ARTICLE IN PRESS + MODEL GEOMOR-02090; No of Pages 10 Geomorphology xx (2006) xxx–xxx www.elsevier.com/locate/geomorph Quaternary sediment in the Yichang area: Implications for the formation of the Three Gorges of the Yangtze River ⁎ Fang Xiang a, , Lidong Zhu a, Chengshan Wang b, Xixi Zhao c, Hongde Chen a, Wenguang Yang a a Institute of Sedimentology, Chengdu University of Technology, Chengdu, 610059, PR China b School of the Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, PR China c Institute of Tectonics, University of California, 1156 High Street, Santa Cruz, CA, 95064, USA Received 29 October 2004; received in revised form 22 April 2005; accepted 29 March 2006 Abstract Sediment data from the Yichang area in the Jianghan Basin of Hubei Province in China suggest deposition in a lacustrine environment prior to 0.75 Ma, B.P., followed by incision of the Yangtze River. The earliest Quaternary Yunchi Formation accumulated in an alluvial fan to fan-delta environment. The subsequent Shanxiyao Formation was deposited in an environment that changed from fan-delta to lacustrine. The distribution of sedimentary facies suggests the presence of a lake in the Yichang area prior to 0.75 Ma, B.P. The lack of sediments contemporaneous with the Yunchi and Shanxiyao Formations in other areas of the Jianghan Basin, suggests that this ancient lake was limited to the Yichang area. This lake predates the present Yangtze River in the Yichang area and the Jianghan basin. Provenance studies of gravels in the Yunchi and Shanxiyao Formations, as well as gravels in terraces and the channel of the Yangtze River indicate a variety of sediment sources, but suggest that no material from the area west of the Three Gorges had been carried into the Yichang area prior to 0.75 Ma, B.P. The Yangtze River cut through Three Gorges area only after 0.75 Ma, B.P. © 2006 Elsevier B.V. All rights reserved. Keywords: Yichang; Quaternary sediments; Three Gorges; Incision; Yangtze River 1. Introduction number of studies on the river (Lee and Chao, 1924; Yei and Xie, 1925; Lee, 1933, 1957; Ren, 1957; Shen, 1965; The evolution of the Yangtze River, the largest river in Yang, 1988; Yang and Lu, 1992; Zhang, 1995; Tian et al., China, has been a research topic for a long time (Chen 1996; Zhao, 1996, 1998; Zhao et al., 1997; Tang and Tao, et al., 2001). Interest tends to focus on two key areas: the 1997; Zhao and Wang, 2000; Li et al., 2001). There is still Yangtze bend near Shigu of Yunnan Province and the no consensus regarding the genesis and formation of the Three Gorges to the east of the Sichan Basin (Fig. 1). The Three Gorges. Researchers, such as Lee and Chao (1924), earliest study of the Three Gorges of the Yangtze was Yei and Xie (1925) and Lee (1957) proposed that the carried out by Willis in 1907 (Zhao, 1996), followed by a Three Gorges were superimposed gorges but others (Lee, 1933; Ren, 1957) thought that the Gorges were antecedent ⁎ Corresponding author. in origin. The age of formation of these gorges, as E-mail address: [email protected] (F. Xiang). suggested earlier, varies: Jurassic to Cretaceous (Lee, 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.03.027 Please cite this article as: Fang Xiang et al., Quaternary sediment in the Yichang area: Implications for the formation of the Three Gorges of the Yangtze River, Geomorphology (2006), doi:10.1016/j.geomorph.2006.03.027. ARTICLE IN PRESS 2 F. Xiang et al. / Geomorphology xx (2006) xxx–xxx Fig. 1. Location map, shaded areas are: A. — Sichuan Basin, B. — Three Gorges, C. — Jianghan Basin. 1933; Ren, 1957), earlier Tertiary (Shen, 1965), later taceous. It changed from a downfaulted basin to a re- Tertiary (Lee and Chao, 1924), 2 Ma, B.P. (Zhang, 1995), gionally subsiding one in the Quaternary (Geological and 1.5 Ma, B.P.(Li and Zhang, 1999), 1.2 Ma, B.P.(Yang and Mineral Bureau of Hubei Province, 1990; Compiling Lu, 1992), 1.16 Ma, B.P. (Li et al., 2001), 0.7–1Ma,B.P Group of the Petroleum Geology of the Jianghan Oil Field, (Tian et al., 1996; Tang and Tao, 1997) and 0.15–0.2 Ma, 1991). The basin is surrounded by the Dabashan fold B.P. (Zhao, 1996). system, Qingling fold system, and Jiangnan uplift zone, Studies of river terraces play a key role in under- and comprise five high areas and depression areas (Fig. 2a). standing the evolution of a river system as sediments in The Jianghan Basin contains sediments from the Creta- terraces are direct records of river behavior (Antoine, ceous to the Quaternary. 