The Great American Bank in Eastern Canada — A Synthesis

Lavoie, Denis; Desrochers, André; Dix, George; Knight, Ian; Salad Hersi, Osman

The rifted margin of Laurentia in eastern Canada had a rugged morphology with salients and recesses formed during the break-up of Rodinia. After short-lived carbonate production during the Early Cambrian, the Great American Carbonate Bank was firmly established in the earliest Middle Cambrian as the last rift-related event was followed by passive thermal subsidence.

Middle to Upper Cambrian carbonates are preserved in the Port au Port Group in Newfoundland. Late Middle to Upper Cambrian carbonates are found in southern and eastern Québec although most of the preserved Upper Cambrian facies in the Reentrant consist of nearshore to fluvial clastics unconformably over the Grenvillian basement. The Cambrian carbonates are dominated by high-energy facies with significant thrombolite reefs at the platform margin. The succession consists in large-scale transgressive-regressive cycles known as Cambrian Grand Cycles. The Cambrian-Ordovician transition occurred at a time of a major sea level lowstand that resulted in a major unconformity in southern Québec and Ontario. In Newfoundland, this sea level fall is recorded in the regressive facies of the last Cambrian Grand Cycle, but did not culminate in sub-aerial exposure.

The duration of the depositional hiatus at the Cambrian-Ordovician transition increases towards the west, from an early Skullrockian gap in the Philipsburg Thrust Slice in Québec, the hiatus covered entire Skullrockian in eastern Ontario. Major sea level rise at or near the base of the Ordovician led to sedimentation on an extensive peritidal mud-dominated, low-energy carbonate platform. This platform is known as the St. George Group (Newfoundland), the Beekmantown Group (Québec and Ontario), the School House Hill Group (southeastern Québec) and the Romaine Formation (Anticosti Island). Two 3rd order cycles are documented in Newfoundland. The presence of such cycles is also proposed further south. Multiple 5th order, metre-scale peritidal dominated cycles have been documented.

A diachronous change in depositional style occurred near the base of the Middle Ordovician. Facies patterns became controlled by faulting and accumulation rates increased significantly. These changes occurred first in late Ibexian in southeastern Québec and in early Whiterockian elsewhere. At most localities, this transition is also expressed in a significant sub-aerial unconformity that is recognized along the entire paleosouthern margin of Laurentia.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013