Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea

Abstract

Multi-phase deepwater fan systems have formed in the Baiyun Sag of the Pearl River Mouth Basin on the northern margin of the South China Sea since the beginning of the Miocene. Integrated analysis of 2D/3D seismic, well log, core, and biostratigraphic data shows that the early Miocene Pearl River deepwater fan deposits consist of four 3rd-order sequences, SQ23.8, SQ21, SQ17.5 and SQ16.5. Each of them is bounded by regional discontinuity surfaces (3rd-order sequence boundaries). Of particular interest are the sandy deepwater fan deposits in the HST of SQ23.8 and LST of SQ21, which are absent in SQ17.5 and SQ16.5. The SQ23.8 deepwater deposits are relatively thin sheets, densely spaced sandy debris flow channels and MTDs, whereas in SQ21 they manifest as thicker sheets and larger channels. In terms of lithofacies, SQ23.8 deepwater deposits are massive sandstones, in contrast to SQ21 deposits of massive sandstones, normally graded sandstones, inversely graded sandstones, and locally interbedded ripple laminated siltstones. It is interpreted that SQ23.8 deepwater sands result from homochronous slumping of the shelf margin delta front, while SQ21 deepwater sands result from the erosion and re-deposition of the ancient SQ23.8 highstand shelf margin delta.The Paleocene-Early Oligocene rift stage (57.5 Ma ∼ 32 Ma) was characterized by extensive development of half-grabens and normal faults. Since 32 Ma, the drift stage has been characterized by thermal subsidence and weak tectonic activity. The distributions of deepwater sands in SQ23.8 and SQ21 are interpreted to have been controlled by seabed topography that was inherited from buried rift-related tectonic features. We propose that the primary controlling factor for the development of the early Miocene Pearl River Deepwater Fan System was likely sediment supply of the Pearl River influenced by the Baiyun Event and the uplift of Qinghai-Tibet Plateau. The specific location of the sandy deepwater fan was due to tectonic influence on paleotopography of the shelf. The interpreted two depositional models can help predict the types of deepwater depositional elements are in the Baiyun Sag of the Pearl River Mouth Basin and where deepwater sands are likely to occur.