The preservation of conglomerates far from mountainous sources are commonly interpreted in terms of tectonic or climatic forcing. To relate a depositional signal to changing conditions in source areas, the process and duration of sediment routing from source to sink need to be determined. For the first time, we quantify sediment transport histories using cosmogenic 21 Ne in quartzite pebbles from modern river gravels and Neogene conglomerates from the modern and ancient North Platte River of the Great Plains of Nebraska. We demonstrate that at ~400 km from the Rockies mountain front, the majority of pebbles must have been stored in older channel deposits for up to several millions of years before being recycled; this is enabled by very slow to zero basin subsidence rates. This implies that upstream tectonic or climatic controls on surface processes are decoupled from the downstream depositional record; a result supported by the similarities in cosmogenic 21 Ne between Miocene, Pliocene and modern river channel pebbles despite known changes in tectonic and climatic forcing.
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