| As an important field of global paleoclimatic and paleoenvionmental studies, Neogene environmental change study is crucial in understanding and explaining correctly many key scientific problems, such as global climatic change processes, formation of Northern and Southern hemispheres icecaps since Miocene. Studies in Neogene environmental change have important value to know aridification in Asia inland, formation of Asia monsoon, and the time, form, scale and environmental effect of the uplift of the Tibetan Plateau.Located in the sensitive northeastern tectonic margin of the Tibetan Plateau and constrained by southern west Qinling Mountain, northern Huajia Hill and eastern Liupan Mountains, Tianshui-Qin' an area in southeast Longzhong Basin is an ideal place to study the tectonics and climatic change since Neogene. Continuous, widely distributed Neogene depositional strata was developed in this area, which provide favorable natural carriers to study the Neogene depositional envionmetal change. However, Neogene environmental study in Tianshui Basin is relatively weak compared with those in sub-basins in the same Longzhong Basin, such as Linxia Basin, Lanzhou Basin and Xining Basin. Since Guo et al. (2002) reported the analog of eolian deposits to Quaternary loess indicating that aridication in the inland of Asia was likely to begin since 22Ma, this area become a hot spot of the study. However, from the view of whole Longzhong Basin in Neogene period, extensive fluvial-lacustrine sediments deposited in sub-basins in Linxia and Lanzhou, could provide analogical tectonic and sedimentary environment background. Paleontological data also showed relative humid climate at that time. In addition, there is large area of typical fluvial-lacustrine stratigraphic sequences in Tianshui Basin during this period, which is inconsistent with eolian viewpoint as reported in Neogene stratum. In order to make clear the incompatibility of sedimentary sequences in this area and understand more precisely the cause of formation of Neogene sediments and environmental change, we have impletmented a series of comparison and analysis of Neogene typical sedimentary sections about stratigraphic sequences, lithological charaters, magnetostratigraphy, evolution of sedimentary facies, physical, chemical and biological indices of the sediments from the south to the north in whole Tianshui Basin on the basis of several years field work and geological, geomorphological and paleontological data collected. Major conclusions have been obtained as follows:1. Based on the stratigraphic comparison of Xiashan, Yaodian and Lamshan sections within the basin from the south to the north, using high-resolution magnetostratigrapic and paleontological mammal fossil chronology, we have established Neogene stratigraphic chronological sequences from 17.02Ma to 1.4Ma in Tianshui Basin. According to the lithological assemblages, stratigraphic conformable contact connection and stratum characteristics in spatial distribution, combined with mammal fossils and lava age below Neogene stratum obtained by previous researches, Neogene strata in Tianshui Basin are macroscopically divided into five sects from top to bottom: "lower red bedding" Ganquan Group, "zebra bedding" Yaodian Group, "upper red bedding" Yangjizhai Group and "upper green bedding" Lamashan Group, and their ages are 22～11.7Ma, 11.70～7.1 Ma, 7.1～4.07Ma and 3.6～1.4Ma respectively.2. The sediments of QA-I section are more similar to those of typical lacustrine origin like Xiashan section, and are different with those of loess, as they obviously not having the characteristics of eolian deposits on composition, which is supported by various analyses on size parameters, constant elements, trace elements and REE geochemistry of sediments from QA-I section, typical lacustrine deposits and Xiashan section. The environmental cause of QA-I section are similar to that of typical lacustrine sediments of Yaodian and Lamashan sections, being possibly resulted from floodplain and lakefront as indicated by organic biomarkers, micromorphological characteristics and diatom indicators.3. Spatial distribution of abundant paleovertebrate mammal fossils, characteristics of the composition and sedimentary environment suggest that there was a more conformable environment of lakebank mudflat for big mammals to exist, and the climate was relatively humid. The taphouomy of abundant mammals and sedimentation rates indicate that whole big mammal fossils were not possible to be buried by eolian deposits at this area before 6Ma.4. The rough distribution range of Neogne paleolacustrine sediments in Tianshui Basin could be mapped on the basis of detailed field investigations. Based on the analyses of lithological assemblages, spatial-temporal distribution of lithological facies in Tianshui Basin, and foreland basin property and tectonic characteristics, we established the Neogene sedimentary mode of Tianshui Basin. Six evolvement stages is divided in Tianshui Basin since Cenozoic: >22Ma, basin rudiment stage; 22～11.67Ma, Neogene tectonic movement and basin formation stage; 11.67～7.1Ma, tectonic movement and basin developing stage; 7.1～<4.07Ma, tectonic uplifting and basin shinking stage; <4.07～3.6Ma, tectonic uplifting and basin perishing stage; 3.6～1.4Ma, intermontane basin stage.5. Analyses on eolian souce, transportation dynamics, accumulational relief, preservation and climatic conditions in Miocene suggest that it was not favorable to form and preserve eolian accumulation in Tianshui Basin during that period. Simultaneously, comparison of the sedimentary sequence of Tianshui Basin with those from Linxia and Lanzhou sub-basins within the whole Longzhong Basin, together with mammal fossils in Longzhong Basin, pollen data of Linxia and Jiuquan basins in Hexi corridor, Miocene eolian accumulation in Tianshui Basin is doubted to a greater extent. The aridification in Asia inland is thought to begin, not since 22 Ma as so-called eolian accumulation indication, but since about 8 Ma.
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