EXTREMELY HIGH RESOLUTION XRF CORE SCANNING REVEALS THE EARLY TRIASSIC EVOLUTION OF THE WESTERN PANGAEAN MARGIN

The Montney Formation is an extensive fine-grained sedimentary unit in Alberta and British Columbia. The distal parts of this formation, where deposition was continuous, represent a complete record of the Early Triassic, showing recovery from the effects of the end-Permian mass extinction along the subtropical western margin of Pangaea. A core of the distal Montney Fm. from northeastern British Columbia was scanned using an ITRAX x-ray fluorescence spectrometer, measuring 30 elements at centimeter-scale resolution. As the Montney Formation is ~400 meters thick at this site, this yielded over a million individual measurements. While these data are semiquantitative, the resolution of the dataset allows for detailed depositional history reconstruction. The reliability of the elemental data series as geochemical response variables was assessed, and elemental ratios were constructed using those with high signal-to-noise ratios. Principal components analysis was used to identify processes controlling element distributions.

The Montney depositional basin was euxinic during the Griesbachian substage, immediately following the end-Permian mass extinction, and this interval contains little to no carbonate. The rapidly eroding margin of North America provided immature sediment. The Dienerian provides evidence for early recovery, with oxic bottom water, a trend toward more mature sediment that may indicate landscape stability from terrestrial recovery, and the deposition of algal calcispheres and a Claraia biostrome. Euxinia and rapid erosion recurred in the mid-Smithian, with tempestites delivering bioclastic carbonate during the late Smithian. Coastal upwelling appears to have resumed in the basal Spathian, but 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. This may reflect the initiation of eastward dipping subduction along the western Pangaean margin. Restricted conditions, with euxinic, likely acidic bottom water, were interrupted by oxic intervals following marine flooding events, when the basin could exchange water with the open ocean. Restriction became less intense in the upper Spathian, and by the end of the Early Triassic algal calcispheres were deposited even during more restricted intervals.