The Upper Cretaceous Nanaimo Group, Georgia Basin

Abstract

The Nanaimo Group comprises up to 4 km of sedimentary rock of Turonian to Maastrichtian age, forming the lower part of the Late Cretaceous to Neogene Georgia Basin of southwest British Columbia. This Upper Cretaceous succession was deposited in a single elongate basin deformed by Eocene compression into a fold and thrust belt. Eleven formations are recognized, comprising conformable and laterally intertonguing successions with sandstone-conglomerate units separated by mudstone and fine grained sandstone formations. Initial alluvial and coastal marine deposits formed on a rugged unconformity. Coal-bearing facies formed in coastal and marginal marine back-barrier environments, associated with fluvial and shallow marine facies. Most of the Nanaimo Group was deposited in marine, generally outer neritic to bathyal depths, by gravity flows and generally as submarine fan deposystems. Initial detritus was from local basement, but most sediment came from the Coast Belt to the east and northwest Cascades, although by latest Cretaceous time the eastern Cordillera was also a source. A forearc basin setting for the Nanaimo Group is only correct in that deposition occurred oceanward of a partly coeval magmatic arc. A foreland basin model is preferred because basin initiation and sedimentation was a direct result of contemporaneous thrusting in the Coast Belt and north Cascades. Nanaimo Group coal resources were historically important, but are now exhausted in most areas. Kaolin-rich deposits on the unconformity may be economically viable. Oil and gas potential is poor, although coalbed methane could be locally present.

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…The detrital zircon pattern of the post‐Campanian Nanaimo and WMB sediments correlates with the signature observed in the circa 79 Ma Skagit Gneiss sample (NC‐772, Figure ). However, most of the Skagit Gneiss metasediments have older MDAs than the Nanaimo Group strata (Mustard, ). If the Nanaimo Group was the source for the Skagit metasedimentary rocks, it would have to be coupled with an Early Cretaceous source, which is not preserved adjacent to the North Cascades.…”

Tectonically controlled initiation of contemporaneous deep‐water channel systems along a Late Cretaceous continental margin, western British Columbia, Canada

“…Continued plate convergence resulted in regional deformation and magmatism along its suture zone forming an extensive metamorphic and plutonic belt (the Coast Plutonic Complex) (Monger et al ., ). The formation of the Nanaimo Basin and deposition of the associated Nanaimo Group occurred during the early Late Cretaceous on Palaeozoic–Jurassic sedimentary and volcanic strata of the Wrangel terrane, west of the plutonic rocks of the Coast Plutonic Complex (Mustard, ; Fig. A and B).…”

“…(B) Map displaying the distribution of exposed Late Cretaceous Nanaimo Group strata. Study areas are located along a north‐west/south‐east oriented, 135 km long transect and are outlined with red boxes (modified from Mustard, ; Bain & Hubbard, ). (C) Stratigraphic chart of the Nanaimo Group (modified from Matthews et al ., ).…”

“…The Nanaimo Basin has been interpreted as either a peripheral foreland (Brandon et al ., ; Mustard, ) or a forearc (Muller & Jeletzky, ; England & Hiscott, ; Matthews et al ., ) basin. Regardless of its origin, the Nanaimo Basin received a continuous supply of zircon‐rich sediment from the adjacent arc during deposition of Nanaimo Group strata (Van der Heyden, ; Mustard, ; Matthews et al ., ).…”

Upper-crustal structure beneath the Strait of Georgia, Southwest British Columbia

“…The Comox and Nanaimo basins contain several kilometres of siliciclastic deposits (mainly sandstones, shales and conglomerates) of the dominantly marine Upper Cretaceous Nanaimo group, which were deposited during a period of basin‐wide subsidence in the Late Cretaceous. In southern Georgia Basin, the sedimentary rocks are in excess of 4 km thick (Mustard 1994; Mustard & Rouse 1994; England & Bustin 1998). Sedimentation in the Whatcom and Chuckanut basins occurred during a second phase of subsidence in the Late Palaeocene to Late Eocene, which resulted in the accumulation of several kilometres of siliciclastic, mainly non‐marine sedimentary rocks (Johnson 1984; Mustard & Rouse 1994).…”