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Book Chapter

Nearshore responses to great storms

By
Robert A. Morton
Robert A. Morton
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Published:
January 01, 1988

Great storms originating in both tropical and extratropical regions represent upper limits in the continuum of physical forces that affect sedimentation in shallow marine basins. The energy expended and sediment transported during a few storm hours may equal many years of non-storm work, or they may surpass thresholds unequaled by less energetic processes; consequently, the volume of storm sediment preserved in the basin fill is disproportionate to the frequency of these convulsive events.

Storms can produce large geomorphic features and exaggerated bedforms, especially on low-energy coasts and adjacent shelves where they commonly exert their greatest influence. During peak intensity, combined wind-driven geostrophic currents and wave-induced oscillatory currents flow alongshore and slightly offshore at up to 2 m/s in relatively shallow water (<50 m). The coarsest nearshore sediments (gravel and sand) are transported and deposited by these strong currents, which have a principal unidirectional component. As the unidirectional current component diminishes, wave orbital velocities assume greater importance, and the subequal oscillatory and unidirectional motion molds fine sand that settles from suspension; in the final phase of deposition, mud accumulates below fair-weather wave base in slack water. The resulting graded beds constitute a single but common class of shelf storm deposit.

Thicknesses of graded storm beds typically range from a few decimeters to a few meters, depending directly on wave and current energy and flow duration, and inversely on distance from the shoreline (water depth). Volumetric estimates suggest that each graded bed is not necessarily a product of sand eroded from the beach and shoreface during a single storm. Instead, they may also result from sand being transported offshore by a series of moderately intense events and later selectively sorted during a single extreme storm lasting several days.

Most amalgamated storm beds are preserved in regressive sequences deposited on broad, moderately stable shelves of passive continental margins; however, some accumulated in thick transgressive sequences deposited on narrow, rapidly subsiding and tectonically active shelf margins associated with converging plates. Episodic storm deposition also accounts for coastal and shelf aggradation under static sea-level conditions. Similarities between ancient storm deposits and their modern analogs suggest that extant marine processes adequately explain the observed sedimentologic properties, textural patterns, and vertical sequences of sedimentary structures. Super storms, which are predicted by analysis of extreme values (e.g., wave heights or wind speeds) may be statistical artifacts or, if they exist, their influence may be unrecognized or unimportant in the geological record.

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Contents

GSA Special Papers

Sedimentologic Consequences of Convulsive Geologic Events

H. Edward Clifton
H. Edward Clifton
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Geological Society of America
Volume
229
ISBN print:
9780813722290
Publication date:
January 01, 1988

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