ABSTRACT

The Jabal Qumayrah area, 50 km ESE of Al Ain and Buraimi, preserves a culmination of Jurassic and Cretaceous continental slope deposits (Sumeini Group) that was emplaced during the Late Cretaceous onto the Oman margin with other Neo-Tethyan units and the Semail Ophiolite. Almost uniquely in the Oman Mountains, Jabal Qumayrah also contains outcrops of gypsum and anhydrite that occur as a central complex from which laterally discontinuous linear and arcuate outcrops extend up to 4 km to the northwest and south. The gypsum and anhydrite bodies contain sedimentary clasts and rafts, which show close affinities with the local Sumeini Group host rock. There are no sedimentary features that indicate the evaporites were deposited in situ, either as part of, or unconformably overlying the Sumeini Group. Boundaries with the host rock are either high-angle faults or steep and intrusive, with significant dissolution of host rock limestones. Two gravity transects across the area indicate the areas of gypsum and anhydrite lie on a gravity low, compatible with an elongated, high-level body concentrated along the main N-S axis of the Jabal Qumayrah dome. Taken together, these features point towards an intrusive origin for the evaporite bodies in Jabal Qumayrah. While the sub-surface is poorly constrained, the central complex is interpreted as representing the deeply weathered top of a salt diapir, whose emplacement had a strong tectonic fault-driven component. The smaller, discontinuous exposures to the northwest and south are interpreted as pods of gypsum and anhydrite that were injected along faults. The absence of other evaporite minerals, in particular halite, is attributed to deep weathering and dissolution similar to that seen at the surface-piercing salt domes of the Ghaba Salt Basin in central Oman.

In the absence of unequivocal dating evidence, the regional context suggests the intrusion may be derived from evaporites within the Ediacaran–Early Cambrian Ara Group. These form large deposits in the Fahud and Ghaba salt basins to the southwest of the Oman Mountains and the Hormuz Salt Basin to the north. The Jabal Qumayrah area may represent another, smaller basin or an extension to the Fahud Basin. The Jabal Qumayrah intrusion does not contain rafts of Ara Group limestones, which characterise the salt diapirs of the Ghaba Salt Basin, but this is not considered diagnostic. Other regional evaporite units of Permian to Jurassic ages do not extend into the area of the Oman Mountains and are thus unlikely potential sources. There is no evidence to suggest the Jabal Qumayrah culmination was thrust over Cenozoic evaporites and this potential source is also discounted. The timing of intrusion is constrained by the boundary faults, which cut across and thus post-date structures related to the Late Cretaceous emplacement of the Sumeini Group of Jabal Qumayrah. There is no evidence of any movement since the unroofing and exposure of the salt intrusion, which began in the Late Miocene.

INTRODUCTION

Two major carbonate-evaporite sequences of significance to the oil and gas industry are developed in the eastern part of the Arabian Plate across Oman and the United Arab Emirates. The Ara Group (Ediacaran–Early Cambrian) is present across much of south-central Oman within a series of NE-trending basins. It comprises cyclic carbonates and evaporites, which form large diapirs that locally pierce the present-day surface and are associated with oil generation and reservoir development (Gorin et al., 1982; Mattes and Conway-Morris, 1990; Peters et al., 2003; Schröder et al., 2003; Al-Siyabi, 2005; Rovira et al., 2008; Reuning et al., 2009; Schoenherr et al., 2010; Kukla et al., 2011). Hormuz Formation salts represent time-equivalent rocks in the northeastern United Arab Emirates and in southern Iran (Kent, 1970, 1979; Edgell, 1991, 1996; Glennie, 2010). Other evaporite sequences are developed in the Permian to Cenozoic of Oman and the United Arab Emirates, of which the Khuff Formation is the most significant. This represents Permian and Early Triassic sedimentation across a broad carbonate ramp that extended across much of the Middle East from Iraq to Saudi Arabia and Oman, with evaporites rich in anhydrite deposited from Kuwait to the United Arab Emirates (Ziegler, 2001; Sharland et al., 2001).

Evaporites that are sufficient in volume either to become mobile along fault planes, or to create diapirs, are absent from coeval basement lithologies and Permian to Mesozoic Neo-Tethyan carbonate platform successions where exposed in the Oman Mountains, in the Musandam Peninsula and the Al Jabal Al-Akhdar and Saih Hatat areas (Blendinger et al., 1990; Allen, 2007; Maurer et al., 2009; Koehrer et al., 2010). Elsewhere in the Oman Mountains, the platform and its underlying basement are concealed beneath the Semail Ophiolite and its structurally underlying thrust sheets comprising mainly Mesozoic, Neo-Tethyan off-margin deepwater sediments. These allochthonous units were obducted onto the Oman continental margin during the Late Cretaceous (Glennie et al., 1973, 1974; Lippard et al., 1986; Bernoulli and Weissart, 1987; Robertson, 1987; Béchennec et al., 1990; Cooper, 1986, 1990; Robertson and Searle, 1990) and now mask the underlying structures. For this reason, the substructure of the northern Oman Mountains sector of the Arabian Plate margin is comparatively poorly understood.

Recently, anhydrite and gypsum bodies have been reported within the Hawasina Window (Csontos et al., 2010a, b) and in Jabal Qumayrah (Cooper et al., 2012) in the central and northern Oman Mountains, respectively (Figure 1). Both are located within the lower structural levels of the allochthonous thrust sheets which themselves are composed of deeper-water sediments. Previous workers have interpreted the Jabal Qumayrah bodies as sub-recent/recent lacustrine deposits (Le Métour et al., 1991) or evaporites deposited as part of the Late Cretaceous Qumayrah Formation (Csontos et al., 2010b). This paper describes the evaporite deposits in the Jabal Qumayrah massif based on detailed mapping, field studies and two gravity transects that run at right angles across the massif. It discusses their possible origin as the surface expression of a salt diapir whose emplacement was strongly influenced by fault tectonics.

GEOLOGICAL SETTING OF JABAL QUMAYRAH

Jabal Qumayrah is located on the western side of the northern Oman Mountains about 50 km from the cities of Al Ain and Buraimi (Figure 1). The backbone of the mountains is formed from the Semail Ophiolite, an exceptionally well-preserved sequence of oceanic mantle and crust dated as Cenomanian (Tippet and Pessagno, 1979; Tilton et al., 1981; 95.3 + 0.2 Ma from U-Pb dating of ophiolitic plagiogranites by Warren et al., 2005). This was obducted onto the Mesozoic Oman continental margin during the Late Cretaceous together with a series of thrust sheets derived from the deeper-water off-margin sedimentary successions (Glennie et al., 1974). These sediments and associated volcanics were deposited in the Hawasina Ocean (Figure 2a), a part of Neo-Tethys that existed between the Late Permian and Late Cretaceous along the edge of the northern Oman section of the Arabian Plate (Glennie et al., 1973, 1974; Béchennec et al., 1990).

The thrust sheets derived from the Hawasina Ocean units are now stacked in a broadly proximal to distal sequence up the structural pile (Figure 2b). Thus the structurally highest thrust sheets beneath the ophiolite comprise the Haybi Complex, a subduction accretionary unit of distal oceanic sediments and volcanics, tectonic and sedimentary mélanges, and greenschist and amphibolite facies metamorphic rocks (Searle and Malpas, 1980; 1982). This unit is structurally underlain by Upper Permian to Cretaceous thrust sheets derived from deep-ocean sediments and oceanic seamounts (Umar, Kawr and Al Aridh groups), then continental rise sediments (Hamrat Duru Group) and, at the base of the structural pile, the continental slope (Sumeini Group). The whole thrust stack overlies the essentially autochthonous Arabian platform which is concealed at depth (Boote et al., 1990; Ali et al., 2008), and foredeep sediments of the Aruma Group. The thrust stack is overlain with a marked angular unconformity along its western edge by post-obduction Maastrichtian and Cenozoic shallow-water and deeper-shelf limestones and marls (Nolan et al., 1990; Skelton et al., 1990).

