clay dune (TXQcd;0)

State	Texas
Name	clay dune
Geologic age	Phanerozoic \| Cenozoic \| Quaternary \| Holocene
Original map label	Qcd
Comments	(from Moore and Wermund, 1993b): dune clay and silt--light-gray, brown, and brownish-gray calcareous silty clay to clayey silt and silty very fine quartz sand. Accumulations of fine to very fine quartz sand and sand-size aggregates of silt and clay deflated from saline flats. Called 'clay dunes' because they appear unusually clayey, but laboratory analyses show them to be sand-silt-clay by Shepard's (1954) classification. Mean grain size of 13 samples from the upper 2 m of a dune west of Baffin Bay was 41 percent sand, 37 percent silt, and 22 percent clay (Garcia and Stetling, 1980). Illite is the main clay mineral, smectite and kaolinite are minor. Buried soils common. Locally deposits contain ostracodes and laminae of clam shell fragments, chiefly Anomalocardia. Active dunes are elongate, crescent-shaped lunettes located on downwind margins of wind-tidal flats and playas which are source areas. Conditions that favor dune growth are arid to dry subhumid climate, persistent unidirectional winds, and expansive flats underlain by saline clay.
Primary rock type	sand
Secondary rock type	silt
Other rock types	clay or mud
Lithologic constituents	Major Unconsolidated > Coarse-detrital > Sand (Bed) Unconsolidated > Fine-detrital > Silt (Bed) Minor Unconsolidated > Fine-detrital > Clay (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	Moore, D.W. and Wermund, E.G., Jr., 1993b, Quaternary geologic map of the Monterrey 4 x 6 degree quadrangle, United States: U.S. Geological Survey Miscellaneous Investigations Series Map I-1420 (NG-14), scale 1:1,000,000. [http://pubs.er.usgs.gov/publication/i1420(NG14)] Garcia, M.N., and Stetling, C.E., 1967, Clay dunes--evaluation of source area and depositional environments, in Guidebook to geology of clay dunes, Baffin Bay and South Texas Sand Sheet: Texas Academy of Science, 83rd annual meeting, Corpus Christi, p. 5 Bureau of Economic Geology, 1976, McAllen-Brownsville Sheet, Geologic Atlas of Texas, Bureau of Economic Geology, University of Texas at Austin, scale 1:250,000.
Geographic coverage	Brooks - Cameron - Hidalgo - Kenedy - Kleberg - Willacy