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Sedimentary Geology 181 (2005) 73 – 91
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Late Ordovician (Ashgillian) glacial deposits in southern Jordan
Brian R. Turner a,*, Issa M. Makhlouf b, Howard A. Armstrong a
a
Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK
b
Department of Earth and Environmental Sciences, Hashemite University, Zarqa, Jordan
Received 29 March 2005; received in revised form 8 August 2005; accepted 26 August 2005
Abstract
The Late Ordovician (Ashgillian) glacial deposits in southern Jordan, comprise a lower and upper glacially incised palaeovalley
system, occupying reactivated basement and Pan-African fault-controlled depressions. The lower palaeovalley, incised into
shoreface sandstones of the pre-glacial Tubeiliyat Formation, is filled with thin glaciofluvial sandstones at the base, overlain by
up to 50 m of shoreface sandstone. A prominent glaciated surface near the top of this palaeovalley-fill contains intersecting glacial
striations aligned E–W and NW–SE. The upper palaeovalley-fill comprises glaciofluvial and marine sandstones, incised into the
lower palaeovalley or, where this is absent, into the Tubeiliyat Formation. Southern Jordan lay close to the margin of a Late
Ordovician terrestrial ice sheet in Northwest Saudi Arabia, characterised by two major ice advances. These are correlated with the
lower and upper palaeovalleys in southern Jordan, interrupted by two subsidiary glacial advances during late stage filling of the
lower palaeovalley when ice advanced from the west and northwest. Thus, four ice advances are now recorded from the Late
Ordovician glacial record of southern Jordan.
Disturbed and deformed green sandstones beneath the upper palaeovalley-fill in the Jebel Ammar area, are confined to the
margins of the Hutayya graben, and have been interpreted as structureless glacial loessite or glacial rock flour. Petrographic and
textural analyses of the deformed sandstones, their mapped lateral transition into undeformed Tubeiliyat marine sandstones away
from the fault zone, and the presence of similar sedimentary structures to those in the pre-glacial marine Tubeiliyat Formation
suggest that they are a locally deformed facies equivalent of the Tubeiliyat, not part of the younger glacial deposits. Deformation is
attributed to glacially induced crustal stresses and seismic reactivation of pre-existing faults, previously weakened by epeirogenesis,
triggering sediment liquefaction and deformation typical of earthquake generated seismites. Deformation, confined to an area of not
more than 4 km wide adjacent to the major fault zone, implies earthquake magnitudes of at least 6 (Mo). The high authigenic chlorite
content of deformed Tubeiliyat sandstones compared to undeformed ones is attributed to a post-seismic hydrothermal system driven
by compactional dewatering and hydrofracturing of the bedrock which acted as a groundwater recharge area, supplied by subglacial
meltwater from beneath the ice sheet. Fluid movement along glacial seismotectonically reactivated faults infiltrated the adjacent
Tubeiliyat sandstones under pressure and elevated geothermal gradient, where chlorite was precipitated from solution.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Ordovician glaciation; Palaeovalleys; Sediment deformation; Jordan
1. Introduction
* Corresponding author.
E-mail address: [email protected] (B.R. Turner).
0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.sedgeo.2005.08.004
The Lower Palaeozoic succession in southern Jordan
(Fig. 1) includes some 750–800 m of well exposed
Ordovician siliciclastic sediments deposited on the margins of the North African (Gondwana) shallow marine
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B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
Fig. 1. Location and geology of the study area in southern Jordan. The inset map (top right) shows the general geology of southern Jordan and the
inset map (bottom left) the location of the study area in Jordan.
shelf (Fig. 2), in terrestrial, subtidal marginal marine
and shelf environments (Powell et al., 1994; Makhlouf,
1995; Amireh et al., 2001). During the Upper Ordovician Jordan was located in a high latitude east Gondwana setting, ~608 S of the equator (Scotese et al.,
1999), and was subjected to the effects of the Late
Ordovician (Hirnantian) glaciation (Fig. 3). Southern
Jordan was situated less than 100 km from the margins
of a terrestrial ice sheet in Northwest Saudi Arabia that
was characterised by two major phases of ice advance
and retreat (Vaslet, 1990).
1.1. Geological background
Abed et al. (1993, Fig. 3) and Amireh et al. (2001)
included all the Late Ordovician (Ashgillian) glacial
sediments in southern Jordan within the Ammar Formation (Fig. 2). They divided it into two units, each underlain by a glacial erosion surface stratigraphically
equivalent to those beneath the Late Ordovician glacial
deposits of the Zarqa and Sarah Formations in Saudi
Arabia (Vaslet, 1990) (Figs. 2 and 4).
The lower unit consists of a sandy channel lag
conglomerate up to 2 m thick, containing glacially
faceted and striated clasts, overlain by 30 m of extensively disturbed, greenish-grey, massive, well sorted
structureless sandy siltstones. The base of the disturbed
sandy siltstones is only recorded from two localities
near Barqa Mountain, some 10 km NNE of Jebel
Ammar (Fig. 1), where it is said to be incised into the
underlying Tubeiliyat Formation (Abed et al., 1993).
The precise location of these two outcrops was not
recorded, but field sketches from 1992 (Makhlouf,
written communication, 2005) have a laterally confined, erosively based, 1 m thick channel lag, overlain
by some 30 m of fine to medium-grained sandstone.
The lower 15 m is a deformed, greenish-grey sandstone, and the upper 15 m a light brown sandstone
containing ripple cross-lamination, flaser and wavy
lamination, correlated with the structureless lower
Ammar Formation at Jebel Ammar by Abed et al.
(1993). The sandstone is erosively overlain by glaciofluvial palaeovalley sandstones, correlated with the
upper Ammar Formation, which is unconformably
overlain by Cretaceous Kurnub sandstone of the Batn
al Ghul Group (Fig. 1).