The outcrops of gypsum and anhydrite in Jabal Qumayrah are found within Jurassic and Cretaceous sediments of the Sumeini Group. These sediments were derived from the adjacent Mesozoic Arabian carbonate platform and deposited in a continental slope environment along the edge of the Hawasina Ocean (Glennie et al., 1974; Watts and Garrison, 1986; Watts, 1990; Le Métour et al., 1991; Cooper et al., 2012).

The Sumeini Group of Jabal Qumayrah (Figure 3) comprises three formations, divided into informal members (Le Métour et al., 1991). The Mayhah Formation comprises thick-bedded limestone conglomerates and oolitic calcarenite turbidites (the informal 1st and 3rd members) separated by a unit of fine-grained limestone turbidites and calcilutites interbedded with shales (the informal 2nd member). The fine-grained limestone turbidites of the informal 4th member are overlain in the east and central part of the Jabal Qumayrah massif by the Huwar Formation, itself comprising regional Tithonian to Lower Cretaceous cherts followed by a return to limestone turbidites. The Huwar Formation is absent in the west, where the informal 4th member of the Mayhah Formation is much thicker and may, in part, also represent time-equivalents of the Huwar Formation (Cooper et al., 2012). These are overlain, mainly unconformably by thick limestone conglomerates, cherts and shales of the Cenomanian to ?Coniacian Qumayrah Formation, which are interpreted as the product of over-steeping then deepening of the edge of the Arabian margin in response to the detachment of the Semail Ophiolite and the onset of thrusting in the Hawasina Ocean (Robertson, 1987; Watts and Blome, 1990).

The structure of the Jabal Qumayrah massif (Figures 4 and 5) has been investigated in detail by Cooper et al. (2012). The area comprises two major superimposed thrust sheets of the Sumeini Group which, following initial stacking, underwent west-directed refolding on a kilometre scale around an elongate west-facing, doubly-plunging antiform that runs north-south through the centre of the massif and that has created a mountain-scale west-facing domed culmination. The north and south sides of the culmination dip to the north and south respectively and along Wadi Ajran and Wadi Qumayrah, where the structurally higher Hawasina and Haybi Complex duplexes are highly attenuated or missing altogether and the Semail Ophiolite is close to, or faulted against the Sumeini Group culmination.

This second phase of shortening was associated with out-of-sequence forward and back-thrusting developed mainly in the lower thrust sheet (the Sumer-Milh Thrust Sheet in Figure 5). This thrust sheet is now mainly exposed by erosion as a window through the upper thrust sheet (the Huwar-Fayyad Thrust Sheet and its extension the Ghashnah-Sanqah Thrust Sheet from which it is separated by later thrusting and erosion (Figures 4, 5a and 5b cross sections A-A’ and B-B’). The northern end of the Ghashnah-Sanqah Thrust Sheet is truncated by a down-to-the-south normal fault, the ‘Sumer Fault’, which runs broadly east to west along Wadi Sumer, through the west-centre of the massif. Parallel and conjugate normal faults that trend NE-SW and ESE-WNW are also developed in the centre of the massif and are the focus of the main gypsum and anhydrite body in Wadi Lisail. Structures in the central part of the Jabal Qumayrah culmination around the Wadi Lisail body are topographically lower when compared with those to north and south, and the sole thrust to the Huwar-Fayyad Thrust Sheet is some 350 m below its altitude along strike (cross-section C-C’ on Figure 5b).

SALT INTRUSIONS IN JABAL QUMAYRAH

The centre of Jabal Qumayrah contains a number of outcrops of gypsum and anhydrite that are here interpreted as representing salt deposits that have undergone deep weathering and dissolution to remove any more soluble evaporitic components that may have been present. Sediment clasts and rafts up to 100 m long float in the gypsum and anhydrite matrix. In what follows, the term “salt” is used loosely to cover all preserved sediments linked to these deposits. Figures 5 and 6 show the locations of the salt exposures. The main body is located in the middle of the Jabal Qumaryah area, bisected by Wadi Lisail. A linear string of isolated exposures extends for 1 km to the WNW, and then 1 km WSW to the point where Wadi Sumer breaches the Jabal Qumayrah mountain front. A second string of isolated exposures stretches southwards along an arcuate line for 2.5 km along the eastern side of Wadi Milh towards the village of Qumayrah.

X-ray diffraction analysis of hand-ground samples using a PANalytical X’Pert Pro Alpha1 instrument with the PANalytical HighScore Plus Search-Match Programme indicates that fresher salt specimens are composed almost entirely of anhydrite, with small amounts of quartz and clay minerals. One sample yielded diffraction peaks consistent with traces of halite. Surface-weathered samples are predominantly gypsum, with significant amounts of calcite and lesser proportions of quartz, dolomite and clay minerals. These latter minerals are interpreted as mainly representing increased concentrations of insoluble residues, although some of the calcite and dolomite is derived from secondary veining and vuggy crystallisation, both of which are observed.

The Central Wadi Lisail Intrusion

The main salt body in Wadi Lisail (Figures 6 to 8) is roughly L-shaped. The central area has a diameter of about 400 m and extends for a further 200 m to the southwest and 500 m to the south. Outcrops on the southern side of Wadi Lisail are well exposed, but those to the north of the wadi are partly obscured by a series of river terraces through which rounded hills of gypsum and anhydrite emerge, typically around the raised margin’s larger sediment rafts. The margins of the Wadi Lisail salt deposit are mostly covered by scree. Locally, however, contacts with the country-rock are well-exposed.

Components of the Salt Deposits

Three broad classes of components are recognised in the salt deposits, namely (1) chaotic salt, (2) salt with sediment ghosts, and (3) sediment rafts (Figures 9 and 10). Each may grade into another laterally or vertically. All outcrops show deep and intense weathering.

(1) Chaotic Salt

Chaotic salt is found mainly around the edges of the larger areas of exposure and makes up most of the smaller hillside outcrops. It has a hard, orange-yellow and pink irregular gypsum-rich crust under which white anhydrite-rich material is granular with no discernible internal structure and, in more weathered parts, it is typically friable with numerous voids. Chaotic salt contains randomly arranged isolated clasts of green and brown shales, marls, grey fine-grained limestone frequently with diagenetic calcite and rhombic dolomite, and hard dark greenish-grey and rust-weathering fine-grained sandstone. These clasts vary in size, but rarely exceed a maximum dimension of 50 cm. Clasts located close to the edge of the intrusion may be aligned sub-parallel to the boundary and associated shear planes. This salt also contains veins and vugs of fibrous and platy anhydrite that locally form dense networks, in which individual books of crystals reach 8 cm long.

Native sulphur is locally preserved in the chaotic salt as irregular crystalline bodies up to one centimetre across. They are typically translucent yellow in colour and occur as small isolated masses, or short disarticulated strings up to about 10 cm long, suggesting the broken remnants of sulphur veining.

Chaotic salt is interpreted as the highly degraded, insoluble residues from the intense weathering of evaporites that have intruded the Sumeini Group of Jabal Qumayrah.

(2) Salt with Sediment Ghosts

Salt with sediment ghosts comprises thin horizons of more coherent sedimentary rock in a matrix of chaotic salt. The sediment ghosts are formed from disjointed blocks, most commonly of dark fine-grained laminated or featureless limestone. Individual blocks can range from a centimetre scale to about 30 cm thick and 2 m long and form disarticulated linear bands that can extend laterally for many tens of metres. Groups of sediment ghosts frequently appear in parallel sets up to a metre apart, separated by chaotic salt. They are often distorted by open folding and can grade laterally and vertically into chaotic salt or more coherent sediment rafts. Much of the ghost bedding is steep and dips of 60° to 90° are common. Where sediment ghosts are close to the edge of the salt, there is a poorly developed sub-parallel relationship between bedding in stringers and the margin.

Salt with sediment ghosts is interpreted as the highly degraded remnants of limestones that have been incorporated with the salt and highly corroded and fragmented, leaving only hints of the original bedding. Some sediment ghosts grade laterally into sediment rafts that have Sumeini Group affinities. Nothing has been identified that could represent unequivocally sediments that were interbedded with the salt when originally laid down. The sediment ghosts were probably re-orientated close to vertical during the emplacement and dissolution processes.

