Central Oman Mountains Research Articles

New radiolarians from the earliest Cretaceous of the Sultanate of Oman (Wahrah Formation, Jebel Buwaydah)

In a section from the central Oman Mountains (Wahrah Formation, Jebel Buwaydah) well preserved radiolarians were extracted. Stratigraphically they represent typical earliest Cretaceous species (Bernasian). Besides well known Tethyan species allowing an exact age determination, unknown morphotypes are common in some samples. Some of these represent new species, which are described herein. The composition and diversity of the faunas hints to a depositional environment in the lower bathyal.

Early Cretaceous Platform-Margin Configuration and Evolution in the Central Oman Mountains, Arabian Peninsula

The Hajar Supergroup (Middle Permian-Lower Cretaceous) of northeastern Oman records rifting and development of a passive margin along the edge of the Arabian platform facing Neo-Tethys. The Jurassic and Lower Cretaceous part, comprising the Sahtan, Kahmah, and Wasia groups, was deposited during the maximum extent of the broad epicontinental sea landward of this margin. These limestone units reach a total of 1500 m in thickness and they correlate with the principal hydrocarbon reservoirs of the Arabian Peninsula. The trace of the Jurassic and Cretaceous margin in northeastern Oman followed a zigzag series of rift segments, resulting in promontories and reentrants that changed in position through time in response to the configuration and differential motion of underlying rift blocks in an overall extensional regime. Synsedimentary normal faulting with 600 m of throw occurred locally during the Middle Jurassic, whereas during the Late Jurassic, the margin was subjected to erosion from variable uplift of up to 300 m before subsiding to below storm wave base. This uplift may have been caused by compression from oceanic crust that obducted along the southeastern side of the platform. Subsequently, Lower Cretaceous strata document varying rates of block subsidence. The Lower Cretaceous succession in the central Oman Mountains and adjacent subsurface began with regional drowning around the Jurassic-Cretaceous boundary. The succession in the east (Saih Hatat) records a single regressive sequence, culminating in the progradation of the shallow-water carbonate platform by the Cenomanian. However, the succession in the west (Jebel Akhdar and interior) is dominated by shallow-water carbonate facies, but punctuated by a second regional drowning in the late Aptian. A third, Late Cretaceous drowning terminated deposition of the Wasia Group in the Turonian and was caused by convergence of oceanic crust and foreland basin formation. Although previously ascribed to eustatic sea level rises, the two Early Cretaceous drowning events can be explained as consequences of the tectonic behavior of the platform. The earlier event (Rayda Formation) occurred when first uplift tilted the eastern side of the platform and induced a relative sea level stillstand and widespread evaporative conditions in the interior (Hith anhydrite), and then the eastern side subsided rapidly to below storm wave base, forming a broad embayment starved of periplatform sediment. By contrast, the second drowning (Nahr Umr Formation) was due to detrital influx generated by uplift on the west side of the platform. Both tectonic episodes led to a reduction in the calcium carbonate generating capacity of the epicontinental sea, causing shallow-water fa ies to be abruptly overlain by deeper water facies. The effects from these two drownings eventually diminished, and shallow-water environments reestablished themselves. The overall succession appears independent of eustatic sea level change.

Mid-Cretaceous ammonites and associated microfossils from the Central Oman Mountains

Mid-Cretaceous ammonites and associated microfossils from the Central Oman Mountains

Cenomanian echinoids, larger foraminifera and calcareous algae from the Natih Formation, central Oman Mountains

