Paleoclimate Of The Late Cretaceous-Paleocene In The Chaling Basin,Hunan,South China

The Cretaceous-Paleogene is one of the most typical greenhouse periods through geological history.The Chaling basin in Hunan is a typical continental red basin formed from the Late Cretaceous though the Paleocene,within which much paleoclimate records are well documented.In this paper,studies on the sedimentology and climate are carried out based on the observation of the Xintiancun Section?PM01?and Dakeng-Dongchongcun Section?PM02?,of which the Daijiapin Formation and Zaoshi Formation are the only two lithostratigraphic units relevant.Lithofacies,clasitic composition index,CL and carbon-oxygen isotopes are used to reconstruct the paleoclimatc change of the Chaling Basin during the Late Cretaceous-Paleocene.Lithofacies analysis shows that the Chaling Basin is characterized by continental red clastic rocks,which have been deposited in environments of allvial fan,river and flooding/fluvial plain.Variety of clastic rocks such as?para-?conglomerates/brecia,sandstones/graywacks,siltstones and silty mudrocks can be observed.The Daijiaping Formation is of arid lithfacies,including dry alluvial fan,gravel desert,aeolian sand dune,flooding plain with fluvial facies;and the Zaoshi Formation is mainly of lacustrine facies.These lithofacies mainly indicate arid-semiarid paleoclimate.Climatic index F/Q and tectonic index L/Q are acquired using grain number ratio of feldspar?F?,lithic clasitic?L?and quartz?Q?.Results show that the index F/Q is significantly affected arid-semiarid climate,and there are two rapid increase of F/Q index possibly inidcating two cooling processes in the Chaling Basin.Pedogenesis is an extensive sedimentation in the Chaling Basin.Besides of lithofacies,stable isotope analyses of pedogenic calcretes are made from the sections PM01 and PM02.Rates of carbon and oxygen isotopes reveal that at least one warming and cool event have happened duing the Late Cretaceous-early Paleocene in the Chaling Basin.The most prominent manifestation is the positive excursion of 5‰?18O and negative excursion of 2 ‰ ?13C at the depth 100 m.The average pCO₂ concentration estimated from pedogenic carbonates is 529 ppmv?1.92 PAL?,and the pCO₂ peak 867 ppmv?3.15 PAL?occurred at the depth 100 m,correlatible with ?18O positive excursion.Combined the pCO₂ with lithofacies and clastic composition index,it is proposed that the Chaling Basin had been in a relatively hot arid and semi-arid during the Late Cretaceous-Paleocene.Particularly,CL and high-resolution stable isotopes of large pedogenic calcretes?>-8 cm in diameter?are analyzed from the Daijiangping Formation.CL analysis shows that,calcretes mainly consist of reddish-brown calcite matrix and light-red calcite vein,corresponding to CL dully reddish-orange or non-luminescence and CL bright orange,respectively.Microdrilling of calcretes is performd for carbon-oxygen analyses under guide of CL images.of reddish-brown matrix ranges from-7.96 ‰ to-11.35 ‰ VPDB,while the ?18O values of calcite vein ranges from-9.16 ‰?-11.16 ‰VPDB.The integration of calcite composition,CL,and oxygen isotope indicates two stages of calcite precipitation have ever undergone for the calcretes.According to the oxygen isotope array,three meteoric calcite lines?MCLs?of ?18O values are recognized as-9.15±0.14 ‰,-9.04 ± 0.18 ‰ and-8.0±0.11‰ VPDB from the sample CL-11C4 and CL-11C2,separately.Subsequently,the ?18Ow values of the late Late Cretaceous paleoprecipitation are estimated from-5.76 to-6.91‰ VSMOW for the Chaling Basin,consistent with those from the similar latitude in North America.These results provide basic reference data for the Cretaceous greenhouse hydrologic cycle model and paleoatmospheric circulation simulation,and verify that the enhanced hydrological cycle may play an important role in greenhouse condition.