(3) Sediment Rafts

Sediment rafts are only found in the largest body in Wadi Lisail (Figure 10). They comprise coherent sedimentary sequences that are embedded in the anhydrite and gypsum matrix. Rafts can vary from 10 m to over 100 m long, and can be up to 50 m thick. They are typically elongate and may be gently folded. More equi-dimensional blocks range from a few metres to over 20 m in diameter. In common with the sediment ghosts, bedding in elongate rafts is often observed to be steep, in particular in rafts that are located close to the edges of the intrusion. Bedding in the more spherical blocks shows no preferred orientation trends.

There are three main lithological associations.

(1) Shales and marls with fine-grained sandstones and limestones: These rafts may have sharp boundaries or grade laterally into salt with sediment ghosts. The sandstones are generally bedded on a 5–15 cm scale, hard, and are structureless or contain parallel-laminations and occasionally poorly preserved ripples. Trace fossils, flutes and load structures are sometimes developed on the underside of bed bases, suggesting a turbidite origin. The grains of finer-grained sandstones are almost entirely of quartz whereas rarer coarser-grained sandstones are well-sorted, sub-mature sublitharenites that contain, in addition to quartz, up to 20% plagioclase feldspar and minor amounts of very fine-grained quartzose lithic fragments, detrital white mica, mud flakes and ferruginous opaque minerals. Quartz grains locally show evidence of quartz overgrowths that pre-date a later phase of heavy corrosion along grain boundaries. The sandstones generally have a sparry calcite cement. They grade upwards to well-cemented then fissile siltstones and marls. The limestones are light to mid grey and bedded on a similar scale, and are typically laminated and fissile or structureless. While no unequivocal dating evidence has been found to tie the rafts to any particular source, the mineralogy of the raft sandstones is essentially indistinguishable from that of sandstones found towards the top of the Qumayrah Formation.

Although circumstantial evidence points towards the upper Qumayrah Formation, there are a number of possible sources for the sandstones. The nearest exposures of the basement to the Permian to Cretaceous carbonate platform occur in the eroded core of the Al Jabal Al-Akhdar anticline 150 km to the southeast. However, the rafts show none of the slatey cleavage characteristic of the basement, which suggests that it is unlikely they were derived from this source. Triassic quartz-bearing sandstones are developed in the Hajar Supergroup carbonate platform, as the Ghalilah Formation exposed in the Musandam Peninsula 200 km to the north and the Lower Mafraq clastics of the subsurface Oman interior. However, the turbidite sedimentology militates against these as a possible source.

The Late Triassic Mahil Formation of the Sumeini Group is exposed 50 km further to the north in Jabal Sumeini (Glennie et al., 1974; Watts and Garrison, 1986; Watts, 1987, 1990), although it is not exposed in the Jabal Qumayrah area and the sole thrust to the upper Huwar-Fayyad Thrust Sheet lies within the Jurassic stratigraphy. It is nonetheless possible that similar sandstones are present at depth, and derivation from these sources cannot thus be ruled out. Quartz-rich sandstones also are developed in the Late Triassic Al Jil Formation and Jurassic Matbat Formation of the Hamrat Duru Group in the Qumayrah area (Cooper, 1990; Le Métour et al., 1991) and the petrology of those of the Al Jil Formation is also similar to that seen in the rafts. However, the Al Jil and Matbat sandstones are not associated with laminated marls and limestones.

(2) Dark grey graded calcisiltites and calcilutites: These rafts form roughly spherical blocks of hard limestone that range from a few metres in diameter to house-sized. Bedding is well-preserved and beds are typically 10–15 cm thick, graded, with fine-grained laminations and ripples concentrated towards their bases characteristic of fine-grained turbidites. Beds may be amalgamated or separated by thin shale partings. Thin calcarenite beds and bases to beds are also locally seen. Fresh surfaces may yield a petroliferous smell and are dark grey, weathering to a lighter blue-grey.

Blocks may show open to tight internal folding on a metre-scale, small-scale fracturing and faulting, and calcite veining. This folding and veining does not extend into the surrounding salt matrix and is significantly more intense than that seen in either the salt with stringers or the association of shales, marls and fine grained sandstones and limestones. It thus pre-dates the inclusion of the blocks in the salt matrix.

The sedimentology and internal structures of these blocks are indistinguishable from the finer-grained parts of the Sumeini Group stratigraphy, and it is highly likely that the rafts were derived from this host rock.

(3) Limestone conglomerates, calcarenites and shales: This assemblage contains mixed shales, dark thin-bedded calcilutites, some graded with very fine-grained rippled bases, fine-grained graded calcarenite turbidites and metre-bedded limestone conglomerates that grade upwards into calcarenites that also contain relic turbidite structures. The margins of the limestone beds are typically silicified into a rust-brown weathering crust. Rafts of this material are comparatively rare. Some show little internal deformation, whereas others are tightly folded.

These rafts are interpreted as blocks of the Qumayrah Formation, which have been incorporated into the salt. Indeed, at the southwestern end of the Wadi Lisail intrusion, a line of thicker limestone conglomerate can be traced laterally from the neighbouring Qumayrah Formation host rock into the salt intrusion where it has become detached to form a sediment raft.

Structural Context of the Wadi Lisail Salt

The Wadi Lisail salt is located in the middle of the Jabal Qumayrah culmination along the crest of the late-stage, doubly plunging N-S anticline that marks the apex of the dome of Jabal Qumayrah (Figure 4). The eastern, and part of northern boundary of the salt are against the structurally higher Huwar-Fayyad Thrust Sheet. Its southern and western sides are against the structurally lower Sumer-Milh Thrust Sheet.

The upper Huwar-Fayyad Thrust Sheet of the Sumeini Group rocks forms the eastern and northern sides of the Jabal Qumayrah culmination. It generally dips away from the N-S axis of the culmination at about 25° along its western edge steepening to about 60° at its margins. Within this, the thrust sheet has been folded on a kilometre-scale into a series of west-facing folds whose steep limbs are close to vertical (Cooper et al., 2012). However, this pattern is modified in three significant ways closer to the salt intrusion, with the depression of the central structures, the reversal of prevailing bedding dips and the development of conjugate faulting.

The sole thrust of the Huwar-Fayyad Thrust Sheet lies at an altitude of about 1,000 m to the north and south of Wadi Lisail, but descends to about 650 m in the area of the salt (Figure 5, cross section C-C’). One consequence of this is that a narrow tongue of the Huwar-Fayyad Thrust Sheet extends to the west, close to the northern side of Wadi Lisail, forming an irregular and comparatively poorly exposed sequence of folded limestones that lie structurally over the upper Qumayrah Formation of the underlying Sumer-Milh Thrust Sheet. The sole thrust itself has been folded comparatively gently along the N-S axis of the culmination.

Similarly, whereas the general dips along the western scarp side of the Huwar-Fayyad Thrust Sheet are generally about 25° to 40° to the east (and, towards the northern end of the culmination, the NE and then north), this reverses in the depressed area of the thrust sheet immediately to the east of the Wadi Lisail salt. Here the bedding in the Mayhah Formation is tilted to the south and SW, in part influenced by faulting. The bedding in the upper Huwar-Fayyad Thrust Sheet along the boundary with the salt also steepens towards the boundary typically in a zone about 100 m and is locally sub-vertical at the contact, dipping away from the salt.

The Wadi Lisail salt body is located at the focal point of a number of significant faults in the upper Huwar-Fayyad Thrust Sheet (Figures 5, 6 and 7), which form a conjugate set. The most important of these faults runs approximately 050° and has been exploited by the headwaters of Wadi Lisail as it cuts into the Huwar-Fayyad Thrust Sheet. The fault is down-to-the-SE, cuts the major SW-verging folding within the thrust sheet and has with a throw of over 100 m close to the salt, although this reduces to the NE and the fault dies out over some 3 km. This fault does not offset the NE edge of the salt body. The conjugate faults trend 195°. Their throws are significantly less and they also die out towards the east. One of the faults offsets the edge of the salt by about 30 m; otherwise they appear to end at the salt, although exposure is poor.

