GEOLOGY AND VOLCANOLOGY OF KIMA’KHO MOUNTAIN, NORTHERN BRITISH COLUMBIA: A PLEISTOCENE GLACIOVOLCANIC EDIFICE

Kima’Kho Mountain is a 1.8 Ma (⁴⁰Ar/³⁹Ar of 1.82 +/- 40 ka) Pleistocene an alkali-olivine basaltic tuya situated in northern British Columbia. The highly-dissected volcanic edifice rises 460 m from its base and comprises: a central vent, dominated by lapilli-tuff and minor pillow lava and dykes; and a lower elevation surrounding plateau underlain by an extensive sequence of dipping (up to 30^o) beds of basaltic tuff-breccia and capped bymore than 11 flat-lying, subaerial lava flows. We present a 1:10,000 geological map for Kima’Kho Mountain that builds on the preliminary work of Ryane et al. (2010) and use the volcanic stratigraphy to explore the implications of three unique features of this glaciovolcano. (1) The central cone comprises massive to crudely-bedded lapilli tuffs that contain abundant armoured lapilli featuring a single highly-vesicular pyroclast core coated with blocky to cuspate vitric ash fragments. These units suggest an explosive origin from within an ice-enclosed lake, and deposited by wet, dilute pyroclastic surge events. (2) The entire stratigraphic sequence hosts at least two "passage zones" (cf. Jones, 1969); the presence and geometry of these passage zones constrain ice thicknersses at the time of eruption and inform on the englacial lake dynamics. (3) Lastly, our field-based stratigraphic relationships are at odds with the classic tuya model (i.e. an effusive onset to the eruption, forming pillow basalts, followed by explosive activity). Our field mapping suggests an alternative model of tuya architecture, involving a highly-energetic, sustained explosive onset creating a tephra cone that become emergent followed by effusive eruption to create lavas and a subaqueous lava-fed delta.

Jones, J. G. Intraglacial volcanoes of the Laugarvatn region, south-west Iceland-I. Geological Society of London Quarterly Journal 124, 197-211 (1969).

Ryane, C., Edwards, B. R. & Russell, J. K. The volcanic stratigraphy of Kima'Kho Mountain: A Pleistocene tuya, northwestern British Columbia. Geological Survey of Canada, Current Research 2011-104, 12p, doi:10.4095/289196 (2011).