Lithostratigraphy and deep-water setting of the upper Nanaimo Group (Upper Cretaceous), outer Gulf Islands of southwestern British Columbia

Abstract

The Upper Cretaceous Nanaimo Group consists of extensive shaly units and lenticular bodies of sandstone and conglomerate deposited in neritic to bathyal marine paleoenvironments. Single sandstone and conglomerate lenses cannot be traced throughout the basin, but instead tend to concentrate at particular stratigraphic levels, defining crudely sheet-like composite units of coarse-grained rocks. Many of the lenticular coarse-grained units rest in major channels, as deep as about 550 m. Sand and gravel were generally transported to the west and northwest by sediment gravity flows through these channels into the basin centre. Recognition of widespread channeling is critical to the definition of formal lithostratigraphic units (four formations and two informal members are defined here for the upper Nanaimo Group).The top of the Nanaimo Group is probably truncated by a major unconformity with local relief of at least 2 km. The effect of the unconformity can only be seen by mapping the orientation of offshore ridges in the Strait of Georgia, which can be traced into the sandstones and conglomerates above the inferred unconformity on Tumbo and Cabbage islands. Large scale cross-bedding, unidirectional paleocurrents, and lack of evidence for marine conditions favour a subaerial, fluvial setting for deposition of these rocks, which correlate to the lithologically similar Paleogene Chuckanut Formation on the Sucia Islands.

Reappraisal ofGwawinapterus beardifrom the Late Cretaceous of Canada: a saurodontid fish, not a pterosaur

“…The reinterpretation of Gwawinapterus thus agrees with the depositional environment of the bed that yielded RBCM.EH2011.003.0001, known to have been a deep-water marine setting (England and Hiscott, 1992;Katnick and Mustard, 2003).…”

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

“…The Geoffrey Formation and other investigated strata were attributed to deposition within a long‐lived submarine channel‐system on the continental slope. Similarly, England & Hiscott () interpreted a submarine channel origin for upper Nanaimo Group units on the outer Southern Gulf Islands (the chain of islands spanning from Gabriola to Saturna Islands, Fig. B).…”

“…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 ., ).…”

“…The Geoffrey Formation is one of three coarse‐grained, deep‐water formations in the Nanaimo Group (Mustard, ). It is notably composed of thick successions of stacked conglomeratic channelform bodies that are bound laterally by out‐of‐channel mudstones, which have been ascribed to the Spray and Northumberland formations (England & Hiscott, ; Katnick & Mustard, ; Bain & Hubbard, ; Fig. D to F).…”

Geochemical evolution of groundwater on Saturna Island, British Columbia

“…The Nanaimo Group sequence was deformed by compression into a fold and thrust belt during the Middle Eocene (Mustard 1994;England and Hiscott 1992) and was later uplifted and eroded during the Neogene, concurrent with uplift of the Coast Belt to the east (Mustard 1994). The resulting fractured syncline-anticline combination is evident in the present outcrop pattern on Saturna Island (Fig.…”