Geophysical investigations in the Dead Sea

Abstract

Bathymetric, magnetic, and continuous seismic measurements were made along approximately 540 km of track in the Dead Sea in July and August of 1974. A new precision bathymetric map was prepared, using these measurements, together with all earlier soundings. The northern basin is shown to be shallower than previously thought, and almost flat bottomed over nearly 40% of its area. The maximum depth, relative to Mediterranean sea level, is 730 m (i.e. a water depth of 332 m below the Dead Sea level at the time of the survey), rather than the 798 m reported for Lt. Lynch's 1848 survey. The basin is asymmetric, with western slopes generally averaging 7° and eastern slopes averaging 30° except in the vicinity of the Arnon River delta where slopes are around 8°. The western flank of the basin is quite corrugated, and along the northwestern flank of the basin the topography becomes steeper, following a linear trend of N27°E which is probably fault controlled. Sediments from the Jordan River delta have built up a smooth and almost planar ramp sloping 3° to depths of 650 m (below MSL), and 1° thereafter into the deep basin. Revised hypsometric calculations show inflection points in the area curve at 425 and 725 m, reflecting the flat-floored nature of the northern basin. The volume of water enclosed with sea level at 400 m is 147 km 3, an increase over earlier estimates. The total field magnetic anomaly map with 5 gamma contour interval shows several interesting features, the anomalous field has an eastward dip with gradients of 5 to 20 gammas per kilometer. Magnetic anomalies (10 to 20 gamma amplitude) were detected east of Ein Gedi, Kallia, and the Jordan River delta as well as in other locations. The E—W trending magnetic anomaly north of Ein Gedi may be an extension of a similarly trending magnetic anomaly noted by Folkman and Yuval (1976) across the Hebron mountains, while the pronounced eastward magnetic gradient may reflect a transition from a less continental type of crust on the west to a more continental type of crust east of the Rift. Diapiric structures induced by the rock salt masses of the Sedom Formation were detected all over the northern basin. They are mainly concentrated along the major western submarine N—S trending border fault, and are probably genetically associated with Mt. Sedom. Similar structures, although smaller in size and fewer, are also found along the steep eastern border fault. Two large, dome-shaped diapiric structures have pierced through the bottom of the median trough: one off Ein Gedi (the Ein Gedi Diapir) and the other at the northeastern segment of the Jordan Delta (the Jordan Delta Diapir). They seem to be genetically related to the diapir of the Lisan Peninsula and to others which were detected south of it in the southern basin. The diapirism processes have been active since deposition of the Amora (Foothills) Formation and especially during deposition of the Lisan Formation (Late Pleistocene). Great thicknesses of younger sediments have been synchronously accumulated in the associated ring depressions. Almost complete desiccation occurred at the end of the Lisan Formation (some 15,000 to 10,000 years B.P.). The rate of deposition which is mostly controlled by the huge quantities of chemical precipitates, have compensated for and exceeded the rate of diapirism. The Jordan Delta was built out and accumulated only since the end of the desiccation phase. The erosional drainage pattern generated during and just after the desiccation phase, was partially masked by the younger Holocene sediments. The eastern border faults exhibit a systematic pattern: a major N—S trending linear fault from which secondary faults periodically branch off to the NNE. The trends of the individual faults in the western border fault system are less regular and uniform. They comprise, however, a system of step-faults. Two NW trending faults border the Lisan Peninsula on its north, and south. No E—W trending fault was found beneath the Jordan Delta to mark the northern edge of the northern basin.