The ICDP-USGS Chesapeake Bay Impact Structure Deep Drilling Project
Gregory S. Gohn U.S. Geological Survey
Christian Koeberl University of Vienna
Kenneth G. Miller Rutgers University
Wolf Uwe Reimold Humboldt University
The Chesapeake Bay Impact Structure Deep Drilling 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 2003 that was attended by 63 scientists from ten countries. In 2004, a funding proposal to ICDP was accepted, and the USGS authorized additional drilling funds. The NASA Science Mission Directorate, ICDP, and USGS provided important supplementary drilling funds in November 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.
Figure 1 – Chesapeake Bay drilling locations.
The late Eocene Chesapeake Bay impact structure (CBIS) is buried at moderate depths beneath continental-margin sediments in southeastern Virginia, USA. It is among the largest and best preserved of the known impact structures on Earth (Poag, et al., 2004; Horton et al., 2005). The CBIS consists of a ~35 to 40-km diameter, strongly deformed central crater surrounded by a ~25-km-wide, less deformed annular trough. Therefore, the full diameter of the structure is ~85 to 90 km. The Eyreville coreholes were drilled into the deepest part of the central crater, as determined from pre-drilling seismic surveys. This structure is perhaps unique among known impact structures as a locality where impact effects in a shallow-marine, rheologically layered, silicic target can be addressed by core drilling. Also, it is the source of one of only four known tektite strewn fields on Earth, the North American tektite strewn field (Koeberl et al., 1996).
Figure 2 – Drilling pad setup, including driller’s trailer (left) and USGS office (right)
Site preparations began in July 2005 at Eyreville Farm in Northampton County, Virginia, USA, and deep coring operations were conducted during September-December 2005. DOSECC, Inc., was the general contractor for the deep drilling operations, and Major Drilling America, Inc., was the contract drilling company. Two connected coreholes were drilled (Table 1). Eyreville corehole A was drilled to a depth of 940.9 m. Problems with swelling clays and the repeated loss of mud circulation led to deviation from Eyreville A to a new corehole, Eyreville B, at 737.6 m. Eyreville B was drilled to a final depth of 1,766.3 m. The USGS, Rutgers University, and the Virginia Department of Environmental Quality conducted shallow coring operations in May-June 2006 to a depth of 140.2 m (Table 1). The final result was a continuously cored section from land surface to a total depth of 1,766 m (Gohn et al., 2006).
Table 1. Cored sections in the Eyreville coreholes
125.6 to 591.0 m, PQ core (85.0 mm diameter)
591.0 to 940.9 m, HQ core (63.5 mm diameter)
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)
0 to 140.2 m, HQ core (63.5 mm diameter)
A total of 1,322 m of crater-fill materials and 444 m of overlying post-impact sediments were recovered in the Eyreville cores (Table 2.) The cored crater section consists of five major lithologic units. The lowest unit consists of 215 m of fractured mica schist, granite pegmatite and coarse granite, and several impact-breccia veins. These rocks could be the autochthonous crater floor, but, more likely, they are parautochthonous basement blocks.
Above these rocks, 158 m of melt-bearing and lithic breccias are considered to be fallback and (or) ground-surge deposits. Above these breccias, a thin interval of quartz sand (22 m) contains an amphibolite block and other lithic clasts of centimeter to decimeter size. This sand occurs below a 275-m-thick granite slab, which is unshocked, and thus must have been transported at least 10 km from the rim of the crater during collapse of the transient cavity. The uppermost and thickest impactite unit consists of 652 m of deformed sediment blocks and overlying sediment-clast breccia. This unit contains clasts of target sediments and crystalline rock, as well as a small component of impact melt, and it is interpreted to represent late-stage collapse of the marine water column and its catastrophic flow back into the crater.
In the post-impact section, the upper Eocene interval consists of silty clays that were deposited in deep water (~200-300 m). There is a major facies shift from the Eocene clays to glauconitic Oligocene sediments that are associated with a >5 myr hiatus that may be attributed to a combination of eustatic fall, tectonic uplift, and sediment starvation. The lower Miocene strata also are highly dissected and thin. Sedimentation rates increase dramatically in conjunction with another facies change to fine-grained biosiliceous middle Miocene sediments. A ~13 to 8 Ma was followed by deposition of shelly, sandy upper Miocene and Pliocene strata. The Pleistocene section consists of two paralic sequences.
