Late Eocene microkrystites and microtektites at Maud Rise (Ocean Drilling Project Hole 689B; Southern Ocean) suggest a global extension of the approximately 35.5 Ma Pacific impact ejecta strewn field

Late Eocene microtektites and microkrystites recovered from Ocean Drilling Project Hole 689B at Maud Rise (Southern Ocean) are stratigraphically and geochemically compared to spherules from the North American and Pacific strewn fields, and to devitrified spherules fiom the Eocene-Oligocene global stratotype section and point section in Massignano, Italy. The ODP 689B microkrystites compare well to the Pacific strewn field microkrystites, which suggests that the geographic extent of the Pacific strewn field was much larger than previously documented. The elemental composition of microtektites of ODP Hole 689B is comparable to tektites of the North American strewn field. Their 87Sr/86Sr ratio, however, is different. We tentatively interpret this to reflect geochemical heterogeneity within the North American strewn field but can not exclude the option that the chemical discrepancies result from the existence of a third late Eocene impact site. INTRODUCTION and microtektites only (Glass et al., 1985). In accordance with their geographical extent, they are referred to as the Pacific strewn field and the North American strewn field, respectively. Tektites from the North American strewn field seem largely restricted to an area around the Gulf of Mexico and the western North Atlantic, whereas the Pacific strewn field extends from the Caribbean across the equatorial Pacific to DSDP Site 216 in the Indian Ocean (Fig. 1). At a number of sites, the Pacific strewn field microkrystite horizon is associated with an Ir anomaly (Glass et al., 1985). Keller et al. (1983) reported at least four stratigraphically different microtektite horizons in the upper Eocene, but two of those horizons could not be confirmed (Glass et al., 1985). Hazel (1989) distinguished even six different microspherule (microkrystite and Late Eocene Strewn Fields Late Eocene tektites, the glassy ejecta from extraterrestrial impacts, are known to occur at a number of locations on the North American continent. These tektites (also known as bediasites or georgiaites) were first recognized in the late 1930s and have since been reported from many sites on the continent (see Koeberl, 1989, and Barnes, 1990, for a historical overview). Microtektites (tektites <I mm in diameter) were found in drill cores from the Caribbean and the Gulf of Mexico, apparently belonging to the same North American tektite strewn field, based on their geographic and stratigraphic occurrence, fission track and 40ArP9Ar age, and chemical composition (Glass et al., 1985; and references therein). John and Glass (1974) reported clinopyroxenebearing spherules, which they initially interpreted to be associated with the North American strewn field. Glass and Bums (1 988) termed such clinopyroxene-bearing spherules microkrystites. Similar microkrystites were found at the Cretaceous-Tertiary boundary (Smit et al., 1992) and in ejecta from a late Pliocene impact (Margolis et al., 1991) A late Eocene Ir anomaly was found at Deep Sea Drilling Project (DSDP) site 149 (Alvarez et al., 1982), and in core RC9-58 (Ganapathy, 1982) in the Caribbean Sea. It was suggested to be associated with the North American microtektite strewn field, although the Ir peak in core RC9-58 is located below the peak occurrence of the microtektites. Also in the late Eocene section in Barbados, an Ir anomaly occurs -27 cm below the microtektite layer (Sanfilippo et al., 1985). Detailed reanalysis of core RC9-58 (Glass and Ganapathy, 1982) I \ showed that, although bioturbational mixing disturbed the FIG. 1. World map showing occurrences of late Eocene impact ejecta. Longitudinally section* the main peak Of microkrystites coincides with elongated hatched area indicates the extension of the Pacific strewn field, not counting the Ir anomaly, -25 cm below the Peak microtektite Massignano and Maud Rise. Black dots are the actual ODP or DSDP sites where ejecta abundance. These data indicate the presence of two have been found. The smaller North American strewn field is indicated by the shaded separate impact events: one with an lr anomaly, area covering the North American east coast, Gulf of Mexico, and Caribbean Sea. Suggested source craters on the North American east coast (Chesapeake Bay), and in abundant microkrystites* and Some microtektites; Siberia (Popigai) are also indicated. 612 and 6898 refer to DSDP Site 612 and ODP followed by a 10-20 ka later impact, producing tektites Hole 689B (Maud Rise), respectively.