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Abstract

During Aptian to Albian times a major influx of terrigenous material from the emerged Arabian Shield led to the deposition of the Nahr Umr Formation within the intracratonic Bab Basin. This argillaceous facies interfingers in northern Oman with sediments shed from reefal complexes. We studied a 100 km transect across the zone of facies intercalation extending from the margin of the Neo-Tethys (northern Jabal Akhdar) into the fringes of the Bab Basin in northern Oman (Foothills). The age of formation boundaries and key beds and surfaces in a reference section (Wadi Bani Kharus; I) was dated by use of the graphic correlation method. Graphic correlation indicates that the Nahr Umr Formation is of uppermost Aptian to Upper Albian age. Two types of stratigraphic markers were recognized: limestone marker beds, successions, and disconformity surfaces. Calcareous marker beds are probably related to increased carbonate production during sea-level lowstands. Investigations of the geochemistry (stable isotopes, fluid inclusions) and sediment petrography of limestones beneath disconformities revealed that they are of combined submarine and subaerial origin. Evidence for a subaerial exposure stage is recognized along the basin margin (northern and southern Jabal Akhdar) but is found less frequently in the basinward settings of the Foothills. Marker-bed successions and disconformity surfaces are correlated throughout the sections and indicate coeval deposition of the Nahr Umr Formation in northern Oman.

Introduction

Throughout the Gulf Region, mid-Cretaceous sedimentary rocks are of primary importance in terms of carbonate petroleum systems. Consequently, the mid-Cretaceous reservoir facies such as the Lower Aptian Shuaiba Formation in the U.A.E. or the Upper Albian to Lower Turonian Natih Formation in Oman have been the subject of numerous studies during the last few decades (cf. Christian, 1997, and references therein).

The Upper Aptian to Upper Albian Nahr Umr Formation of northern Oman, a regionally significant seal, has received comparably little attention, because it is not itself a source or reservoir rock. We have investigated the Nahr Umr Formation in Jabal Akhdar (northern Oman Mountains) and the Foothills to the south. Measured sections reach from the margin of the Neo-Tethys (Jabal Akhdar) into the fringes of the intracratonic Bab Basin to the south (Foothills; Figs. 1, 2).

Fig. 1.

—A) Geologic setting of the study area in northern Oman. The sedimentary cover of the Arabian Platform was buried by the Hawasina Nappes and the Semail Ophiolite in Campanian time and subsequently covered by lower Tertiary deposits. Mid-Tertiary compressional tectonism led to detachment and upfolding of autochthonous sediments, allochthonous nappes, and neo-autochthonous Tertiary deposits in the Jabal Akhdar anticline as well as in smaller anticlines (Foothills) to the south. The Wasia Group, of which the Nahr Ümr Formation is a part, is shown in black. After Béchennec et al. (1992a) and Béchennec et al. (1992b), transect A-B after Mann et al. (1990) and Hanna (1990). Sections studied are indicated I = Wadi Bani Kharus; II = 2.5 km to the West of Wadi Bani Kharus; III = Wadi Mu’aydim; IV = Jabal Salakh; V = Jabal Madar. B) Stratigraphy of the Upper Permian to Lower Turonian Hajar Supergroup, based on Hughes Clarke (1988) and Simmons (1994).

Fig. 1.

—A) Geologic setting of the study area in northern Oman. The sedimentary cover of the Arabian Platform was buried by the Hawasina Nappes and the Semail Ophiolite in Campanian time and subsequently covered by lower Tertiary deposits. Mid-Tertiary compressional tectonism led to detachment and upfolding of autochthonous sediments, allochthonous nappes, and neo-autochthonous Tertiary deposits in the Jabal Akhdar anticline as well as in smaller anticlines (Foothills) to the south. The Wasia Group, of which the Nahr Ümr Formation is a part, is shown in black. After Béchennec et al. (1992a) and Béchennec et al. (1992b), transect A-B after Mann et al. (1990) and Hanna (1990). Sections studied are indicated I = Wadi Bani Kharus; II = 2.5 km to the West of Wadi Bani Kharus; III = Wadi Mu’aydim; IV = Jabal Salakh; V = Jabal Madar. B) Stratigraphy of the Upper Permian to Lower Turonian Hajar Supergroup, based on Hughes Clarke (1988) and Simmons (1994).

Fig. 2.

—A) Reconstruction of the Albian carbonate platform in Arabia. Basement highs are marked A, B, and C. Facies belts: 1, Arabian shield; 2, coastal plain; 3, shallow marine inferred platform, 4, Lurestan Bab Basin. B) An enlargement of the northeastern part of the Arabian carbonate platform. Black dots mark the inferred location of Al Lurestan Formation reefs. Location of transect CD is shown. C) Transect from the continental margin into the intracratonic Bab Basin, with location of sections I to V (Fig. 1). After Murris (1980) and Harris et al. (1984); transect is based partly on Masse et al. (1997).

Fig. 2.

—A) Reconstruction of the Albian carbonate platform in Arabia. Basement highs are marked A, B, and C. Facies belts: 1, Arabian shield; 2, coastal plain; 3, shallow marine inferred platform, 4, Lurestan Bab Basin. B) An enlargement of the northeastern part of the Arabian carbonate platform. Black dots mark the inferred location of Al Lurestan Formation reefs. Location of transect CD is shown. C) Transect from the continental margin into the intracratonic Bab Basin, with location of sections I to V (Fig. 1). After Murris (1980) and Harris et al. (1984); transect is based partly on Masse et al. (1997).

Several aims governed this study: 1) to obtain detailed information on the paleoenvironmental setting of the Nahr Umr Formation; 2) to establish a chronostratigraphic framework by use of the graphic correlation method; 3) to correla te marker beds and surfaces throughout sections in Jabal Akhdar and into the poorly exposed sections in the Foothills. Marker surfaces were studied in detail in order to recognize their origin. The paleoceanographic and sea-level record of the Nahr Umr Formation is discussed in an associated paper (nsect is based partly on Immenhauser et al., 1999).

Regional Geotectonic and Paleoenvironmental Setting

From the mid-Permian through the earliest Turonian (ca. 170 My) an extensive carbonate platform covered large parts of the Arabian craton (Fig. 2). During this period, some 3500 m of mainly carbonate strata accumulated (Hajar Supergroup; Fig. l;Glennie et al., 1974; Murris 1980; Harris and Frost, 1984; Rabu, 1987; Scott et al., 1988; Hughes Clarke, 1988; Scott, 1990; Pratt and Smewing, 1993; Alsharhan, 1995).

The Arabian platform was bordered by basement highs along its southwestern, eastern, and northern margins (Fig. 2). Sedimentary units show a regional thinning and environmental shallowing towards the following structural axes: A) the uplifted and exposed Arabo-Nubian shield to the southwest; B) the uplifted Huqf-Haushi axis to the east (Hughes Clarke, 1988; Ries and Shackleton, 1990); and C) a shallow swell on the continental margin to the north.

During the mid-Cretaceous the Arabian Platform was subdivided into several paleodepositional belts (Fig. 2B). Alluvial to coastal deposits rimmed the Arabo-Nubian Shield in the south-west, grading northeastwards into the shallow marine carbonate shelf. Since the Middle Jurassic the Arabian Shelf was superimposed by the intracratonic Bab Basin, which was deepest to the north (present-day United Arab Emirates) and extended south to Oman (Vahrenkamp, 1996).