Exposures of the structurally underlying Sumer-Milh Thrust Sheet in this area are mostly of the Qumayrah Formation. The chert and shale-rich sequences strike N-S and dip steeply to the east. They are tightly folded, with considerable internal faulting and numerous shear planes. There is a general trend towards higher stratigraphical levels being exposed to the east, although there are major repetitions of the lower chert-rich and upper shale-rich parts of the Qumayrah Formation (Figure 5, cross section B-B’, Cooper et al., 2012). The ‘L’-shape of the Wadi Lisail salt thus means that it cuts across the structural grain of the Qumayrah Formation along Wadi Lisail (northern branch), but the southern extension is parallel to the structural grain of the Sumer-Milh Thrust Sheet.

Boundaries with the “Host Rock”: Wadi Lisail Salt

The boundary between the salt bodies and host rock is generally concealed by surface rubble, but is locally well-exposed. Boundaries may be faulted, with the gypsum and anhydrite abutting the Sumeini Group host rock with little or no evidence of alteration, or intrusive (Figure 11).

Boundaries with the upper Huwar-Fayyad Thrust Sheet

The Lisail salt abuts this thrust sheet along its northeastern and eastern edge. The boundary itself is steep to vertical and marked by a sharp transition from cream to white lower-weathering gypsum and anhydrite to limestone. The immediate boundary zone is up to 3 m wide. The limestone is characterised by intense corrosion and recrystallisation, with numerous small (0.5–1 cm) solution holes and sparry vugs suggesting significant fluid circulation. The adjacent limestone may be brecciated into angular to sub-angular clasts on a 15–30 cm scale. This extends up to 15 m from the contact and is associated with close-spaced steep-to-vertical jointing and faulting. This contact is also very close to the sole thrust of the Huwar-Fayyad Thrust Sheet and, while the brecciation appears to be more intense, it is unclear how much, if any, of the jointing is a product of the salt body as this is also seen along exposed sections of the base of the thrust sheet away from the Lisail salt. In places, bedding in the limestones adjacent to the salt body steepens sharply towards the edge of the salt across a zone up to 100 m wide, dipping away from the salt.

While locally present at the base of the Huwar-Fayyad Thrust Sheet, the salt is not seen to intrude into the limestone-rich Mayhah Formation of that thrust sheet, nor is there evidence that the salt has generally exploited this major thrust plane. The contact between the two thrust sheets is exposed over a wide area in the hillsides to the north of Wadi Lisail (northern branch), where limestones of the Mayhah Formation (1st informal member) lie directly on the shale-rich upper member of the Qumayrah Formation. No salt is seen along this contact even though the closest exposure is just 200 m away from the salt outcrops.

Boundaries with the lower Sumer-Milh Thrust Sheet

Isolated exposures of the contact between the gypsum and anhydrite of the Wadi Lisail salt and the lower Sumer-Milh Thrust Sheet are seen along the southern side of Wadi Lisail (northern branch) and between the northern and southern branches of Wadi Lisail.

Along the southern side of Wadi Lisail (northern branch), a number of small salt outcrops appear adjacent to the hills of the Qumayrah Formation. The bedding in the host rock of tightly-folded and multiply imbricated lower and upper Qumayrah cherts and shales is uniformly steep (70° to 80° dipping to the ENE); the wadi thus cuts perpendicularly across the structural and stratigraphical grain. There is no evidence of folding, steepening or deformation at the boundary with the salt intrusion along the wadi edge, which is interpreted as a slightly curvilinear near-vertical fault. This fault is aligned with the large fault trending 050° that cuts the Huwar-Fayyad Thrust Sheet, and may be an extension of that fault. The fault does not extend much further to the WSW beyond the end of the salt body. Instead, it links with a fault trending to the NW that cuts into the Sumer bowl to the north and is the locus for further, and smaller salt outcrops (see next section).

Small exposures in the north bank of Wadi Lisail (northern branch) at the westernmost end of the salt show a broad zone of altered shales of the Qumayrah Formation. These shales are usually dark grey to red-brown in colour, but this is modified close to the boundary with the salt. The shales become increasingly lighter brown until they are cream-orange and are almost indistinguishable from the weathered and sheared gypsum with its included clasts. The shales are also strongly sheared and show locally pervasive gypsum veining. The boundary with the gypsum itself is not sharp. This is interpreted as a primarily intrusive relationship, further evidence of which is seen in outcrops through the terraces immediately to the north. Here, conglomerate beds in the Qumayrah Formation host rock appear to extend laterally into the salt deposits, where they crop out as large (10 m scale) isolated blocks. While poorly exposed, this suggests an irregular in situ boundary where the host rock has been subsumed into the salt.

The contact with the Qumayrah Formation between Wadi Lisail’s northern and southern branches is sharp. Chaotic salt abuts red cherts with interbedded shale partings of the lower Qumayrah Formation that dip 70° to 80° to the west. The cherts become green coloured within about 0.5–1 m from the salt. There is no evidence of significant brecciation or other alteration along the contact, or of smaller-scale intrusion of the salt into the cherts, although there are small fault offsets that reach about 10 m displacement at their greatest.

Other Salt Exposures in Jabal Qumayrah

Two broadly linear strings of isolated exposures of salt radiate from the central Wadi Lisail salt (Figures 5, 6, and 12). They crop out as irregularly shaped masses on steeper hillsides that reach about 100 m long, although their margins are usually concealed beneath talus. They comprise mostly structureless chaotic salt, locally with small, highly rotted limestone sediment ghosts and anhydrite veining. The largest exposures also contain rare blocks of dark grey platy limestones, similar to those of the Sumeini Group. The salt exposures crop out along mainly scree-covered hillsides in discrete, often widely separated bodies that suggest that they are not continuous at the surface, although it is likely that they are connected at depth.

The northwestern exposures, the “S” series on Figure 6, start at the northern side of the western end of the main Wadi Lisail salt body. They form a straight line, trending 307°, which follows a near vertical fault through the Qumayrah Formation, now exploited by a wadi. At the col between the Lisail and Sumer drainage basins, the fault drops the base of the upper Huwar-Milh Thrust Sheet on its NE side against cherts situated close to the base of the Qumayrah Formation. This suggests a vertical displacement that exceeds the thickness of the folded upper Qumayrah Formation that would otherwise separate these units, i.e. some 100s of metres. Where exposed, the salt has a steep, cross-cutting contact with the host rock. The salt exposures continue over the watershed into the Sumer valley where the upper outcrop areas lie on the same line. The other outcrops (S1, S2 and S3 on Figure 6) appear to follow broadly the line of the fault that runs along Wadi Sumer, that trends 250° with a significant down-to-the-north throw (Cooper et al., 2012). The two faults intersect at the S3 exposure. This has a well-exposed vertical southern boundary with highly folded platy limestones of the Mayhah Formation (4th member). The folds are truncated by the salt. This salt body also shows a northern boundary with shales and thin limestones of the upper Qumayrah Formation, which dip steeply (60°–70°) to the north.

The southern exposures, the “M” series on Figure 6, lie on a gently arcuate south-trending line on the eastern side of the Wadi Milh valley from the tapering southern extremity of the Wadi Lisail salt. They form three main outcrop areas in what is otherwise poorly exposed Qumayrah Formation country rock. One outcrop, M3 on Figure 6, shows a steeply faulted southern boundary with Qumayrah Formation shales and thin limestones, where the greatest thickness of exposure (ca. 50 m) is found. It tapers to the north over about 400 m to just 3 m thick, where the salt intrusion forms a “sill” parallel to the main bedding trends in the Qumayrah Formation shales, but cutting across tight folding picked out by thin limestones. While it is not possible from outcrop data to determine unequivocally the nature of the bodies, the pattern suggests exploitation of a gently curved fault.

GRAVITY INVESTIGATIONS

Gravity data were acquired across Jabal Qumayrah to provide some constraint to the subsurface development, while recognising that, in the absence of corroborative data such as seismic or borehole information, no definitive solution is possible.

