Northeastern Oman Research Articles

Petrogenesis of early cretaceous carbonatite and ultramafic lamprophyres in a diatreme in the Batain Nappes, Eastern Oman continental margin

Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern Oman. The diatreme consists of heterogeneous deposits dominated by ‘diatreme facies’ pyroclastic rocks. These include aillikite and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain nappes. Both rock types are dominated by carbonate, altered olivine, Ti–Al–phlogopite and Cr–Al–spinel and contain varying amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized by low SiO2 (11–24 wt%), MgO (9.5–12.4 wt%) and K2O (<0.3 wt%), but high CaO (18–22 wt%), Al2O3 (4.75–7.04 wt%), Fe2O3tot (8.7–13.8 wt%) and loss-on-ignition (24–30 wt%). Higher CaO, Fe2O3total, Al2O3, MnO, TiO2, P2O5 and lower SiO2 and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite, 66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite, carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep slopes with (La/Sm)n of 6–10 and relative depletion in heavy rare earth elements (Lu = 3–10 chondrite). Initial 87Sr/86Sr ratios vary from 0.70409 to 0.70787, whereas initial 143Nd/144Nd ratios vary between 0.512603 and 0.512716 (eNdi between 2.8 and 3.6). 206Pb/204Pbi ratios vary between 18.4 and 18.76, 207Pb/204Pbi ratios vary between 15.34 and 15.63, whereas 208Pb/204Pbi varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 ± 1 Ma (95% confidence, MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140–160 Ma. Geochemical and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean.

57Fe Mössbauer spectroscopy study of phlogopite megacrysts from an evolved carbonatitic kimberlite in the northeastern Oman Mountains

The Fe oxidation degree determined by 57Fe Mossbauer spectroscopy and microprobe was used to characterize fresh and altered phlogopite megacrysts from an evolved carbonatitic kimberlite from northeastern Oman. The Quadrupole splitting (QS) varies between 2.19 and 2.48 mm/s (Fe2 + ) in the fresh phlogopite samples and between 2.40 and 2.47 mm/s in the altered phlogopite samples. The quadrupole splitting of the Fe3 + doublets varies between 0.66 and 0.85 mm/s in the fresh samples. The altered phlogopite samples show three Fe3 + doublets; the first show a quadrupole splitting between 0.97 and 1.13, the second quadrupole splitting varies between 0.24 and 0.46 mm/s and the third varies between − 0.23 and − 0.35 mm/s. The phlogopite was observed to have an average Fe3 + /Fetotal of 35% to 37%, and corresponds to fresh phlogopite. The second one results from the alteration of the first type, and the Fe3 + /Fetotal ranges between 40% and 57%. Tetrahedral Fe3 + ions were confirmed in the altered phlogopite samples. Quantitative Fe site distributions can be obtained from room-temperature Mossbauer data if the different recoilless factors for octahedral Fe2 + and tetrahedral Fe3 + are considered. The observed isomer shifts are consistent with Mossbauer temperatures of 330 K, reported in the literature for tetrahedral and octahedral Fe3 + and Fe2 + in phlogopite. The results are compared to those obtained for natural and synthetic phlogopite from worldwide.

Paleogeographic significance of Early Permian crinoids and blastoids from Oman

Early Permian crinoids and blastoids from Oman show relationship with Late Paleozoic Tethyan faunas of Timor and Western Australia and support an Early Permian age for part of the Basleo fauna of Timor. The camerate crinoidPlatycrinites omanensis n. sp. and the blastoidsTimoroblastus andDeltoblastus are reported for the first time from Sakmarian strata of northeastern Oman, doubling the known Permian echinoderms from Oman. The blastoids suggest an offshore lower energy shelf environment of deposition.