During Late Aptian to Late Albian time a major influx of terrigenous material from the emerged Arabo-Nubian Shield to the northwest onto the Arabian carbonate platform (Fig. 2; Murris, 1980; Hughes Clarke, 1988) led to the deposition of the argillaceous Nahr Umr Formation within the Bab Basin. Throughout the southern Gulf the Nahr Umr is principally an impermeable unit of clayey facies acting as a seal for the Lower Aptian Shuaiba-Upper Thamama carbonate reservoir rock (Murris, 1980; Hughes Clarke, 1988). In Oman the Nahr Umr is a unit of the parautochthonous Wasia Group of the Hajar Supergroup (Fig. 1; Glennie et al., 1974; Harris and Frost, 1984; Harris et al., 1984; Simmons and Hart, 1987; Rabu, 1987; Hughes Clarke, 1988; Pratt and Smewing, 1993).

Reefal buildups of Aptian to Albian age (Al Hassanat Formation; Masse et al., 1997) rimmed the Arabian Platform to the northeast and probably to the east (Fig. 2, black dots; Immenhauser et al., 1998). These rudist-algal reefs (Masse et al., 1997) grew on a shallow swell trending along the continental margin of Oman. The slope deposits of the Al Hassanat reefs interfingered basinwards (to the south) with Nahr Umr sediments.

The Hajar Supergroup (including the Nahr Umr Formation) was buried by the overthrusted Hawasina Complex and Semail nappes in the Campanian (Glennie et al., 1974). In the Early to Middle Miocene the Afro-Arabian plate collided with Eurasia in the Makran-Zagros area. This led to detachment of cratonic sediments from their basement and uplift of these units as part of the Jabal Akhdar dome and smaller anticlines to its south (Fig. 1; referred as the “Foothills” in this contribution; Rabu, 1987; Rabu et al, 1990; Loosveld et al., 1996).

Methodology

X-ray powder diffraction was measured at 50% relative humidity with CuKα radiation using a custom-built goniometer equipped with automatic divergence and anti-scatter slits and a Si-Li energy-dispersive detector (Kevex). Doubly polished thick sections (150 μm) were analyzed for fluid-inclusion microthermometry (vapor and fluid phase) on a Linkam TP/91-THMS 600 gas-flow heating and freezing stage. The samples were cooled until all of the liquid as well as the vapor phase in the individual inclusions was frozen (approximately -80 to -90°C) and then slowly heated at a rate of 0.5 to 5°C per minute until all of the ice was melted. 415 matrix micrite and 80 carbonate cement samples were analyzed on a VG Prism II ratio mass spectrometer for carbon and oxygen isotope ratios. Repeated analyses of standard material show a reproducibility of better than 0.1 %o for δ180 and less than0.05%o for δ13C. All results are presented as parts per thousand relative to the VPDB standard. Abundance of Ca, Sr, Mg, Mn, Fe, and Ca in calcite and dolomite cement phases were measured at the Institute for Earth Sciences, Vrije Universiteit, Amsterdam by electron microprobe analysis operating at 20 kV, beam current 0.015 μA, and 13 μm beam diameter. Cathodoluminescence on carbonate cements was conducted on a cold cathode luminescence microscope operating under 10–14 kV accelerating voltage, 200–300 μA beam current, and a beam diameter of 4 mm.

Lithostratigraphy and sedimentology

Along the northern slope of Jabal Akhdar, the Nahr Umr Formation is best exposed on the western bank of Wadi Bani Kharus about 1 km south of the village of Awabi (I; Fig. 1). This is the type locality of Rabu (1987) and our reference section.

Along the southern flank of Jabal Akhdar exposures are poor, as in Wadi Mu’aydim (section III), where only about 50% or less of the Nahr Umr is uncovered. Likewise, in the Foothills at Jabal Salakh (IV) only individual beds protrude from the gravel. However, at Jabal Madar (V) approximately 70% of a continuous section is exposed because of a landslide. Sections along the northern flank of Jabal Akhdar (105 m) and those measured in the Foothills (92 m) show a similar stratigraphic thickness, whereas sections in Wadi Mu’aydim reach almost 220 m in thickness. At this locality, an exceptional stratigraphic thickness is also found for the underlying Shuaiba Formation (Pratt and Smewing, 1993) and the overlying Natih Formation (Fig. 1; Philip et al., 1995).

Fades Associations and Related Paleo-Hydrodynamic Regimes

On the basis of sedimentology and microfacies we distinguish three hydrodynamic regimes: a low-energy environment, an intermittently agitated (storm) environment, and a high-energy environment. Low-energy and high-energy environments are end members in terms of hydrodynamic energy and are characterized by their fades and, if present, sedimentary structures. Some of the facies associations were attributed to an intermittently storm-agitated regime because they indicate episodes of agitation in an otherwise calm regime. A main feature of the intermittently agitated environment is the presence of discrete layers, formed by locally coarse bivalve fragments (packstones), in an otherwise argillaceous facies with an in-situ fauna. These layers are interpreted as storm deposits obscured by bioturbation to various degrees.

Low-Energy Environment.—

Unbedded ochre-colored, greenish and reddish clays and, to a minor extent, also marls reflect the calmest hydrodynamic regime. This facies forms about 50% of the paleo-basinal sections measured in Jabal Madar (V) and Salakh (IV; Fig. 1), but is also present at several levels in the Jabal Akhdar sections (paleo-basin margin). The mineral content distinguishes two groups. Clays and marls from the northern slope of Jabal Akhdar contain large amounts of pyrophyllite and chlorite, not present in the Foothill sections. In contrast, samples from the Foothills contain kaolinite and goethite, which are not found at Jabal Akhdar, where illite— mica is mainly present.

A low-energy environment is also interpreted for two other facies types. The first is an ochre-colored, poorly consolidated argillaceous mudstones or fine-grained skeletal wackestone with an in-situ fauna of orbitolinids, few miliolids, ostracods, molluscs, oysters, and echinoderms. Orbitolina texana (Römer), O. subconcava (Lemeyre), and O. aperta (Erman) are most commonly found. For a more detailed list refer to Rabu (1987) and Simmons and Hart (1987). The echinoderm fauna consists of Pseudodiadema carthusianum (Gras) and Coeholecyptus aff. simulis (Desor; cf. Rabu et al., 1986). Locally, beds of this facies are of a condensed facies characterized by abundant iron oxides, phosphate pebbles, and rare ammonites.

The second facies type is ochre-colored and very shaly, and forms beds that are morphologically recessive. Orbitolinid fora-minifera form up to 70 to 80% of the rock volume. Orbitolina packstones are the most characteristic facies of the Nahr Umr and correspond to the “Orbitolina facies” found elsewhere in mid-Cretaceous carbonate platforms (Vilas et al., 1995). Orbitolina packstones are interpreted as an in-situ facies rather than the result of winnowing (Immenhauser et al., 1999).

Intermittently Agitated Environment.—

Sedimentary rocks attributed to an intermittently agitated regime are a predominant facies of the Nahr Umr Formation in Jabal Akhdar, and to a minor degree in the Foothill sections. Beds of this facies type are very commonly nodular and between a few decimeters to several meters thick. Bases of individual beds are commonly shaly but become calcareous towards their upper parts. Bedding surfaces are characterized by a sharp contact between grayish limestones of the underlying bed and ochre-colored, recessive shales at the base of the overlying bed. Two main facies were recognized: a grayish to ochre wackestone to packstone with an in-situ fauna of echinoderms, oysters, serpulids, red algae, orbitolinids, and miliolids, and also fragmented bivalve debris (1–5 mm) in discrete layers of up 5 cm thickness. A second, similar facies type, but with few or no orbitolinids, is found towards the Nahr Umr-Natih formational boundary.

