PROVENANCE OF THE NANAIMO GROUP: A LATE CRETACEOUS LINKAGE TO THE NORTHERN NORTH AMERICAN CRATON

Proponents of large-scale latitudinal terrane translation along the western continental margin argue that there are no definitive linkages between the North American craton and the Insular superterrane prior to ~50 Ma. The provenance of clastic detritus in basins depositionally tied to the Insular superterrane would provide important temporal and spatial constraints on Cretaceous paleogeography.

The Late Cretaceous Nanaimo basin was a peripheral foreland basin deposited on the western edge of the Insular superterrane. The basin contains a distinctly bimodal detrital zircon signature, with a major population of Cretaceous (68 to 100 Ma) zircon and a smaller, yet consistent population of Meso- and Paleoproterozoic (1300-1800 Ma) zircon. Archean zircon is a minor, yet critical, detrital component. The Cretaceous population represents first cycle plutonic detritus eroded during active contractional tectonism within the Coast Plutonic Complex.

The Proterozoic detrital zircon population in the Nanaimo Group is strikingly similar to that of the Mesoproterozoic Belt Supergroup, and represents an important detrital linkage between North America and outboard terranes. Detrital zircon in the Belt Supergroup occurs in distinct suites. A strong 1650-1850 Ma population, probably derived from the Yavapai and Mojave terranes, characterizes most Belt sands. A small 1500-1610 Ma population is unrepresented in North America but is present in the Gawler craton of southern Australia. A 1400-1480 Ma suite represents syn-Belt volcanism. Older Paleoproterozoic and Archean zircons are also present. This ‘Belt signature’ is robust, documented in the Supergroup itself, modern streams draining the Supergroup, and in Cretaceous and Tertiary basins of the northern Rocky Mountains.

This ‘Belt signature’ occurs in the Nanaimo Group, which also contains a 1300-1400 Ma population that is coeval with post-depositional intrusions in the Belt Supergroup. Significantly, quartzite pebbles and cobbles from the upper Nanaimo Group contain the same detrital signature, limiting the probable transport distance. Recognition of Belt Supergroup detritus in the Nanaimo Group establishes a direct link between the Insular superterrane and the North American craton and strongly supports a northern position for the superterrane during the Late Cretaceous