PARTING THE HELDERBERG SEA: CRYPTIC UNCONFORMITIES AND THE SILURIAN - DEVONIAN BOUNDARY IN THE CLASSIC EPEIRIC SEA SEQUENCE OF NEW YORK

Formations of the lower Helderberg Group have served as the "textbook" example of depositional environments in epeiric seas since Laporte (1969). This spectrum of environments was built on the correlations of Rickard (1962), who inferred gradational facies relationships between formations. This reconstruction played a key role in placement of the Silurian-Devonian boundary and assignment of the Manlius and portions of the Rondout Formation to the Devonian System.

Recent field work has documented multiple, cryptic unconformities within the lower Helderberg sequence. The Terrace Mountain Unconformity (TMU) marks the top of the Thacher Member of the Manlius Formation (see also Anderson and Goodwin 1988). We recognize a similar discontinuity at the base of the Kalkberg Formation (Punch Kill Unconformity). The most significant unconformity is the Howe Cave (HCU), which is regionally angular. In west-central NY, HCU progressively bevels older members of the upper Manlius sequence. From Cherry Valley to Schoharie, HCU occurs within the Coeymans Formation, between the Ravena Member and a previously unrecognized extension of the Dayville Member. East of Schoharie, HCU merges with TMU. HCU is marked locally by solutional pitting, small neptunian dikes, lithoclasts, truncation features and borings.

Biostratigraphic evidence corroborates the importance of HCU. Units below HCU have yielded the Silurian (Prídolí) conodonts: Ozarkodina confluens and O. remscheidensis eosteinhornensis. The consistent appearance of I. woschmidti above HCU, at the base of the Coeymans (Ravena and Deansboro members), marks the base of the Lochkovian Stage. Therefore, the Silurian - Devonian boundary occurs within the erosional vacuity of HCU.

The occurrence of multiple, cryptic unconformities in the lower Helderberg sequence invalidates interpretations of gradational facies relationships. Further, because Helderberg facies were not coeval, Laporte's laterally contiguous, onshore-offshore spectrum of environments never existed.