Within the Natih Formation (late Albian-early Turonian) of the Oman Mountains, there occurs a distinctive, correlatable horizon with an abundant and moderately diverse echinoderm fauna. This horizon occurs within Member c of the Natih Formation and can be assigned a latest middle Cenomanian age based on the associated microfossil assemblage which is dominated by alveolinids and gymnocodiacian algae. The depositional environment of this echinoderm-rich bed was probably back barrier, close to radiolitid rudist biostromes and bioclastic shoals which fringed the upper Natih shelf edge. Nine species of echinoid are described, most of which are new records for the Arabian Peninsula. One new species, Pedinopsis sphaerica , is erected. Coenholectypus larteti dominates the assemblage and occurs together with Stereocidaris sarracenarum, Tetragramma variolare, Heterodiadema lybica, Pedinopsis humei, P. desori, P. sphaerica, Hemiaster syriacus and H. cubicus . The composition of the genus Pedinopsis is critically reviewed and a new subgenus, Sinaiopsis , erected. Many of the species have previously been reported from Egypt, the Sinai Peninsula and Israel.

Structures associated with nappe emplacement and culmination collapse in the Central Oman Mountains

Abstract The Fanjah Saddle, in the Central Oman Mountains, forms a structural and topographic depression between two major culminations which expose parautochthonous rocks in windows through the overlying allochthon. Two styles of normal faulting are present in the culminations: high-angle faults which dip toward the saddle core and cut the plunging noses of the culminations, and low-angle faults around the culmination flanks which include bedding-parallel detachments, reactivated thrust planes and surge zones. This second set of faults verge down-dip radially away from the culmination flanks. The high-angle normal faults are interpreted to be accommodation structures resulting from the differential uplift of the parautochthon between the culminations and the saddle. The low-angle normal faults are interpreted to be gravity-driven culmination collapse structures resulting from oversteepening of the culmination flanks. Post-orogenic late Campanian to Maastrichtian sediments adjacent to the culminations indicate that the culminations were initialy emergent during this time, acting as a clastic source. These same sediments, and the underlying nappe pile, are folded by compression from displacements on the normal faults which implies culmination uplift was reactivated after the Maastrichtian, possibly during post-middle Eocene Zagros orogeny.

Jurassic and Early Cretaceous platform margin configuration and evolution, central Oman Mountains

Abstract The Hajar Supergroup is a Middle Permian to middle Cretaceous sequence dominantly composed of shallow-water carbonate rocks which veneer the eastern edge of the Arabian Platform in the Oman Mountains. The Jurassic and Early Cretaceous components of this package were deposited on the passive margin of Neo-Tethys which developed after initial rifting in the Late Permian and Triassic. Strata studied in Jebel Akhdar, Saih Hatat and neighbouring uplifts document low- and high-energy, shallow-water depositional environments punctuated by two drowning events when deeper water, slope conditions migrated onto the shelf margin. The mid-Jurassic event may have been precipitated by synsedimentary down-faulting of the Saih Hatat area as rifting continued. The latest Jurassic, earliest Cretaceous event, depositing the Rayda Formation, is ascribed to a regional decrease in carbonate production because of widespread evaporitic conditions in the Arabian Platform interior. The configuration of the Mesozoic platform margin in northern Oman conforms to a series of rift and transform segments inherited from the underlying crust when it separated in the Early Jurassic. The transform segments became the focus of major structural lineaments during subsequent thrusting and obduction. This provides an especially clearcut example of the structural control of passive margin geometry, as well as its effect on collision-related deformation. A new Jurassic lithostratigraphic unit, the Saih Hatat Formation, is formally described.

