THE SEDIMENTOLOGY OF THE DEAD SEA RAYMOND A. GARBER Chevron Oil Field Research Co.

p.o. Box 446

La Habra, CaJifo17'Ja 90631

YITZHAK LEVY Marine Geology Division Ge01ogk:al Survey ofIsrael JeTUYaIem, Israel GERALD M. FRIEDMAN Department of Geology Brooklyn CoUege of the CiJy University of New York Brooklyn, New York 11210 and

Nottheastem Science Foundation Rensselaer Center of Applied Geology affiliated with Brooklyn CoUege of the CiJy University of New York 15 Third Street, Box 746 Troy, New York 12181-0746 A!lsTRACT: The Dead Sea, one of the most saline lakes in the world, has recently (1979) undergone a major change in its hydrologic regime resulting in the mixing of its once stable meromictis. Prior to, and during this change, a sedimentologic study was undertaken to document the types of sediments in the Dead Sea, covering the entire western half of the lake, to refine ideas on the formation of the evaporite sediments and to explain the distribution of sediments in the Dead Sea Study of the mineralogy and particle-size distribution of sediments of the Dead Sea revealsthat, of the primary minerals, gypsum concentrates in the coarse silt and sand-size fractions whereas aragonite falls in the clay and fine silt fractions. The predominant sediment is a clayey silt The constituents of the bottom sediments fall into two groups: (1) particles of detrital or recycled rocks (limestone, quartz and clay minerals), and (2) crystals of minerals precipitated in surface waters(aragonite, gypsum, and halite). Aragonite concentrations are low in bottomsediments of the northern Dead Sea and increase southward. Gypsum occursin bottomsediments from all water depths. This distribution shows that the rate of sulfate reduction does not keep pace with the rate of sulfate precipitation. Halitewas foundin the southern part of the northern basin. The areal distribution of primary minerals is a result of several processes. High concentrations reflect either periods of restricted circulation in certain areas, leading to massive precipitation, or high influx of saturated brines, leading to precipitation. By contrast, low concentrations reflecthighinputof detritalparticles or periods of minorinflux of highly saturated brine. The lack of large concentrations of evaporite minerals reflects: (1) the abundant supply of detrital particles being carried into the Dead Sea that lower the relative contribution of evaporite minerals; (2) dilution of surface waters by fresh water; and (3) the low valuesof HC01and S()4- in Dead Sea waters.

INTRODUCTION

The Dead Sea is a deep perennial saline lake that lies within a north-south trending graben and is at present precipitating evaporite minerals. This studyexaminesthe characteristics and distribution of evaporite deposits and other sediments in the Dead Sea and, more importantly, the principles under which they are formed prior to a major change in the hydrologic regime caused by a cutoff of the major source of freshwater to the Dead Sea, the Jordan River. The effects this change had on the structure of the water body and the water chemistry are discussed in Steinborn et al. (1979). This sedimentologic study covers the entire western half of the Dead Sea and is an extension of an earlier study by Neev and Emery (1967) that was confined to the southern regionof the Dead Sea south of Ein Gedi (Fig. 1). DEPOsmONAL SETI1NG

The Dead Sea is situated in the lowermost depression of the Jordan-Arava rift valley, an extension of the East African Rift system. It is divided into a northern and southern basin and is 80 km long and up to 20 km wide (Fig. 1) with a CARBONATES AND EVAPORITES, VoL 2,No. I, 1987,p. 43-57 Copyright 1987by the Northeastern Science Foundation, Inc.

surface area of 810 square km and a volume of 144.5 cubic km (Neev and Hall, 1976). The surface of the Dead Sea is 403 m below mean sea level making it the lowest spot on the surface of the earth (Beyth, 1977b). Until recently (1979) the waters of the northern and southern basins were connected across a shallow sill, the LisanStraits(Fig. 1).The water levels have fallen in the last 20 years as a result of a cutoff of the mainsourceof freshwater,theJordanRiver, so that thesouthern basin is dry and the waters in the northern basin are 325 m deep. The floor of the northern part of the northern basin is gently sloping and the sediments from the Jordan River Delta have built out a smooth, planar slope into the northern part (Neevand Hall, 1979).The southern part of the northern basin is topographically complex and believed to have resulted from subaerial erosion during an arid period when the Dead Sea dried almost completely (Neev and Emery, 1967; Neev and Hall, 1979). The Dead Sea is bounded on two sides by steep cliffs that in placescome right to the shore. The main cliff-forming rocks are Cenomanian and Turonian limestones and dolostones with minor exposures of Nubian Sandstone (paleozoic to Early Cretaceous) on the eastern shore. Sedimentation along the rift valley is controlled by a combination of tectonics and climate

RAYMOND A GARBER, ymHAKlEVY, AND GERAllJ M FRIEDMAN

44

120

110

Northern

B••in

100

MEDITERRANEAN

SEA

MASSADA

•

Southern

Bnin

EIN BOOEQo

DMdSMLe"el: 402", .... MSL

"

MT ," SEDOM \ \ ,

_,

-.--

FIG. 1. Bathymetric map of the Dead Sea; inset map shows regional geography. Contours in meters below mean sea level. Data from Neevand Hall(1976).

, I\ ~

;;-::'1;--'"

I

,.

".

