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‰.