Current models for Late Ordovician (Ashgillian)
glaciation in southern Jordan recognise up to two
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
75
Fig. 2. Revised lithostratigraphy and chronostratigraphy for the Ordovician and Silurian in Jordan and Saudi Arabia, showing generalised
depositional environments for outcrops in the southern desert study area. Subdivision of the Ammar Formation into lower and upper Ammar is
based on Abed et al. (1993).
major phases of glaciation (Abed et al., 1993) which
are correlated with two major glacial advances
recorded for Northwest Saudi Arabia (Vaslet, 1990).
The first glacial advance in southern Jordan is thought
to be represented by deformed and structureless glacial
rock flour or loessite siltstones of the lower Ammar
Formation, and the second by a palaeovalley system
of the upper Ammar Formation incised into the deformed siltstones (Abed et al., 1993; Amireh et al.,
2001). In this study we present new field-based data
from southern Jordan, and a reinterpretation of the
Late Ordovician glacial deposits, including the disturbed and structureless glacial loessite or rock flour
siltstones, based on their architecture, field relationships and petrography. The relationship between glacially induced crustal stress and seismic reactivation of
faults, documented for present day and past glacial
regimes, is used to develop a model to explain deformation and neomorphic changes in composition of the
deformed sediments associated with a major fault
zone. The magnitude of the glaciotectonic earthquakes
responsible for deformation can be assessed according
to the relationship between earthquake magnitude and
maximum epicentral distance within which liquefaction deformation can occur.
2. Glacial palaeovalleys
Two types of glacially incised palaeovalley occur
in the Late Ordovician of southern Jordan: a lower
palaeovalley system filled predominantly with marine shoreface sandstones; and an upper palaeovalley system filled with glaciofluvial and shoreface
sandstones.
2.1. Lower palaeovalley
Fig. 3. Late Ordovician palaeogeographical reconstruction of eastern
Gondwana showing the ice sheet (shaded) and the location of Jordan
(redrawn from Sutcliffe et al., 2000).
The lower palaeovalley is locally incised into the top
of the Tubeiliyat Formation (Fig. 4A, Palaeovalley 1).
The palaeovalley-fill is exposed in a series of low, faultbounded hills that can be traced discontinuously for
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Fig. 4. Generalised sections of the glacial succession in the Jebel Ammar area, southern Jordan, and northwest Saudi Arabia, showing the
stratigraphy and sediment fill of the glacially incised palaeovalley systems. Section A is located 0.5 km southwest of Jebel Umeir and section B is
from Jebel Ammar (see Fig. 1). Section C from northwest Saudi Arabia is based on Vaslet (1990).
more than 5 km from the NNW to the SSE in a
direct line with Jebel Ammar and Jebel Amira (Fig.
1). The palaeovalley is 150–200 m wide, 25–50 m
deep and the margins dip at 178–308. The palaeovalley-fill comprises up to 50 m of massive, fine to
medium-grained sandstone (Figs. 5 and 6A) except
along the base where coarse to very coarse, feldspathic, granular and pebbly sandstone occurs (Fig. 5A,B).
The base of individual palaeovalley-fills is often
complex, and includes up to 4 erosion surfaces
(Fig. 5), associated with clast to matrix-supported
conglomerates containing sandstone and siltstone
intraclasts, rare granite clasts, angular to subrounded,
in situ granules, pebbles and occasional small cobbles
of vein quartz and quartzite, many with glacially
faceted and striated surfaces. The sandstone-fill contains small-scale, low angle, intersecting trough crossbeds (5–10 cm thick and 20 cm long), ripple crosslamination, straight-crested symmetrical ripple marks,
medium to large-scale hummocky cross-stratification
(Fig. 5C), local erosion surfaces, water escape structures, burrows and brachiopod moulds. Some sandstones just above the incised palaeovalley base,
contain brachiopods and numerous trace fossils including Harlania, Cruziana and brachiopod resting
traces (Fig. 6B,C,D).
A prominent glaciated surface 25 m above the base
of one palaeovalley-fill (Fig. 5D) can be traced laterally for more than 200 m, as far as the outcrop will
allow. The smoothed and polished surface is locally
striated and grooved (Fig. 5E,F), and embedded with
siltstone and sandstone intraclasts, small glacially faceted quartz pebbles, and a few horizontal burrows.
The striations are 2–3 mm deep, up to 35 cm in lateral
extent, and consist of two intersecting sets: one
aligned E–W and the other NW–SE (Fig. 5E). The
grooves are up to 30 cm long and 1 cm deep, and
aligned NW–SE. Local deformation, in the form of
convolute bedding, is present in the sandstone immediately below this surface, which is overlain by ~5 m
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
Fig. 5. Measured section and photographs of the lower, glacial palaeovalley-fill, 2 km southeast of Jebel Amira (Fig. 1), showing the erosively emplaced palaeovalley base, internal sedimentary
structures and glaciated surface 5 m below the glacially incised upper palaeovalley glaciofluvial sandstones. The position of the features in the photographs A–G are shown on the section.
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Fig. 6. A. Massive cliff-forming sandstone filling lower palaeovalley incised into the top of the Tubeiliyat Formation. B. Brachiopod impressions
covering surface of sandstone from the side of the palaeovalley. C. Brachiopods and trace fossils, including Harlania, on underside of sandstone
block from just above the base of the palaeovalley. D. Cruziana traces on underside of sandstone block from side of the palaeovalley.
of fine to medium-grained hummocky cross-stratified
sandstone.
low angle, wedge-shaped trough cross-bed sets deposited by currents flowing to the east and southeast
(Fig. 5G).
2.2. Upper palaeovalley
3. Disturbed sandy siltstones
The upper palaeovalley system is incised into the
lower one or, where this is missing, it cuts down
directly into the Tubeiliyat Formation (Fig. 4A,B,
Palaeovalley 2). The upper palaeovalley system has
been previously described by Abed et al. (1993) who
considered it to be part of the upper Ammar Formation (Fig. 4). The palaeovalley-fill comprises erosively
based, horizontally to subhorizontally bedded coarse
to medium-grained sandstones containing planar and
trough cross-stratification and reworked glacially faceted and striated clasts, typically concentrated in the
lower 1–3 m (Fig. 4A,B, Palaeovalley 2). These
sandstones fine upwards into thin, marine shoreface
sandstones containing trace fossils and brachiopods
(Abed et al., 1993; Amireh et al., 2001). Where the
upper palaeovalley is incised into the lower palaeovalley shoreface sandstones, above the internally glaciated surface, it comprises 1–2 m of coarse to very
coarse, granular and pebbly sandstone containing
faceted and striated clasts that grade sharply upwards into medium to coarse sandstone (Fig. 5) containing local granular and pebbly streaks and lenses.
Internally the sandstone is structured by large-scale,
Disturbed sandy siltstones beneath the upper palaeovalley in the Jebel Ammar area were placed in the lower
Ammar Formation by Abed et al. (1993) who interpreted
them as a glacial rock flour. Later, Amireh et al. (2001)
interpreted them as glacial loessite derived by erosion
of the underlying Tubeiliyat Formation sandstones.
3.1. Description
Up to 90 m of incompletely exposed disturbed sandy
siltstones occur beneath erosively based, upper palaeovalley-fill sandstones (Fig. 7) adjacent to the Hutayya
graben, a major structural feature in the area (Fig. 1).
The graben trends NW–SE and is bounded by normal
faults which downthrow the base of the Ammar Formation 40 m to the west, with no significant strike slip
component. The beds are tilted 228–288 to the northeast
adjacent to the western boundary fault (Fig. 8A) and
148 to the west along the eastern boundary fault. Although the dip of the beds persists laterally for at least 2
km parallel with the fault plane, it decreases and dies
out completely within a few hundred metres away from
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
79
Fig. 7. Measured section of the disturbed and deformed sandstones at Jebel Ammar. The photograph illustrates some of the more characteristic
features of the section (see text for details). Jebel Ammar lies along the Hutayya fault zone (Fig. 1) and stands over 100 m above the generally flatlying Pleistocene erosion surface. The lower slopes are composed of disturbed pale grey-green sandstone, erosionally overlain at the top by more
resistant, ferruginous-cemented, glaciofluvial upper palaeovalley sandstones containing glacially striated and faceted clasts, that grade upwards into
shoreface sandstones at the top.
the fault plane, whence they pass into undeformed
shoreface and shelf sediments of the Tubeiliyat Formation (Fig. 9).
The disturbed siltstones are a distinctive green-grey
colour, characterised by soft sediment deformation of
varying styles and levels of intensity which occur both
vertically and laterally throughout the succession. Deformation includes: (1) recumbent folding; (2) convolute laminations (Fig. 8B); (3) isolated sand balls
floating in the host sediment and pseudo-nodules
(Fig. 8C); (4) rotated, deformed and rounded ripples
with thin streaky elongate tails resembling tadpoles
(Fig. 8D); and (5) sandstone and siltstone intraclasts,
some of which dfloatT in the host sandstone (Fig. 8E). In
addition rare horizontally segmented burrows, brachiopod moulds and straight- to sinuous-crested ripple
marks (Fig. 8F) occur. Although the internal lamination
is commonly blurred and partially to completely
destroyed, close examination reveals the presence of
diffuse, mm to cm-scale ripple cross-lamination at intervals throughout the succession. Brown-weathered,
rounded, iron-carbonate concretions, a few centimetres
to 2–3 m in diameter occur throughout the succession.
Some of the larger ones, contain intersecting, very lowangle laminae sets of hummocky cross-stratification,
identical to the hummocky cross-stratified concretions
described by Powell (1989) and Makhlouf (1995) from
the Tubeiliyat Formation.
3.2. Texture and petrography
In order to test the glacial loess and rock flour
models for the deformed siltstones, and their derivation
from the underlying Tubeiliyat Formation (Abed et al.,
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Fig. 8. A. Tectonic tilting of beds adjacent to the western boundary fault of the Hutayya graben, Jebel Ammar. B. Convolute laminations, Jebel
Amira. C. Isolated sandstone ball with faint internal laminae interpreted as a rotated and deformed ripple. D. Rotated and deformed ripple with
streaky tail, Jebel Amira. E. Small, irregularly shaped, delicate inclusions of fine sandstone and siltstone (above hammer) in a slurried sandstone that
has undergone total liquefaction, Jebel Ammar. F. Asymmetric current ripples on bedding surface, 1 km southeast of Jebel Amira (see Fig. 1).
1993: Amireh et al., 2001), 5 samples of disturbed
sandy siltstone, 2 samples of lower palaeovalley-fill
sandstone, and 2 samples of Tubeiliyat Formation sandstone were analysed for their textural properties and
mineral composition. The mean grain-size of the disturbed sandy siltstone is 3.11/ with N 80% of particles
in the fine to very fine sand range (2–4/) and b 10% silt
and medium sand (Fig. 10). The sediment is moderately
sorted and texturally submature (Folk, 1968), and the
distribution is skewed towards positive phi values
(+ 0.18).
The disturbed sandstones contain 47% to 56.3%
quartz (Table 1), most of which occur as strained
undulatory grains. The feldspar content, ranges from
5.6% to 10.3%, and comprises fresh or extensively
altered orthoclase with minor amounts of microcline
and plagioclase. Despite their alteration kaolinisation of
feldspars is rare (cf. Abed et al., 1993, Table 1). Igneous
and metamorphic polycrystalline quartz rock fragments
make up b1%. Muscovite and lesser amounts of biotite
are prominent constituents of the sandstones, but pristine biotite is rare, as most grains are partially or
completely altered to chlorite. Chlorite also occurs as
authigenic grain-rimming chlorite (15–20 Am wide
rims), lining pore spaces, as a neomorphic component
of the matrix, and rarely as a matt of small plates
imparting a local felt-like texture to the rock. The
matrix (22.3%–28.6%) comprises quartz silt, muscovite, sericite and chlorite shreds and aggregates, but
volumetrically chlorite is one of the most important
components, and the main reason for the distinctive
greenish colour of the sandstone which increases in
intensity towards the Hutayya fault zone.
XRD analyses of disturbed sandy siltstone by Abed
et al. (1993, Fig. 7) confirms that the matrix is dominated by chlorite, illite and smectite, but with kaolinite,
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
81
Fig. 9. Map of the Jebel Ammar area showing the distribution of the disturbed green-grey sandstone and its passage into transitional and undisturbed
Tubeiliyat sandstone. The map resembles some seismic intensity maps for earthquake activity related to active fault zones (Mohindra and Bagati,
1996, Fig. 2B), except that faulting in the Jebel Ammar area has disrupted the disturbed transitional Tubeiliyat sandstone facies zone.
silt and clay-size grains of quartz, muscovite, and other
non-clay minerals also present. Other minor detrital
constituents include pyroxene, tourmaline, zircon, rutile, garnet and iron oxides. The sandstones are arkosic
wackes (Dott, 1964; Williams et al., 1982), and their
detrital mineral composition and heavy mineral suite,
confirms their primary derivation from predominantly
granitic basement provenance rocks to the south and
west (Fig. 1, inset map), which also includes minor
diorite and gabbro, and numerous dolerite dykes (Bender, 1968). The disturbed sandstones have a similar
composition to the lower palaeovalley-fill sandstones
except for their enrichment in chlorite and chloritised
biotite, and correspondingly lower amounts of biotite
and muscovite (Table 1). Both sandstones differ from
typical Tubeiliyat sandstones in their lower quartz and
kaolinite, and higher biotite, chlorite and chloritised
biotite.
3.3. Interpretation
Fig. 10. Grain-size analyses of 5 samples of disturbed sandstone, 2
samples of lower palaeovalley sandstone cut into the top of the
Tubeiliyat Formation and 2 samples of Tubeiliyat sandstone.
Abed et al. (1993) interpreted the disturbed
denigmaticT sandy siltstones as a rock flour, derived
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B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
Table 1
Petrographic data, disturbed, lower palaeovalley-fill sandstones and Tubeiliyat Formation sandstones
Constituents
Disturbed grey-green sandstone (%)
Monocrystalline quartz
Poly qrtz rock fragments
K-feldspar and microcline
Plagioclase feldspar
Muscovite
Biotite
Chloritised biotite
Chlorite
Matrix
Calcite (patchy cement)
Kaolinite
Heavy minerals
51.0
0.3
6.0
1.6
10.0
1.0
3.0
2.6
22.3
0.3
0.6
1.3
47.0
1.3
9.0
1.3
6.0
0.0
3.6
1.9
28.6
0.0
0.0
1.0
50.0
0.6
8.3
0.6
3.3
1.6
8.3
2.3
24.0
0.0
0.0
0.6
50.6
0.6
6.6
0.3
2.6
1.0
7.3
3.0
26.6
0.0
0.0
1.0
from erosion of the underlying mineralogically identical Tubeiliyat Formation, dumped rapidly on a glacially
incised variable palaeorelief surface. The sandstones
are moderately well sorted, but the quartz grains show
no evidence of grain breakage or grinding consistent
with a glacial rock flour origin. Later Amireh et al.
(2001) interpreted these sediments as glacial loessite
derived from wind erosion of the underlying Tubeiliyat
Formation (Fig. 2). Loess is seldom been recognised in
the pre-Cenozoic rock record (Fischer and Sarnthein,
1988; Johnson, 1989; Soreghan, 1992; Carroll et al.,
1998; Chan, 1999; Kessler et al., 2001), probably because it is difficult to distinguish it from other similar
fine-grained sandstone and siltstone. Comparison of the
main diagnostic characteristics of loess, documented in
the literature (Smalley, 1966; Smalley and Smalley,
1983; Pye, 1984, 1995; Nemecz et al., 2000; Zoller
and Semmel, 2001; Smith et al., 2002), with those of
the disturbed sandstones, show significant differences
in their textural attributes, mineral composition, colour,
abundance of authigenic carbonate and particularly
their bedding characteristics, and absence of interbedded palaeosol horizons (Table 2). Only 7 of the 28
features listed in Table 2 were common to both and, on
this basis, plus the lithological and biogenic characteristics of the disturbed sandstones, it is considered unlikely that they originated as a glacial rock flour or
loessite.
At Jebel Amira 90 m of deformed sandstones are
exposed, without any channel base or channel margins,
and a complete absence of clasts of any type. In addition the disturbed sandstones: (1) were tectonically
deformed; (2) they contain sedimentary structures identical to those in the Tubeiliyat Formation; (3) they can
be traced laterally into undeformed Tubeiliyat Formation; (4) they are located along fault zones; and (5)
56.3
0.6
6.3
0.3
3.6
0.3
4.6
2.3
24.6
0.0
0.6
1.3
Lower palaeovalley-fill
sandstones incised into the
Tubeiliyat Formation (%)
Tubeiliyat Formation
sandstones (%)
48.0
0.3
6.3
0.3
10.3
3.3
1.3
0.6
24.3
0.0
2.0
3.0
71.0
0.6
10.3
0.6
2.3
0.3
0.0
0.0
2.3
3.6
10.2
0.9
52.6
1.0
5.6
1.3
10.6
4.3
2.0
0.3
19.0
0.6
1.3
1.0
69.0
0.6
9.6
1.5
3.3
0.0
0.0
0.0
2.7
2.7
9.6
0.9
according to Abed et al. (1993) they have an identical
mineral composition to the Tubeiliyat. However, Abed
et al. (1993) analysed greenish-grey siltstones and fine
sandstones, not the more typical Tubeiliyat fine to
medium-grained, pale fawn-brown sandstones (Makhlouf, 1995) used in this study, hence the differences in
mineral composition in Table 1. We interpret the sandstones at Jebel Ammar and Amira to be a locally
disturbed facies equivalent of the upper part of the
marine shelf and shoreface sandstones of the 105 m
thick Tubeiliyat Formation, not part of the glacial
Ammar Formation.
