Englacial lake dynamics within a Pleistocene cordilleran ice sheet at Kima' Kho tuya (British Columbia, Canada)

Abstract

Passage zones are stratigraphic surfaces found in littoral settings separating deposits diagnostic of subaqueous environments from overlying sequences of subaerial deposits. In glaciovolcanic settings, passage zone surfaces are unequivocal records of the heights and depths of paleo-englacial lakes at a specific point in time and space, thereby, informing on the presence and nature of the enclosing ice sheet. Kima' Kho, a Pleistocene glaciovolcano (i.e. tuya) located in the northern Cordillera of British Columbia, features multiple and diverse passage zones. The basaltic volcano comprises four main stratigraphic packages: i) subaqueously and subaerially deposited lapilli tuffs (Lt1/Lt2) forming a central tephra cone and representing an explosive onset to the eruption, ii) subaqueously deposited, steeply-inclined beds of tuff breccia dominated by pillow lava fragments (Tb1-3), iii) stacked sheets of subaerial pahoehoe lavas (L1-3), and iv) dykes and sills (I) intruding all units. Stratigraphic and geochemical relationships suggest that Kima' Kho volcanism was continuous and 40Ar/39Ar geochronometry on three samples yields a mean age of 1949 ± 63 ka. Three temporally distinct passage zones record the interplay between growth of the volcanic edifice, syn-volcanic melting of the enclosing ice sheet, and fluctuations in the depth of the englacial lake. The earliest passage zone (PZ1) is expressed in two different ways marking a transition from explosive to effusive eruption: (i) within pyroclastic deposits of the tephra cone (<1800 masl), and (ii) by pillow lava tuff breccia deposits overlain by subaerial lavas. Together they record a peak, sustained lake depth of 320–340 m that constrains the enclosing Cordilleran ice sheet to a minimum thickness of ∼400 m. Two subsequent passage zones (PZ2-3), also defined by sequences of subaerial lavas resting on dipping beds of pillow lava tuff breccias, occur at lower elevations: 1690-1640 masl and 1740-1720 masl, respectively. The latter two passage zones indicate a major draining of the englacial lake followed by refilling to depths of 230–180 m and 260–280 m, respectively. The substantial decline in lake level between PZ1 and PZ2 suggests a massive, catastrophic deluge (i.e. jökulhlaup) of 1–2 km3. Lastly, the reconstructed glaciovolcanic evolution of Kima' Kho demands the presence of a regionally extensive ice sheet overlying the Kawdy plateau at ∼1.9 Ma and representing an earlier incarnation of the Cordilleran Ice Sheet (CIS).