UTILIZING DETAILED PROVENANCE ANALYSIS TO CONSTRAIN VARIATIONS IN BASIN-WIDE DEPOSITIONAL ARCHITECTURE: THE LATE CRETACEOUS NANAIMO BASIN, SOUTHWESTERN BRITISH COLUMBIA, CANADA

The Upper Cretaceous Nanaimo Group accumulated in a peripheral foreland basin on the Insular Superterrane in southwestern British Columbia. Detailed facies studies, abundant paleontologic control and regional provenance analysis facilitate reconstruction of basin depositional architecture.

Basin subsidence began in Turonian time in response to contractional crustal thickening in the southern Coast Belt and Cascade Range to the east and southeast. Initial sedimentation was diachronous, with non-marine, marginal marine, and shallow marine deposition on a complex paleotopography having local relief >100m. Most clastic detritus was locally derived, but the presence of syndepositional Late Cretaceous (ca. 87 Ma) zircon indicates the initiation of rapid unroofing of the arc system on the eastern margin of the basin.

Subsidence dramatically increased in late Santonian time, resulting in the progradation of submarine fan lobes west/northwest across the basin (with the exception of a few paleotopographic highs that remained nonmarine into Campanian time). Santonian to Maastrichtian sedimentation in general produced a thick succession (>4 km) of complexly intertonguing fan lobes resulting from episodic sediment flux. The predominance of volcanoplutonic debris and paleocurrent data indicate derivation from the Coast Plutonic Complex to the east. The presence of syndepositional detrital zircon throughout the section suggests a strong coupling between rapid orogenic exhumation and basin subsidence. By early Campanian time, however, the main continental arc system was locally breached by one or more large rivers, and extraregional sediment from the distal back arc region prograded northward into the basin. Significant (locally >50%) quantities of Precambrian detrital zircon grains derived from uplift of the Belt Supergroup to the east are mixed with Jurassic and Cretaceous grains supplied by the Coast Plutonic Complex. The contribution from the eastern source peaked in late Campanian time, as reflected by a conspicuous component of quartzite pebbles and cobbles in conglomeratic facies. By Maastrichtian time, the Precambrian component was significantly lowered, probably reflecting dilution due to a major pulse of plutonic detritus derived from final unroofing of the Cretaceous arc system.