The packstone layers (fragmented bivalves), interpreted as storm deposits, are in many cases intermingled with the surrounding sediments because of subsequent bioturbation. In some exposures, well-defined storm layers could be traced laterally into heavily bioturbated zones, where they become blurred.

High-Energy Environment.—

The high-energy facies is found only at a few levels in the Jabal Akhdar sections and in the uppermost part of the underlying Shuaiba Formation (Fig. 4F). Bedding surfaces are commonly planar, and in a few cases also slightly nodular. The microfacies types are poorly washed peloidal and skeletal grainstones. In the case of marker succession C (Fig. 3), numerous rudists in growth position are found. They have been determined as Prearadiolites sp. by J.-P. Masse (Fig. 4D). Other beds with a high-energy facies are typified by their coarse (3 mm and larger) skeletal components such as fragments of gastropods, bivalves, oysters, and rudists and display meter-scale cross-bedding. Orbitolinids are sparse in the high-energy environments, and the few found are worn and encrusted by small serpulids. Centimeter-thick layers of bivalve fragments, present in the intermittently agitated environment, are a common feature of this facies. Bioturbation is generally less prominent than in the facies of the low-energy or intermittently agitated environments.

Fig. 3.

—Correlation of hardbottoms (HB1-HB10) and marker-bed successions (A to C). For location of individual sections refer to Figures 1 and 2. A simplified lithostratigraphy is shown and the estimated shale-clay (black) vs. carbonate (limestone signature) composition is given at the left side of each lithostratigraphic column. Note the expanded stratigraphic thickness of the Wadi Mu’aydim Section (III).

Fig. 3.

—Correlation of hardbottoms (HB1-HB10) and marker-bed successions (A to C). For location of individual sections refer to Figures 1 and 2. A simplified lithostratigraphy is shown and the estimated shale-clay (black) vs. carbonate (limestone signature) composition is given at the left side of each lithostratigraphic column. Note the expanded stratigraphic thickness of the Wadi Mu’aydim Section (III).

Fig. 4.

(facing page)—A) Hole of boring bivalve (hardbottom 2; Wadi Mu’aydim, III) encircled with black marker pen. Coin is 2 cm across. B) Bored oyster (hardbottom 3; Wadi Bani Kharus); pen is 15 cm long. C) Thin section of borehole (hardbottom IV; Section II) sealed with a crust of secondary iron oxides (i); width of view is 8 mm. D) Rudist shell filled with blocky cement used for microthermometry studies (hardbottom 3; Wadi Bani Kharus, I); width of view is 7 cm. The approximate location of Figure 7 is indicated. E) Microcodium colony in root molds (hardbottom 4; Wadi Mu’aydim, III); width of view is 6 mm. F) High-energy facies of the uppermost Shuaiba Formation in Jabal Madar (V). Note the secondary intraparticle porosity (p) due to selective leaching during the subaerial exposure stage. Width of view is 6.5 mm. G) Phase 3 dolomites in matrix of burrows and replacing sediment (hardbottom 9; Wadi Bani Kharus, I); width of view is 6.5 mm. H) Phase 4 calcites (arrows) related to the subaerial exposure stage. This cement fills fissures in secondary iron oxides (i). Width of view is 5.5 mm (hardbottom 4, Wadi Mu’aydim, III).

Fig. 4.

(facing page)—A) Hole of boring bivalve (hardbottom 2; Wadi Mu’aydim, III) encircled with black marker pen. Coin is 2 cm across. B) Bored oyster (hardbottom 3; Wadi Bani Kharus); pen is 15 cm long. C) Thin section of borehole (hardbottom IV; Section II) sealed with a crust of secondary iron oxides (i); width of view is 8 mm. D) Rudist shell filled with blocky cement used for microthermometry studies (hardbottom 3; Wadi Bani Kharus, I); width of view is 7 cm. The approximate location of Figure 7 is indicated. E) Microcodium colony in root molds (hardbottom 4; Wadi Mu’aydim, III); width of view is 6 mm. F) High-energy facies of the uppermost Shuaiba Formation in Jabal Madar (V). Note the secondary intraparticle porosity (p) due to selective leaching during the subaerial exposure stage. Width of view is 6.5 mm. G) Phase 3 dolomites in matrix of burrows and replacing sediment (hardbottom 9; Wadi Bani Kharus, I); width of view is 6.5 mm. H) Phase 4 calcites (arrows) related to the subaerial exposure stage. This cement fills fissures in secondary iron oxides (i). Width of view is 5.5 mm (hardbottom 4, Wadi Mu’aydim, III).

Marker-Bed Successions and Disconformity Surfaces

Three important marker-bed successions (A, B, and C, Fig. 3) were recognized in all sections measured except at Jabal Salakh (IV), where the top two marker-bed successions are not exposed. They are characterized by one or several meter-thick limestone beds. These marker beds are widely traceable on wireline logs throughout much of the interior Oman subsurface (Hughes Clarke 1988).

Marker successions A and B consist of massive peloidal packstone beds several meters thick. The diagnostic criteria are the massive beds forming conspicuous cliffs and the low clay content. Marker bed succession A reaches 9 m in thickness on the northern slope of Jabal Akhdar, and is 4.5–5 m thick in the Foothills. Marker succession B is about 4 m thick in the north and decreases to less than 3 m in the south. Marker succession C (2–3.5 m in thickness) is characterized by the rudist Praeradiolites sp. in growth position. This unique facies is recognized in all sections at Jabal Akhdar but is absent in the Foothills.

Numerous disconformity surfaces (hardbottoms) are found throughout all sections measured (Fig. 3). These surfaces are hummocky, often reddish, stained with goethite (FeO(OH)), covered locally with phosphate nodules (a few millimeters in size), and encrusted by black, bored oyster shells. Early lithification is demonstrated by numerous small borings (a few millimeters wide and a few centimeters deep) that truncate allochems, and by holes having the shape of the molluscs that etched them.

Ten surfaces were exposed in the Wadi Bani Kharus section (I), nine were found in section II to the west, six in Wadi Mu’aydim (III), four at Jabal Madar (V), and one at Jabal Salakh (IV; Fig. 3). Sedimentary rocks underneath these surfaces generally are intensely bioturbated, and some burrows are filled with poorly consolidated reddish and ochre micrite rich in secondary iron oxides and dolomite rhombs (Fig. 4G).

Despite careful investigations, no evidence for subaerial erosion was found in the field even though chemical evidence of the influx of light soil carbon and light oxygen from meteoric water is found in several sections. Only the first few centimeters below surfaces were affected by strong dissolution-precipitation processes and iron staining (Fig. 4F). However, dolomitization of micrite-filled burrows may reach several meters into the underlying rocks.

Firmgrounds and bioturbated horizons were recognized in most sections. Firmgrounds are relatively thin domains of well-preserved burrows at the top of an individual bed and are of a clayey, reddish and greenish appearance. Burrows are conspicuous because of their sharp outlines, probably caused by a “stiffness” of the sediment. This incipient submarine cementation confirms an interpretation as firm-grounds. The above characteristics are considered to be the result of slow (condensed) sedimentation, although evidence for early lithification (borings) such as in the case of hardgrounds is absent. Bioturbated horizons, on the other hand, may appear within individual beds. In both casesbioturbation is by far more prominent than in other beds.

