IMPLICATIONS OF AN ALTERNATIVE INTERPRETATION OF THE LOWERMOST BRASSFIELD FORMATION, SOUTHWESTERN OHIO: A RECORD OF FALLING SEA LEVEL DURING THE EARLY SILURIAN

The Brassfield Formation of southwestern Ohio records carbonate deposition during the Early Silurian (Aeronian). Deposition followed the development of an unconformity which marks the Ordovician-Silurian boundary in the area. The Brassfield is particularly important because the upper part contains coral-stromatoporoid patch reefs that are among the oldest Silurian reefs in North America. In the study area at Oakes Quarry Park near Fairborn, Ohio, the Brassfield is approximately 8 m thick. There and elsewhere, the middle and upper parts of the Brassfield exhibit features consistent with deposition in relatively shallow water, above fair-weather wave base (e.g., patch reefs, vortex wave-ripple marks, cross stratification, and crinoidal grainstones and packstones). The lowermost unit, the Belfast Member, previously has been interpreted as the deposits of a shallow, possibly restricted, lagoon or tidal flat. These interpretations were based largely on reports of shallow-water features such as mudcracks, gypsum casts, algal laminites, and birdseye structures. However, the identifications of these sedimentary structures are either suspect or equivocal, and unpublished (reported in M.S. theses and Ph.D. dissertations). It is likely that all of these features have been misinterpreted, most being confused with various effects of bioturbation, weathering, and compaction. Furthermore, a study that used palynomorphs as proxies for Silurian benthic assemblages and water depth estimates also is suspect. Some newly recognized sedimentological features, together with previous information, permit a reinterpretation of the Belfast's depositional environment. The Belfast exhibits hummocks, 3D combined-flow ripple marks and associated swaley cross stratification, planar laminae, dominance of trace fossils characteristic of the Cruziana ichnofacies, glauconite, and strophomenid brachipods. These features are indicative of deposition in an open shelf, ramp-like setting below fair-weather wave base, but above storm wave base. Thus, the Belfast Member together with the remainder of the overlying Brassfield record a sea-level fall, consistent with independently constructed sea-level curves for the Aeronian.