In-situ occurrence of Muong Nong-type Australasian tektite fragments from the quaternary deposits near Huai Om, North-Eastern Thailand

Abstract

There are several reports of Australasian tektites found within a stratum called "laterite” layer widely distributed in Indochina. However, it has been debated whether these tektites are in situ or reworked. This uncertainty is because a detailed description of their field occurrence is lacking. Here, we describe the detailed occurrence of a cluster of tektite fragments recovered from the ”laterite” layer near Huai Om, northeastern Thailand, and demonstrate the evidence of in situ occurrence of the tektites. At least 331 tektite fragments with a total weight of 713 g were found from a 40 x 30 cm area with 10 cm thickness in the uppermost part of the “laterite” layer. The very angular shapes and very poorly sorted nature of the fragments, restoration of larger tektite fragments into one ellipsoidal MN tektite mass, and the similar chemical composition of the fragments suggest that these MN tektite fragments represent a tektite mass that fragmented in situ. The fact that the fragments were found within the “laterite” layer is inconsistent with a previous interpretation that the upper surface of the “laterite” layer is a paleo-erosional surface, on which the tektites are reworked. The size distribution of the fragments is bifractal following two power laws in the range from 10 to 26 mm and from 26 to 37 mm, with fractal dimensions (Ds) of 2.2 and 7.5, respectively. The Ds for the coarse fraction of the tektite fragments is larger than the Ds for rock fragments generated by rockfalls and rock avalanches and similar to the Ds for the coarser fraction fragments generated by high speed impact experiments, suggesting that the tektite fragments were formed through intense fragmentation by a relatively high energetic process. The occurrence of the fragments forming a cluster indicate that the fragments were not moved apart significantly after fragmentation and burial. Based on these results, we concluded that the mass of a tektite was fragmented at the time of the landing on the ground after traveling a ballistic trajectory, and has not been disturbed further.

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