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Stratigraphy of Trinity Rocks (Comanchean) North of Colorado River

Michael E. Ford

The Trinity rocks of north-central Texas are perhaps the best exposed Cretaceous platform deposits in the world. As such, they provide a superior laboratory for the study of Cretaceous shallow-water deposition, for they are exposed now over almost the entire area of their original deposition.

Initial deposition of Trinity rocks on the Central Texas platform was controlled by the topography of the pre-Cretaceous Wichita paleoplain surface. The development of this surface began in the late Paleozoic-early Mesozoic, as subsidence along the Ouachita structural belt caused a reversal of drainage to the southeast and the opening of the Ancestral Gulf of Mexico.

Early Cretaceous clastics were carried across the paleoplain by a highly developed fluvial system and were deposited to form a broad, low-lying alluvial plain lying between the Llano region of central Texas on the south to southern Oklahoma on the north and extending westward an unknown distance. These initial deposits are the basal clastic units of the Trinity Group.

Upon these were deposited the later rocks of the Trinity Group, composed primarily of clastic and carbonate sediments, deposited during two major transgressive periods. During periods of subsidence of the East Texas basin, there occurred deposition of muddy limestones, marls, and shales. Periods of stability resulted in the deposition of relatively clean limestones in the well-circulated water, grading shoreward into marginal marine clastics.

Considered on the grand scale of the platform, the Trinity deposits furnish an ideal model for exploration in tidal and platform deposits.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.