Relative sea‐level control on the building of two distinct shelf‐margin clinothems on the late‐Quaternary Pearl River margin: Insights from numerical stratigraphic forward modelling

摘要

Abstract As one of the most important forcing factors, relative sea‐level changes exert a major influence on the building of shelf‐margin clinothems. However, it is still not well understood how these changes control the growth of shelf edges and the condition of sediments transporting into deep water, especially over the individual‐clinothem scale of several 100 ky. On the late‐Quaternary Pearl River margin, there are two distinct shelf‐margin clinothems: SQ3 and SQ4. They have different shelf‐edge trajectories (slight rising vs. steep rising) and different styles of deep‐water deposition (fan lobes consisting mainly of MTDs vs. fan lobes consisting mainly of turbidites). This work takes those SQ3 and SQ4 as study objects and runs a total of 136 experiments from the Dionisos stratigraphic forward model to investigate how relative sea‐level changes control the trajectories of shelf edges and the volumes of MTDs in deep water over the individual‐clinothem scale. Our quantitative results suggest that under the geological background of high sediment supply on the late‐Quaternary Pearl River margin, the duration of highstand systems tracts (HST) relative to lowstand systems tracts (LST) or forced regressive systems tracts (FST) has a significant influence on the building of individual shelf‐margin clinothems. If the relative duration of HST is either very short or very long, slight‐rising shelf‐edge trajectories and large‐volume MTDs would be formed, whereas if the relative duration of HST is comparable with LST or FST, steep‐rising shelf‐edge trajectories and limited MTDs would be formed. Through the constrains of the model set to the real geological condition of the SQ3 and SQ4 clinothems, it is found that SQ3 was caused by the quite long relative duration of HST, which made highstand deltaic systems advance over the pre‐existing shelf‐slope break, leading to significant accretion and instability of the shelf edge and thus, giving rise to the formation of slight‐rising shelf‐edge trajectories and fan lobes with high MTDs contents. SQ4, however, formed as a result of the comparable durations of HST, LST, and FST, which made highstand deltaic systems advance to but not beyond the previous shelf‐slope break allowing the subsequent FST to be directly perched on the clinoform slope. Such building processes did not drive pronounced accretion and instability of the shelf edge and thus, caused the formation of steep‐rising shelf‐edge trajectories and fan lobes with low MTDs contents.