Terrace Formations And Developments Of Dabujiang River And Late Quaternary Environmental Changes In The Gonghe Basin

Formations of river terrace are sensitive to tectonic activity and climate changes,and therefor are recognized as archives bearing significant information of regional environmental changes.The Gonghe Basin is located in the upper reaches of Yellow River and has experienced intense tectonic uplifts since Late Pleistocene.During the late Pleistocene,the Gonghe Palaeolake disintegrated and the Yellow River flowed through the basin from southwest to northeast,forming a series of river terraces,which in turn transforms the river system of the Yellow River in this area,leading to significant variances both in landforms and environments.Here we focus on the river terraces along with the Dabujiang River,a second tributary of the Yellow River.Eight sections of sediments were selected to study the formation of terrace.Based on geomorphological and sedimentological evidence and optically stimulated luminescence(OSL)dates,three levels of river terraces along the Dabujiang River were determined.We surveyed the terrace distribution and the sedimentary features of deposits on terraces,and further investigated the formation mechanism of the Terraces.The formations of Dabujiang River Terraces are dated at 19.1~17.4 ka BP,15.1~12.8 ka BP and 10.7~7.5 ka BP.By comparing with other palaeoclimatic records from adjacent regions,it is suggested that the down cutting of Dabujiang River generally happened during transitions from cold,dry climate to warm,wet one.Additionally,tectonic uplifts on a regional scale played an important role on the downcutting of this river.Thus,we suggest that the formations of the terraces along the Dabujiang River are the integrate results of climatic changes and tectonic uplifts.The two combined factors produce powerful energy for the river to incise down and further benefit preservations of the terraces.The Dabujiang River was captured by the Mangla River at least prior to 5.2±0.9 ka BP,resulting in the dramatically decrease in runoff.Since then Dabujiang River has been fed by underground water and local precipitation,and there were no more terraces formed along the river.Dabujiang River Terraces were inconsistent with those of the Yellow River,which may be ascribed to the different responses to tectonic activity and climatic changes of rivers with different energy of motion and/or to different preservation conditions for terraces.In general,small rivers,such as the Dabujiang River,would sensitively respond to climatic transitions,i.e.climatic changes with small varying ampltitudes.