Structural evolution of the Suneinah Foreland, Central Oman Mountains

Abstract In the early Mesozoic, the Suneinah Foreland of the central Oman Mountains lay on the flank of a north facing Tethyan rift margin. By the end of the Middle Cretaceous, this had evolved into a carbonate dominated passive margin. Following regional uplift and erosion during the Turonian, it was downwarped, faulted and buried by a deeper water shale facies. At the beginning of the Campanian, the earlier Tethyan sea floor and continental rise prism were rapidly telescoped across the margin, loading and downflexing the crust to form the narrow Suneinah Foredeep and peripheral bulge beyond. By the end of the Cretaceous this foredeep had been filled with marls and coarse detritus shed from the adjacent allochthon, and during the Early Tertiary a transgression swept across and blanketed much of the eroded nappe surface. In the Late Oligocene-Middle Miocene the Arabian plate began to separate from Africa and move northeast towards the Eurasian crustal collage. The Musandam promontory acted as a spur or indentor on this advancing plate, focussing compression and transmitting it back along deep basement fractures in the old Tethyan rift domain. Intense transpressional deformation inverted the entire margin to form the modern Oman Mountains and a new foredeep along its western flank. Reactivated Mesozoic fault blocks fringing the inversion orogen in the Suneinah area, deformed fairly rigidly, but the more plastic Late Cretaceous and Tertiary section above was squeezed into sometimes quite spectacular high amplitude folds. With the onset of sea floor spreading in the Red Sea/Gulf of Aden and northerly movement of the Arabian plate along the Dead Sea transform during the Late Miocene-Pleistocene, tectonic intensity waned in the Oman Mountains and much of its earlier sedimentary cover was stripped off.

The post-Campanian tectonic evolution of the Central Oman Mountains: Tertiary extension of the Eastern Arabian Margin

Abstract This paper documents the structural history of the central Oman Mountains after the obduction of the Semail ophiolite onto the eastern margin of the Arabian continent during the Late Cretaceous (Santonian/Campanian). The post-obduction history is recorded by a Late Campanian/Maastrichian-Tertiary sedimentary cover to the mountain belt. These sediments have been deformed within an extensional tectonic regime which is continuous northeastwards into the Gulf of Oman hinterland basin and southwestwards away from the SW foothills. Large-scale down-to-the-basin normal faults have been recognized on the Batinah Coastal Plain NE of the mountain belt. Localized block-faulting and roll-over folds are associated with major faults that separate the Late Campanian to Tertiary sediments from older basement rocks of the Oman Mountains and with Late Cretaceous thrusts reactivated as normal faults. To the SW of the mountains, large NW-SE striking folds are thought to be gravity structures developed above a decollement in Late Cretaceous shales. Syn-sedimentary tectonic influences are summarized and integrated with the structural analysis to identify the style and timing of tectonism on the S margin of the Gulf of Oman from the Late Campanian/Maastrichtian to present. There is evidence to suggest that penecontemporaneous tectonics controlled rates of subsidence and facies development during this time. A major E-W trending palaeo-high running close to the present-day shoreline affected Palaeogene sedimentation. Unlike the northern Oman Mountains (Musandam Peninsula), there is no unequivocal evidence of mid-Tertiary (Zagros) compressional structures in the central Oman Mountains. Structural development of the cover sediments in the central mountain belt is believed to be controlled by underlying Late Cretaceous basement structures. In particular the Saih Hatat culmination was a positive structure throughout the Late Cretaceous and Early Tertiary, affecting both the stratigraphic and structural history of adjacent areas. The northern end of the Nakhl culmination seems to have had little effect on sedimentation but did influence deformation in the Tertiary.

The Hawasina Nappes: stratigraphy, palaeogeography and structural evolution of a fragment of the south-Tethyan passive continental margin

Abstract Lithostratigraphic and biostratigraphic revision of the Hawasina Nappes in the eastern and central Oman Mountains, including the redefinition of the Hamrat Duru Group and the definition of the three new groups, the Al Aridh, Kawr and Umar Groups, has led to a new interpretation of the palaeogeographic and structural evolution of the south-Tethyan continental margin. Margin history began in the Late Permian with a phase of extension and rifting, accompanied by considerable magmatic activity, and led to the development of the Hamrat Duru Basin separating the Arabian Platform to the south from the Baid Platform to the north. In the Middle to Late Triassic renewed extension led to rifting, again accompanied by important magmatic activity, with the break-up of the Baid Platform, and the development of the Al Aridh Trough, the Misfah Horst and the Umar Basin. These Permian and Triassic tectonic units together constituted the Hawasina Basin. A third phase of extension during the Late Tithonian—Berriasian caused a general foundering of the continental margin. The development of the Hawasina Basin terminated in the Santonian, when compression initiated the first stage of obduction that closed the basin. Ongoing obduction, during the Campanian, led to thrusting of the Hawasina and the Samail ophiolite nappes onto the Arabian Platform by gravitational mechanisms.

