Sedimentary Environment of the Lower CretaceousDuoba Formation of the Cuoqin Foreland Basin in Tibet

Key words: 
• Tibet /
• foreland /
• the Duoba Formation /
• depositional environment

Abstract: Cuoqin foreland basin is located in the north of Tibet, bettwen Gangdisi Arc and 13angong lake一Nu river frac- lure belt. It wnaly extents along east and west from Geji, Nima to Bange. Thousands of meters marine strata of the Mesozoic Tethys is developed in the basin. The Lower Cretaceous T3uoba Formation is a set of mixed sediments of carbonates and terrigenous elastics. In this paper,acmrding to a detailed study for several sedimentary sections,the author expounds sedimentary evolution of the Ihzoba Formation and its main controlling factors The lower part of Duoba Formation is made up of some non一isopach cycles which are composed of elastic rock, especially gray meth，一thin layer feldspathie litharenite.Each cycle represents a depositional phase that de- trital material of tidal一flat fades rapidly progradate and then retrogradate. These cycles can include tidal一flat channel,sand-flat and mud-flat sediments.There are always some carbonaceous fragments on sandstone layers. In the middle part of Duoba Formation, diffrent kind of medium一thin layer carbonate facies and elastic facies mm- pose a series of sedimentary cycles. Carbonate rock increases upward in these cycles. Each cycle is $0一130 meters, showing a transformation from the tip of tidal一flat deposition to carbonate platform deposition. The upper part of Duoha Formation is composed of three sedimentary cycles which is about 130一200 meters. From bottom to top, each cycle includes mudstone of low一energy subtidal facies,sandstone of tidal一flat end facies,mudstorie of suhti dal facies,carbonate rock of platform and its edge fades. Comparing with the middle part, carbonate rock distinctly increase and thicken, and thick layer nudist limestone or orbitolina limestone of shoal facies occurs too. As a whole,progradation of tidal一flat facies is primary in the early of depositional phase of the Duoba Forma- tion,and retrogradation of tidal一flat fades and extention of Carbonate platform is prominent in the late. This depo- sitional character shows not only in the whole Duoba Formation,but also in each Sedimentary cycle of the Duoba Formation.Synthesizing depositional character of the Duoba Formation, a depositional model can be reached. It is a mixed sediments model near shore for carbonates and terrigenous elastics, with plentiful supply of elastic and a gentle slope. From land to sea, there are two parts in the model: one by land is tidal-flat, and the other is carbonate plat- form,bettwen them develops mixed sediments of low-energy subtidai fades. From the south to the north,difference in depositional feature is obvious within narrow basset area of the Duo- ba Formation along east and west in Cucxtin basin. During deposition of the Duoba Formation, there are two car- bonate platforms about Guiya一Lunpola and Dongcuo, and Geji一Yanhu maybe is the third. Palaeotectonic system clearly controls distribution and depositional feature of the Lower Crezaceous including the Duoba Formation in Cuoqin basin. Bouned by.liangma一Geang lake fracture, the south near the Gangdisi Arc develops pyroclastic sediments with 2D00 meters thick near Cunqin, and its sedimentary environment is wedge-top and foredeep of foreland basin. This set of stratum is called Zhenong Formation(Kizl).On the contrary, the north develops mixed sediments of carbonates and terrigenous elastic. Its sedimentary environment is foredeep and fore- lift. This sediments forms the Duoba Formation. Clastic material mainly from Cangdisi Arc and wedge-top. Towards basin uplift near Bangong lake一Vu river fracture belt, carbonate rock in strata increases, and formation thickness reduces and gradully pinch一outs. These obviously show a sedimentary fracture of foreland which is con- troled by thrust belt.