BOTTLE GREEN MICROTEKTITES FROM THE SOUTH TASMAN RISE (ODP SITE 1169): EVIDENCE FOR AN IMPACT EVENT NEAR THE MIOCENE/PLIOCENE BOUNDARY

Ocean Drilling Program Leg 189 recovered a deep-sea core sample (20 cc) from atop the South Tasman Rise (Site 1169; 145º14.2’E, 47º 3.9’S) that contained forty-eight “bottle-green” microtektites. The microtektites are mostly spherical in shape although a few are ellipsoidal, with diameters ranging from 0.10 to 0.90 mm in size. The microtektites are composed of homogeneous glass, exhibiting no vesicles or mineral grains. Average compositions for major oxides show that the microtektites are enriched in alumina and depleted in sodium and potassium relative to volcanic analogs. The study area is located on the extreme, southern fringe of the known geographic range of the Australasian strewn layer(s), making the Australites likely candidates for compositional comparison. Major elemental compositions indicate that the Site 1169 microtektites are, at best, outliers to the Australasian field. Moreover, intensive sampling across the Bruhnes/Matuyama geomagnetic reversal failed to uncover any Australites in the study section. Laser ablation analysis for trace elements indicates also that the Site 1169 microtektites are chemically distinct from other well-studied tektites. Precise age determination of the Site 1169 microtektites is hampered by drilling disturbance and low core recovery of a condensed stratigraphic section plagued by poor microfossil preservation. Nevertheless, a composite microfossil biozonation based on planktonic foraminifera and calcareous nannofossils was used to constrain the age of the Site 1169 microtektites to the Late Miocene – Early Pliocene (6.8 – 4.6 Ma). The geochemical and biochronological evidence suggest that the Site 1169 microtektites were formed by an undocumented impact event.