Saiwan, Gharif and Khuff formations, Haushi-Huqf Uplift, Oman

Saiwan, Gharif and Khuff formations, Haushi-Huqf Uplift, Oman

149


Lucia Angiolini, Sylvie Crasquin-Soleau, Jean-Pierre Platel, Jack Roger,Daniel Vachard, Denis Vaslet and Moujahed Al-Husseini

ABSTRACT

Along the western flank of the Haushi-Huqf Upift in Oman, the upper Palaeozoicsuccession consists of (from oldest to youngest): (1) glaciogenic Upper Carboniferous-Lower Permian Al Khlata Formation; (2) marine Lower Permian Saiwan Formation (=Lower Gharif Member of subsurface Oman); (3) continental Lower and Middle Permianredefined Gharif Formation (= Middle and Upper Gharif members of subsurface Oman);and (4) lower part of the Middle Permian marine Khuff Formation. The successionoverlies lower Palaeozoic-Proterozoic rocks, and the Khuff Formation is truncated byTriassic and younger unconformities. The Al Khlata Formation is about 100 m (328 ft)thick, and consists of a succession of diamictite, sandstone enclosing pebbles to bouldersof sandstone, dolomite, black chert and pink granite clasts (ranging in diametre from afew centimetres to a metre). The overlying Saiwan Formation comprises two bioclasticunits: lower ‘Bellerophon Limestone’ (10-18 m, 33-59 ft thick) and the upper‘Metalegoceras Limestone’ (35-40 m, 115-131 ft thick). A basal Pachycyrtella Bed of theSaiwan Formation yielded Pachycyrtella omanensis associated with subordinatespecimens of the genus Strophalosia indicating a mid-Sakmarian age. Brachiopod,ammonoid and bivalve assemblages in the main part of the Saiwan indicate a lateSakmarian age. The Saiwan Formation contains (5 metres above its base in the typesection) Arabian Plate Maximum Flooding Surface MFS P10 of late Sakmarian age,and based on the latest Permian time scale is recalibrated at about 284 Ma (previously272 Ma).The redefined Gharif Formation (70-100 m; 230-328 ft) lies unconformably abovethe Saiwan Formation, and consists of shale and sandstone deposited in floodplainand ephemeral shallow-lake environments. Uppermost Gharif ‘estuarine’ subunit Blies conformably below the first marine Khuff deposits, and contains a rich macroflorathat is not diagnostic of a precise age, but is considered ?Roadian-?early Wordian. Theincomplete Khuff Formation (30 m, 98 ft) consists of three informal members (1-3 frombase up). The transition from Gharif subunit B to lowermost Khuff member 1 representsan environmental change from a distal fluviatile/estuarine system bordering a coastalplain, to a clastic transgressive shoal/barrier environment. Khuff members 2 and 3reflect a carbonate shelf environment. The lower sequence boundary of the Khufftransgression is interpreted to be at the base of Gharif subunit B. Marine fauna in theKhuff Formation includes cephalopods, brachiopods, conodonts, ostracods andbivalves, which indicate a Middle Permian Wordian age. Conodont fauna from theuppermost levels of Khuff member 3 suggests a late Wordian age. The foraminiferaindicate a Middle Permian age (Wordian and Capitanian).

INTRODUCTION

Along the western flank of the Haushi-Huqf Uplift (also Arch or High) in Oman, Upper Carboniferous-Middle Permian rocks outcrop in a SW-trending belt, and are mapped as the Al Khlata, Saiwan, Gharifand Khuff formations (Figure 1; Dubreuilh et al., 1992; Platel et al., 1992, Roger et al., 1992; hereaftercombined where appropriate as BRGM, 1992). The Permian Khuff Formation, in this region, is truncatedby several unconformities starting with the overlying Triassic Minjur Formation (Figure 2). Theseoutcropping units pass, in part, to the subsurface Al Khlata, Gharif and Khuff formations of InteriorOman (Hughes Clarke, 1988; Osterloff et al., 2004a, b). The outcropping Saiwan Formation becomesthe subsurface Lower Gharif Member; it was renamed by BRGM (1992) to highlight a regional angularunconformity at its top (supra-Saiwan unconformity, Figure 2), which is manifested in south and eastOman (Blendinger et al., 1990; BRGM, 1992; Osterloff et al., 2004b).

The stratigraphic record in this region represents the following four depositional phases: (1) Al KhlataFormation reflecting the Late Carboniferous-Early Permian glaciation of southern Arabia (Arabian Plate

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Angiolini et al., 2004

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Figure 1: The Al Khlata and Saiwan formations (Haushi Group sensu BRGM), and Khuff and redefinedGharif Formation (Akhdar Group sensu BRGM) outcrop along the western flank of the Haushi-HuqfUplift (also Arch or High). The Proterozoic to Cambrian Huqf Supergroup outcrops over the NE-trendingHaushi-Huqf Uplift along the Arabian Sea. The Haushi-Huqf region was mapped in three areas byBRGM in 1992: Duqm and Madraca (18.8°-20°N; Platel et al., 1992); Khaluf (20°-21°N; Dubreuilh et al.,1992); and Mafraq (21°-22°N; Roger et al., 1992).

Khaluf

Sirab

58 30'

57

Mafraqmap area

Khalufmap area

Duqm and Madracamap area

Huqf

JiddatAl Harrasis

Haushi

Arabian Sea

N30

km

0

Duqm

57

Hayy

Fiem

57 30' 58

58 30'

21

20 20

21

Saiwantype section

57 30' 58

19 30'

20 30'

20 45'

20 15'

19 45'

19 30'

20 30'

20 45'

20 15'

19 45'

Norther

n Cliff

Saiwan-1

Haush

i-Nafu

nF

au

lt

Wadi Al Khlata

Fars Group (Miocene-Pliocene)and Quaternary

Dhofar Group (Oligocene-Miocene)

Hadhramaut Group(Palaeocene-Eocene)

Aruma Group

Triassic-Late Cretaceous

Akhdar Group (Permian)(Khuff and redefined Gharif fms)

TERTIARY

END-CRETACEOUS

ARABIAN PLATFORM

Haushi Group (late Palaeozoic)(Al Khlata and Saiwan fms)Haima Supergroup andThumaylah fm (early Palaeozoic)Huqf Supergroup and (latestProterozoic to Cambrian)

Al Jobah granodiorite

Major anticline

Major fault

Limit of sand dunes

PRE-MIDDLE PERMIAN BASEMENT

CRYSTALLINE BASEMENT

Haushi-Huqf Uplift, Oman


151

Glaciation 3, AP G3 of Sharland et al., 2001); (2) Saiwan Formation (subsurface Lower Gharif Member)representing the first Early Permian post-glacial marine flooding (Maximum Flooding Surface MFSP10, Sharland et al., 2001); (3) a late Early and early Middle Permian period of continental depositionseen in the redefined Gharif (subsurface Middle and Upper Gharif members); and finally (4) the startof the Middle Permian transgression (Khuff and coeval rock units). These four phases prevailed overmost of southern Arabia and are recognized in the Wajid, Unayzah and Khuff formations of SaudiArabia (e.g. Evans et al., 1991).

Figure 2: The outcrop stratigraphic scheme of BRGM (Debreuilh et al., 1992; Platel et al., 1992; Roger etal., 1992) that is adopted in this study, is compared to the subsurface stratigraphic scheme (HughesClark, 1988; Osterloff et al. 2004a, b). Note the definitions of the Haushi and Akhdar groups areinconsistent between subsurface and outcrop. The Permian Khuff Formation in this region is truncatedby several unconformities. The geological time scale is adopted from Jin et al. (1997).

CHRONOSTRATIGRAPHY Ma

TRIASSIC ICS (2002) RUSSIA TETHYS

Changh-singian

Sudair Fm

Wuchia-pingian

Capitanian

Wordian

Roadian

Kungurian

Artinskian

Tatarian

Kazanian

Ufimian

Dora-shamian

Dzhulfian

Midian

Murgabian

Kuber-gandian

Bolorian

Mid

dle

(Gua

dalu

pian

)E

arly

(C

isur

alia

n)

PE

RM

IAN

Late

(Lo

ping

ian)

Sakmarian

Asselian

CARBONIFEROUS

251.1± 3.6

253.0± 0.3

264.1± 2.2

272.2± 3.2

280.3± 2.6

290.6± 3.0

INTERIOROMAN HAUSHI-HUQF UPLIFT

KhuffFormation

UpperGharif

Member

member 3member 2

member 1 unit Dunit C

subunit B

subunit A

Triassic andyounger

unconformities

MiddleGharif

Member

?

LowerGharif

Member?

'HaushiLimestone'

supra-Saiwan unconformity

SaiwanFormation

'Metalegoceras Lst''Bellerophon Lst'

Pachycyrtella bed

Rahab shale

Al Khlata Formation

Hau

shi G

roup

Hau

shi G

roup

Akh

dar

Gro

up

Akh

dar

Gro

up s

ensu

BR

GM

(19

92)

rede

fined

Gha

rif F

orm

atio

n

lower unit

uppe

r un

it

DS P17

DS P15

DS P10

DE

PO

SIT

ION

AL

SE

QU

EN

CE

P13

AR

AB

IAN

PL

AT

EG

LA

CIA

TIO

N 3

(AP

G3)

mfs

MFS P10284 Ma

ARABIANPLATE

Carboniferous-Permian Stratigraphy

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This paper starts with a brief review of the Al Khlata Formation (for a more detailed review see Osterloff etal., 2004a). This is followed by a detailed and updated overview of the lithostratigraphy, depositionalenvironment, sequence stratigraphy, fossils and age of the Saiwan, redefined Gharif and lower part of theKhuff formations. This overview provides important stratigraphic constraints on the depositional historyof the hydrocarbon-bearing Unayzah, Gharif and Khuff formations on the Arabian Plate.

