Field Recovery of Layered Tecktites in Northeast Thailand: Evidence of a Large-scale Melt Sheet

Field searches and interrogation of farmers in Thailand has led to the recovery of layered (a.k.a. Muong-Nong) Australasian tektites in situ throughout a region extending from the Laotian border westward to a line connecting Ban Pho Klang near the Mekong River in the northeast through Det Udom to Nam Yun in the south. With two exceptions in sites near the western edge of this region, all recovered fragments are layered. In most cases large layered tektites are found by rice farmers in fields that were forested until the recent past. We were shown exact locations where kilogram-sized tektites had recently been recovered, and we purchased several large specimens. Wherever laterite shot was exposed on the surface, we found many small layered tektites in place. The layering of layered tektites appears to have formed by downslope flow. The structures closely resemble layering found in obsidian flows, and the 20 degree dip of the magnetic remanence relative to the layering shows that the glass masses were in situ on the Earth's surface when they cooled through the Curie temperature. Our observations indicate that a geographical area with minimum dimensions of 40 X 130 km was covered with a sheet of silicate melt. This seems to require that the region was continuously heated by radiation associated with the accretion of extraterrestrial material until the melt had rained out onto the surface; to achieve the observed amount of flow a mean radiation temperature of ~2200 K was required. The highly luminous sky was partly produced by fireballs above the parental impact craters, but also by Tunguska-like atmospheric phenomena produced by infalling projectiles that released their entire energy in the atmosphere and left no crater record. We suggest that the investigated region was covered with a melt sheet. A rough estimate is that the mean thickness of the layer was ~4 mm corresponding to a surface density of 10 kg m^-2. An equally rough estimate of the density of tektites that survived weathering and is still present in pristine regions is ~30 g m^-2. We suggest that an attrition factor of 300 is not inconsistent with that expected from weathering and other surface properties. Large layered tektites are found from Hainan, China to Central Cambodia; we infer that melt sheets covered much of this 1200-km-long region. The ecological consequences of a melt sheet covering this region are enormous. All life within a few centimeters of the surface must have been destroyed.