Gravity Data Acquisition

Gravity measurements were acquired on two profiles (Figure 13) using a Scintrex CG5 gravimeter (with a cited instrument reading resolution of ± 0.001 mGal). Profile I runs southwest to northeast and was acquired in January 2009 with 55 stations and total length of 34 km. Profile II has orientation of north to south and was acquired in January 2010 with 55 stations and total length 15.9 km. The gravity stations were spaced approximately 250 m apart, but some variations in the sampling distance was required due to difficulties in accessing some areas. A base station was established at the centre of the survey area to monitor the drift of the gravimeter and was visited every 3–5 hours. Care was taken to minimize measurement errors, and any ambiguous measurements were double-checked. Latitude, longitude, and elevation were taken at each station using a handheld Garmin GPS receiver. These readings were corroborated with 10 m DEM (Digital Elevation Model) data.

Twelve samples were collected representing each of the main rock types at various locations throughout Jabal Qumayrah area (Table in Figure 14). Although the number of samples available for measurement was limited, some clear trends emerged. The Semail Ophiolite and Sumeini rocks were generally found to have higher densities than the Cenozoic sediments.

Gravity Data Processing and Modelling

The gravity data were corrected for instrumental drift, earth tides, elevation, latitude and terrain. The drift was assumed to be linear between consecutive base station readings and was subtracted from the observed value. The effect of latitude was corrected using the 1967 International Gravity Formula (Mittermayer, 1969). The data were reduced to Bouguer anomaly using an average density of 2,670 kg/m3. The survey was tied to the Seeb International airport (Oman) base station (23°35.00′N 58°17.68′E; g = 978,921.955 mGal). The regional gravity field was assumed to be a linear trend with a low gradient in the survey area. Therefore, it was not removed from the gravity profiles.

Terrain corrections based on Digital Elevation Model (DEM) (Figure 15a) data with a 10 m resolution derived from ALOS PALSAR data (Ali et al., 2012) were applied to obtain the Complete Bouguer anomaly, which corrects for irregularities due to variations in terrain in the vicinity of the gravity stations (Figure 15b). Geosoft MontageTM software was used to calculate the terrain corrections. The software utilises a combination of the methods described by Kane (1962) and Nagy (1966).

We caution that the terrain corrections are based on relatively low resolution DEM data. As a result, some errors may occur in the final terrain corrections. For example, 10 m DEM data may not be sampled finely enough to accurately derive the near-zone terrain corrections over Jabal Qumayrah where the topographic relief could be extreme. However, taking into account the quality of the gravity and positioning measurements, the overall accuracy of the complete Bouguer anomalies are estimated to be less than 0.1 mGal.

Gravity Interpretation

Simple Bouguer Anomaly

The Bouguer anomaly (BA) of Profiles I and II are shown in Figures 16b and 16d. The western area contains low gravity anomalies (< -70 mGal) associated with the Cenozoic and Maastrichtian sediments and thrust Hamrat Duru Group. The central area of Jabal Qumayrah is associated with strong gravity anomalies (ca. 65 mGal), associated with the Sumeini Group. The north-northeast and south of Jabal Qumayrah is associated with very strong anomalies (> -50 mGal) due to the thrust Haybi and Semail Ophiolite.

Additionally, the Bouguer gravity contains a negative anomaly at the centre of Jabal Qumayrah. However, the Bouguer anomaly map contains effects of anomalies caused by the mass of Jabal Qumayrah reducing the gravitational attraction by exerting an upward pull on the gravimeter. This effect is most relevant for the stations within about 3 km of Jabal Qumayrah (Figure 15). Hence, the interpretation was greatly enhanced by applying terrain corrections.

Complete Bouguer Anomaly (CBA)

The Complete Bouguer anomaly was obtained by combining the Bouguer anomaly and the terrain corrections. The CBA profiles (Figure 16b and 16d) show a spectacular negative anomaly that has a range of ca. 5 mGal at the centre of Jabal Qumayrah and provides strong evidence for salt tectonics. Both profiles record this negative anomaly. The anomaly is also coincident with a salt outcrop mapped from the geological mapping of the area. Furthermore, to the north-northeast, the CBA profiles show a steep gradient in the gravitational field where gravity values increase by more than 20 mGal. This is correlated to the Haybi and Semail Ophiolite that crop out in the area.

Gravity Models

The gravity profiles were modelled along two projected profiles using the 2.5-D GM-SYS program to determine deep structure of the area. However, the modelling of the gravity anomalies can be complex due to non-uniqueness (Blakely, 1995). This can be minimised by constraining the models using other geological and geophysical data. In this study the geometry of subsurface structures of the models were constrained by outcrop data at the flanks of the plain and bedrock density measurements. Therefore, the models were initially based on the geological cross sections and subsequently refined to reconcile the differences between the observed and computed gravity data. The depth to the Permian–Mesozoic shelf carbonates, Palaeozoic, Ara Salt, crystalline basement were estimated from regional studies in Central Oman Mountains (e.g. Shelton, 1990; Ravaut et al., 1997; Al-Lazki et al., 2002; Peters et al., 2003; Reuning et al., 2009). Additionally, the densities of formations are based on measured density and published data (Manghnani and Coleman, 1981; Ali et al., 2008, 2009; Searle and Ali, 2009).

The final subsurface models presented in Figures 17 and 18 display a good match between the observed and calculated gravity anomalies. The fit between the observed and calculated anomalies was quantified by a root mean squared (RMS) error computed by the software. The best-fit solutions of the models were determined by reducing RMS error to less than 2.0% of the total dynamic range of the gravity profile. The models indicate that the folded and thrust Sumeini Nappe correlates with the gravity positive anomalies and the foredeep basin with a large negative anomaly. Furthermore, the models suggest that the central area of the profile is underlain by a major salt intrusion that may be related to deep-seated faults causing basement uplift. The absolute size and scale of the salt intrusion is speculative in the absence of corroborative seismic and well evidence; however the model indicates greater consistency with a high-level detached diapir.

DISCUSSION

There are three main possible origins for the salt deposits in Jabal Qumayrah. The first is sedimentary deposition in situ after development in the Late Cretaceous of the structural culmination in the Sumeini Group and its subsequent unroofing. Second, it may derive from sedimentary deposition during the Late Cretaceous as part of the uppermost Sumeini Group (Qumayrah Formation) rocks in which the salt is now found. The third possible origin is intrusion of salt from a lower structural level into the core of the Sumeini Group culmination.

Several lines of evidence suggest that the Jabal Qumayrah salt was not deposited in situ, for example as recent/sub-recent lacustrine deposits, as mapped by Le Métour et al. (1991). The gypsum and anhydrite contains no evidence of the internal stratigraphy that might otherwise be expected, nor is there evidence of lateral or vertical marginal or transitional sedimentary facies. While Jabal Qumayrah has undergone significant uplift since the Late Cretaceous obduction event (Boote et al., 1990) and may have been affected by the Late Cenozoic open folding seen along the western side of the northern Oman Mountains, these could not in themselves obliterate the sedimentary fabric. Furthermore, the area is deep within a geomorphologically immature mountainous region subject to high rates of erosion that would be expected to quickly fill any lacustrine depression. Similarly, there is no clear sedimentary mechanism whereby a comparatively limited number of huge blocks and rafts of Sumeini Group sediments could be introduced into the gypsum and anhydrite matrix. Instead, the salt boundaries cut across bedding within the Sumeini Group host rock without any trace of marginal sedimentary facies and are faulted or intrusive and associated with considerable fluid activity.

