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| Miocene Bahean stratigraphy in the Longzhong Basin, northern central China and its implications in environmental change | |
| 基金项目： | 国家重点基础研究发展计划(973计划);国家自然科学基金 |
| 摘 要： | Fossil mammal-riched Neogene strata are widely distributed in the southeast corner of the huge Longzhong Basin at Tianshui, Gansu Province, northern central China. Hipparion weihoense, a typical member of late Middle Miocene Bahean stage, was recently excavated at Yaodian along a well-exposed outcrop. Owing to the importance of the Bahean stage in the mammalian evolution and its potential for environmental change, we suggested a name of Yaodian Formation for the stratigra- phy, which is correlated to the Bahe Formation at Lantian, Shaanxi. High resolution paleomagnetic dating of the section shows that the Yaodian Formation covers the period between 11.67 Ma and 7.43 Ma, with the site bearing Hipparion weihoense being estimated at about 10.54―10.30 Ma, providing first magnetostratigraphic chronology for the Bahean Stage. The Yaodian Formation consists of fluvial channel deposits (11.67―10.40 Ma) at the bottom, floodplain deposits in the middle (10.40―9.23 Ma) and shallow lake sediments at the top (9.23―7.43 Ma). This upward fining sequence suggests that the relief in nearby mountain ranges such as West Qinling to the south and Huajia Ling to the north was greatly reduced after long-term denudation, fluvial transport capacity was low, and finally the drainage system was disintegrated, replaced with broad-shallow lakes in which only fine sediments like mud and marlite were deposited, indicating an old stage of development of a planation surface. A remarkable shift in ecology and climatic environment was found at 7.4―7.7 Ma when paleoclimate changed from early warm humid to late dry as indicated by sedimentary facies changed from early shallow lake sequence to late eolian red clays and a former coniferous-deciduous mixed forest was replaced by grassland, leading to great growth of Hipparion Fauna of Baodean stage in the region. Therefore, it is estimated that the present high relief of Qinling and drainage pattern did not come into being until Late Pliocene in response to intensive neotectonism and climate change. |
| 收稿时间： | 29 August 2005 |
| 修稿时间： | 5 June 2006 |
Miocene Bahean stratigraphy in the Longzhong Basin, northern central China and its implications in environmental change |
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| Authors: | LI Jijun ZHANG Jun SONG Chunhui ZHAO Zhijun ZHANG Yong WANG Xiuxi ZHANG Jianming CUI Qiaoyu |
| Institution: | 1. Key Laboratory of Western China's Environmental Systems (Ministry of Education) & College of Earth and Environment Sciences,Lanzhou University,Lanzhou,730000,China;Jiangsu Key Laboratory of Environmental Change and Ecological Construction & College of Geography,Nanjing Normal University,Nanjing,210097,China 2. Key Laboratory of Western China's Environmental Systems (Ministry of Education) & College of Earth and Environment Sciences,Lanzhou University,Lanzhou,730000,China 3. Jiangsu Key Laboratory of Environmental Change and Ecological Construction & College of Geography,Nanjing Normal University,Nanjing,210097,China |
| Abstract: | Fossil mammal-riched Neogene strata are widely distributed in the southeast corner of the huge Longzhong Basin at Tianshui, Gansu Province, northern central China. Hipparion weihoense, a typical member of late Middle Miocene Bahean stage, was recently excavated at Yaodian along a well-exposed outcrop. Owing to the importance of the Bahean stage in the mammalian evolution and its potential for environmental change, we suggested a name of Yaodian Formation for the stratigraphy, which is correlated to the Bahe Formation at Lantian, Shaanxi. High resolution paleomagnetic dating of the section shows that the Yaodian Formation covers the period between 11.67 Ma and 7.43 Ma, with the site bearing Hipparion weihoense being estimated at about 10.54–10.30 Ma, providing first magnetostratigraphic chronology for the Bahean Stage. The Yaodian Formation consists of fluvial channel deposits (11.67–10.40 Ma) at the bottom, floodplain deposits in the middle (10.40–9.23 Ma) and shallow lake sediments at the top (9.23–7.43 Ma). This upward fining sequence suggests that the relief in nearby mountain ranges such as West Qinling to the south and Huajia Ling to the north was greatly reduced after long-term denudation, fluvial transport capacity was low, and finally the drainage system was disintegrated, replaced with broad-shallow lakes in which only fine sediments like mud and marlite were deposited, indicating an old stage of development of a planation surface. A remarkable shift in ecology and climatic environment was found at 7.4–7.7 Ma when paleoclimate changed from early warm humid to late dry as indicated by sedimentary facies changed from early shallow lake sequence to late eolian red clays and a former coniferous-deciduous mixed forest was replaced by grassland, leading to great growth of Hipparion Fauna of Baodean stage in the region. Therefore, it is estimated that the present high relief of Qinling and drainage pattern did not come into being until Late Pliocene in response to intensive neotectonism and climate change. |
| Keywords: | Bahean stage magnetostratigraphy Yaodian Formation Neogene Longzhong Basin |
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