ABSTRACT

The Faiyah Range belongs to a group of regional ridges that formed by post-obduction folding of the Upper Cretaceous-Tertiary sedimentary rocks exposed along the western margin of the Northern Oman Mountains. The Faiyah Anticline, generally trends north-northeast to south-southwest with thrust faults striking parallel to the fold axis. The anticlinal hinge was later displaced by a dextral strike-slip fault, named here as the Faiyah Fault, into two segments. The northeastern segment includes Jebels Rumaylah, Faiyah and Mulayhah, and the southwestern segment includes Jebels Buhays and Aqabah. The anticline is interpreted to result from northeast-southwest compression during the Tertiary.

In the Faiyah Range the neoautochthonous sedimentary rocks are the Maastrichtian Qahlah and Simsima formations, and the Eocene Dammam Formation. Stratigraphic evidence shows that the lower part of the Qahlah was deposited in a non-marine environment while the upper part was deposited during a marine transgression. The Simsima was deposited in a shallow-marine environment. These units unconformably overlap the allochthonous Semail Ophiolite. The microfaunal content of the so-called Muthaymimah Formation (?Tertiary), of earlier authors, indicates that it is of Maastrichtian age in the Faiyah Range. This sequence is also conformable to the Simsima and therefore it is considered to be the upper member of the Simsima in this area.

INTRODUCTION

The Faiyah Range constitutes the northernmost outcrop of the western foothills of the Oman Mountains where Maastrichtian and Tertiary sedimentary rocks are exposed in a discontinuous belt of jebels (Figure 1). The range is located between the Dibba and Hatta Zones, and trends parallel to the Northern Oman Mountains Front. It consists of an elongated anticlinal structure, extending for about 21 kilometers (km) in length and 1 to 2 km in width. The range can be subdivided into five jebels (Figure 2): Buhays, Aqabah, Rumaylah, Faiyah and Mulayhah.

In the eastern United Arab Emirates region, several gas/condensate fields, such as Margham and Saja’a (Figure 1), highlight the prospective nature of this province. These fields are trapped in anticlinal structures along the thrust front which constitutes the main boundary between a nearly undeformed foredeep basin to the west, and the fold and thrust belt to the east (Dunne et al., 1990; Mount et al., 1995). The complex nature of these subsurface structures is generally difficult to image with seismic data (Mount et al., 1995; O’Donnell et al., 1995). The study of the Faiyah Range therefore contributes to our understanding of the structural and stratigraphic evolution of this hydrocarbon habitat.

Many recent studies describe the geology of the Northern Oman Mountains (for example, Searle et al., 1983; Searle, 1985, 1988a, b; Dunne et al., 1990; Woodward, 1994; Warrak, 1996; Noweir and Eloutefi, 1997). In addition, several published regional maps cover the entire Oman Mountains (Glennie et al., 1974 at a scale of 1:500,000) and the Northern Oman Mountains (Geological Map of the United Arab Emirates, Sheets 2 and 4, 1979 at a scale of 1:250,000; Open University Oman-Ophiolite Project, 1983 at a scale of 1:250,000). The Upper Cretaceous stratigraphy of the area is also described in several regional studies (for example, Nolan et al., 1990; Skelton et al., 1990; Hamdan and Anan, 1993; Alsharhan and Nasr, 1996; Alsharhan et al., in press). Alsharhan and Nairn (1990, 1994) outlined the regional setting of the Upper Cretaceous deposits around the fringes of the Oman Mountains.

None of the above regional studies, however, provides a detailed description of the Faiyah Range. This paper presents the results of a study which is focused on the structural and stratigraphic setting of this range. We present a geological map (Figure 2, originally constructed at a scale of 1:25,000) based on the examination of vertical aerial photographs and detailed field mapping. The study also presents paleontological results which clarify the age of the stratigraphic units in this region.

GEOLOGICAL SETTING

The Oman Mountains form a prominent arcuate range which is parallel to the Gulf of Oman in southeast Arabia (Figure 1). These mountains formed in response to two main compressional events. The first resulted from the Late Cretaceous (Coniacian-Maastrichtian) obduction of the Semail Ophiolite, and associated sedimentary and volcanic rocks (Sumeini, Hawasina and Haybi groups), onto the eastern margin of the Arabian Platform. As a result of the obduction a flexural foredeep developed along the western flank of the mountains and this basin was filled with Maastrichtian to Tertiary sediments (Glennie et al., 1973, 1974; Coleman, 1981; Searle et al., 1983; Lippard et al., 1986; Patton and O’Connor, 1988; Boote et al., 1990; Nolan et al., 1986, 1990; Robertson et al., 1990; Warburton et al., 1990).

In North Oman, a second compressional post-obduction event occurred in the Late Eocene-Miocene. It was mainly responsible for the formation of foreland folds (Warrak, 1996) and the folding of the Maastrichtian-Tertiary neoautochthonous units in the foredeep (Boote et al., 1990). The effects of this compression are visible in a series of large-scale folds fringing the western foothills of the Northern Oman Mountains, including the Faiyah Range. The second event is correlated by some authors to the Zagros Orogeny in Iran (Ricateau and Riche, 1980; Searle et al., 1983; Searle, 1985; Searle et al., 1990).

Jebel Faiyah trends north northeast-south southwest and is one of three sets of fold axis recognized in the Tertiary outcrops and structures of the Northern Oman Mountains (Figure 1). The second trend is west northwest-east southeast and occurs at the extreme western end of the Hatta Zone (Jebel Rawdah), about 15 km south-southeast of the Faiyah area. This trend is generally at right angles to the north northeast-south southwest trend (Salah and Mersal, 1998). The third trend is more dominant, and trends north northwest-south southeast, parallel to the Central Oman Mountains, near Al-’Ain. Jebel Hafit is an example of this trend.

STRATIGRAPHY

The Semail Ophiolite forms the core of Jebel Faiyah. It is the oldest exposed rock and it consists of a slice of the emplaced Cretaceous oceanic crust and upper mantle which is composed of serpentinites and serpentinized peridotites (Glennie et al., 1974).

The Upper Cretaceous Maastrichtian Qahlah Formation unconformably overlies the peneplaned Semail Ophiolite (Figure 3). The thickness of the Qahlah ranges from 40 meters (m) at Jebel Faiyah and 70 m at Jebel Buhays (Alsharhan and Nasr, 1996). It consists of reddish-brown, lateritic, ferrugenous mudstone at the base, and grades upward into ophiolitic breccia and ophiolite-clast conglomerate derived from the underlying weathered ophiolite, with laminar and cross-bedded sandstone at the top.

A wide variety of non-marine fluviatile to shallow-marine facies are developed in the Qahlah (Nolan et al., 1990; Alsharhan and Nasr, 1996). Fluviatile facies are well-developed in its lower part, while evidence for a marine transgressive facies, in the upper part, is discernable by the presence of Omphalocyclus macroporous (Lamarck) (Figure 4).

The Simsima Formation (Figure 3) is about 140 m thick and is subdivided into two members (Alsharhan et al., in press). The lower member attains a thickness of about 80 m and consists of medium-bedded and shallow-marine bioclastic limestone. It contains common fauna such as Orbitoids medius (d’Archiac) (Figure 5a), Loftusia gr. minor morgani, rhodolithic algae, Rudistids (Figure 5b), Acteonellids as well as Scleractinian corals and echinoids (regular and irregular).

The upper member is about 60 m thick and consists of thin- to medium-bedded, often nodular, creamy white, dolomitic limestone with interbedded conglomerates and yellow marls. It was previously defined by Nolan et al. (1990) as the Muthaymimah Formation in a type section located at E55°49′50″ and N24°06′30″, further south from the study area near Al-’Ain (Figure 1).

The Muthaymimah type section of Nolan et al. (1990) is 300 m thick and consists mainly of limestone, marl and shale. They assign a Paleocene to (Middle?) Eocene age to the Muthaymimah, based on its stratigraphic position above the Maastrichtian “Simsima” and the presence of occasional alveolinid and nummulitic foraminifera and reworked clasts of pene-contemporaneous Paleogene shelf limestones. In the Jebel Faiyah area, Nolan et al. (1990) describe the Muthaymimah as resting unconformably on the “Simsima”. They also indicate that in Jebel Rawdah, the Muthaymimah interdigitates and is overlain by an unnamed Eocene basinal marl, shale and limestone lacking the conglomerates typical of the Muthaymimah.

Our field study in the Faiyah Range area indicates that the upper and lower Simsima members are conformable. Furthermore, the microfaunal studies suggest that the upper Simsima member (Muthaymimah of Nolan et al., 1990) is Late Cretaceous in age and we therefore incorporate it into the Simsima as its upper member. It contains Omphalocyclus macroporous (Lamarck) and Siderolites calcitrapoides Lamarck subsp. nov. (Figures 6a and 6b).

Marl intercalations near the top of the upper member yield a planktonic foraminiferal association which includes Rugoglobigerina and Heterohelicidae (Figure 7) of Maastrichtian age.

An Eocene outcrop in Jebel Faiyah is correlated here for the first time with the Dammam Formation and to Tle5 of Hunting (1979), from Jebel Hafit (reference section), where a complete sequence of the Eocene-Oligocene is exposed.

In Jebel Faiyah, the Dammam Formation is composed of a shallow-marine shelf Nummulitic limestone with thin-bedded marl. It crops out in one locality along the western flank of the Buhays-Aqabah anticline (Figure 2) and attains a thickness of about 10 m. The unit has been dated as latest Middle Eocene (Bartonian) by the presence of Nummulites ptukhiani Kacharava, Nummulites cf. Lyelli d’Archiac and Haime (Figure 8), Sphaerogypsina globula (Reuss), Asterocyclinids and Discocyclinids. The Nummulites were dated following Schaub’s nummulitic scale (1981).

The depositional hiatus between the Eocene (Bartonian) Dammam and the Late Cretaceous (Maastrichtian) upper Simsima may have resulted from an epeirogenic movement which uplifted the Faiyah Range area.

STRUCTURE

Folds, rather than faults, dominate the structural style of the Faiyah Range. A west northwest-east southeast dextral strike-slip fault, named here as the Faiyah Fault, divides the anticline into two unequal segments (Figure 2). The Faiyah Fault is en-echelon to the Hatta Zone and could probably belong to the Wadi Hatta transform fault (Robertson et al., 1990).

The southern Buhays-Aqabah segment is a 5 km long, south-plunging anticline (Figure 2). Cross-section A-A’ (Figure 2) shows that the fold is asymmetric with a moderately-dipping eastern flank (up to 30°) and a steeply-dipping western flank (up to 75° and locally vertical to overturned). The western flank is characterized by the presence of a small, shallow anticline and syncline which folds the Qahlah and Simsima formations. These folds may have formed due to flexural slip as shown by the presence of fibrous slickensides between the bedding in the lower part of the Simsima.

Jebel Aqabah was affected by two main faults. The first is a thrust fault which dips northeast thereby juxtaposing the yellow, lower member of the Simsima against its creamy-white, upper member (Figure 9). The second fault is normal and juxtaposes the down-dropped upper Simsima member against the yellow lower member (Figure 10). South of Jebel Thanais, the Faiyah anticline is cut by a west northwest-east southeast trending dextral strike-slip fault.

The northern Rumaylah-Faiyah-Mulayhah segment is about 16 km long. It is an asymmetric, undulating anticline in which Jebel Faiyah forms the western flank. An isolated outcrop forming Jebel Rumaylah (Figures 2 and 11) represents the eastern flank. The anticline continues northward for about 5 km at Jebel Mulayhah. Five cross-sections constructed across the fold of the study area (Figure 2, cross-sections B-B′ to F-F′) show that the anticlinal ridge has a moderately dipping eastern flank (up to 40°) and a steeper-dipping western flank (up to 65°).

Two thrust faults are recognized in the field in the steep, western flank of Jebel Faiyah. The first thrust fault dips southwest, trends parallel to the fold axis, and causes the repetition of the Semail Ophiolite, and the Qahlah and Simsima formations (Figures 2 (B-B′) and 11). The second thrust fault strikes parallel to the fold axis and juxtaposes the Semail Ophiolite and the Simsima.

Warrak (1996), based on a stratigraphical and structural study of Jebel Hafit, located about 75 km south of Jebel Faiyah near Al-’Ain (Figure 1), concluded that deformation during the second orogenic event started from the east and migrated westwards. Deformation of the folds to the east of Jebel Hafit started in the Paleocene; while deformation at Jebel Hafit itself started just before the Middle Eocene. Growth continued at Jebel Hafit synchronously with sedimentation until the end of Miocene. Warrak (1996) concludes that Jebel Hafit and other foreland folds in the Northern Oman Mountains (for example, Faiyah) formed prior to the main Zagros deformation which started in very Late Miocene and culminated in the Late Plio-Pleistocene (Stöcklin, 1968; Murris, 1980).

Jebel Faiyah formed after the Middle Eocene Dammam Formation was deposited. The similar positions of Jebels Hafit and Faiyah, relative to the Oman Mountains and foredeep, and the similar timing of deformation (Middle Eocene to Miocene for Hafit and post-Middle Eocene for Faiyah in our study) indicates that these two structures share a similar tectonic evolution.

CONCLUSION

The Faiyah Range is a 21 km long, north northeast-south southwest trending anticline, which is parallel to the western front of the Oman Mountains. It is interrupted by a dextral strike-slip fault into a southwestern segment (Buhays-Aqabah Anticline) and a northeastern segment (Rumaylah-Faiyah-Mulayhah Anticline). The anticline is about 2 km wide and asymmetrical, with gently-dipping eastern flanks and steeply-dipping, sometimes vertical to overturned, western flanks. The anticline formed after the Middle Eocene as a result of regional deformation.

The outcrop formations in Jebel Faiyah consist of the Maastrichtian Qahlah and Simsima, and the Eocene Dammam formations. These units unconformably overlie the Semail Ophiolite. Faunal evidence from the so-called Muthaymimah Formation, which was assigned a tentative Paleocene to Middle Eocene age by Nolan et al. (1990), suggests that this sequence is of Maastrichtian age. It is proposed here that the latter unit be considered as the upper member of the Simsima Formation in the Faiyah Range area. The presence of the Dammam in the Faiyah Range was established in this study on the basis of fossil studies.

ACKNOWLEDGEMENTS

The authors wish to thank United Arab Emirates University for providing the necessary facilities for this project and M.D. Simmons (University of Aberdeen), for checking the planktonic forams. The comments of two anonymous reviewers and the Editor-in-Chief greatly improved the manuscript. The authors thank Gulf PetroLink for drafting the figures.

ABOUT THE AUTHORS

M. Atef Noweir has been a Faculty member with the Department of Geology at United Arab Emirates University since 1993. Prior to that and upon returning to Egypt, Atef worked for the University of Tanta. In 1977 and 1983, he received a BSc and MSc in Geology from Tanta University, respectively. He obtained a PhD in Structural Geology from the University of Missouri-Rolla in 1990. Society affiliations include the GSA, AAPG, IASTG, MAS and GSE. Atef is particularly interested in field mapping, structural analysis and balanced cross-sections of foreland fold and thrust belts.

Abdulrahman S. Alsharhan received his MSc (1983) and PhD (1985) in Geology from the University of South Carolina. He is currently Dean of Faculty of Science at United Arab Emirates University. His current research interests include Holocene coastal sabkhas of the UAE, and the geology and hydrocarbon habitat of the Arabian Gulf and adjacent areas. Abdulrahman is the Editor of the “Quaternary Deserts and Climatic Change” Proceedings to be published in 1998 by Balkema. He published his first book “The Sedimentary Basins and Petroleum Geology of the Middle East” co-authored with A.E.M. Nairn earlier this year. Abdulrahman is a member of the Advisory Board of GeoArabia.

Mohamed A. Boukhary joined United Arab Emirates University in 1993 and became the Chairman of the Geology Department in 1996. Prior to that, he worked as Professor of Stratigraphy and Micropaleontology at Ain Shams University in Egypt. He obtained his PhD in 1973 from Ain Shams University. Mohamed is a member of the Egyptian Party of the Paleonile Joint Project between Pierre et Marie Curie University, France, and Ain Shams University. He is particularly interested in Paleogene stratigraphy and the taxonomy of larger foraminifera.