Carbonate Petrology and Syndepositional Facies of the Lower San Andres Formation (Middle Permian), Lincoln County, New Mexico

Abstract

ABSTRACT The lower San Andres Formation (Middle Permian) in Lincoln County, New Mexico consists of dolomite, limestone, and sandstone. This report examines the petrology and syndepositional environments of the carbonate portion of the lower San Andres. The Glorieta Sandstone Member of the San Andres Formation is about 75 m thick at the northern end of the study area. It forms tongues southward as the lower San Andres passes into dominantly carbonate rocks. These lower, middle, and upper Glorieta Sandstone tongues alternate with lower, middle, and upper carbonate tongues, which are described below. Skeletal grains are generally abundant to rare and are divided into a normal, open marine assemblage (articulated brachiopods, crinoids, bryozoans, echinoids, and cephalopods) and a restricted marine assemblage (ostracods, gastropods, Foraminifera, pelecypods, and spirorbid worm tubes). Non-skeletal grains are generally rare to common and consist of peloids, intraclasts, ooliths, and terrigenous grains. Organo-sedimentary structures and emergent desiccation features are rare. Biogenic structures consist of Cruziana type burrows and mottling. Cementation, porosity occlusion and enhancement, dolomitization, iron minerals, dedolomitization, silicification, and stylolitization are the main carbonate diagenetic features present. The patterns of dolomitization are almost entirely consistent with an emergent tidal flat dolomitization model. Dolomitization and silicification appear to have been early diagenetic processes. No preserved evaporite minerals were noted. However, anhydrite nodule molds, evaporite crystal molds, and length-slow chalcedony suggest the former presence of evaporites in the carbonate tongues. Unambiguous evidence of an emergent sabkha or tidal flat origin for these former evaporites was observed only in some rock units. The carbonate tongues consist of the following major syndepositional facies: (1) non-evaporitic tidal flat and shallow lagoonal facies (consisting of supratidal, intertidal, and restricted marine facies), (2) evaporitic emergent tidal flat facies, (3) subaqueous? evaporitic facies, (4) evaporitic restricted marine facies overlain by non-evaporitic supratidal deposits, (5) evaporitic normal, open marine facies, (6) non-evaporitic normal, open marine facies, and (7) undaform-edge carbonate sand facies. These facies are defined on the basis of petrography, paleontology, and sedimentary structures. Subtidal syndepositional facies make up about 95% of the carbonate tongues, whereas supratidal and intertidal facies only about 5%. The carbonate syndepositional facies permit detailed correlat on throughout the study area. Nearly all of the carbonate rock units were deposited in muddy low-energy environments behind the oolitic sand bodies of a high-energy undaform-edge environment.