HAUSHI AND AKHDAR GROUPS

The Haushi Group was first defined in the north of the Haushi-Huqf Uplift by Hudson and Sudbury(1959). Hughes Clarke (1988) and Petroleum Development Oman (PDO) (Osterloff et al., 2004b) adoptedthis group’s definition that combines the Al Khlata and Gharif formations (Figure 2). In contrast, BRGM(1992) define the Haushi Group to comprise the Al Khlata and Saiwan formations, the latter beingequivalent to the Lower Gharif Member (Figure 2). The difference in where to place the upper boundaryof the Haushi Group is caused by the identification of a significant unconformity at the top the SaiwanFormation (supra-Saiwan unconformity, Figure 2) (Blendinger et al., 1990; BRGM, 1992). The supra-Saiwan unconformity constitutes an important surface in understanding the stratigraphic frameworkof the Arabian Plate. According to Le Métour et al. (1995), the unconformity has a regional dip of 10o-15o and possibly marks a hiatus in the Early-Middle Permian, Artinskian-early Roadian (Angiolini etal., 1997, 2001).

The redefinition of the Lower Gharif Member as the Saiwan Formation (BRGM, 1992) cascaded severalother redefinitions that encompass much of the Carboniferous-Permian lithostratigraphic scheme inthe Haushi-Huqf Uplift; namely (Figure 2):

(1) Redefinition of the Lower Gharif Member as the Saiwan Formation;(2) Limitation of the redefined Gharif Formation to the Middle and Upper Gharif members;(3) Inclusion of only the Saiwan Formation together with the Al Khlata Formation in the Haushi

Group sensu BRGM (otherwise the group comprised the Al Khlata and Gharif formations,Hughes Clarke, 1988); and

(4) Inclusion of the redefined Gharif Formation (Middle and Upper Gharif members) in the AkhdarGroup sensu BRGM.

The above redefinitions are carried in parallel with the subsurface definitions; i.e Saiwan = LowerGharif Member, and redefined Gharif Formation = Middle and Upper Gharif members.

AL KHLATA FORMATION

Hughes Clarke (1988) credited B.K. Levell, J.H. Braakman and K.W. Rutten (1982 PDO Report) andLevell et al. (1988) with the definition of the type section of the Al Khlata Formation. Hughes Clarkereported that this type section is located at Wadi Al Khlata near the Huqf Arch (57o25’46”E, 19o46’43”N;Figure 1) where it is about 100 m (328 ft) thick. This type section, however, was not described in Levellet al. (1988), but is referenced to some degree by Besems and Schuurman (1987).

The Al Khlata Formation was mapped in three areas at a 1:250,000 scale by BRGM (1992). In outcropson the western flank of the Haushi-Huqf Uplift, the deposits are considerably thinner than in thesubsurface and possibly no more than 100 m (328 ft), compared to 100-800 m (328-2,624 ft; Osterloff etal., 2004a). Measured Al Khlata Formation sections (Figure 3; BRGM, 1992) indicate a succession ofdiamictites of grey to beige sandstone enclosing pebbles to boulders of white to green sandstone,dolomite, black chert and pink granite, ranging from a few centimetres to a metre in diametre. Thecoarsest deposits are associated with sandy conglomerates and cross-bedded, coarse-grained sandstonethat contains gravel and layers of silty-claystone and locally contorted shale. They are characterised bya topography of gently rounded buttes from which boulders are eroded that can be several metresacross. Ferruginous crusts in the form of festoons highlight the grain-size differences between the verycoarse facies and the commonly grey-beige to blue-grey, claystone-siltstone, fluvio-lacustrine facies(BRGM, 1992). At Saiwan, the uppermost part of the formation consists of red fine-grained sandstonesand green siltstones ascribed to the Rahab shale by Roger et al. (1992, p. 13) and described as an equivalentunit by Angiolini et al. (2003a).

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The Al Khlata Formation unconformably overlies much older Palaeozoic rocks. East of Jebel Murqayshthe Al Khlata Formation rests on a striated pavement scoured across the top of the upper ProterozoicKhufai Formation (Debreuilh et al., 1992; Platel et al., 1992). In the Mafraq map area, the Al Khlatadeposits rest unconformably on the lower Palaeozoic Haima Supergroup, and the Cambrian-ProterozoicHuqf Supergroup. The top of the Al Khlata Formation passes unconformably into fossiliferous sandstoneof the overlying Saiwan Formation.

The Al Khlata Formation consists of a glacial-fluvioglacial complex attributed to the Gondwananglaciation (Braakman et al., 1982). On the basis of palynology, these deposits are attributed to the LateCarboniferous (late Westphalian (Moscovian)) to Early Permian (Sakmarian) (Braakman et al., 1982;Besems and Schuurman, 1987; Hughes Clarke, 1988; BRGM, 1992; Stephenson, 1988; Stephenson andFilatoff, 2000; Stephenson, 2004).

SAIWAN FORMATION

Lithostratigraphy

The Saiwan Formation (Figure 4; Debreuilh et al., 1992) unconformably overlies the diamictite andsilty shale (presumed Rahab shale of the Al Khlata Formation) and combines all the fossiliferous marinebeds informally named the ‘Haushi Limestone’ (Hudson and Sudbury, 1959; Blendinger et al., 1990).

Yellow, fine-grained, laminated sandstone

Diamictite containing large blocks of dolomite, granite, sandstone, stromatolites

White to red, fine-grained sandstone; thin beds of coarse-grained sandstone

Small-block diamictite

Beige siltstone and claystone

Dolomite substratum with striated pavement

Sandstone, red granite and black chert bearing diamictite

Brown siltstone and fine-grained, laminated sandstone

Diamictite with cobbles and boulders of dolomite, sandstone, quartz and black chert; sandstone matrix

Massive, poorly stratified sandstone with lenses and amygdales of clayey siltstone

Very coarse-grained diamictite with white, chert nodule bearing dolomitic blocks 40 to 100 cm across

LOCALSTRATI-GRAPHY

LITHOLOGY and COMMENTSARABIAN PLATE

SEQUENCESTRATIGRAPHY

Al Khlata Formation, Wadi Al Khlata, Oman

Al K

hlat

a F

orm

atio

n

UpperProterozoic

Khufai Formation

late

Wes

tpha

lian

(Mos

covi

an)-

Sak

mar

ian

Car

boni

fero

us-L

ower

Per

mia

n

PAL

AE

OZ

OIC

60

50

40

30

20

10

0 m pre-Khlata unconformity

AP1

AP5

Arabian PlateGlaciation 3

(AP G3) deposits

Figure 3: The Al Khlata Formation outcrops in Wadi Al Khlata in Oman (57o25’46”E, 19o46’43”N), whereit is about 100 m thick (Besems and Schuurman, 1987; Hughes Clark, 1988). The measured section isafter Platel et al. (1992). The Al Khlata Formation overlies the upper Proterozoic Khufai Formation ofthe Nafun Group, Huqf Supergroup. See Figure 1 for location of Wadi Al Khlata.

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Figure 4: Stratigraphy of the Saiwan and redefined Gharif formations in the Khaluf map area, Oman(modified after Dubreuilh et al., 1992). The Saiwan Formation contains Lower Permian SakmarianMaximum Flooding Surface MFS P10 in Depositional Sequence DS P10. The supra-Saiwan unconformityextends across south and east Oman. The type section outcrop is near the Saiwan-1 well (see Figure 1),and is defined between 20o52’04”N, 57 o36’27”E, and 20 o52’26”N, 57 o36’26”E (Angiolini et al., 2003a).

LOCALSTRATIGRAPHY LITHOLOGY and COMMENTS

ARABIAN PLATESEQUENCE

STRATIGRAPHY

Saiwan type section and redefined Gharif formations, Khaluf map area, Oman

rede

fined

Gha

rif F

orm

atio

n

PE

RM

IAN

MID

DLE

Roa

dian

-Wor

dian

LOW

ER

Sak

mar

ian-

Kun

guria

n

PAL

AE

OZ

OIC

uppe

r un

itlo

wer

uni

tS

aiw

an F

orm

atio

n

Al KhlataFm

KhuffFm

Interbedded platy, grey limestone with crinoids, bivalves and brachiopods, and grey marl

Interbedded sandy shale and brown sandstone

Red sandy shale

Grey-white, coarse-grained, cross-bedded sandstone with pebbly beds, containing silicified wood

Channelized, matrix-supported conglomerate

Interbedded coarse-grained, cross-bedded sandstone with grey conglomeratic beds and green silty shale with marbling

Green shale with red marbling

White sandstone

Grey shale with red marbling

Red sandstone with silicified woodPachycyrtella omanensis

Diamictite with large boulders of crystalline rocks

Interbedded hard brown cross-bedded sandstone with brachiopods, bivalves and abundant crinoids, and yellow sandy marl

Green shale

Interbedded yellow sandstone, fine-grained argillaceous sandstone, green shale and bioclastic limestone containing abundant crinoids, large brachiopods, bryozoans and Bellerophon

"Met

aleg

ocer

as li

mes

tone

""B

elle

roph

onlim

esto

ne"

0 m

10

20

30

40

50

60

70

80

90

100

110

120 pre-Khuff unconformity


DS P17

DS P15

DS P13

DS P10

AP G3pre-Saiwan unconformity

MFS P10284 Ma


155

The Saiwan Formation comprises two bioclastic units: the lower ‘Bellerophon Limestone’ (10-18 m, 33-59 ft thick), and the upper ‘Metalegoceras Limestone’ (35-40 m, 115-131 ft thick). Dubreuilh et al. (1992)indicate that in the kilometre-sized domal structures around the Saiwan-1 well (57o39’43”E, 20o54’00”N;2.5 km west of the Saiwan airstrip, Figure 1), the continental siliciclastic sedimentary rocks of theredefined Gharif Formation unconformably overlie the Saiwan Formation.

