Aggradation/incision history of a debris flow/hyperconcentrated flood dominated valley on the eastern margin of the Tibetan Plateau, China

The thesis aims to reconstruct the aggradation and incision history of a tributary valley of the Bailong River on the eastern margin of the Tibetan Plateau. To achieve this goal, geomorphological, sedimentological and chronological methods were employed. Four levels of alluvial terraces (i.e., T1, T2, T3 and T4 ranging from high-elevation to low-elevation terrace levels) were identified via field investigation. The terrace sedimentology was revealed through sedimentary logging of terrace outcrops. Channel fill sediments were investigated from two cores drilled on the channel. These sedimentological analyses indicate that the terraces and channel fill have been primarily deposited by debris flows and hyperconcentrated floods. The numerical ages of the sediments forming the alluvial terraces and channel fill were determined using the single-aliquot regenerative dose (SAR) protocol of optically stimulated luminescence (OSL) dating. By synthesizing the geomorphological, sedimentological and chronological results, the valley evolution history was classified into two aggradation periods (S1 and S3, representing the aggradation of T1 and T2, respectively), and two incision periods (S2 and S4, representing the incision of T1 and T2, respectively). The formation of T3 and T4 terraces is regarded as being due to two short aggradation events punctuating the S4 incision period. The timeframes for these stages were constrained using the OSL ages: S1 occurred before 90.0 ± 10.0 ka; S2 occurred between 90.0 ± 10.0 ka and 46.8± 5.1 ka; S3 occurred between 46.8 ± 5.1 ka and 7.1 ± 0.7 ka; and S4 occurred from 7.1 ± 0.7 ka to present. The short aggradation period which formed the T3 terraces occurred between 3.3 ± 0.3 ka and 0.6 ± 0.1 ka.

Comparison between the aggradation and incision history and regional climatic records suggests that aggradation occurred in cold and/or dry climates, whereas incision was a feature of warm and wet climates. The impact of tectonism on the valley evolution is related to the lateral displacement of a local thrust fault, which caused changes in the depositional loci of the T1 and T2 terraces. The incision during the S2 phase may be related to regional uplift because both climate and the small displacement of the local fault seem unlikely to cause the large incision(~260 m) observed. The base-level effect provided by the Bailong River valley is mainly represented by the lateral migration of the river channel which may have caused deposition and erosion of the T3 and T4 terraces at the valley mouth. The above proposed relationship between climate and valley evolution was applied to assess possible future changes in the present active channel by considering recent climatic records. The results suggest that the valley channel is expected to experience incision in the future.