Role of fluvial and structural processes in the formation of the Wahiba Sands, Oman: A remote sensing perspective

The Wahiba Sands in northeastern Oman are bordered on the north, south and west by highlands. Remote sensing data are used to characterize the region between 19–23.5°N and 56.5–60°E by mapping surface and near-surface drainage, faults and fractures and aeolian features. It is suggested that the sands were originally deposited with surface runoff from the principal wadis and fluvially reworked fault zones, which define the northeastern and southwestern margins. These fluvial processes resulted in the accumulation of the vast groundwater resources now stored there. During dry climates, wind became the principal modification regime and it began to sort and shape the sediments into the dune forms that characterize today's Wahiba region. The thickness of the sands reflects the depth of the basin in which they lie. The center of the basin is filled with the thickest sand (the High Sands) and contains the highest groundwater concentrations. Presently, aeolian reworking dominates in the Wahiba region, although the Low and the Peripheral Sands continue to experience some fluvial action from occasional, seasonal rainfall. Even though dry conditions dominate today, it is clear that similar to the Sahara of North Africa, the surface sands of the Wahiba basin are indicators of groundwater occurrence.

Petrology and Chemistry of Metasomatic Blocks from Bawshir, Northeastern Oman

Several types of Ca-rich, metasomatically altered rocks occur as blocks or reaction zones in a serpentinite-matrix mélange in the Bawshir-Al-Khuwair area, west of Ruwi in northeastern Oman. Type A rodingites contain mainly hydrogarnet, with smaller amounts of diopside, tremolite ± pumpellyite ± prehnite. Type B rocks contain more diopside ± tremolite and smaller amounts of hydrogarnet. Type C nephrites consist chiefly of tremolite and have a distinctive botryoidal texture. Quartzite blocks in this mélange were also affected by Ca-metasomatism, and contain variable amounts of tremolite, hydrogarnet, chlorite, and clinozoisite. Relatively immobile trace and rare-earth element contents of these rocks suggest that the protoliths of types A and B metasomatic rocks were ocean-island basalts, whereas type C nephrites represent metasomatized serpentinites. Isocon diagrams show that all metasomatic rocks formed by the addition of Ca ± Mg ± volatiles, and leaching of almost all other elements to variable degrees. Field and textural relations interpreted with an activity-corrected petrogenetic grid in the system CMASH suggest that metasomatism took place in stages over a range of P-T conditions, beginning at 220-310°C and 3-6 kbar, and ending at 250-310°C, P < 3.2 kbar. Diffusion of Ca2+ out of the serpentinizing peridotites into blocks of basalts and quartzites was the primary cause of early metasomatism, which overlapped with high P/T metamorphism of the same blocks. Infiltration of low-salinity aqueous fluids was partly responsible for the latest stages of metasomatism, which took place during exhumation, and continued after the final emplacement of the ophiolite.

Stratigraphic architecture of the northern Oman continental margin - Mesozoic Hamrat Duru Group, Hawasina complex, Oman

