Upper proterozoic supracrustal rocks of North America: A brief review

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Abstract Well-preserved successions of supracrustal rocks of Upper Proterozoic age are present around the margins of North America but are rare in the continental interior. The thickest and most complete assemblage is present in the Cordilleran region, particularly in the central and northern parts. In these regions, a three-fold subdivision of the supracrustals is possible. The oldest assemblage (Sequence A) is older than 1.2 Ga, the middle assemblage (Sequence B) is between ca. 1.2 Ga and 0.8 Ga and Sequence C is between ca. 0.8 Ga and 0.57 Ga, the base of the Cambrian. Here, Upper Proterozoic is used as roughly equivalent to Sequences B and C. Sequence A is only briefly considered. Detailed lithostratigraphic correlation is possible between the rocks of Sequences A and B and generally thinner counterparts in the Canadian Arctic, on the northern margins of the Canadian shield. Sequence C is not represented by sedimentary rocks on the northern edge of the continent. The break between Sequences A and B, the Racklan orogeny, is not recognised in the southern part of the Cordillera but there is evidence of widespread mafic igneous activity at about the same time (ca. 1.2 Ga ago). Upper Proterozoic rocks are poorly represented in the continental interior; the Keweenawan succession provides an exception. It rocks fill an intracratonic rift, possibly related to early movements during the Grenville orogeny. The Upper Proterozoic history of the eastern part of North America is much simpler in that it comprises a single supracrustal assemblage corresponding to Sequence C and the upper part of Sequence B, all formed after the Grenville orogeny. Thus the different tectonic histories of the eastern and western margins of North America resulted in development of different sedimentary accumulations. With the exception of the Racklan orogeny, which affected the northern part of the Cordillera, the Upper Proterozoic assemblages of both the eastern and western margins of the North American continent reflect sedimentary accumulation and subsidence that culminated in the opening of Phanerozoic oceans.