A detailed total intensity magnetic survey of a local negative magnetic anomaly located in the southern sector of the inner ring in the Ries impact structure was carried out in 2006–2007. As the suevite of the Ries crater is known to have an often strong reverse remanent magnetization causing negative magnetic anomalies, a suevite body lying below shallow lake sediments upon the crystalline basement rocks of the inner ring was suspected to be the cause of the anomaly. A drilling program conducted by the Geological Service of Bavaria offered the opportunity to drill a 100-m-deep core hole into this anomaly in 2006. The core stratigraphy involves from 0 to 4.5 m fluviatile Quaternary lake sediments, from 4.5 to 21 m Neogene clays of the Ries crater lake, and from 21 to 100 m suevite and impact melt rock. The suevite and the impact melt rock have a strong reverse remanent magnetization and very high Koenigsberger ratios. Thermomagnetic and coercivity analyses indicate that magnetite is the dominant carrier of the magnetization. The borehole unfortunately did not penetrate the crystalline basement rocks of the inner ring, but modeling of the magnetic source body indicates that the bottom of the hole could not be far from the contact. A macroscopic survey shows suevite from 21 to 87 m, highly diverse in terms of suevite types, and a gradational transition to massive impact melt rock constituting the lowermost 13 m of the drill core. A detailed macroscopic description and first results of microscopic observations reveal that suevite groundmass is substantially altered to secondary phyllosilicates (mostly smectite, minor chlorite) and locally extensive development of calcite. Crystalline basement–derived lithic clasts and minerals dominate the clast population, and only traces of clastic material derived from the upper sediment parts of the target could be recorded. Macroscopically and microscopically, melt fragments have mostly irregular shapes, which lead to the tentative conclusion that only part of the melt—and by implication suevite—mass is derived from fallout of the ejecta curtain. On the other hand, most melt fragments and larger lithic clasts are seemingly oriented subperpendicular to the core axis. This could be interpreted as being due alternatively to settling through air or lateral movement within the actual crater. The gradational zone between proper suevite and massive impact melt rock is characterized by increasing enrichment of melt component and concomitant reduction of suevitic groundmass, until in the uppermost impact melt rock, only millimeter-wide stringers of groundmass remain between densely packed centimeter- to decimeter-size melt fragments.

This study is a first attempt to compile sedimentological features of synimpact to postimpact marine sedimentary successions from marine-target impact craters utilizing six well-studied examples (Chesapeake Bay, Gardnos, Kärdla, Lockne, Mjølnir, and Wetumpka). The sedimentary formations succeed autochthonous breccias and, in some cases, allochthonous suevites. These late synimpact and early postimpact depositional successions (on top of the suevites) appear to be in comparable stratigraphic developments and facies in marine-impact craters. Their composition reflects common mechanisms of sedimentation; they were developed from avalanches/scree, slides, and slumps through sequences of mass-flow–dominated deposition before ending with density currents and fine-grained sedimentation from fluidal flow and suspension. With detailed study, it may be possible to separate the late synimpact and early postimpact successions based on their clast composition relative to target stratigraphy. The process-related comparisons presented here are highly simplified, including characteristics of moat, central peak, and marginal basin sedimentation of both simple and complex craters.

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 200 Ma, 24-km-diameter Rochechouart impact structure was formed in granitic intrusive and metamorphic rocks of Variscan age (400–300 Ma) close to the margin of the Mesozoic sea. Fractured basement and autochthonous breccias form a several-decameter-thick semicontinuous zone over an 18–20-km-diameter zone. Impact melt rocks, suevite, and polymict lithic breccia are spread over an ~15 km inner zone, forming a centro-symmetric deposit inclined 0.6°N. No topographic expression of the central uplift exists. The crater floor is at the same elevation (~±50 m) over a zone at least 20 km in diameter, corresponding to the central part of the original crater. The pre-erosional diameter of the crater is probably larger than previously thought and possibly reached 40–50 km. The structure appears much less eroded than previously thought, as the sequence of crater fill is complete as exposed near Chassenon. The suevite in Chassenon is capped by an ash-like horizontal deposit of very glass-poor, fine-grained, lithic debris derived from basement rocks. Material with similar grain size and composition is observed in centimeter- to meter-thick multilayered glass-bearing intercalations (dikes) cutting through the suevite. The integrity of the Chassenon sequence strikingly contrasts with the age and morphology of the structure, implying that a rapid and thick sedimentary deposit has covered the crater to protect it from erosion. The impactoclastic top deposit also firmly constrains the thickness and volume of the initial crater fill, which appear extremely depleted (by a factor of 5 or more) compared with similar-sized impact structures and model-based calculations. This anomaly remains unexplained. All the impactites, including the glass-poor and glass-free impactites, are characterized by a prominent K-metasomatism signifying pronounced postimpact hydrothermal activity. Exposed in isolated occurrences from the center to the periphery of the inner 15-km-diameter zone, impact melt rocks are extremely unlikely to have formed a continuous sheet. They display a large variety of textures, grading from pure melt rock into basal suevite, which are distinct in composition, texture, and setting from the main suevite body forming the top of the impact deposit. Heterogeneity and relative inefficiency in mixing are characteristic of the whole impact deposit, resulting in heterogeneous melts at the scale of hand specimens, but also at the kilometer scale, as suggested by close ties between the composition of melt-bearing rocks and the subjacent target rocks.

