Historical sedimentation at an artificial lake margin, Bangudae Petroglyphs site, SE Korea

Abstract

The Bangudae Petroglyphs is Korea's National Treasure No. 285, carved on a vertical rock face on the riverside of a mountain stream (Daegokcheon) near Ulsan. Since the construction of the Sayeon Dam in the downstream area in 1965, the petroglyphs was repeatedly flooded and submerged, raising concerns of erosion and water damage of the prehistoric rock-art motifs. Recent excavation of the fluvial sediments in front of the petroglyphs provided an opportunity to investigate the changes in sedimentation pattern before and after the construction of the dam and the consequences of human activity on natural environments. Sedimentological observations of trench sections and radiocarbon, OSL, and garbage dating of sediments reveal that the sediments comprises gravelly to sandy fluvial sediments in the lower part that accumulated since the early to middle Holocene or early to middle Neolithic Era until 1965. Above this, 25 units of alternating horizontally laminated sand and massive, mottled mud with abundant rootlets occur. This sand/mud sequence is interpreted to have accumulated after the construction of the Sayeon Dam in 1965, which caused repetitive flooding and submergence of the study area every year or two. The alternations of horizontally laminated sand and massive mud are interpreted to have resulted from rainfall-induced floods and subsequent suspension settling of fines in stagnant water after the area was inundated and temporarily transformed into a lacustrine margin setting. Afterwards, the depositional site was exposed and vegetated until the next flood event. Comparison of the deposit features with the precipitation data in Ulsan area, the water level data of the Sayeon Dam, and age data suggests that this flood-related sedimentation occurred almost every year until 2005 when the water level of the Sayeon Dam was lowered permanently because of the construction of another dam in the upstream section of the stream. This study thus provides a good example of a dramatic environmental change of a mountain stream caused by artificial disturbance of a river system due to construction of dams and an example of high-resolution sedimentary records of meteorologically induced floods in an artificial lake margin.