The Williamsport Sandstone: Precession and Ecentricity Forced Cyclicity in the Upper Silurian Strata of Central Pennsylvania, Western Maryland, and Northeastern West Virginia

Abstract

The Upper Silurian Williamsport Sandstone Member of the Bloomsburg Formation, upper Ludlovian to lower Pridolian in age, is the initial deposit of the Salina Group second order supersequence. The Salina Group supersequence includes the Wills Creek and Tonoloway Formations which together may encompass as much as 10 million years. The Williamsport Sandstone interval contains a unique combination of facies, including thick quartz sandstones, hematitic ironstones and fossiliferous limestones, not found together in the rest of the overlying Salina supersequence. The stratigraphy of the Williamsport Sandstone displays a hierarchic cyclic structure consistent with the Milankovitch model of orbital forcing. This distinctive carbonate and quartz sandstone unit is divisible into three 5th order (100 k.y.) short eccentricity cycles representing most of a 4th order (400 k.y.) long eccentricity cycle. Fifth order sequences contain two to five 6th order (20 k. y.) precessional cycles or PACs. Each 5th order sequence is highly asymmetric, with cycles containing deeper facies in the lower part of the sequence and cycles containing shallower facies toward the top. These 5th order sequences and most of their internal cyclic elements can be traced over 100 kilometers from Keyser, West Virginia to Mount Union, Pennsylvania. Their facies are largely marine to the south in West Virginia and Maryland and are predominantly non-marine to the north at Mt. Union, Pennsylvania. At Mt. Union there is a loss of PACs (6th order cycles) in the upper part of each 5th order sequence due to hiatus or vacuity in the onshore direction. Correlation of precessional and eccentricity cycles over significant distances constitutes evidence of a process-determined hierarchy of allocycles based upon the Milankovitch orbital forcing model.