Petrological and Tectonostratigraphic Evidence for a Mid Ordovician Rift Pulse on the Arabian Peninsula

Home , Abas (diatom), East African Orogeny, Hornblende

GeoArabia, Vol. 4, No. 4, 1999 Gulf PetroLink, Bahrain Mid Ordovician Rift, Oman

W. Heiko Oterdoom, Petroleum Development Oman Mike A. Worthing, Sultan Qaboos University, Oman and Mark Partington, Petroleum Development Oman

ABSTRACT

During late Early Ordovician times an increase in the rate of subsidence in the Ghaba Salt Basin and western South Oman Salt Basin is suggested by the thick sequence of continental clastics of the Ghudun Formation. After a phase of rift-shoulder uplift and erosion, related to a renewed pulse of extension which may have initiated diapiric growth of salt structures in the Ghaba Salt Basin, sedimentation resumed again in the Mid Ordovician. During this period, the center of deposition shifted to the Saih Hatat area in North Oman. This paper documents seismic and well data, field investigations and petrological study of potassic mafic rocks from the Huqf area which were intruded in the eastern side of the Ghaba Salt Basin. A Mid Ordovician age of 461 ± 2.4 million years has been established for these rocks by the Argon-Argon step heating method. Analogy with the petrology and setting of similar potassic mafic rocks from the Rio Grande Rift in the western United States of America suggests that they were intruded into the shoulder of an intra-continental rift. The data provide the first clear evidence of a pulse of rift-shoulder uplift in the Huqf area during the Mid Ordovician. The 3-kilometer-thick Mid to Late Ordovician clastic sediments of the Amdeh Formation in North Oman, together with the occurrence of abnormally thick sedimentary sequences and volcanics in the Tabas Graben in Iran, are consistent with a period of break-up of eastern Gondwana. Together, the Ghaba-Saih Hatat and Tabas Basins are considered to be part of a failed rift arm. These observations further improve our regional knowledge of the Early to Late Ordovician tectonic setting of Oman and will assist in unlocking the hydrocarbon potential of classical rift-related structures consisting of early-rift Early Ordovician sand-prone reservoirs sealed by syn-rift Mid to Late Ordovician marine shales.

INTRODUCTION

In Central and South Oman, a series of northeast-southwest to north-northeast–south-southwest trending extensional basins are developed on Precambrian crystalline basement (Figure 1). Episodic basin formation in Gondwana was initiated in Late Precambrian times following the formation of the Rodinia supercontinent (Stern, 1994). In Oman, the western margin of these basins is marked by a structurally complex compressional zone of late Early Cambrian age defined as the “Western Deformation Front” (Loosveld et al., 1996). Deformation increases in intensity towards the southwest across the South Oman Salt Basin into Yemen. This tectonic pulse probably signaled the end of the indentation of West Gondwana into East Gondwana (Stern, 1994) and is associated with regional uplift and erosion over the entire Arabian Plate (Stump et al., 1995). The eastern margin of Oman’s salt basins in contrast is less tectonized and Phanerozoic sediments offlap, onlap onto, or are truncated against, a structural high known as the Huqf High (Gorin et al., 1982). This high is located close to the present east coast of Oman.

Subsequent phases of uplift of the Huqf High resulted in the present-day exposure of rocks ranging in age from the Precambrian granitoid basement to Tertiary sediments (Figure 2). The outcropping sequence contains both major and minor unconformities. Large sections of the stratigraphic column are absent, particularly in the Late Paleozoic due to lack of accommodation space and erosion during a Mid Carboniferous Hercynian event. Documented rift-shoulder uplift pulses in the Late Carboniferous to Early Permian and again in Late Triassic to Early Jurassic times preceded the break-up of Gondwana in Late Jurassic times (Blendinger et al., 1990; Immenhauser et al., 1998). In many cases, the exact nature of the unconformities, particularly the minor ones, remains to be elucidated. In this respect,

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o o o 54Fahud 56 58 Yibal Salt M A ARABIAN Basin H K IG R H A GULF EM N R F O KA L D Figure 3 A Makarem-1 B E L M T ( A C T I V E ) UK Musalim RO Deep-1 AB GULF OF 24o M Saih Ghaba N. Figure 8 Nihayda O OMAN Figure 5 Qarn M UNITED ARABSaih Rawl Alam A Muscat EMIRATES N Wadi Bani Awf M O Semail Gap HAWASINAU THRUST SHEETS Saih N I Transfer Lekhwair T Fault Hatat A Dhulaima I N S Natih W. Qalhat Fahud W. Natih Figure 3 ARUMA TROUGH (Fiqa Foreland Basin) Fahud Fahud Jabel Yibal H Salt IG Ja'alan o N H 22 Basin EM MARADI Maghoul S. AR AK Makarem-1 FAULT ZONE M Andam-1 K Musalim Al- ROU Deep-1 AB Saih Ghaba N. Ashkharah M Nihayda Figure 8 Qarn Figure 5 Saih Rawl Alam Al Jobah SAUDI ARABIA Mafraq Mabrouk

Barik Al Ghubar H G U ) Ghaba Salt S O G Maqtaa IN R T Basin T S U S R H T Zauliyah C ( E N Masirah NT O RA H I E L Figure 13 A T G OM C ID A IR U N D R HI S MASIRAH N Bahja GH B A A O Masirah TRANSFORM o W M L H A Bay 20 D R FAULT U T R S OMAN HIGH I Z N U Zafer-1 I A S Z N F Bahaa O A HUQF T I M Mukhaizna I L South Oman ARABIAN SEA Salt Basin Kulan-1 H NAJD TREND Jalmud IG H Rima H Hawasina Nappes A F Semail Ophiolite S Nimr A Al Noor H Runib (Para-) autochthonous -K N U Ordovician Amdeh Fm. D Amin Irad EASTERN FLANK U Birba H Inqaa G SAWQRAH Basement Marmul Mawhood SIN BA BAY Salt Basin Qaharir IARY RT o Ghudun-1 TE East Oman Ophiolite Complex 18 OBLIQUE OBDUCTION Batain Nappes Dhahaban S. WAGHALD BASIN Huqf Outcrop

AYDIM Jazal Oil Field GRABEN Al Halaniyat S Ordovician mafic QARA MOUNTAINS Sills/dikes Marbat 0 100 Fault Al Hota (Coast) Ain Sarif Km Dry Well (Salalah) CRETACEOUS-TERTIARY (Approximate) Well SALALAH BASIN

Figure 1: Salt distribution coincides approximately with the area of rifting in the Infracambrian. During the Mid Ordovician, the Ghaba Salt Basin and Saih Hatat experienced extension related to a renewed break-up attempt of eastern Gondwana.

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CHRONOSTRAT. SUPER- GROUPS LITHOLOGY/ FORMATIONS CYCLES REMARKS RT Arid Continental and Marginal Marine Sediments FARS Taqa Dammam Collision Arabia Eurasia and Andhur Rus Microcontinents 50 TERTIARY HADHRAMAUT UER Carbonate Evaporite Ramp CENOZOIC Simsima Shamar Arada 2nd Thrusting Event Obduction ARUMA Emplacement of Nappes, Foredeep Fiqa Natih Fore Bulge Unconformity WASIA First-order Flooding Anoxic Events 100 Nahr Umr Shu'aiba Kharib Onset Subduction in Neo-Tethys KAHMAH Lekhwair Regional Unconformity Habshan ? Collision Turkish-Arabian Plate CRETACEOUS Salil Collapse North rift shoulder Rayda 150 Hanifa Tuwaiq Break-away India

Dhruma SAHTAN Onlap onto E-rift-shoulder MESOZOIC INTRACRATONIC SETTING INTRACRATONIC 200 JURASSIC Mafraq Collision to North of Turkey-Iran Plate reorganization PLATFORMS CARBONATE

Jilh EXTENSIONAL COMPRESSIONAL AKHDAR MESOZOIC CYCLE ALPINE CYCLE Sudair Khuff 250 Drifting Neo-Tethys and Rifting Batain Basin Continental Clastics HAUSHI Gharif Al Khlata Glaciation 300 Intracratonic Sag Thermal Doming and Rifting Glacial Erosion

350 HABUR CARBONIFEROUS PERMIAN TRIAS.

MISFAR COMPRESSIONAL 400 DEVONIAN PALEOZOIC Sahmah First-order Flooding SILUR. Anoxic Event Sagging CYCLE PALEOZOIC 450 Glaciation

L SAFIQ SETTING INTRACRATONIC Hasirah Rift Unconformity M Saih Nihayda Rifting Ghudun E Mabrouk HAIMA

MAHATTA EXTENSIONAL ORDOVICIAN Barik Continental to 500 L HUMAID Al Bahsair SUPERGROUP shallow-marine Miqrat Mahwis clastics "Sag" M Amin NIMR Karim "Angudan" Tectonic Event Rifting E ARA Carbonate Evaporite System

CAMBRIAN NAFUN

550 Buah Shuram Khufai HUQF ABU MAHARA Glaciation Rifting SUPERGROUP

Onset escape tectonism VENDIAN 600

Onset East/West Gondwana continent-continent collision PAN AFRICAN CYCLE PAN 650 MARGIN PLATE ACTIVE

PRECAMBRIAN Source STURTIAN Rocks TRANSTENTIONAL COMPRESSIONAL TO Accretion of Island Arcs 850 and Microcontinents Figure 2: Simplified tectono-stratigraphy of Oman interior basins. Time Scales: Harland et al. (1990) for Cenozoic-Silurian; Gradstein and Ogg (1996) for top Ordovician-Cambrian.

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igneous rocks are important as they provide petrological and geochronological evidence pertinent to the plate-tectonic setting and dating of the extensional structural events with which they are associated (Wilson, 1994).

In this paper, we initially review the regional geology and structural history of Oman during the Late Precambrian and Early Paleozoic. The paper then documents the petrology and geochronology of potassium-rich mafic intrusions exposed in the northern Huqf. These volcanics provide evidence for a subtle rift event. We integrate this information with the Early Paleozoic tectonic and sedimentological evolution of Oman and surrounding areas of the Arabian Peninsula.

For the Cambrian to base Silurian, the time scale of Gradstein and Ogg (1996) has been applied (Figure 2). For the younger Phanerozoic we use the time scale of Harland et al. (1990). The main change compared to the time scale of Harland et al. (1990) is the shift in time of the base Cambrian from 570 to 545 million years ago (Ma). Presently, the base Cambrian is assigned an absolute age of 543 Ma (Knoll et al., 1995; Brasier et al., 1997).

REGIONAL GEOLOGY

Onset of Sedimentation onto Basement up to the Angudan Event, Late Early Cambrian

About 560 Ma, a change took place from predominantly transpressional tectonics to a predominantly transtensional regime on the Arabian Peninsula. Sediments older than 560 Ma are present, but their distribution in time and space is poorly known in Oman. The increase in extension occurred at the end of a protracted continent-continent collision between East and West Gondwana (e.g. Stern, 1994; Le Métour et al., 1995; Grunow et al., 1996; Shackleton, 1993 and 1996; Blasband et al., 1997). Uplift and erosion of large areas of continental crust is suggested by the rapid and significant increase in seawater 87Sr/86Sr from 0.708 to 0.709 during deposition of the Huqf Supergroup (Burns et al., 1994). Intra- continental extension is attributed to an extended phase of indentation of West Gondwana into East Gondwana (Schmidt et al., 1979; Stern, 1994). The sinistral northwest-southeast Najd Trend with a cumulative strike-slip component of about 300 kilometers (km) (Moore and Al-Shanti, 1979; Al-Husseini, 1988) is evidence of this continental deformation. In Oman, Loosveld et al. (1996) described two major Infracambrian-Early Cambrian cycles of intracratonic rifting (Figure 2) which formed northeast- southwest to north-northeast–south-southwest trending basins. These rift basins are approximately perpendicular to the Najd Fault Zone and the orogenic collision front and were considered to be related to escape tectonism (Molnar and Tapponnier, 1975; Stern, 1994). An alternative, but not necessarily mutually exclusive, model to explain the Late Precambrian to Early Cambrian extension is based on continental failure and rifting of the Gulf salt basins following a northwest-southeast trend (Al-Husseini and Al-Husseini, 1990; Talbot and Alavi, 1996). Subsequent rifting of Paleo- and Neo-Tethys followed this early trend (Stampfli et al., 1991).

