Red sedimentary units as indicators of Early Pleistocene tectonic activity in the southern Negev desert, Israel

Abstract

Alluvial units (termed “Red Units”) accumulated along the western margin of the Dead Sea Rift (DSR) valley (Southern Negev Desert, Israel) in the Early Pleistocene. These sedimentary units consist of coarse to fine-grained alluvium of local origin, which contain calcic paleosols. The sediments of the Red Units accumulated in stream valleys and newly-forming semi-closed local tectonic basins during a tectonic phase that deformed the western margin of the DSR. As a result of this deformation the Pliocene, northward-flowing, regional drainage system of the southern Negev disintegrated to the present northeast-flowing, ephemeral drainage systems of Nahal Zihor, Nahal Hiyyon and Nahal Paran, all of them draining to the Arava Valley in the DSR. This change was induced by two deformation episodes: (1) reactivation of the Zihor and the Milhan faults—two major, northeast-trending tectonic lines, and (2) east and northeast tilting of the entire region during upwarp (arching) of the central Negev. The paleosols of the Red Units are characterized by pedogenic features such as rhizoconcretions and Bk horizons. The paleosols developed in the alluvial sediments that were deposited and preserved in local, small structural basins along the main fault lines and in reversed or abandoned westward-flowing Pliocene stream valleys. Formation of paleosols was initiated in the stream valleys when they attained a near horizontal gradient due to gradual eastward-directed regional tilting, followed by low rates of deposition. The sections of the Red Units in the Pliocene valleys were incised due to continuing tilting, which completely reversed the primary westward gradient of the valleys, and they were incised by new east and northeast-flowing streams. The Red Units have paleoclimatic implications. Since the Middle Pleistocene the southern Negev has been characterized by gypsic and salic soils typical of an extremely arid climate, while the calcic soils detected in the Red Units point to a semi-arid climate. This indicates a gradual transition from more humid conditions in the Early Pleistocene to the present extreme aridity. Therefore, the Red Units reflect a distinct tectonic and climatic regime and can serve as regional morphostratigraphic marker of Early Pleistocene age. In this paper, we used this marker to reconstruct the tectonic deformation that established the present configuration of the southern Negev Terrain.