ORIGINAL PAPER

Sediment characteristics and microfacies analysis of Jizansupratidal sabkha, Red Sea coast, Saudi Arabia

Mohammed H. Basyoni & Mahmoud A. Aref

Received: 20 May 2014 /Accepted: 24 February 2015 /Published online: 30 April 2015# Saudi Society for Geosciences 2015

Abstract Jizan sabkha extends along the southeastern coastalplain of the Red Sea, Saudi Arabia, and is considered as one ofthe main problems that has a negative impact on infrastructureof buildings. Field examination of the surface of the wetsabkha area indicated the presence of sedimentary surfacestructures produced by physical forces such as adhesion rip-ples, tepee polygonal ridges, efflorescent halite pods, andstructures produced by microbial activities such as peteesand blisters. Microfacies analysis of the siliciclastic and evap-orite lithofacies types has been done for sediment samplesfrom the surface, trenches, and cores. The siliciclasticlithofacies type represents the host sediments in Jizan sabkhaand consists of sand and mud. The evaporite lithofacies type isdistinguished into three microfacies types of gypsum, anhy-drite, and halite. The gypsum microfacies types are represent-ed by diagenetic growth of individual lenticular, twinned len-ticular, twinned complex lenticular, rosettes, nodular,poikilotopic, porphyroblastic, alabastrine, and clastic gypsum.The anhydrite microfacies types are represented by nodularand enterolithic anhydrite. The halite microfacies types arerepresented by primary rafts, cumulates, chevrons and cornets,and diagenetic overgrowth and mosaic halite cement. Thestructural and textural characteristics of the evaporite sedi-ments indicated the formation of primary halite crystals at

the brine surface and floor of saline pans, and the diageneticformation of gypsum and anhydrite below the sediment sur-face as intrasediment displacive, inclusive, and replacivegrowth in the wet sandflat and mudflat areas. Recognition ofsuch structural and textural features of the evaporite sedimentshelps in solving engineering geological problems in Jizan areaand allowed also for interpreting the similar sabkha sedimentsin the rock record.

Keywords Lenticular . Rosette gypsum . Nodular anhydrite .

Basin zonation . Jizan sabkha . Saudi Arabia

Introduction

The coastal plain of the Red Sea of Saudi Arabia contains aseries of isolated coastal lagoons, saline pans, and supratidalsabkhas. The coastal lagoons have salinities slightly higherthan the Red Sea water, and their mineralogic composition,sediment textures, pollution, environmental characteristics,and micro- and macro-faunal assemblages were studied byAbou Ouf and El-Shater (1991), Al-Washmi (1999),Coakley and Rasul (2001), Al-Washmi (2003), Basahamet al. (2006), Abu-Zied et al. (2011), Abu-Zied and Bantan(2013), Rasul et al. (2013), and Basaham et al. (2014). Thesaline pans occur only south of Jeddah and were studied byTaj and Aref (2014, 2015a). The studies carried out on thesupratidal sabkhas of the eastern Red Sea coast were con-cerned mainly with water and sediment chemistry (e.g.,Bahafzullah et al. 1993; Basyoni 1997; Serhan and Sabtan1999; Sabtan et al. 1997; Sabtan and Shehata 2003; Banatet al. 2005; Basyoni and Aref 2014; Taj and Aref 2015b).Several works were concerned with the sedimentology of the-se sabkhas (e.g., Basyoni 2004; Al-Washmi et al. 2005;

M. H. Basyoni (*) :M. A. ArefDepartment of Petroleum Geology and Sedimentology, Faculty ofEarth Sciences, King Abdulaziz University, Jeddah, Saudi Arabiae-mail: mbasyoni50@yahoo.com

M. A. Arefe-mail: m1aref@yahoo.com

M. A. ArefGeology Department, Faculty of Science, Cairo University,Giza, Egypt

Arab J Geosci (2015) 8:9973–9992DOI 10.1007/s12517-015-1852-1

Gheith et al. 2005; Taj and Aref 2009, 2011; Basyoni and Aref2007, 2009, 2010, 2011; Basyoni et al. 2008; Ginau et al.2012; Aref and Taj 2013; Aref et al. 2014). Most studiescarried out on Jizan sabkha were concerned with the engineer-ing geological problems of the sabkha soil (Dhowian et al.1987; Dhowian 1990; Erol 1989; Al-Shamrani and Dhowian1997; Al-Mhaidib 2002; Youssef et al. 2012). However, noneof these works tried to relate the behavior of the sabkha sed-iments to their mineralogy and sediment composition.Therefore, the purposes of the present work are the examina-tion of the sedimentary surface structures, microfacies analy-sis of the evaporite sediments, and distribution of the differentevaporitic basins in the sabkha area. The results of this papermay help in solving the problems of damage of the infrastruc-ture in the sabkha area and interpretation of the depositionalsetting and mechanism of formation of similar evaporitesediments.

Location and geologic setting of Jizan sabkha

Jizan sabkha is located on the southeastern coastal plainof the Red Sea of Saudi Arabia, between latitudes N 16°44′ and N 16° 60′, and longitudes E 42° 32′ and E 42° 42′(Fig. 1). The sabkha area occupies the lowest topographicdepression that gradually increases in height towards theeast. Three topographic zones are defined in Jizan area,which run for approximately 1800 km parallel to eachother in a NW-SE direction (Blank et al. 1987; Husseinand Loni 2011) (Fig. 1b): (1) the dissected highland ofHijaz-Asir Precambrian basement complex; (2) the centralplateau that slopes gently towards the Red Sea coast,which consists of the Cambro-Ordovician WajidSandstone that rests unconformably on peneplainedPrecambrian basement rocks (Powers et al. 1966); and(3) the Tihama coastal plain that forms a strip of landextending approximately 10 km to the foothills of theRed Sea escarpment. The coastal plain is covered withQuaternary aeolian sand, alluvial sand and gravel, loess,and flood plain silt deposits. Recent wet, sabkha sedi-ments are widespread near the shore of the Red Sea(Fig. 1c). The prominent elevated relief (up to 50 m inheight) on the coastal plain is a salt dome at the old cityof Jizan, which has an area of 4 km2 (Fig. 1c).

Climate

Jizan area has a subtropical desert climate, where severalephemeral wadi systems drain to the shelf (Abdelrahmanand Ahmad 1995). Jizan city is characterized by rainfallstorms which vary in intensity and duration. The southern partof the city is sometimes exposed to the risk of flash flood due

to the heavy rainfall intensity which may happen during therainy storms, and the peak runoff flows from the East towardsWest (Elsebaie et al. 2013). Abdelrahman (1997) found thatthe average temperature in Jizan area is 23 °C, the annualprecipitation is 1.3 cm, and the average relative humidityvaries between 45 and 65 % in winter and 25 and 40 % insummer. The annual mean rate of evaporation at Jizan is156 cm/year (Abdelrahman and Ahmad 1995). The prevailingwinds at Jizan blow from west during summer and southwestduring winter, with wind speeds ranging between 2 and50 km/h. These climatic data indicated the recharge of mete-oric water to the sabkha area during winter months and thedeposition of evaporite minerals during summer months.

