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Geophysical investigations in the Dead Sea
Sedimentary Geology, 23 ( 1 9 7 9 ) 2 0 9 - - 2 3 8 © Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
209
GEOPHYSICAL INVESTIGATIONS IN THE D E A D SEA
D A V I D N E E V a n d J O H N K. H A L L
Marine Geology Division, Geological Survey of Israel, Jerusalem 95 501 (Israel) ( R e c e i v e d D e c e m b e r 15, 1 9 7 8 )
ABSTRACT Neev, D. a n d Hall, J.K., 1 9 7 9 . G e o p h y s i c a l i n v e s t i g a t i o n s in t h e Dead Sea. S e d i m e n t . Geol., 23: 2 0 9 - - 2 3 8 . B a t h y m e t r i c , m a g n e t i c , a n d c o n t i n u o u s seismic m e a s u r e m e n t s were m a d e a l o n g a p p r o x i m a t e l y 5 4 0 k m o f t r a c k in t h e D e a d Sea in J u l y a n d A u g u s t o f 1 9 7 4 . A n e w p r e c i s i o n b a t h y m e t r i c m a p was p r e p a r e d , using t h e s e m e a s u r e m e n t s , t o g e t h e r w i t h all earlier s o u n d i n g s . T h e n o r t h e r n b a s i n is s h o w n t o b e s h a l l o w e r t h a n previously t h o u g h t , a n d a l m o s t flat b o t t o m e d o v e r n e a r l y 40% o f its area. T h e m a x i m u m d e p t h , relative t o M e d i t e r r a n e a n sea level, is 7 3 0 m (i.e. a w a t e r d e p t h o f 3 3 2 m b e l o w t h e D e a d Sea level at t h e t i m e o f t h e survey), r a t h e r t h a n t h e 7 9 8 m r e p o r t e d for Lt. L y n c h ' s 1 8 4 8 survey. T h e basin is a s y m m e t r i c , w i t h w e s t e r n slopes generally averaging 7 ° a n d e a s t e r n slopes averaging 30 ° e x c e p t in t h e v i c i n i t y o f t h e A r n o n R i v e r d e l t a w h e r e slopes are a r o u n d 8 ° . T h e w e s t e r n f l a n k o f t h e basin is q u i t e c o r r u g a t e d , a n d a l o n g t h e n o r t h w e s t e r n f l a n k o f t h e b a s i n t h e t o p o g r a p h y b e c o m e s s t e e p e r , f o l l o w i n g a linear t r e n d o f N 2 7 ° E w h i c h is p r o b a b l y fault cot{trolled. S e d i m e n t s f r o m t h e J o r d a n R i v e r d e l t a have b u i l t u p a s m o o t h a n d a l m o s t p l a n a r r a m p sloping 3 ° to d e p t h s of 6 5 0 m ( b e l o w MSL), a n d 1 ° t h e r e a f t e r i n t o t h e d e e p basin. Revised h y p s o m e t r i c c a l c u l a t i o n s s h o w i n f l e c t i o n p o i n t s in t h e area c u r v e at 425 a n d 7 2 5 m, r e f l e c t i n g t h e f l a t - f l o o r e d n a t u r e o f t h e n o r t h e r n b a s i n . T h e v o l u m e o f w a t e r e n c l o s e d w i t h sea level at 4 0 0 m is 147 k m 3, a n increase over earlier estimates. T h e t o t a l field m a g n e t i c a n o m a l y m a p w i t h 5 g a m m a c o n t o u r i n t e r v a l s h o w s several i n t e r e s t i n g f e a t u r e s , t h e a n o m a l o u s field has a n e a s t w a r d dip w i t h g r a d i e n t s o f 5 t o 20 g a m m a s per k i l o m e t e r . M a g n e t i c a n o m a l i e s ( 1 0 to 20 g a m m a a m p l i t u d e ) were d e t e c t e d east o f E i n Gedi, Kallia, a n d t h e J o r d a n R i v e r d e l t a as well as in o t h e r l o c a t i o n s . T h e E--W t r e n d i n g m a g n e t i c a n o m a l y n o r t h o f E i n Gedi m a y b e a n e x t e n s i o n o f a similarly t r e n d i n g m a g n e t i c a n o m a l y n o t e d b y F o l k m a n a n d Yuval ( 1 9 7 6 ) a c r o s s t h e H e b r o n m o u n t a i n s , while t h e p r o n o u n c e d e a s t w a r d m a g n e t i c g r a d i e n t m a y reflect a t r a n s i t i o n f r o m a less c o n t i n e n t a l t y p e o f crust o n t h e west t o a m o r e c o n t i n e n t a l t y p e o f c r u s t east o f t h e Rift. Diapiric s t r u c t u r e s i n d u c e d b y t h e r o c k salt masses o f t h e S e d o m F o r m a t i o n were d e t e c t e d all over t h e n o r t h e r n basin. T h e y are m a i n l y c o n c e n t r a t e d a l o n g t h e m a j o r w e s t e r n s u b m a r i n e N--S t r e n d i n g b o r d e r fault, a n d are p r o b a b l y g e n e t i c a l l y a s s o c i a t e d w i t h Mt. S e d o m . Similar s t r u c t u r e s , a l t h o u g h smaller in size a n d fewer, are also f o u n d a l o n g t h e s t e e p e a s t e r n b o r d e r fault. T w o large, d o m e - s h a p e d d i a p i r i c s t r u c t u r e s have pierced t h r o u g h t h e b o t t o m o f t h e m e d i a n t r o u g h : o n e o f f E i n G e d i ( t h e Ein G e d i Diapir) a n d t h e o t h e r at t h e n o r t h e a s t e r n s e g m e n t o f t h e J o r d a n D e l t a ( t h e J o r d a n D e l t a Diapir). T h e y s e e m t o be g e n e t i c a l l y r e l a t e d t o t h e d i a p i r o f t h e Lisan P e n i n s u l a a n d t o o t h e r s
210 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 c o m prise, 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.
INTRODUCTION
AND PURPOSE
Despite its historical importance, the Dead Sea has been the subject of relatively few marine geological investigations. The first serious survey of the Dead Sea floor was undertaken in 1848 by a United States Navy expedition under the command of Lt. W.F. Lynch {1849). This survey showed the northern basin to be deep and the southern basin to be shallow, and defined the bathymetry for almost all subsequent maps. Neev and Emery (1967) carried out the first modern investigation of the marine geology using direct sampling techniques and echosoundings, but unfortunately their work was constrained to the southwestern quadrant of the Dead Sea. In an effort to enrich this body of knowledge, a cooperative multi
AND ANALYSIS
The survey was carried out from the R /V 'Barcelona', a self-propelled barge provided by the Dead Sea Works Ltd. A brief narrative of the survey, which was marred on the night of 23 July 1974 when the 'Barcelona' {with
_
DEAD
S E A GEOPHYSICAL S U R V E Y
~ig. 1. Profile location map and track chart for the Dead Sea Geophysical Survey of 19 July--1 August 1974. The track is based u p o n ligh resolution electronic Miniranger navigation f r o m precisely surveyed shore-based transponders. T i m e s are a n n o t a t e d every 10 minltes and m a r k e d every 5 m i n u t e s for c o o r d i n a t i o n with the t i m e scales printed above the seismic r e f l e c t i o n profiles. East--west latitutinal profiles have been n u m b e r e d sequentially f r o m 1 to 25, while n o r t h - - s o u t h longitudinal profiles are s e q u e n c e d a l p h a b e t i c a l l y 'tom A to L. All profiles are o r i e n t e d so that south or east is on the right, depending u p o n the o r i e n t a t i o n . Projection is on the local :srael (Cassini) grid, which is in kilometers. Wells are m a r k e d , with total depths. A total of 540 k m o f track is shown.
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212 its equipment) broached to, and was beached and flooded during a strong windstorm, can be found in an earlier report which also contains extensive details concerning the survey methods and data analysis (Hall and Neev, 1975).
Navigation A Motorola Miniranger ® navigation system was used for positioning throughout the survey, Shorebased transponders were sited at a t o t a l of eight locations to provide accurate navigation at almost all of the areas surveyed. Range measurements were recorded twice each minute, and the positioning errors incurred, at the plotting scales used, are completely negligible.
Depth soundings Depth soundings were made with an ELAC portable echosounder. The sounding records were digitized at all slope changes, and a total of 5148 digitized soundings were used in the preparation o f the new bathymetric map (Fig. 2). The value o f the Dead Sea level at the time of the survey was --398.04 m relative to Mediterranean sea level. The echosounder was calibrated for the velocity of sound in the Dead Sea b y making recordings at three locations over the flat-bottomed deep basin at which Neev and Emery had raised cores and accurately measured the depth with a taut vertical coring wire. These calibrations show the round trip sound velocity over the entire water column to be 885.3 m/sec or a b o u t 18% higher than that found in the world oceans.
Continuous seismic reflection profiling Continuous seismic reflection measurements were made with a 1000 joule (watt-second) E.G.&G. Sparker sound source and an eel-type 150 element h y d r o p h o n e array, A measurement was made every 3 sec, with the negative signal phase printed o u t on a 1 sec sweep of a 19" dry-paper recorder. In shallow water the source and receiver were suspended from plastic jerricans to prevent contact with the rough sea floor. The records were interpreted, and then the selected s u b b o t t o m reflectors were digitized and a c o m p u t e r program used to arrive at the data necessary to prepare isopach and structural contour maps for these horizons.
Total field magnetic measurements Total field magnetic measurements were made every 6 see with a Varian direct-reading p r o t o n precession magnetometer. For technical reasons it was not possible to m a k e measurements on three of the eight days of work. The records were smoothed, read every 5 minutes, and then corrected for diurnal
213
variations measured at the Modi'im Forest Magnetic Observatory near Lod. Of the five days during which measurements were made, two could be considered ' s t o r m y ' with m a x i m u m corrections amounting to 72 and 28 gammas. Anomalies were then c o m p u t e d by subtracting the 1965.0 IGRF (Fabiano and Peddie, 1969), which at the Dead Sea surface indicates a regional field increasing in the direction of N20°E at the rate of about 7.8 gammas per kilometer. The I G R F is about 9 gammas higher at --397 m than it is at Mediterranean sea level. Using this reference field all the anomalies are negative. Subsequenct calculations using the updated degree 12 coefficients for epoch 1975.0 (Fabiano and Peddie, 1975) show the updated reference field for the time of the survey to be 275 gammas lower. This gives an identical anomaly pattern but with positive anomalies only. When the anomalies were plotted out, the standard deviation of the differences at the 31 track crossings for which data were available was 8 gammas. In order to present the data at the desired 5-gamma contour interval, further adjustments were made to remove these differences. Considerable confidence m a y be placed in the final result as the required corrections are smooth and well-behaved, suggesting that the diurnal corrections account for the bulk of the short-period temporal changes. RESULTS
Bathymetric mapping Fig. 2 shows the bathymetric map prepared from the new survey data. In this map depths are referenced to Mediterranean mean sea level (MSL) rather than to the variable Dead Sea sea level. A contour interval of 10 m is used except for the shallow regions with depths of less than 410 m, and the 'abyssal' regions with depths greater than 710 m, where a 1-m contour is used. The data from this survey is supplemented by the original soundings of Neev and Emery (1967), which were taken on a 1-kin grid spacing south and east of Ein Gedi, and by the soundings of the American Expedition to the Dead Sea in 1848 (Lynch, 1849). Lynch's soundings, while incorrect (see following section), provided useful information in the areas adjacent to Ain Suweimet, Zerka Mayin, the Arnon River and E1 Mezra'ah, all of which are in parts of the eastern coast which were not covered. Several features are immediately apparent from the map which are absent, or at least not readily apparent, in the earlier map of Neev and Emery (1967). (1) There is a large difference in the slopes on either side of the Dead Sea basin. On the eastern side slopes of 30 ° over a depth range of 200 m are encountered, while the slopes on the western side reach a m a x i m u m of 25 ° in only two places, over a depth range of 50 m, and slopes more generally average 7 ° . Fig. 6a illustrates this difference.
Fig. 2. Bathymetric chart of the Dead Sea. Note: (1) the flat-bottomed box-shaped configuration of the northern basin, whose eastern, western and southern margins are fault controlled, (2) the gradual slope of the Jordan River delta, and (3) the submerged drainage system on the western and southern margins which is partially filled by younger sediments. The contour interval is 10 m, with 1-m co n t o u r between the shoreline and 410 m, and below 710 m. All contours are relative to Mediterranean sea level. The Dead Sea level at the time of this survey was --398 m MSL. Maximum depth is 730 m.
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215 (2) The maximum depths are f o u n d along a longitudinal 'trough' which runs north--south adjacent to the eastern margin of the deep northern basin. This trough consists of a slight depression of the otherwise flat floor of the basin, which appears to result from subsidence (Fig. 6b, Profile 9--9). (3) Along the northwestern side of the northern basin, the topography shows a linear trend of a b o u t N27°E which is probably fault controlled. (4) The western side shows the effects of an earlier drainage pattern to depths of at least 700 m MSL. (5) The maximum depth is in the order of 730 m, or 332 m below the Dead Sea level of 1974, rather than the value of 798 m which is obtained from Lynch's (1849) m a x i m u m of 398 m below the Dead Sea level of 1848 (Klein, 1961). A further finding of this survey, using precision navigation, was that depths observed along one profile do not always agree with depths measured along an intersecting profile at the point of intersection. In the preparation of the final bathymetric map this was primarily seen over the deep basin, where differences sometimes approached 6--8 m. The reason for these discrepancies is still u n k n o w n b u t it cannot be a result of either erroneous navigation or recorder error, for the navigation is self-consistent (identical transponders), the basinal slopes are negligible, and the differences are also observed on the seismic recorder whose time base is stable to about 1 m in 10,000. Short-term hydrographic charges are suspected.
