quaternary sandstones, northeast jordan: age, depositional ...quaternary sandstones, northeast...

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Palaeogeography, Palaeoclimatology, P

Quaternary sandstones, northeast Jordan: Age, depositional

environments and climatic implications

Brian R. Turner a,*, Issa Makhlouf b

aDepartment of Earth Sciences, University of Durham, Durham DH1 3LE, UKbDepartment of Earth and Environmental Sciences, Hashemite University, Zarga, Jordan

Received 10 January 2005; received in revised form 14 June 2005; accepted 16 June 2005

Abstract

OSL dating of weakly consolidated, root-bound, non-calcareous quartz arenites in northeast Jordan, currently assigned

to the Plio–Pleistocene Azraq Formation, suggests a Middle Pleistocene (652F47 ka) age. The sandstones are up to 15.5

m thick and overlain by a 2.5 m thick Holocene gypcrete caprock. Facies and textural analyses suggest that the sandstones

are predominantly aeolian in origin, mainly derived from Tertiary sediments exposed close to the depositional site. The

sands were transported by the prevailing NW winds and deposited in a broad, relatively flat sand sheet environment.

Rhizoliths occur throughout the sandstones, mainly as long, downward tapering, vertical tap roots, rarely branched and

with few laterals. Microscopic examination of root cores replaced by carbonate reveals the presence of alveolar fabrics,

possible needle fibre calcite, calcified organic filaments of fungal, root vessel and root hair origins, characteristic of low

magnesium beta calcretes, typical of humid climates.

Morphologically the roots resemble modern shrub-like species typical of desert environments where water availability at

the surface and in the subsurface was sufficient to support an effective vegetation cover. Plots of stratigraphic variations in

root length, root spacing and root frequency reflect temporal variations in the water table level and precipitation during

sand deposition. All three parameters show a similar crude cyclicity consistent with fluctuations in the level of the water

table with the most moist phase beneath the predominantly waterlain Holocene gypcrete when trees appeared for the first

time. The gypcrete signifies a change to temporary wetter conditions and may mark the boundary between the Pleistocene

and Holocene in this area. Although pedogenic horizonation is poorly developed, especially in desert sands, the beta

calcretes and rhizocretions typically form within active soil zones. Soils do not form where rainfall is b150 mm per year,

and above 350 mm complete leaching of the edaphon occurs. However, above 300 mm per year shrubs are replaced by

grassland, hence rainfall is inferred to have been 150–300 mm per year, much higher than the b50 mm in the area today.

The age of the sandstones may correlate with isotopic event 17, dated at 659 ka, when the Pleistocene climate in Jordan

alaeoecology 229 (2005) 230–250

0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2005.06.024

* Corresponding author.

E-mail addresses: [email protected] (B.R. Turner), [email protected] (I. Makhlouf).

B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 231

was characterised by arid to semi-arid phases interrupted by shorter more humid phases, when the water table was higher and

the precipitation/evaporation balance greater than today.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Quaternary environments and climate; NE Jordan

1. Introduction

Several small, closely spaced exposures of root-

bound sandstone, currently assigned to the Pliocene–

Pleistocene Azraq Formation, occur at Dahikiya in the

southern Badia Region of NE Jordan (Fig. 1). Some

sandstones have been worked for sand aggregate and

glass sand from small opencast pits (785–1571 m3 in

size) during the last 6 years, but only one of these is

currently being mined (Fig. 2). The walls of this

sandpit, provide good exposures and clean surfaces

ideal for studying the sandstone in detail. This study is

based on measured sections, and photomosaics of two

well exposed vertical to subvertical faces: on the

northwest and southeast sides of the pit (Fig. 2).

31o

30o

35o 3

Amman

Zarqa

Ma'an

Irbid

Aqaba

S Y RLake Tiberias

DeadSea J O R D A N

STUDYAREA

S

Azraq

36o

36o

Q

Jordan

Jerusalem

Fig. 1. Map showing the location of the Badia region o

Both faces are now abandoned, and mostly sand

covered, but new working faces, along the northern

and southern sides of the pit, provide excellent lateral

exposures of the sandstone. A white kaolinite layer in

the floor of the pit is thought to lie close to the base of

the sandstone section (personal communication, Ara-

bella Mining Company, 2001).

In this paper we provide the first OSL date for the

sandstones and re-interpret the age of the Azraq For-

mation. The facies architecture, grain textures and the

morphological characteristics and distribution of rhi-

zoliths throughout the succession, are described and

interpreted in terms of the depositional environments

and climatic conditions under which the sediments

were deposited. This in turn allows for comparison

32o

31o

7o

38o

0 100Km

I A

Ruwashidafawi

IRA

Q

SAUDI ARABIA

Badia Region

Highways

Wadi Sirhan

aíFaydat ad Dahikiya

f eastern Jordan and the study area at Dahikiya.

Conveyor belt

Sand pile fortransport

Sandandgravelhummocks

Coalescedbase of slopecolluvial fans

Sand pile

Fine gravel

Fine gravel

S

50 m50 m

Road

B

A

Fig. 2. Plan and photograph, looking south, of the working sandpit showing the location of the two measured sections (A and B) on which this

study is based. Newly excavated faces along the northern and southern margins of the pit provided additional data during the course of this

study.

B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250232

with documented Quaternary climatic changes for

Jordan and the eastern Mediterannean.

2. Geological background

The Badia, a desert region of northeast Jordan (Fig.

1), consists mainly of Palaeogene to Quaternary con-

tinental alkaline–olivine basalts and tuffs, bordered to

the east and southwest by Cretaceous, Palaeogene,

Neogene and Quaternary carbonates and clastics.

The study area, in the southern Badia (Fig. 3), is

dominated by Palaeogene and Neogene to recent clas-

tics, including several small, closely-spaced sandstone

outcrops assigned to the Azraq Formation, located

within a rift zone, bounded by the NW–SE trending

Fuluq and Sirhan faults (Rabb’a, 1997)(Fig. 3). The

base of the Azraq Formation is unconformable on

older strata below, and the top is defined by the

present day erosional and depositional surface. Bore-

hole data indicates that the formation may be up to 80

m thick at Azraq (Fig. 1), although not more than 15

m is exposed, and its extremely variable lithology, and

lack of a chronostratigraphic framework for the depos-

its, makes correlation difficult (Ibrahim, 1996). Thus,

it is impossible to construct a type section for the

formation at any one locality. As a result only a

composite log, based on several localities, is available

(Ibrahim et al., 2001).

