The Geological Features And Sedimentary Enviroment Analysis Of Benbatu Formation Of The Upper Carboniferous In The North Of Alex, Inner Mongolia

This dissertation is based on the study in Alxa right banner, Alxa League, Inner Mongolia autonomous region. The Engelwusu suture zone has divided the palaeozoic erathem into two parts: Tarim-Nanjiang stratigraphic region(the northwest part) and North China stratigraphic region(the southeast part). The Benbatu Formations(C2bb) of upper carboniferous from upper palaeozoic erathem is in North China stratigraphic region, mainly emerged in Engelwusu-Dongdewusu-Sala-Chaganchubu area. The Benbatu Formation in this study area,based on the characters of rock assemblage, can be divided into three members from bottom to top. The bottom member is shallow sea carbonate platform sedimentary deposits, mainly medium thick marble(the original rock is limestone) with fine grained contain in metamorphic lithic sandstone and metamorphic silt-finestone substratum. The middle member is bathyal slope deposits, consist mostly of metamorphic conglomerate, metamorphic containing calcareous silt-finestone and marble. The third member is deep sea-bathyal turbidite fan deposits,including metamorphic sandstone and phyllite. Pyrite crystals and Bouma sequence can be seen in metamorphic sandstones from some layers, which reflects the deep water reduction environment.Mesure the age of zircons by analysing the metasandstone samples collected from Engelwusuben Benbatu Formation(PM122) and the dikes in the metasandstones. The newest zircons in the metasandstones from this Benbatu Formation date back to 407±3Ma while the oldest in the dikes date back to 267±3Ma. Considering the recorded fossil near this area, this Benbatu Formation dates back to late carboniferous epoch.The Si O2 content in the third member of Benbatu Formation in the study area varies between 42.96% and 78.41%, 61.74% on average. The metamorphic sandstones have a high Al2O3 content(7.52%--16.95%), 13.34% on average. This Benbatu Formation also has a relatively higher CIA index, between 58.66 and 76.4%,65.82 on average, which indicates that the sediment comes from areas effected by strong weathering or the sediment has gone through recycling.(Fe2O3t+Mg O)- Ti O2 diagram and(Fe2O3t+Mg O)-Al2O3/(Ca O+Na2O)diagram show that the forming environment of the third member of this Benbatu Formation is associated with active tectonic background. The rare earth elements and their chondrite-normalized patterns in the investigated metamorphic sandstones shows the LREE content enrichment and negative Eu abnormality. ∑REE of the samples is large, for an average of 143.85×10-6, reflect the characteristics of island arc.It can be concluded by synthesizing the lithology assemblages, sedimentary structure and geochemical analysis that the sedimentary environment of the first member of Benbatu Formation is the shallow sea carbonate platform under the stable tectonic background, the second member’s sedimentary environment is bathyal slope which represents the transformation of tectonic background from stability to activity and the third member’s sedimentary environment is pelagic-hemipelagic which is in activity tectonic background.