ABSTRACT The Triassic to Late Cretaceous deep-marine sediments of the Hamrat Duru Group, Oman Mountains, represent a subunit of the Hawasina nappe-complex which was deposited in a deep marine basin. During the Late Cretaceous SSW-directed obduction of the Semail Ophiolite, the Hawasina complex was emplaced onto the autochthonous cover of the Arabian basement, while the original configuration of the basin was destroyed. New lithostratigraphic results and high-resolution radiolarian and conodont biostratigraphy lead to a revised stratigraphic scheme of the Hamrat Duru Group which conforms with the standard stratigraphical nomenclature. The Hamrat Duru Group is divided into six formations: (1) The Early Triassic (Olenekian) to Late Triassic (Upper Norian) Zulla Formation (Limestone and Shale Member, Sandstone and Shale Member, Radiolarian Chert Member and Halobia Limestone Member); (2) The Late Triassic (late Norian to Rhaetian) Al Ayn Formation; (3) The Early Jurassic (late Pliensbachian) to Middle Jurassic (early Callovian) Guwayza Formation (Tawi Sadh Member and Oolitic Limestone Member); (4) Middle Jurassic (Callovian) to Late Cretaceous (Cenomanian?) Sid’r Formation (Lower Member, Upper Member); (5) Late Cretaceous (Cenomanian? to Santonian?) Nayid Formation; and (6) Late Jurassic (early Callovian) to Early (Late?) Cretaceous Wahrah Formation. Most of the lithostratigraphic units (formations and members) show isochronous boundaries between the different outcrop areas. The stratigraphic architecture of the Hamrat Duru Group megasequence is controlled by alternating siliciclastic and carbonate sedimentation possibly related to the second-order sea-level variations. The sediments accumulated on the continental rise of the Arabian margin mostly by submarine sediment-gravity flows and hemipelagic to pelagic rainout. A close relationship of the evolution of the Arabian Platform and the adjoining slope and basinal environments is evident. Changes in carbonate supply, oceanographic circulation and/or variations in silica productivity resulted in two distinct phases of radiolarian sedimentation. The first phase corresponds to the Triassic late Anisian-early Norian time interval; the second started in the Early Jurassic late Pliensbachian and lasted, with some interruptions, up to the Late Cretaceous Coniacian. The litho- and biostratigraphic similarities between the Mesozoic Hamrat Duru Basin of the northern/central Oman Mountains and the Mesozoic Batain Basin of northeastern Oman are seen as related to Neo-Tethys-wide palaeoceanographic changes and suggest a strong interdependence of the two basins with the evolution of the Arabian Platform.

The permian sequence reconstructed from reworked carbonate clasts in the Batain Plain (northeastern Oman)

An almost complete Permian sequence is reworked in the conglomerates and sandstones of the Aseelah Unit forming the lower part of the Batain Croup. This series of Late Permian to Earliest Triassic age overlies conformably the Qarari Unit, dated as Middle Permian by ammonoids. The bioclastic limestone pebbles and boulders deposited on shallow marine-shelf environments yield diversified assemblages of dasyclads, red algae, fusulinids and smaller foraminifers, indicating Yakhtashian, Bolorian, Kubergandian, Early Murgabian, Midian and Dzhulfian ages (systematic work in progress).

Late Palaeozoic to Neogene geodynamic evolution of the northeastern Oman margin

When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ∼80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous–Palaeogene transition (∼65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains.

Detrital chromites in metasediments of the East-Arabian continental margin in the Saih Hatat area: constraints for the palaeogeographic setting of the Hawasina and Semail basins (Oman Mountains)

In Oman, the convergence between Arabia and Eurasia resulted in the Late Cretaceous overthrusting of oceanic crust and mantle lithosphere onto the Arabian continental margin. During this compressional event, a part of the continental plate was subducted to a depth of more than 60 km (0.5 GPa, 250–350 °C to more than 2.0 GPa, 550 °C) resulting in progressive metamorphism of the continental margin sediments, well exposed in the Saih Hatat tectonic window, northeastern Oman Mountains. We attempt to constrain the possibility of one continuous history of extension (starting along the east Arabian continental margin in the Permian) that was followed by one continuous history of convergence starting at 90 Ma near a dead oceanic ridge. This compression resulted in the observed progressive metamorphism by ophiolite overthrusting onto the continental margin. Constraining arguments are the palaeogeographic setting before ophiolite obduction of the As Sifah units and the Hawasina Complex near Ghurba. Detrital chromites in the Triassic–Cretaceous metasediments of the Hawasina Complex are compared with magmatic Semail chromites, and the whole-rock chemistry of these metasediments and associated metabasites are investigated. In contrast to former hypotheses, differences in the chemical composition between detrital and magmatic chromites, and the probable origin of all detrital chromites in the Hawasina Basin from Permian age oceanic rocks, suggest that the high-pressure metamorphic sediments of As Sifah can be considered as part of the basal deposits of the Hawasina Basin.

Exhumation of high-pressure rocks in northeastern Oman

The exhumation of high- P rocks in the northeast Saih Hatat window, Oman Mountains, formed regional nappes associated with greenschist-facies metamorphism. Exhumation resulted in the juxtaposition of two plates with different structural style, stratigraphy, and metamorphic history. A major crustal discontinuity separates the two plates, with lower-metamorphic-grade rocks in the hanging wall. Lower-plate units were exhumed during strong top-to-the-northeast shearing, resulting in the formation of regional closures that evolved by folding and transposition of the high-pressure fabrics while the lithostatic pressure was decreasing. Upper-plate units were later exhumed with the juxtaposition of the two plates, truncation of lower-plate closures, and the formation of upper-plate nappes that are attenuated and sheathlike toward the upper and lower plate boundary. In some high- P terranes, apparent increases in metamorphic-grade down structural section and low-grade C′-type shear bands are often attributed to extensional exhumation. In Oman, however, these exhumation features formed during crustal-scale contraction at a convergent margin. The low-geothermal-gradient regimes afforded by convergent margins are critical for the generation and preservation of high- P –low- T rocks.

Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of P – T conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by &lt;l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant 40 Ar/ 39 Ar apparent ages. Our tectonic model relates all the high- P units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at c . 20–25 km depth, and (ii) exhumation of the entire high- P zone by culmination collapse after obduction.

Radial cracks around α-quartz inclusions in almandine: Constraints on the metamorphic history of the Oman mountains

Radiating tensional cracks around α-quartz inclusions in almandine have been observed in metapelite samples from the southeastern Saih Hatat tectonic window, northeastern Oman Mountains. These almandines show an inclusion-rich (glaucophane + epidote) and strongly deformed core with inclusions of different mineral phases. The rim of the same almandines is inclusion-poor and shows only quartz, apatite, zircon, rutile and Ba Al phosphates as inclusions. Quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks. These radial cracks developed during uplift by the dilation of α-quartz (4–5 vol%) without a phase transformation. Subsequently, these cracks were filled with kaolinite, phengite (Si content 3.4 per formula unit, p.f.u.), chlorite and Fe oxides. We calculated the appearance of radial cracks without phase transformation using the mathematical procedure of Van der Molen and Van Roermund [1]. This calculation involves terms for thermal expansion, isothermal compressibility and shear modulus for the example of α-quartz and almandine for the same P and T interval during a retrograde path. Published geothermobarometric estimates give pressures of between 1.0 and 2.0 GPa and temperatures of between 450 and 600°C for the peak conditions for these rocks of the Saih Hatat tectonic window. On the basis of these P-T data we calculated different retrograde P-T paths in the α-quartz domain. Initiation of garnet fracturing is dependent on the P-T starting conditions and the component of isothermal compression of the retrograde path. The calculations yield a set of smooth monotonic curves whose exact position on the P-T plane between 0.1 and 0.6 GPa and 40 and 500°C depends on the initial P-T conditions and the component of isothermal compressibility of the retrograde P-T paths. This model can be used in general terms to estimate pressure and temperature for the following cases: (1) If independent evidence (such as petrological data) allow the determination of the final pressure at which radial cracks appeared, the initial inclusion pressure can be recalculated. (2) If the initial inclusion pressure is known (e.g. from petrological data), the conditions of radial cracking can be calculated, and the pair initial pressure-final pressure leads to an estimate of the shape of the retrograde P-T path as a function of its component of isothermal decompression. In the example from the northeastern Saih Hatat tectonic window the late syntectonic growth of albite + phengite + kaolinite suggests that the final pressure for fracturing ranged between 0.4 GPa and 0.5 GPa at temperatures of 300°C. These values correspond to high initial pressures of at least 2.0 GPa at a temperature of 550°C. The following geodynamic model is suggested: A regionally extended metamorphism led to the growth of inclusion-rich garnets in the rocks from the northeastern Saih Hatat tectonic window at depths of about 30 km ( < 0.1 GPa, about 450°C). Continuing prograde metamorphism at a depth of more than 60 km with P < 2.0 GPa and T ≈ 550°C affected a metapelite unit that is only exposed immediately south of As Sifah village. In this area, clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite. During uplift along a retrograde P-T path with a large component of isothermal decompression radial cracks around α-quartz inclusions developed in the rims of almandines at a depth of about 12 km (0.4–0.5 GPa, ⩾ 300°C).

