Abstract

The northern margin of the early Middle Jurassic Bowser Basin in British Columbia contains large, solitary shelf-type Gilbert deltas and smaller, multistorey Gilbert deltas stacked against inferred active reverse faults. Analysis of the deltas provides evidence of tectonic structures that can no longer be directly observed.

Bowser Basin developed as a foredeep in response to overthrusting of Stikinia by Cache Creek Terrane (oceanic crust) during accretion of Stikinia to the western margin of North America. The terrane boundary is currently represented by the King Salmon and associated thrust faults.

The shelf-type Gilbert deltas consist of accretional conglomerate packages up to 100 m thick, that are overlain by and laterally associated with cyclothemic shelf facies. Gilbert deltas responded to low accommodation to sediment supply ratios by prograding to the shelf break of the basin, where they fed large volumes of gravel to slope gullies and the deep basin beyond. At one location, fault-block rotation resulted in oversteepening of foresets and internal discordances. Deposition was dominated by non-cohesive debris flows (50% to 60% of foresets) and intervening muddy debris flows. The coarse-grained sediments are texturally and compositionally mature, consisting predominantly of spherical radiolarian chert clasts derived from the overthrust Cache Creek accretionary prism.

A spectacular succession at least 700 m thick near Cartmel Lake records the aggradational stacking of 30 small Gilbert deltas. Like the shelf-type Gilbert deltas, sediment gravity flows predominated. Creation of accommodation space for accumulation of the Gilbert delta stack was repeated many times; the locus of accommodation remained in much the same place over time. This implies a high degree of tectonic control over the geometry of the stacked Gilbert deltas. Accommodation to sediment supply ratios for this kind of Gilbert delta were high.

Based on their thickness, geometry, and general association with shelf lithofacies, Bowser Basin shelf-type Gilbert deltas probably required relatively large drainage catchments, such as those associated with thrust ramps, fault transfer zones or blind thrusts. In one example, delta progradation direction was at a low angle (15 –27°) to the main structural trends and in this particular case we favour delta accumulation at a thrust fault transfer zone. In contrast, the stacked Gilbert deltas are inferred to be footwall deltas that accumulated close to an active, reverse fault where the stacking geometry indicates little or no basinward shift in the locus of deposition. Drainage basins were relatively small and were frequently rejuvenated.

The Bowser Basin Gilbert deltas accumulated at the margin of a foreland thrust wedge, outboard of a major Early to Middle Jurassic, crustal-scale thrust system that carried oceanic Cache Creek terrane rocks in its hanging wall. Coarse-grained sediment, derived either from northwards erosional back-stepping of King Salmon Fault or from older Bowser Basin sediment, was cannibalized and redeposited in the Gilbert deltas and associated lithofacies. Either case provides a mechanism to account for the reworking necessary to produce the high clast sphericity and the monomict chert composition.