17:15 〜 18:30
[BCG04-P12] タイ東北部第四系堆積物から見つかったオーストラリア・アジアテクタイトイベント起源層状テクタイト破片の産状
★招待講演
キーワード：オーストラリア・アジアテクタイト、イジェクタ、天体衝突
There are several reports of Australasian tektites from a stratum called the "laterite layer" widely distributed in Indochina. It has long been debated whether these tektites are in situ or reworked. This uncertainty is due to the lack of a detailed description of their field occurrence and causes a problem on identifying the ejecta deposit derived from the Australasian Tektite Event on-land in Indochina Peninsula. We conducted a field survey and described the detailed occurrence of a cluster of tektite fragments found in the" laterite layer" near Huai Om, northeastern Thailand, demonstrating the evidence of in situ occurrences of these tektite fragments.
At least 331 tektite fragments (total weight of 713 g) were found from a small area (40 x 30 cm area with 10 cm thickness) in the upper part of the "laterite" layer. The very angular shapes of the tektite fragments, the similar chemical composition of the fragments, and the restoration of larger tektite fragments into one ellipsoidal tektite mass suggest that these tektite fragments were formed by fragmentation of one large tektite mass. The fragments' size distribution is bifractal following two power laws in the range from 37 to 26 mm and 26 to 10 mm, with fractal dimensions (Ds) of 7.5 and 2.2, respectively. The Ds for the coarse fraction of the fragments is larger than the Ds for rock fragments generated by rockfalls and similar to the Ds for the coarser fraction fragments generated by high-speed impact experiments. This large Ds value suggests that the tektite fragments were formed through intense fragmentation by a relatively high energetic process rather than a low energetic process such as weathering after deposition. The occurrence of the fragments as a cluster and the distribution of the fragments in sediments revealed by a computer tomography scanning analysis indicate that the fragments were not moved apart significantly after fragmentation and burial. Based on these results, we concluded that a tektite mass was fragmented at the time of the landing on the ground after ejection from the impact site and has not been disturbed further (i.e., in situ).
The result of this study is contrary to the previous interpretation that the upper surface of the "laterite" layer is a paleo-erosional surface, on which the tektites were reworked, and supports the idea that the "laterite layer" is a part of the ejecta deposit of the Australasian Tektite Event.
The main part of this presentation was already published as Tada et al. 2020 from Progress in Earth and Planetary Science (https://doi.org/10.1186/s40645-020-00378-4).
At least 331 tektite fragments (total weight of 713 g) were found from a small area (40 x 30 cm area with 10 cm thickness) in the upper part of the "laterite" layer. The very angular shapes of the tektite fragments, the similar chemical composition of the fragments, and the restoration of larger tektite fragments into one ellipsoidal tektite mass suggest that these tektite fragments were formed by fragmentation of one large tektite mass. The fragments' size distribution is bifractal following two power laws in the range from 37 to 26 mm and 26 to 10 mm, with fractal dimensions (Ds) of 7.5 and 2.2, respectively. The Ds for the coarse fraction of the fragments is larger than the Ds for rock fragments generated by rockfalls and similar to the Ds for the coarser fraction fragments generated by high-speed impact experiments. This large Ds value suggests that the tektite fragments were formed through intense fragmentation by a relatively high energetic process rather than a low energetic process such as weathering after deposition. The occurrence of the fragments as a cluster and the distribution of the fragments in sediments revealed by a computer tomography scanning analysis indicate that the fragments were not moved apart significantly after fragmentation and burial. Based on these results, we concluded that a tektite mass was fragmented at the time of the landing on the ground after ejection from the impact site and has not been disturbed further (i.e., in situ).
The result of this study is contrary to the previous interpretation that the upper surface of the "laterite" layer is a paleo-erosional surface, on which the tektites were reworked, and supports the idea that the "laterite layer" is a part of the ejecta deposit of the Australasian Tektite Event.
The main part of this presentation was already published as Tada et al. 2020 from Progress in Earth and Planetary Science (https://doi.org/10.1186/s40645-020-00378-4).