The deepest part. There appear to be t w o 'deepest' parts, lying along a longitudinal trough in the eastern part of the basin which parallels the 198.5E meridian of the Cassini (Israel) grid system. (1) The northern low appears to have depths o f - - 7 3 0 m over an elliptical area approximately 3.5 km long in the north--south direction (between 110.2 and 107.0N) and a b o u t 2 km wide in the east--west direction (between 197.5 and 199.5E). (2) The southern low appears to be slightly shallower, over 729 m but less than 730 m. It occupies a larger area of approximately 7.5 km length north-south (between 106.0 and 98.5N), and 2.5 km width east--west (between 197.3 and 199.5E). The deepest part is probably in the vicinity of 109.5N 198.5E, with a depth of slightly over 730 m. The accuracy of these measurements, which are subject to some of the errors mentioned above, is probably better than +-2m. These findings are in distinct disagreement with the lead-line soundings obtained b y Lt. W.F. Lynch USN in the 1848 American Expedition to the Dead Sea and the Jordan (Lynch, 1849). Lynch had t w o profiles crossing the deepest parts of the basin mentioned above. The first, taken on Thursday May 4, 1848, b y Lt. John B. Dale with the b o a t 'Fanny Skinner' from Wadi Arnon to tkin Ter~beh (near the present Mizpe Shalem) measured a maxim u m of 195 fathoms (--756 m MSL), and showed depths across the deepest
216 plain generally 24--27 m deeper than what was observed here. A second profile taken by Lt. Lynch himself with the 'Fanny Mason' on Friday May 5, 1848, between Zerqa Mayin and Ain Ter~beh (Mizpe Shalem) produced one sounding of 218 fathoms o r - - 7 9 8 m MSL (Lynch, 1849, pp. 373--374), 68 m deeper than our maximum depth, and a number of other soundings generally 15--20 m greater than our present results. What is the source of these discrepancies? That there was a hole some 70 m deeper which has been filled in the intervening 126 years is ruled o u t b y Neev and Emery's (1967) measured and extrapolated sedimentation rates, as well as by the stratigraphic nature o f the layers seen in the seismic profiles. Instead, it appears that these errors are a result of the rather rudimentary measuring technique used, namely that of lowering a lead weight by hand with a line. In order to avoid measuring a slant depth (due to boat drift or submarine currents), it is generally necessary to use a heavy weight on a very fine line or wire. However, since Lynch had no winch, b u t instead relied upon handling the line b y hand, it seems likely that a smaller weight and thicker line were used, and that there was probably some boat drift and wire angle at the time of measurement. In addition, it is quite possible that such a woven line contracted appreciably (perhaps 10%) due to the effects of the salt, and that the one measurement of --798 meters was a result of the weather that day, described (p. 374) as 'intensely hot, not a breath of air s t i r r i n g . . . ' u p o n the men in the unprotected skiff. Hypsometric curve
In order to s t u d y h o w the surface area and volume of the Dead Sea basin vary as a function o f depth, a hypsometric analysis was carried out. Selected contours in the new bathymetric chart in Fig. 2 were digitized and their areas c o m p u t e d b y a m e t h o d given by Hall (1976). Contours were chosen at the shoreline (--397 m), every 50 m between --450 and --700 m, and then every 5 m to - - 7 3 0 m. Areas were plotted as a function o f depth (Fig. 3), and then a s m o o t h spline curve was carefully passed through the 13,points. Intermediate points were picked o f f and noted, and the area algorithm (Hall, 1976) was again used with a programmable p o c k e t calculator to produce an exact integrated area or volume curve. Comparison o f these results with the earlier curve of Neev and Emery (1967) shows one interesting feature. Despite a 64 m decrease in the maxim u m depth b e t w e e n the t w o bathymetric maps, the volume o f water present with sea level at --400 m i n c r ~ e s some 9% from Neev and Emery's (1967) value o f 135 cubic ~ o m e t e r s to t h e new value of 147 cubic kilometers. This is due to the discovery that the deep northern basin is shaped m o r e like a rectangular box than a long wedge, with the area within the --710 m contour being some 110 k m 2 larger than previously thought. This box shape is reflected in two very pronounced inflection points, marking abrupt changes in average bottom ~ i e n t , which occur at --425 and --725 m.
217
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AREA (Km z) Fig. 3. Dead Sea hypsometric curve prepared from the bathymetric map in Fig. 2. The box shape of the northern basin is reflected in the inflection points on the area versus depth curve at 425 and 725 m. The volume at the time of the survey was approximately 147 km 3.
Magnetic anomaly map T h e m a g n e t i c a n o m a l y m a p is s h o w n in Fig. 4 w i t h a c o n t o u r interval o f 5 g a m m a s (1 g a m m a = 10 -s gauss). A n o m a l i e s seen in t h e s u r v e y r a n g e f r o m - - 8 0 t o - - 2 2 5 g a m m a s . A n u m b e r o f f e a t u r e s are a p p a r e n t f r o m t h e m a p : (1) T h e field has an e a s t w a r d dip w i t h g r a d i e n t s o f 5 t o 20 g a m m a s / k i l o m e t e r . A t t h r e e o f t h e f o u r p o i n t s at w h i c h t h e e a s t e r n s h o r e was a p p r o a c h e d , t h e field d r o p s t o less t h a n - - 2 0 0 g a m m a s . This regional dip m a y be r e l a t e d t o crustal d i f f e r e n c e s across t h e rift. (2) Several a n o m a l i e s are o b s e r v e d . (a) O f f Ein G e d i a m a g n e t i c high (15 t o 20 g a m m a a m p l i t u d e ) w i t h t w o l o b e s t r e n d i n g east a n d n o r t h is a s s o c i a t e d w i t h t h e t o p o g r a p h i c high w h i c h
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Fig. 4. C o n t o u r m a p o f the total field m a g n e t i c a n o m a l y over the Dead Sea. C o n t o u r interval is 5 gammas, a n d tracks a l o n g w h i c h measurements were m a d e are n o t e d as d o t t e d lines. M e a s u r e m e n t s have been c o r r e c t e d for diurnal variations, r e d u c e d t o the 1 9 6 5 . 0 I G R F at an altitude o f - - 3 9 7 m, and t h e n adjusted t o r e m o v e errors remaining at track crossings. N o t e that: ( 1 ) all a n o m a l i e s are negative, ( 2 ) there is an eastward dip o f 5 - - 2 0 g a m m a s / k i n , and ( 3 ) a n o m a l i e s are associated w i t h s o m e o f t h e faults s h o w n in Fig. 12.
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219
projects northeast from Ein Gedi. This high is most likely diapiric (Fig. 5, Profile C), and the northern lobe of the magnetic anomaly is found near a buried diapiric lobe which is seen in Profile 15. It should be noted that such anomalies are probably related to the structures along which diapiric piercement of salt t o o k place, and n o t to the diapirs themselves. (b) There is an interesting saddle point along the coast between Ein Gedi and Mizpe Shalem, where an east--west trending low (--140 gammas) is sandwiched between t w o highs (--80 and --100 gammas) on the north and south. (c) Two small NNW trending anomalies are found in the extreme north and south. A 10 gamma high is f o u n d o f f Kallia where t w o small diapirs were observed while a 20 gamma low occurs in shallow water in the northern part of the Lisan Straits. (3) From Ein Gedi south to the evaporation pools there is good agreement b o t h in direction and gradient b e t w e e n these measurements and those of Domzalski (1967). No aeromagnetic data exists north of Ein Gedi. (4) Inspection of the total field intensity map of Allan and Morelli (1970) for the Gulf of Aqaba indicates that the magnetic activity over the Dead Sea is of the same order of magnitude as that over the Aqaba Gulf, which Allan and Morelli (1970) interpret as indicating that there has been 'no significant extrusion of volcanic material'. Seismic profiles Fig. 1 shows the location of all fifty-one seismic profiles obtained in the survey. The north--south longitudinal profiles have been alphabetically labelled from A to L, and are displayed so that the south is always to the right, as if the observer were looking from the western edge of the Dead Sea. The east--west latitudinal profiles have been numbered from 1 to 25, and displayed with the east on the right, as if the observer were looking north from the southern end of the Dead Sea. All these profiles, together with the short interconnecting profiles, appear in Appendices 1 and 2 of Neev and Hall (1976). The scale on the left side of the profiles gives the depth of the sea floor below MSL. Where available, the total intensity magnetic measurements, corrected for diurnal variations b u t not for the regional field, have been plotted above the profiles. In the seismic survey the deepest reflections recorded were from about 2/3 sec, mainly along the longitudinal axis of the northern basin. In Fig. 5 (north--south profiles A and C) several reflecting horizons are observed. F r o m these, four prominent reflecting horizons were chosen and named, from t o p to b o t t o m , the 'red', 'blue', 'green' and 'brown' markers respectively. These reflectors are equivalent to stratigraphic horizons that can be regionally followed, with the exception of the 'brown' marker (the deepest one) which is observed as fragments giving the appearance of domal structures. The brown horizon marks the t o p of an acoustically transparent unit (one in which little or no internal reflections or reverberations are observed).
220
One example of this acoustic transparency may be observed at the core of the submarine mountain (the Ein Gedi Diapir) which is f o u n d at the southern end of Profile C. In the same profile one can readily observe the overlying three markers ('green', 'blue' and 'red') and the stratigraphic units they enclose. These units thin toward the apex of this high from both the north and the south. This feature is therefore explained as a diapiric intrusion, and is accordingly named the Ein Gedi Diapir. We assume that the transparent stratigraphic unit represents a rock salt body, in the process of plastic movement, which has lost its stratification and is equivalent to the unit which was defined by Zak (1967) as t h e Sedom Formation. In the central part o f Pro-
Fig. 5. Two adjacent north--south longitudinal 1000 joule sparker profiles. North is to the left. Refer to Fig. I for profile locations. In the eastern Profile A (Fig. 5a) the Arnon Sink, which is genetically associated with the Ein Gedi salt diapir (seen in Fig~ 5b ), is fully developed. The fact that the Red reflector (Base Holocene) is almost unaffected by the diapirism can be explained by the exceptionally high rates of deposition which preceded and followed its formation. In Profile C (Fig. 5b) the Ein diapir (at the extreme right) is best presented. The acoustically transparent material below the Brown reflector is probably rock salt in a state of flow. Note the indications for diapiric uplifting since at least the Green reflector (mid-Pleistocene) and continuing to the present. Also note (1) that the Jordan's submarine delta has been accumulating only since the Red marker (base Holocene) and (2) the existence of a deep diapir beneath the delta (extreme left of profile).
221
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Fig. 6. Six transverse (west--east) profiles. East is on the right side. Fig. 6a: These profiles s h o w one steep border fault on the east (Profile 5) and t w o step faults o n the west (the easternmost of these is best seen beneath the flat b o t t o m in Profile 8). Fig. 6b: N o t e : (1) a salt diapir a l o n g the eastern b o r d e r fault (Profile 10), (2) the A r n o n Sink whose t r o u g h is adjacent to the eastern border fault, and (3) the upslope, westward gradual d e v e l o p m e n t o f diapirism. Fig. 6c: Note the gradual s o u t h w a r d d e v e l o p m e n t o f the Ein Gedi diapir (in the central parts of Profiles 14 and 16), which is a 'Central T r o u g h ' structure, similar to the Lisan Peninsula and the J o r d a n delta diapirs (detached f r o m the eastern as well as the western b o r d e r faults).
