Analysis of Small-Scale Cyclicity in the Williamsport Sandstone Member and the Lower Wills Creek Formation in Pennsylvania and Maryland

Abstract

Allocyclic analysis of a 25-meter interval of Upper Silurian peritidal siliciclastic and carbonate facies of Maryland and central Pennsylvania reveals a four-tiered hierarchic cyclic fabric correlative over 125 kilometers. The structure of this correlative framework in the Williamsport Member of the Bloomsburg Formation and the Lower Wills Creek Formation is consistent with predictions of the orbital forcing model, suggesting that precession was the fundamental cycle-producing mechanism and that eccentricity modulated precession at three scales. Fundamental meter-scale cyclicity of the study interval was produced by rapid sea-level changes caused by the 20 thousand­-year precessional cycle of the Earth's orbit. These sixth-order cycles are distinguished by disjunct facies changes at non-depositional surfaces produced by rapid sea-level rises; cycles typically contain sandstone and/or carbonate at the base, and shale at the top. Sixth-order cycles are bundled by the short-eccentricity (100 thousand-year) cycle to produce asymmetric fifth-order sequences, averaging 3 meters in thickness. Eccentricity modulation also produced fourth-order ( 400 thousand-year) and third-order (2 million-­year) sequences consisting of bundles of fifth-order and fourth-order sequences, respectively. The study interval consists of two, 400 thousand-year, fourth-order sequences deposited during a period of relatively constant subsidence in shallow marine and non­marine environments. Lateral facies change within correlative cycles indicates that the southern localities were consistently more marine than the central Pennsylvania locality during the 800 thousand-years of depositional history. While subsidence and paleogeographic position contributed to the thickness of the interval and the nature of the facies, orbitally forced eustatic sea-level fluctuations were responsible for the hierarchic allocyclic framework.