ABSTRACT

Numerous systems of stratigraphic nomenclature are used for the Wilcox Group in South Texas on regional, subregional and localized basis leading to considerable confusion among geologists using different nomenclatures. A broad, regional lithostratigraphic classification is proposed in which other classifications and nomenclatures will fit, thereby standardizing recognition of the various stratigraphic intervals comprising the Wilcox throughout the South Texas area.

Many of the Wilcox depositional systems have been identified and defined in previous studies. The proposed classification is based on the natural subdivisions of the Wilcox progradational sequence and the lithology and depositional systems contained therein.

The North American Stratigraphic Code was followed in preparing the classification. The Wilcox Group is subdivided into lithostratigraphic units as follows: the Upper Wilcox Subgroup, the Middle Wilcox Subgroup, the Lower Wilcox Subgroup, the Carrizo Formation, the Indio Formation and an unnamed downdip shale unit. The Carrizo Formation is further subdivided into the "Carrizo - Bigford" Member, the "Massive Carrizo" Member and the "Wilcox - Carrizo" Member. The Upper Wilcox Subgroup is divided into an Upper Member and Lower Member and the Upper Member is subdivided into the Slick, Luling, Mackhank and Massive sandstone units in the areas where these sandstones are present. Each of the lithostratigraphic units is briefly defined and described as to stratigraphic position, vertical and lateral boundaries, depositional systems and other pertinent information.