A petrographic and geochemical comparison of suevites from the LB-07A and LB-08A cores recovered during 2004 by the International Continental Scientific Drilling Program with suevites from outside of the crater rim of the Bosumtwi impact structure indicates contrasting mechanisms of formation for these respective impact breccias. The within-crater suevites form only a small part of the lithic impact breccia–dominated impactite crater fill, in contrast to the impactites from outside of the crater, which consist solely of suevite. The clasts of suevites from within the crater display relatively low levels of shock (for most material <45 GPa). The numbers of shocked quartz grains, as well as fragments of diaplectic glass of quartz and feldspar in suevites decrease with depth through the LB-07A core (maximum three sets of planar deformation features [PDFs]). In contrast, the out-of-crater suevites sampled north and south of the crater contain up to four PDF sets in quartz clasts, ballen cristobalite, and higher proportions of diaplectic glass than the within-crater suevites. In addition, the suevites from outside of the crater contain significantly more melt particles (18–37 vol%) than the within-crater suevites (<5 vol%). Melt fragment sizes in suevites from outside the crater are much larger than those from suevites within the crater (maximum 40 cm versus 1 cm). The currently known distribution of impactites outside of the crater would be consistent with a low-angle impact from the east. We propose that the within-crater suevites and polymict lithic breccias were emplaced either via slumping off the crater walls or lateral movement of some melted and much displaced target rock within the crater. Limited admixture of fallback material from the ejecta plume is evident in the uppermost impactite deposit encountered in core LB-05B. In contrast, the out-of-crater suevites formed by fallout from a laterally differentiated ejecta plume, which resulted in different clast populations to the north and south of the crater.

Crater-fill impact breccia and basement rock samples from the 1.07 Ma Bosumtwi impact structure (Ghana) were recovered for the first time in 2004 during an International Continental Scientific Drilling Program (ICDP)–sponsored drilling project. Here, we present detailed results of major- and trace-element analyses of 119 samples from drill core LB-08A, together with the chemical compositions of melt particles from suevite. The meta-graywacke and phyllite/slate crater basement rocks can be easily distinguished from each other on the basis of their bulk chemical compositions. A comparison of the chemical compositions of crater-fill and fallout suevites, as well as between proximal and distal impactites, reveals that LB-08A suevites have higher MgO, CaO, and Na 2 O contents than fallout suevites and, similarly, that the CaO and Na 2 O contents are higher by a factor of approximately two in LB-08A suevites than in Ivory Coast tektites. Noticeable differences occur in Cr, Co, and Ni contents between the different impactites; higher abundances are observed for these elements in distal impactites. The observed differences in composition in the various impactites mainly reflect mixing of different proportions of the original target lithologies, as can be seen in the differences in the clast populations between crater-fill and fallout suevites. However, the original impactite compositions may have also been modified by postimpact alteration, particularly in the proximal impactites. Melt particles in suevite show significant differences in major-element compositions between the different samples investigated, but also within a given sample, indicating that they represent melts derived from different lithologies.

The late Eocene Chesapeake Bay impact structure lies buried at moderate depths below Chesapeake Bay and surrounding landmasses in southeastern Virginia, USA. Numerous characteristics made this impact structure an inviting target for scientific drilling, including the location of the impact on the Eocene continental shelf, its three-layer target structure, its large size (~85 km diameter), its status as the source of the North American tektite strewn field, its temporal association with other late Eocene terrestrial impacts, its documented effects on the regional groundwater system, and its previously unstudied effects on the deep microbial biosphere. The Chesapeake Bay impact structure Deep Drilling Project was designed to drill a deep, continuously cored test hole into the central part of the structure. A project workshop, funding proposals, and the acceptance of those proposals occurred during 2003–2005. Initial drilling funds were provided by the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS). Supplementary funds were provided by the National Aeronautics and Space Administration (NASA) Science Mission Directorate, ICDP, and USGS. Field operations were conducted at Eyreville Farm, Northampton County, Virginia, by Drilling, Observation, and Sampling of the Earth's Continental Crust (DOSECC) and the project staff during September–December 2005, resulting in two continuously cored, deep holes. The USGS and Rutgers University cored a shallow hole to 140 m in April–May 2006 to complete the recovered section from land surface to 1766 m depth. The recovered section consists of 1322 m of crater materials and 444 m of overlying postimpact Eocene to Pleistocene sediments. The crater section consists of, from base to top: basement-derived blocks of crystalline rocks (215 m); a section of suevite, impact melt rock, lithic impact breccia, and cataclasites (154 m); a thin interval of quartz sand and lithic blocks (26 m); a granite megablock (275 m); and sediment blocks and boulders, polymict, sediment-clast–dominated sedimentary breccias, and a thin upper section of stratified sediments (652 m). The cored postimpact sediments provide insight into the effects of a large continental-margin impact on subsequent coastal-plain sedimentation. This volume contains the first results of multidisciplinary studies of the Eyreville cores and related topics. The volume is divided into these sections: geologic column; borehole geophysical studies; regional geophysical studies; crystalline rocks, impactites, and impact models; sedimentary breccias; postimpact sediments; hydrologic and geothermal studies; and microbiologic studies.

