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Tectonic Controls on Deep-Water Sediment Routing – Nanaimo Basin, Canada

Abstract

The configuration of deep-water depositional system fairways are inherently linked to the tectonic setting of a basin. Tectonic signals within a deep-water sedimentary succession can be difficult to isolate, however. Outcropping strata of the Cretaceous Nanaimo Basin in western British Columbia, Canada are analyzed to deduce the stratigraphic expression of tectonic influence on sedimentation patterns in ancient turbidite systems. This work details the evolution of deep-water stratigraphy using results from field mapping and maximum depositional age calculations derived from a vast detrital zircon dataset. The Upper Cretaceous – Paleocene stratigraphy of the Upper Nanaimo Group represents a major sediment sink that was active during the final stages of Northern Cordillera assembly. A series of long-lived sediment routing systems deposited in a forearc setting are exposed in an outcrop belt 170 km long and up to 2 km thick. The basin fill records at least two phases of coarse-grained sediment routing. The first phase consists of a series of transverse-oriented channel-levee systems, which transported arc-derived sediment to the basin. After a period of basin reconfiguration, the second phase was dominated by a basin-axial sediment-dispersal system composed of unconfined to weakly confined lobes. The reorganization of the sediment routing systems only occurs in the southern portion of the basin, based on observations of paleocurrent indicators and depositional architecture. It is linked to uplift of the outer-arc high, which either: (1) confined and re-routed flows into a newly formed axial depositional system; or (2) caused a landward (eastward) shift in the axial depocenter. Notably, evidence for this basin reorganization is not preserved in the northern portion of the basin. We speculate that this along strike variation in sediment-routing system evolution is due to oblique convergence at the ocean-ward margin of the basin, driving northward growth and propagation of the outer-arc high. These observations provide a revised perspective of the tectonic setting in the Nanaimo Basin, and evidence for temporal shifts in deep-water stratigraphic architecture comparable to those observed in tectonically active basins globally.