file C, the 'green' and 'blue' reflectors show moderate harmonic folding which is interpreted to be also a result of an initial stage of diapirism. This folding diminishes northward, accompanied b y a slight thinning of the enclosed stratigraphic units. Uplifting of the 'blue' and 'green' markers b y domal structures was observed at the northern end o f Profile C as well as in Profile B which parallels C. In the southern half of Profile A, the 'blue' and 'green' reflectors drop precipitously from b o t h north and south into a deep sink. The center of this sink, herewith named the Arnon Sink, is found northwest of the Arnon River outlet, approximately where Profiles A and 13 cross. In these profiles b o t h the 'blue' and 'green' markers disappear completely into the sink. In Profile F, which is parallel to A, as well as in Profiles 10 + 11 (Fig. 6) and 12, one can study the shape of the Arnon Sink b y observing the characteristics of the 'blue' and 'green' markers. Unlike the 'green' and 'blue' markers, the 'red' marker is usually b o t h
224 horizontal and smooth in Profiles A and C. It is therefore deduced that this marker and the overlying layer were n o t influenced b y the diapirism and other tectonic movements to the same extent as the deeper reflectors. It therefore seems likely that during the period when the s~atigraphic unit, b o u n d e d b y the ~ e d ' and 'blue' markers, was accumulated, the rates o f sedimentation in the deep flat areas o f the northern basin were especially high, and consequently traces of diapiric activity were camouflaged at the upper part of this unit. The thinning of this unit around and over the Ein Gedi Diapir is an exception. The distribution o f t h e sediments above the 'red' marker in the northern part of Profile C as welt as in Profiles B, 3 and 3--4 indicates that the Jordan Delta has only started to accumulate since deposition of this marker. Fig. 6 represents a sequence o f six latitudinal (east--west) profiles arranged in order from north t o south: Profiles 5, 8, 9, 10 + 1 1 , 1 4 and 16. In these profiles, the following features are noticed: (1) Diapiric activity, probably associated with one of the main western fault escarpments o f the graben, which extend along the b o t t o m o f the western flank o f the northern basin, is expressed in Profiles 5, 9, and 14. (2) In Profile 8, along the same flank of the western basin, two eastward stepping faults were observed, instead of diapiric features. (3) On the same western flank m e n t i o n e d above but somewhat further to the west, a series o f more subdued diapirs are observed in Profiles 9, 14 and 16. (4) In Profiles 14 and 16, as well as 13 and 15, the progressive development of the Ein Gedi Diapir from north to south can be followed. This diapir is located east o f the diapirs described in (1) above, and its apex is observed in Profile 16. (5) The development o f the deep Arnon Sink can be seen in the E--W Profiles 10 + 11, 14 and 16, as well as in Profiles 13 and 15. Younger sediments have accumulated in the A m o n Sink contemporaneously with the development o f the Ein Gedi Diapir. The abruptness with which the Arnon Sink begins is accentuated by the fact that there is no trace of the sink in Profile 9 which lies very close to Profile 11. (6) Profile 11 best shows that the 'red', 'blue' and 'green' markers can be easily traced and correlated from the deep basin, across the western basin flank to the narrow shelf margin. In this profile, where diapirism is almost absent, it is clearly shown that all the stratigraphic units b o u n d e d b y these markers drastically thin westward from the deep basin toward the present shoreline. (7) Development of diapirs was also noticed close to the eastern main border fault as can be observed in Profiles 5, 9 and especially 10. (8) Two small needlelike diapirs, herewith named the KaUia Diapirs, were observed in the western segments o f Profiles 1 and 2 in the shallow margin between the Jordan Delta and Kallia in the northwest c o m e r o f the Dead Sea. According to Ben-Avraham (personal communication) a significant heat
225
flow anomaly was observed on t o p of these diapirs. Fig. 7 shows longitudinal profiles D and I which together extend the full distance from Kallia in the north to midway down the southern basin. Profile D extends along the lower part of the western flank of the northern basin and Profile I crosses the Lynch Straits into the southern basin where it runs along the edge of the b o u n d a r y dyke of the northern evaporating pans (see Track Map in Fig. 1). The following features can be noted in these profiles:
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Fig. 7. Two longitudinal profiles along the western margin of the Dead Sea. North is on the left. Fig. 7a: The s u b b o t t o m reflectors indicate that the origin of the rugged topography is both erosionally and diapirically influenced (also noticeable in the northern half of Profile I -- Fig. 7b). Fig. 7b: Note the difference in the acoustic characteristics of the southern basin (dominant reverberations in the skins between the deep diapirs) and adjacent to the Lisan Peninsula. The boundary between the two provinces is fault controlled and is noticed close to the time mark for 1140 (about ¼ of the distance from the right end of Profile I).
226 (1) The different strati~aphic units are much thinner in these profiles compared with those in Profiles A and C. The ' b r o w n ' marker is found relatively close to the sea floor. (2) Diapiric structures are more abundant along Profile D than along Profile C. (3) No diapirs were detected in the northern half o f Profile I, i.e. from the northern b o u n d a r y of the southern basin through the Lynch Straits and Channel up to the northern end o f the profile. (4) There is a p r o n o u n c e d change in the seismic reflection characteristics along the northern segment of Profile I. North of 80N (opposite Massada at the point where the northern basin begins to deepen, or approximately at 0830 Hours on Profile I or at 1625 Hours on Profile L in Fig. 8), the sediments are acoustically transparent, while south of this point and extending to the narrowest part o f t h e S t m i ~ (corresponding to 1140 Hours on Profile I) there is a thick sequence o f highly reflective horizons. It appears that the acoustic transparency o f the northern segment does n o t reflect the diapiric salt b o d y although the exact lithologic nature is n o t y e t clear. It is possible that the d i f f e r ~ c e is due t o lateral facies changes or to an angular unconformable contact between t w o units. (5) The 'red' reflectors in Profiles D and I (Fig. 7), as well as in L and 22 of Fig. 8 represent an erosional morphology. In Profile D this morphology was additionally affected b y diapirism. (6) The transition between the Lisan Peninsula and the southern basin (1140 Hours on Profile I) is fault controlled. South of this point abundant reverberations make the interpretation harder, but indicate a different sedimentary sequence. Despite these difficulties, t w o diapirs, separated by deep sediment-filled sinks, were observed in Profile I at 1205 and 1240 respectively. Two additional diapirs were also observed in Profile J which parallels Profile I in the southern basin. Again deep sinks filled b y a thick sequence of y o u n g sediments were observed adjacent to the diapirs. In Figure 8, north--south Profile L and east--west Profile 22 are presented. Profile L shows the sharp transition from the northern basin, which is characterized b y intensive deposition, to the Lisan Peninsula, with its acoustically transparent nature. It seems that this sharp transition expresses faulting with an appreciable a m o u n t o f throw. Profile 22, which crosses the lower part o f Lynch Channel from west to east as well as the northern projection o f the Lisan Peninsula, shows the following: (1) a thick, post-erosional (post-'red') sedimentary fill at the trough of the Lynch Channel; (2) a relatively small, north-northeast trending fault along the eastern flank of the Lynch Channel; and (3) it is possible that t w o diapirs are found in the northern projection o f the Lisan Peninsula.
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Fig. 8. Profile L is a l o n g i t u d i n a l profile ( N o r t h to t h e left) a l o n g t h e w e s t e r n f l a n k s o f t h e Lisan P e n i n s u l a . N o t e t h e t r a n s i t i o n f r o m t h e flat d e p o s i t i o n a l - b o t t o m a d j a c e n t t o t h e Lisan P e n i n s u l a t o t h e d e e p l y incised e r o s i o n c h a n n e l s a n d t h e fault c o n t r o l l e d t r a n s i t i o n t o t h e d e e p f l a t - b o t t o m e d n o r t h e r n basin. Profile 22 is a west t o east profile t h r o u g h t h e e r o s i o n a l L y n c h C h a n n e l w h i c h is p a r t l y filled b y y o u n g e r ( H o l o c e n e ) s e d i m e n t s . E a s t is o n t h e right. A small ( p r e - e r o s i o n a l ) f a u l t is n o t i c e d a l o n g t h e e a s t e r n slope o f t h e channel.
228 Isopach and structural maps
Using data on the b a t h y m e t r y as well as the elapsed travel time to the four markers mentioned above, isopach and structural contour maps were prepared. The former express the horizontal distribution o f thickness of each of the units, while the structural contour maps represent the m o r p h o l o g y of the markers themselves. In the absence of detailed interval velocities, all thicknesses and depths are expressed in milliseconds o f round trip travel time (in this case assumed to be roughly equivalent to meters). Generally speaking the isopach maps are more expressive than the structural maps in describing the different features which were developed during deposition of that unit. All the maps are presented in Neev and Hall (1976, Appendix 3) while maps o f special interest are presented herewith and described in detail in the text below. Red-Brown isopach map and Brown structural map. As noted above, the 'brown' marker represents the t o p of the Sedom Formation which is composed mostly of rock salt (Zak, 1967). In our seismic profiles the contact between this formation and the overlying units is unconformable and intrusive in nature. The isopach map could be expected to describe the cumulative effect o f the diapiric activity atl through the time during which the overlying units were deposited. It is probable that even greater thicknesses of sediment than are shown in the isopach map were affected b y the diapirism as the seismic reading technique was unable t o see deeper than 1 sec. The Red--Brown interval is shown for illustrative purposes instead of the B o t t o m - - B r o w n interval, because the volume o f the Jordan Delta sediments deposited in post-Red time mask and distort features which were developed prior to Red time. The following features are shown in Fig. 9: (1) A chain o f diapirs protrudes along the contact between the deep floor o f the northern basin and its western flank. This contact probably coincides with a major border fault. A continuation o f this trend toward Mount Sedom is to be expected. In the northern basin additional b u t smaller diapirs were observed on the basin flank west of this line. (2) Two big diapirs are f o u n d east o f this fault line: one is the large Jordan Delta Diapir centered on 128N 203E (Israel Grid) in the norteastern part of the Jordan Delta, and the other is the Ein Gedi Diapir at 96N 195E adjacent to Ein Gedi. These t w o diapirs are surrounded b y pronounced ringsynclines, known in other parts o f the world to be associated with salt domes which are usually n o t bounded b y faults. Other small diapirs are also found along this zone (see also Fig. 12). The deep El Lisan No. I wildcat driiling (Neev and Emery, 1967) may be related to the same group. Another diapir in this group is probably located SE o f Mount Sedom as indicated b y electrical resistivity measurements carried o u t b y the Institute o f Petroleum Research and Geophysics (A. Leva-
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Fig. 9. An isopaeh map of the interval between the Red (base Holoeene) and Brown (top o f the intruded salt of the Sedom Formation
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230 non, personal communication). The diapirs found in Profiles I a n d J in the southern basin are also p r o b a b l y related to the 'trough' chain of diapirs. (3) At least four small diapirs shown on this map also protrude along the trace of the steep eastern border fault of the northern basin. (4) Two WNW trending faults were observed to b o u n d the Lisan Peninsula on the north and south, in agreement with the more speculative scheme proposed b y Neev and Emery (1967). However the fault which they indicated marking the northern limit o f the Dead Sea was definitely not observed in the results of the present survey. Red-Blue isopach map. This map shows t h e following features: (1) Extension o f the piercement process of the Jordan Delta Diapir during that time interval. It must be emphasized that the northward thinrdng o f this unit from the center of the northern basin toward the Jordan River outlet begins a b o u t 13 km south of the rivermouth, approximately opposite Zerka Mayin. (2) Extension o f the diapir piercement process along the b o t t o m of the westward flank o f the northern basin. (3) Continuation of the piercement process of the Ein Gedi Diapir contemporaneously with a very intensive phase of subsidence of the Arnon Sink. During this interval a sequence of a b o u t 450 msec ( ~ 4 5 0 m) of sediments accumulated in this sink. Together with an additional thickness of 200 msec ( 2 2 0 0 m) which accumulated during the preceding interval (Blue to Green -see Appendix 3 of Neev and Hall, 1976) at least 650 msec of sediments were accumulated here. The Arnon Sink, together with other adjacent structural lows, makes an irregular ring syncline around the Ein Gedi Diapir. As noted above (under 'Seismic profiles'), the rate o f sedimentation close to the end of the Red--Blue time interval exceeded the rate of diapir piercement, thereby camouflaging most o f the diapiric activity. This conclusion applies to the deeper parts of the basin (the present flat b o t t o m ) b u t not to its flanks. A north--south trending apparent ridge (area of low sediment accumulation) is expressed in this isopach map, between Cape Costigan on the Lisan Peninsula and the western shoreline of the Dead Sea. Examination of Profile 22 (Fig. 8), which transects this ridge from west to east, shows that this ridge is n o t a diapir b u t rather an erosional channel. Due to the erosional process a b o u t 100 msec of sediments are absent from the trough as compared with its flanks on both the east and west. It is assumed that the erosional process t o o k place toward the end of the Red--Blue interval, i.e. toward the end of the phase when intensified sedimentation occurred in the deeper part of the basin. B o t t o m - - R e d isopach map. The B o t t o m to Red isopach map illustrates the following: (1) Along the western flank o f the northern basin the sediments of this unit have filled up an erosional drainage system. North of Ein Gedi channels
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Fig. 10. An isopach map o f the interval b e t w e e n the Red (base H o l o c e n e ) and Blue ( w i t h i n the Lisan F o r m a t i o n - - Late to mid-Pleistoc e n e ) reflectors. These data s h o w the e x t r e m e rates o f s u b s i d e n c e o f the A r n o n Sink and the c o n t e m p o r a n e o u s diapiric u p l i f t i n g o f the Ein Gedi salt d o m e . The thinned strip b e t w e e n the Lisan Peninsula and the plain o f Massada is o f erosional origin. N o e x c e s s i v e accum u l a t i o n o f s e d i m e n t s is n o t i c e d in this interval in the area o f the Jordan Delta.
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Fig. 11. An isopach map of the interval between the Bottom and the Red (base Holocene) reflectors. The map shows (1) the drainage pattern which was entrenched along the western and southern slopes of the northern basin and (2) the submerged Jordan Delta which has been accumulated only during the Holocene.