In Oman, Rift Cycle I took place in the Late Precambrian forming the Abu Mahara Basin. Geophysical data and the few well penetrations suggest the development of a northeast-southwest trending rift basin segmented by a series of north-south to northeast-southwest-trending basement highs such as the Mabrouk-Makarem High (Figure 1). Rifting was accompanied by volcanism and granite intrusions. Transitional and tholeiitic basalts are known from the Saih Hatat area (Le Métour, 1988), whereas more felsic igneous rocks occur in Central and South Oman. Rhyolitic volcanics at the base of the Abu Mahara Group in the northern Huqf area have Rb/Sr ages of 562 ± 42 Ma and red alkali granite intrusions in the Marbat area were dated at 556 ± 11 Ma (Dubreuilh et al., 1992; Platel et al., 1992).

The Abu Mahara Basin, referred to above, was re-activated during earliest Cambrian Upper Huqf times indicating renewed tectonic activity and rapid subsidence. Loosveld et al. (1996) refer to this stage as Rift Cycle II. However, ongoing evaluation of the western flank of the South Oman Salt Basin suggests that it may be more appropriately described as a foreland basin. (Immerz, personal communication, states that a foreland basin was earlier proposed by Boserio et al., (1995), but the

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(km)

DEPTH

13 14

Mid

uring the

Southeast

HUQF UNITS

THRUSTED ? THRUSTED

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EAST FLANK

050

(Approximately 10 x vertical exaggeration) (Approximately

fault

Maradi

roup Haushi

Amin/Nimr

Ghudun

Safiq G Safiq

Middle Haima

f a

Ara Salt N

field

Qarn Alam

Abu Mahara Abu

PDO ACREAGE

GHABA SALT BASIN GHABA SALT

Sahtan

Wasia/Kahmah

Akhdar

Saih Nihayda field Saih Nihayda

Tertiary

Aruma

limit of Miqrat

Approximate western Approximate

MUSALLIM SLOPE

Abu Mahara Abu

Precambrian (Nafun)

Early Cambrian Ordovician-Late (Middle Haima)

Cambrian (Amin/Nimr) Early Cambrian (Ara)

Precambrian (Abu Mahara) Precambrian (Abu Basement

Makarem-1

Nafun

Al Bashair-1

limit of Barik

Approximate western Approximate

field

Yibal

Tertiary Cretaceous-Triassic (Aruma-Sahtan) Cretaceous-Triassic Triassic-Permian (Akhdar) Triassic-Permian Permian-Carboniferous (Haushi) Permian-Carboniferous Early Silurian-Mid (Safiq) Ordovician

Early (Ghudun) Ordovician-Arenigian

FAHUD SALT BASIN SALT FAHUD

Sahtan

Tertiary

Wasia/Kahmah

Akhdar

Gas Accumulations Oil Accumulations Bitumen Accumulations Potential Prospects/Leads Potential

Oil Seep (Saiwan) Source Rock

Basement

Figure 3: Overstepping of the Infracambrian Huqf rift basins started during deposition of the Amin and Nimr clastics. Renewed Ordovician extension during deposition of the Safiq-Ghudun is difficult to detect on seismic due to erosion, which took place d Ordovician extension during deposition of the Safiq-Ghudun is difficult Northwest Hercynian event and following rift-shoulder uplift. (See Figure 1 for location.)

field

5 2 6

13 14

Dhulaima

(km)

DEPTH

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54° 56° 58° ARABIAN GULF GULF OF OMAN

UNITED ARAB EMIRATES 24° Hawasina Thrust Sheets

¥ ADM-1 Andam-1 ¥ MKM-1 Makarem-1 ¥ SN-3 Saih Nihayda-3 Maradi-Nafun Fault zone Huqf Salt Basins 22° PDO concessions

SAUDI ARABIA

0 250 500 750 1,000 1,350 m

20°

18°

ARABIAN SEA Base Ghudun truncated by the Al Khlata Formation

N 0 100

Km (Approximate)

Figure 4: Isopach map, Ghudun Formation based on well data.

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development of the foreland basin followed the deposition of the Ara Group in their model.) Carbonates, shales, silicilytes and evaporites of the Ara Group are conformably to unconformably overlain by clastics of the Lower Cambrian Nimr Group, recently incorporated within the Huqf Supergroup (Hartkamp-Bakker et al., 1998) (Figure 2). The Angudan Unconformity, separating the Nimr Group from the Haima Supergroup (Figure 2), is considered to represent the end of the East African Orogeny, during which the indentation of West Gondwana into East Gondwana came to a halt, and escape tectonism ceased (Hartkamp-Bakker et al., 1998). Within the clastics of the Nimr Group, many major and minor erosional gaps reflect shifting alluvial fans and pulses of uplift of the hinterland.

The Pan-African cycle, of which the East African Orogeny formed the active part during its waning stages, ended in about Early to Mid Cambrian times and was followed by uplift and cooling of almost the entire Gondwana Continent. This is indicated by apatite fission track ages (AFTA) which do not exceed about 500 Ma versus AFTA ages as old as 1.2 billion years in Laurasia (Veevers, 1995). AFTA data to the southwest of the South Oman Salt Basin measured in mildly anchimetamorphic granitoid basement in Ghudun-1 follow this 500 Ma uplift trend. However, along the eastern flank of the South Oman Salt Basin, AFTA data in pre-Angudan Nimr sediments give ages older than 500 Ma, for example in Rima-1 they range from 670 to 550 Ma (Visser, 1991). For this area, it indicates continued “cooling” without major subsidence after sedimentation.

The large influx of clastics during the late extension phase reflects this last tectonic pulse of the Pan- African cycle and the initiation of salt diapirism in the South Oman Salt Basin. After the extension phase during Ara deposition and the Angudan event, the sag phase resulted in the deposition of the Mid Cambrian to Lower Ordovician Mahatta Humaid Group (Figure 2).

Basin Development following the Angudan Event

Significant overstepping of the Mahatta Humaid clastics from the South Oman and Ghaba Salt Basins onto the Huqf High took place during the late Early Cambrian to Early Ordovician. A flat topography is indicated by the lateral continuity of transgressive-regressive cycles over the entire Fahud-Ghaba Salt Basins (Figure 3). Continental clastics dominate the lower part of the Mahatta Humaid Group, whereas marine clastics characterize the upper part (Droste, 1997). Towards the top of the group, coarsening-upwards deltaic to fluvial clastic sediments of the Arenigian Ghudun Formation dominated once more. In the Ghaba and Fahud Salt Basins, the Ghudun Formation is characterized by thick monotonous proximal braid plain sandstones, thinning over Petroleum Development Oman (PDO) concessions from east (Andam-1, 1,020 meters (m) to west Makarem-1, 300 m, Figure 4). Dipmeter data show dips to the west-northwest to northwest, offlapping from the Huqf axis over Saih Nihayda field. This orientation is consistent with the observation on seismic data of west-northwest to northwest prograding clinoforms and downlap surfaces within the Ghudun Formation (Figure 5).

Ghudun sedimentation was terminated by uplift. The subsequent erosion was more severe in South Oman than in North Oman and more severe over the highs than the lows (Figure 6). Based on the truncation of regionally correlated intra-Ghudun flooding surfaces, up to several hundreds of meters of Ghudun Formation were removed from highs separating the two northern Oman salt basins (Droste, 1997) (Figure 6). The significant thickness of the Ghudun Formation in the lows and subsequent erosion over block-faulted highs is considered to be related to a rifting event. The lack of regional erosion documents the absence of a rift dome or a phase of regional compression. Unfortunately, detailed biostratigraphic control for the poorly fossiliferous Mahatta Humaid and especially for the barren sandy Ghudun is lacking and does not allow precise calculation of an increased rate of subsidence. However, a significant increase during deposition of the late Early Ordovician Ghudun Formation in the Ghaba Basin is evident.

In southwest Oman, west of the Ghudun-Khasfah High and Anzauz-Rudhwan Ridge, the Ghudun Formation onlaps onto the slightly deformed Abu Mahara indicating a westward tilting of the area as the depositional axis shifted away from the South Oman Salt Basin (Loosveld et al., 1996). This westward tilting during Arenigian times, opposite to the direction of subsidence along the western flank of the Late Precambrian-Early Cambrian rift basins, is considered another sign of renewed extension over a

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180

Safiq dip directions

270

Ghudun dip directions

270 90

MLM-DP-A

Ra'an MFS

Musallim Deep-1

Mabrouk MFS

Hanadir MFS

Ghudun Formation

0 2.5

Base Al Khlata Formation Base

Top Natih E Formation Top

6,119 6,200 6,300 6,400 6,500 6,600 6,700 6,800 6,900 7,000 7,100

680

1,000

1,250

1,500

1,750

2,000

2,250

2,500

2,750

3,000

3,250

3,480

Figure 5: Ghudun clinoforms in the Musallim field area (see Figure 1), downlapping onto the Mabrouk Maximum Flooding Surface (MFS). The Hanadir MFS lies at the base of the Saih Nihayda Formation. The Ra’an MFS lies at the base of the Hasirah Formatio The difference between the depositional dips of the Ghudun Formation and the Safiq Group (Saih Nihayda and Hasirah formations) The difference is indicative of an unconformity, difficult to resolve on seismic data. (Regional structural dip removed.) difficult is indicative of an unconformity, Two-Way Time (ms) Time Two-Way

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200

East-Southeast Gamma Ray

Burhaan West-1 Burhaan West-1 (BHW)

Ghaba Salt Basin

Barakat

Wadi Umayri-2 (WU-2) Umayri-2 Wadi

Turbidites

Ghudun, Early Ordovician Safiq, Early Silurian to Mid Ordovician

Al Khlata, Late Carboniferous-Early Permian Al Khlata, Late Carboniferous-Early

200

High

Mabrouk-Makarem

Gamma Ray

MARADI

FAULT ZONE FAULT

(Fiqa Foreland Basin)

Makarem-1 ST2 (MKM-1)

G Burhaan West-1

Wadi Umayri-2 Wadi

Qarn Alam

O Ghaba N.

I H

Mafraq

A TR Al Ghubar M

Natih M

Saih E Saih Rawl

U R Nihayda

AR K

M -

Natih W.

K Makarem-1 U

Al-Bashair-1

Barik Basin O

Mabrouk R

Fahud

Fahud Salt B

A M

Fahud W. Fahud

Yibal

Maghoul S.

Fahud Salt Basin Fahud

200

sst

300m

Barik

Gamma Ray

Al-Bashair-1 (ABH-1)

West-Northwest

Figure 6: Ghudun correlation across north Oman indicating major pre-Safiq uplift and erosion over the Mabrouk-Makarem High related to rifting.

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54° 56° Hawasina Thrust Sheets 58°

UNITED ARAB EMIRATES

Fahud Salt Basin N

22° 22°

0 100 200 300 400 480

meters MARADI FAULT ZONE Huqf Salt Basins PDO concession

SAUDI ARABIA 20° 20°

not Ghaba Salt deposited Basin

h ig H h fa s OMAN a h K n u d observed on seismic datau h Onlap onto the Ghudun KhasfahG High not 18° deposited 18° Not deposited ? Ghudun below Al-Khlata or younger sediments)

not deposited ARABIAN SEA

0 100 Km (Approximate) 54° 56° 58°

Figure 7: Isopach map, Saih Nihayda Formation based on well data.

broad area. Overall tilting of the eastern flank of South Oman to the west-northwest continued during deposition of the Caradocian Hasirah Formation, onlapping onto the Ghudun Formation. In this area, the basal part of the Safiq Group (the Llanvirnian-Llandeilian Saih Nihayda Formation) was not deposited (Figure 7).