100

110

THE SEDIMENTOWGY OFTHE DEAD SEA (Manspeizer, 1985). In places alluvial fans incise the cliffs, building fandeltas intotheDeadSea,andsediments are typically coarse grained and poorly sorted. The toes of the fans that extend into the sea are commonly coated with a crust of aragonite and gypsum in the shallow Dead Sea waters(Garber, 1980). In other areas away from the fans, narrow mudflats borderthe Dead Sea. The Jordan River has built an extensive flood plain in the Rift Valley as it cut through the layered aragonite, calcite, gypsum and detrital sediments of a precursor to the Dead Sea, Pleistocene Lake Lisan. Lisan sediments cover the Rift Valley north and south of the Dead Sea and at various locations along its shore (Begin et aL, 1974). The lake water is a brine in which Mg>Na>ca>K are the major cations (Beyth, 1977b). Relative to seawater concentrated to the same degree, it is depleted in HC03- and S()4=. The specific gravity of the brine is approximately 1.23 g/cm', Throughout mostof its history and during mostof the period of this study (1976-1979), the Dead Sea water body was chemically and thermally stratified (Neev and Emery, 1967). Since 1979 the lake has been chemically homogeneous, with only minor seasonal variations on the surface as a result of the effects of lower inflow volumes of fresh water from the Jordan River (Steinhorn et al., 1979). The overturn of the stratified watercolumn occurred byvertical mixing of the upper water mass, with an excess salinity caused by a cutoffof fresh water from the Jordan River, and lower fossil water mass (Steinborn, 1985). Stiller andChung(1984)suggest thisoverturn can occur natura1Iy and in fact may have occurred 260 years before. The Dead Sea is fed by a number of rivers, wadis and underground springs of varying salinities, ranging from fresh water to hypersaline brines (Bentor, 1961, 1969). Table 1 summarizes somemeteoric data.

45

••

-

e. e..

t

e. e ..

Nort.......

.......

e15

t

-

.17

.31 •. • t

en

ea.. .1a .11 .12 4 5

t

.M

•

FIELD AND LABORATORY STUDIES

Sampling was confined to the sediments of the western part of the northern basin (Fig. 2). A Van Veen grab samplerwas used to collect bottom samples (3-50 em penetration below the sediment surface). A modified FBA(Freshwater Biological Association, England) gravity corer was used to take cores from different partsof thelake.Coresweretakenfromdifferent water depths: (1) < 1 m water depth (sea-margin), (2) 2530 m (shelf), (3) one core from 242 m (slope), and (4) > 300 m (basin). The corespenetrated between 26 ern and almost 1 m of sediment The location of grab samples and cores is shown in Figure 2.

Table 1 . Summary of Hydrologic and Climatic Data Regional Climate Subtropical Desert Basin annual rainfall 47-88 rom 18-40 degrees C air temp. range (summer) air temp. range (winter) 11-21 degrees C ave. water temp. (50 m below surface) 21.5 degrees C watertemp. range (upper50 m) 16-36 degrees C annualevaporation 150 em From Yaffa(1972)

.......... . . . . .UN!:

-

1Mt1r--;

...

_

100

.......... t ........ FIG.2. Location of bottom samples in the Dead Sea.

110

•

46

RAYMOND A GARBER ymHAK LEVY, AND GERALD M FRIEDMAN

Particle sizewas analyzed using settling techniques (Jackson, 1969) andthe mineralogy wasdetermined on each size fraction by x-ray diffraction. The clay fraction was further treated to determine claymineralogy. Carbonate wasremoved byNaOAc (pH=5) and organic matter by H202 treatment (methods of Jackson, 1969). Relative percentages of clay minerals were determined by x-ray diffraction on samples pipetted onto glass slides, driedat roomtemperature andglycolated (Biscaye, 1965) and are expressed as a percentage of the carbonate-free, < 2 #Lm fraction. Carbonate minerals in selected layers of sediments were analyzed for ~ CI3 and ~ 0 18, carefully.

110

DEADSEASEDIMENTS

From the shallows to the deep basin the sediments in the DeadSea are welllayered Individual layers range in thickness from fractions of a millimeter up to 7 em (Fig. 3). The layers possess a wide range of colors: black, gray, green and white. Three bulk sediment types were determined in this study for sediments in the Dead Sea: sand-silt-clay, clayey silt and silty clay (terminology after Shepard, 1954) (Fig. 4). Most layers are fine grained (clayey silt), but lenses of very fine sand occur in several cores.

110

ees

Northern

.....n

IlIA. . .

•

10

Southern

.....n

ElM M)QeQ.

Deed. . Leftl: 4OZ1Il_MSL

FIG. 3. Photograph of core taken from the Dead Sea in 25 metersof water. The sample are is located 2 Ian south of Ein Gedi, 1/2 Ian offshore. Note the alternating laminae of aragonite (white) and mud (dark) composed of clay-size particles of limestone, dolostone and quartz. Scalein cm.

100

LEGEND SC

Iilty clay

CS

clayey lilt

sse

aid-lilt-clay

FIG. 4. Map of sediment types in the Dead Sea according to Shepard(1954).

3 (0-8)

24

5 (0-33)

Sand (>63,...m)

0-51)

19

07-37)

*Values represent 100%of carbonate-free « 2,...m fraction

1. average value and standard deviation 2. range in percent of all samples

(3-65)

(5-45)

(48-67)

(39-66)

23

(2-63,...m)

Silt

26

17 (3-25)

24 (6-43)

56

%Cal

%Arag

55

%Carb

41 (25-55)

%SlImp

40 (24-54)2

Clay (<2,...m)

Sbe

(35-97)

76

(33-52)

(3-9) 3 (0-11)

44

7

%LR.

