ABSTRACT

Three quantitative regional transects across the Saih Hatat and Jebel Akhdar anticlines in the Central and Southern Oman Mountains and the Northern Ghaba Basin have been constructed based on surface, well and seismic data. Interpreted large-scale structural geometries suggest that the Saih Hatat and Jebel Akhdar anticlines are basement-involved compressional structures, underlain by north-dipping, high-angle, blind, reverse faults located beneath their southern limbs. A compressional deformation event initiated in the Oligocene (constrained by apatite fission track data) involving the high-angle reverse faults is interpreted in which pre-Permian strata and Permian-through-Lower Cretaceous strata, exposed in the Saih Hatat and Jebel Akhdar anticlines, were parautochthonous - uplifted over the underlying reverse faults, and not displaced a great distance laterally. The allochthonous Hawasina and Sumeini sedimentary rocks and the Semail Ophiolite complex are interpreted to have been emplaced onto the carbonate platform during the Late Cretaceous, and have subsequently been parautochthonous during the Tertiary deformation.

The upper portion of the pre-Permian section in the Ghaba Basin consists predominantly of a thick (>4 kilometers) sequence of Cambrian-through-Silurian, predominantly non-marine to shallow-marine, clastics of the Haima Supergroup. In contrast, out of the Ghaba Basin proper in the Central Platform or Musallim High region, the Haima Supergroup is generally less than 2 kilometers thick, and interpreted to thin to the north. The fundamental difference in pre-Permian strata exposed in the Saih Hatat and Jebel Akhdar Anticline windows is the thick (>3.4 kilometers) section of Ordovician age, shallow-marine strata (Amdeh Formation) present in the Saih Hatat Anticline, but absent in the Jebel Akhdar Anticline. In our interpretation, the shallow-marine clastics exposed in the Saih Hatat Anticline represent the northern extension of the Early Paleozoic Ghaba Basin, which have been uplifted over a high-angle reverse fault in the Early Tertiary deformation event. The cross-section through Jebel Akhdar is located to the northwest of the Ghaba Salt Basin, along the Musallim High. In this area the thickness of the Ordovician strata deposited is interpreted to be less than in the Ghaba Basin. The Ordovician section is not present in the Jebel Akhdar structure - the thinned section likely eroded in a Late Paleozoic deformation event.

INTRODUCTION

The Oman Mountains are located on the southeast margin of the Arabian Plate. The mountains extend for over 700 kilometers (km) from the Musandam Peninsula to southeast of Muscat, varying in width from 30 km to greater than 125 km, and reaching elevations of up to 3 km above sea-level (Figure 1). Four major tectono-stratigraphic units have been recognized in the Central and Southern Oman Mountains (Figure 2). The oldest unit is a pre-Permian sequence correlatable to the Huqf and Haima Supergroups. The pre-Permian unit is unconformably overlain by Middle Permian to Cenomanian platform carbonates of the Hajar Supergroup. The third major tectono-stratigraphic unit is the allochthonous Semail Ophiolite Complex, including the Hawasina and Sumeini Nappes, which was emplaced onto the Hajar Supergroup platform carbonate sequence in the Late Cretaceous (Mann and Hanna, 1990). The youngest unit consists of Late Campanian-Maastrichtian to Tertiary sedimentary cover.

The frontal thrust sheets of the Hawasina and Sumeini Nappes define the southern boundary of the Central and Southern Oman Mountains (Figure 1). The northeast-southwest trending Ghaba and Fahud Basins are over-thrust by the Hawasina and Sumeini Nappes (Figure 1).

The large-scale structural geometry of the Oman Mountains and foreland is a topic which has received considerable attention in the last three decades. This study proposes a new, quantitative, structural framework for the Central and Southern Oman Mountains. The proposed structural framework is constructed using previous studies (Glennie et al., 1974; Michard et al., 1984; Hanna, 1990; Mann and Hanna, 1990; Cawood et al., 1990), updated geologic maps (1992-1993 BRGM, 1:250,000 scale series), updated industry data, new fission track thermo-chronology data, and a new model describing basement-involved foreland structures (Mitra and Mount, 1998). As with previous interpretations, this study does not solve all of the problems or answer all of the questions regarding the structural geology and evolution of the Oman Mountains. However, the interpretation does provide a relatively simple solution for the large-scale structural geometry of the Central and Southern Oman Mountains which honors the bulk of the observed structural relationships and provides a regional, quantitative, structural framework into which more detailed structural and stratigraphic studies can be incorporated.

