Studies of the chesapeake bay impact structure : Introduction and discussion

摘要: The late Eocene Chesapeake Bay impact structure on the Atlantic margin of Virginia is largest known crater in United States, and it may be Earth's best preserved example a large that formed predominantly siliciclastic continental shelf. 85-kilometer-wide (53-mile wide) also coincides with region saline ground water. It has profound influence ground-water quality flow an area urban growth. USGS-NASA Langley corehole at Hampton, Va., first series new coreholes being drilled crater, to penetrate entire crater-fill sec tion uppermost crystalline basement rock. located southwestern part crater's annu lar trough. A comprehensive effort understand materials, architecture, geologic history, formative processes, as well its water, includes drilling accompanied by high-resolution seismic-reflection seismic-refraction surveys, audio-magnetotelluric related multidisciplinary research. studies core presented this volume provide detailed information outer including basement, overlying impact-modified impact-generated sediments (physical geology, paleontology, shocked minerals, ejecta), upper Quaternary postimpact sedimentary section (stratigraphy, paleoenvironments). total depth below land surface 635.1 meters (m; 2,083.8 feet (ft)). deepest unit Neoproterozoic Granite. top granite 626.3 m (2,054.7 ft) overlain 390.6 (1,281.6 sediments. These materials are beneath 235.6-m-thick (773.12-ft-thick) blanket profile crosses drill site tied borehole geophysical logs, reveals details extensional collapse structures western annular Electrical cross sections based (AMT) soundings image nearly vertical zone high resistivity trough, possibly indicating fresh water location, they show impedance trends match curvature structure. They subsurface contact between conductive resistive showing relatively constant Chemical isotopic data indicate inland saltwater wedge or bulge mixture freshwater seawater, evidence for mixing supports concept differential flushing residual seawater create bulge. Ground-water brine central was produced evaporation, production from heat least theoretically possible.