Bedforms in Salt Deposits of the Dead Sea Brines

Abstract

ABSTRACT Study of salt deposits emphasizes the mineralogical and geochemical aspects of origin with but little attention to processes and features of physical sedimentation. Deposition from saturated brines along one of the main conveyance canals of the Dead Sea Works near Sedom (southern Israel) indicates, however, that in open-channel flow the halite sediment develops bedforms identical to those reported in alluvial channel deposits. The conveyance canal is trapezoidal, with side slopes of 1:2, bottom width of 17 m, bankfull depth of 1.5 m, and a gradient of 0.00022. Design Manning's n is 0.0223, decreasing to about 0.020 after deposition of the fine halite lining. Under normal operating conditions flow velocity is about 30 cm/s and depth is about 1 m. The conveyed brine, with a densi y of 1.29 and viscosity of 5 cp, is supersaturated with respect to halite, and deposition of halite occurs, aided by temperature fluctuations, mixing, and natural and pump-induced turbulence. The sediment consists of halite with minor amounts of carnallite, and it is very poorly sorted. A conventional interpretation of the grain-size distribution is not applicable, however, since the finer fractions, up to about 100 microns, consist of single, cube, block, or prism-shaped crystals, whereas the coarser material includes mainly porous and irregular clusters of crystals, and larger, ball-like aggregates, up to about 3 mm in diameter. Bedforms encountered along the canal include current crescents and shadows due to deformation of flow around obstacles such as bottom irregularities and salt l mps; transverse, linguoidal, and catenary ripples and megaripples, some of which may easily be mistaken for karrenlike products of solution mass transfer and for features of syneresis; and longitudinal ridge patterns due to three-dimensional systems of streamwise vortices. The comparison of results of Stokesian settling, hindered settling, and pipe flow of brines and of suspensions with different concentrations of solids indicates similar hydraulic behavior of salt/brine and quartz/water systems. After adjustment for particle and fluid properties, the observed bedforms fall into the same hydraulic factor fields as determined for bedforms in alluvial channels. Salt deposits, therefore, clearly belong to the wide group of chemical precipitates, ranging from igneous rocks to industri l slurry deposits, which record the process of mechanical transport and deposition.