Frequency and initiation mechanisms of submarine slides on the Fraser Delta front




Stacey, Cooper D.

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The Fraser delta hosts a population of over 500,000 including the municipalities of Richmond and Delta and the Vancouver International Airport. The main arm of the Fraser River has been fixed in place by construction of a jetty focusing sediment deposition on the Sand Heads area. There is a history of submarine slide events at the delta crest which pose substantial risk to coastal infrastructure near the delta front. A submarine channel, characterized by prominent levee deposits, extends seaward from the Sand Heads area. In this study, sand beds in cores from levee overspill deposits are dated using excess 210Pb activity. They are interpreted as the downstream deposits of channelized turbidity currents generated by liquefied slide material. Sedimentation is characterized by sandy mud, interpreted to be deposited continuously by river plume suspension fall-out, and two distinct kinds of sand beds which represent two genetically different processes. The first type of sand bed (Facies 6) is thick, sharp based and clean, often showing classic Bouma turbidite elements including a massive sand base with laminated sands fining up to a mud top and is interpreted as the deposit from slides involving large volumes of material at the upper reaches of the tributary channels. The second type of sand bed (Facies 5) is characterized by muddy sand, has gradational contacts, and is interpreted as a low density deposit from either river generated turbidity currents or distal turbidites from smaller slide events. Facies 6 sand beds often occur as sets of 2 to 4 beds and individual bed sets have been dated to approximately the same ages of known large-scale slide events. Facies 5 sand beds occur more frequently and generally occur after periods with high flow. Sediment cores show three distinct phases of levee growth within the past 100 years approximately. A basal phase consists of very thick beds of medium sand that are interpreted to represent the early stage of channel-levee evolution when continuous overspill occurs during turbidity current events. The second stage is characterized by thick sets of frequent Facies 6 fine grained sand beds separated by less than one year of mud deposition. These sand beds are interpreted as representing a period of levee growth where channel relief is low and overspill events occur often. The third phase is characterized by thick mud intervals with less frequent fine sand beds. Phase 3 is interpreted to reflect a state when levee growth has increased channel relief to a height greater than that of the typical channelized turbidity current. In the third phase, sediment bypass is common and only larger density flows are capable of spilling onto the levees. Deposits interpreted to represent large slides have a return interval of 10 to 15 years during the past 40 years. Deposits of smaller events occur on average every four to five years. Event ages are compared to large spring floods from the Fraser River and seismic activity to determine any causal relationship. There is some relationship between ages of event beds and river flood years, but the largest sand beds do not correspond to unusually large flood years or seismic activity. It is concluded that there are likely a combination of factors which contribute to slope failure including over steepening and increased pore pressure.



Fraser Delta, Fraser River, Marine Geohazards, Sedimentology, Turbidity Current, Submarine Slides