Field recovery of layered tektites in northeast Thailand

Abstract

We recovered Australasian tektites in place throughout a 40X130 km region in northeast Thailand extending from the Laotian border westward to a line connecting Na Pho Klang 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 fragments are layered (muong-Nong-type) tektites. It appears that large layered tektites are mainly found by rice farmers in fields that were forested until the recent past. The presence of layered tektites in this 40X130 km area implies that impact melt that fell in these areas was hot enough to flow if it was deposited on a sloping surface. The absence of splashform tektites from the region indicates that the layer was still molten when masses having shapes (teardrops, dumbbells, etc.) produced by spinning reached the ground. To account for this and to allow time for the melt to flow a few tens of centimeters requires that the atmosphere remained hot (>2300 K) for a few minutes. Tektites that are in place are almost always associated with a widespread 10-cm to meter-thick layer of laterite. In two cases (one involving layered, one involving splash-form tektites), where accurate stratigraphic control demonstrated that the tektites were still in section, they were sited on top of the laterite layer just below a layer of aeolian sand. Introduction Tektites are glasses having chemical and isotopic compositions indicating that they formed by melting well-mixed continental sediments. Four fields are known; common names and approximate ages are North American, 35 Ma; Central European, 15 Ma; Ivory Coast, 1.1 Ma and Australasian, 0.77 Ma (Australasian age by lzett and Obradovich, [ 1992]; other data are from summary by Wasson and Heins, [1993]). We investigated a portion of the youngest strewn field, the Australasian. The Australasian tektites can be divided into two fundamentally different classes: (1) splash-form [more accurately, spinform] that obtained their shapes by solidifying while spinning in a low-pressure environment; and (2) layered (also called Muong-Nong-type) tektites that seem to be fragments of melt sheets, puddles or more complex flow structures that solidified on the Earth's surface [Barnes and ?itakpaivan, 1962]. The largest splash-form tektites (from the Philippines) reach 1 kg in size; the largest splash-form tektites from SE Asia have masses of ~350 g. Splash-form tektites are found on land from Guangxi, China, to Tasmania and as microtektites in sea-sediment cores from Taiwan to Madagascar [Glass, 1982]. Layered tektites have been recovered throughout a 1100-km-long band stretching from Hainan, China [Futrell and Wasson, 1993] to Cambodia [Wasson, 1991; Schnetzler, 1992]. Because those with masses >1 kg are too large Copyfight 1995 by the American Geophysical Union. Paper number 95JE01504. 0148-0227/95/95JE-01504505.00 to have undergone appreciable alluvial transport in low-gradient terrains, discovery sites are inferred to be quite close to the original formation locations. Because most Southeast...

Large Aerial Bursts: An Important Class of Terrestrial Accretionary Events

“…We (Wasson, 1995;Wasson et al, 1995) noted that the necessity to maintain such high temperatures for several minutes implies that the melt did not immediately cool by radiation into a cooler environment. This indicates that the surrounding environment out to an optical depth $1 was at the same temperature or higher.…”

“…Microtektites that are part of this field are found from Okinawa to Madagascar, a distance of ,10,500 km. According to Wasson et al (1995) and Wasson (1995), the properties of layered tektites are best explained by an extensive melt sheet formed by melting surficial continental soils. The large samples are attributed to downslope flow of a thin but variably thick melt sheet to produce ponding in topographic lows.…”

“…The record may, however, be preserved in the glassy objects called layered tektites. We (Wasson, 1995;Wasson et al, 1995) suggested that the layered tektites found in Southeast Asia (SE Asia) may be the record of a melt sheet produced by an aerial burst; despite the large amount of energy deposited only 780 ka ago, no crater can be associated with the Australian tektites. Wasson and Moore (1998) and Wasson and Boslough (2000) ascribed a similar origin to the Libyan desert glass (LDG).…”

Lithium in tektites and impact glasses: Implications for sources, histories and large impacts

“…An important observation to consider is that a lateritic soil profile had developed at the collection site of MN-type tektites in Thailand prior to tektite formation (Wasson et al, 1995). This implies extensive weathering and soil formation for a certain period in a humidity-dominated climate.…”

“…The Muong Nong-type (MN) tektites were collected in NE Thailand (Wasson et al, 1995) together with samples of spatially associated lateritic soil (T191-b5). One pair of samples from a section through a single large (>6 kg) tektite (TS-6 top and TS-6 bottom) was also included.…”