Depositional styles of deltaic systems from the inner shelf to shelf edge and their controlling processes: A case study from the Upper Oligocene to Lower Miocene in the Pearl River Mouth Basin

Abstract Late Oligocene to Early Miocene deltaic systems (the Zhuhai Formation) comprise the most important oil and gas reservoirs in the Pearl River Mouth Basin in the northern South China Sea and their changes in depositional architecture reflect the interplay of basin kinetic energy, sea level change and sediment supply. Based on the analysis of well log, seismic and core data, depositional styles of deltaic systems, sequence stratigraphy and their controlling factors are systematically documented. Three kinds of deltaic systems (braided river, meandering river and incised river deltas) are distinguished from the inner shelf, middle–outer shelf to shelf edge within a dynamic sequence framework. The braided river and braided river deltaic deposits along the inner shelf are mainly composed of vertically-stacked, thick-bedded channel sandstones and broad-lobate shaped delta front sandy deposits. The meandering river deltaic systems distributed in the middle–outer shelf are characterized by relatively thick delta front sandstones, interdistributary bay fills and prodelta mudstones with turbidite sheets in places. Incised river deltaic systems were generally developed at the shelf margin, with clinoform thickness ranging from 200 to 1200 m, and were commonly associated with large-scale slump and slope fan deposits. The temporal–spatial distribution of deltaic systems within the sequence framework shows a distinct pattern: the braided river delta system mainly developed in the early stage of highstand systems tracts; meandering river delta systems occurred in the middle–late stage of highstand systems tracts; and incised river delta systems generally formed in the falling stage systems tract. Depositional architectures of deltaic systems reveal that the basin kinetic condition varied from the inner shelf to shelf edge. Inner shelf braided river deltaic systems were dominated by fluvial processes, whereas middle–outer shelf meandering river delta and shelf edge incised river delta systems were reworked by waves and alongshore currents. The migration of deltaic systems across the entire wide shelf within the transgression–regression cycle is regarded as the result of high-amplitude sea level fluctuations under coolhouse or icehouse climatic condition. High progradation rates of shelf edge deltaic systems reflect enormous sediment supply resulting from the continuous uplift of the drainage area during the early post-rift period and the intense erosion triggered by glacial climate.