Similarly, there is no relic stratigraphy or lateral variation to support a Late Cretaceous sedimentary origin within the upper levels of the Qumayrah Formation, where it is now exposed, as suggested by Csontos et al. (2010b). Clearly, much of the sedimentology could have been destroyed had the salt been involved in the intense folding and thrusting associated with the development of the Jabal Qumayrah culmination, yet the degrees and styles of deformation of the salt and the surrounding rocks are quite different. The shale and chert-rich sediments adjacent to the salt bodies are highly deformed by tight to isoclinal folding, shearing and closely-spaced internal thrusts as well as larger-scale thrust repetition. This extreme deformation is not seen in the salt bodies, notwithstanding their comparative softness. The orientations of the sediment rafts in the salt are random, becoming steep and sub-parallel to the steep faulted margins of the main Wadi Lisail salt centre. This contrasts with the tightly folded bedding in the country rock, which shows a well-developed regional dip that does not transfer into the salt. Furthermore, there are no thrust or fold repetitions of the salt in the Jabal Qumayrah culmination, even though these are common on a variety of scales in the country rock. Nor does the salt form detachment horizons despite its softness and mobility. Instead, the salt is concentrated in the Wadi Lisail centre and in isolated bodies that extend along lines to the NW and south.

The evidence thus points towards an intrusive mode of emplacement of the salt bodies after the development of the Jabal Qumayrah structural culmination. This is further supported by features of the salt itself including the presence of native sulphur, the distribution of sediment rafts, the gravity data and the nature of the boundaries to the salt bodies. Taken together, these results suggest the gypsum and anhydrite outcrops may ultimately have been derived from a salt diapir that has intruded the Jabal Qumayrah massif, albeit with emplacement of the exposed bodies being heavily influenced by faulting.

The presence of native sulphur in the Jabal Qumayrah intrusion is consistent with observations from salt diapirs in other petroleum provinces, even if not diagnostic of such environments. It occurs in particular in the Gulf of Mexico. Elemental sulphur is present in the cap-rocks of many diapirs in Louisiana, Texas and Mexico, and indeed has occurred in commercial quantities in 26 domes, for example the Sulphur Mines, Boling Dome and Main Pass domes (Goldman, 1952; Kyle, 2002; Nehb and Vydra, 2006). Sulphate-reducing bacteria linked with hydrocarbons reduce dissolved sulphate from anhydrite to hydrogen sulphide, which is then re-oxidised to form native sulphur (for example Jones et al., 1956; Feely and Kulp, 1957; Kyle and Posey, 1991; Bechtel et al., 1996; Worden and Smalley, 1996; Machel, 2001). The limestones of the Sumeini Group of Jabal Qumayrah, in particular the Mayhah Formation, are locally fetid and could provide the hydrocarbons to initiate this process. Native sulphur has not been reported from the salt domes in the Ghaba Salt Basin, although Peters et al. (2003) noted sulphurous odours when limestones from rafts in the Qarn Sahmah dome were broken open. A detailed investigation into the origin of the sulphur, and whether it is a product of thermo-chemical or biological sulphate reduction, is beyond the scope of the current investigation.

Surface-piercing salt domes have only previously been reported in Oman from the Ghaba Salt Basin in central Oman, where they are formed from evaporites deposited as part of the Ediacaran–Early Cambrian Ara Group (Peters et al., 2003; Reuning et al., 2009). The gypsum and anhydrite outcrops in Jabal Qumayrah show a range of similarities and differences to those salt domes, as summarised in Figure 19.

The generally disorganised and steep dips of blocks and sediment rafts within the central part of the Jabal Qumayrah intrusion are, in their arrangement, similar to the sediment rafts embedded in gypsum and anhydrite that are a common feature of exposures of the surface-piercing salt domes of the Ghaba Salt Basin in central Oman (Peters et al., 2003; Reuning et al., 2009). In those cases, the rafts are of limestones that were originally deposited within the salt sequence and were brought to the surface by halokinesis. This contrasts with the rafts in Jabal Qumayrah insofar as they consist of the Sumeini Group that surrounds the salt bodies. This is interpreted here as reflecting instead the effects of the interaction with faulting at the very top of the intrusion leading to the detachment and stoping into the salt of blocks from the uppermost structural levels of the Sumeini Group country rock. The abundance of rafts in the salt domes of the Ghaba Salt Basin is attributed to the dissolution of halite, concentrating the rafts in the less soluble gypsum and anhydrite residue (Reuning et al., 2009). A similar concentrating process could also have occurred at Jabal Qumayrah with the stoped blocks. Our investigations and those of Peters et al. (2003) and Reuning et al. (2009) indicated halite is absent from the surface outcrops of at least two of the six surface-piercing salt domes in the Ghaba Salt Basin, even though it may be present below the surface (Peters et al., 2003). The absence of halite in Jabal Qumayrah may thus be a product of dissolution during the deep weathering, which is observed in the outcrops.

The reason for the difference in the composition of the included sediment blocks and rafts in the two localities is unclear. It may reflect an entirely different source for the evaporites. Alternatively, if the Jabal Qumayrah salt is sourced from Ara Group equivalents, similar interbedded limestones may not have been present in the Ara Group of the Jabal Qumayrah area. The shapes of the intrusions are also very different. The Ghaba Salt Basin diapirs are typically circular to oval in shape and both surface expression and sub-surface geophysical investigations show them to be attached plugs or detached tear-drop diapirs that have risen through an essentially layer-cake stratigraphy in which much of the deformation and variation in sediment thickness is salt-induced.

Conversely, the models based on gravity profiles for the Jabal Qumayrah area (Figures 17 and 18) suggest an elongate N-S orientated gravity low that crosses the locus of the Jabal Qumayrah salt intrusion in Wadi Lisail. However, the structure of the Jabal Qumayrah host rock into which its salt has intruded is also quite different. The Jabal Qumayrah culmination has a significantly more complex and asymmetrical structure, being derived primarily from the emplacement in the Late Cretaceous of thrust sheets with small-to-large scale west-directed folding, shearing and faulting. The elongate gravity low follows the main anticlinal axis of the N-S trending, west-facing anticlinal doming of the Jabal Qumayrah massif. Broadly speaking, this represents the structurally deepest levels that are exposed and implies a tectonic control that links the distribution of the subsurface salt to the structural culmination. The isolated salt outcrops that form an arcuate line along the eastern hillside above Wadi Milh (the ‘M’ series on Figure 6), together with the ‘S’ series (also in Figure 6) that first follows a major fault to the NW of Wadi Lisail and then the W-E trending Sumer Fault, collectively indicate that the higher-level arrangement of the salt outcrops has been modified and overprinted by the exploitation of faults. These faults controlled not only most of the boundaries of the Wadi Lisail outcrop area, but also appear to have acted as conduits through which the outlying strings of gypsum and anhydrite were injected. The absence of salt intrusions into the upper Huwar-Fayyad thrust sheet may indicate that the thick limestones at the base of the thrust sheet acted as an upper boundary through which the salt could not penetrate.

Thus the present-day arrangement of the Wadi Lisail evaporite outcrops suggests a dynamic mode of emplacement, rather than simple gravity-induced diapirism, as evidenced by the injection of gypsum and anhydrite along the fault lines to both the NE and south of the central Wadi Lisail area and the faulting along much of the boundary to the central outcrop zone. It is also tempting to speculate that the topographical depression of the sole thrust to the upper Huwar-Fayyad Thrust Sheet (see Figure 5b), and the reversal of the regional E-directed dip of this thrust sheet immediately to the east of the main salt intrusion in Wadi Lisail, may in part reflect halokinetic subsidence driven by the removal of salt at depth as it moved up through the allochthonous units. Evidence to support this idea is currently weak. In addition, the comparatively small scale of both the intrusion, and the gravity anomaly, suggests that the Jabal Qumayrah culmination itself was primarily a product of Late Cretaceous compression and nappe emplacement and was not driven by salt tectonics.

Age of the Intrusion

The precise position of the Jabal Qumayrah salt in the stratigraphical and structural sequence remains poorly constrained. The cross-cutting relationship with folds and thrusts in the Sumeini Group host rocks indicates that salt intrusion post-dated thrust emplacement, and thus was a post-Santonian/Campanian event. This is reinforced by the presence of blocks of Sumeini Group limestones in the salt that had already been intensely folded and veined before their incorporation. However, the absence of any sedimentary cover that post-dates the emplacement and subsequent erosion of the thrust sheets means that there are no outcrop data to define more precisely the timing of intrusion.

