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Sedimentary Geology, 123 (1999), 183-197 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) M.E. BROOKFIELD and I.P. MARTINI Department of Land Resource Science, University of Guelph, Guelph, ON NlG 2Wl, Canada ¡¡  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. |
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