22. RED SEA EVAPORITES: A PETROGRAPHIC AND … · Red Sea, the presence of Miocene salt is indicated by seismic reflection studies (Lowell and Genik, 1972) and confirmed by drilling

22. RED SEA EVAPORITES: A PETROGRAPHIC AND GEOCHEMICAL STUDY

Peter Stoffers, Laboratorium für Sedimentforschung, Universitát Heidelberg, Germanyand

Robert Kuhn, Kali-Forschungs-Institut der Kali u. Salz A.G., Hannover, Germany

INTRODUCTION

One of the major shipboard findings during Leg 23drilling in the Red Sea was the presence of late Mioceneevaporites at Sites 225, 227, and 228 (Figure 1).

RED SEA

SAUDIARABIA

HOT BRINE AREA(SITES 225,226*227)

ETHIOPIA

Figure 1. Leg 23 drill sites in the Red Sea.

The top of the evaporite sequence correlates with astrong reflector (Reflector S) which has been mapped overmuch of the Red Sea (Ross et al., 1969, Phillips and Ross,1970). This indicates that the Red Sea appears to be

underlain by late Miocene evaporites throughout most of itsextent.

Miocene sediments, including evaporites, are knownfrom a few outcrops along the coastal plains of the Gulf ofSuez to lat 14°N (Sadek, 1959, cited in Friedman, 1972;Heybroek, 1965; Friedman, 1972). Along the length of theRed Sea, the presence of Miocene salt is indicated byseismic reflection studies (Lowell and Genik, 1972) andconfirmed by drilling. The recently published data fromdeep exploratory wells (Ahmed, 1972) demonstrate thegreat thickness of elastics and evaporites which weredeposited in the Red Sea depression during Miocene time.

The Red Sea evaporites are of the same age as theevaporites found by deep sea drilling (DSDP Leg 13) in theMediterranean Sea. Therefore, Reflector S in the Red Sea iscomparable to Reflector M in the Mediterranean. It isassumed that during Miocene time a connection betweenthese two basins was established (Coleman, this volume)resulting in a similar origin for the evaporites deposited inthe Red Sea and in the Mediterranean Sea. The origin of theMediterranean evaporites has been discussed in great detail(Hsü et al., 1973; Nesteroff, 1973; Friedman, 1973).

The formation of evaporites may be interpreted by threedifferent hypotheses.

1) Evaporation of a shallow restricted shelf sea orlagoon which receives inflows from the open ocean.

2) Evaporation of a deep-water basin which is separatedfrom the open ocean by a shallow sill (Schmalz, 1969).

3) Evaporation of playas or salt lakes which are situatedin desiccated deep basins isolated from the open ocean (Hsüet al., 1973).

The purpose of this study is to show whether one ofthese models might apply to the formation and depositionof the Red Sea evaporites. Therefore, a detailed petro-graphic and geochemical investigation was carried out.

PETROGRAPHIC DESCRIPTION

The stratigraphy of the evaporite sequence encounteredat Sites 225, 227, and 228 is shown in Figure 2, which alsoindicates the mineralogy of the interbedded black shales.The photographed evaporite cores can be seen in the sitereport chapters for Sites 225, 227, and 228. Forty selectedsamples of the evaporite rocks from the Red Sea boreholeswere studied in thin section under the petrographicmicroscope. All anhydrite samples were X-rayed in order toclarify the presence of gypsum, dolomite, calcite, andmagnesite. The structure and texture of the rocks weredescribed according to Maiklem et al. (1969).

821

P. STOFFERS, R. KUHN

SITE 225 SITE227 S'TE 228

24

25

26

'wy yV1

27

28

29

lüil BLACK SHALE

UH ANHYDRITE

H I HALITE

d = DOLOMITEcr= CRISTOBALITEp = PALYGORSKITEch= CHLORITEq = QUARTZm= MONTMORILLONITEcc= CALCITEi = ILLITE

221(m)

226

235

244

253

262

271

280

289

292

297

305

314

323

332

341

350

359

30

37

38

39

40

44

286(m)

7QS

304

313

3??

35

Λ Λ Λ Λ

36

37

38

BULK

15 10 5

Figure 2. Stratigraphy of the evaporites encountered at Sites 225, 227, 228 and the mineralogyof the interbedded black shales.

822

RED SEA EVAPORITES

Site 225

Site 225 is situated 10 miles east of the Atlantic II Deepin a water depth of 1228 meters. Two hundred and thirtymeters of sediment was continuously cored, including 53meters of evaporites.

At the base of this hole (Samples 29-3, 0-150 cm; 29-2,90-150 cm) white to gray rock salt was recovered. Bedding(3-7 cm apart) of cloudy and clear white halite occurred.Thin anhydrite layers (0.2 to 1 cm) are intercalated withinthese beds. The dip of the anhydrite layers ranges up to15°. Thin section examination (29-3, 100-103 cm; Figure3) reveals medium to coarse crystalline halite (2-3 mm indiameter). Fine polyhalite needles (2CaSC»4 MgS04K2SO4 2H2O) (0.75 mm in length, 0.05 mm in width)are stringlike interspersed in the pore spaces of halitecrystals. Twinning of the polyhalite crystals is found quitefrequently. In larger pore spaces the polyhalite occurs asdensely felted needles (29-3, 100-103 cm; Figure 4). Thecloudy appearance of the gray halite is due to scatteredbrine inclusions. The linear orientation of the inclusionsoften produced crystal structures known as "hopper" and"chevron" which indicate a primary origin (Dellwig, 1953;Wardlaw and Schwerdtner, 1966). These structuresdisappear on recrystallization resulting in more transparentlarger crystals clear of opaque inclusions.

The layering of clear and cloudy salt with intercalatedthin anhydrite bands is typical of the halite recovered onLeg 23 drilling in the Red Sea. No obvious cyclicrelationship is observed between these features. Theabsence of a statistical correlation between clear salt,cloudy salt, and anhydrite layers is reported by Fuller andPorter (1969) from the Devonian and Mississippian saltdeposits of Alberta and North Dakota and by Dellwig(1968) from the Hutchinson salt deposit in Kansas. Theseauthors explain the alteration of clear and cloudy salt bychanges in the rate of evaporation which reflect changes inclimatic conditions. The thickness of the halite laminae isdetermined by seasonal dilution (removal and recrystal-lization), whereas the thickness of the anhydrite laminae isdetermined by seasonal reconcentration.

The halite sequence is topped by a small interval ofslightly layered anhydrite (29-2, 87-90 cm) followed bytypically nodular anhydrite (29-2, 6-81 cm). Black shalesare intercalated within the anhydrite (29-2, 81-87 cm; 29-1,60-67 cm). They are predominantly composed of mont-morillonite and smaller amounts of cristobalite. Theseshales are thought to have originated from interbeddedvolcanic material. The anhydrite structure is described bestas distorted nodular mosaic (29-2, 10-20 cm; Figure 5). Thewhite nodules (up to 3 cm in diameter) are separated bythin greenish seams. Under the petrographic microscope thenodules appear to be a randomly arranged combination ofmicrocrystalline and small lath-shaped crystals (0.05-0.1mm; Figure 6). The nodules are bound by dark brownishstreaks of opaque, probably organic-rich, material and veryfine grained dolomite (~0.003 mm) which were separatedfrom the matrix during growth or deformation of theanhydrite nodules.

