ABSTRACT Rampart craters are omnipresent features on volatile-rich solid planetary surfaces. This raises the question whether, and how many, rampart craters are present on Earth. We reviewed the terrestrial impact crater record with regard to possible rampart morphologies and present detailed morphological analyses of these terrestrial craters here. Our results show that the Ries crater in Germany, Bosumtwi crater in Ghana, Tenoumer crater in Mauritania, Lonar crater in India, and Meteor crater in the United States are terrestrial rampart craters. The Ries and Bosumtwi craters can be classified as double-layer ejecta (DLE) craters, whereas Tenoumer, Lonar, and Meteor craters can be classified as single-layer ejecta (SLE) craters. Tenoumer and Meteor craters show rampart as well as common lunar-like ejecta characteristics within their ejecta blankets and, thus, appear to be hybrid craters. In addition, we discuss seven crater structures that show at least some morphological or lithological peculiarities that could provide evidence for possible ejecta ramparts. Considering the low number of terrestrial impact craters with well-preserved ejecta blankets, the relatively high proportion of rampart craters is astonishing. Obviously, the formation of layered or rampart craters is a common and not a rare process on Earth.

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.