The proposed model for the structural configuration of the Central and Southern Oman Mountains is based on three regional, quantitative, structural transects. The transect locations are shown on the schematic geologic map of Oman in Figure 1. The north-south oriented cross-sections (Transects I and II) extend from the Batinah Coast, across the Central and Southern Oman Mountains, and into the foreland area south of the Hawasina deformation front. The southeast-northwest oriented cross-section (Transect III) extends from the Batain Coast, across the Huqf Arch, and across the Northern Ghaba Basin.

In the proposed model, the Saih Hatat and Jebel Akhdar Anticlines are interpreted as large-scale basement-involved compressional structures, underlain by north-dipping, high-angle, blind, reverse faults located beneath their southern limbs. The latest phase of compressional deformation involving the high-angle reverse faults is constrained as Oligocene by apatite fission track data (Appendix). The model proposes that pre-Permian strata and Permian-through-Cretaceous strata, exposed in the Saih Hatat and Jebel Akhdar anticlines, were parautochthonous (uplifted over the reverse fault, but not displaced a great distance laterally) in the Tertiary deformation event. In addition, the allochthonous Hawasina and Sumeini sedimentary rocks and the Ophiolite Complex (which were emplaced onto the carbonate platform in the Late Cretaceous) are interpreted to have also been parautochthonous during the Tertiary deformation event.

Although the structural style of the Jebel Akhdar and Saih Hatat structures is interpreted to be similar, the pre-Permian stratigraphy exposed in the structural windows generated by the Tertiary deformation event are different. In the proposed model the primary difference in the stratigraphic succession exposed in the windows is that Saih Hatat structure deforms the northern portion of the Ghaba Basin and associated basin stratigraphy and therefore exposes a different sequence of pre-Permian rocks at the surface than are exposed in the Jebel Akhdar window.

STRATIGRAPHY

Figure 2 shows a stratigraphic column, on the left (after the excellent summary by Droste, 1997), which is simplified in the column on the right, showing the stratigraphic color scheme used in the structural transects (Figure 3). Precambrian to Ordovician correlations proposed by Mann and Hanna (1990) are utilized, in which the Ordovician Mahatta Humaid Group, exposed in the Huqf Massif, correlates with the Amdeh Formation in Saih Hatat. The Nafun Group, exposed in the Huqf Massif, is interpreted to correlate with the carbonate and siltstone stratigraphy exposed in the Saih Hatat and Jebel Akhdar windows. In the transect interpretations, the stratigraphy beneath the Nafun Group is schematically represented with constant thickness and is meant to include the Abu Mahara Group, and the Mistal and Hatat formations. Correlation of these units from the Huqf Massif to the Jebel Akhdar and Saih Hatat structures is complicated and needs refinement.

STRUCTURAL TRANSECTS

Figure 1 is a simplified version of the surface geology of the Central and Southern Oman Mountains, and the Oman Mountains foreland area, showing the location of the regional transects. The mapped surface geology indicates that in the Jebel Akhdar windows, the base-Permian unconformity (BPU) overlies Precambrian Huqf Supergroup strata. In contrast, in the Saih Hatat structure, the BPU overlies the Ordovician Amdeh Formation over most of the structure, although BPU overlies the Huqf Supergroup in the northwest portion of the structure (Figure 1). These observed stratigraphic relationships are illustrated in the proposed model.

In general, the southern portions of the transects are constrained by well data (Figure 1) and seismic data (seismic data south of the Hawasina deformation front is of the highest quality). The northern portions of the transects are constrained by surface geology. Also included in the map in Figure 1 are the proposed subsurface Ghaba Basin boundaries (dashed lines). The boundaries are based on the cross-section interpretations.

The structural transects (Figure 3) are displayed at a one-to-one (vertical-to-horizontal) scale, with a vertical scale to 20 km depth and a transect length of approximately 250 km. The stratigraphy is color-coded to the stratigraphic section in Figure 2.

