5.1. Chronostratigraphy of the Menqu section
The Nieniexiongla Formation and the overlain Pupuga Formation can be divided by their distinct characteristics of lithology, bioclastics and δ13C values (Han, Hu, Li, & Garzanti, 2016; Han, Hu, Kemp, & Li, 2018).
The Lithiotis -rich interval occurs in the top of the Pupuga Formation. This horizon appears extensively in the Lower Jurassic successions in the Tethys realm and can be used as a mark of biostratigraphic correlation (Han, Hu, Li, & Garzanti, 2016). The Pupuga Formation consists mainly of medium- to thick-bedded (20-50 cm-thick) limestones with abundant bioclastics, e.g., corals, sponges and bivalves. In contrast, the Nieniexiongla Fm. is composed mainly of thin to medium bedded (~10 cm-thick) oolitic grainstones with a few sandstone intercalations (Han, Hu, Li, & Garzanti, 2016). The upper Nieniexiongla Formation reveals an abundance of storm deposits (Han, Hu, Kemp, & Li, 2018). The base of Nieniexiongla Formation is coincided with the onset of negative excursion of the carbon isotope, which was interpreted as the onset of Toarcian Oceanic Anoxic Event (Han, Hu, Kemp, & Li, 2018). A negative carbon isotope excursion of ~2.5‰ is recorded in organic matter, and a small magnitude of about ~-1.5‰ is recorded in δ13Ccarb (Han, Hu, Kemp, & Li, 2018).
In Menqu section, all the above outlined transitions were found at ~11.7 m. From 0 to 11.7 m of the section, the lithologies are mainly medium- to thick-bedded bioclastic grainstones with a limestone bed yielding rich Lithiotis (Fig. 2d). The radiation of the Lithiotis fauna is a global sedimentary event during the Pliensbachian (Franceschi et al., 2014), which demonstrates an Early Jurassic age of this interval. From 11.7 m to top of the section, the lithologies change dramatically as thin to medium bedded limestones with sandstone intercalations (Fig. 2). Hummocky cross-beddings were found in these sandstone layers, indicating strong storm wave influences. δ13Ccarb values changed significantly at ~12 m to average 2.19‰ in the interval below 11.7 m and 1.40‰ above. These observations allow us to date the Menqu section as Early Jurassic age and reveal that the boundary of Pupuga and Nieniexiongla formations is at ~11.7 m. All the studied oolitic grainstones are from the Lower Jurassic Nieniexiongla Formation.