The study of α-quartz and α-cristobalite ballen in rocks from 16 impact structures (Bosumtwi, Chesapeake Bay, Chicxulub, Dellen, El'gygytgyn, Jänisjärvi, Lappajärvi, Logoisk, Mien, Popigai, Puchezh-Katunki, Ries, Rochechouart, Sääksjärvi, Ternovka, and Wanapitei) shows that ballen silica occurs mainly in impact melt rock and also in suevite, and more rarely in other types of impactites. Ballen α-cristobalite by itself was observed only in samples from the youngest craters studied here (at Bosumtwi and El'gygytgyn), but it occurs in association with α-quartz ballen in impactites from structures with intermediate ages (from ca. 35 to 120 Ma); thus, our observations suggest that α-cristobalite ballen are back-transformed to α-quartz with time. Transmission electron microscope observations show that α-cristobalite and α-quartz ballen have similar microtextures and are formed of several tiny angular crystals with sizes up to ~6 μm. The observation of toasted α-quartz ballen, notably at the Popigai impact structure, further supports the notion that toasting is due to vesicle formation after pressure release, at high post-shock temperatures, and, thus, represents the beginning of quartz breakdown due to heating. Our investigation increases the number of impact structures at which ballen silica has been found to 35.

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 moat of the 85-km-diameter and 35.3-Ma-old Chesapeake Bay impact structure (USA) was drilled at Eyreville Farm in 2005–2006 as part of an International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) drilling project. The Eyreville drilling penetrated postimpact sediments and impactites, as well as crystalline basement-derived material, to a total depth of 1766 m. We present petrographic observations on 43 samples of suevite, impact melt rock, polymict lithic impact breccia, cataclastic gneiss, and clasts in suevite, from the impact breccia section from 1397 to 1551 m depth in the Eyreville B drill core. Suevite samples have a fine-grained clastic matrix and contain a variety of mineral and rock clasts, including sedimentary, metamorphic, and igneous lithologies. Six subunits (U1–U6, from top to bottom) are distinguished in the impact breccia section based on abundance of different clasts, melt particles, and matrix; the boundaries between the subunits are generally gradational. Sedimentary clasts are dominant in most subunits (especially in U1, but also in U3, U4, and U6). There are two melt-rich subunits (U1 and U3), and there are two melt-poor subunits with predominantly crystalline clasts (U2 and U5). The lower part (subunits U5 and U6), which has large blocks of cataclastic gneiss and rare melt particles, probably represents ground-surge material. Subunit U1 possibly represents fallback material, since it contains shard-like melt particles that were solidified before incorporation into the breccia. The melt-poor, crystalline clast–rich subunit U2 could have been formed by slumping of material, probably from the central uplift or from the margin of the transient crater. Melt particles are most abundant near the top of the impact breccia section (above 1409 m) and around 1450 m, where the suevite grades into impact melt rock. Five different types of melt particles have been recognized: (1) clear colorless to brownish glass; (2) melt altered to fine-grained phyllosilicate minerals; (3) recrystallized silica melt; (4) melt with microlites; and (5) dark-brown melt. Proportions of matrix and melt in the suevite are highly variable (~2–67 vol% and 1–67 vol%, respectively; the remainder consists of lithic clasts). Quartz grains in suevite commonly show planar fractures (PFs) and/or planar deformation features (PDFs; 1 or 2 sets, rarely more); some PDFs are decorated. On average, ~16 rel% of quartz grains in suevite samples are shocked (i.e., show PFs and/or PDFs). Sedimentary clasts (e.g., graywacke or sandstone) and polycrystalline quartz clasts have relatively higher proportions of shocked quartz grains, whereas quartz grains in schist and gneiss clasts rarely show shock effects. Rare feldspar grains with PDFs and mica with kink banding were observed. Ballen quartz was noted in melt-rich samples. Evidence of hydrothermal alteration, namely, the presence of smectite and secondary carbonate veins, was found especially in the lower parts of the impact breccia section.

