ABSTRACT: Foraminiferal biofacies and Holocene sediments from Saanich Inlet, British Columbia: Implications for environmental and neotectonic research

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This dissertation has been divided into three separate but related parts. The first describes the Saanich Inlet setting and gives a brief review of possible sedimentary processes in a submarine environment, the second establishes the environmental significance of foraminiferal biofacies from Saanich Inlet and the third, a paleoseismic study, describes and interprets sediments from cores collected in the central part of the basin. Foraminiferal analyses conducted on certain units in the paleoseismic study helped determine their origin. In a reconnaissance study, foraminiferal biofacies identified in Saanich Inlet appear to be closely linked to a variety of environmental parameters including water quality. Five biofacies were defined based on Q-mode cluster analysis and faunal distribution profiles of foraminifera-bearing sediment surface samples from Saanich Inlet. Biofacies 1 (Eggerella advena Biofacies), is found nearshore in two densely populated bays. This assemblage appears to have an affinity to areas contaminated by sewage outfall and septic system drainage. Biofacies 2 (Eggerella advena-Spiroplectammina biformis Biofacies) and 3 (Miliammina fusca Biofacies) characterize shallow, brackish waters, and are distributed in shallow bays near Biofacies 1. Biofacies 4 (Lobatula fletcheri Biofacies), the only biofacies dominated by a calcareous fauna, has been subdivided into two subbiofacies: Sub-biofacies 4A (Stainforthia feylingi Sub-biofacies) and 4B (Buccella frigida Sub-biofacies). Sub-biofacies 4A is found in deep water, low oxygen environments and Sub-biofacies 4B characterizes shallow water, normal marine evironments. The patchy distribution of Sub-biofacies 4B samples is probably due to vagaries of water circulation in the restricted basin. Biofacies 5 (Leptohalysis catella-Spiroplectammina biformis Biofacies) defines a relatively deeper muddy environment with a high proportion of plant debris and low oxygen levels. The main environmental control is restricted water circulation which is guided by the shape of the basin, i.e., presence of the sill. Eight sediment piston cores spanning the last 1500 years were collected from Saanich Inlet to obtain information on sedimentation and prehistoric earthquake activity. The cores consist mainly of silty clay varved sediments and massive beds deposited by debris flows. The debris flows may have been triggered by earthquakes or by the build-up of fine sediment on the walls of the inlet. Cesium-137 and 210Pb data, 14C ages, and varve counts were used to correlate massive layers in the eight cores. The uppermost massive layer in two cores may record a magnitude 7.2 earthquake in 1946 near Comox, B.C., 200 km north-northwest of Saanich Inlet. Five older layers, found in two or more cores are about 200, 550, 800-850, 1050-1100, and 1100-1150 years old. Some of these ages correspond to the times of documented seismic events. There is an average of one massive layer per 116 varves range of 15-336) in the core with the greatest number of such layers. These results are broadly consistent with the expected periodicity of moderate to large earthquakes in the region (on average, one earthquake of local Modified Mercalli Intensity VII r VIII per 100 years). Saanich Inlet may contain a proxy record of all moderate and large earthquakes that have affected southwestern British Columbia during Holocene time, but the set of massive layers likely includes both seismically and nonseismically generated deposits.

Bibliography of Canadian Geomorphology