The thick Proterozoic succession of northwestern North America can be subdivided into three sequences: A (∼1.7 to ∼1.2 b.y.), B (∼1.2 to ∼0.8 b.y.), and C (∼0.8 to 0.57 b.y.). Deposition of the rocks of sequence A was followed by widespread mafic igneous activity about 1.2 b.y. ago. In the Cordilleran region these rocks were then folded during the Racklan (= Grenville?) orogeny. A second, mainly tensional, orogenic event, here called the “Hayhook orogeny, ” occurred near the time boundary between sequences B and C and may have culminated in continental fragmentation in Phanerozoic time. The older Proterozoic rocks of the Cordilleran region (sequences A and B) are considered to have been derived in part from rising orogens in the eastern part of the Canadian Shield. There is no definitive evidence of extensive oceanic areas in the Cordilleran region during the Proterozoic, but local small ocean basins may have existed at the time of deposition of sequence C.

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