The Yellow River (Huang He in Chinese) is one of the largest rivers in the world and flows in the Northeast Tibetan Plateau from the Bayan Har Mountains to the Bohai
Sea. The drainage area of the river is approximately 830,000 km 2 , comparable to rivers such as the Ganges/Brahmaputra and Amazon. The characteristic winding pattern traced on the map by the Yellow River suggests that its history is a complex one and that its course has responded to the tectonic history and deformation of the Tibetan Plateau in addition to climatic changes in the late Cenozoic. In fact, evidence shows that Cenozoic strata in Northeastern Tibet are capped by sediments representing dramatic and potentially catastrophic events that indicate the introduction of the Yellow River onto the
Tibetan Plateau, forced possibly by tectonic reorganization during the Quaternary. The transition from basin fill to degradation in northern Tibet marks the time of a major drainage reorganization that culminated in the through flow of the Yellow River. It is fortunate that the Yellow River’s incision along the sedimentary basins in which it flows provides numerous exposures of basin margin and intrabasin structures and their relations to basin fill.
The purpose of our research is to access the roadless and often trailless Yellow
River canyons by boat to determine the age and stratigraphy of the sediments to further our understanding of the Quaternary history of Plateau uplift, the effect of this on regional paleoclimates, and the development of the Yellow River drainage.
The results of this research are of great interest because the timing of the uplift of the Tibetan Plateau is still controversial. Its uplift is recognized as the cause of major climatic changes, including the affect upon the global monsoonal circulation. The

geologic data collected here would further the understanding of the processes of intracontinental deformation and its relation to uplift of Tibet.
One particular set of sediments southeast of Xinghai, Qinghai Province, is extraordinary in its 500 m thickness and completeness of deposition, representing the entire Plio-Pleistocene depositional history of the region and the timing of the arrival of the river. Documenting the chronology of these sediments can be accomplished through the combination of paleomagnetic stratigraphy, vertebrate paleontology and sedimentology. Access to this sedimentary fill along the Yellow River banks would offer an unprecedented opportunity to establish the necessary high-resolution geochronologic framework. The principal investigator has personally observed this section to note that the only access to the base and middle portion of the section is in the river corridor.
We propose that a crew of five persons use whitewater boats to access the canyon.
The itinerary would initiate at the closest access to the river in the Xiqing Mountains and terminate at the lake upstream of Longyang Gorge, near the town of Gonghe. The applicants envision a total of approximately 150 km of river descent, during which they will conduct geologic investigations including mapping, stratigraphy, paleontological sampling, geomorphological interpretation, and the collection of rock samples for source analysis and paleomagnetic and paleontological age determination. Two of the applicants have extensive experience organizing and conducting geological reconnaissance river expeditions on unrun rivers on the Tibetan Plateau and have access to whitewater rafting equipment suitable for this expedition that is currently located in Tibet.

Geological Setting. The Gonghe Basin, whose surface is at an average elevation of 3,000 m above sea level, and extends over an area of 9,700 km 2 is separated from the bordering Qaidam Basin to the W by the NW-SE trending Wenquan fault (Fig. 1).
Elevations in the basin range from 2,470 m to around 3,500 m (highest peaks, consisting of basement rocks). Incision of the Yellow River has produced a spectacular gorge along its way out of the Gonghe Basin, the Longyang Gorge (2,470 m), which cuts more than
800 m into bedrock and allows a geographic distinction between the Gonghe and Guide basins, although both show very similar structural and sedimentary characteristics. To the north the basin is bordered by the foothills of the Qinghai Nan Shan and Laji Shan, and to the south by the Kunlun range foothills. In the Gonghe basin, faunal assemblages contain abundant Pleistocene fossils and hence the onset of the sedimentary infilling may be younger than that of the neighboring Guide basin, a hypothesis that we will test with magnetostratigraphy.
Fluvial incision due to the Yellow River and its tributaries reveals thick sections through the basin fill. The environmental conditions that prevailed during the deposition of this fill will also be reconstructed by investigating the exposed sediments.
Stratigraphy . We will measure several detailed stratigraphic sections along the
Yellow River banks to document the sedimentology, chronology, and lithofacies assemblages. Because facies progradation is time transgressive, the ages of a conglomeratic sequence will vary between locations within a basin. Consequently, different sections through a basin will produce dissimilarity in ages for the causative tectonic uplift. In order to correctly estimate uplift timing, several sections at various sites across the basin have to be studied and sampled.

