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Biofacies relations in the Cenomanian Buda Limestone have been studied by reference to fossil and rock collections made at more than 40 surface sections and shallow cores in a 55,000 sq mi area of west and Trans-Pecos Texas.
Three east-west trending biofacies bands are distinguished. These facies approximately parallel the Cretaceous shoreline on the north and the axis of the Mexican geosyncline on the south-southwest. The northernmost biofacies is dominated generally by miliolids, Cuneolina, and calcareous algae, and locally by rudists, corals, and stromatoporoids. The southernmost biofacies is characterized by globigerinids, Hoglundina, calcispheres, and siliceous sponges; calcareous algae, rudists, and serpulid worms are abundant locally. The northern and southern biofacies are separated by a transitional biofacies dominated by mollusks and echinoids with microfaunal elements of the adjacent biofacies locally common.
The northern biofacies is coincident with a carbonate grainstone and packstone lithofacies and records very shallow-water (0-30 ft) nearshore environments. Stromatoporoid, coral, and rudist patch reefs are distributed sparsely through the area. The siliceous sponge-globigerinid biofacies is coextensive with a carbonate mudstone and wackestone lithofacies and records a relatively quiet, deep-water (100-300 ft) environment. Algal, rudist, and serpulid assemblages in this biofacies are associated with structurally high areas and mark offshore islands and reefs present during Buda deposition. The transitional biofacies and its coincident wackestone and packstone lithofacies reflect shallow-water (20-100 ft) open-shelf environments.
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