ABSTRACT We studied the following proven as well as hypothetical impact craters (among others), and some of the relevant results are reviewed in this chapter: (1) a hypothetical impact structure in Saginaw Bay, Great Lakes, Michigan; (2) a putative impact crater basin under the ice of Antarctica in Wilkes Land; (3) two recently discovered subglacial impact craters in Greenland; (4) a possible huge impact crater in Kotuykanskaya in a remote area of Siberia near the proven impact crater Popigai; and (5) a hypothetical impact object Burckle on the bottom of the Indian Ocean. They were tested using the gravity data derived from the recent gravity field model EIGEN 6C4 (with ground resolution of ~9 km). Our method is novel; we introduce gravity aspects (descriptors) to augment traditional gravity anomalies. The following gravity aspects were used: (a) gravity disturbances/anomalies, (b) second derivatives of the disturbing potential (the Marussi tensor), (c) two of three gravity invariants, (d) their specific ratio (known as 2D factor), (e) strike angles, and (f) virtual deformations. These gravity aspects are sensitive in various ways to the underground density contrasts. They describe the underground structures (not only the craters) more carefully and in more detail than the traditional gravity anomalies could do alone. Our results support geological evidence of the impact craters found by others in many cases or suggest new impact places for further study.

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).