Depositional environments and facies of the Late Triassic ... · Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan Issa M. Makhlouf a,*, Abdel

Depositional environments and facies of the Late Triassic

Abu Ruweis Formation, Jordan

Issa M. Makhlouf a,*, Abdel Aziz El-Haddad b

a Department of Earth and Environmental Sciences, Hashemite University, P.O. Box 330028, Zarqa 13133, Jordanb Department of Geology, South Valley University, Sohag, Egypt

Received 25 May 2005; received in revised form 27 September 2005; accepted 12 October 2005

Abstract

The Abu Ruweis Formation is composed of carbonates, evaporites, and mudstones, with some locally developed pelletic, oolitic and

stromatolitic limestones. The lateral persistence of bedding, the purity of the evaporite rocks, the alternating arrangement of marine carbonates

and evaporites indicates periodic deposition in subaqueous conditions (salina). Petrographic investigations, X-ray diffraction analysis as well as

chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum, with rare anhydrite relics. Five

microfacies of gypsum were recognized according to their fabrics: porphyroblastic and granoblastic gypsum showing polarization texture,

gypsum pseudomorph after anhydrite laths, and satin spar gypsum. The textures they display indicate a hydration origin of precursor anhydrite,

which is in turn rehydrated from primary gypsum. Some of these anhydrites were formed as a result of replacement processes of the carbonate

sediments associated with the evaporites, as evidenced from the textural relationships of the carbonate and sulfate minerals. The O18 content

ranges from 1.45 to 8.38% PDB and the C13 content ranges from K1.52 to 4.73% PDB. Trace elements analysis has shown that the Abu Ruweis

dolomites are rich in strontium (up to 600 ppm), and sodium (up to 835 ppm). The isotope composition and trace elements content, as well as the

petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the Abu Ruweis dolomites. The evaporites

were deposited during a regressive lowstand systems tract, whereas the carbonates were deposited under shallow water marine conditions during a

highstand systems tract. The Abu Ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin. Restricted

conditions were provided by the development of beach barriers.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Abu Ruweis; Jordan; Evaporites; Triassic; Cyclicity; Salina

1. Introduction

The Triassic strata of Jordan were developed along the

southern margins of the Tethyan seaway. During the late

Triassic, a unique sequence of evaporites, the Carnian Abu

Ruweis Formation, accumulated in northwest Jordan (Fig. 1). It

is the uppermost formation of the marine and fluvial Triassic

succession in Jordan. This formation is considered as the major

evaporite deposit in Jordan. Evaporite depositional environ-

ments of the Abu Ruweis and their vertical succession provide

clues to the depositional history of the basin.

The Triassic succession consists of eight formations. The

Abu Ruweis is the uppermost formation and underlies the

1367-9120/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2005.10.017

* Corresponding author. Tel.: C962 492 5 390 3333; fax: C962 492 5 382

6613.

E-mail address: [email protected] (I.M. Makhlouf).

fluvial Jurassic Hihi Formation (Table 1). The Abu Ruweis

Formation is composed of evaporites, carbonates and shales

(Fig. 2). It is exposed in one locality at the confluence of Wadi

Huni and Wadi Abu Ruweis in Zarqa River, about 35 km

north of Amman and 22 km north of Salt in northwest Jordan

(Fig. 1). The succession was given different names by

different authors, for example, the Formation Gypsifere of

Zarqa (Ionides and Blake, 1939), the Zarqa gypsums

Formation (Basha, 1982), and the Abu Ruweis Formation

(Bandel and Khoury, 1981). Only the uppermost part of the

formation is exposed in the Subayhi area, where the gypsum

deposits are quarried for cement.

The lithostratigraphy of the Abu Ruweis Formation (e.g.

Wetzel and Morton, 1959; Bender, 1974; Ionides and Blake,

1939; Bandel and Khoury, 1981; Basha, 1982; FEJ, 1989;

Bandel and Waksmundzki, 1985; Ahmad, 1989) is well

known. Muneizel and Khalil (1993) prepared a geologic map

.

Journal of Asian Earth Sciences 28 (2006) 372–384

www.elsevier.com/locate/jaes

http://www.elsevier.com/locate/jaes

Fig. 1. Location map showing the outcrops of the Abu Ruweis Formation (modified from Muneizel and Khalil, 1993).

I.M. Makhlouf, A.A. El-Haddad / Journal of Asian Earth Sciences 28 (2006) 372–384 373

(scale 1:50,000) of the Salt area included the study area.

Amireh (1993) described the paleosol horizon at the boundary

between Abu Ruweis Formation and the Jurassic Hihi

Formation. Arikat (2000) studied the geology, sedimentology

and industrial potential of the Abu Ruweis evaporites in the

Subayhi area. The suitability of Abu Ruweis gypsum for

production of gypsum plaster was studied by Saqqa and

Arikat (2003).