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233
of this system trend approximately west to east while from Ein Gedi southward to the Lynch Straits there is one main channel plunging northward, which is joined by secondary channels coming mostly from the west. This main channel, expressed in the red--blue isopach map (Fig. 10) as a 'ridge' or deficiency in sediment thickness, is due to erosion which t o o k place at the end of this interval. The southern half of this drainage system (south of Ein Gedi) was interpreted in this w a y b y Neev and Emery (1967, fig. 8) on the basis of the bathymetric contours. On the other hand, the segment north of Ein Gedi has been almost completely camouflaged by the more intensive younger deposits (probably due to its closer proximity to the Jordan outlet). The depth of the channels in this segment is between 20 and 40 msec as judged by the difference in thickness of this unit above the troughs of these channels (60--80 msec) and above the ridges which separate the channels (30--40 msec). No trace of similar erosional activity was n o t e d in the deeper parts of the basin. Another erosional channel, partly filled by sediments of this unit, was discovered to the east of Cape Costigan, between the Lisan Peninsula and the eastern border fault escarpment, and is herewith named Dale Channel in honor of Lt. John B. Dale, USN. Lt. Dale, who was secondin-command of the U.S. expedition to the River Jordan and the Dead Sea, and who explored the area of the Dale Channel with Lt. W.F. Lynch on April 30, 1848, died on July 24, 1848 of fever contracted during the expedition (Lynch, 1849). (2) Although it is possible that during the period represented by ' b o t t o m - red' sedimentation, the diapirism was more mild in comparison with the previous period (red--blue), its effects are noticeable in influencing and directing the trends of the erosional drainage system. Diapiric piercement activity occurred during this period in the Ein Gedi Diapir and also in the southern basin diapirs. (3) The sediments of the submarine Jordan River Delta were only accumulated following deposition of the red marker. The delta consists of a wedgeshaped block approximately 13 km long by 8 km wide, whose thickness ranges from a b o u t 100 to 330 msec. The maximum thickness of this wedge by the present Jordan River outlet is a b o u t 300 msec. (4) South of the Jordan Delta, within the deep northern basin, the thickness of this unit is rather uniform: a b o u t 90--100 msec with local thickening of up to 130 msec. (5) The volume of sediments in this unit in the northern basin was calculated b y numerically integrating the area within the various contours times the thickness in meters assuming a sound velocity within the unit of 2000 m/sec (1 msec = 1 m), using the methods outlined in Hall (1976). Approximately 49 cubic kilometers of sediment are present in this unit, of which some 16 cubic kilometers comprise the wedge-shaped delta.
234 G E N E R A L DISCUSSION AND CONCLUSIONS
Stratigraphy and geological history The red marker appears t o express the beginning o f the present Dead Sea. Prior to, and close to this time, two different processes t o o k place: the first was the very intensive deposition o f t h e sediments in the central parts of the northern basin, and the second was intensive erosion which dissected the basin flanks. It is considered likely that due to the extreme shrinkage of Lake Lisan which almost reached the point of complete desiccation and marked the end of the Lisan period, most o f the dissolved salts were precipitated in the lowest part of the basin.'In addition, an u n k n o w n quantity of sediment was eroded and transported to this part of the lake during its shrinkage. It is assumed that these processes ended approximately 13,000 years ago, since according to Th--U and 14C m e t h o d s the youngest Lisan sediments in the Dead Sea area on the Amatsyahu and Amiaz Plateaus (south and west o f Mount Sedom respectively) range from 13,000 to 17,000 years in age (Kaufman, 1971 ). It is evident that the climate during this transitional period was extremely dry. The sediments o f the Jordan Delta and the rest of the unit above the red marker were therefore accumulated and deposited during the younger pluvial period which followed. This makes it possible to calculate approximate rates of deposition of 30 m / 1 0 0 0 years and 10 m/ 1000 years for the Jordan Delta proper and the floor of the deep basin respectively. The salts dissolved in the present Dead Sea water were mainly derived from t w o sources: (1) The end brines which remained o n the b o t t o m of the lake in solution when it had been complete desiccated, due to their high hygroscopy and to the fact that some drainage always occurred into this terminal lake. These end brines were mixed with the fresh water which entered during the subsequent pluvial period. (2) The Lisan Lake brines which penetrated into the adjacent mountainous reservoir rocks and which were later displaced and expelled b y the rising water tables which accompanied the more pluvial period. In view o f the physical relationship between the red, blue and green markers (mostly conformable contacts, at least in the deep parts of the northern basin) we assume that the blue marker was deposited sometime within the Lisan F o r m a t i o n and that the green marker occurs within one of the upper m e m b e r s o f the Amora (Foothills} Formation. As n o t e d above, t h e brown marker expresses the unconformable and intrusional c o n t a c t between the rock salt Sedom Formation and the overlying younger formations.
Structure The data on which Fig. 9 (tectonic element sketch map) is based are as follows:
~'ig. 12. A sketch map of the tectonic elements and physiographic features in the Dead Sea region: Fault systems, salt domes, the krnon Sink, the Jordan Delta, and the Lynch and Dale Channels.
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236
( 1 ) t h e physiographic features as expressed in the bathymetric map (Fig. 2), the adjacent topographic maps, as well as air photographs and satellite imagery; (2) the stratigraphic--structural relationships as expressed in the seismic profiles and the isopach and structural contour maps. Three groups of faults are identified in and around the Dead Sea: (1) The Eastern Border Faults. (2) The Western Border Faults. (3) The WNW trending faults within the graben. The latter group of faults b o u n d the horst of the Lisan Peninsula on the north and south, and also t h e southern Dead Sea Basin on its southern side (The Amatsyahu F a u l t - Neev and Emery, 1967). The northwest trending graben o f Kerak (Transjordan) m a y be genetically related to these faults. This m a y therefore be another argument in favor of the northward shift due to the sinistral m o v e m e n t along t h e eastern border faults. The Western Border Faults form a system of step faults and step blocks. Several deviations from the main fault on land (the westernmost one) were noted in the form o f crescentic faults trending b o t h to the northwest and southwest (Begin, 1975; Kashai, 1976). Along the easternmost fault o f this western system, many salt diapirs pierce upward. It appears that Mount Sedom is one of the diapirs along this fault although the fault segment between Mount Sedom and the northern basin is not always identified. South o f Mount Sedom this fault was observed b y Neev and Emery (1967) to extend as far south as the Amatsyahu Fault. Another fault in this system, which is of special interest, is identified mostly on the basis of b a t h y m e t r y ; it extends diagonally NNE from o f f Ein Gedi in the south, toward Kallia in the north. According to airphotos, satellite imagery and field observations, this fault continues in the same direction into the lower Jordan Valley where it joins the Eastern Border Fault. In this w a y it forms a fault-bounded, wedge shaped block enclosing the northern end o f t h e Dead Sea. The Eastern Border Faults form a more systematic group than the Western Border Fault system in that NNE to NE trending faults systematically branch o u t from the main border fault. Some o f these faults marked on our sketch map are newly identified. T h e y were traced from air photos, satellite imagery as well as b y direct observations from the sea. It is possible that in places the vertical displacement along these branching faults is small to absent * The northward extension of the westernmost fault o f the Eastern Border * See following LANDSAT imagery: (1) Image 81054074215G0, 15 September lg72, MSS Band 5. (2) Image 81162074255G2, 1 January 1973, False Color Composite. (3) Image 81342074245A0, 30 June 1973, MSS Band 6. (4) Image 81145074305G2, 14 December 1972, False Color Composite.
237 Fault System is hypothetical. It extends from the southern basin through the topographic trough of E1 Mezra'ah (NE part of the Lisan Peninsula), continues through the Dale Channel and the structural trough along the northern basin (including the Arnon Sink), and leaves the Dead Sea via the Jordan Delta Diapir.
Basement tectonic features From the total intensity magnetic anomaly map two inferences regarding the deep basement configuration may be drawn: (1) The anomaly map shows a pronounced eastward gradient of approximately 20 gammas per kilometer which suggests a change in regional crustal characteristics. Consideration of the known structural movements along the Jordan Rift would suggest that this magnetic gradient may reflect a transition from a less continental type of crust on the west to a more continental type on the east beyond the Eastern Border Fault system of the graben. This idea is in agreement with the conclusion of Knopoff and Belshe (1965) based upon their compilation of gravity and magnetic measurements across the graben to the north. We do not believe that this gradient can reflect a change in the depth to the basement as three very close approaches of the magnetometer to within tens of meters of the Eastern Border Fault where the basement is known to be very close to the surface resulted in a decrease rather than an increase in the field. (2) The east--west trending negative magnetic anomaly located between Ein Gedi and Mizpe Shalem may be an extension of another predominant magnetic anomaly of similar trend, crossing the Hebron Mountains between the Dead Sea and Gaza, Which is found in the aeromagnetic map of Folkman and Yuval (1976). ACKNOWLEDGEMENTS The authors gratefully acknowledge the enthusiastic assistance and encouragement given to this work by our many colleagues from a number of different institutions. The Dead Sea Works Limited provided manifold logistical and financial support. The Mapping, Research, and Photogrammetry Divisions of the Survey of Israel, Ministry of Labour, provided assistance in preparing the positioning stations. The Israel Oceanographic and Limnological Research Limited provided service personnel and equipment, and other equipment was lent by the UN/UNDP-GSI Offshore Dredging Project and by the Steinitz Laboratory of the Hebrew University, and by Sherman Engineering Co. Hospitality was provided by the Youth Hostel at Ein Gedi, the National Parks Authority, and by the Israel Defense Forces. In addition, our colleagues from the Marine Geology Division of the Geological Survey of Israel provided valuable help during all stages of the work. We gratefully thank Dr. Alexander Klung of Oil Exploration (Investments)
238 Ltd. f o r giving a p r e v i e w o f r e c e n t l y a c q u i r e d m u l t i c h a n n e l seismic d a t a f r o m t h e western p a r t o f t h e area p r e s e n t l y investigated. This d a t a r e i n f o r c e d s o m e o f o u r c o n c l u s i o n s d r a w n f r o m o u r o w n r e v e r b e r a t i o n - l a d e n shallow w a t e r data. This w o r k was f u n d e d b y C o n t r a c t 2 9 0 9 0 with t h e D e a d Sea Works Ltd. REFERENCES Allan, T.D. and Morelli, C,, 1970. The Red Sea. In: A.E. Maxwell (Editor), The Sea, IV (part 2). New York, N.Y., pp. 493--542. Begin, Z.B., 1975. Structural and lithological constraints on stream profiles in the Dead Sea region, Israel. J. Geol, 83 (1): 97--111. Domzalski, W., 1967. Aeromagnetic survey of Israel--Interpretation. Inst. Pet. Res. Geophys. Rep., No. SMA/482/67, December 1967, 66 pp. Fabiano, E.Bj and Peddie, N.W., 1969. Grid Values of Total Magnetic Intensity IGRF1965. U.S. Dept. of Commerce, Environmental Science Services Administration Technical Report C & G S -- 38. Fabiano, E.B. and Peddle, N.W., 1975. U.S. World Chart Coefficients for 1975: E.O.S. Trans. Am. Geophys. Union, 56 (9): 587--588. Folkman, Y. and Yuval, A., 1976. Aeromagnetic Map, 2 sheets on Geological Map o f Israel (1 : 250,000), 1 Sheet of Offshore Israel (1 : 250,000). IPGR, May 1976, The Survey of Israel, Tel Aviv. Hall, J.K., 1976. Algorithms and programs for the rapid computation of area and center of mass. Comput. Geosci., 1 : 203--205. Hall, J.K. and Neev, D., 1975. The Dead Sea Geophysical Survey 19 July--1 August 1974, Final Report No. 1, GSI-MGD Internal Report No. 2/75, December 1975, 27 pp. Kashai, E., 1976. The 'Ami'az Oil Prospect, Oil Exploration (Investments) Ltd, Tel Aviv, 22 pp. Kaufman, A., 1971. U-Series dating of Dead Sea Basin carbonates. Geochim. Cosmochim. Acta, 35: 1269,1281. Klein, C., 1961. On the fluctuations of the level of the Dead Sea since the beginning of the 19th century. Ministry of Agriculture Hydrological Service, Israell Hydrological Paper No. 7, pp. 1--73. Knopoff, L, and Belshe, J.C., 1965. Gravity observations of the Dead Sea Rift: In: The World Rift System, a Symposium. Geol. Surv. Can. Pap., 66--14, pp. 5--21. Lynch, W.F., 1849. Narrative of the United States' Expedition to the River Jordan and the Dead Sea. Lea and Blan~ard, Philadelphia, Pa., 508 pp. Neev, D. and Emery, K.O., 1967. The Dead Sea: Depositional Processes and Environments of Evaporites, Geol, Surv. Isr., Bull. No. 41,147 pp. Neev, D. and Hall, J.K., 1976. The Dead Sea Geophysical Survey 19 July--1 April 1974. Final Report No. 2, GSI-MGD Internal Report No; 6/76, September 1976, 21 pp. Zak, I., 1967. The GeolOgy o f Mount Sedom. Thesis, the Hebrew University, Jerusalem, 208 pp. (unpublished, in Hebrew with English abstract).
