Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230 – 250
www.elsevier.com/locate/palaeo
Quaternary sandstones, northeast Jordan: Age, depositional
environments and climatic implications
Brian R. Turner a,*, Issa Makhlouf b
a
Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK
Department of Earth and Environmental Sciences, Hashemite University, Zarga, Jordan
b
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 (652 F 47 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 b 150 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
* Corresponding author.
E-mail addresses: [email protected] (B.R. Turner), [email protected] (I. Makhlouf).
0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2005.06.024
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
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, Arabella Mining Company, 2001).
In this paper we provide the first OSL date for the
sandstones and re-interpret the age of the Azraq Formation. The facies architecture, grain textures and the
morphological characteristics and distribution of rhizoliths 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
1. Introduction
Several small, closely spaced exposures of rootbound 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).
38
SYRIA
o
Q
IRA
36o
Lake Tiberias
Jordan
Irbid
Ruwashid
Safawi
Zarqa
Jerusalem
32
Amman
Azraq
QaíFaydat ad Dahikiya
irh
iS
JORDAN
an
o
I AR
D
U
SA
ad
W
31
IA
AB
STUDY
AREA
Dead
Sea
o
31
o
Badia Region
Ma'an
30
Highways
o
0
Aqaba
35
o
36
o
37
Km
100
o
Fig. 1. Map showing the location of the Badia region of eastern Jordan and the study area at Dahikiya.
232
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
Fine gravel
Sand pile
Fine gravel
Sand
and
gravel
hummocks
B
Road
Coalesced
base of slope
colluvial fans
A
S
Conveyor belt
Sand pile for
transport
50 m
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.
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 continental 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 clastics, 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 deposits, 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 caprock 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 conglomerate 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-
233
aa
nti
cli
ne
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
N
Da
hik
iy
10
FFuul
uq
Um
ar
au
iF
lt
5
Sirh
Fa
7
ult
5
an
a
yd
Fa
iki
7
ah
dD
ta
Fau
ya
lt
DAN
JOR
Study area
I
AUD
S
BIA
ARA
Plio-Pleistocene
Miocene
2 km
Middle-Late Eocene
5
Horizontal strata
Dip and strike
Alluvium/wadi sediments
Pleistocene gravels
Azraq
Azraq
Sandstone Formation
Calcareous Qirma
Sandstone Formation
Sandstone Wadi Shallala
Formation
Chalk
}
}
}
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; Qaı́adan, 1992), where interbedded 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-
234
B Southeast
Cross-bedding
13
15
Calcretised tree trunks
N
11
Gypsum-cemented, brown
sandstone and fine conglomerate.
Finer sandy, rippled middle zone;
Laminated, saucer-shaped heave
structures; high angle ripple laminae;
horizontally laminated crust
14
13
Abundant
roots
12
12
10
7
5
Facies 5
4
Facies 4
3
Facies 3
2
Facies 2
1
Facies 1
0
sets
Rip
Coarse trough set
Inset 3
50 c
m
ple
s
cm
0
OSL sample 652 ± 47Ka
Deformed
zone
Trough cross-bedding; deformed zone; low angle
lamination in upper part
Sharply-based, cross-bedded sandstone, containing
shale intraclasts and granules and small pebbles of
quartz, chert,chalcedony and calcrete
Ripple cross-lamination; laminae dipping in opposite
directions; irregular calcite-cemented sand nodules;
fining-upward trend
Small, low angle troughs and ripple cross-lamination;
coarser sand and quartz granules concentrated along
foresets and base of trough sets
{
Clay
Silt
n = 13
Deformed Small trough cross-lamination;
foresets
local chert clasts
6
Facies 4
5
Intersecting
roots
Small-scale, ripple cross-lamination
Abundant roots
4
Subhorizontal lamination; locally
developed trough foresets; carbonate
cemented sandstone concretions
Moderately abundant roots
3
Facies 3
2
Facies 2
Facies 1
0
FMC
Sand
N
Horizontal to subhorizontal laminations;
cross-bedding; cross-bedded pebbly zone
at top; greenish shale intraclasts; sandstone
nodules
Moderate to poorly rooted
8
7
Sequence of sedimentary structures comprises
from the base up: (1) small-scale trough cross-lamination;
(2) larger scale trough cross-bedding; (3) small-scale
trough cross-lamination; (4) larger scale trough
cross-bedding; (5) small-scale trough cross-lamination;
(6) convolute lamination and ripple cross-lamination.
