A SEQUENCE STRATIGRAPHIC FRAMEWORK FOR TERRESTRIAL TO SHALLOW MARINE STRATA OF THE LOWER NANAIMO GROUP, VANCOUVER ISLAND, CANADA

The Upper Cretaceous Nanaimo Group is a 4 km thick sedimentary succession that is presently subdivided into lithostratigraphic units. This lithostratigraphic framework is based on the position of coarse- and fine-grained units relative to the basal nonconformity and to each other. Unfortunately, this framework does not account for spatial or temporal variations in lithology and substantial topography on the basal unconformity. As a result, strata deposited millions of years apart may be correlated together. This study aims to develop a sequence stratigraphic framework for the lower Nanaimo Group via identification of sequence stratigraphically important surfaces and correlation of time-equivalent strata. To this end, thick and continuous outcrops of the lower Nanaimo Group are logged and compared to a 400 m long core through similar strata. The outcrop and core logs are correlated regionally, with the assistance of maximum depositional ages (MDA) and age distributions from detrital zircon (DZ) data.

Key stratigraphic surfaces include flooding surfaces and possible transgressive surfaces of erosion marked by the Glossifungites Ichnofacies. Glossifungites surfaces commonly cap coal beds and appear to be regionally extensive. The identification of regional stratigraphic surfaces is used to demonstrate substantial topography on the basal unconformity, and enables sequence stratigraphic correlation of time-equivalent units regardless of topography. The overall stratigraphy of the lower Nanaimo Group reflects drowning of the unconformity, with an upwards and intermittent shift from terrestrial and coastal deposits into deep marine strata; this observation agrees with previous studies. However, a second trend of deepening towards the southeast is also observed with terrestrial and shallow marine strata grading laterally into marine mudstones.

Seven DZ samples from the lower Nanaimo Group are analyzed for MDAs and age distributions. MDAs show greater than ten million years difference from samples defined as belonging to the same formation, while samples with similar MDAs exhibit different age distributions. This suggests that a more complex depositional architecture exists than previously thought. Analyses of DZ is ongoing, and will be used to refine depositional models of the lower Nanaimo Group.