Geochemistry

Carbon Isotopes.—

Light carbon-isotope ratios were obtained from matrix samples directly underneath hardbottoms (Fig. 5). Across these contacts δ13C values fall from a range of +4 to +2.5%o to values between +0.9 to -1.2%o. Besides a clear negative excursion associated with the disconformity at the Shuaiba-Nahr Umr Formation boundary, eight other excursions are present in Wadi Bani Kharus (I). A bundle of three minima appears between meters 0 and 20, respectively, and five of them are grouped into a second bundle between meters 65 and 104 (Fig. 5). The distance between individual hardbottoms within these bundles is remarkably uniform and on the order of 10 to 12 m. Similar but less pronounced negative excursions are observed in sections II and III. In Wadi Mu’aydim (III), however, the shift in δ13C isotope ratios is much reduced to values of +1 to +2.5%o. Only the basal Shuaiba-Nahr Umr unconformity shows a negative excursion of over +5%o (Fig. 5).

Fig. 5.

—Chemostratigraphy of the Nahr Umr Formation and the top of the underlying Shuaiba Formation in wadis Bani Kharus (I), Mu’aydim (III), and Jabal Madar (V). Note the negative excursions in δ13C and partly in δ180 isotope values beneath hardgrounds in the Wadi Bani Kharus Section (I). The negative excursion in δ18O values within the top few meters in Wadi Bani Kharus (I) is related to metamorphism beneath the overriding hot sole of the Semail Ophiolite.

Fig. 5.

—Chemostratigraphy of the Nahr Umr Formation and the top of the underlying Shuaiba Formation in wadis Bani Kharus (I), Mu’aydim (III), and Jabal Madar (V). Note the negative excursions in δ13C and partly in δ180 isotope values beneath hardgrounds in the Wadi Bani Kharus Section (I). The negative excursion in δ18O values within the top few meters in Wadi Bani Kharus (I) is related to metamorphism beneath the overriding hot sole of the Semail Ophiolite.

Microsamples drilled from matrix material display δ13C values ranging from +2.5 to -1.2%o. These values are slightly heavier than background data from bulk rock samples (Fig. 6). δ13C values measured in samples drilled from orbitolinids are well within the values of bulk-rock samples, but samples of dolomite-rich burrows underneath hardgrounds may show very light δ13C values on the order of -3.8%o.

Fig. 6.

—Comparison of carbon and oxygen isotope composition in selected microsamples obtained from sedimentary rocks directly beneath surfaces. Note the isotopically light composition of cement phase four, related to a subaerial exposure stage.

Fig. 6.

—Comparison of carbon and oxygen isotope composition in selected microsamples obtained from sedimentary rocks directly beneath surfaces. Note the isotopically light composition of cement phase four, related to a subaerial exposure stage.

Heavy δ13C values (+4 and +5%o) occur within the top ~ 20 m of the Shuaiba Formation in Wadi Bani Kharus (I). In the equivalent units of the Wadi Mu’aydim Section (III) δ13C values of up to +5.5%o were measured, whereas at Jabal Madar (V) δ13C values range from +4 to +4.2%o (Fig. 5).

Oxygen Isotopes.—

In the lower 65 m of the Wadi Bani Kharus Section (I), oxygen isotopes largely follow the trend of carbon isotopes described above (Fig. 5). δ180 values reach minima underneath hardbottom surfaces except at surface 3. The magnitude of drops in δl80 values is on the order of 2.25 to 2.5%o to values as low as -5 to -6.5 %o. Oxygen isotope values become more homogeneous from meter 65 upward, and excursions associated with hardgrounds are on the order of 0.5% only. A pronounced negative shift to values of -7.6 %o is found in limestones in the top few meters of the section, which have been altered during initial burial beneath the hot metamorphic sole of the Semail Ophiolite.

Microsamples drilled from dolomite-rich burrows beneath hardgrounds show δ18O ranging from -4.3 to -11.8%o. These values are in part more negative than the background values of -3.2 to -6.5 %o measured from bulk-rock samples of identical hand specimen (Fig. 6).

Fluid Inclusions.—

The coarse blocky calcite that fills druses in the rudist Praeradiolites sp. (Fig. 4D) contained two-phase (liquid-vapor) fluid inclusions that were in part suitable for microthermometric measurements. Samples were taken beneath the hardbottom atop marker-bed succession C in the Wadi Bani Kharus (I) and the Wadi Mu’aydim (III) sections (Fig. 3). The vapor phase was most likely induced during Late Cretaceous burial beneath the hot metamorphic sole of the overriding Semail Ophiolite and indicates temperatures of about 70°C during formation. The size of most inclusions is small, between 1 and 10 μm. The shapes of inclusions are round to square, rectangular or irregular and rather flat. A preferred orientation of inclusions was not observed (Fig. 7).

Fig. 7.

—Detailed sketch of a clear calcite crystal rich in primary liquid-vapor inclusions. Inclusions show no preferred orientation and are randomly dispersed. Adjacent crystals in the lower left corner are cloudy and grayish and yield few inclusions. The position of Figure 7 is indicated in Figure 4D.

Fig. 7.

—Detailed sketch of a clear calcite crystal rich in primary liquid-vapor inclusions. Inclusions show no preferred orientation and are randomly dispersed. Adjacent crystals in the lower left corner are cloudy and grayish and yield few inclusions. The position of Figure 7 is indicated in Figure 4D.

Criteria summarized in Goldstein and Reynolds (1994) were used to recognize the origin of the inclusions that represent the fluid composition at the moment of trapping (under the assumption that no reequilibration has taken place). Fluid inclusions studied are very likely primary because they form clusters bound to specific growth zones within the blocky calcites, indicating that no recrystallizarion has occurred (Fouke and Reeder, 1992). The inclusion-rich cements are nonluminescent, typical of a near-surface environment of precipitation and thus are unlikely of burial origin.

Determination of the final melting of ice in small-sized inclusions, such as the case in this study, is difficult to recognize because the relief of the ice is almost identical to that of calcite. Zwart (1995) described a method for liquid-vapor inclusions using the tendency of the last remaining ice to stick to the walls of vapor bubbles. The sudden movement of the vapor bubbles associated with the disappearance of the last ice is commonly the only way to determine the melting-point depression.

The results of microthermometric analyses of final ice melting temperatures (freezing-point depression) of 31 inclusions is shown in Figure 8. Conversion of the temperature range between -1.4 and -1.6°C into salinity (wt. % NaCl) after Bodnar (1993) indicates brackish water (less than 2.8% salinity). Inclusions with a depression of the freezing point below values of -1.6°C comprise normal marine to hypersaline fluids.

Fig. 8.

—Histogram showing the distribution of final melting temperatures measured in fluid inclusions and the calculated weight percent of NaCl (Bodnar, 1993).

Fig. 8.

—Histogram showing the distribution of final melting temperatures measured in fluid inclusions and the calculated weight percent of NaCl (Bodnar, 1993).

Carbonate Cements Beneath Hardbottoms

Six major cement phases were recognized. The geochemical data are presented in Table 1. These phases are, from old to young: (1) early marine isopachous cement; (2) coarse, iron-stained, blocky sparite; (3) zoned euhedral dolomite; (4) sparry equant calcite; (5) mosaic sparite in root molds; and (6) late calcite veins.

Table 1.

