日本地球惑星科学連合2019年大会

講演情報

[J] ポスター発表

セッション記号 H (地球人間圏科学) » H-CG 地球人間圏科学複合領域・一般

[H-CG32] 堆積・侵食・地形発達プロセスから読み取る地球表層環境変動

2019年5月26日(日) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:清家 弘治(産業技術総合研究所・地質調査総合センター)、山口 直文(茨城大学 広域水圏環境科学教育研究センター)、成瀬 元(京都大学大学院理学研究科)、高柳 栄子(東北大学大学院理学研究科地学専攻)

[HCG32-P07] Petrographic characteristics of Middle Permian sandstones in Southern Mongolia and their geological implications

*Enkhbayar Bolormaa1Dashpuntsag Nansalmaa1Bat-Orshikh Erdenetsogt1Baigalmaa Norov1Bat Bold1Luvsanchultem Jargal1Avirmed Tulga1 (1.National Unniversity of Mongolia)

キーワード:sandstone provenance, Middle Permian, Mongolia

Middle Permian Tavantolgoi coal deposit lies in southern Mongolia and has more than 6 billion tons of coal including significant tonnage of coking coal. In 2017, exploration bore holes were drilled at Borteeg, the largest syncline of the deposit. Eighteen sandstone samples were collected from coal-bearing sequences that were intersected by 900 m deep bore holes. Standard thin sections were prepared from core samples and studied under polarizing microscope and point-counted using the Gazzi-Dickinson method.

Detrital grains of the studied samples consist of mineral grains and rock fragments. They are generally well-rounded and moderately sorted. Size of detrital grains ranges from 0.1 mm to 1.0 mm. Detrital minerals quartz and feldspar comprising 7-31% of the total constituents. Quartz comprises 13-24% and monocrystalline quartz dominates over polycrystalline quartz. In feldspar grains, plagioclase is more abundant than K-feldspar. Lithic fragment comprises 49-60%, of which volcanic rock fragments predominates. Cement is carbonates partly enriched in iron oxide, resulting in reddish color.

The sandstones of Middle Permian in Borteeg syncline are classified as litharenite suggesting that weathering intensity in source area was weak, probably due to relatively arid to semiarid climate conditions. Provenance analysis (Q-F-L, Qm-F-Lt, and Qp-Lvm-Lsm diagrams) suggests that the sandstones are recycled sediments derived from transitional arc and undissected arc that were formed during the closure of Paleoasian ocean between southern Mongolia and North China block.