Sedimentology and Paleoenvironments of the Oriskany group of Eastern Pennsylvania

Abstract

The Lower Devonian (Deerparkian) Oriskany Group in eastern Pennsylvania has been shown to have been deposited under conditions of marine regression (Epstein etal, 1967). This study integrates field evidence and cumulative size ­frequency dat a in an investigation of the stratigraphic expression of marine regression. Lower Devonian paleogeo­graphy is interpreted, and the central Appalachian Basin shore line is adjusted from recently published maps (Boucot, 1968; Head, 1969) to include a land ·mass near Andreas, Pa. The Oriskany Group in eastern Pennsylvania is divided into three lithologic units based on bedding sequence, sed­imentary textures, and sedimentary structures. Lithologic unit one is a sequence of poorly bedded silty shales and siliceous argillaceous limestones which thickens to the northeast. The unit is interpreted as occupying an environmental position offshore and below wave base. Lithologic unit two is a sequence of thinly bedded coarse sandstones disconformably overlying, and interbedded with, rocks identical to those in unit one. Unit two represents depo­sition of sand by ripple trains or sand waves during storms in near (but below) -wave base environments of unit one. Lithologic unit three is a sequence of thick bedded sandstones which were deposited in a nearshore bar and beach environment. Analysis of cumulative size-frequency data supports the contention of Spencer (1963), that all sediments are mixtures of two or three log-normally distributed populations of sediment, and that sorting can only be defined by the degree of truncation of the size-frequency curve. Although the Oriskany is usually called a well sorted sand, only sand beds from Kunkletown (interpreted as the most nearshore of the three sections studied) show any evidence of sorting based on the above definition. Sand beds become coarser in a nearshore direction, but wide vertical fluctuations in coarseness measures· are present in any one section. The sequence of lithologies, textures, and structures of the Oriskany Group of eastern Pennsylvania are suggested as a model of the condition of marine regression.