209
GEOPHYSICAL INVESTIGATIONS IN THE D E A D SEA
D A V I D N E E V a n d J O H N K. H A L L
Marine Geology Division, Geological Survey of Israel, Jerusalem 95 501 (Israel) ( R e c e i v e d D e c e m b e r 15, 1 9 7 8 )
ABSTRACT Neev, D. a n d Hall, J.K., 1 9 7 9 . G e o p h y s i c a l i n v e s t i g a t i o n s in t h e Dead Sea. S e d i m e n t . Geol., 23: 2 0 9 - - 2 3 8 . B a t h y m e t r i c , m a g n e t i c , a n d c o n t i n u o u s seismic m e a s u r e m e n t s were m a d e a l o n g a p p r o x i m a t e l y 5 4 0 k m o f t r a c k in t h e D e a d Sea in J u l y a n d A u g u s t o f 1 9 7 4 . A n e w p r e c i s i o n b a t h y m e t r i c m a p was p r e p a r e d , using t h e s e m e a s u r e m e n t s , t o g e t h e r w i t h all earlier s o u n d i n g s . T h e n o r t h e r n b a s i n is s h o w n t o b e s h a l l o w e r t h a n previously t h o u g h t , a n d a l m o s t flat b o t t o m e d o v e r n e a r l y 40% o f its area. T h e m a x i m u m d e p t h , relative t o M e d i t e r r a n e a n sea level, is 7 3 0 m (i.e. a w a t e r d e p t h o f 3 3 2 m b e l o w t h e D e a d Sea level at t h e t i m e o f t h e survey), r a t h e r t h a n t h e 7 9 8 m r e p o r t e d for Lt. L y n c h ' s 1 8 4 8 survey. T h e basin is a s y m m e t r i c , w i t h w e s t e r n slopes generally averaging 7 ° a n d e a s t e r n slopes averaging 30 ° e x c e p t in t h e v i c i n i t y o f t h e A r n o n R i v e r d e l t a w h e r e slopes are a r o u n d 8 ° . T h e w e s t e r n f l a n k o f t h e basin is q u i t e c o r r u g a t e d , a n d a l o n g t h e n o r t h w e s t e r n f l a n k o f t h e b a s i n t h e t o p o g r a p h y b e c o m e s s t e e p e r , f o l l o w i n g a linear t r e n d o f N 2 7 ° E w h i c h is p r o b a b l y fault cot{trolled. S e d i m e n t s f r o m t h e J o r d a n R i v e r d e l t a have b u i l t u p a s m o o t h a n d a l m o s t p l a n a r r a m p sloping 3 ° to d e p t h s of 6 5 0 m ( b e l o w MSL), a n d 1 ° t h e r e a f t e r i n t o t h e d e e p basin. Revised h y p s o m e t r i c c a l c u l a t i o n s s h o w i n f l e c t i o n p o i n t s in t h e area c u r v e at 425 a n d 7 2 5 m, r e f l e c t i n g t h e f l a t - f l o o r e d n a t u r e o f t h e n o r t h e r n b a s i n . T h e v o l u m e o f w a t e r e n c l o s e d w i t h sea level at 4 0 0 m is 147 k m 3, a n increase over earlier estimates. T h e t o t a l field m a g n e t i c a n o m a l y m a p w i t h 5 g a m m a c o n t o u r i n t e r v a l s h o w s several i n t e r e s t i n g f e a t u r e s , t h e a n o m a l o u s field has a n e a s t w a r d dip w i t h g r a d i e n t s o f 5 t o 20 g a m m a s per k i l o m e t e r . M a g n e t i c a n o m a l i e s ( 1 0 to 20 g a m m a a m p l i t u d e ) were d e t e c t e d east o f E i n Gedi, Kallia, a n d t h e J o r d a n R i v e r d e l t a as well as in o t h e r l o c a t i o n s . T h e E--W t r e n d i n g m a g n e t i c a n o m a l y n o r t h o f E i n Gedi m a y b e a n e x t e n s i o n o f a similarly t r e n d i n g m a g n e t i c a n o m a l y n o t e d b y F o l k m a n a n d Yuval ( 1 9 7 6 ) a c r o s s t h e H e b r o n m o u n t a i n s , while t h e p r o n o u n c e d e a s t w a r d m a g n e t i c g r a d i e n t m a y reflect a t r a n s i t i o n f r o m a less c o n t i n e n t a l t y p e o f crust o n t h e west t o a m o r e c o n t i n e n t a l t y p e o f c r u s t east o f t h e Rift. Diapiric s t r u c t u r e s i n d u c e d b y t h e r o c k salt masses o f t h e S e d o m F o r m a t i o n were d e t e c t e d all over t h e n o r t h e r n basin. T h e y are m a i n l y c o n c e n t r a t e d a l o n g t h e m a j o r w e s t e r n s u b m a r i n e N--S t r e n d i n g b o r d e r fault, a n d are p r o b a b l y g e n e t i c a l l y a s s o c i a t e d w i t h Mt. S e d o m . Similar s t r u c t u r e s , a l t h o u g h smaller in size a n d fewer, are also f o u n d a l o n g t h e s t e e p e a s t e r n b o r d e r fault. T w o large, d o m e - s h a p e d d i a p i r i c s t r u c t u r e s have pierced t h r o u g h t h e b o t t o m o f t h e m e d i a n t r o u g h : o n e o f f E i n G e d i ( t h e Ein G e d i Diapir) a n d t h e o t h e r at t h e n o r t h e a s t e r n s e g m e n t o f t h e J o r d a n D e l t a ( t h e J o r d a n D e l t a Diapir). T h e y s e e m t o be g e n e t i c a l l y r e l a t e d t o t h e d i a p i r o f t h e Lisan P e n i n s u l a a n d t o o t h e r s
210 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 c o m prise, 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.
INTRODUCTION
AND PURPOSE
Despite its historical importance, the Dead Sea has been the subject of relatively few marine geological investigations. The first serious survey of the Dead Sea floor was undertaken in 1848 by a United States Navy expedition under the command of Lt. W.F. Lynch {1849). This survey showed the northern basin to be deep and the southern basin to be shallow, and defined the bathymetry for almost all subsequent maps. Neev and Emery (1967) carried out the first modern investigation of the marine geology using direct sampling techniques and echosoundings, but unfortunately their work was constrained to the southwestern quadrant of the Dead Sea. In an effort to enrich this body of knowledge, a cooperative multi
AND ANALYSIS
The survey was carried out from the R /V 'Barcelona', a self-propelled barge provided by the Dead Sea Works Ltd. A brief narrative of the survey, which was marred on the night of 23 July 1974 when the 'Barcelona' {with
_
DEAD
S E A GEOPHYSICAL S U R V E Y
~ig. 1. Profile location map and track chart for the Dead Sea Geophysical Survey of 19 July--1 August 1974. The track is based u p o n ligh resolution electronic Miniranger navigation f r o m precisely surveyed shore-based transponders. T i m e s are a n n o t a t e d every 10 minltes and m a r k e d every 5 m i n u t e s for c o o r d i n a t i o n with the t i m e scales printed above the seismic r e f l e c t i o n profiles. East--west latitutinal profiles have been n u m b e r e d sequentially f r o m 1 to 25, while n o r t h - - s o u t h longitudinal profiles are s e q u e n c e d a l p h a b e t i c a l l y 'tom A to L. All profiles are o r i e n t e d so that south or east is on the right, depending u p o n the o r i e n t a t i o n . Projection is on the local :srael (Cassini) grid, which is in kilometers. Wells are m a r k e d , with total depths. A total of 540 k m o f track is shown.
s~----~.~
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212 its equipment) broached to, and was beached and flooded during a strong windstorm, can be found in an earlier report which also contains extensive details concerning the survey methods and data analysis (Hall and Neev, 1975).
Navigation A Motorola Miniranger ® navigation system was used for positioning throughout the survey, Shorebased transponders were sited at a t o t a l of eight locations to provide accurate navigation at almost all of the areas surveyed. Range measurements were recorded twice each minute, and the positioning errors incurred, at the plotting scales used, are completely negligible.
Depth soundings Depth soundings were made with an ELAC portable echosounder. The sounding records were digitized at all slope changes, and a total of 5148 digitized soundings were used in the preparation o f the new bathymetric map (Fig. 2). The value o f the Dead Sea level at the time of the survey was --398.04 m relative to Mediterranean sea level. The echosounder was calibrated for the velocity of sound in the Dead Sea b y making recordings at three locations over the flat-bottomed deep basin at which Neev and Emery had raised cores and accurately measured the depth with a taut vertical coring wire. These calibrations show the round trip sound velocity over the entire water column to be 885.3 m/sec or a b o u t 18% higher than that found in the world oceans.
Continuous seismic reflection profiling Continuous seismic reflection measurements were made with a 1000 joule (watt-second) E.G.&G. Sparker sound source and an eel-type 150 element h y d r o p h o n e array, A measurement was made every 3 sec, with the negative signal phase printed o u t on a 1 sec sweep of a 19" dry-paper recorder. In shallow water the source and receiver were suspended from plastic jerricans to prevent contact with the rough sea floor. The records were interpreted, and then the selected s u b b o t t o m reflectors were digitized and a c o m p u t e r program used to arrive at the data necessary to prepare isopach and structural contour maps for these horizons.
Total field magnetic measurements Total field magnetic measurements were made every 6 see with a Varian direct-reading p r o t o n precession magnetometer. For technical reasons it was not possible to m a k e measurements on three of the eight days of work. The records were smoothed, read every 5 minutes, and then corrected for diurnal
213
variations measured at the Modi'im Forest Magnetic Observatory near Lod. Of the five days during which measurements were made, two could be considered ' s t o r m y ' with m a x i m u m corrections amounting to 72 and 28 gammas. Anomalies were then c o m p u t e d by subtracting the 1965.0 IGRF (Fabiano and Peddie, 1969), which at the Dead Sea surface indicates a regional field increasing in the direction of N20°E at the rate of about 7.8 gammas per kilometer. The I G R F is about 9 gammas higher at --397 m than it is at Mediterranean sea level. Using this reference field all the anomalies are negative. Subsequenct calculations using the updated degree 12 coefficients for epoch 1975.0 (Fabiano and Peddie, 1975) show the updated reference field for the time of the survey to be 275 gammas lower. This gives an identical anomaly pattern but with positive anomalies only. When the anomalies were plotted out, the standard deviation of the differences at the 31 track crossings for which data were available was 8 gammas. In order to present the data at the desired 5-gamma contour interval, further adjustments were made to remove these differences. Considerable confidence m a y be placed in the final result as the required corrections are smooth and well-behaved, suggesting that the diurnal corrections account for the bulk of the short-period temporal changes. RESULTS
Bathymetric mapping Fig. 2 shows the bathymetric map prepared from the new survey data. In this map depths are referenced to Mediterranean mean sea level (MSL) rather than to the variable Dead Sea sea level. A contour interval of 10 m is used except for the shallow regions with depths of less than 410 m, and the 'abyssal' regions with depths greater than 710 m, where a 1-m contour is used. The data from this survey is supplemented by the original soundings of Neev and Emery (1967), which were taken on a 1-kin grid spacing south and east of Ein Gedi, and by the soundings of the American Expedition to the Dead Sea in 1848 (Lynch, 1849). Lynch's soundings, while incorrect (see following section), provided useful information in the areas adjacent to Ain Suweimet, Zerka Mayin, the Arnon River and E1 Mezra'ah, all of which are in parts of the eastern coast which were not covered. Several features are immediately apparent from the map which are absent, or at least not readily apparent, in the earlier map of Neev and Emery (1967). (1) There is a large difference in the slopes on either side of the Dead Sea basin. On the eastern side slopes of 30 ° over a depth range of 200 m are encountered, while the slopes on the western side reach a m a x i m u m of 25 ° in only two places, over a depth range of 50 m, and slopes more generally average 7 ° . Fig. 6a illustrates this difference.
Fig. 2. Bathymetric chart of the Dead Sea. Note: (1) the flat-bottomed box-shaped configuration of the northern basin, whose eastern, western and southern margins are fault controlled, (2) the gradual slope of the Jordan River delta, and (3) the submerged drainage system on the western and southern margins which is partially filled by younger sediments. The contour interval is 10 m, with 1-m co n t o u r between the shoreline and 410 m, and below 710 m. All contours are relative to Mediterranean sea level. The Dead Sea level at the time of this survey was --398 m MSL. Maximum depth is 730 m.
b~
215 (2) The maximum depths are f o u n d along a longitudinal 'trough' which runs north--south adjacent to the eastern margin of the deep northern basin. This trough consists of a slight depression of the otherwise flat floor of the basin, which appears to result from subsidence (Fig. 6b, Profile 9--9). (3) Along the northwestern side of the northern basin, the topography shows a linear trend of a b o u t N27°E which is probably fault controlled. (4) The western side shows the effects of an earlier drainage pattern to depths of at least 700 m MSL. (5) The maximum depth is in the order of 730 m, or 332 m below the Dead Sea level of 1974, rather than the value of 798 m which is obtained from Lynch's (1849) m a x i m u m of 398 m below the Dead Sea level of 1848 (Klein, 1961). A further finding of this survey, using precision navigation, was that depths observed along one profile do not always agree with depths measured along an intersecting profile at the point of intersection. In the preparation of the final bathymetric map this was primarily seen over the deep basin, where differences sometimes approached 6--8 m. The reason for these discrepancies is still u n k n o w n b u t it cannot be a result of either erroneous navigation or recorder error, for the navigation is self-consistent (identical transponders), the basinal slopes are negligible, and the differences are also observed on the seismic recorder whose time base is stable to about 1 m in 10,000. Short-term hydrographic charges are suspected.
