Inlet sequence: a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets

Tidal inlets, narrow, comparatively deep gaps between barrier islands, are channels through which tidal currents flow vigorously as the water enters and leaves the backbarrier lagoons, bays, and intertidal flats and marshes. Many geologists have compared tidal inlets to river channels and have speculated that if an inlet shifts laterally, it should deposit a distinctive sequence of sediments, analogous to the point-bar sequence left by a meandering stream channel. Fire Island Inlet, located 56 km east of New York City, has migrated WSW at a mean rate of 64 m/year during the period 1825–1940. Waves approaching the coast, predominantly from the southeast, have shifted sediment along the shore toward the WSW. Deposition of sediment on the east side of the inlet has forced the tidal currents to erode the west side of the inlet, thus causing lateral migration. Because hydraulic conditions vary from the channel floor to the subaerial part of the spit which is present on the ENE side of the channel, sedimentary structures and textures vary systematically with depth. We have determined the various sedimentary environments associated with the modern Fire Island Inlet, sampled and described the sediments from these environments, and have collected samples from corings made on that part of Fire Island through which the inlet has migrated. On the basis of our studies, we propose an inlet sequence which is formed by the lateral migration of a tidal inlet. The sequence includes five major units, as follows on p. 492. The sediments belonging to various units in this sequence have been identified in four borings made on those parts of Fire Island through which the Fire Island Inlet has migrated since 1825. This sequence should be applicable to other inlets also. We think that the boundary between deep channel and shallow channel units remains relatively fixed at −4.5 m, whereas the thickness of the deep channel unit is determined by the depth range between −4.5 m and the total depth of the inlet. Hence, the main source of variation in the inlet sequence will be the thickness of the deep-channel unit. Most of the sediments of the inlet sequence are incised below mean low water; hence they will almost certainly be preserved in the geologic record, even if all other associated sediments from barrier environments located above mean low water are not preserved. Because of the great variability possible in rates of lateral migration of inlets along the shore compared with the rates of barrier displacement perpendicular to the shore, inlet sediments may be preserved as elongate lenses, or as widespread blankets. The shape of inlet deposits reveals much about the behaviour of barriers during a submergence. Hence, inlet sediments should shed new light on sediments of the continental shelves and on basal transgressive sands in the geologic record.