ABSTRACT

Detailed field mapping and structural studies in the Jebel Auha-Jebel Huwayyah area northeast of Al-Ain indicate that folding of neoautochthonous sedimentary rocks produced the north-northwest-trending Huwayyah Anticline. The anticline at the surface is composed of the Maastrichtian Qahlah and Simsima formations unconformably overlain by shallow-marine carbonate rocks that are correlated on faunal grounds with the Middle Eocene Dammam Formation. The investigation of the Huwayyah Anticline has identified three microfacies of bioclastic packstone, nummulitic packstone, and nummulitic packstone-grainstone in the local Dammam Formation. Diagenesis in the form of silicification, cementation, recrystallization, dissolution, compaction and neomorphism is widespread.

The Huwayyah Anticline is a fault-propagation fold above a thrust ramp. The ramp developed from a pre-existing Late Cretaceous basal thrust within the Semail Ophiolite on the Oman Mountain Front. The anticline was formed as a result of regional compressive deformation due to rejuvenation of the Late Cretaceous thrust in post-Middle Eocene times. Westward-directed high-angle reverse faults of Jebel Auha trend parallel to the fold axis of the anticline. The Auha faults probably originated as west-dipping thrusts on the western flank of the anticline and were subsequently rotated to their present attitude as the flank of the anticline became steeper due to compression from the east.

INTRODUCTION

Maastrichtian to Eocene neoautochtonous sedimentary rocks crop out as a discontinuous belt around the westernmost foothills of the Northern Oman Mountains (Figure 1). They were deposited unconformably on nappes of the obducted Late Cretaceous Semail Ophiolite and associated sedimentary and volcanic rocks of the Hawasina and Haybi complexes (Glennie et al., 1974; Searle et al., 1983; Nolan et al., 1990; Skelton et al., 1990). The neoautochthonous sequence exposed near the mountain front is folded, faulted and uplifted in a complex pattern due to Tertiary-age deformation. The Jebel Auha-Jebel Huwayyah area (Figure 2), 15 kilometers (km) northeast of Al-Ain provides examples of structures (named here as the Al-Ain Tertiary Folds) produced by the post-obduction deformation of the Late Cretaceous-Tertiary neoautochthonous sedimentary rocks.

Until now, deformation accompanying these neoautochthonous ‘post-nappes’ sedimentary rocks has received little attention. The Al-Ain area contains good exposures of the post-nappes rocks that are easily accessible and offer opportunities to investigate the Tertiary deformation. An understanding of the structural configuration of this leading-edge area is of interest from the perspective of hydrocarbon exploration as several gas-condensate fields, such as Margham and Saja’a, occur in this geological environment (Figure 1). The fields are trapped in anticlines along the Mesozoic thrust fault that separates a nearly undeformed foredeep basin to the west from the fold and thrust belt to the east (Dunne et al., 1990; Mount et al., 1995). Investigations by Mount et al. (1995) and O’Donnell et al. (1995) have shown that the complex nature of these subsurface structures is difficult to image seismically. Therefore, the field investigation of areas such as Jebel Auha-Jebel Huwayyah contributes to our understanding of the structural and stratigraphic evolution of this hydrocarbon habitat. Other parts of the Northern Oman Mountain Front that have been investigated in this manner are Jebel Malaqet-Jebel Mundassa (Noweir and Eloutefi, 1997), the Faiyah Range (Noweir et al., 1998), Jebal Rawdah (Sayed and Mersal, 1998), and Jebel Hafit (Noweir, 2000).

The main objective of this investigation was to describe the structural deformation in the Jebel Auha-Jebel Huwayyah area as a representative of the Al-Ain Tertiary Folds. In addition, the petrology and paleontology of the Middle Eocene Dammam Formation were also studied. The field study was carried out through detailed field mapping using aerial photographs at a scale 1:20,000 as base maps.

Previous investigations of the Jebel Auha-Jebel Huwayyah area were part of the general geologic mapping of the Al-Ain area (Hunting Geology and Geophysics Ltd., 1979; Warrak, 1986), or regional stratigraphic studies (Skelton et al., 1990; Hamdan, 1990; Smith et al., 1995; and Alsharhan and Nasir, 1996). This earlier work provided useful information on the lithology of the Maastrichtian Qahlah and Simsima formations but did not deal with the detailed structure of the area.