59 (45-75)

0

%DoIo

0

rare

absent

= =

X

o

XXX = abundant XX = present

0

X

X

0

08-38)

07-52)

X

28

Kao*

32

Smect*

40 (23-57)

0*

Dimibution of Minerals in Varioos Particle-Size Fractions

Table 2

XXX

XXX

XX

Quartz

XX

X

0

Felds

XX

X

0

Gypswn

X

0

0

Halite

~

-...I

~

~

~

~

"<

§

~ C3

~

~ ~

48

RAYMOND A. GARBER, YITZHAK lEVY, AND GERAW M FRIEDMAN

The distribution of minerals in various particle-size fractions are summarized in Table 2 and can be considered in two fractions, the clay-size and the coarser than clay-size fractions. The two major groups of minerals in the clay-size fraction are carbonates (aragonite and calcite) and clays (illite, smectite, kaolinite and traces of palygorskite). Aragonite and calcite are present in the clay-size fraction roughly in the sameproportion and together make up 40 percent of the clay-size fraction. The concentration of carbonate is higherin the fine siltfraction (2-20 #Lm) than in the coarse silt fraction (20-64 #Lm) due to a significantly larger contribution by aragonite. Gypsum occurs sporadically in the coarse silt fraction (20-64 #Lm) in some samples from the Lisan straits. The mineralogy of the sand-size fraction is variable throughout the Dead Sea and is dependent on the contribution of detrital versus chemogenic sediments. Table3 summarizes the mineralogy within thecores. Crusts of aragonite and gypsum were found in some cores and grab samples from the shore areas, similar to indurated crusts found on the modem Dead Sea beaches (Druckman, 1981,and Garber et aL, 1981).

Evaporite Minerals in the Dead Sea Aragonite.-Aragonite is a common component of DeadSea sediments. It occurs as needles and needle aggregates concentrated in specific layers and rarely as a mixture with other sediments. The needles are 5 to 10 #Lm in length and to 1 #Lm wide (Fig. 7). The areal distribution of aragonite in bulk samples of the DeadSeabottomsediments shows twoprominent features (Fig. 5): (1) the northern part of the Dead Sea, down to a line connecting Mitzpe Shalem with Zirque Ma'yin, shows low concentrations of aragonite (5-10%); and (2) the southern area, down to the Lisan straits, shows a complicated pattern of distribution which coincides witha complex andrugged bottom topography. This complex distribution pattern may be related to bottom currents.

The distribution of aragonite (and gypsum) in cores is summarized in Figure 6. Measurements on individual layers show thathigh aragonite concentrations areusually encountered in white layers (85 to 99%), whereas in dark layers aragonite concentrations are low (0-25%). The aragonite concentrations in the cores were determined by analyzing intervals consisting of several layers and are reported as an average. This was necessary because many layers were so thin they were impossible to separate, and the DeadSeamudswerecommonly so unconsolidated that they had the consistency of thick soup. The amounts of aragonite present correlate with the ratios of white to dark laminae within the intervals; intervals dominated by light layers are dominated by aragonite. This may explain large fluctuations from interval to interval and the lack of clear trends within the cores(Fig. 6). In cores from the deep basin, aragonite concentrations generally range between 15% and 30%, though a few zones in the cores contain up to 48%, and some are as low as 5%. The aragonite concentration in the single core from the slope facies increases from 7% at the sediment-water interface to a maximum concentration of 33% at 19 em, Below 19 em the concentration levels off to a range between 26 to 32%. In general, the concentration of aragonite in cores from the shelf facies increases with depth, although intercalations of layers showing relatively low concentration are also encountered Aragonite concentrations are high (up to 58%) in the shallow and restricted Lisan straits. Aragonite isof primary origin in the DeadSea. The aragonite appears as fragile stellate aggregates of needles radiating out from a common center (Fig. 7). The excellent state of preservation of these crystals and their concentration within white layers ruleout transportation or reworking fromtheLisan Formation. A primary origin of aragonite is further supported by isotopic evidence. Aragonite is enriched in heavierisotopes of carbon and oxygen (Fig. 8). Aragonite layers average +0.7°/00 for () cn (range: -2.0 to +2.5°/00) and +0.60/00 for.

Table 3 Summary of sediments in coresfrom the DeadSea

I

,

Sea-Marginal

Shelf

Slope

Basin

0-41%

2-58%

7-33%

5-48%

Range of Concentrations

14tll%

25t17%

2519%

26t12%

Average Concentration

<5%

17-25%

19%

4-9% (N) to 15% (S)

Thickness of White Layers as a Percentage of TotalLength

0-1%

0-49%

1-13%

0-63%

Range of Concentrations

0.3tO.3%

13t8%

615%

6t12%

Average Concentration

Individual Crystals Ind Crystals and Crusts

Individual Crystals

Individual Crystals

Occurrence

59-100%

34-98%

65-81%

29-90%

Range of Concentrations

86%tll %

65t17%

70!5%

69t15%

Average Concentration

THE SEDIMENTOLOGY OF THE DEAD SEA

o '" ..

...... 1.".1

49

o '" ..

020 . .

50

I

110

••••••••••

GYPSl.M

tLJ

Northern

8llsin

.... 10

~

•

10

"ICENJ" 0

--

...

20 . . T

''----'

0

20 ..

0

..

Iouthern IOQIQ.

-"

l{ I

1••1

1111

0

.... "' ....

IV

..

DMd_ .. LewI: _ MIL 402 10

i ••••

- - -

ot

_

I I ••

"-

•

ItO

LEGEND

~

0-1~

II

11-20

II

•

21-30 31- «)

FIG.5. Distribution of aragonite in the bottom sediments of the Dead Sea

FIG. 6. Variations in distribution of aragonite and gypsum below the sedimentwaterinterface. Depth in em.