The exposed succession in the study area comprises

up to 15.5 m of weakly consolidated, root-bound

sandstone, locally overlain by a harder gypcrete cap-

rock up to 2.5 m thick (Fig. 4). The sandstones have

been interpreted as marine (Wentzel and Morton,

1959), fluvial and/or lacustrine (Kady, 1983; Ibrahim,

1996; Ibrahim et al., 2001), but their stratigraphic

position and correlation are uncertain. The conglom-

erate at the base of the formation noted by Ibrahim et

al. (2001), is not seen in the study area, and only the

uppermost part of the 16 m maximum recorded thick-

Fuluq

SAUDI ARABIA

Faydat ad Dahikiya

Umari Fault

2 km

Alluvium/wadi sedimentsPleistocene gravelsAzraqSandstone

AzraqFormation

}}

CalcareousSandstone

QirmaFormation

SandstoneChalk

Wadi ShallalaFormation

Plio-Pleistocene

Miocene

Middle-Late Eocene

Fuluq

Dah

ikiy

aan

ticlin

e

Sirhan

Fault

JORDAN

7

10

57

5

Study area

Horizontal strata

5 Dip and strike

Fault

N

}

Fig. 3. Generalised geological map of the southern Badia along the border with Saudi Arabia showing the location of the study area.


ness is sandy, and this is described as calcareous and

rootless (Ibrahim et al., 2001).

3. Age of the Azraq formation

The formation is thought to overlie the Middle

Eocene Wadi Shallala Formation in the Dahikiya

Fig. 4. Northern face of the sand pit showing crudely bedded, rooted

sandstones capped by darker, more resistant gypcrete, and base of

slope colluvium sand and gravel fans with large fallen blocks of

gypcrete. Face is 13.5 m high (see Fig. 5A).

area (Hamdan et al., 1998), and the Miocene Qirma

Formation at Azraq (Fig. 3), some 40 km NW of the

study site, where two gastropods of Miocene age were

recovered from sandstones penetrated by groundwater

wells drilled in 1975 (Hamdan et al., 1998). Two types

of post-Miocene freshwater diatoms (pennate and

centric types) were found in the Azraq Formation at

Azraq (Kady, 1983; Qaıadan, 1992), where inter-

bedded lava flows have been dated as upper Miocene.

The bivalve Cardium edule paludosa, recovered from

the Azraq Formation at Dahikya, close to the study

site, was assigned by Wentzel and Morton (1959) to

the Neogene. Acheulian and Levalloiso–Mousterian

period artifacts were reported from the formation,

indicating a possible Middle to Late Pleistocene age

(Bender, 1974). Although most workers consider the

age of the formation to be Pliocene–Pleistocene, the

evidence is equivocal and Hamdan et al. (1998, Table

1 and p.25) consider the Azraq Formation to be

Pleistocene and Pliocene–Pleistocene in age, whilst

Ibrahim et al. (2001) gives a Pliocene–Pleistocene age

on the geological and mineralogical map of the Badia,

but a range from Upper Miocene to Late Pleistocene

in the accompanying report. Optical Stimulated Lu-

Met

res

Cla

yS

ilt

Sand

{

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Rareroots

Calcretised tree trunks

Abu

ndan

troo

tsA

bund

ant r

oots

Deformedzone

Rar

ero

ots

Abu

ndan

tro

ots

Sequence of sedimentary structures comprisesfrom the base up: (1) small-scale trough cross-lamination;(2) larger scale trough cross-bedding; (3) small-scaletrough cross-lamination; (4) larger scale troughcross-bedding; (5) small-scale trough cross-lamination;(6) convolute lamination and ripple cross-lamination.Two closely spaced clay-rich surfaces

Trough cross-bedding; deformed zone; low anglelamination in upper part

Sharply-based, cross-bedded sandstone, containingshale intraclasts and granules and small pebbles ofquartz, chert,chalcedony and calcreteRipple cross-lamination; laminae dipping in oppositedirections; irregular calcite-cemented sand nodules;fining-upward trendSmall, low angle troughs and ripple cross-lamination;coarser sand and quartz granules concentrated alongforesets and base of trough sets

Gypsum-cemented, brownsandstone and fine conglomerate.Finer sandy, rippled middle zone;Laminated, saucer-shaped heavestructures; high angle ripple laminae;horizontally laminated crust

Small-scale trough cross-laminationand ripple cross-lamination

15

14

13

12

11

10

9

8

7

6

5

4

3

2

0M

etre

s

Cla

yS

i lt F MCSand

{

Gypsumcrystals

Intersectingroots

Deformedforesets

Gypsumcrystals

Pebbly zone

Hard, brown, blockycrust; carbonaceous,non- living modern roots

N

n = 13Horizontal to subhorizontal laminations;cross-bedding; cross-bedded pebbly zoneat top; greenish shale intraclasts; sandstonenodules

Small trough cross-lamination;local chert clasts

Small-scale, ripple cross-lamination

Subhorizontal lamination; locallydeveloped trough foresets; carbonatecemented sandstone concretions

Trough cross-bedding;some ripple cross-lamination

Gypsum-cementedcaprock

Abundant roots

Poorly defined bedding

Wavy laminae; local ripplecross-lamination

Moderately abundant roots

Abundant roots

Moderate to poorly rooted

Abu

ndan

tro

ots

Abu

ndan

tro

ots

Mod

erat

ely

root

ed

; green shaleintraclasts

Base not seen

Facies 1

Facies 2

Facies 3

Facies 4

Facies 5

Facies 6

N

n = 8

FMC

A Northwest B Southeast

Fine trough set

Coarse trough set

Coarse foreset

50 cm

ForesetsRipples

Inset 1

Inset 3

Inset 2

Facies 7 Facies 7

Facies 6

Facies 5

Facies 4

Facies 3

Facies 2

Facies 1

Ho

loce

ne

Mid

dle

Ple

isto

cen

e

Mod

erat

ely

abun

dant

root

s

OSL sample 652 ± 47Ka

Cross-bedding

Horizontal tosubhorizontallaminations

Ripplecross-laminationDeformation

Clasts

Roots

100

0

cm

Fig. 5. Detailed sections of the Pleistocene sandstones and Holocene gypcrete caprock measured at the northwestern (A) and southeastern (B) ends of the sandpit at Dahikiya.

B.R.Turner,

I.Makhlouf/Palaeogeography,Palaeoclim

atology,Palaeoeco

logy229(2005)230–250

234


minescence (OSL) dating on a sample of sandstone

taken 9 m below the top of the succession on the

northwest side of the pit (Fig. 5A) produced an early

Middle Pleistocene minimum age of 652F47 ka. This

is a preliminary age based on a single sample, and

further dates are required to determine any age–depth

trend.

4. Sedimentary facies

The walls of the sand pit have vertical to steeply

inclined faces up to 13 m high (Fig. 4), composed

of weakly consolidated, friable, root-bound, non-

calcareous quartz arenites, containing b3% kaolinite

clay (Turner and Makhlouf, 2002), uncemented except

for local, irregularly-shaped carbonate concretions.

The sandstone is locally overlain on the northern and

southeastern sides of the pit by a more resistant,

brownish-weathered, vertical, gypsum-cemented sand

and gravel-dominated caprock (Fig. 5). Elsewhere the

sandstone is directly overlain by Holocene gravels and

pebbly sandstones.

The sandstone has been divided into 7 lithofacies,

based mainly on a detailed measured section on the

northwest side of the pit (Fig. 5A) and a second

detailed reference section, measured some 70 m

away, on the southeast side of the pit (Fig. 5B). The

sections differ in thickness between individual facies,

and in the presence of finer grained intervals in Facies

2 and 4 on the northwest side, otherwise they have a

broadly similar internal facies architecture (Fig. 5),

except where specifically mentioned. New working

faces on the south and north sides of the pit (Fig. 2),

opened up during the course of this study, reveal new

sedimentological features which have been included

here for completeness.

