190-3 Expression of the M1A-M1B Sequence Boundary in Non-Biohermal Facies of the North American Type Chazyan (Middle-Upper Ordovician; Northeastern New York, USA)

Session: Global Stratigraphy (Posters)

Presenting Author:

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Abstract:

The Middle-Upper Ordovician (Darriwilian-Sandbian) Chazy Group of northeastern New York, USA, consists of three formations. These are, in ascending order, the Day Point, Crown Point, and Valcour formations, with the Middle-Upper Ordovician boundary occurring cryptically within the Day Point Formation. This interval is renowned for containing one of the oldest examples of metazoan-based bioherms associated with diverse metazoan communities. Consequently, previous stratigraphic and paleoecological studies have focused primarily on biohermal successions, with little attention paid to inter-bioherm strata, leaving such intervals poorly integrated into sequence stratigraphic frameworks. The present study describes a stratigraphic interval exposed in the Upstone Materials quarry in northern Plattsburgh, Clinton County, northeastern New York, that represents a topographic depression between bioherm-bearing highs. Here, the Day Point Formation is buried but the lower portion of the Crown Point Formation is exposed, revealing a highly fossiliferous, open-marine fauna preserved within biomicrites displaying rhythmicity interpreted as potential parasequences. Previous regional sequence stratigraphic analyses have placed the position of the North American Mohawkian M1A-M1B third-order sequence boundary within the lower Crown Point Formation, but given the historical attention to biohermal successions, the presence of originating horizons for such buildups has been used as a primary indicator of stratigraphic position. The current study records an angular unconformity within the Crown Point Formation at the Plattsburgh Upstone quarry site that marks the M1A-M1B sequence boundary. This suggests a form of minor (regional?)  tectonic event during the latest M1A sequence to generate this stratigraphic discordance. The fossiliferous carbonates overlying this angular unconformity represent the transgressive systems tract of the M1B sequence, which extends through the remainder of the Crown Point Formation.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11196

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