Conversely, there is no evidence from the current outcrop and drainage patterns to suggest that active salt movement is currently occurring or that it has taken place since exhumation of the salt. Drainage patterns suggest that a major phase of uplift followed the deposition of the Cenozoic limestones which form the western mountain front in this area and which have been folded along a NNW-SSE axis (Figure 4). The salt is cut and exposed by tributaries to Wadi Qumayrah, which itself cuts from east to west through the southern part of Jabal Qumayrah where it has eroded a gorge up to 400 m deep through the Sumeini Group across the structural grain of the mountains. Wadi Lisail itself also cuts through the western side of the Jabal Qumayrah massif (Jabal Ghashnah, Figure 5). These wadis join and form of one of a number of major wadis that run towards the west and have carved gorges that slice through the folded and uplifted Cenozoic mountain front, which now reaches an altitude of over 1,000 m in places.

Wadi Fatah, Wadi Sidarat, Wadi Shukkayah and, further south, Wadi Khubayb form a classic antecedent drainage pattern which suggests they had developed before the main phase of folding and uplift, and were able to keep pace with the uplift by cutting down across the growing structures. The main phase of Cenozoic folding and uplift in the northern Oman Mountains is linked with the Zagros event in Iran and took place from the Late Miocene (Boote et al., 1990; Warrak, 1996; Searle, 2007; Searle and Ali, 2009). Down-cutting of the tributaries to Wadi Qumayrah (Wadi Sumer and Wadi Milh, Figure 5) that cut into the salt therefore indicates that exhumation of the salt is also post-Miocene in age.

Age of the Source of the Jabal Qumayrah Salt

Gypsiferous intrusions within the area of the Oman Mountains are rare. They appear to be limited (at the surface) to the Jabal Qumayrah massif and the Hawasina Window about 75 km to the ESE. There, they crop out as elongate 100 m scale bodies along faults in newly excavated road cuts through allochthonous Hawasina (Hamrat Duru Group) sediments (Csontos et al., 2010a, b; Figure 1). Our investigations of the Hawasina Window bodies point towards fault-related tectonic emplacement, with little evidence of large-scale diapirism detected from the initial analysis of a gravity survey of the area. Small, poorly-exposed gypsiferous deposits have also been observed close to the western side of Jabal Sumeini, another major culmination of the Sumeini Group located about 100 km to the north of Jabal Qumayrah, but their origin, age and geological relationships are unclear.

Beyond the Oman Mountains, the SE part of the Arabian Plate has experienced a number of periods during which evaporites were deposited. The main evaporite units comprise the Ediacaran–Early Cambrian Ara Group, the Permian Khuff Formation, the Middle Triassic Jilh Formation, and Late Jurassic Arab and Hith formations, the Early Eocene Rus Formation and horizons within the Oligocene–Miocene Fars Group. Their potential as a source for the Jabal Qumayrah salt is considered below but, in the absence of unequivocal dating or detailed sub-surface evidence, the origin of the Jabal Qumayrah salt intrusion remains unproven.

The Ediacaran–Early Cambrian Ara Group as a Possible Source

The Ediacaran–Early Cambrian Ara Group of the upper part of the Huqf Supergroup, contains the only significant salt deposits in northern and central Oman. Ara Group salts are concentrated to the southwest of the Oman Mountains in three main NNE-trending basins, the Fahud, Ghaba and South Oman salt basins (Figure 1). These basins are parallel to the major Hormuz Salt Basin, where there is also extensive salt diapirism, and the northern and southern Gulf salt basins of the present-day Arabian Gulf (Figure 20a, Loosveld et al., 1996). Collectively, the Ara Group represents sedimentation along the edge of the Pangean supercontinent (Gorin et al., 1982; Wright et al., 1990; Allen, 2007; Bowring et al., 2007). Although these areas are affected to a degree by halokinesis, the development of salt diapirs is limited to the Hormuz Salt Basin and the Ghaba Salt Basin, about 300 km to the SSE of Jabal Qumayrah, where six diapirs pierce the surface. They are absent from the South Oman Salt Basin where NE-trending salt walls prevail, and in the Fahud Salt Basin where the Ara Group salt appears not to have been thick enough or stressed enough to generate diapirs (Heward, 1990).

Seismic studies (Mount et al., 1998) have shown that the circular outcrop of shelf carbonates at Jabal Madar is also underlain by a salt intrusion. Although it has not breached the surface, Mount et al., (1998) suggest this salt probably extends northwards beneath the allochthonous Hawasina and Semail Ophiolite to the south of Saih Hatat (Figure 1). However, no evaporites are seen in the windows through the allochthon at either Al Jabal Al-Akhdar or Saih Hatat. Instead, the interval is represented by pelagic re-sedimented carbonates, cherts, fine-grained turbidites and volcano-clastics of the Fara Formation, which is also Ediacaran–Early Cambrian (Allen, 2007; Rieu et al., 2007).

Allen (2007) notes that the basement structure of Eastern Arabia is dominated by N- to NNE-trending structural highs and lows that have wavelengths of 50–100 km, with the Ara Group salts concentrated in topographic lows. This pattern (Figure 20a) suggests the Arabian hinterland to the Qumayrah area may have formed a tectonic high during the Ediacaran and Cambrian, explaining why seismic sections across the foreland of North Oman show no evidence of salt-related structures (Boote et al., 1990; Dunne et al., 1990; Ali and Watts, 2009). Nonetheless, the Jabal Qumayrah salt could represent a NE equivalent of the Ara Group evaporites either an extension of the Fahud Salt Basin, perhaps either linking the central Oman and Hormuz salt basins, or existing as a remnant of a smaller Infra-Cambrian salt basin which is now situated on the northeast edge of the Oman continental margin following the Permian and Triassic rifting events.

Other Regional Evaporite Intervals

The other main evaporite deposits in the Oman and southern Gulf region that pre-date the Late Cretaceous obduction of the Semail Ophiolite and emplacement of thrust sheets of Hawasina Ocean sediments are the Permian Khuff Formation, the Middle Triassic Jilh Formation, and Late Jurassic Arab and Hith Formations (Ziegler, 2001; Figure 20b). However, these are confined to the interior of the Arabian Peninsula and the Arabian Gulf areas, where they are mainly anhydrite and are not prone to halokinesis (Al-Jallal, 1994; Angiolini et al., 2003; Alsharhan, 2006). No evaporites are developed in the outcrop equivalents in the Oman Mountains, in either the Musandam Peninsula (the Bih and Hagil formations) or the Al Jabal Al-Akhdar area (Saiq Formation and lower part of the Mahil Formation), which comprise more open marine carbonate platform facies related to early rifting of the proto-Neo-Tethyan Ocean (Lee, 1990; Rabu et al., 1990; Maurer et al., 2009). This does not rule out the possibility that small, as yet unknown evaporite basins developed locally along the margin edge as part of the incipient Neo-Tethyan rift, and are now preserved in the deeply buried and unexplored carbonate platform. However, the absence of any evidence of similar basins in the surrounding geology suggests a Permian source is unlikely for the Jabal Qumayrah salt intrusions.

Similarly, broadly shallow marine conditions of part of the Mahil Formation persisted along the Oman Margin during the period of deposition of the Middle Triassic Jilh Formation, with terrigenous clastics being deposited in the Musandam Peninsula (Glennie et al., 1974, Ziegler, 2001). The Late Jurassic intra-basin evaporitic Arab Formation and, in particular, Hith Formation pass into more open-marine facies in western UAE. The Hith Formation is broadly synchronous with the Rayda Formation of the Oman Mountains which marks the foundering of the platform edge further to the southeast (Rabu et al., 1990; Pratt and Smewing, 1990). Again, the geology of the Mesozoic carbonate platform in areas adjacent to the northern Oman Mountains militates against the Jilh and Hith formations being the source of the Jabal Qumayrah salt.

