Shore-Oblique Sand Bars and Sediment Heterogeneity: Precursors to Storm-Generated Sand Ridges and Indicators of Transgression?

Seafloor sediment heterogeneity and three-dimensional seabed morphology complicate our understanding of sediment transport processes occurring in the nearshore, defined here as the swash, surf, and breaker zones. Though there are many examples of the interaction between patches of coarse and fine sediments and seafloor morphology in the recent literature, most of these investigations have focused on short-term (i.e. days to months) changes at the seabed surface. In sediment-limited systems, the source of shoreface heterogeneity is often shallow, subsurface strata that can be exposed or buried. The timescales over which these changes are important and the influence of exposed and/or buried strata on the genesis of seabed morphologies is the focus of this research.

Eighteen vibracores were collected from the nearshore of the Outer Banks, North Carolina from within and outside a previously identified shore-oblique bar field. Cores from shore-oblique bars/troughs show greater variability in median grain size than those taken from a convex shoreface. Geophysical data show a coarser underlying stratum that is exposed from the axis of the trough and on flanks of shore-oblique bars. Sediment analysis indicates that this stratum is coarser within the field of shore-oblique bars. Changes in the cross-sectional morphology of bars through time indicate a possible feedback between bar-associated sand and coarser trough sediment over interannual timescales. The observed lithologic heterogeneity is directly related to an underlying paleofluvial channel. A literature review reveals an association of paleochannels with sand ridges from sites worldwide. Based on the combination of our data and existing observations, we suggest that shore-oblique bars serve as precursors to storm-generated sand ridges. In our conceptual model, heterogeneous sediments derived from relict strata mediate the formation and preservation of the “ridge precursor” and may help to explain the existence of sand ridges in locations where their presence is unsupported by existing models.