Print this pageMicrobially Precipitated Micrite in the Capitan Formation, Guadalupe Mountains, Texas and New Mexico
KIRKLAND, BRENDA L.,RACHEL A. WOOD, and TONY DICKSON
Within the Massive Member of the Permian Capitan Formation, many accumulations of micrite defied gravity at the time of deposition. Vaguely laminated micrite encrusts the steep sides of in situ organisms and micrite caps on organisms have steep angles from horizontal (up to 90 degrees)- in excess of the angle of repose for cohesion less solids. Thin sections reveal that these micritic accumulations contain peloids (10-50 micrometers in diameter), rare Girvanella, Ephipyton- and Renalcis-like structures, and may be bounded by thin layers of micrite. Study with SEM reveals that small spheres (average 0.1 microns) are abundant in micritic accumulations, but rare in radiaxial calcite cements or in late spar. These gravity-defying peloidal micrites are interpreted as having precipitated within microbial biofilms.
Micritic features previously termed Archaeolithoporella are divided into at least two groups. Multiple parallel lines of homogeneous micrite commonly form the initial encrustation on in-situ organisms and may be fossil red algae. Clotted layers of micrite forming on the upper surfaces of large (>0.5 cm) botryoids may be of microbial origin and are best excluded from Archaeolithoporella.
Microbially precipitated micrite records a complex series of microbial communities and defies classification by simple terminology. Microbial micrite bound, lithified, and preserved the Capitan reef. It may prove to be key to lithification and preservation of primary porosity in other carbonate buildups.