LATE CRETACEOUS EVOLUTION AND SEDIMENT PROVENANCE OF THE NANAIMO BASIN: DEFINITIVE LINKAGE TO NORTHERN LATITUDES

The Turonian to Maastrichtian Nanaimo Group provides an excellent record of orogenic exhumation and basin evolution in the southern Canadian Cordillera, and provides critical constraints on tectonism along the western continental margin in Late Cretaceous time. The Nanaimo basin is a contractional forearc basin formed along the western edge of the combined Insular/Intermontane Superterranes. Excellent biostratigraphic control demonstrates that subsidence initiated in Turonian time in response to contractional crustal thickening in the southern Coast Belt to the east. Initial basin sedimentation occurred on a complex paleotopography, as non-marine, marginal marine, and shallow marine sediments onlapped on Wrangellia to the west and onto Jurassic plutons of the Coast Plutonic Complex (CPC) to the east. Subsidence increased dramatically during the late Santonian to Maastrichtian, producing a thick succession (>4 km) of intertonguing submarine fan deposits. Paleocurrent data, abundant volcanoplutonic debris and the presence of syndepositional detrital zircon link rapid orogenic exhumation of the CPC and basin subsidence.

The basin contains a distinctly bimodal detrital zircon signature, with a major population of Cretaceous (68 to 100 Ma) zircon representing first cycle plutonic detritus from active contractional tectonism within the Coast Plutonic Complex, and a significant component of extraregional detritus that floods the basin in early Campanian time. Multiple lines of evidence link the Nanaimo basin to northern latitudes in the Late Cretaceous, including 1) abundant MesoProterozoic detrital zircon, including magmatic gap (1450-1610 Ma) grains, that closely correspond to the Belt Supergroup detrital zircon signature; 2) quartzite cobbles with detrital zircon spectra that match the Belt and Windermere Supergroups and the Cambrian Flathead Sandstone; 3) muscovite Ar/Ar ages that correspond to muscovite ages from the Idaho Batholith; 4) abundant 70-75 Ma detrital zircon grains that do not match plutonic assemblages in the southern CPC, but closely match ages of the Idaho and Boulder batholiths. This northern linkage requires that the Nanaimo Basin has always been located north of the Sierra Nevada system, and invalidates paleomagnetically-based tectonic reconstructions.