QUANTIFYING SEDIMENT TRANSPORT RATES IN THE LOWER CRETACEOUS KURNUB RIVER USING COSMOGENIC 21NE

The relationship between denudation of continents and the rivers that drain them have been extensively researched over a wide range of timescales and stream-sizes. However, our understanding of fluvial geomorphology in the deep geological past is extremely limited. Recent developments in the field of cosmogenic nuclides have established stable nuclides (primarily ³He and ²¹Ne) as a valuable tool in the study of landscape evolution and continental tectonics in the geological past.

During the Lower Cretaceous, a thick mantle of quartz-arenitic sandstones covered much of North Africa and Arabia, deposited by a large-scale fluvial system emptying into a shallow-marine shelf on the passive northern margin of Gondwana. Here we attempt to evaluate rates of sediment transport for this paleo-river and examine it in comparison to modern large rivers.

We sampled quartz sand from five boreholes along an ~400 km transect following the general downstream direction of the Lower Cretaceous Kurnub River: the oldest clastic sedimentary unit examined to date using in-situ cosmogenic nuclides (~140 Myr). The excess of ²¹Ne measured ranges from ~1-3·10⁶ atoms/g quartz. Interpreting the rates of surface processes from these relatively low ²¹Ne concentrations is not a trivial task, especially when considering the long burial time compared to exposure. We consider nucleogenic and muogenic ²¹Ne and correct for Ne diffusion of these samples to evaluate cosmogenic ²¹Ne produced during exposure. Next, we examined the changes in cosmogenic Ne along the ancient river. The results show little difference in cosmogenic ²¹Ne along the transect, suggesting that transport within this system was rapid enough so that sediments spent a relatively short time exposed in the Lower Cretaceous fluvial system (<60 kyr). Sediment residence times evaluated here are similar to those measured in the Ganges tributaries and Narmada and Tapti Rivers using uranium-series isotope signatures. The results presented here are the first attempt at investigating the dynamics of a fluvial system in the deep geologic past, and hold promise that this novel application of cosmogenic Ne will provide new insights into rates of surface processes throughout the geologic record.