The Late Ordovician Dawson Point Formation (Timiskaming outlier, Ontario): key to a new regional synthesis of Richmondian–Hirnantian carbonate and siliciclastic magnafacies across the central Canadian craton

Abstract

Current paleogeographic reconstructions extend Late Ordovician Taconic-derived siliciclastics across the central Canadian craton prior to the terminal Ordovician glacioeustatic lowstand. Revision of the Late Ordovician Dawson Point Formation of the Timiskaming outlier greatly reduces the distribution of these siliciclastics, and documents a greater spread of shallow-water carbonate of Richmondian age. As revised, the Dawson Point Formation contains two informal members: a deep-water graptolitic shale that grades upward into shallow-water siliciclastic redbeds, and an upper member of shallow-water, muddy, crinoidal limestone with interbedded shale, likely representing low-energy shoals on a muddy shelf. Deep-water shale accumulation began in the upper manitoulinensis graptolite Zone following foundering of the regional foreland carbonate platform. Basin development documents a northward-younging (~1 million years) from southern Ontario foreland basins, in keeping with regional tectonic-driven transgression along eastern North America. The shale-to-carbonate succession of the Dawson Point Formation correlates with the Georgian Bay Formation on Manitoulin Island, wherein the upper carbonate-dominated divisions of both formations are equivalent to the siliciclastic Queenston Formation of southern Ontario. In absence of additional biostratigraphic information, the upper member of the Dawson Point Formation is likely Richmondian (or late Ashgillian) in age. The revised Late Ordovician history of the Timiskaming outlier may identify a once significant volume of shallow-water carbonate across the central Canadian craton, with related sequestration of carbon dioxide possibly aiding global cooling. Erosion of the carbonate, driven by developing glacioeustatic lowstand conditions, was likely contemporaneous with early Hirnantian peritidal deposition of the uppermost Queenston Formation in southern Ontario.