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).