0 18 (range -0.6 to +2.5°/(0) confirming the earlier study of Friedman (1965). He attributes the enrichment of heavier isotopes intheDead Sea aragonites to strong evaporation during the formation of aragonite fromsurface waterswhenthe lighter isotopes are preferentially removed as part of CO2, Dark layers, in which carbonates are almost completely calcite, have an isotopic composition of lj CI3 averaging -3.00/00 (range: -3.8 to +0.4°/(0) and of lj 0 18 averaging -4.1°/00 (range: -5.1 to -2.7°/(0). Theseranges fallwithin therangeofmarine limestones that have undergone freshwater diagenesis (Friedman, 1964; Gross, 1964;Hudson, 1977) and indicate that most, if not all, of the calcite in the Dead Sea dark layers has been derived from the surrounding Cenomanian and Turonian limestones. Further supporting the isotopic data is the condition of the. particles ofcalcite inthedarklayers; theyarecommonly abraded and show no crystal form, lj

50

RAYMOND A GARBER YlTZHAK LEVY, AND GERAllJM FRIEDMAN

Thus it is inferred that the aragonite forms in the surface waters due to supersaturation byevaporation and settles through the water column to the bottom. This most likely occurs yearround and would explain the presence of minor amounts of aragonite in the darker detrital layers. The white layers are formed during massive precipitation of aragonite at the time Ofhighest water temperatures (Neev and Emery, 1967). These events, termed "whitenings", which tum the watera milky white, do not occur annually, but at irregular intervals (once or twice every 15 years; Shalem, 1949). None were observed during this study. In the northern part of the northern basin the influx of large quantities of detrital particles from the Jordan River leads to "dilution" of aragonite (Fig. 9). This dilution occurs elsewhere opposite rivers, streams and wadis but is more localized (e.g. Nahal Hever, NahalKidron andNahalDarringer). Therelatively high concentration of aragonite in the southern part of the Northern Basin reflects both the smaller influx of detrital

5 4 -

•

aragonite layer

•

dark layer

3

•

2

~ !:!

•• • •

0

•

V

co

- 1

• • •

-2

• •

,"...-

-3 -4

•

-5 -7

-6

-5

-4

·3

-2

-1

0018

(%.)

0

234

FIG. 8. Graphof ()018 vs ()CI3 of carbonatesin the Dead Sea.

FIG. 7. A. Scanning electron photomicrograph of aragonite in white laminae from core sampled near KaIlia. Individual aragonite needles radiate out from a central point Note coccolith (rare) in lower center as a detrital particle. Scale - 10 ~m. B. Scanning electron photomicrograph. Close-up of A. Scale- 4~m.

particles and the more severe evaporitic conditions due to the shallow conditions which resulted inhigher ratesofprecipitation. No straightforward explanation is proposed for the complex pattern of aragonite distribution in the southern part of the northern basin. Such a complex patternmayhaveresulted from the composite effects of many processes. In some areas, the effect of evaporation, influx of bicarbonate by springs (Amit and Bentor, 1971; Stiller and Chung, 1984), and addition of reworked aragonite from recently deposited sediments (although considered negligible) may haveled to relatively high concentrations of aragonite. In neighboring areas an influx of detrital material, paralleled by dilution by these same streams, could have resulted in low concentrations of aragonite. The complex topography of the southern part of the northern basin is another factor which could have determined differential distributions of reworked sediments in the pattern observed Gypsum.-Gypsum occurs in the Dead Sea in three forms: (1) continuous hard layers of lenticular crystals of gypsum; individual crystals are generally smallerthan I mm in diameter; (2) clusters of gypsum with individual crystals ranging from 1 to 5 mm in diameter (Fig. lOA); and (3) flat, hexagonal crystals ranging from 40 to 100 J.Lm in diameter dispersed through the surrounding sediment (Fig. lOB). Percentages ofgypsum in samples ofbottom sediments range from < 1% up to 50% by weight (Fig. 11). Aside from large concentrations of gypsum in three local areas (opposite Ein Feshkha, southeast of Ein Gedi, and west of the tip of the Lisan Peninsula), the concentration of gypsum in the Dead Sea is lessthan 20%. Detailed studies of the distribution of the kinds of gypsum found in cores reveal the following: (1) deep basin and slope: theconcentration ofgypsum variesgreatly throughout thelength of each core(0 to 63%)and each core shows a different vertical distribution of gypsum (Fig. 6). Gypsum occurs as dispersed fine-grained lenticular crystals. (2) shelf likewise the concen-

THESEDIMENTOWGY OF THEDEADSEA

51

110

Northern

Baaln

10

- 10

FIG. 10. A. Photomicrograph of clusters of gypsum crystals from sea-marginal core south of Ein Gedi, taken at a water depth of 25 m, 21 ern below the sediment-water interface. B.Photomicrograph of flathexagonal gypsum crystals fromcoretakenat a water depthof 35 m,9 em belowsediment-waterinterface. Scale- 50 ~m.

Southern

Baaln

ElM IOQI!Q.

DMcI SMlII...l: 402m _IISL 10

I ••••

0 1224& k.

110

110

200

LEGEND

~

91 - 100X

m

81 - 90

II

71 - 80

-

10

210

•

61-70

mill <60 ,'i '

FIG. 9. Distribution of detrital particles in bottom sediments of the Dead Sea.

tration and vertical distribution of gypsum are highly variable (Fig. 5); however, gypsum is present both as lenticular crystals dispersed within the sediments and as hard crusts: gypsum formed a discrete layer in the core at Station9. (3) sea margin: concentrations of gypsum are very low « 1%); it occurs as mm-sizedlenticularcrystals dispersed within the muddybottom sediments. Thus, to generalize, gypsum rarely forms discrete laminae but rather is disseminated in the terrigenous (dark) layers that contain mainly limestone fragments with lesser amounts of quartz, dolostone and feldspar. Furthermore, when present in measurable concentrations, gypsum is not evenly distributed throughout the cores. Hard layers of gypsum were only encountered in cores from shelf areas. Alternatively, clusters of gypsum crystals were found in grab samples throughout the Dead Sea. It is thought that the absence of clusters in cores may be due to difficulty in penetrating hard layers by our coring device. Previous studies failed to find significant concentrations of gypsum in the bottom sediments of the Dead Sea and attributed this to a degradation of gypsum to form calcite in the presence of sulfate-reducing bacteria(Friedman, 1965;Neevand Emery, 1967). The results of this study indicate that the Dead Sea

