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The growth history of a phylloid algal biohermal complex of Late Carboniferous (Westphalian) age, outcropping within the La Pasada Formation in northeastern New Mexico, U.S.A., is described in relationship to paleogeography, biostratigraphy, and microfacies associations. This phylloid algal biohermal complex occurs within typical Late Paleozoic cyclical sediments, in a paleogeographic setting along a narrow shelf margin bordering a deep geosynclinal trough to the northwest. Fusulinid foraminifers suggest that the bioherm is of Middle Pennsylvanian (lower Des Moines) age, whereas the overlying sediments are of middle Des Moines age. Both field and petrographic evidence suggest that the bioherm grew upward from relatively shallow water depths, into extremely shallow water where it probably was subaerially exposed. Biohermal growth appears to have been initiated on a ‘hard-ground’ surface on which lithified clasts and abundant clusters of robust linoproductid brachiopods served as a foundation. Four major microfacies have been identified from the biohermal complex. These are: (I) phylloid algal wackestones-packstones (mound proper), (2) marly limestones (sediments laterally adjacent to and overlying mound), (3) pelletal-foraminiferal wackestone (partially capping the mound), and (4) crinoidal packstones-grainstones (post-mound). Comparison is made with other surface and subsurface occurrences of phylloid algal complexes of similar age in the southwestern United States.