Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities. To fully establish the size of the crater, a total field magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature, expressed as a 100 nT low, 2–3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic field, up to a maximum diameter of 3 km. Dikes

Fifteen impactites from various intervals within the Eyreville cores of the Chesapeake Bay impact structure were sampled to measure siderophile element concentrations. The sampled intervals include basement-derived rocks with veins, polymict impact breccias and associated rocks, and crater-fill sediments. The platinum group element (PGE) concentrations obtained are generally low (e.g., iridium concentrations less than 0.1 ng/g) and are fractionated relative to chondrites. There is no clear distinction in concentration between the different impactite units. So far in the Chesapeake Bay material, only the impact melt rocks from the 823-m-deep Cape Charles test hole, drilled over the central uplift of the structure, have generated a bulk chondritic signature of 0.01–0.1 wt% meteoritic contribution based on a mixing model of 187 Os/ 188 Os isotopic ratios and Os concentrations. However, none of the samples studied shows PGE abundances that enable identification of the type of projectile responsible for the formation of the structure. Hence, it is at present not possible to link the Chesapeake Bay impact to the proposed ordinary chondrite falls by projectiles recorded for other late Eocene craters, namely the 100-km-diameter Popigai impact structure in Siberia and 7.5-km-diameter Wanapitei structure in Canada. The absence of a clear projectile signature hinders further discussions on the existence and the nature of the late Eocene shower event (asteroid versus comet).

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.