1994; Hancock et al., 1999; Antoine et al., 2000; Colls The Yichang area is located in western Hubei (Yichang et al., 2001). Terraces are also closely linked with depression of the Jianghan Basin) and to the east of geological and topographical conditions, and are often Huangling Dome (Figs. 2a and 3). The elevation in this deformed by tectonic movements or physical and chem- area decreases from west to east, from mountains (about ical weathering processes. Consequently, they often do 1700 m) in the northwest to the Jianghan Plain (b50 m) in not exhibit the complete evolutionary history of a river the southeast. The terrain between these two areas is hilly system. In contrast, sediments in a loading river basin with an elevation between 100 and 350 m. can be more continuous and intact, offering more infor- Quaternary sediments in the Yichang and adjacent mation about the evolution of a fluvial system (Alam, areas were previously divided (Fig. 3) into Yunchi 1989; Uddin and Lundberg, 1998; Cliff and Christoph, Formation (Early Pleistocene), Shanxiyao Formation 2000). In this paper, we discuss palaeocurrent measure- (Middle Pleistocene), Yidu Formation (Late Pleistocene), ments, gravel characteristics, provenance studies, and and Pingyuan Formation (Holocene), and the underlying sedimentary facies from 7 measured stratigraphic out- marlstone of Neogene Duodaoshi Formation (Nd) or crops from the Yichang area, located immediately below interbedded sandstone, siltstone and mudstone of Eogene the Three Gorges (Figs. 1 and 2a). Fangjiahe Formation contacted with the Yunchi Forma- tion by an unconformity (Regional Geology Survey Team 2. Geological background of Hubei Province, 1970; Geological and Mineral Bureau of Hubei Province, 1990). The Yunchi and Shanxiyao The Yichang area is on the western edge of the Jianghan Formations, especially the Yunchi, are mainly distributed Basin (Figs. 1 and 2a). The Jianghan Basin is a large inland in the Yichang area, and are generally absent in rest of the basin in Hubei Province, formed during the Early Cre- Jianghan Basin (Fig. 2b) except to cover a small area near Please cite this article as: Fang Xiang et al., Quaternary sediment in the Yichang area: Implications for the formation of the Three Gorges of the Yangtze River, Geomorphology (2006), doi:10.1016/j.geomorph.2006.03.027. ARTICLE IN PRESS F. Xiang et al. / Geomorphology xx (2006) xxx–xxx 3 Fig. 2. Tectonic background of the Jianghan Basin and sedimentary characters of some Quaternary sedimentary sections. a) tectonic background of the Jianghan Basin; b) stratigraphic character of sections A, B, C and D. Sections are selected from three depressions: Jiangling Depression (sections A and B), Qianjiang Depression (section C) and Mianyang Depression (section D). Although sections A, B and D are located near the modern Yangtze River, the Yunchi Formation and Shanxiyao Formation of early Pleistocene are absent (A–D locations refer 2a. N — Neogene; Qyd — Yidu Formation; Qp — Pingyuan Formation. Data source: The Geological Bureau of Hubei Province, 1973, 1977a, 1977b, 1977c). Please cite this article as: Fang Xiang et al., Quaternary sediment in the Yichang area: Implications for the formation of the Three Gorges of the Yangtze River, Geomorphology (2006), doi:10.1016/j.geomorph.2006.03.027. ARTICLE IN PRESS 4 F. Xiang et al. / Geomorphology xx (2006) xxx–xxx Fig. 3. Sediment distribution in the Yichang area, locations of 17 outcrops observed in the field and paleocurrent directions for the selected 7 outcrops (Data source: Geological and Mineral Bureau of Hubei Province, 1990). fold systems and uplift zones (Geological and Mineral data were obtained from the dip of the largest flat plane of Bureau of Hubei Province, 1990). These Quaternary gravels, and verified by dip of the foreset beds in cross sediments have been poorly or inadequately described. bedded sands in the outcrop. Their provenance was Furthermore, there is no consensus on the facies designation of these sediments (Chen and Ma, 1987; Table 1 Geological and Mineral Bureau of Hubei Province, 1990). ESR ages of samples from the Yunchi and Shanxiyao Formations Sampling Sampling Serial number Mineral for Age Ma, 3. Methodology number location in lab dating B.P. Y-01 Outcrop S2 Quartz 1.08±0.11 Seventeen outcrops were observed in the field (Fig. 3). 015, Y.F. Seven of these (053, 00B, 021, 022, 024, 017 and 015; Y-02 Outcrop S3 Quartz 0.87±0.09 listed in the downstream direction) were selected for 017, S.F. detailed study on basis of their sedimentary characteristics Y-03 Outcrop S4 Quartz 1.15±0.12 021, Y.F. and previous research reports (Chen and Ma, 1987; Yang Y-04 Outcrop S1 Quartz 0.82±0.08 and Lu, 1992; Tian et al., 1996). One hundred gravels in 022, S.F. sediments of each of these seven outcrops were point- Y-05 Outcrop S5 Quartz 0.75±0.08 counted for their composition, grain size, sphericity, and 024, S.F.