After uplift and erosion of the Ghudun Formation, a renewed period of extension took place in the Ghaba Salt Basin during Mid Ordovician times. Seismic evidence of rift-shoulder uplift and erosion over the Mabrouk-Makarem High is shown in Figure 8. This tectonic event is based on the observed truncation of the Ghudun Formation towards the high and it is followed by rapid Mid Ordovician subsidence (‘collapse’) as indicated by the drape of the anoxic Hanadir Shale over the continental Ghudun Formation. Clearly expressed on the seismic data is the downlap (‘outbuilding’) of the deltaic highstand deposits of the Saih Nihayda Formation onto the Hanadir flooding surface. The onlap of the Hanadir Shale onto the Ghudun Formation is suggested, but not expressed, on this line because of truncation by the Al Khlata Formation beyond about shot point 4,300.

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South Line A979002C, Mig North

1,075 1,250 1,500 1,750 2,000 2,250 2,500 2,750 3,000 3,250 3,500 3,750 4,000 4,250 4,500 4,750 5,000 5,250 5,500 5,750 6,000 6,250 1,432

1,500

1,600

1,700

1,800 Near Top Gharif

1,900 Near Top Al-Khlata

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2,100

Two-way Time (ms) Two-way Near Top Ghudun 2,200

2,300

2,400

2,500 0 2.5

2,600 Km 2,640

Flattened on Reflection ‘Near Top Gharif’ 1,075 1,250 1,500 1,750 2,000 2,250 2,500 2,750 3,000 3,250 3,500 3,750 4,000 4,250 4,500 4,750 5,000 5,250 5,500 5,750 6,000 6,250 1,432

1,500

1,600

1,700

1,800

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Near Top Gharif 2,000

Near Top Al-Khlata 2,100

Two-way Time (ms) Two-way Safiq Group 2,200 Hanadir Shale 2,300 Near Top Ghudun

2,400

2,500

0 2.5 2,600 2,640 Km

Figure 8: (Top) Seismic evidence for a tectonic event during deposition of the Ghudun and Saih Nihayda formations (late Early to early Middle Ordovician). (Bottom) Rift-shoulder uplift and erosion of the Ghudun Formation over the Makarem High is followed by renewed subsidence as indicated by the downlap of the Saih Nihayda Formation (Safiq Group) onto the Hanadir maximum flooding surface (tie to Musallim Deep-1, Figure 5). Seismic line: courtesy of P. Senycia.

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Southwest Northeast

Chronostratigraphy Palynological Zones Makarem-1 Hazar-2 Barik-6 Ghaba-1 Barakat-3 Andam-1 Najah-1 (Gradstein and Ogg, 1996) North South (Projected) (Projected) 424 Tectonic Events Wenlockian

Llandoverian 1003

Early Sahmah Decrease in

SILURIAN subsidence rate 443 Qusaiba Hot Shale MFS Ashgillian Glaciation

Caradocian Hasirah roup Late 1005 1012 1012 1005 Ra'an Shale Maximum Flooding

Surface (MFS) Safiq G 458 Llandeillian 1098 MFS Middle Llanvirnian Hanadir Hot Shale Saih Nihayda 470 MFS

ORDOVICIAN Rift-shoulder uplift and erosion Arenigian

Ghudun roup Barakat High

Early Member subsidence rate MFS 3 Mahatta Tremadocian 1108 Humaid G

Marine Claystone Shallow-marine/Deltaic Fluvial Sandstone Oil Discovery Mass Flow Sands Sands Lower shoreface High API Claystone Shallow-marine/Deltaic Sands Main observed Sequence Local down-cutting Rift-related erosion Upper shoreface/shoreline Boundaries Figure 9: Schematic chronostratigraphic cross-section, Upper Haima Supergroup in North Oman.

In addition, analysis of the seismic and well data indicates that the rate of halokinesis increased during this period as shown by an increased rate of downbuilding and ponding of deepwater mass flows against diapir flanks. These mass flows belong to the Saih Nihayda and Hasirah formations (Figure 9). Notwithstanding continued halokinesis in the Ghaba Salt Basin during Cambrian to Early Ordovician times (Boserio et al., 1995), in the following period salt grew diapirically over reactivated basement faults related to the fault pattern of the former Huqf extension cycles. Regional extension has been shown to trigger the rise of diapirs by faulting the brittle overburden (Vendeville and Jackson, 1992). The trap formation of the Saih Nihayda field is related to this upper Ghudun-lower Safiq period of extension and is indicated by en-echelon faulting of its crest (Figure 10). The tilted fault blocks are onlapped by transgressive basal sands and eventually sealed by shoreface mudstones of the Saih Nihayda Formation during a renewed pulse of extension following rift-shoulder uplift and erosion (Figures 9 and 11).

The Mid Ordovician increase in the rate of subsidence is probably the result of the pulse of extension that initiated a basin-wide change in depositional environment. During deposition of the Arenigian Ghudun Formation in the Ghaba Salt Basin, the environment varied only slightly from alluvial plain to deltaic, whereas during Mid Ordovician to Early Silurian times the environment changed significantly from alluvial plain in the south to marine (outer shelf) in the north (Figure 9). This rise in relative sea- level on the Arabian Peninsula occurred at the onset of Safiq deposition during a period of a significant global sea-level fall (Stump et al., 1995; Vail et al., 1977). This is further evidence for an extensional tectonic pulse under the assumption of rather constant sediment supply.

The Safiq Group has been subdivided into three major transgressive-regressive (TR) cycles (Droste, 1997; Partington et al., 1998). A major flooding occurred at the base of each TR-cycle, followed by progradation of braid-plain deltas flowing into deep water, resulting in mass flow deposits in the Saih Nihayda and Hasirah formations. The youngest cycle, the Early Silurian Sahmah Formation, escaped erosion only along the west flank of the Ghudun-Khasfah High and Anzauz-Rudhwan Ridge, and locally in paleo-lows within the Fahud and Ghaba Salt Basins formed by salt withdrawal.

During the Caradocian, at the end of the deposition of the Hasirah Formation, the impact of a short- lived, intense pulse of polar glaciation was felt. The advance of ice was at its maximum in the Ashgillian (Vaslet, 1990; Al-Husseini, 1990). This glaciation was responsible for a eustatic sea-level fall, resulting in significant erosion along the basin flanks and a temporary return to coarse clastic shallow-marine sedimentation as observed in outcrops of the Saih Hatat area (Le Métour et al., 1986). Erosion of the

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8,500 8,550 8,600 8,650 8,700 8,750

SN-35 Y

Location of cross- section, Figure 11

SN-3 RE

SN-33 SN-24

SN-15 Y'

est Transtensional Fault SN-36

SN-14

Saih NihaydaW

SN-34 N

Saih Nihayda EastTranstensional Fault

Figure 10: Saih Nihayda field, mapped at the maximum flooding level of the Saih Nihayda Formation (equivalent of the Hanadir Formation). Evidence for extension during Lower Safiq times based on an echelon faulting.

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Y Y' Saih Nihayda-3 RE North South

Khuff Upper Gharif

Late Reactivation MiddleM. Gharif Gharif

Lower L.Gharif Gharif Hasirah Basin-Floor Fans Al Khlata Al Khlata

Safiq Group Mudstones Saih Nihayda Deltaics

Hanadir Mudstone Equivalents Bas al Sandstone

Top Ghudun Unconformity

Ghudun Braid-Plain Sands

Mabrouk Mudstones Oil accumulations 0 500 Gas accumulations Meters Figure 11: Tilted fault blocks with the sandy Ghudun Formation as the prime reservoir were formed during a rift pulse and draped by lower shoreface mudstones of the Saih Nihayda Formation.

Hasirah Formation due to the Late Ordovician glaciation appears not to be significant in the Ghaba Salt Basin (Figure 9), although incised channels have been observed on seismic sections. In South Oman along the East Flank, evidence for the deposition of the Safiq Group and the impact of the Late Ordovician glaciation is destroyed by Mid to Late Carboniferous Hercynian uplift and erosion, followed by the onset of rifting. During the latest Carboniferous-Early Permian, the East Flank was part of a pronounced rift shoulder of the Batain Basin, a proto-Indian Ocean Basin (Belushi et al., 1996; Immenhauser et al., 1998). Rift-shoulder uplift was accompanied by Late Carboniferous to Early Permian highland glaciation, again removing major amounts of the Paleozoic.

In North Oman, the Ghudun depocentre coincides approximately with the depocentre of the Early Cambrian Ghaba Salt Basin. The northern extension of this Ghudun Basin can be traced into the southeastern Oman Mountains. Here, the Early to Late Ordovician Amdeh Formation (Figure 12) was deposited in a shallow-marine to an outer-shelf environment, unconformably overlying the Lower Huqf Hiyam Formation which is equivalent to the Khufai Formation (Lovelock et al., 1981; Le Métour, 1988). The basal, 80-240 m thick Lower Siltstone Member Am1 of the Amdeh Formation is not dated, but is thought to be late Early Ordovician (Arenigian) in age. This interpretation is based on a similar geologic setting compared to the Ghaba Basin. The conglomerates (Figure 12) in the top part of the Lower Siltstone Member Am1 (Villey et al., 1986) indicate a phase of rift-shoulder uplift following initial extension and resulting in the formation of tilted fault blocks. The conglomerates, including carbonates of the Late Precambrian Hiyam Formation, were derived predominantly from the west. Evidence of rift-shoulder activity is also based on the architecture of the unconformably overlying Lower Quartzite Member Am2, wedging out completely to the northwest (Villey et al., 1986). In Saih Hatat, this event is the signal for increased subsidence resulting in the deposition of about 3,000 m of Mid to Late Ordovician clastic sediments (Figure 12). The Lower Quartzite Member Am2 above the unconformity represents the coarse transgressive systems tract before the major transgression of the

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Correlation with CHRONO- approximately time-equivalent TECTONOSTRATIGRAPHY STRATIGRAPHY sections in the LITHOSTRATIGRAPHY Ghaba Salt Basin REMARKS

LATE Saiq Fm Major uplift and erosion PERMIAN 3,400 m u related to Neo-Tethys rifting. Massive to cross-bedded white or dark Marine, coarse-clastics T grey-purple conglomerates and coarse- grained sandstones; green and red shales related to glacially induced 3,200 Fe and shaly siltstones (shell beds, trilobites) sea-level fall. Conglomerates and angular shale clasts in silty matrix, dropstones? Mafic intrusion. Age? Upper 3,000 Maximum Flooding Surface Siltstone (3rd-order) Member White or pink quartzitic sandstones, often Amdeh Fm laminated (common load structures), Am5 bioturbation and dark green purple or grey

CARADOCIAN ? T poorly bedded (intense bioturbation)

LATE ORDOVICIAN LATE micaceous or shaly siltstones, shale horizons; shell beds, trilobite marker bed (T) HASIRAH FORMATION 458 Ma

2,400 White light-brown quartzitic sandstones, load structures RIFTING RELATED Alternating white quartzitic sandstones, EXTENSION and greenish-grey micaceous or shaly 2,000 siltstones, common bioturbation Some 3 km Mid- to Late Ordo- vician shallow-marine foreshore to distal shoreface deposits. Tuff: not (yet) observed

Upper 1,800 White and light grey ridge forming Quartzite quartzitic sandstones, common load Member structures, channels Amdeh Fm Am4

Thin-bedded shaly or micaceous sandstones, common bioturbation (Skolithos, Daedalus)

1,200 Massive or laminated quartzitic SAIH NIHAYDA FORMATION SAIH NIHAYDA sandstones, and well-bedded MIDDLE ORDOVICIAN 1,000 sandstones and shaly siltstones

Brown sandstones often ferruginous T (Cruziana). Green shaly siltstones and shales (shell beds, trilobites, lamelli-

L L A N V I R N I A N - L L A N D E I L I A N A I N D E I L A L N - L A N V I R A L L Middle Shale branchs and brachiopods) Member Amdeh Fm Grey and green shales, shaly siltstones and sandstones, intense bioturbation Maximum Flooding Surface Am3 600 (Skolithos, Daedalus) (3rd-order)

480 Mafic intrusion. Age? Lower Quartz Member 360 White to pale brown well bedded Amdeh Fm ? quartzitic sandstone (wave ripples) Sharp boundary. Unconformity 470 Ma Am2 related to rift-shoulder uplift 240 u Conglomerate horizons and lenses and erosion. Lower Siltstone below Am2 Greenish-grey pebbly Member Onset of transtension, faulting ? ARENIG. 120 siltstones and sandstones

Fm. Amdeh Fm observed. Purple-grey siltstone, sandstone Ghudun ORDO. EARLY Am1 conglomerate lenses LATE 0 m u PRECAMB. VENDIAN Hiyam Fm u = unconformity T = Trilobite Fe = Ironstone Figure 12: Tectonostratigraphy of the Amdeh Formation, Saih Hatat area. Lithostratigraphy from Le Métour et al. (1986) with data from Lovelock et al. (1981). Time scale from Gradstein and Ogg (1996).

