MICROBIALITES AND CEMENTSTONES IN DOWNSLOPE MOUNDS OF THE HOLDER FORMATION (UPPER PENNSYLVANIAN, VIRGILIAN), SACRAMENTO MOUNTAINS, NEW MEXICO, AND THEIR SIGNIFICANCE TO LATE PALEOZOIC REEF EVOLUTION

While the classic Upper Pennsylvanian (Virgilian) shelf-margin phylloid algal mound complexes exposed in Dry and Yucca Canyons of the Sacramento Mountains, New Mexico, were being formed, there was also a series of smaller mounds growing in adjacent, slightly deeper-water, upper slope settings. Those more downslope mounds are also predominantly composed of phylloid algal bafflestones, but they also contain common thrombolitic microbialites intermixed with botryoidal radial fibrous cementstone, and have locally common calcisponges. The association and apparent intergradation of the thrombolitic microbialites and botryoidal radial fibrous cementstone suggests that the cements were also microbial in origin, and that they did not just fill cavities, but also grew on the open seafloor. The microbialite/cementstone facies is dominant in the lower mound cores and represents initial stabilization and mound growth on the upper slope seafloor during sealevel transgression. Phylloid algae colonized that early mound surface and became continuously more common upward, dominating the upper mound core facies during sealevel highstand. A similar mound facies growth pattern also occurs in uppermost Pennsylvanian mounds of the Laborcita Formation in the Sacramento Mountains.

During the subsequent Lower Permian (Wolfcampian), shelf-margin phylloid algal-dominated mound communities combined and integrated with communities from smaller calcisponge-bryozoan boundstone mounds of adjacent upper slope facies. That newly integrated pioneer Permian reef community continued to expand and diversify, and reached an acme of development in North America with the growth of the Middle Permian Capitan reef of the Permian Basin region. Many Permian reefs are rich in radial fibrous cement and microbialites, and some workers have concluded that both were mostly post-depositional in origin. The Virgilian downslope mounds of the Sacramento Mountains exposures suggest that the microbialites and radial fibrous cements were intimately associated, and that they played a critical role in the stabilization of substrates, initiation of mound growth and topography, and the evolution of Late Paleozoic organic carbonate buildups, particularly in upper slope depositional settings.