—Trace-element and isotopic composition of calcite and dolomite cements in the sediments beneath hardbottoms.

Cement phase Mineralogy δ18O %„PDB δ18C %„PDB MgO (ppm/%) MnO (ppm) FeO (ppm) SrO (ppm) CaO (wt %) 
I. Isopachous, early marine cement fringes calcite n.d. n.d. n.d. n.d. n.d. n.d. n.d. 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.55 1.12 1120 (±80) 810 (±200) 8640 (±220) b.d.l. 56 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.8 0.83 720 (±80) 1900 (±200) 7610 (±220) 60 (±130) b.d.l. 56 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.59 1.09 1890 (±80) 840 (±200) 3890 (±220) b.d.l. 56 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -4.38 2.06 2400 (±80) 970 (±210) 840 (±210) b.d.l. 55.5 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -5.69 2.68 1670 (±80) 560 (±210) 7810 (±220) b.d.l. 55.1 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -4.33 2.03 1430 (±80) 750 (±210) 410 (±220) b.d.l. 56.2 
III. Zoned, euhedral dolomite rhombs dolomite n.d. n.d. 21.4 (%) 540 (±190) b.d.l. b.d.l. 29.9 
III. Zoned, euhedral dolomite rhombs dolomite n.d. n.d. 21.3 (%) 710 (±190) b.d.l. b.d.l. 30 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -7.3 -1.4 5020 (±80) 40 (±200) 1630 (±220) 110 (±130) b.d.l. 56 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -7.19 -1 3470 (±80) b.d.l. 1060 (±220) b.d.l. 56 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -4.89 -2.92 2010 (±80) b.d.l. b.d.l. 120 (±130) b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite -6.17 1.24 730 (±80) 10 (±200) 2130 (±220) b.d.l. b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite -7.27 0.77 103 (±80) 200 (±200) 850 (±220) b.d.l. b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite n.d. n.d. 81 (±80) 130 (±200) b.d.l. 2890 (±220) 20 (±130) b.d.l. 56 
Rudist shell, iron stained calcite -4.94 2.44 2440 (±80) 750 (±200) 11.730 (±220) b.d.l. 54.6 
Cement phase Mineralogy δ18O %„PDB δ18C %„PDB MgO (ppm/%) MnO (ppm) FeO (ppm) SrO (ppm) CaO (wt %) 
I. Isopachous, early marine cement fringes calcite n.d. n.d. n.d. n.d. n.d. n.d. n.d. 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.55 1.12 1120 (±80) 810 (±200) 8640 (±220) b.d.l. 56 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.8 0.83 720 (±80) 1900 (±200) 7610 (±220) 60 (±130) b.d.l. 56 
IIa. Coarse, iron-stained, blocky cement calcite, goethite -7.59 1.09 1890 (±80) 840 (±200) 3890 (±220) b.d.l. 56 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -4.38 2.06 2400 (±80) 970 (±210) 840 (±210) b.d.l. 55.5 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -5.69 2.68 1670 (±80) 560 (±210) 7810 (±220) b.d.l. 55.1 
IIb. Coarse, inclusion-rich, blocky sparite in rudists calcite -4.33 2.03 1430 (±80) 750 (±210) 410 (±220) b.d.l. 56.2 
III. Zoned, euhedral dolomite rhombs dolomite n.d. n.d. 21.4 (%) 540 (±190) b.d.l. b.d.l. 29.9 
III. Zoned, euhedral dolomite rhombs dolomite n.d. n.d. 21.3 (%) 710 (±190) b.d.l. b.d.l. 30 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -7.3 -1.4 5020 (±80) 40 (±200) 1630 (±220) 110 (±130) b.d.l. 56 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -7.19 -1 3470 (±80) b.d.l. 1060 (±220) b.d.l. 56 
IV. Clear, equant sparite-quartz intergrowth calcite, quartz -4.89 -2.92 2010 (±80) b.d.l. b.d.l. 120 (±130) b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite -6.17 1.24 730 (±80) 10 (±200) 2130 (±220) b.d.l. b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite -7.27 0.77 103 (±80) 200 (±200) 850 (±220) b.d.l. b.d.l. 56 
V. Mosaic sparite filling root molds calcite, goethite n.d. n.d. 81 (±80) 130 (±200) b.d.l. 2890 (±220) 20 (±130) b.d.l. 56 
Rudist shell, iron stained calcite -4.94 2.44 2440 (±80) 750 (±200) 11.730 (±220) b.d.l. 54.6 
n.d. = no data; b.d.l. = below detection limit

Early Marine Isopachous Cements.—

Early marine cements are found in the high-energy grainstone facies of the topmost Shuaiba Formation and, less commonly, in the upper few meters of the Nahr Umr Formation. They form isopachous fringes, 50 μm thick, of short bladed crystals coating bioclasts. Early marine cements are nonluminescent, and individual crystals are too small to be sampled by conventional methods for stable-isotope analysis.

Coarse, Iron-Stained Blocky Sparites.—

Ochre-colored to slightly reddish, ferroan and nonferroan, blocky calcite cements fill solution cavities and occlude part of the pore space in rudist shells. The characteristic reddish color is due to an Fe content up to ten times higher than in other cement phases. This phase is also enriched in Mn relative to other cements (Table 1). This cement phase is very dull to nonluminescent. Stable-isotope analyses reveal mean δ180 values of -7.6 %o and δ13C values of 1.0%o. Secondary iron oxide (goethite) replaces this cement in many thin sections.

Enhedral Dolomites.—

Euhedral dolomites (Fig. 4G), about 200 μm in length, replace matrix and bioclasts as well as cement phases one and two. They appear in great number in burrows underneath hardbottoms and fill voids of dissolved bioclasts in the grainstone facies towards the top of sections in the northern Jabal Akhdar. Dolomite crystals are nonluminescent with the exception of an outer, bright luminescent rim of goethite and dolomite, 10–25 μm thick. No stable-isotope data could be measured from individual dolomite crystals, because of their small size. Data obtained are from bulk rock microsamples drilled from dolomite-rich zones. The resulting isotope data are light relative to background values of the matrix. The range in δ180 values is -8.3 to -4.4%o, and δ13C values of 1.54 to -0.13% were found (Fig. 6).

Sparry, Equant Calcite-Quartz Intergrowth.—

Inclusion-free, equant sparite occludes voids, covers walls of small fissures in sediments or secondary iron oxides, and replaces blocky cement of phase two (Fig. 4H). Individual crystals in fissures are on average 0.5 mm long. This cement locally replaces phase three dolomites. Phase four calcites are commonly enriched in Mg relative to earlier cement phases and are nonluminescent (Table 1). Stable-isotope analyses reveal mean δ180 values of -6.3%, and δ13C values of -1.8% (Fig. 6). As observed under the electron microprobe, calcites of phase 4 form a complex commingling with authigenic quartz. The average calcite:quartz ratio is about 7:3. The coexistence of calcite and quartz cements was not observed in other phases. A fine-grained phase of the calcite-quartz intergrowth is also present in the goethite crusts that seal the hardbottoms.

Mosaic Sparite in Root Molds.—

Root molds are characterized e.g., by the appearance of Microcodium (Fig. 4E) and are filled with a poorly consolidated reddish to ochre micritic sediment. Patches of mosaic, inclusion-rich spa rites replace Microcodium and matrix. Stable-isotope analyses of this cement phase display mean δ180 values between -6.2 and -7.3 %o and δ13C values between + 0.8 and +1.2 %o. This cement phase is nonluminescent and differs geochemically from the preceding phase four by its lower Mg content.