Core 28 and the lower part of Core 27, Section 2 consistof bedded clear white and dusty gray halite (about 2 to 4cm apart) which is intercalated by thin anhydrite layers

(0.5-1 cm). The anhydrite layers show dips between 10-25°.The thin sections (28-3, 95-105 cm; 27-2, 110-112 cm)show medium to coarsely crystalline halite (2 to 4 mm)with partly mosaic structure (Figures 7, 8). Within thehalite there are thin layers of needle-like anhydrite whichlocally forms rounded aggregates. In the dusty grayhalite-oriented liquid, inclusions are very abundant.

The halite is topped by a small interval of light gray,slightly laminated anhydrite (27-2, 69-87 cm) followed bydistorted nodular to bedded nodular mosaic anhydrite(27-2, 0-60 cm; 27-1, 90-150 cm; Figure 9). Petrographicmicroscope examination (27-1, 119-122 cm) shows that thematrix is a fine crystalline anhydrite. Besides fine-graineddolomite and streaks of opaque, probably organic-rich,material, clusters of coarse crystalline anhydrite occurwhich sometimes display sheath-like to stellate patterns(Figure 10). The top of Core 27, Section 1 consists of amore laminated anhydrite. A thin layer of greenish grayfine-grained dolomite (27-2, 60-69 cm) is intercalated in theanhydrite, indicating a regressive stage of evaporation.

Core 26 is mainly composed of white anhydrite withbedded nodular mosaic structure (26-2, 133-136 cm; Figure11). The nodules are separated by black opaque, probablyorganic-rich, matter and small amounts of carbonatematerial. The matrix again is a very fine crystallineanhydrite of aligned felted texture (~0.005 mm). Withinthis ground mass there are porphyrotopes of coarsecrystalline anhydrite (26-1, 140-144 cm; Figure 12). Theseaggregates are thought to have formed by diageneticmetablastese.

The anhydrite is topped by a black shale sequence (26-1,1-34 cm). The thin section (26-1, 13-16 cm) reveals thepresence of slightly laminated algal oncolites (Figure 13 a,b, c). The oncolites consist of very fine-grained dolomite(0.005-0.01 mm) impregnated with opaque material,probably rich in organic matter, and some pyrite. Thecavities within the oncolites are often filled with secondaryanhydrite, sometimes with chalcedony.

A finely laminated massive anhydrite of alternatingmedium gray and white layers is found in the lower sectionof Core 25 (137-150 cm). The thickness of the individuallayers is about 1-1.5 mm. The upper part of the coreconsists of dark shales. X-ray analyses show that the shalesare mainly composed of dolomite (up to 60%), layersilicates, palygorskite, analcite, quartz, feldspar, and somepyrite. The dominant clay mineral is an illite-montmorillonite mixed layer. The formation of the blackdolomite shales is related to regressive evaporation causedby a strong dilution of the brine.

The top 10 cm of Core 25 and the major part of Core 24(67-150 cm) consist of wavy laminated massive anhydrite(Figure 14). The laminations are described best as stromato-lites. They are composed of alternating white 2-4 mm andmedium gray 0.5-1 mm thick layers. The thin section (24-1,70-74 cm; Figure 15) shows very uniform fine crystallinelath-shaped anhydrite (~0.015 mm) in subparallelorientation. The darker layers consist of scattered micronsized opaque, probably organic, material and very finegrained dolomite.

A dark gray to black semilithified to lithified dolomiticsilty claystone (Core 24-1, 0-70 cm; Cores 23, 22, 21) tops

823


-

• * •

> • -

1

•

iFigure 3. Cloudy halite with oriented liquid inclusions and

interspersed polyhalite (225-29-3, 100-103 cm). Scalebar represents 4 mm.

f rf f

~ \ * • : •

:- v *

Figure 4. Clusters and fine needles of polyhalite in halite(225-29-3, 100-103 cm). Partially crossed nicols. Scalebar represents 1 mm.

the evaporite section of Site 225. The increasing marinefauna found in the upper cores of this sequence indicatesthe change to open marine conditions.

Site 227

Site 227 is situated about two miles east of the edge ofthe Atlantis II Deep in a water depth of 1795 meters and 359meters of Quaternary to late Miocene sediments werecored, including 133 meters of late Miocene and probablyolder anhydrite and halite.

The lowermost core (45-1, -2) consists of beddedalternating white and light gray rock salt. In general, theindividual layers vary between 2 and 5 cm. Anhydrite layers0.2 to 10 mm thick (2 to 6 cm apart) are intercalatedwithin this halite sequence. The thin section (45-1, 112-117cm; Figure 16) shows coarse crystalline halite (5-8 mm)interspersed with polyhalite needles (0.05-0.08 mm).Liquid inclusions are abundant within the gray haliteforming typical chevron and hopper structure.

Anhydrite is found in Core 44, Sections 1 and 2. In thelower part of Section 2 (70-150 cm), the anhydrite is ofdistorted bedded nodular structure. The white nodules aresurrounded by greenish gray dolomite seams. The top ofSection 2 and the entire Section 1 consist of thin layers(1-2 mm) of massive whitish gray anhydrite interbeddedwith layers of medium gray material (~0.03 mm; Figure17). Petrographic microscope examination (44-2, 65-70 cm;Figure 18) reveals very fine laminated anhydrite of alignedfelted texture (0.03-0.06 mm). The lath-shaped crystals areoriented parallel to the bedding planes. The darker layersare composed of dolomite impregnated with organic matterand very fine grained pyrite.

The entire Core 43 (Sections 1 to 5) and Core 42(Sections 1 to 3) are represented by irregularly bedded clearwhite and gray halite. Very distinct layers of anhydrite(~1.5 cm) interbedded within the halite occur in Sections3, 4, and 5, whereas the anhydrite layers in the uppersection of Core 43 and in Core 42 are only slightly

indicated. Clear white halite dominates this upper sequence.The thin section (43-2, 20-25 cm; Figure 19) showsmedium coarse crystallized halite (1-2 mm) of generallymosaic structure. The halite is interbedded by a 1.5 cmthick anhydrite layer. The subfelted anhydrite crystals(~0.07 mm) are in random orientation. Within theanhydrite there are dark stringers of opaque, probablyorganic-rich, material.

The lowermost 4 cm of Core 41, Section 1 is composedof bedded massive anhydrite topped by alternating whiteand gray halite (4-8 cm apart) with intercalated 2-10 mmthick anhydrite layers. Under the microscope (41-1,105-110 cm; Figure 20), the halite is coarse crystalline andshows a mosaic structure. The microcrystalline anhydrite(0.02-0.05 mm) is often aligned with the halite particles.This concentration of anhydrite along the halite rims iscaused by anhedral halite growth which seems to replaceeuhedral halite crystals.

Core 40, Sections 1 and 2 consist of black shales whichwere completely fractured by drilling. X-ray diffractionshows that the shales are composed, in general, of mont-morillonite and smaller amounts of calcite, feldspar, quartz,clinoptilolite, analcite, pyrite, dolomite, gypsum, andanhydrite. The abundant montmorillonite and the presenceof zeolites suggest a strong volcanic influence.

Core 39, Section 1 and Core 38, Section 1 arerepresented by massive anhydrite with a slight indication ofbedding. The thin section (38-1; Figure 21) shows alath-shaped anhydrite matrix interspersed with large fibrousor sheath-like anhydrite aggregates. The recrystallization ofthe anhydrite has caused the scattered dolomite and opaquematerial to migrate to the boundaries of the aggregates.

