A TRANSPORT STRATIGRAPHY IN THE UPPER CRETACEOUS NANAIMO GROUP, BRITISH COLUMBIA ? – THE EVIDENCE AND IMPLICATIONS TO LATE CRETACEOUS LARGE LATITUDINAL TRANSLATION HYPOTHESES

The Nanaimo Group is a Late Cretaceous peripheral foreland basin which overlies the western Insular Superterrane in a stratigraphic succession >4 km thick. This succession is dominated by submarine fan deposits, an ideal third order type deposystem for preserving regional provenance signatures. The basin is part of “Baja B.C.” entity, a tectonic entity which paleomagnetic evidence suggests is far-translated from the south (>3000 km in some models). The Nanaimo basin’s position on the west side of the tectonic entity suggests it should show the maximum amount of any translation. A continuous record of deposition during the majority of the possible “window of translation” also suggests the basin might record a change in non-local provenance during rapid north translation: a “transport stratigraphy”.

A dataset compiled from 8 years of detrital zircon studies shows a signature dominated in all formations by Cretaceous and minor other Mesozoic ages, reflecting a local first order source from active thrust belts in the adjacent Coast and Cascade rocks. However, Precambrian zircons are present in all samples and must be derived from a non-local eastern source. We suggest this is likely the Belt Supergroup, which was within a few hundred kilometers of the Nanaimo basin position when documented post-basin fault motions are restored. Significant to this study is that there is no systematic change in the Precambrian detrital zircon signature from oldest to youngest formations sampled (~85-65 Ma). This refutes interpretations that a transport stratigraphy within the Nanaimo Group supports a large translation model. The youngest (~65 Ma) sample has only rare Precambrian zircons, suggesting that the Precambrian source area may become more distal (or otherwise unavailable) during late stages of Nanaimo basin deposition. This is the opposite of the expected result for a large translation model. The consistent Precambrian zircon signature is even present in pebble and small cobble clasts of orthoquartzite from at least one formation. This strongly supports conclusions that the tectonic entity must have been directly linked to North America during the time spanned by this study, and that 1000’s of km of either long shore drift or fluvial transport are very unlikely methods for transporting the non-local zircons to the basin.