Journal of Petroleum Geology, 3, 2, pp. 139-152, 1980 I39

GEOLOGY OF THE MUSANDAM PENINSULA (SULTANATE OF OMAN) AND ITS SURROUNDINGS

R. Ricateau" and P. H. Richk""

The Musandam peninsula (Sultanate of Oman) is a vast outcrop of carbonate rocks of Permian to Middle Cretaceous age. The facies of these formations relates them without doubt to the contemporaneous formations of the Arabian platform and there is a good analogy with the formations encountered in the wells drilled in the Persian Gulf west of the Musandam outcrops. After the deposition of these platform formations, several tectonic events deepb affected this area, and led to the present configuration: at the end qf the Middle Cretaceous, a wide area emerged on the E edge of the Arabian platform. u.herea.r on the W side of the Peninsula important halokinetic movements of the structures of the Persian Gulfpersisted.

Following that tectonic event, the sedimentary domain of the platform withdrew towards the Arabian shield during the Upper Cretaceous, whereas the orogenic activity spread out into the oceanic domain. The Musandam peninsula was located between those two domnins, in a sedimentary basin which successively received first the turbidites of'the Muti Formation (Coniacian - Santonian), mainly produced by the erosion of the Middle Cretaceous emerged area, and then the Hawasina nappes, mainly formed by abyssal sediments of earlier Mesozoic age.

I t is not firmly established whether the Semail nappe of the ophiolitic complex rvas emplaced over the Musandam as it was over the greater part of the Oman mountains. During and after the setting of the nappes, E-W compression caused the series of overthrusts, creating in the Musandam area a ridge edged on the W by a subsidmr foredeep which received materials from the ridge, and on the E by an oceanic zone in which the sedimentation was far more reduced. The same pattern continued until the end of the Lower Miocene, when a very important compressive movement from E to Wgave their present shape to most structures.

During the Upper Miocene, vertical movements of compensation took place and continued to thepresent time overMusandam, while on the Eflankof the Peninsula, in the subsiding Gu(fof Oman, sedimentation became very important.

~ ~ ~ ~~

*Exploration Manager. Elf Aquitaine Oman, P.O. Box 33S2 Ruwi, Sultanate of Oman. (Current address: E!f Aquitaine Colombia, Apartado Aero 91343, Bogota, Colombia). **Exploration Manager for Middle East, SNEA(P), 92088 Paris Ladefense, Cedex 22. France. {Current address: c /o IFP, ENSPM, 4 Ave de Bois-Priau, 92500 Rueil Malmaison, France). The authors would like to thank: H.E. the Minister of Agriculture, Fisheries, Petroleum and Minerals of the Sultanate of Oman who kindly authorised the publication of this article, the SNEAIP) Board of Directors who gave its agreement to issue the present results, the team of geologists who took part in the field survey. Mr. J. Biehler and Mu. C. Chevalier, and the geologists of SNEA(P) Central Exploration Laboratoty who studied the samples.

1-40 R. Ricareatc and P. H. Richh

INTRODUCTION 'The Musandam peninsula, which partially

bars the Straits of Hormuz and separates the Persian Gulf from the Indian Ocean {Gulf of Oman). has received little study because of topographical difficulties which confronted geutogical field parties. Also. from 1872 up to 1960. expeditions reconnoitred only the regions near the coast or frontiers. During the winters o f 1966 and 1967. a geological field party working for Petroleum Development (Oman). during the course of a general study of the Oman mountains. carried out a helicopter reconnaissance and deciphered the main structural framework of the region.

Elf Aquitaine Oman. in the course of a petroleum prospecting programme over the Peninsula. conducted a geological helicopter survey which allowed detailed studies of the stratigraphic series and also the tectonics affecting them. A geological map has been published. During the same campaign. the Post-Nappes Series and their relationships with the underlying beds were observed in the Buraiini region.

Offshore. marine geophysical surveys (seismic. gravity and magnetic) and the three deep wells drilled by the Elf Aquitaine group. have greatly extended the geological know- ledge of the region.

A synthesis of this information is presented below.

1. THE STRATIGRAPHY OF THE MUSANDAM PENINSULA (Figs. 2 and 4)

The nomenclature adopted is that used in the publication of K. W. Glennie el u1. 1974(2) and also that used in the Gulf and in Iran. The wries dre described. in order of deposition. from Permian to Tertiary.

