鄂西清江中上游高海拔砾石层ESR定年及地质意义

摘要

Quite a few high elevation gravels were found in the upper and middle reaches of the Qingjiang River within Jianshi, Badong and Lichuan areas, west Hubei Province. Characteristics of these gravels suggest that they are probably the "high terrace" of the NNE-trending ancient drainage system. ESR dating of the high elevation gravels indicates that the gravels were formed between the early and middle Pleistocene (677±67)-(789±78) ka in the middle reaches of the Qingjiang River, and in the middle period of middle Pleistocene (371±37)-(551±55) ka in the upper reaches, implying that there existed a NNE-trending ancient drainage system which was developed in the middle reaches of the Qingjiang River in early and middle Pleistocene, and this development lasted until the middle period of middle Pleistocene in the upper reaches of the Qingjiang River. This probably implies that the capture of the middle reaches by the Qingjiang River probably occurred between the early and middle Pleistocene, and the capture of the upper reaches took place in the middle period of middle Pleistocene. The modern Qingjiang River has been formed since the middle period of middle Pleistocene. The same geological background (last rapid uplift of the Tibetan Plateau) indicates that the phenomenon that the Yangtze River cut through the Three Gorges is probably attributed to the constantly attack from east to west, which, however, took place a bit earlier than things of the Qingjiang River.%在鄂西清江中游建始、巴东地区、上游利川地区的野外地质调查中,发现多处残留的高海拔河流相砾石层,其特征指示砾石层可能为NNE向古水系残留的“高阶地”.对高海拔砾石层进行了ESR测年,清江中游建始、巴东地区高海拔砾石层形成于早更新世末中更新世初(677:±67)～(789±78) ka,上游利川地区高海拔砾石层形成于中更新世中期(371±37)～(551±55) ka,说明早更新世末中更新世初,清江中游地区尚发育NNE向古水系,而直至中更新世中期,清江上游利川地区仍发育NNE向古水系.这可能指示清江袭夺中游水系发生于早、中更新世之交,袭夺上游水系发生于中更新世中期,现代清江水系形成于中更新世中期之后.由于清江贯通、长江贯通三峡都与青藏高原末次快速隆升有密切关系,从而指示长江贯通三峡的方式总体上可能也是自东向西的不断袭夺,而贯通时间则可能要稍早于清江.