Abstract
Abstract. Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development.For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al-Haditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area.The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.
Highlights
Since around 1980 until today, thousands of sinkholes have affected specific areas along the Dead Sea shoreline (Yechieli et al, 2006; Shalev et al, 2006; Abelson et al, 2017), appar-Published by Copernicus Publications on behalf of the European Geosciences Union.U
This study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings
We further show that shear wave reflection seismics have advantages for studying highly porous, partly saturated alluvial fans with complex compositions, e.g. compared to P-wave reflection and refraction, which is partly controlled by the pore fluids, and multi-channel analysis of surface wave (MASW), which is only valid in a 1-D layer case
Summary
Since around 1980 until today, thousands of sinkholes have affected specific areas along the Dead Sea shoreline (Yechieli et al, 2006; Shalev et al, 2006; Abelson et al, 2017), appar-U. Since around 1980 until today, thousands of sinkholes have affected specific areas along the Dead Sea shoreline (Yechieli et al, 2006; Shalev et al, 2006; Abelson et al, 2017), appar-. Wachs et al, 2000) and eastern (El-Isa et al, 1995) shorelines of the Dead Sea. An early map of the main sinkhole sites was published by Yechieli et al (2002). Arkin and Gilat (2000) defined two different classes of sinkholes: (a) gravel holes on alluvial fans that consist of highly permeable gravel and sand layers including some silt, clay, and evaporites and (b) mud holes on Dead Sea mudflats that consist of very fine marl, silt, clay, and evaporitic minerals like aragonite, gypsum, and halite The sinkholes typically appear in clusters on either alluvial fans or mudflats. Arkin and Gilat (2000) defined two different classes of sinkholes: (a) gravel holes on alluvial fans that consist of highly permeable gravel and sand layers including some silt, clay, and evaporites and (b) mud holes on Dead Sea mudflats that consist of very fine marl, silt, clay, and evaporitic minerals like aragonite, gypsum, and halite
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