METEORIC DIAGENESIS BELOW A SUBMERGED PLATFORM - IMPLICATIONS FOR DELTA-C-13 COMPOSITIONS PRIOR TO PRE-VASCULAR PLANT EVOLUTION, MIDDLE ORDOVICIAN, ALABAMA, USA

The Middle Ordovician ''Chickamauga'' Formation at Red Mountain Expres sway (RME) and Tidwell Hollow (TWH) sections in Alabama (U.S.A) has a complex early diagenetic history. Early diagenetic history of the buil dup facies consisted of two phases of marine cementation separated by two generations of meteoric, equant calcite with moldic dissolution se parating equant calcite generations. A third generation of meteoric, e quant (drusy) calcite is present after secondary marine diagenesis. Eq uant (drusy) calcite is interpreted as meteoric because its stable iso topic composition (deltaC-13 = -0.1 to -1.6 parts per thousand, deltaO -18 = -4.8 to -7.3 parts per thousand) is too variable to be accounted for by marine or mixing zone precipitation. Equant (drusy) calcite fr om buildup facies is mostly non-luminescent and non-ferroan. This is u nlike early, ferroan equant calcite (dull to bright luminescent) from below the unconformity that overlies the ''Chickamauga'' Formation at RME. Meteoric diagenesis below the unconformity is unrelated to early diagenesis in the underlying buildup facies. Equant (drusy) calcite fr om the buildup facies at RME has a covariation of deltaC-13 and deltaO -18 values that forms a truncated inverted ''J'' and has elevated Mg v alues. These geochemical signatures indicate precipitation at a site t hat was distal from the recharge zone. Meteoric diagenesis at RME occu rred during rising relative sea level and flow of meteoric fluids unde r a submerged platform is the only mechanism able to reconcile availab le stratigraphic, petrologic, and geochemical data. Ordovician meteori c deltaC-13 values (from TWH and other formations) are slightly deplet ed (2-4 parts per thousand) relative to coeval marine carbonate, espec ially when compared with post-Silurian meteoric carbonate. Minor deple tion of Ordovician carbonate is the result of limited generation of li ght soil-gas CO2 before the development of vascular plants.