4.1. Facies 1.0–1.5 m

This incompletely exposed facies, comprises

coarse-grained, soft, friable, moderately well-sorted,

white to very light grey (Munsell rock colours N8–

N9) sandstone containing a few granules of quartz and

chert, and elongate, greenish shale intraclasts, up to

2.5 cm long, with their long axes aligned parallel to

crudely defined horizontal to subhorizontal bedding

surfaces. The sandstones are structured internally by

low angle, small-scale trough cross-bedding, (indivi-

dual sets up to 12 cm thick), and ripple cross-lamina-

tion. Cross-bed foresets dip predominantly towards

the north-northwest (Fig. 5) at steep angles of up to

40–458. Coarser sand and quartz granules concentrate

along the base of cross-bed sets, and along the base of

individual, foresets. Low contrast sedimentary struc-

tures in the upper 50 cm reflect poor internal grain

segregation.

Vertical to sub-vertical in situ fossilised roots, and

subordinate horizontal to subhorizontal roots occur

throughout the sandstones (Fig. 6A). These occur as

sandy-coated root moulds, and less commonly as soft,

friable, brownish, Fe-oxide impregnated sandy root

structures, a few millimetres to 1.2 cm in diameter,

and a maximum exposed length of 45 cm. They

closely resemble tap root rhizoliths figured by Este-

ban and Klappa (1983, Figs. 53, 59). Although most

roots have a submillimetre thick sandy outer coating,

slightly harder and better-cemented than the host

sandstone, they are still fragile and break easily. A

few roots have a harder, calcareous-cemented, root

core. Root frequency was assessed by placing a metre

square frame against the outcrop face and counting

the number of roots within the frame. Root frequency

is similar throughout the facies but with a maximum

of 31/m2, 0.5 and 1 m, respectively, above the base of

the facies. Some root infills contain occasional quartz

granules and slightly coarser sand than the host sand-

stone, whilst others occur as reddish-brown root

moulds (dikaka).

4.2. Facies 2. 1.5-2.3 m

This facies is slightly coarser than Facies 1, and

locally it shows a slight fining-upward trend (Fig.

5A). The finer grained upper part may be equivalent

to the finer grained interval in Facies 2 in the south-

east (Fig. 5B), except that in the southeast the sand-

stone is harder, better consolidated and contains mm

to sub-mm, slightly wavy laminae. The top 3–4 cm,

which is very hard and cemented, overlies a 5–6 cm

thick structureless layer. The sandstone in the north-

west contains whitish (N9), irregularly-shaped, cm-

scale patches of hard, carbonate-cemented sandstone

concretions, which weather out from the softer, unce-

mented host sandstone. These concretions, some of

which resemble carbonate-cemented root structures

Fig. 6. (A) Rhizolith-rich horizon showing closely spaced vertical, in situ taproot rhizoliths exposed in wind eroded face of sand pit. Note the in

situ cross-cutting rhizoliths in the centre of the photograph (Pen is 14 cm long). (B) Calcareous-cemented root structure (rhizocretion) weathered

out from softer host sandstone (Pen is 14 cm long). (C) Hard, resistant rhizocretion on floor of sand pit (Pen is 14 cm long). (D) Vertical, in situ,

hard, calcretised tree trunks with lateral root structures. The top of the trunks stop abruptly at the base of the overlying gypcrete (Hammer is 33

cm long).


(rhizocretions)(Fig. 6B,C) are uncompacted and un-

deformed, and they occur sporadically throughout

other parts of this facies. The lower 65 cm contains

ripple cross-lamination, comprising 3–4 cm thick,

trough shaped sets arranged in alternating coarser

and finer sets. Foresets within individual sets also

have alternating finer and coarser laminae (Fig. 5A,

Inset 1). The ripple cross-lamination dips at b108 weston one side, and b108 east on the other side, over a

distance of about 1 m, with the laminae continuous

across the change in dip. The upper 60 cm of this

facies comprises low angle (128) sub-mm to mm-thick

laminae dipping in opposing directions. When traced

laterally some laminae are discontinuous, have very

low angle dips, and pass down dip into small trough-

shaped ripple cross-stratification within a distance of

1.5 m (Fig. 5A, Inset 2). Soft, friable, sandy roots, up

to 1.2 cm in diameter and 30 cm in length occur

throughout this facies. Up to 34 roots/m2 were

counted in the lower rhizolith-rich 65 cm of the facies,

whereas in the upper 60 cm only 19/m2 were counted.

4.3. Facies 3. 2.3–2.6 m

This comprises a coarse-grained slightly harder,

better cemented and darker yellowish-grey (5Y 8/1)

sandstone than the facies below. It contains greenish

shale intraclast-rich zones, scattered granules and

small pebbles of white quartz, rose quartz, greenish

quartz, zoned chalcedony, shale, chert and calcrete.

Some black chert clasts, up to 1 cm in diameter, have

been polished by wind abrasion. The sandstone trun-

cates the foresets below (Fig. 5A), and is internally

structured by foresets, indicating palaeoflow to the

northwest (3358)(Fig. 5). In the northwest it contains

small to medium-scale, root-penetrated trough cross-

bedding, in sets up to 40 cm thick, and well rounded,

carbonate-cemented sandstone concretions up to 5 cm

in diameter, which occur as individual concretions or

pairs of concretions. This facies in the southeast is

almost 3 m thick and more variable in its internal

architecture (Fig. 5B).

4.4. Facies 4. 2.6–3.55 m

This is a fine to medium-grained, white to light

grey (N8–N9) sandstone internally structured by

small-scale cross-bedding in the lower 20–30 cm

(Fig. 5A). These occur within a coset, comprising at

least 7 sets, in which the individual sets are typically

lens shaped, up to 15 cm thick and defined by coarser

sandstone concentrated along the base of sets. The


uppermost sets, which show locally deformed fore-

sets, are overlain by a 45–50 cm thick zone of hori-

zontal to subhorizontal, slightly wavy lamination, that

becomes deformed in the upper 15–20 cm (Figs. 5A,

7A). The upper 40 cm consists of very low angle

lamination and some slightly higher angle, ripple

cross-lamination, and coarser sandstone lenses con-

taining scattered quartz and chert granules. Roots are

very abundant throughout this upper zone (N35/m2),

which contains some of the largest recorded: over 90

cm long and 1.5 cm in diameter. Some roots are

carbonate-cemented, especially the root core. Strati-

graphically this facies occurs at about the same level

as the upper part of Facies 3 in the southeast, except

that it is finer grained (Fig. 5). Attempting to correlate

Fig. 7. (A) Cross-bedded units overlain by horizontal to subhorizontal, s

Facies 4, northwest face of pit. (B) Root penetrated aeolian cross-bedding

of the top of the foresets beneath rippled zone, Facies 5, northwest face of

face of pit. (D) Granular and pebbly sandstone capped by a thin horizontal

northern face of pit showing coarse-grained sandstones with scattered gra

clast-supported fine conglomerate. The contact between these two zones is

Note the prominent saucer-shaped laminated structures in the upper part o

mats. (F) Fluvial channel with lateral wing incised into gypcrete and fille

long).

individual facies, even across a distance of 50 m, is

difficult, and attests to the dynamic nature of the

depositional environment.

