Abstract

The paleolatitudinal distribution of bivalve rudists has important significance for the Baja British Columbia (Baja BC) hypothesis that western Canadian superterranes from British Columbia have been displaced 3000 km since Cretaceous time. Rudists are not observed in Baja BC sedimentary rocks, yet they are common in Late Cretaceous strata in California and Baja California, which have the same paleomagnetically determined paleolatitudes (approximately 25°N) as Baja BC rocks of Late Cretaceous age. In order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of Baja BC, paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the California margin. Compaction-corrected paleomagnetic data from the Peninsular Ranges and Salinia terranes indicate that rudists were restricted to paleolatitudes between 34° and 40°N. Evidence of coastal upwelling in the latest Cretaceous Marca Shale may explain the northern limit of the rudist distribution. These data suggest that Baja BC was no farther south than 40°N in the Late Cretaceous, thus limiting its post-Cretaceous displacement to less than 1500 km, and that burial compaction has also affected the paleomagnetism of Nanaimo Group sedimentary rocks from Vancouver Island. This result also helps resolve the conflict between paleomagnetic results, which show 1500 km of post–Late Cretaceous offset between the Insular–Coast Plutonic Complex superterrane and the Intermontane superterrane and geologic observations, which can allow only tens of kilometers of offset between these terranes in the Methow- Tyaughton basin.

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