STRATIGRAPHY

The Jebel Auha-Jebel Huwayyah area is the only part of the Northern Oman Mountains where rocks of the Maastrichtian neoautochthonous sedimentary sequence occupy the core of a Tertiary structure. The exposed rocks range in age from Late Cretaceous (Maastrichtian) to Middle Eocene (Bartonian). The sequence consists of the Qahlah Formation, overlain by the shallow-water to deep-water marine deposits of the Simsima Formation, and by the shallow-marine Dammam Formation (Figure 3). The following descriptions of the Qahlah and Simsima formations are mainly based on the work of Skelton et al. (1990), Hamdan (1990), Smith et al. (1995), and Alsharhan and Nasir (1996). The description of the Dammam Formation is based on petrologic and paleontologic studies made during the present investigations.

Qahlah Formation

The oldest rocks in the study area belong to the Maastrichtian Qahlah Formation. The Formation was formally defined by Glennie et al. (1974). In the type section, the Formation is about 140 meters (m) thick. At Jebel Auha-Jebel Huwayyah only the upper 24 m of the Qahlah is exposed (Figure 3). The lowest exposure consists of poorly exposed and weathered khaki-colored shales, overlain by cross-bedded pebbly brown sandstones and chert conglomerates. The succession passes into thinly bedded, richly fossiliferous marls and limestones that contain abundant specimens of the benthic foraminifera Loftusia.Glennie et al. (1974) assigned a Maastrichtian age to the Qahlah Formation based on the presence of Loftusia sp. A two- to three-meter-thick bed of unfossiliferous, red chert-pebble breccia-conglomerate caps the Qahlah Formation (Figure 4a). The Formation shows a general and gradual decrease in thickness from south to north and it thins out completely toward the extreme north of the study area. This lateral variation in thickness is interpreted to be the result of structural growth.

Simsima Formation

Nolan et al. (1990) designated Jebel Faiyah as the surface type section of the Simsima Formation. According to Sayed and Mersal (1998), the Formation is 85 m thick in the type section but Noweir et al. (1998) measured its thickness as 140 m at Jebel Faiyah. In the study area, as in many parts of the northwestern foreland of the Oman Mountains, the Maastrichtian Simsima Formation conformably overlies the Qahlah Formation (Figure 4b). Here the Simsima Formation is 20 to 26 m thick and consists of two units. The lower 5 to 8 m is composed of shelly, foraminiferal packstone of shallow-water origin overlain by fine-grained deep-water packstone 15 to 18 m thick, that contains chert nodules and scattered orbitoids (Figure 3). The variations in lithology may reflect deposition on a shelf and distal outer shelf to slope area. The lower unit is exposed in the southern part of the area, whereas the upper unit crops out in the north.

Dammam Formation

The Dammam Formation was named by Bramkamp in 1941 (see Powers et al., 1966) for a unit of outcropping shale, limestone and marl in the Dammam Dome of eastern Saudi Arabia.

In the Jebel Auha-Jebel Huwayyah area, the Simsima Formation is unconformably overlain by limestone intercalated with thin-bedded marl that grades upward into soft yellow marl for a total thickness of 30 to 40 m (Figure 3). The yellow marl is, in turn, overlain with sharp contact by 10 to 20 m of hard, gray, dolomitic limestone (Figure 4c). The succession has been identified on faunal evidence and lithological characteristics as a stratigraphic equivalent of the Dammam Formation. The microfauna (Figures 5a–d) include Nummulites sp. and Discocyclina sp. and planktonic foraminifera (for example, Truncorotaloides sp.) indicating a Middle Eocene (Bartonian) age.

The main constituents of the carbonate rocks are larger foraminifera, planktonic forams, pelecypods and echinoderms. In addition, intraclasts, a few algal and bryozoan fragments, and micritized shell fragments were recorded. The fossil material and clasts are for the most part embedded in a fine-grained micrite that has been partly recrystallized to microsparite. Microscopic examination of 18 thin-sections representing various lithologies of the Dammam Formation led to the recognition of three microfacies associations. These are bioclastic packstone, nummulitic packstone, and nummulitic packstone-grainstone. These facies changes within the limestone of the Dammam Formation are attributed to the relative abundance of larger foraminifera (for example, Nummulites) and dwarfed megafossils (mainly pelecypods and echinoderms).

