Extremely high resolution XRF core scanning reveals the Early Triassic depositional history of the Montney Formation in northeastern British Columbia, Canada

Abstract

The Montney Formation is a laterally extensive, fine-grained sedimentary unit in western Alberta and northeastern British Columbia, and a major target of hydrocarbon exploration. It also retains a complete record of the Early Triassic epoch, a time when the biosphere was recovering from the effects of the end-Permian mass extinction. An industry well provides a full-diameter core of the complete distal Montney Formation in northeastern British Columbia (over 350 m thick), with stratigraphic age control provided by conodont biostratigraphy and regional correlations. For this study, the core was scanned using an ITRAX X-ray fluorescence spectrometer, at centimeter-scale resolution. The resulting dataset (over 37,000 individual measurements) provides an extremely high level of resolution for a deep-time geochemical study. While the data generated by this instrument are semiquantitative, they can be calibrated by correlating them with more conventional ICP-MS and XRF measurements of select samples. Using this method, we assessed the reliability of each of the elemental data series, and where possible converted them to quantitative geochemical measurements. These calibrated measurements serve as the basis for our detailed reconstruction of the Montney Formation's depositional history.The distal Montney depositional basin, including the location of the c-65-F core, was euxinic immediately following the end-Permian mass extinction, and accumulated sediments from the rapidly eroding margin of western North America. Conditions stabilized somewhat in the Dienerian, with oxic bottom water and a trend toward more compositionally mature sediment, but conditions of widespread euxinia and rapid erosion of the North American craton recurred in the early Smithian, earlier than this global crisis has been recognized elsewhere. Above the middle-upper Montney sequence boundary, in the overlying Spathian succession, local effects predominate. Coastal upwelling resumed in the basal Spathian, though the Montney depositional basin was soon cut off from the global ocean by uplift of the Yukon-Tanana arc bounding the basin to the west. Restricted conditions, with euxinic bottom water, were punctuated by oxic intervals, when rising sea level allowed the basin to freely exchange water with the open ocean. Intervals of restriction became progressively less acute in the upper Spathian.