IMPACTITES OF THE GOW LAKE IMPACT STRUCTURE, CANADA: A UNIQUE CRATER-FILL SEQUENCE
We focus here on the Gow Lake impact structure located in the Precambrian Shield of northern Saskatchewan (56o 27’ N, 104o 29’W). It was visited only once in the early 1970’s for a reconnaissance visit. Gow Lake is ~5 km in diameter and the country rocks are predominantly quartzofeldspathic gneisses. We conducted fieldwork at Gow Lake in July 2011, during which we were able to map an almost complete sequence of impactites from the fractured crater floor through the allochthonous crater-fill. From the crater floor upwards, this sequence is as follows:
1) Fractured basement: Quartzofeldspathic gneiss target rocks, ranging from slightly fractured to very heavily brecciated, in situ. No shock effects present.
2) Lithic impact breccia: Massive impact breccia with a clastic matrix and very rare shock effects (PDFs on one sample, PFs in two samples)
3) Clast-rich impact melt rock: Pink, massive unit with a groundmass dominated by k-feldspar and clasts commonly displaying PDFs and PFs. Clast content typically >25 %.
4) Brown/red clast poor-impact melt rock: Brown/red massive unit with a groundmass dominated by k-feldspar and clasts commonly displaying PDFs and PFs, with rare ballen siliar. Clast content typically <25 %.
5) Green clast poor-impact melt rock: Large, massive outcrops with crude to well-defined columnar jointing. Groundmass of plagioclase + k-feldspar. Common presence of PDFs, ballen silica, and diaplectic quartz.
6) Impact melt-bearing breccia: Occurs as lenses in green impact melt rocks and possibly on top of this unit. Clastic groundmass containing elongate cm-long pale yellow glass bodies.
In summary, exposed at Gow Lake is a relatively unique stratigraphic sequence through s small 5 km diameter impact structure. Despite its small size, close to ~40 m of impact melt rocks and melt-bearing breccias have been identified, providing constraints on melt volumes during impact events. There is a clear differentiation in the shock level of clastic material within different units.