Christian KÖBERL - Chesapeake Bay impact structure: ICDP/USGS deep drilling project

The Chesapeake Bay Impact Structure Deep Drilling Project (CBIS Project) completed its coring operations during September-December 2005 and April-May 2006. Cores were collected continuously to a total depth of 1,766 m. The recovered section consists of 1,322 m of impactites beneath 444 m of post-impact continental-shelf sediments.
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The CBIS Project is a joint venture of the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS). Project activities began with a planning workshop in September 2003 that was funded by the ICDP, hosted by the USGS, and attended by 63 scientists from 10 countries. A resulting funding proposal to ICDP was accepted in late 2004, and additional drilling funds were authorized by the USGS. Field operations began with site preparation in July 2005, and coring began in September 2005. DOSECC, Inc., was the general contractor for the drilling operations throughout 2005. The NASA Science Mission Directorate, ICDP, and USGS provided important supplementary drilling funds in November-December 2005 that permitted coring of the deeper part of the impact structure. Studies of post-impact sediments were supported by the U.S. National Science Foundation (NSF), Earth Science Division, Continental Dynamics Program.
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Buried at shallow to moderate depths beneath continental-margin sediments in southeastern Virginia, USA (Fig. 1), the late Eocene Chesapeake Bay impact structure is among the largest and best preserved of the known impact structures on Earth (Poag et al., 2004). It is the second largest among only a handful of known impact structures that formed in a marine setting (surpassed in size only by the Chicxulub structure in Mexico, the subject of an ICDP drilling project in 2001-2002). It is the source of one of only four tektite strewn fields that are currently known on Earth, the North American tektite strewn field (Koeberl et al., 1996). The Chesapeake Bay impact structure consists of a 38-km-wide, strongly and deeply deformed central zone surrounded by a shallower outer zone of sediment collapse known as the annular trough. Collectively, these two zones have a diameter of about 85 km and a distinctive shape that is generally referred to as an “inverted sombrero.”
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The Chesapeake Bay impact structure is an inviting target for corehole studies of impact phenomena. This structure is perhaps unique in presenting a drilling target where principles of impact effects in a shallow-marine, siliciclastic, rheologically layered target can be addressed and where the potential exists to recover a complete corehole record through the impact-breccia fill of a crater and through the post-impact sedimentary cover.
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This project also provided the opportunity to study other topics of societal and scientific interest, including the sea-level and climate history, the effects of the impact on the regional hydrologic framework and resources, and the ancient and modern microbiota of a deep subsurface environment. The post-impact upper Eocene to Pleistocene sediments that cover the impact structure consist primarily of fine-grained marine sediments that document the middle to late Cenozoic sea-level history, stratigraphic sequences, and climate variability of the Mid-Atlantic segment of the U.S. Atlantic continental margin. Development of a global sea-level estimate (Miller et al., 2005) allows quantitative evaluation of tectonic subsidence in complex regions such as the impact structure. We will backstrip the Eyreville corehole stratigraphic data to account for the effects of sediment loading, compaction, paleowater depth, and basin subsidence. Comparison with coreholes from outside of the crater (e.g., Miller et al., 2005) will allow us to quantify the effects of tectonics and global sea level.
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The presence of salty groundwater throughout the impact structure is of significant interest to hydrologists studying the future availability of fresh water in the densely populated and rapidly growing Hampton Roads-Norfolk-Virginia Beach urban corridor located along the southwestern and southern margins of the structure. Topics of immediate interest that are addressed by the deep corehole include the physical disruption of the aquifer system by the impact, the entrainment and alteration of Eocene seawater, and the effects of the impact-related hydrothermal system and later diagenesis on groundwater chemistry. Core samples were collected from the Eyreville cores for the determination of groundwater chemistry and hydrogeologic properties of the sediments and rocks.
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The CBIS Project provided an opportunity for deep biosphere research in a variety of environmental and paleo-environmental settings to elucidate such basic parameters as subsurface microbial diversity and abundance. For example, although specialized life forms exist in Earth’s hydrothermal systems, the biota of terrestrial hydrothermal systems related to impact events are essentially unstudied compared to those related to volcanic activity. The long history of impact cratering throughout the solar system suggests the possibility that impact-related hydrothermal systems might be common habitats on other solar system bodies and the need for the study of similar systems on Earth. In addition, cores from the post-impact section at Eyreville provide an opportunity to study fossil microbial traces at significant depths within geologically young marine sections, in addition to the hydrothermal environment. Microbiota samples were collected from the three Eyreville cores using appropriate drill-site anti-contamination protocols, including halon gas and microbeads as tracers in the drilling mud during core retrieval.

The drilling program was designed to continuously sample the entire section of post-impact sediments and crater-filling impactites, and a short section of autochthonous breccias in the crater floor, to a depth of about 2.2 km. However, problems with lost mud circulation, trapped drill rods, and locally slow penetration rates in the impactite section ultimately limited the total depth to 1.766 km. The drill site is located on private land, known locally as Eyreville Farm (Fig. 2), in Northampton County, Virginia, about 7 km north of the town of Cape Charles (Fig. 1).
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Three coreholes were drilled at the Eyreville site (Table 1). In late July 2005, Somerset Drilling, Inc., began the process by rotary drilling (no coring) to a depth of about 128 m and installing large-diameter steel casing to a depth of 125 m in what would become the Eyreville A corehole.

