Preservation of Thick, Transgressive Shallow-Marine Successions in a Forearc Basin; Nanaimo Basin, British Columbia

Abstract

Preservation of shallow-marine deposits is directly linked to the generation of accommodation space, net sedimentation rate, and paleo-topography. In low-accommodation settings, such as foreland basins, transgressive successions tend to be thin and are manifested as coarse-grained lags (10-50 cm thick). In settings where subsidence and relative base-level rise is rapid, preservation of thick transgressive successions is possible. The Cretaceous-aged Nanaimo Basin, British Columbia, Canada is interpreted as a forearc basin that developed when the Farallon and Kula plates were subducted beneath North America during the accretion of the Wrangellia composite terrane. The fill of the Nanaimo Basin reflects rapid subsidence and high sedimentation rates over a highly variable basal nonconformity, and thick transgressive deposits are commonly preserved. The first major transgressive unit is the Comox Formation. In most locations, the Comox Formation forms a conglomerate deposit (up to 2 m thick) that rapidly transitions upwards into offshore to shelf shales. However, where the Comox Formation is developed against a steep-slope of the underlying nonconformity, it is over-thickened and comprises foreshore through lower shoreface deposits that form successions up to 80 m thick. Younger strata unrelated to the nonconformity also comprise thick transgressive units (up to 40 m thick), with offshore to foreshore progradational sequences grading upwards into foreshore to offshore. The combined thickness of the shallow-marine regressive-transgressive sequence ranges up to 65 m thick with approximately 25 m for the regressive succession. The preservation of thick transgressive deposits in forearc basins suggests that exploration strategies derived for foreland basins (and other intracratonic basins) cannot be applied to those developed in basins on tectonically active margins (e.g. forearc, backarc, peripheral foreland basins). In particular, elevated subsidence rates, increased sedimentation rates due to proximity to source and rapid uplift, and significant paleo-topographic relief all impact preservation potential of shallow-marine strata, and hence have a significant influence on the size of geobodies. High sedimentation rates and rapid subsidence enable preservation of thicker reservoir units.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016