Most of the recorded carbonate facies of the Dammam Formation have a grain-supported fabric. The grains are generally fine- to coarse-grained and moderately sorted and, in places, show a marked orientation that indicates an active shallow-marine environment. Various types of grain contacts are present, for example, long, point and concavo-convex. Porosity is generally poor and is represented mainly by small irregular vuggy pores. Diagenesis has had a profound affect on the carbonates and has modified their texture, structure and petrophysical properties. The diagenetic phases of silicification, cementation, recrystallization, dissolution and compaction were recorded. Evidence of aggrading neomorphism was observed as microsparite calcite that occurs in larger foraminifera and surrounds the micritic groundmass in most samples. Silicification can be traced by the presence of fibrous chalcedonic quartz partially replacing bioclasts, especially Nummulites.

The Middle Eocene Dammam Formation overlies the Maastrichtian Simsima Formation at Jebel Auha. The absence of Paleocene to Lower Eocene sedimentary rocks may suggest the presence of a paleohigh that did not receive sedimentation during Maastrichtian to Early Eocene times. The same unconformable relationship between the Dammam and Simsima formations has been reported by Noweir et al. (1998) in the Faiyah Range, which is located about 100 km north of the study area.

STRUCTURE

Late Maastrichtian and Tertiary neoautochthonous sediments and the underlying allochthonous rocks of the Semail Ophiolite and the Hawasina complex were involved in Tertiary-age folding along the western leading edge of the Northern Oman Mountains. In the Al-Ain area (see Figure 2), the folding produced sets of folds that have a strong north-northwesterly trend parallel to the mountain front. The style of the Al-Ain Tertiary Folds varies according to the type and stratigraphic position of the rock units involved in the folding. Folds were developed in both the allochthonous and neoautochthonous units at Jebel Malaqet and Jebel Mundassa. Elsewhere, for example at Jebel Huwayyah, Maastrichtian and Tertiary neoautochthonous rocks were folded but at Jebel Hafit and Jebel Qatar only the neoautochthonous Tertiary sediments were involved.

Macrostructures

The structure of the Jebel Auha-Jebel Huwayyah area is dominated by the north-northwest-trending Huwayyah Anticline that involves the Qahlah, Simsima and the Dammam formations. High-angle reverse faults and strike-slip faults affect the western part of the area.

Huwayyah Anticline

The Huwayyah Anticline is a U-shaped structure (Figures 6 and 7) from 8 to10 km in length and 6 to 8 km wide. It has the form of a broad, southerly plunging anticline. Its asymmetric box form is seen in cross-sections A-A’, B-B’, and C-C’ of Figure 8. The geological map (Figure 6) and the cross-sections show that the anticline has a steep western flank (sometimes vertical to overturned), a less-steep eastern flank and a flattened crest that plunges gently southward. Mesostructures (see below) are present in the steep western flank of the anticline.

High-angle Reverse Faults

Two high-angle reverse faults, post-Maastrichtian in age, are present in the study area. Both faults occur on the steep western flank of the anticline. They trend in a north-northwesterly direction parallel to the fold axis and dip as much as 75° to the west and west-northwest. The main reverse fault, named here the Auha Fault, can be traced for about 8 km along the eastern margin of the Jebel Auha strike ridge (Figures 6 and 9). The Auha Fault causes the repetition of the Maastrichtian Qahlah and Simsima formations. A second reverse fault (Figure 6 and Figure 8, B-B’), lies east of the Auha reverse fault and also causes the repetition of the Simsima Formation. These faults probably originated as ordinary west-dipping thrusts on the western flank of the anticline and were subsequently rotated to their present attitude as the flank of the anticline became steeper due to compression from the east.

Strike-slip Faults

Two strike-slip faults cut the Qahlah and Simsima formations in the steep western flank of the Huwayyah Anticline in Jebel Auha (Figure 6). They are oriented east-northeast approximately perpendicular to the general trend of the Huwayyah structure. Another strike-slip fault occurs near the southern end of Jebel Haqalah (Figure 6) affecting the Eocene Dammam Formation. The displacements of these faults are of the order of a few meters to several meters and the sense of rotation of bedding into the faults suggests dextral movement.