Sedimentary evolution and palaeogeographical reconstruction of the Mesozoic continental rise in Oman: evidence from the Hamrat Duru Group

Abstract The mainly Mesozoic Hamrat Duru Group represents deep-water, primarily continental rise Neo-Tethyan sediments derived from the Oman continental margin and later emplaced onto the Arabian carbonate platform during the Late Cretaceous obduction of the Semail ophiolite. Sediment patterns reflect fluctuations in sea level relative to the platform, with carbonate input dominant during periods of platform stability. High and low sea-level stands, controlled by eustacy or tectonism, were accompanied by significant reductions of carbonate production and input into the basins. Raised calcite compensation depths associated with high sea levels resulted in the deposition of cherts whereas, during low stands, terrigenous clastics prograded across or by-passed the platform into the Hawasina basin. The Hawasina basin was divided into two sedimentary basins separated by a submarine high. The shale-rich ‘Al Ayn sub-basin’ contained coarse-grained sediments that formed either small fans, or laterally coalescing sediment bodies that were fed by multi-source input of carbonate and terrigenous clastics. This is preserved as the Hamrat Duru Group of the northern and central Oman Mountains. The carbonate-rich ‘Duru sub-basin’ was fed exclusively from the south and large sediment bodies with sheet-like morphologies extended northwards parallel to the margin, outboard from the Al Ayn sub-basin, with thinning and fining trends to the NW and NE. Duru basin units are found in the central and southern Oman Mountains. Contrasting sedimentation patterns in the Al Ayn subbasin and the Duru sub-basin may reflect the influence of prevailing SE trade winds on the margin.

Evolution of the Triassic Hawasina Basin, Central Oman Mountains

Abstract Sediments of Late Triassic age preserved in the upper Hawasina nappes of the Central Oman Mountains document the early sedimentary and oceanographic evolution of a segment of the Triassic seaway bordering the northern Gondwanian continental margin. Within the Zulla Formation of Glennie et al. four members can be distinguished. 4. Halobia limestones (Lower to ?Upper Norian) 3. Radiolarian cherts (Ladinian to Lower Norian) 2. Sandstone-shale member 1. Calcarenite-shale member Changes in composition and abundance of turbiditic sediments in the Zulla Formation reflect environmental changes in the source areas, probably linked to sea-level fluctuations, whereas changes in the pelagic facies reflect basin-wide oceanographic fluctuations. Episodic low-oxygen conditions are indicated by laminated dolomites in the calcarenite-shale member. An increase in radiolarian abundance may record an increase in surface-water productivity possibly related to a sea-level rise and corresponding changes in the circulation pattern. A deepening of the CCD during the Early Norian is documented by the Halobia limestones. Similar facies types are encountered along the South-Tethyan seaway (Lagonegro-Pindos-Antalya-Pichakun). However, diachroneity of facies changes from radiolarite to limestone point to differences in the bathymetric and/or oceanographic evolution of the basins.

