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)

Abstract

Sequence architecture is controlled by two fundamental factors; the water level relative to the depositional surface (which controls accommodation space), and the points or areas of sediment injection (which control how this space is filled in). Relative water level, accommodation space and injection points normally vary together and various genetic terms are used to define systems tracts. But in glacially influenced basins, accommodation space is controlled not only by water level, but also by the glacier. Injection points are similarly dually controlled — by water level and by the terminus of the glacier. During high lake levels the injection point of the glacier may be underwater at the base of the slope. Genetic terms like `highstand' and `lowstand' therefore are meaningless in successions controlled by sediment input points which are independent of water levels and accommodation space. 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 glacier input-point system controlled by the position of the subaqueous glacier front. And both can form juxtaposed systems passing laterally into each other. A bewildering and incompatible terminology is needed if the terms of genetic sequence stratigraphy are used for both water-level and glacier input-point systems, because the sedimentary injection points and sequence boundaries of the two systems fluctuate out of phase with one another. Sequences 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 needs separate genetic terminologies to describe them, which leads only to confusion. However, descriptive sequence stratigraphy (allostratigraphy) can be readily applied to successions like the Quaternary of Lake Ontario. We therefore suggest that genetic sequence terms be discarded and simpler allostratigraphic terminology used in glacially influenced basins.