Peter S. Mustard

Archean zircons in Cretaceous strata of the western Canadian Cordillera: The “Baja B.C.” hypothesis fails a “crucial test”

The magnitude of late Mesozoic terrane translation along the western North American continental margin is controversial. Paleomagnetic data suggest more than 3000 km of dextral displacement of the Insular superterrane and associated assemblages between ca. 74 and 50 Ma. Conversely, geologic evidence is more compatible with less than 500 km of offset during this time. Precambrian cratonic rocks older than 2.5 Ga are restricted to northern latitudes (above 40°N) in North America, and the presence or absence of Archean zircons within easterly derived Cretaceous strata has been proposed as a “crucial test” of the major translation hypothesis. There is no plausible source of Archean and Early Proterozoic zircons found in the Queen Charlotte, Nanaimo, and Methow basins other than the Canadian Shield or closely associated Proterozoic strata. The coexistence of Archean–Early Proterozoic, Middle Proterozoic (1.5–1.6 Ga) and Mississippian detrital zircon in these basins indicates deposition in northern latitudes, incompatible with the Baja B.C. hypothesis.

Oriented Clastic Dike Swarms as Indicators of Paleoslope? An Example from the Upper Cretaceous Nanaimo Group, Canada

ABSTRACT An impressive oriented clastic dike swarm (> 165 dikes) occurs within 3.25 m of strata of a submarine-fan turbidite succession of the Upper Cretaceous Nanaimo Group, British Columbia. The dikes are remarkably parallel in orientation, are consistently subvertical, and strike southwest. Slump-fold vergence, paleocurrent indicators, and other evidence indicate that local paleoslope strike was northwest, with a downslope direction to the southwest. Thus the clastic dikes strike downslope, indicating that extension occurred perpendicular to the regional downslope direction, not parallel as generally expected. Two possible models are proposed: (1) Rapid downslope compression caused by sudden loading, perhaps due to slumping events or deposition of thick turbidity current beds. This downslope compression caused minor extension perpendicular to the downslope direction, resulting in small extensional joints that accommodated clastic injection from buried, overpressured sands. (2) Chaotic sedimentary breccias laterally adjacent to the dike swarm formed as part of a large slumping event, which created a local slump channel. Minor slumping into the main channel from the sides caused extension parallel to the channel-margin slope and formation of the clastic dikes parallel to the channel margin but perpendicular to the regional slope. Oriented clastic dike swarms may represent a new type of paleoslope indicator, but they may be created by a number of processes and should therefore be used only in conjunction with supporting evidence.