4.5. Facies 5. 3.55–5.75 m

This facies shows the following sequence of sedi-

mentary structures from the base upwards (Fig. 5A):

(1) small-scale trough cross-bedding (sets up to 5 cm

thick); (2) larger scale trough cross-bedding (sets up

to 30 cm thick)(Fig. 7B); (3) small-scale trough cross-

bedding identical to (1); (4) larger scale trough cross-

bedding identical to (2); (5) small-scale cross-bed-

ding; (6) convolute lamination (Fig. 7A) (deformed

foresets occur at a similar level in Facies 4 in the

lightly wavy lamination, that becomes deformed in the upper part,

with thin intervening rippled sandstone bed. Note slight overturning

pit. (C) Aeolian dune cross-bedding at the top of Facies 6, southern

ly laminated crust, gypcrete, northern face of pit. (E) Gypcrete along

nules and small pebbles coarsening-upwards into a matrix- to local

marked by a thin, finer grained ripple cross-laminated sandstone bed.

f the gypcrete, interpreted as evaporitic adhesive structures or algal

d with rooted, aeolian sand, northern face of pit (Notebook is 9 cm


southeast) (Fig. 5B); and (7) ripple cross-lamination.

Scattered chert clasts, up to 1 cm long, occur in parts

of the facies, which is well rooted in the lower part

(30/m2) but less well rooted in the upper part (b20/

m2). Two laterally extensive, darker coloured beds, 95

cm apart, occur close to the top of this facies in the

northwest (Fig. 5A, Inset 3), where they dip at about

58 to the south–southwest. The lower bed is a 10 cm

thick, pale olive (10Y 6/2), clay-rich layer with a

variable silt content and a sharp base and top (Fig.

5A, Inset 3). It contains rare small roots, and forms a

useful marker around the western and northern sides

of the pit. The upper bed is a light greenish-grey (5GY

8/1), rippled siltstone and fine sandstone, 7–10 cm

thick, with a 1 cm thick, darker pale olive, silty clay

layer at the top and bottom (Fig. 5A, Inset 3). A

laterally impersistent, 20 cm thick mottled zone

occurs just above and to the left of the rippled silt-

stone and fine sandstone bed.

4.6. Facies 6. 5.75–12.0 m

The internal architecture of this facies is dominated

by ripple cross-lamination and small-scale trough

cross-bedding. Stratigraphically it includes the very

top of Facies 4, the whole of Facies 5 and most of

Facies 6 in the southeast (Fig. 5B). On the more

accessible northwest side, the upper 4 m contains

northerly dipping foresets and chert pebbles up to 5

cm long, with their long axes parallel to the foresets.

Two superimposed large-scale cross-bed sets occur at

this level in the southern face of the pit (Fig. 7C). The

lower one comprises medium to coarse-grained, thin,

concave-up foresets, dipping at up to 208 to the west.

These occur within a 2.5 m thick wedge-shaped set

that shallows and thins to the west away from the crest

of the structure. The upper, 1.7 m thick set, has a more

complex internal architecture. The base, defined by a

more resistant sandstone bed, has a concave-up geo-

metry, disconformable with the foresets below (Fig.

7C). Internally it is characterised by bedding surfaces

dipping at 58 to the east, which enclose steeper fore-

sets dipping 208 west (Fig. 7C). Roots are less com-

mon in the cross-bedded sandstones compared to the

rest of the facies which shows an increased root

frequency towards the top (N34/m2), which decreases

sharply in the top 2 m with the first appearance of in

situ, calcretised, hard, vertical to subvertical, tree

trunks up to 30 cm in diameter and 2 m long, some

showing lateral root offsets (Fig. 6D).

4.7. Facies 7. 12.0–13.5 m caprock

Gypcrete forms a hard, resistant, erosionally-based

caprock up to 2.5 m thick, above the rooted sand-

stones around the northern and southeastern rim of the

pit (Figs. 4, 5A,B). It is locally overlain by Holocene

alluvial gravel and pebbly sandstone (Fig. 8), but

elsewhere the gypcrete is missing and the gravel and

pebbly sandstones rest directly on rooted sandstones.

(Turner and Makhlouf, 2001). The gypcrete comprises

a lower gypsum-cemented, pale brown, coarse sand-

stone, containing scattered granules and small pebbles

of quartz, chert and shell material, overlain by a

coarser upper part of matrix-to clast-supported fine

conglomerate (Figs. 5A, 7D,E). The clasts, set within

a medium to coarse-grained sandstone matrix, are

mostly angular to subangular chert, with the more

elongate clasts showing a crude long axis alignment.

The contact between these texturally distinct zones is

marked by a finer grained, ripple cross-laminated

sandstone bed (Fig. 7E). Roots are absent in the

upper part of the gypcrete and rare in the lower part,

the undersurface of which contains local mud poly-

gons up to 20 cm across.

When traced around the outcrop the gypcrete

shows the following lithological variations (Fig.

5A): (1) a granular and pebbly upper part capped by

a horizontally laminated, 5 cm thick crust (Fig. 7D);

(2) a granular and pebbly upper sandstone containing

faint, internal ripple laminae dipping at about 258,with the more elongate clasts aligned with their long

axes parallel to the laminae; (3) an upper conglome-

ratic part characterised by large (30–50 cm diameter),

crudely laminated, gypsiferous saucer-shaped struc-

tures (Fig. 7E); and (4) a sequence of up to 9 vertically

stacked, thin, irregularly-bedded gypsiferous mud-

stone–sandstone cycles, cut by gypsum veins and

lenses (Fig. 8). The cycles decrease in thickness up-

wards from 44 to 7 cm, accompanied by an increase in

the sand–mud ratio by a factor of two. The mudstone

has a variable silt content, is typically moderate to

dusky red (5R 3/4, 5R 4/6) and, like the sandstone, it

contains mm thick, white displacive fibrous gypsum

lenses, imparting a distinct blocky character to the

outcrop face. The sandstones are greyish-orange

Erosion surfaceLocal mudcracks

Cla

y

Silt

Sand

F M C{

0

20

40

60

80

Cm

100

120

140

Maroon-chocolate browngypsiferous silty mudstone

Gyp

sum

v

eins

an

d

lens

esG

ypsu

m

vei

ns

and

le

nses

Pale pink-fawn, medium-grained

rippled, gypsiferous sandstone withrare cross-beds

Pale pink-fawn,medium-grainedrippled, gypsiferous sandstone

Fin

e to

med

ium

-gra

ined

,st

ruct

urel

ess

san

dsto

ne

Gravels

Poorly defined mudstone-sandstonecycles

Fig. 8. Measured section of mudstone–siltstone/sandstone cycles in gypcrete along the northern face of the pit where the gypcrete is overlain

locally by younger gravels.


pink (5YR 7/2), fine- to coarse-grained, and internally

structured by ripple cross-lamination, or less com-

monly small trough cross-beds, which are most clearly

seen in the lower 3 cycles where the sandstones are

slightly coarser grained (Fig. 8).