Anhydrite of bedded mosaic structure is found in Core37, Sections 1 and 2. The thin section (37, CC; Figure 22)shows fine crystalline dolomite layers (0.005-0.02 mm)impregnated with opaque material. The dolomite layers(0.05-0.1 mm thick) are intercalated within the lath-shapedanhydrite matrix.

824

RED SEA EVAPORITES

Figure 5. Nodular anhydrite (225-29-2, 10-20 cm). Scalebar represents 1 cm.

*•*u.

Figure 6. Nodules consisting of microcrystalline and smalllath-shaped anhydrite (225-29-2, 10-20 cm). Scale barrepresents 4 mm. Oriented thin section. Crossed nicols.

Φ". •*

r•• .

Figure 7. Medium to coarsely crystalline halite intercalatedwith thin anhydrite layers (225-28-3, 95-105 cm). Scalebar represents 4 mm.

Figure 8. Medium to coarsely crystalline halite with round-ed anhydrite aggregates (225-27-2, 110-112 cm). Scalebar represents 4 mm.

825


Figure 9. Anhydrite of distorted nodular mosaic structure(225-27-1, 119-122 cm). Scale bar represents 2 cm.

Figure 11. Anhydrite of bedded nodular mosaic structure(225-26-2, 133-136 cm). Scale bar represents 2 cm.

Figure 10. Fine crystalline anhydrite matrix, dark graysemi-opaque, probably organic-rich, streaks, fine-graineddolomite, clusters of coarse crystalline anhydrite (225-27-1, 119-122 cm). Scale bar represents 4 mm. Crossednicols.

Figure 12. Fine crystalline anhydrite of aligned felted tex-ture, streaks of opaque material, aggregates of coarsecrystalline anhydrite (225-26-1, 140-144 cm). Scale barrepresents 4 mm. Thin section is oriented. Crossednicols.

826

RED SEA EVAPORITES

Figure 13. Oncolite structure (225-26-1): (a) scale bar represents 4 mm; partially crossed nicols; (b) partially crossednicols; some cavities filled with anhydrite and chalcedony; scale bar represent 0.5 mm; and (c) fine grained dolo-mite impregnated with opaque, probably organicrich material; some anhydrite in the cavities; scale bar repre-sents 0.1 mm.

827


Figure 15. Fine crystalline anhydrite of aligned feltedtexture with alternating layers of opaque materialinterspersed with dolomite (225-24-1, 70-74 cm).Crossed nicols. Scale bar represents 4 mm.

Figure 14. Wavy laminated anhydrite (stromatolites) of al-ternating white anhydrite and dark layers composed ofdolomite and opaque material (225-24-1, 80-85 cm).Scale bar represents 2 cm.

Figure 16. Coarse crystalline halite interspersed withpoly halite needles, oriented liquid inclusions (227-45-1,112-117 cm). Partially crossed nicols. Thin section isoriented. Scale bar represents 4 mm.

Figure 17. Bedded mosaic anhydrite with intercalated darklayers (227-44-2, 65-70 cm). Scale bar represents ~2 cm.

828

RED SEA EVAPORITES

Figure 18. Fine laminated anhydrite of aligned feltedtexture. Dark layers consist of dolomite impregnatedwith opaque material (227-44-2, 65-70 cm). Crossednicols. Thin section is oriented. Scale bar represents4 mm.

Figure 19. Coarse crystalline halite with interbeddedanhydrite layer (227-43-2, 20-25 cm). Partially crossednicols. Thin section is oriented. Scale bar represents4 mm.

.*>-'- * . . « • - . .

Figure 20. Coarse crystalline halite with micro crystallineanhydrite aligned with the halite particles (227-41-1,105-110 cm). Partially crossed nicols. Thin section isoriented. Scale bar represents 4 mm.

Λ -• . - > ' . ' • . * ^ i ;è.•V ; > ,^ / fò ^ :

Figure 21. Lath-shaped anhydrite matrix interspersed withlarge fibrous or sheath-like anhydrite aggregates(227-38-1, 130-134 cm). Crossed nicols. Thin section isoriented. Scale bar represents 4 mm.

829


Interbedded black shales occur within the anhydrites ofCore 36 (36-3, 70-150 cm; 36-2, 95-150 cm). The shaleswere completely broken up by the drilling operation. X-rayexamination reveals disordered cristobalite as the pre-dominant constituent (>80%) besides traces of paly-gorskite, zeolites, dolomite, quartz, feldspars, and pyrite.Nannofossils {Discoaster quinqueramus) and benthonicforaminifera are present in the core catcher of Core 36.Under the petrographic microscope, the microcrystallinecristobalite is finely laminated by opaque material probablyrich in organic constituents (Figure 23). Scanning micro-graphs show typical silica spherules (5-1 Oµ) of disorderedcristobalite. The spherules consist of irregularly orientedtubes and needles (Figures 24 a, b) described as lussatite(Berger and von Rad, 1972). The mineralogy of the blackshales suggests volcanism as a source of the silica.

The anhydrite in Core 36, Section 3 has a massivestructure with slight indications of bedding. Under thepetrographic microscope (36-3; Figure 25), the anhydrite isof microcrystalline texture (~0.05 mm). Intergrownclusters of coarse recrystallized anhydrite and authigenicquartz crystals (up to 3 mm) are found within thisfine-grained matrix. The quartz crystals often showtwinning and frequently contain inclusions of anhydrite.These inclusions are oriented parallel to the crystal edgesdue to rhythmic growing of the quartz crystal (36-3; Figure26). Quartz is generally common in saline deposits. Itoccurs partly as clastic grains and partly as an authigenicmineral with well developed idiomorphic forms. Accordingto Grimm (1962), quartz frequently forms in salinesediments during an early stage of diagenesis. Idiomorphousquartz crystals with inclusions of anhydrite demonstratethat this anhydrite rock probably was of primary origin andnot derived from gypsum by diagenesis. Due to the volumeincrease accompanying the transition to anhydrite, theinclusions of primary gypsum would have produced somefracturing. However, microscopic investigations show thatthe inclusions of anhydrite in the quartz completely fill theavailable space. No fractures could be observed.

Anhydrite of streaky structure is found in Core 36,Section 2 (Figure 27). Tails and pieces of greenish graycarbonate material are intercalated with dark grayanhydrite beds. The carbonate material was identified asmagnesite by X-ray diffraction. Besides magnesite there aretraces of pyrite. The thin section (36-2; Figure 28) showsmicrocrystalline anhydrite (~0.05 mm) with slightlyaligned orientation. The very fine crystalline magnesite(0.003-0.007 mm) occurs as clusters or streaks within theanhydrite. Core 36, Section 1 is also composed of streakyanhydrite. Petrographic microscope examination (36-1;Figure 29) reveals anhydrite crystals of aligned feltedtexture. Lumps of anhydrite (up to 10 mm) with sharpedges or straight sides are separated from one another by amagnesite matrix (0.007-0.02 mm). Some of these featuresmight be deformed gypsum pseudomorphs (Borchert andBaier, 1953). The microcrystalline magnesite occurs ashypidiotopic to xenotopic grains. Frequently, magnesiteforms ringlike aggregates which surround a single magnesiteor pyrite grain. Ringlike magnesite aggregates withinZechstein anhydrites of Northern Germany have beendescribed by Langbein (1961) and Kosmahl (1969). It is

thought that these ringlike structures originated fromrecrystallization due to diagenesis.

