Explosive glaciovolcanism at Cracked Mountain Volcano, Garibaldi Volcanic Belt, Canada

Cracked Mountain (CM) is a basaltic volcano within the northern extension of the Cascade volcanic arc into southwest, British Columbia, Canada (i.e. Garibaldi Volcanic Belt) and is dated at 401 ± 38 ka (40Ar/39Ar). The edifice covers an area of ~1.5 km2 and has a volume of 0.18 km3. The volcano features steep margins with local relief of ~250 m (1,650 m a.s.l), is dissected by abundant 0.5 to ~20 m wide extensional cracks with depths up to 30 m, and has a highly eroded top. The edifice is dominated by massive to poorly stratified, moderately to pervasively palagonitized lapilli tuffs, comprising vitric fine ash to lapilli. Juvenile pyroclasts have blocky to highly vesiculated shapes consistent with a phreatomagmatic (i.e. explosive) origin. The lapilli tuffs are intruded by and mingled with coherent to disaggregated lobes of peperitic pillowed lavas. Contacts between lapilli tuff and peperitic pillowed lava show soft-sediment deformation and in-situ quench-fragmentation indicating that the tephra was both unconsolidated and water-saturated at the time of intrusion. Local stacks of pillow lava are found on the margins of the edifice. More than fifty, 0.5 to 3 m wide dykes intrude CM stratigraphy and display, either, peperitic pillowed margins or sharp, chilled margins and columnar-jointing. Measurements of paleomagnetic directions (9 sites) were made for all CM lithofacies and record a single-pole direction indicating a single monogenetic eruption. A glaciovolcanic origin is strongly suggested by the abundant subaqueous lithofacies (i.e. palagonitized tephra, pillows, peperites) and the physiographic setting of the edifice which is well above any drainage that could sustain a standing body of water. The elevation of CM and depth of surrounding valleys indicate syn-eruptive confinement by a paleo-ice sheet that was ≥850 m thick. The edifice shape, size, and surrounding topography suggest a ‘leaky’ paleolake system capable of supporting ~0.36 km3 of water. The glaciovolcanic origin and the absolute (40Ar/39Ar) age of CM represent an important record of the Cordilleran Ice sheet (CIS) in southwestern BC during the mid-Pleistocene.