The gypcrete is cut out locally by the steep (358)eastern margin of a 1.3 m deep channel, aligned NE–

SW, filled with soft, friable, well-rooted, trough cross-

bedded, medium to coarse-grained, well to moderately

well-sorted sandstone (Fig. 7F). An unusual feature of

the channel is a lateral wing, typical of fluvial chan-

nel-fills (Alexander, 1992). The wing extends from

the channel across the top of the adjacent gypcrete for

2–3 m before wedging out (Fig. 7F).

5. Roots

5.1. Outcrop description

Most roots in the gypcrete caprock, and the top of

Facies 6 have a carbonaceous coating and contain

preserved, non-living, modern carbonaceous root tis-

sue. Throughout the rest of the succession the roots

preserve no organic material and occur mainly as

long, slightly downward tapering, vertical tap roots

more than 1 m long and 2.5 cm in maximum (neck)

diameter, rarely branched and with few preserved

laterals (Figs. 6A, 9A). Many roots have well formed

circular cross-sections and in order of abundance they

Fig. 9. Part of a hard, calcareous-cemented, tapered, vertical rhizolith (A), circular horizontal section (B) and photomicrograph of part of the

calcareous-cemented root core (C). Note the slightly darker (brownish) tone of the carbonate cemented area formerly occupied by the root core

(Xylem and Phloem) in (B) and the ovate to irregular pores lined with microspar (arrowed) in (C). The microcavities in (C) produce root moldic

porosity which resembles alveolar textures.


are preserved as: (1) soft, poorly compacted, unde-

formed root structures replaced by sandstone; (2) root

structures replaced by sandstone with the root core

area cemented by calcite (Fig. 9B); and (3) hard,

carbonate concretions, many of which retain the mor-

phology of the original, uncompacted root structure

(Fig. 6B). According to the classification of Klappa

(1980) and Esteban and Klappa (1983) the roots

comprise the following types of rhizoliths: root

molds, root casts and rhizocretions. Type 2 rhizoliths

have a brownish, more tightly cemented, calcareous

root core, identical to that figured by Esteban and

Klappa (1983, Fig. 54).

5.2. Microscope description

Cylindrical to ovate and irregular pores, up to 0.2

mm in length, lined with microspar, occur within the

root core (Fig. 9C). These microcavities produce a

root moldic porosity and closely resemble the alveolar

textures of Esteban (1974) and Amit and Harrison

(1995, Photo 10). SEM analysis of carbon-coated

samples shows the cavities to be: (1) filled or partial-

ly-filled with calcite cement; (2) open and lined with

microspar, with the spar oriented mainly normal to the

cavity walls; or (3) they have a very thin, darker,

micritic collar, aligned around the walls of the cavity,

partly filled with scattered, poorly-sorted quartz and a

few calcite grains (Fig. 10A). Some individual, well-

rounded quartz grains show conchoidal fracture sur-

faces, and a typical wind abraded punctuate surface

(Fig. 10B).

Associated with the cavities are a number of

organic filaments. Most filaments have smooth

walls without ornamentation, and are unbranched or

very rarely branched. X-ray microprobe (EDAX)

analysis of the chemical composition of the filaments

indicates that they are composed predominantly of

low magnesium calcite (b4 mol x Mg). The fila-

ments are up to 3 Am wide and have a maximum

recorded length of 450 Am. They occur as individual

filaments, and less commonly as coiled pairs (Fig.

10C) and radiating clusters, which includes one rare

example of a branched filament (Fig. 10D). Although

the internal structure of most filaments is difficult to

see, a few comprise open tubes surrounded by a

calcified wall up to 0.5 Am thick (Fig. 10D). The

filaments show varying degrees of surface calcifica-

tion. Most filaments have only minor, local growths

of calcite on the surface of the walls (Fig. 10C,D), or

Fig. 10. (A) Cylindrical pore left after root decay with a thin, darker, fine micritic collar around the walls of the cavity which contains scattered

poorly-sorted quartz and a few calcite grains. (B) Well rounded quartz grain showing conchoidal fracture surfaces and a typical wind abraded,

punctuate surface. (C) Coiled pair of fungal filaments showing local encrustations of calcite on the surface. (D) Branched, radiating pipe-like

cluster of open ended hollow filaments surrounded by a calcified wall up to 0.5 Am thick. (E) Extensively calcified filament resembling needle-

shaped calcite illustrated by Guo and Federoff (1990, Fig. 2). (F) Root hair or fungal hyphea.


rarely they are more completely calcified, mainly by

well-formed encrusted calcite crystals up to 0.3 Amlong (Fig. 10E).

5.3. Interpretation

The generally well-formed cylindrical nature of

many roots in cross-section, and their dominant, in

situ, vertical growth position indicates an unrestricted

growth environment and little or no post-depositional

compaction. Alveolar fabrics are common and gene-

rally considered to be indicative of a root origin

(Wright, 1990). Although needle-fibre calcite, a typi-

cal component of alveolar-septal fabrics, was not

observed in this study, the extensively calcified fila-

ment in Fig. 10E, resembles the needle-shape calcite

illustrated by Guo and Federoff (1990, Fig. 2) and

Loisy et al. (1999, Fig. 4a). The tubular filaments are

interpreted as: (1) calcified organic filaments of prob-

able fungal origin, similar to those illustrated by Jones

(1988, Fig. 5), Amit and Harrison (1995, Photo 3) and

Chenu and Stotzky (2002, Fig. 8); (2) radiating, or-

ganic, pipe-like clusters of open ended hollow fila-

ments (Fig. 10D) which may correspond to parts of

the root vessel (Alonso-Zarza and Arenas, 2004, Fig.

4e); and (3) irregular organic filaments that more

closely resemble a rootlet or root hair than fungal

hyphea (Fig. 10F). The preserved roots and root cavi-

ties clearly supported micro-organisms, which may

have aided the calcification process (Jones, 1994).


The variety of well preserved microstructures

reflects the almost complete absence of post-deposi-

tional diagenesis in the sandstone apart from root

calcification. Wright (1990) recognised two micro-

morphologically distinct types of calcrete: alpha cal-

crete developed through physio-chemical processes in

arid and semi-arid climates; and beta calcretes deve-

loped through the activity of micro-organisms under

wetter climatic conditions. Thus, most pedogenic

carbonates in arid environments are alpha forms.

The roots display many fabrics, including alveolar

textures, fungal filaments, and calcium carbonate-

coated and cemented root structures (rhizocretes),

characteristic of the beta calcretes of Wright (1986),

which are typically composed of low magnesium

calcite. Beta calcretes form through the activity of

soil microorganisms, especially fungi (Wright, 1990)

and, although typical of humid climates, they have

been recorded from arid desert environments (Amit

and Harrison, 1995).

