Sedimentary record of Devonian-Mississippian carbonate and black shale systems, southernmost Canadian Rockies and adjacent Montana: Facies and processes

Age-environment relationships and depositional conditions of uppermost Devonian and Lower Mississippian strata in the southernmost Canadian Rockies were evaluated. During early and middle Famennian time, the region was the site of a westward-deepening carbonate ramp that was bordered to the west by a deep basin. The middle-late Famennian exhibited the termination of carbonate ramp sedimentation and the ultimate deposition of organic-rich sediments under oxygen-stressed conditions is shelfal to basinal environments (Exshaw Formation). Widespread oxygen deficiency in the late Famennian accompanied an aerially extensive transgression. Sedimentation of anaerobic to marginally aerobic, deep-water, lower Banff facies occurred intermittently into the middle Tournaisian, and locally into the late Tournaisian prior to the westward progradation of ramp carbonates of the middle and upper Banff. Deep-water units include sequences of biogenic chert, black and gray shale, phosphatic and siliceous mudstone, and glauconitic and phosphatic clastics. The upper Banff bioclastic carbonates contain late Tournaisian bactrognathid-hindeodid faunas indicative of shallower ramp/shelf settings. Quartzofeldspathic clastics in the Exshaw and Banff reflect the periodic influx of detritus from a likely western orogenic source from the late Famennian into the late Tournaisian. Degree of bottom water oxygenation during deposition of the low-oxygen facies of the Exshaw and Banff formations was recognized using ichnofabric, organic carbon and sulfur relationships, and degree of pyritization (DOP). Anaerobic, laminated, and amorphinite-rich 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. A framework 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 the late Famennian transgression. Within this framework, the complex interaction of factors such as increased shelf area, promotion of organic productivity, and relative sea level changes controlled the environmental gradients and distribution of low-oxygen facies. (Abstract shortened with permission of author.