Petrology and Provenance of Modern Sands from Cascade Range Forearc and Canadian Rocky Mountain Fold-Thrust Belt

Andrea G. Kretchmer, Raymond V. Ingersoll

The Cascade Range volcanic arc and forearc, and the Canadian Rocky Mountain fold-thrust belt represent the two sides of a continental margin arc-trench system. Sands from these areas show clear compositional differences. The most significant discriminating parameters are volcanic lithic grains, metamorphic lithic grains, plagioclase-to-feldspar ratio, and quartz. Variable sediment composition is also evident within each setting. Cascade sands are volcaniclastic and have high plagioclase-to-feldspar ratios. They divide into three categories (volcanic arc, alluvial forearc, and coastal forearc) that differ in their lithic contents and plagioclase-to-feldspar ratios. These changes reflect the attrition of volcanic lithics with distance from the arc and the input of recycled ediment and subduction-complex lithologies. Rocky Mountain sands are sedimenticlastic. They are of two types, a miogeocline-shelf provenance and a clastic-wedge provenance. These linear belts differ in clastic-carbonate content, plagioclase-to-feldspar ratio, and quartz content. The compositional differences reflect interstratified petrofacies of fold-thrust belts. Just as we can use detrital modes of modern sands to characterize provenance and tectonic setting, modes of ancient sandstones help us recognize provenance terranes and reconstruct paleotectonic settings.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.