3-D Geomorphology of Pleistocene Fluvial Systems in the Northern Shelf of the South China Sea: Implications for the Mid-Pleistocene Climate Transition

Abstract

Multiple successions of buried fluvial channel systems were identified in the Quaternary section of the mid-shelf region of the northern South China Sea. Using three commercial 3D seismic surveys, accompanied by 2D lines and shallow boreholes, the sequence stratigraphy, seismic geomorphology and the stratal architecture of the fluvial channels were 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 stable channels (Class 6). Trunk channels and 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 shallow boreholes indicate that the landscapes associated with these fluvial systems changed significantly near the completion of the mid-Pleistocene Climate Transition (MPT), which approximately corresponds to 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 fluvial systems. We speculate that completion of the MPT as well as the onset of 100 ky climate cycles at ∼ 0.6 Ma, during which the duration of cycles and magnitude of sea level change both increased, are considered to trigger the extensive development of incised fluvial systems. The intensification of the East Asia Monsoon at 0.9 Ma and 0.6 Ma driven by the episodic uplift of the Tibetan Plateau may have also enhanced the amplitude of sea level falls and thus the fluvial incisions of the northern shelf of the South China Sea.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016