Comparative sedimentology of shelf carbonates of the middle ordovician St. Paul group, central appalachians

Abstract

Observations of present-day environments are commonly used to aid in the interpretation of ancient sedimentary sequences. This approach is used to determine the paleoclimate and depositional processes operating during formation of the limestones and dolostones of the lower Middle Ordovician St. Paul Group (Row Park and New Market Formations) of Maryland and southern Pennsylvania, a 200 m thick carbonate platform deposit. Within the group, six facies defined by suites of sedimentary structures, faunal assemblages, and early diagenetic features are arranged in a large-scale regressive-transgressive sequence. These consist of: Facies I—lower burrowed, peloidal facies; Facies II—burrowed, skeletal facies; Facies III—lower laminated, peloidal facies; Facies IV—stromatolitic, peloidal facies; Facies V—upper laminated, peloidal facies; and Facies VI—upper burrowed, peloidal facies. A comparison of the St. Paul Group deposits with those of the modern peritidal carbonate environments of Shark Bay, Western Australia, the Abu Dhabi and Trucial Coast areas of the Persian Gulf, and Andros Island and the Great Bahama Bank reveals (1) an overall similarity in features between the St. Paul Group and the sediments of Andros Island and the Great Bahama Bank and (2) a dissimilarity between those two deposits and the sediments of Shark Bay, Western Australia and shoreline areas of the Persian Gulf. For example, the supratidal zones of the latter two typically contain evaporite minerals (e.g., gypsum and anhydrite) whereas the supratidal deposits of Andros Island and the St. Paul Group are freshwater algal marsh and lake sediments. The subtidal deposits of Shark Bay and the Persian Gulf generally are mechanically layered skeletal sand; the subtidal deposits of the Great Bahama Bank and the St. Paul Group are bioturbated peloidal micrites. The St. Paul Group and the Great Bahama Bank deposits are similar on a facies scale down to fine details observed microscopically. Bahamian modern analogs of the facies of the St. Paul Group are: for Facies I—shallow restricted bank and subtidal pond environments; for Facies II—a “normal” marine subtidal environment similar to the shoals which rim parts of the Great Bahama Bank; for Facies III and V—tidal channel, levee, pond, and marsh subenvironments of the channeled-belt environment (Hardie and Garrett, 1977); for Facies IV—levee, marsh, and freshwater-lake subenvironments of the interior of Andros Island; and for the Facies VI—a semirestricted subtidal-bank environment. Based on the features of the St. Paul Group, and by analogy with modern deposits, the St. Paul Group was deposited in a “low-energy”, rainy environment where deposition was controlled by storm processes. Many facies display penecontemporaneous marine cements and dissolution features.