Million-year-scale changes in the provenance of the Miocene Doumsan fan-delta system, Pohang Basin, SE Korea: Separating the effects of eustasy and tectonic subsidence

Abstract

The relationships between fan deltas and their drainage basins have been extensively studied for the present day, but as far as we are aware of, studies have not yet been performed for the geologic record. This study examines the provenance of the Miocene Doumsan fan-delta system (17–10.5 Ma), Pohang Basin, southeast Korea, using combined detrital zircon and whole-rock geochemistry to understand the long-term development of the fan-delta system and its drainage basin. Of the lower five sequences of the Doumsan fan-delta, which were interpreted to have been controlled solely by eustasy, the lower sequences (sequences 1–3) show gradual expansion of the drainage basin without tectonic activity. In contrast, sequence 4 records an abrupt shrinkage of the drainage basin induced by uplift of the source area at ca. 15 Ma, which could have resulted from a regional collisional tectonic event that has not yet been recognised from the fan delta systems of the Pohang Basin, although palaeomagnetic studies and compositional changes of basalts of the basin suggest this event. Sequence 5 records a subsequent gradual expansion of the source area during tectonic quiescence. Warm and humid climatic conditions in East Asia during the middle Miocene would have resulted in a relatively large drainage basin. This study thus demonstrates how eustasy and tectonic subsidence as well as climate can control the drainage basins of fan-delta systems over timescales of several million years.