Three important conditions for the formation of

beta calcretes, especially in arid environments, are a

high permeability parent material such as sand, inten-

sive biogenic activity, such as fungi and bacteria, and

a dense vegetation cover (Amit and Harrison, 1995).

All these conditions appear to have been met during

deposition of the sandstones at Dahikya. Thus, the

morphology and fabric of the calcareous root struc-

tures, the dominance of low magnesium calcite (in the

absence of sandstone diagenesis), and the presence of

rhizocretions in the succession are all consistent with

a biogenic origin as pedogenic carbonate.

6. Textural characteristics—grain size, sorting and

roundness

The mean grain-size of the sandstones ranges from

medium to coarse sand-size (0.4–1.2 mm), but with

60% of the samples falling within the coarse sand class

(Fig. 11). A few of the coarse sand samples show a

weak bimodal signature, and with the exception of one

very poorly-sorted sample (2.22, on the sorting scale

of Folk and Ward, 1957) they are all well to moder-

ately well sorted (0.463–0.526). In thin section the

rhizocretions, (Fig. 6C,D) consist predominantly of

rounded to subrounded quartz grains with minor sub-

angular and well-rounded grains tightly cemented by

secondary calcite spar crystals filling most of the

primary pore space. Many quartz crystals have irreg-

ular, calcite-corroded margins, and some have minor,

remnant secondary quartz overgrowths. Grain-size

analysis reflects the corroded nature of the grains in

that calcite cemented sandstones are medium-grained,

whilst uncemented sandstones are coarse-grained (Fig.

11). Quartz grains in uncemented sandstones have

wind-abraded, frosted surfaces; an observation con-

firmed by SEM analysis (Fig. 10B). In addition to

quartz (N95%), the sandstone contains minor chert

and polycrystalline quartz rock fragments (b2%),

some with metamorphic internal grain boundaries,

rare feldspar and pyroxene (b1%), and b3% kaolinite

clay. A variety of accessory heavy minerals occurs

throughout the sandstones dominated by zircon, tour-

maline and iron oxides.

7. Depositional environmental

7.1. Sandstones

The maturity and sorting of the sandstone, paucity

of clay (b3%), the high degree of grain rounding, and

their wind abraded, punctate surfaces, all point to a

predominantly aeolian origin for these sandstones,

which have a wider range of grain-sizes, poorer sorting

and weaker bimodal signature than that normally

found in dune sands (Nickling, 1994; Lancaster,

1996). Significant amounts of sand, derived from

eroded Palaeogene and Neogene source rocks ex-

posed 2–3 km to the south and southeast of the

depositional site, were transported by the dominant

NW winds. These winds have not deviated signifi-

cantly from present day sand-moving winds which

have an average velocity of 8–15 knots, increasing to

20–25 knots with occasional gales (Allison et al.,

1996). Since present day winds are able to erode

sandstone faces in the pit and transport the loose

sand and fine gravel several kilometres, the winds

operating in the past at Dahikiya must have been at

least as strong as those operating today. However,

wind velocities must have been modified by the

moderate to substantial vegetation cover which may

have reduced surface and near surface wind velocity

and wind erosion, thereby helping to stabilize the

desert surface (Bullard, 1997).

Coarse sand Mediumsand

Fine sand1.0 0.6 0.4 0.3 0.2 0.1

0

5

1620

30

40

50

60

70

8084

9095

100

Per

cent

mm2.04.0 3.08.0

GranulesPebbles

A

B

C

E

D

Sample A B C D ESmall pebbles % 1.8Granules % 8.2Coarse sand % 18.0 22.0 78.0 92.0 85.0Medium sand % 74.0 64.0 21.32 7.0 5.0Fine sand % 8.0 14.0 0.68 1.0Mean size mm 0.410 0.403 0.600 0.800 1.235Sorting value 0.463 0.522 0.378 0.526 2.22Sorting descriptor(Folk and Ward,1957)

Well sorted ModeratelyWell sorted

Well sorted Moderatelywell sorted

Very poorlysorted

Fig. 11. Grain-size distribution curves, grain-size data and sorting values for five samples of sandstone at Dahikiya. Samples (A) and (B) are

calcareous cemented sandstone nodules.


Three-dimensional ripples were the dominant

bedforms. These are the most common bedform

in aeolian environments, especially in sand with

mean grain-sizes of 0.3–2.5 mm (Nickling, 1994).

The ripples existed as discrete forms or part of

larger bedforms. The presence of laminae with

opposing dips, subhorizontal wind ripple laminae,

and the packaging of cosets of cross-strata accord-

ing to scale (small trough cross-strata overlain by

ripples), suggests that they may represent parts of

the following dune types (Breed and Grow, 1979):

(1) discrete mounds of sand (simple dunes); (2)

small superimposed dunes (compound dunes); and

(3) incipient dune structures, such as coppice or

shadow dunes. These form from the trapping and

fixing of saltating sand around vegetation and give

rise to low angle (3–108) cross-strata, often dipping

in opposing directions, comprising mainly wind

ripple lamination. The horizontal to subhorizontal

laminae with traces of low angle (b108) foresets are

interpreted as tractional deposition of subcritical

translatent strata, with rare low angle foresets at-

tributed to wind ripple cross-lamination from a

heterogeneous sediment mix. The two large cross-

stratified units in Facies 6, one with concave-up

foresets typical of grainflow and grainfall deposits

on dune lee faces (Gaylord, 1990), are interpreted

as dune bedforms.


The laterally extensive, darker clay-rich layers sep-

arated by subhorizontal and low angle rippled sand-

stone near the top of Facies 5 probably represent

suspension deposition from ponded surface waters in

response to clay-rich water from surface run-off or a

rising water table and groundwater discharge, with the

ripples formed in shallow surface waters, possibly

from wind-driven bottom traction currents. A high

water table has the ability to trap finer material such

as clay and precipitate diagenetic cements such as

calcite. Thus, the rhizocretions may be a result of

these high water table conditions and periodic wetting

of the sands, possibly along preferred flow paths, and

the preferential precipitation of CaCO3 around plant

roots (Esteban and Klappa, 1983). Contorted and

deformed laminae also require wetter conditions for

their formation. These may be related to surface water,

and shifts in position of the water table (Turner and

Smith, 1997).

The lack of significant dunes reflects the: (1)

evenly spaced vegetation cover, which inhibits aeolian

activity and dune construction, whilst encouraging

accretion of low angle and wavy laminae (Gaylord,

1990); (2) the predominantly coarse sand-size which

does not readily form dunes; (3) periodic or seasonal

flooding which inhibits dune development (Thomas,

1997); and (4) a high groundwater table. We infer the

depositional environment to have been part of a rela-

tively flat, well vegetated, aeolian sand sheet or broad

sandy wadi with minor scattered dunes, having a near

surface water table. However, the periodic flooding,

significant coarse sand population, presence of vege-

tation and high water table at Dahikiya, all favour

sand sheet formation (Kocurek and Nielson, 1986),

possibly within a depression which is a favoured site

for aeolian sand sheet accumulation (Christiansen et

al., 1999).