Methods of study

The results achieved in this paper are based on the fieldworks, trenches, cores, and thin sections. Ten-day fieldtrips were made to Jizan sabkha in August 2012 andMay 2013 for examining the surface features of the wetand dry parts of the sabkha. Several shallow trenches, upto 150 cm in depth, that meet the water table have beenexcavated in Jizan sabkha. In the field work, the salinity,temperature, and pH values of the brines in the trenchesdug in the sabkha were measured. The salinity was deter-mined by hydrometer glasses taking into account the mea-suring of standard sea water. The hydrometers measurethe Mass % NaCl in the brine up to 25 %. The densityof the brine samples was measured by using two portablehydrometer glasses; the first measures density from 1.00to 1.10 g/cm3 and the second measures density from 1.10to 1.2 g/cm3. The pH value of the brine was measured inthe field by a portable pH meter. Seven PVC plastic tubeshave been drilled to a depth of 120 cm with a hammer toextract core samples (Fig. 1d). The evaporite sedimentswere selected for preparation of thin sections from thesurface, trenches, and cores in the sabkha area. The wetand loose sediment samples were dried in the oven andimpregnated with epoxy resin. The thin sections wereprepared under dry cool condition using paraffin oil andepoxy cement in the laboratory of Cairo University.Petrographic examination and mineralogic identificationof 51 thin sections were investigated by Meiji polarizingmicroscope adapted with digital camera. The relativeabundance of the siliciclastic components in thin sectionswas made in relation to a comparison chart.

�Fig. 1 a Location map for Jizan area. b Geology of Jizan area (afterBlank et al. 1987). Note: the insert box is the studied area shown in (c).c Surface lithology and location of the sediment samples and cores drilledin the studied Jizan sabkha. d Lithologic interpretation of the cores drilledin Jizan area

9974 Arab J Geosci (2015) 8:9973–9992

C

JZ-1 JZ-7 JZ-2 JZ-3 JZ-4 JZ-6 JZ-5

10 cm

0

Shell fragments

Mud

Sand

Lenticular gypsum

Clastic gypsum

Sand & mud layers

Anhydrite nodules

Gypsum nodules

Rosette gypsum

Poikilotopic gypsum

Microbial filaments

RED SEA

YEMEN

JizanB

D

Quaternary surficial deposits

Pleistocene basalt

Mesozoic & Paleozoic sedimentary rocks

Granite pluton

Proterozoic rocks

Hijaz-Asir complex

SAUDI ARABIA

C A

RED SEA

N 16 58

E 42 30´

N 16 54

N 16 50

N 16 46

5

4

6

7

2

1

Y0 250 km

NORTH

EGYPT

SUDAN

SUDI ARABIA

Jeddah

Duba

Jizan

RED

SEA

0 3 km

Al

Ji

11

3 5

8

12

14

13

6

7910

1526

18

20

23

24

25

1

2

4

19

2221

Core location (JZ-7)

Wet mudflat/sandflat

Dry sandflat

Salt dome

Sediment sample10

7

A

°

´°

´°

´°

´°

16 17 3

E 42 34 E 42 38 E 42 42´° ´° ´°

Arab J Geosci (2015) 8:9973–9992 9975

Discussion

Facies description

(a) Salt dome

The highest topographic area (50 m) in the coastal plain ofJizan is a salt dome at the old Jizan city (Fig. 2a). The saltdome consists of a massive and thin bedded clear, colorless,mosaic, halite crystals on fresh surface and gray on weatheredsurface. The halite mass has vertical dissolution lines withsharp ridges in between due to their recent dissolution byrainfall (Fig. 2a). Several dissolution sinkholes with a diame-ter less than 4m and depth exceeding 6mwere observed at thefloor of the quarries made in the area of salt dome (Fig. 2b).The halite of the salt dome is usually covered with duricrust(salcrete) due to re-precipitation of halite crust on theweathering surface that may form polygonal ridges (Fig. 2c).

The cap rocks of the salt dome are gypsum, anhydrite, dolo-mite, shale, and sandstone layers (Fig. 2a).

(b) Surface sediments

The low land surrounding the salt dome is distinguishedinto three zones with different sediment compositions,groundwater depth, and presence or absence of evaporite min-erals. These are (Fig. 1c) (1) the dry, partially vegetated zoneat east; (2) the wet, sabkha sediments near the coastal plain,and (3) few saline pans at the lowest topographic depressionsin the sabkha area. The dry land contains loess and somescattered dune fields, where the groundwater is deep(>150 cm in depth). Loess forms small hills, 170 cm in height,and consists of homogenous and compact silt to fine sand-sized quartz, feldspar, and mica grains (Fig. 2d). Near thetop of the hills, displacive, white anhydrite nodules and occa-sional rootlets were recorded. Sand dunes (barchan type) and

a b

c d

e f

Fig. 2 Salt dome and surfacesediments in the coastal plain ofJizan area. a A quarry forextraction of halite in the saltdome showing verticaldissolution lines in the halite massat bottom that are overlain with acap of anhydrite and clastics. b Asinkhole at the floor of the quarrydue to recent dissolution byrainfalls. c A salcrete crust on saltdome showing polygonal ridges.d A loess composed ofhomogenous silt with finerootlets. e Coastal, barchans duneover the wet sabkha area. f Adense population of mangroveforest in the shallow intertidalarea

9976 Arab J Geosci (2015) 8:9973–9992

sand sheet form most of the eastern part of the coastal plain.The dunes range in height from 120 cm up to 250 cm, andlength up to 30 m, and the down wind direction of the dunespoint towards southeast (Fig. 2e). Wind-induced ripples coverthe stoss-side of most dunes and show elongation in NE-SWdirection.

