Dated maps show that Fire Island inlet, with a present channel maximum depth of 10 m, migrated 8 km WSW between 1825 and 1940 (when a jetty was built to stop migration). During those 115 years a substantial sand body, lying entirely below modern sea level, and hence having a high probability for preservation, was created by filling of the shifting inlet channel. The dimensions of this sand body are: length, 8 km; width, ~500 m; and maximum thickness, ~10 m. Assuming a triangular cross section with flat top at modern sea level the volume is 2 × 10⁷ cu m. If average porosity is 25%, the pore space of the inlet filling would accommodate 3.3 × 10⁶ bbl of fluids.

Study of the modern inlet from the bottom of the active channel to the spit at Democrat Point permits recognition of 3 major environments of sedimentation having 10 subdivisions; sediments of each subdivision display distinctive structures. The channel environment extends from 10.0 to 3.75 m below modern sea level. Its subdivisions are: (a) channel floor (-10.0 m), (b) deep channel (-10.0 to -4.5 m), and (c) shallow channel (-4.5 to -3.75 m). The spit-platform environment (-3.75 to -0.6 m) consists of bottomset, foreset, and topset subdivisions. The spit environment (-0.6 to +2 m) includes washover delta, bay (to -2 m), berm, and beachface subdivisions.

Despite the fact that the basal gravel layer is overlain by large sand waves (60-100 m long and 0.5-2 m high), the only large-scale cross-strata occur on washover deltas that occur locally at the top of the inlet sequence. In sands from the deep parts of the channel, where large-scale cross-strata might be expected because of the large sand waves, only small-scale, lenticular cross-strata occur. This absence of large-scale cross-strata results from the effect on the sand waves of reversing tidal currents.

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