Aragonite depositional facies in a Late Ordovician calcite sea, Eastern Laurentia

The Black River (Upper Ordovician – Sandbian) and Trenton (Upper Ordovician – Katian) groups are traditionally interpreted as a deepening‐upward succession deposited in a progressively subsiding Appalachian Basin margin that contained warm‐water, marine, photozoan deposits that pass upward into cool‐water, marine, heterozoan carbonates. This succession is customarily interpreted to reflect an incursion of cold, high‐latitude ocean waters into the area. This view is herein confirmed for coeval carbonates in the northern part of the basin, particularly the St. Lawrence Platform. They are now well explained in this study on the basis of recent studies of cool‐water carbonates and calcite–aragonite seas. Overall the succession is one of Sandbian photozoan ramp deposits succeeded by Katian heterozoan ramp carbonates that changed back to photozoan ramp deposits prior to the Hirnantian glaciation. The current interpretation, that deposition took place throughout a calcite sea time, seems at odds with this series of strata. Instead it is herein proposed that deposition took place during an aragonite sea time wherein calcite sea‐like sediments accumulated under cold ocean‐water temperatures. Such an interpretation is supported by recent experimental data that supports the importance of seawater temperature on CaCO3 polymorph precipitation. If correct, this means that some of the evidence for calcite sea deposition through time brought about by global tectonics, should be re‐evaluated to make sure it was not simply cool‐water carbonate production.