SILICATE DIAGENESIS IN MICROBIALLY ACTIVE SEDIMENTS OF THE ATLANTIC COASTAL PLAIN, SC

Microbial acetogenesis has been shown to be the most significant organic-acid producing mechanism during low temperature diagenesis of Atlantic Coastal Plain sediments. Microbial processes have an important role in mineral dissolution and providing material for mineral precipitation. Field and laboratory evidence show that sediments at Lake City, SC contain viable microorganisms actively producing organic-acids (formate and acetate). A description of the K₂O-SiO₂-AlO₂ diagenetic phases formed in Atlantic Coastal Plain sediments at Lake City, SC is provided.

To evaluate low temperature diagenetic products in microbially contaminated sediments at Lake City, SC, 40 m of core material was examined by light microscopy, scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction analysis (XRD) and electron microprobe (EM). Geochemical modeling of the present-day groundwater composition provided mineral saturation indices for select silicate phases. Air permeability and Hg-porosimetry measurements were obtained to quantify the heterogeneity in fluid transport mechanisms between sands and clays.

Lake City, SC sediments are deltaic sands, silts and clays, with minor carbonate and silica cemented zones. Quartz, k-feldspar, mica, and illite/smectite are the dominant detrital minerals. Scanning electron microscopy supports XRD findings of opal-CT, clinoptilolite, and authigenic k-feldspar. Authigenesis is most dominant in fine-grain silts where opal-CT and clinoptilolite occur as pore-filling cements and syntaxial to detrital species. In the silts, multiple episodes of authigenic k-feldspar and opal-CT observed in SEM. A complex precipitation history is to be expected where microbial processes are active in mineral dissolution and providing material for mineral precipitation.