No abstract

The Nanaimo Group comprises up to 4 km of sedimentary rock of Turonian to Maastrichtian age, forming the lower part of the Late Cretaceous to Neogene Georgia Basin of southwest British Columbia. This Upper Cretaceous succession was deposited in a single elongate basin deformed by Eocene compression into a fold and thrust belt. Eleven formations are recognized, comprising conformable and laterally intertonguing successions with sandstone-conglomerate units separated by mudstone and fine grained sandstone formations. Initial alluvial and coastal marine deposits formed on a rugged unconformity. Coal-bearing facies formed in coastal and marginal marine back-barrier environments, associated with fluvial and shallow marine facies. Most of the Nanaimo Group was deposited in marine, generally outer neritic to bathyal depths, by gravity flows and generally as submarine fan deposystems. Initial detritus was from local basement, but most sediment came from the Coast Belt to the east and northwest Cascades, although by latest Cretaceous time the eastern Cordillera was also a source. A forearc basin setting for the Nanaimo Group is only correct in that deposition occurred oceanward of a partly coeval magmatic arc. A foreland basin model is preferred because basin initiation and sedimentation was a direct result of contemporaneous thrusting in the Coast Belt and north Cascades. Nanaimo Group coal resources were historically important, but are now exhausted in most areas. Kaolin-rich deposits on the unconformity may be economically viable. Oil and gas potential is poor, although coalbed methane could be locally present.

The Upper Cretaceous Nanaimo Group of southwest British Columbia is a >4 km-thick succession consisting mostly of deep marine siliciclastics deposited directly on the Insular Superterrane. As such, this succession has been the focus of several paleomagnetic, isotope geochemistry, paleontology, and sedimentology studies in attempts to elucidate the tectonic history and paleolatitude of the Insular Superterrane and associated entities during the critical time of Nanaimo Group deposition (ca. 90–65 Ma). However, disagreement as to whether deposition occurred into a single or multiple basins has led to confusion concerning the formal stratigraphy and formation names for the succession, and has resulted in problems with both local and regional correlations. The upper two-thirds of the succession is continuously and well exposed on Denman and Hornby islands and represents the best example of this part of the succession in the northern half of what we consider the single Nanaimo Basin. This area includes the previously only informally defined type areas for the Geoffrey and Spray formations, defined here formally for the first time with type sections and detailed descriptions. New interpretations of the geology of these islands demonstrate that previously interpreted major faults do not exist, resulting in stratigraphic and age controls that are both different and simpler than previously interpreted. The redefined stratigraphy of the northern part of the basin is remarkably similar to that of southern areas in both type and age, affirming both a single basin evolution and a single stratigraphic nomenclature.

The Baja B.C. model has the Insular Superterrane and related entities of the Canadian Cordillera subject to >3000 km of northward displacement with respect to cratonic North America from ~90 to ~50 Ma. The Upper Cretaceous Nanaimo Group (on and about Vancouver Island, British Columbia) is a prime target to test the model paleomagnetically because of its locality and age. We have widely sampled the basin (67 sites from seven islands spread over 150 km, Santonian to Maastrichtian age). Most samples have low unblocking temperatures (<450°C) and coercivities (~10 mT) and strong present-field contamination, forcing us to reject three quarters of the collection. Beds are insufficiently tilted to provide a conclusive fold test, and we see evidence of relative vertical axis rotations. However, inclination-only analysis indicates pretilting remanence is preserved for many samples. Both polarities are observed, and reversals correlate well to paleontological data, proving that primary remanence is observed. The mean inclination, 55 ± 3°, is 13 ± 4° steeper than previously published results. Our new paleolatitude, 35.7 ± 2.6° is identical to that determined from the slightly older Silverquick and Powell Creek formations at Mount Tatlow, yet the inferred displacement is smaller (2300 ± 400 km versus 3000 ± 500 km) because North America was drifting southward starting around 90 Ma. The interpreted paleolatitude conflicts with sedimentologic and paleontologic evidence that the Nanaimo Basin was deposited near its present northern position.

Geologic and paleomagnetic data lead to two contradictory hypotheses regarding the paleoposition of the Insular and Intermontane Superterranes that presently constitute the western Canadian Cordillera. Paleomagnetic data from the Insular and Intermontane superterranes suggest a southerly origin coinciding with the latitude of Mexico and the northwest United States, respectively, during the mid‐Cretaceous. Geologic evidence points to a northerly origin for these same tectonic entities during this period; both models cannot be correct. Geologic and paleomagnetic data from the Empire Valley–Churn Creek area in south central British Columbia (51.5°N, 122.5°W) are critical to resolving these contradictory hypotheses. Late Cretaceous rocks correlated to the Insular Superterrane with large paleomagnetic displacements unconformably overlie mid‐Cretaceous rocks correlative to the Spences Bridge Group of the Intermontane Superterrane. We provide paleomagnetic evidence of this correlation based on similar magnetic properties, opaque mineral assemblages, demagnetization behavior, fold test results, mean inclinations, clockwise vertical axes rotations, and statistically indistinguishable paleomagnetic poles and displacement estimates. This correlation and the observed geologic relationships in the Empire Valley–Churn Creek area indicate that the Insular and Intermontane Superterranes were linked by the mid‐Cretaceous. Sites from the two previous Spences Bridge Group studies are combined with their correlatives in the Empire Valley–Churn Creek area to give 81 sites that yield a paleomagnetic pole of 60.5°N, 304.5°E, dp = 3.7°, dm = 5.5° which corresponds to 1050 ± 450 km of displacement from the south. This new displacement estimate suggests that the Spences Bridge arc formed at the latitude of southern Oregon during the mid‐Cretaceous.