INFLUENCE OF STORM WAVE BASE FLUCTUATIONS ON CARBONATE SHELF FACIES IN THE ORDOVICIAN POINT PLEASANT FORMATION (CENTRAL OHIO)

The Ordovician Point Pleasant Formation is interpreted as a deepening-upward carbonate shelf sequence in central Ohio, based on analysis of the Chevron 1A Prudential core (#3410120196) at the Ohio Geological Survey H.R. Collins Core Laboratory. Lithofacies of the Point Pleasant Formation are a mixed siliclastic and carbonate depositional system. The base of the Point Pleasant Formation (above the Trenton Limestone) is dominated by carbonate tempestites, which are interpreted as evidence for shallow-water shelf conditions (above storm wave base). The carbonate tempestites are composed of undulated skeletal grainstones (Cgu) that are 2 cm thick, light grey, undulated, and are composed primarily of brachiopod shells. Above the skeletal grainstones are carbonaceous calcareous wackstones (Cw) that are 2-4 cm thick, organic rich, dark brown/black, and contain sparse amounts of fossil fauna. The Point Pleasant Formation then transitions upward to mostly mixed siliclastic and carbonate pelagic rhythmites, which is interpreted as evidence for deeper water conditions (below storm wave base). The rhythmites are composed of siltstone (SS) that is light grey, fine grained, and thinly bedded (<1 cm). Above the siltstone is carbonaceous, calcareous mudstone (Cm) that is dark brown/black thinly bedded (1-2 cm), highly organic and contains little or no fossil fauna. Within the pelagic rhythmites there are also intervals of bioturbated siltstone (SSb) which are light grey, thinly bedded (<1 cm) and discontinuous. The pelagic deposits are interrupted by intervals dominated by carbonate tempestites, mostly skeletal packstones (Cpu), that are light brown/grey, undulated, 5-10 cm thick and are composed primarily of fragmented brachiopod shells and calcareous mud which then transitions back to pelagic rhythmites. This could represent changing water depth conditions on the shelf or else the recurrence of larger storms (deeper storm wave base) at certain times. At the top of the Point Pleasant Formation, an erosional surface overlain by skeletal packstones (shell hash intermixed with coarse-grained sand particles that is highly cememted) is interpreted as evidence for shoreface deposits. This is interpreted as evidence for water shallowing, possibly a sequence boundary, prior to deposition of the overlying Utica Shale.