Extension of lithofacies and conodont biofacies models of Late Devonian to Early Carboniferous carbonate ramp and black shale systems, southern Canadian Rocky Mountains

Abstract

Uppermost Devonian and Lower Mississippian strata in the Rocky Mountains of southwestern Canada and northwestern Montana record widespread oceanographic changes during middle to late Paleozoic time associated with the termination of a carbonate ramp system, the onset of a deep-water, low-oxygen event and possible marginal tectonism, and the later reestablishment of a carbonate ramp. Integrated lithofacies and conodont biofacies developed previously for these strata between the Bow Valley and the international border have been extended northward to the Athabasca region of the Alberta Rocky Mountains. During early-middle Famennian time, the southern Canadian Rocky Mountains region was the site of a westward-deepening and westward-thickening carbonate ramp system (Palliser Formation). By late Famennian time carbonate ramp deposition ended and was followed by widespread deposition of organic-rich, low-oxygen facies in shelf to basinal environments (Exshaw Formation and correlative units). The overlying Banff Formation consists of anaerobic to marginally aerobic, starved-basin to deep-ramp lithofacies succeeded by shallower water carbonates; this sequence records basinward (westward) progradation of the Banff ramp in middle to late Tournaisian time. Distinct conodont biofacies representative of shallow-ramp to deep-basin settings that were previously recognized in the southernmost Canadian Rocky Mountains and Montana have also been identified to the north between the North Saskatchewan and Athabasca valleys. Upper Palliser carbonates contain low-diversity conodont faunas of indigenous to transported palmatolepid-, polygnathid-, and apatognathid-dominated assemblages. Exshaw deposits contain indigenous and reworked palmatolepid- and bispathodid-dominated assemblages and reworked or transported polygnathids. Lower Banff biofacies include transported and indigenous assemblages of siphonodellids, polygnathids, and pseudopolygnathids representative of the deep-middle Banff ramp. Polygnathid-hindeodid biofacies of shallower middle-ramp environments occur higher in the Banff Formation in the North Saskatchewan and Athabasca valleys.