Paper No. 7
Presentation Time: 10:30 AM
THE THERMO-HYDRAULIC REGIME OF THE CHESAPEAKE BAY IMPACT STRUCTURE
WILHELM, Helmut
1, BURKHARDT, Hans
2, POPOV, Yuri
3,
HEIDINGER, Philipp4, MAYR, Sibylle I.
5, ROMUSHKEVICH, Raisa
6 and GOROBTSOV, Denis
6, (1)Geophysical Institute, Karlsruhe Institute of Technology, Hertzstr. 16, Karlsruhe, 76187, Germany, (2)Geophysikalisches Institut, TU Berlin, Berlin, 13355, Germany, (3)Schlumberger Moscow Research Center, Moscow, 119285, Russia, (4)Geophysics Institute, Universität Karlsruhe, Hertzstr. 16, Karlsruhe, 76187, Germany, (5)Department Section Geophysics, Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, 12249, Germany, (6)Technical Physics and Rocks Physics, Russian State Geological Prospecting University, Miklukho-Maklai str., 23, Moscow, 117997, Russia, heidinger@gmx.net
The Chesapeake Bay Impact Structure (CBIS) was created approximately 35 Ma ago when a large comet or asteorite struck the coastal waters off Virginia, USA, and intruded the unconsolidated sediments and even the granitic basement of the continental shelf. The impact destroyed the existing Cretaceous and Eocene aquifers and excavated a crater which was filled with Cretaceous megablocks breccia, containing saltwater and brines, overlain by sediment-clast breccia which was deposited by ocean-resurge currents over the whole impact area. After the impact unconsolidated sediments were deposited again in the shallow waters forming a new system of aquifers and aquicludes. Due to the brines contained in the crater fill a 50 km wide saltwater wedge exists within the Cretaceous aquifers around the crater. With increasing tendency, urbanisation and industrialisation lead to extraction of great amounts of freshwater from the coastal aquifers and so to a contamination hazard.
High resolution temperature measurements in seven boreholes, within and outside the impact crater, including the ICDP-CBIS borehole Eyreville, and laboratory investigations of core samples from these boreholes regarding their thermo-physical and hydraulic properties have provided a data set which yields information about vertical groundwater motion in the breccia crater fill and the Cretaceous and post-impact aquifers of the region. With a stationary cylinder-symmetric thermo-hydraulic finite-element model of the CBIS region, the seven measured temperature profiles are simulated and interpreted with respect to the thermal and hydraulic conditions in the region. The vertical velocity components of groundwater in the aquifers, determined from the curvature of the measured temperature profiles, are in the range of 0.1 - 0.2 m/year, whereas in the breccia crater fill the vertical heat transport is purely conductive in view of the linear temperature-depth variation.
