During Late Devonian through mid-Mississippian time thick sequences of elastics flooded the Canadian Cordilleran miogeocline. Chert conglomerate, chert-quartz sandstone, and pebbly mudstone of the westerly derived Earn sequence formed local massive channel deposits up to 200 m thick on submarine fans and in narrow grabens within a broad region of shale deposition. Extension and local contraction of the miogeocline during rifting or strike slip outboard of present preserved limits can explain (1) the derivation of Earn elastics from older miogeoclinal units, (2) local alkaline volcanism, (3) exhalite mineralization, and (4) tremendous thickness variations related to steep syndepositional faults.

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