Alluvial Fans

Evolution, shape, deposition of and secondary processes on alluvial fans and their distinction from river delta and braided rivers are discussed in detail by Blair and McPherson (1994a) and Blair and McPherson (1994b). The following characteristics for developing a depositional model were taken into account (taken mostly from Blair and McPherson (1994a) and Blair and McPherson (1994b)):

Primary processes on alluvial fans

Sediment accumulation on alluvial fans, referred to as primary processes, take place as catastrophic events in high magnitudes, and, as much as sediment deposition is concerned , low frequency.

Sediment deposition happens over the whole area either on active depositional lobes or, in case of small fans over the whole fan (not in channels like braided river systems). Two classes, fluid-gravity and sediment gravity flows are responsible for sediment accumulation on alluvial fans.

Secondary processes on alluvial fans

Processes reworking the primary deposition of fan sediments are called secondary processes. These processes follow the catastrophic discharge events and take place during a much larger time span (up to thousands of years) compared to the active deposition which happens during a very short time span.

Such secondary processes are erosive overland flow, which results in the production of rills and gullies, and calcite sedimentation, which can be observed nowadays at the northeastern part of the Dead Sea. Very shallow groundwater tables downfan lead to evaporation, which results in the development of salt crusts or travertine. bioturbation, neotectonics, and weathering soil development complete the picture of secondary processes.

4. Aquifer Geometry

Fig. 4.2-1: Drainage net of the major catchment areas based on 1 : 50,000 scale topograhic maps provided by the Royal Geographic Center of Jordan (center map). Surface bedrock of the different catchments based on the 1 : 50,000 scale geological maps provided by the National Resources Authority of Jordan. Note, that the scale of the bedrock catchments differ from the drainage net map. Lower Right: Table of the surface bedrock area of each catchment for each geological Formation.

Alluvial fan forms

Alluvial fans are semi-circular, but with lateral constrictions, i.e. the influence of neighbouring fans, lead often to elongated fans perpendicular to the mountain front. On elongated fans incised channels may transport sediment masses away from the mountain front to the active depositional lobes of their respective fans. Their presence highly influences the form of the fan. Alluvial fans generally have a planconvex geometry and slopes range from 2 to 6° in sheetflood dominated environments and up to 30 to 40° in the zone of freefall accumulations (Blissenbach 1954). But slopes within the study area might be lower due to the deposition of the lacustrine facies which deposited to a higher extent in the mid basin and to smaller extend near the mountain fronts, thus reducing the general slope of the topography.

Case studies from the western side of the Dead Sea combined with field observations

According to Blair and McPherson (1994) mainly five key factors control the sedimentary processes and deposits on alluvial fans. These key factors are: lithology, shape and evolution of drainage basins, neighbouring environments, climate, and tectonism. All key factors are essentially the same on the western side of the Dead Sea. Therefore findings within these areas can be used to set up a principal sedimentary model for the study area. Several articles describe alluvial fan sedimentation on the western side of the Dead Sea directly or indirectly (i.e. Sneh 1979; Machlus et al. 2000; Bartov et al.

2002; Klinger et al. 2003). Due to the lowering of the Dead sea level (base-level), erosive overland flow cut channels (secondary process) deeply into the alluvial fan sediments. These channel outcrops reveal valuable information about the depositional nature of these fans.

Sneh (1979) reported coarse clastic sediments that interfinger with fine- grained lacustrine sediments.

With increasing distance from the fan apex, the proportion of alluvial to lacustrine sediments decreases. The distal fan area is characterized by interlayered sand/mud facies. According to Frostick and Reid (1989) none of the alluvial fan sediments studied on western side of the Dead Sea can be regarded as debris flow deposits. Apart from boulders observed directly at the apex of the fan, as observed within the Holocene sediments of the study area, Frostick and Reid (1989) observed no downstream decline of grain-size: “neither is there any noticeable change in other sedimentary parameters, as bed thickness, distance of the apex or height in sedimentary sequence”. An abrupt change from an alluvial to a lacustrine sequence was observed by Frostick and Reid (1989). Only seven incursions into the lacustrine Lisan Formation have been described, which gives a recurrence interval of these catastrophic events of around 1,000 years. Frostick and Reid (1989) explained the sharp change from fine laminated lacustrine sediments to alluvial sedimentation by the deposition of high magnitude discharge events that happen at low frequencies. On the base of the lake level curve of Lake Lisan Frostick and Reid (1989) attributed these high magnitude discharge events to climate change. Bowman (1971) studied the geomorphology of fans deposited on the western margin of the Dead Sea and describes the proximal part next to the Jordan Valley boundary escarpment as crudely stratified with a slightly cemented conglomeratic facies. According to Bowman (1971) the thickness of the studied fans reaches several tens of metres and has a sheet-like structure (indicator for fluid-gravity flow). The so-called rift escarpment is mantled by Pleistocene talus veneers. Klinger et al. (2003) studied three major alluvial fans near and at the Dead Sea. The Wadi Dahal fan, located to the southeast of the Dead Sea, shows Pleistocene fan segments covered by the lacustrine Lisan Formation, which is itself overlain by Holocene fan segments. Unlike the fans deposited within the study area, the present day morphology of the Wadi Dahal fan is around –100 m above sea level (the maximum lake level of lake level was around –160 m (chapter 2.2.2.3)). Therefore the Lisan Formation in the area of Wadi Dahal covers only the mid- and distal fan area. To verify the deposition of the whole Lisan section above the Pleistocene fan segments Klinger et al. (2003) dated lacustrine samples by using the

4. Aquifer Geometry

U/Th method and the alluvial deposits by using its rock varnish (¹⁰Be isotope method). Since no agriculture is practiced in the area, the different fan segments are easy to identify on satellite images and their extension and facies distribution of the fan can be mapped by remote sensing techniques. The other fan sites, the Hazeva area and Wadi Hever, are located to the southwest of the Dead Sea and to the west of the Dead Sea, respectively. The effect of slip rate on alluvial fan sedimentation for the areas southeast of the Dead Sea is discussed by Klinger et al. (2003). They studied alluvial fans whose feeder channels are continuously displaced by left-lateral strike-slip faults. Klinger et al. (2003) show, that the location of the major transform fault can strongly influence the shape and extent of alluvial fans. The effect of earthquakes on a coastal alluvial deposit is addressed in Enzel et al. (2000). They described the displacement of sediment accumulations caused by Holocene to recent earthquakes.