Sea-Level Changes and Formation of Cyclic-Sequence of Cambrian-Ordovician in Akesu-Bachu Area,Xinjiang

Key words: 
• cyclic-sequence /
• system /
• tract /
• sea-level /
• changes

Abstract: The Cambrian-Ordovician sequences are the most widespread ones in Tarim basin.More and more results of explorations and exploitations indicate that they are most important source and reservoir rocks in the basin. It is a set of shallow-bathyal deposits of carbonate and clastic rocks. The sedimentary facies of the study area were a kind of local platform in Cambrian,and platform-open sea shelf in Early-Middle Ordovician,at the same period,it was shelf facies in Keping area and open platform facies in Bachu area.In late Ordovician,the bulk areas were rapidly uplifted and changed into land by the influences of Caledonian orogeny. So the Upper Ordovician was hiatus or truncation in most areas of the basin. The variations of sea level had varying scales or orders during the long geological history and it led to formation of various sedimemtary characteristics in which it were objectively recorded in sedimentary rocks and sequences.For this reason,by means of studying the sedimentary features,sequences and boundary characters in the rocks and strata,it is able to estimate the laws of sea level changes during the period of sedimentation,analyse the conditions of paleogeography and paleoclimatology,and provide some scientific basis for exploration of oil and gas.Through study on the deposition characteristics,sequence stratigraphic framework and sea level changes,two second-order cyclic sequences,four third-order cyclic sequeces and nine fouth-order cyclic sequences have been recognized.Detailed analysis has focussed on the third-order cyclic sequences. The study shows that the formation of every cyclic sequence is caused by the relatively rapid rise,slowly rise,stagnant,slowly fall and rapid fall of the sea-level,in which the maximun sea-level rise occured during late Qioulitage period-early Tumuxiouke Period and,thereafter,for the sake of tectonic activity and global sea-level fall,the sea water withdrawed rapidly from the study area,resulting in the loss of the Upper Ordovician deposition. In brief,according to the evolutionary history of sea level changes and cyclic-sequence,there were two times large scale transgression-regression during the period,and it was the Early-Middle Cambrian and Late Cambrian-Orovician,in which the period of maximum transgression was the Middle Ordovician in the Tarim basin.In the long period(Ⅲ-order) of sea level changes,it contained many high-frequency short-period variations of sea level that it was named composite sea level change.