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Llanvirnian Middle Shale Member Am3. A second major marine transgression took place in the Upper Siltstone Member Am5 during Early Caradocian times. Its timing is based on the presence of possible Caradocian trilobites (Le Métour, 1988) and a regional correlation with the Hasirah Formation in the Ghaba Salt Basin. The coarse conglomerates within the marine siltstones at the top of the Upper Siltstone Member Am5 could represent dropstones related to the advancing ice sheets and the accompanying eustatic sea-level fall.

In contrast to the Ghaba Basin where extension is primarily an Arenigian phenomenon, major extension in Saih Hatat took place predominantly in Mid Ordovician times, 10 to 15 million years (m.y.) later. Diachronous development of individual sub-basins belonging to one major extensional system is well known; for example, the diachronous Late Jurassic basin formation in the North Sea. Often, the boundaries between the (sub) basins are enigmatic structural zones consisting of transfer faults and accommodation zones (Chapin and Cather, 1994). The boundary between the Ghaba and Saih Hatat basins is located below the Hawasina-Semail Nappes without reliable seismic coverage and has thus not been studied.

Evidence for a Mid Ordovician extensional pulse was discussed by Loosveld et al. (1996). These authors noted that although the end of the sag phase should have been reached at the latest some 50 m.y. after Huqf rifting, extension resumed again with the deposition of shallow- to deep-marine sediments of the Safiq Group. This phase of extension was poorly understood due to lack of well data and poor seismic imaging. The recent drilling campaign for Haima gas yielded new well data, and seismic imaging improved considerably. This phase of late Early to Mid Ordovician extension, following a period of uplift and erosion, was also observed in other parts of the Arabian Peninsula (Stöcklin, 1968; Connally and Wiltse, 1996; Jones et al., 1996; Jones and Stump, 1999), and was hitherto poorly understood. The observation of potassium-rich mafic dikes, related to this renewed pulse of extension, enables a more complete picture of the geology to be obtained.

Petrology and Setting of the Mafic Rocks

Several outcrops of mafic intrusives occur in the Huqf area (Figure 13) (Dubreuilh et al., 1992). These rocks were described as alkali-olivine basalts or trachytes and in some cases were erroneously correlated with Tertiary basalts from Al-Ashkarah further north (Figure 1), which were intruded at the onset of the Gulf of Aden rifting some 38 to 40 Ma (Bott et al., 1992; Worthing unpublished 39Ar/40Ar data). During the course of fieldwork for this paper, three of the Huqf occurrences were sampled: Qalilat Suwaidi, Qarn Aswad, and a small body west of Sirab (Figure 13). The Sirab rocks are clinopyroxene and olivine-phyric basanites that petrographically and geochemically closely resemble the Tertiary basalts of Al-Ashkarah and are thus correlated with them. They will not be considered further here. Rocks from the other two outcrops are petrographically and geochemically distinct and are discussed below.

Qalilat Suwaidi (Little Black Hill) The intrusive rocks cropout on three contiguous knoll-shaped hills about 400 m south of the Duqm to Sinaw road. They were mapped as basalt dikes by Dubreuilh et al. (1992) at locality 20°27.5'N, 57°52.0'E (Figure 13). The highest knoll rises about 40 m above the surrounding plain. The sub-circular shape of the knolls suggests a plug or plugs. However, outcrops on the northerly knoll suggest that the mafic rocks are underlain by purple shales of the late Precambrian Shuram Formation. Contact dips appear to be shallow and radial from the center of the outcrop and thus it is possible that the body was intruded as either a sill, dike or laccolith. The fine-grained, dense, black mafic rock is strongly fractured into sub-horizontal joints on a three to five centimeter scale. The joints dip outwards parallel to the contour of the slope on one knoll and are interpreted as cooling fractures. The shallow intrusions contain xenoliths of crustal material such as quartzite, red granite, and stromatolitic dolomite and, more rarely, euhedral crystals of a pink K-feldspar up to 2 centimeters long. No ultramafic xenoliths were seen. In coarser facies, phlogopite phenocrysts (phlogopite: the magnesium-rich variety of biotite) are visible to the naked eye, though generally the rock is aphanitic in hand specimen.

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In thin section, the rocks contain abundant microphenocrysts of lath-like, red-brown sieved and castellated phlogopite, euhedral laths of colorless clinopyroxene, and sparse olivine microphenocrysts partly or completely altered to a pale-colored serpentine. The groundmass contains lath-like K-feldspar, abundant granular ilmenite and colorless glass. The rock also contains euhedral to rounded pseudomorphed phenocrysts of brown amphibole locally in reaction relationship with clinopyroxene. A more comprehensive petrographic description is given in the Appendix.

Qarn Aswad (Black Horn) Here dense black mafic rocks form a roughly elliptical ridge some 300 m by 150 m (20°32.1'N., 57°53.8' E., Figure 13). The core of the Qarn Aswad structure is occupied by low ground in which there is no exposure. In the inner walls of the ridge, at the exposed lower contact, it is clear that the mafic intrusion is discordant to the bedding in the enclosing purple shales of the Shuram Formation. There is also a narrow zone of contact metamorphism. Where exposed, dips of the contact are shallow to moderate

Gulf of Oman

Muscat N Fahud 020 Area Enlarged Km OMAN Huqf Qarn Zaza 0 150 Maradi-Nafun Km Fault

Qarn Aswad

Qalilat Suwaidi

Ras Bintot Jabel Qariha

Khufai Anticline "Masirah Trough" Gulf Basement Fault of

Masirah Masirah Fault Zone

Cenozoic sediments Mesozoic sediments As Sirab Carboniferous-Triassic sediments

Early Cambrian-Early Ordovician (= Nimr Group + Haima Super Group)

Buah Fm Huqf Shuram Fm Supergroup

Khufai Fm Infra-Cambrian/ Abu Mahara Fm Cambrian

Tertiary Basalt Intrusions Middle to Late Ordovician Nafun Basalt Dikes

Anticlinal axis Fault

Unconformity Duqm Synclinal axis line

Figure 13: Simplified geological map of the Huqf area (modified from Gorin et al., 1982).

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and appear to be radial to the core of the structure. The intrusion thus has an umbrella-shaped form similar to that seen at Qalilat Suwaidi. This distinctive form may have been caused by local updoming of competent horizons within the Shuram Formation by a rising plug of mafic magma accompanied by lateral intrusion into the updomed sediments. Petrographically the rocks are similar to those from Qalilat Suwaidi except that pseudomorphed olivine is more abundant, though still comprising only about 10% by volume of the rock and phlogopite is present in only minor amounts.

The long axis of the elliptically shaped Qarn Aswad outcrop is aligned northwest to southeast indicating the local principal horizontal stress direction during intrusion. Based on Landsat satellite maps, the Qarn Zaza-Qarn Aswad-Qalilat Suwaidi intrusions are aligned in a zone trending north-northeast to south-southwest (Figure 13). This direction is considered to represent the regional principal horizontal stress direction during the Mid to Late Ordovician, parallel to the Ghaba Basin axis (Figure 4).

Geochemical Analyses and Mineralogical Classification of the Mafic Rocks

Whole rock, trace element and rare earth element (REE) analyses were determined at the University of Copenhagen, Denmark, and the University of Tasmania, Australia. One sample (HG 10) was run for whole rock and trace elements at both laboratories with excellent agreement. Analytical methods are given in the Appendix and representative analyses are presented in Table 1. The mafic rocks are

undersaturated (SiO2: 44.42 to 46.67 weight percentage [wt%]) and alkaline, with total alkalis ranging

from 6.65 to 7.60 wt%. K2O values range from 3.63 to 5.02 wt% and Na2O from 2.51 to 3.45 wt%. K2O/

Na2O ratios range between 1.31 and 1.94 (mean 1.51). They are nepheline normative (range 4.51 to 9.10%) and strongly enriched in large ion lithophile elements (LIL) such as Sr, K and Rb. MgO ranges from 5.12 to 6.31 wt% and the Mg number (100 (Mg/Mg + Fe)), from 48.73 to 50.27. Cr and Ni range from 55 to 103 parts per million (ppm) and 68 to 92 ppm, respectively. The high total alkalis and high

K2O/Na2O ratios indicate that the rocks are potassic (e.g. Peccerillo, 1992).

Geochemically, the rocks from Qalilat Suwaidi and Qarn Aswad are almost identical, however the mineralogical differences require some comment. The presence of abundant phlogopite microphenocrysts at Qalilat Suwaidi suggests that the rocks may be lamprophyres (Carmichael et al., 1974). Streckeisen (1979) and Rock (1987) defined lamprophyres as hypabyssal, melanocratic igneous rocks with porphyritic textures carrying only mafic phenocrysts, essentially phlogopite and/or amphibole with minor olivine. According to the classifications of these two authors and Muller and Groves (1995), the Qalilat Suwaidi mafic rocks would be classified as minettes. It is interesting to note that Platel et al. (1992, 1993) have described as “semi-lamprophyric” a suite of northwest to southeast trending dikes intruded into the basement metamorphics in the Marbat area east of Salalah (Figure 1). These rocks contain phlogopite and/or hornblende as the main mafic minerals. They are also potassium-

rich with a K2O/Na2O ratio of 1.71. The Qarn Aswad rocks plot in the basanite-tephrite field of a TAS (Total Alkalis Silica) diagram. The rarity of phlogopite and the presence of normative olivine at <10% indicates that they should be classified as potassic tephrites.

Geochronology

Potassium-Argon (K/Ar) and Argon-Argon (Ar/Ar) age determinations on the Huqf intrusive rocks are summarised in Table 2. Although the accuracy of K/Ar dates is generally regarded as suspect, the data set as whole points to a tectonomagmatic event in the Middle to Late Ordovician. The northwest to southeast trending “semi-lamprophyric” dikes (Platel et al., 1993) in the Marbat area (438 ± 22 Ma) are also included in Table 2 although sericitization as well as resetting of unstable minerals was noted. Based on overall setting, they are considered not to belong to the same Mid to Late Ordovician tectonomagmatic event as the Huqf suite. The Marbat mafic dikes were clearly eroded before the deposition of the Marbat Sandstone Formation which is regionally correlated with the Late Precambrian Abu Mahara Formation (personal communication of one of the reviewers). The Ordovician age may have been imposed on the Marbat dikes by resetting during the heat pulse associated with the Mid Ordovician extensional event described in this paper.

39 40 In the present study, one sample (HG 10, wt% K2O = 3.93) was selected for Ar/ Ar dating at Oregon State University(OSU), Corvallis, U.S.A. The sample was selected because of its freshness and relative

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Table 1 Selected analyses: mafic dike samples from Qalilat Suwaidi and Qarn Aswad, Huqf area.

