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The glaciovolcanic origin of Kima'Kho mountain, northern British Columbia Turnbull, Marie


Kima’Kho is one of several englacially erupted volcanoes located on the Tuya-Kawdy plateau in northwestern British Columbia. Evidence that Kima’Kho erupted into ice is recorded by lithofacies and stratigraphical features indicating deposition into a sustained but fluctuating englacial lake. Mapping of the edifice alongside analysis of an extensive sample suite, has elucidated the first in-depth model of physical growth for this glaciovolcano within the context of a reconstructed englacial lake. Four major stratigraphic units were mapped: lapilli tuffs (Lt₁/Lt₂), altered pillow bearing tuff breccias (Tb₁/Tb₂), basalt lavas (L₂/L₃/L₄), including pillow basalts (L₁), and basalt intrusions (IDS). This work also introduces new geochronological data and integrates stratigraphical relationships alongside geochemical data, to reconstruct volcanological evolution through time. ⁴⁰Ar/³⁹Ar data indicate Kima’Kho formed ~1980 ka with an unexpected late phase of intrusive activity. This is demonstrated by cross-cutting relationships, variation in petrography and a chemical evolution describing an outlying group of evolved dykes, distinct and separate from other stratigraphical units. Mapping of Kima’Kho has defined six points in stratigraphic time where the englacial lake level can be established during the growth of the volcanic edifice. The first lake level recorded is associated with the initial explosive phase of eruption which formed a pyroclastic tephra cone that hosts a ‘passage zone’ – an inferred surface delineating the transition between sub-aqueous and sub-aerial deposition. Dynamic fluctuations in lake level are recorded by two later passage zones exposed at lower elevations on the flanks of the cone and also within effusive units. Effusive eruption is represented by basaltic pillow tuff breccias and subaerial lavas. The three lattermost passage zones are marked by the contact between these tuff breccias and lavas and record a second significant decline in lake level followed by relative stability, at which point the effusive lavas were able to cap the brecciated deposits. The decryption of the stratigraphy to this complex ancient glaciovolcano provides many insights that cannot be recovered from modern glaciovolcanoes, which are obscured by their present-day ice fields. This study demonstrates the feasibility of reconstructing glacial conditions through the proxy of englacial lake fluctuations recorded in the glaciovolcanic deposits.

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