52

RAYMOND A GARBER, YI1ZHAK LEVY, AND GERAW M FRIEDMAN

water was saturated with respect to gypsum throughout the period represented by the samples. Otherwise, the gypsum that formed would have been dissolved. Likewise, the average rate of growth of gypsum crystals exceeded the rate of sulfate reduction. If not, only trace amounts of gypsum would have remained in the sediment and the gypsum would have had a corroded appearance. In core it is found as pristine lenses and discs. . The precipitation of gypsum is related to: (1) Directcrystallization from the water column as a result of supersaturation and subsequent settling to the bottom as reported from sediment trapstudies byNeev and Emery (1967). Such precipitation would explain the preponderance of finegrained crystals of identical morphology found in bottom sediments. This probably explains the silt and fine sand-size gypsum butnotthe largercrusts. Theirsizeranges are probably related to the rate of growthof crystals as well as to the span Northern of time during which the Dead Sea water was saturated with n respect to gypsum. (2) The formation of gypsum crystals resulted from the upward diffusion through the bottom sediments of brines supersaturated with respect to gypsum. This could explain the formation of larger gypsum crystals since these large crystals commoI1ly show displacive textures with little or no bottomnucleated orientation. Underground springs saturated with sulfate are known to exist along the Dead Sea (Mazor et aL, 1969).Likewise partialdissolution ofdiapiric structures at depth underlying theDead Sea, as described by NeevandHall(1976), led to the formation of suchhighly supersaturated brines. Brine seeps on the deep water floor of the basin may have induced the growth of clusters at depth and would also help to explain the highly irregular pattern of gypsum concentrations found in the DeadSea.Studies of radium concentrations in the Dead IIASUDo\ Sea waters strongly suggest an external source of water, • presumably brines that carried radium and other solutes into the DeadSea(Stiller and Chung, 1984). Contours representing equal concentrations of gypsum cross both bottom topography and water depth. Low gypsum concentrations could also be explained by dilution withdetrital sediments. Southern (3) The occurrence of crusts in shelf areas resulted from periodic desiccation events. The crusts resemble those found ElM Baln lIOQEQ. along the shoreline today (see Druckman, 1981, and Garber et aL, 1981). An alternative suggestion isthatsamples ofgypsum that occur at depths of 30 m result from transport of gypsum crusts from the shore by occasional floods (Neev and Emery, Deed 1967). 402m _IIISL HaJiJe.-Only 8 out of 40 sediment samples contained halite. Semiquantitative estimations show that halite comprises no morethan 10 percentof the total sediment Most of the halite eM • • • _ (and gypsum clusters) occurs south of a line connecting Ein Gedi and the Amon River (Fig. 12). The sparsity of halite 100 observed in this study differs from reported occurrences by Neev and Emery (1967) who found abundant crystalline rock LEGEND salt and cubes of halite. However, their study was confined to the southern basin and the southernmost part of the northern basin. 21- 30 ~ 0-10~ The following kinds of halite were identified from grab samples: • >30 11- 20 (1) Solid limpid cubesof halite, "suspended" in the sediment, are several millimeters across. Six out of the eight halite grab samples were of this kind Four of these samples occurred in the southern part of the northern basin, extending from the Lisan Straits to the deepest part of the lake opposite Ein Gedi FIG. l l. Distribution of gypsum in thebottomsediments of the Dead Sea.

110

tOO

10

10

"""I:

II

II

10

THE SEDIMENTOLOGY OFTHE DEAD SEA Two samples were located in the northern part of the Dead Sea, south of Ein Feshkha. Unlike occurrences reported by Neev and Emery (1967), no laminated mud was found as inclusions in any of the halitecubessampled. (2) Solid crystalline blocks of halite were collected in only two samples, both located in the southern part of the northern basin. One sample opposite Nahal Hever contained a block of halite over 30 em in diameter showing signs of dissolution (Beyth, 1977b). In at least 4 other stations, all south of the line between Ein Gedi and the Arnon River, massive hard rock salt was encountered, as indicated by very small chips of salt adhering to the grab sampler. (3) Salt "mush" was encountered in the Southern Basin of the Dead Sea. Neev and Emery (1967) reported extensively on these occurrences. Microscopic examination revealed that the "mush" consisted of cubes of halite with a size range of 40 to 500 #Lm. Weiler et aL (1974) recorded the formation of ooids composed of halite in this part of the Dead Sea. Halite was only found in core 7 (water depth 320 m) as cubes (1 to 2 rom) "suspended" in the mud These formed lessthan 10%of a layer 3.3 em thick and occurred at a depth of 10.5 em below the sediment-water interface. No other halite was found in cores. The formation of the various halite morphologies reflects both (1) precipitation of halite from the water column and, (2) infiltration of highly concentrated brines into the sediment and subsequent interaction with Dead Sea interstitial water. The following possibilities as to the origin of thesedensebrines are suggested: (1) The dense brines may be end brines flowing down the slope into the deep basin from the shallow and restricted Southern Basin. The extreme southern part of the Northern Basin is nowalmost completely dryand massive halite deposits have formed (Beyth, 1977a;Druckman and Beyth, 1977). The local salt works has, in the past, dumped dense brines directly into the Northern Basin via the Lisan Straits. Any mixing between these endbrines and Dead Seawaterwill causemassive precipitation (Starinsky, 1974). Beyth 0977a and b) and Druckman and Beyth (1977)reporton the occurrence of halite "reefs"in the Lisan Straits in such a mixing zone. (2) Upwelling brines associated with diapirs may interact withthe Dead Sea water. (3) Springs underneath the Dead Sea interacting with the Dead Sea water. Increased discharge of saline underground springs directly into the already saturated Dead Sea water as a result of drastic lowering of the Dead Sea waterlevel (Stiller and Chung, 1984) may cause precipitation of halite (Beyth, 1977a).