The Saiwan Formation was adopted by many authors (e.g. BRGM, 1992; Béchennec et al., 1993; LeMétour et al., 1995; Broutin et al., 1995; Angiolini et al., 1997, 2001, 2003a), and corresponds, in part, tothe Lower Gharif Member (Osterloff et al., 2004b). The unpublished type section in outcrop of theLower Gharif Member is located in the western Huqf region (57o22’55”E, 19o48’26”N) and northern Huqfregions (57o37’29”E, 20o50’45”N), and is 200 m (656 ft) thick in composite section (Hughes Clarke, 1988).

The Saiwan Formation was mapped as part of the Haushi Group sensu BRGM in three geological mapareas at 1:250,000 scale in Oman (BRGM, 1992), shown in Figure 1. In the southeast of the Mafraq maparea, the limited exposures of the Al Khlata and Saiwan formations in folded and faulted structures,makes it necessary to combine the Al Khlata and Saiwan formations in the 1:250,000 scale maps, into asingle undifferentiated unit (CPak-swn), even though they are quite easily distinguished on the ground.

The type section of the Saiwan Formation occurs in outcrop near the Saiwan-1 well in the Khaluf area(Figures 1 and 4; Dubreuilh et al., 1992), and is defined between 20o52’04”N, 57o36’27”E, and 20o52’26”N,57 o36’26”E (Angiolini et al., 2003a). In its type section, the formation attains a thickness of 50 m (164 ft)and the boundaries with overlying and underlying units are exposed. In the Duqm and Madraca areas(Platel et al., 1992), the Saiwan Formation is 20-40 m (66-131 ft) thick (Photo 1). In the Mafraq area(Figure 5; Roger et al., 1992), the Saiwan Formation is probably some 50 m (164 ft) thick, as in the type-section, although poor exposure make this difficult to ascertain.

In the Mafraq area, the Saiwan Formation consists of two separate units (Roger et al., 1992; Angiolini etal., 2001, 2003a) that are similar to the type section (Figure 4).

Photo 1: The Saiwan Formation outcrops in Jebel Gharif of Oman. Bioclastic hybrid limestones showstrong laminations; among the bioclasts, brachiopods are dominant. (Photo by Lucia Angiolini)

Photo 1



156

The two units of the Saiwan Formation are described as follows: Saiwan lower unit ‘BellerophonLimestone’: about 5-10 m (16-33 ft) thick, consists of rusty-brown, cross-bedded quartz sandstonecontaining brachiopods, cephalopods, bivalves, gastropods, bryozoans, conularids and crinoids. Thesandstone beds are 1-2 m (3-7 ft) thick, and include rounded quartz and granite pebbles reworked fromthe Al Khlata Formation, interbedded with several layers of green silty-claystone. The lowermost 40-60cm of the Saiwan lower unit consist of cross-laminated bioclastic sandstone, named the Pachycyrtellabed (Angiolini et al., 2003a) (Photos 2-3). It contains silicified fossil wood as well as large spiriferinidbrachiopods.

Figure 5: The section of the Saiwan Formation in the Mafraq map area in Oman(modified after Roger et al., 1992).

MFS P10284 Ma



STRATIGRAPHY

Saiwan and redefined Gharif formations, Mafraq map area, Oman

rede

fined

Gha

rif F

orm

atio

nS

aiw

an F

orm

atio

nK

huff

Fm

Al K

hlat

aF

m

Bioclastic limestone

Sandy limestone tocalcareous sandstone

Bioclastic marl

ClaystoneSandy claystoneto siltstone

Sandstone

Conglomeratic lag

Diamictite

Plant remains(silicified wood)

Brachiopods

Crinoids

Bivalves

Coal flakes

Burrows

Ammonoids andNautiloids

Ripple marks

Cross-bedding

120

100

80

60

40

20

0 m

Alternating, cross-bedded, coarse- to medium-grained whitish sandstone with gravel lenses and silicified fossil logs, and red or green silty claystone.

Inter-bedded, cross-bedded sandstone, sandy claystone and highly bioclastic sandy limestone with bivalves, gastropods, brachiopods, crinoids, and bryozoans.


pre-Saiwan unconformity

Grey bioclasticlimestone andmarl

Transitionalboundary

LEGEND(Figures 5 and 6)

DS P10

DS P13

DS P17

DS P15

AP G3

PE

RM

IAN

MID

DLE

Roa

dian

-Wor

dian

LOW

ER

Sak

mar

ian-

Kun

guria

n

PAL

AE

OZ

OIC


157

Photo 3: Detail of the Pachycyrtella bed at the base of the Saiwan Formation (Figure 4), showing thecharacteristic large spiriferinids which colonised, during the Sakmarian time, the shallow marineenvironments at the end of the Gondwanan deglaciation. (Photo by Lucia Angiolini)

Photo 2: The base of the Saiwan Formation in Saiwan area of Oman showing cross-laminated bioclasticsandstones of the Pachycyrtella bed (see Figure 4). (Photo by Lucia Angiolini)

Photo 2

Photo 3


158

Saiwan upper unit ‘Metalegoceras Limestone’: about 40 m (131 ft) thick, is made up of beds of alternatingcross-bedded calcarenitic sandstone and laminated sandy calcarenite, bioclastic limestone and marlstone.The sandstones are yellow-brown to white, in units several metres thick (2-6 m; 7-20 ft), commonlycoarse-grained and cross-bedded. The deposits are coarse- to fine-grained and contain a mixture ofquartz, feldspar and lithoclastic grains. These units are intercalated with highly fossiliferous limestonebeds about 10-30 cm thick along with subordinate micaceous green shale. They include an abundant,fragmented and highly diverse macrofauna of brachiopods, cephalopods, bivalves, gastropods, corals,conularids, ostracods, crinoids and trilobites, recording a significant increase of biodiversity.

Depositional Environment and Sequence Stratigraphy

The Saiwan Formation represents the first post-glacial Permian transgression (BRGM, 1992), the marginof which flooded the western part of the Haushi-Huqf palaeohigh. The shallow-marine deposits(shoreface) of this formation are characteristic of a high-energy environment. During this period, stronginfluxes of terrigenous material were derived from the emergent Haushi-Huqf Uplift.

Sharland et al. (2001) picked Arabian Plate Maximum Flooding Surface MFS P10 within bioturbatedshales immediately below the ‘Haushi Limestone’ within the Lower Gharif Member (Saiwan Formation).The maximum flooding surface is recorded by bioclastic sandstone located about five metres above thebase of the formation in the Saiwan type-section (Figure 5).

Fossils and Age

The very rich faunal content of the Saiwan Formation includes brachiopods, bivalves, gastropods,conularids, bryozoans, crinoids, barnacles, ostracods, fishes, asterozoans, trilobites, corals, nautiloidsand few ammonoids. The significant biodiversity coupled with the spectacular preservation of themacrofossils make them the object of numerous palaeontological studies since the fifties (Miller andFurnish, 1957; Hudson and Sudbury, 1959; BRGM, 1992; Angiolini et al., 1997, 2001; Jell and Willink,1993), besides those provided by Petroleum Development Oman.

Brachiopods dominate the fossil associations throughout the formation (Photo 4); only in its upperpart articulate crinoids and molluscs are locally very abundant. Based on the Unitary Association methodof Guex (1991), two brachiopod biozones have been established by Angiolini et al. (1997, 2003a): (1)Pachycyrtella omanensis Zone at the base of the formation; and (2) Reedoconcha permixta-Punctocyrtellaspinosa Zone immediately above. The basal Pachycyrtella Bed yields Pachycyrtella omanensis Angiolini,2001 associated with usually subordinate specimens of Strophalosia sp., bivalves (Cypricardinia? aff.elegantula, Myalinella sp., Nuculopsis cf. bangarraensis, Aviculopectinidae gen. et sp. indeterminate),gastropods, conularids, crinoids, bryozoans and barnacles. A few metres above, the brachiopodassemblage abruptly changes, recording a higher taxonomic diversity with the brachiopods Derbyiaharoubi Angiolini, 1997, Arctitreta cf bioni (Reed, 1932), Reedoconcha permixta (Reed, 1932) Neospirifer aff.hardmani (Foord, 1890), Neospirifer sp., Trigonotreta sp., Subansiria sp., Punctocyrtella spinosa Plodowski,1968, Cyrtella sp., Punctospirifer sp., Callispirina sp., Fletcherithyris sp., and ?Gilledia sp. (Reedoconchapermixta-Punctocyrtella spinosa Zone) associated with bivalves, gastropods, crinoids, conularids, rareammonoids (Metalegoceras hudsoni) and nautiloids. At the top of the Saiwan Formation, the taxonomicdiversity decreases again, the fossiliferous content chiefly consisting of Neospirifer, bivalves, gastropodsand spectacular articulated crinoids (Texacrinus sp.).

BRGM (1992) reported the following list of molluscs from the Saiwan lower unit (‘BellerophonLimestone’): Bellerophon cf., triangularis, Bellerophon spp., Bembexia sp., Murchisonia sp., Procrassatellaamarassiensis, Pseudomonotis sp., Schizodus sp., Stutchburia pallasi sp., and Nucula sp. Accordingly, theupper part of the Saiwan Formation (‘Metalegoceras Limestone’) is endowed with a comparablemolluscan fauna but is characterised by very abundant crinoid parts and a slightly different brachiopodassociation, and more common occurrence of ammonoids (Metalegoceras hudsoni) and orthoconicnautiloids. An ostracod association occurs in some of the shaley horizons with Hollinella emaciata,Roundyella aff. simplicissima, Monoceratina cf. parvula and Cavelinella nebrascensis.


