ABSTRACT: The Lesser Slave dune ridge barrier complex, north central Alberta: depositional processes, paleogeography and paleoclimate.

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Abstract

Barriers with dune ridges are one of the few remaining unstudied deposits within lacustrine coastal depositional systems. The Lesser Slave Lake barrier complex, located in north central Alberta, is composed of one east-to-west spit and four north-to-south barriers. West to northwesterly winds blow along 90 km of fetch moving sediments by longshore transport to the eastern shore and the barrier complex. Ground penetrating radar was used to infer internal architecture of the barriers and deduce paleoprogradational processes and direction of deposition. Vibracoring was used to identify the stratigraphy for each of the dune ridges on top of the barrier, which in turn, was used to infer lake level at the time of deposition and understand shore processes that formed the barrier and infer paleoclimate. The eastern-most dune ridges for each barrier have foresets dipping to the south, which implies the barriers initially formed as recurved spits towards the south. To the west of the eastern-most dune ridge there are westward dipping accretionary wedges, implying the entire shore of the barrier prograded westward after the initial recurved spit progradation. Processes that form the barriers are mainly controlled by lake level variation, which changes the offshore profile causing onshore and offshore erosion and deposition. Lake level rise is associated with recurved spit formation to the south and lake level fall is associated with progradation of the entire shore to the west as accretion or clinoform wedges of sand. Lake levels measured from the base of the foreshore deposits within vibracores show there have been climatic changes occurring during the past 6,665 years. High lake levels are associated with higher precipitation and lower temperatures and low lake levels are associated with low precipitation and high temperatures. Data suggest there was a cool wet period from 6,665 to 6,250 BP a warm dry period from 6,250 to 5,550 BP, and a cold wet period from 5,550 to 3,000 BP. After 3,000 BP the climate was warmer and dryer until 1,600 BP, when climate became slightly cooler and wetter. Paleogeographic interpretation of the data indicate that the Lesser Slave valley was inundated by glacial Lake Peace/Edmonton by 10,700 BP, at an elevation of 655 m above mean seal level (a msl). The glacial lake was followed by a lower phase called the Poplar Lake phase between 9,000 and 7,000 BP, an an elevation of 586 m asl. Lake level then dropped to the spit phase, which is the first phase of the barrier sequence at 579 m asl. Following the spit phase, between 6,665 BP and the present, all four barriers of the complex formed at lake elevations varied from 579 to 575.3 m asl.

Bibliography of Canadian Geomorphology