AAPG Southwest Section

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Geology of T* Ranch, Tom Green and Irion Countries, West Texas

Abstract

The T* Ranch is located 15 miles northwest of San Angelo, Texas within the NW Edwards Plateau in the North Concho River drainage basin. The Cretaceous Antlers Sand of the Trinity Group, Cretaceous Fort Terrett Formation of the Edwards Group, and older and recent Quaternary alluvium are exposed on the ranch. The mapping area covers ~16 sections. Widespread Lower Cretaceous rocks exposed were deposited in shallow-marine, lagoon, beach shoal, and peritidal environments within in the Cretaceous Interior Seaway during a global transgressive sequence (Rose, 1972). Rocks in the subsurface include Permian mixed siliciclastic, carbonate, and evaporate strata of the Eastern Shelf of the Midland basin (Henderson, 1928). The Laramide orogeny affected Cretaceous sedimentation patterns and the geometry of the seaway (Blakey, 2014). Late Cenozoic Basin and Range extension in Trans-Pecos Texas may have affected Cretaceous and older rocks.

This project applies several techniques to collect field data and create digital maps. Detailed, 1:12,000-scale geologic mapping involves walking out formation contacts and marker beds, precisely locating contacts with GPS, measuring fracture orientations and cross-bed foresets with a Brunton compass, and then transferring the field map to a GIS map. Several detailed stratigraphic sections within limestone quarries are measured with a Jacobs staff and Brunton compass. Lineaments interpreted from aerial photographs and satellite imagery are overlapped with (4-3-2) Near Infrared mosaic images from Landsat+ETM using ESRI ArcGIS.