159

Photo 4: Lower Permian, Sakmarian brachiopods of the Saiwan Formation. (Photos by Lucia Angiolini)(1) Pachycyrtella omanensis Angiolini, 2001; dorsal view of an articulated specimen (natural scale).(2) Pachycyrtella omanensis Angiolini, 2001; dorsal valve showing barnacles borings (natural scale).(3) Pachycyrtella omanensis Angiolini, 2001; ventral valve bearing an attached specimen of Strophalosia sp.(4) Pachycyrtella omanensis (Angiolini, 2001), several articulated specimens with variable orientation.(5)Trigonotreta sp. dorsal valve (magnification x2.6)(6) Callispirina sp. ventral valve (magnification x2.6).

1 2

34

65

Photo 4


160

In addition, the following fauna is mentioned in published literature, along with that provided byPetroleum Development Oman: bryozoans Fenestella sp., Leiocloma cf. globosa; brachiopods Spiriferellinasp., Dielasma sp., Asyrinx haushiensis (=Pachycyrtella omanensis); gastropods Meekospira sp., Soleniscus(Strobeus) mucronatus; bivalves Astartella permocarbonica, Myalina sp., Nucula sp., Procrassatellaamarassiensis, Pseudomonotis sp., Schizodus sp., Stuchburia pallasi; cephalopods Eoasianites sp.;pseudorthoceratids; various orthocones; the nautiloid Gzheloceras uralense; the trilobite Pseudophillipsialipara and the ostracod Roundyella aff. simplicissima.

According to Miller and Furnish (1957), this fossil fauna indicates that the Saiwan Formation is of EarlyPermian, Sakmarian-Artinskian age; whereas Hudson and Sudbury (1959) indicate a Sakmarian agefor the formation.

Angiolini et al. (1997, 2001, 2003a) identified Pachycyrtella omanensis in the basal Pachycyrtella Bed of theSaiwan Formation which is associated with a smaller number of specimens of the genus Strophalosia.The latter genus straddles the early-late Sakmarian boundary, and they therefore attributed a mid-Sakmarian age for the base of the Saiwan Formation. They add that brachiopod, ammonoid and bivalveassemblages in the main part of the Saiwan indicate a late Sakmarian age, as documented by theoccurrence of genera such as Arctitreta, Punctocyrtella, Subansiria, Fletcherithyris and species of Reedoconcha,Trigonotreta and Neospirifer, all belonging to the Reedoconcha permixta-Punctocyrtella spinosa Zone. This issupported by correlation with the western and eastern Australian Permian biostratigraphic zonations(Archbold, 1998; Briggs, 1998). According to Love (1994), Stephenson and Osterloff (2002), andStephenson et al. (2003) the subsurface Lower Gharif Member is Artinskian based on palynologicalevidence.

Sharland et al. (2001) follow the age interpretation of Angiolini et al. (1997, 2001) and consider MFS P10as late Sakmarian with an age of about 272 Ma according to the scale of Gradstein and Ogg (1996).Based on the latest Permian age calibrations (Jin et al., 1997; Menning et al., 1997; Menning, 2001a,b;International Commission Stratigraphy website, 2002) the Sakmarian spans the period 287-280.3 Ma, suchthat if MFS P10 is indeed late Sakmarian, then it should be dated at about 284 Ma (rather than 272 Ma).

REDEFINED GHARIF FORMATION

Lithostratigraphy

The redefined Gharif Formation of BRGM (1992) corresponds in part to the subsurface Middle andUpper Gharif members (Hughes Clarke, 1988; Guit et al., 1985; Osterloff et al., 2004b). It comprises thedeposits that lie unconformably above the Saiwan (Lower Gharif Member or Haushi Limestone) or AlKhlata formations, and conformably below the first marine deposits of the Khuff Formation. Theredefined Gharif Formation is mapped in the Mafraq, Khaluf, and Duqm-Madraca areas (BRGM, 1992).It is some 70-100 m (230-328 ft) thick at outcrop in the Haushi area (Figures 4-6). The Middle andLower Gharif members are proposed as Depositional Sequences DS P13 and DS P15 (Osterloff et al.,2004b). Near the Saiwan-1 well (Figures 1 and 4) and in the western Saiwan area, the formation consistsof two units (BRGM, 1992).

Redefined Gharif Formation, lower unit

This unit is 30-40 m (98-131 ft) thick, and consists of mottled shale layers, intercalated with severalmetre-thick beds of white, cross-bedded sandstone. The sandstone is typically coarse-grained andincludes lenses of fine conglomerate with rounded centimetre-sized clasts, plus fragments of silicifiedfossil wood in some beds. The finer-grained layers grade from claystone to siltstone and are grey-greento brick-red in color. It seems probable that the outcropping redefined Gharif lower unit is equivalentto the subsurface Middle Gharif Member subunit 3 (Osterloff et al., 2004b).


161

Redefined Gharif Formation, upper unit

The upper unit is some 30-35 m (98-115 ft) thick, and consists of white-grey to locally brown, generallycoarse-grained, cross-bedded sandstone. It contains conglomerate lenses with rounded polygeneticclasts of mainly quartz and granite, and typically includes fossil wood, determined to include theginkgophyte Baieroxylon cf. implexum. The sandstone sequences are broadly upward-fining and thinning,and terminated by commonly lenticular, green or red-mottled shale. This becomes more common and



STRATIGRAPHY

Redefined Gharif and Khuff formations, Haushi region, Mafraq map area, Oman

TRIASSICMinjur Formation

Khu

ff F

orm

atio

nre

defin

ed G

harif

For

mat

ion

80

70

60

50

40

30

20

10

0 m

Varicolored claystone and sandstone with ferruginous crust

Interbedded grey bioclastic limestone, calcarenite and bioclastic marl

Bioclastic sandstone

Alternating sandstone with coal flakes and pyritic shale

Alternating cross-bedded coarse- to medium-grained whitish to reddish sandstone with gravel lenses, silicified fossil logs and red or green silty claystone

DS P17

AP6

AP5

DS P15

DS P13

PAL

AE

OZ

OIC

PE

RM

IAN

MID

DLE

Roa

dian

-Wor

dian

pre-Minjur unconformity

Figure 6: Lithostratigraphy of the redefined Gharif and Khuff formations in Mafraq map area, Oman(modified after Roger et al., 1992) shows the base of the marine Khuff Formation is picked at the base ofthe bioclastic sandstone (Khuff unit C in Figures 7 and 8). The sequence boundary, however, may occursome 15 m lower at the top of Depositional Sequence P15 (base Gharif subunit B in Figures 7 and 8).


162

Figure 7: The top sequence boundary of Depositional Sequence Permian 15 (DS P15) in the Haushioutcrop is picked at the base of the Upper Gharif subunit B (Crumeyrolle et al., 1997; Angiolini et al.,2001). Subunit B (also red beds) corresponds to the ‘middle unit’ of Broutin et al. (1995) and representsan estuarine meandering channel complex as shown in the Northern Cliff (Figure 8).

LITHOLOGYLOCAL STRATIGRAPHY ENVIRONMENTSEQUENCE

STRATIGRAPHY

low

er

subu

nit A

uppe

r G

harif

san

dsto

ne

basa

l uni

t

red

beds

red

beds

mid

dle

unit

uppe

r un

it

subu

nit B

Mem

ber

1M

embe

r 2

MID

DLE

Roa

dian

rede

fined

Gha

rif F

orm

atio

n

Wor

dian K

huff

For

mat

ion

Bro

utin

et a

l.(1

995)

Cru

mey

rolle

et

al.

(199

7);

Ang

iolin

i et a

l. (2

001)

Mar

ine

faun

a

CoastalPlain

Fluvial

Marine

AP6

AP5

PAL

AE

OZ

OIC

PE

RM

IAN

SequenceBoundary

Cla

ysto

ne

Silt

ston

e/M

arl

Fin

e sa

ndst

one

Med

. san

dsto

ne

Coa

rse

sand

ston

e

Con

glom

erat

e

50 m

Boundary of Gharif and Khuff formations, Huqf, Oman

C

D

DEPOSITIONALSEQUENCE P15

DEPOSITIONALSEQUENCE P17

mfs

Gha

rifpa

laeo

flora

Con

tinen

tal f

lora

Muscat

Arabian Sea

Gulf of Oman

OMAN

SAUDIARABIA

UAE23°

21°

19°

585654

21

19

23

54 56 58 60

0 150

km

NorthCliff

Hau

shi-H

uqf U

plift


163

more silty or sandy through the top 15 m (49 ft) of the formation, immediately beneath the lowestfossiliferous beds of the Khuff Formation. The uppermost few metres consist of thinly-interbedded,dark grey carbonaceous shale and gritty sandstone lenses.

Located east of the Haushi anticline (Figures 1, 7 and 8), in the ‘Northern Cliff’ (about 20-30 m or 67-100ft high; 21o00’34” N, 57 o39’35” E and 21 o02’09” N, 57 o42’22” E), the redefined Gharif upper unit hasbeen further subdivided into subunits A and B (Figures 7 and 8; Broutin et al., 1995; Crumeyrolle et al.,1997; Razin and Platel, 1998; and Angiolini et al., 2001).

Redefined Gharif upper unit, subunit A has a maximum exposure of 50 m (164 ft) in the NorthernCliff, and consists of plurimetre-thick, fining-upward sequences. It comprises channel bodies of coarse-grained sandstone, showing festoon cross-bedding, interbedded in variegated silt clays. Red palaeosolhorizons with numerous root traces occur between the sequences and indicate prolonged periods ofemersion. The sandstone occurs as single or multi-storey channels locally showing erosive scours. Thebase of each sand body is sharp and erosive, and shows very coarse to pebbly sand with large-scaletrough cross-bedding.

Redefined Gharif upper unit, subunit B is about 1-15 m (3.3-49 ft) thick and overlies subunit A alongan irregular surface showing incised channels up to 10 m deep (33 ft) deep (Figure 8).

Osterloff et al. (2004b) consider the outcropping redefined Gharif upper unit to correspond to the UpperGharif Member of subsurface. In particular, subunit B may correspond to the subsurface uppermostCycle 8 of the Upper Gharif Member, beneath the main Khuff transgressive unit.

Figure 8: The top sequence boundary of Depositional Sequence P15 is interpreted in the Northern Cliff,east of the Haushi structure (Crumeyrolle et al., 1997). The sequence boundary (red) is placed at theunconformity below the estuarine meandering channel complex (subunit B) that cuts into the UpperGharif braided fluvial channel sand sheet (subunit A).

Gharif and Khuff formations, Haushi Northern Cliff, Oman

Shallow marine carbonates and clastics

Transgressive shoal/barrier clastics

Braided fluvial channel sand sheet

Khuff Formation

unit D

unit C

estuarine meandering channel complex

10 m

1 2 km

subunit BRedbeds

subunit Aor basal

unitupper Gharif Formation

Southwest Northeast


164


The depositional environment of the redefined Gharif lower unit is characterised as floodplain and/orephemeral shallow-lake environments as indicated by clayey sand flats with the development ofpalaeosols and overbank and/or minor sand channels (e.g. Angiolini et al., 2001). The interbeddedsandstones are in places bioturbated indicating over bank deposits within a flood plain and/orsubaqueous environment. Current directions are to the north and to 100o N.

Angiolini et al. (2001) add that some sections show cosets of 2-D and 3-D megaripples and ripples;however, their paucity suggests deposition did not occur in a large water body. These sandstone bodiesare sheet-like deposits with no discernible lateral or downstream accretion. Restricted areas of cross-bedding are separated by wider zones of tabular sets which possibly define areas of more persistentchannel flow and areas of sand flat in the inferred braid plain. These overlie a floodplain facies withmud cracks and palaeosols.

In the south Mafraq and north Khaluf map areas, the redefined Gharif lower unit gradually disappearsso that in places the redefined Gharif upper unit rests directly on the Al Khlata Formation (BRGM,1992). This pattern indicates progressive onlap across the Haushi-Huqf Uplift. In south Mafraq, thesandstone beds reach a cumulative thickness of some 15 m (49 ft) and consist of cross-bedded, coarse-grained sandstone, containing lenses of gravel, silicified wood fragments and large seeds (Gharif subunitA). The sandstone beds have a well-defined channel form marked by oblique bedding related to thelateral migration of point bars. The channels cut into predominantly red-coloured claystones of afloodplain facies (Gharif lower unit).

The sandstone bodies of the redefined Gharif upper unit, subunit A (BRGM, 1992; Angiolini et al.,2001) form extensive channel belts interpreted as sandy sheets of broad braided to low-sinuosity riversystems. Large-scale accretion surfaces and low-angle dipping surfaces in a down-current directionare seen locally in massive, coarse-grained facies that are interpreted as longitudinal or transversealluvial bar deposits. The depositional environment is interpreted as an alluvial floodplain.

The uppermost subunit B of the redefined Gharif upper unit is made up of three types of interbeddedfacies:

Subunit B, facies 1 is composed of coarse to very coarse sandy lenticular bodies developed along largelow-angle dipping surfaces. Bi-directional cross-bedding along the main accretion surfaces, as well asnumerous clay chips and drapes, indicate that flood tidal currents were active.

Subunit B, facies 2 consists of heterolithic sandstone-claystone bodies, some 100 to 1,000 m in width,characterised by large inclined cross-stratifications; these heterolithic facies are made up of coarse- tofine-grained, sheet like, graded, sandstone sets varying in thickness from a decimetre to severaldecimetres, interbedded with dark to variegated laminated silty claystone with scattered plant debris. Theseare interpreted as coastal plain deposits associated with distribution channels filling vast incised valleys.

Subunit B, facies 3 is a dark laminated silty shale with subordinate interbeds of medium to fine-grainedsandstone lenses. These deposits are considered as swamp claystones with very rich organicaccumulations made up of plant debris and pollen.

Angiolini et al. (2001) consider Upper Gharif subunit B deposits as representing a system of coalescentestuarine meander bars filling a vast incised valley. The occurrence of cyclic beds with climbing ripples,and upper flow regimes parallel laminations, suggests fluvial, flood-dominant regime periods duringpoint bar migration. The occurrence of episodic mud flasers and drapes, sigmoidal cross-bedding andbi-directional cross-bedding, also suggests some tidal influence. Periods of weak fluvial discharge, followedby abandonment, correspond to the infill of organic shales within laterally juxtaposed clayey marshes.

Fossils and Age

The age of the redefined Gharif lower unit is not well constrained because of poor fossil remains(Angiolini et al., 2001). Silicified tree trunks, in places several metres long, are common in the redefined


165

Gharif Formation. Subunit B of the uppermost Gharif Formation contains a rich macroflora (Gondwanantaxas as Glossopteris, Cathaysian taxas as Gigantopteris, Euramerian taxas as Otovia (Walchia) hypnoides),and microfloral assemblage including numerous spores and monosaccate, non striate and striatebisaccate pollen grains (Broutin et al., 1995; Angiolini et al., 2001). This flora is not diagnostic of aprecise age, but considered by these authors as ?Roadian-?early Wordian. According to P. Osterloff(2003, written communication) the sandstone bodies of the redefined Gharif upper subunit in outcropmay most likely be ?Kungurian-?Roadian and as young as Kazanian (Roadian-early Wordian).

The following taxa were determined by J. Broutin (1995):

Trilete spores: Laevigatosporites vulgaris, Leiotriletes spp., Retusotriletes cf. simplex, Calamaspora spp.,Verrucosisporites sp., Cyclogranisporites arenosus, Polypodiisporites cf. mutabilis, Cristatisporites sp., Brazileasp., Chordecystia sp.

Monosaccate pollen: Florinites pellucidus, F. eremus, Potonieisporites novicus, Potonieisporites sp.,Playfordiaspora cancellosa, Scheuringipollenites ovatus, Scheuringipollenites spp.

Not-striate bisaccate pollen: Vitreisporites pallidus, Vesicaspora schemeli, Falcisporites sp., Gardenasporitessp., Cedripites priscus, Platysaccus sp.

Striate bisaccate pollen: Protohaploxypinus amplus, P. devinensis, P. hartii, P. limpidus, P. microcorpus, P.pennulatus, P. perfectus, Lunatisporites cf. pellucidus, Lunatisporites sp., Lueckisporites singhii, L. virkkiae,Corisaccites alutas, Striatoabietites multistriatus, S. richteri, Striatopodocarpites cancellatus, S. fusus,Weylandites lucifer.

KHUFF FORMATION

Lithostratigraphy

The Khuff Formation (Steineke et al., 1958; Angiolini et al., 2001, 2003a,b; Osterloff et al., 2004b) is bestexposed in the homoclinal western flank of the anticlinorium, from the east of the Haushi fold beltnorthwards as far as Jebel Tharay (southwest of Saiwan). The Khuff Formation corresponds to the‘Lusaba Limestone’ (Goldring, 1957; Miller and Furnish, 1957; Morton, 1959; Hudson and Sudbury,1959), and is mapped in the Haushi-Huqf region by BRGM authors (Dubreuilh et al., 1992; Platel et al.,1992; Roger et al., 1992).

Photo 5: The Jurf, Khuff and Gharif formations outcrop in Jebel Gharif of Oman (see Figure 11). Thegrey limestones and marlstones of the Khuff Formation unconformably overlie the red sandstones ofthe Gharif Formation and are, in turn, sharply truncated by the dolostones of the Cretaceous JurfFormation. Height of the cliff is about 50 metres. (Photo by Lucia Angiolini)

Photo 5

Jurf Formation

Khuff Formation

Gharif Formation


166

G. g

hab

aens

is-K

. tes

coru

m Z

one

Acr

itosi

a sp

.-G

lob

osob

ucin

a Z

one

N. (

N.)

arab

icus

-C. m

anar

olla

i Zon

e

mem

ber

3m

embe

r 2

mem

ber

2m

embe

r 1

Sub

unit

BU

nit C

Khu

ff F

orm

atio

n

Khu

ff F

orm

atio

nre

defin

ed G

harif

Fm

Uni

t D

Maximumflooding surface

marineflooding surface

1 m

1 m

Ammonites

Bivalves

ConodontsCorals

Crinoids

Brachiopods

Fish

Gastropods

Scaphopod

Nautiloid

Bryozoa

Trilobites

Redefined Gharif and Khuff formations, Oman

Bioturbatedsandstone

Cross-laminatedsandstone

Marlstone

Marlstone

Marlstone

Mudstone

Mudstone

Mudstone

Bioclasticlimestone

Bio-intraclasticlimestone

Sandylimestone

Limestone

Bioclasticsandstone

Ostracod

Figure 9


167

The Khuff Formation lies everywhere conformably on the Gharif Formation. The base is picked at thefirst calcareous bed immediately beneath the marine marl, containing bivalve and brachiopod debris(Figures 5-10). This bed overlies the continental deposits of grey, sandy shale with sandstone lenses, atthe top of the Gharif Formation (Figures 7 and 8).

In the Khaluf and Mafraq areas, the top of the Khuff Formation is an unconformity with continentalalteration where rapid emersion and in situ brecciation took place. The uppermost Khuff carbonate-marl layers are normally reddened, indicative of subaerial weathering, and are abruptly overlain byvariegated, red shale and sandstone of the Triassic Minjur Formation. In the southwestern Duqm andMadraca areas (Figure 11, Platel et al., 1992), deep erosion after a period of deformation has completelyremoved the Jurassic Sahtan Group. Throughout this area, the yellow dolomitic limestone of theCretaceous Jurf Formation (Thamama Group) unconformably overlies the dark grey flags of the KhuffFormation (Photo 5).

In the Haushi-Huqf area, the Khuff Formation is about 30 m (98 ft) thick (Photos 6 and 7). It comprisessequences, several metres thick, of grey marl showing worm tubes and well-preserved fossils withinterbeds (about 10 cm thick) of granular, grey to yellow-brown limestone, rich in large fossil remains.At the base, the first few metres consist of limestone beds that contain a fine-grained sandy fraction(Photo 8). Higher in the formation, yellow calcarenite beds appear; while the most common type oflimestone, forming the tops of sequences, consists of biomicritic wackestone followed by highly

Photo 6: The Khuff Formation members 1-3 outcrop in the Haushi Northern Cliff of Oman (see Figures7-9). The three Khuff members are easily distinguished by their sharp lithological contrast. Member 2chiefly consists of marls and shows a very low topographic relief; its base marks a flooding surface.Height of the cliff is about 30 metres. (Photo by Lucia Angiolini)

Figure 9 (facing page): Stratigraphy of the redefined Gharif and Khuff formations, Haushi region(modified after Angiolini et al., 2001). The lower sequence boundary of the Middle Permian transgressionis placed below Gharif subunit B. The flora in subunit B is not diagnostic of a precise age, but considered?Roadian-?early Wordian. The marine fauna of the exposed lower Khuff Formation support a MiddlePermian Wordian age. Foraminifera confirm a Middle Permian age (Wordian and Capitanian). Conodontfauna from Khuff member 3 suggests a late Wordian age.

Photo 6

Khuff Formationmember 3

Khuff Fmmember 2

Khuff Fmmember 1

Gharif FormationGharif Formation


168

Photo 8: Khuff Formation members 1 and 2 in the Haushi Northern Cliff in Oman seen along thesection drawn in Figure 9. Height of the cliff is about 12 metres. (Photo by Lucia Angiolini)

Photo 7: Khuff Formation members 1-3 outcrop in the Haushi Northern Cliff in Oman (see Figures 7-9).Khuff member 1 marks the passage of the foreshore facies of the basal Khuff Formation. Khuff members2 and 3 were deposited in a carbonate shelf. Height of the cliff is about 20-25 metres. (Photo by LuciaAngiolini)




Gharif Formation

mbr

3 K

huff

Fm

mbr

2m

br 1

Photo 7



Photo 8


169

fossiliferous grainstone with well-preserved fossil fauna (condensed sequences). The end-of-sequencediscontinuities are commonly characterised by large-scale symmetrical ripple-marks indicative of high-energy conditions in the subtidal environment of deposition.

In the outcrop of this Mafraq Map area (Figure 6; Roger et al., 1992), the Khuff Formation is relativelythin, averaging around 20-30 m (66-98 ft) thick. The formation consists of grey-green, fossiliferous,marl interbedded with several decimetre-thick layers of highly bioclastic, argillaceous limestone. Aninflux of terrigenous material is visible in the first four metres (13 ft) of the formation, shown by thinbeds of quartzitic sandstone that alternate with white, ripple-marked argillaceous limestone. This levelencloses an abundant macrofauna of ammonoids, nautiloids, brachiopods, serpulids, etc. Above this,the coarse-grained fraction is replaced by grey-white to greenish-coloured marl. This succession formsrepeat-sequences that start with thick marl that becomes progressively burrowed, and end with adecimetre-thick resistant bed of ochre-coloured, bioclastic limestone.

The individual sequences become noticeably thinner (average thickness of 2 m/7 ft) in the topmost 10 m (33ft) of the formation, resulting in the section showing a relative increase in number of carbonate intervalsof a wackestone-packstone texture. The carbonate beds display large-scale symmetrical ripple marksformed by wave action; their upper surface is generally marked by a hard ground or the preferentialaccumulation of an abundant and diverse fauna including cephalopods, crinoids, bryozoans,brachiopods and asteroids. Most of the fauna occurs within this upper interval. The macrofauna ispreferentially concentrated within the packstone-textured carbonate intervals, and microfauna(ostracods) is also present.

Figure 10:Stratigraphy ofthe redefinedGharif and Khuffformations,Duqm andMadraca maparea (Platel et al.,1992).

Brecciated grey-beige limestone

Bioclastic limestone

Yellow marl and marly limestone

Alternating grey marl and marly limestone

Grey bioclastic limestone with brachiopods, bryozoans and molluscs

Green marl with white palustrine calcareous nodules

Yellow and white to purple argillaceous sandstone

Whitish argillaceous sandstone and green marbled shale

Palaeosol

Interbedded yellow to grey cross-bedded sandstone, yellow sand, whitish marl and bioclastic limestone with abundant brachiopods, bryozoans, echinoids and molluscs

LOCALSTRATI-GRAPHY



Redefined Gharif and Khuff formations, Duqm and Madraca map area, Oman

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The Khuff Formation consists of shallow-water carbonate deposits laid down across the Arabian Platformduring the widespread transgression that started in the mid-Permian. The transgression covered thewestern edge of the Haushi-Huqf Uplift (Hughes Clarke, 1988). In the Northern Cliff, above subunit Bof the redefined Gharif Formation, the basal part of the Khuff Formation is characterised by members 1(units C, D), 2 and 3 (Angiolini et al., 2001, 2003b) (Figures 7-9; Photo 6). The lowermost Khuff unit Chas a sharp erosional contact with the underlying shale of Gharif subunit B.

Khuff member 1, unit C is about 2-4 m (6.6-13.2 ft) thick, and consists of stacked graded sequences,several decimetres thick, of coarse to fine-grained, laminated clayey sandstone and contains an increasingfraction of highly abraided marine bioclasts. These beds form a very tabular continuous horizon, andare considered fluvial-infuenced; the intense bioturbation of these facies is related to lagoonal or bayenvironments. Thin to medium sandstone beds with NNE-directed straight-crested wave ripples areintercalated at the base.

Khuff member 1, unit D, about 3 m (9.9 ft) thick and fossiliferous at the base (brachiopods, conodonts,crinoids, ostracods) (Photo 9), is made up of shallow marine yellowish to whitish carbonate mudstoneshowing interbedded rippled bioclastic sandy layers (Photo 10). Khuff units C and D mark the passageof the foreshore facies of the basal Khuff Formation.

Khuff member 2 is 12 m (39 ft) thick, and consists of burrowed marlstones and marly limestones,interbedded with thin sandy bioclastic tempestites (Photo 11). The upper part consists of bioclasticgrainstone beds in which wave ripples occur regularly. It lies conformably on unit D. Marlstones andlimestones are rich in ostracods.

Photo 9: Khuff Formation member 1 (upper part) in the Haushi Northern Cliff in Oman, showing avery rich bioclastic bed, full of the convex-up valves of the brachiopod Celebetes manarollai Angiolini,1999. (Photo by Andrea Tintori).

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Photo 10: Unit D of Khuff Formation member 1 in the Haushi Northern Cliff in Oman showing wave-rippled, sandy bioclastic bed along the section drawn in Figure 9. Height of the cliff is about 4 metres.(Photo by Lucia Angiolini)

Photo 11: Member 2 of the Khuff Formation in the Haushi Northern Cliff in Oman showing stronglyburrowed marlstones. (Photo by Lucia Angiolini)

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Khuff member 3 is 21 m (69 ft) thick, and ends the Khuff Formation with an erosional surface at its top,below the red siliciclastic continental deposits of the poorly constrained Triassic-Jurassic MinjurFormation (Photo 12). The major facies of this unit displays an inflection toward marly-carbonate micritewith cephalopods, brachiopods, ostracods, and conodonts (Roger et al., 1992; Broutin et al., 1995;Angiolini et al., 1998, 2003b; Angiolini and Bucher, 1999; Crasquin-Soleau et al., 1999; Crasquin-Soleau,2003). These facies exhibit interbedded grainstone layers with wave ripples that have been interpretedby Angiolini et al. (2001, 2003a, b) as tempestites, including mixed autochthonous and allochthonousfossils (algae, bivalves, brachiopods, conularids, crinoids, benthic foraminifera, gastropods, conularids,bryozoans, ostracodes). Vertebrate remains are also common, mostly represented by durofagous fishesfeeding on shallow-water molluscs. The interbedded marlstones, instead, contain chiefly infaunalbivalves and quasi-infaunal productids.

The transition from Gharif subunit B to the Khuff Formation is a trangressive trend; from base to top:(1) Gharif subunit B: distal fluviatile system bordering a coastal plain; (2) Khuff member 1, units C and D:clastic transgressive shoal/barrier; and (3) Khuff members 2 and 3: carbonate platform. The lowersequence boundary of the transgression is interpreted at the base of subunit B (Broutin et al., 1995;Angiolini et al., 2001). It corresponds to a relative sea-level fall that resulted in fluvial incision (Figure8). They interpret Gharif subunit B, and Khuff units C and D as the TST and a marine flooding surface(mfs) at the base of member 2 of the Khuff Formation.

Grey micritic palustrine limestone with flint nodules

White micritic limestone, brecciated at base

White limestone with burrows

Brecciated rusty brown dolomite

Alternating yellow dolomitic limestone and dolomitic marl

Black silty shale

Grey palustrine limestone

Alternating marl and clay

Grey calcareous paleosol

Marl with bivalves

Grey cross-bedded sandstone with abundant molluscs (beach-rock)

Interbedded beige to grey laminated marl, rusty brown sandstone, and bioclastic limestone with corals and abundant bivalves

LOCALSTRATI-GRAPHY



Khuff Formation, Duqm and Madraca map area, Oman

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Figure 11: The top of theKhuff Formation in theDuqm and Madraca maparea is eroded by the pre-Jurf unconformity (afterPlatel et al., 1992).


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Photo 13: Ammonoids (Pseudohalorites arabicus Miller and Furnish 1957, determined by M. Balini inAngiolini et al., 2003b) in the upper part of member 2 of the Khuff Formation, Haushi Northern Cliff,Oman. Each specimen is about 15-20 mm in diametre. (Photo by Andrea Tintori)

Photo 12: Member 3 of the Khuff Formation in the Haushi Northern Cliff in Oman, showing bioclasticlimestones and biointraclastic limestones below the unconformable red beds of the Triassic-LowerJurassic Minjur Formation. Height of the cliff is about 7-8 metres. (Photo by Lucia Angiolini)

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Fossils and Age

The Khuff Formation is characterised by an abundant and diverse fauna, much of which is beautifullypreserved intact and unbroken, particularly within the marly intervals (BRGM, 1992). Even thoughbrachiopods, bivalves, gastropods and cephalopods predominate, the macrofauna also includesbryozoans, echinoderms, trilobites, corals, conularids and vertebrate remains. Foraminifers, algae andostracods are also present. An abundance of faunal material has been gathered, determined and

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published by Goldring (1957), Miller and Furnish (1957), Hudson and Sudbury (1959), Angiolini et al.(1998), Angiolini and Bucher (1999), Crasquin-Soleau et al. (1999), Crasquin-Soleau (2003), Dickins(1999), Angiolini et al. (2001, 2003b). A recent synthesis of most of the fauna has been undertaken byAngiolini et al. (2003b), who listed:

Cephalopods: Pseudohalorites arabicus Miller & Furnish, 1957, Stacheoceras sp., nautilocon nautiloids(Photo 13).

Brachiopods: Lingula sp., Perigeyerella raffaellae Angiolini, 1999, Derbyia cf. diversa Reed 1944, Neochonetes(Nongtaia) arabicus (Hudson and Sudbury, 1959), Celebetes manarollai Angiolini 1999, Haydenella sp.,Dyschrestia rugosa Angiolini 1999, Juresania omanensis Hudson and Sudbury 1959, Kozlowskia tescorum(Hudson and Sudbury, 1959), Bilotina yanagidai Angiolini 1999, Vediproductus sp. [= Calliprotonia sp. inAngiolini and Bucher (1999)], Linoproductus aff. kaseti Grant 1976, Magniplicatina sp., Grandaurispinaghabaensis Angiolini 1999, ?Cyclacantharia sp., Globosobucina sp., Acritosia sp., Edriosteges sp., ?Darlinuriasp., Orthotichia cf. bistriata Reed 1944, Terebratuloidea sp., Cleiothyridina cf. seriata Grant 1976, Squamulariasp., Martiniopsis sp., Spiriferellina sp., Callispirina sp., Hemiptychina sp., and Dielasma spp. (Photo 14).

Conodonts: Hindeodus wordensis Wardlaw, 2000, Merrillina praedivergens Kozur and Mostler 1976 andSweetina sp.

Ostracods: Acratia cf. subularis Gusseva 1972, Acratia n. sp.2, Acratia n. sp.3, Acratina diffusa Schneider1959, Bairdia cf. acrisa Stepanaitys 1975; Bairdia sp.1, Bairdia omanensis Crasquin-Soleau 1999, Birdsallela?spp., Carinaknightina braccinii Crasquin-Soleau 1999, Cavellina boomeri Crasquin-Soleau 1999, Cavellinan. sp.4, Cavellina gerryi Crasquin-Soleau 1999, Cavellina? huqfensis Crasquin-Soleau 1999, Fabalicyprisparva Wang 1978, Geffenina wangi Crasquin-Soleau 2003; Healdianella? n. spp. Crasquin-Soleau 1999,Hollinella (H.) benzartiae Crasquin-Soleau 1999, Hollinella (H.) herrickana Girty 1909, Hollinella (H.) martensiCrasquin-Soleau 1999, Jordanites lordi Crasquin-Soleau 1999, Kindella sp.1, Knightina cf. fildari Payne1937, Knightina unnoda Wang 1978, Langdaia hornei Crasquin-Soleau 1999, Moorites cf. minitus Warthin1930, Moorites cf. suboblonga Wang 1978, Perprimitia brevirostris Schneider 1958, Perprimitia sp., Placideacf. petschorensis Schneider 1958, Roundyella cf. bellatula Bradfield 1935, Roundyella suboblonga Wang 1978,Sargentina minuta Wang 1978, Sargentina transita Kozur 1985, Sargentina woutersi Crasquin-Soleau 1999,Sulcella arabica Crasquin-Soleau 1999, Sulcella sulcata Coryel & Sample 1932, Sulcella superpermiana Kozur1985 (Photo 15).

Vertebrates: Menaspoid scales, Cladodont teeth, Hybodont teeth and denticles, Deltodus teeth,Actinopterygian teeth, scales and vertebrae.

Foraminifera: Vachard in Angiolini et al. (1998) preliminarily identified some smaller foraminifera.An updated list (Photo 16) is as follows: Diplosphaerina ex gr. inaequalis (Derville, 1931), Earlandia ex gr.minor (Rauzer-Chernousova, 1948); Calcitornella sp., Multidiscus cf. arpaensis Pronina, 1988, Nodosaria

Photo 14 (facing page): Middle Permian, Wordian, brachiopods of the Khuff Formation. Scale asindicated. (Photo by Lucia Angiolini)(1) Derbyia cf. diversa Reed, 1944, ventral valve, x1.(2) Neochonetes (Nongtaia) arabicus (Hudson and Sudbury, 1959), ventral valve, x1.(3) Bilotina yanagidai Angiolini, 1999, interior of dorsal valve, x1.(4) Celebetes manarollai Angiolini, 1999, ventral view of an articulated specimen, x1.(5) Linoproductus aff. kaseti Grant, 1976, ventral valve, x1.(6) Juresania omanensis Hudson and Sudbury, 1959, articulated specimen with gastropods, x1.(7) Perigeyerella raffaellae Angiolini, 1999, two ventral valves with bivalves, x1.(8) Acritosia sp., articulated specimen, x1.(9) Juresania omanensis Hudson and Sudbury, 1959, ventral view of an articulated specimen, x1.(10) Vediproductus sp., ventral valve, x1.(11) “Dielasma” sp., ventral view of an articulated specimen, x1.(12) Magniplicatina sp., ventral valve, x1.(13) Globosobucina sp. , ventral valve with spines, x1.


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Photo 15: Examples of ostracod species of the Khuff Formation in Oman. Scale bar is 100 micrometre.(Photo by S. Crasquin-Soleau)(1-2) Langdaia hornei Crasquin-Soleau 1999; (1) holotype, left lateral view; (2) paratype right lateral

view.(3) Carinaknightina braccinii Crasquin-Soleau 1999; holotype, left lateral view, x 45, MGL74654, sample

A18A.(4) Knightina cf. fildari Payne 1937 sensu Crasquin-Soleau and Orchard 1994; left lateral view.(5) Knightina unnoda (Wang 1978); left lateral view.(6-7) Hollinella (H.) benzartiae Crasquin-Soleau 1999; (6) holotype, right lateral view; (7) right lateral

view.(8-9) Hollinella (H.) martensi Crasquin-Soleau 1999; (8) holotype, right lateral view; (9) holotype, right

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sp., Geinitzina spp., Pachyphloia robusta Miklukho-Maclay 1954, P. cukurkoyi Sellier de Civrieux andDessauvagie 1965, Lingulonodosaria (?) sp., Frondinodosaria n. sp. aff. plena Sosnina 1976, Rectoglandulina(?) sp., Langella n. sp. aff. venosa Lange 1925, L. cf. ocarina Sillier de Civrieux and Dessauvagie 1965, L. sp.

The fusulinids are completely absent, probably due to a warm temperate climate, and not to evolutionarycontrol, such as the post-keriotheca phase (Sheng, 1992). These smaller forminifera are accompaniedby algae Permocalculus cf. tenellus (Pia, 1937) and pseudo-algae Stacheoides sp.

Bivalves: Dickins (1999) described 10 species from the genera Nuculopsis, Phestia, Edmondia, Dyasmya,?Janeia, ?Liebia, Vnigripecten, Cyrtorostra, Schizodus and ?Astartella from members 2 and 3 of the KhuffFormation.

The following faunal descriptions were added to by Petroleum Development Oman with a number ofunpublished determinations (Aldridge on conodonts; Lethiers on ostracods) and material collectedduring BRGM field work (brachiopod studies by Boullier; cephalopod studies by Glenister). The listnow consists of:

Bryozoans: Fenestella sp., Polypora sp.

Cephalopods, nautiloids: orthoconic nautiloids, Tylonautilus sp., Mooreoceras sp.

Ammonoids: Foordiceras sp., Licroceras sp., Tainoceras sp., Domatoceras? sp., Stacheoceras sp. andPseudohalorites arabicus.

Gastropods: Bellerophon cf. triangularis

Trilobites: Pseudophillipsia lipara, P. stealopyga (Goldring, 1957)

Conodonts: Hindeodus cf. minutus

Ostracods (subsurface): Hollinella aff. herrickana, H. emaciata, H. ulrichi, H. gibbosa

The marine fauna of the exposed lower Khuff Formation notably includes cephalopods, brachiopods,conodonts, ostracods and bivalves, which support a Middle Permian Wordian age (Angiolini et al.,1998, 2001, 2003b; Angiolini and Bucher, 1999; Crasquin-Soleau et al., 1999). The updated smallerforaminifera confirm a Middle Permian age (Wordian and Capitanian, Murgabian and Midian), asindicated by the appearance of Multidiscus (see Reitlinger, 1965; Kotlyar et al., 1989; Vachard et al.,2002), and the specific diversity of the Langella. In addition, conodont fauna from uppermost levels(Khuff member 3) suggests a late Wordian age (Angiolini et al., 2003b).

Photo 15 continued:(10-11) Jordanites lordi Crasquin-Soleau 1999; (10) holotype, right lateral view; (11) right lateral view.(12) Perprimitia brevirostris Schneider 1958; left lateral view.(13) Sargentina minuta Wang 1978; left lateral view.(14-15) Hollinella (H.) herrickana (Girty 1909); (14) right lateral view; (15) left lateral view.(16) Sargentina wautersi Crasquin-Soleau 1999; holotype, left lateral view.(17-18) Sargentina transita (Kozur, 1985); (17) left lateral view; (18) left lateral view(19-20) Cavellina huqfensis Crasquin-Soleau 1999; (19) left lateral view; (20) holotype, left lateral view.(21-22) Cavellina boomeri Crasquin-Soleau 1999; (21) holotype, left lateral view; (22) paratype, left lateral

view.(23) Cavellina gerryi Crasquin-Soleau 1999; holotype, left lateral view.(24) Bairdia omanensis Crasquin-Soleau 1999; holotype, right lateral view.(25) Sulcella arabica Crasquin-Soleau 1999; holotype, left lateral view.(26) Acratina diffusa Schneider 1959; left lateral view.(27-28) Fabalicypris parva Wang 1978; (27) right lateral view; (2) right lateral view.


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PALAEOCLIMATIC EVOLUTION

The Lower to Middle Permian succession of the Haushi-Huqf Uplift records a significant change fromglacial-arid to warm-humid climatic conditions in about 25 million years, inferred from palaeontological,sedimentological and petrographical indicators.

The Al Khlata diamictites were deposited during the earliest Permian climax of the Gondwananglaciation, whereas the topmost interval of the unit is related to glacial retreat and concomitant sea-level rise in the late Sakmarian, with rapid evolution from glacial to temperate climates around theTastubian-Sterlitamakian boundary (Angiolini et al., 2003a, b). The overlying Sakmarian Saiwanbrachiopod fauna is dominated by large and thick-shelled spiriferids with Gondwanan affinity, andhas a generally low to moderate diversity, with a Permian Ratio of 0.57 (Angiolini et al., 1997). Ecosystemdevelopment in the Saiwan Formation records a rapid increase in temperature at the end of theGondwanan deglaciation, with a shift towards temperate conditions in the late Sakmarian (Angioliniet al., 2003a). Such a climatic evolution was also observed in subsurface palynofloras by Stephensonand Osterloff (2002).

The palaeoflora contained in the Upper Gharif Formation suggests much warmer and humid settingsat the beginning of the Middle Permian (Broutin et al., 1995), with permanent precipitation and shortdry season (Fluteau et al., 2001). Furthermore, Middle Permian sandstones of the Haushi-Huqf Upliftdocument a marked increase in mineralogic stability which is chiefly ascribed to more intense chemicalweathering with time, fostered by progressively warmer and humid climates after the end of theGondwanan glaciation and onset of spreading in the Neo-Tethys Ocean.

Such a climatic trend towards higher temperatures continued during deposition of the Khuff Formation,unconformably capped by Triassic laterite/bauxite palaeosols. The Middle Permian Khuff brachiopods(predominantly thin-shelled productids) are significantly diversified (Permian Ratio of 1.57), with amixture of wide-ranging, Paleoequatorial, Cimmerian, and Gondwanan genera indicating a consistent

Photo 16 (facing page): Wordian algae and smaller foraminifera of the Khuff Formation (Photos byDaniel Vachard).(1) Permocalculus cf. tenellus (Pia, 1937). Oblique section. Sample 13A, Photo 3.32, magnification x32.(2) Langella n. sp. aff. venosa Lange, 1925. Axial section. Sample 13A, photo 3.25, magnification x55.(3) Langella cf. ocarina Sillier de Civrieux and Dessauvagie 1965. Subaxial section,. Sample 13, photo

3.20, magnification x78.(4) Frondinodosaria n sp. aff plena Sosnina, 1976. Subaxial section. Sample 13A, photo 3.27, magnification

x78.(5) Langella cf. Oblique section. Sample 13A, photo 3.29, magnification x60.(6) Rectoglandulina (?) sp. Oblique section passing by an aperture. Sample JR 95.6, photo 3.24,

magnification x60.(7) Rectoglandulina (?) sp. Oblique section. Sample JR 95.6, photo 3.21, magnification x60.(8) Pachyphloia robusta Miklukho-Maclay, 1954. Subsagittal section. Sample 13, photo 3.19, magnification

x75.(9) Pachyphloia robusta Miklukho-Maclay, 1954. Subfrontal section. Sample 13, photo 3.19, magnification

x75.(10) Pachyphloia robusta Miklukho-Maclay, 1954. Oblique section. Sample 13, photo 3.10, magnification

x75.(11) Pachyphloia cukurkoyi Sellier de Civrieux and Dessauvagie, 1965. Subsagittal section. Sample 13A,

photo 3.26, magnification x73.(12) Multidiscus cf. arpaensis Pronina, 1988. Axial sparitized section (bottom left) with an ostracod

valve (top right) and Permocalculus sp. (center). Sample JR 95-6, photo 3.22, magnification x75.(13) Fertile Permocalculus cf. tennellus (left) and Langella sp. (right) with several fragments of mollusks

(bottom and top left). Sample 13A, photo 3.31, magnification x47.


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increase of water temperatures (Angiolini et al., 2003b). Of great significance is the invasion of a largenumber of Palaeoequatorial genera (about 35%), which are good indicators of warm-water temperatures(Archbold and Shi, 1995). Khuff bivalves comprise cosmopolitan genera and suggest mild and equablewater temperatures (Dickins, 1999). Finally, Khuff ostracodes belong to the thermosphere, and are typicalof the intertropical zone.

All these climatic indicators suggest warm temperatures, in apparent contrast with the low diversity offoraminiferal and coral assemblages. The absence of fusulinids may be explained by the occurrence ofanomalously high water temperature and nutrient increase. This may also cause a drastic drop in coralbiodiversity. High quartz content at the base of the Khuff Formation indicates that alteration andpedogenesis were progressively more active with time on a wide Arabian area, including all possiblesources of Khuff detritus (Angiolini et al., 2003b).

Climatic changes inferred from palaeontologic and petrographic indicators in Interior Oman has beenascribed to the northward latitudinal shift of Northern Gondwana coupled with the global warmingdocumented from Early to Late Permian (Angiolini et al., 2003b).

CONCLUSIONS

The Al Khlata, Saiwan, redefined Gharif and lower Khuff formations in the Haushi-Huqf outcrops ofOman present the most complete exposure of the Lower and Middle Permian succession in the ArabianPeninsula. They show the passage from the glacial environments of the Al Khlata Formation to theMiddle Permian Khuff carbonate platform. In this study two marine formations are biostratigraphicallydated with some degree of confidence. The older Saiwan Formation (= Lower Gharif Member) is EarlyPermian late Sakmarian in age, and contains MFS P10 (late Sakmarian, Sharland et al., 2001). Thisflooding event is recalibrated at about 284 Ma based on the Sakmarian stage boundaries (betweenabout 287 to 280.3 + 2.6 Ma; Jin et al., 1997). The second marine formation corresponds to the basalcarbonates of the Khuff Formation that are Middle Permian Wordian in age (about 270-260 Ma; Jin etal., 1997). Between the two dated marine formations, the age of the continental redefined GharifFormation is not well constrained, but would range from Early Permian (Artkinsian) to Middle Permian(Roadian/early Wordian).

The Upper Carboniferous-Lower Permian Al Khlata Formation produces oil in Oman, and correspondsto the hydrocarbon-bearing Unayzah C and B members in Saudi Arabia (Stephenson et al., 2003). ThePermian Saiwan and redefined Gharif formations pass laterally (in part) to the subsurface GharifFormation of Interior Oman, and the Unayzah A Member of Saudi Arabia (Stephenson et al., 2003).These subsurface rock units also constitute important hydrocarbon sandstone reservoirs. The presentedage estimates, together with the lithofacies and interpreted depositional environments, provideconstraints for the subsurface sequence stratigraphic architecture and chronostratigraphic correlationsin the Arabian Peninsula.

ACKNOWLEDGEMENTS

The authors wish to thank Professors Marco Balini, Eduardo Garzanti, Alda Nicora and Andrea Tintorifor sharing their results and interpretations in this study. The authors also thank Peter Osterloff andMike Stephenson for their useful comments on the manuscript.

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Saiwan, Gharif and Khuff formations, Haushi-Huqf Uplift, Oman

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