Abstract

Recognition of two types of enviornmentally significant discontinuities in the Helderberg Group has led to the conclusion that Late Silurian-Early Devonian paleogeography of the Appalachian Foreland Basin developed episodically, not gradually. All stratigraphic sections consist entirely of punctuated aggradational cycles (PACs) whose boundaries are synchronous discontinuities produced by rapid sea-level rises with a recurrence interval of thousands or tens of thousands of years. At each PAC boundary the existing paleoenvioronmental spectrum was modified significantly causing the superposition of environmentally disjunct facies when deposition resumed. Within individual PACs paleoenvironments evolved gradually in response to vertical aggradation throughout the full extent of the cycle. Therefore Walther’s Law can be applied only within individual PACs. In thick stratigraphic sections correlation of PACs reveals cryptic small scale unconformities with a recurrence interval of hundreds of thousands of years. Produced by major sea-level falls (and subsequent rises), these discontinuities are associated with significant reorganizations of paleogeography. Recognition of this kind of unconformity at formation boundaries previously considered to be the stratigraphic result of lateral migration of contiguous paleoenvironments significantly alters previous gradualistic interpretations of Helderbergian palegeography. The pervasive presence of these two stratigraphic discontinuities, PAC boundaries and cryptic unconformities indicates that Helderbergian palegeography was modified abruptly by allogenic events at two scales and at two frequencies.