The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville drill cores from the Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure. This paper presents a series of geologic columns and descriptive lithologic information for the lower impactite and crystalline-rock sections in the cores. The lowermost cored section (1766–1551 m depth) is a complex assemblage of mica schists that commonly contain graphite and fibrolitic sillimanite, intrusive granite pegmatites that grade into coarse granite, and local zones of mylonitic deformation. This basement-derived section is variably overprinted by brittle cataclastic fabrics and locally cut by dikes of polymict impact breccia, including several suevite dikes. An overlying succession of suevites and lithic impact breccias (1551–1397 m) includes a lower section dominated by polymict lithic impact breccia with blocks (up to 17 m) and boulders of cataclastic gneiss and an upper section (above 1474 m) of suevites and clast-rich impact melt rocks. The uppermost suevite is overlain by 26 m (1397–1371 m) of gravelly quartz sand that contains an amphibolite block and boulders of cataclasite and suevite. Above the sand, a 275-m-thick allochthonous granite slab (1371–1096 m) includes gneissic biotite granite, fine- and medium-to-coarse–grained biotite granites, and red altered granite near the base. The granite slab is overlain by more gravelly sand, and both are attributed to debris-avalanche and/or rockslide deposition that slightly preceded or accompanied seawater-resurge into the collapsing transient crater.

The Eyreville A and B cores, recovered from the “moat” of the Chesapeake Bay impact structure, provide a thick section of sediment-clast breccias and minor stratified sediments from 1095.74 to 443.90 m. This paper discusses the components of these breccias, presents a geologic column and descriptive lithologic framework for them, and formalizes the Exmore Formation. From 1095.74 to ~867 m, the cores consist of nonmarine sediment boulders and sand (rare blocks up to 15.3 m intersected diameter). A sharp contact in both cores at ~867 m marks the lowest clayey, silty, glauconitic quartz sand that constitutes the base of the Exmore Formation and its lower diamicton member. Here, material derived from the upper sediment target layers, as well as some impact ejecta, occurs. The block-dominated member of the Exmore Formation, from ~855–618.23 m, consists of nonmarine sediment blocks and boulders (up to 45.5 m) that are juxtaposed complexly. Blocks of oxidized clay are an important component. Above 618.23 m, which is the base of the informal upper diamicton member of the Exmore Formation, the glauconitic matrix is a consistent component in diamicton layers between nonmarine sediment clasts that decrease in size upward in the section. Crystalline-rock clasts are not randomly distributed but rather form local concentrations. The upper part of the Exmore Formation consists of crudely fining-upward sandy packages capped by laminated silt and clay. The overlap interval of Eyreville A and B (940–~760 m) allows recognition of local similarities and differences in the breccias.

A 443.9-m-thick, virtually undisturbed section of postimpact deposits in the Chesapeake Bay impact structure was recovered in the Eyreville A and C cores, Northampton County, Virginia, within the “moat” of the structure's central crater. Recovered sediments are mainly fine-grained marine siliciclastics, with the exception of Pleistocene sand, clay, and gravel. The lowest postimpact unit is the upper Eocene Chickahominy Formation (443.9–350.1 m). At 93.8 m, this is the maximum thickness yet recovered for deposits that represent the return to “normal marine” sedimentation. The Drummonds Corner beds (informal) and the Old Church Formation are thin Oligocene units present between 350.1 and 344.7 m. Above the Oligocene, there is a more typical Virginia coastal plain succession. The Calvert Formation (344.7–225.4 m) includes a thin lower Miocene part overlain by a much thicker middle Mio-cene part. From 225.4 to 206.0 m, sediments of the middle Miocene Choptank Formation, rarely reported in the Virginia coastal plain, are present. The thick upper Miocene St. Marys and Eastover Formations (206.0–57.8 m) appear to represent a more complete succession than in the type localities. Correlation with the nearby Kiptopeke core indicates that two Pliocene units are present: Yorktown (57.8–32.2 m) and Chowan River Formations (32.2–18.3 m). Sediments at the top of the section represent an upper Pleistocene channel-fill and are assigned to the Butlers Bluff and Occohannock Members of the Nassawadox Formation (18.3–0.6 m).

During 2005–2006, the International Continental Scientific Drilling Program and the U.S. Geological Survey drilled three continuous core holes into the Chesapeake Bay impact structure to a total depth of 1766.3 m. A collection of supplemental materials that presents a record of the core recovery and measurement data for the Eyreville cores is available on CD-ROM at the end of this volume and in the GSA Data Repository. The supplemental materials on the CD-ROM include digital photographs of each core box from the three core holes, tables of the three coring-run logs, as recorded on site, and a set of depth-conversion programs. In this chapter, the contents, purposes, and basic applications of the supplemental materials are briefly described. With this information, users can quickly decide if the materials will apply to their specific research needs.

Chesapeake is a 35-Ma-old shallow-marine, complex impact structure with a diameter of ~85 km. The structure is completely buried beneath several hundreds of meters of postimpact sediments. Therefore, subsurface information can be obtained only from geophysical surveys and drill holes. Recently, deep drilling into the inner crater zone, at Eyreville near Cape Charles, was carried out in order to provide constraints on geophysical modeling and cratering processes in a multilayered marine target. We analyzed samples of the Eyreville core including postimpact, impact- produced, and basement-derived units in order to clarify the magneto-mineralogy, to provide physical parameters for better understanding the influence of the impact on the petrophysical and rock-magnetic properties, and to provide rock-magnetic data for magnetic modeling. Results show a complex behavior of physical properties of the lithologies in the Eyreville core due to different lithologies having been affected by shock-induced changes. Our data suggest that pyrrhotite and magnetite carry the magnetic properties in most of the core samples, whereas hematite is present in oxidized clays from the uppermost impact-generated unit (Exmore beds) and related sediment megablocks. The granitic megablock appears to be undeformed based on lack of brittle deformation in magnetite and petrophysically appears as a single block. In contrast, the impactite sequence below the megablock shows brittle deformation and magnetic fabric randomization, and the pyrrhotite in the associated schist fragments is strongly fractured. Thus, the Chesapeake Bay deep core provides an extraordinary opportunity to study the effect of impact on magnetite and pyrrhotite, the two main magnetic minerals creating crustal magnetic anomalies.