Facies architecture and sequence stratigraphy in glacially influenced basins: basic problems and water-level/glacier input-point controls (with an example from the Quaternary of Ontario, Canada)

Sequence architecture is controlled by two fundamental factors; the water l evel relative to the depositional surface (which controls accommodation spa ce), and the points or areas of sediment injection (which control how this space is filled in). Relative water level, accommodation space and injectio n points normally vary together and various genetic terms are used to defin e systems tracts. But in glacially influenced basins, accommodation space i s controlled not only by water level, but also by the glacier. Injection po ints are similarly dually controlled - by water level and by the terminus o f the glacier. During high lake levels the injection point of the glacier m ay be underwater at the base of the slope. Genetic terms like 'highstand' a nd 'lowstand' therefore are meaningless in successions controlled by sedime nt input points which are independent of water levels and accommodation spa ce. Distinct water-level and glacier input-point systems can occur side by side: the water-level system controlled by lake level or sea level, the gla cier input-point system controlled by the position of the subaqueous glacie r front. And both can form juxtaposed systems passing laterally into each o ther. A bewildering and incompatible terminology is needed if the terms of genetic sequence stratigraphy are used for both water-level and glacier inp ut-point systems, because the sedimentary injection points and sequence bou ndaries of the two systems fluctuate out of phase with one another. Sequenc es developed by movement of the glacier are completely independent of those controlled by water levels. If genetic sequence stratigraphy is applied to a basin with both lake-level and glacier input-point systems, then each ne eds separate genetic terminologies to describe them which leads only to con fusion. However, descriptive sequence stratigraphy (allostratigraphy) can b e readily applied to successions like the Quaternary of Lake Ontario. We th erefore suggest that genetic sequence terms be discarded and simpler allost ratigraphic terminology used in glacially influenced basins. (C) 1999 Elsev ier Science B.V. All rights reserved.