Table 1

Rock formations and lithostratigraphy of the Triassic succession in Jordan

Era Period Epoch Lithology Formation Depositional environments

Mesozoic Jurassic Bathonian Sandstone, claystone and Fe-paleosol Hihi Fluvial

Carnian Gypsum, anhydrite, and claystone Abu Ruweis Marine (supratidal)

Ladinian Limestone and dolomite Umm Tina Marine (subtidal)

Limestone and dolomite Iraq el Amir Marine (subtidal)

Triassic Anisian Sandstone, shale, marlstone and limestone Mukheiris Fluvial and marine

Limestone and shale Hisban Marine (subtidal)

Scythian Sandstone, shale and marlstone Ain Musa Marine (intertidal–subtidal)

Sandstone and marlstone Dardur Marine (intertidal–subtidal)

Sandstone, shale and marlstone Ma’in Marine (intertidal–subtidal)

Paleozoic Permian Conglomerate, sandstone and shale Umm Irna Fluviatile

I.M. Makhlouf, A.A. El-Haddad / Journal of Asian Earth Sciences 28 (2006) 372–384374

The present work aims to shed some light on the origin of

Abu Ruweis evaporite deposition and their diagenetic

evolution. The depositional model evolved for the Abu Ruweis

evaporites may help interpreting related sedimentary basins in

the Middle East.

1.1. Geological and stratigraphical setting

The uppermost part of the Abu Ruweis Formation is

exposed in the Subayhi area, at the confluence of Wadi el Huni

andWadi al Azab with the Zarqa River (N 32810 0 and E 35843 0)

(Figs. 1 and 2). A combination of domal uplift, denudation and

incision by the Rivers Zarqa and Huna has created local

exposures of the Abu Ruweis Formation. The quarry excavated

by the Jordanian Cement Factory during the last 60 years

produced excellent exposures. The Abu Ruweis Formation

overlies the carbonates of the Triassic Umm Tina Formation.

The base of the formation is not exposed and has only been

reported from the subsurface, where it is defined by the first

appearance of a thick anhydrite bed. It is unconformably

overlain by the Jurassic limestones of Azab Group (Bandel and

Khoury, 1981).

At outcrop, the top of the formation is defined by the first

appearance of the paleosol horizon that marks the base of the

Jurassic Hihi Formation (Khalil and Muneizel, 1992). The Abu

Ruweis Formation has been dated as Carnian in age on the

basis of diverse palynomorph-assemblages identified by

Keegan et al. (1987a,b) including Taeniaesporites noviavlen-

sis, Pseudenzonalasporites summus, T. acutus, Camerosporites

secatus, Enzonalasporites subgranulatus and Duplicicsporites

granulatus.

All the evaporite units in the Zarqa River basin are

horizontal (Fig. 3(A)), although locally they exhibit some

deformation, such as gentle flexures, folds and faults. Although

the basal part is unexposed, complete stratigraphic sections

occur in the subsurface, where they reach a maximum

thickness of about 500 m (Andrews, 1992). Dalqamuni

(1995), Sadooni and Dalqamouni (1998) studied the sequence

stratigraphy and oil prospects of the Abu Ruweis Formation in

the subsurface in three wells, Ajlun-1 (AJ-1), Northern

Highland-2 (NH-2), and Risha-2 (RH-2). The measured

thicknesses were 230, 287, and 492 m, respectively.

1.2. Methodology

Detailed field investigations of the succession were carried

out and representative samples collected for further laboratory

investigations (Fig. 2). Thin sections were prepared from the

evaporites and carbonates for petrographic analysis. Miner-

alogical composition was determined by X-ray diffraction

(XRD), and dolomite samples were analyzed for dO18

and dC13.

1.3. Regional setting

Jordan as a part of northern Arabian Plate witnessed an

evaporitic stage during the Late Triassic. Therefore, the Abu

Ruweis Formation is considered to be the equivalent of the

Mohilla Formation (Druckman, 1974; Bandel and Khoury,

1981), and was equated with the upper member of the

Saharonim and the Mohilla Formations of southern Palestine

(Ahmad, 1989). Northwards, the Abu Ruweis Formation is

equivalent to the Kurra China Formation in Iraq (Sadooni,

1995), and to the Kurra Chine Anhydrite Formation in Syria

(Beydoun and Habib, 1995).

2. Lithofacies

The exposed thickness of Abu Ruweis Formation gypsum in

the quarry area (Subayhi) is about 90 m (Figs. 2 and 3(A)). It is

composed mainly of three lithofacies; evaporites, carbonates

and shales. The evaporites comprise gypsum are laminated,

banded, massive, enterolithic and nodular types. Carbonate

lithofacies includes dolomite (most prominent), oolitic, pelletic

and stromatolitic rocks, and shale lithofacies includes

carbonaceous shales and mudstones (Figs. 2 and 3(B)–(D)).

