NGWA Event and Education Recordings Archive

Eocene to Miocene age marine deposits of the Coastal Plain of Delaware are typically weakly consolidated in most areas. This is not surprising as modern groundwaters are strongly to moderately undersaturated with respect to calcite. In the Smyrna, Delaware area, however, carbonate and silica cemented beds of sand and shell, carbonate concretions, silicified wood and shells, and shell molds, ghosts, and casts have been reported since the 19th century. Recently acquired cores of limestone from aquifer units at depths > 200 feet show micro-karst features.

Coreholes and outcrops in these stratigraphic units in the Smyrna area have yielded evidence of faults and fractures at depths between 40 and 250 feet below land surface. Structural and stratigraphic cross sections through this area show the dip and thickness of several units changes markedly. Deep seismic reflection profiles in the adjacent Delaware River have indicated that Smyrna is located near the edge of a buried Mesozoic-age rift basin structure located more than 1500 feet below land surface.

It had long been thought that epigenetic processes were responsible for the cementation, concretion, and dissolution features. However, the coincidence of cementation and microkarst with shallow and deep-seated structural features are evidence that hypogene processes may have been active in this area. Spurred by a Groundwater Summit field trip to the Edwards Aquifer, additional petrographic and geochemical investigations have recently begun to determine the relative importance of epigene and hypogene processes in the formation of cements, concretions, and dissolution features.