Huus a1 Jibal Group Two formations. of 700 m thickness. outcrop

mainly along the east coast. 'The Hagil Forn~ution is formed of thinly

bedded argillaceous dolomites whereas the Ghail Formution is formed of massive microcrystalline dolomites. The presence of numerous hard-grounds. of ghosts and debris of gastropods. of banded bzds with stroniarolites indicates intertidal or supratidal deposits. Rare foraminifera have been observed /Gcinir:;jza sp.?). The most probable age by comparison with the known succession in Iran. is Permian.

Elphinstone Group The Mililha Fornzution (95100 m ) consists

at its base of alternations of reddened to yellow shale and banks of dolomitic stromatolitic limestone. these deposits again corresponding to a supratidal environment. The upper part of the formation comprises thick beds of black lime mudstone-wackestone with gastropods and lamellibranchs liWegulodon). It has been dated as Triassic by its microfauna lTrocholinu crussu. T. niultispiru, Frondiculuriu woodwurdi).

The Ghaliluh Formution is mainly shaly sands. dominantly coloured orange-red, with intercalations of limestone beds with corals and large lamellibranchs, and is terminated by if massive, feature-forming, bioclastic limestone unit.

The group forms a terrace characterised by its colour. The sedimentation is subcontinental, not far removed from the tideline. Its Lower Lias age is indicated by its microfauna (Litigulinu tenera, Paleodusycladus medifer- t-meus).

Musandam Supergroup This is a monotonous, massive carbonate

interval ( 1800 m), generally well bedded, which has been divided into nine members: -Mem- bers A and B (270 m ) comprise, a t the base, regular alternations of lime mudstone- wacke- stone with brachipods and Lithiofis sp. and with numerous algae of the Cylindroporella sp. type. Above, the massive cliff. which is grey and striped, with banks of light-coloured dolomite. is formed from bioturbated lime- stones with abundant valvulinids.

-Member C (37 m ) composed of argillaceous nodular limestones; it forms a negative feature in the topography.

-Member D and E (480 m), dark-coloured cliff formed of thick banks of lime mudstone, often bioturbated, with Algae (Sjmngioslrorna) and with common Succocoma. The uppermost part has well-defined rhythmic bedding of marly horizons, lumachelles or algal limestones. The rich microfauna comprises valvulinids, Inr~olutinu liusica, Haurunia umiji. H. deseum, Vidulina martana.

The suite of members A to E is attributed to the Middle and Upper Lias.

-Member F (370 m ) corresponds to a cliff, capped with a ferruginous layer and constitutes a transgressive sedimentary sequence includ- ing. at its base. lime mudstones with Algae (Oncholites) and Gastropods and, a t the top, lime packstones with corals. The microfauna gives the following ages:

Geology of’the Musandam Peninsula 141

At the base: Middle Jurassic, with Meyendorffina, Protopeneroplis striata. In the middle: Bathonian, with Pfenderina salernitana. At the top: Calloviun-Oxfordian, with Kurnubia palestiniensis, Psezidocj~clammina jaccardi.

The Malm is not faunally recognised, but it could be condensed into several metres of thickness.

Member G (325 m ) begins with a “siliceous marker” (15 m): i t is composed of lime mud- stones-wackestones with siliceous and Cerruginous nodules and bands. The pelagic microfauna indicates a Lower Cretaceous to Berriasian age with Calpionella alpina and C. elliptica.

The series that follows makes a cliff (1 10 m) of lime mudstones, followed by a bench of argillaceous limestone with terebratulids, rare belemnites and ammonites, overlain by another cliff ( 170 m) of lime mudstones-wackestones and packstones to grainstones with a tendency to boundstones. The cliffs contain a rich microfauna with Calpionella darderi, C. incinata. and Haplophregmoides catenui, pointing to a Berriasian to Valanginian age.

Members H and I (300 m) comprise, in the lower parts, alternations of metre-thick beds of

argillaceous, nodular limestones, and bio- turbated limestones with bryozoans, gastro- pods, solitary corals and lamellibranchs. The upper part of the sequence is made up of lime mudstones and wackestones with orbitolinids, terminated by an irregular, bored surface.

The enclosed microflora, found at the base, includes Salpingoporellu annulata, S. appenica, S. dirzarica. Lithoeodium aggregatum. The microfauna comprises: Choffhtella decipiens. Evercyclammina sp., Orbitolina sp., Dic- tyoconus arabicus, giving a BarremiadAptian age.

