GEOLOGICAL SIGNATURE OF MARINE GEOHAZARDS IN COASTAL NORTHWEST BRITISH COLUMBIA

Recent work in the vicinity of Douglas Channel, British Columbia, Canada, regarding timing and distribution of palaeotsunami deposits has given us the opportunity to better appreciate the unique conditions of Holocene “event” preservation in rugged, bedrock dominated depositional environments. Our search for tsunami deposits was significantly influenced by the dynamic nature of high energy conditions prevalent along the Pacific Northwest coast. The coves, bays, lagoons, channels, inlets and fjords of north coastal British Columbia rarely support sufficient soft sediment accumulations capable of preserving a geological record of activity during the Holocene. Moreover, those rare deposits that contain measurable amounts of unconsolidated sediment are usually no more than a few centuries old and typically reflect short-term fluvial accretion on fans and deltas; remobilization of older deposits by landslides; monotonous gravel beach accumulations from tidal wave action; or reworking of marine and fluvial sediments during storm surges. The incidence of tsunami-laid deposition is extremely rare and not always physically visible. Only a few soil and peat deposits provided access to features of palaeotsunami deposition. These deposits differ from storm and flood features by their position away from fluvial inputs; significant distance landward beyond storm-wave berms; laterally extensive distribution; distinct upper and lower horizontal contacts; and textural composition that reflect adjacent deeper water sediment conditions. In contrast, flood deposits are typically stratified and dip seaward, whereas storm deposits are generally discontinuous and stratigraphically feathered in appearance with terrestrial soils and intertidal sediments. Herein, we provide examples of the types of Holocene event features that would be typically encountered in bedrock-dominated high energy environments in coastal northwest British Columbia. We contrast the sedimentological and stratigraphic characteristics of these features to better emphasize the need for careful study and discrimination in this type of research study.