The Alpine deformation of the Central Oman Mountains

Abstract The Saih Hatat, Jebel Nakhl, Jebel Akhdar and the Hawasina Window structures of the Oman Mountains developed as culminations above a major sole thrust associated with late stage emplacement of the ophiolite from the northeast onto the Arabian margin. The outcrop of a pre-Permian sequence in the cores of Jebel Akhdar, Jebel Nakhl and Saih Hatat implies that the major sole thrust is located within the pre-Permian succession. This thrust is emergent in the southern foothills of the mountains south of the Jebel Salakh Range. In the Hawasina Window, however, the thrust is at a higher level and is located within the slope and oceanic sediments. An extensional regime due to the gravity sliding of the Semail ophiolite is recognized from the occurrence of surge zones, extensional duplexes, extensional cleavage and gravity folds. The development of the extensional faults, which occur in the frontal, dorsal, lateral and oblique walls of the above culminations, may account for the absence and locally thinning of the Tethyan thrust sheets around the flanks of these structures. The extensional tectonic regime is also believed to be responsible for the final extensional nature of the ‘Semail Thrust’. The Oman Mountains foreland fold and thrust belt mostly consists of a series of thrust sheets of oceanic sediments (Hawasina) together with widespread exposures of syntectonic (Aruma) deposits. The belt is dominated by imbricate stacks of both Hawasina and Aruma units. These imbricate stacks have been folded around major culminations arranged in a characteristic en echelon pattern. Late Alpine (Tertiary) deformation is largely attributed to an extensional regime of culmination collapse. Folds and thrusts were developed at the leading edge of the extensional faults (surge zones). Tertiary folding and thrusting may reactivate the pre-existing Early Alpine (Late Cretaceous) structures.

The Hawasina Basin: A fragment of a starved passive continental margin, thrust over the Arabian Platform during obduction of the Sumail Nappe

A complete revision of the lithostratigraphy of the Hawasina Nappes in the Eastern and Central Oman Mountains, as compared to that of the previous stratigraphy ∗ ∗ Glennie et al. (1974). , leads to a new palaeogeographical reconstruction of the basin. Five very distinct zones have been determined, representing five principal structural units that succeed one another from the external to the internal zones as follows: 1. (1) the Hamrat Duru Basin, 2. (2) the Baid Platform, 3. (3) the Al Avidh Trough, 4. (4) the Misfah Platform, and 5. (5) the Umar Basin. The history of the Hawasina Basin began in the Late Permian with the formation on the northeastern edge of Gondwana of a vast intracontinental basin, the Hamrat Duru Basin. However, this basin became a passive continental margin only during the Middle-Late Triassic. Formation of the Neo-Tethys by crustal spreading and accretion began at this time, in the internal part of the basin and in continuity with the Umar Basin, the Misfah Platform and the Al Aridh Trough which formed at the same time within the Hawasina Basin. From this period onwards, the main geomorphological features of the basin and of the margin were fixed and persisted up to the Late Cretaceous. The phase of extension by which the basin was subsequently affected during the Late Tithonian-Berriasian was mainly manifested by generalized subsidence. These main stages in the formation and evolution of the Hawasina Basin are marked by extensive development of in general alkaline, but in places transitional and locally tholeiitic MORB magmatism. The sediments of the Hamrat Duru Basin are dominated by turbidites composed of material mainly derived from the Arabian Platform, whereas the Al Aridh Trough and Umar Basin are characterized by pelagic sediments. However, the Al Aridh and Umar grabens also contain layers of breccia of variable extent, containing clasts and blocks of Late Permian and Late Triassic shallow-water carbonates derived from the adjacent Baid and Misfah platforms. Sedimentation in the Hawasina Basin thus began locally in the Late Permian and by the Triassic it was taking place in the remaining parts of the basin, continuing up to the Late Turonian-Early Senonian. When the basin closed and overthrusting during obduction of the Sumail Ophiolite occurred, sedimentation ended. The overthrusting took place in three stages: 1. (1) a southerly directed, compressive, pre/early obduction phase during which the Hawasina Basin was detached from its substratum to form an obduction prism, 2. (2) an obduction phase sensu stricto which, by gravitational mechanisms, induced major displacement of the principal Hawasina units and their piling up on the Arabian Platform (with the outermost units at the base and the innermost at the top), and 3. (3) a late obduction phase which caused local north-northeastward backfolding, possibly associated with the development of a fracture cleavage which, in the basal structural levels, may pass into true schistosity.

