Environmental record of Devonian-Mississippian carbonate and low-oxygen facies transitions, southernmost Canadian Rocky Mountains and northwesternmost Montana

Abstract

Uppermost Devonian and Lower Mississippian strata in the southernmost Canadian Rocky Mountains and adjacent Montana record part of a widespread low-oxygen episode in middle Paleozoic epicontinental and shelf seas and provide critical information concerning the oceanographic setting of the western continental margin of Euramerica at this time. During early-middle Famennian time, the region was the site of a westward-deepening carbonate ramp (Palliser Formation) that was bordered to the west by a shale basin (Lussier region). Sedimentation patterns changed in middle-late Famennian time with the termination of carbonate ramp sedimentation and ultimate deposition of organicrich sediments in an oxygen-stressed, deep-water setting (Exshaw Formation). A model for the widespread deposition of these sediments entails the initial flooding of epicontinental/shelf seas with water derived from an expanded and intensified oxygen-minimum zone during a late Famennian transgression. Sedimentation of anaerobic to marginally aerobic, deep-water, lower Banff facies occurred into middle Tournaisian and, locally, late Tournaisian time prior to the westward progradation of carbonate ramp sediments of the middle and upper Banff. Quartzofeld-spathic clastic rocks in the Exshaw and Banff reflect the periodic influx of detritus from a likely western (Antler-age) orogenic source from late Famennian into late Tournaisian time. Degree of bottom-water oxygenation during deposition of the Exshaw and Banff low-oxygen facies was recognized using ichno-fabric, organic C and S relationships and degree of pyritization (DOP). Anaerobic, laminated black shales with high DOP values lack a macrobenthos, with the notable exception of rare concentrations of inarticulate brachiopods on bedding planes. Dysaerobic to marginally aerobic facies have intermediate DOP values and a gradient of increasing degree of bioturbation.