The coastal plain is separated from the Red Sea with densestands of mangrove forests that grow in the intertidal zone(Fig. 2f). The floor of the intertidal zone is usually mud- andsilt-sized bioclasts and aeolian sand grains. The wet sabkhaarea runs parallel to the coast with a width that ranges from 4

to 10 km (Fig. 1c), and the water table varies from 20 to150 cm in depth. The salinity of the water table ranges from170 to 180‰, the density ranges from 1.13 to 1.142 g/cm3,and the pH value ranges from 6.01 to 7.45. Generally, thesubsurface sediments of the wet sabkha area are composedof interbedded yellow and brown sand and gray mud layersthat form the sandflat and/or mudflat areas on their exposure(Fig. 3a). Aggregates of dark gray lenticular and rosette gyp-sum and white nodular gypsum are recorded at the depths of10, 40, and 90 cm (Figs. 1d and 3b). Spherical, elliptical andlenticular, nodular mosaic anhydrite, 2–7 mm in size, are

a b

c d

Fig. 3 Subsurface sediments ofthe wet sabkha area. a Thin layersof green, brown mud and yellowsand, with white gypsum layer.The floor of the trench showsplastic, wet clay. b Aweatheredsurface showing displacivegrowth of lenticular and rosettegypsum in soft mud between sandlayers. c White mosaic ofanhydrite nodules growdisplacively in brown sand. d Atrench in the wet sabkha showingblack, organic-rich mud,enterolithic and nodular anhydritein sand layers

Arab J Geosci (2015) 8:9973–9992 9977

common near the sediment surface of the sabkha (Fig. 3c).Sometimes, the nodules coalesce to form enterolithic folds ofmilky white, soft anhydrite at a depth of 15 cm (Fig. 3d).

The sedimentary surface structures of the wet sabkha areaare adhesion ripples, tepee polygonal ridges, efflorescent ha-lite pods, petees, and blisters. The adhesion ripples, tepeeridges, and halite pods are found in areas where the water tableis located at depths exceeding 40 cm. Petees and blisters areassociated with green microbial mats and exist in areas withshallow (<10 cm) water table condition. Most of the wetsabkha area is covered with adhesion ripples that consist ofsmall undulation of sand mounds. Below the sediment sur-face, numerous, fine gypsum nodules are widespread betweenthe sand grains (Fig. 1d). In slightly lower topographic areas inthe wet sabkha, the sediment surface evolves into small(<3 cm in height) and large (>20 cm in height) tepee polygo-nal ridges (Fig. 4a). The tepee ridges are composed of gypsumand/or halite crusts that form buckling, inverted V-shapedstructures (Fig. 4b). The tepee ridges form the non-orthogonal (120°) polygonal pattern (Fig. 4a). At the slightlyhigh topographic area, the tepee structure is of the immaturetype (Assereto and Kendall 1977) that consists of thin (<2 cm)crusts low (<3 cm) in height and with small (<10 cm) diameterbetween polygons. At slightly low topographic areas towardthe center of the sabkha, the surface evaporite layer evolvesinto mature tepee structure that consists of thick crusts (4 cm),of high height (<20 cm), and with a broad area (120 cm)between polygons (Fig. 4b). Below the tepee crust is brownishsand or gray sand due to the decay of subrecent microbial mats(Fig. 4b). Efflorescent halite pods are scattered on the wetsabkha surface in small depressions between the tepee gyp-sum ridges. They consist of milky white aggregates of spher-ical nodules of spongy halite crystals (Fig. 4c).

Petees and blisters exist in areas either covered with a thinfilm of water or the water table that is a few centimeters belowthe sediment surface. The sediment surface may evolve intobuckling petee crusts less than 3 cm in height (Fig. 4d). Thecrusts form elongated, hollow, twisted petee ridges (Fig. 3d)and consist of interlaminations of black and green microbialmats and gypsum layers. Blisters also occur in areas offlourished microbial mats. They consist of miniatures of smalldomes, <3 cm in height and width (Fig. 4e), and consist of thingypsum crust over hollow centers. The lower surface of thesebuckling features has numerous vugs of escaped gases(Fig. 4f) from subrecent decayed microbial mats.

The lowest topographic areas in the wet sabkha containsaline pans filled with a high salinity brine >250‰ and adensity >1.26 g/cm3. The pans are less than 5 m in diameterand <30 cm in depth. Halite crystallized in these pans at thebrine surface as thin rafts and pyramidal hoppers (Fig. 4g, h).Rapid lowering of the brine surface favors the formation ofsuccessive rafts that form 5–10-cm-thick layers. Halite alsocrystallized on the floor of the pan as aggregates of chevrons,

cornets, and cumulus crystals that may increase in thickness tocoalesce with the halite rafts. The halite layer may show se-vere dissolution by dripping water to form comb-like halitefibers.

Microfacies analysis

Petrographic examinations of 51 thin sections for the evapo-rite sediments from the surface, trenches, and cores dug in thesabkha area allowed the recognition of two lithofacies types:(a) siliciclastic lithofacies type and (b) evaporite lithofaciestype. Because of the importance of the evaporite lithofaciestype, it is subdivided into several microfacies types based ontheir mineralogy, textures, and fabrics that are identified underthe microscope. The following is the description of theselithofacies types:

(a) Siliciclastic lithofacies type

The siliciclastic lithofacies represents the host sediments inJizan sabkha, where the evaporite minerals were crystallizedin the capillary evaporation zone from the groundwater by theevaporative pumping mechanism of Hsü and Siegenthaler(1969). Bioclasts and calcite cement were recorded occasion-ally, in addition to microbial mats in some locations (Tables 1and 2). Sand-sized grains form laminated, rippled-cross lam-inated or even massive structures (Fig. 5a). The sand grainsare medium to fine size, subangular to subrounded, and mod-erately to poorly sorted (Fig. 5b). They consist of quartz (60–80 %), mica (10-30 %), feldspar (5–10 %), hornblende, andother igneous and metamorphic grains (<10 %) (Fig. 5b). Themica flakes are represented by biotite flakes that generallyshow preferred orientation (Fig. 5c) or form random orienta-tion when they are associated with growth of evaporite min-erals. Feldspar is represented dominantly by fresh plagioclasegrains (Fig. 5b). Mud clasts of coarse sand-sized grains arerarely recorded between the quartz grains (Fig. 5d). The typeof the sand-dominated sediment is a lithic arenite because ofthe dominance of mica and lithic grains over the feldspargrains.

Most of the sand grains are loose, without appreciableamount of cement. However, some samples have a matrix ofbrown clay (Fig. 5b), which decrease their compositional andtextural maturity of the lithic arenite sediment. Patches of cal-cite cement are scarcely observed (Fig. 5e). The calcite crys-tals may corrode and engulf the quartz and feldspar grains.

Mud-sized sediment is next in abundance, which is mas-sive and brownish. It is composed of angular, poorly sorted,silt-sized quartz grains, mica flakes, and brown clay minerals(Fig. 5f). The mica flakes are relatively finer in size than thatrecorded in the sand fraction. The mica may form preferred orrandom orientation. Sometimes, the mud-sized sedimentforms thin lamina that grades into laminae composed of silt-

9978 Arab J Geosci (2015) 8:9973–9992

sized quartz grains and mica flakes. Brownish, massive claysediment is recorded in some samples (Tables 1 and 2). It iscomposed mainly of clay minerals and few quartz and micagrains. The clay may be recorded as random, angular clastsdispersed in siltstone (Fig. 5f) or form thin laminae that inter-calated with silt laminae.

