PALEOGEOMORPHOLOGY OF EARLY MIDDLE MIOCENE IN THE XISHA SEA AREA AND ITS CONTROL FACTORS

The study of tectonic paleogeomorphology, which controls the sediment sources and depositional system, is helpful to oil and gas exploration. Based on the abundant seismic, wells and high-resolution multi-beam data we have in hand so far, the early Middle Miocene paleogeomorphology in the Xisha Sea Area is recovered in this paper, in particular, after the treatment of decompaction and palaeobathymetric correction. Results show that in early Middle Miocene, there were two nearly northeast-trending uplifts surrounded by deep water areas in this region. The uplifts were favorable for development of shallow water carbonate platforms. Volcanism was active in the Oligocene leaving a quantity of topographic highs on the Xisha Uplift, which provided opportunities for the growth of reefs. With the sea level rise, many volcanoes to the south of this Uplift also evolved into small isolated carbonate platforms and the slopes around the Xisha Uplift were usually very steep. However, the terrain on the Guangle Uplift was very broad and flat, and its slopes were characterized by the framework of "steep in south and smooth in north". The northern slope was rather complicated, where exist at least three roughly parallel channels which were favorable for transportation and deposit of carbonate clasts. There are two main controlling factors on the paleogeomorphology in the area in early Middle Miocene, i.e. tectonic movement and sedimentation, including the simultaneous faulting in Oligocene, the development of reef and carbonate platform, and the transport and deposition of carbonate clasts.