Paleoclimate Changes Of The Late Cretaceous-Early Paleocene In The Nanxiong Basin, South China

The Cretaceous-Paleogene is the hottest, most typical greenhouse period in the Phanerozoic. Documented a series of the continuous terrestrial red succession, the Datang profile of the Nanxiong Basin provide lots of materials for researching the climate changes of the continental Cretaceous-Paleogene. This thesis focused on study of sedimentology and geochemistry of the Upper Cretaceous Zhutian Formation and of the Paleocene Shanghu Formation at the Datang profile. Study of sedimentology is involved in basic lithofacies analysis, climatic and tectonic indexes of debris ratio from clastic composition, and relative content of clay mineral component. Geochemistry study is on carbon and oxygen isotope analysis of pedogenic calcrete.By observation of field cross-section and underscope thin-section from the Upper Cretaceous-Lower Paleogene lithologies show loose red continental clastic rocks, and mainly sedimentary rocks are argillaceous siltstones and silty mudstones intercalated with sandstones and conglomerates. The grain-size texture of the Datang profile upward changes as a pattern of fine-coarse-fine clastic. Totally, sedimental facies of the Zhutian Formation are of alluvial plain to shallow lake facies, the Zhenshui Formation are of river-dominated delta facies, and the Shanghu Formation is of lacustrine facies.Based on the climate index F/Q and the structure index L/Q of debris statistical analysis of sandstone thin sections under the microscope, relative content of clay minerals, illite crystallinity index and illite chemistry index by clay mineral X-ray diffraction analysis, and the value of δ13C (VPDB) and δ18O (VPDB) of pedogenic calcrete, it is proposed that three stages of climatic changes are classified: arid-semiarid during the Maastrichtian, warm and moist semiarid during the early Danian, and the semiarid during the late Danian.1) The first stage------the Maastrichtian (70.4-65.0Ma), the Zhutian Formation to the bottom of Pingling Member of the Shanghu Formation (beds1-41). During the stage, illite is the dominant clay mineral by a mean content of83.4%, chlorite is11.6%in average, illite crystallinity index and chemical index are relatively stable. The climate index F/Q changes near0.1, and the structure index L/Q upward decreases by a fluctuation in value. The carbon and oxygen isotopes of paleosol carbonate nodules remain relatively stable, only a small fluctuation in Zhutian Formation. These indicate that the stage is mainly warm/cool semiarid-arid climate with an overall stability2) The second stage------the early Danian of the Paleocene (65.0-63.8Ma), the bottom of Pingling Member-Xiahui Member of Shanghu Formation (beds41-54). In this stage, the relative content of illite suddenly drops to68%from ca.80-90%, the relative content of smectite increases to the maximum in the three stages by small rising of the relative content of chlorite and kaolinite, and the illite crystallinity index and chemical index increases significantly to the mean0.36and0.62, respectively. In another hand, the mean value0.1of F/Q index and the mean0.27of the tectonic index L/Q shows a decline to the first stage. Comparison to the first stage, the carbon and oxygen isotope values change in opposition:carbon isotope value decreases sharply and then increases slowly; oxygen isotope value slowly decreases after a sharp increase. These indicate it is in hot-humid semiarid in the stage.3) Third stage------the Danian of the Paleocene (63.8-62Ma), Xiahui Member of the Shanghu Formation (beds55-70). The relative content of clay minerals are few changed in average:illite76%, chlorite16%, kaolinite7%, little smectite, the illite crystallinity index and chemical index are mean value0.31and0.48respectively, showing a smooth and steady change. Both climate index and tectonic index change little. The carbon and oxygen isotope values fluctuate with a slowly rising. It implies that this stage is in semiarid, approximately consistent with the climate change indicated by spore-pollen.In summary, the Nanxiong Basin is in arid-semi-arid climate interening some hot-humid climate during the Late Cretaceous-Early Paleocene. According to global palaeogeographic and palaeoclimatic distribution of the Late Cretaceous-Early Paleocene period, the Nanxiong Basin is in tropical arid climatic zones, being consistent with the clay mineral implications in this study. Three kinds of indexes above at the bottom of the Shanghu Formation show sudden climate change near the K/Pg boundary, implying a correspondence South China to the global climate change event of the Cretaceous-Paleogene, which are determined by paleontological, paleomagnetic and stable isotope method.