Microbial mats are recorded in most of the surface sedi-ment samples, and in only one sample of the cores (Tables 1and 2), associated with quartz grains and gypsum crystals.They consist of slightly and highly irregular, thin, yellowishor brownish microbial filaments (Fig. 5g). These filaments arerecorded enveloping quartz grains and lenticular gypsum

g h

a b

c

f

d

e

Fig. 4 Surface features in thesaline sandflat and saline panareas. a Polygonal tepeestructures of halite crusts withpartial dissolution of the top ofridges. b Inverted, V-shaped tepeehalite ridges on the sedimentsurface that overlie black,massive sand. c Milky whitegypsum forms efflorescentnodules between the buckled dirtyhalite crust. d Highly contorted,small, inverted U-shaped peteestructure due to microbialinfluence on the sediment, withwhite efflorescent gypsumcrystals. e Blistered surface ofthin gypsum crust due to gasmigration from sub-recentdecayed microbial mats. fView ofthe underside of the blistersshowing numerous circular vugsdue to escape of gases. g Thinhalite rafts floating at the brinesurface in a shallow depression. hBedding plane view showing thevariable sizes of pyramidal halitethat form the raft

Arab J Geosci (2015) 8:9973–9992 9979

Tab

le1

Facies

andmicrofacies

analyses

ofthesurfaceevaporite

sedimentsin

Jizansabkha

Petrographic

types

Hostsedim

ents

Individuallenticulargypsum

Rosette

gypsum

Nodular

gypsum

Com

plex

lenticular

gypsum

Nodular

anhydrite

Halite

cement

Halite

chevron

andcornet

Microbial

mats

Secondary

anhydrite

Alabastrine

gypsum

Porphyrotopic

gypsum

Sam

ple

number

Clay

Mud

Sand

Silt-

sized

Sand-

sized

Gravel-

sized

2m

md

f

5lower

md

ff

5middle

df

dm

d

5upper

mm

ff

6Af

dm

ff

6Bm

mm

mf

ff

7Ad

ff

7Bm

d

14A

df

fd

14B

dd

m

16f

rm

df

17A

dm

mf

dm

17B

df

dd

fm

17C

dm

d

22d

dd

24d

df

f

Forsamplelocatio

n,seeFig.1c

ddominant,mmoderate,ffew

,rrare

9980 Arab J Geosci (2015) 8:9973–9992

Tab

le2

Faciesandmicrofacies

analyses

ofthesubsurface

evaporite

sedimentsfrom

corestakenin

Jizansabkha

Petrographic

types

Hostsedim

ents

Individual

lenticulargypsum

Rosette

gypsum

Nodular

gypsum

Com

plex

lenticular

gypsum

Nodular

anhydrite

Halite

cement

Halite

chevron

andcornet

Microbial

mats

Secondary

anhydrite

Clastic

gypsum

Poikilo

topic

gypsum

Depth

(cm)

Calcitecement

Clay

Mud

Sand

Silt-

sized

Sand-

sized

Gravel-

sized

Jz1(16–21)

Matrix

dm

m

Jz1(39–44)

fd

Jz1(53–59)

fd

Jz2(1–7)

dBioclast

fd

dm

Jz2(14–20)

fd

fd

Jz2(32–38)

fd

fd

Jz3(18–25)

fd

Jz3(34–39)

fd

m

Jz3(56–61)

fd

dm

Jz4(13–18)

dm

m

Jz4(42–47)

mf

dm

d

Jz4(85–91)

dd

md

Jz5(12–17)

mm

fd

fd

f

Jz5(23–27)

mm

f

Jz5(52–57)

mm

dd

fm

Jz5(70–76)

df

md

mm

fm

Jz6(12–17)

df

fd

f

Jz6(38–42)

mm

f

Jz6(44–49)

df

ff

Jz7(18–22)

dm

fd

Jz7(27–31)

mf

m

Jz7(53–57)

fm

Jz7(68–71)

fd

m

Forsamplelocatio

n,seeFig.1c

ddominant,mmoderate,ffew

,rrare

Arab J Geosci (2015) 8:9973–9992 9981

crystals, which led to their biostabilization (Noffke 2000,2010), or surrounding nodules of lenticular and granular gyp-sum crystals (Fig. 5h). Sometimes, the microbial filaments aremicritized and consist of dense aggregate of micrite grains thatalso enclose the quartz grains and gypsum crystals.

(b) Evaporite lithofacies types

The evaporite lithofacies types are subdivided according totheir dominant mineral composition into (1) gypsummicrofacies types, (2) anhydrite microfacies types, and (3)

100 µm

h

200 µm

c

200 µm

d

200 µm

e

200 µm

g

b

200 µm

a

100 µm

f

200 µm

Fig. 5 Siliciclastic hostsediments in Jizan sabkha. aTrough, cross-laminated, veryfine sand. Polarized Light. bPoorly sorted, angular quartz andfeldspar grains in brownish claymatrix. Polars crossed. c Preferredorientation of biotite flakes in finesand. Polars crossed. d Coarse,mud fragments between poorlysorted quartz grains withbrownish clay matrix. Polarscrossed. e Patch of calcite cementbetween quartz grains. Polarscrossed. f Clay fragmentsscattered in silt-sized quartzgrains. Polars crossed. g Brown toorange microbial filaments in finesand. Polarized Light. hDisplacive lenticular gypsum inbrown microbial filaments.Polarized Light

9982 Arab J Geosci (2015) 8:9973–9992

halite microfacies types. Each of these microfacies types aredistinguished into several types (Tables 1 and 2) according tothe fabrics and textures of the deposited crystals that are con-trolled by the processes acting in the depositional and diage-netic environments.

