LATE CRETACEOUS BASIN EVOLUTION ALONG THE WESTERN MARGIN OF THE INSULAR SUPERTERRANE: THE NANAIMO GROUP, BRITISH COLUMBIA

The Turonian to Maastrichtian Nanaimo Group provides an excellent record of orogenic exhumation and basin evolution, and constrains the tectonic evolution of the combined Insular and Intermontane superterranes. The Group was deposited on the western edge of the superterranes, flanked on the west by Wrangellian basement and on the east by the Coast Plutonic Complex. Basin subsidence began in Turonian time in response to contractional crustal thickening in the Coast Belt and Cascade Range to the east/southeast. Initial basin sedimentation occurred on a complex paleotopography, resulting in accumulation of locally derived sediment in non-marine, marginal marine, and shallow marine environments. However, the presence of syndepositional Late Cretaceous (ca. 87 Ma) zircon indicates the initiation of rapid unroofing of the arc system to the east. Subsidence dramatically increased in late Santonian time, resulting in progradation of submarine fan systems to the west/northwest across the basin, resulting in a thick succession (>4 km) of complexly intertonguing fan lobes that reflect episodic sediment flux corresponding to contractional tectonism in the Coast Range Thust Belt to the east. Paleocurrent data, abundant volcanoplutonic debris and the presence of syndepositional detrital zircon demonstrate a strong linkage between rapid orogenic exhumation of the CPC and basin subsidence.

The tight linkage between orogenic exhumation and basin subsidence was interrupted in early Campanian time, as the arc system was breached by at least one major fluvial system that delivered extraregional sediment from the back arc region into the basin. Significant (locally >50%) quantities of Precambrian detrital zircon (ca. 1300-1800 Ma), apparently derived from the Belt and Windemere Supergroups to the east, mixed with JuraCretaceous grains derived from the CPC. The timing of this older zircon flux corresponds to uplift of Mesoproteroic and Neoproterozoic successions along the Sevier orogenic system to the east. Extraregional sedimentation peaked in late Campanian time, with a distinct increase in quartzite pebbles and cobbles in conglomeratic facies. The extraregional component was substantially diluted by Maastrichtian time by a major pulse of plutonic detritus derived from final unroofing of the Cretaceous arc system.