Facies Development in a Lower Cretaceous Coragrudist Patch Reef (Mural Limestone, Southeast Arizona)

Abstract

Patch reefs within the Mural Limestone (Bisbee Group, Cretaceous, S. E. Arizona) are dominated by scleractinian corals, algal stromatolites, and rudist bivalves. Reef development occurred during the Early Albian transgression into the Chihuahua Trough, a north-west extension of the Tethyan seaway. One patch reef (270 m x 120 m) was studied in detail. Species abundance and growth form were estimated in each of 57 quadrats of l m2 along ten transects. Peels and slabs were used for detailed identification and for lithologic determination. Facies development is controlled by wave energy. The crescent-shaped front is dominated by branching and mas­ sive corals. Behind this fades a zone of laminar Microsolena (coral) along with algal stromatolites form the crest facies. Rudists dominate the back reef. The Petalodontia subfacies is closest to the reef crest and consists of thickets of gregarious, thick-walled Petalodontia, a few corals, and interbedded molluscan debris. The caprinid subfacies is farthest from the reef crest and contains corals, bivalves, and molluscan fragments. A debris facies of molluscan fragments and foraminifera surrounds the reef. Reefs of roughly correlative age in Texas are dominated by rudists, not corals. The Texas rudist frameworks are often associated with evidence of abnormal salinity conditions which were presumably intolerable for corals.