MAGNETIC SIGNATURE OF EXMORE BRECCIA AND GRANITIC MEGABLOCKS, OF THE 35 MA OLD CHESAPEAKE IMPACT STRUCTURE, VIRGINIA, USA

Introduction: The ICDP-USGS deep core at Eyreville, VA, was drilled in 2005-2006 into the late Eocene Chesapeake Bay impact structure, the largest (~85km) known impact crater in the U.S. The Chesapeake Bay impact structure is a complex crater with the inner central crater (35km) which is surrounded by outer annular trough. The Eyreville core was drilled into inner crater zone near Cape Charles and includes three holes with total depth of 1766m (Gohn et al. 2006), penetrating through post-impact sediments as well as impact modified rocks.

Results: We are presenting magnetic properties of Exmore breccia (the matrix-supported sedimentary deposit) and granite megablocks from Eyreville core. Laboratory measurements of petrophysical parameters, such as density, porosity, magnetic susceptibility (k) and NRM; together with thermomagnetic measurements (including thermal behavior of magnetic susceptibility and magnetic hysteresis) were carried out on core samples. Additionally, the susceptibility of whole core was measured in USGS prior to sampling in 50-60cm intervals whenever core conditions allowed. Results show that the sandy matrix of Exmore breccia is dominated by paramagnetic (k=100*10^-6SI) behavior. Small ferromagnetic component is present in clay-silt and diamicton intercalations and is caused possibly by low concentration of titanomagnetites. Granitic megablock is magnetically distinctly more susceptible. In upper part of the megablock the fine-grained biotite granite has occasionally higher magnetization values than medium-to-coarse grained granite. Altered biotite granite in lower part of the core holds lower magnetizations. The most scattered are results from layered gneissic granites, holding both highest (k=86000*10^-6SI) and lowest (200*10^-6SI) values. Studies so far show no differences in magneto-mineralogy within granite megablock, indicating the presence of only magnetite. Slight differences in magnetic properties can be therefore linked either to differences in magnetic grain sizes or concentration of magnetic fraction.

References: G. S. Gohn, C. Koeberl, K. G. Miller, W. U. Reimold, C. S. Cockell, J. W. Horton, W. E. Sanford, M. A. Voytek, 2006. Chesapeake Bay Impact Structure Drilled. EOS, vol 87, nr 35.