Major Elements Qalilat Suwaidi Qarn Aswad in Oxide wt% HG10 HG23 HG18 HG17

SiO2 44.42 44.81 45.45 43.92 TiO2 2.80 2.63 3.05 3.25 Al2O3 14.26 14.42 14.20 13.89 FeOT 10.73 10.76 11.92 12.77 MnO 0.18 0.18 0.19 0.20 MgO 6.19 5.12 5.83 6.31 CaO 8.11 7.22 7.57 8.49

Na2O 3.09 3.45 3.34 2.75 K2O 4.11 4.34 3.56 3.63 P2O5 0.93 0.88 0.97 1.08 Volat 3.46 4.46 2.27 2.22 Total 99.71 99.47 99.68 99.93

Trace and Rare Qalilat Suwaidi Qarn Aswad Earth elements in ppm HG10 HG23 HG18 HG17 V 134 120 158 154 Cr 57 57 87 73 Ni 69 62 75 74 Rb 81 105 47 70 Sr 1,799 1,266 1,334 1,490 Y 30 41 39 39 Zr 599 747 583 574 Nb 167 162 130 129 Ba 1,036 981 1,050 1,128 La 138 150 120 not determined Ce 249 273 216 not determined Pr 27.9 30.0 25.0 not determined Nd 96.5 101 89.6 not determined Sm 17 17.3 16.5 not determined Eu 5.14 5.12 4.79 not determined Gd 13.1 13.5 12.9 not determined Tb 1.84 1.95 1.88 not determined Dy 8.90 9.43 9.26 not determined Ho 1.57 1.68 1.65 not determined Er 3.82 4.08 4.03 not determined Tm 0.48 0.51 0.52 not determined Yb 2.77 2.95 3.00 not determined Lu 0.37 0.41 0.42 not determined

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500.0 Qalilat Suwaidi

460.0 461.2 ± 2.4 Ma

420.0 Age (Million years) 380.0

340.0 10 20 30 40 50 60 70 80 90 100 % Ar39 Cumulative Released

Figure 14: 40Ar/39Ar age spectrum, mafic dike Qalilat Suwaidi (sample HG 10), Huqf area. Most of the different age components represent volcanic activity during Mid Ordovician times.

absence of xenoliths. Rock chips 0.5-1 millimeter in size were irradiated for 6 to 8 hours in the core of the OSU TRIGA reactor and conversion from 39K to 39Ar by neutron capture was monitored with hornblende standard Mmhb-1 (Samson and Alexander, 1987). Six incremental heating steps were conducted. The isotopic composition of Ar released at each step was measured using an AEI-MSIOS mass spectrometer at OSU. Plateau age estimates are based on consecutive incremental heating steps that overlap within analytical uncertainties and cumulatively include at least 50% of the 39Ar released from the sample.

The whole-rock spectrum is presented in Figure 14 and indicates an age of 461 ± 2.4 Ma. According to the time-scale of Gradstein and Ogg (1996), the event took place during the Mid Ordovician (Llandeilian) (Figure 14). The excellent plateau suggests that the age is reliable and there is no clear evidence of a younger thermal event. The date is thus consistent with the tectonomagmatic event suggested by the data in Table 2. The low temperature step possibly represents degassing of the K-feldspar which is susceptible to Ar loss, particularly from crystal rims (closure temperature Tc = 125° to 350°C). The plateau age probably represents degassing of the biotite (Tc = 260° to 350°C) and thus indicates the point at which the rock cooled below Tc for biotite. The high temperature step possibly represents degassing of the amphibole (Tc = 450° to 525°C), (Spear 1993).

DISCUSSION The Origin of Potassic Magmas in the Rio Grande Rift A worldwide survey by Thompson et al. (1989) suggested that minettes are closely associated in space and time with heating and or thinning of the sub-continental mantle. This conclusion is partly based on their study of minettes occurring within a zone of intracontinental extension in Northern Colorado, U.S.A. This area is part of the northerly extension of the Oligocene-Miocene Rio Grande Rift and studies of the same area by Alibert et al. (1986) and Gibson et al. (1993) also record potassic rocks including minettes. The evolution of the Rio Grande Rift took place in two distinct phases; an early phase (32 to 27 Ma) characterised by the formation of broad, shallow basins and low-angle faulting. During this phase, crustal extension took place in a broad zone up to 170 km across. In the second phase, which took place in the Miocene (16 to 10 Ma), the zone of extension narrowed to about 50 km and faulting was high-angle resulting in the formation of narrow grabens that define the present morphology of the rift zone (Chapin and Cather, 1994). Gravity and teleseismic studies have shown that the width of thinned lithospheric mantle underlying the Rift is about 750 km and this distance corresponds to the surface expression of magmatism.

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preferred

eld by

FM- Consultants, 1983 Calvez, BRGM Calvez, report 22/11/85 JICA, 1981

alteration suggests alteration

1983 1983 1983 1985 1985 1980

FM FM FM BRGM BRGM MMR

Lab

3 3

12 14 22

± ±

± ± ±

470 Age Year Reference 440 449

378 433 438

K/Ar Ar/Ar Ar/Ar K/Ar K/Ar K/Ar

Method

(whole rock)

Table 2 Table

#1, mafic dyke #2, mafic dyke #3, mafic dyke #4, mafic dyke #5, mafic dyke #6, dolerite dyke

Age Data of Mafic Rocks, Huqf Area and Marbat (South Oman)

Huqf Group, Shuram Huqf Group, Huqf Group, Shuram Huqf Group, Huqf Group, Shuram Huqf Group, Huqf Group, Shuram Huqf Group, Basement

Huqf Group, Shuram Huqf Group,

Host Formation

possible argon loss discrepancy and presence of xenoliths indicates the possibility of excess argon discrepancy. indicates the possibility of excess argon loss discrepancy and presence of xenoliths possible

feldspathoids. Effect: conventional ages too high. conventional Effect: feldspathoids.

by FM Consultants. by

Location Sample Lithology

Qarn Aswad Qarn Aswad Qarn Aswad Qarn Aswad Qalilat Suwaidi

Marbat Area Original sample description and comment: Sample 1: phonolitic basanite to basanitic foidite; and xenocryst-rich, fine-grained, altered, xenolith- Sample JH 675. Patchy,

Sample 2: h to some extent possibly argon from older xenoliths, initial radiogenic extraneous Same sample JH 675, xenolith-rich;

Sample 3:This age estimate argon. initial radiogenic of the presence of extraneous evidence Same sample JH 675, xenolith-poor;

Sample 4: Slightly altered microlithic basalt. Sample 5: Slightly porphyritic, microlithic with augititic groundmass. Sample 6: Age considered to represent a reset age (see text). heating. by minerals Sericitization, resetting of unstable

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Gibson et al. (1993) undertook a 670 km geochemical traverse across the axis of the Rio Grande Rift, sampling volcanics from the flanks, shoulders and rift trough. Their work which includes K/Ar dates, showed that the earlier magmatic activity on the rift flanks and shoulders is dominated by potassic mafic rocks including minettes. In the trough on the other hand, the volcanics are tholeiitic and alkaline in character with OIB-like affinities (OIB = oceanic-island basalt) in both phases of activity. Potassic magmas are absent from the trough. They concluded that the potassic magmas were derived from a metasomatised layer in the lithospheric mantle that melted by decompression during the early stages of continental extension (McKenzie, 1989).

Comparison between Interior Oman and the Rio Grande Rift

The intracontinental Rio Grande Rift is of similar size to the rifted basin described in this paper and its geophysical setting, tectonomagmatic evolution, and igneous geochemistry are well-documented (e.g. Keller and Cather, 1994; Rogers et al., 1982; Alibert et al., 1986; Thompson et al., 1989; Gibson et al., 1993). Thus, in the following section the geochemical characteristics of potassic rocks from both areas will be compared with the aim of drawing possible conclusions about the tectonomagmatic setting of the Huqf area during the Middle Ordovician.

In order to compare the potassic rocks from both areas, analyses were abstracted from Rogers et al. (1982), Alibert et al. (1986), Thompson et al. (1989), and Gibson et al. (1993) and plotted on major element and chondrite normalized trace element and REE variation diagrams. As Gibson et al. (1993)

do not clearly identify the rock types of all their samples, all analyses with K2O/Na2O>1 were plotted including two rocks specifically identified as minettes. Analyses from the other papers are identified by the authors as minettes. Selected examples of these diagrams are shown in Figures 15a to 15f and 16a and 16b. The diagrams show that the geochemical signatures of the Huqf intrusions are broadly similar to the minettes and other potassic rocks from the Rio Grande Rift, particularly the specimens from Gibson et al. (1993). However, there are some differences, particularly in the major element plots (Figures 15a to 15f). For example, compared with the Rio Grande suite, the Huqf mafic rocks are very depleted in MgO and slightly depleted in CaO (Figures 15e and 15f). Although many of the analyses lie close to the Rio Grande Trend, they plot at the low silica end of the diagrams suggesting that they are primitive. However the low MgO (Figure 15e) which is particularly anomalous does not support this. This will be discussed below. On the other hand, chondrite-normalised trace element and REE signatures (Figures 16a and 16b) are very similar to those from the Rio Grande Rift, the latter showing strong, light REE enrichment. The mild Nb enrichment shown in Figure 16b may be explained by the high modal content of ilmenite as the Nb is probably partitioned in this mineral.

The origin of lamprophyres and other potassic rocks is controversial. In igneous provinces where they occur in abundance, their composition may vary greatly. This compositional variability is explained as being the result of a variety of processes, such as melting of different mantle sources, magma mixing, assimilation and fractional crystallisation (e.g. Rock, 1987). Without further isotopic studies it is impossible to draw firm conclusions about the origin of the Huqf rocks. However, their broad geochemical similarity to those from the Rio Grande Rift suggests that they have a comparable origin. It is possible that some of the differences outlined above may be explained by fractionation. For example, the large phenocrysts of amphibole and clinopyroxene in a magmatic reaction relationship may represent a residual CaMg-rich phase that has largely fractionated out of the magma, thereby depleting these elements. Similarly, the low MgO may also be due to olivine fractionation. Thus, in summary, the Huqf rocks have a similar geochemical signature to the potassic rocks from the Rio Grande Rift and differences may be explained in terms of simple fractionation models, but the operation of other petrogenetic processes cannot be discounted.

Potassic igneous rocks occur in a wide variety of geological settings including continental arcs, post- collisional arcs, initial and late oceanic arcs and within-plate settings (Muller and Groves, 1995). Since the standard basalt tectonic discriminant diagrams are misleading for this suite, Muller and Groves (1995) have proposed a set of diagrams that discriminate between rocks from these different tectonic

settings. Using their Zr/Al2O3 against TiO2/Al2O3, TiO2 against Al2O3 and Y against Zr diagrams, the

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4 20 a b

3 3 2 O

2 2 10 TiO Al

0 0 40 50 60 70 4050 60 70 SiO2 SiO2

20 0.3 c d

0.2

10 MhO FeOT

0.1

0 0.0 4050 60 70 40 50 60 70 SiO2 SiO2

20 20 e f

10 10 CaO MgO

0 0 4050 60 70 4050 60 70 SiO2 SiO2

Qalilat Suwaidi, Huqf North Colorado, rift shoulder (Thompson et al., 1989) Qarn Aswad, Huqf Rio Grande, rift shoulder and flanks (Gibson et al., 1993) Navajo rift shoulder (Rogers et al., 1982) Average minette composition (140 analyses) (Herriot, 1989) Navajo rift shoulder (Alibert et al., 1986) Figure 15 (a-f): Selected whole-rock major element variation diagrams for the Huqf mafic rocks and potassic rocks from the Rio Grande Rift. The data suggest broad geochemical similarity between potassic rocks from Huqf and shoulders and flanks of the Rio Grande Rift.

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1,000 a

100

10 Sample/C1 Chondrite

Light REE Heavy REE 1 La CePr Nd Sm EuGd Tb DyHo Er Tm Yb Lu

10,000 b

1,000

100 Sample/C1 Chondrite

1 Ba RbKCeZrY Nb La Sr P Ti

Qalilat Suwaidi, Huqf North Colorado, rift shoulder (Thompson et al., 1989) Qarn Aswad, Huqf Rio Grande, rift shoulder and flanks (Gibson et al., 1993) Denotes overlapping Rio Grande Rift symbols

Figure 16: Variation diagrams of potassic Rio Grande rift-shoulder volcanics and Huqf mafic rocks: (a) rare earth elements (REE); (b) chondrite-normalized trace element. The data show that the rocks are geochemically very similar suggesting a comparable origin.

