关键词: 忻定盆地, 河流阶地, 山麓剥蚀面, 隆升, 黄土

Abstract: Xinding Basin is a typical Cenozoic graben-type basin in the northern Fenwei graben system,which is surrounded by mountains of 2000 to 3000m high above sea level. Downward from the peak to the piedmont on the basin side are respectively peneplanes of Beitai stage and Dianziliang stage in Paleogene,peneplane of Tangxian stage in Neogene,and seven-level river terraces and six-level pediments in Quaternary. Loess and paleosol sequences on the landforms are correlative to typical loess section,which is an advantage for landform dating.A study has been made of the paragenetic relations of the pediments and river terraces on the northern piedmonts of Wutai Mountains and Xizhou Mountains via geomorphologic mapping on 1:10000 scale,and an analysis on the role of tectonic,climatic,and drainage factors in the parageneses. The Quaternary pediments and river terraces on the north piedmont of Wutai Mountains are united to constitute six steps of geomorphic surfaces. The episodic uplifting of fault blocks is deemed to be the dominant factor in the formation of the unified surfaces.Episodic uplifts result in longitudinal profiles of riverbeds alternated by cascade and gentle reaches one in mountainous areas. Terraces disappear upstream near the vertex of cascade reach. Cascade reaches represent quickly uplifting stages,and gentle ones represent stable stages. On condition that waterfalls between adjacent terraces recede at same speed,durations of active or stable stages can be estimated by ages of terraces and relative length of cascade reach and gentle one,and then ages of uplifting stages since 1.20Ma are estimated. The four quickly uplifting stages are from 1200 to 1059ka BP,from 600 to 501ka BP,from 130 to 103ka BP,and from 20ka BP to present. Other intervals are stable stages.

Key words: Xinding Basin, river terrace, pediment, uplift, loess