Seismic morphology and infilling structure of the buried channel system beneath the inner shelf off western Long Island, New York: Accessing clues to palaeo-estuarine and coastal processes

Abstract

Buried channel systems are important sedimentary repositories for documenting environmental changes in response to eustatic events. Here we investigate the morphology, infilling structure, and evolutionary history of buried channel systems offshore of Long Beach Island, New York, through dense, ultra-high resolution seismic profiles. On the basis of stratigraphic superposition, characteristic of bounding seismic surfaces, seismic geometries as well as previous studies, we identify four seismic units interpreted, top-to-bottom, as: Holocene marine sand sheet (SU1); Pleistocene-Holocene channel system (SU2); younger Pleistocene outwash lobe (SU3); and older Pleistocene outwash sediments (SU4). Two sets of channel systems are observed, one shallower (SU2) and the other deeper (inside SU4). The shallower channel system, consists of both fluvial/glaciofluvial incised valley and tidal channels/inlets, incised during or near Last Glacial Maximum and was filled during the ensuing Pleistocene-Holocene transgression, associated closely with estuarine process. The channel fill sediments, the only preserved stratigraphic record of the last sea-level cycle around this area, comprises six seismic facies, which are interpreted as: fluvial/glaciofluvial lag deposits (SF1); relict estuarine complex (SF2); retrogradational barrier system (SF3); later-formed bayhead delta (SF4); an undetermined facies which has the potential to be either barrier, central basin, bayhead delta deposits, or their complex inside the incised valley (SF5); and tidal channel/inlet/flat complex outside the valley (SF6). These facies are bounded by either ravinements (bay flooding, tidal, shoreface) or sedimentary surfaces. This interpretation and seismic geometry reveal evolution of the shallower channel system experienced (1) fluvial/glaciofluvial incision and lag preservation, (2) back-filling of estuarine complex (bayhead delta and central basin sediments), (3) landward rollover of barrier system (river mouth complex), (4) progradation of a new bayhead delta that onlaps the barrier sediments, and (5) reworking and partial erosion resulting in the transgressive ravinement (T surface). This suggests that the estuarine process was the primary factor that resulted in the infilling structure. Our results indicate the deeper channel system comprises two discontinuous channel segments. Although several seismic horizons and facies are identifiable in association with this channel system, we are unable with these data to determine if these segments are connected, nor can we confidently determine the origin and filling structure of the deeper channel system.