The upper Hawasina nappes in the central Oman Mountains: stratigraphy, palinspastics and sequence of nappe emplacement

In the Oman mountains, a succession of sedimentary decollement nappes, the Hawasina nappes, is sandwiched between the Samail ophiolite nappe and its underlying melange and the “autochthonous” sequences of the Arabian platform. The sediments of the Hawasina nappes document the Mesozoic evolution of the northeastern Arabian continental margin and the adjacent Tethys Ocean. In earlier paleogeographic reconstructions, based on simple telescoping of the tectonic units, the upper Hawasina nappes represent the distal part and the lower nappes the proximal part of the margin. New stratigraphic data suggest a revision of the paleogeography and a more complex model for nappe emplacement in the central Oman mountains. The lower Hawasina nappes with their Jurassic and Cretaceous base of slope and basin sediments (Hamrat Duru, Wahrah) form the original cover of part of the upper Hawasina nappes. In the latter (Al Ayn, Haliw), Triassic pelagic sediments, locally overlain by massive sandstone successions are preserved. Complete Mesozoic sequences with pelagic Cenomanian sediments as youngest dated elements are found in the highest Hawasina units (Al Aridh and Oman Exotics). The stratigraphic data indicate polyphase thrusting in the central Oman mountains. Downward propagation of thrusting in front of the Samail is responsible for cutting the original stratigraphie sequence into a number of thrust-sheets, involving successively older and more external formations. This kind of thrust propagation eventually leads to the observed superposition of originally lower stratigraphie units onto their original cover. Regional deformation of the nappe contacts in post-nappe culminations (J. Akhdar, Saih Hatat) is related to ramp-flat-systems in the Arabian foreland.

Structure of the Hawasina Window culmination, central Oman Mountains

ABSTRACTDetailed mapping, stratigraphic logging and structural analysis of the Hawasina Window culmination in the central Oman Mountains of Arabia reveals an extremely complex thrust geometry and structural history. The initial thrust sequence involved a southwestward propagating stacking during telescoping of the Arabian continental margin slope (Sumeini complex), and time-equivalent, more distal Tethyan basin (Hawasina and Haybi complexes) facies rocks. The Semail thrust, carrying the 12 km-thick ophiolite sequence, progressively overlaps Haybi and Hawasina duplexes towards the SW. Late stage “leap-frog” thrusts have punched Sumeini duplexes higher up into the earlier thrust stack locally reversing the normal stacking order. SW-directed thrusts and SW-facing folds in the SW and NE-directed backthrusts and NE-facing backfolds in the NE have created a fan structure cored by the Jebel Rais “pop-up” composed of Sumeini slope facies rocks.The palaeogeographic presence of a large NE-facing promontory in the Cretaceous shelf edge is inferred beneath the Window. The frontal ramp of this promontory was sufficiently large to inhibit the overthrusting of large volumes of Hawasina and Haybi complex rocks. The whole central part of the Window shows NE-facing and verging backfolds and backthrusts affecting all duplexes from the lowest Sumeini up to the Semail ophiolite. The promontory is bounded by major lateral ramps to the NW (Wadi Shafan area) and SE (Jebel Milh area). A map and four balanced cross-sections are presented here to promote a complex model for the structural evolution of the Hawasina Window.

Sequence of thrusting and origin of culminations in the northern and central Oman Mountains

Detailed mapping and structural analysis of three large-scale culminations (Sumeini and Asjudi half-windows and Haybi-Hawasina window) in the Oman Mountains shows a considerably more complex history of deformation than a simple foreland (or downward) sequence of thrust development. Early thrusting processes tended to create a regular stacking order of imbricate slices and major thrust sheets, complying with the “rules’ of thrust propagation, assembled progressively downwards and forwards in the direction of translation. ‘Out-of-sequence’ thrusts can also be demonstrated in places by truncation of footwall structures (folds, imbricate slices, etc.), gross strain differences between thrust sheets, downward-facing structures in footwall units and elimination of thrust sheets beneath. Late stage thrusts frequently cut up-section through the previously assembled stack putting previously younger, lower thrust sheets over previously older, higher ones. Many of the culminations in the northern and central Oman Mountains were formed by ramping associated with this late-stage leap-frog rethrusting event.

