Diagenetic Evolution of a Silurian Limestone Reef: Geochemical Documentation of Mixed-Water Dolomitization: ABSTRACT

Examination of a partially dolomitized Silurian platform reef (Pipe Creek Jr.) indicates that the timing of dolomitization is tightly constrained within the calcite diagenetic sequence. Cathodoluminescence and isotopic analysis of calcite diagenetic components define the diagenetic environment and fluid chemistries at the time of dolomitization. Abundant syntaxial and fibrous calcite marine cements suggest synsedimentary lithication of the reef complex. Multiple episodes of subaerial exposure are recorded both in the petrologic fabric as well as in the geochemical signatures of altered marine components and equant calcite cements. In the upper reef, marine components are corroded, and resulting porosity is partially infilled by red geopetal vadose silts. Resubmergence and continued marine cementation establishes a Silurian timing for this initial episode of exposure. Evidence for subsequent exposure is contained in the isotopic and cathodoluminescent patterns of later marine cements and overlying equant calcite spars. Marine cement isotopic compositions deviate from an initial marine composition (-4.0 ^pmil ^dgr18O; + 2.0 ^pmil ^dgr13C PDB) along covariant trends that reflect alteration by diagenetic pore waters. Equant calcite cements are restricted to the upper reef and partially occlude primary porosity. These cements exhibit invariant oxygen compositions between -7.0 to -8.0% and variable carbon which shows progressive depletion toward the upper reef. On this basis we interpret these cements as having formed within a shallow meteoric phreatic lens with light carbon derived from soil-gas CO2 at the exposure surf ce. Cathodoluminescent patterns and sulfide mineralization indicate progressive reduction of pore waters in the meteoric lens. The latest stage of phreatic cementation records anaerobic fermentation during which the oxygen composition remains unchanged with carbon varying up to + 7.0 ^pmil following precipitation of pyrite. Dolomitization occurs within this sequence of meteoric phreatic diagenesis. Early zoned meteoric spars, reflecting initial fluctuations in reducing conditions, are corrosively overlain by dolomite. The earliest formed dolomite is luminescent, signifying reduction of manganese, and is overlain by an outer ferroan dolomite. A progressive increase in reducing conditions is further indicated by the reduction of sulfate and concomitant precipitation of pyrite. The heavy carbon equant calcites, which postdate pyrite, suggest anaerobic fermentation during the final stages of phreatic cementation. Because dolomitization is bracketed by events of meteoric phreatic cementation and diagenesis, and its chemical evolution mimics the progressive reduction of the pore-water system, we suggest a mechanism of meteoric-marine water mixing to account for partial dolomitization of the Pipe Creek Jr. reef. This does not, however, imply a similar mechanism for the pervasive dolomitization of adjacent shelf reefs. Rather, this study emphasizes the complex early diagenetic history of shelf sequences with implications for late Silurian sea level fluctuations. End_of_Article - Last_Page 457------------