Mesostructures

Two types of mesostructures were recognized in the Jebel Auha-Jebel Huwayyah area mainly on Jebel Haqalah. These are mesofolds and minor thrusts that have affected the Eocene Dammam Formation.

Mesofolds

Mesofolds have affected mainly the marl and limestone beds of the Dammam Formation. The folds are symmetric to asymmetric, plunging, steeply dipping and crumpled (Figure 10a) and generally have a near box shape (Figure 10b). The small-scale box-shaped folds perhaps mimic the shape of the Huwayyah Anticline (Figure 8). Some chevron-shaped mesofolds were also found at Jebel Haqalah in the Dammam Formation.

Minor Thrusts

Several minor thrusts affect the Dammam Formation in the steep western limb of the Huwayyah Anticline. They have a general strike direction of N20°E and an average dip of 35°NW.

MODE OF FORMATION

The leading edge of the Oman Mountains fold and thrust belt northeast of Al-Ain is marked by a series of gentle broad folds, such as the Huwayyah Anticline, that deform the outcropping Maastrichtian-Tertiary neoautochthonous sedimentary rocks.

We propose a structural model to explain the relationship between thrusting and leading-edge folds (see Boyer, 1986), where a thrust dying up-section, as well as along strike, forms a box-shaped anticline (Figure 8). Fault-propagation folding described by Suppe and Medwedeff (1990) and Mitra (1990) is a widely accepted kinematic explanation for folds that form during lateral and up-section propagation of thrust faults (Figure 11). A ramp fault propagated upward from a pre-existing Late Cretaceous basal thrust that is assumed to lie within the Semail Ophiolite and which originated during emplacement of the Ophiolite. The ramp fault is a blind thrust that does not reach the surface. The upward propagation of the fault led to the folding of the overlying neoautochthonous sedimentary rocks that form the steep flank of the fault-propagated Huwayyah Anticline. The fault-propagated anticline is characterized as a strongly asymmetric and west-verging fold with steeply dipping to overturned beds in the leading (western) limb due to compression from the east. A similar geometry has been recognized by Noweir and Eloutefi (1997) in the Malaqet-Mundassa Anticline (Figure 2), about 20 km southeast of the Huwayyah Anticline, and by Noweir (2000) in the Hafit Anticline south of Al-Ain. In the case of the Malaqet-Mundassa structure, the imbricate fault that splayed from the basal thrust reaches the surface.

The geometry of leading-edge folds requires the presence of a basal detachment zone. Various studies indicate that detachment zones are present in the Oman Mountains and its western foreland. For example, deep seismic soundings in the foreland in the United Arab Emirates provide evidence of extensive post-Cretaceous tectonism in the form of asymmetric folds and westward-directed thrusting (Ricateau and Riche, 1980). An inferred subsurface system of folds and thrust faults that borders the western flank of the Oman Mountains near Al-Ain is based on a structural interpretation of seismic reflection data (Boote et al., 1990; Dunne et al., 1990; Warburton et al., 1990). Similarly, moderate to steep easterly dipping imbricate thrust faults and tight, double-plunging folds have been documented by Woodward (1994) based on detailed mapping of the subsurface structures in the Al-Ain area of the eastern United Arab Emirates (Figure 12). Thus, the evidence exists for the presence of thrust faults in the study area necessary for the generation of thrust-induced box folds.

TIMING OF DEFORMATION

In the Northern Oman Mountains, two major compressional events have been recognized. The first resulted from the Late Cretaceous (Coniacian-Maastrichtian) obduction of the Semail Ophiolite and associated sedimentary and volcanic rocks (Sumeimi Group and Hawasina, and Haybi complexes) onto the eastern margin of the Arabian Platform (Glennie et al., 1974; Lippard et al., 1986). As a result of the obduction, a flexural foredeep developed along the western margin of the Oman Mountains and was filled with Maastrichtian to Tertiary sediments (see Noweir et al., 1998). Following the obduction phase, a second compressional event occurred in Late Eocene to Miocene time. This event was responsible for the folding and short-distance thrusting of the allochthonous units and of the Upper Cretaceous and Tertiary neo-autochthonous cover in the foredeep basin (Searle et al., 1983; Searle, 1985; and Dunne et al., 1990).

