Facies and Paleogeography of Middle Silurian Bisher and Lilley Formations, Adams County, Ohio: ABSTRACT

Abstract

Six facies are recognized in the conformable Bisher and Lilley Formations of south-central Ohio. Areal distribution of microfacies indicates biohermal accumulation of the Lilley Formation, suggests new paleogeographic interpretation of the formations, and may explain morphologic differences among some Silurian reefs. The Bisher Formation is subdivided by bedding and silicification. Isopachous mapping indicates that the lower Bisher was deposited as a carbonate sand blanket which was covered by north-northeast-trending upper Bisher silty carbonate sediment ridges. Intervening troughs received finer grained sediments and were populated by a moderately diverse fauna. Highly diverse faunas of the Lilley Formation occur in bioclastic, biohermal, and biostromal facies which are subdivided by bedding, grain composition, and apparent transport of grains. Isopachous and lithofacies maps display a distinct relationship between Lilley and Bisher facies distributions. Bioclastic Lilley facies fill Bisher troughs, biohermal facies overlie Bisher ridges, and biostromal facies blanket the filled troughs and populated ridges. The Bisher and Lilley Formations represent part of a Silurian regressive sequence on the northeast margin of the Cincinnati arch. As easterly derived clastic input decreased, Lilley fauna populated Bisher ridges. Rate of drop in sea level nearly equaled rate of subsidence in the area. Circulation was curtailed by basin infilling and shallowing seas in late Lilley time. These new conditions favored the relatively restricted fauna and lithologies of the conformably superjacent Peebles formation, a massive dolomite of Middle Silurian age. End_of_Article - Last_Page 486------------