1. Gypsum microfacies types

The terminologies used to describe the size of the gypsumcrystals into gypslutite (silt-sized gypsum crystals),gypsarenite (sand-sized gypsum crystals), and gypsrudite(gravel-sized gypsum crystals) follow the terms used byWarren (1982a), Aref (1998), Varol et al. (2002), and Lugliet al. (2007). Also, the terminologies used to describe themorphology of the gypsum crystals such as lenticular, tabular,tabular-prismatic, twinned, rosette, etc., follow the terms usedbyCody (1976, 1979), Cody and Cody (1988), andMees et al.(2012). The dominant microfacies types of gypsum areformed by diagenetic growth of displacive and/or inclusivegypsum during desiccation of the saline pans, or in the salinesandflat/mudflat depositional setting of Lowenstein andHardie (1985); Casas and Lowenstein (1989). The gypsummicrofacies types are represented by (Tables 1 and 2;Fig. 6a) individual lenticular gypsum, twinned gypsum,twinned complex lenticular gypsum, rosette gypsum, nodulargypsum, poikilotopic gypsum, porphyroblastic and alabas-trine gypsum, and clastic gypsum. Microbial mats are occa-sionally associated with some of the gypsum crystals underthe less saline water conditions close to the sea side or conti-nental side of the sabkha area. The following is the descriptionof the microfacies types of gypsum:

Individual lenticular gypsum

Lenticular gypsum of curved crystal faces are the most com-mon type of the evaporite minerals in Jizan sabkha (Tables 1and 2). The lenticular gypsum crystals range in size fromgypslutite (<80 μm in length and <10 μm in width) togypsarenite (200–500 μm in length and <50 μm in width) togypsrudite (2 mm to 3 cm in length and 500–1000 μm inwidth). The lenticular gypsum may increase in number anddensity to form aggregates of random orientation (Fig. 6b) ormore commonly form individual crystals floating in the hostsediment (Fig. 6c, d). The host sediment may be representedwith mud rich in mica flakes that commonly form preferredorientation (Fig. 6e, f) to sand-sized quartz, feldspar, and lithicgrains (Fig. 6c). The lenticular gypsum crystals may be veryclear and free from inclusions of the host sediment (Fig. 6d),which indicate their displacive origin. The displacive growthorigin of such gypsum crystals are also evidenced by the dis-turbance of the preferred orientation of the mica flakes(Fig. 6d). Lenticular gypsum may also grow by the inclusivegrowth of materials from the host sediment. Such inclusive

gypsummay entrapmica flakes, claymineral, or silt and sand-sized quartz grains (Fig. 6c, e). The lenticular gypsum mayshow growth bands marked by impurities from the host sedi-ment (Fig. 6g).

Individual gypsum crystals may form tabular crystals withstraight crystal faces (Fig. 6f). Mees et al. (2012) found thatthe tabular gypsum crystals form lozenge shape in cross-sections perpendicular to (010) and elongated parallelogramin cross-sections parallel to (010). These tabular gypsum crys-tals are recorded in a host of clay sediment (Fig. 6f) or a sandsediment. These tabular gypsum crystals may disturb the pre-ferred orientation of mica flakes or aligned parallel to thepreferred orientation of the mica flakes. The gypsrudite len-ticular crystals are occasionally observed to be replaced byfibrous anhydrite crystals that form parallel sheaves crossingthe gypsrudite crystal (Fig. 6h).

Twinned gypsum

Twinned gypsum is equivalent to type Ill penetration twins ofCody and Cody (1988). The twinned gypsum is composed ofeither two individuals of lenticular gypsum crystals (Fig. 7a)or hemi-bipyramidal individuals of gypsum crystals of equalor subequal size. Twinning occurs along the (100) plane. Thetwinned gypsum crystals occur displacively and inclusively ina host clay matrix or a sand matrix. These twinned gypsumcrystals are characterized by the dominance of inclusions ofclastic materials within the growing crystals, or they are freefrom inclusions of the matrix material.

Twinned complex lenticular gypsum

The twinned gypsum may also be more complex and haveadditional smaller crystal faces parallel to the two main com-ponent individuals (Fig. 7b). This type was referred by Codyand Cody (1988) as twin complexes. They are recorded spo-radically in the surface and subsurface sediments of thesabkha (Tables 1 and 2). The individual gypsum crystalsmay have multiple terminations along only one side of thecrystals or have numerous, small terminations in both sides(Fig. 7b). The twinned complex gypsum crystals are mostlyenclosed matrix material from the host sediment.

Rosette gypsum

The rosette gypsum is equivalent to type V rosettes of Codyand Cody (1988). The rosette gypsum consists of large, parentgypsum crystals, from which smaller, individual, lenticulargypsum crystals fan out (Fig. 6a–c and d). The smaller lentic-ular crystals resemble the “petals” of a rose. Such small gyp-sum petals fan out from the central area of the large gypsumcrystals, and its margin is clearly visible (Fig. 7c). Sometimes,the component lenticular gypsum crystals appear randomly

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intergrown to form aggregates of lenticular gypsum, ratherthan arranged to form the rosette shape. Such aggregates oflenticular gypsum are highly varied, but some of themmay besimilar to well-defined rosettes.

The petals of the smaller gypsum crystals appear as a radialgrowth of lenticular gypsum from a common center that col-lectively form the rosette pattern (Fig. 7c). More than six

lenticular crystals of similar sizes form the rosette gypsum ofapproximate symmetrical appearance. However, the radialgrowth of asymmetric aggregates of lenticular gypsum of var-iable sizes and orientations are also observed (Fig. 6a–c). Thehost sediment of the gypsum rosettes and aggregates are com-monly sand- and silt-sized quartz grains. All gypsum crystalsof the rosettes and aggregates are grown displacively and

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Fig. 6 Gypsum microfaciestypes. a Different crystalmorphologies of lenticulargypsum: a individual lenticular, btwinned, c rosettes, d aggregates.b Random, lenticular gypslutitebelow brownish microbial layer.Polars crossed. c Single,gypsarenite lenticular crystalbetween quartz and mica. Polarscrossed. d Disturbance of thepreferred orientation of micaflakes by the displacive growth oflenticular gypsum. Polars crossed.e Preferred orientation ofbrownish, mica (biotite) flakeswith displacive growth oflenticular gypsum. Polars crossed.f Tabular (T) and lenticular (L)gypsum in a clay matrix. Polarscrossed. g Growth zones inlenticular gypsum is marked byimpurities from the host sediment.Polars crossed. h Fibrous andfelted anhydrite replacing gypsumthat enclose quartz grains. Polarscrossed

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enclose sand- and silt-sized quartz grains, mica flakes, andclay material.