The deepest part. There appear to be t w o 'deepest' parts, lying along a longitudinal trough in the eastern part of the basin which parallels the 198.5E meridian of the Cassini (Israel) grid system. (1) The northern low appears to have depths o f - - 7 3 0 m over an elliptical area approximately 3.5 km long in the north--south direction (between 110.2 and 107.0N) and a b o u t 2 km wide in the east--west direction (between 197.5 and 199.5E). (2) The southern low appears to be slightly shallower, over 729 m but less than 730 m. It occupies a larger area of approximately 7.5 km length north-south (between 106.0 and 98.5N), and 2.5 km width east--west (between 197.3 and 199.5E). The deepest part is probably in the vicinity of 109.5N 198.5E, with a depth of slightly over 730 m. The accuracy of these measurements, which are subject to some of the errors mentioned above, is probably better than +-2m. These findings are in distinct disagreement with the lead-line soundings obtained b y Lt. W.F. Lynch USN in the 1848 American Expedition to the Dead Sea and the Jordan (Lynch, 1849). Lynch had t w o profiles crossing the deepest parts of the basin mentioned above. The first, taken on Thursday May 4, 1848, b y Lt. John B. Dale with the b o a t 'Fanny Skinner' from Wadi Arnon to tkin Ter~beh (near the present Mizpe Shalem) measured a maxim u m of 195 fathoms (--756 m MSL), and showed depths across the deepest
216 plain generally 24--27 m deeper than what was observed here. A second profile taken by Lt. Lynch himself with the 'Fanny Mason' on Friday May 5, 1848, between Zerqa Mayin and Ain Ter~beh (Mizpe Shalem) produced one sounding of 218 fathoms o r - - 7 9 8 m MSL (Lynch, 1849, pp. 373--374), 68 m deeper than our maximum depth, and a number of other soundings generally 15--20 m greater than our present results. What is the source of these discrepancies? That there was a hole some 70 m deeper which has been filled in the intervening 126 years is ruled o u t b y Neev and Emery's (1967) measured and extrapolated sedimentation rates, as well as by the stratigraphic nature o f the layers seen in the seismic profiles. Instead, it appears that these errors are a result of the rather rudimentary measuring technique used, namely that of lowering a lead weight by hand with a line. In order to avoid measuring a slant depth (due to boat drift or submarine currents), it is generally necessary to use a heavy weight on a very fine line or wire. However, since Lynch had no winch, b u t instead relied upon handling the line b y hand, it seems likely that a smaller weight and thicker line were used, and that there was probably some boat drift and wire angle at the time of measurement. In addition, it is quite possible that such a woven line contracted appreciably (perhaps 10%) due to the effects of the salt, and that the one measurement of --798 meters was a result of the weather that day, described (p. 374) as 'intensely hot, not a breath of air s t i r r i n g . . . ' u p o n the men in the unprotected skiff. Hypsometric curve
In order to s t u d y h o w the surface area and volume of the Dead Sea basin vary as a function o f depth, a hypsometric analysis was carried out. Selected contours in the new bathymetric chart in Fig. 2 were digitized and their areas c o m p u t e d b y a m e t h o d given by Hall (1976). Contours were chosen at the shoreline (--397 m), every 50 m between --450 and --700 m, and then every 5 m to - - 7 3 0 m. Areas were plotted as a function o f depth (Fig. 3), and then a s m o o t h spline curve was carefully passed through the 13,points. Intermediate points were picked o f f and noted, and the area algorithm (Hall, 1976) was again used with a programmable p o c k e t calculator to produce an exact integrated area or volume curve. Comparison o f these results with the earlier curve of Neev and Emery (1967) shows one interesting feature. Despite a 64 m decrease in the maxim u m depth b e t w e e n the t w o bathymetric maps, the volume o f water present with sea level at --400 m i n c r ~ e s some 9% from Neev and Emery's (1967) value o f 135 cubic ~ o m e t e r s to t h e new value of 147 cubic kilometers. This is due to the discovery that the deep northern basin is shaped m o r e like a rectangular box than a long wedge, with the area within the --710 m contour being some 110 k m 2 larger than previously thought. This box shape is reflected in two very pronounced inflection points, marking abrupt changes in average bottom ~ i e n t , which occur at --425 and --725 m.
217
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AREA (Km z) Fig. 3. Dead Sea hypsometric curve prepared from the bathymetric map in Fig. 2. The box shape of the northern basin is reflected in the inflection points on the area versus depth curve at 425 and 725 m. The volume at the time of the survey was approximately 147 km 3.
Magnetic anomaly map T h e m a g n e t i c a n o m a l y m a p is s h o w n in Fig. 4 w i t h a c o n t o u r interval o f 5 g a m m a s (1 g a m m a = 10 -s gauss). A n o m a l i e s seen in t h e s u r v e y r a n g e f r o m - - 8 0 t o - - 2 2 5 g a m m a s . A n u m b e r o f f e a t u r e s are a p p a r e n t f r o m t h e m a p : (1) T h e field has an e a s t w a r d dip w i t h g r a d i e n t s o f 5 t o 20 g a m m a s / k i l o m e t e r . A t t h r e e o f t h e f o u r p o i n t s at w h i c h t h e e a s t e r n s h o r e was a p p r o a c h e d , t h e field d r o p s t o less t h a n - - 2 0 0 g a m m a s . This regional dip m a y be r e l a t e d t o crustal d i f f e r e n c e s across t h e rift. (2) Several a n o m a l i e s are o b s e r v e d . (a) O f f Ein G e d i a m a g n e t i c high (15 t o 20 g a m m a a m p l i t u d e ) w i t h t w o l o b e s t r e n d i n g east a n d n o r t h is a s s o c i a t e d w i t h t h e t o p o g r a p h i c high w h i c h
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TOTAL FIELD MAGNETIC ANOMALY MAP |
DEAD SEA GEOPHYSICAL SURVEY \
Fig. 4. C o n t o u r m a p o f the total field m a g n e t i c a n o m a l y over the Dead Sea. C o n t o u r interval is 5 gammas, a n d tracks a l o n g w h i c h measurements were m a d e are n o t e d as d o t t e d lines. M e a s u r e m e n t s have been c o r r e c t e d for diurnal variations, r e d u c e d t o the 1 9 6 5 . 0 I G R F at an altitude o f - - 3 9 7 m, and t h e n adjusted t o r e m o v e errors remaining at track crossings. N o t e that: ( 1 ) all a n o m a l i e s are negative, ( 2 ) there is an eastward dip o f 5 - - 2 0 g a m m a s / k i n , and ( 3 ) a n o m a l i e s are associated w i t h s o m e o f t h e faults s h o w n in Fig. 12.
bO Oe
219
projects northeast from Ein Gedi. This high is most likely diapiric (Fig. 5, Profile C), and the northern lobe of the magnetic anomaly is found near a buried diapiric lobe which is seen in Profile 15. It should be noted that such anomalies are probably related to the structures along which diapiric piercement of salt t o o k place, and n o t to the diapirs themselves. (b) There is an interesting saddle point along the coast between Ein Gedi and Mizpe Shalem, where an east--west trending low (--140 gammas) is sandwiched between t w o highs (--80 and --100 gammas) on the north and south. (c) Two small NNW trending anomalies are found in the extreme north and south. A 10 gamma high is f o u n d o f f Kallia where t w o small diapirs were observed while a 20 gamma low occurs in shallow water in the northern part of the Lisan Straits. (3) From Ein Gedi south to the evaporation pools there is good agreement b o t h in direction and gradient b e t w e e n these measurements and those of Domzalski (1967). No aeromagnetic data exists north of Ein Gedi. (4) Inspection of the total field intensity map of Allan and Morelli (1970) for the Gulf of Aqaba indicates that the magnetic activity over the Dead Sea is of the same order of magnitude as that over the Aqaba Gulf, which Allan and Morelli (1970) interpret as indicating that there has been 'no significant extrusion of volcanic material'. Seismic profiles Fig. 1 shows the location of all fifty-one seismic profiles obtained in the survey. The north--south longitudinal profiles have been alphabetically labelled from A to L, and are displayed so that the south is always to the right, as if the observer were looking from the western edge of the Dead Sea. The east--west latitudinal profiles have been numbered from 1 to 25, and displayed with the east on the right, as if the observer were looking north from the southern end of the Dead Sea. All these profiles, together with the short interconnecting profiles, appear in Appendices 1 and 2 of Neev and Hall (1976). The scale on the left side of the profiles gives the depth of the sea floor below MSL. Where available, the total intensity magnetic measurements, corrected for diurnal variations b u t not for the regional field, have been plotted above the profiles. In the seismic survey the deepest reflections recorded were from about 2/3 sec, mainly along the longitudinal axis of the northern basin. In Fig. 5 (north--south profiles A and C) several reflecting horizons are observed. F r o m these, four prominent reflecting horizons were chosen and named, from t o p to b o t t o m , the 'red', 'blue', 'green' and 'brown' markers respectively. These reflectors are equivalent to stratigraphic horizons that can be regionally followed, with the exception of the 'brown' marker (the deepest one) which is observed as fragments giving the appearance of domal structures. The brown horizon marks the t o p of an acoustically transparent unit (one in which little or no internal reflections or reverberations are observed).
220
One example of this acoustic transparency may be observed at the core of the submarine mountain (the Ein Gedi Diapir) which is f o u n d at the southern end of Profile C. In the same profile one can readily observe the overlying three markers ('green', 'blue' and 'red') and the stratigraphic units they enclose. These units thin toward the apex of this high from both the north and the south. This feature is therefore explained as a diapiric intrusion, and is accordingly named the Ein Gedi Diapir. We assume that the transparent stratigraphic unit represents a rock salt body, in the process of plastic movement, which has lost its stratification and is equivalent to the unit which was defined by Zak (1967) as t h e Sedom Formation. In the central part o f Pro-
Fig. 5. Two adjacent north--south longitudinal 1000 joule sparker profiles. North is to the left. Refer to Fig. I for profile locations. In the eastern Profile A (Fig. 5a) the Arnon Sink, which is genetically associated with the Ein Gedi salt diapir (seen in Fig~ 5b ), is fully developed. The fact that the Red reflector (Base Holocene) is almost unaffected by the diapirism can be explained by the exceptionally high rates of deposition which preceded and followed its formation. In Profile C (Fig. 5b) the Ein diapir (at the extreme right) is best presented. The acoustically transparent material below the Brown reflector is probably rock salt in a state of flow. Note the indications for diapiric uplifting since at least the Green reflector (mid-Pleistocene) and continuing to the present. Also note (1) that the Jordan's submarine delta has been accumulating only since the Red marker (base Holocene) and (2) the existence of a deep diapir beneath the delta (extreme left of profile).
221
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Fig. 6. Six transverse (west--east) profiles. East is on the right side. Fig. 6a: These profiles s h o w one steep border fault on the east (Profile 5) and t w o step faults o n the west (the easternmost of these is best seen beneath the flat b o t t o m in Profile 8). Fig. 6b: N o t e : (1) a salt diapir a l o n g the eastern b o r d e r fault (Profile 10), (2) the A r n o n Sink whose t r o u g h is adjacent to the eastern border fault, and (3) the upslope, westward gradual d e v e l o p m e n t o f diapirism. Fig. 6c: Note the gradual s o u t h w a r d d e v e l o p m e n t o f the Ein Gedi diapir (in the central parts of Profiles 14 and 16), which is a 'Central T r o u g h ' structure, similar to the Lisan Peninsula and the J o r d a n delta diapirs (detached f r o m the eastern as well as the western b o r d e r faults).
