Breakup unconformities at the end of the early Oligocene in the Pearl River Mouth Basin, South China Sea: significance for the evolution of basin dynamics and tectonic geography during rift-drift transition

摘要

The widely distributed unconformity (T7) in Pearl River Mouth Basin, formed at 30 Ma, is one of the most important unconformities of northern South China Sea related to the evolution of basin dynamics. Based on seismic data and logs, this study documents the distribution of the unconformity in Pearl River Mouth Basin (PRMB) and discusses the mechanism of the breakup unconformity in the evolution of the basin. The study verifies that the T7 boundary is a breakup unconformity: (1) most fault tips end at T7 unconformity or the offset diminishes markedly across the unconformity, and (2) these faults control the stratal thickness, reflected by the wedge shape formation. The T7 unconformity can be divided into three zones according to different structures and contact relationships: (1) the angular unconformity zone, (2) the local unconformity and onlap zone, and (3) the conformity zone, these three correspond to strong erosion zone, weak erosion zone, and non-erosion zone respectively. The distribution pattern of the breakup unconformity in this study area is controlled by the tectonic and geomorphology in the different zones of a basin: The angular erosion zone in the north is controlled by fault activity, but in local unconformity and onlap zone the erosion is mainly caused by diapir or local uplift. The conformity zone is located at the depression center in an underwater environment. This pattern indicates that the mechanism of the breakup unconformity is related to the erosion on shoulder uplift structure at the basin margin in rift stage.