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.

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continental shelf deposits in the Saih Hatat tectonic window of northeastern Oman, where they structurally underlie the Semail ophiolite. These eclogites and blueschists constitute part of a coherent high-P/T metamorphic terrain comprised of interlayered metabasites, metapelites, and quartzofeldspathic and calcareous mica schists. The metabasites record a continuous regional trend of increasing P and T from west to east, where crossite-epidote schists in central and western Saih Hatat grade into blueschists and eclogites at As-Sifah, allowing for the subdivision of this terrain into three metamorphic zones. P-T conditions range from 4·5 to 5·5 kb atT <340°C for zone A to >10·2 kb at 500–580°C for zone C. Zoning patterns of garnets, pyroxenes, and phengite, and the compositions of pyroxene and amphibole inclusions in garnet, indicate that the eclogites and enclosing high-grade blueschists of zone C followed a ‘clockwise’ P-T rpath of increasing P and T followed by increasing T and decreasing P and a final stage of retrogression. Garnets and pyroxenes crystallized at different stages of P-T evolution of the terrain in the various eclogite pods and boudins. This conclusion and the contrasting mineralogies of the eclogites and the enclosing blueschists are attributed to differences in bulk-rock chemistry, fluid availability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a result of A-type subduction and crustal thickening of the Oman continental margin along an east-northeast-dipping thrust or ‘subduction zone’ which was initiated by the change in plate motion between Africa and Eurasia at 131 Ma. High P/T metamorphism continued until the final stages of ophiolite emplacement in the late Cretaceous, as indicated by the metamorphism of the thrust sheets overlying the coherent terrain.

Evaluating the Potential for Ground-Water Recharge in an Arid Climate: Sultanate of Oman

For centuries, the Sultanate of Oman, located along the eastern coast of the Arabian peninsula, has obtained its water supplies from springs and hand-dug well systems. When the government of Oman embarked upon a nationwide modernization program in the early 19709s, their water resources became badly stressed and overextended. Aware that continued economic development would require more extensive and better managed water resources, the Omani government contracted for a series of water resource studies to identify present resources and long range requirements needed for continued growth. These studies were hampered by a very poor data base, and often reached conclusions which could not be substantiated by detailed review or later studies. However, from the efforts of these early studies, the nucleus of a data base and the means to expand it were developed. During the period 1978–1980, the Corps of Engineers was requested by Oman through the AID program to study and identify the feasibility of enhancing water supplies in the capital area of Muscat, and in the mountain wadis flowing to the Batinah Plain of northeastern Oman. This study, paralleled by studies made by the Public Authority for Water Resources (PAWR), recommended a water recharge project on the Wadi Sama9il between the village of Al Khawd and Seeb, about 20 miles northwest of Muscat. The discussion will outline the traditional methods and systems of developing water supplies in Oman, the influence of modern technology on disruption of these systems, and the role of ground water recharge projects for mitigating water deficiency problems.

Late Cretaceous Eugeosynclinal Sedimentation, Gravity Tectonics, and Ophiolite Emplacement in Oman Mountains, Southeast Arabia

