www.elsevier.com/locate/palaeo
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 233
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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 235
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).
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250236
(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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 237
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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250238
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 239
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250240
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 241
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).
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250242
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 243
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250244
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 245
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.
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250246
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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250 247
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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250248
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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