STratigraphic Petrology as Aid to Mapping Calcarenite Bodies in Lower and Middle Trinity Cretaceous Rocks of San Marcos Platform, Texas: ABSTRACT

Abstract

Lower Trinity rocks (Sycamore sandstone, Hosston sandstone, and Sligo limestone) and middle Trinity rocks (Hammett shale and Cow Creek limestone) represent two depositional cycles, bounded by regional disconformities, in the basal Lower Cretaceous section of south-central Texas. The cyclic units contain alluvial sandstone, alluvial and lagoonal terrigenous mudstone, lagoonal carbonate mudstone, and oolitic and skeletal calcarenites. Study of outcrops, cores, well cuttings, and electric logs demonstrates that these lithofacies are in ordered belts parallel with paleoslope contours on the depositional shelf. Calcarenites formed where highly agitated marine water impinged on the shelf surface. At different times, belts of calcarenite were formed at the shoreline, about 50 mi offshore, and 75-90 mi offshore. The resulting bodies are 5-25 ft thick, 5-10 mi wide, and tens of miles long. The approximate location of each offshore body can be determined by mapping lithofacies belts landward. Seaward from the calcarenite bodies, ooliths and well-rounded skeletal fragments are present as exotic grains in mud. These round grains range from 0.05 to 0.1 mm in diameter and are in quiet-water, open-marine sediments characterized by calcareous mud, terrigenous silt, glauconite silt, large angular shell fragments, and fragile whole fossils. Careful correlation of electric logs on the basis of scattered cores, plus thorough examination of well cuttings and cores, provides the ability to predict locations of calcarenite belts within these cyclic deposits. End_of_Article - Last_Page 600------------