Punctuated aggradational cycles: Implications for stratigraphic analysis

Abstract

Application of the hypothesis of punctuated aggradational cycles (PACs), a model of episodic stratigraphic accumulation, has yielded results fundamentally in conflict with those predicted and generated by gradualistic models. The assumption that stratigraphic accumulation is controlled principally by rapid small‐scale base level fluctuations in the Milankovitch band leads to the adoption of the PAC, a thin time‐stratigraphic unit, as the fundamental unit of stratigraphic description and interpretation; the thicker generally diachronous formation is inadequate for stratigraphic analysis because it is an artifact of stratigraphic gradualism. Correlation of stratigraphic sections is accomplished by matching patterns of facies change, particularly the abrupt changes at the synchronous surfaces that mark PAC boundaries. This kind of physical event correlation is far more accurate than traditional biostratigraphic methods. Focus on synchronous PAC boundaries separating environmentally disjunct facies reveals discontinuities that preclude application of Walther's law across those surfaces. When viewed from this episodic perspective, the Manlius Formation of the Helderberg Group in New York state becomes a highly ordered set of time‐stratigraphic cycles rather than a disordered mosaic of facies representing short‐term autogenic processes in a single nearshore environment. Not only was Manlius deposition episodic with no gradual upward deepening as is predicted by stratigraphic gradualism, but also the Manlius‐Coeymans formational boundary is interpreted as a small‐scale unconformity rather than a paleoenvironmental boundary created by gradual transgression. Each of these interpretations is the direct result of assuming and applying a model of episodic stratigraphic accumulation.