Browsing by Author "Fruchter, N"

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    Revealing sediment sources, mixing and transport during erosional crater evolution in the hyperarid Negev Desert, Israel
    (Elsevier, 2011-11-15) Fruchter, N; Matmon, A; Avni, Y; Fink, D
    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 (10)Be 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, (10)Be 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 I Be exposure ages of terrace sediments give deposition ages between similar to 340 ka to similar to 50 ka, at which time the youngest alluvial terrace was incised. Deposition was interrupted twice, at similar to 300 ka and similar to 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, (10)Beconcentrations 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, (10)Be 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 (10)Be 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. (C) 2011 Elsevier B.V.
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    Sediment source and mixing and the cycle of sediment transport: an example from NE Negev Desert, Israel
    (Copernicus Publications, 2010-05-02) Fruchter, N; Matmon, A; Zilberman, E; Avni, Y; Fink, D
    Alluvial terraces represent the end product of sedimentary cycles; each includes sediment generation, transport, accumulation, and the ultimate incision that forms abandoned alluvial surfaces. We examine the middle Pleistocene to recent drainage system evolution in Makhtesh Hazera, Negev Desert, southern Israel and compare the characteristics of erosion and sediment transport in the present system with those expressed by the alluvial terraces. The Hazera drainage basin lies at the margins of the arid to hyper-arid Dead Sea rift (DSR). Makhtesh (crater) Hazera is a deeply incised erosional structure (5X7 km) that has been excavated since the early Pliocene into the crest of the Hazera asymmetric anticline. The Makhtesh floor is surrounded by cliffs rising more than 400 meters high. The cliffs are built of Upper Cretaceous hard carbonates caprocks overlying Lower Cretaceous friable quartz sandstones. Bedrock knickpoints isolate the drainage basin in the Makhtesh and above it from a direct influence of the terminal base level of the DSR. Thus, the accumulation of sediment and abandonment of terraces are controlled by climate and bedrock barriers located at the Makhtesh outlet. We use cosmogenic isotope concentrations to determine bedrock denudation rates, ages of alluvial terraces, and basin wide erosion rates in different channels throughout the basin. The use of cosmogenic isotopes enables us to determine sediment sources and reconstruct sedimentary cycles. OSL dating was used to determine the accumulation ages of alluvial sediment in alluvial terraces. These two methods enable quantitative evaluation of fluvial processes. Bedrock erosion rates suggest a strong dependence of erosion on lithology. While the Lower Cretaceous sandstone erodes at >100 mm ky-1, the overlying hard carbonate caprock yielded cosmogenic isotope concentrations that correspond to erosion rates of 1-3 mm ky-1. This significant difference in erosion rates maintains the dramatic relief of the Hazera drainage basin. We find that the quartz sediment in the present fluvial system of Makhtesh Hazera originates from two predominant sources. One is the Lower Cretaceous sandstone that crops out along the base of the Makhtesh cliffs. The second source are un consolidated Miocene sands that fill the syncline which is located north west of the Makhtesh and is drained into it. 10Be concentrations in successive samples indicate that the Miocene sand is gradually diluted by Lower Cretaceous sand as it flows down stream and the mixing of sediment from both sources is good. Alluvial terraces and bedrock units exposed inside the Makhtesh do not contribute a significant amount of sediment to the present drainage system. Three major alluvial terrace levels were identified. The highest terrace level (MKT0) was abandoned at 279±19 ky. This level probably covered most of the Makhtesh surface. The deposition of the two lower levels, MKT1 and MKT2 (which were abandoned at 160±6 and 47±9 ky, respectively), was confined to the present drainage system. Analysis of cosmogenic depth profiles from the terraces suggests significant recycling of sediment within the Mekhtesh. This is in contrast to the present system that lacks recycled sediment. We explain this difference by the fact that the terraces are the final product of a sedimentary cycle while the present drainage system presents a “snapshot” in time which does not represent the entire cycle only the present state of the system which is expressed by rapid incision and very little lateral migration. © Author(s) 2009