A study of the available analyses of carbonate in saltrocks of Central Europe (Kuhn, 1968) shows that along themargins of the basin, calcite, and more commonly,dolomite occurs, derived from relatively diluted brines;whereas, magnesite occurs exclusively in the sediments ofthe more highly concentrated brines of the interior of thebasin. The distribution of carbonates is evidently related tothe process of progressive concentration during which theheavier magnesium chloride solution accumulated in thedeepest part of the basin. According to Link (1938, 1942),the dolomite and the magnesite have to be considered asearly diagenetic.

The bottom of Core 35 consists of bedded massive lightgray anhydrite interbedded with thin dark gray layers about1 mm apart (Figure 30). Petrographic microscope examina-tion (35-5, 115-123 cm; Figure 31) shows microcrystallineanhydrite (~0.05 mm) interspersed with sheath-likeanhydrite (1-2 mm). Dark layers (0.02-0.1 mm) areintercalated in the fine crystalline matrix and are composedof opaque, probably organic-rich, material and smalldolomite grains (~0.004 mm). The anhydrite is topped bybedded alternating clear white and medium gray halite(Sections 1-5). The thickness of the individual beds,generally, is about 3 to 7 cm with the exception of thelower part of Section 5 (60-120 cm), which is representedby a sequence of medium gray halite. Thin brownish greenanhydrite layers (0.5-1 cm) are intercalated in the halite(Figure 32) at intervals of 1 to 7 cm. In the medium grayhalite of Section 5, these laminations are only 0.5 cm apart.The thin anhydrite layers show dips up to 60°. The thinsection (35-3, 53-60 cm; Figure 33) reveals medium tocoarse crystalline halite (~l mm) with clusters and streaksof lath-shaped anhydrite (0.1-0.2 mm). Recrystallization ofthe halite has caused the anhydrite to concentrate along thecrystal boundaries of the halite.

Core 34 (Sections 1 to 4), Core 33 (Sections 1 to 4), andCore 32 (Sections 1 to 5) consist of alternating clear whiteand medium gray halite with interbedded thin anhydritelayers (0.2 mm). In Cores 32 and 33, the anhydrite layersare only slightly indicated. Clear white halite predominatesthis sequence. Under the petrographic microscope (33-1,80-84 cm; Figure 34), the medium to coarse crystallinehalite (> 1 mm) of mosaic structure is oriented parallel tothe bedding plane. Polyhalite needles are found in clustersor stringers interspersed within the halite.

The halite sequence is topped by dark brown to blackshales (Core 31, Section 1, 0-70 cm) which are broken upby the drilling operation. The extremely hard finelylaminated black shale pieces found in the core catcher ofCore 31 are brecciated and fractured (Figure 35). Thecracks are filled with halite. According to X-ray diffraction,this black shale is predominantly composed of disorderedcristobalite with traces of pyrite and quartz. Under thepetrographic microscope (31, CC; Figures 36 a,b,c), angularfragments of cristobalite containing laminated streaks oforganic material are cemented by halite. Silica of colloformtexture occurs within the halite matrix and forms chainsand rings which show undulatory extinction. The textureclearly indicates a colloidal origin. The 2-3 mm thick

830

RED SEA EVAPORITES

Figure 22. Bedded mosaic anhydrite with thin layers of dolomite impregnated with opaque mate-rial (227-37, CC). Scale bar represents 0.1 mm.

'$Mt, *i ,,- , . • ^ / • .

2* « •* ' ^ r al

ft? a r % " 12 ~•Λ

H.... #0.- V* . » t "* ZjSmF••

/-J KM•• < l*Jj ^ i ^ H

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~m

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: c*3f ; ?4 f ; J

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• r " • " - j

Figure 23. Microcrystalline cristobalite finely laminated by opaque, probably organic rich, materi-al and pyrite (227-36-2). Scale bar represents 0.1 mm.

831


Figure 24. Scanning micrographs of cristobalite spherules (227-36-2): (a) scale bar repre-sents lOµ (micron), and (b) scale bar represents 2µ.

Figure 25. Microcrystalline anhydrite matrix, clusters ofcoarse crystalline anhydrite and authigenic quartz (227-36-3, 5-7 cm). Crossed nicols. Scale bar represents 4 mm.

832

RED SEA EVAPORITES

Figure 26. Authigenic quartz with oriented anhydrite inclusions in fine crystalline anhydrite matrix (227-36-3). Scale bar represents 1 mm. Crossed nicols.

Figure 27. Streaky laminated anhydrite (227-36-2, 73-76cm). Scale bar represents 2 cm.

833


Figure 28. Microcrystalline anhydrite with slightly alignedorientation intercalated with dark fine crystallinemagnesite (227-36-2, 73-76 cm). Crossed nicols. Thinsection is oriented. Scale bar represents 4 mm.

Figure 29. Anhydrite of aligned felted texture. The sharpedges of the anhydrite lumps suggest deformed gypsumpseudomorphs. Dark material is microcrystalline magne-site (227-36-1). Crossed nicols. Scale bar represents4 mm.

Figure 30. Bedded massive anhydrite (227-35, CC). Scalebar represents 1 cm.

Figure 31. Microcrystalline anhydrite interbedded withdark layers composed of opaque material and smalldolomite grains (227-35-5, 115-123 cm). Scale barrepresents 4 mm.

834

RED SEA EVAPORITES

fractured vein, which crosses perpendicular to the beddingplane of the shale, is filled with sheave-like partly radiatingbundles of length-slow fibers of quartzine and somemegaquartz. The silica of this black shale might beoriginated from devitrification of volcanic glass or from thedissolution of opaline skeletons. The absence of volcanicalteration products such as clinoptilolite and montmoril-lonite suggests a biogenic source for the silica. Redepositionof the silica formed disordered cristobalite. Quartzine wasprecipitated in veins. Due to some kind of tectonicmovement, probably caused by salt flowage, the black shalewas intensely brecciated. The halite, which filled the cracks,precipitated from the pore solution. Some of the silica wasprobably dissolved again because of the hypersalineenvironment and was redeposited within the halite intypically colloform texture. The thin section might demon-strate a diagenetic process leading from probably opalineskeletons via disordered cristobalite and quartzine tomegaquartz. The significance of length-slow chalcedony insedimentary rocks is discussed in great detail by Folk andPittman (1971). According to these authors, the occurrenceof quartzine is characteristic of a silification in highly basicor sulphate-rich environments and is most commonlyobserved in sequences of shallow-marine or playa lakes andevaporitic, sabkha-type depositions.

X-ray diffraction shows that the dark brown shalefragments in Core 31 consist mainly of dolomite withsmaller amounts of layer silicates, quartz, feldspar, pyrite,zeolites, and palygorskite. The white chips found in theshale sequence are completely composed of well-orderedpalygorskite. Abundant nannofossils (Discoaster quin-queramus) and rare neritic benthic foraminifera, whichoccur in the black shale of Core 31, suggest fluctuationsfrom evaporitic to marine conditions.

The lower part of Core 30, Section 2 (100-150 cm) isrepresented by white halite with thin grayish greenanhydrite layers. The overlying rock is bedded massiveanhydrite of alternating white (0.2-0.6 mm) and dark layers(0.02-0.05 mm). Under the petrographic microscope (30-2,43-48 cm; Figure 37), the anhydrite is of aligned feltedtexture (0.015-0.2 mm). The dark brown layers consist ofdolomite and micron-sized opaque material.

The anhydrite is topped by dark gray to black semi-lithified to lithified dolomitic silty claystone (Cores 26-29).The occurrence of nannofossils and foraminifera in Core 26indicates the change to open marine conditions.