7.2. Gypsum caprock

The poor sorting of the sandstone and conglom-

erate, the clast fabric and angularity, suggest that the

gypsum-cemented caprock may be partly waterlain.

The erosional base of the gypcrete signifies a major

change in depositional environment from aeolian to

fluvially-dominated, and the mudcracked surface

provides clear evidence of shallow water subaqeous

deposition, drying and subaerial exposure. The sand-

stones in the sandstone–mudstone cycles likewise

record shallow water deposition, possibly by wind-

driven currents, generating mostly three-dimensional

ripple bedforms, whereas the structureless mudstones

were deposited from suspension in surface standing

water, which had high levels of salinity. Cycle stack-

ing implies a regular repetition of these conditions

possibly related to seasonal shifts in position of the

water table, and the periodic development of ponded

surface water.

The most likely source of gypsum was the de-

flation and aeolian transport of Eocene and Pliocene

gypsiferous sandstone source rocks to the south and

east (Ibrahim et al., 2001). Leaching of gypsum into

the subsurface may play a role in subsurface gyp-

sum precipitation, especially in the nearby presence

of saline surface and near surface groundwater

which reduces gypsum solubility and promotes pre-

cipitation (El Sayed, 2000). An aeolian interlude

between the lower pebbly sandstone and upper

conglomerate is indicated by the intervening finer

grained, better sorted sandstones containing low

angle lamination, which resembles subcritical trans-

latent strata. The crudely laminated gypsiferous-rich

structures in the upper part of the conglomerate

may be algal mats or evaporitic adhesive structures.

They also resemble laminar calcrete structures fig-

ured by Kosir (2004, Fig. 6A). The steep side of

the erosively emplaced channel suggests that it was

incised initially by alluvial (wadi) processes, possi-

bly during a flash flood event, and then filled by

wind-blown sand.

8. Climate

The abundant, closely-spaced, long roots through-

out the sandstones demonstrates that the climate was

able to support a substantial vegetation cover, which

in turn is a function of the balance between precipi-

tation and evapotranspiration. The roots are morpho-

logically similar and correspond to the Type 2 roots

(prominent tap roots with few laterals) of Cannon

(1911) characteristic of desert environments where

water is available at depth. Most roots did not follow

a tortuous pathway but grew straight down, consistent

with a damp or periodically wet substrate conducive

to root growth (Rundel and Nobel, 1991).

R2 = 0.4302

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Root spacing cm

Max

imu

m r

oo

t d

iam

eter

cm

Series1Linear (Series1)

Fig. 12. Graph showing the relationship between maximum roo

diameter and root spacing.


Provided sand movement is not too intense, sandy

desert areas are better habitats for plants than non-

sandy areas because they provide better aeration

(Groeneveld and Crowley, 1988) and rapid rates of

rainfall infiltration, often to relatively deep levels

(30–90 cm by vertical penetration), thereby reducing

evaporation loss (Prill, 1968; Orshan, 1986; Tsoar,

1990; Amit and Harrison, 1995; Bullard, 1997).

Roots and microorganisms, such as fungi, also create

significant microporosity thereby enhancing the mois-

ture capacity of the sands (Jones, 1988).

Root morphology and plant density effect root effi-

ciency (Rundel and Nobel, 1991; Volis and Shani,

2000). Thus, deep tap roots are most typical of stressful

desert environments, where the distance between plants

increases with increasing aridity (Orshan, 1986; Bul-

lard, 1997). The morphology and size of the roots at

Dahikiya is inconsistent with grass roots and more

closely resemble the roots of shrub and shrub-like

species with few laterals recorded from modern desert

environments (Rundel and Nobel, 1991, Fig. 6). The

similar root morphology further implies a low species

shrub-like vegetation cover. The abundance and close

spacing of plants with similar root architecture, in a

consistent sandy substrate, suggests that interference

competition for moisture was not a major factor (Bar-

ber, 1979; Fonteyn andMahall, 1981; Caldwell, 1987),

compared with the Badia today which receives b50

mm per annum of rain, with an evaporation rate of

1500–2000 mm per annum. As a consequence of this

aridity the very sparse, low species, shrub vegetation,

dominated by Achillea fragrentissima and Capparis

orata, has deep tap roots, spaced 0.5 to several metres

apart, spatially concentrated into sand rich-patches

such as within and along shallow wadis (Gimingham,

1955; Zohary, 1962; Rundel and Nobel, 1991; Bul-

lard, 1997). Around Azraq (Fig. 1) where more water

is available the shrubs are more closely spaced and

small trees and bushes occur locally.

The abundance of well preserved roots throughout

the succession provides an opportunity to test root

spacing or density, as a measure of competition, against

root neck diameter and compare the results with those

from modern desert environments. Root neck diameter

increases with root spacing, (Fig. 12) but the R2 value

of 0.43 is less convincing than that found by Volis and

Shani (2000) for the desert annual Eremobium aegyp-

tiacum in Israel (r=0.66). Eremobium aegyptiacum

t

provides a useful comparison because, like the pre-

served roots, it possesses one main root with few

laterals, and it has a comparable maximum root length

of 75 cm. The weaker correlation between root dia-

meter and root spacing recorded here may reflect post-

burial increases in the original root diameter as they

become encased and replaced by sandstone.

Variations in root length between and within facies

is attributed to temporal variations in wetness and

fluctuations in the level of the water table (Rundel

and Nobel, 1991). Stratigraphic variations in root

abundance (frequency) relate to seasonal variations

in rainfall, whilst root spacing positively correlates

with rainfall (Woodell et al., 1969) and serves as a

proxy for the availability of surface water during sand

deposition. Stratigraphic variation in root abundance

(frequency), root length and root spacing have been

plotted against inferred shifts in the water table level,

and hence local depositional base level (Fig. 13). The

decrease in root length at the top of Facies 6 suggests

a rise in the water table level. A similar decrease in

root spacing supports this view and suggests a possi-

ble overall increase in rainfall and moisture availabil-

ity towards the top of the sandstone succession. The

decrease in root frequency and introduction of trees at

the top of Facies 6 is consistent with this high water

table and increased precipitation in that trees require

more moisture than shrubs. A crude cyclicity can be

seen in the root frequency, similar to that for root

Rootfrequency

Root lengthWater table

Rising Falling>503010Facies

Met

res

7

6

5

1

23

4

10 20 30

cm cm2 4 6 8 10 12

5

10

14

Number

Root spacingAge

HO

LO

CE

NE

PL

EIS

TO

CE

NE

Trees

Clay-rich layer

Fig. 13. Stratigraphic variation in root abundance (frequency), root length and root spacing in relation to inferred changes in the level of the

water table.


length, and to a lesser extent to root spacing (Fig. 13).

These patterns are consistent with fluctuations in the

level of the water table and availability of surface

moisture during sand deposition, with the most

moist phase occurring just below the Holocene gyp-

crete caprock where trees appear for the first time.

