Luminescence dating of fluvial terrace and karst cave deposits in the Upper Beijiang River, Guangdong Province, and its geomorphological implications

Fluvial deposits in river terraces and alluvial deposits in karstic caves are important archives for geomorphological study of river valleys. However, there is still a lack of direct chronological evidence supporting the relationship between the developing processes of multi-layer caves and river terraces in basin scale. The upper reaches of Beijiang River run through both sandstone basins and limestone areas, resulted in multilevel river terraces and cave systems. This unique setting provides an ideal opportunity to investigate the evolutionary relationship between rivers and stratified caves. In this study, optically stimulated luminescence (OSL) dating was applied to constrain the age of the fluvial deposits from the second-level terrace in Pingshi Town in the upper reaches of Beijiang River in Guangdong Province, as well as from alluvial sediments in the second-level karst cave of Gufoyan Cave system in the neighboring area. The dating results reveal that the second terrace at Pingshi dates back to 101～79 ka, while the deposits in the second layer of Gufoyan Cave date back to 121～71 ka, both falling within the Marine Isotope Stage (MIS) 5. These findings indicate that during the last interglacial period, an incision occurred in the upper reach of the Beijiang River, leading to a drop in regional groundwater table. Therefore, it resulted in abandonment of the second layer of Gufoyan Cave by underground river. The cave alluvial sediments and river terrace deposits in the area were synchronous at the glacial-interglacial time scale, which provided direct chronological evidence for co-evolution between karst and river landforms in the upper reaches of Beijiang River. This study provided supports to further studies on regional Quaternary geology and river landform evolution through dating alluvial sediments found within karst caves.