AN EARLY SILURIAN PATCH REEF IN THE ROSE HILL FORMATION OF CENTRAL PENNSYLVANIA

Three limestone caves are exposed in the Rose Hill Fm. along the north limb of the Nippenose Anticline in Lycoming Co., PA. The caves are located at different elevations in relation to each other and allowed access to a significant cross section of the Rose Hill Fm. due to the regional dip of ~30° N. In this location, the formation is mapped as shales, siltstones, and interbedded sandstones with thinly-bedded fossiliferous limestone appearing only in the upper Rose Hill. Neither karst nor limestone outcrops are common to the Rose Hill in this region. To determine the depositional environment that formed this rare limestone lens, we conducted a detailed thin-section analysis of samples from the walls and roofs of the caves. Our analyses indicate that this limestone lens represents a diverse patch reef community. Areas of the rock are extensively fossiliferous with bryozoans, calcareous algae, trilobites, and other benthic organisms dominating. Thin sections were primarily bioclastic packstones, pelloidal wackestones, and microbial mudstones - indicating that deposition in many locations was microbially mediated. Other researchers have identified similar Silurian patch reefs in the upper Midwest, U.S.A. and eastern Canada. The reefs occur in swarms and are interpreted as a response to the generally unstable climate and tectonics of the early Silurian. The finite extent of this carbonaceous lens and the great variety in reef biodiversity may also indicate localized environmental responses to the changing conditions of the early Silurian.