Site 228

Site 228 lies close to the axial valley of the Red Seaabout lat 19°N in a water depth of 1038 meters. Threehundred and fifteen meters of sediment was continuouslycored, including 35 meters of brecciated anhydrite.

Core 39, Section 1 consists of intensely brecciatedanhydrite in a black siltstone matrix (Figure 38). Accordingto X-ray analyses, the siltstone is composed of layersilicates (chlorite and illite, with smaller amounts of mixedlayer illite-montmorillonite and kaolinite), anhydrite,quartz, Plagioclase, microcline, pyrite, and traces ofgypsum. Under the petrographic microscope (39-1, 120-122cm; 39-1; Figure 39 a,b), the felted anhydrite crystals(0.1-0.3 mm) are in random orientation. Dolomite crystals

(0.1-0.3 mm) and authigenic quartz with anhydriteinclusions are scattered in the anhydrite. This siltstoneanhydrite breccia is characterized by a high content ofheavy metals, especially zinc (Manheim, this volume). Thethin section (39-1; Figure 40), in general, shows euhedralsphalerite crystals aligned with the bedding plane. Thesphalerite crystals only occur in the anhydrite layers. Thebrownish, probably organic-rich, semi-opaque layers arefree of sphalerite inclusions.

Core 38 is represented by dark greenish gray to darkbrown siltstone. Fragments of anhydrite were found in thecore catcher. X-ray analyses of siltstone reveal a mineralogysimilar to that found in Core 39. No pyrite and gypsumwere detected. Palygorskite is abundant in the greenishmaterial. The dark brown color of the sediments isprobably due to amorphous iron. The thin section of theanhydrite (38, CC; Figure 41), in general, shows roundedanhydrite grains (0.1-0.5 mm) with abundant liquidinclusions. The grains are cemented by pyrite impregnatedwith carbonate material. The relatively uniform grain sizeand the scattered occurrence of detrital quartz grainssuggest reworked and resedimented anhydrite (Ogniben,1955).

Core 37, Sections 1 and 2 consist of brecciatedanhydrite in a greenish dark siltstone matrix. According toX-ray analyses, the siltstone is composed of layer silicates(illite, illite-montmorillonite and chlorite-montmorillonitemixed layer minerals, kaolinite, and chlorite), quartz,feldspars, and traces of dolomite. In Section 2, palygorskiteis locally very abundant. Distorted bedded mosaicanhydrite is found in Section 2 (20-35 cm). The thinsection (37-1, 145-147 cm; Figure 42) shows feltedanhydrite (~0.05-0.5 mm). The large aggregates surroundedby opaque, probably organic-rich, material seem to indicategypsum pseudomorphs.

Partly nodular and partly bedded anhydrite with inter-calated siltstone (36-1, 108-120 cm) is represented in Core36, Section 1.

White bedded massive anhydrite (35-2, 125-150 cm)topped by brecciated anhydrite in a black siltstone matrixand black siltstone (35-2, 0-125 cm) is found in Core 35.Under the petrographic microscope, the lath-shapedanhydrite crystals (—1-1.5 mm) are oriented parallel to thebedding plane (35-2, 120-125 cm; Figure 43). Salt tectonicsis suggested by the fractured and bent anhydrite layers(35-2, 116-117 cm; Figure 44). The thin section (35-2,116-117 cm; Figure 45) shows anhydrite of microcrystal-line texture (~O.Ol mm). The thin dark layers, which areabout 0.5 mm apart, consist of opaque, probably organic-rich, material, pyrite, and some tiny dolomite crystals.Bedded massive anhydrite is found at the top of Core 35,Section 2 (35-2, 14-17 cm; Figure 46). Darker layers ofcoarse-grained material are intercalated in the anhydrite.Under the petrographic microscope (35-2, 14-17 cm; Figure47), the anhydrite is of microcrystalline texture (~O.Olmm) and shows bedding due to the intercalated thinopaque layers. Abundant clastic material is interspersed inthe anhydrite. The clastic material is generally rounded toangular and consists of quartz, feldspar, and heavy minerals(green hornblende and epidote). In the upper part of thesection, the clastic material is fine grained (~O.l mm),

835


6 7 8 9 1 2 5 6 7

Figure 32. Alternating clear white and gray halite with intercalated thin anhydrite layers (227-35-4).

Figure 33. Coarse crystalline halite with clusters andstreaks of lath-shaped anhydrite (227-35-3, 53-60 cm).Scale bar represents 4 mm.

Figure 34. Coarse crystalline mosaic halite with inter-spersed polyhalite (227-33-1, 80-84 cm). Partiallycrossed nicols. Scale bar represents 4 mm.

Figure 35. Fractured laminated black shalewith a chalcedony filled vein (227-31, CC).Scale bar represents 1.25 cm.

836

RED SEA EVAPORITES

Figure 37. Anhydrite of aligned felted texture. Dark brownlayers consist of fine-grained dolomite and micron-sizedopaque material (227-30-2, 43-45 cm). Thin section isoriented. Scale bar represents 4 mm.

Figure 36. Fractured cristobalitic shale, cracks are filledwith halite, colloform silica is within the halite. (227-31,CC): (a) scale bar represents 4 mm; partially crossednicols, (b) scale bar represents ~ 1 mm; partially crossednicols, and (c) fractured length-slow chalcedony veinwith megaquartz; scale bar represents ~ 1 mm; partiallycrossed nicols.

837


Figure 38. Intensely brecciated anhydrite in a black siltstone matrix.

Figure 39. Felted anhydrite crystals with dolomite and authigenic quartz (228-39-1, 120-122 cm): (a) scale bar 4 mm,crossed nicols; and (b) scale bar 0.5 mm.

838

RED SEA EVAPORITES

Figure 40. Sphalerite andpyrite crystals in anhydrite (228-39-1). Scale bar represents 0.4 mm.

\**r , - * > - % - * . . • -

Figure 41. Rounded anhydrite grains cemented by pyriteand some dolomite grains (228-38, CC). Scale bar repre-sents 4 mm.

Figure 42. Felted anhydrite. Large aggregates seem to indi-cate gypsum pseudo-morphs (228-37-1, 145-147 cm).Crossed nicols. Thin section is oriented. Scale bar repre-sents 4 mm.

839


Figure 43. Lath-shaped anhydrite crystals oriented parallelto the bedding plane (228-35-2, 120-125 cm). Crossednicols. Thin section is oriented. Scale bar represents 4mm.

•> • .

,?•r•. * 4^

;-

'M\, ••&r '3S;

Figure 44. Fractured and bent massive anhydrite layers(228-35-2, 116-117 cm). Scale bar represents 4 mm.

Figure 45. Microcrystalline anhydrite. Dark layers consistof opaque material, pyrite and small dolomite crystals(228-35-2, 116-117 cm). Crossed nicols. Scale bar rep-resents 4 mm.

840

RED SEA EVAPORITES

whereas, the grain size in the lower section is about 0.5 mwhich suggests graded bedding. Medium gray distortedbedded to nodular mosaic anhydrite occurs at the bottomof Core 35, Section 1. Frequently, the anhydrite masses areseparated by a dark matrix. The thin section (35-1, 133-135cm; Figure 48) shows anhydrite of aligned felted texture.The bedded character is demonstrated by intercalatedlayers of opaque and clastic material (quartz, feldspar)(~0.07 mm). According to X-ray analyses, the shales ofSection 1 are composed of layer silicates, quartz, feldspar,clinoptilolite, pyrite, and traces of dolomite. The clayfraction is dominated by montmorillonite and smalleramounts of chlorite and illite. Palygorskite is locallyabundant.

