Bio- and lithofacies of modern sediments in Knight and Bute inlets, British Columbia

Abstract

The high sedimentation rates that occur in fjords make them an ideal setting for studying sediment transfer from proximal first cycle sources. These environments, and their associated sediment transport processes, are good analogues to some small hydrocarbon-bearing basins. Knight and Bute Inlets, on Canada's west coast, are fords that display textural facies of recent sediments and distributions of benthonic foraminifera, thecamoebians and clay-size minerals that reflect freshwater discharge, bottom-water renewal and sediment gravity flow processes. In Knight and Bute, a fiord head prodelta mud facies shaped by fluvial depositional processes is defined by unimodal (5-60) sediments and by low-diversity arenaceous foraminifera assemblages that include a large proportion of displaced freshwater thecamoebian specimens. A comparatively distal basinal mud facies reflects a flocculation process that gives rise to a unimodal (80) sediment. The clay-size fraction of this basinal facies is enriched in quartz and feldspar compared to the prodelta mud. Arenaceous foraminifera predominate in upper and middle fiord basinal mud facies environments but are replaced by calcareous types near fiord mouths because of the seasonal incursion of marine water from the continental shelf The prodelta mud and basinal mud facies interfinger with a gravity flow and overspill transition facies. The grain size distributions of the transition facies includes types having 1.50 or 4-60 modes that reflect a combination offluvial, hemipelagic, and gravity flow processes. Channels cut by the downslope transport of coarse sediment as cohesionless mass flows focus deposition in distal basins. Inherent in this process is sediment bypassing and the passive transport of large numbers of thecamoebians to distal prodelta marine environments. Sediment on sills near the mouth of Knight and Bute Inlets are reworked morainal deposits facies that are presently being modified by tidal currents that also serve to concentrate indigenous calcareous foraminifera populations into foraminiferal lag deposits. INTRODUCTION