Transect I

Transect I (Figure 3, top section) is a dip-section across Jebel Akhdar. The section extends from the Barka-1 well near the Batinah Coast (Figure 1), across Jebel Akhdar (through the Mistal and Saiq window outcrops, and Wadi Mi’aidin on the south flank of Jebel Akhdar), through the Jebel Madmar-1 well, and south to the Habiba-2 well along the Musallim High.

The Jebel Akhdar Anticline is interpreted to be a basement-involved compressional structure. The southern limb of Jebel Akhdar is underlain by a high-angle, north-dipping, blind, reverse fault. Apatite fission track analysis of a granite cobble from the Mistal Formation in the Kharus window (~10 km west of the transect) provides a well-constrained Oligocene age for the latest denudation of the Jebel Akhdar structure (ARCO Exploration and Production Technology Company, Internal Report, 1998). The Semail Ophiolite Complex, including the Sumeini and Hawasina groups, are interpreted to be far-displaced (>200 km) allochthonous units that were emplaced in the Late Cretaceous. In the Tertiary basement-involved compressional event, the Semail Ophiolite complex is interpreted to have been parautochthonous (uplifted over the reverse fault, but not displaced a great distance laterally).

Likewise the Jebel Madmar Anticline is interpreted as a basement-involved structure developed in response to a small amount of slip over a high-angle (~45°) reverse fault which extends into the basement. Recent models of foreland basement-involved structural styles (Mitra and Mount, 1998), which will be discussed in the next section, indicate that structures which are highly asymmetric to overturned with geometries similar to Jebel Madmar, are commonly associated with basement-involved structures.

Transect II

Transect II (Figure 3, middle section) is a dip-section across the Saih Hatat structure. The section extends from Muscat in the north, across the Saih Hatat structure (through the Saih Hatat window and Wadi Amdeh on the south flank of Saih Hatat), through the Al-Hammah-1 well, across Jebel Madar and south into the Wadi Andam area along the axis of the underlying Ghaba Basin (Figure 1).

Similar to the Jebel Akhdar interpretation, the Saih Hatat Anticline is interpreted as a basement-involved compressional structure. The southern limb of the Saih Hatat structure is underlain by a high-angle, north-dipping, blind, reverse fault. The Ordovician Amdeh Formation, exposed in Wadi Amdeh on the south flank of the Saih Hatat structure, is correlated to the thick Ordovician section observed in well and seismic data to the south, implying that the Ghaba Basin extended to the north, at least into the Saih Hatat area. The northern end of Transect II crosses the Ghaba Basin boundary at an oblique angle (Figure 1) and is structurally similar to the northwest basin boundary in Transect III in the subsurface between Wadi Andam and the Semail Gap.

The southern portion of Transect II extends into the Ghaba Salt Basin. Note that there are a series of diapiric structures cored by the Cambrian Ara Formation. The northernmost diapiric structure is the Jebel Madar structure - a domal feature 8 km across - exposing Permian through Cretaceous shelf carbonates at the surface with approximately 500 meters (m) relief above the desert plain (Figure 4).

Transect III

Transect III (Figure 1) is an oblique dip-section across the northern end of the Ghaba Basin (Figure 3, bottom section). The section extends from the Batain Coast and the Jebel Fayah-1 well, across the Huqf Arch, through two wells on the western-flank of the Huqf Arch (Remlat Al-Wahiba-1 and Najah-1), into the Ghaba Basin and through the Al-Hammah-1 well (where Transect III ties Transect II), across Wadi Andam, across the southern end of the Semail Gap, terminating at Jebel Akhdar (where Transect III ties Transect I).

The geometry of the western flank of the Huqf Arch (the eastern flank of the Ghaba Basin) is tightly-constrained by well and seismic data in Transect III. The western flank of the Ghaba Basin is less constrained as it is overlain by the Semail Ophiolite complex and not well-imaged in seismic data. Projection of the basin margin from the south, where it is better-defined in seismic data, suggests that the basin extends to the north to the east of the Semail Gap (Figure 1). The western margin of the Ghaba Basin was likely controlled by an early high in the Jebel Akhdar vicinity - similar to, but not as significant as, the Huqf Arch.