Conclusions on the Jurassic and the Lower Cretaceous

The Jurassic begins with a shalehand episode which although quite thick can be correlated to the Marrat formation known on the Arabian platform. It presents after that a type of limestone sedimentation of restricted charac- ter, but the Lias is very strongly developed, testifying to an important subsidence; in contrast, the Middle Jurassic and particularly the Malm are much reduced, with possible lacunae.

The Lower Cretaceous begins with a very deep facies with tintinnids. and the “siliceous

R. Ricateatc arid P. H. Rich6

t

STRATIGRAPHIC COLUMN - MUSANDAM PENINSULA AREA

PPER R E T A G

IDDLE R E T A C

*_

__

Ln 3 0

w u < t W

u

a W

a

3 0 A

-Zp HAWASINA NAPPE

- PPE R U R A S . __

Ln * a 2 W A n P I __

P W

3 0 4

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ALAN6 ~avcerir

ERRIA , I A N

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mrmbrr I (127 m )

500 m mrmbrr 6 I323 m )

Figure 2

Prr-Smonkn

Unconlormily

-0101101 \no ,5p lc" l t s

mtmtrr 0

Mcyrndotllina ' -

- Vidalino rnarlana

Orb,loprella precursor

Geolog~ of‘the Musandam Peninsula 143

Figure 3

POST NAPPES FORMATIONS BURAlMl AREA

N. S.

J . RAUDHA J. MALAQAT J . AWAYNAH

marker”cou1d correlate with the “porcellanites” of the Oman mountains (Rayda formation). The later facies changed towards platform cover deposits. The top Member, I , is equivalent to the Kharaib and Shuaiba formations.

The thicknesses oEthe Lower Cretaceous are also comparable to those drilled by wells along the edge of the Oman Mountains, but with more carbonate in the facies (the Salil argillaceous episodes being unrepresented).

The Lower Cretaceous sub-group constitutes a regressive megasequence.

Wasia formation p.p. Above the bored erosion surface, 130 rn of

marls with orbitolinids and yellow-stained shell limestones with echinoids have been dated Albian to Lower Cenomanian by the contained microfauna with Orbitolina cf. concava, rare Lithocodium aggregatum and Orbitolina sp., and also by comparison with the formations known in Iran.

Only the lower part of this formation has been recognised on the Peninsula, erosion having removed the younger deposits. I t is in part equivalent t o the Nahr Umr formation, which shows the same criteria on the Arabian platform. and to the Kazdhumi formation as defined in Iran and in the Gulf.

Formation equivalent to the Muti Formation Many outcrops that can be attributed to this

formation have been recognised. although their microfaunas consist solely of sponge spicules and radiolaria which d o not permit dating. These are yellow and green shales and marls with intercalations of quartzitic sandstones, testing on a karstified hard-ground on the substratum, persisting from Ruus a1 Jibal (south of the peninsula, in front of the nappes) to the Musandam limestone (synclinal block with evidence of nappes) and marking a strong discordance which could be tfie pre- Senonian discordance; in this hypothesis these formations are comparable to the conglo- meratic Muti, better known in the South in the Oman Mountains, and to the Laffan shales as known in the Gulf.

Hawasina and Semail Groups These are the overthrust nappes coming

from the east, which cover the autochthonous sedimentary series. They are absent now over most of the Peninsula, although evidence of them exists in two localities in the synclinal trough of Qawah Sibi.

Their study is not the main purpose of this article (cf. K. W. Glennie et al. 1974 and F. Alleman et af . 1972). We note only that the Hawasina series is formed from turbiditic

I44 R. Ricnteau arid P. H. Richb

sediments of bathyal type and that the nappes of the Seinail are grouped into a submarine volcanic complex of ophiolite type.

"Post-nappes" formations (Fig. 31 Studied in the Buraimi region. their thickness

and their facies change very rapidly. for the transgression is over a structured substratum possessing marked topographical relief.

The base isgenerally formed by a transgres- sive hrecciated horizon containing ophiolite elements (Jsbel Rauha; Jebel Malaqatl some- times with ructists (Jebel Rauha and Sunxinil. Thew horizons have been dated Maestrichtian b y the presence of Sidero1ite.r culcicr-apoideJ. 0i?ifJhnfoL:\.cIi~.( mucr-oporus and Orbitoides upicitluru. The cement binding the breccias contains Globotr-rcticcrnci and heterohelicids.

