PETROGRAPHY AND DIAGENESIS OF AN OLIGOCENE CARBONATE SUCCESSION AT ST. STEPHENS QUARRY, ALABAMA

St. Stephens Quarry exposes a section of interbedded Eocene to Lower Oligocene bioclastic limestones, marls and siliciclastic units. This study is examining the petrofacies and diagenetic history of the bluff-forming, carbonate-rich portion of the Oligocene interval (Marianna Limestone to Glendon Limestone Member of the Byram Formation). The Marianna Limestone is predominantly composed of variably argillaceous, fine to medium grained, skeletal grainstone and packstone. The lower portion of the formation is rich in planktonic foraminifera and sporadic skeletal remains (e.g., bryozoa, echinoderms). This grades up to more coarsely grained skeletal grainstones containing abundant large Orbitoides foraminifera. The upper portion of the Marianna Limestone grades up into well sorted, coarse grained skeletal grainstones of the Glendon Limestone Member. Dominant fossils in this unit include foraminifera, bivalves, particularly Ostrea sp. and Chlamys sp., corals and echinoderms. The Lower Oligocene interval has been variably cemented by one or two phases of calcite cement. The first phase is non-ferroan, and equant in character. It formed isopachous rinds around allochems throughout the succession, and may be equivalent to a first phase drusy pore-filling cement within the Glendon Limestone. In the Glendon Limestone, a second more dominant phase consisting of scalenohedral to drusy, moderately ferroan calcite, pervasively filled much of the intergranular pore space. This phase of cement is the cliff-forming agent in this portion of the St. Stephens Quarry site. That it is chemically zoned, suggests that minor changes in pore water chemistry were occurring during diagenesis. The interval under investigation represents a cyclothem produced during a period of sea level change. This study hopes to link the diagenetic history of the limestones with the Oligocene sea level record.