ABSTRACT: Stratigraphy and paleoseismic implications of cores from Saanich Inlet, British Columbia.

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Abstract

Hydraulic piston cores up to 118 m long were collected at two sites in Saanich Inlet, British Columbia, in August 1996 during Ocean Drilling Program (ODP) Leg 169S. The cores provide a detailed record of sedimentation in the inlet over the last 15,000 years. The ODP cores are missing about 1-2 m of water-richsediment directly underlying the seafloor, but this sediment is preserved in shorter piston cores which were collected in 1989 and 1991 to study late Holocene earthquake activity in southwestern British Columbia. The two sets of cores overlap, thus the relatively precise chronology established for the shortcores could be extended to the ODP cores. Sediments in the ODP cores include, from youngest to oldest, three major units: (1) rhythmically laminated (varved) marine mud with intercalated massive beds, interpreted to be debris flow deposits; (2) indistinctly laminated, bioturbated marine mud; and (3) glaciomarine mud. Two conspicuous marker beds occur within the indistinctly laminated mud unit: Mazama volcanic ash, about 7500 years old; and a grey silty clay bed, about 11,000 years old. Some debris flow deposits in the laminated mud unit are linked to past earthquakes, including the 1946 Vancouver Island earthquake (M7.2), a great (M8-9) plate-boundary earthquake in 1700, and a large crustal or plate-boundary earthquake about 1000 years ago. Earthquakes may also be responsible for debris about 400 years ago and between 1980 and 2550 years ago. Other debris flow deposits may also have a seismic origin, although nonseismic triggers cannot be ruled out. As a rule of thumb, an extensive debris flow deposit produced by a single large failure or a series of smaller, coincident failures probably was triggered by an earthquake.

Bibliography of Canadian Geomorphology