Palynostratigraphy, lithostratigraphy and thermal maturity of the Lower Triassic Toad and Grayling, and Montney formations of western Canada, and comparisons with coeval rocks of the Sverdrup Basin, Nunavut

Abstract

Abstract In western Canada samples obtained from outcrops of the Lower Triassic Toad and Grayling formations and core samples from coeval subsurface sections of the Montney Formation contain well-preserved palynofloras. These are generally dominated by taeniate bisaccate (e.g. Lunatisporites noviaulensis) and polyplicate gymnosperm pollen (e.g. Ephedripites steevesiae). Acanthomorph acritarchs (e.g. Filisphaeridium sp. cf. F. deunffii, Micrhystridium breve, M. setasessitante, Wilsonastrum colonicum) are often common to abundant. In addition, the stratigraphically and environmentally significant alga(?) Chordecystia chalasta occurs. These western Canadian Lower Triassic assemblages correlate with the Griesbachian Chordecystia chalasta–Striatoabieites richteri Assemblage Zone of the Sverdrup Basin, Canadian Arctic Archipelago, Nunavut. The environment of deposition, based on the abundance of pollen and spores and common occurrence of acanthomorph acritarchs, is near-shore marine. At a few horizons in the Montney Formation acritarchs are dominant, and this may indicate deeper water facies. Trace fossils in core and outcrop, lithofacies and sedimentary structures indicate fully marine, proximal offshore to lower and upper shoreface settings. The paleoclimate at the site of plant growth, based on the dominance of taeniate bisaccate pollen and polyplicate pollen, was probably arid. The Lower Triassic samples commonly include reworked Middle and Upper Devonian spores and less common Lower Carboniferous spores, demonstrating that significant erosion of Paleozoic rocks took place in early Triassic time in this area; recognition of such erosional events is relevant to sequence stratigraphic reconstructions. The thermal maturity of the in situ organic matter in the study areas is generally low (close to the threshold or within the oil window). This, along with the abundant amorphous and common exinous material, suggests a potential for the generation of liquid hydrocarbons. At one locality in southeast Yukon within the deformation belt the thermal maturity is high, suggesting some potential for dry gas generation. Reworked material generally has a slightly higher maturity, a fact that should be taken into consideration when assessing other thermal maturity indicators such as vitrinite reflectance.