Late Calcite Veins.—

Late diagenetic features such as calcite veins and authigenic precipitates in the pressure shadows of bioclasts crosscut and thus postdate all previously mentioned cement phases. All veins show complex patterns of numerous subsequent phases of calcite, most of which display a bright luminescence.

Age of the nahr Umr Formation in Northern Oman

Graphic Correlation

Graphic correlation is a quantitative stratigraphic technique that compares the fossil ranges in a section with the total ranges of these taxa composited in numerous sections. A result is that ranges of different groups are integrated. Graphic correlation of sections generates hypotheses of age relationships by testing which taxa in a section are near to their true appearance or extinction times. The fossil ranges in the section are compared with their ranges in a composite standard database of many sections (Shaw, 1964; Carney and Pierce, 1995). The mid-Cretaceous composite standard (MIDK CS) in this study is a database of 40 sections located in the Tethyan Realm in Europe, Africa, the Mid-East, and North America. Over one thousand taxa and event marker beds form this database. Stage boundaries are defined by European reference sections that have been graphed into the database and calibrated to the Harland et al. (1990) time scale. During the last few years new ages for the Aptian-Albian and Albian-Cenomanian boundaries have been proposed (cf. Obradovich, 1993; Gradstein et al., 1994), but until these proposals are fully evaluated, the Harland time scale is retained here.

Although the age-diagnostic biota in the Wadi Bani Kharus Section (I) are not diverse, they are typical of the Albian in Oman. The following biota were used to graphically correlate this section: Orbitolina texana (Roemer), the red algae Permocalculus irenae Elliott, Orbitolina subconcava Leymerie, Orbitolina aperta (Erman), Eoradiolites lyratus (Conrad), and Hemicyclammina whitei (Henson) (Fig. 9).

Fig. 9.

—Graphic correlation of the Wadi Bani Kharus Section (I). The position of individual marker biota and the resulting line of correlation are shown. The vertical axis is the stratigraphic thickness of the section, and the horizontal axis is the time line. The chronostratigraphic result of this graph suggests a lower formational boundary at 113.8 Ma and an upper boundary at 99.9 Ma. This refers to a Late Aptian age for the base of the Nahr Umr Formation and a Late Albian age for its top in Wadi Bani Kharus (I); (Harland et al., 1990, time scale).

Fig. 9.

—Graphic correlation of the Wadi Bani Kharus Section (I). The position of individual marker biota and the resulting line of correlation are shown. The vertical axis is the stratigraphic thickness of the section, and the horizontal axis is the time line. The chronostratigraphic result of this graph suggests a lower formational boundary at 113.8 Ma and an upper boundary at 99.9 Ma. This refers to a Late Aptian age for the base of the Nahr Umr Formation and a Late Albian age for its top in Wadi Bani Kharus (I); (Harland et al., 1990, time scale).

Our first data point is provided by Orbitolina texana found above the basal discontinuity at the Shuaiba-Nahr Umr Formation boundary, dated in the MIDK CS at 113.72 Ma (Fig. 9). This, however, does not date the onset of subaerial exposure but dates the initial sedimentation related to sea-level rise and drowning of this surface. The base of the Nahr Umr here is as old as latest Aptían, using a date of 112 Ma for the Aptian-Albian boundary. This is consistent with the Aptian age of the Shuaiba-Nahr Umr Formation boundary in Wadi Mu’aydim (III) and in good agreement with Late Aptian nannoplankton recovered from the basal Nahr Umr (Shaffer, in Scott 1990). On the basis of the sediment accumulation rate discussed below, the Aptian-Albian boundary is approximately located at meter 14 in the Wadi Bani Kharus Section (I; Fig. 9).

The Nahr Umr-Natih Formation boundary plots at 99.9 Ma in the graphic correlation (Fig. 9). The implication of this is that the top of the Nahr Umr Formation in Wadi Bani Kharus (I) is Late Albian in age. This age is somewhat older than that found for the top of the Nahr Umr Formation in Wadi Mu’aydim (III; Scott, 1990). The Albian-Cenomanian boundary was placed at 97 Ma by Harland et al., (1990).

On the basis of this graphic correlation experiment, a simple calculation of the age vs. thickness in the Wadi Bani Kharus Section (I) provides a rough scale for the duration of sedimentary cycles. Each meter of section represents about 123 ky on average (8.13 m/miilion years = 8.13 bubnoffs; Fischer 1969). This value, however, includes the time represented by hiatuses, and also it assumes uniform depositional rates (which probably was not the case), and it assumes no erosion during deposition of the entire Nahr Umr Formation. The ages implied by graphic correlation of the other hardbottoms found in the Wadi Bani Kharus Section (I) are shown in Figure 9.

The above results are at variance with some previous work suggesting a Middle Albian to Cenomanian age for the Nahr Umr Formation (see Le Métour et al., 1995, and references therein). Hoogkamer (1979), Simmons and Hart (1987), Hughes Clarke (1988), and Scott (1990) attributed an Albian age to the Nahr Umr in Oman. Harris et al. (1984), Alsharhan and Nairn (1988), and Scott (1990) indicate a Late Aptian to Albian age for the Nahr Umr Formation in the United Arab Emirates, whereas Alsharhan (1991) suggested a Mid-Albian age for this unit.

Ammonite Biostratigmphy

Few ammonites have been reported from the intracratonic basin of the Arabian Platform. We have found Knemiceras uhligi (Choffat) at meter 82 of our section in Wadi Bani Kharus (I) above HB 7 (Figs. 3, 10). Our specimens are described in the Appendix. This level is dated to be Upper Albian by graphic correlation. However, the group of Knemiceras is known generally to be of Lower to Mid-Albian age, although the substage boundaries are not accurately identified in the sections yielding specimens of Knemiceras. A range into the Upper Albian cannot be ruled out at this time.

Fig. 10.

Knemiceras uhligi (Choffat 1886) from the upper part of the Nahr Umr Formation (meter 82) in Wadi Bani Kharus (I). A) Ventral, B) lateral views. Scale bar is 20 mm.

Fig. 10.

Knemiceras uhligi (Choffat 1886) from the upper part of the Nahr Umr Formation (meter 82) in Wadi Bani Kharus (I). A) Ventral, B) lateral views. Scale bar is 20 mm.

In Lebanon and Israel Lewy and Raab (1976) reported two horizons with ammonites of this genus that are upper Lower Albian and probably Middle Albian in age (compare Kennedy and Simmons 1991). Kennedy and Simmons (1991) described K. dubertreti (Basse) from the lower part of the Nahr Umr Formation from Jabal Madar (V; Fig. 1; see Fig. 3 of these authors) and dated the ammonite horizon following Lewy and Raab (1978) as upper Lower or lower Middle Albian, which is consistent with microfau-nal data. According to our stratigraphy, the ammonite horizon of Kennedy and Simmons (1991) lies above of marker bed succession A in Wadi Bani Kharus (I) between meter 36–40 and directly below HB 4, which is also dated by graphic correlation to be upper Lower or lower Middle Albian at 108.2 Ma.