Two closely spaced clay-rich surfaces
Roots
Moderately
rooted
Facies 5
Clasts
Trough cross-bedding;
some ripple cross-lamination
Pebbly zone
Gypsum
crystals
9
Coarse foreset
100
6
10
Inset 2
Fore
11
Hard, brown, blocky
crust; carbonaceous,
non- living modern roots
Ripple
cross-lamination
Deformation
Abundant
roots
Inset 1
Fine trough set
Abundant
Abundant roots Moderately abundant roots
roots
Rare roots
Middle Pleistocene
Metres
8
Small-scale trough cross-lamination
and ripple cross-lamination
n=8
Gypsum Wavy laminae; local ripple
crystals cross-lamination
Poorly defined bedding ; green shale
intraclasts
Abundant roots
Base not seen
F MC
Sand
Clay
Sitl
9
Metres
Abundant roots
Facies 6
Facies 6
Horizontal to
subhorizontal
laminations
Gypsum-cemented
caprock
Facies 7
Rare
roots
{
Facies 7
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, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
Holocene
A Northwest
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
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 652 F 47 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, noncalcareous 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
235
low angle, small-scale trough cross-bedding, (individual sets up to 12 cm thick), and ripple cross-lamination. 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 structures 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 Esteban 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 sandstone, 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 southeast (Fig. 5B), except that in the southeast the sandstone 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 northwest contains whitish (N9), irregularly-shaped, cmscale patches of hard, carbonate-cemented sandstone
concretions, which weather out from the softer, uncemented host sandstone. These concretions, some of
which resemble carbonate-cemented root structures
236
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
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 undeformed, 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 west
on one side, and b 108 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 troughshaped 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 truncates 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 crossbedding, 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
uppermost sets, which show locally deformed foresets, are overlain by a 45–50 cm thick zone of horizontal 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 containing 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. Stratigraphically 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
237
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 sedimentary 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 crossbedding identical to (1); (4) larger scale trough crossbedding identical to (2); (5) small-scale cross-bedding; (6) convolute lamination (Fig. 7A) (deformed
foresets occur at a similar level in Facies 4 in the
Fig. 7. (A) Cross-bedded units overlain by horizontal to subhorizontal, slightly wavy lamination, that becomes deformed in the upper part,
Facies 4, northwest face of pit. (B) Root penetrated aeolian cross-bedding with thin intervening rippled sandstone bed. Note slight overturning
of the top of the foresets beneath rippled zone, Facies 5, northwest face of pit. (C) Aeolian dune cross-bedding at the top of Facies 6, southern
face of pit. (D) Granular and pebbly sandstone capped by a thin horizontally laminated crust, gypcrete, northern face of pit. (E) Gypcrete along
northern face of pit showing coarse-grained sandstones with scattered granules and small pebbles coarsening-upwards into a matrix- to local
clast-supported fine conglomerate. The contact between these two zones is marked by a thin, finer grained ripple cross-laminated sandstone bed.
Note the prominent saucer-shaped laminated structures in the upper part of the gypcrete, interpreted as evaporitic adhesive structures or algal
mats. (F) Fluvial channel with lateral wing incised into gypcrete and filled with rooted, aeolian sand, northern face of pit (Notebook is 9 cm
long).
238
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
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 (b 20/
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 siltstone 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 geometry, disconformable with the foresets below (Fig.
