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Thermal Maturation and Organic Petrology Study of ICDP-USGS Deep Corehole in the Central Crater of the Chesapeake Bay Impact Structure: MaryAnn Love Malinconico

Project Title: Thermal Maturation and Organic Petrology Study of ICDP-USGS Deep Corehole in the Central Crater of the Chesapeake Bay Impact Structure
Mendenhall Fellow: MaryAnn Love Malinconico, (703) 648-6423, mmalinconico@usgs.gov
Duty Station: Reston, VA
Start Date: October 2, 2006
Education: Ph.D. (Earth and Environmental Sciences), Columbia University, 2002
Research Advisors: J. Wright Horton, (703) 648-6933, whorton@usgs.gov; Greg Gohn, (703) 648-4382, ggohn@usgs.gov; David Powars, (703) 648-4325, dspowars@usgs.gov; Ward Sanford, (703) 648-5882, wsanford@usgs.gov

Project Description: The Chesapeake Bay Impact (CBI) structure was formed 35 million years ago (late Eocene) during the collision of an approximately 3-km asteroid or comet nucleus on the Atlantic continental margin of North America. It is the largest known impact crater in the United States (central crater diameter ~38 km; diameter including annular trough ~85 km), seventh largest in the world, and the best-preserved example of a large impact in a triple-layered (ocean water, sediment, silicic basement) target. The basement floor of the crater is approximately 2.0 km below sea level. The CBI structure is not exposed at the surface but has been buried and preserved under Eocene to Pleistocene Coastal Plain sediments. The crater was discovered in the early 1990s, and is the site of an inland wedge of highly saline ground water that has affected growing population centers around its margin.

Investigative drilling in the structure, prior to 2004, had been mostly in the annular trough and around the margins of the crater. In 2004, a partially cored test hole was drilled at Cape Charles, Va., over the central uplift, as a test preceding the late 2005–2006 drilling of a deep (1.8 km) corehole at Eyreville farm, Northampton County, Va., by the International Continental Scientific Drilling Program (ICDP) and the USGS (http://geology.er.usgs.gov/eespteam/crater/). The Eyreville core successfully sampled (with increasing depth) post-impact marine sediments, sediment clast breccia and megablocks from resurge of ocean water and target sediments into the crater, suevite (breccia that contains clasts of impact melt rock) and lithic breccia deposited from fallback or ground-surge, and fractured schist and pegmatites derived from the crater floor (Gohn and others, 2006).

The objectives of the Mendenhall research project are to measure thermal maturity and low-grade metamorphism in the crater fill and post-impact continental margin sediments of the Eyreville core for collaborative modeling of the conductive and advective thermal history of the crater and to document the particulate organic carbon facies of the post-impact sediments, particularly any evidence of impact-related wildfire. Samples from previous shallow cores may also be included in the study. These objectives support the goals of the USGS Chesapeake Bay Impact Crater Project and the related ICDP–USGS Chesapeake Bay Impact Structure Deep Drilling project, especially:

    1. understanding the thermochemical evolution of a large marine impact, including thermal gradients,
    2. understanding the evolution of saline ground water in the crater that affects overlying communities,
    3. investigation of the deep biosphere in the crater, particularly presence of any extremophile microbes and the temperature conditions they may have endured, and
    4. sedimentological and petrologic studies of impact-related sediments.

The analytical methods of the proposed research are two microscope-based thermal maturity techniques: vitrinite reflectance and spore-based thermal alteration index (TAI); and, for organic facies analysis, conventional petrographic point-counting with supportive geochemical quantitative assessment of kerogen type and elemental carbon.

Regional map showing the location of the ICDP-USGS Eyreville drill site in the Chesapeake Bay impact structure

 

Schematic radial cross section showing half of the Chesapeake Bay impact structure

(ABOVE) Schematic radial cross section showing half of the Chesapeake Bay impact structure (modified from Horton and others, 2005).

(LEFT) Regional map showing the location of the ICDP-USGS Eyreville drill site in the Chesapeake Bay impact structure (from Gohn and others, 2006).

References Cited
Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Cockell, C.S., Horton, J.W., Jr., Sanford, W.E., and Voytek, M.A., 2006, Chesapeake Bay impact structure drilled: Eos, v. 87, p. 349, 355.

Horton, J.W., Jr., Powars, D.S., and Gohn, G., 2005, Studies of the Chesapeake Bay impact structure—The USGS-NASA Langley corehole, Hampton, Virginia, and related coreholes and geophysical survey: U.S. Geological Survey Professional Paper 1688.

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