Interpretation and Discussion

Disconformity Surfaces? Marine versus Subaerial Origin

On the basis of evidence presented in the previous chapters several stages in the evolution of disconformity surfaces in the Nahr Umr Formation of Northern Oman are distinguished. The marine stage(s) of most hardbottoms in the Nahr Umr Formation in Jabal Akhdar is easily recognizable and predominant in the field. Conspicuous features clearly relate to the marine environment. Pétrographie evidence indicates early submarine lithification accompanied by phosphatization, penetration of these surfaces by numerous boring bivalves, and encrustation by well-preserved biota such as oysters (Fig. 4B) and serpulids. From the following lines of evidence, however, we propose that a subaerial exposure stage is superimposed on many of these surfaces in Jabal Akhdar.

  1. The carbon and, in part, the oxygen chemostratigraphy of the Nahr Umr Formation is characterized by a shift towards lighter values beneath hardbottoms (Fig. 3). Following previous authors (Allan and Matthews, 1982; Humphrey et al., 1985; Beier, 1987; Marshall, 1992), we interpret the sharp decreases in carbon and oxygen isotope ra tios associated with hardgrounds to be the result of diagenetic overprinting during subaerial exposure and associated paleosol formation (Immenhauser et al., 1999). The light δ13C value is the opposite of the heavy δ13C value commonly found in submarine hardgrounds (Marshall and Ashton 1980). The presence and magnitude of light excursions associated with subaerial exposure depends on the climate and the duration of exposure. Formation of these hardgrounds related to sea-level lowering is also indicated by a statistical study of the functional morphology of orbitolinids, which indicates a very shallow environment beneath and a rapid deepening above hardground surfaces (Immenhauser et al., 1999).

  2. Our study of fluid inclusions in blocky calcites beneath the marker-bed succession C in Wadi Bani Kharus (I) and Wadi Mu’aydim (III) shows the presence of a large number of inclusions with brackish water. This is considered to be the result of mixing of meteoric water with residual salt water during subaerial exposure (Fig. 8; Immenhauser et al., 1999).

  3. Indirect evidence for paleosols above these hardgrounds is also provided by root molds, some of which still contain Microcodium, calcitized mycorrhizae which are interpreted to be the symbiotic association between soil fungi and cortical cells of roots of higher land plants (Fig. 4E; Klappa, 1978). The cakite-quartz intergrowth of cement phase four is possibly another indication for a pedogenic overprint of these surfaces. Authigenic quartz cements are precipitated during burial diagenesis because many deeper subsurface waters contain several tens of ppm of silica. This interpretation seems unlikely in this case, however, because the crystals are overlain by marine sediments (Fig. 4H). We therefore favor the interpretation that this calcite-quartz intergrowth is incipient silcrete, which is well known from many fossil soils forming mainly under semiarid to arid climatic conditions (Meyers, 1977; Summerfield, 1983).

All of the above lines of evidence are in contrast to the scarcity of macroscopic features, such as deep-cutting karst, commonly found at exposure surfaces (Wright, 1988; Mylroie and Carew, 1995). Locally, faint superficial karst penetrates a few centimeters into the limestones. It is also important to note that few upward-shoaling cycles were recognized beneath these surfaces, resting in many cases directly upon sediments indicating a low to intermittently agitated paleo-hydrodynamic regime (Immenhauser et al., 1999). For the time being, it is not understood why major karst features are absent. This applies especially to the Shuaiba-Nahr Umr Formation boundary, which was subjected to subaerial conditions for several millions of years but also lacks large karst features. Possible reasons for the absence of deep-cutting karst is discussed in Immenhauser et al. (1999).

Another key problem that merits discussion is whether subaerial exposure preceded a marine hardground stage or whether a marine hardground stage preceded subaerial exposure. Pétrographie and field evidence strongly suggests that two marine hardground stages were separated by the subaerial exposure stage described above. Circumstantial evidence for a marine hardground stage following the subaerial exposure are the well preserved encrusting biota found in many locations (Fig. 4B). The preservation potential of such delicate features under conditions related to subaerial exposure is indeed negligible. It thus seems likely that these shells, as well as an unknown proportion of the borings, postdate subaerial exposure. But equally compelling evidence suggests a marine hardground stage before subaerial exposure. The following chain of arguments is based mainly on petrographic observations. As observed in thin sections, many of the borings were later sealed with crusts of goethite (Fig. 4C). The growth of large goethite crystals fractured the lithified sediments beneath hardground surfaces. These crusts fill and therefore postdate the borings. They are related either to a late phase of a marine hardground stage or, more likely, to a subsequent subaerial exposure stage. Late fault systems are locally also iron stained, but these thin veneers cannot be compared with the centimeter-thick crusts sealing hardgrounds. The assumption that crusts of secondary iron oxides are the product of subaerial exposure is supported by the following observation. In many cases, phase four calcite-quartz (Table 1) occludes pore space within the iron-oxide crusts that seal bore holes and also fills fissures created by the growth of large goethite crystals. The isotope composition of the cement phase four yields the light isotope signal measured underneath hardbottoms (Table 1, Fig. 6). The light isotope composition is related to subaerial exposure, soil formation, and meteoric water.

We conclude, in summary of the above discussion, that the composite surfaces of the Nahr Umr Formation initially formed as genuine marine hardgrounds, subsequently were turned into subaerial exposure surfaces during lowstand, and became marine hardgrounds again during subsequent sea-level rise. This accounts for the surfaces in Jabal Akhdar but probably not for those in the Foothills. Disconformity surfaces in the Foothills, with the exception of the Shuaiba-Nahr Umr Formation boundary, show only very faint or no signs of subaerial exposure and may thus be marine in origin.

Origin of Composite Surfaces

The formation of marine hardgrounds is related to periods of nonsedimentation and early lithification (Purser, 1969; Kennedy and Garrison, 1975; Bromley, 1978; Brett and Brookfield, 1984). These environmental conditions may be related to either a rapid sea-level rise (maximum flooding surfaces), sea-level fall, and/or bottom currents (Tucker and Wright, 1990; Schlager et al., 1994).

In the case of the hardbottoms recognized in the Nahr Umr Formation of northern Oman it seems likely that the winnowing of these surfaces is related to a drop in sea level. The concomitant lowering of the effective wave base exposes the sea floor to wave action (Fig. 11). Waves and currents sweep the sea floor clean, pump water through pores of the sediment, and cause precipitation of early marine cements. The first result is a marine hardground stage. Under continuously falling sea level the hardground eventually emerges and becomes exposed to subaerial conditions, and soil forms and land plants leave root molds (Fig. 10). Eventually transgression takes place and removes the soil layer. Waves inhibit sediment deposition (second hardground stage) until the surface has subsided below wave base and sedimentation atop of the hardbottoms continues.

Fig. 11.

—Origin of composite hardgrounds in the Nahr Umr Formation in Jabal Akhdar. 1) Sedimentation below effective wave base during highstand. 2) Sea-level fall lowers the effective wave base. Wave and currents winnow the seafloor and circulate water through pores, causing early marine cementation (hardground stage I). 3) Continuous sea-level drop leads to exposure of the hardground surface. Sediments beneath the surface are altered due to isotopically light soil C02 and meteoric water. 4) Sea level rises again and erodes the soil atop hardground surface. 5) Hardground stage II. Wave and currents initially inhibit sediment deposition. 6) Sea level and effective wave base rise above the sea floor; sedimentation continues.

Fig. 11.

