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Characteristics of Shoreface Storm Deposits: Modern and Ancient Examples
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1976
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Ocean DynamicsEngineeringGeomorphologyCoastal GeomorphologyCoastal ProcessCoastal HydrodynamicsNew YorkFire IslandEarth ScienceShoreface SedimentsNearshore ProcessNearshore ProcessesShoreface StormMarine GeologyCoastal DepositCoastal ProcessesSedimentologySediment TransportCoastal Sediment TransportCoastal SystemsMorphodynamicsStructural GeologyCivil EngineeringGeomechanics
Shoreface sediment cores off Fire Island, New York, display a sharp‑bounded three‑part sequence of basal lag, finely laminated sand, and burrow‑mottled sand, a pattern attributed to wave‑ripple laminae or burrow‑mottled deposition during fair weather and replicated in numerous modern and ancient coastal settings worldwide. During a storm, maximum intensity jostles coarse gravel into a basal lag while intense bottom shear during the waning phase rapidly deposits the finely laminated sand. The upper, coarser mottled sand reflects fair‑weather shoreface processes, whereas the lower basal lag and laminated sand are storm‑derived, indicating that most shoreface sediment records are dominated by storm deposits.
ABSTRACT Cores of shoreface sediments off Fire Island, Long Island, New York, in water depths ranging between 5 and 21 m, contain a distinctive three-part sequence; from base upward consisting of: (a) basal lag, up to 50 cm thick, containing coarse gravel; (b) finely laminated sand up to 2 m thick, having median diameters ranging from 0.210 to 0.125 ram, and (c) burrow-mottled sand coarser in size than unit (b), up to 30 cm thick. The boundaries between (a), (b), and (c) are sharp. The relatively coarser, mottled sand composing the upper part of this sequence results from processes operating in the shoreface zone during fair weather. Such processes create either wave-ripple laminae (between wave base and the breaker zone) or burrow-mottled sediment (seaward of wave base). The lower part of the sequence is inferred to result from storms. The basal gravel is interpreted as a lag formed during maximum storm intensity when the coarse material is jostled at the bottom and the finer sediment is kept dispersed or is in turbulent suspension. The fined laminated sand is inferred to have been rapidly deposited under conditions of intense bottom shear as the storm waned. Examples in the geologic record of what we regard as comparable storm-deposited sequences occur at the following locations: Relict Holocene sediments off Fire Island, New York; Pleistocene sediments underlying Fire Island barrier, Long Island, New York; Norfolk Formation (Pleistocene), Benns Church, southeastern Virginia; Eocene and conglomerates, Tehachapi and San Emigdio Mountains, southern California; and Cambrian and conglomerates, Parfrey's Glen, Baraboo Range, Wisconsin. In addition, other workers have identified characteristic sequences very similar to the one described here in the following localities: Recent sediments off Virginia; Recent sediments, southern North Sea; upper part of Sundance Formation (Jurassic) of Wyoming and Montana; Upper Cambrian, Ba aboo district, Wisconsin; and sublittoral sheet sandstones described from several locations. We speculate that the geologic record of shoreface sediments contains a minimal proportion of sediments deposited during fair-weather conditions and consists largely of storm deposits.