As in Sites 225 and 227, the evaporites of Site 228 aretopped by a semilithified black dolomitic silty claystone.Marine fossils first occur in Core 33 and increase towardsthe top of the hole, thus indicating the change to openmarine conditions.

TABLE 1Analyses of Halite Samples from Sites 225 and 227

GEOCHEMICAL STUDIES

Introduction

Basic research concerning the geochemistry of evaporiteswas carried out by Van't Hoff and coworkers (1912). Sincethen, geochemical studies, of trace elements in particular,have gained increasing significance. Bromine, strontium,and rubidium are of special interest for the study of halite,anhydrite/gypsum, and potash deposits, respectively.

MethodsBromine (Br) and Rubidium (Rb)

For the Br determination, the analytical methoddescribed by D'Ans and Höfer (1934) was used. Approxi-mately lOg of sample was dissolved and buffered withphosphate solution, oxidized with NaOCl, and the bromatetitrated with n/100 thiosulfate solution after the additionof K solution. No other chloridic mineral was detectedunder the petrographic microscope. Cl was determinedaccording to the Mohr method (AgNO3) and the Br datacould be referred to pure halite.

Potassium was detected flame photometrically and Rbby atomic absorption spectrophotometer. Under the petro-graphic microscope, polyhalite was determined as the onlypotassium salt. Therefore, the K content was referred topolyhalite. Rb was detected in two samples which have arelatively high polyhalite content. The Rb values werereferred to pure polyhalite. Results are given in Table 1.

Strontium (Sr) and Calcium (Ca)

Analyses were carried out to determine the Sr and Cacontent of the anhydrite samples as well as the anhydriteobtained as "water insoluble residue" from one of thehalite samples, lg of sample was dissolved in 1 liter of waterwith the addition of HC1 and La2C»3 a nd the Sr measuredby atomic absorption spectrophotometry.

Ca was determined titrimetrically by means of TitriplexIII (CioH14N2Na2 C»8 2H2O). In most samples, anhydritewas the only Ca mineral detected under the petrographicmicroscope; thus, the Ca could be considered as anhydriteand the Sr be referred to it. Carbonate minerals (dolomiteand magnesite) were found in four samples. SO4 wasdetermined gravimetrically as BaS04 in these samples.

Sample

Site 225

27-2, 110-11228-1, 100-10528-3, 95-105 (top)28-3, 95-105 (bottom)29-2, 125-13029-3,40-45 (top)a

29-3,40-45 (bottom)29-3, 100-103

Site 227

30-2,97-10130-2, 120-12432-2, 135-14032-5, 80-8533-1,80-8433-1, 130-13333-2, 100-10534-2,77-8035-1, 143-150 (top)35-1, 143-150 (medium)

35-1, 143-150 (bottom)35-3,53-6035-5, 115-12341-1,-105-11042-1, 100-10342-2, 125-13043-2, 20-25 (top)43-2, 20-25 (bottom)43-5, 128-13645-l,75-80b

45-1, 112-117

NaCl(%)

98.8599.2884.3399.3395.3373.9185.7889.44

97.3594.5595.1698.9098.8197.0299.1098.4499.2796.74

97.8093.8136.7998.2299.8494.9898.9097.5991.6390.7992.80

Polyhalite(2CaSO4MgSO4

K2SO

42H2θ)(%)

0.160.160.160.083.52

25.5614.071.56

0.160.163.521.291.252.751.150.360.060.06

0.060.000.160.040.160.380.080.160.008.777.16

Br per pureNaCl(%)

0.00610.00590.00490.00550.01260.01370.01280.0129

0.00350.01240.01610.01600.01670.01650.01590.01410.00970.0093

0.00940.00850.00600.00840.00830.00780.00720.00690.00540.01320.0098

Note: All values given in weight-%.aRb content is 0.00037 weight-% per total sample or 0.0014weight-% Rb per pure polyhalite.

bRb content is 0.00013 weight-% per total sample or 0.0014weight-% Rb per pure polyhalite.

Anhydrite was calculated on the basis of SO4 while the Caexcess was bound as CaC03 Results are given in Table 2.

Results

Bromine (Br)

The use of bromine as a geochemical indicator of themarine evaporite environment has been discussed in anumber of publications (Boeke, 1908; D'Ans and Kuhn,1940; Kuhn, 1953, 1955; Braitsch and Herrmann, 1962,1963; Valyashcko, 1956; Holser, 1966a; Wardlaw andSchwerdtner, 1966; Herrmann, 1972; etc.). Bromine valuescan be used to indicate the relative balance between influxand reflux into or out of the basin. Fairly constant brominevalues during salt deposition suggest that the salinity of thebasin brines remained constant. Bromine profiles whichshow an increase in the bromine content during saltdeposition indicate a reduced influx or reflux whichcorrespond to a higher salinity of the basin brine. Decreas-ing bromine content points to increasing influx or refluxwhich lowered the salinity of the brines in the basin.

841


Figure 46. Bedded massive anhydrite with layers of clasticmaterial (228-35-2,14-17 cm). Scale bar represents 2 cm.

Figure 47. Micro crystalline anhydrite intercalated withangular clastic material. Clastic material shows grading(228-35-2, 14-17 cm). Crossed nicols. Thin section isoriented. Scale bar represents 4 mm.

^#lè^^3^?|j|

‰.'i.Å

i

Figure 48. Anhydrite of aligned felted texture withintercalated opaque, probably organic-rich, material andclastic detritus (228-35-1, 133-135 cm). Crossed nicols.Thin section is oriented. Scale bar represents 4 mm.

842

RED SEA EVAPORITES

TABLE 2Analyses of Anhydrite Samples

Sample

225-24-1, 70-74225-24-1, 80-85225-26-1, 140-144225-26-2, 133-136225-27-1, 119-122225-27-2, 85-87225-29-1, 133-136225-29-2, 10-20

227-30-2, 43-45227-36-2, 73-76227-35, CC227-38-1, 130-134227-41-1, 147-150227-44-1, 90-92227-44-2, 48-50227-44-2, 65-70

228-35-1, 133-135228-35-2, 14-17228-35-2, 116-119228-35-2, 120-125228-36-1, 133-134228-37-1, 145-147228-39-1, 120-122

225-24-1, 60-63225-26-1, 13-16227-31, CC227-36-3, 5-7

225-28-3, 95-105water insoluble<0.05 mm Φ225-28-3, 95-105water insoluble<0.2 mm Φ

CaSO4

(%)

96.8798.2199.8499.5096.5198.67

100.2099.3399.8267.5096.80

100.0894.5999.8681.97

100.28

82.9283.9196.5698.4199.6991.9690.76

1.9211.101.17

74.12

94.57

98.97

CaCO3

(%)

--------

-------

------

49.2831.49

3.551.32

..

..