DETRITAL SEDIMENTS IN THEDEAD SEA

53

130

•

ElM

'\4-,-

RIt«HA

••

~

120

•• • ••

110

Northern

Basin

100

10

.

MASIADA 10

Southern

Basin

ElM IOOEQ-

DHdS-l...I: 402m _IISL

-

~;-;

fUM.

~

k.

100

110

LEGEND

Detrital sediments make up a large part of the totalsediment in the Dead Sea (Fig. 9). Many of the layers in deep basin THIS StuDY ..IV .... IMIIY (''In cores are dark, mostly detrital layers containing particles of . . .i .,._ o ~i m-quartz, feldspar, limestone and dolostone derived from the ......1Ie . o .'yoIO"l heli .. surrounding rocks. The material that makes up these layers A _.i"e helile f:::,. heli.. crUll wascarriedto theDeadSeaby majorrivers, streams and wadis. ..........Iy o ..........Iy Some of the aragonite and gypsum in the deep basin is also detrital. The Dead Sea is surrounded by the alternating layers ofaragonite, gypsum anddetrital sediments ofPleistocene Lake Lisan. During storms, wave action was observed by us FIG. 12. Occurrences of halite and gypsum in the bottom sediments of the to rework some of this material, and many alluvial fans have Dead Sea.

54

RAYMOND A GARBER, ymHAKlEVY, AND GERAW M FRIEDMAN

cutdown intotheLisanFormation carrying theerodedsediment into the lake. We believe that aragonite and gypsum provided from the Lisan is insignificant compared to the total volume of aragonite and gypsum precipitated in the Dead Sea. Clayminerals are a significant partof allDeadSeasediments. The percentage of illite in the carbonate-free clay fraction « 2 ~m) is above 30% almost everywhere in the basin, and averages about 40% (Fig. 13). Higher concentrations of illite are found at the inlets of the Jordan River, Nahal Darringer, and Nahal Tze'elim. Only one occurrence opposite the Arnon River in the centerof the lakewasbelow 30%. Theexceedingly high concentration of illite in the southern part of the northern basin corresponds with high gypsum concentrations. Smectite accounts for 32% of the carbonate-free clay fraction (Fig. 14). The highest smectite concentrations of > 40% are located in the deepest parts of the Dead Sea. Values are below 20% at the inlet of the Jordan River, Nahal Darringer, and in the southern part of the northern basin. Kaolinite has a relatively homogeneous distribution in the Dead Sea with an average concentration of28% (Fig. 15). Higher concentrations (> 30%) are found at the inlets of the Jordan River, Zirque Ma'yin, NahalHever, and Nahal Tze'elim, Although it has been suggested that clay minerals have undergone diagenesis in the Dead Sea (Amit, 1966; Amit and Bentor, 1971), the results of this study suggest that most, if not all, the clay minerals are of detrital origin. The three dominant clay minerals in the Dead Sea are also found in sediments along its western shore and in suspended matter within the Jordan River. In both areas, they occur in relatively the same proportions as in the Dead Sea sediments (Garber, 1980). The x-raydiffraction peaksof the clay minerals alsosuggest a detrital origin. lllite, when in a degraded or weathered form shows broad, very weak peaks as is the case of the Dead Seaillites (Millot, 1970,p. 326). Thekaolinite peaks aredistinct and well crystallized, a characteristic of detrital kaolinite, and the smectite peaks are broad, also an indication of a detrital origin (Millot, 1970). The areal distribution of clay minerals in the Dead Sea also suggests a detrital origin due solely to mechanical transport. lllite and kaolinite are concentrated at the inlets of rivers and wadis, whereas smectite concentrates in the deeperareas. This distribution pattern is consistent with results of experiments by Whitehouse et aL (1960). They found that kaolinite and illite had similar settling rates which far exceeded the settling rates of montmorillonite (smectite) in solutions of up to a chlorinity of 25 parts per thousand It would therefore be expected that illite and kaolinite would settle out close to the inlets of rivers and wadis whereas smectite would disperse throughout the Dead Sea DISCUSSION

III

Northern

B sin

MASSAOA

•

Southern

Baaln

ElM BOQEQ.

70

0Md llMle.el : 402m _MSl

-

The Dead Sea, during the period of this study and for the previous 300 hundred years (Stiller and Chung, 1984), was <30% a stratified lake similar in structure to Schmaltz's hypothetical euxinic stage (Schmalz, 1969). A lower water mass or fossil waterbodywas near or at halite saturation, whereas the upper 30-40 water mass was undersaturated with respect to aragonite, gypsum and halite, except at the air-water interface. During mostof theyear,but especially in thesummermonths, evaporite minerals (aragonite and to a ·Iesser extent gypsum and halite) FIG. 13. Distribution of illite would precipitate at the surface as a result of evaporation. of the Dead Sea.

~r:nt*·~ kill

200

210

LEGEND

m

40-50

>50 in the clay fraction of the bottom se
THE SEDIMENTOWGY OFTHE DEAD SEA

55

-

-

-

-

•

•

Northern BalIn

•

.

MASUDA

•

...

IOQRQ.

-

.--.

....

L .;r

•

-

-

-

.. ..

~MnF·'71

LEGEND

AG. 14. Distribution of smectite in the clay fraction of the bottom sediments of theDeadSea.

. . . . . . LMeI:

f\

111' IBIC* \ \

LoP \ ,

),1\ _ _-

- - -

_~

•

. ..

