RECONSTRUCTION OF THE PRE-TRANSGRESSION TOPOGRAPHY OF A COASTAL MARSH, LEWES, DELAWARE AND ITS IMPLICATIONS OF MODELING THE HOLOCENE SALT MARSH ACCRETION PROCESS

Sculptured by last glaciation, the pre-transgression topography of coastal Delaware is a regional unconformity surface that separates the Holocene transgressive lithosome (less compacted or loose silty sand and sandy silt) from any other older underlying units like Beaverdam Formation (formed in fluvial and estuarine environments, highly compacted medium to coarse clayey/silty sand and Miocene to Late Pliocene in age) and Omar Formation (formed in lagoonal or tidal/estuarine environments, compacted silty sand or sandy silt with braided rusty colors and Pleistocene in age). Reconstruction of such a surface at a higher spatial resolution, therefore, is the initial step toward modeling the coastal salt marsh changes through time.

The study area is the Great Marsh in Lewes. It is located in the lower Delaware Bay and has been a place of many research interests. Two methods were used for the field data acquisition: hand-driven augers and refraction seismic survey. The depth to the pre-Holocene surface was determined based on the lithologic differences of the above-mentioned lithosomes. The final products include a 3D TIN view of the pre-Holocene surface of the region and a 3D TIN view of the drainage basin discharging to the Great Marsh. These maps were generated using more than 760,000 data points, which include data points from the quad maps, previous data, and some 220 new core data and refraction seismic data acquired by the author in the field for this project. Based on the reconstructed basin topography, the volumetric changes of the basin and the planimetric area of sea flooding due to transgression were plotted against sea level positions, which help understand the process of basin geometric adjustment to sea-level changes and valley infill process. Based on the topographic analysis and stratigraphic study of the area, it suggests that the initiation and development of the salt marsh tidal rivers and interfluves are strictly controlled by the pre-existing topographic features. Even the modern tidal river distribution still follows the general topographic frames of the pre-Holocene surface. The meandering process of tidal rivers during the later part of the Holocene period had contributed a great deal to the spatial dislocation of initial valley streams and played a dominant role in the salt marsh accretion process.