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Kohki Yoshida, Shinshu University (Japan)
Akiko Yamanaka, Shinshu University (Japan)
Toshio Kawamura, Miyagi University of Education (Japan)
Shigeyuki Suzuki, Okayama University (Japan)
Megh Raji Dhital, Trivuwan University (Nepal)
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Records of incisive paleoenvironmental changes during the Early-Middle Triassic period are documented in the sedimentary sequence exposed in the central area of Nepal Himalayas. In this region, the Triassic sequence comprises the Tamba Kurkur Formation (Lower Triassic: Scythian-Anisian), Mukut Limestone Formation (Middle Triassic: Anisian-Carnian), Tarap Shale Formation (Upper Triassic: Upper Carnian-Norian) and Quartzite Formation (Thini Formation, Most Upper Triassic; Norian-Rhaetian) in the ascending order. These strata were deposited in the southern margin of the Neotethys Sea.
The Lower Triassic Tamba Kurkur Formation, which rests directly on the Permian Thini Chu Formation, comprises weakly bioturbated mudstone and limestone with a large amount of ammonoids in the lower part. In the upper part of the formation, moderately bioturbated mudstone prevails. The transitional horizon correlates to the Dienerian-Smithian period between the lower and upper parts of the formation; although the exposure is limited to sporadic occurrences, it appears to reveal an upward increase in the bioturbation, suggesting a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. The Middle Triassic Mukut Limestone Formation comprises alternate beds of completely bioturbated mudstone and marly limestone. Mudstone contains several fossils such as ammonoids, brachiopods, bryozoans and foraminiferas.
For sedimentary successions, the element geochemical proxies, which are chemical indices of alternation (CIA value), Zr/TiO2, Cr/Zr, Y/Ni, Zn/TiO2, V/Cr and V/V + Ni indicate a pronounced variation in the sedimentary conditions during the Early to Middle Triassic periods. For the period form the early Triassic until the Spathian, these geochemical proxies exhibit fluctuating geochemical settings. A relatively low CIA value that is affected by various paleoweathering regimes in arid conditions is detected. The bottom-water redox and salinity conditions and provenance assemblage exhibited significant fluctuations, and temporal dysaerobic conditions occasionally occurred in the shelf environments. In particular, in early Triassic strata in Dienerian-Smithian record punctuated episodes of low oxygenated bottom water condition. In the Middle Triassic, the geochemical signals suggest decreasing aridity and increasing weathering in the hinterland. Slight dysaerobic bottom-water conditions are recorded suggesting a relative increase in sea levels followed by the stabilization of climatic and bottom-water conditions without significant anoxia and salinity changes.
It is considered that the southern margin of the Neotethys Sea was oxygenated until the low oxygen conditions reached their maximum extent during the Late Griesbachian (Woods, 2006). There is a possibility that the poorly oxygenated bottom water may have persisted or existed sporadically until the Smithian in this area, and the dysaerobic water appeared to have retreated until the Spathian.
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