ABSTRACT

Jebel Rawdah is a west-northwest to east-southeast trending, post-obduction fold located at the western edge of the Hatta Zone of the Northern Oman Mountains. The main syncline plunges about 5 kilometers to the northwest and it is flanked to the west by a minor anticline. The outcrops in the area consist of: (1) allochthonous Semail Ophiolite, consisting of slices of oceanic crust and upper mantle, together with the Haybi Complex of volcanic rocks and associated metamorphics; (2) the parautochthonous Sumeini Group consisting of shelf edge and slope carbonates and clastics; and (3) the post-obduction neoautochthonous clastics and carbonates of the Qahlah, Simsima and Muthaymimah formations (Maastrichtian to Early Tertiary).

Two stages of folding were detected in the Jebel Rawdah area. The older folds affect the allochthonous rocks and result from shearing deformation along the westward extension of the Hatta Zone. The younger deformation is manifested in drape folds in the neoautochthonous rocks which was caused by differential vertical movements of fault blocks in the underlying allochthonous rocks. Three sets of faults were observed: (1) northwest-southeast trending vertical to steeply-dipping scissor faults; (b) reverse faults which form flower structures; and (c) northeast-southwest trending normal faults.

Field observations, biostratigraphic studies and petrographic examination suggest three stages in the development of the stratigraphic units in Jebel Rawdah. The first stage occurred during the Early Maastrichtian when the Oman Mountains emerged and were subsequently exposed to subaerial erosion. In the second stage a transgression occurred during the gradual subsidence of the area which led to the deposition of the Qahlah Formation in a fluviatile to shallow-marine environment, and the overlying Simsima in a shallow shelf setting. In the final Tertiary stage the Muthaymimah Formation was deposited in a subsiding basin and slope setting.

INTRODUCTION

Jebel Rawdah is a west northwest-east southeast trending fold located at the western end of the Hatta Zone, along the western margin of the Northern Oman Mountains (Figure 1). Robertson et al. (1990a) interpreted the Hatta Zone as a left-lateral transform lineament associated with the Late Cretaceous and Tertiary tectonic evolution of the Oman Mountains.

The regional geological evolution of the Oman Mountains has been described elsewhere (for example, Hudson et al., 1954; Glennie et al., 1973, 1974; Ricateau and Riche, 1980; Searle, 1985, 1988; Searle et al., 1983, 1990; Patton and O’Connor, 1988; Nolan et al., 1986, 1990; Dunne et al., 1990; Robertson et al., 1990a, 1990b; Skelton et al., 1990; Warburton et al., 1990; Warrak, 1996). These studies provide an overview of the region but do not describe in great detail smaller features such as Jebel Rawdah.

This paper reports on the lithostratigraphy, paleogeography and structural geology of Jebel Rawdah. The objective is to provide more detailed field observations on the complex geology of the western end of the Hatta Zone. Two stratigraphic sections at the southwestern and northern sectors of Jebel Rawdah were sampled, measured and correlated (sites A and B in Figure 3). Also several structural sections were constructed across the area. Hundreds of samples were petrographically examined in hand specimens, under binocular microscope and by light microscopy of 47 thin sections. Aerial photographs and a satellite composite of the area were used to distinguish and map the main formations.

STRATIGRAPHY OF JEBEL RAWDAH

The outcrops in the Jebel Rawdah (Figures 2 and 3) are: (1) allochthonous Semail Ophiolite, consisting of slices of oceanic crust and upper mantle; (2) the allochthonous Haybi and Hawasina groups consisting of metamorphic and volcanic rocks; (3) the parautochthonous Sumeini Group, consisting of shelf edge and slope carbonates and clastics; and (4) neoautochthonous Upper Cretaceous (Maastrichtian) Qahlah and Simsima formations, and Tertiary (?Paleocene-Eocene) Muthaymimah Formation. In the following discussion the Upper Cretaceous-Tertiary formations are described.

Qahlah Formation

Lithology

The Qahlah Formation was first described by J. Horstink (unpublished report for Petroleum Development Oman, 1967) and formally defined by Glennie et al. in 1974 (Nolan et al., 1990). In the type section (described in Nolan et al., 1990) the Qahlah is around 140 meters (m) thick, and consists of conglomerates, thick-bedded sandstones and minor limestones and basalt. The formation rests unconformably upon sub-vertical Hawasina, while the contact with the overlying Simsima may be conformable. Glennie et al. (1974) assigned a Maastrichtian age to the Qahlah based on the presence of Loftusia sp.

