Late Ordovician (Ashgillian) glacial deposits in …Late Ordovician (Ashgillian) glacial deposits in...

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Sedimentary Geology 1

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, UKb 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

0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sedgeo.2005.08.004

* Corresponding author.

E-mail address: b.r.turner@durham.ac.uk (B.R. Turner).

1. Introduction

The Lower Palaeozoic succession in southern Jordan

(Fig. 1) includes some 750–800 m of well exposed

Ordovician siliciclastic sediments deposited on the mar-

gins of the North African (Gondwana) shallow marine

81 (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.

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9174

shelf (Fig. 2), in terrestrial, subtidal marginal marine

and shelf environments (Powell et al., 1994; Makhlouf,

1995; Amireh et al., 2001). During the Upper Ordovi-

cian Jordan was located in a high latitude east Gond-

wana 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 Forma-

tion (Fig. 2). They divided it into two units, each un-

derlain 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 exten-

sively 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 con-

fined, 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 sand-

stone, 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 glacio-

fluvial 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

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).

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 75

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 de-

formed siltstones (Abed et al., 1993; Amireh et al.,

2001). In this study we present new field-based data

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).

from southern Jordan, and a reinterpretation of the

Late Ordovician glacial deposits, including the dis-

turbed and structureless glacial loessite or rock flour

siltstones, based on their architecture, field relation-

ships and petrography. The relationship between gla-

cially induced crustal stress and seismic reactivation of

faults, documented for present day and past glacial

regimes, is used to develop a model to explain defor-

mation 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 liquefac-

tion 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 ma-

rine shoreface sandstones; and an upper palaeoval-

ley system filled with glaciofluvial and shoreface

sandstones.

2.1. Lower palaeovalley

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, fault-

bounded hills that can be traced discontinuously for

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).

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9176

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 palaeoval-

ley-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, feldspath-

ic, 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 con-

tains small-scale, low angle, intersecting trough cross-

beds (5–10 cm thick and 20 cm long), ripple cross-

lamination, straight-crested symmetrical ripple marks,

medium to large-scale hummocky cross-stratification

(Fig. 5C), local erosion surfaces, water escape struc-

tures, burrows and brachiopod moulds. Some sand-

stones just above the incised palaeovalley base,

contain brachiopods and numerous trace fossils in-

cluding 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 later-

ally 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 fac-

eted 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 imme-

diately below this surface, which is overlain by ~5 m

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.

B.R.Turner

etal./Sedimentary

Geology181(2005)73–91

77

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.

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9178

of fine to medium-grained hummocky cross-stratified

sandstone.

2.2. Upper palaeovalley

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 Forma-

tion (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 fac-

eted 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 palaeo-

valley shoreface sandstones, above the internally gla-

ciated surface, it comprises 1–2 m of coarse to very

coarse, granular and pebbly sandstone containing

faceted and striated clasts that grade sharply up-

wards into medium to coarse sandstone (Fig. 5) con-

taining local granular and pebbly streaks and lenses.

Internally the sandstone is structured by large-scale,

low angle, wedge-shaped trough cross-bed sets depos-

ited by currents flowing to the east and southeast

(Fig. 5G).

3. Disturbed sandy siltstones

Disturbed sandy siltstones beneath the upper palaeo-

valley 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 palaeo-

valley-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 For-

mation 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. Al-

though 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

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 flat-

lying 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.

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 79

the fault plane, whence they pass into undeformed

shoreface and shelf sediments of the Tubeiliyat Forma-

tion (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. De-

formation includes: (1) recumbent folding; (2) convo-

lute 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). Inaddition rare horizontally segmented burrows, brachio-

pod 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 inter-

vals 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 low-

angle 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.,

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).

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9180

1993: Amireh et al., 2001), 5 samples of disturbed

sandy siltstone, 2 samples of lower palaeovalley-fill

sandstone, and 2 samples of Tubeiliyat Formation sand-

stone were analysed for their textural properties and

mineral composition. The mean grain-size of the dis-

turbed sandy siltstone is 3.11/ with N80% of particles

in the fine to very fine sand range (2–4/) and b10% 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 pris-

tine 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, musco-

vite, 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 domi-

nated by chlorite, illite and smectite, but with kaolinite,

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.

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.

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 81

silt and clay-size grains of quartz, muscovite, and other

non-clay minerals also present. Other minor detrital

constituents include pyroxene, tourmaline, zircon, ru-

tile, 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 (Bend-

er, 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

Abed et al. (1993) interpreted the disturbed

denigmaticT sandy siltstones as a rock flour, derived

Table 1

Petrographic data, disturbed, lower palaeovalley-fill sandstones and Tubeiliyat Formation sandstones

Constituents Disturbed grey-green sandstone (%) Lower palaeovalley-fill

sandstones incised into the

Tubeiliyat Formation (%)

Tubeiliyat Formation

sandstones (%)

Monocrystalline quartz 51.0 47.0 50.0 50.6 56.3 48.0 52.6 71.0 69.0

Poly qrtz rock fragments 0.3 1.3 0.6 0.6 0.6 0.3 1.0 0.6 0.6

K-feldspar and microcline 6.0 9.0 8.3 6.6 6.3 6.3 5.6 10.3 9.6

Plagioclase feldspar 1.6 1.3 0.6 0.3 0.3 0.3 1.3 0.6 1.5

Muscovite 10.0 6.0 3.3 2.6 3.6 10.3 10.6 2.3 3.3

Biotite 1.0 0.0 1.6 1.0 0.3 3.3 4.3 0.3 0.0

Chloritised biotite 3.0 3.6 8.3 7.3 4.6 1.3 2.0 0.0 0.0

Chlorite 2.6 1.9 2.3 3.0 2.3 0.6 0.3 0.0 0.0

Matrix 22.3 28.6 24.0 26.6 24.6 24.3 19.0 2.3 2.7

Calcite (patchy cement) 0.3 0.0 0.0 0.0 0.0 0.0 0.6 3.6 2.7

Kaolinite 0.6 0.0 0.0 0.0 0.6 2.0 1.3 10.2 9.6

Heavy minerals 1.3 1.0 0.6 1.0 1.3 3.0 1.0 0.9 0.9

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9182

from erosion of the underlying mineralogically identi-

cal 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 be-

cause 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 inter-

bedded 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 character-

istics of the disturbed sandstones, it is considered un-

likely 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 addi-

tion the disturbed sandstones: (1) were tectonically

deformed; (2) they contain sedimentary structures iden-

tical to those in the Tubeiliyat Formation; (3) they can

be traced laterally into undeformed Tubeiliyat Forma-

tion; (4) they are located along fault zones; and (5)

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 (Makh-

louf, 1995) used in this study, hence the differences in

mineral composition in Table 1. We interpret the sand-

stones 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 sand-

stones 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 proxim-

ity 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 pat-

terns but was characterised by epeirogenic movements

Table 2

Comparison of the properties of disturbed Tubeiliyat sandstone with those of typical loess

Loess Disturbed Tubeiliyat sandstone facies

Silt content >50% Silt content <10%Quartz grains predominate Quartz grains predominateMean grain size range: 5.75 to 4.60φ Mean grain size 3.11φWell sorted Moderately sortedPositively skewed in the range: +0.30to +0.70

Positively skewed: +0.18

Texturally mature Texturally submature

Texture

Quartz grains rounded to subrounded Quartz grains angular to subangularQuartz (46.3%–52.6%)Feldspar (8.6%–11.6%)Mica (1.0%–0.3%)No primary carbonateClays <1%Authigenic carbonate (41%–34%)Rock fragments (1%)ChloriteHeavy minerals (1–2%)Compositionally immature

Mineral composition Quartz (40%–80%)Feldspar (5%–30%)Mica (5%–10%)Primary carbonate (0–30%)Clays (5%–20%)Heavy minerals (1%–5%)Authigenic carbonate rareCompositionally submature toimmatureQuartzose to slightly feldspathicsiltstone Subarkosic fine sandstone

Colour Plae yellow but can be buff, grey, redor brown

Grey-green

Bedding Homogeneous; non-stratifiedStructurelessTypically interbedded with thinpalaeosolsNo quartz pebbles or pebble stringers

Locally developed horizontal bedding;disrupted bedding; convolute laminations;ripple cross-lamination; hummocky cross-stratification; vertically stacked coarsening-upward parasequences; no palaeosolsSparse, small, angular quartz pebblesCalcium carbonate nodules and concretionsPost-depositional changes Calcium carbonate nodules and

concretions Authigenic pore-filling carbonate

The shaded areas highlight the differences between the disturbed Tubeiliyat sandstone facies and typical loess.

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 83

along reactivated existing Pan-African fault systems

(Echikh and Sola, 2000). In some areas, such as west-

ern Libya and southern Jordan, glacially incised palaeo-

valleys are preferentially located along these reactivated

Pan-African basement faults (Glover et al., 1999). Dur-

ing the early Palaeozoic Jordan was subjected to peri-

ods 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 contempo-

raneous 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 concen-

trated 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 ac-

tivity 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 gen-

erated 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-in-

duced soft sediment deformation features (Davenport

and Ringrose, 1989). In Scotland these features

formed during or immediately after deglaciation

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9184

(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 adja-

cent 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 pro-

vides a trigger for sediment deformation through re-

duced overburden pressure and increased hydrostatic

fluid pore pressure (Johnson, 1987). Consequently, the

poorly consolidated, fine sands, adjacent to the reacti-

vated Hutayya fault zone were liquefied and extensive-

Fig. 11. Simplified conceptual model showing the relationship between th

responsible for chloritisation of the deformed sandstones along the Hutayya g

hydrofractures in the bedrock which acted as a groundwater recharge area fo

fluids leached Mg and Fe from the rocks through which they passed as they

Infiltration of these hydrothermal fluids, under pressure and elevated geot

alteration of existing minerals and the precipitation of chlorite.

ly deformed, leading to a partial to complete loss of

lamination. This may have involved some internal mix-

ing, 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 grain-

size (sand-silt). Small siltstone clasts floating in ungrad-

ed sandstone are consistent with a non-Newtonian flow

rheology (Gani, 2004).

Kuribayashi and Tatsuoka (1975) proposed a rela-

tionship between the Richter earthquake magnitude

and the maximum epicentral distance in which soft

sediment deformation, produced by liquefaction,

occurs. Earthquake magnitudes of b5 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 earth-

quake 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

e ice sheet in Northwest Saudi Arabia and the hydrothermal system

raben in the Jebel Ammar area. Glacial meltwater infiltrated subglacial

r the initiation of a convective hydrothermal system. The hydrothermal

moved towards the reactivated Hutayya fault zone in southern Jordan.

hermal gradient, into the sandstones adjacent to the fault led to the

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 85

activity was noted by Youd (1977) and Youd and

Prentice (1978), who measured the distance of lique-

faction 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 af-

fected 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 deforma-

tion of primary sedimentary structures with little pre-

served evidence of non-deformed zones (Scott and

Price, 1988).

Fig. 12. Schematic three-dimensional stratigraphic block diagram showing th

Hutayya fault zone. Compactional dewatering of the sediment pile and gl

fluids, enriched in Fe and Mg, which preferentially moved upwards alo

sandstones.

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 de-

formed 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 authi-

genic addition of chlorite during or following deforma-

tion. 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 (al-

kaline) pore solutions enriched in Fe2+ and Mg2+(Fosco-

los, 1985; Jahrend andAngaard, 1989; Small et al., 1992;

e relationship of Jebel Ammar–Amira and other associated hills to the

acial meltwater initiated chemically reactive, reducing, hydrothermal

ng the reactivated fault zone causing chloritisation of the adjacent

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 8Cisotherm 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 rich-

water, 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) 73–9186

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 neo-

morphism 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 pres-

sure (Cox and Etheridge, 1989) as well as for heat and

mass transfer (Deming, 1993). Where these fluids in-

filtrate 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 com-

pactional 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 (Carls-

son, 1979) where hydrofractures may penetrate bedrock

to depths of 100 metres (Muir-Wood, 1989; M. Ham-

brey, 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 water-

driven, convective, low temperature hydrothermal sys-

tem (Fig. 11). These reducing hydrothermal fluids may

have leached out Fe and Mg from localised diorites,

gabbros and dolerite dykes along the sediment-base-

ment contact and from the sandstone-dominated sedi-

ment 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, especial-

ly if they were injected into the sandstones adjacent to

the fault under pressure and elevated geothermal gradi-

ent, 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 Tubei-

liyat 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

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 87

glacial incision. Ice loading is also rejected as the

source of burial. Simple isostatic modelling, assuming

a normal continental geothermal gradient, predicts un-

realistic 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 gra-

dient 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

b100 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,

Fig. 14. Schematic model of ice sheet evolution during the first major i

environments, and its effect on southern Jordan. The lithostratigraphic sectio

(see Fig. 4 and text for further details).

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 For-

mation. 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 lithifica-

tion 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

ce advance and retreat in NW Saudi Arabia, showing depositional

ns show the glacial successions in Saudi Arabia and Southern Jordan

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–9188

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 deglaci-

ation seismotectonics and deformation. Postglacial

melting, together with a concomitant rise in relative

sea level and southerly transgression across the Gond-

wana 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 sec-

ond 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-depo-

sition. 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 palaeovalley-

fill 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 load-

ed, Tubeiliyat shoreface and nearshore shelf deposits,

preferentially excavating NW–SE trending major fault-

controlled 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, over-

lain 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, pro-

duced 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. Fol-

lowing melting the palaeovalley filled with transgres-

sive glaciofluvial sandstones and shoreface sandstones.

The upper palaeovalley is incised into deformed sand-

stones 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 sed-

imentary structures. The same sediments were later

interpreted as a structureless glacial loessite derived

from reworking of the underlying mineralogically iden-

tical Tubeiliyat Formation (Amireh et al., 2001). How-

ever, the lack of grain breakage and grinding, and

significant differences in their textural and composi-

tional attributes compared to typical loess argues

against either of these interpretations.

The deformed sandstones, which are non-channe-

lised 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 epeiro-

genic movement. Seismic reactivation of the fault zone

B.R. Turner et al. / Sedimentary Geology 181 (2005) 73–91 89

triggered ground shaking, sediment liquefaction and

deformation adjacent to the faults. The increased authi-

genic chlorite content of the fault-related deformed

Tubeiliyat sandstones, compared to undeformed ones,

can be explained by compaction-driven low tempera-

ture 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 preferen-

tially 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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