3.3.1. Deformation
No mechanism has so far been suggested for the
origin of the post-depositional deformation of the sandstones and, why these are confined to the Jebel Ammar
area on either side of the Hutayya graben. Soft sediment
deformation must have occurred before the sediments
were lithified, whereas the same sediments must have
been sufficiently lithified in order for ice to incise deep,
steep-sided palaeovalleys into the Tubeiliyat Formation.
The Upper Tubeiliyat (upper anceps B2 zone) was
deposited contemporaneous with, and in close proximity to, the advancing ice sheet in southern Jordan and
Northwest Saudi Arabia (Armstrong et al., 2005). Thus,
there is no significant hiatus between the Tubeiliyat and
overlying glacial deposits in southern Jordan. Indeed,
Armstrong et al. (2005) consider that much of the
deposition of the Tubeiliyat was glacially forced and
marks the onset of glacial isostatic subsidence.
The mid-Ordovician in North Africa and Arabia was
characterised by tectonic instability with the main phase
of tectonism occurring between the Llandeilo–Caradoc.
This regional tectonism did not create new fault patterns but was characterised by epeirogenic movements
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
83
Table 2
Comparison of the properties of disturbed Tubeiliyat sandstone with those of typical loess
Texture
Mineral composition
Colour
Bedding
Post-depositional changes
Loess
Disturbed Tubeiliyat sandstone facies
Silt content >50%
Quartz grains predominate
Mean grain size range: 5.75 to 4.60φ
Well sorted
Positively skewed in the range: +0.30
to +0.70
Texturally mature
Quartz grains rounded to subrounded
Quartz (40%–80%)
Feldspar (5%–30%)
Mica (5%–10%)
Primary carbonate (0–30%)
Clays (5%–20%)
Heavy minerals (1%–5%)
Authigenic carbonate rare
Compositionally submature to
immature
Quartzose to slightly feldspathic
siltstone
Silt content <10%
Quartz grains predominate
Mean grain size 3.11φ
Moderately sorted
Positively skewed: +0.18
Plae yellow but can be buff, grey, red
or brown
Homogeneous; non-stratified
Structureless
Typically interbedded with thin
palaeosols
No quartz pebbles or pebble stringers
Grey-green
Calcium carbonate nodules and
concretions
Texturally submature
Quartz grains angular to subangular
Quartz (46.3%–52.6%)
Feldspar (8.6%–11.6%)
Mica (1.0%–0.3%)
No primary carbonate
Clays <1%
Authigenic carbonate (41%–34%)
Rock fragments (1%)
Chlorite
Heavy minerals (1–2%)
Compositionally immature
Subarkosic fine sandstone
Locally developed horizontal bedding;
disrupted bedding; convolute laminations;
ripple cross-lamination; hummocky crossstratification; vertically stacked coarseningupward parasequences; no palaeosols
Sparse, small, angular quartz pebbles
Calcium carbonate nodules and concretions
Authigenic pore-filling carbonate
The shaded areas highlight the differences between the disturbed Tubeiliyat sandstone facies and typical loess.
along reactivated existing Pan-African fault systems
(Echikh and Sola, 2000). In some areas, such as western Libya and southern Jordan, glacially incised palaeovalleys are preferentially located along these reactivated
Pan-African basement faults (Glover et al., 1999). During the early Palaeozoic Jordan was subjected to periods of epeirogenic movement which ended in the Late
Ordovician sometime prior to glaciation (Sabbah and
Ramini, 1996). This led to the development of two
major lineament trends: one NW–SE and the other
ENE–WSW, similar to the trends of the two subsidiary
glacial advances.
Since there is no evidence of tectonism contemporaneous with Late Ordovician glaciation in southern
Jordan crustal stress and deformation were probably a
result of glaciation. Glaciation induces crustal stress
and seismicity, particularly in the upper 5–10 km of
the crust, which is sensitive to very small changes
(b0.1 Mpa) in stress (Thorson, 2000). These stresses,
and moderate levels of seismicity, are mainly concentrated around the perimeters of present day continental
scale ice sheets, such as Greenland and Antarctica; the
interior is largely aseismic because loading by large
ice sheets stabilises faults and suppresses seismic activity beneath the load in the underlying brittle crust.
(Johnson, 1987, 1989; Wu and Hasegawa, 1996).
Although crustal stresses occur during glacial advance
and postglacial retreat, most glacial seismotectonic
models favour the more significant crustal stress generated during postglacial retreat as the most likely
cause of seismic activity (Johnson, 1987; Adams and
Basham, 1989; Muir-Wood, 1989; Arvidsson, 1996;
Stewart et al., 2000). Large palaeofaults and increased
levels of seismicity in Britain and Fennoscandia are
thought to have been triggered by postglacial rebound
(Gregersen and Basham, 1989); a view supported by
the numerical modelling work on postglacial rebound
stress by Wu and Hasegawa (1996) in eastern Canada.
Postglacial crustal stress is known to produce a pulse
of seismotectonic activity and palaeofault reactivation
which commonly leads to associated earthquake-induced soft sediment deformation features (Davenport
and Ringrose, 1989). In Scotland these features
formed during or immediately after deglaciation
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B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
(Firth and Stewart, 2000). This association has been
well documented for the 1811–1812 earthquakes in
the Upper Mississippi valley (Penick, 1976) and the
1886 Charleston earthquake (Dutton, 1889), amongst
others (Sieh, 1981). In southern Jordan the Hutayya
fault zone trends NW–SE, similar to the trend of
basement and Pan-African faults which formed 630–
580 Ma (Ibrahim and McCourt, 1995). Modern studies
demonstrate that faults are able to propagate through
weak, poorly lithified sediments (Moran and Christian,
1990), not involving fracture of the grains, during
tectonism (Moore and Byrne, 1987; Maltman, 1994).
Thus, as the ice began to melt and retreat southwards,
following the first glacial incision, crustal stresses
caused seismic shocks and reactivation of the Hutayya
fault zone, triggering soft sediment deformation adjacent to the faults. These palaeofaults, weakened by
earlier tectonism, would be particularly susceptible to
reactivation by later postglacial seismicity (Hicks et
al., 2000).
Ground shaking concentrated along these faults provides a trigger for sediment deformation through reduced overburden pressure and increased hydrostatic
fluid pore pressure (Johnson, 1987). Consequently, the
poorly consolidated, fine sands, adjacent to the reactivated Hutayya fault zone were liquefied and extensive-
ly deformed, leading to a partial to complete loss of
lamination. This may have involved some internal mixing, local sediment flowage and movement (Lowe,
1975) and the development of more locally complex,
deformed and chaotically disturbed zones in response to
local differences in viscosity and density due to grainsize (sand-silt). Small siltstone clasts floating in ungraded sandstone are consistent with a non-Newtonian flow
rheology (Gani, 2004).
Kuribayashi and Tatsuoka (1975) proposed a relationship between the Richter earthquake magnitude
and the maximum epicentral distance in which soft
sediment deformation, produced by liquefaction,
occurs. Earthquake magnitudes of b 5 cause little or
no liquefaction whereas earthquake magnitudes of 6
cause liquefaction within a radius of 4 km, extending
to up to 20 km from the epicentre at magnitudes of 7
(Morgenstern, 1967; Kuribayashi and Tatsuoka, 1975,
Fig. 5; Scott and Price, 1988; Mohindra and Bagati,
1996). This relationship implies that the Jebel Ammar
area lay within 4 km of the epicentre of an earthquake of magnitude 6 or more, concentrated along
the reactivated Hutayya fault zone. Seismic shocks
travel vertically and laterally, and vertical transit may
be amplified if concentrated along a major fault
plane. This relationship between faulting and seismic
Fig. 11. Simplified conceptual model showing the relationship between the ice sheet in Northwest Saudi Arabia and the hydrothermal system
responsible for chloritisation of the deformed sandstones along the Hutayya graben in the Jebel Ammar area. Glacial meltwater infiltrated subglacial
hydrofractures in the bedrock which acted as a groundwater recharge area for the initiation of a convective hydrothermal system. The hydrothermal
fluids leached Mg and Fe from the rocks through which they passed as they moved towards the reactivated Hutayya fault zone in southern Jordan.
Infiltration of these hydrothermal fluids, under pressure and elevated geothermal gradient, into the sandstones adjacent to the fault led to the
alteration of existing minerals and the precipitation of chlorite.
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
activity was noted by Youd (1977) and Youd and
Prentice (1978), who measured the distance of liquefaction from the fault rather than the epicentre of the
earthquake.
Cohesiveless silts and fine to medium sands are
particularly prone to seismogenic liquefaction (Lowe,
1975) when the intensity of ground shaking exceeds a
critical threshold level (Seed and Idris, 1971). Above
this critical threshold level variations in intensity of
ground shaking are associated with different styles of
soft sediment deformation (Mohindra and Bagati,
1996). However, the entire sediment pile at Jebel
Ammar–Amira (at least 90 m thick locally) was affected by a similar style of deformation throughout,
suggesting a single deformation event or possibly
several shocks of high magnitude and duration.
These can lead to extensive liquefaction and deformation of primary sedimentary structures with little preserved evidence of non-deformed zones (Scott and
Price, 1988).
85
3.3.2. Sandstone composition
Differences in composition between the deformed
and undeformed Tubeiliyat sandstones (Table 1) may
provide clues to the chemistry of circulating pore fluids
and the amount of sediment cover removed by glacial
erosion. The differences in composition, especially the
increased amounts of authigenic chlorite in the deformed sandstone, could be explained by: (1) mixing
and homogenisation of Tubeiliyat shales, siltstones and
sandstones during liquefaction; (2) diagenetic alteration
of kaolinite; (3) breakdown of biotite; or (4) the authigenic addition of chlorite during or following deformation. The homogenisation model seems least likely
given that, whereas the Tubeiliyat shales and siltstones
are chloritic (Table 1), most sandstones are chlorite free
(Makhlouf, 1995) and also the most susceptible to
liquefaction (Lowe, 1975). Chlorite precipitation, and
the alteration of kaolinite to chlorite, implies basic (alkaline) pore solutions enriched in Fe2+ and Mg2+(Foscolos, 1985; Jahrend and Angaard, 1989; Small et al., 1992;
Fig. 12. Schematic three-dimensional stratigraphic block diagram showing the relationship of Jebel Ammar–Amira and other associated hills to the
Hutayya fault zone. Compactional dewatering of the sediment pile and glacial meltwater initiated chemically reactive, reducing, hydrothermal
fluids, enriched in Fe and Mg, which preferentially moved upwards along the reactivated fault zone causing chloritisation of the adjacent
sandstones.
86
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
Bjørlykke, 1999). Some of the Fe2+ and Mg2+ may have
been derived from the breakdown of biotite and its
alteration to chlorite, but a more likely source of
these basic waters, given the extensive chlorite neomorphism in the matrix, is seawater which is typically
buffered between pH 8.0 and 8.4 (Brownlow, 1979;
Cookenboo and Bustin, 1999). However, any reduction
in alkalinity leads to chlorite dissolution and favours a
change from chlorite to illite to kaolinite. Thus, for
chlorite to precipitate basic Fe2+ and Mg2+ rich pore
solutions are required at low temperatures of at least 15
but less than 100 8C (Cookenboo and Bustin, 1999).
Because Fe2+ is less abundant in seawater than Mg2+ an
additional source of Fe2+ is required.
3.3.3. Fault-controlled hydrothermal fluids
Seismically reactivated tectonic faults can act as
pathways for post-seismic fluid flow under high pressure (Cox and Etheridge, 1989) as well as for heat and
mass transfer (Deming, 1993). Where these fluids infiltrate the adjacent sandstones (Knipe, 1992), new
minerals such as mica, chlorite and smectite may
form (Cookenboo and Bustin, 1999; Warr and Cox,
2001). The source of these fluids may have been compactional dewatering and dehydration of hydrous
minerals in the sediment pile. A possible additional
fluid source is subglacial meltwater beneath the nearby
wet-based temperate ice sheet which was forced
through openings in the bedrock (hydrofracturing)
under high pressure from the overlying ice (Davison
and Hambrey, 1996). Hydrofracturing is an important
process beneath glaciers, and has been documented
from beneath the Fennoscandanavian ice sheet (Carlsson, 1979) where hydrofractures may penetrate bedrock
to depths of 100 metres (Muir-Wood, 1989; M. Hambrey, written communication, 2003). Ice loading of
deep seated aquifers accelerates groundwater flow
and, with increasing depth (~250 m) the groundwater
becomes more saline (England et al., in press). Thus,
the base of the melting glacier may have acted as a
groundwater recharge area (Boulton and Dobbie, 1993)
for the initiation and maintenance of a meteoric waterdriven, convective, low temperature hydrothermal system (Fig. 11). These reducing hydrothermal fluids may
have leached out Fe and Mg from localised diorites,
gabbros and dolerite dykes along the sediment-basement contact and from the sandstone-dominated sediment pile through which the fluids moved along the
Hutayya fault zone (Fig. 12). Although sandstones
contain few trace metals, compaction of interbedded
muds, such as the Hiswa mudstones (Fig. 2), flushed
seawater enriched in Fe and Mg through the sandstones,
with the Hutayya fault zone acting as the conduit for
this fluid movement. These fluids may also have been
involved in the sediment deformation process, especially if they were injected into the sandstones adjacent to
the fault under pressure and elevated geothermal gradient, where they precipitated chlorite from solution.
Given a normal continental geothermal gradient of
30 8C km 1 chloritisation would occur between 500
and 3330 m depth, hence the upper part of the Tubeiliyat Formation would have to be buried to at least 500
m to be chloritised. This is unrealistic given the time
available between deposition of the Tubeiliyat and
Fig. 13. Alternative conceptual models to explain the observed zone
of chloritisation adjacent to the Hutayya graben. Both models assume
a regional continental geothermal gradient of 30 8C km 1 and chlorite
precipitates between 15 and 100 8C. Crosses denote Pan-African
crystalline basement. (A) A simple 2-D isostatic model to demonstrate
the effects of ice loading. The model assumes the density of ice and
sedimentary rock is 1 and 2.4 g cm 3 respectively. The model is
loaded by 1160 m of ice to generate 500 m of equivalent rock
subsidence. If the geothermal gradient is maintained this has the effect
of raising the top of the zone of chloritisation (15 8C isotherm) to
coincide with the base of the ice within the graben. Outside the graben
the predicted temperature at the base of the ice is 10.5 8C, the 0 8C
isotherm would lie ~350 m within the ice sheet. In all cases the
predicted temperatures are unrealistic. (B) Preferred bhydrothermal
modelQ in which ground water, illustrated as ice meltwater, reaches the
zone of chloritisation at 500 m depth. The faults bounding the
reactivated Hutayya graben act as conduits to Fe and Mg richwater, and in the vicinity of the faults the geothermal gradient is
raised. Zones of chloritised upper Tubeiliyat Formation occur along
the faults.
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
glacial incision. Ice loading is also rejected as the
source of burial. Simple isostatic modelling, assuming
a normal continental geothermal gradient, predicts unrealistic temperatures at the base of the ice sheet (Fig.
13). Our preferred bhydrothermal modelQ is driven by
compaction. Ground water, likely ice meltwater,
reaches the zone of chloritisation at 500 m depth. The
faults bounding the Hutayya graben act as conduits to
Fe and Mg rich-water and locally the geothermal gradient is raised. Zones of chloritised upper Tubeiliyat
Formation occur along the faults. The temperature at
the cover-basement contact is 24 8C and in this model
the zone of chloritisation could theoretically extend
~2.5 km into the basement.
4. Depositional model
Southern Jordan in the Late Ordovician was located
b 100 km from the margin of a terrestrial ice cap in NW
Saudi Arabia (Fig. 3), which formed part of a more
extensive ice sheet of similar size to the present day
Antarctic ice sheet (Deynoux, 1985; Vaslet, 1990; Eyles
and Young, 1994). The ice cap was characterised by
two major phases of ice advance and retreat (cf. Vaslet,
87
1990). Abed et al. (1993) were of the opinion that both
these glacial phases, covered parts of southern Jordan
with ice, whereas Powell et al. (1994) considered that
only the first glacial phase, correlated with the incision
beneath the Zarqa Formation in Saudi Arabia, affected
southern Jordan.
We propose a four stage glacial model in which the
top of the Tubeiliyat shoreface and nearshore shelf
sediments was incised and truncated in response to
the first major glacial advance from Saudi Arabia
(Fig. 14). The ice preferentially excavated NW–SE
trending major fault-controlled depressions cutting
steep-sided U-shaped valleys into the Tubeiliyat Formation. This implies that the sediments must have been
sufficiently lithified to facilitate glacial incision. In
view of the relatively short time between sediment
deposition and glacial incision the degree of lithification may have been limited. However, temperature
simulations for the Late Ordovician (Crowley and
Baum, 1995) indicate a low temperature range from
5–0 8C for the 608 S latitude of Jordan at this time.
Moreover, the ground surface in front of large ice sheets
is characterised by permafrost (Fowler and Noon, 1999)
which may extend down to depths of a few metres to
Fig. 14. Schematic model of ice sheet evolution during the first major ice advance and retreat in NW Saudi Arabia, showing depositional
environments, and its effect on southern Jordan. The lithostratigraphic sections show the glacial successions in Saudi Arabia and Southern Jordan
(see Fig. 4 and text for further details).
