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Depositionalenvironmentsandsequencestratigraphyofparalicglacial,paraglacialandpostglacialUpperOrdovician...

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DOI:10.1016/j.sedgeo.2005.02.006

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Sedimentary Geology 17

Depositional environments and sequence stratigraphy of

paralic glacial, paraglacial and postglacial Upper Ordovician

siliciclastic deposits in the Murzuq Basin, SW Libya

Mohamed Ali Kalefa El-ghaliT

Department of Earth Science, Uppsala University, Villavagen 16, SE 75236 Uppsala, Sweden

Received 17 March 2004; received in revised form 30 December 2004; accepted 15 February 2005

Abstract

The application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic

deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat,

and concomitant isostatic loading and rebound of the continental shelf. The study of outcrop and subsurface sections of the

Upper Ordovician glaciogenic Melaz Shuqran and the Mamuniyat formations, Murzuq Basin SW Libya, yielded important

information on understanding changes in the relative sea level related to glacier movements. Three depositional sequences were

recognized. Depositional sequence one (DS-1) corresponds to the entire Melaz Shuqran Formation that was bounded below by

subglacial erosion surface (i.e. sequence boundary, SB) formed during glacier advance into shallow water areas. This surface

correlates with a transgressive surface (TS) in deep water areas and records an initial rise in the relative sea level owing to

glacier advance and loading of the continental shelf. Transgression occurred due to the slower rate of eustatic sea-level fall than

the isostatic loading. The transgressive systems tract (TST) comprises shoreface sandstones and offshore mudstones/diamictites

with ice-rafted debris. Further rise in relative sea level was associated with glacial retreat, which resulted in sediment starvation

in deep water areas and the formation of a condensed section. These deposits represent glacial depositional systems. The

highstand systems tract (HST) comprises prograding deltaic deposits, which was formed when the rate of sediment supply

exceeds the rate of relative sea level rise. The HST deposits represent paraglacial depositional systems.

Depositional sequence two (DS-2) corresponds to the lower and middle part of the Mamuniyat Formation, and is bounded

below by deep erosional surface (SB) that cuts into DS-1 as an incised valley, which was formed during glacial advance across

the continental shelf. Incision occurred due to the faster rate of eustatic sea-level fall than the rate of isostatic loading. Glacier

retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems

tract (LST) and tide-dominated estuarine sediments in a TST. These deposits represent glacial depositional systems. The HST

sediments represent prograding foreshore to shoreface deposits, which were formed when the rate of sediment supply exceeded

the rate of relative sea-level rise. These deposits represent paraglacial depositional systems.

Depositional sequence three (DS-3) corresponds to the upper part of the Mamuniyat Formation and is bounded below by an

erosional surface (SB) that was formed as a result of isostatic rebound, associated with relative sea-level fall. DS-3 LST

0037-0738/$ - s

doi:10.1016/j.se

T Correspondi

E-mail addr

7 (2005) 145–173

ee front matter D 2005 Elsevier B.V. All rights reserved.

dgeo.2005.02.006

ng author. Tel.: +46 18 4712552; fax: +46 18 4712591.

ess: mohamed.kalefa@geo.uu.se, elghali_geo@yahoo.co.uk.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173146

comprises Gilbert-type, deltaic conglomeratic sandstones grading upwards into sandstones. Subsequent relative sea-level rise

during the late Ordovician resulted in deposition of shoreface TST sandstone. Further rise in the relative sea level resulted in

sediment starvation in deep water areas and the formation of thin hardground layer (i.e. condensed section) that marks the upper

boundary of late Ordovician deposits. This study provides predictive model for the spatial and temporal distribution of ancient

glaciogenic depositional facies that may have important implications in hydrocarbon explorations.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Sequence stratigraphy; Glaciated intracratonic basin; Murzuq Basin; Upper Ordovician; Glacial, paraglacial and postglacial

sediments

1. Introduction

Sequence stratigraphy is a powerful tool for

predicting the spatial and temporal distribution of

reservoir, seals and source rocks, which is con-

trolled by the interplay between the rates of

sediment supply, basin-floor physiography and the

rate of changes in relative sea level (Posamentier

and Allen, 1993, 1999). Sequence stratigraphic

models are relatively well constrained for non-

glaciogenic, paralic deposits (e.g. Posamentier et

al., 1988a,b; Galloway, 1989; Van Wagoner et al.,

1990; Einsele et al., 1991; Posamentier and Allen,

1993, 1999; Schwarzacher, 1993) because changes

in relative sea level are better recognized in such

environments. Conversely, the sequence stratigraphic

framework for glacial and glacial-related deposits is

complicated by the impact of glacial advance and

retreat (i.e. glacio-isostatic loading and rebound,

respectively) on relative changes in sea level.

Despite these difficulties, the interest to establish a

sequence stratigraphic framework for glaciogenic

deposits has accelerated considerably since their

importance as hydrocarbon reservoirs in several

Paleozoic basins of the world was recognized (e.g.

Marini and Glooschenko, 1985; Levell et al., 1988;

Boulton, 1990; Framca and Potter, 1991; Eyles and

Eyles, 1992; Eyles, 1993; O’Brien et al., 1998; Powell

and Cooper, 2000).

The upper Ordovician deposits in the Murzuq

Basin, SW Libya have traditionally been subdivided

into two formations, namely the mud-dominated

Melaz Shuqran and the sand-dominated Mamuniyat

formations. These two formations represent an

important oil exploration target, as hydrocarbon

reservoirs and source rocks in the Murzuq Basin.

Attempts to construct sequence stratigraphic

schemes for these deposits have been proposed by

Blanpied et al. (2000) and McDougall and Martin

(2000). Blanpied et al. (2000) suggested that the

sequence stratigraphic model proposed for the upper

Ordovician rocks in the Taoudeni Basin, Mauritania

could be used as an analogue for the upper

Ordovician rocks in the Murzuq Basin, SW Libya.

This model was limited to the Al-Qarqaf area. The

sequence stratigraphic model proposed for the upper

Ordovician rocks in the Murzuq Basin by McDou-

gall and Martin (2000) were not provided in detail,

and was limited to the Ghat and Al-Qarqaf areas. In

this study we attempt to extend the study area in the

Murzuq Basin to include the Ghat, Al-Qarqaf and

Wadi Anlalin areas (Fig. 1A) where extensive

outcrops of the upper Ordovician deposits are

exposed at the basin margin, in addition to drill

cores from the subsurface of NC 174 (Fig. 1B). In

each location, rocks were studied in detail to

provide an evaluation of the depositional history,

which resulted in alternative sequence stratigraphic

schemes for these deposits when compared with

those of Blanpied et al. (2000) and McDougall and

Martin (2000).

This paper aims to constrain the spatial and

temporal distribution of depositional facies in the

upper Ordovician glaciogenic rocks of the Murzuq

Basin, SW Libya within a sequence stratigraphic

context, and to highlight the effects of glacial,

paraglacial and postglacial depositional systems on

sediment distribution. The sequence stratigraphic

framework provided herein allows the prediction of

the spatial and temporal distribution of glaciogenic

sedimentary facies and sheds new light on the

interactions between relative sea-level fluctuations

and glacial cycles. Changes in the relative sea level,

which were concomitant with glacier movements

Fig. 1. (A) Location map of the Murzuq Basin southern west Libya (study area is shaded), which is bounded by present-day tectonic structures, namely, the Al Qarqaf Arch, the Brak

Bin Ghanimah Uplift, and the Tihimbukah High in the northern, eastern and western part, respectively. The basin extends southwards toward the Republic of Niger, where it is known

as The Djado Basin. (B) Geological map of the Murzuq Basin showing the distribution of the basement, Palaeozoic, Mesozoic and Cenozoic rocks, locations of the main studied areas

of the upper Ordovician glaciogenic sediments (A, B, and C areas), which represent shallower, intermediate and deeper water areas, respectively, and oil wells in the Murzuq Basin.

M.AliKalefa

El-g

hali/Sedimentary

Geology177(2005)145–173

147

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173148

during the upper Ordovician allowed the recognition

of three depositional sequences within the Murzuq

Basin. Each depositional sequence comprises numer-

ous facies successions that resulted from amalgamated

facies associations related to a spectrum of deposi-

tional environments.

2. Geological setting of the Murzuq Basin

The Murzuq Basin is located on the North

African Saharan platform, SW Libya, and represents

a huge Paleozoic intracratonic basin that was

glaciated in the Upper Ordovician. The present day

basin is triangular to sub-circular in shape and

extends southward towards the northern Republic

of Niger (Fig. 1A). The basin forms one structural

unit and covers an area in excess of 400,000 km2.

The present day basin margins are defined by three

prominent tectonic elements that were developed

during mid-Palaeozoic to Tertiary times. These

tectonic elements are Al-Qarqaf Arch, the Brak Bin

Ghanimah Uplift, and the Tihemboka Arch, which

form the northern, eastern and western margins of the

basin, respectively (Fig. 1A and B).

The Murzuq Basin is filled with continental

Mesozoic and Cenozoic sediments lying unconform-

ably on the marine Paleozoic rocks, which reach a

maximum thickness of about 4000 m in the basin

centre (e.g. Mamgain, 1980; Grubic et al., 1991;

Pierobon, 1991; Davidson et al., 2000; Echikh and

Sola, 2000; Sutcliffe et al., 2000a; Figs. 1B and 2).

Development of the Murzuq Basin started by con-

solidation of North Africa during the Pre-Cambrian,

followed by a long arid period characterized by

intensive erosional and peneplanation processes

(Unrug, 1996; Grunow, 1999).

During the late Ordovician (i.e. Hirnantian), west

Gondwana was located close to the South Pole at

high latitude (c658 S; e.g. Kent and Van der Voo,

1990; Smith, 1997; Scotese et al., 1999; Fig. 3), and

the Murzuq Basin was lying along its continental

margin (Davidson et al., 2000). Based on glacio-

genic features such as soft-sediment striated surfaces

and ice-rafted debris and glaciogenic structures

(Beuf et al., 1971; Deynoux, 1998; Sutcliffe et al.,

2000a,b), it has been suggested that the ice sheet

was centered in present-day Central Africa and

South America. In the Murzuq Basin, the possible

maximum extent of the ice-sheet during glacier

advance events approached north Al-Qarqaf and

Wadi Anlalin areas (Figs. 1B and 3). Therefore, the

Ghat represents the ice-proximal (shallow water)

areas, whereas the Wadi Anlalin represents the ice-

distal (deep water) area and Al-Qarqaf was located

in the intermediate area with respect to the ice-sheet

center.

An episode of a short-lived glaciation (0.5–1

million years; Blanpied et al., 2000) resulted in the

deposition of glacial and glacial-related sediments,

characterized by their heterogeneous nature and the

presence of glacier features such as soft-sediment

striated surfaces and striated and/or faceted outsized

clasts. The glaciogenic rocks are exposed along the

basin margins with a total thickness of about 200–250

m (Fig. 4). In the subsurface, these deposits represent

the main hydrocarbon target of the Murzuq basin.

Traditionally, these deposits have been subdivided

into two formations: (i) the mud-dominated Melaz

Shuqran Formation, and (ii) the sand-dominated

Mamuniyat Formation (Echikh and Sola, 2000;

Sutcliffe et al., 2000a; Figs. 2 and 4). These

glaciogenic formations occur above the middle

Ordovician, shallow marine sandstones and below

the Silurian deep marine, offshore graptolitic mud-

stones and are bounded by transgressive surfaces (Fig.

2). These transgressive surfaces were probably formed

during the isostatic loading of the continental shelf

during glacial advance and a global sea-level rise

during late Ordovician to early Silurian time (Sutcliffe

et al., 2000b; Fig. 2). Sea-level rise during the late

Ordovician to early Silurian time was a consequence

of decay of a terrestrial ice sheet.

Subsequent changes in the paleogeographic set-

ting of the Murzuq Basin, as an integrated part of

the Gondwana, and consequent changes in the

paleoclimate during the lower Silurian resulted in

drastic changes in sedimentary facies from glacio-

genic to deep marine, Graptolitic mudrocks with

intercalated siltstones and sandstones of the Tanzuft

Formation (Grubic et al., 1991; Pierobon, 1991;

Figs. 1B and 2). This formation is overlain by

intercalated shallow marine mudrocks and sand-

stones grading upwards into conglomeratic sand-

stones of the Akakus Formation (Grubic et al.,

1991; Pierobon, 1991; Abugares and Ramaekers,

PreCa

Pale

ozoi

cM

esoz

oic

Ceno

zoic

Ages Formations Lithology Depositional EnvironmentsTectonic / Glacioeustatic- Isostatic Events

Pan African Orogeny

Extension pull-apart basins

Passive margin sag basin

Arenigian hiatus

Glacial advance

Glacial re-advanceGlacio / isostatic uplift

Llandoverian transgression

Caledonian ( Ardenian phase )

Mid Devonian inversion

Early Carboniferous hiatus

Hercynian Orogeny

Mid Cenozoic uplift and erosion

Fig. 2. Lithostratigraphy of the Murzuq Basin, SW Libya.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 149

1993; Fig. 2), which was deposited during the late

Llandoverian regression.

3. Materials and methods

Three principal study localities are situated along

the basin margins and include eighteen well-exposed,

representative stratigraphic sections from shallow

water (Ghat area) to deep-water (Wadi Anlaline area)

areas with respect to the margins of an ancient

Paleozoic depocenter selected for this study (Fig.

1B). The Al-Qarqaf study area was situated in an

intermediate position relative to the margin of the

ancient depocenter (Fig. 1B). Additionally, nearly 150

m of cores from two boreholes drilled in the basin

from Concession NC174 (Fig. 1B) were logged and

correlated with the surface sections. Sedimentological

Africa

Possible South Pole

Baltica

Laurentia

close to 65 S

possible ice-sheet extension boundary

The Murzuq Basin

Fig. 3. Paleogeographic map showing location of the Murzuq Basin, and the ice-sheet extension during the upper Ordovician (Modified after

Ghienne, 1998).

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173150

analyses were carried out on these outcrop sections and

drilled cores that resulted in recognition of different

lithofacies (Table 1). These lithofacies are grouped into

facies associations, facies succession and depositional

sequences (Table 2). Terms such as glacial, paraglacial

and/or postglacial are used herein within facies

succession to refer to sediment packages that were

deposited during glacial advance and retreat, after

glacier retreat and, post-glacial when the area was not

influenced anymore by direct glacial processes.

4. Facies associations and depositional

environments

Sedimentological analysis of the selected outcrop

sections and drill cores revealed nine facies associa-

tions within the upper Ordovician succession (e.g.

FA1-FA9; Table 2). These associations represent

sharp, transitional and intercalated series of individual

facies that record specific depositional environments

(Fig. 4).

4.1. Facies associations

4.1.1. Facies association 1 (FA1, upper shoreface)

4.1.1.1. Description. Facies association 1 (FA1; ca.

10 m thick) is restricted to the base of the lower part of

the Melaz Shuqran Formation (Fig. 4), and occurs in

outcrops of the deep-water areas (i.e. Wadi Anlaline

area; Fig. 1B) only. FA1 tends to form laterally

extensive bodies (ca. hundreds of meters wide) of

fine-to-medium-grained, trough cross-stratified (St)

sandstone beds (Fig. 5A). These beds are 1–1.5 m

thick and display low angle, tangentially dipping

foresets. The sandstone beds are devoid of mudstones,

and the bed tops are sparsely bioturbated by Skolithos

ichnofabric of Arenicolities, Diplocraterion and

Skolithos (Fig. 5A). This facies association rests upon

Isos

tatic

rebo

und

cycl

e

Sequ

ence

Stra

ta

F

acie

sA

ssoc

iati

ons

F

acie

s su

cces

tion

Form

atio

n

Scal

e in

(m

)

Lit

holo

gy Grain sizeSedimentary structure

Facies Codes

F S CDm&

HST

Mel

az S

huqr

an

CS

TST

mfs

SB

FA 2

FA 3

SB

FA 1

TST

LST

SB

Mam

uniy

at

HST

TST

ts

mfs

Tanzuft Formation

FA 4

FA2

/ FA

5FA

6 /

FA 7

FA 8

LST

tsFA

9

II

II

I

0

100

150

200

250

300

50

1 2 3 4 5 6 7 8 9 10 11

Cambrian-Ordovician

FS-1

FS-2

FS-1

FS-2

FS-3

Firs

t gla

cial

/par

agla

cial

cyc

le

Seco

nd g

laci

al/p

arag

laci

al c

ycle

G

laci

al c

ycle

s

Murzuq Basin, SW - Libya

CS

I

Loadingpebbles

BioturbationStriation

Iron nodules

diamictite (Dm)

conglomerate (C)

sandstone (S)

fine grained (F)

Lithofacies

Legend

planar bedding

parallel lamination

current ripples

massive

wave ripples

trough cross-stratification

hummocky cross-stratification

syn-sedimentary deformationstructuresoutsized clastic

Sedimentary structure

1 = clay2 = silt3 = very fine sand4 = fine sand5 = medium sand6 = coarse sand

fine grained

sand

ston

eco

nglo

mer

ate7 = very coares sand

8 = granules9 = pebbles10 = cobbles11 = bouldres

Grain size

LST = Lowstand systems tracts

TST = Transgressive systems tracts

HST = Highstand systems tractsCS = Condensed sectionmfs = Maximum flooding surface

ts = Transgressive surface

SB = Sequence boundary

Sequence stratigraphy

Others

lenticular bedding

flaser bedding

mud drapes

Coarsing and shallowing

fining and deepning

Fig. 4. Schematic lithostratigraphic and sequence stratigraphic summary for the Melaz Shuqran and the Mamuniyat formations of upper

Ordovician glacial/glacial-related deposits of the Murzuq Basin, SW Libya. See Tables 1 and 2 for facies code.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 151

Table 1

Descriptive lithofacies abbreviations used in the upper Ordovician

glaciogenic formations, Murzuq Basin, SW Libya

Main lithofacies Sedimentary structure

Diamictite (Dm) Massive (m)

Conglomerate (G) Stratified (s)

Sandstones (S) Cross stratification (cs)

Fine grained (F) Trough cross stratification (t)

Hummocky cross stratification (hcs)

Parallel lamination (pl)

Parallel bedding (pb)

Laminated (l)

Wavey ripples (wr)

Current ripples (cr)

Bioturbated massive (bm)

Bioturbated parallel lamination (bpl)

Bioturbated trough cross stratification (bt)

Flaser (f)

Lenticular (ln)

Deformed (d)

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173152

Cambrian–Ordovician strata, and has basal lag

deposits, which are ca. 0.75–1 m thick (Fig. 6A).

These lags are composed of conglomeratic to

medium-grained sandstones with a rippled uppermost

layer.

4.1.1.2. Interpretation. The laterally extensive

bodies of trough cross-stratified sandstones with

Arenicolities, Diplocraterion and Skolithos and lack

of mudstones are interpreted to be deposited above

fair weather wave base as upper shoreface settings

(Fig. 7A). The sparse bioturbation possibly indicates

low-water temperature in ice-influenced areas (c.f.

Table 2

Description of depositional settings, and facies successions in upper Ordo

Formation Facies successions Basal bounding surface Facies

associa

Mamuniyat FS-3 Isostatic

rebound

Transgressive lags (ts) FA 9

Uplift erosion surface (SB) FA 8

FS-2 Paraglacial 2 Maximum flooding

surface (mfs)

FA 7

FA 6

FS-1 Glacial 2 Transgressive surface (ts) FA 5

Deep incision surface (SB) FA 4

Melaz Shuqran FS-2 Paraglacial 1 Maximum flooding

surface (mfs)

FA 3

FS-1 Glacial 1 Transgressive lags (ts)+

subglacial erosion

surface (SB)

FA 2

FA 1

See Table 1 for facies code.

Syvietski and Farrow, 1989). Deposition of the basal

lag is attributed to a marine transgressive event.

Therefore, FA1 records sedimentation above fair-

weather wave base as upper shoreface settings.

4.1.2. Facies association 2 (FA2, lower shoreface–

offshore with IRD)

4.1.2.1. Description. Facies association 2 (FA2, ca.

15–20 m thick) occurs in the lower part of the Melaz

Shuqran and lower Mamuniyat formations (Fig. 4),

and is well preserved in outcrops throughout the

studied areas (Fig. 1B). In the deep-water areas (i.e.

Wadi Anlaline area), this facies association rests on

FA1. The rocks of this association tend to form

laterally extensive, bodies of muddy and silty

diamictites (Fig. 5A). The diamictites are composed

of clasts supported in a mud and/or silt matrix (Fig.

6B). The clasts in the diamictites occur as scattered

subrounded to well-rounded pebbles or boulders

(0.1–3 m in diameter) and/or rare poorly organized

conglomeratic layers. These clasts are composed of

intrabasinal lithologies of fine-to-coarse-grained

sandstones and, less commonly, extrabasinal granite,

gneiss, quartzite and schist, which rarely exhibit

striated and/or faceted features. The clasts display a

preferred horizontal orientation in the uppermost

part of this facies association in the shallow water

areas (i.e. Ghat area). The bases of the outsized

clasts rarely show evidence of loading into the

underlying strata. In the shallow water areas (i.e.

Ghat area), the muddy diamictites are overlain by a

vician glaciogenic formations, Murzuq Basin, SW Libya

tions

Dominant facies Depositional settings

St, Sbt Upper shoreface

Gm, Gt, Gpb, Sb, Stb Gilbert-type delta

Shcs, St Upper to lower shoreface

Sm, Spl, Spb, Scr, Swr Foreshore

Sc, Spl, Sm, Swr, Fl Tide-influenced estuarine

Gpf m-t, St, Spl, Spb, Sc, Sm, Fl Braided fluvial

Spb, Spl, Sm, Sd, Sbpl,Sf, Sln, Fl Deltaic

Dm (m), Dm (s), Shcs,

Swr, Fwr, Fl

Offshore glaciomarine

with ice rafted debris

Sbt, St Upper shoreface

111 2 3 4 5 6 7 8 9 10

Sm

Spl

Spb

Spb

Fld

Sbpl

Spl

SbplI

Spb

Spl

Spb

Sm

Sd

Sm

Spl

0

10

20

30

40

50

Sd

I

I

I

I

I

I

I

FA 3

Dm (m)

Dm (s)

Dm (m)

Dm (s)

Dm (s)

Swr

Spl

Fl

10111 2 3 4 5 6 7 8 9

St

Sbt

Sm

I

I

I

Cs-sr

20

0

10

30

FA 1

FA 2

A) B)

Spl

SmbFf

SmSm

Ff

Sm

Sm

FlSplSmFmSm

Sm

Sm

Fl

Ff

Sm

Sm-pl

Sm

Sm

Fl

Sm

Ff

Ff

SmSpl

Sm

Ff

Ff

Ff

Sm

SmFf

Fl

1 234 567891011

2370

2360

2350

2340

FA3

C)m

etre

s

met

res

met

res

Wadi Anlalin area Al-Qarqaf areaoil well B1

ig. 5. (A–B) Graphic logs showing the typical facies and facies associations of the Melaz Shuqran Formation in the studied outcrops. (C)

raphic logs showing the typical facies and facies associations 3 in the Melaz Shuqran Formation on the subsurface of the Murzuq Basin. See

ig. 1 for location, Fig. 4 and Tables 1 and 2 for facies code and legend.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 153

F

G

F

C

FA 2

FA 3

A

B

Fig. 6. First depositional sequence: (A) Base of the Melaz Shuqran

Formation at Wadi Anlaline area (i.e. deeper water areas, see Fig. 1

for location) showing ca. one meter thick, conglomeratic to

medium-grained sandstones with ripples at top, which are inter-

preted as transgressive lag deposited formed due to a relative sea-

level rise that was coeval with a glacial advance and loading the

shelf. This lag deposit is resting on Cambrian–Ordovician strata. (B)

Uppermost lower Melaz Shuqran Formation showing muddy and

silty diamictite deposits (FA2) with ice-rafted debris (arrows) up to

3 m in diameter. This facies association forms part of TST that has

been deposited during a relative sea-level rise coeval with glacier

advance and loading the shelf and subsequent glacial retreat. (C)

Upper Melaz Shuqran Formation showing extensive syn-deposi-

tional deformation features. This facies (FA3) is regarded as HST

that has been deposited in a paraglacial system.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173154

silty diamictite, and locally by layers of chaotic

clasts, which are traceable for ten of meters in

outcrops.

The diamictites are interbeded with thin beds of

mudstones and sandstones (5A). The latter comprising

fine-to-medium-grained, current rippled (Scr), hum-

mocky cross-stratified (Shcs) sandstone beds (0.1–

0.15 m thick; 5A). The sandstone beds fine upwards

from medium to fine-grained sands. The mudstones

have planer laminea (Fl), sparsely bioturbated (Fb),

and devoid of outsized clasts. In the shallow water

areas (i.e. Ghat area), these mudstones, which are

preserved in the uppermost part of this association,

have a lower surface lined by imbricate clasts (i.e.

shallow water areas).

In Wadi Anlalin area (i.e. deep water areas), the

rocks of this association rest on top of the underlying

FA1 with the boundary being marked locally by

loading, whereas in the shallow water areas (i.e. Ghat

area), the basal boundary is scoured and/or are

subglacially eroded surface on top of the Cambrian–

Ordovician strata (i.e. Ash Shabiyat and/or Hasawnah

formations). The upper boundary of this facies

association is caped by a thin Fe-rich layer of 0.1–

0.15 m thick.

4.1.2.2. Interpretation. The presence of striated and/

or faceted outsized clasts and distinct chaotic layers

that load and disrupt the underlying laminated mud-

stones are indicators of sedimentation in a glaciomar-

ine setting (c.f. Crowell and Frakes, 1971; Herbert,

1980; Eyles et al., 1998). The depositions of the

striated and/or faceted outsized clasts in mudstones

are interpreted to be ice-rafted debris (IRD) from

icebergs. Variations in the amount of IRD in the

diamictites are attributed to the rate of hemipelagic

clay sedimentation relative to deposition from ice-

bergs. The occurrence of relatively large amount of

IRD interbedded with thin layers of mudstones may

reflect higher water temperature during the summer

when increased melting of icebergs releases more

sediment into the ocean (c.f. Elverhoi and Henrich,

1996). Therefore, deposition of IRD exceeded the rate

of hemipelagic sedimentation. Conversely, the rela-

tively small amounts or absence of the IRD reflects

the lowering of water temperature during the winter so

that hemipelagic deposition exceeded the sedimenta-

tion of IRD. Diverse sources of IRD are indicated by

the diverse lithologies of these clasts.

In the shallow water areas (i.e. Ghat area), the local

preferred horizontal orientation and imbrication of the

relative sea level fa

ll Delta (FA 3)

Cambria

n-Ord

ovici

an

(Pre

-glac

ial S

trata

)

offshore mud (FA 2)

relative sea-level ris

e

NW

glacier advancelan

dward

icerbergs

subglacial erosion surface and subsidence

Cambr

ian-

Ord

ovici

an

(Pre

-gla

cial S

trat

a)

IRD

proglacial subsidence

isostatic rebound

B

A

offshore mud (FA 2)

Shoreface (FA 1)

Shoreface (FA 1)

Basindward

glacier retreat

Ghat area

Wadi Anlaline area

Al-Qarqaf area

Ghat area

Wadi Anlaline area

Al-Qarqaf area

Fig. 7. Schematic depositional model for first depositional sequence DS-1. (A) First glacial advance and isostatic loading the shelf that was

accompanied by a relative sea-level rise, which resulted in the formation of a subglacial erosion surface (SB) in ice-proximal and shallow water

areas and transgressive surface (ts) in ice-distal and deep water areas. The base of TST comprises upper shoreface deposits (FA1) that are

overlain by offshore–shoreface deposits including ice-rafted debris (FA2). (B) During glacial retreat, the system started to prograde due to a high

flux of sediment, which exceeded the rate of accommodation creation, resulting in the deposition of a HST comprising a tidal influenced-delta

(FA3).

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 155

relatively small outsized clasts ( ca. b0.1 m in

diameter) at the contact with mudstone facies in the

shallow water areas (i.e. Ghat area) suggest current

reworking after deposition (cf. Eyles et al., 1998)

during relative sea-level rise associated with deglaci-

ation (Miller, 1996; Fig. 7A). The absence of IRD

within the upper mudstone facies (above the imbri-

cated clasts) suggests that this facies was deposited

during final stages of glacier retreat (Fig. 7A).

In the diamictites, the hummocky cross-stratified

sandstones were deposited by strong oscillatory flows

under storm-dominated wave conditions (e.g. Makh-

louf, 2002). The effects of strong, storm-generated

currents are recognized on the surfaces by the

presence of wave-formed gravel ripples (cf. Eyles et

al., 1998) and imbricate clasts. Such facies are

typically deposited in water depths greater than those

of the trough cross-stratified sandstone facies (i.e.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173156

FA1; e.g. Walker and Plint, 1992), and may record

lower shoreface settings. Therefore, FA2 records

sedimentation below fair weather wave base in

transition offshore settings that was influenced by

the deposition of ice-rafted debris (IRD) from icebergs

(Fig. 7A).

4.1.3. Facies association 3 (FA3, deltaic)

4.1.3.1. Description. Facies association 3 (FA3) is

ca. 50-60 m thick and occurs in the upper part of the

Melaz Shuqran Formation (Fig. 4). The rocks of this

facies association crop out throughout the deep-

water areas (i.e. Wadi Anlaline area) and the

intermediate area (i.e. Al-Qarqaf area; Figs. 1B and

5B) and are recognized in the drill cores too (Fig.

5C). The rocks of FA 3 are comprised of ca. 0.25–

0.75 m thick beds of fine to medium-grained,

parallel bedded (Spb), trough cross-stratified (St),

and massive sandstones (Sm) that coarsen and

thicken upwards, and are interbeded with mudstone

facies (Fl; Fig. 5B and C) in the lower part of the

succession. These sandstones have bed tops that are

parallel laminated (Spl) near the base of the FA and

become wave rippled (Swr) upwards. Bioturbation in

sandstones within this facies association is rare. The

sandstone facies exhibit abundant mud drapes, and

have extensive syn-sedimentary deformation features

such as slumping and sliding (Fig. 6A) particularly

in the upper part of FA 3 (Fig. 5B). These facies and

features are also recognized in the drill cores, in

addition to 0.75–1 m thick, lenticular and flaser

bedded (Sln and Sf, respectively, Fig. 5C), fine-

grained sandstones and mudstones. The mudstone

facies (ca. 1–2.5 m thick) are planar laminated (Pl),

and interbeded with silty sandstones and sandstones,

and display rare syn-sedimentary deformations fea-

tures (Fig. 5B). FA3 in the Wadi Anlaline area (i.e.

deep water areas; Fig. 1B) rests on top of the ca.

0.1–0.15 m thick oxidized layer, whereas in the

intermediate area (i.e. Al-Qarqaf area), its basal

surface is erosional.

4.1.3.2. Interpretation. The presence of mud drapes,

flaser bedding, wavey ripples and lenticular bedding

suggest sedimentation in tidal influenced settings

(Reineck and Singh, 1980). The coarsening-upward

succession from lower energy, finer-grained deposits

(i.e. mudstones) to higher energy, coarser-grained

deposits are typically indicating deltaic settings (Fig.

7B). The presence of rare bioturbated sandstones and

the absence of IRD within FA3 suggest the amelio-

ration of depositional conditions from glacial domi-

nated to paraglacial dominated settings (Fig. 7B).

Conversely, the presence of syn-sedimentary defor-

mation features (e.g. slumps) is attributed to high

sedimentation rates induced by isostatic rebound of

the continental shelf in response to glacier retreat (Fig.

7B). Therefore, FA3 represents tide-dominated deltaic

deposits.

4.1.4. Facies association 4 (FA4, braided fluvial)

4.1.4.1. Description. Facies association 4 (FA4),

which is ca. 40–60 m thick, occurs in the lower part of

the Mamuniyat Formation (Fig. 4) and crops out

throughout the study area (Fig. 1B). The rocks of FA4

are composed predominately of massive to trough

cross-stratified, conglomeratic sandstones (Gpf m-t),

trough cross-stratified sandstones (St), planer-bedded

(Spb) to planar-laminated (Spl) and current-rippled

(Scr) sandstones, interbedded with mudstone facies

(Fl, Fm; Fig. 8A).

The conglomeratic sandstone facies (Gpf m-t)

occurs in the lower part of the lower Mamuniyat

Formation (Figs. 4 and 8A), and is comprised of

scattered clasts supported by conglomeratic sand-

stones to coarse-grained sandstones matrix (Fig. 9A

and B). The clasts range in size from granules to

large boulders (up to 3 m in diameter), vary in

shape from subrounded to well rounded, and are

composed predominantly of very fine-to-coarse-

grained sandstones and rarely granite. These clasts

are often intrabasinal, and are rarely striated and/or

faceted and display preferred orientation only in the

deep-water areas (i.e. Wadi Anlaline area; Fig. 1B)

where they form lateral continuous like layer and

decrease in size up to 10 mm in diameter basinward

(Fig. 9B).

Individual beds of massive conglomeratic sand-

stones and coarse-grained sandstones (ca. 2–5 m

thick) grade vertically and laterally into fine-to-

coarse-grained, trough cross-stratified and parallel

bedded to laminated sandstones (Fig. 8A). These

sandstones rarely pass upward into thin (N20 cm

thick), very fine-to-fine-grained, current-rippled and

0

10

20

30

40

50

60

0

10

1 2 3 4 5 6 7 8 91011

St-plFlSplSwr

St

Sm

Sg

1 2 3 4 5 6 7 8 9 1011

Sm

Spl

Sm

Swr

Spb

Spb

Spl

Swr

Spb

Swr

Spl

Spb

SplScr

SwrSpl

0

10

20

30

40

50

60

Spb

FA 6

FA 5

1 2 3 4 5 6 7 8 9 1011

Spl

St

Spl

Gpf

St

SHcsStSt

Sm

St?Sm

Spb

Scr

Spl

Sm

Spb

SmFlSplSt ?FlSmFlSplFlSplFlSt?

Fl

FA 4

A) B) C)

met

res

met

res

met

res

FA 2

Wadi Anlalin area Ghat area Al-Qarqaf area

Fig. 8. (A–F) Graphic logs showing the typical facies and facies associations of the Mamuniyat Formation in the studied outcrops. (G) Graphic

logs showing the typical facies and facies associations in the Mamuniyat Formation on the subsurface of the Murzuq Basin. See Fig. 1 for

location, Fig. 4 and Tables 1 and 2 for facies code and legend.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 157

1 2 3 4 5 6 7 8 910 11

Sbt

St0

10

Swr

0

Sbt

Sbt

Sb

Sbpb

GSbp

GStGSt

1 2 3 4 5 6 7 8 9 10 11

10

20

GSt

Sm

GSm

1 2 3 4 5 6 7 8 9 10 110

10

30

20

I

ShcsSpl

Sm

Spl

StFA 7

FA 8FA 9

D) Wadi Anlalin areaE) Ghat area F) Al-Qarqaf area

Tanzuft shale

Shcs

met

res

met

res

met

res

I

I

I

I

I

I

I

I

I

Fig. 8 (continued).

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173158

parallel-laminated sandstones that are interbedded

with laminated and current rippled, silty sandstones

and mudstones. The basal bounding surface of this

facies association has 50–70 m deep erosional

surfaces incised into the lower shoreface and offshore

deposits of FA2, and the deltaic deposits of FA3, and

pre-glacial Cambrian–Ordovician rocks (Fig. 9C).

4.1.4.2. Interpretation. The nature of the sandbod-

ies, which commonly fine upwards from basal erosion

surface that overlain coarse-grained sandstones with

little or no preserved fine-grained deposits, indicate

sedimentation in high-energy bedload braided streams

(Collinson, 1996). These fluvial deposits fill incised

valley as indicated by the large and deep erosional

surface that cuts down into underlying rocks (i.e. FA2

and FA3 of the Melaz Shuqran Formation and/or

preglacial Cambrian–Ordovician strata). These incised

valleys are believed to have been formed during

relative sea-level fall. The presence of striated (Fig.

9A) and faceted outsized clasts within the lower part

of this facies association suggests a glacial origin. The

interbedded mudstones and fine-grained, wave-

rippled, and laminated sandstones are interpreted to

be floodplain deposits. The upward fining of the

sandstones, disappearance of the striated, and facetted

outsized clasts may indicate that deposition occurred

during glacier decay.

4.1.5. Facies association 5 (FA5, estuarine)

4.1.5.1. Description. Facies association 5 (FA5) is

ca. 15–20 m thick, overlies FA4 (Fig. 4), and is

preserved in the shallow water areas (i.e. Ghat area)

and intermediate area (i.e. Al-Qarqaf area; Fig. 1B).

The base of this facies association is marked by multi-

scoured channels defined at their base by the presence

of pebble lags, overlain by fining upward succession

from medium-to-fine-grained sandstone interbedded

with mudstones (Fig. 8A). These sandstones are

comprised of fine- to medium-grained, trough cross-

stratified (St), massive (Sm), and wave rippled sand-

1700

1690

1680

Csm

SplSplSbSbt

Sbt

Sbt

StSbSbFlSmCsg

CsmFlCsmCsm

Csm

CsmSmCsmCsmSmCsm

CsmCsmSm

CsmSwrSpl

SplSm

Sm

Sm

SplSwrSmSpl

Spl

Sm

1740

Sm

Spl

Spl

Sm

SplFlSplSmSmFlSmSmSm

Sm

SmSm

SwrSpl

1730

1720

Sm

SmSmSmSmSplSm

Sm

Sm

SmSm

Spl

SwrSmSm

Spl

1710

1780

1750

1770

Sm

Swr

Sm

Sm

Sm

Sm

Sm

SmSmSmSpl

Sm

Sm

SmSmSm

Spl

Spl

Spl

Scr

Sm

1790

Sm

Sm

Sm

Sm

Sm

SmSmSm

Sm

Spl

Spl

1760

1 2 3 4 5 6 7 8 91011

FA 6

FA 6

FA 6

FA 8

G) oil well F2m

etre

s

not c

ored

Fig. 8 (continued).

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 159

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173160

stones (Figs. 8A and 9A). The sandstones contain

abundant mud drapes. The mudstones display dis-

continues laminated and weak massive structure.

C

A B

FA 4

FA 2

FA 4

E

ice-rafted debris

FA 6

FA 4

F

4.1.5.2. Interpretation. The small scoured channel,

which has basal erosional surface with lags followed

by fining upward succession that overlain braided

FA 4

FA 5

D

4 m

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 161

fluvial deposits in incised-valley landward position is

interpreted to have been in estuarine setting. The

presence of mud drapes and discontinuous laminated

muds indicate tidal influence (Nichols, 1999). How-

ever, there is no evidence of reverse flow as might be

expected in tidal influenced settings due to the

presence of multi-channels, which force the ebb and

tidal currents to follow different pathways. Thus, the

rocks FA5 record sedimentation in tide-dominated

estuarine environment (Fig. 10B).

4.1.6. Facies association 6 (FA6, foreshore)

4.1.6.1. Description. Facies association 6 (FA6),

which is up to 70 m thick, occurs within the middle

part of the Mamuniyat Formation, and occurs in the

intermediate area (i.e. Al-Qarqaf area) and in the drill

cores (Figs. 1B and 4). The rocks of this association

tend to form laterally extensive sandstone bodies,

which comprise 0.5–1 m thick beds of medium-to-

coarse-grained, with flat to low angle cross-stratifica-

tion (Scs) and planar bedding (Sp) sandstones. The

bed tops in the upper part of this association (Fig. 8C

and G) are, in some cases, fine-grained parallel

laminated (Spl), wave rippled (Swr), and current

rippled (Scr) sandstones. The rocks of this association

coarsen upwards from fine-to-medium-grained sand-

stones, and are devoid of mudstones. The basal

surface of this facies association with the underlying

FA5 is erosional.

4.1.6.2. Interpretation. The dominant of planar to

low angle cross-stratified, medium-to-coarse-grained

sandstones indicate sedimentation above fair weather

wave base in foreshore settings (Nichols, 1999). The

lack of mudstones within this facies association

indicates deposition under high-energy current, in

shallower water compared to those, which have

hummocky and trough cross-stratified sandstones.

Fig. 9. Second depositional sequence: (A and B) the lower Mamuniyat Form

in diameter) supported by conglomeratic and coarse-grained sandstone m

shallower water areas, see Fig. 1), or form laterally extensive units in t

association represents braided fluvial sediments of LST deposited duri

represented by FA2 below and DS-2 represented above (arrows), which for

The boundary is interpreted as SB. This surface also separates the Melaz Sh

of lower Mamuniyat Formation (FA5) showing tidal influenced estuarine se

area (shallow water areas, see Fig. 2). This facies association forms par

Formation. (F) Flame structure preserved within foreshore sediments (FA6

4.1.7. Facies association 7 (FA7, upper to lower

shoreface)

4.1.7.1. Description. Facies association 7 (FA7),

which is up to 40 m thick, occurs in the middle part of

the Mamuniyat Formation (Fig. 4), in the deep-water

areas (i.e. Wadi Anlaline area; Fig. 1B). The rocks of

this association tend to form laterally extensive sand

bodies. The sandstone beds are ca. 0.2 m up to 1 m

thick that are fine to medium-grained, trough cross-

stratified (St) and hummocky cross-stratified (Shcs)

with rare parallel laminated (Spl) at tops (Fig. 8D).

The rocks of this association coarsen upwards from

fine to medium-grained sandstones, and rests on top

of FA4 with a sharp contact. The sandstone beds are

characterized by sharp base surface with local loading

features with the underlying fine-grained sediments

(Fig. 9F).

4.1.7.2. Interpretation. The occurrence of trough

and hummocky cross-stratified sandstones suggests

that FA7 was deposited above the storm wave base in

upper to lower shoreface settings (Fig. 10C), under the

influence of high-energy, oscillatory flow during

storm events (Makhlouf, 2002).

4.1.8. Facies association 8 (FA8, Gilbert-type delta)

4.1.8.1. Description. Facies association 8 (FA8),

which is ca. 15 to 20 m thick, forms the upper part of

the Mamuniyat Formation (Fig. 4), and occurs in the

shallow water areas (i.e. Ghat area) and in the drill

cores (Fig. 1B). The rocks of this association are

composed of massive conglomeratic sandstones

(Gb), and trough cross-stratified, conglomeratic

sandstones (Gt) that pass upwards into bioturbated,

trough cross-stratified sandstones (Sbt; Fig. 8E and

G). This facies association has an erosional uncon-

formity with the underlying FA2 in the shallow

ation (FA4) showing the outsized intraformational clasts (up to 3 m

atrix. These deposits are localized outcrops in the Ghat area (i.e.

he Al-Qarqaf area (i.e. intermediate areas, see Fig. 1). This facies

ng glacial stillstand and retreat. (C) the boundary between DS-1

med during glacial advance and the associated relative sea-level fall.

uqran Formation from the Mamuniyat Formation. (D) The upper part

diments resting on top of braided fluvial sediments (FA4) in the Ghat

t of TST. (E) Subglacial striated surface within lower Mamuniyat

) that form part of a HST.

Cambria

n-Ord

ovici

an

stra

ta an

d firs

t glci

al

braided fluvial (FA 4)

Tidal influencedEustuarine (FA 5)

B

relative sea-level fall

Cambria

n-Ord

ovici

an

strat

a and fi

rst g

lcial

Shoreface (FA 7)

foreshore (FA 6)

C

relative sea level fall

due to glacial advance

landward

Cambria

n-Ord

ovici

an

strat

a and fi

rst g

lcial

A

subglacial erosion surfacebasinward

relative sea-level rise

due to ice melt

braided fluvial (FA 4)

glacier retreat

Ghat area

Wadi Anlaline area

Al-Qarqaf area

glacier advance

NW

Fig. 10. Schematic depositional model for the second depositional sequence DS-2 in the upper Ordovician glaciogenic deposits. (A) Second

glacial advance accompanied by relative sea-level fall, which resulted in the development of subglacial erosion surface (SB) that bounds the

base of the FA4. This erosional surface is regarded as a SB. (B) The incised-valley fills, which was developed above the SB, comprise a LST of

braided fluvial deposition (FA4) that are overlain by a TST comprised of tidal-influenced estuarine (FA5) sandstones. Deposition of LST and

TST occurred during a relative sea level rise associated with deglaciation. (C) The system started to prograde as the rate of sediment supply

exceeded the rate of accommodation creation, which caused the deposition of paraglacial HST comprised of foreshore–shoreface sediments

(FA6 and FA 7).

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173162

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 163

water areas (i.e. Ghat area; Fig. 11A), and progrades

away from palaeohighs with a dip angle N158. Thesandstone beds are medium- to coarse-grained,

massive to trough cross stratified, and extensively

bioturbated (Fig. 11B and C).

4.1.8.2. Interpretation. The prograding and coars-

ening-upward succession of coarse-grained deposits

with dips N158 from palaeohighs suggest that the

rocks of FA8 represent Gilbert-type delta setting

(Fig. 12). The presence of a paleohigh with a basin-

margin gradient is believed to be formed as a result

of isostatic rebound of the continental shelf that

postdates the glacier retreat (Sutcliffe et al., 2000a,b;

Fig. 12). The lack of glacial features and the

presence of bioturbation suggest amelioration of the

depositional environment from glacial, paraglacial to

postglacial environments. The erosional surface with

the lack of any incision and the presence of this

facies association on top of FA2 (Fig. 11A) in the

shallow water areas (i.e. Ghat area; Fig. 1B) implies

the removal of the lower and middle part of the

Mamuniyat Formation, which probably occurred

during uplift of the continental shelf as a result of

isostatic rebound during deglaciation (Sutcliffe et al.,

2000a).

4.1.9. Facies association 9 (FA9, upper shoreface)

4.1.9.1. Description. Facies association 9 (FA9), ca.

10 m thick, which occurs in the intermediate area (i.e.

Al-Qarqaf area; Fig. 1B), forms the uppermost part of

the Mamuniyat Formation (Figs. 4 and 11D) and rests

on top of lag deposits. FA9 tends to form laterally

extensive bodies of fine to medium-grained sand-

stones. These sandstones, which show large-scale,

trough cross-stratifications (St), occur as beds up to

0.75 m thick that are associated with rare wave rippled

surfaces (Fig. 11D). The uppermost part (ca. 1 m

thick) of this association comprises fine-grained,

bioturbated sandstones that show small-scale, trough

cross stratifications (Sbt) and herringbone cross

stratifications (Sh; Figs. 8F and 11C). These sand-

stones are covered by a distinctive hardground (12 cm

thick) of iron-oxide rich, very fine-grained sandstones

(Fig. 11D). The sandstones of FA9 thin and fine

upwards from medium-to-fine-grained sandstones

(Fig. 8F).

4.1.9.2. Interpretation. The presence of the laterally

extensive bodies of bioturbated, trough cross-stratified

sandstones and the absence of mudstones within FA9

suggest deposition above fair-weather, wave-base in

upper shoreface environments (Fig. 12). The occur-

rence of herringbone cross stratifications at the top of

this facies association indicates tidal influenced shore-

face settings. The thin distinctive iron-oxide rich

hardground (12 cm thick) of very fine-grained sand-

stones is attributed to be formed owing to sediment

starvation associated with a late Ordovician/early

Silurian global sea-level rise.

4.2. Paleoenvironments of the late Ordovician in the

Murzuq Basin

Upper Ordovician sediments of the Murzuq Basin

were deposited during the late Ashgillian glaciation of

Gondwana when the late Ordovician ice sheet was

centered over central Africa and expanded outward

onto the surrounding continental shelves during

extraordinarius zone of the early Hirnantian (Sutcliffe

et al., 2000a,b). The wide variety of the upper

Ordovician facies associations in the basin provides

important information on the depositional conditions

across a glaciated and glacial-related, paralic con-

tinental shelf at high latitudes.

The trough cross-stratified sandstones of FA1

record deposition above fair weather wave base in

upper shoreface environments close to an ice margin

(Fig. 7A). The overlying FA2 is dominated by

deposition of IRD and hemipelagic mud in offshore

to lower shoreface environments (Fig. 7A). The

transition from FA1 to FA2 represents a glacio-marine

transgressive event that resulted from isostatic loading

of the shelf during glacial advance and subsequent

glacial retreat (Figs. 7A and 13). The thin iron-oxide

rich layer on top of FA2 indicates sediment starvation

concomitant with relative sea-level rise during glacier

retreat (c.f. Miller, 1996), and thus represents the final

stage of glacial retreat event (Fig. 13). FA3 is

dominated by mud drapes, parallel to cross-bedded

and bioturbated sandstones that are interbedded with

mudstones, which show an increase in the sand/mud

ratio upwards and record deposition in tide-dominated

delta (Fig. 7B). Syn-sedimentary deformation feature

are common (Fig. 7C) and reflect high sedimentation

rates and deposition on unstable paleohigh. The

A

D

FA 2

FA 8

Tanzuft shale Formation (Lower Silurian)

Mamuniyat Formation

E

B

FA 8

FA 9

FA 9

C

0.5 metre

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173164

relative sea level rise

time 2

landward

basinward

fi

rst a

nd/or se

cond

g

lacial

depos

its

shoreface (FA 9)

rela

tive

sea

leve

l fal

ldu

e to

isos

tatic

upl

ift

Gilbert delta (FA 8)

time

1

Uplift erosion surface due to rebound

Ghat area

Al-Qarqaf area

Fig. 12. Schematic depositional model for the third depositional sequence DS-3 in the upper Ordovician glaciogenic sediments. Isostatic

rebound was accompanied by a relative sea-level fall, which resulted in the formation of an uplift erosion surface due to rebound, the base of the

FA8. This erosional surface is regarded as a SB and is overlain by LST of Gilbert-type deltas capped by a TST of shoreface sandstone (FA9).

These TST sediments were deposited during a global sea-level rise during late Ordovician–early Silurian time.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 165

substantial differences in the sedimentological char-

acter between FA2 and FA3 suggest changes from a

glacial to paraglacial setting and amelioration in the

depositional condition (Figs. 7A,B and 13).

Conglomeratic sandstones of FA4 with outsized

clasts (Fig. 9A and B) and trough cross-stratified

sandstones grading upward into parallel bedded and

current rippled sandstones that are interbedded with

mudstones, are interpreted to be the braided fluvial

infill of an incised valley (Fig. 10A and B). The

significant change in the sedimentological character

from FA3 and FA4 and the presence of deep incised

valley are attributed to change from paraglacial into

glacial conditions (Figs. 7B and 10A and B), and thus

record the onset of new glacial cycle (Fig. 13). The

presence of soft sediment striation surfaces above the

main subglacial erosion surface (i.e. at the base of the

incised-valley) is not considered evidence of a new

glacial surface and hence a new glacial cycle, because

Fig. 11. Third depositional sequence: (A) The boundary between DS-3 repr

is an unconformity shown by the absence of DS-2. This erosional surface su

deglaciation, and is regarded as a SB. (B) Gilbert-delta deposits of the lowe

(shallow water areas, see Fig. 2) that prograde away from paleo-highs of

isostatic rebound coeval sea-level fall. These deposits form part of a LST.

very coarse-grained sandstones. (D) Upper shoreface deposits (FA9) of th

(intermediate area, see Fig. 2) that was formed during the latest Ordovician

boundary (arrows) is marked by condensed section. (E) Close up view of FA

implies deposition under tidal processes.

these surfaces are limited to the ice-proximal area and

have no impact on the changes in the relative sea level

and thus represent a local glacial re-advance. FA5

comprises wave rippled and cross-stratified sand-

stones that contain mud drapes, which indicate

deposition in tidal influenced environment (Figs. 8D

and 11B). Therefore, the boundary between FA4 and

FA5, which is marked by pebble lags, represents

marine transgression (TS) during deglaciation. FA6,

which is dominated by parallel bedded sandstones

and current rippled sandstones, is interpreted to

indicate deposition in foreshore environments (Fig.

10C). The presence of hummocky and trough cross-

stratified sandstones within FA7 is interpreted to

indicate deposition in upper to lower shoreface

environments (Fig. 10C). Therefore, the gradual

change from FA4 to FA7 indicates a gradual change

in the depositional condition from glacial to para-

glacial (Figs. 4 and 13).

esented by FA9 above and DS-1 represented by FA2 below (arrows)

ggests erosion of the DS-2 during the time of isostatic rebound after

rmost upper Mamuniyat Formation (FA8), preserved in the Ghat area

the uplifted Melaz Shuqran Formation. FA8 was deposited during

(C) Close up view of FA8, which is composed of conglomeratic to

e upper part of upper Mamuniyat Formation in the Al-Qarqaf area

–earliest Silurian global sea-level rise, and forms part of a TST. The

9 showing herringbone trough cross-stratification sandstone, which

II

II

? ?

?

I

I

I

I

I

I

I

I

I

I

CS

transgressive lags

1 2 3 4 5 6 7 8 9 10110

50

100

150

200

Sequ

ence

Stra

ta

Form

atio

n

Scal

e in

(m)

Lithology

Grain sizeSedimentary structure

Facies CodesF S CDm&

HST

Mel

az S

huqr

anM

amun

iyat

TST

mfs

Tanzuft

FA

2 F

A 3

TST

LST

SB

HST

TST

ts

mfs

FA

4 F

A 5

FA

6 F

A 9

II

II

FS-3

FS-1

FS-2

FS-1

Fa

cies

Ass

ocia

tions

Faci

es S

uce.

Al - Qarqaf Arch area

SB

1 2 3 4 5 6 7 8 9 10 110

50

HST

mfs

Mel

az S

huqr

anM

amun

iyat

100

150

TST

LST

ts

SB

HST

mfs

FA 7

FA 4

FA 3

FA 2

FA

1

Cambrian-Ordovician

FS-1

FS-2

FS-1

FS-2

Wadi Anlaline area

Sequ

ence

Stra

ta

Form

atio

n

Scal

e in

(m)

Lithology

Grain sizeSedimentary structure

Facies CodesF S CDm&

Faci

esA

ssoc

iatio

ns

Faci

es S

uce.

1 2 3 4 5 6 7 8 9 10 110

50

Ghat area

Sequ

ence

Stra

ta

Fa

cies

Ass

ocia

tions

Faci

es S

uce.

Form

atio

n

Scal

e in

(m)

Lithology

Grain sizeSedimentary structure

Facies CodesF S CDm&

TST

Melaz

Shuq

ranM

amun

iyat

FA

2

LST

TST

LST

ts

SB

FA

8FA

5 F

A 4

FS-3

FS-1

FS-1

Cambrian-Ordovician

SB

SB

? ?

?

BasinwardLandward

TST

SB+tsI

Loadingpebbles

BioturbationStriation

Iron nodules

diamictite (Dm)

conglomerate (C)

sandstone (S)

fine grained (F)

Lithofacies

Legend

planar bedding

parallel lamination

current ripples

massive

wave ripples

trough cross-stratification

hummocky cross-stratification

syn-sedimentary deformationstructuresoutsized clastic

Sedimentary structure

1 = clay2 = silt3 = very fine sand4 = fine sand5 = medium sand6 = coarse sand

fine grained

sand

ston

eco

nglo

mer

ate7 = very coares sand

8 = granules9 = pebbles10 = cobbles11 = bouldres

Grain size

LST = Lowstand systems tracts

TST = Transgressive systems tracts

HST = Highstand systems tractsCS = Condensed sectionmfs = Maximum flooding surface

ts = Transgressive surface

SB = Sequence boundary

Sequence stratigraphy

Others

I

I

I

I

I

I

I

I

CS

I

CS

transgressive lags

FS-2

lenticular bedding

flaser bedding

mud drapes

?

?

?

Coarsing and shallowing

fining and deepning

FA 2

Fig. 13. Sequence stratigraphic correlation of the upper Ordovician glaciogenic deposits in the Murzuq Basin of Southwestern Libya. See Fig. 1 for location, Tables 1 and 2 for facies

code and legend. Scale in meters.

M.AliKalefa

El-g

hali/Sedimentary

Geology177(2005)145–173

166

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 167

FA8, which is dominated by trough cross-stratified,

conglomeratic sandstones and extensively bioturbated

trough cross-stratified sandstones that prograde away

from palaeohighs record deposition as Gilbert-type

delta on an erosional surface (Figs. 11A and 12).

These deposits imply significant shallowing across

this unit. FA9, which is dominated by large scale,

trough cross-stratified sandstones, records deposition

in upper shoreface environments (Fig. 12). FA8 and

FA9 together record deposition during a period of

isostatic rebound (Figs. 12 and 13).

5. Facies succession: recognition of glacial,

paraglacial and postglacial isostatic rebound

Based on the interpretations of facies association

and the nature of the boundaries that separate them

(Fig. 13), the upper Ordovician deposits of the

Murzuq Basin display changes in depositional con-

ditions that reflect changes in the rates of sediment

supply and in relative sea level owing to glacial

advance and retreat and concomitant loading and

rebound of the continental shelf. The boundaries

between facies associations are either gradational or

sharp (Fig. 13). The recognition of these major

bounding surfaces at the base of some facies

associations allows recognition of the base of facies

successions (i.e. packages of genetically strata or

allomembers; Walker, 1992), which record large-scale

changes in relative sea level and deposition in glacial,

paraglacial and postglacial settings. Three different

types of facies successions have been recognized

including glacial, paraglacial, and isostatic rebound

cycles (Fig. 13 and Table 2).

5.1. Facies succession 1 (FS-1, glacial deposits)

Two examples of facies succession 1 (FS-1) occur

within the upper Ordovician glaciogenic rocks of the

Murzuq Basin that reflect two cycles of glaciations.

The first glacial cycle (Figs. 4 and 13) comprises FA1

and FA2 and correlates with the lower part of the

Melaz Shuqran Formation. The second glacial cycle

(Figs. 4 and 13) comprises FA2, FA4 and FA5, and

corresponds to the lower part of the Mamuniyat

Formation. The first example of FS-1 represents a

gradual deepening upward succession that deepens

from upper shoreface (FA1) to offshore (FA2) during

the first glacial advance and isostatic loading the shelf

and subsequent glacial retreat (Fig. 7A). The second

example of FS-1 records a shift from braided fluvial,

incised-valley fills (FA4) into tidally influenced

estuarine environments in shallow water areas and

into offshore in deep water areas, during second glacial

advance and subsequent retreat (Fig. 10A and B).

In the first glacial cycle, the basal boundary is a

transgressive lag in deep-water areas that may

correlate with a subglacial erosion surface in shallow

water areas (Figs. 7A and 13). This basal boundary

was formed during a time of glacial advance, loading

the continental shelf and concomitant relative sea-

level rise. The overlying deposits of FA2 were

accumulated during glacial advance and subsequent

glacial retreat. In the second glacial cycle, the basal

boundary of FS-1 is interpreted to represent a

subglacial erosion surface that was formed during

glacial advance (Figs. 11A and 13) in the shallow

water areas, while the overlying deposits were

accumulated during glacial standstill and subsequent

retreat (Figs. 7A,B and 11A,B).

The upper boundary of FS-1 is interpreted as MFS

that was formed during maximum sea level rise due to

glacial retreat, and marks the end of glacial cycles and

the onset of the paraglacial cycle (Fig. 13). The

difference in the sedimentological character of FS-1

between the first and second glacial cycle implies that

the extent of glacial advance was greater during

second glacial cycle, or that glacial retreat was faster

during the first glacial cycle.

5.2. Facies succession 2 (FS-2, paraglacial deposits)

Two examples of facies succession 2 (FS-2) that

reflect two paraglacial cycles in the Murzuq Basin

(Figs. 4 and 13) occur within the late Ordovician

glaciogenic rocks. FS-2 consists of FA3 in the first

paraglacial cycle (Figs. 4 and 13) that correlates with

the upper part of the Melaz Shuqran Formation, and of

FA6 and FA7 in the second paraglacial cycle that

correlate with the middle part of the Mamuniyat

Formation (Figs. 4 and 13).

FS-2 represents a shallowing upward succession as

evidenced by an overall prograding and coarsing

upward of tide-dominated deltaic deposits (FA3) in

the first paraglacial cycle, and progradational fore-

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173168

shore-shoreface deposits (FA6 and FA8) in the second

paraglacial cycle. The basal boundary of FS-2 is

interpreted to represent MFS, which separates the

deepening upwards succession below from the shal-

lowing upwards succession above. This MFS marks

the end of glacial cycle and the onset of deposition of

paraglacial cycles (Fig. 13). These two paraglacial

deposits are omitted in the shallow water areas (i.e.

Ghat area) due to erosion during second glacier

advance and isostatic rebound (Fig. 13).

5.3. Facies succession 3 (FS-3, isostatic rebound

deposits)

Facies succession 3 (FS-3) occurs above the

second paraglacial cycle, within the upper Mamuniyat

Formation, and consists of Gilbert-type delta (FA 8)

and upper shoreface deposits (FA9, Figs. 4 and 13).

The basal erosional boundary of this facies succession

is interpreted to represent uplift and erosion of the

underlying strata during isostatic rebound (Sutcliffe et

al., 2000a; Fig. 12) and a concomitant relative sea-

level fall that favored deposition of the Gilbert-type

deltaic sediments (FA8). Subsequent global sea-level

rise during the late Ordovician to early Silurian

resulted in the deposition of overlying upper shoreface

sediments (FA9; Fig. 12). The upper boundary of FS-

3 is marked by a distinctive hardground horizon (12

cm thick) of Fe-oxide rich, very fine-grained sand-

stones (Figs. 9D and 13). The top of this bed marks

the boundary between the upper Ordovician and the

overlying hot shale of the lower Silurian Tanzuft

Formation (Fig. 9D).

6. Sequence stratigraphy of the late Ordovician

glacial deposits

An attempt is made below to extend and apply the

methodology of sequence stratigraphy, which were

originally developed for non-glaciated paralic deposits

(e.g. Posamentier et al., 1988a,b; Galloway, 1989; Van

Wagoner et al., 1990; Einsele et al., 1991), to the

upper Ordovician glaciogenic sediments of the Mur-

zuq Basin. The sequence stratigraphic framework

proposed for this basin is based on careful sedimento-

logical and stratigraphic analysis in both outcrop

sections and borehole drilled cores.

Three depositional sequences, which represent

amalgamated units of different facies successions

have been recognized within the upper Ordovician

succession, which contains sequence boundaries,

maximum flooding surfaces and systems tracts

defined for each depositional sequence. Sequence

boundaries are placed at regional unconformities that

have resulted from subglacial or uplift erosion, while

maximum flooding surfaces are placed at surfaces that

define the maximum marine incursion and/or by the

presence of condensed section in the distal basinward

areas.

6.1. Depositional sequence 1

Depositional sequence 1 (DS-1) corresponds to the

entire Melaz Shuqran Formation (Fig. 4), and is

composed of: (i) transgressive systems tract (TST)

comprised of upper shoreface, lower shoreface, and

offshore deposits (i.e. FS-1), and (ii) highstand

systems tract (HST) with deltaic deposits (i.e. FS-2).

The sequence is bounded below by a subglacial

erosion surface that was formed in shallow-water

areas and is correlated with a transgressive surface

(TS) in deep-water areas and at its top by a subglacial

erosion surface. These two subglacial erosion surfaces

are the sequence boundaries of this depositional

sequence (SB; Fig. 13).

Depositional sequence 1 was deposited during a

period of overall transgression related to relative sea-

level rise during glacial advance and loading the

continental shelf and subsequent glacial retreat (Fig.

9A and B). The rise in the relative sea level during

glacial advance is attributed to the slower rate of

eustatic sea level fall than isostatic loading. The

timing of changes in the relative (i.e. transgression–

transgression) relative to glaciation and deglaciation

are very difficult to predict, because of isostatic effects

associated with the same glaciation–deglaciation

event. However, relative sea level rise owing to

glacial advance and loading of the continental shelf

is anticipated to occur subsequent to eustatic sea level

fall during glaciation (Miall and Turner-Keizer,

personal communication, 2004). The formation of a

regional subglacial erosional surface at the base of

DS-1, which cuts into the lower to middle Ordovician

formations (i.e. the Hawaz and the Hasawnah

formations) in the shallow water areas (i.e. Ghat area;

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 169

Figs. 1B and 13) during glacial advance is also

associated by erosion of the sediment that is expected

to be deposited in front of the glacier. Thus, the

continental and shallow marine LST deposits are not

preserved in the stratigraphic record of the basin. The

subglacial erosion surface extends and merges with a

transgressive surface marked by lag deposits in deep

water areas (i.e. Wadi Anlalin area; Figs. 1B and 13),

which were formed during initial stages of relative sea

level rise concomitantly with glacial advance and

loading of the shelf.

Deposition of the early TST occurred as a

consequence of relative sea level rise caused by

glacial advance and loading of the continental shelf,

and deposition of upper shoreface sandstones on top

of the transgressive lag deposits (FA1; Figs. 6A and

13). Further rise in the relative sea level occurred as a

consequence of melting of the ice sheet during glacial

retreat, which resulted in the deposition of late TST

that is comprised of silty and muddy diamictite

alternating with layers of current-rippled and hum-

mocky cross-stratified, fine-grained sandstones (FA2;

Fig. 13), and finally mudstones. Rising relative sea

level during glacial retreat (Miller, 1996) was asso-

ciated with sediment starvation in the distal shelf areas

(i.e. deep water areas; c.f. Loutit et al., 1988) and

resulted in the deposition of ca. 0.1–0.15 m thick, of

thin Fe-oxide rich layer of very fine-grained sand-

stones (Figs. 4 and 13). This Fe-oxide layer is

interpreted to represent a condensed section, which

has traditionally been correlated with the maximum

flooding surfaces (Emery and Myers, 1996).

The occurrence of prograding deltaic deposits on

retrogradational, shoreface to offshore deposits sug-

gests regression above the maximum flooding surface.

These regressive deposits, which have been accumu-

lated when the rate of sediment supply ultimately

outstripped the rate of relative sea level rise and

accommodation creation is interpreted to be HST

deposits (FA3; Fig. 9B).

6.2. Depositional sequence 2

Depositional sequence 2 (DS-2) corresponds to the

lower and middle part of the Mamuniyat Formation

(Fig. 4), and is composed of: (i) lowstand systems

tract (LST) comprised of braided fluvial deposits, (ii)

transgressive systems tract (TST) comprised of

estuarine deposits (e.g. FS-1), and (iii) high systems

tract (HST) comprised of foreshore to shoreface

deposits (e.g. FS-2, Figs. 10A,B,C and 13). The

sequence is bounded at its base by a subglacial

erosion surface and its top by uplift erosion surface

(Figs. 4 and 13). These erosional surfaces represent

the sequence boundaries of this depositional sequence

(Fig. 13).

The overall formation and infilling of the incised

valley are consistent with a rapid relative sea level fall

followed by a stillstand and then a slow progressive

rise (e.g. Allen and Posamentier, 1993). The initial

rapid relative sea level fall is believed to have

occurred due to glacial advance and resulted in valley

incision (Smart, 2000; Deynoux and Ghienne, 2004;

Le Heron et al., 2004), and the formation of regional

unconformity (SB) at the base of the valley (Fig. 10A)

because the rate of eustatic sea level fall was faster

than isostatic loading. The valley has incised into the

TST of DS-1 and/or the middle Ordovician formations

(i.e. the Ash Shabiyat and Hawaz formations, Figs. 10

and 13). Deposition of LST occurred due to stabiliza-

tion of the relative sea level at the glacial maximum,

and the subsequent sea level rise was a result of

glacial retreat, which favored the deposition of

braided-fluvial conglomeratic sandstones and parallel

to trough cross-stratified, medium-to-coarse-grained

sandstones.

When the rate of relative sea level rise and

accommodation creation ultimately outstripped the

rate of sediment supply, the shelf was transgressed

and the incised valley was converted into an estuary

(Fig. 10B), and the fluvial deposits were subjected to

reworking by tidal currents. These tidal reworking

processes resulted in the deposition of transgressive

lag deposits at the base of estuary and formation of

transgressive surface (Fig. 13). Thus, the tidally

influenced, incised valley deposits mark the onset of

overall transgression (e.g. Allen and Posamentier,

1993) and the onset of deposition of the TST in DS-2

(Fig. 13). The TST sediments are dominated by wave

rippled and cross-stratified, fine-to-medium-grained

sandstones with mud drapes, and are intercalated with

mudstone (FA5; Figs. 6B and 7D).

The shift from retrogradational estuary into pro-

gradational foreshore to shoreface deposits has been

used as a criterion to recognize the MFS (Emery and

Myers, 1996), and the beginning of overall shoreline

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173170

regression, at the base of the HST in DS-2 (Figs. 4 and

13). These sediments were formed when the rate of

sediment supply ultimately outstripped the rate of

accommodation creation due to increasing sediment

supply from the ice melting (Fig. 10C). Deposition of

the HST is dominated by parallel, wave and current

rippled, fine-to-medium-grained sandstones of pro-

gradational foreshore, and trough cross-stratified and

hummocky cross-stratified, fine-to-medium-grained

sandstones of progradational lower to upper shoreface

deposits (FA6 and FA7).

6.3. Depositional sequence 3

Depositional sequence 3 (DS-3) corresponds to the

upper part of the Mamuniyat Formation (Fig. 4), and

represents: (i) a lowstand systems tract (LST) that is

comprised of Gilbert-type deltaic deposits, and (ii) a

transgressive systems tract (TST) that is comprised of

upper shoreface deposits. This sequence is bounded at

its base by an erosional surface (SB) and at its top by

condensed section that marks the boundary between

the Mamuniyat Formation below and the Silurian

Tanzuft Formation above (Fig. 13). The initial rapid

fall in relative sea level is believed to be a

consequence of isostatic rebound, which postdated

the last glacial retreat (e.g. Sutcliffe et al., 2000a,b)

and resulted in the formation of regional unconformity

at the base of DS-3 (Fig. 12). This erosional surface

extends down to the lower part of the Melaz Shuqran

Formation (i.e. TST of DS-1) and is devoid of

incision, and thus suggesting removal of the under-

lying facies subsequent to deglaciation and rebound of

the shelf (Figs. 8A and 12).

Subsequent to isostatic rebound, erosion and

formation of a SB, deposition of the LST occurred

during relative-sea level lowstand, which favored the

deposition of Gilbert-type deltaic sediments that

include trough cross-stratified, conglomeratic sand-

stones and bioturbated, trough cross-stratified sand-

stones (FA 8; Fig. 13). As a result of subsequent

relative sea level rise during the late Ordovician and

early Silurian, the accommodation creation ultimately

outstripped the rate of sediment supply and result in

the deposition of retrogradational upper shoreface

deposits (Fig. 12) that marked the onset of the TST in

DS-3 (Fig. 13). The TST within DS-3 comprises

trough cross-stratified, fine-to-medium-grained sand-

stones (FA 9; Fig. 13). Further rising in the relative

sea level was associated with sediment starvation in

the basinward area and resulted in the deposition of

distinctive thin hardground layer. This hardground is

interpreted to be a condensed section (Fig. 13), which

marks the top of the Mamuniyat Formation. The

condensed section is traditionally correlated with the

maximum flooding surfaces (Emery and Myers,

1996).

7. Implication for hydrocarbon exploration

The rocks of the Melaz Shouqran and Mamuniyat

formations represent the main hydrocarbon in the

Murzuq Basin. Therefore, applying sequence strati-

graphic concepts should help to better elucidate and

predict the spatial and temporal distribution of the

depositional facies, and hence of seal and reservoir

rocks. LST and HST deposits are often important

hydrocarbon reservoir targets because they are com-

monly composed of coarse-grained, and thus porous

and permeable deposits (Posamentier and Allen,

1999). Most discoveries of hydrocarbon in the upper

Ordovician rocks occurred in the tide-dominated,

deltaic HST sandstones of the DS-1, and foreshore

to shoreface HST and Gilbert-type deltaic LST sand-

stones of the DS-2 and DS-3. However, successful

exploration of these reservoir rocks in the basin can be

achieved when the impact of changes in the relative

sea level and glacier movements are considered,

which leads to successful prediction of the spatial

and temporal distribution of the depositional facies,

and hence reservoir rocks.

8. Comparison with other sequence stratigraphic

models

Comparison between the sequence stratigraphic

model proposed here and other models for glaciated

basins are of profound important in order to gain better

understanding of facies distribution as a result of

changes in relative sea level owing to glacier move-

ments and their impact on the spatial and temporal

distribution of reservoir and seal rocks. The model

proposed here agrees with that presented by McDou-

gall and Martin (2000) regarding consideration of; (i)

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 171

the Melaz Shouqran Formation (i.e. DS-1) as one

depositional sequence formed during relative sea level

rise due to glacial advance loading the continental

shelf and subsequence melting of glacier during

retreat, and (ii) the upper Mamuniyat Formation (DS-

3) as one depositional sequence, which was formed

during isostatic rebound associated with relative sea

level fall and formation of LST sandstones overlain by

TST that formed as a result of global sea level rise

during early Silurian. The disagreement between these

two models is limited to the lower and middle

Mamuniyat Formation. The model in our study regards

the lower and middle Mamuniyat Formation as one

depositional sequence (DS-2), whereas McDougall

and Martin (2000) divided them into three depositional

sequences. Observations of our study do not count the

presence of any subglacial erosion surface as a major

sequence boundary unless it occurs as regional surface,

because such surfaces are localized and preserved in

the ice-proximal areas, which represent local glacial

re-advance of glacier.

The model of this study has several similarities to

the model proposed by O’Brien et al. (1998) for the

Permian glacial succession in Canning Basin, Western

Australia. These authors conclude that: (i) extreme

glacial advance can produce relative sea level at

highstands because the ice sheet loading of the crust

even though eustatic sea level is at lowstand and result

in deposition of TST and HST, which is similar to the

formation of DS-1 in the Murzuq Basin, and (ii)

Glacial advance produces relative sea level fall and

major erosion surface that cuts downward into under-

lying deposits, followed by relative sea level rise

owing to glacial retreat and deposition of LST, TST

and HST, which is similar to the formation of DS-2 in

the Murzuq Basin.

9. Conclusion

This study shows that the spatial and temporal

distribution of upper Ordovician deposits in the

Murzuq Basin can be constrained within a sequence

stratigraphic framework. Important findings of this

work include:

(1) Basin-fill sedimentary facies architecture of

glacial, paraglacial and postglacial deposits

were strongly controlled by changes in the

relative sea level that were induced by

glacial advance and retreat and concomitant

loading and unloading of the continental

shelf and changes in the rate of sediment

supply.

(2) The glaciogenic deposits have three prominent,

regional erosion surfaces that were formed

during glacial advance and/or isostatic rebound,

concomitant with major relative sea level fall.

These erosional surfaces mark the base of three

distinct depositional sequences.

(3) The first depositional sequence (DS-1) was

formed during a time of overall transgression.

Rising relative sea level was caused by glacial

advance and loading the continental shelf (i.e.

the rate of eustatic sea level fall was slower than

isostatic loading) and subsequent glacial retreat,

which resulted in the deposition of transgressive

systems tract (TST).

(4) A further rise in the relative sea level during

glacial retreat was associated with sediment

starvation in the deep-water areas and the

formation of a condensed section that correlates

with a MFS. Progradation of the highstand

systems tract (HST) sediments occurred above

the maximum flooding surface (MFS) when the

rate of sediment supply exceeded the rate of

relative sea level rise.

(5) Deposition of the second depositional sequence

started during an overall relative sea level fall

associated with glacial advance (i.e. rate of

eustatic sea level fall was faster than isostatic

loading) that caused the incision of DS-1 and

the formation of the sequence boundary (SB).

(6) Subsequently, the glacial retreat resulted in

relative sea level rise. Deposition during a glacial

stillstand favored the deposition of braided

fluvial deposits within the incised valley

(LST). The subsequent retreat results in depo-

sition of an estuarine transgressive systems

tract sediment.

(7) Progradation of foreshore to shoreface deposits

occurred when the rate of sediment supply

exceeded the rate of relative sea level rise.

These progradational deposits, which cover the

TST represent regression above MFS and

interpreted to belong to HST.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173172

(8) Deposition of the third depositional sequence

occurred during isostatic rebound that postdated

glacial retreat and was concomitant with a rela-

tive sea level fall and the formation of a se-

quence boundary, followed by slow relative sea

level rise and deposition of Gilbert-type deltaic

sediments of LST. Subsequent relative sea level

rise during late Ordovician to early Silurian re-

sulted in the deposition of shoreface TST

sediments.

Acknowledgment

I would like to thank my supervisor Professor Sa-

doon Morad for his critical reading of several versions

of the manuscript. Special thanks go to the Petroleum

Research Center, Tripoli Libya, especially Professor

Ali Sbeta, Dr. A. Bourima and Dr. A. El-Harbi for

supporting the field trip. I also would like to thank my

colleagues K. Ashibani, J. Mayouf, A. Abd-Alkariem

and A. Bedada who accompanied me to the field.

Thanks also go to the National Oil Corporation es-

pecially B. El-Mejrab and data section for providing us

with access to the drill core samples. Thanks go to the

editor, editor assistant and the reviewers for their

comments, which have profound improvement on the

manuscript.

References

Abugares, Y.I., Ramaekers, P., 1993. Short notes and guidebook on

the Palaeozoic geology of the Ghat area, SW Libya, Field trip,

October 14–17, 1993. Earth Science Society of Libya. Interprint

Ltd., Malta. 84 pp.

Allen, G.P., Posamentier, H.W., 1993. Sequence stratigraphy and

facies models of an incised valley fill: the Gironde Estuary,

France. J. Sediment. Petrol. 63, 378–391.

Beuf, S., Biju-Duval, B., DE Charpal, O., Rognon, P., Gariel, O.,

Bennacef, A., 1971. Les gres du Paleozoique inferieur du

Sahara. Sci. Tech. Petrole. Editions Technip, Paris, 18, 464 pp.

Blanpied, C., Deynoux, M., Ghienne, J.F., Rubino, J.L., 2000. Late

Ordovician glacially related depositional systems of the Gargaf

Uplift (Libya) and comparisons with correlative deposits in the

Taoudeni Basin (Mauritania). In: Sola, M.A., Worsley, D.

(Eds.), Symposium on Geological Exploration in Murzuq Basin.

Elsevier, Amsterdam, pp. 485–507.

Boulton, G.S., 1990. Sedimentary and sea level changes during

glacial cycles and their control on glacimarine facies architec-

ture. In: Dowdeswell, J.A., Scourse, J.D. (Eds.), Glacimarine

Environments: Processes and Sediments, Special Publication,

vol. 53. Geological Society of London, pp. 15–52.

Collinson, J.D., 1996. Alluvial sediments. In: Reading, H.G. (Ed.),

Sedimentary Environments: Processes, Facies and Stratigraphy,

3rd edition. Blackwell Science Publication, Oxford, United

Kingdom. 706 pp.

Crowell, J.C., Frakes, L.A., 1971. Late paleozoic glaciation: Part IV.

Australia. Geol. Soc. Amer. Bull. 82, 2515–2540.

Davidson, L., Beswetherick, S., Craig, J., Eales, M., Fisher, A.,

Himmali, A., et al., 2000. The structure, stratigraphy and petroleum

geology of the Murzuq Basin, southwest Libya. In: Sola, M.A.,

Worsley, D. (Eds.), Symposium on Geological Exploration in

Murzuq Basin. Elsevier, Amsterdam, pp. 295–320.

Deynoux, M., 1998. Contribution to the stratigraphy and sedimen-

tology of the upper Ordovician- glacial formations of the

Murzuq Basin, Libya. Unpublished report, CNRS-EOST. Univ.

Louis Pasteur, Strasbourg. 52 pp.

Deynoux, M., Ghienne, J., 2004. Late Ordovician glacial pavement

revisited: a reappraisal of the origin of striated surface. Terra

Nova 16, 95–101.

Echikh, J., Sola, M.A., 2000. Geology and hydrocarbon occurrences

in the Murzuq basin, SW Libya. In: Sola, M.A., Worsley, D.

(Eds.), Symposium on Geological Exploration in Murzuq Basin.

Elsevier, Amsterdam, pp. 175–222.

Einsele, G., Ricken, W., Seilacher, A., 1991. Cycles and events in

stratigraphy—basic concepts and terms. In: Einsele, G., Ricken,

W., Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. St.

John’s: Springer-Verlag, pp. 1–19.

Emery, D., Myers, K.J., 1996. Sequence stratigraphy. B.P explora-

tion. Blackwell Press, London. 297 pp.

Elverhoi, A., Henrich, R., 1996. Glaciomarine environments

ancient glaciomarine sediments. In: Manses, John (Ed.), Past

Glacial Environments: Sediments, Forms and Techniques,

Glacial Environments, vol. 2. Butterworth-Heinemann Ltd,

Oxford, UK, pp. 179–211.

Eyles, N., 1993. Earth’s glacial record and its tectonic setting. Earth

Sci. Rev. 35, 1–248.

Eyles, N., Eyles, C.H., 1992. Glacial depositional systems. In:

Walker, R.G., James, N.P. (Eds.), Facies Models: Response to

Sea Level Change. Geological Association of Canada, St John’s,

pp. 73–100.

Eyles, C.H., Eyles, N., Gostin, V.A., 1998. Facies and allostrati-

graphy of an early Permian polar continental margin: southern

Sydney Basin, Australia. Sedimentology 45, 121–161.

Framca, A.B., Potter, P.E., 1991. Stratigraphy and reservoir potential

of glacial deposits of the Itare Group (Carboniferous to Permian),

Parana basin, Brazil. AAPG Bull. 75, 62–85.

Galloway, W.E., 1989. Genetic stratigraphic sequence in basin

analysis I: architecture and genesis of flooding surface bounded

depositional units. AAPG Bull. 73, 125–142.

Grubic, A., Dimitrijevic, M., Galecic, M., Jakovljevic, Z., Komar-

nicki, S., Protic, D., et al., 1991. Stratigraphy of western Fezzan

(SW Libya). In: Salem, M.J., Belaid, M.N. (Eds.), The Geology

of Libya, vol. IV. Academic Press, London, pp. 1529–1564.

Grunow, A.M., 1999. Gondwana events and palaeogeography: a

palaeomagnetic review. J. Afr. Earth Sci. 28, 53–69.

M. Ali Kalefa El-ghali / Sedimentary Geology 177 (2005) 145–173 173

Herbert, C., 1980. Evidence of glaciation in the Sydney Basin

and Tamworth synclinorial zone. Geol. Surn. NSW Bull. 26,

275–293.

Kent, D.V., Van der Voo, R., 1990. Palaeozoic paleogeography from

palaeomagnetism of Atlantic bordering continents. In: McKer-

row, W.S., Scotese, C.R. (Eds.), Palaeozoic Paleogeography and

Biogeography Geol. Soc. Mem., vol. 12, pp. 49–56.

Le Heron, D., Sutcliffe, O., Bourgig, K., Craig, J., Visentin,

C., Whittington, R., 2004. Sedimentary architecture of

upper Ordovician tunnel valleys, Gargaf Arch, Libya:

implications for a genesis of a hydrocarbon reservoir.

GeoArabia 9, 137–160.

Levell, B.K., Braakman, J.H., Rutten, K.W., 1988. Oman glacia-

tions well log response. AAPG Bull. 72, 775–796.

Loutit, T.S., Hardenbol, J., Vail, P.R., Baum, G.R., 1988.

Condensed sections: the key to age dating and correlation

of continental margin sequence. In: Wilgus, C.K., Hastings,

B.S., Kendall, C.G.S.C., Posamentier, H.W., Ross, C.A., Van

Wagoner, J.C. (Eds.), Sea-level Changes: An Integrated

Approach SEPM Society For Sedimentary, Geology Special

Publication, vol. 42, pp. 183–216.

Makhlouf, I., 2002. Sea-level fluctuations and storm-wave influence

on Ordovician shelf sediments, Jordan. Littoral III, 319–324.

Mamgain, V.D., 1980. The pre-Mesozoic (pre-Cambrian to Paleo-

zoic) stratigraphy of Libya: a reappraisal. Bulletin, vol. 14. IRC,

Tripoli, Libya. 104 pp.

Marini, I.P., Glooschenko, W.A., 1985. Cold climate peat formation

in Canada and its relevance to lower Permian coal measures of

Australia. Earth Sci. Rev. 22, 107–140.

McDougall, N., Martin, M., 2000. Facies models and sequence

stratigraphy of upper Ordovician outcrops, Murzuq Basin,

Libya. In: Sola, D., Worsley, D. (Eds.), Symposium on

Geological Exploration in Murzuq Basin. Elsevier, Amsterdam,

pp. 223–236.

Miller, J.M.G., 1996. Glacial sediments. In: Reading, H.G. (Ed.),

Sedimentary Environments: Processes, Facies and Stratigraphy.

Blackwell science, pp. 454–484.

Nichols, G., 1999. Sedimentology and stratigraphy. Blackwell

Science Publication. 355 pp.

O’Brien, P.E., Lindsay, J.F., Knauer, K., Sexton, M.J., 1998.

Sequence stratigraphy of a sandstone-rich Permian glacial

succession, Fitzroy Trough, Canning Basin, Western Australia.

Aust. J. Earth Sci. 45, 533–545.

Pierobon, E.S.T., 1991. Contribution to the stratigraphy of the

Murzuq Basin, SW Libya. In: Salem, M.J., Belaid, M.N.

(Eds.), The Geology of Libya, vol. V. Elsevier, Amsterdam,

pp. 1767–1783.

Posamentier, H.W., Allen, G.P., 1993. Variability of the sequence

stratigraphic model: effects of local basin factors. Sedimentol.

Geol. 86, 91–109.

Posamentier, H.W., Allen, G.P., 1999. Siliciclastic sequence

stratigraphy—concepts and applications. Special Publication,

vol. 7. Society of Economic Palaeontologists and Mineralogists.

210 pp.

Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988a. Eustatic controls

on clastic deposition. I—Conceptual framework. In: Wilgus,

C.K., et al. (Eds.), Sea Level Changes: An Integrated Approach,

Special Publication, vol. 42. Society of Economic Palaeon-

tologists and Mineralogists, pp. 109–124.

Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988b. Eustatic controls

on clastic deposition. II—Sequence and systems tract models.

In: Wilgus, C.K., et al. (Eds.), Sea Level Changes: An Integrated

Approach, Special Publication, vol. 42. Society of Economic

Palaeontologists and Mineralogists, pp. 125–154.

Powell, R.D., Cooper, J.M., 2000. A glacial sequence

stratigraphic model for temperate, glaciated continental

shelves. Special Publication, vol. 203. Geological Society,

London, pp. 215–244.

Reineck, H.E., Singh, I.B., 1980. Depositional environments, 2nd

ed. Springer-Verlag, New York. 549 pp.

Schwarzacher, W., 1993. Cyclostratigraphy and the Milankovitch

theory. Developments in Sedimentology, vol. 52. Elsevier,

Amsterdam. 140 pp.

Scotese, C.R., Boucot, A.J., McKerrow, W.S., 1999. Gondwana

paleogeography and palaeoclimatology. J. Afr. Earth Sci. 28,

99–114.

Smart, J., 2000. Seismic expression of depositional processes in

the upper Ordovician succession of the Murzuq Basin, SW

Libya. In: Sola, M.A., Worsley, D. (Eds.), Symposium on

Geological Exploration in Murzuq Basin. Elsevier, Amsterdam.

p. 397–415.

Smith, A.G., 1997. Estimates of the Earth’s spin (geographic) axis

relative to Gondwana from glacial sediments and palaeomag-

netism. Earth Sci. Rev. 41, 161–179.

Sutcliffe, O.E., Harper, D.A.T., Ait Salem, A., Whittington, R.J.,

Craig, J., 2000a. The development of a typical Hirnantian-

brachiopod fauna and the onset of glaciation in the late

Ordovician of Gondwana. Trans. R. Soc. Edinb. Earth Sci. 92,

1–14.

Sutcliffe, O.E., Dowdeswell, J.A., Whittington, R.J., Theron, J.N.,

Craig, J., 2000b. Calibrating the Late Ordovician glaciation and

mass extinction by the eccentricity cycles of the Earth’s orbit.

Geology 18, 967–970.

Syvietski, J.P.M., Farrow, G.E., 1989. Fjord sedimentation as an

analogue for small hydrocarbon-bearing fan deltas. In: Whate-

ley, M.K.G., Pickering, K.T. (Eds.), Deltas Sites and Traps for

Fossil Fuels. Geological Society of London, pp. 21–43. Special

publication.

Unrug, R., 1996. The assembly of Gondwanaland: scientific results

of IGCP project 288: gondwanaland sutures and mobile belts.

Episodes 19, 11–20.

Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahmanian,

V.D., 1990. Siliciclastic sequence stratigraphy in well logs, cores,

and outcrops: concepts for high-resolution to correlation of time

and facies. American Association of Petroleum Geologists

methods in Exploration Series, vol. 7. 55 pp.

Walker, R.G., 1992. Facies, facies models and modern stratigraphic

concepts. In: Walker, R.G., James, N.P. (Eds.), Facies Models:

Response to Sea Level Change. Geological Association of

Canada, St. Johns, Newfoundland, pp. 1–14.

Walker, R.G., Plint, A.G., 1992.Wave- and storm-dominated shallow

marine systems. In: Walker, R.G., James, N.P. (Eds.), Facies

Models: Response to Sea Level Change. Geological Association

of Canada, St. Johns, Newfoundland, pp. 219–238.