Nodular gypsum

Gypsum crystals may be selectively aggregated to form nod-ules of variable sizes and shapes in a host of fine siliciclastic

and argillaceous sediments, such as silt and clay sediment, andless commonly in sand sediment (Fig. 7d, e). Gypsum nodulesmay occur as individual, spherical nodules (Fig. 7d) that areexclusively isolated from the neighboring one or show planeor concavo-convex contact with the adjacent nodules(Fig. 7e), or may coalesce with each other to form theenterolithic nodular structure. The nodules consist generally

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Fig. 7 Morphologies of gypsumcrystals. a Clear, twinned gypsumcrystals surrounded with finergypsum. Polars crossed. bTwinned complex lenticulargypsum with inclusion of micaflakes. Polars crossed. c Rosettegypsum consists of radial growthof variable sizes of lenticularcrystals. Polars crossed. dEllipsoid gypsum nodule consistsof random, lenticular gypsum inmud sediment with preferredorientation of mica flakes. Polarscrossed. e Two gypsum nodulesslightly merged along concavo-convex boundary in clay matrix.Polars crossed. f Large,poikilotopic gypsum crystalsencloses finer, lenticular, gypsumcrystals. Polars crossed. gPorphyrotopic gypsum crystalswith irregular interpretationboundaries. Polars crossed. hLocal reworking of lenticulargypsum and formation ofmatching fragments. Polarscrossed

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of gypslutite crystals of variable shapes and densities. Thenodules may consist of scattered, random lenticular gypsumcrystals (Fig. 7d) or dense aggregates of tabular gypsum, orfine, granular gypsum crystals (Fig. 7e). Other nodules arecomposed of random aggregates of twinned complex lenticu-lar gypsum crystals or a mixture of fine granular and lenticulargypsum crystals.

Poikilotopic gypsum

Poikilotopic gypsum consists of large (>500 μm) individualcrystals that enclose either finer, lenticular gypsum crystals(Fig. 7f) or quartz sand and mica flakes from the surroundingmatrix. The poikilotopic gypsum crystals have generally irreg-ular outline with the surrounding gypsum crystals and clasticmaterial. They exist as individual crystals scattered in the ma-trix or may form aggregates of adjacent poikilotopic crystalsof variable optical orientations. The enclosed, fine gypsumcrystals are commonly found with perfect lenticular morphol-ogy in the poikilotopic gypsum (Fig. 7f), which indicates thatthe large gypsum crystals grow by inclusive growth and en-trapment of the finer lenticular gypsum and not by thereplacive growth.

Porphyrotopic and alabastrine gypsum crystals

Porphyrotopic and alabastrine gypsum crystals are recorded inonly three sediment samples in Jizan sabkha (Tables 1 and 2).Porphyrotopic gypsum commonly occurs as coarse(>600 μm) crystals that form mutually interfering aggregates(Fig. 7g) or as floating crystals within finer granular gypsumcrystals. The porphyrotopic gypsum crystals are generallyclear and free from inclusion of clastic materials that are com-mon in the aforementioned gypsum microfacies types.

Alabastrine gypsum is recorded as irregular or elongatedpatches adjacent to or within coarse lenticular andporphyrotopic gypsum crystals. The alabastrine gypsum con-sists of aggregates of microcrystalline gypsum with a size lessthan 50 μm. The boundaries between the finer alabastrinegypsum and the coarser gypsum are usually gradational inter-penetrating contact, indicating the replacement of the coarsergypsum crystals by finer gypsum. This process of replacementof the coarse gypsum crystals by finer gypsum has been de-scribed before from the Miocene evaporites of the Red Sea(Aref et al. 2003; Mandurah and Aref 2010), from theMessinian evaporites of Italy (Testa and Lugli 2000), and frompedogenic gypsum crusts (Aref 2003).

Clastic gypsum

During periods of aridity, intensive rainfall, or occasionalflood, the displacive gypsum crystals in the sabkha sedimentare subjected to wind or water erosion and slight reworking.

This resulted into the formation of clastic gypsum grains thatare composed of local reworking of lenticular gypsum(Fig. 7h). The gypsum fragments can be easily matched, indi-cating a short transportation distance. However, transportationof the clastic gypsum for a relatively long distance is evi-denced by the occurrence of cleavage fragments or terminalparts of the lenticular crystals mixed with sand grains, or in amatrix of mud sediment.

2. Anhydrite microfacies types

Anhydrite microfacies types are rarely recorded in the sur-face and subsurface sediments of Jizan sabkha (Tables 1 and2). They are represented by nodular and enterolithic anhydritemicrofacies types. The morphology and composition of nod-ular anhydrite and enterolithic anhydrite nodules are similar.They consist of spherical and ellipsoidal, dense aggregates ofanhydrite crystals, where the boundary of the nodules are gen-erally highly irregular (Fig. 8a), in contrast to the relativelysmooth boundary of gypsum nodules (Fig. 7e). These individ-ual and coalesced anhydrite nodules are scattered in brownishmud and fine sand-sized quartz grains (Fig. 8a). Mica flakescommonly show preferred orientation parallel to the outerboundary of the anhydrite nodules. Generally, the nodulesare composed of prismatic, elongated anhydrite crystals thatshow random orientation at the center of the nodules (Fig. 8b)or throughout the nodular structure. However, radial, fan-likeanhydrite crystals may exist at the boundary of the nodules(Fig. 8c). In large nodules, 100-μm-thick zone consists ofparallel sheaves of prismatic anhydrite crystals that are ar-ranged parallel to the boundary of the anhydrite nodules(Fig. 8d). The anhydrite nodules are generally clear and freefrom inclusion of the mud and sand sediments (Fig. 8a, b).However, few, scattered quartz grains are occasionally foundat the margin of the nodules. Lenticular gypsum and twinnedlenticular gypsum may exist close to the anhydrite nodule(Fig. 8e). The existence of lenticular gypsum and the clearnature of the anhydrite nodules indicated the presence of pri-mary anhydrite crystals that were formed displacively in thehost siliciclastic sediment.

3. Halite microfacies types

Halite microfacies types are recorded in the saline panswithin Jizan sabkha. They are represented by primary halitecrystals (chevrons and cornets) and diagenetic halite (over-growth and cement).

Primary halite (chevrons and cornets)

Chevron and cornet halite crystals are composed of fluidinclusion-rich bands and fluid inclusions-poor bands(Fig. 9a, b). The chevron halite forms an upturned V-shaped

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pattern that is usually pointing upward (Fig. 9a), whereas thecornet halite crystals consist of horizontal halite bands thatincrease in width upward (Fig. 9b). The fluid inclusion-richbands in chevron and cornet halite crystals are formed duringdaytime due to the increase in temperature and excess evapo-ration which favor rapid rate of halite crystallization and theentrapment of fluid inclusions. During night time, the decreasein temperature and evaporation favor slow crystallization ofhalite without fluid inclusions (Smoot and Lowenstein 1991).

Diagenetic halite (overgrowth and cement)

Diagenetic halite crystallized from the capillary evaporation ofthe saline groundwater as overgrowth and mosaic halite ce-ment. The halite overgrowth consists of large, clear halite thatsharply truncates the fluid inclusion-rich bands in chevron andcornet halite (Fig. 9a). Mosaic halite cement consists ofequigranular cubic and plate-like halite crystals that are

displacively grown between quartz grains (Fig. 9c). The mo-saic halite may contain clay materials, quartz, and gypsumfragments from infiltrating groundwater.

