Browsing by Author "Dence, Michael"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Investigation of impact melt in allochthonous crater-fill deposits of the Steen River impact structure, Alberta, Canada
    (2018) MacLagan, Ebberly A.; Walton, Erin L.; Herd, Christopher D. K.; Dence, Michael
    The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix-supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K2O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter-sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale-derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile-rich nature of the shale protolith. The wide compositional range of impact-melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile-rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed-target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).
  • Thumbnail Image
    Item
    Thermal metamorphic signature in melt-bearing polymict breccias from the Steen River Impact Structure, Canada
    (2015) Walton, Erin L.; Dence, Michael; Herd, Christopher D. K.
    Melt-bearing polymict breccia, similar to so called “crater suevite” described from Ries, has been documented from other small to medium size impact craters and those formed in mixed targets. This breccia type typically forms a volumetrically significant portion of allochthonous impactites; however, their mechanism(s) of formation is still debated, with recent studies calling on fuel-coolant interaction to explain certain characteristics. Detailed petrographic studies of these and related impactites can help to shed light on their genesis. In this study we describe a 127 m thick, remarkably uniform deposit of polymict breccia containing co-genetic impact melt clasts from the Steen River impact structure (SRIS) in NW Alberta, Canada.