Eyreville A:	125.6 to 591.0 m, PQ core (85.0 mm diameter) 591.0 to 940.9 m, HQ core (63.5 mm diameter)
Eyreville B:	737.6 to 1,100.9 m, HQ core (63.5 mm diameter) 1,100.9 to 1,766.3 m, NQ core (47.6 mm diameter)
Eyreville C:	0 to 140.2 m, HQ core (63.5 mm diameter)

Table 1. Cored sections in the Eyreville coreholes.

The principal contract driller, Major Drilling America, began coring at 125 m depth in Eyreville A on September 15th using a CP-50 wireline coring rig. Coring with a PQ sampling system continued to a depth of 591.0 m when mud circulation was lost on September 23rd, and the CHD-134 (“PQ”) rods became trapped in the hole. Coring resumed on September 24th using an HQ coring system and continued to a depth of 940.9 m on October 8th when mud circulation again was lost. At that time, the HQ rods and bit were pulled up to a depth of 591.0 m. Repeated attempts to ream the hole back to 940.9 m depth continued for nearly two weeks because of repeated loss of mud circulation due to expanding and sliding red-clay sections.
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The bit finally returned to a depth of 940.9 m on October 20th. However, during the reaming process, the bit had deviated from the original hole at a depth of 737.6 m. As a result, duplicate cores were collected between depths of 737.6 m and 940.9 m. The new corehole below the deviation point at 737.6 m was designated as the Eyreville B corehole (Table 1).
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Coring with the HQ system continued to a depth of 1,100.9 m in Eyreville B, at which point the HQ bit was deliberately stuck within a section of granite on October 26th, leaving the HQ rods in the hole to serve as casing against the red clays. Coring with an NQ sampling system started on October 27th at 1,100.9 m and continued without major problems to the final depth of 1,766.3 m within Eyreville B on December 4th.
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Project members from the USGS, Rutgers University, and the Virginia Department of Environmental Quality returned to the Eyreville site in April 2006 and cored a third hole, Eyreville C, using a USGS truck-mounted Mobile B-61 wireline coring rig. HQ cores were recovered to a total depth 140.2 m between April 29 and May 4 (Table 1). As a result, the upper part of the post-impact sedimentary section was sampled in Eyreville C to complement the deeper section of post-impact sediments recovered in Eyreville A.
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Limited suites of geophysical logs (natural gamma, spontaneous potential, resistivity) were acquired from the upper 125 m of the Eyreville A corehole and from the 140-m-deep Eyreville C corehole. Unfortunately, planned interim and final geophysical logging programs for the deeper section of combined coreholes A and B were compromised because of trapped drill rods, logging equipment malfunctions, and bridging of the open hole after the NQ rods were removed. Three logs were acquired after the coring was completed on December 4th. The USGS logger collected a natural gamma log and a temperature log inside the NQ rods for nearly the entire length of combined holes A and B. A temperature log also was collected using a probe from Karlsruhe University, Germany. Additional temperature logs were collected in the A-B corehole in May 2006 using the Karlsruhe temperature logger. Supplementary measurements of petrophysical properties using a multisensor core logger are planned.

The 1,322-m-thick section of impactites consists of four major lithologic units (Gohn et al., 2006; Reimold et al., 2006). The lowest unit consists of about 216 m of mica schist and pegmatite with minor gneiss and a few impact-generated breccia veins (Table 2). About 157 m of suevitic and lithic impact breccias (Fig. 4) overlie the schists and pegmatites and underlie a 275-m-thick megablock (or megablocks) of granitic rock. The upper part of the sediment section consists of a sedimentary breccia that contains sediment clasts and crystalline-rock clasts. A wide variety of mineralogic, petrologic, geochemical, radiometric, and structural studies of the impactite section is now underway.

0 to 444 m	Post-impact sediments
444 to 1,096 m	Sediment-clast breccia and sediment megablocks
1,096 to 1,371 m	Granitic megablock(s)
1,371 to 1,393 m	Lithic blocks in sediment
1,393 to ca. 1,550 m	Suevitic and lithic breccia
ca. 1,550 to 1,766 m	Schist and pegmatite; breccia veins

Table 2. Preliminary composite geologic section for the Eyreville coreholes.

The huge unexpected megablock of granite encountered at Eyreville presents an example of the need for perseverance and “thinking big” during the drilling of large impact structures. The coring of hundreds of meters of granite across numerous days eventually lead some to believe that the crater floor had been penetrated and that drilling operations should cease, whereas scientists more familiar with the project’s geophysical data suggested that the coring should continue. The coring did continue and ultimately the base of the granite was reached and the important section of suevitic breccias was encountered below (Table 2).
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The 444-m-thick section of post-impact sediments consists of upper Eocene, Oligocene, Miocene, and Pliocene marine sediments and Pleistocene paralic sediments. Lithologic (including grain size, composition, and clay mineralogy), sequence stratigraphic, biostratigraphic (including studies of calcareous nannofossils, foraminifers, dinocysts, diatoms, and pollen), and chemostratigraphic (including Sr- and stable isotopes) studies are ongoing. Preliminary studies indicate thick middle Miocene to Pliocene and upper Eocene successions, with a relatively thin lower Miocene and Oligocene sections.

