4.1 Lithostratigraphy and δ13C isotopic stratigraphy
The Menqu section, which has a total thickness of 179.7 m, is located near the Menqu village in the Nyalam area (Fig. 1). In the lower 0-11.7m, the carbonate rocks are composed of medium- to thick-bedded bioclastic grainstones (Fig. 2a), bioclastic wackestones (Fig. 2b) and framestones (coral reef, Fig. 2c). This part is characterized by high content of fossils, e.g., Lithiotis (Fig. 2d), corals (Fig. 2c), bivalves, gastropods (Fig. 2a), sponges and echinoderms (Fig. 2a). At the interval of 11.7-179.7m, the section is mainly composed of thin- to medium-bedded mudstones. Besides, texture is frequently interrupted (Fig. 2e) by siltstones, grainstones, wackestones and packstones. This part is characterized by rare fossils and much finer grains. Sedimentary structures include erosive base (Fig. 2f), sand lens (Fig. 2f) and hummocky cross-bedding.
Carbon-isotope data of the Menqu section is shown in Fig. 3. At the interval of 0-12.8m, δ13C values recorded by five samples range between 1.76 and 2.67‰, with average value 2.19‰. At the interval of 12.8-179.7m, δ13C values vary from 0.28 to 2.04‰, with average value of 1.40‰.