2.1. Laminated gypsum facies

This facies occurs as thick to very thick-bedded pale

white to gray gypsum. Beds range in thickness from 80 to

160 cm. Very thin evaporite beds and laminae are inter-

bedded/interlaminated with thinner dark lime mud laminae

(organic material possible algal mats) and thin-bedded

dolomites (Fig. 4(A)). Evaporite laminae display enterolithic

Thi

ckne

ss(m

)

For

mat

ion

Epo

ch

Per

iod

Age

10

20

30

40

50

60

70

80

90

100

0

(cm)

AB

U R

UW

EIS

CA

RN

IAN

TR

IAS

SIC

Sha

le

Gyp

sum

Dol

omite

Stro

mat

olite

Ool

ite

Lithology

Laminated gypsum

Nodular gypsum

Dolomite

Stromatolite

Oolite

Shale

Mudcracks

LEGEND

0

Fig. 2. Measured vertical section of the Abu Ruweis Formation exposed in the

Subayhi area, North Jordan.


features. The laminated structure of this facies shows local

deformation and overturning.

2.2. Nodular gypsum facies

Nodular gypsum commonly overlies the laminated gypsum

facies, and both are separated by sharp and conspicuous

contacts (Fig. 4(A)). The nodules are elliptical in shape and

aligned almost parallel to the laminated gypsum below with a

preferred orientation (Fig. 4(B) and (C)). They consist of

closely packed, pale white nodules set in a faint gray–black

organic lime mud, forming the chicken-wire (Fig. 4(B) and

(C)) or mosaic structure (Fig. 4(D)) of Maiklem et al. (1969).

The individual beds range in thickness from a few

centimetres to 1 m.

The change of gypsum fabrics from laminated to nodular

suggests that the latter results from the mobility of sulfate

during diagenesis (Hussain and Warren, 1989; Aref et al.,

1997). The formation of nodular structures is probably the

result of fluctuations in salina water level, and periodic

precipitation of gypsum during seasonal concentration of

marine water (Ogniben, 1955).

3. Description of gypsum microfacies

Microscopic investigation as well as X-ray diffraction has

shown that the evaporites of the Abu Ruweis Formation are

composed primarily of microcrystalline gypsum (Fig. 5), The

microcrystalline gypsum displays a variety of textures ranging

from xenotopic to idiotopic. Five microfacies were recognized;

(1) gypsum pseudomorphs after anhydrite, including gypsum

pseudomorphs after felted anhydrite and gypsum pseudo-

morphs after radiated anhydrite, (2) porphyroblastic gypsum,

(3) granoblastic gypsum, (4) amiboid gypsum, and (5) fibrous

satin spar gypsum. Of these, the fibrous (satin spar) gypsum

variety is the only type which formed from sulfate-rich

solutions, developed as a result of the rehydration of anhydrite,

whereas, the other microfacies were formed by rehydration of

precursor anhydrite.

3.1. Gypsum pseudomorph after anhydrite

This is the most common type in the Abu Ruweis gypsum.

Two varieties were recorded. The first is composed of

elongated, randomly arranged thin laths showing a felted

appearance (Fig. 6(A) and (B)). The second variety is

composed of prismatic crystals arranged in a fibro-radiated

texture (Fig. 6(C)). It is believed that these two varieties

formed by the rehydration process of precursor anhydrite. This

is supported by the presence of satin spar gypsum veins which

are the bi-products of the rehydration process of anhydrite

(Holliday, 1970; Testa and Lugli, 2000).

3.2. Porphyroblastic gypsum

This is composed of large euhedral to subhedral crystals which

showing sharp extinction (Fig. 6(D)). These large crystals occur as

individual crystals or in groups of two or three crystals (Fig. 6(D)).

The large crystals are surrounded by fine gypsum crystals and are

characterised by abundant anhydrite relics (Fig. 6(D)). The

porphyroblastic gypsum was formed by the rehydration of

anhydrite as indicated by anhydrite relics (Holliday, 1970).

Fig. 3. (A) General view showing the gypsum quarry face in the Subayhi area. Note the lateral persistence of strata and regularity of the bedding. The arrow shows

contact with Jurassic strata. (B) Thick bedded evaporites alternating with carbonates and shales. (C) Thin bedded evaporites alternating with carbonates and shales.

Note the distortion of bedding (upper right). Hammer is 28 cm long. (D) Carbonaceous shale sandwiched between two gypsum beds. Gypsum nodules are developed

within the shale (Hammer is 28 cm long).