I n some other areas. as at Jebel Awaynah. i t seems that the limestone at the base must be attributed to the Paleocene. The overlying series is either argillaceous and conglomeratic (Juweiza faciesl o r made up of carbonates {Umrn er Radhuma formation); its age is Paleocene to Middle Eocene. and it is 450 to 500 m thick.

The main micropaleontological findings in the different sections are reported on the currelaticin plate (Fig. 3 ) .

11. STRATIGRAPHY OF THE ADJACENT MARINE REGIONS Gulf Region (Fig. 4 )

The type sequence is taken from the wells of Sofiran - Henjam 1 and Elf Aquitaine Oman - Khassab 1. At the base. the sequence corresponds with that of the Peninsula (Fahliyan. Gadvan. Dariyan formations. corresponding t o members G . H, 1 from Berriasian to Aptian), with which there are many similarities. including the change of environment of the deposits (regressive megasequence). the lithology. the biostrati- graphic markers (Tintinnids at the base. Sulpitigoporella annulatu and S. dinaricu. Dicgoconirs arubicus) and by the thickness of units.

Kazdhumi Formation The thickness is in the order of 130 m. The

shales with orbitolinids and Hemicyclammina .riga/i indicate a neritic environment.

Sarvak Group This group is formed of two rnegasequences:

At the base (Mauddud Formation) a trans- gressive sequence begins with intertidal lime mudstones-wackestones with green algae. gastropods. ostracods and orbitolinids, and ends with a platform to open-sea environment.

The succeeding (Khatiyah and Mishrif Formations) regressive sequence begins with lime-wackestones-ppackstones with pelagic foraminifera, followed by lime-grainstones with rudists. alveolinids and much organic debris. and ends with limestones with green algae, ostracods and gastropods. A 60 m thickness of this last sequence is eroded from the Khassab well and 1-10 m from the Bukha well beneath the break at the base of the overlying forma- tion. The thickness of the Sarvak is therefore incomplete, but can attain 200 m: it isattributed to the Upper Albian-Cenomanian, and includes the equivalents of the Mishrif, Khatiyah and Mauddud Formations of other areas.

Laffan Formation This formation is reduced in the wells to 1 m

of black to red shale, without proof of age. Its environment of deposition seems to be of open sea type: this sharp change of environment is eniphasised by the strong discordance of these shales.

Ilam Formation In the Henzam well, the Ilam is made up of

20 to 25 ni of bioclastic lime-packstones-grain- stones with echinoids, dkbris of rudists, and Rotaliu skourensis, Dicyrlina sp., Valvulam- miria. Montcharmontia appenica, giving a Santonian age.

In the Bukha well, the Ilam has not been differentiated and it could have been elimin- ated by erosion preceding deposition of the Gurpi Formation.

In the case where the Laffan shales, updated, could represent part of the Coniacian, the hiatus involving absence of Turonian and perhaps partly Coniacian sediments is noteworthy.

Gurpi Formation This formation marks the beginning of a

cycle of basinal argillaceous carbonate sedimen- tation. About 600 m thick, the argillaceous limestones and dark shales contain:

In the lower part, a predominantly argillaceous series with a mixture of pelagic Foraminifera (globigerinids), Heterohelix and arenaceous Foraminifera characteristic of the outer platform environment; its age is Santonian to Campanian.

Geologj, ofthe Musandam Peninsula 14.5

W X w w

X s w

6 g s o u a + O W U

2

LlGOCENl

W

X W

U

0

W

ALEOCENI

j t c r . m E ; > a . i t ;:E - ! C E N O N .

1 ALBIAN

i APTIAN

! BARREM

-

; HAUTER: : VALAN6- I BERRIA! ---

Figure 4 STRATIGRAPHIC COLUMN

STRAIT OF HORMUZ AREA

f

\

'.

nsula

MU S A N DA M P EN1 NSU LA A G E

In the upper part. a rhythmically bedded series of sandy limestones. sometimes silicified. with arenaceous Foraminifera and argillaceous limestones with pelagic formscorresponds to ;I carbonate flysch facies. of Campanian t o Maestrichtian age / G ' ~ c J ~ o / / I ~ I I ~ u / I u /ijrriiecircl.

