Stratigraphic Architecture and Significance of Mass Transport Deposits in the Deep-Marine Strata of the Windermere Supergroup, Southern Canadian Cordillera

R.W.C. Arnott
University of Ottawa, Ottawa, ON

Deep-marine strata of the Windermere Supergroup, which currently are exposed in an area over 35,000 km² in the southern Canadian Cordillera, were deposited on the passive margin of Neoproterozoic western North America. Stratigraphic evidence of slope instability is indicated by common mass wasting (slump and slide deposits), and cohesive sediment-gravity flow deposits (debris flow deposits). Collectively these mass transport deposits, or MTDs, occur in two ways: thin successions consisting of a single to a small number of beds; or thick complexes more than 100 m thick. Thin successions tend to be stratigraphically layered and compositionally simple (i.e. composed of non exotic kinds of sediment). Thick units, on the other hand, show rapid lateral facies changes caused by cut and fill or compensational stacking, and consist commonly of exotic, shallow-marine carbonate clasts and dispersed, well rounded quartz pebbles. In addition, unlike those associated with the thin deposits, channel deposits in the thick successions commonly lack channel-margin levees, and instead fill negative topography along the top of the underlying MTD. A major and stratigraphically important difference between thin and thick MTDs, is that thin units are events that are superimposed on a general depositional theme. In contrast, thick MTDs overlie strata that are texturally and compositionally different from those that overlie them. Thick units, therefore, are interpreted to be associated with falling relative sea level that caused widespread instability on the upper part of the slope and downslope export of voluminous amounts of debris, which commonly is of shallow-marine origin.