Facies data will be collected on a bed-by-bed basis, using Miall’s (1978) lithofacies system. Paleocurrent and conglomerate clast-size data will be collected in the context of measured sections. Conglomerate provenance data in the context of measured sections will allow us to document temporal and spatial changes in composition. The location of each measured section will be chosen to permit construction of stratigraphic cross sections across the basin to illustrate variations in thickness and lithofacies. Fabric measurements will be taken from gravel horizons, on pebbles with a greater than 3:1 elongation ratio. Degree and direction of dip of the long axis and the a-b plane will be measured following Andrews’ (1971) procedure. In addition, Anisotropy of Magnetic
Susceptibility (AMS) will be used to determine paleocurrent directions in the sandy layers
(Tarling and Hrouda, 1993).
Sampling procedures : We propose to develop chronostratigraphy based on magnetostratigraphic profiles from selected sections along the Yellow River banks.
Paleomagnetism constitutes the core of the analytical part of this project. Samples will either be drilled in the field when sediments are cohesive or hand samples will be collected and impregnated with a sodium-silicate solution, which allows the friable sediments to be laboratory-heated and manipulated without disintegration. We anticipate collecting approximately 3,000 samples from the five stratigraphic sections with about 3 samples per stratigraphic horizon. Average separation of horizons will be around 2-3 m, depending on the lithology. Permission for bringing the samples from China to the US will be obtained through Prof. X. Fang, University of Lanzhou, with whom we have had a collaboration since 1994 (see enclosed letter in ‘supplementary documents’ and publication list).

Laboratory measurements for paleomagnetic analysis : Standard cylindrical or cubic specimens will be prepared in the laboratory for paleomagnetic measurements.
Specimens will be processed using routine paleomagnetic techniques at the
Paleomagnetism Laboratory of the University of Michigan. It has been our experience that thermal demagnetization satisfactorily isolates the characteristic remanent magnetization (Li et al., 1997a,b; Pares et al., 2002; Yan, 2002) although the choice between TH and AF will be done on a basis of specific lithologic assemblages.
Directional analysis will be carried out by standard laboratory programs involving
Principal Component Analysis after progressive step-wise demagnetization. Bedding of the proposed sections is typically horizontal or sub-horizontal, preventing the application of fold tests to determine the magnetization age. Only along major faults it is possible to find some minor scale drag folds. In the absence of other tests, we will carry out reversal tests to establish the stability of the obtained paleomagnetic directions (McFadden and
McElhinny, 1990; Tauxe, 1998).
Laboratory measurements for rock magnetism: Isothermal Remanent
Magnetization acquisition and demagnetization, Anhysteretic Remanent Magnetization acquisition and demagnetization, frequency-dependent susceptibility and low-temperature measurements are the general tools that will be used. Anisotropy of Magnetic
Susceptibility will also be measured to determine the presence of paleocurrent direction distribution.
Robustness of the magnetic polarity stratigraphy and correlation to the
Geomagnetic Polarity Time Scale : Fossil assemblages in Gonghe (Zheng et al., 1985) will help in correlating the obtained magnetic stratigraphy to the GPTS. In addition,

robustness of the produced magnetic polarity stratigraphies will be evaluated by the jackknife statistical procedure as adapted for magnetostratigraphy by Tauxe and Gallet
(1991), which we already have used in our preliminary study in Guide (Pares et al., 2002).
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