Analysis Of Carbonate Microfacies From The Late Cretaceous To Oligocene In Wuqia Area,the Northwestern Tarim Basin

The Tarim Basin is one of the few areas in China where Meso-Cenozoic marine strata are exposed, which is rich in oil and gas resources. From the world’s ancient oceanographic point of view, it is controled by the evolution of Tethys Ocean.The predecessors have researched these layers ten years and achieved some important progress of classification and paleontology from the Cretaceous to Oligocene in the areas.In this study, Kuzigongsu section of Late Cretaceous to Oligocene strata is selected to investigate the lithology and biological assemblage and particle composition and substrate type and sedimentary structure of the Late Cretaceous to Oligocene marine stratum by the means of sedimentary microfacies analysis. Thus, this will surely provide some references for finding out evolution of the Tethys Ocean from the Cretaceous to Oligocene and reconstructing regional paleoenvironment and researching oil-gas exploration in Tarim Basin. Through the study obtain some understandings.1. The Tarim Basin of Northwestern margin Wuqia area of Late Cretaceous-Oligocene develop some the carbonate microfacies types, including biogenic limestone (oyster bio-limestone), spar oolite limestone, biodetritus limestone (spar bioclastic limestone, micrite biodetritus limestone), biodetritus micrite limestone, with biodetritus micrite limestone, micrite limestone (bird’s-eye limestone), microcrystalline sphedrulite dolomite, oolitic microcrystalline dolomite, fine crystalline dolomite and microcrystalline dolomite. Combining with sedimentary characteristic of the section, we identify five sedimentary environments (supratidal zone, intertidal zone, subtidal zone, shallow of platform edge, and reefs) and restored the sedimentary evolution process from the late Cretaceous to Oligocene in the section.2. From the late Cretaceous to Oligocene, the Kuzigongsu section’s sedimentary evolution of the process is as follows:the lower part cycles of the kezilesu group is priority to continental sedimentary facies and river sedimentary facies. The upper part cycle is near the continental margin of the shore. The medium term of Kukebai formation was formed subtidal zone, the early term and late term of the formation develop low-energy intertidal zone. With the increased degree of regression, Wuyitake formation is formed subtidal zone deposition. A small-scale transgressive is founded in Yigeziya formation, which form intertidal-supratidal carbonate mud flat deposits. Tuyiluoke formation is deposited in the supratidal sabkha zone. Aertashi formation continues the feature of Tuyiluoke, which form a thick gypsum layer deposition. A mainly sedimentary assemblage of dark mudstone rich in planktonic foraminifera is formed in Qimugen formation. Its sediment characteristics are subtidal zone-shallow water deposition. And the transgression is the biggest in this section. With the period of large-scale regression Gaijitage formation formed on the tide with a red gypsum mudstone deposition. The third transgression happens in the Kalataer formation which is formed reef and shoals. The early term of Wulagen formation formed shoal facies, and the late happen regressive. Early in Bashibulake formation has supratidal sediment characteristics, and found a layer of oyster shell on top, forming a short intertidal deposition. Based on the distribution of sedimentary facies and the regional information, we infer that there are five cycles of marine transgression along the northwestern margin of Tarim Basin from the Cretaceous to Paleogene. Three large-scale transgressions are founded in Kukebai group, Qimugen group and Kalataer group and two small-scale transgressions are founded in Yigeziya group and the late of Bashenbulake group.3. The upper strata of Bashibulake formation is Wuqia group which mainly composed of mudstone and siltstone interbedded sandstone and marl, the typical marine fossils disappear, the Tethys final exit layer of the northwestern margin of the Tarim Basin is the top of the the Oligocene Bashibulake formation. After the area transform the marine environment into the residual sea (lagoon)-salt lake environment.