At the northwest end of Transect III the Semail Gap structure is interpreted as being underlain by a west-dipping, high-angle, reverse fault. In essence the southern flank of Jebel Akhdar and the Semail Gap dip panel are generated by drape of Permian-through-Cretaceous strata over the frontal and lateral edges of a corner formed by the intersection of high-angle, deep-seated basement faults. In the interpretation very little strike-slip deformation is required along the Semail Gap, the predominant deformation being uplift along the reverse fault.

DISCUSSION

Basement-involved Structural Styles

In the proposed model for the Central and Southern Oman Mountains regional structural framework, the latest major deformation event (Oligocene) is interpreted as a basement-involved compressional event with the southern limb of the Jebel Akhdar and Saih Hatat structures underlain by north-dipping, high-angle, blind, reverse faults, similar to the model in Figure 5a. The basement-involved structural model (Mitra and Mount, 1998) was developed to assist seismic interpretation in foreland areas of foldbelts, such as the Rocky Mountains of the Western United States and the foreland of the Andes Mountains in South America. In the model, deformation over a deep-seated detachment results in a large-scale anticlinal structure. Slip over a synclinal bend creates a backlimb dip panel and folding at or near the fault-tip results in a forelimb dip panel. The forelimb dip panels associated with basement-involved structures are commonly steeply-dipping to overturned, and can show thickening or thinning of the strata involved depending on the stratigraphic level and the amount of slip on the fault.

In Figure 5b the foreland basement-involved model is applied to the Tertiary deformation event in the Oman Mountains. The Ophiolite complex, including the Hawasina and Sumeini Nappe complex, are interpreted to have been emplaced in the Late Cretaceous (Mann and Hanna, 1990) and deformed essentially as another stratigraphic layer during the Tertiary basement-involved deformation. The present-day erosion level is indicated in Figure 5b. In the Saih Hatat structure, Ordovician and Huqf Supergroup units are exposed beneath the BPU. In the Jebel Akhdar structure, only Huqf Supergroup units are exposed beneath the BPU.

The application of the basement-involved structural model to the Oman Mountains is intended to satisfy observed, large-scale, first-order structural and stratigraphic relationships. The data available are not sufficient to definitively position the blind reverse faults underlying the southern limbs of the Jebel Akhdar and Saih Hatat structures. The interpretations presented show that displacement on the reverse faults have decreased to zero near the basement-cover interface. However, similar basement-involved solutions in which displacement extends further into the sedimentary cover, as well as interpretations in which displacement decreases to zero prior to the fault-tip reaching the basement-sediment interface, have been considered and are viable. The detailed structural situation is far more complicated with reactivated faults, unconformities, stratigraphic complexities, etc. (for example, Mann and Hanna, 1990).

Basement-involved Model Versus Detached Fault-bend Fold Model

The basement-involved interpretation for the Central and Southern Oman Mountains contrasts with some earlier interpretations in which the Jebel Akhdar and Saih Hatat structures are interpreted as detached fault-bend folds. In the detached model (Figure 6a), a fold is created by slip over a bend in a fault: a fault-bend fold. In the fault-bend fold model, slip along the fault is displaced to the left. The slip displaced to the left (or into the foreland) must be accounted for if a detached fault-bend fold model is used in a structural interpretation. In the basement-involved model (Figures 5 and 6b), a fold is again created by slip over a bend in a fault; however, all of the slip is consumed in folding near the fault-tip and no slip is sent off to the left, or into the foreland.

Figure 7 shows three examples of previous interpretations in which a fault-bend fold model is applied to the Central and Southern Oman Mountains. In each case a substantial amount of slip is propagated to the south from the Jebel Akhdar or Saih Hatat structures. In the Hanna (1990) and Cawood et al. (1990) interpretations of Saih Hatat (Figure 7), the slip propagated to the south generates Jebel Madar as a frontal fold structure. Seismic data indicates that the Jebel Madar structure is a salt diapir structure and not a frontal fold (Figure 4). Therefore, the slip propagated to the south on a low-angle detachment in these interpretations needs to be accounted for by some other mechanism. In the proposed basement-involved interpretation presented in Transects I and II (Figure 3), the observed data are honored without requiring a large amount of slip in the Tertiary deformation event.

