--> ABSTRACT: Slope Failure on the Jordan River Delta in the Dead Sea, by G. Almagor; #90097 (1990).

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ABSTRACT: Slope Failure on the Jordan River Delta in the Dead Sea

G. Almagor

The Dead Sea is a longitudinal (80 km × 20 km) terminal lake that was formed by the active Syrian-African transform fault. Considerably saline inflow from the Jordan River and a very high evaporation rate have led to the excessive salinity (360 g/L) and water density (1.226-1.232 g/cc) of the lake. The rate of evaporite precipitation is 630-2000 mg/cm2/yr.

Clastic sediments from the Jordan River have formed a very moderately sloping (about 2°) delta that covers the northern lake to a distance of 8 km offshore and to a depth of 300 m. Many irregularly shaped mounds of sediment that are acoustically transparent and that contain numerous discontinuous reflectors cover the delta slope. They extend over several hundred square meters each and rise 2-5 m above their surroundings. These sediments indicate active downslope slumping processes.

The small-scale sedimentary pattern and the engineering properties of the sediments were examined by testing 6 core samples collected at various depths along the Jordan delta. Lengths range from 0.90 to 3.20 m. Depth of coring was restricted by an extremely hard, impenetrable layer of halite, and the topmost 10-25 cm consists of unconsolidated, recently deposited grainy halite. Euhedral crystals of halite are disseminated throughout the entire length of the core samples, especially in the deep-water cores. The high salinity of the pore water and the presence of large amounts of disseminated halite within the cores required modifications of the standard soil mechanics laboratory testing procedures.

The sediments are predominantly dark brown, layered kaolinitic silty clays. They contain numerous conspicuous black organic-matter and sulfide-bearing layers and white aragonite and gypsum layers. Occasionally, slump structures are encountered. Numerous small voids (1 mm3) within many of the core samples indicate the presence of

gas. The specific gravity of the salt-washed solids ranges from 2.59 to 2.70. Unit weights of 1.64-1.74 g/cc and water content of 47-56% (before correction for salt content) were measured. The cohesion is 2.0-4.6 kPa, and the sensitivity is 4. Consolidation tests indicate that the material is normally consolidated. Total and effective angles of internal friction of 16.5-19.1° and 35.2-38.8°, respectively, were measured by triaxial consolidated-undrained shearing, and an angle of 32° was measured by direct shearing under drained conditions indicating stable delta slopes. Similar results were obtained by tests made on cylindrical samples prepared with their cylinder axis inclined at 60° (45° + ^phgr/2) to the field vertical axis.

The high effective angles of internal friction are caused by the high salt content of the sediments. A preliminary analysis of the force equilibrium within the sediments indicates that horizontal accelerations generated by earthquakes, which are frequent in and around the lake, cause delta slope failures followed by episodic slumping, mass movement and sediment redistribution.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990