Chronology and provenance of the Yichang Gravel Layer deposits in the Jianghan Basin, middle Yangtze River Valley, China: Implications for the timing of channelization of the Three Gorges Valley

Abstract

The Three Gorges Valley, linking the upper and the middle-lower Yangtze River areas is a key area for understanding the geomorphological evolution of the Yangtze River valley, especially the timing of its formation, which remains controversial and occurred following the authors to from the pre-Miocene to Late Pleistocene periods. The 100 m thick Yichang Gravel Layer (YGL), extending over an area >100 km2, is immediately observed downstream of the Three Gorges Valley in the middle Yangtze River Basin also named Jianghan Basin. Studying heavy minerals assemblage and chronology of the YGL deposits can shed lights not only on the geological origin of the sediments and the morphological evolution of the Three Gorges Valley, but also the formation of the Yangtze River Valley. In this study, 18 sediment samples carried out from six representative sections of the unconsolidated YGL and 2 samples from the underlying Neogene and Paleogene bedrocks were analyzed in order to examine their heavy mineral assemblage compositions. These assemblages were compared with modern Yangtze River sediments sampled in Yichang area (n = 5) and present-day sediments from other rivers of the Jianghan Basin (n = 6) to further identify the sediment provenance of the YGL. The statistic results indicate that the heavy minerals identified the YGL dominated by zircon, epidote, titanite, hematite, limonite and ilmenite, are similar to the assemblages observed in the 5 modern Yangtze River sediment samples but differ obviously from both underlying bedrock assemblages, containing kyanite, garnet, rutile and monazite, and other modern river sediments (Jianghan Basin), including hornblende, chlorite, tourmaline, rutile, anatase, spinel and garnet). In addition, 8 sandy samples from the YGL were dated by Electron Spin Resonance (ESR) method using Ti–Li center, showing a deposition age ranging between 1123 ± 139 and 734 ± 183 ka. Based on these observations, we propose that the YGL sediments derived from the upper Yangtze River Basin above the Three Gorges Valley, indicating that the Three Gorges Valley was channelized before 1.12 Ma ago. This result is consistent with previous provenance tracing studies of continuous deposited late Cenozoic sediment from central Jianghan Basin (since 3.0 Ma), which also constrains the incision time of the Three Gorges to ~1.1 Ma.