SEDIMENTOLOGY OF THE BELFAST MEMBER OF THE BRASSFIELD FORMATION (SILURIAN, WESTERN OHIO AND NORTHERN KENTUCKY, U.S.A.): IMPLICATIONS FOR REGIONAL SEA-LEVEL CHANGES AND TECTONICS

The lowermost Silurian Brassfield Formation, Cincinnati Arch area, records mid-continent deposition on a carbonate ramp that sloped into the Taconic foreland basin. The Brassfield is significant because it represents earliest Silurian deposition in the area resulting from sea-level rise following the late Ordovician glacial maximum, it records the faunal transition from the terminal Ordovician extinction, and its upper part contains tabulate coral-stromatoporoid reefs that are among the oldest Silurian reefs in North America. The lowermost unit of the Brassfield, the Belfast Member, is the focus of this study. Previous interpretations of the Belfast as having been deposited in relatively shallow water (intertidal to shallow subtidal, including lagoonal) associated with the initial early Silurian transgression are equivocal. The Belfast exhibits 3D ripple marks and associated cross laminae, hummocks, hummocky and swaley cross stratification, and planar stratification, with thin interbeds of planar laminated mudstone, all consistent with deposition in an inner shelf-type setting, below fair-weather wave base, but above storm wave base. Trace fossils, including Chondrites, Rosselia, Teichichnus, Thalassinoides, and Palaeophycus, indicate that during rapid sedimentation associated with storms, organisms adjusted vertically. Under subsequent fair-weather conditions, organisms recolonized upper portions of some beds, while totally bioturbating others. Hence, the Belfast represents relatively deep-water deposition on the Ordovician-Silurian unconformity associated with a relatively rapid sea-level rise. The transition from the Belfast to the overlying Brassfield appears to record a sea-level fall. As such, the Belfast is a low-order sequence not previously recognized. Sea-level changes recorded by the Belfast and overlying Brassfield are consistent with glacio-eustatic variations evident on published sea-level curves. An increase in subsidence also may have contributed to the rapid deepening recorded by the Belfast. Increased subsidence may have been associated with hydrostatic and/or sediment loading, Taconic orogenesis, dynamic subsidence associated with northward directed subduction along the southern margin of Laurentia, or movement on regional structures.