ABSTRACT: The lowermost Silurian Brassfield Formation of the Cincinnati Arch area records mid-continent deposition on a carbonate ramp that sloped gently 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 Formation, the Belfast Member, is the focus of this study. The Belfast previously has been interpreted as representing relatively shallow water deposition associated with the initial early Silurian transgression. Newly recognized sedimentological features, however, together with previous information, indicate that the Belfast was deposited in water deeper than previously recognized. The purpose of this paper is to evaluate previous studies of the Belfast that have lead to interpretations of shallow water deposition, present new sedimentological information indicative of deeper water deposition, and discuss the implications of those results. Previous interpretations of the Belfast as having been deposited in relatively shallow water, intertidal to shallow subtidal, including lagoonal, 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 (possibly on the order of 10-60 m deep). Trace fossils, including Chondrites, Rosselia, Teichichnus, Thalassinoides, and Palaeophycus, indicate that during rapid sedimentation associated with storms, organisms adjusted vertically. Under fair-weather conditions following storms, settling of suspended sediment formed planar laminae and organisms recolonized upper portions of some beds, while totally bioturbating others. Hence, the Belfast represents relatively deep-water deposition on the Ordovician-Silurian unconformity (which developed primarily under subaerial conditions) associated with a relatively rapid sea-level rise. The transition from the Belfast Member to the overlying part of the Brassfield Formation appears to record a sea-level fall. As such, the Belfast is a low-order sequence. The sea-level changes recorded by the Belfast and overlying part of the 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 (vis-a-vis dynamic topography) associated with northward directed subduction along the southern margin of Laurentia, or movement on regional structures, reflecting the influences of events at the southern plate margin or induced by hydrostatic loading.


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