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The Great American Carbonate Bank in Eastern Canada: An Overview

By
Denis Lavoie
Denis Lavoie
Geological Survey of Canada-Quebec Division, Quebec City, Quebec, Canada
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André Desrochers
André Desrochers
Department of Geology, University of Ottawa, Ottawa, Ontario, Canada
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George Dix
George Dix
Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada
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Ian Knight
Ian Knight
Geological Survey of Newfoundland and Labrador, St. John’s, Newfoundland, Canada
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Osman Salad Hersi
Osman Salad Hersi
Department of Earth Sciences, Sultan Qaboos University, Sultanate of Oman
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Published:
January 01, 2012

Abstract

The postrifted margin of Laurentia in eastern Canada had a rugged paleomorphology, with major salients and recesses formed during the long-lasting (Ediacaran to late Early Cambrian) breakup of Rodinia. After short-lived carbonate production during the Early Cambrian, the great American carbonate bank (GACB) was firmly established in the earliest Middle Cambrian as the last rift-related event (Hawke Bay event, late Early Cambrian), and was followed by mostly passive thermal subsidence of the continental crust of Laurentia.

Middle to Upper Cambrian carbonates are well preserved in the Port au Port Group in western Newfoundland (St. Lawrence promontory). Scattered outcrops of upper Middle to Upper Cambrian sedimentary rocks are found in southern and eastern Quebec (Quebec reentrant), although most of the preserved Upper Cambrian facies in the reentrant consist of nearshore to fluvial clastics unconformably overlying the Grenvillian basement. The Cambrian shallow-marine carbonates are dominated by high-energy facies with significant thrombolite reefs at the platform margin. The succession consists of large-scale transgressive-regressive cycles known as Cambrian grand cycles. Some anomalies in stacking patterns are suggestive of local tectonic events that were hypothesized based on the nature (facies and age) of carbonate clasts that accumulated on the continental slope. The Cambrian–Ordovician transition occurred at a time of a major sea level lowstand that resulted in a significant unconformity in southern Quebec and Ontario. In western Newfoundland, this sea level fall is recorded in the regressive facies of the last Cambrian grand cycle but did not culminate in subaerial exposure.

The duration of the depositional hiatus at the Cambrian–Ordovician transition increases toward the west from an early Skullrockian gap in the Philipsburg thrust slice in southeastern Quebec; the hiatus covered the entire Skullrockian in eastern Ontario. A major sea level rise at or near the base of the Ordovician resulted in sedimentation on an extensive peritidal, mud-dominated, low-energy carbonate platform. This platform is known as the St. George Group (western Newfoundland), the Beekmantown Group (southwestern Quebec and Ontario), the School House Hill Group (southeastern Quebec), and the Romaine Formation (Anticosti Island). The carbonate facies are characterized by large- and small-scale depositional cycles. Two third-order cycles are well documented inwestern Newfoundland. The presence of such cycles is also proposed farther south, although their precise character still has to be documented. Multiple fifth-order meter-scale peritidal-dominated cycles have been documented in the Lower Ordovician carbonates.

A diachronous change in depositional style occurred along the margin of Laurentia near the base of the Middle Ordovician. Facies patterns became controlled by faulting and accumulation rates increased significantly. These changes occurred first in the late Ibexian in southeastern Quebec and in the early Whiterockian elsewhere. At most localities, this transition is also expressed in a significant subaerial unconformity that is recognized along the entire eastern (paleosouthern) margin of Laurentia. This subaerial event is interpreted as resulting from lithosphere upwarping in front of the migrating Taconic orogenic wedge. The west-directed migration of the tectonic peripheral bulge resulted in the final destruction of the GACB as sedimentation resumed in a tectonically active foreland basin.

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Contents

Memoir

Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia

James Derby
James Derby
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Richard Fritz
Richard Fritz
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Susan Longacre
Susan Longacre
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William Morgan
William Morgan
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Charles Sternbach
Charles Sternbach
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American Association of Petroleum Geologists
Volume
98
ISBN electronic:
9781629810201
Publication date:
January 01, 2012

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