The Mackenzie Delta: Sedimentary processes and facies of a high-latitude, fine-grained delta

Abstract

The Mackenzie Delta is a large fine-grained delta deposited in a cold arctic setting. The delta has been constructed upon a flooding surface developed on a previous shelf-phase delta. There are three principal depositional zones: the subaerial delta plain, the distributary channel mouth region and the subaqeous delta. The subaerial delta plain is characterized by an anastomosing system of high-sinuosity channels and extensive thermokarst lake development. This region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels, ice-jamming and flooding in the early spring and declining river stage during the summer and autumn. Deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable. In the channel mouth environment, deposition is dominated by landward accretion and aggradation of mouth bars during river- and storm surge-induced flood events. The subaqeous delta is characterized by a shallow water platform and a gentle offshore slope. Sediment bypassing of the shallow-water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine-grained sediments. Facies of the shallow platform include silty sand with climbing ripple lamination. Offshore facies are dominated by seaward-fining fine sand to silt tempestites. Sea-ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies. The low angle profile of the shallow-water platform is interpreted to be the combined response of a fine-grained delta to (1) storm sediment dispersal; (2) autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley; (3) limited water depth above the underlying flooding surface; and (4) efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge. Several indicators of the cold climate can be used as criteria for the interpretation of ancient successions, including thermokarst lake development, submarine channel scours, freeze-thaw deformation and ice-scour deformation structures. Permafrost inhibits compaction subsidence and, together with the shallow-water setting, also limits autocyclic lobe switching. The cold climate can thus influence stratal architecture by favouring the development of regional-scale clinoform sets rather than multiple, smaller scale lobes separated by autocyclic flooding surfaces.