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 Expressway (RME) and Tidwell Hollow (TWH) sections in Alabama (U.S.A) has a complex early diagenetic history. Early diagenetic history of the buildup facies consisted of two phases of marine cementation separated by two generations of meteoric, equant calcite with moldic dissolution separating equant calcite generations. A third generation of meteoric, equant (drusy) calcite is present after secondary marine diagenesis. Equant (drusy) calcite is interpreted as meteoric because its stable isotopic 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 from buildup facies is mostly non-luminescent and non-ferroan. This is unlike 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 from 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 values. These geochemical signatures indicate precipitation at a site that was distal from the recharge zone. Meteoric diagenesis at RME occurred during rising relative sea level and flow of meteoric fluids under a submerged platform is the only mechanism able to reconcile available stratigraphic, petrologic, and geochemical data. Ordovician meteoric deltaC-13 values (from TWH and other formations) are slightly depleted (2-4 parts per thousand) relative to coeval marine carbonate, especially when compared with post-Silurian meteoric carbonate. Minor depletion of Ordovician carbonate is the result of limited generation of light soil-gas CO2 before the development of vascular plants.