Abstract: Silica Diagenesis of Mississippian Carbonates of Northern Alabama

NOLTE, R. A., and D. J. BENSON
Department of Geology, University of Alabama, Tuscaloosa, AL

Diagenetic silica is a common component of Mississippian carbonates from northern Alabama. Five megascopic silica types have been recognized from core and outcrop samples: disseminated, convoluted-nodular, rounded-nodular, anastomosing-bedded, and bedded.

There is a strong correlation between silica type and enclosing lithology. Disseminated and convoluted-nodular silica are preferentially concentrated within mudstones and wackestones, whereas rounded-nodular, anastomosing-bedded, and bedded silica occur chiefly within packstones and grainstones.

Petrographically, nine replacement and three cement textures have been observed in the silica. The replacement textures include: microcrystalline quartz, mosaic quartz, spherulitic chalcedony, euhedral quartz, fibrous quartz, lutecite, microflamboyant quartz, and megaquartz. The cement textures include: mosaic quartz, spherulitic chalcedony, and chalcedonic overlay.

Based on isotopic composition of the silica and paragenetic relationships, silicification is interpreted to have occurred in a mixing zone environment where active replacement of carbonate was frequently associated with penecontemporaneous dolomitization. As a result of eustatic sea level falls or aggradational carbonate cycles, the Mississippian carbonates were subjected to three main episodes of silicification. Each episode resulted from the exposure of portions of

the carbonate sequence which led to the formation of freshwater lenses and attendant mixing zones.

The silica was sourced through dissolution of siliceous sponge spicules which are present throughout the Mississippian carbonate interval. The principal controls on the distribution and shape of the megascopic silica types included: concentration of sponge spicules, bioturbation, grain size, orientation of the mixing zone, and permeability and porosity of the host lithology. The principal controls on microscopic texture were the number of nucleation sites and the concentration of dissolved silica.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas