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The AAPG/Datapages Combined Publications Database

CSPG Special Publications


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 99-119
Storm-Dominated Shelves - Processes

Fluid Processes and Sea-Floor Response on a Modern Storm-Dominated Shelf: Middle Atlantic Shelf of North America. Part I: The Storm-Current Regime.

Donald J. P. Swift, Gregory Han, Christopher E. Vincent


Analysis of ancient shelf deposits requires a thorough understanding of shelf sedimentary processes. However, fluid and sediment dynamics with the large time and space scales of a continental shelf cannot be extrapolated from the fluid dynamics of flumes or rivers.

Shelf sedimentary regimes are driven by storm-generated currents (80% of the world’s shelves), tide-dominated currents (17%) or by intruding oceanic currents (3%). The Middle Atlantic Continental Shelf of the United States is a typical storm-dominated shelf. Its fluid and sediment dynamics are probably better studied and better understood than those of most others, and arc therefore used in this paper to develop a model for sedimentation on storm-dominated shelves.

Sediment transport on the Atlantic Shelf is forced by combined-flow bottom currents, in which a high frequency (6–12 s) wave orbital component is superimposed on a slowly varying wind-driven mean flow component. Combined-flow currents are highly efficient transporters of sediment because the two components together create a more intense bottom shear stress than the sum of the stresses that would be created by either separately.

Sustained, intense, storm flows on the Atlantic Shelf are generally responses to pressure fields created by the pile-up of water against the coast by winds (storm set-up) or by the withdrawal of nearshore water in response to winds (storm set-down). In the equation of motion describing such flows, the pressure term is balanced by the Coriolis term; these are referred to as geostrophic (earth-turning) flows. Geostrophic flows are predominantly coast-parallel and contour-parallel flows. Although there is frequently an onshore or offshore component of flow near the bottom, its strength is usually less than the strength of the alongshore component. Shelf flows at a high angle to the shoreline are generally of brief duration.

Atlantic shelf storms are mainly mid-latitude low pressure systems (northeasters). They are most intense in winter, when they may create sustained southwesterly flows along 1000 km of shelf. Hurricanes also occur on the Middle Atlantic Shelf. They are intense, rapidly moving, localized storms of tropical origin. They tend to occur during the late summer period of maximum thermal stratification of the water column. They are generally less efficient than winter storms in generating sustained shelf currents and transporting sediment, although they frequently do considerable damage to the beach.

In both hurricanes and winter storms, onshore-directed storm-surge currents and offshore-directed “storm-surge ebb” currents contribute to the fluid discharge, but are of relatively minor importance in comparison with the along-shelf geostrophic component of flow. When a storm moves ashore, the mass of water left piled up against the coast does not simply rush back out to sea under the impetus of gravity. Instead it moves along the coast in response to the Coriolis force, as a topographically trapped shelf wave, until it is finally dissipated by the frictional drag of the bottom. Because of the dominance of such along-shelf, southwesterly storm flows, there is a systematic pattern of southwesterly (along-shelf) transport of sediment.

The surface of the Atlantic Shelf is mantled with sand. It is in a sense anomalous with respect to the rock record in that muddy, prograding shelf sequences volumetrically dominate the thin transgressive sandstones that mark the sequence boundaries. However, shelf lithology depends not only on shelf dynamics, but on the rate of sediment input and the rate of rise in sea level. Comparison with other modern shelves suggests that the modem Middle Atlantic Shelf is generally suitable as a dynamic (process) model regardless of the lithological response. Storm-dominated shelves like the Middle Atlantic Shelf are characterized by along-shelf flows that (in the northern hemisphere) flow counterclockwise with respect to the ocean basin centre.

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