Depositional environments in the middle part of the Glen Rose Limestone (Lower Cretaceous), Blanco and Hays Counties, Texas




Cleaves, A. W. (Arthur Wordsworth)

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A 70-foot interval from the middle part of the Glen Rose Limestone (Lower Cretaceous) has been sampled at 35 localities in central Texas for the purpose of reconstructing vertical and lateral changes of depositional environment on the San Marcos Platform. A marker unit, the Corbula Interval, crops out in the center of the stratigraphic section. The middle Glen Rose was deposited as a mosaic of shoal-water lithotopes in a broad lagoon behind the Gulf Coast Reef Trend. Over part of the Platform the sea was sufficiently shallow to permit the development of local offlap sequences. As a result, intertidal and supratidal units comprise a significant proportion of many local facies successions. In Blanco and Hays counties there are two distinct patterns of vertical facies succession. Closer to the Llano Uplift (Blanco County) 3 to 5 offlap cycles are seen in the 70-foot interval. These involve a gradational trend from subtidal through supratidal facies. Each cycle is bracketed by sharply-defined bedding planes. The cycles are regressional and result from the progradation of carbonate mud flats into a shelf sea. Further to the east and more distant from the Llano Uplift (Hays County) the facies tract lacks the imbricated succession of regressive cycles. Subtidal units comprise the bulk of the section. The difference in the vertical facies pattern for the two areas may result from their relationship to the ancient shoreline. Because the Llano Uplift was emergent during deposition of the middle Glen Rose, the outcrops closest to the Uplift contain abundant evidence of tidal flat sedimentation. Mud mounds and small islands adjacent to land may have served as nuclei for the development of local offlap sequences. To the southeast (Hays County) the shelf sea may have been slightly deeper and probably lacked the nuclei necessary to initiate these sequences. One complete cycle, the Corbula Cycle, crops out in both areas and may record a brief period of emergence for most of the San Marcos Platform