AMnF

'71

LEGEND

AG. 15. Distribution of kaolinite in the clay fraction of the bottom sediments of the Dead Sea.

56

RAYMOND A GARBER, YI1ZHAK LEVY, AND GERAW M FRIEDMAN

The minerals would then settle through the upper water mass, and those that survived would sink to the bottom to be incorporated into the bottom detrital sediments. The results of this study indicate that halite rarely survived, whereas aragonite and to a lesser extent gypsum found their way to the bottom. Irregular periods of intense evaporation resulted in massive precipitation of aragonite to form the white layers in the Dead Sea,but theseevents wereuncommon. Towards the end of this study the Dead Sea underwent an overturn and mixing that resulted in a loss of stratification (Steinborn et al., 1979). The event might have been expected to cause massive precipitation of evaporite minerals but this did not occur. Mostofthe modemDeadSeasediments are of detrital origin. They are supplied to the Dead Sea by rivers and streams and, during periodic flooding, by wadis. The Jordan River alone hasbuilta deltaintothe DeadSeaextending several kilometers. The lack of thick evaporite sediments in the modem Dead Sea maybe explained by: (1) The proximity to large sources of terrigenous sediment The Dead Sea is of small areal extent and is bounded on all sides by alluvial fans which are fed by sediments of several rivers, streams and wadis. (2) Up until recently an abundant supply of fresh water was carried to the Dead Sea which diluted surface waters. These waters may have caused the dissolution of evaporites, which wereprecipitated at the surface as a resultof intense evaporation near the air/water interface, before they could settle to the bottom. Thus it might be expected that evaporites would precipitate and be preserved if less fresh water were supplied to the Dead Sea. In fact, a series of diapiric structures along the majorwestern submarine north-south trending borderfault reflects a period in the history of the rift valley whenconditions were more favorable for evaporite formation (Neev and Hall, 1976). The Phillips Petroleum Lisan #1 well, drilled on the Lisan Peninsula, penetrated 4600 m of Pleistocene evaporites (Friedman, 1980) and was precipitated when Lake Lisan became almostcompletely desiccated. (3) The low values of the Dead Sea waters in HC03- and S()4=. Locally high concentrations of evaporite minerals in the bottom sediments maybe theresultof(1)undersea brinemixing, and (2) dissolution at the surface of diapirs that may pierce the bottom of the Dead Sea. CONCLUSIONS

streams, while its massive precipitation during periods of whitenings isprobably theresult of intense evaporation. Gypsum formation may be related to the influx of sulfate-rich waters from streams, springs, and brines. Gypsum occurs in bottom sediments from all water depths. This finding is in contrast to that of others(Neev and Emery,1967)andshows that sulfate reduction is of minorimportance in the DeadSea sediments. Halite was found chiefly in the southern part of the northern basin (south of Ein Gedi), where it occurs mainly as cubes dispersed within the bottom sediment Several processes may have led to the formation of halite: (i) end brines returning from the shallow and restricted southern basin; (ii) brine upwelling from the interface of diapirs; and (iii) springs underneath the Dead Sea discharging and mixing with Dead Sea waters. (4) The areal distribution of primary minerals is a result of several processes. Their relatively high concentrations may reflect periods of either excessive evaporation or high influx of brines leading to their precipitation. Relatively low concentrations may reflect high detrital input and/or periods of low evaporation and small influx of highly saturated brines from springs and other sources. ACKNOWLEOOMENTS

The authors wish to express their thanks to the Dead Sea Works for use of their boat and a special thanks to Yosef Sasson, the Captainofthe boat We alsogratefully acknowledge the assistance in the field of Dr. Mira Stiller, Aminadov Nishri, Dana Steinborn, and Rami Matmoni. The senior author also wishes to thank the Weizmann Institute of Science and especially Y001 Gat for providing logistical support, and office and lab spaceforthe duration of the research study. The writers are grateful to J. Warren, A. Reeckmann and P. M. Harris for critical reading of an earlier version of the manuscript. We also wish to acknowledge L. Hardie, Y. Bentor and J. Hudson for helpful comments. This research was supported by a grant from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel, to GeraldM. Friedman and Professor Y001 Gat

REFERENCES

AMrr, ORA, 1966, The behavior of clays in the Dead Sea: Unpubl. Masters Thesis, Hebrew University, 32 p. (in Hebrew)

(1) The most common kind of sediment in the Dead sea AMrr, ORA, and 8ENToR, Y. K., 1971, pH-dilution curves of saline waters: is clayey silt Onlyin one small area is a finersediment present Chemical Gool., v. 7, p. 307-313 (2) Samples from cores and grab samples down to 50 em BEGIN, Z 8., EHRUCH, A., and NATHAN, Y., 1974,Lake Lisan, the Pleistocene below the sediment-water interface are composed of fineprecursor of the Dead Sea: 0001 Survey of Israel Bull., v.63, 30 pp. grained sediments (clayey silt or silty clay) which display alternating dark detrital-rich and white aragonite-rich laminae BENTOR, Y. K., 1961, Some geochemical aspects of the Dead Sea and the ranging in thickness from fractions of a millimeter to 7 em. question of its age: Geochim. Cosmocbim. Acta, v.25, p. 238-260. Intercalations of sand lenses, gypsum crystals, and halite cubes BENTOR, Y. K., 1969, On the evolution of subsurface brines in Israel: Chern. are alsopresent 0001., v. 4, p. 83-110. (3) Theparticles ofwhich thebottomsediments arecomposed BEY1lI, M., 1977a,Recent evolution of Dead Sea brines: Geol, Survey Israel fall into two groups: internal report MB/3177, IIp. (a) detrital sediments, and BEYTH, M, 1977b, Present stageof DeadSeabrines: Geol, Survey Israel internal (b) primary minerals. report MGII 1/77,5 p. Detrital sediments include particles of limestone, quartz and clayminerals. Feldspar and dolostone occur in minoramounts. BJscAYE, P. E., 1965,Mineralogy and sedimentation of Recent deep-sea clay The primary minerals include aragonite, gypsum, and halite. in the Atlantic Oceanandadjacent seasand oceans: Bull. Geol, Soc.America, v. 76, p. 803-832. Aragonite formation is enhanced by influx of bicarbonate from

THE SEDIMENTOWGY OF THE DEADSEA

57

DRUCKMAN, Y., 1981, Sub-recent manganese-bearing stromatolites along shorelines of the Dead Sea: in Monty, C, 00., Phanerozoic stromatolites: Springer-Verlag, NewYork,p. 197·208.

