Paper No. 87-16
Presentation Time: 9:00 AM-5:30 PM
HIGH FREQUENCY SEQUENCES WITHIN THE UPPER MISSISSIPPIAN ALLENSVILLE MEMBER, LOGAN FORMATION, SOUTH-CENTRAL OHIO
The Allensville Member of the Logan Formation (Late Tournaisian; 341-359 Ma) in south-central Ohio is an anomalously coarse-grained unit deposited in a shallow marine setting when tectonic subsidence rates were low and after the onset of Late Paleozoic glacial eustasy. The Allensville Member consists of very fine- to very coarse-grained arenites 0.05-2.5 m thick interbedded with mudstone (0.04-0.20 m thick). Less common are moderately well sorted, well rounded, granule to pebble conglomerates (0.14-0.50 m), as well as pebbly sandstones (0.03-0.80 m thick) composed of quartz clasts. Average clast size is 0.27 cm with a maximum clast size of 3.2 cm. Vertical grain size trends in the measured sections show similar trends of coarsening and fining upwards that can be correlated along strike for 30 km. Fossils within the member consist of disarticulated crinoid ossicles and rare brachiopods. Vertical burrows up to 3 cm wide and filled with coarse-grained sandstone and granules are present at the same stratigraphic position within each measured section. The texture of the conglomerates, pebbly sandstones, and coarse-grained sandstones suggests deposition as transgressive lags. The horizons with vertical burrows are interpreted to be the Glossifungites ichnofacies indicating a firmground that was infilled by the coarse lag deposits. Vertical changes in grain size and the location of fossils were used to subdivide the measured sections into at least four sequences that vary in thickness from 2-3 m thick with boundaries placed at the top of the Glossifungites horizons. The grain size trends combined with the fossil and ichnofacies data are interpreted to represent 4th order sequences within a 3rd order transgressive systems tract represented by the Logan Formation.