Contrasting fluvial styles across the mid-Pleistocene climate transition in the northern shelf of the South China Sea: Evidence from 3D seismic data

• Published in: Quaternary science reviews 2015-12, Vol.129, p.128-146
• Main Authors: Zhuo, Haiteng, Wang, Yingmin, Shi, Hesheng, He, Min, Chen, Weitao, Li, Hua, Wang, Ying, Yan, Weiyao
• Format: Article
• Language: English
• Subjects: East Asian monsoon; Fluvial system; Mid-Pleistocene climate transition; Sea level; Seismic geomorphology; South China Sea
• ISSN: 0277-3791
• DOI: 10.1016/j.quascirev.2015.10.012

Multiple successions of buried fluvial channel systems were identified in the Quaternary section of the mid-shelf region of the northern South China Sea, providing a new case study for understanding the interplay between sea level variations and climate change. Using three commercial 3D seismic surveys, accompanied by several 2D lines and a few shallow boreholes, the sequence stratigraphy, seismic geomorphology and stratal architecture of these fluvial channels were carefully investigated. Based on their origin, dimensions, planform geometries and infill architectures, six classes of channel systems, from Class 1 to Class 6, were recognized within five sequences of Quaternary section (SQ1 to SQ5). Three types of fluvial systems among them are incised in their nature, including the trunk incised valleys (Class 1), medium incised valleys (Class 2) and incised tributaries (Class 3). The other three types are unincised, which comprise the trunk channels (Class 4), lateral migrating channels (Class 5) and the stable channels (Class 6). The trunk channels and/or the major valleys that contain braided channels at their base are hypothesized to be a product of deposition from the “big rivers” that have puzzled the sedimentologists for the last decade, providing evidence for the existence of such rivers in the ancient record. Absolute age dates from a few shallow boreholes indicate that the landscapes that were associated with these fluvial systems changed significantly near the completion of the mid-Pleistocene climate transition (MPT), which approximately corresponds to horizon SB2 with an age of ∼0.6 Ma BP. Below SB2, the Early Pleistocene sequence (SQ1) is dominated by a range of different types of unincised fluvial systems. Evidence of incised valleys is absent in SQ1. In contrast, extensive fluvial incision occurred in the successions above horizon SB2 (within SQ2–SQ5). Although recent studies call for increased incision being a product of climate-controlled increase in river discharge, the down-dip location of our study area suggests that relative sea level change was the most important control of the evolution of fluvial systems. However, it is acknowledged that climate change was also important through its role in regulating glacio-eustasy. We speculate that the small amplitude and periodicity of sea level cycles before and during the MPT were not sufficient to fully expose the shelf and cause extensive fluvial incisions. Completion of the MPT as well as t