Allocyclic Analysis Of The Upper Silurian Tonoloway-Keyser Formational Contact In Central Pennsylvania

Abstract

Allocyclic analysis of the stratigraphic interval encompassing the Tonoloway­-Keyser formation boundary in Central Pennsylvania reveals a complex stratigraphic relationship produced by the superposition of orbitally forced hierarchic eustatic sea-level fluctuations on a differentially subsiding basin. To the north (Tyrone, Pennsylvania) the formation boundary is an unconformable third-order sequence boundary at which open shelf Keyser carbonate facies lie directly on peritidal carbonates of the Tonoloway Formation. Below the unconformity differential subsidence and/or uplift resulted in various amounts of vacuity throughout the study area. To the south (Everett, Pennsylvania) additional Keyser cycles occur above the unconformity, indicating decreasing hiatus in regions of greater subsidence following exposure and erosion. Beneath the unconformity, peritidal Tonoloway facies to the north grade laterally into subtidal Keyser facies to the south within individual sixth- and fifth-order cycles as a result of higher subsidence rates toward the basin axis. A correlative hierarchic allocyclic fabric of fourth- and fifth-order sequences permits a chronological analysis of these lateral facies changes and of the unconformity. Immediately before accumulation of the study interval, peritidal (Tonoloway) facies existed throughout the study area. Above this datum, over a period of 400,000 years, differential subsidence generally produced Keyser facies to the south and east, a pattern interrupted every 100,000 years by fifth-order lowstands in sea level that produced Tonoloway facies. At localities closest to the basin margin Tonoloway facies persisted throughout the study interval. If the interpretation of the hierarchic cyclic fabric is correct, at least ten fifth-order sequences ( 1 million years) are lost by erosion at some localities beneath the unconformity (vacuity) and at least five fifth-order sequences (500,000 years) are missing by non-deposition above the unconformity (hiatus). Marked differences in vacuity at Tyrone and the localities immediately to the south and east suggest Upper Silurian movement along a structural lineament. The demonstration of a basinwide correlative allocyclic framework allows quantification of the roles of tectonics and eustasy in the accumulation of stratigraphic sequences.