Abstract

This study compares the three-dimensional sedimentologic and stratigraphic architecture of Green Run and Sinepuxent inlets, two former, historically documented, wave-dominated tidal inlets along Assateague Island, Maryland, USA. The two inlets are only separated by 10 km along the same barrier island, but they exhibit distinctly different geomorphic expressions. The former Green Run Inlet formed from a single breaching event, and once opened, the inlet migrated 680 m to the southwest and rotated counterclockwise. In contrast, the former Sinepuxent Inlet was likely a breach zone, hosting multiple breaching events with little to no lateral inlet migration. Textural analysis of sediment cores collected from the two former tidal inlets includes the first sedimentological description of an inlet-closure ridge. Moreover, textural analyses revealed first-order and second-order vertical grain-size trends in tidal-inlet fill. First-order vertical grain-size trends generally follow the well-accepted signature of a fining-upward succession characterizing inlet-fill deposits, while second-order trends tend to be more variable. However, two cores (one from an inlet-closure ridge and one from an intermediate inlet-closure ridge) within the former Green Run Inlet exhibit first-order coarsening-upward successions, having a similar sedimentological signature of a prograding barrier-island deposit. Thus, an expanded sedimentologic and stratigraphic model for wave-dominated inlet-fill deposits is proposed. Three styles of inlet-channel fill were documented: 1) laterally accreted tidal-inlet fill: first-order fining-upward succession with subtle second-order variations, 2) laterally accreted and prograding tidal-inlet fill: first-order fining-upward succession with pronounced second-order coarsening- and fining-upward trends, and 3) prograding tidal-inlet fill: first-order coarsening-upward succession with subtle second-order variations. Short-term increases in flow velocity likely produced these second-order grain-size trends, probably in response to increases in tidal prism related to spring tides (i.e., syzygy and perigee syzygy) and/or storm impacts (i.e., extratropical and tropical cyclones). Findings from this study provide an efficient link comparing modern sedimentary processes to ancient sedimentary deposits. Additionally, results from this study may lead to reinterpretations of ancient sedimentary depositional environments because tidal inlets may rework a significant amount (30 to 85%) of coastal barrier deposits.

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