Despite the vegetation cover preserved evidence of

pedogenic horizonation is lacking. However, some

pedologists classify sands, including dunes, as rego-

sols, provided they support vegetation, even though

horizonation is poorly developed (Orshan, 1986). This

lack of horizonation is typical of many desert soils

where the pedogenic overprint is weakly developed

(Zohary, 1962; Blume et al., 1995). The only evidence

of pedogenesis is the carbonate coated and cemented

roots, interpreted as beta calcretes, and the rhizocre-

tions which typically form through root transpiration

within an active soil zone, aided by root fungi, bac-

teria and microbes (Hall et al., 2004).

The fact that the sandy substrate at Dahikiya was

able to support a substantial vegetation cover implies

rainfall in excess of 150 mm per annum, with maxi-

mum values up to 350 mm per annum, above which

the rainfall is sufficient to cause complete leaching of

the edaphon (Orshan, 1986). Moreover, the 350 mm

isohyet corresponds broadly to the borderline between

arid and non-arid territories, and the 100 mm isohyet

between arid and semi-arid regions, below which rain-

fed vegetation hardly exists. According to Meigs’s

(1964) classification, deserts receive N100 mm of

annual rainfall; a value in good agreement with that

of Orshan (1986). Root morphology, consistent with a


dominant shrub-like vegetation, further implies that

precipitation was b300 mm per annum since above

this value shrubs are replaced by grassland. Thus,

rainfall during deposition of the Dahikiya sands in

the southern Badia is inferred to have been 150–300

mm per annum, significantly higher and wetter than at

present. Goudie (1992) noted that where the average

rainfall exceeds 100–300 mm per year the vegetation

cover may be too dense for significant aeolian activity

and dune formation.

The abrupt appearance of well preserved in situ,

trees at the top of Facies 6, beneath the gypcrete, is

an ecological change of potential climatic significance.

The eroded tops of the trees along the base of the

gypcrete (Fig. 6D), indicates a possible hiatus and a

change towards an overall drier climate in the later

Holocene, interrupted by wet phases that became gra-

dually smaller and less wet through time in Jordan (De

Jaeger, 2001). This change may mark the Pleistocene–

Holocene boundary in this area, but with the possibility

that part of the Pleistocene section is missing, given the

ease with which aeolian deposits are reworked. Thus,

the Holocene gypcrete caprock may correspond to the

well documented early Holocene wet period that

peaked about 9000 years BP (Aqrawi, 2001) when

rainfall was 100–400 mm more than now (Wilson et

al., 2000). Evidence of this increased wetness is the

mudcracked surface, alternating water-influenced

sandstone–mudstone cycles and the sudden appearance

of in situ trees beneath the gypcrete. Unimpeded tree

growth tends to occur where the annual rainfall exceeds

300–350mm in theMiddle East today, due to the whole

soil and rock profile being recharged and leached with

water each year (Orshan, 1986). The inferred fluvial

depositional processes operating during gypcrete de-

position, punctuated by flash floods, favour such an

interpretation. Nevertheless, the gypcrete signifies an

overall change to a more arid climate and the onset of

saline groundwater, typical of hot desert climates with

an annual rainfall of 50–175 mm, although rarely it

occurs in deserts, with up to 300 mm of rain per annum

(English et al., 2001).

9. Conclusions

Well rooted, weakly consolidated, uncemented,

Middle Pleistocene aeolian sandstones, at Dahikiya

in northeast Jordan, were mainly sourced from Palaeo-

gene and Neogene clastics to the south and southeast.

The locally derived sand, transported by the pre-

vailing NWwinds, was deposited on a broad, relatively

flat sand sheet or sandy wadi environment charac-

terised by a fluctuating near surface water table, able

to support a moderate to substantial vegetation cover.

Three-dimensional, discontinuous, curved-crested rip-

ples were the dominant bedforms, but significant dune

development was inhibited by the vegetation cover, the

coarse sand-size and periodic or seasonal flooding of

the environment. Ponded surface water and periodic

wetting of the sand during deposition promoted the

preferential precipitation of calcium carbonate around

root structures.

The gypcrete is mainly a water-lain deposit with

aeolian influences, cemented by subsurface precipita-

tion of gypsum, within a desert environment, char-

acterised by arid and less arid (wetter) climatic phases

within an overall increasingly arid climate, punctuat-

ed by flash floods. Both the sandstones and gypcrete

at Dahikiya therefore, bear the imprint of past climat-

ic and hydrological regimes, particularly in the

morphology, size and distribution of preserved root

structures which resemble modern desert shrub root

systems.

Abundant, closely-spaced, large tap roots, are

most typical of sandy desert environments where

competition for moisture was not a significant factor,

unlike the Badia today. However, the dominance of

one root type and the low species shrub-like vege-

tation cover suggests a possible scale problem in that

the study area is small. Nevertheless, water availabil-

ity at the surface and in the subsurface was sufficient

to support an effective vegetation cover. Variations in

root length, root spacing and root frequency reflect

fluctuations in the water table level and variations in

rainfall. The water table shows a general increase

towards the top of the succession, with the most

moist phase just below the caprock, where abundant

roots are largely replaced by the first appearance of

substantial trees. This change is of potential strati-

graphic and climatic significance given that unim-

peded tree growth in the eastern Mediterranean today

occurs where rainfall exceeds 300–350 mm per

annum.

Evidence of pedogenesis is restricted to the car-

bonate coated and cemented roots, interpreted as beta


calcretes typical of humid climates, and possibly the

mottling in Facies 5. Soils do not form where rainfall

is less than 150 mm per annum, and above 350 mm

complete leaching of the edaphon occurs. However,

above 300 mm per annum shrubs are replaced by

grassland, hence rainfall during deposition of the

Dahikiya sandstones is inferred to have been between

150 and 300 mm per annum, significantly higher than

the b50 mm today.

Because the age of the sandstones, based on a

single sample, may not be absolute, it is difficult to

link it with specific glacial or interglacial intervals,

especially as the sandstones may reflect local rather

than global scale climate change. However, the age of

the sandstones (652F47 ka) suggests a possible cor-

relation with isotopic event 17, dated at 659 ka

(Bassinot et al., 1994, Fig. 7). Global climate was

cool to cold at this time during the build-up to a

major glaciation dated at 625 ka (Bassinot et al.,

1994). During build-up to major glaciations the desert

climate over Arabia in the Middle Pleistocene alter-

nated between more humid and more arid phases

(Glennie, 1998). According to De Jaeger (2001) the

Pleistocene in Jordan was characterised by arid to

semi-arid climates interrupted by several wet phases

with higher amounts of precipitation. The association

of subaqeous and deformation deposits with predom-

inantly aeolian strata containing abundant rhizoliths

suggests a more humid phase with a high water table

(Blum et al., 1998). Calcrete formation, a moderate to

abundant vegetation cover and landscape stabilization

typically occur under more humid phases when the

water table must have been higher, and the precipita-

tion/evaporation balance greater than in the Badia

today. Fluctuations in the level of the water table

were probably one of the major controls on deposition

and the vegetation cover.

Acknowledgments

We should like to thank the Arabella Mining

Company for their support and hospitality whilst

working in the sandpits. We gratefully acknowledge

financial support from the British Council, the Jorda-

nian Higher Council for Science and Technology and

the University of Durham, to whom we are most

grateful.

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