Abstract
Upper Devonian-Lower Mississippian siliciclastic miogeoclinal strata in the southern Canadian Rocky Mountains provide a partial record of the geographic and tectonic setting of the Paleozoic western margin. Although these marine strata (Exshaw and lower Banff formations) contain extensive shale and mudstone, and some quartzofeldspathic sandstone and siltstone, as well as volcanic ash layers, the provenance of these clastic units has, thus far, remained poorly or incompletely understood. The integration of sandstone petrofacies analysis with neodymium isotopic and trace-element (rare earth element, Th, and Sc) analyses of mudrock facies was undertaken in this study to constrain better the sedimentary provenance.
Coarse siltstones and sandstones in the Exshaw and Banff formations are chiefly subfeldspathic and feldspathic (quartzose-arkosic) arenites, with a complex sedimentary history involving derivation, at least in part, from an uplifted plutonic and volcanic source, such as possible western "Antler" orogenic highlands. The presence of monocrystalline quartz (inclusionless and with unit extinction) and sanidine in Exshaw sandstones at some sections suggests at least a partial acidic volcanic or shallow intrusive source component. The Exshaw and Banff petrofacies reflect the possible combined effects of regional tectonic complexity (different and/or mixed source terranes), sediment transport and recycling, complex sediment dispersal paths, and weathering and diagenesis. Mudrock elemental and isotope geochemistry also support a provenance involving potentially multiple sources (mixed provenance). Chondrite-normalized REE distribution patterns and La-Th-Sc ratios for Exshaw and Banff mudrocks resemble patterns for sediments derived from old continental crust (e.g., passive margins, etc.) as well as some arc settings. Nd isotopic compositions of the Exshaw and Banff formations do not support a specific provenance, but they are consistent with addition of material younger than Archean in age.
