Stratigraphy of Lower Cretaceous Trinity Deposits of Central Texas

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Date

1971

Authors

Stricklin, Jr., F.L.
Smith, C.I.
Lozo, F.E.

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Publisher

University of Texas at Austin. Bureau of Economic Geology

Abstract

The stratigraphic record of the Lower Cretaceous Trinity Division in Central Texas, as revealed by extensive outcrop investigation, is that of a shallow sea transgressing the southern flank of the ancient Llano Uplift. This history is demonstrated by the overlap of marine carbonates on terrigenous facies representative of nearshore or onshore deposition and by sedimentary features indicative of various shallow-water environments of the marine shelf. Internally, the Trinity is comprised of three elastic-carbonate couplets, separated by disconformities, that reflect a pattern of cyclic sedimentation superimposed on the overall transgressive regimen. These couplets, made up of terrigenous formations overlain by carbonate formations, are regarded as lithogenetic time-stratigraphic units and are designated lower, middle, and upper Trinity. Formations constituting these Trinity subdivisions are, in ascending order of deposition, Sycamore Sand and Sligo Limestone, Hammett Shale and Cow Creek Limestone, and Hensel Sand and Glen Rose Limestone. Trinity deposits are particularly illuminating from an environmental point of view because features of stratification and sedimentation are exposed in unusual detail. Included among the environmentally diverse strata are blanket-like beach deposits, rudist reefs, widespread tidal-flat deposits, shallow-water evaporites with an association of unusual diagenetic features, and shore deposits of caliche and alluvium. Of these strata, the beach and tidal-flat deposits are of prime importance because (1) they illustrate the diversity of environmental conditions that existed on the marine shelf, and (2) certain of their features allow interpretations of water depth, degree of water circulation, and morphology of the depositional environment. The formations, bedding sequences, and sedimentary features that are genetically associated with these deposits should aid in the recognition of similar deposits elsewhere. Among the numerous sedimentary features that have proved valuable in the recognition of Trinity depositional environments are sequences of cross-bedding, ripple marks, and organic features distributed along bedding surfaces. The latter include stromatolitic mounds and ridges of algal origin, clam borings, levels of bored pebbles, dinosaur tracks, mudcracks, and oyster shells cemented in growth position to bedding surfaces. If these features occur in a succession of marine beds, they indicate shallow-water, probably intertidal, deposition, and the surfaces bored by clams and incrusted by oyster shells are very likely related to brief subaerial exposure of marine substrates and consequent hardening of sediments. On the other hand, if such borings and incrustations occur on top of continental sediments, they are associated with a disconformity and mark a former land surface transgressed by the sea. The overall character of the Trinity and the nature of the land over which the transgression occurred indicate that a mild degree of land erosion favored the extensive deposition of low-to-high-energy bioclastic limestones along a shore of hummocky relief. This differs markedly from the setting of the modern low-lying Gulf Coastal Plain and its deposits. Except in some areas of very low land relief, such as along the Florida peninsula, the shelf today is being veneered predominantly by land-derived sediments.

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