G. r i , i c . ~ ~ r r i ? i u l , e r . i and G. tnugdu1erisi.s /.

Pabdeh Formation This is an argillaceous carbonate series. of

1.500 m thickness. of which the base is formed of rhythmically bedded argillaceous limestone with cherts and shales. The base. which follows the Maestrichtian flysch series. is dated as Paleocene /Glohisgc~ritiu rr.i/oczilirioiiir.c. Koruliii reii.wiii/.

In higher parts o f the formation the following ages have been determined:

Eocene. Di.woc>.c/iiio sp.. t i i i / i i r n ~ i / i / I d ~ .

F/o.cc,ulina sp.. (;lohorotu/;u cer/.occ:arleri.\;.\. G. c ~ e r i r / x i i i c , Hurilkeniriu br~e~~i.spirin.

Oligocene: Chilogzinihehiu sp.. Globigeriiici c$erorn.s is.

The bathyal environment. manifested at the base. changes progressively towards external platform with open influence towards the top: this is therefore ii weakly regressive megasequence. 'The last marine faunas give a 3liddIe Oligocene age before the sharp change t o an eLaporitic regime in the overlying fommation.

Lower Fars Formation Its thickness is 6.50 to 7.50 m: it is an

evaporitic episode comprising a basal saliferous unit overlain by an anhydritic unit. Its age is reputed t o range from Upper Oligocene t o Lower M ic wen e .

Guri Formation The dolomitic shales and bioclastic lime-

stones ibrachiopods. crinoids, corals. lamelli- branchsl of this 50 m thick unit rest discordantly 0 1 1 older series and have been correlated with the Guri Formation, defined in Iran. where it is dated Middle Miocene. In the Bukha area the faciex is already more shaly and the carbonate epismie is tery reduced.

T h e Formation overlies ii major uncon- f<mnii\ which may truncate. in an easterly direction. the whole of the Fars and the Pabdzh iBukha I well). so that the Guri ma! rest. o n the edge of the Peninsula. on eroded Musandam limestones; this formation is transgressive and over the surroundings of the Musandam peninsula its age could be Upper Miocene.

Mishan and ?ounger Formations ~Tht .ser iescni ls~i thshalcs . with sandy mark

interbedded with banhs of shelly limestones. These deposits arc of Upper Miocene t o Quaternac age.

Gull of Oman Region 'Phis is much less well explored than the

Arabian G u l f region discussed above, since onl? four wells have been drilled along the coast of the Gulf of Oman. the nearest to the Musandam Peninsula being situated offshore Fujairah.

This is ii geologic province totally different f r o m the Persian Gulf in both its sedi- mentological and structural aspects. The explored series is argillaceous and detrital throughout the section. From geophysical data, the base of this series is formed into large ridges. parallel to the coast. which seem to be attributable to clay diapirism (domes of high pressure shales): these diapiric shales could be of Upper Cretaceous age (equivalent possibly t o the Gurpi series) or they could be of considerably older origins (equivalent to the Hawasina series).

Between the ridges, the filling is formed by a turbiditic series passing into conglomerates, generally coarse. The direction of progradation is mostly from north to south (running parallel to the edge o f the platform constituted by Musandam). These turbidites range in age from Paleocene t o Lower Miocene.

The discordance at the base of the Upper Miocene. noted in the western region. is also well marked here (seismic marker H1. Figs. 7 and 8) but the carbonate episode of the Guri is absent. The overlying sediments are rhythmic alternationsof silts andshalesorof sandy shales and conglomerates. which thicken towards the open sea, attaining 2500 rn at the edge of the permit. The mode of deposition is that of a deltaic complex.

For the whole of this post-Upper Cretaceous series. the material was provided by the erosional dismantling of the ophiolite complex (Semail Formation) and of radiolarites t Hawasina Formation). The derived fauna frequently found in the series renders true age determination a matter of great difficulty. Only the top of the high-pressure shales have been penetrated in the Fujairah well (seismic marker H5. Fig. 6 ) .

Below the plastic shales, seismic results are nullified and the substratum to the high pressure shales has not been defined exactly.

Geologj, of' the Musandurvi Petrinsula 137

LL--L Normal toult

-6

9- Thrust laul l

Reverse t a u l t w l l h dip o l l h e fault plone

Anttcl inal (rend

- - S y c l ~ m l t r e n d

Figure 6 SCHEMATIC STRUCTURAL CROSS -SECTIONS

*a WNW ESE’W E

SU# + ft 4 X . l Q**N

m” ‘0

Lwo Po0 mm

. i__.

WNW E

i ..

1 wo cc)ntr;iclictorT hypotheses can be ad\. anced: thc high-pressure shales ma! o\ erlie cratonic shield cohered w i t h c;rrbonates. mw-e or less r r - c ) r l ~ ~ l . 01- t he! m;r! he laid don II o v e r oceanic lxisement. niort‘ or less obducted along the N N E line of the Dihba f a u l t . with its c(i\er of HaKasina series. From the regional i n f o r n i a o o n i i i their possession the authors ilefinitelq preler the second of these hk pothcws i c t . l.ic, 01.

111. TEClONICS AFFECTING THE PEYINSL’LA (Figs. 5 and 61

north of the Peninsula. Chronologically. these art. the youngest faults.

Flc.\-ihk~ D@iviutioirs Between the faults. the carbonate series d o

not lend themselves t o deformations of this type. and the blocks have a tabular, monoclinal aspect. However. along the pinched synclinal and anticlinal axes in the eastern zone. there is an :inticlinal b o x fold (Ribat axis) that extends lor more than I 0 kms: in the southern part it undergoes a torsion of axis to the W , whereby the W flank of the fault disappears. effectively becoming a thrust zone. I n thc plastic beds (Elphitistone and Ghalilah Formations). tight folds. a metre t o ten metres across. generally incline tuwarcls the W. emphasising the existence of strong compression: they are accompanied by reverse faults. inclined in the siiiiie direction.

7-c~r.r o r r ic .\lice.\ In the region of Dibba. the limestone thrust

slices mark the contact between the nappe and the Ruus al Jibal dolomites. The schistosity ile\eloped indicates a direction of N . l 10° lo r the senw of constraint. I n the Shimal thrust /.one. the slices have been described by Hudson i ’ I trl. I 10041.

I SEISMtC SECTKIN IN THE MUrAaAM WESTERN OFFSHORE Figure 7 (d- t - nonmiqrotcd ) I , .,. ... ... ... ,.. ,.. .. ,.. ,.. ... ,,, ,.. ... ,., ,.. ,.. ,.. ,.. ... .,. ... ... ... ,.. .,. c

W

0

1-

peee

3-

Overthrust Nuppes These are limited to the S part of the

Peninsula. Their structure is very complex. with both large and small-scale folds and the often chaotic superposition of wedges of different types. The thrust planes, slightly inclined to sub-horizontal. are striated and the rose diagram of the direction of sliding ranges from N.25O to N.245'. Boundary evidences of two nappes are seen in the synclinal axis of Qawah Sibi; the nappe series are composed of grey-green radiolarites, violet argillo-siliceous rocks and argillites with cobbles derived from Barremian to Cenomanian limestones. They may be compared with the Dhera Formation, defined in the region of Dibba by K. W. Glennie et ul. (1974). They rest either on the equivalents of the Muti formation. or on the Musandam limestone. This evidence indicates that the absence of nappes over the Peninsula at the present time is due to erosion.

Anothernappeoutcrop occurs in the tectonic window of Hagil. These nappes are themselves covered here by the thrust of the autochthonous Musandam series.

Thrusts Difficult to prove, thrusts become apparent

only when mapping a marker (e.g. the siliceous rock marker of Lower Cretaceous age). They

L

-

have been observed in the W part, where the same marker can be found superimposed four times, and in the Hagil tectonic window. The thrust planes are either sub-horizontal or inclined 130-50°). Horizontal displacements are impossible to measure; in the case o f Shimal the overthrust is visible for many kilometres. In the W part. the thrust planes become vertical and turn into reverse faults without much separation. The principal zones of thrusting are at Shimal. Bayah, Jabal Bi Aya. Rahabah, Sham Bukha and Grubbali.

Chronology and the Age of the Different Tectonic Periods

Erosion having removed the Recent series from over the Peninsula. it is in the Buraimi region that the first dating is given to the sediments of the Post-Nappe Series. which are of Maestrichtian age at the base. This confirms the Upper Cretaceous age of the emplacement of the nappes which has been demonstrated in other parts of the Oman mountains. On the other hand. seismic sections of the Gulf. of which an example is given in Fig. 7. generally show that:-

The thrust faults are sealed over by the flyschoid series (interval H6-HX) of Upper Cretaceous to Paleocene age. showing that consecutive thrust movements of the nappu

150 R. Rirateair arid P. H. Rich6

/ POSlTlCN SKETCH MAP Figure B

SElSYlIC SECTION ACROSS THE NORTHERN PART OF XISANDAM PENINSULA

( d - t - nonmigrotea - horizontal scale highly compressed 1

G U M OF OMAN

emplacement continued into the Paleocene. The thrust of the Musandam series over the nappes o f the Hagil window is ii comparable phenomenon and should be attributed t o the same phase of conipression.

In the series up t o the Miocene. there are anticlinal and synclinal axes which are eroded and unconformably covered by the Guri limestone cseismic marker H1) : this second phase of compression is therefore pre-Upper Miocene in age. I t is t o this phase that the folding o f the post-nappe outcrops m d the foothills situated along the edge of the Oman mountains is mainly due. To this phase. also. must he credited the great anticlini~l swells o f the Oman mountains of which the Musandam peninsula forms a part. The peneplanation surface seen in the outcrops prubably originated immediately after this upswelling.

The normal faults die out in the argillaceous Pabdeh series. These faults were created at the cnti of this second phase of compression.

Over the Peninsula. as i t plunges N (Fig. 8 ) . normal faults affect series of more and more recent age. showing frequent rejuvenation u p t o the present time. Fig. H shows the sharp breahs of tipper Miocene - Pliocene horizons against the Knolls surrounding the Peninsula. 21s well its their very rapid transgressive character: one can deduce a synsedimentary sinking of

the Peninsula during the Mio-Pliocene, but the central part as shown by the peneplanation was not submerged. Above the horizon HO, which probably corresponds to a level in the Plio-Quaternary. the aspect of the deposits indicates submergence of surfaces of very marked relief: these surfaces are comparable tu those which can be seen in outcrop, and which owe their relief to the rejuvenation of NE-SW faults which have broken the erosion surface of the pre-Upper Miocene anticlinal swell. creating a succession o f horsts. T h e submergence of this relief in recent times is demonstrated by the drowned valleys (rias) of the present North and East coasts. This, and the presence of uplifted Quaternary beaches o n the West coast. testify to very recent tilting movements of the Peninsula towards the North- East.

IV. PALEO-STRUCTURAL AND PALEO-SEDIMENTARY

RECONSTRUCTION

From Permo-Trias until now. the history of the zone under consideration can be recon- structed in the following f a s h i m -

I n the Permo-Triassic. sedimentation of slightly deep to sub-aerial types. indicated the

Geology ofthe Musandam Peninsula 151

continental margin that limited the Arabian platform;

The Lias was marked by subcontinental sedimentation close to the tide-line, then by low-energy carbonate sedimentation. A strong subsidence found at the present site of Musandam already testifies to the instability of this zone;

In the Middle to Upper Jurassic, very low- energy sedimentation continued, but in contrast to the Liassic situation, the subsidence was very weak: lacunae and condensation mark this active margin of the Arabkanplatform;

The Lower Cretaceous began with marine transgression which submerged this edge, then the sedimentation became that of a sheltered platform type;

In the Middle Cretaceous less sheltered conditions of deposition and marine pulsations are indicated by a rapidsuccessionof sequences.

At the end of the Middle Cretaceous, a tectonic phase uplifted the whole of the edge of the platform and caused salt movements in the Gulf (Hormuz salt of Cambrian age). These movements correspond to the Turonian unconformity and to the erosion of part of the Musandam series along the edge of the Arabian platform.

Up to the end of Middle Cretaceous time, the limit of the platform coincided, adven- titiously, with the edge of the Peninsula. The oceanic domain (situated in the Gulf of Oman) received bathyal, open sea or slope talus: radiolarites, siliceous limestones, cherts and turbidites forming the Hawasina complex, sedimentation of which commenced in the Triassic.

The Upper Cretaceous commenced with discordance at the base of the Laffan (transgression), succeeding sediments being the non-sheltered, high-energy platform de- posits of the Ilam. Above this, the sediments change abruptly to lime mudstones indicating that there was a further strong subsidence during which open sea sediments were deposited on both sides of the Peninsula; the pelagic environment was thus shifted abruptly towards the W and SW, spreading over the whole edge of the Arabian platform. This shift of the axis of the basin towards the W can be attributed to the start of the closing of the two regions, which led to the collision of the edge of the continental plate with the oceanic domain and caused: -The overthrust of oceanic sediments (Hawasina) then of the peridotites and gabbros

of the crust (Semail) over the edge of continent during the Upper Cretaceous; -An imbrication of the substratum (thrusts, reverse faults) lasting to the Paleocene; -The creation, on the edge of the plate. of high relief due to stacking of nappes and thrust slices; this relief was immediately attacked by erosion to give flyschoid products which filled a subsiding foredeep parallel to and in front of the ridge lormed by isostatic compensation. In a W direction the flyschoid series diminishes rapidly in thickness as a result of sedimentary cut-out. On the Musandam E flank, behind the ridge, the well data and geophysics of the Gulf of Oman seem to indicate much-reduced pelagic sedimentation during this period. The formation of shale ridges from sediments at great depths (Musandam) could have started at this time as a consequence of the E-W compression. -From Upper Paleocene to Lower Miocene. there was continuation of argillaceous sedi- mentation, with slightly different developments in the two domains: in the Gulf of Oman, turbidite deposition was prevalent whereas, in the Persian Gulf, a regressive cycle, filling up the foredeep, ended with the evaporitic episode of the Lower Fars.

The Musandam area, situated on the emergent ridge, separated two domains: that in the E, which remained oceanic, with steep slopes at its edge, and that in the W which, sheltered and stable, tended to become a lagoon.

The second phase of compression, general for the whole Middle East. then folded all the existing series and rejuvenated faults associated with certain old alignments. --In the Persian Gulf. remobilisation of salt which flowed with different intensities accord- ing to the disposition and extent of the fault network. -Along the edge of the platform, folding of the series; because of mechanical incompetence of the affected formations, the sharp "shale diapiric" folds are localised in the argillaceous cover and the series which overlie them (Gulf of Oman), whereas in the carbonate substratum the formation of large wave flexure folds involved the upheaval of the edge of the plate (Musandam Peninsula, Oman Mountains).

At the end of this phase, a network of faults was created by release from constraint; this distension may have reactivated first phase reverse faults. The peneplanation surface. clearly visible on the outcrops of the Peninsula.

1.52 R. Ricnteciu arid P. H. Rich6

could correspond t o an erosional phase after this paroxysmal folding. ---From Upper- Miocene to Pleistocene. the sedimentation was argillaceous/silty o r argillaceous/calcareuus. accompanying a strong subsidence in the Gulf of Oman. The peneplaned Peninsula was progressively sub- merged. followed by a strong uplift at the end o f the period. with rejuvenation of normal faults a t this time. the rejuvenation being rcstricted t o a narrow band along the edge of the platform. where all t h e tectonic movements have their maximum effect. These movements tiii\e heen recently Eolicn\ced hq ii i i l t with very rapid upheaial of the southern part. This last nioct'mrnt is emphatically indicated by the formation of fluvial terraces. the existence of Quaternary raised beaches 1 Ras A1 Khaimah. cf. K. W. Glennie et [ I / . . 19741. the evidence of

an inclined peneplane varying in altitude by more than 1.500 m. and by the submergence o f the valleys over the whole of the NE coast.

REFERENCES

ALLEMAN. F. and PETERS. Ti.. 1972. T h e Ophiolite-Radiolarite belt of the North Oman

RIEHLER. J.. CHEVALIER. C. et RICATEAU. R.. 1375. Carte geologique d e la peninsule de Musandam. Edition B.R.G.M.. Orleans i France).

GLENNIE. K . W.. BOEUF. M. G. A.. HUGUES CLARKE. M. W.. MOODDY-STUART. M.. PILAAR. M. F. and REINHARDT. B. M.. 1973. Geology of the Oman Mountains. Verh. K o n . Deded. Geol. f t f ~ ' j n . Genoor.. 31, Pts. 1 , 3.

HL'DSON. R. G. S.. McGUGAN. A. and MORTON. D. M.. 1964. The structure of the3ibal Hagabarea, Trucial Oman. Quart. Journ. Geol. Soc. London, 110,2. No. 438. 121-152.

nl\)urlt:[it1\. Ec/og. G<>O/. Hcltr. 65, 3. 057-697.