Culmination Collapse in Fanjah Saddle, Oman: ABSTRACT

Culmination collapse structures in the central Oman Mountains have overprinted and clouded the surface expression of deep-seated regional structures, as typified by the Fanjah Saddle. The saddle forms a domain of low structural relief situated along the trend of a series of major structural culminations. It is bounded on the east and west by the plunging noses of the parautochthonous Mesozoic shelf carbonates of the Saih Hatat and Jabal Nakhl culminations, respectively, and contains a stack of Late Cretaceous basinal and ophiolitic thrust sheets. Culmination collapse, initiated by backslip movement of the dense overlying nappes, has resulted in a saddle geometry containing both extensional and compressional regimes. Listric normal faults in the culminations converge toward the saddle and serve as feeder zones to the saddle core which acts as a sink for the thrust sheets. Smaller surge zones with extensional trailing edges and compressional fronts defined by recumbent folds radiate outward from the culminations. This has been superimposed over earlier isoclinal folding produced by the emplacement of the overlying nappes. The final result is a complex internal geometry and distribution pattern of structures. Distinguishing these shallow deformational events from deep seated structural styles is essential for the correct extrapol tion of surface data to the subsurface in any exploration program. End_of_Article - Last_Page 463------------

Late Cretaceous Nappes in Oman Mountains and Their Geologic Evolution

The rocks of the central Oman Mountains can be divided into five main units. These comprise, from bottom to top: a pre-Permian basement that stratigraphically is overlain by mostly shallow-marine carbonate rocks ranging in age from Middle Permian to Late Cretaceous (Senonian). Above this, several thin and imbricated nappes, together with several large (mountain-size) exotics, form the allochthonous Hawasina unit. The age range of the strata within each of these nappes covers a part of the time span from Permian to Cenomanian. The sediments vary from the Permian and Triassic shallow-marine limestones of the exotics that were deposited over a substrate of pillow lavas, to limestone turbidites and cherts which locally are associated with minor amounts of basaltic pillow avas. In the northern mountains the Hawasina tectonically overlies Permian to Cenomanian sequences of reefal limestones, slump conglomerates, turbidites, and calcareous mudstone of the Sumeini Group. The Hawasina is overlain by the Semail nappe, an enormous mass of ophiolites. Minor amounts of pelagic sediment dated as Cenomanian and early Senonian are associated with the submarine lavas of the ophiolite suite. Both the Hawasina and the Semail are overlain unconformably by Maestrichtian and early Tertiary sedimentary rocks. From the sedimentary and structural complexities of this mountain range, the following simplified interpretations have been made. 1. The autochthonous Permian to Senonian shallow-marine carbonate sediments were deposited on the continental margin of Arabia. 2. The Sumeini Group was deposited on the northeastern edge and continental slope of Arabia. 3. The age range of each of the Hawasina sedimentary sequences is based partly on the recognition that many of the microfossil assemblages from turbidite sequences were derived penecontemporaneously (rather than reworked into a younger sequence) and form part of a normal stratigraphic succession. 4. The sedimentary rocks of the Hawasina nappes were deposited northeast of the present Oman Mountains between the Permian and the Cenomanian. 5. The rocks that now constitute the Semail nappe were generated in an upwelling part of the earth's mantle (peridotites and gabbros) and on the ocean floor (lavas). The upwelling is thought to have been associated with ocean-floor spreading and to have given rise to an oceanic ridge which formed the distal limit to that part of the basin in which the Hawasina sediments were deposited. 6. During the Late Cretaceous (late Campanian) the Sumeini and Hawasina sedimentary rocks and the Semail ophiolites were emplaced tectonically into their present location on the outer part of the Arabian continental margin. 7. After emplacement, the Hawasina and Semail nappes were covered unconformably by shallow-marine Maestrichtian and early Tertiary limestones. 8. The outer part of the Arabian continental margin (together with its cover of nappes and younger marine limestones) was uplifted during the Oligocene and early Miocene to form the present Oman Mountain range.