Microfacies analysis and paleoenvironmental significance of palustrine carbonates in the Thakkhola-Mustang Graben (Nepal Himalaya)

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• 作者：Basanta Raj Adhikari ; Michael Wagreich
• 关键词：Nepal Himalaya ; Thakkhola-Mustang Graben ; Carbonate sedimentology ; Stable isotopes
• 刊名：Journal of Asian Earth Sciences
• 出版年：2013
• 出版时间：15 November, 2013
• 年：2013
• 卷：77
• 期：Complete
• 页码：117-126
• 全文大小：4354 K

文摘

The Thakkhola-Mustang Graben represents the extensional tectonic phase of the Tibetan Plateau uplift and whole Himalayan orogeny. It is situated at the northern side of the Dhaulagiri and Annapurna Ranges and south of the Yarlang Tsangpo Suture Zone. Stratigraphically, the oldest sedimentary units are the Tetang and Thakkhola Formations (Miocene), while the Sammargaon, Marpha and Kaligandaki Formations lying disconformably above these formations represent Plio-Pleistocene units. In this study, different lacustrine carbonates and calcretes were investigated within different lithological units and depositional environments to interpret the palaeoenvironmental and palaeoclimatological evolution of the area.

Geological mapping, construction of columnar sections and carbonate sampling were carried out in the field, and stable oxygen and carbon isotope analyses and thin section analyses were done in the laboratory. Lacustrine facies contained abundant pelletal, charophytic algae, oncolitic algal micritic palustrine limestones with ostracods, and micritic mudstones with root traces. Stable carbon and oxygen isotope analysis from the carbonates show a range of ¦Ä¹³C values from ?0.6¡ë to 11.1¡ë (V-PDB) and ¦Ä¹⁸O values from ?13.5¡ë to ?25¡ë (V-PDB).

Discontinuous growth of oncolites and spherical pellets (25-40 ¦Ìm in diameter) in micritic limestone, algal mats and charophyte algae indicate the presence of both shallow and deep water carbonates. Ostracods in dark micritic carbonates indicate quiet and calm water conditions. Microfabrics of the carbonates suggest that they were deposited in a flat and shallow lacustrine environment. The ¦Ä¹⁸O values of the investigated limestones of the Thakkhola-Mustang Graben suggest that it attained the current elevation level prior to the east-west extension of the Himalaya.