file C, the 'green' and 'blue' reflectors show moderate harmonic folding which is interpreted to be also a result of an initial stage of diapirism. This folding diminishes northward, accompanied b y a slight thinning of the enclosed stratigraphic units. Uplifting of the 'blue' and 'green' markers b y domal structures was observed at the northern end o f Profile C as well as in Profile B which parallels C. In the southern half of Profile A, the 'blue' and 'green' reflectors drop precipitously from b o t h north and south into a deep sink. The center of this sink, herewith named the Arnon Sink, is found northwest of the Arnon River outlet, approximately where Profiles A and 13 cross. In these profiles b o t h the 'blue' and 'green' markers disappear completely into the sink. In Profile F, which is parallel to A, as well as in Profiles 10 + 11 (Fig. 6) and 12, one can study the shape of the Arnon Sink b y observing the characteristics of the 'blue' and 'green' markers. Unlike the 'green' and 'blue' markers, the 'red' marker is usually b o t h
224 horizontal and smooth in Profiles A and C. It is therefore deduced that this marker and the overlying layer were n o t influenced b y the diapirism and other tectonic movements to the same extent as the deeper reflectors. It therefore seems likely that during the period when the s~atigraphic unit, b o u n d e d b y the ~ e d ' and 'blue' markers, was accumulated, the rates o f sedimentation in the deep flat areas o f the northern basin were especially high, and consequently traces of diapiric activity were camouflaged at the upper part of this unit. The thinning of this unit around and over the Ein Gedi Diapir is an exception. The distribution o f t h e sediments above the 'red' marker in the northern part of Profile C as welt as in Profiles B, 3 and 3--4 indicates that the Jordan Delta has only started to accumulate since deposition of this marker. Fig. 6 represents a sequence o f six latitudinal (east--west) profiles arranged in order from north t o south: Profiles 5, 8, 9, 10 + 1 1 , 1 4 and 16. In these profiles, the following features are noticed: (1) Diapiric activity, probably associated with one of the main western fault escarpments o f the graben, which extend along the b o t t o m o f the western flank o f the northern basin, is expressed in Profiles 5, 9, and 14. (2) In Profile 8, along the same flank of the western basin, two eastward stepping faults were observed, instead of diapiric features. (3) On the same western flank m e n t i o n e d above but somewhat further to the west, a series o f more subdued diapirs are observed in Profiles 9, 14 and 16. (4) In Profiles 14 and 16, as well as 13 and 15, the progressive development of the Ein Gedi Diapir from north to south can be followed. This diapir is located east o f the diapirs described in (1) above, and its apex is observed in Profile 16. (5) The development o f the deep Arnon Sink can be seen in the E--W Profiles 10 + 11, 14 and 16, as well as in Profiles 13 and 15. Younger sediments have accumulated in the A m o n Sink contemporaneously with the development o f the Ein Gedi Diapir. The abruptness with which the Arnon Sink begins is accentuated by the fact that there is no trace of the sink in Profile 9 which lies very close to Profile 11. (6) Profile 11 best shows that the 'red', 'blue' and 'green' markers can be easily traced and correlated from the deep basin, across the western basin flank to the narrow shelf margin. In this profile, where diapirism is almost absent, it is clearly shown that all the stratigraphic units b o u n d e d b y these markers drastically thin westward from the deep basin toward the present shoreline. (7) Development of diapirs was also noticed close to the eastern main border fault as can be observed in Profiles 5, 9 and especially 10. (8) Two small needlelike diapirs, herewith named the KaUia Diapirs, were observed in the western segments o f Profiles 1 and 2 in the shallow margin between the Jordan Delta and Kallia in the northwest c o m e r o f the Dead Sea. According to Ben-Avraham (personal communication) a significant heat
225
flow anomaly was observed on t o p of these diapirs. Fig. 7 shows longitudinal profiles D and I which together extend the full distance from Kallia in the north to midway down the southern basin. Profile D extends along the lower part of the western flank of the northern basin and Profile I crosses the Lynch Straits into the southern basin where it runs along the edge of the b o u n d a r y dyke of the northern evaporating pans (see Track Map in Fig. 1). The following features can be noted in these profiles:
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Fig. 7. Two longitudinal profiles along the western margin of the Dead Sea. North is on the left. Fig. 7a: The s u b b o t t o m reflectors indicate that the origin of the rugged topography is both erosionally and diapirically influenced (also noticeable in the northern half of Profile I -- Fig. 7b). Fig. 7b: Note the difference in the acoustic characteristics of the southern basin (dominant reverberations in the skins between the deep diapirs) and adjacent to the Lisan Peninsula. The boundary between the two provinces is fault controlled and is noticed close to the time mark for 1140 (about ¼ of the distance from the right end of Profile I).
226 (1) The different strati~aphic units are much thinner in these profiles compared with those in Profiles A and C. The ' b r o w n ' marker is found relatively close to the sea floor. (2) Diapiric structures are more abundant along Profile D than along Profile C. (3) No diapirs were detected in the northern half o f Profile I, i.e. from the northern b o u n d a r y of the southern basin through the Lynch Straits and Channel up to the northern end o f the profile. (4) There is a p r o n o u n c e d change in the seismic reflection characteristics along the northern segment of Profile I. North of 80N (opposite Massada at the point where the northern basin begins to deepen, or approximately at 0830 Hours on Profile I or at 1625 Hours on Profile L in Fig. 8), the sediments are acoustically transparent, while south of this point and extending to the narrowest part o f t h e S t m i ~ (corresponding to 1140 Hours on Profile I) there is a thick sequence o f highly reflective horizons. It appears that the acoustic transparency o f the northern segment does n o t reflect the diapiric salt b o d y although the exact lithologic nature is n o t y e t clear. It is possible that the d i f f e r ~ c e is due t o lateral facies changes or to an angular unconformable contact between t w o units. (5) The 'red' reflectors in Profiles D and I (Fig. 7), as well as in L and 22 of Fig. 8 represent an erosional morphology. In Profile D this morphology was additionally affected b y diapirism. (6) The transition between the Lisan Peninsula and the southern basin (1140 Hours on Profile I) is fault controlled. South of this point abundant reverberations make the interpretation harder, but indicate a different sedimentary sequence. Despite these difficulties, t w o diapirs, separated by deep sediment-filled sinks, were observed in Profile I at 1205 and 1240 respectively. Two additional diapirs were also observed in Profile J which parallels Profile I in the southern basin. Again deep sinks filled b y a thick sequence of y o u n g sediments were observed adjacent to the diapirs. In Figure 8, north--south Profile L and east--west Profile 22 are presented. Profile L shows the sharp transition from the northern basin, which is characterized b y intensive deposition, to the Lisan Peninsula, with its acoustically transparent nature. It seems that this sharp transition expresses faulting with an appreciable a m o u n t o f throw. Profile 22, which crosses the lower part o f Lynch Channel from west to east as well as the northern projection o f the Lisan Peninsula, shows the following: (1) a thick, post-erosional (post-'red') sedimentary fill at the trough of the Lynch Channel; (2) a relatively small, north-northeast trending fault along the eastern flank of the Lynch Channel; and (3) it is possible that t w o diapirs are found in the northern projection o f the Lisan Peninsula.
TOTAL
MAGNETIC
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Fig. 8. Profile L is a l o n g i t u d i n a l profile ( N o r t h to t h e left) a l o n g t h e w e s t e r n f l a n k s o f t h e Lisan P e n i n s u l a . N o t e t h e t r a n s i t i o n f r o m t h e flat d e p o s i t i o n a l - b o t t o m a d j a c e n t t o t h e Lisan P e n i n s u l a t o t h e d e e p l y incised e r o s i o n c h a n n e l s a n d t h e fault c o n t r o l l e d t r a n s i t i o n t o t h e d e e p f l a t - b o t t o m e d n o r t h e r n basin. Profile 22 is a west t o east profile t h r o u g h t h e e r o s i o n a l L y n c h C h a n n e l w h i c h is p a r t l y filled b y y o u n g e r ( H o l o c e n e ) s e d i m e n t s . E a s t is o n t h e right. A small ( p r e - e r o s i o n a l ) f a u l t is n o t i c e d a l o n g t h e e a s t e r n slope o f t h e channel.
228 Isopach and structural maps
Using data on the b a t h y m e t r y as well as the elapsed travel time to the four markers mentioned above, isopach and structural contour maps were prepared. The former express the horizontal distribution o f thickness of each of the units, while the structural contour maps represent the m o r p h o l o g y of the markers themselves. In the absence of detailed interval velocities, all thicknesses and depths are expressed in milliseconds o f round trip travel time (in this case assumed to be roughly equivalent to meters). Generally speaking the isopach maps are more expressive than the structural maps in describing the different features which were developed during deposition of that unit. All the maps are presented in Neev and Hall (1976, Appendix 3) while maps o f special interest are presented herewith and described in detail in the text below. Red-Brown isopach map and Brown structural map. As noted above, the 'brown' marker represents the t o p of the Sedom Formation which is composed mostly of rock salt (Zak, 1967). In our seismic profiles the contact between this formation and the overlying units is unconformable and intrusive in nature. The isopach map could be expected to describe the cumulative effect o f the diapiric activity atl through the time during which the overlying units were deposited. It is probable that even greater thicknesses of sediment than are shown in the isopach map were affected b y the diapirism as the seismic reading technique was unable t o see deeper than 1 sec. The Red--Brown interval is shown for illustrative purposes instead of the B o t t o m - - B r o w n interval, because the volume o f the Jordan Delta sediments deposited in post-Red time mask and distort features which were developed prior to Red time. The following features are shown in Fig. 9: (1) A chain o f diapirs protrudes along the contact between the deep floor o f the northern basin and its western flank. This contact probably coincides with a major border fault. A continuation o f this trend toward Mount Sedom is to be expected. In the northern basin additional b u t smaller diapirs were observed on the basin flank west of this line. (2) Two big diapirs are f o u n d east o f this fault line: one is the large Jordan Delta Diapir centered on 128N 203E (Israel Grid) in the norteastern part of the Jordan Delta, and the other is the Ein Gedi Diapir at 96N 195E adjacent to Ein Gedi. These t w o diapirs are surrounded b y pronounced ringsynclines, known in other parts o f the world to be associated with salt domes which are usually n o t bounded b y faults. Other small diapirs are also found along this zone (see also Fig. 12). The deep El Lisan No. I wildcat driiling (Neev and Emery, 1967) may be related to the same group. Another diapir in this group is probably located SE o f Mount Sedom as indicated b y electrical resistivity measurements carried o u t b y the Institute o f Petroleum Research and Geophysics (A. Leva-
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Early to m i d - P l e i s t o c e n e ) r e f l e c t o r s , a n d a s t r u c t u r a l c o n t o u r m a p o n t h e t o p o f t h e B r o w n r e f l e c t o r . C o n t o u r values are in millis e c o n d s b e n e a t h t h e sea surface at t h e t i m e o f t h e survey. These m a p s s h o w very clearly t h e f a u l t c o n t r o l l e d diapirs w h i c h are associated with t h e w e s t e r n b o r d e r fault s y s t e m ( w h i c h also i n c l u d e s M o u n t S e d o m ) as well as t h e g r a b e n ' s t r o u g h diapirs (the Lisan P e n i n sula, Ein Gedi, a n d J o r d a n Delta diapirs - - see Fig. 12 for l o c a t i o n s ) .
Fig. 9. An isopaeh map of the interval between the Red (base Holoeene) and Brown (top o f the intruded salt of the Sedom Formation
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230 non, personal communication). The diapirs found in Profiles I a n d J in the southern basin are also p r o b a b l y related to the 'trough' chain of diapirs. (3) At least four small diapirs shown on this map also protrude along the trace of the steep eastern border fault of the northern basin. (4) Two WNW trending faults were observed to b o u n d the Lisan Peninsula on the north and south, in agreement with the more speculative scheme proposed b y Neev and Emery (1967). However the fault which they indicated marking the northern limit o f the Dead Sea was definitely not observed in the results of the present survey. Red-Blue isopach map. This map shows t h e following features: (1) Extension o f the piercement process of the Jordan Delta Diapir during that time interval. It must be emphasized that the northward thinrdng o f this unit from the center of the northern basin toward the Jordan River outlet begins a b o u t 13 km south of the rivermouth, approximately opposite Zerka Mayin. (2) Extension o f the diapir piercement process along the b o t t o m of the westward flank o f the northern basin. (3) Continuation of the piercement process of the Ein Gedi Diapir contemporaneously with a very intensive phase of subsidence of the Arnon Sink. During this interval a sequence of a b o u t 450 msec ( ~ 4 5 0 m) of sediments accumulated in this sink. Together with an additional thickness of 200 msec ( 2 2 0 0 m) which accumulated during the preceding interval (Blue to Green -see Appendix 3 of Neev and Hall, 1976) at least 650 msec of sediments were accumulated here. The Arnon Sink, together with other adjacent structural lows, makes an irregular ring syncline around the Ein Gedi Diapir. As noted above (under 'Seismic profiles'), the rate o f sedimentation close to the end of the Red--Blue time interval exceeded the rate of diapir piercement, thereby camouflaging most o f the diapiric activity. This conclusion applies to the deeper parts of the basin (the present flat b o t t o m ) b u t not to its flanks. A north--south trending apparent ridge (area of low sediment accumulation) is expressed in this isopach map, between Cape Costigan on the Lisan Peninsula and the western shoreline of the Dead Sea. Examination of Profile 22 (Fig. 8), which transects this ridge from west to east, shows that this ridge is n o t a diapir b u t rather an erosional channel. Due to the erosional process a b o u t 100 msec of sediments are absent from the trough as compared with its flanks on both the east and west. It is assumed that the erosional process t o o k place toward the end of the Red--Blue interval, i.e. toward the end of the phase when intensified sedimentation occurred in the deeper part of the basin. B o t t o m - - R e d isopach map. The B o t t o m to Red isopach map illustrates the following: (1) Along the western flank o f the northern basin the sediments of this unit have filled up an erosional drainage system. North of Ein Gedi channels
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Fig. 10. An isopach map o f the interval b e t w e e n the Red (base H o l o c e n e ) and Blue ( w i t h i n the Lisan F o r m a t i o n - - Late to mid-Pleistoc e n e ) reflectors. These data s h o w the e x t r e m e rates o f s u b s i d e n c e o f the A r n o n Sink and the c o n t e m p o r a n e o u s diapiric u p l i f t i n g o f the Ein Gedi salt d o m e . The thinned strip b e t w e e n the Lisan Peninsula and the plain o f Massada is o f erosional origin. N o e x c e s s i v e accum u l a t i o n o f s e d i m e n t s is n o t i c e d in this interval in the area o f the Jordan Delta.
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Fig. 11. An isopach map of the interval between the Bottom and the Red (base Holocene) reflectors. The map shows (1) the drainage pattern which was entrenched along the western and southern slopes of the northern basin and (2) the submerged Jordan Delta which has been accumulated only during the Holocene.
