Englacial lake dynamics within a Pleistocene Cordilleran ice sheet at
Kima Kho tuya (British Columbia)
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) features multiple and
diverse passage zones. The basaltic volcano comprises 4 main
stratigraphic packages: i) subaqueously and subaerially deposited
lapilli tuffs 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, iii) stacked sheets of subaerial lavas , 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 3 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 is expressed in two different ways indicating a
transition 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
ice sheet to a minimum thickness of ~400 m and a minimum
radial extent >7 km relative to Kima Kho. Two subsequent
passage zones, 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 passage zones suggests a
massive, catastrophic deluge (i.e. jökulhlaup) of 1-2 km3. Lastly, the
reconstructed evolution of Kima Kho demands the presence of a regionally
extensive, cold-based ice sheet on the Kawdy plateau at
~1.9 Ma.