Csontos et al. (2010b) have suggested that, after adjustment for possible sulphate reduction effects, strontium and sulphur isotope measurements from gypsum outcrops in Jabal Qumayrah and the Hawasina Window point towards a possible Late Cretaceous age for these deposits, although they include no details of their underlying sampling or methodology. However, there is no evidence from Jabal Qumayrah or elsewhere that evaporitic conditions could have existed during the Late Cretaceous period of the closure of the Hawasina Ocean and the development of a foredeep linked with nappe emplacement. This argues against a Late Cretaceous age for the Jabal Qumayrah evaporites. Indeed, the observed deep and pervasive weathering of all outcrop areas, and the mobilisation of sulphate minerals recorded (in particular in the pervasive later-stage gypsum veining), suggests the original isotope signatures may have been subject to significant overprinting by fluid circulation and later diagenetic events and weathering.

A post nappe-emplacement Cenozoic origin for the Jabal Qumayrah gypsum and anhydrite is unlikely from both sedimentological and structural perspectives. Cenozoic evaporites were deposed in southeast Arabia during the Early Eocene (Rus Formation) and the Oligocene–Miocene (Fars Group), but the time-equivalent sedimentation in the foreland to the Jabal Qumayrah area does not contain significant evaporites. The Early Eocene is represented by chalky shales, wackestones and calcarenites of the shallow-water to tidal Rus Formation that pass northwards into deeper-water redeposited limestone conglomerates, breccias interbedded with calcarenites and lime-mudstones of the Muthaymimah Formation (Nolan et al., 1990; Le Métour et al., 1991). Miocene sediments now exposed in Jabal Hafit to the NW are predominantly limestones, marls and gypsiferous clays (Warrak, 1996), but major evaporite deposits are not present. As previously noted, the relationships between the Jabal Qumayrah salt and the surrounding country rock suggests it is unlikely the salt was deposited in situ unconformably above the Sumeini Group. Similarly, there is no evidence for large-scale late Cenozoic thrusting in the Jabal Qumayrah area that could have emplaced the Sumeini Group over later Cenozoic evaporites before their subsequent injection into the core of the Jabal Qumayrah structural culmination.

CONCLUSIONS

The Jabal Qumayrah salt intrusion is a rare example of evaporites outcropping in the Oman Mountains. There are no sedimentological features to suggest they were deposited in situ, either as part of the Late Cretaceous Sumeini Group which forms their country rock, or as later deposits lying unconformably over the Sumeini Group rocks. Gravity investigations point towards a salt concentration along the anticlinal axis of the N-S Jabal Qumayrah dome. The central outcrops of gypsum and anhydrite in Wadi Lisail, which contain large rafts of the Sumeini Group country-rock, are collectively interpreted as the highly weathered cap of a salt diapir. The absence of halite is consistent with observations in the Ghaba Salt Basin of central Oman, where it is attributed to weathering and dissolution. The surface expression and emplacement of the Jabal Qumayrah outcrops has been strongly determined by faulting, both around the margins of the main Wadi Lisail outcrop zone, and in the development of pods of evaporites injected along fault lines that extend up to 4 km to the NW and south from this central zone.

The depositional age of the evaporites is unclear, but contextural factors suggest that they may have originated in an unknown basin or basin extension of the Ediacaran–Early Cambrian Ara Group. There are large salt basins to the south and west, one of which, the Ghaba Salt Basin, also contains surface-piercing salt diapirs. This is preferred over an alternative origin from other Permian to Jurassic evaporitic units of southeastern Arabia. The time-equivalent units in the Oman Mountains do not contain evaporites. While it is not impossible that small evaporitic basins developed, in particular during initial rifting along the Oman sector of Neo-Tethys, their absence elsewhere suggests it is unlikely that these could have formed the source for the Jabal Qumayrah salt. Salt intrusion post-dates the Late Cretaceous emplacement of the Hawasina Nappes and obduction of the Semail Ophiolite. There is no evidence that there has been any salt intrusion since the area was unroofed and the evaporites exhumed following the main period of post-Miocene uplift of the massif.

Circumstantial evidence thus points towards intrusion of the salt during the period between the Late Cretaceous emplacement of the Hawasina thrust sheets and Semail Ophiolite onto the Oman margin and the main phase of uplift and erosion of the Northern Oman Mountains during the Miocene. It is thus tempting to suggest that the salt movement was initially triggered by the major loading along the Oman margin during the final stages of ophiolite obduction with diapirism facilitated during the Early Cenozoic in an extensional environment as the margin relaxed and was subject to isostatic uplift. Final emplacement into the Jabal Qumayrah massif may have taken place in a compressional regime linked with the onset of the Zagros event, with the salt injected along faults in the core of the structural culmination. Detailed geophysical investigations in difficult terrain will be needed to constrain the subsurface configuration of Jabal Qumayrah and to test this hypothesis, and we recognise the precise subsurface arrangement must remain speculative in the absence of seismic and well data.

ACKNOWLEDGEMENTS

We are grateful to The Petroleum Institute, Abu Dhabi and Emirates Foundation for sponsoring the project, and to Ian Wood (University College, London) for XRD analyses. Deborah Rees provided logistical support in the field. We would like to thank Alan Heward, Paul Nicholson and an anonymous reviewer for their very helpful comments on earlier drafts of this paper. We appreciate the assistance of the Editor and staff at GeoArabia and in particular Nestor “Nino” A. Buhay IV for the design of the paper and final preparation of the figures.

ABOUT THE AUTHORS

David J.W. Cooper is an independent Consultant Geologist working out of the UK. He obtained a BA in Geology from Oxford University (1982) and a PhD from Edinburgh University in the sedimentology of the Hamrat Duru Group in Oman (1986). This was followed by a NERC research fellowship at Leicester University working on the sedimentology and structure of the Neo-Tethyan continental margin and deepwater sediments in Ladakh, NW India. He has recently returned to geological research after a diverse career working for the UK tax authorities.

djwcooper@gmail.com

Michael P. Searle is a Lecturer in the Department of Earth Sciences, Oxford University, United Kingdom, and is a Senior Research Fellow at Worcester College, Oxford. He obtained his PhD in 1980 working on structures and metamorphism beneath the Semail Ophiolite in northern Oman. Since then he has worked along the length of the Oman Mountains from the Musandam to Masirah Island and the Batain coast. Michael has also worked over 25 years along the Himalayan and Karakoram Ranges in Pakistan, India, Nepal, Bhutan and Sikkim as well as in Tibet, Burma, Thailand and Vietnam. His research integrates field mapping, structural geology, metamorphic and igneous geology, and geochronology with the aim of unraveling the large-scale evolution of mountain belts.

mikes@earth.ox.ac.uk

Mohammed Y. Ali has a BSc in Exploration Geology from Cardiff University, an MSc in Geophysics from Birmingham University, a Postgraduate Certificate in Education from UWCN, and a PhD in Marine Geophysics from Oxford University, UK. His current research projects are focused on exploration geophysics in the areas of passive seismic, seismic stratigraphy and reservoir characterization and modelling. Other research interests include basin analysis, crustal studies, and the structure of passive margins. Mohammed joined the Petroleum Institute in 2003 and currently he is an Associate Professor of Geophysics. He is a fellow of the Geological Society of London and a member of the SEG, EAGE and AGU

mali@pi.ac.ae

Ali Al-Lazki is a Reservoir Geophysicist in the Exploration Department of Petroleum Development Oman (PDO), Oman. He obtained his PhD in Geophysics and Earthquake Seismology from Cornell University, USA, MSc in Geophysics from the University of Tulsa, USA, and BSc in Earth Sciences from Sultan Qaboos University, Muscat, Oman. After receiving his PhD Ali was an Assistant Professor at the Department of Earth Sciences, College of Science, Sultan Qaboos University from 2002–2012. He was the primary author of the paper A crustal transect across the Oman Mountains on the eastern margin of Arabia (GeoArabia, 2002, v. 7, no. 1, p. 47–78). Ali’s current research interests involve the integration of regional scale seismology, geophysics and geology to further understand crust and upper mantle rheology and structure. His research includes using techniques such as Pn-velocity and anisotropy tomography, receiver function analysis, shear wave splitting, and earthquake location analysis. His research in Exploration Geophysics includes the applications of seismic interpretation, gravity modeling, well data analysis, AVO modeling and studying microseismicity associated with hydrocarbon withdrawal.

ali.al.lazki@pdo.co.om.