Table 2. Generalized composite geologic section for the Eyreville cores (Gohn et al., 2008).
0 to 444 m
444 to 1,096 m
Sediment-clast breccias and sediment blocks
1,096 to 1,371 m
1,371 to 1,393 m
Lithic blocks in sediment
1,393 to 1,551 m
Suevite, melt rock, lithic breccia, cataclasite
1,551 to 1,766 m
Schist, pegmatite and coarse granite, impact-breccia veins
Research Program and Selected Preliminary Results
The project has wide ranging scientific goals that include studies of impact processes and products, post-impact continental margin tectonics and sedimentation, hydrologic consequences of the impact, and the effects of the impact on the microbial biosphere (Gohn et al., 2008). International science teams established at the planning workshop began the research phase of the project in March 2006 with a sampling party at the USGS National Center in Reston, VA. About thirty project scientists from seven countries attended the sampling party, and over 1,800 samples were marked for future study. These samples were in addition to hundreds of samples collected at the drill site for hydrologic and biologic study. Suevitic and lithic impact breccias were popular intervals for sampling, as well as the thin interval that records the transition from syn-impact to post-impact sedimentation.
The release of detailed research results began in October 2007 with 53 oral and poster presentations in three sessions at the Geological Society of America Annual Meeting. A 43-chapter volume of research results is nearing completion (December 2008) and a 2009 publication date is expected.
Major-ion and stable-isotope chemical analyses for >100 pore-water samples from the Eyreville cores yielded salinities of 40-64 ppt over most of the 1,322-m-thick section of impact deposits. Chloride and 18O values in the post-impact sediments show a steady trend with depth that indicates vertical mixing of fresh and saline waters. The Ca/Mg ratio increases regularly with depth, suggesting water-rock interaction related to increasing temperatures with depth. Overall, the pore-water chemistry suggests that much of the ground water in the central crater has been there since the impact, and that much of the brine was likely present in the target coastal plain sediments before impact.
Microbiological enumeration and other methods, combined with geochemical data, suggest that three microbiological zones are present in the Eyreville cores. The upper zone (0-700 m) is characterized by a logarithmic decline in microbial numbers from the surface through the post-impact section across the transition into the upper layers of the crater-fill sediments. The middle zone (700-1,400 m) corresponds to a region of low hydraulic conductivity that may have been sterilized, in part, by the impact thermal pulse. A lack of culturable organisms and extractable DNA, and microbiological enumerations below the limits of detection, suggest that the middle zone is a biologically impoverished environment. The lowest zone (>1,560 m) coincides with fractured, hydrologically conductive schist, pegmatite, and granite. It has microbial cell numbers that are higher than the middle zone, and heterotrophic organisms have been cultured. These results support the hypothesis that impact events cause disruption to the subsurface biosphere that can result in well-defined zones of microbiological colonization linked to the process of impact cratering.
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 Transactions, American Geophysical Union, v. 87(35), p. 349, 355.
Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Browning, J.V., Cockell, C.S., Horton, J.W., Jr., Kenkmann, T., Kulpecz, A.A., Powars, D.S., Sanford, W.E., and Voytek, M.A., 2008, Deep drilling into the Chesapeake Bay impact structure: Science, v. 320, p. 1740-1745.
Horton, J.W., Jr., Powars, D.S., and Gohn, G.S., 2005, Studies of the Chesapeake Bay impact structure—Introduction and discussion, in Horton, J.W., Jr., Powars, D.S., and Gohn, G.S., Studies of the Chesapeake Bay impact structure—The USGS-NASA Langley corehole, Hampton, Virginia, and related coreholes and geophysical surveys: U.S. Geological Survey Professional Paper 1688, p. A1-A23.
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.
Poag, C.W., Koeberl, C., and Reimold, W.U., 2004, The Chesapeake Bay Crater—Geology and Geophysics of a Late Eocene Submarine Impact Structure: New York, Springer-Verlag, 522 p., CD-ROM.