Basin zonation

Based on field examination of the sediment surface andtrenches in Jizan sabkha, and the structural and textural char-acteristics of the evaporite sediments, the supratidal sabkha inJizan area is distinguished into three depositional basins; theseare the central ephemeral saline pan, the wet sandflat/mudflat,and dry sandflat. Each subenvironment is characterized byspecific textural and structural features of the evaporite min-erals that have been formed during flooding, evaporative con-centration, and desiccation stages. Deposition of the evaporiteminerals in Jizan sabkha took place in a shallow, flat basin thatis normally dry in summer and partially flooded with seepage

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Fig. 8 Anhydrite microfaciestypes. a Several anhydritenodules grow displacively in mudsediment. Polars crossed. bRandom prismatic anhydrite atcenter and preferred orientation ofprismatic anhydrite at theboundary of anhydrite nodule.Polars crossed. cRadial growth ofparallel sheaves of prismaticanhydrite at the boundary of thenodule. Polars crossed. d Parallelarrangement of prismaticanhydrite at the boundary ofanhydrite nodule. Polars crossed.e Lenticular gypsum at top andanhydrite nodule at bottom in ahost mud sediment. Polarscrossed

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seawater and meteoric water from direct rainfalls or floods inwinter time.

(a) Ephemeral saline pans

The saline pans in Jizan sabkha represent the ephemeralsaline lake subenvironment of Hardie et al. (1978) and theephemeral saline pans of Lowenstein and Hardie (1985).Ephemeral saline pans exist in the lowest topographic depres-sion in Jizan sabkha and are filled with halite saturated brinethat is highly saline (salinity >250‰ and a density >1.26 g/cm3) with respect to the surrounding brines of the saline mud-flat and sandflat areas (salinity 170 to 180‰, density 1.13 to1.142 g/cm3). Primary halite crystallized in the saline pan atthe brine-air interface as rafts and cumulus crystals and at thefloor of the pan as cornets and chevrons.

Rafts and cumulates halite

During summer months, when the evaporation of the brine inthe pans is more than inflow of marine and/or meteoric water,the salinity of the brine increases to the range of halite satura-tion level. Halite crystallized at the brine-air interface asmillimeter-sized square-shaped plates (Arthurton 1973) andpyramidal hoppers (Dellwig 1955) that are suspended hori-zontally by surface tension. With continuous growth of halitecrystals at the brine-air interface, they merge with adjacentcrystals to form continuous rafts (Fig. 4g, h). When the weightof the suspended rafts overcomes the surface tension, they

sink to the bottom under the effects of gravity, similar to ob-servation by Handford (1991) and Taj and Aref (2015a). Therepeated process of raft formation and sinking of the rafts ledto the formation of thick (7 cm) halite layers composed ofsequences of halite rafts. When the brine is slightly agitatedat the early stage of nucleation of the halite crystals, the wavesdisturb the surface tension (Smoot and Lowenstein 1991), andthe individual halite crystals settle to the bottom as aggregatesof cumulus crystals (Fig. 4h). These cumulus crystals accu-mulate between sinking halite rafts.

Chevrons and cornets

Continuous concentration of the brine by evaporation, togeth-er with mixing with the shallow pan waters, eventually pro-duces a supersaturated brine from which halite crystals growon the earlier settled rafts and cumulus crystals. Syntaxialgrowth on the earlier formed crystals and the competitivegrowth of halite produce vertically oriented crystals that re-semble the halite “teeth” (Fig. 9a) described by Valyashko(1952). The morphology of the upward growing crystals de-pends on the attitude of the parent crystals. When syntaxialovergrowth begins on a halite cube lying on the edge, theresulting overgrowth will be chevron-shaped with an upwardpointing corner (Fig. 9a) (Wardlaw and Schwerdtner 1966;Arthurton 1973; Lowenstein and Hardie 1985). When the par-ent cube is oriented with its face upwards, syntaxial over-growth results in the formation of flat-topped, upward widen-ing cornet-shaped crystals (Fig. 9b) (Arthurton 1973).

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Fig. 9 Halite microfacies types. aChevron halite with inverted V-shaped pattern of fluid inclusion-rich bands and fluid inclusion-poor bands (black arrows) thattruncated with overgrowth ofclear halite cement (red arrows).Polarized Light. b Cornet haliteconsists of fluid inclusion-richbands and fluid inclusion-poorbands. Polarized Light. c Mosaichalite cement with clay impuritiesbetween cubic halite crystals.Polars crossed with mica plate

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(b) Saline mudflat/sandflat zone

Saline mudflat and sandflat are broad extensive flats ofsiliciclastic sediments that cover most of the coast of Jizanarea (Fig. 1c). Saline mudflat and sandflat deposits consistof wet silt and sand in which intrasediment, diagenetic gyp-sum, anhydrite and halite, and surface efflorescent halite wereformed from the evaporation of shallow water table. Upwardmovement of moisture from the water table saturated withrespect to gypsum and/or halite, probably by the evaporativepumping mechanism of Hsü and Siegenthaler (1969), led tothe formation of (a) polygonal tepee structures, (b) diageneticgrowth of lenticular and rosette gypsum crystals, (c)displacive growth of mosaic and enterolithic anhydrite nod-ules, and (d) petee structures.

Polygonal tepee structures

With increases in the salinity of the brine during summermonths, pervasive, displacive growth of halite cement(Fig. 9c) in the subsurface causes the surface of the salinesandflats to fracture and buckle (Fig. 4a) by volume expan-sion, resulting in the development of a network of polygonalsutures (Warren 1982b). With continuous growth of subsur-face halite, the surface crust is altered into an overthrust tepeestructure (Fig. 4b). The non-orthogonal pattern of the tepeestructure reflects a homogeneous behavior of the salt crust andan expansion in a homogeneous stress field. Similar interpre-tations were mentioned by Lachenbruch (1962), Warren(1982b), Kendall and Warren (1987), and Aref (1998, 2014).

Several mechanisms were described by Kendall andWarren (1987) which cause crusts to expand and crumple intotepee structure. The fracturing of the crust may be a responseto thermal expansion and contraction or a response to thebuoyancy effects of a fluctuating water table. Both mecha-nisms may take place in the sabkha of Jizan area. The resultingcracks may be filled or covered with wind-blown sandy ma-terial or filled with halite cement.