7C). Internally it is characterised by bedding surfaces
dipping at 58 to the east, which enclose steeper foresets dipping 208 west (Fig. 7C). Roots are less common 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 sandstones 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 sandstone, 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 polygons 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 conglomeratic part characterised by large (30–50 cm diameter),
crudely laminated, gypsiferous saucer-shaped structures (Fig. 7E); and (4) a sequence of up to 9 vertically
stacked, thin, irregularly-bedded gypsiferous mudstone–sandstone cycles, cut by gypsum veins and
lenses (Fig. 8). The cycles decrease in thickness upwards 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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
239
Gravels
140
Gypsum veins and lenses
Fine to medium-grained,
structureless sandstone
120
80
Pale pink-fawn,medium-grained
rippled, gypsiferous sandstone
Pale pink-fawn, medium-grained
rippled, gypsiferous sandstone with
rare cross-beds
lenses
Cm
100
veins and
60
Gypsum
40
20
Poorly defined mudstone-sandstone
cycles
Maroon-chocolate brown
gypsiferous silty mudstone
Erosion surface
Local mudcracks
F M
C
{
Silt
Clay
0
Sand
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 commonly 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 crossbedded, 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 channel-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 tissue. 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
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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, undeformed 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 morphology 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 partially-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, wellrounded quartz grains show conchoidal fracture surfaces, 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 (b 4 mol x Mg). The filaments 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 calcification. Most filaments have only minor, local growths
of calcite on the surface of the walls (Fig. 10C,D), or
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
241
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 needleshaped 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 Am
long (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 generally considered to be indicative of a root origin
(Wright, 1990). Although needle-fibre calcite, a typical 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 probable 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, organic, pipe-like clusters of open ended hollow filaments (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 cavities clearly supported micro-organisms, which may
have aided the calcification process (Jones, 1994).
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The variety of well preserved microstructures
reflects the almost complete absence of post-depositional diagenesis in the sandstone apart from root
calcification. Wright (1990) recognised two micromorphologically distinct types of calcrete: alpha calcrete developed through physio-chemical processes in
arid and semi-arid climates; and beta calcretes developed 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 carbonatecoated 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, intensive 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 structures, 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 moderately well sorted (0.463–0.526). In thin section the
rhizocretions, (Fig. 6C,D) consist predominantly of
rounded to subrounded quartz grains with minor subangular and well-rounded grains tightly cemented by
secondary calcite spar crystals filling most of the
primary pore space. Many quartz crystals have irregular, 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 confirmed by SEM analysis (Fig. 10B). In addition to
quartz (N 95%), the sandstone contains minor chert
and polycrystalline quartz rock fragments (b 2%),
some with metamorphic internal grain boundaries,
rare feldspar and pyroxene (b 1%), and b 3% kaolinite
clay. A variety of accessory heavy minerals occurs
throughout the sandstones dominated by zircon, tourmaline and iron oxides.
7. Depositional environmental
7.1. Sandstones
The maturity and sorting of the sandstone, paucity
of clay (b 3%), 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 exposed 2–3 km to the south and southeast of the
depositional site, were transported by the dominant
NW winds. These winds have not deviated significantly 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).
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Sample
Small pebbles %
Granules %
Coarse sand %
Medium sand %
Fine sand %
Mean size mm
Sorting value
Sorting descriptor
(Folk and Ward,
1957)
A
B
C
D
18.0
74.0
8.0
0.410
0.463
Well sorted
22.0
64.0
14.0
0.403
0.522
Moderately
Well sorted
78.0
21.32
0.68
0.600
0.378
Well sorted
92.0
7.0
1.0
0.800
0.526
Moderately
well sorted
100
95
90
243
E
1.8
8.2
85.0
5.0
1.235
2.22
Very poorly
sorted
A
84
80
D
B
70
Percent
60
50
40
30
20
16
5
C
E
0
8.0 4.0 3.0 2.0
Pebbles Granules
0.6
1.0
Coarse sand
0.4 0.3
Medium
sand
0.2
Fine sand
0.1 mm
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 according 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 (b 108) foresets are
interpreted as tractional deposition of subcritical
translatent strata, with rare low angle foresets attributed to wind ripple cross-lamination from a
heterogeneous sediment mix. The two large crossstratified 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.
