A review of Middle Ordovician sedimentation in the St. Lawrence Lowland, eastern Canada

Abstract

AbstractMiddle Ordovician sediments of the St. Lawrence Lowland, eastern Canada, and its northeastward extension to St‐Siméon, are subdivided into the numerous formations of the Chazy, Black River and Trenton Groups. Details of each formation and interpretation of environments of deposition are presented and a coherent model for the development of the upper Middle Ordovician Trenton Group throughout the region is presented. In the southwest, around Montreal, a complete and continuous Middle Ordovician sequence is present and Trenton Group sediments overlie well‐developed tidal flat and lagoonal (Black River Group) and mixed shallow subtidal (Chazy Group) sediments. This sequence was deposited on a slowly subsiding, essentially flat, broad shelf environment. Northeastward from Montreal, toward the Montmorency Promontory of the Quebec City area, basal Middle Ordovician sediments become younger and the extent of the shelf area narrowed significantly. The latter resulted in skeletal shoal sediments (lower Trenton Group) developing closer to shore and concomitant less well‐developed clastic‐rich lagoonal sediments (Black River Group and basal Trenton Group). At Montmorency Promontory the shoal sediments (basal Trenton Group) accumulated along an irregular and rugged coastline. Northeast of the Promontory a steep onshore to offshore profile and rapidly deposited basal inshore clastics (Black River Group) precluded the deposition of skeletal shoals and rapid submergence promoted the early development of deeper shelf (middle and upper Trenton Group) and slope and basin (top Trenton Group, Saint Irénée Formation) sediments. In contrast, corresponding offshore sediments (middle and upper Trenton Group) in the southwest reflect a lower depositional gradient and more gradual subsidence. These patterns of deposition were determined by the interaction of the changing nature of the Ordovician coastline southwest, at, and northeast of the Montmorency Promontory and the variable subsidence rates influenced by the eastward evolving Taconic Orogen.