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 sandstones and conglomerates, Tehachapi and San Emigdio Mountains, southern California; and Cambrian sandstones 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, Baraboo 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.

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