The Study On The Drainage Evolution And Fluvial Terraces In The Jinshaan Yellow River During Late Cenozoic

The Yellow River between the Shanxi and Shaanxi provinces flows across the Jinshaan Gorge and the Fenwei Basin, constructing the main reach of the Middle Yellow River. Numerous previous studies have focused upon this reach of the Yellow River, especially the formation time and evolution of the Yellow River in the Jinshaan Gorge is generally regarded as a significant scientific question, attracting wide attention in earth sciences. However, some views for it diverge so greatly that we can not reach a consensus for this challenging issue until now. Moreover, the competing role of climate change and surface uplift in the Yellow River terrace development is also controversial in fluvial geomorphology. Whereas the research on terrace development in the Jinshaan Yellow River by understanding the geomorphic response of different tectonic reaches (the uplifted Jinshaan Gorge and the subsidence Fenwei Basin) to climate change and surface uplift (or subsidence) unfortunately remains a gap. Taking into account these problems, this paper firstly on the basis of previous work makes a series of supplementary study on the geochronology of the Tangxian Planation Surface in the middle reach of the Yellow River, and establishing a reliable chronological framework for these fluviolacustrine strata and fluvial gravel layers covered by aeolian Red Clay on this planation surface in the northern Jinshaan Gorge. In addition, a chronological sequence for these Yellow River terraces below the Tangxian Planation Surface is also constructed by paleomagnetic dating, loess-paleosol sequences and optical stimulated luminescence (OSL). Under the control of these established chronological frameworks, we reconstruct the time-slice of fluvial evolution in the Jinshaan Gorge on the basis of gravel fabric and lithology in these fluviolacustrine sediments and Red Clay covered fluvial layers. Secondly, this paper also evaluates the response of the Yellow River in the uplifted Jinshaan Gorge and subsidence Fenwei Basin to allogenic controls, and then presents a uniform pattern for the Yellow River terrace development in different tectonic reaches as a combined response to climate change and tectonic movements (surface uplift and subsidence).In the northern Jinshaan Gorge, a continuous fluviolacustrine strata with a chronological framework of>8.3-3.7Ma were accumulated in the Baode region (Shanxi Province), being correlated sediments of the Tangxian Planation Surface. The statistics of gravel fabric and lithology in these fluviolacustrine sediments reveals that a paleo-lake occupying the Baode area was fed by some local streams from surrounding uplifted regions, and gradually growing into a local lacustrine drainage. In the northern Jinshaan Gorge, two fluvial gravel layers covered by aeolian Red Clay were stacked on the Tangxian Planation Surface. Their formation times were dated prior to4.9Ma and3.7Ma respectively. The statistics of gravel fabric and lithology indicates that these gravel layers represent a northward flowing paleo-river, which is different from the southward flowing Yellow River. It linked the paleo-lake occupying the Baode region and the drainage in the Hetao Basin. A dramatic surface uplift initiating prior to3.7Ma not only interrupted the fluviolacustrine sedimentation, but also leaded to lifting of the Tangxian Planation Surface. These exposed surfaces were then immediately covered by continuous aeolian Red Clay. Owing to this drastic uplift, hypsographic relief was enlarged, and resulting in fluvial head erosion and reorganization of river system. In the northern Jinshaan Gorge, the northward flowing river linking the Baode paleo-lake and the drainage in the Hetao Basin was pirated by the river in the southern Loess Plateau, and the Yellow River appeared prior to1.2Ma. An episode of drastic uplift initiating at1.2Ma forced the Yellow River to continuously downcut along the Jinshaan Gorge, and developing a series of fluvial terraces. The modern spectacular down-cutting valley has been shaped gradually by the Yellow River since this uplift.The loess-paleosol sequence on each terrace of the Jinshaan Yellow River shows a remarkable uniform pattern, i.e., each terrace deposits were immediately overlied by a paleosol layer. This stratigraphic relationship suggests that the abandonment of these treads due to incision by the Yellow River in the uplifted Jinshaan Gorge and the subsidence Fenwei Basin all occurred at the transitions from glacial to interglacial climates. However, the Jinshaan Yellow River can not develop a terrace at every transition from glacial to interglacial climates since1.24Ma, implying that climate cycle probably has a temporal control on the fluvial behaviour of deposit-incision alternation, even though the Yellow River develops in the subsidence Fenwei Basin, but it has no bearing on whether terrace staircases are formed. Surface uplift is also an essential factor for the genesis of the Yellow River terrace staircase. It can force the Jinshaan Yellow River to downcut deeply enough during each climate cycle to separate terrace levels adequately, favouring the generation and subsequent preservation of large terrace staircase flight (e.g.,0.24-0Ma in the Jinshaan Gorge and Fenwei Basin). If surface uplift presents a relatively low rate (e.g.,1.24-0.4Ma in the Jinshaan Gorge), the Yellow River can downcut deeply enough and develop terrace staircases only during these climatic transitions with large amplitude. However, when the surface stays in a subsidence stage (e.g.,1.24-0.24Ma in the Fenwei Basin), no terrace staircases are recorded but aleatory stacked terraces representing the fluvial behaviour of deposit-incision alternation, although numerous large amplitude climate cycles occurred in this period. Therefore, the formation of the Jinshaan Yellow River terrace sequence (the Yellow River terraces in the Jinshaan Gorge and Fenwei Basin) can be attributed to a combined response of the Yellow River to glacial-interglacial climate cycle and surface uplift or subsidence. Only coupling between climate change and surface uplift can force the Jinshaan Yellow River to downcut deeply enough at the erosion stage of terrace staircase formation to separate terrace levels adequately, favouring the generation and subsequent preservation of large terrace staircase flight.