lennox Hills Formation, Leonardian Hess, Skinner Ranch (Psr), and Cathedral Mountain (Pcm) Formations, and the Leonardian-Guadalupian Road Canyon Formation. This project applies traditional and modern mapping techniques and descriptive structural analysis to make 1:12,000-scale geologic maps of key areas, refining classic geologic mapping by P.B. King (most detailed scale 1:62,500). Traditional techniques include: documenting fault and unconformity contacts, measuring many bedding attitudes, thin-section and hand-sample description, applying three-point method to calculate contact orientations, and constructing cross- sections. Modern techniques include: using 3-D Google Earth images to refine contact locations, locating key contacts with GPS, and correlating map units with recent sequence-stratigraphic work. Geologic mapping produced several recent results. The original SE Delaware basin slope in the Leonardian was 9° NW, calculated by using a stereographic projection technique to remove late Permian and Laramide tilting from Psr slope clinoform attitudes. The calculated Psr- Pcm contact orientation on Leonard Mountain is 052 7NW, similar to bedding orientations in Pcm and different from underlying clinoform bedding in Psr. Point-counted Gaptank and Road Canyon sandstones yielding dated detrital zircons are lithic arenites containing differing amounts of carbonate, volcanic, and chert rock fragments.