In Jebel Rawdah, the Qahlah lies unconformably on the Semail Ophiolite and associated volcanic and sedimentary rocks (Haybi, Hawasina and Sumeini groups, Figure 4). The contact with the overlying Simsima is an erosional surface. The Qahlah consists of upward-fining sequences formed of lateritic ironstone, clast-supported conglomerates and lithic sandstones (Figures 5 and 6). The Qahlah is absent in the north and its thickness increases from 25 m at A, to 42 m at B to the southwest (Figure 3). Skelton et al. (1990) identified four facies in the Qahlah, three of which are found in Jebel Rawdah. These are:

The Lateritic Ferruginous Mudstone Facies (Figure 6a) overlies the thick volcanic unit of the Semail Ophiolite with a clear, irregular erosional surface that truncates the steeply-dipping thrust unit. It consists mainly of interbedded red and purple colored ferruginous mudstone, sandstone and siltstone, with gritty and pebbly hematitic ironstone. This facies increases from 12 m in the southeast (A) and to 17 m in the southwest (B) (Figure 3).

The Ophiolite-Clast Conglomerate Facies (Figures 6b and 6c) consists of red-stained, sandy conglomerate with boulders of ophiolitic and serpentinitic conglomerate and local calcareous lithic sandstone interbeds. This facies decreases from 13 m in the southeast (A) to 10 m in the southwest (B) (Figure 3).

The Lithic Sandstone Facies (Figure 6d) is exposed near the top of the Qahlah and consists mainly of interbedded green, reddish-colored, cross-bedded sandstone and clast conglomerate. This facies is 8 m thick in the southeast (A) and 12 m in the southwest (B) (Figure 3). Near the top of the Qahlah, micritic calcite with large foraminifera (Orbitoids) and rudist fragments were found.

Depositional Environment

McFarlane (1983) interpreted the preservation of a lateritic-saprolitic residual cap to the basement (as in the case of the lower facies of the Qahlah in Jebel Rawdah) to be due to chemical mobilization under warm, wet, at least partly-oxidizing conditions. These conditions correspond to subaerial weathering in a warm, humid climate. The bouldery conglomerate and the admixture of locally-derived, fractured and exfoliated serpentinite clasts and marine shells, indicates a marginal marine environment (i.e. intertidal beach) for the middle facies of the Qahlah. The restriction of these conglomerates to the bottom part of the section indicates that there was greater continental input in the earlier stage of deposition.

A petrographic examination of this facies showed well-rounded grains, fibrous cements of marine character with no indications for any compaction features, indicating deposition in a beach setting (Dunham, 1970). The presence of silt-size quartz within the matrix of this facies suggests that it was relatively close to the paleo-shoreline. Nolan et al. (1990) interpreted the rounding of large boulders, the well-sorted coarse matrix and the direct contact with fresh serpentinite beneath Jebel Rumaylah (approximately 13 kilometers (km) north of Jebel Rawdah) as indicators for high-energy conditions. Most of the Qahlah lithofacies are interpreted to represent deposition in continental and marginal to shallow-marine settings.

Simsima Formation

Lithology

The Simsima Formation was first described by W. Sugden (unpublished report, Qatar Petroleum, 1956) in the Dukhan-1 well in Qatar. The type sections in Oman and Qatar were respectively defined by Glennie et al. (1974) and Sugden and Stranding (1975). Due to the inaccessibility of the type sections, Nolan et al. (1990) designated Jebel Faiyah (Figure 1), Northern Oman Mountains (19 km northwest of Jebel Rawdah) as the surface type section for the Simsima Limestone. This formation is 85 m thick in the type section and consists of a slightly dolomitic, nodular, bioturbated, highly-fossiliferous packstone. The section becomes less fossiliferous upward and changes to finer-grained foraminiferal packstone and wackestone (Nolan et al., 1990). The Simsima conformably overlies the Qahlah and is overlain unconformably by the Muthaymimah. It was deposited in a shallow-marine setting and was assigned an early to late Maastrichtian age, based on the mollusks, corals, echinoids and foraminifera (Skelton et al., 1990).

The Simsima varies in thickness from 15 m in the southeast (A) to nearly 65 m in the northwestern (B) (Figure 3). It consists of three main facies (Figures 7 and 8) which are described by Nolan et al. (1990) and Skelton et al. (1990):

The Lithic Bioclastic Sandstone Facies (Figure 8a) consists of well-laminated, well-sorted sandstones with clasts of the underlying red Qahlah sandstone, along with pebbles of Semail serpentinite and Hawasina chert. Its thickness increases from 3 m in the southeast to 7 m in the northwest.

The Durania Limestone Facies (Figures 8b and 8c) consists of slightly dolomitic, nodular, bioturbated, orbitoid rich, foraminiferal packstone, with abundant rudists, rhodoliths, gastropods, and occasional corals and echinoids. The main rudist type in this facies is the Durania sp. This facies increases in thickness from 6 m in the southeast (A) to 24 m in the northwest (B). It rests conformably on the lower facies.

The Dictyoptychus Limestone Facies (Figures 8d, 8e and 8f) consists of medium-grained bioclastic grainstone and finer, nodular packstone. This facies contains more fauna than that at the type locality. Some nautiloids, rare ammonites and several large hippuritids are recorded. Its thickness increases from 7 m in the southeast (A) to 30 m in the northwest (B).

Depositional Environment

The Simsima lithic sandstone was deposited in the same setting as the Qahlah, reflecting sustained siliciclastic input. The algal facies of the Durania limestones indicates a highly-agitated environment. This is supported in part by the presence of coralline limestone. In addition, the flat-bedded and cross-bedded grainstones in the Durania indicate a lower foreshore to shallow shoreface zone. Nolan et al. (1990) interpreted the channel forms and possible lateral accretion surfaces of the Durania to be suggestive of meandering tidal channels migrating across the lower shore. The upper facies of the Simsima show a more fine-grained packstone matrix and breakage of many of the macrofossils, relative to that of the underlying facies, indicating less persistent current reworking than in the Durania facies.

Muthaymimah Formation

Lithology

The Muthaymimah Formation was defined by Nolan et al. (1990) to describe the dip-slope section on the northwest side of Sayh Muthaymimah, located near Buraymi in Oman. At the type section (E55°49'50" and N24°06'0", Figure 1) this formation is 300 m thick and consists mainly of interbedded limestone and shale with some marls and conglomerates. Rare ophiolite and chert clasts are present at the upper part of the section. The Muthaymimah unconformably overlies the Semail Ophiolite in the type section.

In Jebel Rawdah, the Muthaymimah consists mainly of yellow to yellowish-gray colored flaggy, laminated, cherty limestone with thin interbeds of shale and marl. A petrographic examination revealed one microfacies association, peloidal foraminiferal packstone/grainstone with an abundance of lithophyllum algae (Figure 9). The packstone is typical of the Lower Tertiary rocks exposed along the northwestern Oman Mountains (for example Jebel Hafit in Abu Dhabi; Mersal, 1995). It is exposed only in the northwestern part of the jebel with an average thickness of 13 m. The Muthaymimah lies unconformably over the Simsima (Figure 10a). This boundary is marked by the presence of a 2-meter thick conglomeratic bed at Jebel Thanayes (north of Jebel Rawdah).

Depositional Environment

The thin interbedded limestone and argillaceous mudstones within the Muthaymimah in Jebel Rawdah reflects more open-marine deposition. The absence of conglomerate and/or ophiolite clasts, as seen in the Qahlah and lower Simsima, further reflects a deeper marine environment.

STRUCTURE OF JEBEL RAWDAH

Jebel Rawdah is located at the western end of the Hatta Zone which is a 50 km long, 10 km wide tectonic feature (Figure 1). The distribution and structural geometry of the stratigraphic units of Jebel Rawdah are affected by the evolution of this zone. The Hatta Zone is interpreted by Robertson et al. (1990a) as a left-lateral offset of the North Oman passive continental margin that was generated by the spreading of the Neo-Tethys Ocean. Jebel Rawdah is dominated by a long west northwest-east southeast trending ridge consisting of Hawasina and Haybi groups. The ridge slices through the Semail Ophiolite succession which is unconformably overlain to the west by Maastrichtian-Tertiary sediments.

The syncline of the main Jebel Rawdah fold is symmetric and splits the jebel nearly in half (Figure 3 and 10a). Its axis trends west northwest-east southeast and its limbs dip at nearly 50°. The corresponding main anticline lies in the western part of the jebel and extends nearly 1.5 km along its plunge (Figure 10b). Its eastern and western limbs dip at 49° and 45°, respectively. Several small-scale steeply-plunging folds, as well as crenulations, are present at the eastern end of Jebel Rawdah.

The faults in Jebel Rawdah consist of three types. The first are steeply-dipping thrust faults which bound the uplifted blocks along the main trend of the jebel (Figure 3). The movement on these faults, which dip at more than 75°, is mostly oblique. The second type are less common. They consists of vertical reverse faults which at shallower depth resemble flower structures or “palm trees”. The third consists of northeast-southwest trending normal faults, some of which have minor offsets relative to the other two types of faults. The normal faults juxtapose the lower unit of the Simsima against the underlying upper part of the Qahlah (Figure 4b).

In addition to the erosional unconformity recorded at several places between the Maastrichtian Simsima and Lower Tertiary Muthaymimah, a widespread non-conformity is recorded almost everywhere between the Qahlah basal clastics and the underlying Semail Ophiolite or Sumeini Group. An angular unconformity between the Simsima and Qahlah is present at the southeastern front of the jebel, facing the main road to Hatta (Figure 4a).

Warrak (1990), in a study of the structural style of Jebel Rawdah concluded that lateral displacement by a basement fault divided, the overburden into elongate blocks. With continued strike-slip movement of the basement fault, the blocks were displaced both vertically and horizontally, and commonly with a component of rotation on a transverse axis. The overlying sedimentary sequence, above these rising and sinking blocks, folded accordingly.

The structural geometry of the Hatta Zone and its western Jebel Rawdah extension suggests that the observed faults are part of a transpressional strike-slip or oblique-slip movement. The oldest folds in the allochthonous rocks may have formed in response to shearing deformation, whilst those involving the autochthonous rocks are mainly the product of forced folding in response to differential vertical movements of blocks cut in the allochthonous rocks below.

DISCUSSION

The geologic evolution of Jebel Rawdah started in the Late Cretaceous when allochthonous and parautochthonous rocks were obducted onto the Oman continental margin causing a foredeep to developed along the western flank of the obducted allochthon. Erosion of the allochthon followed and the lower part of the Qahlah was deposited as paleosols. This was followed by a transgression during the Maastrichtian resulting in the deposition of the Qahlah and lower part of the Simsima in beach and shorefaces settings and the upper Simsima in an open marine shelf. Near the end of the Cretaceous, an erosive episode occurred, possibly due to emergence, which marked an unconformity between the Cretaceous and Tertiary sequences. During the Early Tertiary, a rapid transgression submerged the area resulting in the deposition of pelagic slope limestone of the Muthaymimah.

Finally, the Upper Cretaceous and Tertiary rocks which form Jebel Rawdah were deformed as part of a large-scale system of periclinal folds which developed along the western edge of the Northern and Central Oman Mountains. Jebel Rawdah formed in response to shearing deformation and differential vertical movements of blocks cut in the allochthonous rocks below.

ACKNOWLEDGMENTS

Dr. G. Whittle (University of South Florida) is thanked for reviewing the manuscript. Special gratitude is due to Professors M. Warrak (Imperial College), A. Ziko (United Arab Emirates University), K. Glennie (University of Aberdeen), Dr. M. Al-Husseini, Editor-in-Chief, and two anonymous reviewers for their constructive comments and suggestions which improved our paper.

ABOUT THE AUTHORS

M.G. Salah Abou Sayed received his MSc (1989) and PhD (1995) in Petroleum Geology and Reservoir Characterization from Cairo University, Egypt. He joined BP Petroleum Development in 1984 and moved to Gulf Canada Resources Ltd., as a senior geologist in 1990. Salah worked with the Desert and Marine Research Center at United Arab Emirates University between 1992 and 1996. After a post-Doctoral Fellowship at the University of Windsor, Canada, he rejoined UAE University in the Geology Department until June 1998 when he joined Landmark Graphics Corporation in Houston. Salah has published 23 papers and presented numerous papers at international professional meetings. His research interests include reservoir characterization, basin modeling and computer applications in petroleum geology.

Mohamed A. Mersal received a MSc in 1982 and PhD in 1995 in Sedimentology from Alexandria University, Egypt. He worked as an instructor at the same University from 1976 to 1982, before joining Baroid N.L. International. Soon after he joined United Arab Emirates University. His main interests include carbonate petrography and diagenesis.