Carboniferous stratigraphy of the Brady area, McCulloch and San Saba counties, Texas
Creators: name: Freeman, Thomas Jewell, 1932-
Publisher: name: The University of Texas at Austin
Year: 1962
Subjects: subject: Carboniferous stratigraphy
subject: Brady area
subject: McCulloch County geology
subject: San Saba County geology
subject: Texas geology
subject: Stratigraphic geology
subject: Barnett Formation
subject: Marble Falls Formation
subject: Geology--Texas--McCulloch County
subject: Geology--Texas--San Saba County
subject: Geology, Stratigraphic--Carboniferous

description: Every traceable interval with lithic or topographic distinction was mapped, and these were nomenclatorially arranged into genetically distinct units. Abnormally thick deposits of Chappel limestone occur in structural sinks, two of which have undergone post-Chappel collapse. Elsewhere the Chappel was probably reduced by pre-Barnett erosion, but removal of Chappel beds cannot be conclusively demonstrated. At the top of the Barnett Formation is an interval of glauconitic and phosphatic limestone that represents a retarded rate of sediment accumulation. No erosional vacuity can be shown, but it is suggested that a hiatus separates the Barnett from the Marble Falls Formation. Deposition of cherty microsparite and lenses of [oo]lite, which constitute the Lower Member of the Marble Falls, was followed by a period of emergence. Prior to the return of the sea, relief that was produced by post-Lower Marble Falls faulting was reduced by erosion. Onkolites and other conglomerates locally mark this unconformity throughout the area. The post-Lower Marble Falls sea was characterized by a near-shore terrigenous mud lithotope and an off-shore carbonate lithotope that transgressed from east to west. As these environments reached the western part of the area, there were generated to the east great banks of coralline algae (Komia) with tubular algal (Donezella) deposits on their flanks. The last event in Marble Falls deposition was the accumulation of codiacean algae (Ivanovia) and carbonate mud to the west simultaneously with the accumulation of probable shale (Smithwick?) to the east. The name Smithwick, as applied to a map unit, is restricted to a sequence of shale and conglomerate in the northeastern part of the area. The Strawn overlaps the Smithwick southwestward along regional strike and rests on an eroded surface of Marble Falls limestone. The Canyon Formation in turn overlaps the Strawn southwestward and appears to truncate a post-Marble Falls fault. Major faulting is interpreted as having occurred sometime between the deposition of the Marble Falls and Canyon Formations. The conglomerate at the top of the Smithwick probably represents landslides of structurally elevated detritus, and suggests movement of the adjacent Hall Uplift at that time. Eleven species belonging to seven genera of fusulinids are described and illustrated from collections systematically taken from Marble Falls rocks. In addition, five species are described from higher units; these, together with the Marble Falls species, represent the North American zones of Millerella, Profusulinella, Fusulinella, Fusulina, and Triticites. Time-rock correlations and depositional history are based on a combination of lithic and faunal evidence