Punctuated Aggradational Cycles (Small-Scale, Upward Shallowing Units) Of The Thacher Member, Manlius Formation (Lower Devonian) In New York State

Abstract

The facies mosaic within the Thacher Member of the Manlius Forma­tion in central New York State can be subdivided into a sequence of ten upward-shallowing carbonate units. Each unit or Punctuated Aggradational Cycle (PAC), is bounded by non-depositional transgressive (deepening) surfaces. Between these surfaces, basal subtidal facies grade upward into shallow subtidal, intertidal and/or supratidal facies. These PACs exhibit facies which were deposited in several environments including an open shelf, a subtidal stromatoporoid patch reef, a restricted shelf, a bioclastic sand shoal and sand shoal fringe, and gradational shallow subtidal environments into intertidal and supratidal flat environments. Each PAC can be correlated between closely spaced outcrops by tracing key beds, facies lithosomes, cycles of similar thickness and internal facies arrangement. Longer distance correlation across the study area can be accomplished by matching shallowing sequences of PACs, matching major vertical facies changes across transgressive surfaces (indicating major deepening events) and by matching laterally persistent facies lithosomes. Analysis of the facies mosaic within a PAC provides a detailed interpretation of the paleogeographic changes accompanying its deposition. The Thacher Member consists of a sequence of ten paleo­environmental units consisting of facies which exhibit gradational transitions within and distinct facies changes at their boundaries.