Reconstruction of a Relict Inlet System and Historical Storm Signatures along Southern Assateague Island, Maryland

Abstract

Assateague Island is a classic example of a retrograding barrier island, with its recent geological history punctuated by episodes of overwash and breaching. However, in addition to a number of historical inlets, parts of the island owe their origin to relict (pre-historic) channels. The present study was conducted north of the Virginia-Maryland border, focusing on a narrow segment of the island fronting the Green Run Bay. The site lies north of the historical Green Run Inlet that was active until 1880; however, there is no geological evidence of its migration along the island. More than 4 km of high-resolution (250 MHz) ground-penetrating radar (GPR) images, complemented with sediment cores and multi-dating techniques, were used to reconstruct the geological legacy of the Green Run Bay segment of the barrier and to test whether it was the site of an older channel. My findings indicate that a backbarrier paleo-channel still visible within the Green Run Bay corresponds to a large (>380 m wide, 3.0-3.5 m thick) channel cut-and-fill structure revealed in GPR images. The channel fill consists of tangential- to sigmoidal-oblique, southward-dipping reflections downlapping onto channel lag facies, which overlie subhorizontal bay-fill strata. Hummocky reflections in a shore-normal channel transect suggest partial preservation of inlet-related bedforms, believed to be associated with the channel closure. Mollusk shells from the bay fill yield radiocarbon ages of 4630-2400 cal BP (calibrated years before 1950). The paleo-channel facies overlying the bay deposits exhibit a fining-upward sequence, with a mean grain size range of 0.44-2.43 phi. The first set of optical dates indicates that the inlet fill is 660 +/- 70 cal BP (AD 1220-1360). The paleo-channel fill does not extend to the south and therefore is a separate relict feature that predates the historical Green Run Inlet. Based on geophysical and core data, the paleo-tidal prism of the relict channel is 17x10 6 m3. Following the closure of the inlet, a series of beach ridges have developed across the Green Run Bay segment and exhibit signatures of storm erosion in shore-normal GPR profiles. This punctuated barrier progradation took place during the historical period (post-1600), with optical dates of beach ridge and dune generations ranging from AD 1680 to 1920. In addition to geological evidence, dendrochronological records were examined for occurrences of abrupt thinning of tree rings as a proxy for intense environmental stress. Tree samples that exceed 50 years in age (n=7) display an abrupt ring thinning in 1962 that coincides with the Ash Wednesday extra-tropical storm of record. This study demonstrates that the historical stability of the Green Run Bay segment of Assateague Island is likely due to the influx and preservation of substantial sand volume related to a relict tidal inlet.