Fig. 4. (A) Regular alternation of laminated gypsum (white) and dolomites (gray) rich in organic matter, passing upward into nodular gypsum. (B) Alternating

laminated and nodular gypsum displaying chicken-wire pattern of the aligned nodules. (C) Banded nodular gypsum displaying chicken-wire pattern. (D) Aggregate

of gypsum nodules forming a botryoidal pattern (Hammer is 28 cm long).


10 20 30 40 500

2000

4000G-GypsumA-Anhydrate

G

G

G

G

AA

G

Fig. 5. X-ray diffraction pattern of the Abu Ruweis gypsum, Subayhi area,

North Jordan.


3.3. Granoblastic gypsum

The granoblastic gypsum is of limited distribution in the

Abu Ruweis succession. It exhibits well defined euhedral to

subhedral crystals with sharp and homogenous extinction

(Fig. 7(A)). The crystals being almost equigranular in size (10–

50 mm) are interlocking and form a mosaic texture (Fig. 7(A)).

This gypsum microfacies is characterized by the absence of

anhydrite relics. It is believed that the granoblastic gypsum

represents an advanced stage of anhydrite rehydration (West,

1964, 1965; Holliday, 1970; Warren, 1999).

Fig. 6. Photomicrographs showing the different microfacies of the Abu Ruweis gyps

anhydrite. (C) Gypsum pseudomorph after fibro-radiated anhydrite. (D) Porphyrob

3.4. Amiboid gypsum

The distribution of this microfacies in the Abu Ruweis

gypsum is limited, and is similar to the granoblastic gypsum.

It is composed of gypsum crystals which display cloudy

amiboid texture (Fig. 7(B)), similar to the supra individual

amiboid texture of Ogniben (1957), and amiboid texture of

Holliday (1970). The crystals have very irregular boundaries

and undulose extinction. No anhydrite relics were observed

associated with this type of gypsum. The cloudy amiboid

microfacies of gypsum has been interpreted by many authors

as an early stage of anhydrite rehydration (Ogniben, 1957;

Holliday, 1970; Lugli and Testa, 1993; Testa and Lugli,

2000).

3.5. Fibrous (satin spar) gypsum

The fibrous satin spar gypsum occurs as gypsum veins

interbedded within the evaporites as well as the carbonates of

the Abu Ruweis Formation. The veins are composed of

elongated crystals arranged perpendicular to the walls of the

veins (Fig. 7(C) and (D)). Occasionally, the gypsum crystals

are arranged randomly (Fig. 7(C)). Many authors consider satin

spar gypsum as a bi-product of the rehydration process of

anhydrite (Shearman et al., 1972; Testa and Lugli, 2000). It is

believed that the gypsum veins, which are composed of

randomly oriented crystals, were originally formed as

anhydrite at depth and at high temperature and pressure,

prior to their later rehydration to gypsum.

um. (A) Gypsum pseudomorph after anhydrite laths. (B) Gypsum pseudofelted

lastic gypsum (C.N.100!), scale is 250 mm.

Fig. 7. Photomicrographs showing the different microfacies of the Abu Ruweis gypsum. (A) Granoblastic gypsum. (B) Ameboid gypsum. (C and D) Satin spar

gypsum veins within the Abu Ruweis dolomites (C.N.100!), scale is 250 mm.

D-Dolomite

G-GypsumC-Calcite

Q-Quartz

5040302010

D

DD

D

DDG+

Q

CCC

C

G0

400

600

800

200

Fig. 8. X-ray diffraction pattern of the Abu Ruweis dolomites, Subayhi area,

North Jordan.


4. Interpretation of gypsum microfacies

The microscopic investigations indicate that most gypsum

microfacies of the Abu Ruweis Formation resulted from

hydration of precursor anhydrite. Experimental studies and

modern sedimentary analogies indicate that primary gypsum is

more commonly precipitated under normal surface conditions

(Haridie, 1967; Shearman, 1985; Testa and Lugli, 2000). The

precursor anhydrite could be interpreted as a diagenetic

product derived from rehydration process of primary gypsum

(Murray, 1964; Holliday, 1970).

It is believed that the Abu Ruweis evaporites were deposited

originally as gypsum under restricted shallow marine salina

conditions. This is evidenced by their regular bedding and

interbedding with marine carbonates. These conditions may

have changed, for some time, into a desiccated basin, as

evidenced from the presence of a thick succession (up to 50 m)

of rock salt in the subsurface (Andrews, 1992; Kendall, 1989).

The primary gypsum was transformed diagenetically into

anhydrite during burial (Murray, 1964; Holliday, 1970; Testa

and Lugli, 2000; Kirkland, 2003), or by solar heating similar to

the Miocene evaporites of the northwestern Red Sea (Aref et

al., 2003).