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233
of this system trend approximately west to east while from Ein Gedi southward to the Lynch Straits there is one main channel plunging northward, which is joined by secondary channels coming mostly from the west. This main channel, expressed in the red--blue isopach map (Fig. 10) as a 'ridge' or deficiency in sediment thickness, is due to erosion which t o o k place at the end of this interval. The southern half of this drainage system (south of Ein Gedi) was interpreted in this w a y b y Neev and Emery (1967, fig. 8) on the basis of the bathymetric contours. On the other hand, the segment north of Ein Gedi has been almost completely camouflaged by the more intensive younger deposits (probably due to its closer proximity to the Jordan outlet). The depth of the channels in this segment is between 20 and 40 msec as judged by the difference in thickness of this unit above the troughs of these channels (60--80 msec) and above the ridges which separate the channels (30--40 msec). No trace of similar erosional activity was n o t e d in the deeper parts of the basin. Another erosional channel, partly filled by sediments of this unit, was discovered to the east of Cape Costigan, between the Lisan Peninsula and the eastern border fault escarpment, and is herewith named Dale Channel in honor of Lt. John B. Dale, USN. Lt. Dale, who was secondin-command of the U.S. expedition to the River Jordan and the Dead Sea, and who explored the area of the Dale Channel with Lt. W.F. Lynch on April 30, 1848, died on July 24, 1848 of fever contracted during the expedition (Lynch, 1849). (2) Although it is possible that during the period represented by ' b o t t o m - red' sedimentation, the diapirism was more mild in comparison with the previous period (red--blue), its effects are noticeable in influencing and directing the trends of the erosional drainage system. Diapiric piercement activity occurred during this period in the Ein Gedi Diapir and also in the southern basin diapirs. (3) The sediments of the submarine Jordan River Delta were only accumulated following deposition of the red marker. The delta consists of a wedgeshaped block approximately 13 km long by 8 km wide, whose thickness ranges from a b o u t 100 to 330 msec. The maximum thickness of this wedge by the present Jordan River outlet is a b o u t 300 msec. (4) South of the Jordan Delta, within the deep northern basin, the thickness of this unit is rather uniform: a b o u t 90--100 msec with local thickening of up to 130 msec. (5) The volume of sediments in this unit in the northern basin was calculated b y numerically integrating the area within the various contours times the thickness in meters assuming a sound velocity within the unit of 2000 m/sec (1 msec = 1 m), using the methods outlined in Hall (1976). Approximately 49 cubic kilometers of sediment are present in this unit, of which some 16 cubic kilometers comprise the wedge-shaped delta.
234 G E N E R A L DISCUSSION AND CONCLUSIONS
Stratigraphy and geological history The red marker appears t o express the beginning o f the present Dead Sea. Prior to, and close to this time, two different processes t o o k place: the first was the very intensive deposition o f t h e sediments in the central parts of the northern basin, and the second was intensive erosion which dissected the basin flanks. It is considered likely that due to the extreme shrinkage of Lake Lisan which almost reached the point of complete desiccation and marked the end of the Lisan period, most o f the dissolved salts were precipitated in the lowest part of the basin.'In addition, an u n k n o w n quantity of sediment was eroded and transported to this part of the lake during its shrinkage. It is assumed that these processes ended approximately 13,000 years ago, since according to Th--U and 14C m e t h o d s the youngest Lisan sediments in the Dead Sea area on the Amatsyahu and Amiaz Plateaus (south and west o f Mount Sedom respectively) range from 13,000 to 17,000 years in age (Kaufman, 1971 ). It is evident that the climate during this transitional period was extremely dry. The sediments o f the Jordan Delta and the rest of the unit above the red marker were therefore accumulated and deposited during the younger pluvial period which followed. This makes it possible to calculate approximate rates of deposition of 30 m / 1 0 0 0 years and 10 m/ 1000 years for the Jordan Delta proper and the floor of the deep basin respectively. The salts dissolved in the present Dead Sea water were mainly derived from t w o sources: (1) The end brines which remained o n the b o t t o m of the lake in solution when it had been complete desiccated, due to their high hygroscopy and to the fact that some drainage always occurred into this terminal lake. These end brines were mixed with the fresh water which entered during the subsequent pluvial period. (2) The Lisan Lake brines which penetrated into the adjacent mountainous reservoir rocks and which were later displaced and expelled b y the rising water tables which accompanied the more pluvial period. In view o f the physical relationship between the red, blue and green markers (mostly conformable contacts, at least in the deep parts of the northern basin) we assume that the blue marker was deposited sometime within the Lisan F o r m a t i o n and that the green marker occurs within one of the upper m e m b e r s o f the Amora (Foothills} Formation. As n o t e d above, t h e brown marker expresses the unconformable and intrusional c o n t a c t between the rock salt Sedom Formation and the overlying younger formations.
Structure The data on which Fig. 9 (tectonic element sketch map) is based are as follows:
~'ig. 12. A sketch map of the tectonic elements and physiographic features in the Dead Sea region: Fault systems, salt domes, the krnon Sink, the Jordan Delta, and the Lynch and Dale Channels.
ja
236
( 1 ) t h e physiographic features as expressed in the bathymetric map (Fig. 2), the adjacent topographic maps, as well as air photographs and satellite imagery; (2) the stratigraphic--structural relationships as expressed in the seismic profiles and the isopach and structural contour maps. Three groups of faults are identified in and around the Dead Sea: (1) The Eastern Border Faults. (2) The Western Border Faults. (3) The WNW trending faults within the graben. The latter group of faults b o u n d the horst of the Lisan Peninsula on the north and south, and also t h e southern Dead Sea Basin on its southern side (The Amatsyahu F a u l t - Neev and Emery, 1967). The northwest trending graben o f Kerak (Transjordan) m a y be genetically related to these faults. This m a y therefore be another argument in favor of the northward shift due to the sinistral m o v e m e n t along t h e eastern border faults. The Western Border Faults form a system of step faults and step blocks. Several deviations from the main fault on land (the westernmost one) were noted in the form o f crescentic faults trending b o t h to the northwest and southwest (Begin, 1975; Kashai, 1976). Along the easternmost fault o f this western system, many salt diapirs pierce upward. It appears that Mount Sedom is one of the diapirs along this fault although the fault segment between Mount Sedom and the northern basin is not always identified. South o f Mount Sedom this fault was observed b y Neev and Emery (1967) to extend as far south as the Amatsyahu Fault. Another fault in this system, which is of special interest, is identified mostly on the basis of b a t h y m e t r y ; it extends diagonally NNE from o f f Ein Gedi in the south, toward Kallia in the north. According to airphotos, satellite imagery and field observations, this fault continues in the same direction into the lower Jordan Valley where it joins the Eastern Border Fault. In this w a y it forms a fault-bounded, wedge shaped block enclosing the northern end o f t h e Dead Sea. The Eastern Border Faults form a more systematic group than the Western Border Fault system in that NNE to NE trending faults systematically branch o u t from the main border fault. Some o f these faults marked on our sketch map are newly identified. T h e y were traced from air photos, satellite imagery as well as b y direct observations from the sea. It is possible that in places the vertical displacement along these branching faults is small to absent * The northward extension of the westernmost fault o f the Eastern Border * See following LANDSAT imagery: (1) Image 81054074215G0, 15 September lg72, MSS Band 5. (2) Image 81162074255G2, 1 January 1973, False Color Composite. (3) Image 81342074245A0, 30 June 1973, MSS Band 6. (4) Image 81145074305G2, 14 December 1972, False Color Composite.
237 Fault System is hypothetical. It extends from the southern basin through the topographic trough of E1 Mezra'ah (NE part of the Lisan Peninsula), continues through the Dale Channel and the structural trough along the northern basin (including the Arnon Sink), and leaves the Dead Sea via the Jordan Delta Diapir.
Basement tectonic features From the total intensity magnetic anomaly map two inferences regarding the deep basement configuration may be drawn: (1) The anomaly map shows a pronounced eastward gradient of approximately 20 gammas per kilometer which suggests a change in regional crustal characteristics. Consideration of the known structural movements along the Jordan Rift would suggest that this magnetic gradient may reflect a transition from a less continental type of crust on the west to a more continental type on the east beyond the Eastern Border Fault system of the graben. This idea is in agreement with the conclusion of Knopoff and Belshe (1965) based upon their compilation of gravity and magnetic measurements across the graben to the north. We do not believe that this gradient can reflect a change in the depth to the basement as three very close approaches of the magnetometer to within tens of meters of the Eastern Border Fault where the basement is known to be very close to the surface resulted in a decrease rather than an increase in the field. (2) The east--west trending negative magnetic anomaly located between Ein Gedi and Mizpe Shalem may be an extension of another predominant magnetic anomaly of similar trend, crossing the Hebron Mountains between the Dead Sea and Gaza, Which is found in the aeromagnetic map of Folkman and Yuval (1976). ACKNOWLEDGEMENTS The authors gratefully acknowledge the enthusiastic assistance and encouragement given to this work by our many colleagues from a number of different institutions. The Dead Sea Works Limited provided manifold logistical and financial support. The Mapping, Research, and Photogrammetry Divisions of the Survey of Israel, Ministry of Labour, provided assistance in preparing the positioning stations. The Israel Oceanographic and Limnological Research Limited provided service personnel and equipment, and other equipment was lent by the UN/UNDP-GSI Offshore Dredging Project and by the Steinitz Laboratory of the Hebrew University, and by Sherman Engineering Co. Hospitality was provided by the Youth Hostel at Ein Gedi, the National Parks Authority, and by the Israel Defense Forces. In addition, our colleagues from the Marine Geology Division of the Geological Survey of Israel provided valuable help during all stages of the work. We gratefully thank Dr. Alexander Klung of Oil Exploration (Investments)
238 Ltd. f o r giving a p r e v i e w o f r e c e n t l y a c q u i r e d m u l t i c h a n n e l seismic d a t a f r o m t h e western p a r t o f t h e area p r e s e n t l y investigated. This d a t a r e i n f o r c e d s o m e o f o u r c o n c l u s i o n s d r a w n f r o m o u r o w n r e v e r b e r a t i o n - l a d e n shallow w a t e r data. This w o r k was f u n d e d b y C o n t r a c t 2 9 0 9 0 with t h e D e a d Sea Works Ltd. REFERENCES Allan, T.D. and Morelli, C,, 1970. The Red Sea. In: A.E. Maxwell (Editor), The Sea, IV (part 2). New York, N.Y., pp. 493--542. Begin, Z.B., 1975. Structural and lithological constraints on stream profiles in the Dead Sea region, Israel. J. Geol, 83 (1): 97--111. Domzalski, W., 1967. Aeromagnetic survey of Israel--Interpretation. Inst. Pet. Res. Geophys. Rep., No. SMA/482/67, December 1967, 66 pp. Fabiano, E.Bj and Peddie, N.W., 1969. Grid Values of Total Magnetic Intensity IGRF1965. U.S. Dept. of Commerce, Environmental Science Services Administration Technical Report C & G S -- 38. Fabiano, E.B. and Peddle, N.W., 1975. U.S. World Chart Coefficients for 1975: E.O.S. Trans. Am. Geophys. Union, 56 (9): 587--588. Folkman, Y. and Yuval, A., 1976. Aeromagnetic Map, 2 sheets on Geological Map o f Israel (1 : 250,000), 1 Sheet of Offshore Israel (1 : 250,000). IPGR, May 1976, The Survey of Israel, Tel Aviv. Hall, J.K., 1976. Algorithms and programs for the rapid computation of area and center of mass. Comput. Geosci., 1 : 203--205. Hall, J.K. and Neev, D., 1975. The Dead Sea Geophysical Survey 19 July--1 August 1974, Final Report No. 1, GSI-MGD Internal Report No. 2/75, December 1975, 27 pp. Kashai, E., 1976. The 'Ami'az Oil Prospect, Oil Exploration (Investments) Ltd, Tel Aviv, 22 pp. Kaufman, A., 1971. U-Series dating of Dead Sea Basin carbonates. Geochim. Cosmochim. Acta, 35: 1269,1281. Klein, C., 1961. On the fluctuations of the level of the Dead Sea since the beginning of the 19th century. Ministry of Agriculture Hydrological Service, Israell Hydrological Paper No. 7, pp. 1--73. Knopoff, L, and Belshe, J.C., 1965. Gravity observations of the Dead Sea Rift: In: The World Rift System, a Symposium. Geol. Surv. Can. Pap., 66--14, pp. 5--21. Lynch, W.F., 1849. Narrative of the United States' Expedition to the River Jordan and the Dead Sea. Lea and Blan~ard, Philadelphia, Pa., 508 pp. Neev, D. and Emery, K.O., 1967. The Dead Sea: Depositional Processes and Environments of Evaporites, Geol, Surv. Isr., Bull. No. 41,147 pp. Neev, D. and Hall, J.K., 1976. The Dead Sea Geophysical Survey 19 July--1 April 1974. Final Report No. 2, GSI-MGD Internal Report No; 6/76, September 1976, 21 pp. Zak, I., 1967. The GeolOgy o f Mount Sedom. Thesis, the Hebrew University, Jerusalem, 208 pp. (unpublished, in Hebrew with English abstract).