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Abstract:

The Nanaimo Group of southwest British Columbia overlies the Wrangellia terrain and the western Coast Belt and is in fault contact with the northwestern margin of the Cascades to the southeast. Generally interpreted as deposits of a Late Cretaceous forearc basin, a foreland basin model is preferred for the Nanaimo Group, in large part due to the recent recognition of major, westerly-directed thrust systems in the Coast Belt to the east and northwestern Cascades, coupled with new age constraints which indicate that thrusting in part overlaps with Nanaimo Group sedimentation. As a test of the foreland basin model, U-Pb ages of twenty-two detrital zircons from three formations of the Nanaimo Group provide new evidence about changing source areas with time for Nanaimo Group deposition. Zircons from the lower Campanian Extension and Protection Formations indicate that Coast Belt and San Juan thrust systems were the dominant source areas, and Wrangellia was not a major source of detritus. Submarine fan sandstone of the uppermost Gabriola Formation (Maastrichtian age) contains abundant detrital titanite, epidote, and zircon with ages as follows: Precambrian zircons, zircons of late Mesozoic age with Precambrian inheritance, concordant 87 Ma zircons, and a predominant 72–73 Ma population of zircons. These results indicate derivation from varied sources which may include the eastern Coast Belt (87 Ma), Paleozoic or late Precambrian sedimentary rocks (recycled Precambrian zircons), and a combination of the Idaho Batholith and Omineca Belt plutons (72–73 Ma grains and grains with inheritance). The Idaho Batholith as a possible source is consistent with the position of the basin prior to latest Cretaceous-(?)-Eocene transcurrent dextral faulting. Thus, the source areas for upper Nanaimo Group sediments were considerably more widespread than previously believed, suggesting that major fluvial drainage systems were active in the western Cordillera during the late Maastrichtian.

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