Linking framework geology and nearshore morphology: Correlation of paleo-channels with shore-oblique sandbars and gravel outcrops

This work describes a newly-discovered spatial correlation between paleo-channels and nearshore morphology along the barrier islands of northeastern North Carolina and southeastern Virginia, U.S. mid-Atlantic coast. These nearshore morphological features include regions of shore-oblique sandbars and coincident gravel outcrops that previously have been linked to erosional hotspots at the shoreline. These new findings have direct bearing on both nearshore morphodynamics and shoreline management. Paleo-channels were identified in the chirp seismic record and compared to shore-oblique sandbar locations discerned from bathymetric data and corroborated by gravel outcrops depicted in the side scan record. Both graphical correlation and statistical cross-correlation analyses revealed a significant spatial relationship between these features, with higher statistical correlation for channels > 500 m wide. Seismic records also indicate that the gravel outcrops seen on the updrift flanks of the shore-oblique bars are surface expressions of the underlying geology. We hypothesize that these outcrops are composed of relict channel fill sediments that interact with the hydrodynamic regime to produce sorted bedforms. The bathymetric anomalies then serve to alter incident wave energy and affect shoreline behavior. The spatial and temporal stability of these features over several years and through a variety of wave conditions suggests some degree of underlying geologic control. While the mechanisms responsible for this relationship remain speculative, these results may bridge the gap between studies focusing on framework geology and its influence on shoreline change and those that investigate bar morphodynamics and the initiation of sorted bedforms.