During uplift, the exposed part of the Abu Ruweis

anhydrites changed to gypsum. These evaporites are still in

the form of anhydrite in the subsurface as mentioned by

Andrews (1992), Dalqamuni (1995). All gypsum microfacies

of Abu Ruweis Formation in the Subayhi area (e.g. gypsum

pseudomorphs after anhydrite, porphyroblastic gypsum, gran-

ular gypsum and amiboid gypsum) are of secondary origin.

This is confirmed by the widespread satin spar gypsum veins

scattered within the Abu Ruweis Formation (Fig. 7(C) and

(D)), which are considered by many authors to be the result of

the hydration process of anhydrite (Holliday, 1970; Kendall,

1989; Testa and Lugli, 2000).

5. Carbonate lithofacies

The carbonates are composed primarily of dolomites with

subordinate oolitic and stromatolitic limestones.

5.1. Dolomite facies

The microscopic investigations, X-ray analysis and staining

technique have revealed that the carbonate beds interbedded

with the Abu Ruweis evaporites are mainly composed of the

mineral dolomite (Fig. 8). The Abu Ruweis dolomites are

characterised by their fine crystallinity, thin laminated

dolomicrite (Fig. 9(A)), and the abundance of organic matter.

Some bioclastic dolomites are composed of pelecypod

Fig. 9. Photomicrographs showing the different microfacies of the Abu Ruweis dolomites. (A) Laminated dolomicrite. (B and C) Bioclastic dolomites. (D) Peletal

dolomites (C.N.100!), scale is 250 mm.

Table 2

Stable isotope content and Sr and Na content of some samples taken from the Abu Ruweis dolomites at Subayhi area (ppm)

Sample No. 1 2 3 4 5 6 7 8 9 10 11 12

dO18 5.36 6.11 3.71 4.78 7.23 8.21 6.32 5.45 8.38 7.41 4.49 1.45

dC13 3.27 K0.57 2.45 K1.52 K1.21 4.73 2.72 3.17 4.22 K1.16 4.16 K0.72

Sr 515 421 600 412 482 588 621 600 342 561 322 411

Na 425 672 351 785 835 812 734 611 575 442 566 779

10 8 6 4 2 0

2

7

δC13


fragments and crushed shells of arenite size (Fig. 9(B) and (C)),

and other dolomites are pelletal (Fig. 9(D)).

Oxygen and carbon isotope analysis of the dolomites

associated with the Abu Ruweis gypsum show that these

dolomites are isotopically heavy. The O18 content ranges from

1.45 to 8.38‰ which is higher than the Standard Mean Ocean

Water (SMOW) value which is 0‰. The C13 content range

fromK1.52 to 4.73‰ PDB (Table 2, Fig. 10). The Oxygen

isotope values are too high to have formed from normal sea

water. Therefore, these isotopically heavy dolomites must have

been precipitated from fluids enriched in O18, probably as a

result of evaporation (Gill et al., 1995). Trace elements analysis

has shown that the Abu Ruweis dolomites are rich in

strontium (up to 600 ppm) and sodium (up to 835 ppm)

(Table 2). The isotope composition and trace elements content

as well as the petrographic characteristics, all point to

penecontemporaneous hypersaline dolomitization for the

Abu Ruweis dolomites.
––3
δO18

Fig. 10. dO18 and dC13 of the Abu Ruweis dolomites at Subayhi area, North

Jordan.

5.2. Stromatolite facies

These rocks are composed of closely spaced, thin algal

laminations. Some of these rocks are thick bedded reaching

up to one metre thick. The stromatolite structure is composed

of interlaminated light coloured clastic-rich and dark-

coloured organic-rich laminae that are highly contorted

(Fig. 11(A)). During hot conditions, brine is saturated, and

gypsum is deposited. During the next wet season, water

influx dilutes salina waters promoting algal mats, to develop

on top of the preceded precipitated gypsum (Arkal, 1980;

Aref et al., 1997).

The laminated mats formed in a shallow marginal marine

salina along a supratidal flat. The interlamination with

gypsum indicates that the growth of algal mats took place

Fig. 11. (A) Regular interlaminations of slightly undulating, dark coloured stromatolitic laminae and light coloured gypsum laminae. (B) Mudcracks infilled by

gypsum (Hammer is 28 cm long). (C) Laminated gypsum passing upward into claystones (note the brecciation at the boundary between both facies and the

associated embedded gypsum layer which is completely broken into pieces) (Hammer is 28 cm long). (D) Gypsum and claystones showing contortion of the strata

(the ruler top left is 20 cm long).


on the bottom during phases of dilution of salina water and

low salinity rates, when gypsum precipitation ceased

(Schreiber et al., 1982; Aref et al., 1997). Some workers

restrict the term ‘stromatolite’ to structures, such as domes

and columns, with primary relief, but stromatolites may be

originally flat deposits (Riding, 2000).

