--> Abstract: Detrital Mineral Geochronology of the Alberta Foreland Basin: Reconstructing Exhumation and Orogen Evolution, by Gerald M. Ross, P. Jonathan Patchett, and Mike Hamilton; #90039 (2005)

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Detrital Mineral Geochronology of the Alberta Foreland Basin: Reconstructing Exhumation and Orogen Evolution

Gerald M. Ross1, P. Jonathan Patchett2, and Mike Hamilton3
1 Kupa'a Farm, Kula, HI
2 University of Arizona, Tucson, AZ
3 University of Toronto, Toronto, ON

We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trace and rare earth (REE) geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Detrital zircon and monazite from the Fernie-Kootenay clastic pulse (154-142 Ma) indicate derivation from Triassic-Ordovician sandstones imbricated within the thrust-and-fold belt, consistent with the Nd tracer results and petrography. U-Pb zircon ages from the Blairmore Group (115-103 Ma) indicate a provenance from Triassic and Jurassic arc rocks of Quesnel terrane with only minor contributions from older miogeoclinal rocks. The upper part of the Blairmore Group shows a transition to less juvenile Nd isotopic signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Milk River-Porcupine interval (78-58 Ma) with juvenile material occurring early in the sequence, accompanied by zircon grains from syndepositional volcanic sources, and by more continental material in the upper part of the sequence. The Fernie-Kootenay clastics record the erosion of thrust-imbricated miogeoclinal rocks during the creation and erosion of the foreland thrust-and-fold belt with no detectable material derived from the deeper parts of the hinterland. A significant unconformity of about 27 m.y. duration at the base of the Blairmore Group led to redistribution of the foreland basin fill and erosion of the adjacent thrust-and-fold belt. Renewed contraction within the erosionally-modified thrust wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnel terrane) to become the dominant source for foreland (Blairmore Group) to the exclusion of miogeoclinal material. The third pulse of sediment records significant input of wind-blown ash from juvenile sources in the Coast Belt mixed with bedload components derived from more local sources in the eastern Cordillera.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005