10:00 〜 10:15
[BCG04-05] オーストラリア・アジアテクタイトイベント：インドシナ半島におけるイジェクタ層の層厚分布と礫種組成に基づく衝突地点推定
キーワード：オーストラリア・アジアテクタイト、天体衝突、イジェクタ
Tektites are glass spherules formed by large impacts and reported from 4 strewn fields on the Earth. Australasian tektites distributed widely from Southeast Asia to Antarctica are considered to be formed by an impact event that occurred ca. 0.8 Ma in Southeast Asia (Australasian Tektite Event; AATE). This impact event is the youngest among large impact events. Because of its young age, it is expected that evidence of the impact (impact crater and ejecta deposits) is well preserved.
However, the biggest mystery is that no crater has been found yet despite intensive search efforts. The location of the impact has been considered as somewhere in Eastern Indochina, mainly based on the distribution of the tektites. For more precise constraints on the impact location, the distribution of the ejecta deposit is important since an ejecta deposit becomes thicker toward the impact location. The ejecta deposit of this impact event, however, has not been identified on the Indochina Peninsula. To identify the ejecta deposit of the AATE and clarify its distribution, we have conducted field surveys across a wide area of Eastern Indochina, including northeastern Thailand, central Vietnam, southern Laos, and northern Cambodia.
We identified the ejecta deposit associated with the AATE based on the in-situ occurrence of the tektite fragments (Tada et al. 2020 PEPS) and the discovery of shocked quartz grains with planar deformation features (PDFs) from the Quaternary deposit at Huai Om and HO06 sections in northeastern Thailand. PDFs were confirmed by the measurement of their orientation and SEM/TEM observations.
The ejecta deposit can be divided into three lithostratigraphic units (units 1–3 in ascending order). Unit 1 is composed of gravely silt to sand layers or a sandy gravel layer containing materials reworked from the local basement and occasionally showing cross-lamination. Unit 2 is a tektite-bearing breccia, and Unit 3 is a massive sand to silt layer. Unit 2 and 3 are laterally traceable in eastern Indochina. Based on their stratigraphy and lithological and sedimentological evidence, Unit 1 is probably fine ejecta carried by impact-induced winds. Unit 2 and 3 are considered to be the ejecta curtain deposit breccia and fallout fine ejecta. The thickness of Unit 2 increases toward southwestern Laos, where a breccia of over 9 m thick was found. The regression analysis of the distribution of the thickness of Unit 2, assuming that the log thickness is negatively correlated with log distance from a source area, indicates that the Bolaven Plateau area (15.2°N, 106.1°E) is the most probable impact site.
Unit 2 is composed of weathered basalt fragments, sandstone and mudstone fragments, and fragments of rounded quartzite gravels. This gravel composition of Unit 2 indicates that the target rock of the impact includes basalt, sedimentary rocks, and possibly river gravels. This estimation of the target rocks is consistent with the estimation of the impact site in southwestern Laos, where Neogene to Quaternary basalt lava and Mesozoic sedimentary rocks (mudstone, sandstone, and conglomerate) are widely distributed.
The result of this study strongly supports the hypothesis of Sieh et al. (2019) that the AATE occurred in the basalt field of the Bolaven Plateau, and the crater was buried by younger basalts that erupted after the impact.
However, the biggest mystery is that no crater has been found yet despite intensive search efforts. The location of the impact has been considered as somewhere in Eastern Indochina, mainly based on the distribution of the tektites. For more precise constraints on the impact location, the distribution of the ejecta deposit is important since an ejecta deposit becomes thicker toward the impact location. The ejecta deposit of this impact event, however, has not been identified on the Indochina Peninsula. To identify the ejecta deposit of the AATE and clarify its distribution, we have conducted field surveys across a wide area of Eastern Indochina, including northeastern Thailand, central Vietnam, southern Laos, and northern Cambodia.
We identified the ejecta deposit associated with the AATE based on the in-situ occurrence of the tektite fragments (Tada et al. 2020 PEPS) and the discovery of shocked quartz grains with planar deformation features (PDFs) from the Quaternary deposit at Huai Om and HO06 sections in northeastern Thailand. PDFs were confirmed by the measurement of their orientation and SEM/TEM observations.
The ejecta deposit can be divided into three lithostratigraphic units (units 1–3 in ascending order). Unit 1 is composed of gravely silt to sand layers or a sandy gravel layer containing materials reworked from the local basement and occasionally showing cross-lamination. Unit 2 is a tektite-bearing breccia, and Unit 3 is a massive sand to silt layer. Unit 2 and 3 are laterally traceable in eastern Indochina. Based on their stratigraphy and lithological and sedimentological evidence, Unit 1 is probably fine ejecta carried by impact-induced winds. Unit 2 and 3 are considered to be the ejecta curtain deposit breccia and fallout fine ejecta. The thickness of Unit 2 increases toward southwestern Laos, where a breccia of over 9 m thick was found. The regression analysis of the distribution of the thickness of Unit 2, assuming that the log thickness is negatively correlated with log distance from a source area, indicates that the Bolaven Plateau area (15.2°N, 106.1°E) is the most probable impact site.
Unit 2 is composed of weathered basalt fragments, sandstone and mudstone fragments, and fragments of rounded quartzite gravels. This gravel composition of Unit 2 indicates that the target rock of the impact includes basalt, sedimentary rocks, and possibly river gravels. This estimation of the target rocks is consistent with the estimation of the impact site in southwestern Laos, where Neogene to Quaternary basalt lava and Mesozoic sedimentary rocks (mudstone, sandstone, and conglomerate) are widely distributed.
The result of this study strongly supports the hypothesis of Sieh et al. (2019) that the AATE occurred in the basalt field of the Bolaven Plateau, and the crater was buried by younger basalts that erupted after the impact.