6. Shales lithofacies

This facies displays sharp contact with the evaporite beds

throughout the study area (Fig. 3(C) and (D)). Individual beds

range in thickness from a few centimetres up to 150 cm. This

facies shows vertical to oblique cracks that are partially or

completely infilled with clear white transparent gypsum from

the overlying subaqueous gypsum layer (Fig. 11(B)), similar

to those described by Aref et al. (1997) from the Gulf of

Suez, Egypt. The mudstone layers also enclose white

lenticular gypsum nodules up to 25 cm long and 10 cm

thick, showing local deformation structures (Fig. 11(C) and

(D)). Organic-rich shales are common in the Abu Ruweis

succession, especially as part of the evaporite–carbonate

cycles.

Black shale samples from RH-1 well (well depth at 850–

1380 m) were analyzed by the Amoco Oil Company (1987).

They indicated that the samples were slightly carbonaceous,

non to slightly calcareous. The kerogen content consists mainly

of amorphous organic matter, associated with small amounts of

miospores and plant tissues. Therefore, the Abu Ruweis

Formation may be considered as a potential source rock

(Dalqamuni, 1995).

7. Gypsum–dolomite relationship

The microscopic investigations of the dolomites inter-

bedded within the Abu Ruweis evaporites have revealed that

they contain gypsum as veins and scattered crystals amongst

their constituents. The gypsum occurs either as elongate thin

laths, scattered randomly within the dolomite crystals, or as

small irregular areas composed of fine aggregates of gypsum

(Fig. 12(A)–(C)). These gypsified areas usually occur very

close to the previously described gypsum veins. Occasionally,

the walls of the veins are eroded and became irregular and pass

laterally to the gypsified areas.

It is believed that this type of gypsum was formed by the

replacement of carbonate minerals (calcite or dolomite) by

sulfate-rich solutions (Fig. 13). The textural characteristics of

this gypsum indicate that the carbonate minerals (calcite or

dolomite) were replaced originally by anhydrite, which in turn

was replaced by secondary gypsum (Fig. 13). If this is the case,

which is more likely, the replacement of carbonate minerals

took place at depth under high temperature and pressure,

thereby favouring the formation of anhydrite rather than

gypsum (Murray, 1964; Holliday, 1970; Kendall, 1989; Testa

and Lugli, 2000). Locally, the gypsum occurs as lenticular

crystals scattered within the dolomite crystals (Fig. 5(F)) which

probably indicates a direct replacement of carbonate minerals

by gypsum under near surface conditions (Warren, 1999).

Fig. 12. Photomicrographs showing: (A and B) sulfatization of the Abu Ruweis

dolomites (the gypsum occurres as separated areas as in A or as lenticular crystals

as in B), (C) sulfatized dolomites in the form of gypsum vein and irregular areas.

Note that the gypsum laths are arranged randomly (C.N.100!), scale is 250 mm.

Satin spar gypsum

Pseudomor-ph Gypsum

PorphyroblasticGypsum

GanoblasticGypsum

AmeboidGypsum

Epigenetic felted Anhydrite

Primary Gypsum

Fig. 13. The diagenetic stages of the Abu Ruw


8. Gypsum–dolomite cyclicity

The alternation of gypsum and dolomite as couplets

continues upward, and displays a ‘dolomite–gypsum rhythmic

pattern’ at different levels of the outcropping succession. Each

couplet consists of a dark-gray, thin to thick-bedded dolomite

overlain by a thick-bedded white gypsum. Rhythmic couplets

are distinct, but are irregularly spaced, and occur in multiple

sequences commonly capped with carbonaceous shales. When

the three components are together the term ‘dolomite–gypsum–

shale tripartite’ is used. This tripartite style of composition also

occurs in the subsurface, where it is composed of an alternation

of laminated dolomite and anhydrite with rare shale intercala-

tions (Andrews, 1992). In the subsurface, a thick salt bed (22–

57 m) was recorded (Andrews, 1992). It is noted that the

dolomite interbeds increase in thickness and frequency at the

lower and upper parts of the section, and decrease in the middle

part of the section.

9. Environment of deposition

Evaporite sequences have been generated in a variety of

geographic settings: (1) coastal intertidal and supratidal

environments (marine sabkhas), (2) along coasts, (3) large

basins with marine inflow (subaqueous marine/salina), and

(4) non-marine interior basins (continental sabkha-playa)

(Kendall, 1984). No single facies model can be applied to

such heterogeneous rocks as the evaporites, and their

identification may depend more upon associated facies than

upon internal characteristics (Kendall, 1984).

