Reconstruction of coarse sediment recycling in rivers using cosmogenic nuclides in the Great Plains, USA

Fluvial systems are regarded as a critical component of earth surface processes because they link the sediment source and the basin. The characteristics of luvial sediments are influenced by tectonic and climatic processes. Thus, sediment grains, as the basic element of fluvial sediments, are often used to reconstruct changing tectonic and climatic conditions. One common observation of fluvial sediments is a tendency for the bedload to become finer downstream. In many studies, large pebbles located far away from the sources have been interpreted to be caused by tectonic or climatic influences. In the Great Plains, USA, large pebbles can be found more than 1400 km away from the sources. In this area, it has been demonstrated that slow subsidence resulted in multiple cycles of fluvial incision and aggradation. A hypothesis can then be proposed that large pebbles downstream came from the recycling of the paleosediments. To test this hypothesis, three main geological tools were applied: grain sizes analysis, cosmogenic nuclides analysis and numerical modelling of cosmogenic 21Ne. In the grain sizes analysis, downstream fining curves were generated, and similarity statistics were done. Based on the result, three locations of probable lateral sediment input from recycled paleo-channel deposits were recognised. For each of these locations, grain size distributions with and without lateral sediment input were generated by combining the grain size fitting curves and the similarity distributions. From the comparison between the grain size distributions with and without lateral sediment input, it can be concluded that almost all the pebbles collected from these three locations are from recycling of palesediments. Cosmogenic nuclides accumulated when the sediments were exposed, so that they can provide information about the exposure and burial history of fluvial sediments. In this thesis, we analysed eighty-two samples (in total) for cosmogenic 21Ne and five samples for 10Be. A “steady-state” model and a “simplified migration” model were built to estimate the maximum concentration of cosmogenic 21Ne accumulated from the source to the target location (Keystone) with a constant migration rate. From the comparison between this maximum concentration of cosmogenic 21Ne and the concentrations of cosmogenic 21Ne measured from the pebbles, it can be concluded that most pebbles collected from Keystone have experienced a long time of storage. This provides support for the existence of recycling in the Great Plains. Next, based on the Ne/Be ratio, the oldest age of recycled sediments in the Great Plains was defined as at least Miocene in age. The result of cosmogenic nuclides analysis introduced an interesting apparent conflict that cosmogenic 21Ne concentrations contained within the samples collected from upstream are higher than those from downstream. To resolve this conflict, numerical models were built to test the controls of cosmogenic 21Ne accumulated during recycling. Two geological parameters, elevation and age of paleosediments, were tested in these models. As suggested from the results, elevation plays a dominant role in the accumulation of cosmogenic 21Ne during recycling. And, the paleosediments of Pliocene/Miocene are the main sources of the recycled pebbles in the Great Plains, but the portion of them plays a minor role in the distributions of cosmogenic 21Ne contained within the pebbles collected along the North Platte River. For the influence of grain size, the data show no relationship between the grain size and the concentration of cosmogenic 21Ne. Lastly, a previous study which used grain size to reconstruct paleo-river profile was tested to evaluate the impact of introducing recycling on the previous interpretations. As the result shows, regional grain size fining rate cannot be used directly to reconstruct the gradient of the whole channels, when recycling is dominant. To conclude, I have shown that in the Great Plains, recycling is the main reason for the existence of pebbles 1400 km away from the sources, and recycling happened in modern time can affect the paleosediments as old as Miocene in age. Besides, it was concluded that elevation is the main factor that influences the concentration of 21Ne accumulated during recycling, and the paleosediments of Miocene/Pliocene are the main sources of recycling. In the end, it was recommended that recycling should be taken into consideration in geological studies