2. Geological setting
The southern Tibet can be subdivided into five tectonic units, which are Xigaze Forearc Basin, Yarlung-Zangbo Ophiolite, Tethyan Himalaya, Greater Himalaya and Lesser Himalaya from north to south (Fig. 1). Mesozoic strata are well-developed in the Tethyan Himalayan zone, which was divided into the Northern and the Southern subzones (Wang et al., 2005) separated by the Gyirong-Kangmar Thrust. Our study section, the Menqu section (GPS: 28°39′08″N, 86°02′57″E), is located in the Tethyan Himalayan Southern subzone. During the Mesozoic, the Tethyan Himalaya belongs to the north margin of Indian subcontinent. The Jurassic sediments in Southern subzone are characterized by shallow-water carbonates and terrigenous rocks (Jadoul, Berra, & Garzanti, 1998). The Jurassic sedimentary succession is composed of thick-bedded bioclastic limestones of Pupuga Formation (Fm.), ooidal limestones with sandstone intercalations of Nieniexiongla Fm., marls of Lanongla Fm., ooidal ironstones of Dingjie Fm. and clastic rocks of Menkadun Fm. from the bottom to the top (Jadoul, Berra, & Garzanti, 1998; Wang et al., 2005; Li & Wang, 2005; Han, Hu, Li, & Garzanti, 2016).
The Nieniexiongla Formation consists of gray to light-gray, thin- to medium-layered mudstones or oolitic grainstones rhythmically interbedded with sandstones (Li & Wang, 2005) and is Toarcian-Bajocian aged as indicated by ammonites (Yin, 2010). Middle to outer ramp was indicated by microfacies analysis (Han, Hu, Li, & Garzanti, 2016) and foraminiferal ecological research (Wan, 1989). A rapid transgressive and negative carbon isotope excursion initiated at the boundary of the Pupuga and Nieniexiongla formations is recently interpreted as the responses to the Toarcian Oceanic Anoxic Event (T-OAE, Han, Hu, Kemp, & Li, 2018).