Abstract

The concept of microbial microstratigraphy allows us to unravel the complex history of layered microbial growth and sequential diagenetic events that fill cavities in reefs, build stromatolites, and create microbial bio-herms. Microbial layers sometimes defy gravity and the stratigraphic rule of horizontality, but the rules of stratigraphic superposition and cross cutting relationships can be applied to interpretation of thin sections and polished slabs. Diagenesis can be considered as one of the events in the stratigraphic micro-sequence. In the Permian Capitan Reef, sponges and bryozoans formed a framework, which was encrusted by microbial biofilms, infilled by peloidal micrite, cemented by aragonitic botryoids, radiaxial calcite, and generations of spar. Application of the microstratigraphy concept to lacustrine stromatolites from the Rita Blanca Formation of the Texas panhandle reveals three distinct generations of microbial growth. In the Laborcita Formation of central New Mexico, layers of microbial growth formed large (10-30 cm) oncoliths and eventually sedentary mini-bioherms (50 cm in diameter). In both the Holder and Laborcita formations, microbial growth formed complex layers, many of which defied gravity, and stromatactis fabrics (irregularly shaped voids and geopetal in-fillings) are present at multiple scales. Large (>20 km long), microbial buildups in the Smackover Formation of Alabama contain similar features including some sponges serving as framework, layers of growth, and stromatactis fabrics at multiple scales. In each of these settings, just as in the Capitan Formation, documenting the succession of microbial communities and diagenetic events promotes a greater understanding of depositional environment and porosity formation.