Deconstructing the High–Latitude Characteristics of the Mackenzie Delta, Arctic Canada
Philip R. Hill
Natural Resources Canada, Sidney, BC
The Mackenzie Delta is a large, fine-grained, highstand delta that prograded over a shelf-phase delta platform during the Holocene. Several indicators, including a very low angle subaqueous profile and rapid retreat of the shoreline, suggest that it is presently in a state of transition between highstand and transgressive systems tract geometries. This transition is attributed to the process of autoretreat. The low angle subaqeous profile favours frictional dissipation of wave energy, intense reworking of the seabed and development of a tempestite facies association. High –latitude climate effects are superimposed on these “regular” delta characteristics. A low angle nearshore profile is typical of high latitude deltas and is associated with the development of bottomfast sea ice, to water depths as great as 2 m. In the Mackenzie, a shallow nearshore platform is present and is crossed by submarine extensions of distributary channels. These channels facilitate the relatively modest winter flow of the river. Offshore sediments, outside of the intensely reworked tempestite zone, are highly deformed by sea-ice gouging. Cold climate indicators are also well developed on the subaerial delta plain. The most obvious large-scale morphological features of the Mackenzie Delta are the myriad thermokarst lakes formed by freeze-thaw and differential ground subsidence. These features would be recognized in the geological record as steep-walled depressions containing rhythmically bedded seasonal layers of mineral clastics and allocthonous organic matter that show syn-sedimentary deformation. Smaller-scale freeze-thaw deformations, such as ice wedge structures, are typically present in subaerial deposits of levee and subaerial delta plain environments.