The research phase of the project began on March 19-22, 2006, with a international sampling party at the USGS National Center in Reston, Va., USA. At that time, the cores from the Eyreville A and B coreholes were displayed for examination by the project science-team members (Fig. 5). About thirty project scientists from seven countries attended the sampling party, and about 1,800 samples were marked for future study. Popular targets for sampling were the suevitic and lithic impact breccias as well as the short section that records the transition from late syn-impact to post-impact sedimentation and bio-recovery. The cutting and shipping of samples was underway in April and May and will be completed by June 2006.
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The project’s publication plan calls for presentation of preliminary results at an international meeting in 2007, followed closely by a published scientific report. A comprehensive, multi-chapter volume is planned for 2008.

We thank DOSECC, Inc., for their excellent handling of the field operations, and Major Drilling America for their professionalism and the successful completion of the deep coring. We also thank the Buyrn family for the use of their land for the drilling operations and for their enthusiastic interest in the project. We thank ICDP, USGS, the NASA Science Mission Directorate, and NSF for funding the drilling operations. Finally, the Principal Investigators thank the international group of site geologists and technicians for their dedication and hard work at the drill site.

O. Abramov, W. Aleman Gonzalez, N. Bach, A. Blazejak, J. Browning, T. Bruce, C. Budet, L. Bybell, E. Cobbs, Jr., E. Cobbs, III, C. Cockell, B. Corland, C. Durand, H. Dypvik, J. Eckberg, L. Edwards, S. Eichenauer, T. Elbra, A. Elmore, J. Glidewell, G. Gohn, A. Gronstal, A. Harris, P. Heidinger, S.-C. Hester, W. Horton, Jr., K. Jones, A. Julson, D. King, J. Kirshtein, C. Koeberl, T. Kohout, T. Kraemer, D. Kring, A. Kulpecz, M. Kunk, D. Larson, U. Limpitlaw, M. Lowit, N. McKeown, P. McLaughlin, K. Miller, S. Mizintseva, R. Morin, J. Morrow, J. Murray, J. Ormö, R. Ortiz Martinez, L. Petruny, H. Pierce, J. Plescia, D. Powars, A. Pusz, D.B. Queen, D.G. Queen, U. Reimold, W. Sanford, E. Seefelt, J. Self-Trail, D. Vanko, M. Voytek, B. Wade, J. Wade, D. Webster, B. Zinn, V. Zivkovic.

1Browning, J.V., Miller, K.G., McLaughlin, P.P., Kominz, M.A., Sugarman, P.J., Monteverde, D., Feigenson, M.D., and Hernandez, J.C., 2006, Quantification of the effects of eustasy, subsidence, and sediment supply on Miocene sequences, mid-Atlantic margin of the United States. Geological Society of America Bulletin, v. 118, p. 567-588.
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Dowsett, H.J. and Cronin, T.M, 1990, High eustatic sea level during the middle Pliocene: Evidence from southeastern U.S. Atlantic coastal plain. Geology, v. 18, p. 435-438.
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Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., et al., 2006, Preliminary site report for the 2005 ICDP-USGS deep corehole in the Chesapeake Bay impact crater (abst.). 37th Lunar and Planetary Science Conference, Houston, TX, March 13-17, 2006, abstract no. 1713.
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Koeberl, C., Poag, C.W., Reimold, W.U., and Brandt, D. (1996) Impact origin of Chesapeake Bay structure and the source of North American tektites. Science v. 271, p. 1263-1266.
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Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E., Sugarman, P.J., Cramer, B.S., Christie-Blick, N., and Pekar, S.F., 2005, The Phanerozoic record of global sea-level change. Science, v. 310, p. 1293-1298.
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Poag, C.W., Koeberl, C., and Reimold, W.U., 2004, Chesapeake Bay Crater: Geology and Geophysics of a Late Eocene Submarine Impact Structure. Impact Studies, vol. 4, Springer, Heidelberg, 522 pp. (+ CD-ROM).
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Reimold, W.U., Gohn, G.S., Koeberl, C., and Miller, K.G., 2006, Report on the 2005 ICDP-USGS deep corehole in the Chesapeake Bay impact structure (abst.). Workshop - Impact Craters as Indicators for Planetary Environmental Evolution and Astrobiology, Östersund, Sweden, June 8 - 14, 2006..

Gregory S. Gohn, U.S. Geological Survey, 926A National Center, Reston, VA, USA
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Christian Koeberl, University of Vienna, Vienna, Austria
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Kenneth G. Miller, Rutgers University, Piscataway, NJ, USA
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Wolf Uwe Reimold, Univ. of the Witwatersrand, Johannesburg, South Africa, & Humboldt University, Berlin, Germany