Shoreline migration paths and depositional architecture of early–mid Miocene deltaic clinoforms in response to sea‐level changes in the north‐eastern shelf margin, South China Sea

Abstract

Shoreline trajectory analysis improves the understanding of the correlation between clinoform architecture, shoreline and shelf-edge development, and sea-level changes. This study uses cores, well-logs and seismic data to investigate the depositional architecture and shoreline migration paths of earlyÀmid Miocene deltaic clinoforms in the northern Pearl River Mouth basin, South China Sea. Five clinoform types are observed, including sigmoidal, oblique, tangential oblique, parallel oblique and sigmoidal-oblique clinoforms. Both the rollovers of deltaic clinoforms in seismic sections and the transitional points between delta plain and delta front deposits in well correlation cross-sections can be used as the indicators for shoreline positions, provided that there are no significantly extended subaqueous deltas. This allows for the identification of three shoreline trajectory types from a single clinoform set and three major stages from the net shoreline trajectory for multiple clinoform sets. A landward-directed net shoreline trajectory and ascending shoreline trajectories dominate in Stage 1 (23.8 to 17.5 Ma) with sigmoidal clinoforms transitioning into parallel oblique clinoforms, suggesting a rise in third and fourth-order relative sea level. Stage 2 (17.5 to 13.8 Ma) displays flat or descending trajectories and an overall seawarddirected net trajectory with a change from parallel to tangential oblique clinoform, indicating a relative sea-level fall. The reoccurrence of sea-level rise in Stage 3 (13.8 to 10.5 Ma) was inferred by a landward migration of parallel oblique clinoforms. This study shows that shoreline trajectory and net shoreline trajectory plotted from both seismic and well data is consistent with the regional sea-level curves estimated from foraminiferal fossil records and the shelf-edge development, which can be used to estimate the third and fourthorder relative sea-level changes in passive margins with wide, low-gradient shelves. Furthermore, clinoform architecture and migration may have been controlled by the variations of subsidence/uplift and sediment supply produced by tectonic events.