CONTROLS ON QUATERNARY AGGRADATION-INCISION CYCLES IN A HYPERARID WATERSHED, NORTHEASTERN NEGEV, ISRAEL

Active since the early Pliocene, the Dead Sea rift (DSR) dictates overall incision of the northeastern Negev streams. The overall incision is punctuated by long intervals of aggradation recorded by fluvial terraces. Previous studies in the Negev attempted to associate the formation of these landforms with climate. However, the manner in which climate triggers aggradation and incision in the hyperarid watersheds of this region is yet poorly understood. Here, we use geomorphic mapping, together with luminescence and cosmogenic nuclides methods, to (a) investigate aggradation-incision cycles of the hyperarid Nahal Zafit (40 km²), and (b) construct a chronological and environmental framework for the Quaternary landscape evolution along the DSR western margins. Beveled bedrock surfaces, coated by mature desert pavement, constitute Nahal Zafit’s water divide. Exposure ages of these surfaces confine the formation of the current watershed to the past 1-2 Ma. In its upstream reaches, the stream entrenches the elevated Hazera anticline, forming a canyon with a narrow outlet. The canyon hosts landslides, taluses and terraces, forming a significant sediment storage. Despite high availability of sediments within the canyon, current slope and stream gradients, coupled with the hyperarid climate, promote substantial incision rather than sediment transport. OSL ages of terraces in downstream reaches of Nahal Zafit, occupying the Mazar syncline, indicate aggradation intervals at ~550-350 ka, prior to ~125 ka, and at ~65-25 ka. These cycles are roughly synchronous with aggradation-incision cycles in adjacent watersheds (Nahal Tahmas, Nahal Peres, and Nahal Hazera) operating under the same climatic, tectonic, and lithologic conditions. The sedimentary cycles in the northeastern Negev watersheds are a-synchronous with other climatic proxies such as periods of increased speleothem growth, hence, reflecting a lag time between climatic triggers, aggradation, and incision.