MrLLOT, G., 1970,Geology of clays-weathering, sedimentology, geochemistry (trans. from French by W. R Ferrand and Helene Paquet): New York and Berlin, Springer-Verlag, 429 p.

DRUCKMAN, Y. and BEYTII, M., 1977, "Salt reefs" a product of brine mixing, Lisan Straits, Dead Sea (a preliminary study): Geol. Survey of Israel report, MGm77, 15p.

NEEV, D. and EMERY, KO., 1967, The Dead Sea, depositional processes and environments of evaporites: Geo1. Survey IsraelBull., v. 41, 147p.

FRIEDMAN, GM., 1964,Earlydiagenesis andlithification incarbonatesediments: Jour. Sedimentary Petrology, v. 34, p. 777-813. FRIEDMAN, G.M., 1965, On the origin of aragonite in the Dead Sea: Israel Jour.Earth-Sciences, v. 14,p. 79-85. FRIEDMAN, GM., 1980, Review of depositional environments in evaporite deposits andtheroleofevaporites inhydrocarbon accumulation: Bull., Centres de Recherches Exploration-Production Elf-Aquitaine, v. 4., p. 589-608. GARBER, R A, 1980, The Sedimentology of the Dead Sea: unpubl. Ph.D. Thesis: Rensselaer Polytechnic Institute, Troy N.Y., 169p. GARBER, R A, NISHRI, A, and NISSENBAUM, A, 1981, Recent deposition of manganese alongthe Dead Sea shore: Sed. Geo1., v. 30, p. 267-274. GROSS M.G.,1964,Variations in the0 18/016 and,CI3/CJ2 ratiosindiagenetically altered limestones in the Bermuda Islands: Jour. Geology, v. 72, p. 170194. HUDSON, J. D., 1977, Stable isotopes and limestone lithification: Jour. Geol. Soc.London, v. 133,p. 637-660. JACKSON, M L, 1969,SoilChemicalAnalysis-Advanced Course 2nd edition, 8th printing, 1973, Published by the author, Dept of Soil Science, U. of Wise., Madison, 895 p.

NEEv, D., and HALL, J-K, 1976, The Dead Sea Geophysical Survey, 19 JulyI August 1974:Geol,Survey lsraellntemal Report MGD 6176, 21p. NEEV, D. and HALL, J. K, 1979, Geophysical investigations in the Dead Sea: Sediment Geol., v, 25, p. 209-238. SCHMAlZ, R F., 1969, Deep-water evaporite deposition: A genetic model: Am Assoc. Petroleum Geo1. Bull., v. 53, p. 798-823. SHALEM, N., 1949, Whitening of the waters of the Dead Sea: Nature, v. 164, p.72. SHEPARD, F. P., 1954, Nomenclature based on sand-silt-clay ratios: Jour. Sedimentary Pet, v. 24, p, 151-158. STARINSKY, A, 1974,Relationship between Ca-chloride brines and sedimentary rocks in Israel: Unpub1. Ph.D. Thesis, Hebrew University, Jerusalem, 176p. (in Hebrew). STEINHORN, I., 1985, The disappearance of the long term meromictis stratification of the Dead Sea: Limnol, and Oceanogr., v. 30, p. 451-472. STEINHORN, I., AssAF, G., GAT, 1. R, NlSHRl, A, NISSENBAUM, A., STILLER, M, BEYTII, M.,NEEV, D.,GARBER, R, FRIEDMAN, G.M., and WEISS, W., 1979, The DeadSea: deepening of themixolimnion signifies the overture to overturn of the watercolumn: Science, v. 26, p. 55-56. STILLER, M. and CHUNG, Y C; 1984, Radium in the Dead Sea: A possible tracerfor the durationof meromixis: Limno1. Oceanogr., v. 29, p. 574-586.

MANSPEIZER, W., 1985,The Dead Sea Rift: impactof climate and tectonism on Pleistocene and Holocene sedimentation: in Biddle, K T. an N. ChristieBlick(eds.), Strike-slip deformation, basinformation, and sedimentation: Soc. Econ.Paleontologists Mineralogists Spec. Publ. No.37, p. 143-158.

WEll.ER, Y, SASS, E., and ZAK, I., 1974,Halite oolites and ripples in the Dead Sea,Israel: Sedimentology, v. 21, p. 623-632.

MAzoR, E., ROSENTIiAL, E., and EKSTEIN, J., 1969, Geochemical tracing of mineralwatersources inthesouthwestern DeadSeaBasin, Israel: Jour.Hydrol., v. 7, p. 246-275.

WHITEHOUSE, U. G., JEFFERY, L. M., and DEBBRECHT, J.D., 1960, Differential settling tendencies of clay minerals in saline waters: Proc. Conf. ClaysClay Min., 7th,Washington, D. c, 1958,5: p. 1-79. YAFFA, SHMUEL, 1972,The DeadSea climate: Meteorological Service of Israel, Publication No. 12,8 p. (in Hebrew). Manuscript received March 16, 1987 Manuscript acceptedJuly2, 1987