OCCULT ARAGONITIC FABRICS AND STRUCTURES WITHIN MICROBIOLITES, PENNSYLVANIAN PANTHER SEEP FORMATION, SAN ANDRES MOUNTAINS, NEW-MEXICO, USA

Black microbiolites (stromatolites and oncoids) are common constituent s within the 800m thick Pennsylvanian Panther Seep Formation. This for mation, which is composed of cyclically repeated sandstones, carbonate s, and shales, crops out in south-central New Mexico, U.S.A. The depos its range from those that accumulated within semi-arid to arid tidal f lats, cut by small fluvial channels, to evaporitive lagoonal and open shallow marine depositional environments. The microbiolites accumulate d predominantly as tidal flat and very shallow subtidal deposits. The black color of these microbiolites is prominent on the outcrop. Unfort unately, on the microscopic scale, the organic matter as well as diage netic alteration obscures many of the internal structures and fabrics of these organosedimentary features. However, the use of the very simp le ''white card'' technique clearly brings out the occult structures a nd fabrics and thus permits detailed description of the original fabri c of the individual crystals as well as their aggregate morphology. Th e relict fabric indicates that the microbiolites were originally compo sed of aragonitic crystals. This interpretation is supported by the hi gh relict Sr content of the present diagenetic low-magnesian calcitic material. Thus, initially, these microbiolites were essentially totall y composed of aragonitic crystals which precipitated perpendicularly t o the mucilaginous substrate within the microbiolites.