Sr(%)

0.0990.1590.1660.1700.1950.2430.2350.232

0.4420.4950.2320.2050.2120.2120.2270.193

0.2560.1630.2230.1550.4510.1500.313

0.1340.1130.0150.156

0.142

0.277

Sr 1000Ca

3.475.505.655.806.868.367.907.93

15.0424.91

8.146.967.617.219.416.54

10.496.607.855.35

15.375.55

11.71

6.607.128.476.98

5.10

9.51

Sr per pureanhydrite

(%)

0.1020.1620.1660.1700.2030.2460.2300.2330.4430.7330.2390.2050.2240.2120.2770.192

0.3080.1940.2300.1570.4520.1630.344

(-2.2)-0.82-1.02-0.19

0.150

0.279

Note: All values given in weight-%

According to D'Ans and Kuhn (1940) and Kuhn (1955),the following marginal values for the different evaporiteshave been established:

boundary anhydrite/haliteboundary halite/polyhalite

0.003-0.017 %Br/NaCl0.017-0.023 %Br/NaCl

The bromine values found in the Red Sea halites vary fromthe anhydrite up to the beginning polyhalite region(227-33-1). In some samples, a remarkable polyhalitecontent is noticed, though the Br value still corresponds tothe anhydrite region (225-29-3; 227-45-1). It is assumedthat polyhalite formed synsedimentarily from anhydrite ina relatively shallow basin corresponding to the polyhaliteformation described by Holser (1966b) for the recentshallow lagoon Ojo de Liebre. Under such conditions,polyhalite can even occur at a relatively low Br concentra-tion in the brine.

The Br content of Site 225 clearly indicates a regressiveevaporation, i.e., the Br values decrease towards the top ofthe evaporite sequence due to a gradual dilution of thebrine.

The Br values of Site 227 show an evaporation profileranging from a first halite up to a beginning polyhaliteregion. The Br values occasionally vary up to a first haliteprecipitation (227-43-5; 227-35-5). This might be indicativeof brine fluctuations and dilutions caused by influx ofseawater into the basin. The low Br content of Sample227-30-2, 97-101 cm suggests a very strong dilution. Thissalt seems to be re crystallized completely.

In general, the bromine content found in the clear whitehalite is higher than that of the medium gray haliteintervals. This was also observed in the prairie evaporiteformation (Saskatchewan) by Wardlaw and Schwerdtner(1966). Mineralogical studies show that the clear whitehalite seems to be younger and to replace the medium grayhalite. This observation is difficult to reconcile with thebromine data. According to Schmalz (1969), the markeddensity (or salinity) stratification is responsible for thedifference in the bromine content. The gray halite ("hoppersalt") is thought to have formed at the surface, whereas, theclear halite is assumed to have precipitated in situ on thebasin floor.

The bromine content in the halite is a useful means ofassessing the brine depth. Bromine may be accommodatedin the halite crystal lattice replacing chlorine in solidsolution. Progressive evaporation causes a concentration ofbromine in the residual brine. Consequently, the haliteprecipitated from such a residual brine is characterized by ahigh Br content. Therefore, the Br content of the residualbrine in a shallow basin should exceed that in a deep-waterbasin. This principle was applied by Kuhn (1953)1 todevelop an equation to calculate the brine depth fromwhich the halite had precipitated. The samples were takenfrom a distinct evaporation stage within an "annual layer."The values are given in the following table.

Br Content

Sample

LowerSample

UpperSample

Cb2)

Intervalin meters

(m)

CalculatedDepth in

meters (T)

225-29-3227-35-1227-43-2

0.01280.0930.069

0.01370.0970.072

0.0250.0200.025

4.7 (3.04)5.9 (4.0)7.4 (5.0)

In this case, factor f = 12.3 for the polyhalite region hasbeen applied. Using factor f = 8 for the anhydrite region,the depth values decrease even more.

The above table shows that the brine basin was relativelyshallow. It is emphasized that the applicability of thismethod is limited due to the influx and reflux into or out ofthe basin. Nevertheless, these values may serve as a roughestimate.

T = f =2.17 100

2.17 = specific gravity of NaCls = specific gravity of the brinec medium NaCl concentrationin oceanic salt systems in

weight-%

843


Rubidium (Rb)

Rubidium, which can replace potassium diadochically inpotassium minerals, is also useful in studying natural saltsolutions.

The Rb content (0.0014 weight-%) found in Samples225-29-3, 40-45 cm and 227-45-1, 75-80 cm is the highestRb value described for polyhalites up to now (cf. Kuhn,1972). Only a polyhalite from Hallstatt, Austria has acomparable Rb content. The high Rb values in the Red Seapolyhalites cannot be explained by the normal evaporationof seawater. It is assumed that the brine was relativelyenriched in Rb due to local conditions such as activevolcanism accompanied by Rb rich thermal springs orexhalations. Volcanism is indicated by the abundantvolcanic material which is intercalated within the evaporiteand thus explains the high Rb values. For the Hallstadtsalts, melaphyre and fine tuffitic layers give evidence ofvolcanism (Schauberger, 1960) and may be responsible forthe high Rb content.

Strontium (Sr)

The close geochemical relationship between Sr and Cafavors the ionic substitution of Ca by Sr in calcium sulphateminerals. The Sr data furnish valuable information on thegenetic character of the salts. The Sr values (referred to pureanhydrite) and the factor Sr/Ca 1000 were used to discussthe Red Sea anhydrite deposits. Abundant data fromEuropean evaporite deposits concerning the Sr/Ca 1000factor are given by German Muller (1962) and GerhardMuller (1964). According to Gerhard Muller, Sr/Ca 1000data from 1 to 3 are indicative of isomorphous Srincorporation; Sr minerals (mainly strontianite) may formalready. Strontium minerals are found at values >7, whileonly celestine should occur in the anhydrite at Sr values>20. Sr values varying less than 20 percent suggest uniformevaporation; whereas, scatter greater than 30 per cent isindicative of irregular changes of the brine, perhaps due tofreshwater influx. The Sr/Ca 1000 data from the Red Seasamples range between 3.5 and 24.9. Excluding the highvalue of 24.91 (227-36-2, 73-76 cm), an average value of7.8 ± 2.7 (scattering of 35 percent) is found. This valuecorresponds to the oceanic anhydrite region in which abeginning precipitation of Sr minerals is expected. The highscattering may be explained by irregular sedimentationcaused bv effluents or brine fluctuations.

According to Usdowski (1973), the Sr content ofgypsum at the beginning of the CaSθ4 precipitation variesbetween 0.11 and 0.12 weight-%. At the beginning and atthe end of the NaCl phase, the Sr content is 0.21 to 0.23and 0.89 to 0.975, respectively. The data at the end of theNaCl precipitation are somewhat uncertain because theisomorphous replacement of Ca with Sr takes placesimultaneously with the formation of celestine.

If gypsum changes early diagenetically into anhydritedue to a relatively high temperature in the CaSC>4 phase ofthe evaporation, this anhydrite contains 0.229-0.202weight-% Sr according to Usdowski; whereas, primaryanhydrite should already have a Sr value of 0.229 weight-%.

In general, the Red Sea anhydrite samples are of massivecharacter which suggests their deposition in the CaSC»4

phase. On the basis of Usdowskfs data, it might beconcluded that part of the anhydrite (anhydrite sampleshaving a Sr content of < 0.229) originated diageneticallyfrom gypsum. The part of the anhydrite having a higher Srcontent (> 0.229 weight-%) must be considered to be ofprimary origin.

The Sr values of the anhydrites which occur togetherwith carbonate material were calculated according toUsdowski with the distribution factor b = 1200 foranhydrite and b = 100 for CaCO3. The high value of Site225-24-1 is probably indicative of the presence of Srminerals. The two samples of the halite phase (225-28-3,95-105 cm; 0.05 mm 0 and 0.2 mm 0) show a relatively lowSR content; therefore, the thin anhydrite layers within thehalite might be conceived as short recurrences back intothe CaSθ4 phase of evaporation. The Sr values varyingwith the grain size might correspond to different genera-tions of CaS04 growth.

