Before the Great North American Carbonate Bank: A Complex Cambrian-Lower Ordovician Transgressive History Recorded in Siliciclastic Strata of the Potsdam Group, Southeast Laurentia
Lowe, David G.; Arnott, Bill
In the Ottawa Embayment (OE) and Quebec Basin (QB) rocks of the Potsdam Group form the base of the Cambrian-Ordovician Sauk Megasequence and record siliciclastic deposition during early marine transgression that preceded deposition of marine carbonates here and in basins across most of southern Laurentia. The OE is a physiographic embayment bounded to the north and south by the limits of a Neoproterozoic rift, and to the west and east by two transcontinental arches, the Frontenac and Oka-Beauharnois (OkBA) arches, respectively. During the Cambro-Ordovician the OE was sandwiched between the two arches whereas the QB was bounded to the west by the OkBA and to the east by the open Iapetus Ocean. Recent field investigations show that the Potsdam isopach and its constituent lithofacies change dramatically across the OkBA.
In the OE the Potsdam is about 10-50 m thick and consists of three unconformity-bounded, mostly continental units of quartz arenite and conglomerate. Unit 1 comprises aeolian and fluvial deposits; Unit 2, alluvial and fluvial sandstone and conglomerate, including probable syn-rift deposits; and Unit 3, marginal- and shallow-marine sandstone. In contrast, the Potsdam succession in the QB is 100-500 m thick and comprises three conformably stacked, mostly marine units: Unit 1, made up of marine mudstone, sandstone and dolostone; Unit 2, marginal marine arkose; and Unit 3, marginal- and shallow-marine quartz arenite.
Dramatic differences in thickness and nature of the internal bounding surfaces within the Potsdam Group across the OkBA reflects major differences in accommodation space, most likely related to differences in rates of subsidence between the OE and QB. Moreover, although three regionally extensive stratal units can be identified on opposite sides of the OkBA, it is currently uncertain how the lower two units in the OE and QB correlate, if in fact they do. This suggests that during the earliest stages of transgression of the Sauk Megasequence the OkBA acted as a physiographic barrier that fully separated continental conditions in the OE from marine and marginal marine conditions in the QB to the east. Later, however, marine and marine-influenced strata of Unit 3 spread progressively westward from the QB breaching the OkBA and into the OE. Continued transgression eventually replaced siliciclasic with carbonate sedimentation.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013