| The Chinese Loess Plateau (CLP) in central China forms an important archive for regional and global climate change. The distinct alternations between loess layers and paleosols have been linked to variations in the East Asian Monsoon, which in turn is the most important climate phenomenon in East Asia. To provide additional information on monsoonal climate change in China, core LYH-1 was drilled in a fault-controlled depression near the northern margin of the Weihe Basin, directly adjacent to the CLP.The basin is an active rift basin, which subsided in response to the collision between India and Eurasia, the resulting uplift of the Tibetan Plateau and the associated eastward extrusion of South China. The sediments stored in the basin might therefore also contain valuable information regarding tectonic activity, related to the uplift of the Tibetan Plateau. A third topic that can be addressed by analyzing the sediments in the core is the evolution of the Yellow River, China's second largest river and esential in the historical development of the Chinese civilization. The integration of its middle and lower reaches is associated with the drainage of a large lake (Sanmen Lake) in the Weihe Basin. However, before climatic and tectonic signals, as well as signals linked to a large scale hydrological reorganization can be obtained from the sediments in the core,a well-established understanding of the sedimentation in the basin is essential. In this thesis, I analyzed the uppermost 221 m of core LYH-1 (corresponding to 1 Myr) and carried out a morphological and sedimentological analysis on the Luo River, flowing proximal to core site, in order to reconstruct its sediment contribution history.The first chapter of this thesis provides a general introduction, followed by a chapter on the geological and hydrological background. In the third chapter, the core was studied using detailed macroscopic description of the core, alongside micro-paleontological content and various sedimentary properties including grain size distributions. In the following chapter the observations are extended by geochemical and mineralogical analysis. To put our observations in a broader perspective, chapter 5 reports on the investigation of the influence of the Luo River on the depositional history at the site where core LYH-1 was taken. In the sixth chapter, we refined the preliminary age model as established in chapter 3. This is carried out through a comparison between sedimentary characteristics in core LYH-1 to the nearby CLP sequence.Subsequently, we explored periodicities in the sediment core in order to extract possible climatic signals.The core analyses show rapid alternations between several distinct lithofacies,which can be interpreted as the result of sedimentation in alluvial, shallow lacustrine,playa and eolian environments. These are alternated by periods of minor pedogenic alteration. The reconstructed environments and the tectonic setting led us to conclude that the drill site is located at a subsiding,distal alluvial fan setting,originating from rivers eroding the CLP from the north.In general, fan progradation takes place during colder periods, when the CLP lacks sufficient protective vegetation cover. Conversely, fan retreat takes place during warmer periods, when soils protect the catchment from erosion. Shallow lakes and playas develop in response to increased precipitation and evaporation. Spectral analysis(FFT and wavelet) on magnetic susceptibility,carbonate and two different grain size proxies (derived from end-member analysis) reveal that there are indeed Milankovitch cycles (eccentricity, obliquity and precession) present in the sediment sequence. This implies a climatic forcing on the sedimentation in the northern Weihe Basin. However,the cycles are discontinuous and dependent on tectonic subsidence rates at the core site.When there is not enough accommodation space created, climate signals are not well preserved. In addition to Milankovitch cycles, higher frequency cycles are also present,which can be attributed to autocyclicity of the fan system (rhythmic refocusing of sediment lobes). Long term climate change is revealed by geochemical analyses:increased clay deposition in the basin is recorded from 0.7 - 0.6 Ma onwards. This timing coincides with the start of the intensified summer monsoon, which promoted soil formation and authigenic clay production in the CLP.Based on the distribution of the lithofacies, the studied interval of the core was divided into three main units (1 - 0.69 Ma; 0.69 - 0.33 Ma; 0.33 - 0 Ma), corresponding to two periods with relative extensive ponding at the drill site which are interrupted by a drier middle interval. The latter interval is likely the result of temporarily diminished subsidence rate. This in turn implies that tectonic activity resumed or increased during the start of the last wetter interval. This is confirmed by morphological and sedimentological analyses of the Luo River,which nowadays flows in an incised position proximal to the core site, but in the past delivered sediments through its alluvial fans. The terraces and fans of the Luo River show that the river started to incise,and therefore stopped its sediment contribution, due to increased faulting at around 200-240 ka. Due to fault-forced folding, the relative subsidence rate at the core site increased, which resulted in the (re)appearance of relative intensive flood deposits and the widespread development of shallow lakes with associated floral and faunal diversity.This result indicates that after incision of the Luo River the sediments in the core were dominantly derived from northerly directions.Our results reveal a complex interaction between internal dynamics and tectonic and climatic influences on the sedimentation in the northern Weihe Basin. We found no evidence for the existence of a large lake,implying that the drainage of the Sanmen Lake occurred before the investigated time span. |
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