The Abu Ruweis evaporites formed in shallow seawater,

and occasionally in desiccated environments subject to floods.

The lateral persistence of beds over large areas with only minor

changes in thickness and facies indicates that they formed on

broad, very low relief areas that were affected by rapid

transgressions that led to major changes in brine chemistry

(Peryt, 2001).

Satin spar gypsum

Secondary gypsum

Anhydrite Anhydrite Anhydrite

Dolomite Gypsum

Calcite Stage I

Stage II

Stage III

Stage IV

eis evaporites at Subayhi, North Jordan.

Subtid

al (o

pen m

arin

e she

lf)

Beach

barsIn

tertid

al

Supert

idal (

Sabkh

a)

(peri

odic

expo

sure)Allu

vial p

lain

Arabian

Nubian

Shield

DolomiteDolomite

LimestoneLimestone

GypsumGypsum

HaliteHalite

SandstoneSandstone

Shale

SStromatolite

OoidsOoids

Pelloids

Mudcracks

FossilsN

Influx

Open m

arine s

ource

Fig. 14. Schematic block diagram of the coastal salina setting showing generalized subenvironments and their respective lithofacies of the Abu Ruweis Formation.


The marine origin of Abu Ruweis evaporites–carbonates

cyclic pattern is explained by Braitsch’s model (1962). As

water evaporates, salinity progressively increases, calcium

carbonate and calcium sulfate precipitate in the order of their

solubility, and an evaporative couplet results. Another influx of

seawater begins a new cycle.

During deposition of the Abu Ruweis Formation, beach bars

may have developed toward the sea and shoals further landward

(Fig. 14). This setting shows considerable variation in facies

characteristics and represents the transition from subtidal shelf

conditions through intertidal to supratidal, as evidenced by the

presence of pelletic, oolitic and stromatolitic facies. Based on

our studies, this formation is believed to have been deposited in

three types of setting (Fig. 14): (1) shallow marine shelves, (2)

intertidal flats, and (3) supratidal sabkhas. The supratidal

environment was dominated by sabkhas during an arid climate.

As a result algal mats were developed along the tidal flats and

stromatolites were formed. Carbonate stromatolites vary

considerably in origin and components, and also in the quality

of lamination. Episodic, even accretion promotes layering,

including lamination, and may be related to seasonal growth,

periodic sedimentation or both (Braga et al., 1995). Modern

marine stromatolites are well known in tidal flat settings, and

the most famous examples are restricted to hypersaline

embayments of Shark Bay in Western Australia (Logan, 1961).

Carbonaceous shales also formed in the supratidal zone and

are interbedded between the stromatolites and evaporites.

Water agitation in the intertidal zone was responsible for the

formation of oolitic beds. Pellets were also accumulated along

the intertidal and subtidal zones. This setting is also suitable for

the deposition of mudstones and stromatolites (Fig. 13).

Shallow-water evaporative environments are associated with

strong microbial activity promoting dolomite formation and

early dolomitization of other carbonate sediments at low

temperature (Schreiber and El Tabakh, 2000). Primary

dolomite deposits may have formed in supratidal and upper

intertidal environments, such as those reported from the

modern coastal lagoon environment in Brazil (Vasconcelos

and McKenzie, 1997).

The dolomite beds containing marine fauna in the form of

bioclasts (Fig. 9(B) and (C)) indicate an open-marine setting

for the original limestones (dolomitised grainstones). The

dolomite beds are finely laminated and rich in organic material,

indicative of restricted conditions. The dolomite–gypsum

cyclicity is the result of periodic deepening (transgressive

shoal) and shallowing (regressive supratidal) conditions.

During deepening carbonates developed. Periodical inundation

of the supratidal sabkha surface during high tides was also

common. This situation is similar to that described by

Alsharhan and Whittle (1995) in the Arabian Gulf. A shallow

carbonate shelf was proposed by Dalqamuni (1995), starting

with a subtidal foraminiferal bioclastic limestone grading into

oolitic shoals, passing into lagoonal pelletal and peloidal

limestone, followed by intertidal stromatolitic limestone that

shifted at the top to nodular anhydrite. The succession

represents a shallowing upward carbonate sabkha sequence,

whereas, a salina was developed in the subsurface where thick

halites were precipitated (Dalqamuni, 1995).


It seems that the Abu Ruweis Formation experienced a

remarkable periodicity; a dry season during which more water

left the basin when the evaporation rate is high and

consequently gypsum was deposited; and a humid season

when water is available and the evaporation rate is low. During

the humid season, carbonates (calcite and dolomite) precipi-

tated resulting in distinct lamina of carbonates and admixed

organic matter.