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100 a Huqf Mafic Rocks

Within-Plate, Intracontinental Setting 3 O 2 10 Continental Arc Qalilat Suwaidi, Huqf

Zr/Al Post Collisional Arc Qarn Aswad, Huqf Navajo rift shoulder (Rogers et al., 1982) Navajo rift shoulder (Alibert et al., 1986) Initial Oceanic Arc North Colorado, rift shoulder (Thompson et al., 1989) Late Oceanic Arc Rio Grande, rift shoulder and flanks (Gibson et al., 1993)

1 Average minette composition (140 analyses) (Herriot, 1989) 0.02 0.1 0.8

TiO2/Al2O3

6 100 b c

2 3 Y 10 TiO

Within-Plate, Intracontinental Setting 2 Within-Plate, Intracontinental Setting

1 1 6132010 100 1,000 Al2O3 Zr Figure 17 (a-c): Tectonic discriminant diagrams for potassic rocks after Muller and Groves (1995). The Huqf mafic samples clearly plot in the Within-Plate field, suggesting intrusion during extension in an intra-continental setting.

Huqf mafic intrusives clearly plot in the Within-Plate fields, confirming intrusion within an intracontinental setting (Figures 17a, b, c). Potassic rocks including minettes from the Rio Grande Rift are also plotted on this diagram.

The magmatic activity recorded in the Huqf appears to be very minor when compared with the Rio Grande Rift. However, more volcanics may still be buried or have been stripped off during subsequent erosion that may have taken place during Mid to Late Carboniferous times resulting in the removal of up to 2 km of sediments in the Huqf (Visser, 1991). It is probable that lithospheric stretching in Oman did not progress beyond the initial stages represented in the Rio Grande Rift by the early phase of magmatic activity that resulted in the intrusion of the potassic rocks on the rift flanks and shoulders (Gibson et al., 1993). This is also consistent with the style of Mid Ordovician rifting seen in central Oman that appears to have involved formation of a broad, shallow basin not a narrow, steep graben.

Thus in summary, minettes and other potassic rocks from the Rio Grande Rift were intruded into the flanks and shoulders of an intracontinental rift during an early phase of broad extension involving the

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crust and lithospheric mantle. Potassic rocks from the Huqf are geochemically similar and their geographical location suggests that they were intruded into the eastern rift shoulder of the intracontinental Ghaba Salt Basin during a pulse of extension in the Middle Ordovician. These observations are consistent with the known continental to deep-marine environment of the Mid to Late Ordovician sediments in Oman that lack any plate margin or island arc affinity. This is the first direct evidence based on shallow intrusions, that active rift extension took place during deposition of the Upper Ghudun Formation and the Safiq Group. A transtensional tectonic setting explains the faulting of the Ghudun Formation across the basin and the onlap of the unconformably overlying Safiq Group as observed on seismic sections. Tilted Ghudun fault blocks associated with the extensional event provide first-class reservoirs and are efficiently sealed by draping lower shoreface shales of the Saih Nihayda Formation.

Early Paleozoic Rifting on the Arabian Peninsula

Following the amalgamation of Gondwana in the protracted Pan-African Orogeny, crustal separation within the Arabian Peninsula had already started during the waning stages of the continent-to-continent collision. Rifting in the Late Precambrian-Early Cambrian can be considered as the first break-up attempt of the combined rift basins on the Arabian Peninsula, including Iran, and Greater India which have been defined as Proto-Tethys (Talbot and Alavi, 1996). The next phase of Gondwana break-up started during the Mid to Late Cambrian and led to the development of the Rheic Ocean, separating Western Gondwana (North Africa) from Baltica (Paris and Robardet, 1990). The Rheic Ocean was the continuation of Paleo-Tethys to the west. During Cambrian-Ordovician times, Northern Gondwana terrain slivers separated from Gondwana (e.g. Ziegler, 1988) and terrain slivers drifted away from Iran towards the north during the Ordovician (Stampfli et al., 1991). Such a terrain sliver is exposed in the Lesser Caucasus (Sengör, 1990). Rifting and drifting was accompanied by igneous activity. In northern Iran, the earliest phase of mafic volcanism started in the Ghelli region, some 350 km north of Tabas, during the Early Ordovician. Volcanic activity continued in the Talesh Mountains, southwest of the Caspian Sea, during the Mid Ordovician. It was followed by major basalt outpouring during the Late Ordovician to Early Silurian, especially in the northeastern Alborz Range and on the Lut Block (Berberian and King, 1981) (Figure 18).

Recent deep exploration drilling in Central Saudi Arabia also shows a Mid to Late Ordovician phase of extension (e.g. Jones et al., 1996). Following a phase of compression sub-parallel to the Huqf axis in the Early Ordovician, as expressed by uplift of the Central Arabian and Qatar Arches, subsequent extension was characterized by broad grabens filled with marine Mid to Late Ordovician siliciclastics belonging to the Qasim Group (Connally and Wiltse, 1995, 1996). So far, Ordovician volcanism has not been reported in Saudi Arabia. In this area, Mid Ordovician to Early Silurian sedimentation appears to have taken place in a rim basin related to rifting to the north in Iran and to the east in Oman (for the concept of rim basin or intracratonic sag, see Stampfli et al., 1991). The eastern flank of this rim basin is formed by the Ghudun-Khasfah and Mabrouk-Makarem Highs. The latter is the western rift shoulder of the Ghaba Salt Basin. This basin is considered to be an aulacogen, one arm of a triple-rift junction that failed to drift to an ocean. The other two rift arms successfully developed as part of the northwest to southeast trending Paleo-Tethys and were located to the north of the Alborz Range (Figure 18). Extensive mafic volcanism (Berberian and King, 1981) is observed in the area of the triple-rift junction (Figure 18).

Although the concept of a failed rift is conjectural, the northern extension of the Ghaba Salt Basin has been shown to be located in Saih Hatat, within the southeastern Oman Mountains (Le Métour, 1988), (Figure 1). The extension of the Ordovician basin to the north of Oman is not fully constrained and reflects the unresolved paleo-plate reconstruction of platelets such as the Central Iranian and Helmund Blocks (e.g. Scotese and McKerrow, 1990). The Central Iranian Block (CIB) consists of the Tabas Graben flanked to the east by the Lut Block and to the west by the Yazd Block or Nain Horst (CIB as defined by Sengör, 1990). Based on the presence of Ordovician carbonates on the Central Iranian Block (Stöcklin, 1968; Berberian and King, 1981), Le Métour (1988) suggested that this platelet must have been located further north, closer to the equator, when compared to non-carbonate conditions in Oman further south. This proposed location of the CIB is consistent with the extreme thickening of the Ordovician-

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Regional High Continental and PALEO-TETHYS Marine clastics Talesh Continental Mountains N V Volcanism Marine clastics T Tuff V Triple Deep marine Ghelli A V Junction clastics/oceanic Oceanic Ridge L Region B Carbonates - mixed O V Regional Arch R V siliciclastics (thick) Z V R V V Carbonates - mixed NO A Basin outline RTH - W N G siliciclastics (thin) (certain - uncertain) E Teheran ZAGROS MARGINALEST IRAN RIDGE

TABUK WIDYAN BASIN BLOC V BASIN K Hail NAINI HORST LUT BLOCK

TABAS GRABEN RIM OMAN HIGH BASINS T

Jabal Qamar, 80m Mid Ord. Riyadh Sandstone- Shale Central Muscat Arabian SAIH HATAT Arch Fahud Basin Arabian Mabrouk- Shield Makarem High GHABA SALT BASIN X RUB ’AL KHALI BASIN WITH HORST - GRABEN V X’ Red EXTENSION Sea V HUQF HELMUND AL WAJID (KABUL) Ghudun- South Oman Khasfah Salt Basin BLOCK High Marbat

0 500

Aden SOMALIA Km

X ARABIAN RUB ’AL KHALI MAKAREM GHABA HUQF X’ SHIELD AL WAJID BASIN HIGH BASIN V

Broad, shallow grabens

Schematic Cross-section

Figure 18: Inferred paleogeographic setting based on Le Métour (1988) with additional data from Stöcklin (1968); Berberian and King (1981); Soffel and Förster (1984); Al Laboun (1986); Al-Husseini (1990); Sengör (1990); Connally and Wiltse (1996); and Alsharhan and Nairn (1997).

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Late Ordovician-Early Silurian (Ashgillian-Llandoverian)

Regional Onlap Al Halaniyat Islands

Arabian Rub 'Al-Khali Fahud-Ghaba Huqf Present Shield Basin Basins Area Coastline

DEPOSITION OF

Top Hasirah and

SAG-PHASE Sahmah formations Moho in Oman Zarqa-Sarah and Qalibah formations Cooling in Saudi Arabia Asthenosphere

0Approx. 500

Mid to early Late Ordovician (Llanvirnian-Caradocian) Extension Restricted to Local Grabens INITIAL BREAK-AWAY strong westwind Plateaus basalts

DEPOSITION OF

Lower crust Lower Plate Upper Plate Saih Nihayda and Moho Hasirah formations in Oman Lithosphere Hot Asthenosphere Qasim Group in Saudi Arabia Mafic dikes and sills

RIFT PHASE Late Early Ordovician (Arenigian)

Regional Transtension North-Northeast

WIDESPREAD DEPOSITION OF

Ghudun Formation in Oman

Sajir Member (Upper Saq Fm) in Saudi Arabia RUB 'Al-KHALI GHABA HUQF Time Figure 19: Geo-dynamic evolution of the Rub 'Al-Khali and Ghaba basins during late Early Ordovician to Early Silurian times. Profile location (X-X') on Figure 18 (adapted from Stampfli et al., 1991).

Silurian sediments in the Tabas Graben (Figure 18) which was probably the extension of Oman’s aulacogen to the north. In Iran, the thickness of the Infracambrian to Middle Triassic is, with the above exception, remarkably constant (Stöcklin, 1968). The present paleogeography of the CIB agrees well with the locations proposed by Soffel and Förster (1984) and Le Métour (1988). Following Ordovician times, left-lateral strike-slip faulting during the Late Triassic-Early Jurassic Cimmerian event resulted in the displacement and counter-clockwise rotation of the Central Iranian Block with respect to Oman (Sengör, 1990).

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The contrast in amount of volcanics on opposite sides of the Tabas Graben in Iran and the Ghaba Salt Basin (Figure 18) can be explained by the asymmetric rift configuration of a simple shear model (e.g. Stampfli et al., 1991; Talbot and Alavi, 1996). The upper plate above the asthenospheric upwelling is suggested to have been located in the Huqf and Lut areas, where significant amounts of volcanics are still present notwithstanding erosion (Figure 19). The paucity of tuffs both over Central Iran and within the Ghaba Basin may be partly due to westerly winds and partly due to subsequent erosion over Central Iran and Huqf as well as difficulties in detecting tuff mixed with other fine-grained lithologies at great depth in the Ghaba Salt Basin. In contrast, the lower plate, characterised by a lack of volcanics, contains the Ghaba Rift Basin and its western flank. The morphology of the western and eastern rift shoulders is consistent with the above model. The eastern flank, formed by the Huqf High, is morphologically expressed by a flexure as observed on seismic data, in contrast to the western flank where the Ghudun-Khasfah and Mabrouk-Makarem Highs are developed as a sharp rift shoulder representing the initial break-away (Figure 19).

The late Early to early Late Ordovician rift basins have similar locations and trends compared to those developed in the Late Permian-Early Triassic, when Neo-Tethys was taking shape to the northwest of Oman (Stampfli et al., 1991) and the Batain Basin, a Proto-Indian basin, to the east of Oman. It was not until the Late Jurassic that rifting in the southern rift arm progressed into drifting with the formation of oceanic crust (Immenhauser et al., 1998).

CONCLUSIONS

Rifting of the Arabian Peninsula during the late Early to early Late Ordovician has been suspected for some time based on an increase in rate of deposition in Central Oman, and a change in facies from fluvial to lower shoreface. Efforts to more clearly document this extensional phase however, were hampered by lack of well data and poor seismic imaging. Further north in Saih Hatat, rift-shoulder uplift and deposition of some 3 km of sediments during the Mid to early Late Ordovician was considered to be further evidence for such an extensional phase.

