LATE EOCENE IMPACT MICROSPHERULES: STRATIGRAPHY, AGE AND GEOCHEMISTRY

• Published in: Meteoritics
• Main Authors: Keller, G., D'Hondt, S.L., Orth, C.J., Gilmore, J.S., Oliver, P.Q., Shoemaker, E.M., Molina, E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1987
• Subjects: Northwest Atlantic
• Online Access: http://dx.doi.org/10.1111/j.1945-5100.1987.tb00883.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1945-5100.1987.tb00883.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.1987.tb00883.x

Recent discoveries of microtektite and related crystal bearing microspherule layers in deep‐sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off‐shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al 2 O 3 and FeO values in SiO 2 equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non‐crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer ...