Abstract:
A deep-sea sediment core from the Ocean Drilling Program Hole 1144A was examined for the presence of Australasian microtektites. Previous authors predicted that the source crater of the Australasian strewn field is located in central Indochina. The close proximity of Hole 1144A to the predicted source crater as compared to previously examined core sites within the Australasian strewn field, implied that the microtektite concentration would be high. Biostratigraphic data indicate an age of 0.79 Ma for the microtektite layer, which is consistent with the microtektites belonging to the Australasian strewn field. Over 40 samples from Hole 1144A were processed for study. The samples were oven dried and weighed, and then they were placed in beakers with water and disaggregated using ultrasonics, wet sieved into three size fractions of (<63, 63-125, >125 microns), dried, and weighed. The >125 microns size fraction was searched for microtektites using a binocular microscope. A preliminary study of the samples revealed that the peak abundance of the impact layer was in sample 37X/6, 65-66 cm, which contained more than 3,500 microtektites. The number of microtektites per square cm was estimated to be at least 7100 microtektites/cm2. Approximately 80% of the microtektites are fragments; the remainder are mostly spherical, with a few percent discs, teardrops, dumbbells, and prolated spheres. The largest spherical microtektite is 720 microns in diameter. Partially melted rock fragments up to 1400 microns across and abundant rock and mineral grains were associated with the microtektite layer. X-ray diffraction data of several white opaque grains indicate that they are shocked quartz containing coesite and occasionally stishovite. The microtektite concentration at Site 1144A is higher than all previously studied sites. The high concentration of microtektites at Site 1144A, which is located in the northern part of the South China Sea, suggests that the source crater is further to the northeast than previously predicted. The high percentage of microtektite fragments is consistent with the site being closer to the source crater than previously studied sites. No major changes in foraminifera taxa were observed to coincide with the peak abundance of microtektites.