In this paper we have assembled geochemical and geochronological evidence based on potassic mafic dykes from the Huqf which suggest that they were intruded into the shoulder of an intracontinental rift during the Mid Ordovician. The igneous activity together with stratigraphic, sedimentological and tectonic data have produced a compelling picture of rift-related extension and rift-shoulder uplift in the late Early to early Late Ordovician. The diachronous development of the Ghaba Salt and Saih Hatat Basins indicates an extended time of rifting of not less than 15 million years. Together with the Tabas Graben in Iran, these two basins can be considered as sub-basins of a failed rift arm. This rift phase represents an early stage in the break-up between Africa/Arabia and India. Thus, an additional phase of tectonic activity prior to the Late Carboniferous to Early Cretaceous extensional and Alpine compressional events has been documented. Structures formed during such phases are important for the trapping of hydrocarbons. Therefore, the tilted fault blocks of the sand-prone Ghudun Formation sealed by onlapping marine shales of the Safiq Group are promising new targets for hydrocarbon exploration.

ACKNOWLEDGEMENTS

The authors wish to thank the Ministry of Oil and Gas, Sultan Qaboos University and Petroleum Development Oman (PDO) for their permission to publish this paper. Mike Worthing is grateful to Oregon State University, Corvallis, for supplying age data of the mafic dike and to Kent Brooks of the University of Copenhagen and Phil Robinson of the University of Tasmania for facilitating the X-ray fluorescence and REE analyses. Mike Worthing also thanks Ron Berry of the University of Tasmania, Australia for discussions on the geochronology and Samir Hanna who proposed the project on which this paper is based. The PDO authors would like to thank H. Droste, T. Faulkner, E. Hoogerduin Strating, A. McCoss, Ide van der Molen, M. Naylor, P. Richard and Paul Senycia for stimulating discussions and contributions. The comments of two referees have been valuable in improving this paper.

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REFERENCES

Al-Husseini, M.I. 1988. The Arabian Infracambrian Extensional System. Tectonophysics v. 148, p. 93-103. Al-Husseini, M.I. 1990. The Cambro-Ordovician Arabian and Adjoining Plates: A Glacio-Eustatic Model. Journal of Petroleum Geology, v. 13, p. 267-288. Al-Husseini, M.I. and S.I. Al-Husseini 1990. Origin of the Infacambrian Salt Basins of the Middle East. In J. Brooks (Ed.), Classic Petroleum Provinces. Geological Society of London Special Publication, No. 50, p. 279-292. Alibert, C., A. Michard and F. Albarede 1986. Isotope and Trace Element Geochemistry of Colorado Plateau Volcanics. Geochemica and Cosmochimica Acta, v. 50, p. 2735-2750. Alsharhan, A.S. and A.E.M. Nairn 1997. Sedimentary Basins and Petroleum Geology of the Middle East. Elsevier, Amsterdam, 843 p. Belushi, J.D., K.W. Glennie and B.P.J. Williams 1996. Permo-Carboniferous Glaciogenic Al Khlata Formation, Oman: A New Hypothesis for the Origin of its Glaciation. GeoArabia, v. 1, no. 3, p. 389-403. Berberian, M. and G.C. King, 1981. Towards a Paleogeography and Tectonic Evolution of Iran. Canadian Journal of Earth Sciences, v. 18, p. 210-265. Blasband, B., P. Brooijmans, P. Dirks, W. Visser and S. White 1997. A Pan-African Core Complex in the Sinai, Egypt. Geologie en Mijnbouw, v. 76, p. 247-266. Blendinger, W., A. Van Vliet, M.W. Hughes Clarke 1990. Updoming, Rifting and Continental Margin Development during the Late Palaeozoic in Northern Oman. In A.H.F. Robertson, M.P. Searle and A.C. Ries (Eds.), The Geology and Tectonics of the Oman Region. Geological Society of London Special Publication No. 49, p. 27-37. Boserio, I.M., C. Kappellos and H. Priebe 1995. Cambro-Ordovician Tectono-stratigraphy and Plays in the South Oman Salt Basin. In M.I. Al-Husseini (Ed.), Middle East Petroleum Geosciences, GEO’94. Gulf PetroLink, Bahrain, v. 1, p. 203-213. Bott, W.F., B.A. Smith, G. Oakes, A.H. Sikander and A.I. Ibrahim 1992. The Tectonic Framework and Regional Hydrocarbon Prospectivity of the Gulf of Aden. Journal of Petroleum Geology, v. 15, p. 211-243. Brasier, M., O. Green and G. Shields 1997. Ediacarian Sponge Spicule Clusters from Southwestern Mongolia and the Origins of the Cambrian Fauna. Geology, v. 25, p. 303-306. Burns, S.J., U. Haudenschild and A. Matter 1994. The Strontium Isotopic Composition of Carbonates from the late Precambrian (~560-540 Ma) Huqf Group of Oman. Chemical Geology (Isotope Geoscience Section), v. 111, p. 269-282. Calvez, J.Y. 1985. Report on Geochronological Study. Bureau de Recherches Géologiques et Minières (unpublished report, PDO). Carmichael, A.S.E., F.J. Turner and J. Verhoogen 1974. Igneous Petrology. McGraw-Hill Book Company, New York, 739 p. Chapin, C.E. and S.M. Cather 1994. Tectonic Setting of the Axial Basins of the Northern and Central Rio Grande Rift. In G.R. Keller and S.M. Cather (Eds.), Basins of the Rio Grande Rift: Structure, Stratigraphy, and Tectonic Setting. Geological Society of America, Special paper 291, p. 5-25. Connally, T. and E. Wiltse 1995. Correlation of Ordovician Sandstones in Central Saudi Arabia. In M.I. Al- Husseini (Ed.), Middle East Petroleum Geosciences, GEO'94. Gulf PetroLink, Bahrain, v. 1, p. 321- 333. Connally, T. and E. Wiltse 1996. Saudi Sandstone Correlations. Middle East Well Evaluation Review, no. 16, p. 27-41. Droste, H.J. 1997. Stratigraphy of the Lower Paleozoic Haima Supergroup of Oman. GeoArabia, v. 2, no. 4, p. 419-472. Dubreuilh, J., F. Béchennec, A. Berthiaux, J. Le Métour, J.P. Platel, J. Roger and R. Wyns 1992. Explanatory Notes to the Geological Map of Khaluf, Sheet NF 40-15, scale 1:250,000. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals. FM Consultants 1983. An Argon-40/Argon-39 Age Spectrum study of sample JH 675, Qarn Aswad, Huqf. Geochronological Report No. FMK/1593 (unpublished report, PDO). Gibson, S.A., R.N. Thompson, P.T. Leat, M.A. Morrison, G.L. Hendry, A.P. Dickin and J.G. Mitchell 1993. Ultrapotassic Magmas Along the Flanks of the Rio Grande Rift, USA: Monitors of the Zone of Lithosperic Mantle Extension and Thinning Beneath a Continental Rift. Journal of Petrology, v. 34, p. 187-228. Gorin, G.E., L.G. Raz, and M.R. Walter 1982. Late Precambrian – Cambrian Sediments of Huqf Group, Sultanate of Oman. American Association of Petroleum Geologists Bulletin, v. 66, p. 2609-2627.

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Gradstein, F.M. and J. Ogg 1996. A Phanerozoic Time Scale. Episodes, v. 19, p. 3-5. Grunow, A., R. Hansom and T. Wilson 1996. Were Aspects of Pan-African Deformation Linked to Iapetus Opening? Geology, v. 24 , p. 1063-1066. Harland, W.B., R.L. Armstrong, A.V. Cox, L.E. Craig, A.G. Smith and D.G. Smith 1990. A Geological Timescale 1989. Cambridge University Press, New York, 265 p. Hartkamp-Bakker, C., L. Kuilman and W.H. Oterdoom 1998. The Karim Oilplay: Cambrian Alluvial- Lacustrine Deposits in South-Central Oman. GeoArabia, v. 3, no. 1, GEO’98 Abstracts, p. 97. Herriot, A. 1989. Lamprophyre. In D.R. Bowes (Ed.), The Encyclopedia of Igneous and Metamorphic Petrology. Van Rostrand Reinhold, New York, p. 272-275. Immenhauser, A., G. Schreurs, T. Peters, A. Matter, M. Hauser and P. Dumitrica 1998. Stratigraphy, Sedimentology and Depositional Environments of the Permian to Uppermost Cretaceous Batain Group, Eastern-Oman. Eclogae geologie Helvetiae, v. 91, p. 217-235. Japan International Cooperation Agency (JICA) 1981. Report on the Geological Survey of the Sultanate of Oman (Salalah Area). Metal Mining Agency of Japan/Japan International Cooperation Agency, 56 p., Unpublished Report. Jones, P.J., T.E. Stump and M.I. Al-Khan 1996. The Depositional and Tectonic Setting of the Early Silurian Hydrocarbon Source Rock Facies of Central Saudi Arabia. GeoArabia, v. 1, no. 1, Abstracts GEO’96, p. 152. Jones, P.J. and T.E. Stump, 1999. Depositional and Tectonic Setting of the Lower Silurian Hydrocarbon Source Rock Facies, Central Saudi Arabia. American Association of Petroleum Geologists Bulletin, v. 83, p. 314- 332. Keller, G.R. and S.M. Cather (Eds.), 1994. Basins of the Rio Grande Rift: Structure, Stratigraphy, and Tectonic Setting. Geological Society of America, Special Paper 291. Knoll, A.H., A.J. Kaufman, M.A. Semikathov, J.P. Grotzinger and W. Adams 1995. Sizing-up the sub- Tommotian Unconformity in Siberia. Geology, v. 23, p. 1139-1143. Le Métour, J. 1988. Géologie de l’Autochtone des Montagnes d’Oman: la Fenêtre du Saih Hatat. Bureau de Recherches Géologiques et Minières Documents, No. 129, 425 p. Le Métour, J., J.C. Michel, F. Béchennec, J.P. Platel and J. Roger (BRGM) 1995. Geology and Mineral Wealth of the Sultanate of Oman. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals. Le Métour, J., M.J. Villey and X. de Gramont, 1986. Geological Map of Quryat, Sheet NF 40-4D, scale 1:100.000, with Explanatory Notes. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals. Loosveld, R., A. Bell and J. Terken 1996. The Tectonic Evolution of Interior Oman. GeoArabia, v. 1, no. 1, p. 28-51. Lovelock, P.E.R., T.L. Potter, E.B. Walsworth-Bell and W.M. Wiemer 1981. Ordovician Rocks in the Oman Mountains: The Amdeh Formation. Geologie en Mijnbouw, v. 60, p. 487-495. McKenzie, D. 1989. Some Remarks on the Movement of Small Melt Fractions in the Mantle. Earth and Planetary Science Letters, v. 95, p. 53-72. Molnar, P. and P. Tapponnier 1975. Cenozoic Tectonics of Asia: Effects of a Continental Collision. Science, v. 189, p. 419-426. Moore, J.M. and A.M. Al-Shanti, 1979. Structure and Mineralization in the Najd fault System, Saudi Arabia. In S.Talhoun (Ed.), Evolution and Mineralization of the Arabian-Nubian Shield. Pergamon Press, New York, v. 2, p. 17-28. Muller, D and D.I. Groves 1995. Potassic Igneous Rocks and Associated Gold Copper Mineralisation. Lecture Notes in Earth Sciences. Springer Verlag, Berlin, 210 p. Paris, F. and M. Robardet 1990. Early Palaeozoic Palaeobiogeography of the Variscan Regions. Tectonophysics, v. 177, p. 193-213. Partington M., T. Faulkner, A. McCoss and E. Hoogerduijn Strating 1998. The Seismic Sequence Stratigraphy of the Ghudun/Safiq (Ordovician and Silurian) of North Oman. GeoArabia, v. 3, no. 1, GEO’98 Abstracts, p. 139-140. Peccerillo, A. 1992. Potassic and Ultrapotassic Rocks: Depositional Characteristics, Petrogenesis, and Geological Significance. Episodes, v. 15, p. 243-251. Platel, J.P., J. Roger, Tj. Peters, I. Mercolli, J.D. Kramers and J. Le Métour 1992. Explanatory Notes to the Geological Map of Salalah, Sheet NE 40-09, 1:250.000. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals. Platel, J.P., A. Berthiaux, J. Roger and A. Ferrand 1993. Geological map of Marbat, sheet NE 40-9E, scale 1:100,000. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals.

