--> Abstract: Reconstructing the Architecture and Sequence Stratigraphy of the Preserved Fluvial Record: A Reality Check, by Andrew D. Miall; #90039 (2005)

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Reconstructing the Architecture and Sequence Stratigraphy of the Preserved Fluvial Record: A Reality Check

Andrew D. Miall
University of Toronto, Toronto, ON

Much valuable work is now being carried out on modern rivers and their deposits, aided by such techniques as ground penetrating radar (GPR), but studies of modern and recent systems cannot address the question of long-term preservability. Only studies of the rock record itself can explore this issue. Two studies of ancient fluvial systems illustrate some of the problems.

A seismic time-slice study of Plio-Pleistocene fluvial systems in the Gulf of Thailand revealed major changes in channel size and fluvial style over short vertical intervals. Braided and meandering systems (meander-belt widths 4 to >10 km) are separated by a few tens of metres of section, or less, and are interbedded with the deposits of much smaller rivers, showing straight, meandering and anastomosed patterns. Incised valleys and underfit streams are also present. These variations can be interpreted in terms of a sequence model, but they indicate the problems that could arise from the use of a single suite of dimensional variables as input into numerical reservoir heterogeneity and flow models.

A study of the Hawkesbury Sandstone (Triassic, Sydney Basin, Australia), highlighted the difficulty in interpreting the dimensions of large sand bodies from comparisons with a modern analog. The traditional comparison of this system with the Brahmaputra River was re-examined, making use of recent GPR work on that river. Even where entire large bars can be imaged, the question of their long-term preservability limits the reliability of the ancient-to-modern comparison, and the very large coastal outcrops of the Hawkesbury Sandstone are, nonetheless, inadequate for much more than order-of-magnitude architectural reconstructions. The marked variability of paleocurrent patterns of the Hawkesbury Sandstone illustrates the limitations inherent in subsurface predictions of channel-body orientation, which imposes another constraint on numerical models.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005