Climatically Triggered Gilbert-Type Lacustrine Fan Deltas, the Dead Sea Area, Israel

The marginal facies of the Upper Pleistocene, lacustrine Lisan Formation in the Dead Sea area consists of a moderately dipping, slightly cemented and crudely bedded boulder conglomerate, composing a sequence of fan deltas and alluvial fans at canyons exits. Overlying these are Gilbert-type sequences which demarcate the very top marginal facies of the Lisan Formation. Various sources, such as pollen, archaeology, paleosols and lacustrine sediments provide evidence of the wetter character at the end of the Lisan stage 18000-14000 yr bp. Gilbert-type sequences were observed in small catchment areas (Wadi Mor, W. Boqeq) and in medium ones (W. Rahaf). However, these sequences were not observed in the largest catchment areas as those of W. Zeelim and W. Arugot. It is suggested that the small drainage basins were most sensitive to the increased humidity, i.e. through their denser vegetative cover, erosion was retarded and run-off decreased, resulting in deposition of Gilbert-type units. The large catchment areas, notwithstanding the vegetative cover, retained their flashfiood regime, thus filtering out any evidence of hydrological change. The sharp textural and structural differences between basal fan deltas and the superimposed Gilbert-type sequences reflect the change in the hydrological regime, i.e. confinement of sediment traction and decrease in the dispersal energy. The unbridging wedge-shaped geometry of the Gilbert-type units further precludes a tectonic origin and emphasizes the climatic trigger for depositing Gilbert-type sequences.