497

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/4/467/4553195/oterdoom.pdf by guest on 30 September 2021 Oterdoom et al.

Rock, N.M.S 1987. The Nature and Origin of Lamprophyres: An Overview. In J.G. Fitton and B.G.J. Upton (Eds.), Alkaline Igneous Rocks. Geological Society of London Special Publication No. 30, p. 191-226. Rogers, N.W., S.W. Bachinski, P. Henderson and S.J. Parry 1982. Origin of Potash-rich Basic Lamprophyres: Trace Element Data from Arizona Minettes. Earth and Planetary Science Letters, v. 57, p. 305-312. Samson, S.D. and E.C. Alexander 1987. Calibration of Interlaboratory 40Ar/39Ar Dating Standard, Mmhb-1. Chemical Geology, v. 66, p. 27-34. Schmidt, D.L., D.G. Hadley and D.B. Stoeser 1979. Late Proterozoic Crustal History of the Arabian Shield, Southern Najd Province, Kingdom of Saudi Arabia. In S. Talhoun (Ed.), Evolution and Mineralization of the Arabian-Nubian Shield. Pergamon Press, New York, v. 2, p. 41-58. Scotese, C.R. and W.S. McKerrow 1990. Revised World Maps and Introduction. In W.S. McKerrow and C.R. Scotese (Eds.), Palaeozoic Palaeogeography and Biography. Geological Society of London, Memoir 12, p. 1-21. Sengör, A.M.C. 1990. A New Model for the Late Palaeozoic – Mesozoic Tectonic Evolution of Iran and Implications for Oman. In A.H.F. Robertson, M.P. Searle and A.C. Ries (Eds.), The Geology and Tectonics of the Oman Region. Geological Society of London Special Publication No. 49, p. 797-831. Shackleton, R.M. 1996. The Final Collision Zone between East and West Gondwana: Where is it? Journal of African Earth Sciences, v. 223, p. 271-287. Shackleton, R.M. 1993. Tectonics of the Mozambique Belt in East Africa. In H.M. Prichard, T. Alabaster, N.B.W. Harris and C.R. Neary (Eds.), Magmatic Processes and Plate Tectonics. Geological Society of London Special Publication No. 76, p. 345-362. Soffel, H.C. and H.G. Förster 1984. Polar Wander Path of Central-east-Iran Microplate including New Results. Neues Jahrbuch für Geologische und Paläontologische Abhandlungen, v. 168, 2/3, p. 165-172. Spear, F.S. 1993. Metamorphic Phase Equilibria and Pressure Temperature Time Paths. Mineralogical Society of America, 779 p. Stampfli, G., J. Marcoux and A. Baud 1991. Tethyan Margins in Space and Time. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 87, p. 373-409. Stern, R.J. 1994. Arc Assembly and Continental Collision in the Neoproterozoic East African Orogen: Implications for the Consolidation of Gondwanaland. Annual Review Earth and Planetary Sciences, v. 22, p. 319-351. Stöcklin, J. 1968. Structural History and Tectonics of Iran, A Review. American Association of Petroleum Geologists Bulletin, v. 52, p. 1229-1256. Streckeisen, 1979. Classification and Nomenclature of Volcanic Rocks, Lamprophyres, Carbonatites and Melalitic Rocks. Neues Jahrbuch für Mineralogie Abhandlungen, 134, p. 1-14. Stump, T.E., S. Al-Hajri, and J.G.L.A. Van der Eem 1995. Geology and Biostratigraphy of the Late Precambrian through Palaeozoic Sediments of Saudi Arabia. Review of Palaeobotany and Palynology, v. 89, p. 5-17. Talbot, C.J. and M. Alavi 1996. The Past of a Future Syntaxis Across the Zagros. In G.I. Alsop, D.J. Blundell and I. Davison (Eds.), Salt Tectonics. Geological Society of London Special Publication No. 100, p. 89-109. Thompson, R.N., P.T. Leat, M.A. Morrison, G.L. Hendry and S.A. Gibson 1989. Strongly Potassic Mafic Magmas from Lithospheric Mantle Sources During Continental Extension and Heating: Evidence from Miocene Minettes of Northwest Colorado, U.S.A. Earth and Planetary Science Letters, v. 98, p. 139-153. Vail, P.R., R.M. Mitchum, Jr. and S. Thompson III 1977. Seismic Stratigraphy and Global Changes of Sea Level, Part 4: Global Cycles of Relative Changes of Sea Level. In C.E. Payton (Ed.), Seismic Stratigraphy – Applications to Hydrocarbon Exploration. American Association of Petroleum Geologists, Memoir 26, p. 83-97. Vaslet, D. 1990. Upper Ordovician Glacial Deposits in Saudi Arabia. Episodes, v. 13, no. 3, p. 147-161. Veevers, J.J. 1995. Emergent, long-lived Gondwanaland vs. submergent, short-lived Laurasia: Supercontinental and Pan-African heat imparts long-term buoyancy by mafic underplating. Geology, v. 23, p. 1131-1134. Vendeville, B.C. and M.P.A. Jackson 1992. The Rise of Diapirs during Thin-Skinned Extension. Marine and Petroleum Geology, v. 9, Special Issue, p. 331-353. Villey, M., J. Le Métour and X. de Gramont, 1986. Geological Map of Fanjah, Sheet NF 40-3C, scale 1:100,000, with explanatory notes. Directorate General of Minerals, Oman Ministry of Petroleum and Minerals. Visser, W. 1991. Burial and Thermal History of Proterozoic Source Rocks in Oman. Precambrian Research, v. 54, p. 15-36. Wilson, M. 1994. Magmatism and Basin Dynamics within the Peri-Tethyan Domain. In F. Roure (Ed.), Peri- Tethyan Platforms. Proceedings of the IFP/Peri-Tethys Research Conference, Arles, France, March 23-25 1993. Editions Technip, Paris, p. 9-23. Ziegler, P.A. 1988. Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists Memoir, v. 43, 198 p.

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APPENDIX

Petrographic Descriptions

In order to verify petrographic mineral identifications, particularly of the finer-grained phases, qualitative EDS spectra were obtained using a LINK Isis 3 attached to a Jeol JSM 840A scanning electron microscope at Sultan Qaboos University.

Qalilat Suwaidi In thin section the mafic rocks have a fresh appearance with a hypocrystalline and micro-porphyritic texture. They contain abundant microphenocrysts of lath-like red-brown to almost colourless sieved and castellated phlogopite ranging in length between 0.5 and 1.5 mm. The mineral is locally altered to a pale-colored chlorite. Euhedral laths of a colorless clinopyroxene about 0.1 mm long are also abundant. They have low birefringence and strongly oblique extinction and some hourglass structure. There is a faint flow structure in the rock defined by alignment of the clinopyroxene laths. The rock also contains sparse olivine phenocrysts up to 0.5 mm in diameter which are now partly or completely pseudomorphed with a pale-colored serpentine showing typical meshing after olivine. Rare phenocrysts of clinopyroxene also occur which enclose phlogopite laths. Coarser facies contain K-feldspar with Carlsbad twinning that poikilitically enclose groundmass minerals. In finer grained facies the groundmass minerals are difficult to resolve but include lath-like K-feldspar, phlogopite, abundant granular ilmenite and a colorless glass. Rounded xenoliths of strained quartz up to 0.25 mm are present. These may be derived from the underlying gneissic basement or the Abu Mahara Formation.

Another notable feature of the rock is euhedral to rounded pseudomorphed phenocrysts composed mainly of aligned blebs of ilmenite and a colourless mineral. The largest is 2.5 mm across. Isolated grains of brown K-Ti-rich amphibole within the phenocryst are in optical continuity suggesting that the area was once occupied by a single large K-Ti-rich amphibole crystal. In one case, this crystal is in contact with a grain of pale-colored clinopyroxene that appears to be in equilibrium with the amphibole and is interpreted as a primary inclusion inside the amphibole probably in a magmatic reaction relationship. The remaining area is filled by blebs of granular ilmenite and primary phlogopite laths plus a pale-colored mineral that possibly represents the breakdown of the primary amphibole to a low-temperature amphibole plus ilmenite.

Qarn Aswad These rocks are similar to those from Qalilat Suwaidi except that pseudomorphed olivine is more abundant, though still comprising only about 10% of the mode, and phlogopite is present in only minor amounts.

Geochemical Analytical Methods

At Copenhagen University, an automatic PW 1606 x-ray fluorescence machine was used. Samples were first ignited to 950oC and volatile loss measured and corrected for oxidation (Fe2+ to Fe3+). Ignited samples were then mixed with sodium tetraborate and fused. Sodium was determined by atomic absorption after solution in HF. FeO was determined on a glass disc.

At the University of Tasmania, samples were run on a PW 1480 machine. Samples were heated to 1,000oC overnight to determine volatile loss. Ignited samples were then mixed with Norrish flux and lithium nitrate and fused at 1,050oC for ten minutes and poured into Pt moulds. Na was determined

on the fusion disc and Fe was determined as Fe2O3. One sample (HG10) was run at both laboratories with excellent agreement. Pressed pellets for trace elements analysis were prepared at 32 tonnes cm-2 using 6 gms of powder and PVA binder. The pellets were backed with boric acid.

REE determinations were done on an ELEMENT HR-ICP-MS Funnigan MAT machine at the University of Tasmania. Deionised water was used for rinsing and solution preparation. Samples were digested

in HF and BDH Analar ® HNO3 purified by double distillation. One hundred milligrams of powdered sample were weighed into Sarvillex vials and moistened with ultrapure water. Two milliliters of HF

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o and 0.5 ml of HNO3 were added and the sealed vial placed on a hotplate for 48 hours at 130 C. The vials were removed twice in every 24 hours, cooled and placed in an ultrasound bath for 2 hours. After 48 hours the vials were opened and evaporated to incipient dryness and the residuum dissolved in 2

ml HNO3 followed by 10 to 20 ml of water. The resulting solution was transferred to a polycarbonate container and diluted to 100 ml. An Indium internal standard was also added to give a final concentration of 10 mg gm-1. At least two reagent blanks for each sample were prepared.

ABOUT THE AUTHORS

W. Heiko Oterdoom joined Petroleum Development Oman in 1995 as a Senior Exploration Geologist in diverse functions ranging from operational stratigraphy and geological studies to frontier exploration. Heiko has 18 years exploration experience as Regional Geologist with Shell in oil shales, the North Sea – Norwegian Sea and the Far East. Heiko was educated at Berkeley and Zürich and holds a PhD in Petrography from the Federal Institute of Technology (ETH), Zürich. He is a licensed guide of the Dutch tidal flats.

Mike A. Worthing is Head of the Earth Science Department at Sultan Qaboos University, Oman where he has worked since 1992. He completed a PhD in Structural Geology and Metamorphic Petrology at London University in 1971 based on fieldwork in Arctic Norway. Since then Mike has worked as an academic in the UK, Uganda, Papua New Guinea, Australia and Oman. His main research interests are mineralogy and the petrology and geochemistry of mafic igneous and metamorphic rocks. He enjoys sports, wadi-bashing and violin music.

Mark Partington is a Senior Seismic Interpreter in the Exploration Department at Petroleum Development Oman. Mark’s main responsibilities lie in the field of prospect evaluation, applied stratigraphy and integration with seismic data. Before joining PDO in 1996, he was employed as Stratigrapher by Shell International and Shell Research in The Netherlands. He has a MSc from University College London and a PhD from Aberdeen.

Manuscript Received March 7, 1999

Revised May 5, 1999

Accepted June 19, 1999

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