A hydrostratigraphic framework has been developed for the Indian River Bay, Delaware, watershed in support of a larger research project that will model groundwater interactions between the upland and the estuary. Four hydrostratigraphic units have been defined base on stratigraphy from high-resolution seismic surveys within Indian River Bay, supported by previously collected cores and gamma ray logs. The seismic stratigraphy is correlated with complementary marine resistivity surveys that primarily show the subsurface distribution of fresh and saline groundwater. This combined dataset is used to infer different modes of submarine groundwater discharge along the margins of Indian River Bay.
Additional geophysical surveys were conducted on land at three field sites; Holts Landing State Park, James Farm Ecological Preserve, and Fresh Pond Field Site, which are adjacent to Indian River Bay. Ground penetrating radar and electrical resistivity transects at these sites provided the geometry of shallow stratigraphy and groundwater interactions near the estuary margins.
The hydrostratigraphic units were defined from major seismic stratigraphic surfaces and their overlying depositional units that could be traced throughout the study area. The deepest of these units is the upper Bethany Formation that is a regional confining unit and is taken as the lower boundary for the modeling effort. The next unit up, Unit II, is composed dominantly of the coarse clastic materials of the lower Beaverdam Formation; however this unit does include some beds of finer-grained sediments. Unit II has a characteristic variable signature in gamma logs, and the seismic character implies back-barrier and estuarine deposits. The base of Unit II is taken as a prominent reflecting surface found throughout the area at 15-20 m below sea level. From the data available, it is not possible to assign age to the deposits in this unit, however, they may be the uppermost Beaverdam Formation or Pleistocene deposits infilling an ancestral Indian River. Capping the entire sequence, Unit I, is composed of Holocene sediments deposited during sea level rise that fill the lowstand drainage-system valleys of Indian River and its tributaries.
Several major geologic findings were a product of the research. Evidence of multiple occupations of an ancestral Indian River valley, probably during the highstand of the middle to late Pleistocene exists within the seismic stratigraphy. Also, evidence of late Pleistocene margins of an ancestral Indian River Bay, both submerged along the flanks of the bay and beneath the land surface adjacent to the bay, are present in the seismic stratigraphy and ground penetrating radar data. There is also evidence of multiple late Pleistocene shorelines associated with the Cedar Neck and Fresh Pond ridges. These shorelines are most likely associated with late marine isotope stage 5. The seismic stratigraphy also provided evidence of an older, inferred fluvial channel trending northwest to southeast that appears to be near the base of the Beaverdam Formation and erodes into the underlying top of the Bethany Formation.