Punctuated Aggradational Cycles And Their Stratigraphic Implications In The Upper Silurian-Lower Devonian Carbonate Sequence (Tonoloway And Keyser Formations) In Central Pennsylvania

Abstract

An application of the Hypothesis of Punctuated Aggradational Cycles to the Upper Silurian-Lower Devonian carbonate sequence (Tonoloway and Keyser formations) at seven localities in central Pennsylvania reveals that the interval is entirely divisible into as many as 22 PACs which are correlative throughout the study area. Of these 22 PACs, 5 groups of PACs of similar facies are recognized in the interval. These groups of PACs are initiated and terminated by major deepening events which produce the primary surfaces of correlation in the interval. Paleoenvironments develop episodically throughout the interval, each punctuation event initiating and terminating a unique spectrum of small-scale environments. The Tonoloway portion of the interval represents a sequence of tidal-flat PACs. The transition to the shelf environments of the Lower Keyser Formation occurs episodically through 2 PACs consisting of nearshore, subtidal facies. The Upper Keyser is a shallowing sequence of PACs representing the episodic return from shelf to tidal-flat facies. Correlations reveal that as much as 13.5 meters of stratigraphic section, representing seven PACs, is missing at the Tonoloway-Keyser formation boundary at Tyrone, Pennsylvania. This missing section is interpreted to be the result of non-deposition because of differential uplift to the north of a proposed crustal-block boundary, represented by the Tyrone-Mt. Union lineament, while the basin to the south was differentially subsiding and PACs 1-7 were being deposited. Recognition of two paleoisotopographic surfaces (sea-level surfaces) at the base of PAC 1 and the top of PAC 22 facilitates the discernment of 17.5 meters of stratigraphic thickness difference attributable to differential subsidence between Tyrone and Cessna, Pennsylvania.