“Corner Problem” and Semail Gap Interpretation

Another observation that is consistent with the proposed basement-involved model for the Tertiary deformation event in the Central and Southern Oman Mountains is the geometry of the southeast corner of Jebel Akhdar. The exposed southeast corner of the Jebel Akhdar structure, in the vicinity of Wadi Mi’aidin (Figure 1), consists of a nearly perpendicular intersection of a northwest-southeast striking, southwest-dipping panel of Permian to Cretaceous age carbonates (the southern limb of Jebel Akhdar) with a northeast-southwest striking, southeast-dipping panel of the same units (the northern continuation of this panel forms the eastern boundary of the Jebel Akhdar structure). A topographic low, the Semail Gap, runs immediately east of the northeast-southwest striking dip panel. In previous studies, the Semail Gap has been interpreted as being underlain by normal, as well as strike-slip faults. In our basement-involved structural model for the Jebel Akhdar structure it is interpreted to be underlain by a high-angle, reverse fault.

In previous normal fault models for the Semail Gap, observed thinning of the Hawasina was attributed to the presence of a northeast-southwest striking, southeast-dipping normal fault in the subsurface beneath the dip panel. However, as described earlier in the basement-involved structural styles section, both thinning and thickening of the cover sequence are predicted in the steep-limb, dip panel of basement-involved structures. In the model proposed here, the observed Hawasina thinning is interpreted as the response of the cover sequence to displacement on a northeast-southwest striking, northwest dipping, high-angle, basement reverse fault underlying the Semail Gap dip panel (Transect III, Figure 3).

In the past, the Semail Gap has also been interpreted as being underlain by a strike-slip fault, in part due to the apparent offset in map view of the southern limbs of the Saih Hatat and Jebel Akhdar structures. Furthermore, if the Jebel Akhdar structure is underlain by a south-vergent, large displacement, low-angle thrust fault, then a large displacement, strike-slip fault is required as a lateral termination of the structure at depth along the Semail Gap. However evidence of large strike-slip displacement in the exposed Permian through Cretaceous carbonate section is not observed along the Semail Gap dip panel. Further, it is difficult to reconcile the geometry of the Jebel Akhdar southeast “corner” (a sharp, yet continuous, 90° bend in the trend of the Permian through Cretaceous carbonate section) as being generated by the intersection of a low-angle thrust fault and a large displacement strike-slip fault. If, however, the Jebel Akhdar structure is underlain by a corner created by high-angle, deep-seated faults, then drape over the frontal and lateral edges is predicted. This is consistent with Stearn’s (1978) analysis of Western United States basement-involved structures. The mapped geometry of the southeast corner of the Jebel Akhdar structure is consistent with the proposed basement-involved structural style interpretation for the Jebel Akhdar structure and suggests that it is underlain by a high-angle, reverse fault.

Timing of Deformation - Apatite Fission Track Analysis Results

In an attempt to constrain the timing of the Tertiary basement-involved deformation event in the Central Oman Mountains, apatite fission track analysis and modeling were performed on a granite cobble collected from the Precambrian Mistal Formation exposed in the core of Jebel Akhdar (Wadi Beni Kharus - approximately 10 km west of Transect I). Results of the modeling indicate that the most likely thermal history consists of two stages: an initial rapid cooling phase at 30 million years before the Present (Ma) from >120 degrees Centigrade (°C) to 60°C (at ~20°C/million years (my)), followed by a second phase of slow cooling from 25 Ma to Present at ~2°C/my. Therefore, the Wadi Beni Kharus granite cobbles in the Oman Mountains experienced more than 100°C of Middle-to-Late Cenozoic rapid cooling likely related to Oligocene uplift and erosion, followed by slower Neogene cooling. Greater than 3 km of denudation is interpreted, most of which occurred from 30 to 25 Ma. Fission track data and interpreted model results are summarized in the Appendix.

Previous Interpretations

The large-scale structural configuration of the Central and Southern Oman Mountains was recognized in the early 1970’s by Glennie et al. (1974) (Figure 8a). Their schematic interpretation is similar to our proposed quantitative interpretation in that there is no large-scale shortening in the Tertiary deformation event, and there is an inferred relationship between the pre-Permian strata exposed in the Huqf outcrop area and those exposed in the windows of Jebel Akhdar. Michard et al. (1984) advanced the schematic interpretation a step further by inferring a relationship between the Cambrian-Silurian strata in the Ghaba Basin and the Ordovician Amdeh Formation exposed on the south flank of Saih Hatat. Our proposed structural framework model builds on the Glennie et al. and Michard et al. interpretations in presenting a scaled, quantitative, solution for the Central and Southern Oman Mountains in which stratigraphic relationships between the foreland and the mountains, and between the exposures in the Akhdar and Hatat windows, are proposed.

