The occurrence of impact products in the stratigraphic record such as impact glass, shocked minerals, Ir anomalies, impact breccia or tsunami deposits is less than what is expected from the present estimates of terrestrial cratering rates [ Grieve, 1984; Shoemaker et al., 1990]. However, in the last few years impact products have been identified at several key stratigraphic horizons.
Debris produced by the oceanic impact of a small asteroid (
0.5 km diameter) occur
in the late Pliocene sediments of the South Pacific Ocean about 1400 km west of Cape Horn.
Grains of vesicular impact melt, unmelted fragments of the impacting mesosiderite asteroid and
glassy spherules containing Ni-rich magnesioferrite spinels have been found associated with a
20 ng/g Ir anomaly [ Kyte et al., 1988; Margolis et al., 1991]. The preserved liquid,
solid and vapor impact ejecta phases make this small impact an interesting recent analog for the
study of oceanic impact events.
Impact products and Ir occur at several closely spaced stratigraphic intervals within the late
Eocene but a detailed understanding of the sequence of events is hampered by lack of
stratigraphic resolution. The North American microtektite strewn field is dated at 35.4 
0.6 Ma [ Glass et al., 1986]. A small Ir anomaly is found associated with the apparently
slightly older late Eocene microkrystite (or crystal-bearing spherule) layer [ Glass et al.,
1985]. Montanari et al., [1993] recently also detected two Ir anomalies in the upper part
of Chron 16N in the Massignano section (Italy) and at Ocean Drilling Project site 689B on Maud
Rise (Antarctica) and showed that the succession of impact events seems to spread in the late
Eocene over a period of a million years between 35.7 and 34.7 Ma.
Quartz grains that appear to have been shock-metamorphosed have been found within three
closely spaced shale beds from the uppermost Triassic in the Northern Apennines of Italy [
Bice et al., 1992]. This repetitive occurrence of potential impact products spans 
150 ky.
The uppermost shocked-quartz layer appears to coincide with the Triassic-Jurassic mass
extinction [ Bice et al., 1992].
Impact products have also been recently identified at another major mass extinction horizon
365 Ma ago in the Late Devonian. Wang [1992] found glassy spherules
morphologically and chemically similar to microtektites, containing lechatelierite in a basal
Famennian carbonate section in Qidong, China at a stratigraphic level equivalent to the 0.3 ng/g
Ir anomaly detected in the Caning Basin (Western Australia). Similar glass spherules, but
roughly 1 to 2 million years older,have been found associated with the Frasnian-Famennian
boundary in two Late Devonian sections in Belgium [ Claeys et al., 1992; Claeys and
Casier, 1994]. The water content of the glass is in the range of impact glasses values and the
major glass chemistry seems compatible with the target rock at the 368 
1 Ma Siljan Ring
crater in Sweden [ Claeys and Casier, 1994]. The spherule layer is not marked by an Ir
anomaly [ Claeys et al., 1994].
The early Archean 3400 Ma old Fig Tree Group, Barbeton Greenstone Belt in South Africa, also contains impact debris in the form of sand-sized clay altered spherules and Ni-rich spinels associated with an enrichment in Ir and noble metals [ Lowe et al., 1989; Kyte et al., 1992; Byerly and Lowe, 1994). Noble metal concentrations are fractionated relative to chondritic abundance but more compatible with an impact origin than with an enrichment by magmatic or hydrothermal processes [ Kyte et al., 1992]. These deposits may represent the first tangible terrestrial record of the Archean bombardment of the Earth and the great thickness of the spherule layer supports a very large size for the Archean impactor [ Lowe et al., 1989].