Fluviatile deposits and morphology of alluvial surfaces as indicators of Quaternary environmental changes in the southern Negev, Israel

Abstract

Summary Quaternary environmental changes in the southern Negev are recorded in the alluvial deposits and the geometry of depositional surfaces in the Timna Valley. There are some distinct differences between the alluvial deposits of the Holocene to modern flood plains and earlier (Pleistocene) ones. The Holocene to modern alluvium is characterized by sand, fine- to medium-sized gravel and very coarse gravel in large gravel bars. The sediments indicate environments ranging between extremely arid and semi-arid dominated by floods of low to moderate magnitude but with occasional short large floods. The Pleistocene sediments include: (1) debris-flow deposits indicating talus activity under a moderately arid to semi-arid climate; (2) alternating fine-grained and gravelly layers reflecting floods of low to moderate magnitudes. The nature of the soils supports these conclusions. Thick and argillic Reg soils are typical of the Pleistocene surfaces whereas shallow loamy ones are characteristic of the Holocene terraces. The alluvial surfaces usually converge downstream. The intersection points migrate upstream, following stages in the recession of a major escarpment. Broad pedimented surfaces and bajadas are characteristic of the Pleistocene period, whereas long narrow terraces of gentle declivity were formed during the Holocene. The latter suggest depletion of the available debris and in the effectiveness of large floods.