Diagenetic growth of lenticular and rosette gypsum crystals

Gypsum and/or anhydrite were diagenetically grown belowthe surface in brine-soaked sediments of the saline sandflatsand mudflats (Fig. 3), when concentrated brines diffused up-ward from shallow water table by capillary action, because ofthe high rates of evaporation that prevailed during the summermonths (Gornitz and Schreiber 1981), a process termed “evap-orative pumping” (Hsü and Siegenthaler 1969). Precipitationwould occur at the level where gypsum, anhydrite, or halitebecomes supersaturated. The saline minerals grow by twoprocesses: (1) by displacive growth where the surroundingsediment is pushed aside by the force of crystallization and

(2) by incorporative (Smoot and Lowenstein 1991) or inclu-sive (Rouchy et al. 1994) growth.

Displacive growth of lenticular, tabular, and rosette gyp-sum (Figs. 6 and 7) occur when nucleation of the gypsumcrystals pushes aside the host mud and fine sand sediments,whereas the poikilotopic growth of lenticular gypsum crystalsoccurs when the host siliciclastic sediments are entrappedwithin the growing gypsum crystals (Fig. 6g). The incorpora-tive or inclusive growth of gypsum is interpreted by Rouchyet al. (1994) to take place within relatively coarse-grainedsediment by rapid growth of gypsum in partially lithified hostsediment. Gypsum crystals may nucleate on the surface ofdetrital grains or may push or separate the sandy grains fromthe cementing material. The displacive and inclusive growthof lenticular and rosette gypsum took place in a brine-soakedmud that contains the type of organic matter which inhibits thegrowth of (111) face (Cody and Cody 1988; Rosen andWarren 1990; Aref 1998).

The environmental conditions that control gypsumnucleation and crystal morphology have been studiedexperimentally by Cody (1976, 1979) and Cody and Cody(1988). They observed that organic compounds promote thelenticular habit only in an alkaline environment. The lenticularhabit is most common in warm, chloride-rich mud (Cody1979); however, the mineral content, structure, and environ-mental setting of the mud do not influence the gypsum crystalhabit (Cody 1979; Rosen and Warren 1990).

Displacive growth of nodular mosaic and enterolithicanhydrite nodules

Close to the sediment surface of the saline mudflat/sandflatzones of Jizan sabkha, milky white nodular mosaic andenterolithic anhydrite nodules were recorded in the vadoseand upper phreatic zones (Fig. 8a, b), similar to observationsbyNeev and Emery (1967), Gornitz and Schreiber (1981), andHandford (1982). The displacive growth of anhydrite tookplace in the high-salinity brine in the capillary evaporationzone as mosaic of nodular aggregates or as enterolithic folds.The association of nodular anhydrite and lenticular gypsum insome samples (Fig. 8f) may point to the primary origin of theanhydrite nodules from highly concentrated brine, whereasthe lenticular gypsum crystals may be formed from brine witha relatively lower salinity.

Petee structures

During winter time, some parts of the saline mudflat/sandflatsediments were perennially moistened by seepage of less sa-line groundwater or rainfall. This led to extensive growth ofmicrobial mats on the sediment surface. The existence ofgreen and black microbial mats are usually developed on thefloor of very shallow (<10 cm in depth) water body. Their

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locations are probably controlled by a nearby flow of ground-water seepage as their existence required a water salinity rangefrom 60 to 150‰ (Cornée et al. 1992). During summermonths, the shallow water table and the increase in evapora-tion favor the growth of microbial mats to exhibit “petee”structure (Fig. 4d). According to Reineck et al. (1990),Noffke (2010), Aref et al. (2014), and Taj et al. (2014), thepetee structure was formed due to accumulation of escapedgases beneath surficial mat layers that generate folds in thesoft, wetted microbial substrate prior to consolidation. Thisgas has resulted from bacterial degradation of buried organicmatter. Initial escaping of such gases may form blisters on thesediment surface (Fig. 4e) or may be entrapped in the subsur-face sediments forming spherical vugs (Fig. 4f).

(c) Dry sandflat zone

Dry sandflat is located in the topographically high areaadjacent to the saline mudflat/sandflat zone towards the eastand grade outward into the high mountain of the Red Searegion. It is subaerially exposed most of the time and thegroundwater level is too deep to allow the precipitation ofevaporite minerals, similar to observation and interpretationby Smoot and Lowenstein (1991). Therefore, the dry sandflatis severed from the process of erosion more than the salinepans or saline mudflat and sandflat zones. The dry sandflatzone is characterized by the abundance of halophytes.Repeated wetting and drying episodes of the dry sandflat zonelead to stabilization and fixation of the sandy particles by thinfilm of water to form a pavement of sand sheet. In windyareas, the dry sand flat surface is covered with sand dunes(Fig. 2e) that consist dominantly of quartz grains.

Conclusions and recommendations

The recharge of seawater seepage and/or meteoric water viarainfall and surface flood, coupled with excessive evaporationin summer months, favors deposition of primary and diage-netic evaporite minerals in the sabkha area. The lowest topo-graphic depressions were filled with water and formed salinepans, where primary halite crystals were deposited as rafts,cumulates, chevrons, and cornets. At the slightly higher topo-graphic area, the water table was shallow (<120 cm), wherediagenetic growth of gypsum and anhydrite was formeddisplacively and inclusively below the sediment surface ofthe sabkha. Seasonal fluctuation of the water table in thesabkha area led to further diagenetic growth of gypsum andanhydrite below the sediment surface and buckling of thesurface crusts into tepee polygonal ridges. The variable mor-phologies of the gypsum crystals were related to the variablesalinities of the groundwater and contribution of organic acidsfrom the mangroves in the tidal flats and bushes in the sabkha.

The highest topographic area, east of Jizan sabkha, was char-acterized by a deeper water table and formation of sand sheetsand sand dunes in the dry sandflat zone.

The damage of the infrastructures of buildings in Jizansabkha may be caused by (1) the crystallization pressureexerted from the diagenetic growth of gypsum and anhydritebelow the sediment surface and possibly within pores of theinfrastructure, and (2) the corrosive effect of the sulfate ionswhich reacted with steels from the foundation of buildings andcaused their corrosion. The factors that may have great effectson the deterioration of foundation in Jizan sabkha are surfacetopography of the sabkha, depth of the water table, chemicalcomposition of the brine, nature and density of the diagenet-ically grown evaporite minerals, and type of siliciclastic sed-iments. Studies of these geomorphic and sedimentologic fac-tors together with the degree of the actual damage of infra-structures may help in future planning of the best locations forexpansion of Jizan city.

Acknowledgments This project was funded by the Deanship of Scien-tific Research (DSR), King Abdulaziz University, Jeddah, under grant no.(307/145/1432). The authors, therefore, acknowledge with thanks DSRtechnical and financial support. We thank two anonymous reviewers forthe careful reading of the manuscript and their valuable comments.

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