Sedimentary facies and palaeogeography of the Lhozhag region in southern Xizang during the Jurassic and Cretaceous

Nine sedimentary facies and nine sedimentary subfacies have been distinguished for the Jurassic and Cretaceous strata in the Lhozhag region,southern Xizang.From south to north,the sea water becomes progressively deeper in the Ritang Formation,where three facies belts may be recognized.The Lure Formation lies totally in the shelf environment,and the Zhela Formation in the shelf and deep-sea basin environments,where there occur the intermediate and basic volcanic rocks in the extensional settings north of the Rongpu fault and radiolarian siliceous rocks in the deep-sea basin facies.In the Weimei Formation in the deep-water environment,there exist the siliceous rocks and muddy sediments intercalated with shallow-water sediments in the shelf-basin zone,and the coexistence of incised valley deposits and turbidity current deposits.The intense extension and intension once took place in the Sangxiu Formation.The intermediate and basic volcanic eruption along the Rongpu fault brought about diatreme breccias,pillow lavas and andesite with developed prismatic joints and slump breccias on the northern side of the fault while on the southern side of it,there deposited a succession of shelf muddy sediments.The geomorphologic framework of "one uplift and two depressions" appears in the Jiabula Formation:the open shelf environment north of the Rongpu fault and restricted lagoon environment south of Lhozhag.