TO WHAT EXTENT WAS THE NANAIMO BASIN OF SOUTHERN BRITISH COLUMBIA DERIVED FROM THE ADJACENT COAST PLUTONIC COMPLEX?

Much of the current debate surrounding the Nanaimo Group of southern British Columbia centers on potential extraregional sources for the Late Cretaceous–early Paleogene sedimentary basin. Given this focus, it is important to quantify the extent to which the Nanaimo Group received sediment locally from the tonalitic Jurassic–Paleogene southern Coast Plutonic Complex. Thermochronology studies of the northern Nanaimo Group performed on detrital K-feldspar document three distinct periods of deposition. The lowermost formations of the Nanaimo Group received detrital material from shallowly denuded basement with Cretaceous K-feldspar Ar/Ar cooling ages ranging from 87 Ma to 128 Ma. The overlying Cedar District Formation shows a change in sediment to more deeply denuded basement. The stratigraphically highest formations of the basin overwhelmingly consist of ages below 80 Ma, including a significant basement source with 63–68 Ma cooling ages.

The Coast Plutonic Complex experienced periods of high magmatic flux in the Late Jurassic, mid- to Late Cretaceous, and early Eocene. Previously compiled biotite cooling ages from the BC Age database indicate that the southern Coast Plutonic Complex yields a broad range of pre-Cenozoic cooling ages with maxima at 90–100 Ma and 145–160 Ma. This study augments that compilation with 36 additional K-feldspar and biotite Ar/Ar cooling ages along a transect from Squamish to Lillooet. Representative results from the mid-Cretaceous Squamish pluton provide cooling ages of 77 Ma and 97 Ma, for K-feldspar and biotite respectively. These results correspond with detrital K-feldspar results from the stratigraphically lowermost units of the Nanaimo Group. Moreover, these Nanaimo Group formations contain abundant albite that is characteristic of greenschist facies gneissic Jurassic plutons of the Coast Plutonic Complex. Conversely, the Coast Plutonic Complex does not provide K-feldspar cooling ages that correspond to the 63–68 Ma peak from the Nanaimo Group’s muscovite-bearing, stratigraphically highest formations. We conclude that the detrital material from the lowermost formations of the Nanaimo Group correlate with mid-Jurassic plutons from the Coast Plutonic Complex, and that the basin’s youngest detrital contributions originate from an extraregional location.