INVESTIGATING HOLOCENE SEDIMENTATION IN CHINCOTEAGUE BAY, VA THROUGH VIBRACORING AND CHIRP SONAR DATA

Chincoteague Bay (CB) is a shallow back bay located between the mainland coast of the Delmarva Peninsula and the barrier islands of Assateague and Chincoteague. In order to investigate Holocene sedimentation we collected three sediment cores from CB along with Chirp sonar sub-bottom data in Aug. 2014. The cores were extracted using a vibracore system along a NW-SE transect. CB VC3, 11 m from the mainland coast, was collected in 0.5 m of water from a site which recently (post-1994) transitioned from a salt marsh to a bay environment as the promontory marsh eroded. CB VC1 was extracted from a 1 m deep eel grass shoal 2.4 km NW of Chincoteague Island (CI). CB VC2 was collected 0.5 km offshore of the northern tip of CI in 1 m of water.

VC3, 2.75 m long, consists entirely of silts and back bay muds. Comparison with cores taken from the adjacent salt marsh (see Cooper et al., 2015) suggest that the uppermost marsh sediments have been eroded. Stratigraphic heterogeneity and grain size increase toward the SE. In the center of the bay, VC1, 1.75 m long, consists of alternating layers of back bay muds and mica-rich sandy silts with interbedded fine (phi 2-3) sand layers 2-7 cm thick. At the base of the core is a 38 cm-thick layer of medium-grained (phi 1-2) quartz sand. VC2, 2.15 m long, is composed primarily of mica-rich silty sand and organic-rich silt. From 21-31 cm there is layer of quartz sand comparable to that seen at the base of VC1. At the base of VC2 there is a 50 cm layer of fine quartz sand.

The silts and back bay muds are reflective of modern depositional environments in CB, however further work is needed to determine the origin of the massive sand layers and the heterolithic sequences. VC1 and 2 likely contain evidence of the geomorphic evolution of Assateague and Chincoteague Islands as a result of both barrier island roll-over and longshore sediment transport. The sand layers in VC1 and 2 are likely storm overwash deposits but may also contain wind-blown sediment and/or sediment deposited during the opening/closing of the Assateague Inlet which was reported on early maps of the region. Future work will focus on correlating the sedimentological units observed in the cores to the sub-bottom records. We will also perform macro and micro fossil analysis to constrain the environment of deposition. These data will allow us to create a stratigraphic model for CB.