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Austin Chalk

Austin Chalk
State Texas
Name Austin Chalk
Geologic age Phanerozoic | Mesozoic | Cretaceous-Late [Gulfian]
Comments In Eastern part of Trans-Pecos Texas and High Plains- Chalk and marl; chalk mostly microgranular calcite with minor foraminifera tests and Inoceramun prisms, averages ab out 85 percent calcium carbonate, ledge forming, grayish white, white; alternates with marl, bentonitic seams locally recessive; medium gray; pyrite nodules common, weather to limonite; thickness 325-420 ft. In East Texas and Gulf Coast to Rio Grande and In north, central, and south Texas including Quaternary for all of west Texas- Chalk, massive, some interbeds and partings of calcar. clay, lt. gray; middle part mostly thin-bedded marl with interbeds of massive chalk, hard lime mudstone to soft chalk; lt. gray; weathers white; marine megafossils scarce; chalk is mostly microgranular calcite with minor foraminifer tests and Inoceramus prisms, local thin bentonitic beds in lower part, thickness 600 +- ft. thickness 1,200 ft in Crystal City-Eagle Pass Sheet (1976). Ouachita tectonic belt province, Fort Worth syncline. Unit is below Taylor Marl, Ozan Formation, Upson clay, Anacacho Limestone, or Pecan Gap Chalk and is above Eagle Ford Formation or Boquillas Flags.
Primary rock type limestone
Secondary rock type mudstone
Other rock types clay or mud; bentonite; mudstone
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone > Chalk (Bed)Chalk.
Sedimentary > Carbonate > Marlstone (Bed)chalk predominate
Minor
Sedimentary > Clastic > Mudstone > Claystone (Bed)
Map references
Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000
Unit references
Bureau of Economic Geology, 1976, Crystal City-Eagle Pass Sheet, Geologic Atlas of Texas, University of Texas, Bureau of Economic Geology, scale 1:250,000.
Shumard, B.F., 1860, Observations upon the Cretaceous strata of Texas: St. Louis Academy of Science Transactions, v. 1, p. 583-590.
Bureau of Economic Geology, 1974, San Antonio Sheet, Geologic Atlas of Texas, University of Texas, Bureau of Economic Geology, scale 1:250,000.
Bureau of Economic Geology, 1974, Austin Sheet, Geologic Atlas of Texas, University of Texas, Bureau of Economic Geology, scale 1:250,000.
Bureau of Economic Geology, 1977, Del Rio Sheet, Geologic Atlas of Texas, University of Texas, Bureau of Economic Geology, scale 1:250,000.
Counties Bell - Bexar - Comal - Dallas - Edwards - Ellis - Falls - Grayson - Hays - Hill - Kinney - McLennan - Maverick - Medina - Terrell - Travis - Uvalde - Val Verde - Williamson