CONCLUSIONS

A new model for the structural framework of the Central and Southern Oman Mountains, based primarily on three regional quantitative transects, is presented. The model proposes that Jebel Akhdar and Saih Hatat are basement-involved compressional structures created in an early-to-middle Tertiary deformation event. The southern limb of each structure is underlain by a north-dipping, high-angle, blind reverse fault. The southeast-dipping Semail Gap dip panel is also underlain by a high-angle reverse fault and was generated through predominantly dip-slip deformation (minimal strike-slip deformation).

The primary difference between the Saih Hatat and Jebel Akhdar structures is that the Saih Hatat structure deforms the northern portion of the Ghaba Basin, exposing the Ordovician Amdeh Formation. In contrast, the Jebel Akhdar structure deforms the western-bounding flank of the Ghaba Basin, and only Precambrian strata are observed beneath the base-Permian unconformity (no Paleozoic strata).

The timing of the basement-involved deformation in the Central Oman Mountains as constrained by apatite fission track analysis and modeling of results is interpreted to be Oligocene.

The quantitative structural framework presented here builds on earlier schematic models (Glennie et al., 1974; Michard et al., 1984) for the Central and Southern Oman Mountains.

APPENDIX: Fission Track Data and Interpreted Model Results

ACKNOWLEDGEMENTS

The Sultanate of Oman Ministry of Oil and Gas is thanked for providing data and insight into the large-scale framework of the Central and Southern Oman Mountains. The authors thank Salim Al-Maskery (Oman Geotours) for introducing them to the Paleozoic and Precambrian of the Huqf outcrop and Saih Hatat. ARCO Oman Inc. and Partex (Oman) Inc. (Carlos Neves, in particular) are thanked for providing ideas, support, and logistics for the study. Dr. David S. Campbell, ARCO Middle East Exploration Manager, is thanked for sacrificing time to collect the Wadi Beni Kharus apatite fission track sample. Jeff Corrigan (ARCO Exploration and Production Technology Company, Exploration Research and Technical Services) is thanked for initiating the apatite fission track analysis project and John Murphy (University of Wyoming Fission Track Lab) provided the fission track data. Two anonymous reviewers are thanked for providing suggestions to improve the paper. The authors thank Dr. Husseini and Heather Paul-Pattison for providing input and suggestions to improve the paper and Gulf PetroLink for redesigning the figures.

ABOUT THE AUTHORS

Van S. Mount received a BA degree in Geology from Hamilton College and a PhD in Structural Geology from Princeton University. He joined the Structural Geology Research Group at ARCO Exploration and Production Technology Company in 1989 where his work concentrated on quantitative analysis and interpretation of complexly deformed prospect-scale structures. Van is presently at ARCO International Oil and Gas Company where he is working as an Exploration Geologist in the Middle East Exploration Group.

Roderick I.S. Crawford graduated from the University of Wales with a BSc in Geophysics. Initially he was employed in seismic acquisition and processing at Geoteam Ltd., UK, later moving into seismic interpretation. As a Consultant, Roderick specialized in mapping structurally complex areas. He joined ARCO British Ltd. in 1991 where he has continued to work challenging exploration and development projects. Presently Roderick is seconded to the Middle East New Ventures and Operations Group, ARCO International Oil and Gas Company.

Steven C. Bergman is a Volcanologist-Geochronologist-Hard Rock Petrologist-Tectonicist who has worked at the ARCO Exploration Research Laboratory in Plano, Texas for the last 18 years. He received a BSc in Geology from the University of Dayton in 1977, and MA and PhD degrees in Geology from Princeton University in 1979 and 1982, respectively. Steve was a visiting scholar at the University of Cambridge Bullard Laboratory in 1996-1997. He is mainly interested in increasing hydrocarbon exploration efficiency by the integration of field geology and high-tech laboratory data to characterize basin evolution, thermal evolution, and other elements of the petroleum system.