Microbialite and microstratigraphy; the origin of encrustations in the middle and upper Capitan Formation, Guadalupe Mountains, Texas and New Mexico, USA

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ABSTRACT The Permian Capitan reef of the Guadalupe Mountains was bound, lithified, and preserved by microbially precipitated micrite. A microbial origin or association is based on the distribution, texture, fossil content, and order of microstratigraphic succession. In samples from the middle Capitan Formation, micrite layers (about 1 cm thick) encrusted the top, sides, and undersides of in situ organisms and also piled above organisms in thick (0.5 to > 3 cm) accumulations, with high angles of repose (up to 90°). Primary encrustations commonly consist of thin, homogeneous, subparallel layers of Archaeolithoporella--interpreted to be a red alga that probably lived in dimly lit cavities. Secondary encrustations (0.5-30 mm thick) contain 10-50 m diameter peloids and fossil cyanophytes, and are usually bound by thin layers of micrite. Secondary encrustations of gravity-defying peloidal micrites are interpreted as having precipitated within mucilaginous microbial aggregations, probably in dark cavities. The secondary microbial micrites were followed by cavity-filling, aragonitic botryoids. Thin (< 1 mm) layers of clotted micrite were deposited on the first botryoids, probably by microbial biofilms. In contrast to samples from the middle Capitan Formation, samples from the upper Capitan Formation taken near the transition from reef to outer shelf are composed of smaller (1-2 mm) framework organisms and are bound by layers of thinner (< 1 mm) primary encrustations. In the upper Capitan Formation samples, dense, clotted micrite forms secondary encrustations ( 1 mm thick), commonly underneath primary framework. Secondary encrustations are succeeded by more abundant tertiary encrustations (1-4 mm thick) of sparse, clotted micrite with vague dendritic structures. The secondary and tertiary encrustations are interpreted as microbial. The samples from the upper Capitan Formation were deposited in the shallowest part of the reef (< 1 to 10 m deep), and most of the samples from the middle Capitan were deposited in deep water (10-140 m). The difference in the encrustations and in the overall scale of the two FIG. 1. Map of the study area. Observations were made on samples from each of the canyons shown, but samples from McKittrick and Walnut canyons are the focus of this study. End_Page 956------------------------ fabrics is probably related to differences in environmental energy during deposition.