Deep Biosphere Studies at the Chesapeake Bay Impact Structure
This project was funded by the USGS and the ICDP. In the fall of 2005, continuously cored sections were acquired from two holes drilled to a composite depth of 1,766 m at a site within the central part of the structure near Cape Charles, Virginia, USA. (Fig. 1)
The buried CBIS is the seventh largest, and one of the best preserved, of the known impact structures on Earth. The structure consists of a 38-km-wide, highly deformed central zone, which approximates the dimensions and location of the transient impact crater, surrounded by a shallower outer zone of sediment collapse known as the annular trough. Together, these zones have a diameter of about 85 km and a distinctive shape that is generally referred to as an "inverted sombrero."
The primary objectives of this project are to: understand the deep subsurface biosphere at this unique site; obtain detailed information on the subsurface structure and crater fill of one of the best preserved large impact structures; to correlate these data with the geophysical studies that were recently completed or are in progress; to determine the presence and composition of melt bodies in the crater fill; and to perform comparative geochemical (including isotopic) and petrographical studies of the crater fill breccias, possible melt rocks and basement rocks.
The cored impactite section consists of five major lithologic units. (Fig. 2) The lowest unit consists of about 216 meters of fractured mica schist and pegmatite with minor gneiss and several veins of impact-generated breccia.. About 157 meters of suevitic breccias (breccias containing clasts of impact-generated melt rock) overlie the schist and pegmatite and are considered to be fallback and (or) ground-surge deposits. Above these breccias, a thin interval of quartz sand (22 meters) contains large and small lithic clasts and underlies a 275-meter-thick megablock (or megablocks) of granitic rock. The uppermost and thickest impactite unit consists of about 652 meters of deformed sediment megablocks and overlying sedimentary breccia.The post-impact sedimentary section consists of 444 meters of upper Eocene to Pliocene (~5.3 to ~1.8 Ma) marine sediments and Pleistocene (~1.8 to ~0.01 Ma) non-marine sediments.
Findings to Date
Microbiological enumeration and culture and culture-independent methods coupled with geochemical data suggest the presence of three major microbiological zones.
- The upper zone (0-700 m) is characterised by a logarithmic decline in microbial numbers from the surface through the post-impact section of upper Eocene to Pliocene marine sediments and Pleistocene non-marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments.
- The middle zone (700-1400 m) corresponds to the region that has low hydraulic conductivity and in its deeper sections may have been sterilized by the thermal pulse delivered during impact. Lack of culturable organisms, no extractable DNA and microbiological enumerations below the limits of detection throughout the middle zone show a biologically impoverished environment, which may be sterile in some locations.
- The lowest zone (>1,560 m) coincides with a region of heavily impact -fractured, hydrologically conductive target schist/pegmatite in which microbial cell numbers are higher than the middle zone and heterotrophic organisms have been cultured.
Figures
(click to enlarge)
Fig. 1
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| Regional map showing the location of the ICDP-USGS Eyreville drill site in the CBIS. | Details of the core holes and lithological sequence through the Chesapeake impact crater. |
Photographs
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| Julie Kirshtein with Halon Tank and Gunner, the drill dog. | Adding fluorescent microbeads to core barrel to track contamination in core sections. |
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| Core sectioning machine cutting a section for the biologists | Core sections | Coring rig set up in Eyreville, VA |
