Revealing sediment sources, mixing, and transport during erosional crater evolution in the hyperarid Negev Desert, Israel

Abstract

To better understand the sedimentary history of the erosional crater of Makhtesh Hazera in the hyper-arid Negev Desert of southern Israel we have measured concentrations of in situ 10Be in alluvial sediments from the active drainage system and from abandoned alluvial terraces and dated them using optically stimulated luminescence (OSL). These sedimentary sequences suggest changes in the drainage system behavior over time and show a periodic pattern in which periods of sediment aggradation alternate with periods of degradation through incision and erosion. In alluvial terrace sediments, 10Be concentrations were combined with OSL ages to gain insight of the temporal framework of sediment deposition and shed light on the process of sediment storage. OSL and simple 10Be exposure ages of terrace sediments give deposition ages between ~ 340 ka to ~ 50 ka, at which time the youngest alluvial terrace was incised. Deposition was interrupted twice, at ~ 300 ka and ~ 160 ka, when periods of rapid incision caused the abandonment of the active fluvial surface and the stabilization of a younger and lower fluvial surface. In the active drainage system, 10Be concentrations suggest several possible quartz sources, of which the Lower Cretaceous sandstone bedrock exposed at the base of the cliff enclosing the Makhtesh is the most dominant. The results suggest that sediment eroding form these cliffs are conveyed through the active alluvial channels without significant sediment contribution from alluvial terraces or from eroded bedrock exposed within the Makhtesh. In contrast to our measurements in the active drainage system, 10Be concentrations in alluvial terrace sediments record significant storage within the Makhtesh. We suggest that the geometry of the Makhtesh, mainly the disproportion between the size of the Makhtesh and its narrow outlet, leads to significant accumulation of sediment within the Makhtesh. Episodic breaching of the barrier is followed by rapid and short-lived incision into the stored sediment. The long residence time of sediment within the Makhtesh results in high measured 10Be concentrations. At the end of each of these short incision episodes, when channels are already carved into the alluvial sediment and terraces are formed, the channels only convey sediment directly eroded from the bounding cliff of the Makhtesh; a situation presently observed.