DETRITAL K-FELDSPAR THERMOCHRONOLOGY OF THE NANAIMO GROUP: EVIDENCE FOR LATE CRETACEOUS DENUDATION OF ARC CRUST AND THE INFLUX OF EXTRAREGIONAL SEDIMENT

Late Cretaceous-Early Paleogene forearc basin strata of the Nanaimo Group overlie the contact between the early Wrangellia terrane and Jurassic arc crust of the Coast Plutonic Complex (CPC) of southern mainland British Columbia. Previously reported detrital zircon U-Pb age data indicate that (1) the Santonian-Early Campanian sediment of the Nanaimo Group eroded from the adjacent CPC, and (2) a Late Campanian influx of extraregional sediment enriched in middle Proterozoic zircon filled the basin. The source of this extraregional sediment is disputed and may have originated either from the Belt Supergroup and Idaho/Boulder batholith to the east or from a tectonically-displaced southern location alongside the Mojave/Salinia segment of the southern California margin.

New single crystal ⁴⁰Ar/³⁹Ar laser fusion ages from seven detrital K-feldspar samples from Santonian-Maastrichtian strata of the northern Nanaimo Group help constrain the denudation history of potential source regions. Santonian-Early Campanian strata from the Comox and Extension Formations yield evenly distributed 85-130 Ma cooling ages consistent with shallow erosion of the CPC. In contrast, 84-88 Ma maxima appear in the detrital K-feldspar age distributions from the overlying Cedar District and De Courcy Formations. We interpret this peak as a denudation-induced signal produced by deep erosion into the CPC by Upper Campanian time. Results from the base of the Upper Campanian Geoffrey Formation signal the abrupt appearance of a new sediment source dominated by 72-82 Ma K-feldspar that is coeval with the local-to-extraregional shift indicated by detrital zircon provenance. This 72-82 Ma signal persists in overlying sediments and is accompanied by systematically younger age maxima at 68 Ma (Spray Fm.) and 65 Ma (Gabriola Fm.) that may reflect more youthful intrusion and/or deeper erosion of the extraregional source. Based upon existing data, the Idaho/Boulder batholith area presently appears more capable than the Mojave/Salinia segment of having supplied the Nanaimo Basin with detrital K-feldspar that display the observed cooling age distributions. Further work is underway to better evaluate these and other possibilities.