The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville boreholes through the annular moat of the Chesapeake Bay crater recovered polymict impact breccias and associated rocks from the depth range of 1397–1551 m. These rocks record cratering processes before burial beneath resurge deposits. Quantitative analyses of clast sizes, matrix contents, and distribution of impact melt reveal a shock metamorphic gradient in these impactites. The reason for the low estimated quantity of impact melt in the crater (~10 km 3 ) remains elusive. Possible causes may relate to increased excavation efficiency due to a high ratio of water column and sedimentary target to depth of excavation, an oblique impact, or a buried melt sheet at depth. A plausible petrogenetic scenario consists of a lower block-rich section that slumped from an outer region of the transient cavity into the annular moat ~1.5 min after impact. This blocky debris was mixed with the remains of the excavation flow, which contained a pod of melt entrained in ground-surge debris on top. Subsequently, melt-rich suevites were emplaced that record interaction of the expanding ejecta plume with fallback material related to the evolving central uplift. A clast-rich impact melt rock that likely shed off the central uplift covers these suevites. Incipient collapse of the ejecta plume is recorded in the uppermost subunit, but the arrival of resurge flow terminated its continuous deposition ~6–8 min after impact. Limited thermal annealing allowed preservation of glassy melt and high-pressure polymorphs. Mild hydrothermal overprint in the central crater was likely driven by the structural uplift of ~100 °C warmer basement rocks.

The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville B core hole, drilled into the 35.5-Ma-old Chesapeake Bay impact crater, Virginia, has recovered postimpact sediments, crater-fill breccias, megablocks of the crystalline basement, and suevites with fresh glass shards. Bulk rock analyses of 2 glass shards, 21 crystalline target rocks, and microchemical analyses of 7 glass shards and 3 bediasites (tektites of the North American strewn field) were performed in order to contribute to the understanding of formation processes and to better constrain the precursor materials of these glasses as well as of the bediasites. Statistical treatment (hierarchical cluster analyses) yielded an assignment of the data for the crystalline basement samples into four groups; two of those (various schists, meta-graywackes, and gneisses) display characteristics similar to the impact glasses in the suevites and the bediasites. However, the suevitic glasses show a broad range in composition at the micrometer scale. These data show the frequent presence of schlieren, and in particular, enhanced TiO 2 contents that require admixture of an “amphibolitic component” to the melt. Evidence for such a process is provided by the occurrence of relict, in-part thermally corroded grains of rutile and ilmenite, and by formation of Ti-rich tiny mineral aggregates in the glass. The three studied bediasites show only minor inter- and intrasample heterogeneity, and their chemical composition agrees well with previously published data. The new data for the bediasites are compatible with heating of the “tektite melt” to extreme temperatures, followed by quenching.

This paper documents an attempt to detect a meteoritic component in both wash-back (resurge) crater-fill breccia (the so-called Exmore breccia) and in suevites from the Eyreville core hole, which was drilled several kilometers from the center of the 85-km-diameter Chesapeake Bay impact structure, Virginia, USA. Determining the presence of an extraterrestrial component and, in particular, the projectile type for this structure, which is the largest impact structure currently known in the United States, is of importance because it marks one of several large impact events in the late Eocene, during which time the presence of extraterrestrial 3 He and multiple impact ejecta layers provide evidence for a comet or asteroid shower. Previous work has indicated an ordinary chondritic projectile for the largest of the late Eocene craters, the Popigai impact structure in Siberia. The exact relation between the Chesapeake Bay impact event and siderophile element anomalies documented in late Eocene ejecta layers from around the world is not clear. The only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic osmium isotopic signature in impact melt rocks recovered from a hydrogeologic test hole located on Cape Charles near the center of the structure, and confirmation of a similar signature in suevitic rocks would have been desirable in order to place constraints on the type of projectile involved in formation of the Chesapeake Bay crater. Unfortunately, the current data show no discernible differences in the contents of the platinum group elements (PGEs) among the suevite, the Exmore breccia, and several crystalline basement rocks, all from the Eyreville core hole. Abundances of PGEs are uniformly low (e.g., <0.1 ppb Ir), and chondrite-normalized abundance patterns are nonchondritic. These data do not allow unambiguous verification of an extraterrestrial signature. Thus, the nature of the Chesapeake Bay projectile remains ambiguous.

The alteration or transformation of impact melt rock to clay minerals, particularly smectite, has been recognized in several impact structures (e.g., Ries, Chicxulub, Mjølnir). We studied the experimental alteration of two natural impact melt rocks from suevite clasts that were recovered from drill cores into the Chesapeake Bay impact structure and two synthetic glasses. These experiments were conducted at hydrothermal temperature (265 °C) in order to reproduce conditions found in melt-bearing deposits in the first thousand years after deposition. The experimental results were compared to geochemical modeling (PHREEQC) of the same alteration and to original mineral assemblages in the natural melt rock samples. In the alteration experiments, clay minerals formed on the surfaces of the melt particles and as fine-grained suspended material. Authigenic expanding clay minerals (saponite and Ca-smectite) and vermiculite/chlorite (clinochlore) were identified in addition to analcime. Ferripyrophyllite was formed in three of four experiments. Comparable minerals were predicted in the PHREEQC modeling. A comparison between the phases formed in our experiments and those in the cores suggests that the natural alteration occurred under hydrothermal conditions similar to those reproduced in the experiment.