中国科学院南京地质古生物研究所机构知识库

A Middle Ordovician limestone succession, representing part of the Alai Formation (middle part of the Chiatsun Group) of the Himalayan Orogenic Belt, is well exposed at Jiacun section in Nyalam County to the west of the Mount Jolmo Lungma (Mount Everest), southern Xizang (Tibet), China. The succession, though reported previously, has yet to be investigated in detail, especially in the aspects of precise dating, stratigraphic correlation and its paleogeographical affinities. In this contribution we document for the first time the conodont biostratigraphy, carbon isotope chemostratigraphy and microfacies analyses of this carbonate succession. The conodont fauna indicates a middle Darriwilian (Middle Ordovician) age corresponding to the Histiodella holodentata Biochronozone. The carbon isotope records have shown minor fluctuations of delta C-13 values, with only a small negative carbon excursion in the lower part. The microfacies analysis has resulted in the identification of seven microfacies, including micritic limestone, lithoclastic packstone, non-skeletal grainstone, bioclastic limestone, bioclastic-lithoclastic limestone, bioclastic-lithoclastic-oncoid limestone, and bioclastic-lithoclastic-oncoid-ooid limestone. From thin sections, fragments of crinoids, brachiopods, gastropods, trilobites, bryozoans and calcareous algae are identified. Abundant lithoclasts, cortoids, peloids, oncoids and ooids, together with the occurrences of dasyclad algae and hardgrounds, are also recognized, indicating a shallow- and warm-water marine environment, and a likely tropical or subtropical affinity. Comparison of the lithofacies and faunas of this limestone succession of the Alai Formation with coeval strata in the Zanskar-Spiti area (northern India), Nepal, Bhutan, Canning Basin (West Australia), Lhasa, Qiangtang, Sibumasu, Tarim and North China, suggests that during the Darriwilian these regions or terranes may have possessed a warm-water marine environment and were located in low-latitudinal zones. It seems likely that this region of the northeastern Gondwana and peri-Gondwana was protected from substantial influx of cool-water currents derived from high latitudes at that time.