The Oman Mountains border the Gulf of Oman from the Arabian Sea to the mouth of the Persian Gulf. The Jebel Akhdar anticline forms the central core of the mountains and exposes a section of massive limestones which range in age from Permian (transgressive on a Precambrian nucleus) to late Tertiary; the latter fringe the uplift. Intercalated in this sequence, between well-dated latest Cretaceous sediments, is a thick contorted assemblage of rocks composed, from base to top, of turbidite limestone and radiolarian chert (Hawasina Group), massive exotic blocks of Permo-Triassic limestone, and a thick sheet of serpentinic igneous rock (Semail igneous series). This association of rocks, which is comparable to that known from many complex Laramide orogenic belts, can be studied with advantage in Oman where the sequence has been little disturbed since its emplacement during the latest Cre aceous (Campanian-Maestrichtian). The absence of recognizable latest Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted features of the strata have led some geologists to postulate that these sediments were deposited outside of their present location during a prolonged pre-Late Cretaceous interval and then tectonically emplaced during the latest Cretaceous. However, it is concluded from field data presented herein that the Hawasina was deposited in its present location during latest Cretaceous time. Regional stratigraphic correlations show that northeast Oman was situated far out on the Arabian platform where quiet carbonate sedimentation persisted from Permian to middle Cretaceous time during prolonged regional tectonic quiescence. Sedimentary and tectonic quiescence ended during latest Cretaceous time when the thick Aruma pelagic shale, originating from a northerly source, was deposited across northeastern Oman concomitant with major normal faulting. Hawasina facies, north of the Aruma province, give evidence in the form of basal boulder beds, turbidites, and massive terminal gravity slides of sharp contemporaneous tectonism. Lower Hawasina deep-water turbidite sediments have yielded an inverted faunal sequence of shallow-water fossils, whereas the overlying cherty facies contains only pelagic Radiolaria. Thus the whole sequence cannot be older than the reworked youngest middle Cretaceous fossils present in the basal beds. Distribution, grading, and constitution of the carbonate clastic material in the Hawasina, and the alignment of exotic limestone blocks indicate that the sediment source area was a northwest-southeast-trending uplift of Permian to middle Cretaceous carbonate rocks north of the Oman Mountains. The absence of terrigenous clastic material and terminal submarine volcanism suggest that the source area was a submerged seamount. Erosion from this high is believed to have been by means of turbidity currents activated by repetitive and at times strong block-fault movement. Hawasina sediments compare with present deep-water sediments in the Puerto Rico Trench where faunally barren siliceous oozes of abyssal facies are interbedded with calcareous turbidites rich in reworked older and contemporaneous shallow-water fauna. The absence of contemporaneous (latest Cretaceous) shelf fauna in the Hawasina is attributed to the seamount source area being deeper than neritic, whereas the absence of contemporaneous pelagic calcareous fauna was the result of dissolution during descent to abyssal depths. The latest Cretaceous Aruma Shale is compared with Holocene pelagic globigerinal sediment occurring above 4,500-m depth. The deep trough which received Hawasina sediments appears to have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the neritic Aruma belt to the Arabian carbonate platform. At the close of the time of Hawasina deposition, volcanism and catastrophic tension-relief faulting dislodged the remnants of Permo-Triassic limestone sedimentary rocks from the seamount, and these descended into the trough as huge gravity slides (e.g., Jebel Kawr, 250 sq mi), leaving the seamount as a denuded basement uplift. Regional tension relief was accomplished finally by crustal separation and flood eruption of Semail ultrabasic magma which blanketed the abyssal landscape and cooled slowly, under great hydrostatic pressure, along the axi of the trough. According to this interpretation, the sedimentation of Hawasina beds is considered to have been contemporaneous with the deposition of the latest Cretaceous Aruma Shale, and major block-fault tectonism in the orogenic belt is correlated with known latest Cretaceous tension faulting on the southwest basin slope. Compressional features in the Hawasina are attributed to contemporaneous gravity slumping of the beds on the steep northeast limb of the trough, combined with the subsequent End_Page 626------------------------------ bulldozing effect which massive slides of exotic blocks and the thick Semail eruptives had on the incompetent sediments. Oil prospects in the Hawasina belt are difficult to explore and assess for several reasons, not the least of which is the masking effect which contorted chert has on seismic and gravity exploration for more simple structures in the competent sub-Hawasina limestone succession, which is hydrocarbon-bearing below Aruma Shale at Fahud, Natih, and Yibal oil fields.