U-PB GEOCHRONOLOGY OF DETRITAL ZIRCON AND MONAZITE IN THE FORELAND BASIN OF ALBERTA: RECONSTRUCTING WEDGE EVOLUTION FROM PROVENANCE PATTERNS

Patterns of U-Pb ages of zircon and monazite, in tandem with whole rock Sm-Nd and trace element geochemistry, provide a multifaceted fingerprint of provenance evolution in the foredeep basin to the Canadian Cordillera in SW Alberta. Foredeep clastics comprise three major pulses of sediment delivery into the basin: the Fernie-Kootenay (154-142 Ma), the Blairmore (110-95 Ma), and Belly River-Porcupine (78-55 Ma). Detrital zircon and monazite from the first clastic pulse indicate derivation from Triassic-Ordovician sandstones of the miogeocline imbricated within the thrust wedge, consistent with the Nd results. U-Pb zircon ages from the Blairmore Group (pulse 2) confirm a provenance from Quesnellia with only minor components from the older miogeocline and also detect the presence of syndepositional magmatic material. The upper part of the Blairmore Group shows a transition to less juvenile Nd signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Belly River-Porcupine interval with juvenile material occurring early in the sequence, accompanied by syndepositional grains of magmatic affinity, and more continental material in the upper part of the sequence. Pulse 1 records the erosion of imbricated miogeoclinal rocks during the creation and erosion of retroarc topography with no detectable material derived from the deeper parts of the orogenic hinterland. A significant unconformity of about 15 m.y. duration led to regrading of the foreland fill and adjacent wedge and corresponds to a period of tectonic quiescence in the Cordillera. Renewed contraction of the erosionally-modified foreland wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnella) to become the dominant source for foreland (pulse 2). 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. The final sediment deposited in the basin (circa 58 Ma) is characterized by a cosmopolitan provenance that likely records cannibalization of older parts of the foreland as previously deposited foreland strata became important components of the thrust wedge.