PALEOGEOGRAPHIC IMPLICATIONS OF LONG-LIVED SUBMARINE CANYON SYSTEMS IN THE LATE CRETACEOUS NANAIMO GROUP, DENMAN AND HORNBY ISLANDS, BRITISH COLUMBIA

The results of recent sedimentologic and stratigraphic analysis shed insight into the paleogeographic setting of the Georgia Basin during the Late Cretaceous. Nanaimo Group strata of the northern Gulf islands, Straight of Georgia, consist of immense channelform sedimentary bodies composed of thick-bedded sandstone and conglomerate. At least 27 channelform units are present, which are up to 3 km wide and 150 m thick. The channelforms can be amalgamated into one another, or separated by mudstone-dominated strata. At their margins, steep erosion surfaces laterally separate channel deposits from fine-grained, out-of-channel strata. Large-scale sedimentary rock distribution patterns, coupled with detailed sedimentary facies analysis, indicate processes of erosion, sediment bypass (of abundant coarse-grained material further basinward), and deposition in submarine canyons or slope valleys. Paleoflow was variable, with mean values amongst the distinct channelforms ranging between 160 and 300°, attributed to the sinuous nature of the conduits.

The channelforms are present along a depositional-strike-oriented transect ~20 km in length; this relatively consistent geographic position of channelform bodies suggests a fixed zone of down-slope sediment transfer for up to 8-10 Ma, recorded in 1.5-2 km of stratigraphy. Based on comparison to other ancient examples of deep-water depositional systems, what paleogeographic insight can be drawn about the enigmatic, up-dip catchment for Nanaimo Group sediments? Can a paleo-latitudinal position for the easterly-derived fluvial feeder system(s) be better constrained?

Submarine conduit deposits characterized by comparably stacked, composite channelform bodies that persisted over millions of years are common to continental slope systems outboard of major fluvial, sometimes continental scale, drainage systems (e.g., paleo- Congo, Niger and Nile rivers). In tectonically active areas, catchments and associated subaerial drainage patterns tend to be more ephemeral, impacting the longevity of the up-dip extension of submarine conduits like canyon heads. However, down-dip portions of these slope systems can persist for millions of years in instances where they are captured in deep and underfilled elongate basins such as a foredeep or forearc.