SEDIMENTARY DYNAMICS OF THE INNER CONTINENTAL SHELF OF DELAWARE: DO OFFSHORE VIBRACORE RECORDS PROVIDE INSIGHT INTO SHELF GEOMORPHOLOGY?

Vibracore records from offshore Delaware were studied for insight into decadal-scale shelf morphodynamics. The dataset comprises 466 sediment cores, which were collected over the past 40 years off the coast of Delaware for sand resource-assessment projects in state and federal waters. Descriptions of seafloor-sediment composition and recorded water depths were evaluated in context of geomorphic change.

While all records aided in geologic mapping of the shelf, 366 core locations offered sufficient detail of seafloor position to ensure tide correction and normalization to the NAVD88 datum. Vertical change metrics were calculated as the difference in seafloor elevation between time of coring and a 2007 NOAA bathymetric dataset. These were evaluated in context of time, shelf geology, and the morphologic compartmentalization of the seafloor.

The arithmetic mean of vertical change was calculated to be around -0.2 m. While the envelope of change spanned from -6.0 m to +5.4 m, around 87% of data points fell within ±2 m, corresponding to the relief of most shoal fields within the study area. Approximately 75% of data points were associated with shoal and sheet sands of the inner shelf platform, where water depths were less than 15 m. Data points exceeding 2 m of accretion appeared to relate spatially to isolated areas of the shelf with sand ridges up to 4 m in amplitude. Seafloor-elevation differences between 2007 and 2008 were consistent with the placement of bedforms, as ridge crests and troughs favored erosion and accretion, respectively. No such distinctions could be made for longer timespans, which may reflect the influence of high-energy events.

Surface lithology, as mapped from core data, showed no distinct correlation with vertical change, likely due to geomorphic decoupling of surficial sands from underlying units (muddy Pleistocene valley fills and late Pliocene fluvio-deltaic sands and gravels), which are thinly veiled or crop out in the swales between sand ridges. Findings adhere to our conceptual understanding of shelf dynamics and indicate that core records are potentially useful indicators for measuring seafloor bathymetric change in areas of offshore sand resource and infrastructure projects.