Punctuated Aggradational Cycles In The Thacher Member Of The Manlius Formation, Hudson Valley Region, New York

Abstract

In the Hudson Valley region the Thacher Member of the Manlius Formation consists entirely of PACs, thin shallowing­ upward cycles separated by correlative sharp non-depositional surfaces. Vertically, within each PAC, facies represent aggradational shallowing; laterally adjacent facies in each PAC represent contiguous paleoenvironments. Between PACs facies change abruptly at PAC boundaries in response to rapid episodic base-level rises. This small-scale stratigraphic framework permits detailed paleoenvironmental analysis of specific facies with respect to depth and lateral position relative to other facies. In this analysis, the Manlius Formation consists of 11 PACs each of which contains variable but coeval subtidal facies in its lower portion and intertidal or supratidal facies in the upper portion. In previous studies (e.g. Laporte, 1967), the assumption of a gradualistic model of stratigraphic accumulation resulted in the conclusion that the Manlius Formation comprised a disordered facies mosaic in which any facies could occur at any time. In contrast, this analysis, which assumes episodic stratigraphic accumulation at a small scale, has produced a highly ordered interpretation. Specifically, the PAC approach indicates that facies patterns were controlled by abrupt allogenic events (base-level rises) followed by sedimentary aggradation resulting in a series of correlative PACs containing predictable facies patterns.