The Marine Jurassic and Lower Cretaceous of southern Xizang (Tibet): bivalve assemblages, correlation, paleoenvironments and paleogeography

Abstract

This study is based on 15 measured Jurassic to Lower Cretaceous sections and their abundant bivalve faunas distributed mainly in the Nyalam and Gamba districts of southern Xizang. It addresses the establishment of bivalve assemblages, stratigraphic subdivision and correlation, analysis of sedimentary environments, sea-level change and paleogeography in light of the tectonic framework and geological evolution of the Xizang Plateau. The Plateau consists of three terranes: Qangtang, Gangdise and Himalaya They are separated by the sutures: Kunlun-Hoh Xil-Jingsha Jiang, Banggong Co-Nu Jiang, and Yarlu Zangbo Jiang, which represent three closed oceans: Paleo-Tethys, Neo-Tethys and South Xizang Sea. Stratigraphic development of the Jurassic and Lower Cretaceous of southern Xizang is especially closely related to the evolution of the South Xizang Sea Bivalves are one of the most common and important Jurassic and Early Cretaceous fossil groups in the Plateau. From 15 measured sections, 51 bivalve species within 26 genera are described, including 3 new genera and 14 new species. New genera: Vanustus, Yoldioides, Antipectenoides New species: Nuculoma oriens (Nuculidae) Yoldioides jurianoides (Malletiidae) Mesosaccella gangbaensis (Nuculanidae) M. Orienta Grammatodon (Indogrammatodon) sinensis (Parallelodontidae) Oxytoma jiabulensis (Oxytomidae) Meleagrinella minima M. dongshangensis M. sinensis Entolium dongshangensis (Entoliidae) Antipectenoides sinensis Ctenoides shizangensis (Limidae) Anisocardia shizangensis (Arcticidae) Protodiceras lanonglaensis (Megalodontidae) A stratigraphic sequence of ten different bivalve assemblages and four Buchia faunas from the Jurassic to Lower Cretaceous of Nyalam district, and four Jurassic bivalve faunas from the Gamba district are recognised. In addition, the Oxytoma jiabulensis bed and two Meleagrinella beds are also recognised. These assemblages can be seen to be of two types, high and low diversity, indicating different sedimentary environments. These assemblages along with co-existing ammonites also provide a good basis for stratigraphic division and correlation of the sequences. Special attention is given to nomenclatural and other problems associated with the important genus Buchia:. Australobuchia Zakharov is believed to be inseparable generically, and some Xizang species of Buchia an placed in synonymy. On this basis the evolution and migration of the genus is discussed From measured sections in southern Xizang, a comparatively complete Jurassic- Lower Cretaceous sequence has been established, which includes six formations: (in ascending order) Pupugar. Nieniexiongla. Lanongla. Menbu. Xuomo and Gucocun Formations. A new stratigraphic unit, the Gangdong Formation, is established, for Mid-Upper Jurassic strata of the Gamba district. Direct and indirect biostratigraphic correlation within China and between southern Xizang and other places around Gondwanaland is well established. Lithologic features, sedimentary structures and bivalve assemblages allow four environments including nine lithofacies to be recognised in southern Xizang: 1. Inner shelf environment 1] Shoreface sandstone facies 2] Neritic terrigenous clastic facies 3] Open shelf carbonate facies 4] Protected barrier carbonate facies 5] Reef limestone facie 2. Outer shelf environment 6] Siltstone-shale-carbonate facies 3. Slope environment 7] Fine clastic facies 8] Pelagic limestone facies 4. Bathyal environment 9] Dark Buchia-ammonite-lutite facies They can be seen to be organised into sedimentary cycles, which clearly show a huge transgression with three peaks in the Upper Jurassic reaching a maximum at the top of the Jurassic (Upper Tithonian). In the Lower and Middle Jurassic, sea-level changes fluctuated between coastal and shelf-sea environments, whereas in the Upper Jurassic, as a result of sea-floor spreading in the South Xizang Sea and the formation of fault basins, the area descended into bathyal depths. Paleogeographically, the northern part of the Plateau (i.e. the Northern Branch, or Neo-Tethys) underwent a shallowing process along with areal extension during Jurassic times, but shrank in the Early Cretaceous, whereas in the Southern Branch, during Jurassic-Early Cretaceous times, the South Xizang Sea developed a wide variety of sedimentary environments, including inner and outer shelf sea, continental slope and bathyal fault basin.