Origin of the Evaporites

It is assumed that separate great uplifts on either side ofthe Red Sea during late Eocene and early Oligocene timeproduced the Ethiopian and Arabian swells with theproto-Red Sea depression separating them (Coleman, thisvolume). The Miocene transgression of the MediterraneanSea into this closed depression led to thick evaporitedeposits.

Two main problems occur concerning the depositionalenvironment of evaporites. The first problem is whether theevaporites were precipitated under shallow- or deep-waterconditions; the second is the question of the depth of thebasin in which the evaporites were formed.

The investigation of recent sabkha environments(Tunisia, Libya, Baja California, etc.), especially at AbuDhabi on the Arabian side of the Persian Gulf (TrucialCoast) (Shearman, 1963; Kinsman, 1966, 1969; Evans,1966; KendaU and Skipwith, 1969; Butler et al., 1965;Butler, 1969; and others), have provided new models forthe interpretation of ancient evaporite deposits. The recentsabkha or tidal flat sediments are characterized by distinctstructures such as stromatolites, nodular gypsum andanhydrite, planar and ripple cross-lamination, flat pebbleconglomerates, polygonal shrinkage cracks, to mention themost important features. Using recent sabkhas as a guide,much evidence for the presence of former sabkha environ-ments is found in the salt deposits of the geological past(Fuller and Porter, 1969; West, Brandon, and Smith, 1968;Hardie and Eugster, 1971; Shearman and Fuller, 1969;Friedman, 1973).

The petrographic study of the Red Sea evaporitesrevealed abundant evidence for a sabkha deposit. Undu-lating laminations composed of semi-opaque or opaque,probably organic-rich, material impregnated with very finegrained dolomite are found within the anhydrites at Site225 (Cores 24, 25; Figure 15) and Site 227. Finelylaminated spheroidal forms consisting of dolomite, organicmaterial, and traces of pyrite are abundant at Site 225,Core 26 (Figure 13). These features are interpreted as algalstromatolites and oncolites which accumulated in shallowwater.

844

RED SEA EVAPORITES

The nodular anhydrites found in the Red Sea evaporitesequence (especially at Site 225, Core 29; Figure 5) aresimilar to the supratidal anhydrite nodules of recent sabkhadeposits. The nodules grow in carbonate sediments abovethe ground water table and displace the overlying sedi-ments. At a more advanced stage the nodules coalesce. Theoriginal boundaries become distorted which results in amosaic or distorted mosaic structure. Due to continuousgrowth, the nodules finally merge into one another (entero-lithic structure, Shearman, 1963). These different structurescan be seen in the Red Sea anhydrites.

The frequent alternations of halite, anhydrite, anddolomitic black shales reflect strong dilutions and recon-centrations of the brine, which seems most likely to berelated to a shallow-water environment.

Further support for a salina model for the Red Seaevaporites is given by geochemical studies. Depth calcula-tion by means of the bromine content revealed a veryshallow brine depth in the range of some meters. The dataobtained by isotope studies (Friedman and Hardcastle, thisvolume) show a strong influence of fresh water which canonly be explained by a salina model. The great variations inthe pore water data (Manheim, this volume) also agree witha shallow-water deposit.

Both the petrographical and the geochemical datasuggest that the Red Sea evaporites recovered during Leg 23originated from very shallow water with occasional sub-aerial exposure.

This assumption, however, does not permit drawing anyconclusions on the depth of the basin. The main difficultywith the explanation of ancient salt deposits is thediscrepancy between the rate of salt deposition in compari-son to the possible rate of basin floor subsidence. Theseproblems have been discussed very carefully by Schmalz(1969). Schmalz showed that this discrepancy can be solvedwith a deep water-deep basin model. The salts weredeposited in basins several hundreds to thousands of metersdeep. (The discovery of salts under the Gulf of Mexico(DSDP Leg 1, Ewing et al., 1969) is in excellent agreementwith this model). Indeed, the paleogeographic reconstruc-tion of the Red Sea suggests that the proto-Red Seadepression represented a topographic depression of con-siderable depth. The stratigraphic and structural recordindicates that the NW-SE trending depression developed asa trough between the Arabian and African swells rather thanas a down-faulted block (Brown and Coleman, 1972;Coleman, this volume). During Miocene time, a greatthickness of elastics (2-3 km) and evaporites (3-4 km) wasdeposited in this proto-Red Sea depression as demonstratedby deep exploratory wells (Ahmed, 1972). It is concludedthat a "shallow water-shallow basin" model for the evapor-ite deposits does not agree with this geological informationwhich evidently favors a deposition in a deep basin.

The question which remains to be answered is whetherthe salts were precipitated in deep water or in very shallowwater on the floor of a desiccated deep basin. This problemhas been discussed in great detail by Hsü et al. (1973) forthe Mediterranean evaporites (DSDP Leg 13). They con-sider a desiccated deep basin as the only logical alternativeto explain the Mediterranean evaporites. The argumentsused by these authors against the Schmalz model are not

entirely satisfactory. As in the Red Sea, the evaporitesrecovered in the Mediterranean penetrate only the terminalpart of the evaporite sequence. At this final stage, one canalso expect a salina or sabkha depositon according to thedeep water-deep basin model (see terminal stage II, Figure 8[nearly filled basin] Schmalz, 1969).

The sedimentological and geochemical studies of theevaporites recovered on Leg 23 reveal convincing evidenceof a shallow-water evaporite deposit. However, this evi-dence should be used with caution. It cannot, necessarily,be taken as an explicit indicator for the total amount ofRed Sea evaporites (3-4 km) to be of salina or sabkhaorigin. Further evidence of the deep strata is necessary andcould be provided by the data obtained from deepexploratory wells. Unfortunately, most of these data havenot been released for publication by the oil companies.

Leg 23 has proven that Reflector S in the Red Sea is thetop of a Miocene evaporite deposit comparable to ReflectorM in the Mediterranean. Assuming a connection betweenthe two basins, the "waterfall" descending from the Straitof Gibraltar must be held responsible not only for the up totwo kilometer thick evaporite sequence in the Mediter-ranean but also for the three to four kilometer thickevaporites in the Red Sea. Hsü et al. (1973) concluded thateight or ten marine invasions could have been sufficient toaccount for all the salts found under the Mediterraneanabyssal plain. Considering the idea of a repeated refilling ofthe basin during Miocene time, it seems that a desiccateddeep-basin hypothesis is not in contrast to the deepwater-deep basin model postulated by Schmalz (1969). Thedesiccated deep-basin model may be regarded as a specificstage in a cycle of repeated refilling and evaporation.Therefore, it is concluded that the origin of the Mediter-ranean and Red Sea evaporites could be accounted for bySchmalz's "Deep-Water Model" modified by periods ofdesiccation caused by occasional isolation of the Mediter-ranean from the Atlantic.

ACKNOWLEDGMENTS

The senior author is indebted to the entire Leg 23scientific and technical party for their cooperation aboardthe Glomar Challenger. Special thanks are due D. A. Ross,P. R. Supko, and O.E. Weser. The assistance given by F.T.Manheim leading to improvement of the manuscript isgreatly appreciated. Part of the work was carried out atWoods Hole Oceanographic Institution; financial supportcame from NATO. We thank K. H. Ritter for doing thechemical analyses and W. Kücking for preparing the thinsections.

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Documents

22. RED SEA EVAPORITES: A PETROGRAPHIC AND … · Red Sea, the presence of Miocene salt is indicated by seismic reflection studies (Lowell and Genik, 1972) and confirmed by drilling