Abstract: Middle Glen Rose (Cretaceous) Facies Mosaic, Blanco and Hays Counties, Texas

Arthur W. Cleaves

An 80-ft (24 m) interval from the middle part of the Glen Rose Limestone has been sampled at 35 surface localities in Central Texas for the purpose of reconstructing vertical and lateral changes of depositional environment on the northwestern part of the San Marcos platform. A marker unit, the Corbula interval, crops out in the center of the stratigraphic interval and serves as a datum for interpreting carbonate facies changes above and below the marker.

The middle Glen Rose was deposited as a mosaic of shoal-water lithotopes in a broad, shallow lagoon behind the Gulf Coast reef trend. Over much of the platform the sea was sufficiently shallow to allow the formation of local tidal-flat offlap sequences. Hence, intertidal and supratidal units make up a significant proportion of the individual local facies successions.

In Blanco and Hays Counties, there are two distinct patterns of vertical facies change. Closer to the Llano uplift (Blanco County) three to five offlap cycles are present within the 80-ft (24 m) interval. In all sections, the highest of the cycles contains the Corbula interval. These cycles involve a gradational trend from shallow-subtidal through supratidal facies. Each cycle commonly is bracketed by sharply defined bedding planes. The cycles represent local marine offlap sequences and result from the progradation of carbonate-mud flats into the shelf sea. Farther east and more distant from the Llano uplift (Hays County), the facies mosaic lacks the superposed succession of local regressional cycles. Subtidal mudstones and fossiliferous marls compose most of the section both above an below the Corbula interval.

The difference in the vertical facies pattern for the two areas may result from their position in relation to the ancient shoreline. Because the Llano uplift was emergent during the accumulation of middle Glen Rose lithotopes, the outcrops closer to the uplift could be expected to 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. Florida Bay is an excellent Holocene analog for these deposits. Farther southeast, the shelf sea was slightly deeper and lacked the nuclei necessary to initiate offlap sequences. One complete cycle, the Corbula interval cycle, crops out in both areas and may record a brief period of emergence for part of the San Marcos platform. Above th Corbula interval, no complete cycles are present for approximately 150 ft (46 m), with subtidal marlstone and mudstone being the dominant lithologies for both areas.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas