Facies analysis and palaeoclimatic signiﬁcance of ironstones … 2020-05-31 · terrestrial...

Facies analysis and palaeoclimatic significance of ironstonesformed during the Eocene greenhouse

WALID SALAMA*† , MORTADA EL AREF* and REINHARD GAUPP‡*Department of Geology, Faculty of Science, Cairo University, Giza, Egypt (E-mail: [email protected])†CSIRO Earth Science and Resource Engineering, ARRC, PO Box 1130, Bentley, WA, 6102, Australia‡Institute of Earth Sciences, Friedrich-Schiller University, Jena, Germany

Associate Editor – Peir Pufahl

ABSTRACT

Lower and middle Eocene ironstone sequences of the Naqb and Qazzun

formations from the north-east Bahariya Depression, Western Desert, Egypt,

represent a proxy for early Palaeogene climate and sea-level changes. These

sequences represent the only Palaeogene economic ooidal ironstone record of

the Southern Tethys. These ironstone sequences rest unconformably on three

structurally controlled Cenomanian palaeohighs (for example, the Gedida,

Harra and Ghorabi mines) and formed on the inner ramp of a carbonate plat-

form. These palaeohighs were exposed and subjected to subaerial lateritic

weathering from the Cenomanian to early Eocene. The lower and middle

Eocene ironstone sequences consist of quiet water ironstone facies overlain

by higher energy ironstone facies. The distribution of low-energy ironstone

facies is controlled by depositional relief. These deposits consist of lagoonal,

burrow-mottled mud-ironstone and laterally equivalent tidal flat, stromato-

litic ironstones. The agitated water ironstone facies consist of shallow subtid-

al–intertidal nummulitic–ooidal–oncoidal and back-barrier storm-generated

fossiliferous ironstones. The formation of these marginal marine sequences

was associated with major marine transgressive–regressive megacycles that

separated by subaerial exposure and lateritic weathering. The formation of

lateritic palaeosols with their characteristic dissolution and reprecipitation

features, such as colloform texture and alveolar voids, implies periods of

humid and warm climate followed major marine regressions. The formation

of the lower to middle Eocene ironstone succession and the associated later-

itic palaeosols can be linked to the early Palaeogene global warming and

eustatic sea-level changes. The reworking of the middle Eocene palaeosol and

the deposition of the upper Eocene phosphate-rich glauconitic sandstones of

the overlying Hamra Formation may record the initial stages of the palaeocli-

matic transition from greenhouse to icehouse conditions.

Keywords Bahariya Depression, climatic changes, Egypt, ooidal ironstones,palaeosols.

INTRODUCTION

The Eocene is one of the most significantCenozoic epochs, with climatic changes givingrise to the early Palaeogene greenhouse world,

a period characterized by high temperaturesand no ice sheets (e.g. Greenwood & Wing,1995; Zachos et al., 2001, 2008). During theearly Cenozoic, a series of short durationhyperthermal events (less than a few tens of

1594 © 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists

Sedimentology (2014) 61, 1594–1624 doi: 10.1111/sed.12106

thousands of years each) involved extreme glo-bal warmth and massive additions of carbondioxide to the atmosphere over a wide rangeof scales of forcing and response. These eventsinclude the Palaeocene–Eocene thermal maxi-mum (PETM; ca 55 Ma), the early Eocene Cli-matic Optimum (EECO; ca 51 to 53 Ma) andthe middle Eocene Climatic Optimum (MECO;ca 41�5 Ma). The PETM coincided with amajor perturbation of the carbon cycle, asindicated by a sharp negative carbon isotopeexcursion (e.g. Dickens, 1999). Microfossilrecords also show extinctions and diversifica-tions in pelagic and open marine ecosystems(e.g. Thomas, 1998; Kelly, 2002; Crouch et al.,2001). Larger foraminifera are the most com-mon constituents of upper Palaeocene–lowerEocene carbonate platforms, and this groupshows a turnover at the PETM (Scheibneret al., 2005). The PETM is simultaneouslymarked by a well-documented rejuvenation ofterrestrial vertebrates (Maas et al., 1995). Thesetransient events were followed by graduallycooling temperatures through the Eocene andthe development of permanent ice sheets onAntarctica around the Eocene–Oligoceneboundary (Zachos et al., 1996; Coxall et al.,2005). The long-term shift in Earth’s climaticstate resulted, in part, from differences involcanic emissions, which were particularlyhigh during parts of the Palaeocene andEocene epochs (ca 40 to 60 Ma). Changes inchemical weathering of silicate rocks and for-mation of palaeosols were also important (Kra-use et al., 2010). As the atmospheric CO2

concentration rose, temperature and precipita-tion increased and enhanced chemical weather-ing (Zachos et al., 2008).Cenozoic ooidal ironstones (COIS) are located

in 39 districts, mostly in the Northern Hemi-sphere between the equatorial zone and lati-tude 60°N (Van Houten, 1992; Petranek & VanHouten, 1997). The COIS and lateritic palaeo-sols are most abundant in the lower and mid-dle Eocene (ca 50%) because of the majorpalaeogeographic changes that occurred(Shackleton, 1986; Prothero et al., 1990; VanHouten, 1992; Retallack, 2010). The early andmiddle Eocene greenhouse world was markedby high sea-level, and most of the low-eleva-tion coastal plains, such as those on theAtlantic and Gulf Coast of North America orthe margins of Africa facing the TethysSeaway, were drowned (Prothero, 2003; Sluijset al., 2008). In North Africa, COIS occur in

mixed siliciclastic–carbonate sequences and areassociated with manganiferous mineralizationand lateritization (Petranek & Van Houten,1997). The main iron sources for the COISwere lateritic terrains; few sedimentarysequences contain beds of volcanic ash, but agenetic link to volcanism is evident (VanHouten, 1992; Petranek & Van Houten, 1997).This study presents a detailed description of a

shallow marine ironstone facies and sequencesof Eocene ironstones of the Bahariya Depressionin the Western Desert of Egypt. The objectives ofthis work are: (i) to develop a depositionalmodel for the Bahariya ironstones; (ii) to studydiagenetic and subaerial weathering processesaffecting the primary mineralogy and textures;and (iii) to infer the palaeoclimatic conditionsthrough marine and terrestrial proxies and sea-level changes prevailing during the early andmiddle Eocene in the Bahariya Depression.Results are considered within a global context.

STUDY AREA AND GEOLOGICALSETTING

The Bahariya district is of special interest due tothe presence of large reserves of high grade ironore (ca 380 million metric tons). The lower tomiddle Eocene ironstone succession of the Baha-riya Depression represents the only economicooidal ironstone record of the palaeo-Tethyanshoreline of early Cenozoic in North Africa andSouthern Europe (El Aref et al., 2001, 2006).These economic deposits represent the mainexploitable iron ore deposits of Egypt since1973.The Bahariya Depression lies between lati-

tudes 27°48′ and 28°30′N and between longi-tudes 28°35′ and 29°10′E. It is a large, ovalshaped NE-oriented depression in the centre ofthe Western Desert of Egypt (Fig. 1A). It islocated ca 370 km south-west of Cairo and190 km west of the Nile Valley. Its maximumlength from north-east to south-west is ca 94 kmand its greatest width is ca 42 km. It is sur-rounded on all sides by a plateau of highly kars-tified Cretaceous and lower to middle Eocenenummulitic limestones. The floor and the basalpart of the surrounding scarps of the BahariyaDepression are formed of lower Cenomaniansandstones and mudstones of the Bahariya For-mation. The Bahariya Depression is the onlylocality in the northern and central parts of theWestern Desert where the Cretaceous rocks are

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 61, 1594–1624

Palaeoclimatic significance of Eocene ironstones 1595

exposed as an inlier within the practically flat-lying Palaeogene rocks.The study area includes three iron ore mines

in the north-eastern part of the BahariyaDepression, i.e. the Ghorabi-Nasser (3�5 km2),Harra (2�9 km2) and Gedida (15 km2) (Figs 1Band 2). In each of the three mine localities, thelower to middle Eocene ironstone successionforms an unconformity bounded condensedsection (10 to 15 m thick) and shows a facieschange towards the surrounding, equivalentthick nummulitic dolomitic limestones (70 to80 m). The ironstone succession is composedmainly of autochthonous/para-autochthonousfacies, rich in ferruginous ooids, peloids, onc-oids and various ferruginized skeletal particles(Helba et al., 2001; El Aref et al., 2001, 2006).The Ghorabi-Nasser area is located at the

extreme north-eastern corner of the Bahariya

Depression ca 25 km west of the Bahariya-Cairoroad (Fig. 2). The Ghorabi mine is a plateau-likestructure and is completely separated from thesurrounding lower to middle Eocene carbonatescarp by deep structurally controlled valleys. Tothe north, the structurally controlled valleyseparates the Ghorabi mine from the scatteredhills of the Nasser area and the karstified Eocenecarbonates. In the south, the Ghorabi minedirectly overlooks the Bahariya Depression andthe surrounding valleys open into the maindepression (Fig. 2). It attains a maximumelevation of ca 316 m above sea-level The coreof the Ghorabi plateau consists of the lowerCenomanian Bahariya Formation, which isunconformably overlain by the lower to middleEocene ironstones.The Harra mine is located in the north-eastern

part of the Bahariya Depression, 14 km SW of

A

B

Fig. 1. (A) Geological map of the Bahariya Depression showing the distribution of the main rock units (modifiedafter Hermina et al., 1989; detailed structural elements are shown in Sehim, 1993). (B) Location map of the ironore mines.


1596 W. Salama et al.

the Ghorabi-Nasser area (Figs 1B and 2). Stratain the Harra mine consist mainly of the lowerCenomanian Bahariya Formation overlain bykarstified carbonates of the lower to middleEocene Naqb and Qazzun formations andQuaternary playa sediments.Morphologically, the Gedida mine is a semi-

closed depression and its central part is a highcentral area surrounded by marked structurallycontrolled valleys. These valleys separate thehigh central area from the surrounding karsti-fied lower to middle Eocene carbonate plateau.The high central area is built up of the lowerCenomanian Bahariya Formation, overlain bythe main lower to middle Eocene ironstones.In the eastern and western valleys, the iron oresuccession is truncated unconformably by theupper Eocene green glauconitic sands of theHamra Formation, Oligocene siliciclastics ofthe Qatrani Formation and Quaternary sanddeposits.

METHODS

Five stratigraphic sections were measured,described in detail and correlated. One sectionwas constructed for the eastern valley of theGedida mine and one for the Harra mine, as wellas three sections for the different sectors of theGhorabi mine (Fig. 2).The goal of the stratigraphic measurements is

to define the main ironstone units and discrimi-nate their lower and upper boundaries, internallithological subdivisions and discontinuity sur-faces, as well as their lateral and vertical facieschanges. The sedimentological analyses anddepositional environments of the different iron-stone facies were determined through detailedfield observations and megascopic and micro-scopic examinations. The terminology of theironstone microfacies was adopted from theclassification of Young (1989a). The petro-graphic analyses of the different ironstones were

The Ghorabi mine

The Harra mine

The Gedida mine Playa deposits

Bahariya Fm.

Karstified Carbonate plateauNaqb Fm.

Nasser

The Ghaziya area

Ghorabi sand dunes Abu Mohariqsand dunes

Bahariya Depression

250 m

Playa deposits

Bahariya Formation Bahariya

Depression

HamraFormation

NaqbFormation

The Harra mine

SC

N

SX

XX

X

Fig. 2. Satellite images showing the three iron ore mine areas; the Ghorabi, Harra and Gedida. These images alsoshow the main structural elements, the distribution of the main landforms and the locations of the measured sec-tions (X) in the Ghorabi and Harra mines.



performed on polished thin sections under atransmitted and reflected light microscope (Zeissmodel; Carl Zeiss AG, Oberkochen, Germany),with a Hitachi HV-C20 digital camera (Hitachi,Tokyo, Japan) attached.Samples from the different marine ironstone

facies and lateritic iron ores were collected fromthe three mine areas and investigated for theirmicro-morphological and nano-morphologicalcharacteristics, mineralogical composition andmicrobial forms. Carbon coated samples wereinvestigated by using a JEOL JSM-7001 fieldemission scanning electron microscope (SEM;JEOL Limited, Tokyo, Japan) with an energy-dispersive X-ray spectroscopy (EDAX) unitattached. The analyses were carried out in theOtto-Schott Institute of Glass Chemistry, Fried-rich-Schiller University, Jena, Germany.

STRATIGRAPHIC AND TECTONICFRAMEWORK

Deformation in the Bahariya district is related towrenching-type stress (Sehim, 1993, 2000;Moustafa et al., 2003). The Bahariya Depressionwas deformed by a NE right-lateral wrench sys-tem, accompanied by the development of severaldouble plunging folds and extensional faults(Sehim, 1993, 2000). Regional mapping of mostof the depression revealed the presence of threefault belts of ENE-trend (Fig. 1). The maximumdeformation is encountered in the two northernbelts of the Ghorabi-Ghaziya fault and theGedida-Harra fault (Figs 1 and 2). The areasaround the main faults show double plungingfolds represented by early Cenomanianpalaeohighs in the north-eastern plateau of theBahariya Depression. The Ghorabi, Harra andGedida mine areas represent three major earlyCenomanian palaeohighs that are aligned alongthe main NE wrench faults. The central parts ofthese palaeohighs are highly eroded, whereastheir flanks are preserved by normal faults form-ing horst structures. These faults divided theGhorabi mine into three main sectors; southern,central and northern. In the Gedida mine, thesefaults divided it into a high central area andeastern and western valleys. The Harra mine isbounded by two main faults trending NE–SWgiving it an extended graben form. This grabentraps the Hamra Formation and the underlyingiron ores. Quaternary playa deposits also occupythe lowland area of this graben. The wrenchdeformation happened mainly during the late

Cretaceous, but was occasionally re-activatedduring the late Eocene (Sehim, 1993; Moustafaet al., 2003). The reactivation process resultedin deformation of the lower to middle Eoceneironstones by a series of east–west normal faults(Fig. 3A).The Ghorabi-Ghaziya and the Gedida-Harra

structural belts played an important role in caus-ing the drastic variations in both facies and thick-nesses of the Eocene carbonates and ironstones.The lower to middle Eocene ironstone successionof the Bahariya Depression overlies differentstratigraphic horizons of the lower Cenomanianclastic sediments of the Bahariya Formation. Inthe southern and northern downthrown sectorsof the Ghorabi mine, the Bahariya Formationconsists of thickly bedded, white, kaolinitic andgreen glauconitic mudstones intercalated withglauconitic ironstone bands, comprising theupper member of the Bahariya Formation. In thecentral upthrown sector, the Ghorabi ironstonesuccession rests upon a thick succession of whitebarite-bearing and ferruginous fluvial sandstonesof the lower member of the lower CenomanianBahariya Formation. The Ghorabi ironstone suc-cession is exposed and not covered by the Hamraand Qatrani formations.In the Harra and Gedida mines, the ironstone

succession is unconformably overlain andtruncated by the Bartonian–Priabonian glauco-nitic sediments of the Hamra Formation and/orcross-bedded sandstones of the OligoceneQatrani Formation (Fig. 3B). The upper uncon-formable contact has a scouring relief reachingup to 5 m and is dominated by basal conglome-rates with locally reworked ironstone gravels aswell as reworked nummulitids and silicifiedlimestone nodules of the Qazzun Formation.The whole ironstone succession is channelledby cross-bedded fluvial sandstones of theOligocene Qatrani Formation, especially in theGedida mine (Fig. 3B).The ironstone succession consists of two main

(lower and upper) sequences. These twosequences correspond to the lower and middleEocene Naqb and Qazzun formations, respec-tively (El Aref et al., 1999; Helba et al., 2001).The lower boundary of the lower sequence isthe lower Cenomanian–lower Eocene unconfor-mity, whereas the upper boundary of the uppersequence is the middle–upper Eocene unconfor-mity (Fig. 3B). The two sequences are separatedby the lower–middle Eocene disconformity.The lower Cenomanian–lower Eocene boun-

dary is an angular unconformity that shows an



onlap relation between the Bahariya Formationand the overlying middle Eocene ironstones.This boundary is also marked by the presence ofa basal intraformational conglomerate layer thatis only 10 cm thick. It consists of moderatelysorted and matrix-supported rounded fragmentsof ooidal ironstones cemented by glauconiticclay matrix.The lower ironstone sequence displays lateral

and vertical variations in both thickness andfacies. These variations are related to the palaeo-topography of the underlying lower CenomanianBahariya Formation as well as the tectonic ele-ments that were active during the early Eocene.In contrast, the upper ironstone sequence showsa homogenous composition in all mines.In all of the mine areas, the lower and upper

ironstone sequences were exposed to subaerial

weathering during and after the Eocene andwere terminated with palaeosols. The weather-ing processes are more intensive on top of theupper ironstone sequences.

FACIES ANALYSES

The lower ironstone sequence

The lower ironstone sequence belongs to thelower Eocene Naqb Formation based on thebenthic foraminifera identified (Said & Issawi,1964). Based on the correlation scheme of theplatform, shallow benthic foraminiferal bio-zones (SBZ) and pelagic sequences for the Pal-aeocene–Eocene Tethys (Serra-Kiel et al., 1998;Boukhary et al., 2011), the lower Eocene ben-

A

B

Oligocene Qatrani FormationUpper Eocene Hamra Formation

Lower and middle Eocene ironstones

Lower and middle Eocene ironstones

Cenomanian Bahariya Fm.

Fig. 3. (A) Panoramic field view of the post-middle Eocene deformation of the middle Eocene ironstones in theextreme southern sector of the Ghorabi mine. (B) Stratigraphy of the eastern valley of the Gedida mine showingthe lower to middle Eocene ironstones overlain by the green sands of the upper Eocene Hamra Formation and thesandstones of the Oligocene Qatrani Formation.



thic foraminifera of the Naqb Formation areAlveolina ellipsoidalis (SBZ: 6), Nummulitesatacicus (SBZ: 8), N. subramondi (SBZ: 8 and9), Alveolina oblonga (SBZ: 10 and 11), N. cail-liaudi (SBZ: 10 to 12) and Alveolina frumenti-formis (SBZ: 12 and 13). The lower ironstonesequence is composed of two, three or all fourof the following facies in any given location: (i)mud-ironstone facies; (ii) stromatolitic ironstonefacies; (iii) fossiliferous ironstone facies; and(iv) nummulitic–ooidal–oncoidal ironstone facies(Fig. 4). The mud-ironstone and ooidal–oncoi-dal–nummulitic ironstone facies are the domi-nant facies in all mine areas (Figs 4 and 5). Thestromatolitic ironstone facies is laterally equiva-lent to the mud-ironstone facies in the centralsector of the Ghorabi mine (Fig. 4). In the south-ern sector of the Ghorabi mine, the lower iron-stone sequence is dominated by mud-ironstonefacies, fossiliferous ironstone facies and ooidal–oncoidal–nummulitic ironstone facies, whereasin the northern sector, it is dominated by mud-ironstone and nummulitic ironstone facies.

Facies 1: Mud-ironstone facies (1 to 4 m inthickness)

DescriptionThis facies rests on the lower Cenomanian–lower Eocene unconformity and is well-deve-loped in the basal part of the lower ironstonesequence in all mines. It shows lateral varia-tions in thickness and mineralogical composi-tion. The variation in thickness of this faciesis attributed to the palaeotopography of theBahariya Formation and the erosive nature ofthe overlying facies. The mud-ironstone faciesattains up to 4 m in thickness in the southernsector of the Ghorabi mine. It consists of bur-row-mottled sandy manganiferous mud-iron-stone in the basal part that grades upward intobedded brick red hematitic mud-ironstones(Fig. 6A and B). The hematitic mud-ironstoneshave fenestral cavities, rhizoliths and desicca-tion cracks. The desiccation cracks are 3 to5 cm wide, 5 to 10 cm deep, V-shaped andfilled with kaolinitic clays from the overlyingthin kaolinitic mudstone bed (Fig. 6B). In thenorthern sector of the Ghorabi mine, this faciesis up to 1 m thick and consists of nummuliticmanganiferous mud-ironstones. In the Ghorabimine, the mud-ironstone facies is overlain by abioturbated fossiliferous ironstone facies(Fig. 6C and D). In the Harra mine, the mud-ironstone facies attains up to 1�5 m in thickness.

It consists of burrow-mottled manganiferousmud-ironstone. In the Gedida mine, the mud-ironstone facies are thinly laminated.Microscopically, the manganiferous mud-iron-

stone facies consists of ellipsoidal aggregates ofhematite surrounded by nanocrystalline goethite(Fig. 7A). These aggregates are mixed withdetrital sub-angular sand-sized quartz, feldsparand heavy mineral grains (for example, rutile).Authigenic clay minerals such as kaolinitebooklets and illite flakes together with manga-nese minerals (for example, todorokite, birnes-site, aurorite, manjiroite and psilomelane) arediagenetically developed in the pore spacesbetween the ellipsoidal hematite aggregates(Fig. 7B to D). The identification of these man-ganese minerals by Raman Spectroscopy wasdescribed in Ciobot�a et al. (2012) and Salamaet al. (2012).

InterpretationThe fine grain size, mud-supported fabric,burrow-mottled textures and massive internalstructures are all indicative of deposition undergenerally quiet water conditions in lagoonaland tidal flat environments. The lagoonal envi-ronment was well-developed in low-lying areasbetween palaeohighs (i.e. the Ghorabi, Harraand Gedida mines). The burrow-mottled fabricof this facies indicates a low sedimentation rateduring deposition. The vertical gradation fromthe burrow-mottled to the brick red hematiticmud-ironstone with its characteristic desicca-tion cracks, rhizoliths and fenestral cavitiesindicates change through time from a lagoonalto a tidal flat depositional environment. Thepresence of such features strongly suggests sub-aerial emergence of the mud-ironstone facies,especially in the southern sector of the Ghorabimine. The northward lateral change towardsnummulitic mud-ironstone facies in thenorthern sector of the Ghorabi mine reflects atransition to open marine conditions. Theabsence of marine fauna in the mud-ironstonefacies in the southern sector of the Ghorabimine may indicate that the depositional basinreceived fresh water influx by surface run-offand/or groundwater.

Facies 2: Stromatolitic ironstone facies (1�5 to2 m thick)

DescriptionThe stromatolitic ironstone facies is well-deve-loped in the central sector of the Ghorabi mine.



Fig.4.Lithostratigraphy,faciesanalysesanddepositionalenvironments

ofthelowerto

middle

Eoceneironstonesu

ccessionoftheGhorabimine.



Green sand andmudstone with

patches of ironstone

Low

er E

ocen

eU

pper

Eoc

ene

The Harra mine section Eastern valley of the Gedida mine area

Middle Eocene

5

10

M W P G

5

10

M W P G

Bahariya Fm.

Ooi

dal-N

umm

uliti

c iro

nsto

ne fa

cies

Seq.

Mud

-iron

ston

e

Low

er ir

onst

one

sequ

ence

Fm.

Naq

b Fo

rmat

ion

Facies

Qazzun Fm.

Age

14 H

amra

For

mat

ion

Foss

ilife

rous

lim

esto

neG

lauc

oniti

c gr

een

sand

s

Bahariya Fm.

Seq.

Upp

er ir

onst

one

sequ

ence

Low

er ir

onst

one

sequ

ence

Fm.

Naq

b Fo

rmat

ion

Facies

Late

ritiz

ed (k

arst

) iro

n or

e

Qaz

zun

Form

atio

nA

ge

Mud

-iron

ston

eO

oida

l-Num

mul

itic

irons

tone

faci

es

G

lauc

oniti

c gr

een

sand

s

Ham

ra F

orm

atio

n

Mud

-iron

ston

e

Ooidal pack-/grain-ironstone


Oncoidal float-/rud-ironstone

Ooidal-nummulitic grain-ironstone







Nummulitic limestone

Mottled mud-ironstone

Intraformational conglomerate00



Ironstone breccia Ironstone breccia

Nummulitic-bioclastic wacke-/pack-ironstone

Laminated and bioturbated mud-ironstone

Green sand andmudstone with

patches of ironstone

Bioturbated fossiliferous limestone

m14m

14m

Shallow subtidal-intertidal shoal environment

Tidal flat environment

Low

er L

utet

ian

Mid

dle

Eoce

neU

pper

Eoc

ene

Fig. 5. Lithostratigraphy, facies analyses and depositional environments of the lower to middle Eocene ironstonesat the upstream end of the Harra valley and at the eastern valley of the Gedida mine.



It rests unconformably on the periphery of thenorthern flank of the Ghorabi palaeohigh. It isseparated from the underlying tilted ferruginoussandstones of the Bahariya Formation by acoastal onlap surface marked by intraformationaloligomictic conglomerates (Fig. 8A and B). Thestromatolitic ironstone facies represents the lat-eral facies equivalent of the lagoonal mud-iron-stone facies in the southern sector and thenummulite-dominated mud-ironstone facies inthe northern sector.Three stromatolitic morphotypes are recorded

from base to top as follows (Fig. 8C): (i) irregular

oncoids up to 1�5 cm in diameter withconcentric to semi-concentric wavy cortical lam-inae coating skeletal and/or non-skeletal cores.The cores are composed of reworked ironstonefragments and bioclasts of various macrofossilsand large benthic foraminifera (LBF); (ii) strati-form stromatolites consisting of planar to wavyand crinkled laminae (Fig. 9A); and (iii) digitate(small-scale columns and domes) stromatolitesform as irregular fingers on top of the planarstromatolites (Fig. 9B).The stromatolitic ironstone facies is overlain

by thickly bedded oncoidal rud-ironstone and

Cenomanian Bahariya Fm.

Mud-ironstone

Fossiliferous-ironstone

Nummulitic-ooidal-oncoidal ironstone Mudstone

Low

er se

quen

ce

Upp

er se

quen

ce

Lateritized iron ore

A

B D

Bedded brick red mud-ironstone

Burrow-mottled mud-ironstone

C Fossiliferous ironstone

Mudstone

Fig. 6. (A) An exposure in the southern sector of the Ghorabi mine showing the lower to middle Eocene ironstonessubdivided into two sequences; the lower one consists of mud, fossiliferous and nummulitic–ooidal–oncoidal iron-stones and the upper one consists of mudstones and deeply weathered ironstone (lateritic ores). The lowersequence is 9�5 m and the upper sequence is 5�5 m. This section represents the southern sector in Fig. 4. (B) Bur-row-mottled manganiferous mud-ironstone overlain by brick red bedded hematitic mud-ironstone. The hammer isca 32 cm. (C) A hand sample from the fossiliferous ironstone facies showing the replaced echinoid spines andpelecepod casts. (D) An exposure of the upper part of the fossiliferous ironstone facies contains numerous burrowsfilled with mud-ironstones. The marker pen is ca 15 cm.



ooidal–nummulitic grain-ironstone (Fig. 8C).The ferruginous oncoids have stromatoliticironstone cores with multiphase encrustationsseparated by multiple microunconformities(Fig. 9C and D). Rare oncoids incorporate num-mulite tests and ooids within their cortical lam-inae, forming hybrid coated grains similar tothose described by Burkhalter (1995). Themegascopic and microscopic characteristics ofthese ferruginous microbial structures aredescribed in detail in Salama et al. (2013). Thestromatolitic microstructures consist of rhyth-mic alternations of microbial and iron-rich lam-inae. The microbial laminae are composed offilamentous iron-oxidizing bacteria, similar tothe present-day Leptothrix sp. The iron-richlaminae consist of yellowish-brown amorphousiron oxyhydroxides, which changed during dia-genesis into goethite; they are characterized by

laminoid fenestral cavities and desiccationcracks.

InterpretationThe oncoid cortical laminae coat a variety of skel-etal particles (nummulites and skeletal algae).These skeletal particles were reworked from shal-low subtidal areas near to the north during agi-tated tidal and/or storm conditions (El Aref et al.,2006). The gradual upward changes from the onc-oidal type to the stratiform and digitate types mayindicate a shift to quiet water conditions in a tidalflat environment. Analogous stratiform stromato-lites in carbonates with their characteristic fenes-tral cavities and the reworked LBF and skeletalalgae are common in an intertidal zone (Wilson,1975; Gerdes et al., 1994). The random internalmicrofabrics of the cortical laminae of the ferrugi-nous oncoids and their gradational contacts with

Illite

Kaolinite

B

1 µm

A

1 µm

Todorokite

D

Hematitespindle

6 µm

Psilomelane

C

6 µm

Fig. 7. (A) Ellipsoidal hematite aggregates surrounded by yellow amorphous iron oxyhydroxides, PPL. (B)Authigenic kaolinite booklets and illite flakes concentrated in the pore spaces between hematite ellipsoids, SEM.(C) and (D) Secondary authigenic psilomelane fibres (C) and todorokite (D) crystallized in pore spaces betweenhematite spindles, SEM. PPL, plane polarized light; SEM, scanning electron photomicrograph.



the surrounding matrix indicate that the oncoidsgrew at the sediment–water interface in quietwater conditions, probably during the inter-stormperiods. The incorporation of LBF and bioclasticdebris of echinoderm and algae in the cortices ofsome oncoids points to an oxic shallow marineenvironment (Bayer, 1989; Burkhalter, 1995).The vertical change from the stromatolitic

ironstone to the overlying oncoidal rud-ironstoneand nummulitic grain-ironstones indicates anincrease in agitated water conditions due tostorm waves and/or tidal currents. These condi-tions were responsible for local erosion andreworking of the underlying stromatolites, whichconstitute the most common cores of the overly-ing ferruginous oncoids. Moreover, the multi-phase encrustation of the cortical laminae andmicrounconformities inside the oncoid struc-

tures are evidence supporting their local rework-ing and stepwise formation.

Facies 3: Fossiliferous ironstone facies (3 to6 m thick)

DescriptionThis facies is well-developed in the southern andnorthern sectors of the Ghorabi mine (Fig. 6A).The fossiliferous ironstone facies attains 6 m inthickness in the eastern part of the southern sec-tor, whereas in the western part of the southernsector, this facies is reduced in thickness to 3 mand is truncated by the overlying megaripplednummulitic–ooidal–oncoidal ironstone facies.The fossiliferous ironstone facies is generallyreduced in thickness to 50 cm in the northernsector of the Ghorabi mine.

Stromatolitic mud-ironstone

Oncoidal rud-ironstone

Ooidal-nummulitic grain ironstone

SW

Stromatolitic ironstone facies

Nummulitic-ooidal-oncoidal ironstone facies

2 cm

Bahariya Formation

1

2

123

A

B C

Fig. 8. (A) Panoramic field view of the lower ironstone sequence of the central sector of the Ghorabi mine. Thissequence onlaps and truncates the underlying tilted Bahariya Formation. Photograph is looking due south-west.(B) Stromatolitic ironstone facies consists of oncoidal and stratiform morphotypes; they are overlain by oncoidalrud-ironstone and ooidal–nummulitic grain-ironstones. The hammer is ca 32 cm. (C) A hand sample showing onc-oidal (1) and stratiform stromatolitic (2) morphotypes.



In the southern sector, it shows graded beddingfrom poorly to moderately sorted, grain-sup-ported fossiliferous rud-ironstone at the base tocrudely laminated pack–grain-ironstones towardsthe upper part. The upper surface of the fossilife-rous ironstone facies is intensively bioturbatedand mottled, whereas the lower surface is erosive(Fig. 6D). The primary sedimentary structureswere obliterated by the bioturbation. This faciesconsists mainly of hematitic moulds and castsand, rarely, hard skeletal parts of burrowing echi-noderms, bivalves, gastropods, skeletal algae andlarge benthic foraminifera (Fig. 6C).

InterpretationThe abundance and sorting of open marinefossils in the southern sector of the Ghorabimine indicate transportation under stormconditions. The storm waves and currentsinduced erosion and reworking of the skeletalgrains. An erosional scoured lower surface andnormal graded bedding of the shell-beds

represent initial deposition from traction bedload followed by the accumulation of finersediments from waning turbulent flow. The skel-etal components were transported from nearbyshallow subtidal areas to the depositional sitevia washover fans and tidal channels. Duringthe waning stage of the storm, the reworkedskeletal grains settled to form winnowed shelllags over a basinal erosional surface. The iron-rich sediments filled the inter-skeletal and intra-skeletal pore spaces and gradually replaced theoriginal calcareous wall structures of the shelllags. Post-storm disturbance is indicated by bio-turbation that destroyed most of the primarysedimentary structures.

Facies 4: Nummulitic–ooidal–oncoidalironstone facies (6 to 17 m)

DescriptionThis facies is the most abundant and thickestironstone facies in almost all mine areas. It

800 µm1 mm

400 µm

D

C

A

D

Stromaoliticcore

B

800 µm800 µm

Oncoliticcore

Fig. 9. (A) Stratiform stromatolites with laminoid fenestral cavities. (B) Microcolumnar ferruginous stromatolites.(C) and (D) Ferruginous oncoids with non-skeletal cores [oncoid fragment in (C) and stromatolitic fragment in (D)]surrounded by multiphase encrustations (phases I and II cortical laminae) that are separated by microunconformi-ties (UC). The oncoids from the photomicrographs (C) and (D) are the main components of the rud-ironstones inFig. 8B. All photomicrographs are in plane polarized light.



varies in thickness from 17 m in the eastern val-ley of the Gedida mine to 11 m in the centralsector of the Ghorabi mine and 6 m in the Harramine (Figs 4, 5, 8A and 10). It rests on the stro-matolitic ironstone facies in the central sector ofthe Ghorabi mine and on the mud-ironstonefacies in the other mine areas. The upper surfaceof this facies is generally a widespread subaerialweathering surface terminating the lowersequence, along which pedogenic ironstonebreccias are developed (Fig. 10). These brecciasconsist of gravel-sized yellowish-brown ooidal–nummulitic ironstone fragments cemented bydark brown colloform goethite cement. In theGhorabi mine, this facies consists of fivesuccessive shallowing-upward cycles, each ofwhich attains a thickness of 1 to 3 m. Each cyclestarts with yellow bioturbated matrix-supportedooidal and oncoidal mud-ironstones passingupwards into well to moderately sorted and

grain-supported ooidal, oncoidal and nummu-litic ironstones (Fig. 10). The main frameworkcomponents comprise ferruginous ooids, onc-oids, peloids, ferruginized and silicified skeletalgrains such as LBF (nummulites, miliolids andalveolinids), as well as bioclastic debris of echi-noderms, bivalves and skeletal algae.This facies shows low-angle planar cross-

bedding with a general dip of 5 to 10° due north.The uppermost cycle of this facies exhibitstrough-shaped ripple cross-lamination, normalgraded bedding, scour structures and fill struc-tures. In the southern sector of the Ghorabimine, this facies truncates the underlying fossil-iferous ironstone facies and is reduced in thick-ness to 3 m. The nummulitic–ooidal–oncoidalironstone facies can be subdivided into fourmicrofacies based on the proportional abun-dance of the ooids, oncoids and nummulites.These microfacies are oncoidal rud-ironstone,

Oncoidal rud-ironstone

Oncoidal-ooidal grain-ironstone

Oncoidal grain-ironstone

2 cm

2 cm

2 cm

4

Ooidal grain-ironstone

Ironstone breccia on the top of cycle 5

3

2

1

5

Fig. 10. Nummulitic–ooidal–oncoidal ironstone facies in the central sector of the Ghorabi mine where it consistsof five shallowing-upward cycles (1 to 5). The samples (1 to 5) are from the top of the cycles for which they arenumbered. Sample (5) is ironstone breccia formed at the expense of the ooidal–nummulitic ironstone microfacieson top of the lower ironstone sequence. Field of view for the overview panel is 10�5 m.



peloidal–ooidal grain-ironstone, ooidal–oncoidalgrain-ironstone and ooidal–nummulitic grain-ironstone.

Peloidal–ooidal grain-ironstoneThe peloidal–ooidal grain-ironstone microfacies(cycle 1) forms the base of the nummulitic–ooidal–oncoidal ironstone facies in the centralsector of the Ghorabi mine. It also forms thebulk of the nummulitic–ooidal–oncoidal iron-stone facies in the Harra mine. It has internal

megaripple-scale cross-bedding and a scouredupper contact with the overlying oncoidal rud-ironstone microfacies (Fig. 11A).Petrographically, this microfacies consists of

well-sorted and grain-supported yellowish-brown ooids, peloids and less commonlynummulite tests. The ferruginous ooids (0�5 to2 mm in diameter) are generally spheroidal,although some have composite shapes. Themajority of the ooids consist of cortical laminaewithout obvious nuclei (Fig. 11B).

6·5 mm

A

800 µm

B

5 mm

D

1 cm

C

Fig. 11. Photomicrographs of ironstone microfacies. (A) Rippled contact between ooidal grain-ironstone andoverlying silicified oncoidal rud-ironstone, OL. (B) Well-sorted and grain-supported peloidal–ooidal ironstonemicrofacies with goethite and silica cement (cycle 1), PPL. (C) Polished slab of the grain-supported oncoidalrud-ironstone microfacies (cycle 2), OL. (D) Cross-laminated ooidal–nummulitic grain-ironstone microfacies, OL.OL, ordinary light; PPL, plane-polarized light.



Oncoidal rud-ironstoneThe oncoidal rud-ironstone microfacies formsthe lower part of the nummulitic–ooidal–oncoi-dal ironstone facies (cycle 2) in the centralsector of the Ghorabi mine. It is also present inthe eastern valley of the Gedida mine at the baseof the lower nummulitic ooidal–oncoidalironstone facies (Helba et al., 2001).Petrographically, the essential framework

components are moderately sorted yellowishbrown ferruginous oncoids (mostly 0�5 to 1 cmin diameter, but rarely over 2 cm) and ooids (0�5to 1�5 mm), showing grain-supported fabrics(Fig. 11C). The ferruginous oncoids havespheroidal, discoidal or ellipsoidal shapes andthey have no nuclei. These frameworkcomponents are bioturbated and cemented bysiliceous cement.

Ooidal–oncoidal grain-ironstoneThe ooidal–oncoidal grain-ironstone microfaciesforms the middle part of the nummulitic ooidal–oncoidal ironstone facies (cycles 3 and 4) in thecentral sector of the Ghorabi mine. It containslow-angle cross-bedding and is scoured into theunderlying peloidal–ooidal grain-ironstone alongits basal contact. It is also best developed in theeastern valley of the Gedida mine, forming len-ticular ironstone bodies. These ironstone bodiesrange in thickness from 40 cm to 1 m and areseparated from one another by thin discontinu-ous layers or laminae of mud-ironstone.Petrographically, this microfacies consists

mainly of moderately sorted ferruginous ooidsand oncoids with a few scattered peloids andskeletal particles in a grain-supported fabric.The framework components have a bimodalgrain-size distribution; ooids do not exceed2 mm in diameter and the oncoids are 0�5 to1 cm in diameter. The ooids and oncoids areidentical in their mineralogical composition,grain morphology and their internal micro-fabrics.

Ooidal–nummulitic grain-ironstoneThis microfacies is recorded in all mine areas ofthe Bahariya Depression. It covers the nummu-litic–ooidal–oncoidal ironstone facies in thecentral sector of the Ghorabi mine (cycle 5). Theabundance of the nummulite tests increases inthis microfacies relative to other microfacies atthe expense of both ferruginous ooids andoncoids. In the eastern valley of the Gedidamine and Harra mine, it forms the bulk of thenummulitic–ooidal–oncoidal ironstone facies

and consists of five to six megarippled beds (30to 80 cm thick, for each). These megaripplesoverlap and sometimes truncate one another;they have sharp to scouring basal contacts andare almost all internally graded and cross-lami-nated (Fig. 11D). This microfacies extends intothe western part of the southern sector ofthe Ghorabi mine, where it scours into theunderlying fossiliferous ironstone facies. It alsoforms megaripple-scale bed forms, whosetroughs are filled by bioturbated mud-ironstones.In the Harra section, they form festoon cross-bedding. The top of this microfacies is alwaysmasked by lower Eocene pedogenic features andprecipitates.Microscopically, it is composed mainly of

ferruginized and silicified microbored skeletalparticles with a few scattered ferruginous ooids,oncoids and peloids (Fig. 11D). The mainskeletal components are LBF, bioclasts ofbivalves, echinoderms and skeletal algae. Thismicrofacies shows thin cross and graded lamina-tions and scour structures. The nummulite testsare completely replaced by amorphous ironoxyhydroxides and still preserve the originalinternal radial wall architectures. Almost allferruginized skeletal and bioclastic particles arecoated by thin massive rims forming ferruginouscortoids (Fig. 12A and B) or thickly laminatedand non-isopachous iron oxyhydroxide cortices,forming ferruginous oncoids (Fig. 12C and D).The cortices are often crowded by fossilizedmicroborings and microbial remains.

InterpretationThe main allochemical component of thenummulitic–ooidal–oncoidal ironstone faciesare LBF, which thrived on shallow, oligo-trophic, circum-Tethyan carbonate platforms(Buxton & Pedley, 1989). These facies are abun-dant in Palaeocene to upper Eocene sedimentsof the Mediterranean (especially North Africa)and the Arabian Peninsula. This fossilassemblage indicates open and normal marineconditions (Wilson, 1975). The nummulitids,which are the main fossil allochems in allfacies, may have originally lived on soft muddysubstrate (Aigner, 1985a) and their develop-ment was optimal in well aerated, clear, warm(25°C) and shallow marine water (<120 m;Blondeu, 1972). The nummulite deposits in theTethyan Palaeogene were generally associatedwith ramps, and they were deposited in shal-low water, shoaling, inner ramp environments(Buxton & Pedley, 1989). The ramp model is



proposed by Helba et al. (2001) as the deposi-tional environment of the middle Eocene car-bonates of the north-eastern part of theBahariya Depression. Nummulite tests can betransported by storm currents (Aigner, 1985b),and their accumulations may be further modi-fied by winnowing and other sedimentologicaland biological processes. The nummulites gen-erally occur in packstones–grainstones whichform distinctive low-amplitude banks and me-garipples that show scouring, test abrasion,imbrication and winnowing. This may suggestan increase in energy due to a shallowing-upward tendency and a change from current-dominated to wave-dominated processes (Ra-cey, 2001).The generally moderate sorting of the

ooidal–nummulitic grain-ironstone microfacies

suggests that the reworking of nummulitesoccurred without large-scale transport. Theselocally reworked fossils represent parautochth-onous allochems (Aigner, 1985b; Racey, 2001).The abundance of nummulites increasestowards the top of the lower ironstonesequence in all mines. This increase indicatesthat the palaeotopographic variation of theBahariya Formation was damped out and thedepositional surface became nearly planarthrough time.During the intermittent low-energy periods

(inter-storm events) of low velocity, the iron-rich fine materials settled from suspensionwith a low sedimentation rate as confirmed bythe presence of burrowing, iron-precipitatingmicrobes and ferruginous bored bioclasts(Gehring, 1989; Kidwell, 1991; Burkhalter,

800 µm

800 µm 800 µm

800 µm

Echinoid plate

Echinoid plate

Meliolids

C

BA

D

Fig. 12. Ferruginous microbially coated grains with different skeletal cores and different cortex thicknesses andmorphologies. (A) Hematitized nummulite core with thin goethitic cortex. (B) Ferruginous cortoid consisting of athin cortical lamina around an echinoid plate. (C) Thick even organic-rich cortex arounds a meliolida core. (D) Aferruginous oncoid with a thick wavy cortex around an echinoid plate. All photomicrographs are in PPL.



1995; Salama et al., 2013). The iron-rich mate-rials deposited from suspension infiltrated intoand were adsorbed onto the inter-skeletal andintra-skeletal pore spaces assisted by the activi-ties of microboring organisms. The original cal-careous fossil particles were initially stainedand then gradually replaced by amorphousiron oxyhydroxides. These ferruginized fossillags provide a mobile substrate for iron-encrusting organisms to develop ferruginousooids and oncoids. These grains have thinto thick, poorly developed, crenulated andmicrostromatolitic cortical laminae surroundingcores of ferruginized nummulite tests and bio-clastics.The second essential category of allochems is

the ferruginous ooids and oncoids. Theseallochems are identical in mineralogy, but differin morphology and internal microfabric from

one another and the associated stromatoliticironstone facies. They originally consisted ofnanocrystalline iron oxyhydroxides, and recrys-tallized during early diagenesis into goethite(Fig. 13A to C). A biogenic role in the origin ofthe ferruginous ooids and oncoids (Salamaet al., 2013) is confirmed by the wavy andoverlapping nature of the cortical laminae,sometimes showing club-shaped microstromato-litic structures, as well as by the presence offerruginized algal filaments and bacterial cells inthe cortices (Fig. 13D).The association of ferruginous ooids and onc-

oids with ferruginized fossils, occurring as bothfree particles and nuclei, indicates their growthand accumulation at or just below the sediment-water interface in an oxygenated environment.Ferruginous ooids are generally thought to beautochthonous in origin when they exhibit no

B

D

1 µm10 µm

10 µm3 µm

A

C

Fig. 13. (A) Gradational contacts between successive cortical laminae. (B) Nanocrystalline iron oxyhydroxidesprecipitates of the iron-rich laminae. (C) Early diagenetic crystalline goethite. (D) A microbe-rich lamina consist-ing of fossilized iron-oxidizing leptothrix-like bacterial filaments and their casts (Salama et al., 2013).



signs of abrasion or fracturing or when thenuclei are composed of material resembling thatof the surrounding matrix, and when similardebris is found in both the cortex and the matrix(Gygi, 1981; Burkhalter, 1995; Collin et al.,2005). The internal truncation of the corticallaminae, as well as the overlapping nature andincorporation of small ooids and fossilfragments into the cortices, indicates intermit-tent erosion and renewed encrustation. Inaddition to intra-particle erosion, the grain-sup-ported fabric, moderate sorting, bimodal grainsizes and the physical sedimentary structures(both scour and fill and cross-lamination) of thenummulitic–ooidal–oncoidal ironstone faciessuggest local reworking and concentration of theferruginous ooids and oncoids under episodicagitated water conditions.

The upper ironstone sequence

The upper ironstone sequence belongs to themiddle Eocene Qazzun Formation based on theidentified benthic foraminifera (Said & Issawi,1964). Based on large foraminifera biostratigra-phy of the Tethyan Palaeogene (Serra-Kiel et al.,1998), the middle Eocene benthic foraminiferaof the Qazzun Formation are Assilina praespira(SBZ: 13), N. discorbinus (SBZ: 14 to 16), N.gizehensis (SBZ 14 to 16), Orbitolites complana-tus (SBZ: 13 to 15), Discocyclina sp., Operculinadiscoidea, Assilina sp., Discocyclina sp., Rotaliasp. and Miliolidae sp.The upper ironstone sequence in the Ghorabi

mine begins with the deposition of a basinalgreen mudstone facies changing laterally intomud-ironstone facies (Fig. 14A). The mud-iron-stone facies grades upward into shallow subti-dal–intertidal nummulitic–bioclastic ironstonesrepeated in three cycles. The mudstone faciesrepresents a marker stratigraphic horizon up to1 m thick (Fig. 14B). It consists of thinly lami-nated kaolinitic mudstones with stratiform dia-genetic barite nodules, which range in lengthfrom 0�5 to 15 cm (Fig. 14C). The deposition ofa uniform thickness of the mudstone facies atthe base of the upper ironstone sequence in theGhorabi mine indicates that the subaerial weath-ering surface of the lower ironstone sequencewas nearly peneplained. Such sharp and abruptchanges in facies suggest that the Ghorabi minewas drowned by a new marine transgressionaccompanied by rapid deepening (Van Wagoner,1995; Taylor et al., 2000). These mudstoneswere deposited in a relatively deep basinal area

with low-energy, anaerobic to dysaerobic condi-tions below wave base (Davis & Byers, 1989;Sageman et al., 1991).In the eastern valley of the Gedida mine, the

upper ironstone sequence consists ofunburrowed variegated mud-ironstones in thelower part, grading upward into nummulitic-ironstone facies rich in mouldic cavities andferruginized tests of the large N. cailliaudiwith a few gastropods and pelecypods.Silicified and ferruginized nummuliticlimestone nodules of the Qazzun Formationare scattered in the upper ironstone sequencein the Gedida mine. This indicates that theupper ironstone sequence belongs to the mid-dle Eocene Qazzun Formation (Helba et al.,2001). In the present work, the fieldinvestigations indicated that the silicifiednummulitic limestone nodules characterizingthe Qazzun Formation are also found as weath-ering remnants of scattered boulders on top ofthe lower ironstone sequence in the Harramine.

MINERALOGY AND DIAGENESIS

Goethite is the primary mineral phase of theancient ooidal ironstones (Burkhalter, 1995;Sturesson et al., 1999; Collin et al., 2005). Itmay result from transformation of chamosite byoxidation (Gygi, 1981; Cotter, 1992). Moderniron ooids demonstrate that goethitic (limonitic)ooids can form as primary precipitates (Heikoopet al., 1996). In the nummulitic–ooidal–oncoidaland stromatolitic ironstone facies studied, notrace of precursor clay minerals has been identi-fied by X-ray diffraction, and no clay particleshave been observed by SEM (Salama et al.,2012). Moreover, bulk and microchemical analy-ses show low to negligible concentrations ofaluminium (Salama et al., 2012), which rule outany replacement of a precursor clay mineral bygoethite (Maynard, 1986; Gehring, 1989; Cotter,1992). These arguments suggest that the goethiteis of primary origin in this case. Goethite precip-itated in oxidizing conditions as indicated bythe abundance of benthic fauna and the rework-ing and bioturbation of the sediment at the timeof deposition (Gygi, 1981; Gehring, 1989; Burk-halter, 1995; Collin et al., 2005). The iron isthought to be derived from the migration of Fe2+

ions in pore fluids from the deeper reducing andacidic groundwater to the surface layer of the sed-iment, where iron oxidizes and precipitates as



goethite (Dabous, 2002) at the boundary of theiron reduction and oxidation zones (Heikoopet al., 1996). Dissolved available iron must have

been abundant because the goethite mineralogyof the ferruginous ooids and oncoids of theBahariya Depression is interpreted as primary in

100 µm 50 µm

B

D

F

Mudstones

Barite nodule

Cavity-filling Botryoidal goethite

Colloform goethiteCavity-filling acicular goethite

Lateritic palaeosol

A

C

E

Fig. 14. (A) An exposure of highly lateritized and brecciated upper ironstone sequence in southern sector of theGhorabi mine. Field of view is 4�5 m. (B) Exposure of green mudstone facies at the base of the upper ironstonesequence that overlies a palaeosol at the top of the lower ironstone sequence. Hammer is 32 cm. (C) An exposureof diagenetic barite nodules in the mudstone facies. (D) Solution cavity lined with reniform (botryoidal) goethite.Lens cap is ca 6 cm. (E) Photomicrograph in PPL showing colloform goethite developed inside a dissolution cav-ity. (F) Photomicrograph in PPL showing cavity filled with secondary acicular goethite.



origin. The influence of microbial activity in theprecipitation of ferric oxyhydroxides was dis-cussed in detail by Salama et al. (2013). Somestudies have indicated that microbial activityplays a significant role in the genesis of ironcoated grains (Burkhalter, 1995; Pr�eat et al.,2000). Here, the iron-oxidizing bacteria pro-moted the precipitation of amorphous iron oxy-hydroxides as amorphous ferric gel on theirsurfaces (Salama et al., 2013). It was later trans-formed into goethite during early diagenetic re-cystallization without any compositionalchanges. Goethite was transformed to hematiteby dehydration during late-stage diagenesis.

Lateritic iron ore

Lateritic iron ores and palaeosols were formedon top of these sequences and demarcate thesequence boundaries. This iron ore constitutesthe main and thickest stratigraphic ore unit inthe Gedida mine (up to 22 m thick). These pal-aeosols are overlain by the green sand of theupper Eocene Hamra Formation as a result of anew marine transgression.The main pedogenic features recorded at these

surfaces include the formation of cockade struc-tures (collapsed ironstone breccias encrustedwith crustified rhythmic layers of iron oxyhydrox-ides), solution cavities and caves developedwithin the original marine ironstone facies andlined with botryoidal goethite and hematite(Fig. 14D to F) or filled with goethite stalactitesand stalagmites and vadose pisoids cementedwith meniscus goethite cements. The formation ofthese pedogenic features may indicate dissolutionand reprecipitation of iron from colloidal, super-saturated solution under neutral pH conditions(El Aref & Lotfy, 1989; El Aref et al., 1999).Locally, the groundwater introduced sulphate

and silica-rich water, which contaminated thelower ironstone sequence by precipitation ofboth sulphate and silica minerals, like jarositeand quartz, under oxic and acidic conditions.The effects of the groundwater contaminationduring the subsurface alteration were discussedin detail in Salama et al. (2012, 2013).

DISCUSSION

Origin of ancient ferruginous ooids

Depositional environments suggested for ancientferruginous ooids include: (i) shallow marine

environments (Maynard, 1986; Taylor & Curtis,1995; Macquaker et al., 1996; Donaldson et al.,1999; Sturesson, 2003); (ii) calm and occasion-ally agitated offshore transitional environments(Gygi, 1981; Madon, 1992; Burkhalter, 1995);(iii) continental environments (Siehl & Thein,1989); (iv) restricted lagoonal environments(Bayer, 1989); and (v) coastal and deltaicenvironments (James & Van Houten, 1979;Collin et al., 2005). Ferruginous ooids formedduring relative sea-level lowstands, and somewere reworked during transgressive episodes(Chan, 1992). For marine ironstones, differentbathymetries and hydrodynamics, ranging fromlow energy (Knox, 1970; Gygi, 1981) to agitatedregimes (Hallam, 1975; Germann et al., 1987;Siehl & Thein, 1989; El Aref et al., 1996) havebeen suggested. The ooidal ironstones are alsoconsidered to have accumulated either duringtransgressions (Van Houten & Purucker, 1984;Young, 1989b), regressions (Hallam & Bradshaw,1979; Teyssen, 1989) or both (Gehring, 1989;Burkhalter, 1995).Several depositional mechanisms have been

proposed for the formation of ancient ferrugi-nous ooids. These include: (i) via replacementof carbonate ooids (Kimberley, 1979); (ii) as inplace microconcretions (Gygi, 1981; Maynard,1986); (iii) via crystallization from precursoriron oxyhydroxide gels (Harder, 1989); (iv) amechanical accretion of clay particles(Bhattacharya & Kakimoto, 1982; Van Houten &Purucker, 1984; Madon, 1992); (v) via intrasedi-mentary and snowball accretion mechanisms(Chauvel & Guerrak, 1989); (vi) by formation inlaterite soils and subsequent transportation toand deposition in marine settings (Siehl &Thein, 1989); and (vii) having a biogenic originvia unspecified processes (Dahanayake & Krum-bein, 1986; Burkhalter, 1995). Modern ferrugi-nous ooids are formed by volcanic activity in ashallow marine setting offshore of a volcanicisland in Indonesia (Heikoop et al., 1996). Ironooid formation there has been linked to volcanicactivity, with volcanic ash and hydrothermalfluids enriching sea water in Fe, Al and Si(Sturesson et al., 2000). However, that mecha-nism cannot be invoked in this case. Not onlywere the ferruginous ooids studied heredeposited on a normal shallow marine innerramp during times of condensed sedimentation(El Aref et al., 2001; Helba et al., 2001), there isalso no evidence of volcanic activity in theBahariya Depression in Eocene times whenthese ooids formed (El Aref et al., 1999). The



depositional or diagenetic origin of the ancientooids based on internal microfabrics and tex-tures remains a matter of controversy (Hughes,1989; Young, 1989a).

Origin of the Bahariya ironstones

The field, megascopic and microscopic investi-gations presented here indicate that the ferrugi-nous ooids and oncoids are of primarydepositional origin. Here, a continental originproposed by Siehl & Thein (1989) for ferrugi-nous ooids can be excluded because largebenthic foraminifera (LBF) and skeletal algae areassociated with or sometimes incorporated intothe cortices of the ooids. Also, the near absenceof any calcareous ooids and oncoids from thesurrounding equivalent carbonates indicates thatthe ferruginous ooids and oncoids are plausiblyprimary depositional grains. They neither inher-ited calcareous coated grains that were subse-quently replaced by iron-bearing solution (e.g.El Hinnawi, 1965; Basta & Amer, 1969) norformed as lateritic ooids and pisoids in hinter-lands that were subsequently reworked into theshallow marine environments (Siehl & Thein,1989).The ferruginous ooids and oncoids of the

ironstones studied are subdivided into coredand uncored types according to their internalstructures and outer morphology. The internalnanostructures of the uncored ferruginous ooidsand oncoids reveal that they are primary,autochthonous grains, as confirmed by the pres-ence of gradual contacts between the corticallaminae and the surrounding matrix, which areof the same composition. The textural and inter-nal microfabrics reveal neither tangential norradial internal structures, but instead a randomnanocrystalline microfabric. The ferruginousooids and oncoids, associated with the stromato-litic build-ups, consist of repeated doublets ofiron-oxyhydroxides and microbe-rich laminae,and they have a wavy to highly crenulated andoverlapping nature, sometimes with club-shapedmicrostromatolitic shapes. The microbe-richcortical laminae are intensively crowded withmineralized filamentous iron-oxidizing bacteria,which indicate an in situ biogenic origin(Salama et al., 2013).Ooids and oncoids with ferruginized fossil

and non-skeletal cores and multiphase corticalencrustations indicate a local reworking alongthe sea bottom by tidal currents and/or stormwaves. These reworked grains are considered

parautochthonous grains. The multiphasecortical encrustations are separated by breaks ormicrounconformities which reflect successivestages of growth. Encrusting foraminifera andskeletal algae have been observed on thesurfaces of cortical laminae. This clearly indi-cates stepwise formation where the ooids andoncoids were exposed at the water–sedimentinterface. Similar ferruginous ooids and oncoidshave been recorded in other deposits (Burkhal-ter, 1995; Collin et al., 2005). The cored ooidsand oncoids are of variable sizes and shapes,and their cortical laminae (sheaths) may encrusta variety of skeletal and non-skeletal cores. Thenon-skeletal particles include angular mud-iron-stone clasts, stromatolitic chips, ferruginouspeloids and ooids. The skeletal particles com-prise ferruginized and/or silicified microboredLBF and bioclasts of echinoderms, bivalves andskeletal algae. The skeletal and non-skeletalcores were brought to the area of deposition byreworking during storm waves and/or tidal cur-rents and acted as substrates for the cortexdevelopment. The development of corticesoccurred during the waning stages in quietwater conditions. Consequently, it is importantto distinguish the formation site from the depo-sitional site.

Source of iron

Several possible sources of iron for the Baha-riya ironstones have been discussed in previousliterature. These include: (i) direct precipitationof iron from a shallow water lagoonal andlacustrine environment and replacement of theunderlying middle Eocene limestone (Akkad &Issawi, 1963); (ii) karstification of host lime-stone (El Aref & Lotfy, 1989); (iii) weathering ofthe glauconitic sandstones of the HamraFormation (El Sharkawi et al., 1984; Dabous,2002); (iv) iron leaching from the sandstone ofthe Nubian aquifer by upward-moving ground-water which was then deposited in the overly-ing pre-existing limestone (Dabous, 2002); and(v) metasomatic replacement of the underlyinglimestone by hydrothermal solutions (Basta &Amer, 1969).In the present work, the lateritic materials

produced by the weathering of the CenomanianBahariya Formation (hinterlands) are consideredto be the main source of iron for the ironstonesin the Bahariya Depression. The iron was proba-bly transported as colloidal solution from southto north by surface run-off to the shallow marine



coastal environment. The continental source ofiron is indicated by the decrease of clays anddetrital quartz from the southern to the northernsector of the Ghorabi mine. This mechanism isconsistent with the vast increase in sedimentsupply from the continent to the shelf duringthe Palaeocene-Eocene thermal maximum(PETM; Sluijs et al., 2008). The iron was depo-sited mostly during the initial marine floodingpulse that resulted in the formation of the quietwater mud-ironstones and stromatolites. Ironcontinued to be introduced throughout the suc-cession by fluvial discharge from the stillexposed Bahariya Formation, as well as by thereworking and resuspension of sediment depo-sited in the initially low-energy environmentthat was then redistributed upsection as shoal-ing occurred. Salama et al. (2012) proposedrecent subsurface alteration of the primary mar-ine ironstones by acidic groundwater to accountfor cavity-filling jarosite, which forms underacidic and arid climatic conditions.

Proxies for climatic changes

Evidence of climatic changes during theformation of the lower and middle Eoceneironstones can be extracted from marinedepositional and terrestrial chemical weather-ing proxies. During the late Cretaceous to latePalaeocene/early Eocene, the southern Tethyanmargin was characterized by a warm andhumid climate with high precipitation (Bolle &Adatte, 2001; Scheibner et al., 2005; Ernstet al., 2006; Scheibner & Speijer, 2008). Duringthis time and under these climatic conditions,the Cenomanian Bahariya Formation in theGhorabi, Harra and Gedida mines was tectoni-cally uplifted and stood as structurally con-trolled palaeohighs in the north-eastern part ofthe Bahariya Depression (Fig. 15). It was sub-jected to intensive denudation and pedogenesisunder warm and humid climatic conditions(Said, 1990; El Aref et al., 1999; Helba et al.,2001). Therefore, the upper Cretaceous toupper Palaeocene/lower Eocene palaeosolformed during superimposed cycles ofweathering and erosion, as well as hyperther-mal climatic events such as the PETM. ThePETM is represented in both northern (GalalaMountains, 20°N) and southern (WesternDesert, 19°N in the Kharga Oasis, FarafraOasis, Dakhla Oasis and central Egypt) parts ofEgypt, where the upper Cretaceous/lowerPalaeogene succession is preserved (Scheibner

et al., 2005; Scheibner & Speijer, 2008). ThePETM in Egypt has been investigated inten-sively mainly on the basis of smaller benthicforaminifera (e.g. Speijer et al., 1997; Ernstet al., 2006), larger benthic foraminifera (e.g.Scheibner et al., 2005), planktonic foraminifera(e.g. Obaidalla, 2000; Ouda et al., 2003), nanno-plankton (e.g. Youssef, 2004), ostracods (e.g.Speijer & Morsi, 2002) and geochemical andmineralogical parameters (e.g. Bolle et al., 2000;Speijer & Wagner, 2002). The time gap in thestratigraphic succession decreases further southin the Bahariya Depression, where the upperCenomanian Heiz Formation, the CampanianHefhuf Formation and the Maastrichtian Kho-man Formation are well-represented (Fig. 1).

Sea-level changes

The character of global eustatic sea-level changeduring the early Eocene has direct relevance tothe origin of hyperthermals (Sluijs et al., 2008).Two proposed causes for the hyperthermals,which both invoke a significant drop in sea-level, are: (i) the oxidation of organic matter insubaerially exposed marine deposits (Higgins &Schrag, 2006); and (ii) the release of microbiallyderived methane from the shelf (Schmitz et al.,2004).The late Cretaceous to early Palaeogene is

characterized by warm, generally ice-free condi-tions (Zachos et al., 1993; Sloan & Thomas,1998); however transient glaciations may haveoccurred in inland Antarctica, to account forsea-level changes up to ca 30 m (e.g. Speijer& Morsi, 2002). These warm conditionsculminated in the early Eocene Climatic Opti-mum (EECO; Zachos et al., 2001). The earlyCenozoic sea-level variations can be assessedthrough analyses of sedimentary sequencesdeposited on continental margins. In Tethyanmargin sequences, benthic foraminifera and lith-ological evidence indicate transgression duringthe PETM (e.g. Speijer & Wagner, 2002). Sluijset al. (2008) suggested a link between global sea-level and ‘hyperthermal’ intervals; this may bedue to the melting of small Alpine ice sheets onAntarctica, thermal expansion of sea water, orboth. These authors also proposed that transgres-sion occurred despite increasing sediment sup-ply from the continents, which would normallylead to basin filling and relative sea-level fall.Schmitz et al. (2004) have argued that lithologi-cal changes across the PETM support a majorsea-level fall.



During the beginning of the early Eocene inthe Bahariya Depression, the sea partiallydrowned the Cenomanian palaeohighs in thethree mine areas and, as a consequence, aninner ramp carbonate environment was devel-oped (El Aref et al., 2001). A mud-ironstonefacies was deposited in quiet water, semi-restricted lagoons that were developed in low-lying areas between palaeohighs (Fig. 15). Thesummits of these palaeohighs were still emer-gent, forming shallow tidal flats along which astromatolitic ironstone facies was deposited.This ironstone facies association interfingeredlaterally with an equivalent nummulitic lime-stone facies (Helba et al., 2001). Upon theseexposed tidal flats, iron-rich solutions were lea-ched from the latertitized Cenomanian siliciclas-tics via surface and subsurface pathways intothe lagoonal areas. With continued sea-level risein the early Eocene, the entire area was drownedand became a subtidal carbonate ramp settingecologically favourable for the prolific growth ofthe LBF (mainly nummulitids and alveolinids),pelecypods, gastropods, echinoderms andskeletal algae. On the submarine Cenomanianpalaeohighs, shallow subtidal–intertidalnummulitic–ooidal–oncoidal ironstones weredeposited in a shoaling environment (Fig. 15).When shoaling was at its peak, LBF colonizedthe shallow sea bottom and increased in abun-dance and diversity. The predominance of LBFin the Bahariya area is associated with a parallelabsence of reef-building corals. These characte-ristics support the ramp depositional setting forthe Eocene limestones (El Aref et al., 2001).Because of the vulnerability of corals to highsurface-water temperatures and enhanced CO2

levels, the late Palaeocene to early Eocene,global warming may have favoured larger fora-minifera at the expense of reef-building corals asthe main carbonate-producing component onoligotrophic carbonate platforms at lower lati-tudes (Kiessling, 2002; Sheppard, 2003; Scheib-ner et al., 2005). The LBF rapidly diversifiedunder sea surface temperatures between 28°Cand 32°C (Pearson et al., 2007); they remainedalmost unchanged from the larger foraminiferalturnover at the PETM, through the EECO and aslow but gradual cooling that followed,interrupted by the middle Eocene Climatic Opti-mum (MECO) in the early Bartonian (Zachoset al., 2001, 2008; Bohaty & Zachos, 2003).According to Bohaty et al. (2009), the MECOwas followed by a sudden return to a gradualcooling trend in the late middle Eocene. This

trend continued until the Eocene/Oligoceneboundary, which is marked by a drastic decreasein the temperature caused by permanentAntarctic glaciation (e.g. Zachos et al., 1996).The gradual cooling of the Earth after the MECOin the early Bartonian until the drastictemperature decrease at the Eocene/Oligoceneboundary caused the gradual disappearance ofthe lower–middle Eocene larger benthicforaminifera.Nummulites in the Bahariya Depression occur

as reworked grains in high-energy shoal envi-ronments (El Aref et al., 2006). From a sedimen-tological point of view, nummulite-richironstone facies resulted from the accumulationof tests transported by storm-induced or tide-induced waves and currents, and the residualconcentration of tests after repeated local win-nowing and condensed events (Aigner, 1985b).This facies is consistent with the presence ofplanar and trough cross-laminations, small-scalescour and fill structures, and the imbrications ofnummulites. The resulting sedimentary texturesindicate para-autochthonous to allochthonousdeposits. During calm periods between storms,the sedimentation rate was minimal as con-firmed by the presence of microbored bioclastsand burrowing organisms. The net result of theshoaling-upward conditions was a generalreduction in the Cenomanian palaeotopographyand a significant northward regression of thesea. As a consequence of the sea-level fall, thelower Eocene ironstone sequence was com-pletely exposed to subaerial lateritic weatheringand pedogenesis under a warm and humid cli-mate (Fig. 15).The beginning of the middle Eocene was

characterized by a global high sea-level, wheremost of the low-elevation coastal plains weredrowned (Prothero, 2003). A rise in the sea-levelduring the early middle Eocene led to the redr-owning of the exposed lower Eocene ironstonesequence. As a consequence of this new marinetransgression, a green mudstone facies wasdeposited under shallow subtidal conditions,forming a blanket of uniform thickness. Thisfacies extends all over the study area and isobserved in all three mines. Subsequently, theupward shoaling led to a gradual verticalchange into shallow subtidal bioturbated mud-ironstones and intertidal bioclastic–nummuliticironstones. Similar to the lower Eocene ironstonesequence, the shoaling-upward tendency of themiddle Eocene ironstone sequence led to amajor regression. This, in turn, led to intensive



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subaerial weathering and enrichment of the ironore, especially in the Ghorabi and Gedida mines.

Subaerial weathering and palaeosolformation

During the early Palaeocene, the southern Teth-yan region was characterized by a warm andhumid climate with high precipitation (Bolleet al., 2000; Bolle & Adatte, 2001). From the latePalaeocene onwards, a seasonal climate existedin Egypt (Bolle et al., 2000) and the PETM wascharacterized by a warm and humid climate(Ernst et al., 2006). This increased terrigenousinput and nutrients from continental areas to

the marginal marine basins, whereas the openocean may have remained oligotrophic(Scheibner & Speijer, 2008). An increase in theclay mineral contents in North and SouthAtlantic oceanic deposits from late Palaeoceneto middle Eocene indicates increased humidityand thus more continental run-off in low to midlatitudes, while increased evaporation tookplace in low-latitude coastal areas (Robert &Kennett, 1994; Kelly et al., 2005).In the Bahariya Depression, the northward sea

regressions culminating the early and middleEocene resulted in the exposure of the lowerand upper ironstone sequences to subaerialweathering and pedogenesis. The formation of

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Fig. 16. A comparison between the stratigraphic section of the Bahariya ironstones and (A) the global d18O curvesof palaeotemperatures (Miller et al., 2005); (B) the eustatic sea-levels (Miller et al., 2005); (C) the larger benthicforaminifera (Serra-Kiel et al., 1998); (D) the 39Ar–40Ar age of the lateritic palaeosols in western Africa (Beauvaiset al., 2008). (E) The time distribution of the Cenozoic ooidal ironstones (COIS) (Van Houten, 1992). The dottedcurve in panel (B) is the long-term fit of the sea-level curve.

Fig. 15. A depositional model of the lower ironstone sequence in the Ghorabi mine as an example of the complexinferred history of deposition and subaerial weathering.



the lateritic palaeosols on top of ironstonesequences was coeval with the karstification ofthe surrounding equivalent limestones. Thekarstification of the limestone resulted in theformation of cave carbonate deposits (El Aref &Lotfy, 1989). Lateritic weathering took placeunder warm and humid conditions and led tosupergene enrichment of iron via infiltration ofsurface meteoric fluids. Repeated mineral disso-lution and reprecipitation along fractures andwithin cavities resulted in formation of: (i) goe-thite stalactites and stalagmites with botryoidaland colloform textures; (ii) cockade textureswhere ooidal ironstone breccias are cemented bycrustified layers of colloform goethite; (iii)small-scale alveolar voids at sites of former plantroots; and (iv) vadose meniscus cement. Thesubaerial weathering also partly obliterated thesedimentary structures, fabrics and textures ofthe original marine ironstones.

Global palaeoclimatic significance of theBahariya ironstones

Comparing the global d18O and eustatic sea-levelcurves (Miller et al., 2005) and the biozonationof LBF (Serra-Kiel et al., 1998) with the39Ar–40Ar ages of the lateritic palaeosols fromwestern Africa (Beauvais et al., 2008)contributes to a better understanding of therelation between of early Palaeogene changes inclimate and sea-level (Fig. 16A to D). From thePalaeocene (ca 60 Ma) to middle Eocene (ca45 Ma), d18O was low and the eustatic sea-levelwas high (Zachos et al., 2001; Miller et al., 2005;Fig. 16). These conditions led to drowning of thelow-elevation coastal plains of northern Africafacing the Tethys Seaway. In the BahariyaDepression, the formation of lateritic palaeosolsat the end of the early and middle Eocenerequire a major fall in sea-level and subaerialexposure of the marine ironstones and the sur-rounding equivalent limestones at those times.Palaeosols such as those studied here arethought to have formed under a warm, sub-tropi-cal climate (Retallack, 2010); thus, they may con-stitute evidence of the early and middle Eoceneglobal warming. On a global scale, the formationof the Bahariya palaeosols can be compared withthose formed in Africa, Europe, South Americaand Australia during this time (Beauvais et al.,2008; Retallack, 2008, 2010; Krause et al., 2010;Thorne et al., 2012). The 40Ar/39Ar ages of K–Mnminerals in lateritic soils of western Africa (forexample, Burkina Faso) indicate that the lateritic

weathering commonly took place in short inter-vals (<100 ka) from 40 to 50 Ma, correspondingto the early and middle Eocene (Beauvais et al.,2008; Retallack, 2010). In Europe, the formationof nickel laterite in western Turkey and southernAlbania was also common in the early Eocene(Thorne et al., 2012). In SE Australia, bauxiticpalaeosols from the lower Eocene Monaro Volca-nics have been formed under a humid, warmclimate (Retallack, 2008). In South America,lateritized tephric palaeosols have formed in theCentral Patagonia, Argentine (Krause et al.,2010). These laterite peaks were coeval withtransient times of high levels of warmth, precipi-tation and atmospheric carbon dioxide (Retal-lack, 2010). The global significance of theBahariya ironstones lies in their link betweensea-level and climatic changes. This link mayexplain the global abundance of Cenozoic ooidalironstones (Van Houten, 1992) and lateriticpalaeosols (Retallack, 2010) in the lower andmiddle Eocene (Fig. 16).According to Zachos et al. (2001, 2003), the

early Palaeogene global warming had begun bythe early Palaeocene and continued into theearly Eocene, culminating at the EECO, and thenchanged in the middle Eocene to progressivelycooler conditions (e.g. Zachos et al., 2001). Thelateritic palaeosols capping the lower Eoceneironstone sequences may have been closelyrelated to the EECO. The palaeoclimatic trend ofthe middle Eocene sequence of the QazzunFormation is very similar to the lower Eoceneironstone sequence of the Naqb Formation; thissuggests that another spike of warm climate isrelated to the MECO.Following the eustatic change, the progres-

sive northward retreat of the sea at the end ofthe middle Eocene led to the incision and ero-sion of the lateritic palaeosol and the formationof channel iron ore conglomerates in the Gedi-da mine (El Aref et al., 1999). The mechanicalweathering of the middle Eocene lateritic palae-osol followed by deposition of upper Eoceneglauconitic sandstones rich in phosphate maycoincide with the first stages of the palaeocli-matic transition from greenhouse to icehouseconditions.

CONCLUSIONS

The north-eastern part of the BahariyaDepression hosts the main economic ooidalironstone succession in Egypt. Based on large



benthic foraminifera, this succession is subdi-vided into lower and middle Eocene shallowing-upward ironstone sequences. These sequencesrepresent condensed sections deposited on topof deeply weathered Cenomanian palaeohighs inthe Ghorabi, Harra and Gedida mine areas.Based on facies analysis, the ironstone

sequences were deposited during two majortransgressive–regressive events. These sequencesconsist of two main facies associations: (i) aquiet water lagoonal mud-ironstone facies andits laterally equivalent tidal flat stromatoliticironstone facies; and (ii) a shallow subtidal–intertidal nummulitic–ooidal–oncoidal (shoal)and storm-related fossiliferous ironstone facies.The lower Eocene sequence shows great lateral

and vertical variations in thickness and lithology,whereas the middle Eocene ironstone sequence isuniform in lithology. Large benthic foraminifera,ooids and oncoids are the main components ofthe ironstones. Ooids and oncoids are subdividedinto cored (parautochthonous) and uncored(autochthonous) types. The autochthonous ooidsand oncoids consist entirely of cortical laminae;they show an internal random microfabric andgradual contacts between the cortical laminaeand the surrounding matrix. The parautochtho-nous grains consist of locally reworked fossilclasts or fragments of the non-skeletal grains fromthe nearby shallow subtidal setting. Nummulitiesincrease in abundance at the top of the lower iron-stone sequence forming a regional marker horizonthat reflects maximum shallowing conditions.Palaeoclimatic and sea-level changes can be

deduced from marine and terrestrial proxies.Among the marine palaeoclimatic proxies is thedominance of large benthic foraminifera, inparticular nummulitids and alveolinids, at theexpense of coral-building reefs. This indicates anincrease in sea surface temperature that cannot betolerated by coral reefs. Among the terrestrial pal-aeoclimatic proxies is the formation of lateriticpalaeosols on top of the lower and middle Eocenemarine ironstone sequences. These palaeosolsrepresent a transition from normal marine to sub-aerial weathering associated with major marineregression under warm and humid conditions;they provide evidence of the early Palaeogeneglobal warming events.

ACKNOWLEDGEMENTS

The authors acknowledge the financial supportfrom the Deutsche Forschungsgemeinschaft

(Graduate School 1257 ‘Alteration and elementmobility at the microbe-mineral interface’). Inaddition, this work was supported by theGerman Academic Exchange Service (DAAD)through a two year PhD grant to the first author.The authors are indebted to Dr. G€unter V€olkschfor SEM and EDAX analyses. The authors areindebted to Ignacio Gonzalz-Alvarez and RobertThorne from CSIRO and the two anonymousreviewers of sedimentology for reviewing themanuscript and their extremely helpfulcomments and annotations.

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Manuscript received 26 February 2013; revisionaccepted 10 January 2014



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