Middle Ordovician of Central Pennsylvania

Abstract

Middle Ordovician formations outcrop in several anticlinal areas in central Pennsylvania between Path Valley and the Allegheny Front. There are about 1,500 feet of limestone and shale representing the Chazyan and Mohawkian series. A number of conclusions have been reached, and many problems remain to be solved. The area lies in the eastern part of the Allegheny stratigraphic belt, extending to the Adirondack axis, which passes along Path Valley; rocks in the Cumberland Valley on the southeast were formed in the Champlain miogeosynclinal belt. Of the Chazyan formations, the stratigraphy of the Loysburg formation is insufficiently known; the Hatter formation thins southeastward by overlap. The lower Mohawkian, Black River, Benner, and Curtin limestones thin southeastward to extinction beyond the Adirondack axis, principally through offlap and truncation. The higher Mohawkian, Trenton, formations are the Nealmont, Salona, and Coburn limestones and the Antes black shale. The Nealmont, lower Trenton, lies with pronounced regional unconformity on older rocks and thins by overlap from a maximum in a northeast-trending belt within the area of outcrop. The Salona, middle Trenton, is rather constant in lithology and thickness in most of the area, converging rapidly as it approaches the Adirondack axis. The distribution of the upper Trenton Co-burn and Antes formations is insufficiently known. The stratigraphy of the economically important Valentine "chemical" limestone, the upper member of the Curtin formation, has an important bearing on the understanding of the behavior and distribution of the unit. Metabentonites, present in the Benner, Curtin, Nealmont, and Salona limestones, permit more precise correlation of sections and interpretation of relationships than are normally possible.