Abstract-Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95-17957-2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm 2 ) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22° N and 104° E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 ± 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ∼800 ka ago and that it is spread over an area of at least 3.7 × 10 7 km 2 .

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Geographic variations in the concentration of Australasian microtektites in 42 cores from the Indian Ocean, western equatorial Pacific Ocean, and the Philippine, Celebes, and Sulo Seas were used to predict the location of the Australasian tektite source crater and to estimate its size. The location that seems to best explain the geographic variations in microtektite concentrations is located in central Cambodia at about 12°N latitude and 106°E longitude. The diameter of the source crater is estimated to be between 32 and 114 km based on thickness of the microtektite layer at each site and distance from the predicted source area in Cambodia. The large range in estimated size of the source crater is due to a lack of knowledge about the value of the exponent that describes the radial decrease in ejecta thickness. Additional microtektite/ejecta‐bearing sites closer to the source region could help resolve this problem.

▪ Abstract  A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers. Secular changes in surface environments and/or the nature of the impactors striking Earth through its history could also be reflected in differences in spherules and spherule layers as a function of geologic age. In this paper, we briefly review what spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers.