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Lower Cambrian to Middle Devonian miogeoclinal strata were deposited along a passive margin of western Ancestral North America which formed as a result of rifting in Late Proterozoic time. Local anomalous thicknesses and facies of sedimentary rocks and the presence, in all systems, of minor volcanic rocks suggest repeated episodes of extension. The miogeoclinal sedimentary prism thickens markedly west of hinge lines which vary slightly in position for different rock units. Farther west thick carbonate sequences grade abruptly into much thinner argillaceous strata which, perhaps, reflect deposition on significantly attenuated continental crust. Thus, the main characteristics of the Lower Cambrian to Middle Devonian rocks in themiogeocline may be attributed to a complex interplay of continental rifting, attenuation, drifting, thermal subsidence and flexuring along the western margin of the continent. Rocks of similar ages, in part volcanogenic, mainly in the Alexander Terrane and forming minor components of several other terranes seem to have had both volcanicisland arc and possibly miogeoclinal affinities.

The Cambrian System of the Cordillera can be subdivided into four series, the Lower Cambrian Placentian and Waucoban Series, Middle Cambrian, and Upper Cambrian. The Placentian Series is strikingly different from the other three in its predominance of clastic sediments. Various water depths are indicated, but no major carbonate body was deposited, which suggests deposition in relatively cool waters. Placentian basinal shale and siltstone were probably deposited in British-Barn Basin and are present in much of Selwyn Basin. Between the two basins was the ancestral Yukon Platform, which

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