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The ages of formations and larger groupings of Precambrian stratified rocks in the Cordillera are, in general, poorly constrained. Only two useful macrofossil assemblages are known: an Ediacaran, non-skeletal fauna found only in the youngest formations, and an older, Chuaria-Tawuia associationwhose duration of several hundred million years provides age control little more constrained than that provided by the few available radiometric dates. Stromatolite biostratigraphy to date has failed to provide useful chronocorrelation with the Riphean of the U.S.S.R. Only the youngest formations have been characterized palynologically.

Few reliable radiometric dates have been published, but bracketing dates and structural relationships permit the assignment of most major successions of Cordilleran Proterozoic strata to one or other of the sequences recognized by G.M. Young and co-workers:

  • Sequence A: =1.7 to =1.2 Ga

  • Sequence B: =1.2 to =0.78 Ga

  • Sequence C: =0.78 to =0.57 Ga

These sequences, of which A and B are the subject of this chapter, provide a useful, though imprecise, framework for discussion. Sequence C nconformably overlies Sequence B and is discussed in Chapter 6.

Sequence A (Purcell-Wernecke, Cap Mountain-Hornby Bay, Muskwa assemblages) conspicuously lacks the longitudinal Cordillera continuity of Upper Proterozoic strata (Sequence C) and is exemplified by the classical Purcell (Belt) Supergroup of southeastern British Columbia, southwestern Alberta, Montana and Idaho. Its patterns of sedimentary facies and thickness (up to 20 km) have generally been interpreted as those of a continental-marginsuccession, but for the Belt-Purcell Basin at least, work in the northwestern United

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