The basal unconformity of the Nanaimo Group, southwestern British Columbia: a Late Cretaceous storm-swept rocky shoreline

Abstract

The Turonian to Santonian Comox Formation forms the basal unit of the Nanaimo Group. In the southern Gulf Islands of British Columbia, the Comox Formation nonconformably overlies Devonian metavolcanic and Jurassic intrusive rocks and is interpreted to reflect a rocky foreshore reworked by waves and ultimately drowned during transgression. The nonconformity displays a relief of metres to tens of metres. Basal deposits vary in thickness, as does the facies character along the several kilometres of paleoshoreline studied. In the study area, three distinct but related environments are expressed, typical of a complex rocky shoreline with headlands and protected coves. Crudely stratified conglomerates represent gravel-dominated fans characterized by debris-flow processes, building out from local coastal cliffs and gullies directly onto the rocky shoreline. Fine-grained basal units represent shoreline environments protected from higher energy shoreline processes, presumably in small embayments. Sandstone facies associations reflect stormdominated shoreface environments. The unusual thickness and coarseness of these shoreface intervals suggest a combination of increasing accommodation space, proximal and high sediment supply, and high frequency and energy of storm activity. This, in turn, suggests that the majority of the shoreline was exposed to the full effects of large, openocean storms. This interpretation differs from most previous models for the lower Nanaimo Group, which suggest that deposition occurred in more sheltered strait or bay environments.Résumé : La Formation de Comox (Turonien à Santorien) comprend l'unité de base du Groupe de Nanaimo. Dans les îles du sud du golfe de la Colombie-Britannique, la Formation de Comox repose en discordance sur des roches méta-volcaniques dévoniennes et sur des roches intrusives du Jurassique; cette formation est interprétée comme le reflet d'un estran rocailleux retravaillé par les vagues et finalement submergé au cours d'une transgression. La discordance pré-sente un relief de mètres à des dizaines de mètres. Les dépôts à la base et le faciès ont une épaisseur variable le long des kilomètres de paléorivage étudié. Dans la région à l'étude, trois environnements distincts mais reliés sont définis; ils sont typiques d'un rivage rocailleux complexe avec des caps et des anses protégées. Des conglomérats à stratifications mal définies représentent des éventails de gravier caractérisés par des écoulements de débris; ils sont construits à partir de falaises côtières et de ravins, directement sur le rivage rocailleux. Les unités basales à grain fin représentent des environnements de rivage protégés contre des processus de rivage à énergie plus élevée, probablement dans de petites baies. Les faciès associés au grès reflètent des environnements d'avant-plage dominés par des tempêtes. L'épaisseur et la grosseur inhabituelles des sédiments de ces intervalles d'avant-plage suggèrent plus d'espace, un approvisionnement proximal et abondant de sédiments et une grande fréquence de tempêtes...

Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

“…It is possible that chert in the lower Nanaimo Group derived from minor chert units within strata of Wrangellia (Pacht, 1984;Brandon et al, 1986), which is thought to have formed basement highs that controlled sedimentation of the lower Nanaimo Group (Pacht, 1984;Mustard, 1994;Kent et al, 2019), as well as acted as a sediment source (Huang et al, 2019). The lack of chert in the upper Nanaimo Group in these regions could signal burial of Wrangellian basement during progressive onlap of the basal unconformity during deposition of lower Nanaimo Group (Johnstone et al, 2006;Kent et al, 2019 1-13) and potential source terranes in the NCTS (i.e., PDFs 14-24). Note scale break at 300 Ma.…”

U-Pb and Hf Isotopic Analysis of Detrital Zircons From Paleozoic and Cretaceous Strata of Southern Vancouver Island, British Columbia

“…Formation is interpreted to consist, in some areas, of fluvial sediments that were trapped by topographic highs during deposition, whereas in other areas consists of deltaic deposits (Johnstone et al, 2006).…”

Plant-Arthropod Interactions inAcanthostrobus edenensis(Cupressaceae), a New Conifer from the Upper Cretaceous of Vancouver Island, British Columbia

“…Traditionally, the Comox Formation was thought to be Santonian in age (Muller and Jeletzky 1970;Bickford and Kenyon 1988;Kenyon et al 1992), but deposits in the Gulf and San Juan Islands have been dated to the Turonian (Haggart 1994;Haggart et al 2005). Although Haggart (1994) and Haggart et al (2005) argue that Turonian sediments in the Gulf and San Juan Islands ought to be considered Sidney Island Formation, Johnstone et al (2006) recognize similar lithology and sedimentology between these and the Vancouver Island sites. As such, the Comox Formation is understood to be diachronous in nature, encompassing the Turonian to early Campanian (Mustard 1994;Johnstone et al 2006;Karafit 2008;Karafit and Stockey 2008).…”

“…Although Haggart (1994) and Haggart et al (2005) argue that Turonian sediments in the Gulf and San Juan Islands ought to be considered Sidney Island Formation, Johnstone et al (2006) recognize similar lithology and sedimentology between these and the Vancouver Island sites. As such, the Comox Formation is understood to be diachronous in nature, encompassing the Turonian to early Campanian (Mustard 1994;Johnstone et al 2006;Karafit 2008;Karafit and Stockey 2008). The Comox Formation beds from which this specimen was obtained are exposed along the Eden Main logging road (Schweitzer et al 2003;Karafit 2008), which is 35 km northwest of the city of Comox (lat.…”