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

• Published in: Canadian journal of earth sciences Vol. 43; no. 8; pp. 1165 - 1181
• Main Authors: Johnstone, P. D, Mustard, P. S, MacEachern, J. A
• Format: Journal Article
• Language: English
• Published: Ottawa, Canada National Research Council of Canada 01.08.2006; NRC Research Press
• Subjects: bedding; British Columbia; Canada; clastic rocks; coastal environment; Comox Formation; conglomerate; Cretaceous; depositional environment; Environmental aspects; high-energy environment; Invertebrata; lithofacies; Mesozoic; Nanaimo Group; paleogeography; planar bedding structures; sediment supply; sedimentary rocks; sedimentary structures; Shorelines; southwestern British Columbia; storms; Stratigraphy; unconformities; Upper Cretaceous; Western Canada; British Columbia; INE, Canada, British Columbia, Vancouver I., Comox
• ISSN: 0008-4077; 1480-3313
• DOI: 10.1139/e06-046

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, open-ocean storms. This interpretation differs from most previous models from the lower Nanaimo Group, which suggest that deposition occurred in more sheltered strait or bay environments.