88
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
several hundred metres (Burgess and Smith, 2000). Ice
loading also increases the geotechnical strength of the
beds beneath the ice (Stewart et al., 2000) and this,
together with the permafrost-hardened ground, provides
a resistant substrate for glacial incision and the cutting
of the steep-sided lower palaeovalley prior to deglaciation seismotectonics and deformation. Postglacial
melting, together with a concomitant rise in relative
sea level and southerly transgression across the Gondwana shelf, deposited thin, transgressively reworked
glaciofluvial, locally conglomeratic sandstones in the
bottom of the glacially incised valley. As melting and
marine transgression continued shoreface sediments
were deposited in the valley (Fig. 14).
The intersecting glacial striations suggest that a second and possibly a third subsidiary glacial advance
interrupted deposition and final filling of the lower
palaeovalley. This implies a fall in relative sea level
in response to renewed ice build-up to allow for the
cutting of the glacial surface, followed by a brief rise in
sea level as shoreface deposition continued. The source
of this ice, which lay to the west and northwest (Fig. 5),
may have been subsidiary lobes of the major ice sheet
in northwest Saudi Arabia. Final filling of the lower
palaeovalley by shoreface sandstones was followed by
a short hiatus, which may correlate with the interglacial
between the two major ice advances in Saudi Arabia
(Vaslet, 1990). This was followed by a further 4th
major ice advance, correlated with the 2nd major ice
advance in northwest Saudi Arabia. Turner et al. (2002)
interpreted the upper palaeovalley as low stand tunnel
valleys of the upper Ammar Formation (Abed et al.,
1993) (Fig. 4). The final ice advance and formation of
the upper palaeovalley may have removed interglacial
sediment, or the interglacial was a period of non-deposition. The palaeovalley was subsequently filled with
outwash plain sediments as the ice melted, concomitant
with a transgressive rise in sea level and the deposition
of shoreface sands over the glaciofluvial palaeovalleyfill sands, thereby defining a post-glacial transgressive
surface (Armstrong et al., 2005).
5. Conclusions
Southern Jordan in the Late Ordovician was located
b100 km from the margin of a terrestrial ice sheet in
Northwest Saudi Arabia characterised by two major
phases of ice advance and retreat (cf. Vaslet, 1990),
which have been correlated with similar events in
southern Jordan (Abed et al., 1993). In additional,
two subsidiary ice advances are now recognised in
southern Jordan. During the first major glacial advance
ice incised into permafrost-hardened and glacially loaded, Tubeiliyat shoreface and nearshore shelf deposits,
preferentially excavating NW–SE trending major faultcontrolled depressions, cutting a steep-sided U-shaped
valley. This first glacial advance, correlates with the
first glacial advance in Northwest Saudi Arabia, and
was followed by deglaciation, a rise in base level and
transgressive filling of the palaeovalley with a thin,
reworked bottom lag of glaciofluvial sandstones, overlain by thick, transgressive, shoreface sandstones. Late
transgressive filling of the palaeovalley was interrupted
by a 2nd and possibly a 3rd subsidiary glacial advance
producing a glacially polished and grooved surface
with intersecting glacial striations, indicating ice flow
from the west and northwest.
A further 4th glacial advance, which correlates with
the second major ice advance in Saudi Arabia, produced a regionally extensively low stand tunnel valley
beneath the ice sheet (Turner et al., 2002). This was
subsequently preserved as a palaeovalley incised into
the lower palaeovalley-fill deposits or, where this is
missing, into the top of the Tubeiliyat Formation. Following melting the palaeovalley filled with transgressive glaciofluvial sandstones and shoreface sandstones.
The upper palaeovalley is incised into deformed sandstones adjacent to the Hutayya graben in the Jebel
Ammar area. These sandstones were placed in the
lower Ammar Formation by Abed et al. (1993) who
interpreted them as a glacial rock flour devoid of sedimentary structures. The same sediments were later
interpreted as a structureless glacial loessite derived
from reworking of the underlying mineralogically identical Tubeiliyat Formation (Amireh et al., 2001). However, the lack of grain breakage and grinding, and
significant differences in their textural and compositional attributes compared to typical loess argues
against either of these interpretations.
The deformed sandstones, which are non-channelised and at least 90 m thick, contain no glacial features.
However, they contain sedimentary structures similar to
those in the Tubeiliyat Formation, and they can be
traced laterally away from the fault zone over a distance
of 4 km into undeformed Tubeiliyat. Thus, we interpret
the sandstones in the Jebel Ammar area to be a locally
disturbed facies equivalent of the pre-glacial marine
Tubeiliyat Formation, not part of the glacial Ammar
Formation, which now comprises the lower and upper
regionally extensive glacial palaeovalley-fill deposits.
Southern Jordan may have been subjected to postglacial
seismotectonism, concentrated along the Hutayya fault
zone, which had been previously weakened by epeirogenic movement. Seismic reactivation of the fault zone
B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91
triggered ground shaking, sediment liquefaction and
deformation adjacent to the faults. The increased authigenic chlorite content of the fault-related deformed
Tubeiliyat sandstones, compared to undeformed ones,
can be explained by compaction-driven low temperature hydrothermal fluids, with a possible additional
fluid source from subglacial hydrofracturing of the
bedrock setting up a hydrothermal system beneath the
ice sheet which acted as a groundwater recharge area
for the hydrothermal system. Iron and Mg were leached
from the sediment pile as the fluids moved preferentially up the reactivated Hutayya fault zone. This acted
as a conduit for hydrothermal fluid movement and
where these hydrothermal fluids infiltrated adjacent
sandstones chlorite was precipitated from solution.
Acknowledgements
This research was supported by the UK Natural
Environmental Research Council (NERC Grant NER/
B/2000/000068), the University of Durham and the
Hashemite University. We are grateful to the Natural
Resources Authority of Jordan for their logistical help
and support, and Dr. Belal Amireh for his help with the
petrography through the loan of a thin section. We are
indebted to two anonymous reviewers, and the journal
editor, for their helpful comments and suggestions.
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