CHANGING PROVENANCES REFLECTED BY ND ISOTOPES, TRACE ELEMENTS, AND FRAMEWORK GRAINS IN THE CORDILLERAN FORELAND SEQUENCE OF WESTERN CANADA

Clastic sedimentary rocks of the Cordilleran Foreland in Alberta, also known as the Western Canada Sedimentary Basin, comprise three major pulses of sediment delivery, each named after prominent formations that occur within them. They are the Fernie-Kootenay (154-142 Ma), the Blairmore (110-95 Ma), and the Belly River-Porcupine (78-55 Ma). Nd isotopes, trace elements and sandstone framework grains show a remarkably consistent picture of changing provenance for these pulses. In the Fernie-Kootenay pulse, clastics show epsilon Nd values -8 to -11, consistent with derivation from earlier miogeoclinal sedimentary units, plus possible contributions from older rocks present in inboard terranes of the Cordillera. Trace elements in the Fernie-Kootenay pulse demonstrate a firmly recycled geochemistry, while sandstone grains consist overwhelmingly of quartz/chert assemblages. In the Blairmore pulse, sedimentary units with a more recycled origin still occur, but clastics with much more juvenile Nd (epsilon values up to +1) are very common. These rocks show abundant plagioclase and volcanic detritus in their grain populations, and carry distinctive trace element characteristics such as low total Rare-Earth Elements, absence of Eu anomalies, and Th/Sc ratios down to 0.2. In the Belly River-Porcupine pulse, this scenario is repeated almost exactly in the analytical data, but with the difference that whereas many Blairmore pulse sandstones appear to be dominated by intermediate-chemistry detritus, the Belly River-Porcupine pulse seems to contain volcanic detritus that is dominantly felsic. Both between the Blairmore and Belly River pulses, and in the latter stages of the third pulse, the Nd isotopic values returned to a background signal (epsilon values of -8 to -12) that is similar to the Devonian through mid-Jurassic miogeoclinal sequence. We interpret the changing delivery of recycled versus juvenile detritus as reflecting (1) uplift of differing terrane and/or fold and thrust belt components in the developing Cordillera, and (2) the related extent to which drainage catchments extended westwards into the Cordillera.