The Huwayyah Anticline is a unique example of the Al-Ain Tertiary Folds, in which evidence of Tertiary compressional structures was found. The fold is cored by the Upper Maastrichtian Qahlah Formation and flanked by the Upper Maastrichtian Simsima Formation and the Middle Eocene Dammam Formation. Evidence exists of Tertiary thrusting that affected the neoautochthonous sucession which post-dated the emplacement of the Semail Ophiolite. This phase of thrusting of the Late Cretaceous-Tertiary rocks is seen on seismic profiles west of the Northern Oman Mountain Front (Dunne et al., 1990; Boote et al., 1990) and demonstrated in the Jebel Sumeini area (Figure 12) by Searle et al. (1990). The Tertiary compressional event caused the reactivation of the Late Cretaceous basal ramp fault to form the Huwayyah Anticline and associated thrust faults and strike-slip faults. It is possible that the second event can be correlated with the Zagros Orogeny in Iran (Ricateau and Riche, 1980; Searle et al., 1983; Searle, 1985; Searle et al., 1990).

The Dammam Formation in the Huwayyah Anticline is affected by the mesostructures and macrostructures. This indicates that the deformation took place after the rocks of the Middle Eocene Dammam Formation were deposited and that the formation of the Huwayyah Anticline occurred in post-Middle Eocene times. Noweir et al. (1998) obtained a similar deformational age for the Faiyah anticline, about 100 km to the north. Detailed mapping and structural analysis of the Upper Cretaceous-Tertiary rocks of the neighboring structures in the Al-Ain region, have revealed that deformation occurred mostly in post-Middle Eocene times. The exception is the Hafit structure, south of Al-Ain (Figures 1 and 12), that was formed by post-Miocene movements (Hunting Geology and Geophysics Ltd., 1979; Boote et al., 1990; Noweir, 2000).

CONCLUSION

The Huwayyah Anticline is unique in the Al-Ain area by being developed in Maastrichtian-Tertiary neoautochthonous sedimentary rocks. The outcropping formations in the anticline are the Maastrichtian Qahlah and Simsima formations unconformably overlain by shallow-marine carbonate rocks correlated on faunal grounds with the Middle Eocene Dammam Formation. This investigation of the Huwayyah Anticline has identified three microfacies in the local Dammam Formation. They are bioclastic packstone, nummulitic packstone, and nummulitic packstone-grainstone. Diagenesis in the form of silicification, cementation, recrystallization, dissolution, compaction and neomorphism is widespread.

The Huwayyah Anticline is an asymmetric structure having a steep western flank, a nearly flat crest, and a gently dipping eastern flank. The structural interpretation suggests that the anticline is a fault-propagation fold, whereby the fold developed above a ramp thrust that propagated from a pre-existing basal thrust within the Semail Ophiolite. The steep western flank has been affected by westward-directed high-angle reverse faults due to westerly directed compression. The anticline was formed by compressional deformation in post-Middle Eocene time by the reactivation of Late Cretaceous thrusts within the Semail Ophiolite.

ACKNOWLEDGMENTS

The authors thank United Arab Emirates University, Al-Ain for providing the necessary facilities for this work and O.A. Osman (UAE University) for identifying the fossils. The helpful comments of anonymous reviewers and the editors of GeoArabia have greatly improved the manuscript. The graphics were designed by Gulf PetroLink.

ABOUT THE AUTHORS

M. Atef Noweir has recently returned to Tanta University, Egypt as Lecturer in Structural Geology. Since 1993, Atef had been on secondment to the Department of Geology of the United Arab Emirates University, Al-Ain. He has a BSc (1977) and MSc (1983) in Geology from Tanta University and in 1990 the University of Missouri-Rolla awarded him a PhD in Structural Geology. His is a member of the GSA, AAPG, IASTG, MAS, and GSE. Atef is interested in field mapping, structural analysis, and balancing cross-sections of foreland fold and thrust belts.

Abdulrahman S. Alsharhan is Dean of the Faculty of Science at the United Arab Emirates University, Al-Ain. He received his MSc (1983) and PhD (1985) in Geology from the University of South Carolina. His current research interests include Holocene coastal sabkhas of the UAE, and geological environments of hydrocarbons in the Arabian Gulf and adjacent areas. In 1998, he co-authored with A.E.M. Nairn, Sedimentary Basins and Petroleum Geology of the Middle East, and edited Quaternary Deserts and Climatic Change. He is a member of the Advisory Board of GeoArabia.