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Book Chapter

Modern Shelf Sands

Published:
January 01, 1985

Abstract

A wide variety of processes have operated on the seabed of the shelves of the world in the past including perhaps storms, permanent currents, wind induced alongshore currents, wave modified currents, subtidal tidal currents and turbidity currents. These processes generated sand bodies with different geometries which commonly contain different sedimentary structures or different sequences of sedimentary structures. On ancient shelves the most common sedimentary structures observed in vertical sections of sandstones are planar-tangential to planar-tabular cross beds, horizontal to subhorizontal laminations, current ripples, wave rippleswave modified current ripples and burrowed and bioturbated (>75% burrowed) sandstones. This sequence is in approximate order of decreasing energy (fluid power). Where consistent vertical sequences of sedimentary structures are observed, one of the most common reflects upward increase in depositional energy. However, a sequence reflecting upward increasing energy and consequent increase in grain size is not unique to shelf sandstones; a similar coarsening upward pattern is reflected in subsurface log patterns in both river- and wave-dominated deltas and in beach/ barrier dominated shorelines.

Ancient sandstone examples used to characterize a variety of these processes, geometries, and shelf locations include the “Gallup” (Tocito), Shannon, Fales and Frontier Sandstones from the Cretaceous of the Western Interior. In addition modern Atlantic shelf and North Sea systems are discussed.

Shelf sandstones may be classified on the basis of their position on the shelf (shoreface-attached, inner shelf, middle shelf, outer shelf) and on the basis of whether they are deposited during a transgression, regression, or a stillstand. Both vertical and lateral sequences of lithologies vary with position on the shelf, processes of deposition, and position within a transgression-regression spectra. On the middle and outer shelf, shelf sandstones are almost always surrounded by shale.

On the inner shelf, and where attached to the shoreface, shelf sandstones overlie a variety of lithologies (sandstone, siltstone and shale) dependant in part on whether they were deposited during a transgression, regression or stillstand. Lithologies deposited lateral to shelf sandstones also vary with the position of the sand body within the spectra of transgression-regression. Vertical and lateral sequences of lithologies areprobably the most variable on the inner shelf.

Local topography also may affect the distribution of shelf sandstones. Winnowing of the seabed in areas which are topographically high may concentrate sand into sand ridges. Depressions in the shelf sea floor may result from erosion of lithologies which have different susceptabilities to erosion (i.e., strike valley sands), during sea level drops, by shoreface retreat or as a result of submarine erosion. Depressions on the sea floormay fill with fine- to coarse-grained sand.

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Contents

SEPM Short Course Notes

Shelf Sands and Sandstone Reservoirs

R. W. Tillman
R. W. Tillman
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D. J. P. Swift
D. J. P. Swift
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R. G. Walker
R. G. Walker
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SEPM Society for Sedimentary Geology
Volume
13
ISBN electronic:
9781565762428
Publication date:
January 01, 1985

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