NEOPROTEROZOIC OF THE MACKENZIE MOUNTAINS, NORTHWESTERN CANADA

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km t hick, and comprise two supergroups of differing style. The Mackenzie M ountains Supergroup (4-6 km thick), an epicratonic succession of mainl y shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-77 8 Ma. The overlying Windermere Supergroup (5-7 km thick) comprises bas al rift deposits and glacial diamictites that are overlain by three, k ilometre-scale, siliciclastic to carbonate ''Grand Cycles'' deposited on a passive margin. ''Grand Cycles'' in the eastern Mackenzie Mountai ns were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope ; mapping of successive shelf-margin positions reveals southwestward p rogradation. The Windermere Supergroup is unconformably overlain by th e Ingta Formation, which contains the Proterozoic-Cambrian boundary. S ubtrilobite Cambrian strata are approximately 1.5 km thick, and are ov erlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi For mation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and micro bial structures occur throughout the entire succession, but are especi ally well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Ediacara-type fossils first appear below the second Winde rmere glacial unit (Ice Brook diamictite), and range upwards through 2 .5 km of strata to near the top of the Windermere; three distinctive a ssemblages corresponding to the Twitya, Sheepbed, and Blueflower forma tions can be recognized. The Neoproterozoic-Cambrian boundary occurs w ithin the Ingta Formation, and is marked by an abrupt change from simp le, subhorizontal burrows (Planolites-Torrowangea Assemblage) to compl ex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glaci al diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record show s three hair-pin curves that should be recognizable in continents atta ched to Laurentia. Carbonate interbeds throughout the succession typic ally yield relatively unaltered carbon isotope ratios; C and Sr chemos tratigraphy shows marked excursions that appear to be globally correla table.