Anatomy of a Single Punctuated Aggradational Cycle, Keyser Formation, Central Pennsylvania: ABSTRACT

Abstract

Detailed facies analysis of a single punctuated aggradational cycle (PAC) in the Silurian-Devonian Keyser Formation of central Pennsylvania over an outcrop distance of over 200 km has provided confirmation of certain aspects of the PAC hypothesis. The lower PAC boundary represents a paleoenvironmental discontinuity between shallow subtidal facies below and deeper subtidal facies above. The upper PAC boundary separates supratidal laminites below from intertidal and shallow subtidal facies. Uniformity of the basal facies suggests that the PAC was deposited on a nearly flat surface. The cryptalgal laminites at the top of the PAC at almost all localities suggest that paleotopography at the close of deposition of the PAC was also flat. Facies within the PAC generally reflect shallowing of paleoenvironments during deposition of the PAC. More detailed study reveals that the PAC is divided by a surface of abrupt facies change into two upward-shallowing units. Facies analysis and a simple computer model suggest that this correlative surface interval to the PAC is actually a new kind of PAC boundary - one which separates shallower facies above from deeper facies below, in contrast to normal PAC boundaries, which separate shallower facies below from deeper above. The computer program CYC, which integratesmore » three out-of-phase sea level curves based on the astronomical theory of climates, explains this peculiar surface with a combination of a major sea level drop followed by a minor sea level rise so that water depths are never as deep as before the initial drop. According to CYC, such situations are relatively infrequent and are always associated with conditions of low subsidence. If further testing confirms the existence of this kind of PAC boundary, then revisions to the PAC hypothesis may be necessary.« less