Arenig and Llanvirn graptolite biostratigraphy, Canadian Cordillera

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Abstract
The proto-Pacific margin of North America remained passive for the main part of 1.4 Ga. The stratigraphy that accumulated can be split into four major time-rock sequences; each may be considered a separate passive-margin sequence. The margin, initiated at about 1500 Ma, was remodelled by extensional (rift) episodes. Geological evidence for a 780 Ma extension event is documented here; subsidence analysis supports a 550 to 600 Ma separation event (Bond & Kominz), and stratigraphic and structural evidence from the northern Canadian Cordillera suggests 370 Ma extension. Each event is interpreted as the precursor to, and initiator of, passive-margin sedimentation.