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The Cretaceous lower Nanaimo Group in the Georgia Basin, Canada comprises multiple depositional phases with distinct depocentres that accumulated in a tectonically active forearc basin setting. Basal coarse-clastic strata are preserved in paleotopographic depressions and grade upwards into coal-bearing coastal plains and shallow-marine deposits. Coal-bearing and shallow-marine strata grade laterally into and are overlain by, regionally extensive mudstones and turbidites deposited in deep water. A glauconitic sandstone bed within the deep-water strata is interpreted as a condensed section and underlies a major disconformity that developed during a pause in the deposition of the lower Nanaimo Group. A second major coarse-clastic succession occurs hundreds of metres above the glauconite bed in the central Georgia Basin and comprises conglomerate, sandstone, mudstone and coal deposited in continental depositional environments. The shift in sedimentation from the northern Georgia Basin to the central Georgia Basin is interpreted to record the emergence of an island (Nanoose Uplift) in the central Georgia Basin that acted as a major sediment source to the adjacent depocentres. The stratigraphic break between the coal-bearing coarse-clastic strata in the northern Georgia Basin and the significantly younger coal-bearing coarse-clastic strata in the central Georgia Basin indicates that the lower Nanaimo Group was deposited in distinct depocentres. Between the older, coarse-clastic strata in the north and younger, coarse-clastic strata in the central Georgia Basin, we hypothesize that a major deepwater canyon system (Qualicum Canyon) existed and transferred sediment from the semi-restricted Georgia Basin to the Pacific Ocean to the west. Development of the Qualicum Canyon and exposure of the Nanoose Uplift during deposition of the younger, central coarse-clastic strata suggests that syntectonic activity drove basin uplift and erosion and this occurred throughout the deposition of the lower Nanaimo Group. [ABSTRACT FROM AUTHOR]

The lower Nanaimo Group was deposited in the (forearc) Georgia Basin, Canada and records the basin's initiation and early depositional evolution. Nanaimo Group strata are subdivided into 11 lithostratigraphic units, which are identified based on lithology, paleontology, texture and position relative to both the basal nonconformity and to each other. Significant topography on the basal nonconformity, however, has resulted in assignment of lithostratigraphic units that are not time correlative, and hence, cannot reliably be used to accurately reconstruct basin evolution. Herein, we present a sequence stratigraphic framework for lower Nanaimo Group strata in the Comox Sub‐Basin (northern Georgia Basin) that integrates both facies analysis and maximum depositional ages (MDAs) derived from detrital zircon. This stratigraphic framework is used to define significant sub‐basin‐wide surfaces that bound depositional units and record the evolution of the basin during its early stages of development. Seven distinct depositional phases are identified in the lower 700 m of the lower Nanaimo Group. Depositional phases are separated by marine flooding surfaces, regressive surfaces, or disconformities. The overall stratigraphy reflects net transgression manifested as an upwards transition from braided fluvial conglomerates to marine mudstones. Transgression was interrupted by periods of shoreline progradation, and both facies analysis and MDAs reveal a disconformity in the lowermost part of the Nanaimo Group in the Comox Sub‐Basin. Stratigraphic reconstruction of the Comox Sub‐Basin reveals two dominant depocenters (along depositional strike) for coarse clastics (sandstones and conglomerates) during early development of the Georgia Basin. The development and position of these depocenters is attributed to subduction/tectonism driving both subsidence in the north‐northwest and uplift in the central Comox Sub‐Basin. Our work confirms that in its earliest stages of development, the Georgia Basin evolved from an underfilled, ridged forearc basin that experienced slow and stepwise drowning to a shoal‐water ridged forearc basin that experienced rapid subsidence. We also propose that the Georgia Basin is a reasonable analogue for ridged forearc basins globally, as many ridged forearcs record similar depositional histories during their early evolution. [ABSTRACT FROM AUTHOR]