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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 303-312
Comparison of Storm- and Tide-Dominated Shelves

Depositional Sequences in Ancient Shelf Sediments: Some Contrasts in Style

Douglas J. Cant, Frances J. Hein

Abstract

Three major types of shelf sediments are: 1. cyclic coarsening-upward sequences; 2. thick accumulations lacking grain-size trends; and 3. patchy, stratigraphically isolated units of sandstone or conglomerate. The cyclic sequences (Type 1) were deposited as regressive pulses separated by transgressions. The shelf sandstones in these are simple seaward-tapering wedges. The offshore variation from sandstone to shale results from deposition on a graded shelf. These deposits are common in the western foreland basin of North America because large amounts of sediment were periodically supplied as a result of tectonic events in the orogenic belt, resulting in progradational regressive sequences.

The thick accumulations (Type 2) are areally extensive bodies of sediment that lack well developed cycles. In the Gog Group (Cambrian) of the Canadian Rocky Mountains, up to 1500 m of shallow marine sandstone and shale accumulated, with no progradational shoreline deposits being preserved. Strong storm and tidally driven currents redistributed sediment uniformly over a wide, shallow marine shelf. The Gog Group was deposited on the Lower Cambrian western passive margin of North America, which underwent continual and uniform subsidence. Sediment supply was probably more constant than in the foreland basin.

The patchy sandstone and conglomerate bodies (Type 3), such as the Viking and Cardium Formations of Alberta, occur at discrete stratigraphic horizons within thick marine shales. Numerous isolated bodies of coarse sediment are surrounded by shale. The Viking and Cardium units were deposited contemporaneously with sea level low stands. During the fall in sea level, a regression occurred and coarse sediment was spread over the basin as coarsening-upward sequences. At the lowest stand, channels and disconformities were cut. During the rise in sea level, transgressive reworking of littoral, deltaic, and estuarine deposits formed shelf sands and conglomerate. The differences in sandstone geometry were probably partly a result of the pre-existing depositional environments and partly created during the transgression. Thus, it is not necessary in these examples to account for long-distance transport of sand and gravel across a muddy shelf. All the types of sand bodies formed on modern shelves may be present in these transgressive intervals. The sequences of these deposits result from superimposition of the transgressive units on the regressive units because of the fall and subsequent rise in sea level.


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