The regularity and lateral persistence of evaporite bedding

(Fig. 3(B)), as well as the alternation of marine carbonates

containing bioclasts, oolites and pellets indicate deposition

under subaqueous marine conditions similar to those happen-

ing today at the Ras Mohamed pool at the northern end of the

Red sea (Friedman, 1982). It is believed that the depositional

conditions changed periodically from a normal marine phase

during when the carbonates were deposited, to an evaporative

phase when the evaporites were deposited. The persistent

stratification was developed due to high seasonal evaporation

rate (Kirkland, 2003).

Mudstone layers were deposited periodically when muddy

flood water inundates the marginal marine salina setting, where

it ponds and settles (Aref et al., 1997) during major flooding

stages and high tides (Lowenstein and Hardie, 1985). The

presence of desiccation cracks (Fig. 11(B)) suggests that

periods of desiccation and exposure may have occurred in the

hypersaline basin, regardless of its initial depth (Schreiber and

El Tabakh, 2000). The presence of vertical cracks filled with

gypsum is most probably related to the formation of

subaqueous shrinkage cracks in an environment subjected to

large salinity fluctuations (Astin and Rogers, 1991; Plummer

and Gostin, 1981). The absence of detrital materials to fill

cracks and their filling by subaqueous gypsum from the

overlying laminated gypsum layer confirm a subaqueous

shrinkage of the cracks (Aref et al., 1997). The thick halite

precipitation encountered in the subsurface reflects greater

evaporation and greater brine concentration (Kendall, 2000).

The frequent occurrence of the organic matter (OM) indicates

the euxinic conditions prevailed throughout the deposition of

Abu Ruweis Formation. The source of some of the organic

debris in such a barred and restricted basin is probably marine

plankton swept in by influx currents (Peterson and Hite, 1969).

It seems that the Abu Ruweis Formation is one of the few

marine evaporite settings where the amount of organic matter

accumulating in the evaporitic facies has been established.

10. Sequence stratigraphy

A proposed sequence stratigraphic framework for the Abu

Ruweis Formation may indicate that the bulk volume of

evaporites formed during lowstand conditions, whereas the

carbonates represent the transgressive and highstand con-

ditions. During evaporative phases, the basin subsided in order

to accommodate the thick evaporite succession. Areas of

sufficient evaporation and low water influx are developed

during periods of low sea level, mainly during a low stand

systems tract. High rates of accumulation of organic matter

indicate that bottom waters were anoxic (Kirkland, 2003). The

development of a barrier caused restriction of the basin,

associated with relatively stable arid climatic conditions,

allowing deposition of a thick prograding succession, similar

to that described by Schroder et al. (2003) in the Early

Cambrian Ara Group in Oman. Kendall (1992) indicated that

low-lying basins with a flat basin floor are flooded rapidly

leading to rapid establishment of open-marine conditions.

Nevertheless, shallow water deposition persisted throughout

the deposition of the Abu Ruweis Formation (Fig. 14).

11. Conclusion

The Abu Ruweis Formation in the Subayhi area has a

tripartite arrangement: thick evaporite beds, thin dolomite beds

and a few shale beds. Several evaporite phases took place

during the deposition of the Abu Ruweis Formation. At the

beginning of each phase; progradation took place and a shallow

water carbonate ramp gradually evolved into a series of

shallow sulfate and halite salinas. Several carbonate–evaporite

sequences are recognized, each consisting of a lower,

carbonate part, containing minor amounts of evaporites,

representing transgressive and highstand systems tracts.

These are overlain by an evaporite part formed mainly during

a lowstand systems tract.

Most of the Abu Ruweis gypsum lithofacies are of

secondary origin as a result of a successive series of

rehydration and hydration processes. The satin spar gypsum

veins scattered within the Abu Ruweis evaporites and

carbonates are considered as a bi-product of the hydration

processes of the precursor anhydrite. The carbonates associated

with the evaporites of the Abu Ruweis Formation are mainly

composed of the mineral dolomite resulted from hypersaline

dolomitization.

Acknowledgements

The authors are greatly indebted to the following institutes

for facilitating analyses. X-ray diffraction analyses were

performed at the laboratories of Assut University, chemical

analyses were performed at the laboratories of the South Valley

University, and isotope analyses were performed at the English

Academic Laboratories through the industrial petroleum

consultancy (FJA) to whom we are most grateful. The

Deanship of Scientific Research at the Hashemite University

is also thanked for providing logistic support. The paper has

been substantially improved as a result of suggestions and

criticisms by Abdulkader Abed, Charlotte Schreiber and Khalil

Ibrahim. We thank them for their advice. Brian Turner and

Basim Moh’d are thanked for their critical reviewing of the

manuscript.

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Depositional environments and facies of the Late Triassic ... · Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan Issa M. Makhlouf a,*, Abdel