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B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
The laterally extensive, darker clay-rich layers separated by subhorizontal and low angle rippled sandstone 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 relatively 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 vegetation 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 conglomerate, 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 winddriven 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 stacking 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 deflation 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 gypsum precipitation, especially in the nearby presence
of saline surface and near surface groundwater
which reduces gypsum solubility and promotes precipitation (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 translatent 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 figured by Košir (2004, Fig. 6A). The steep side of
the erosively emplaced channel suggests that it was
incised initially by alluvial (wadi) processes, possibly during a flash flood event, and then filled by
wind-blown sand.
8. Climate
The abundant, closely-spaced, long roots throughout the sandstones demonstrates that the climate was
able to support a substantial vegetation cover, which
in turn is a function of the balance between precipitation and evapotranspiration. The roots are morphologically 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).
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
3
Maximum root diameter cm
Provided sand movement is not too intense, sandy
desert areas are better habitats for plants than nonsandy 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 moisture capacity of the sands (Jones, 1988).
Root morphology and plant density effect root efficiency (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; Bullard, 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 (Barber, 1979; Fonteyn and Mahall, 1981; Caldwell, 1987),
compared with the Badia today which receives b 50
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; Bullard, 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 R 2 value
of 0.43 is less convincing than that found by Volis and
Shani (2000) for the desert annual Eremobium aegyptiacum in Israel (r = 0.66). Eremobium aegyptiacum
245
R2 = 0.4302
2.5
2
1.5
1
0.5
Series1
Linear (Series1)
0
0
10
20
30
40
50
60
Root spacing cm
Fig. 12. Graph showing the relationship between maximum root
diameter and root spacing.
provides a useful comparison because, like the preserved 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 diameter and root spacing recorded here may reflect postburial 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 possible overall increase in rainfall and moisture availability 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
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
HOLOCENE
Age
Facies
7
Metres
246
Root
frequency
Number
10 20 30
Root length Root spacing
cm
cm
10
30 >50
2 4 6 8 10 12
Water table
Rising Falling
14
Trees
PLEISTOCENE
6
10
Clay-rich layer
5
5
4
3
2
1
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 gypcrete 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 regosols, 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, bacteria 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 maximum 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 rainfed vegetation hardly exists. According to Meigs’s
(1964) classification, deserts receive N 100 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
dominant shrub-like vegetation, further implies that
precipitation was b 300 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 gradually 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
300–350 mm in the Middle 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 deposition, 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
247
in northeast Jordan, were mainly sourced from Palaeogene and Neogene clastics to the south and southeast.
The locally derived sand, transported by the prevailing NW winds, was deposited on a broad, relatively
flat sand sheet or sandy wadi environment characterised by a fluctuating near surface water table, able
to support a moderate to substantial vegetation cover.
Three-dimensional, discontinuous, curved-crested ripples 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 precipitation of gypsum, within a desert environment, characterised by arid and less arid (wetter) climatic phases
within an overall increasingly arid climate, punctuated by flash floods. Both the sandstones and gypcrete
at Dahikiya therefore, bear the imprint of past climatic 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 vegetation cover suggests a possible scale problem in that
the study area is small. Nevertheless, water availability 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 stratigraphic and climatic significance given that unimpeded tree growth in the eastern Mediterranean today
occurs where rainfall exceeds 300–350 mm per
annum.
Evidence of pedogenesis is restricted to the carbonate coated and cemented roots, interpreted as beta
248
B.R. Turner, I. Makhlouf / Palaeogeography, Palaeoclimatology, Palaeoecology 229 (2005) 230–250
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 b 50 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 (652 F 47 ka) suggests a possible correlation 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 alternated 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 predominantly 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 precipitation/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 Jordanian Higher Council for Science and Technology and
the University of Durham, to whom we are most
grateful.
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