—Origin of composite hardgrounds in the Nahr Umr Formation in Jabal Akhdar. 1) Sedimentation below effective wave base during highstand. 2) Sea-level fall lowers the effective wave base. Wave and currents winnow the seafloor and circulate water through pores, causing early marine cementation (hardground stage I). 3) Continuous sea-level drop leads to exposure of the hardground surface. Sediments beneath the surface are altered due to isotopically light soil C02 and meteoric water. 4) Sea level rises again and erodes the soil atop hardground surface. 5) Hardground stage II. Wave and currents initially inhibit sediment deposition. 6) Sea level and effective wave base rise above the sea floor; sedimentation continues.

The above scenario (Fig. 11) applies to the Nahr Umr Formation in Jabal Akhdar only. Hardbottoms in the Foothills, situated in a more basinward and thus deeper setting, underwent only very short-lived subaerial exposure or remained submerged. The amplitude of sea-level drop was probably too small to expose the sea floor in the deeper domains of the intracratonic basin.

Origin of Marker-Bed Successions

It seems likely that the marker-bed successions of the Nahr Umr Formation are the product of a change in environmental conditions related to a drop in sea level. Light intensity and organic production are, among other factors, related predominantly to water depth (Bosscher and Schlager 1992) and thus linked to sea-level fluctuations. A sea-level drop consequently leads to increased illumination and, linked to this, increased carbonate production by reefs.

Marker-bed succession C in Jabal Akhdar is recognized by the massive appearance of the rudist Praeradiolites sp. The same rudist is characteristic of the Aptian to Albian Al Hassanat Formation, which was deposited along the margins of the Bab Basin of northern Oman. A sea-level drop as a controlling factor for the formation of the marker-bed successions is consistent with an inferred progradation of the Al Hassanat reefs towards the intracratonic basin. An alternative explanation for the origin of the marker beds would be the temporary reduction of clay input into the basin.

Spatial and Temporal Correlation of the Nahr Umr Formation

Some of the surfaces in sections of the Nahr Umr Formation occupy a characteristic stratigraphic position at the top of marker-bed successions (Fig. 3). A one-dimensional sequence stratigraphic interpretation suggests that many of the hardbottoms between marker-bed successions can be correlated throughout sections in Jabal Akhdar and in part also between Jabal Akhdar and the Foothills (Immenhauser et al., 1999). Marker-bed successions and hardbottoms are considered to be quasi-time-parallel event markers and allow the correlation of measured sections. They are the product of very specific paleoenvironmental conditions at a given time.

The correlation of two stratigraphie boundaries demands further attention: the Shuaiba-Nahï Umr boundary (hardbottom HB 1, Fig. 3) and the Nahr Umr-Natih boundary. The Shuaiba-Nahr Umr boundary is an important regional unconformity in northern Oman. This surface defines a major change in depositional style from thick bedded shallow-shelf limestones below (Shuaiba Formation) to a shaly succession of mixed clastic-carbonate deposits above (Nahr Umr Formation) and represents a hiatus of probably several million years duration. In northern Jabal Akhdar, where this surface can be traced over many kilometers, it remains parallel to bedding planes of the underlying Shuaiba and therefore is a disconformity or a very low-angle unconformity. The facies of sediments underlying the disconformity indicate an upward increase of wave energy and thus a shoaling of the environment. The Shuaiba-Nahr Umr Formation boundary may record both a pronounced global regression followed by rapid transgression (Murris, 1980) as well as regional tilting and uplift in the west (Burchette and Britton, 1985; Scott et al., 1988; Scott, 1990; Wagner, 1990).

The sedimentary rocks underneath the Shuaiba-Nahr Umr disconformity are characterized by relatively heavy carbon-isotope values (Fig. 5) and were dated as upper Lower Aptian (M. Simmons, personal communication, 1997; Scott et al., 1988, Scott, 1990). This is younger than the positive carbon-isotope excursion related to OAE la and older than OAE lb (Erbacher et al., 1996). Stable-isotope curves from the Shuaiba in the Bab Basin (Vahrenkamp, 1996) and elsewhere (Grötsch et al., 1998) show similarly heavy isotope values for the Lower Aptian Shuaiba cycle 2, which is below a shift to “normal” carbon-isotope compositions and a drop in sea level in the Late Aptian. According to the Harland et al. (1990) time scale the boundary between Lower and Upper Aptian is interpolated at 118 Ma, and Gradstein et al. (1994) date this boundary at 117 Ma by assuming that the ammonite zones are of equal duration. Scott et al. (1988) dated this boundary at 116 Ma by graphic correlation and in the MIDK CS it is dated at about 114.6 Ma. The basal sedimentary rocks of the Nahr Umr above the basal unconformity were dated at 113.8 Ma by our graphic correlation model (Fig. 9). Thus, the duration of the hiatus in Wadi Bani Kharus (I) may range from 1 to 4 My. This upper range is a longer period of nonsedimentation than suggested by previous authors (Harris et al., 1984; Simmons and Hart, 1987; Pratt and Smewing, 1993), although Scott (1990) suggested that the hiatus ranged from 1 to 10 My. Scott (1990) suggested that the Shuaiba-Nahr Umr hiatus is diachronous and becomes increasingly longer towards the east and northeast. Within the 100 km scale of the transect, this assumed diachroneity is not visible in our data.

The second important marker is the Nahr Umr-Natih boundary (Fig. 3). In fact, it is not a surface but rather a zone, several meters thick, in which the Nahr Umr Formation grades into the Natih Formation. In the field, the Nahr Umr-Natih boundary was placed where the shaly, ochre-colored beds of the Nahr Umr grade into dark-gray, massive limestones of the basal Natih Formation. This change in facies is also related to a last appearance of important accumulations of orbitolinids. In Wadi Bani Kharus (I) and Wadi Mu’aydim (III) surface 10 (Fig. 3) coincides approximately with the position of the formation boundary. In Section II and in the Foothills the contact is a gradual change in facies and depositional pattern but no hardbottom was found. In both Sections I and II the top few meters of the Nahr Umr consist of high-energy, partly cross-bedded grainstones.

Paleobathymetry of the Bab Basin in Northern Oman

Plate tectonic reconstructions (Scotese et al., 1988) indicate a subequatorial to equatorial paleo-latitude for Oman in the Albian. During the mid-Cretaceous, wind fields in Oman were probably very weak (Banner and Simmons, 1994), implying low-energy surface waves and maybe a low background water energy for the Bab Basin. Reefs of the Al Hassanat Formation (Masse et al., 1997) rimmed the outer margin of the Arabian Platform in Oman and protected the shelf setting from the impinging oceanic swell of the Neo-Tethys (Fig. 2). However, as discussed before much of the sediments of the Nahr Umr Formation in Jabal Akhdar were deposited below fair-weather wave base but within the reach of storm waves. This led to sediment deposition in an environment that we termed “intermittently agitated” (Immenhauser et al., 1999). The resulting wackestone facies is common in most sections. A very tentative estimate of the paleo-water depth for the Nahr Umr during highstand can be made by assuming a fair weather-wave base at 10 to 20 meters and a storm wave base at 30 to 50 meters. In northern Jabal Akhdar the paleo-water depth was therefore possibly in the range of 30 to 40 meters. Towards the south, the basin deepened. This is indicated by the argillaceous facies of the Foothill sections, which were deposited in water depths below the limit of even the largest storm waves.

The present-day Persian Gulf, 1000 km long and 200–300 km wide, is probably a modern analogue of the Bab Basin. The sea floor on the Arabian side of the Gulf averages 35 m deep and about80min the basin center, which may wel1 be comparable with the Bab Basin in Jabal Akhdar. Given a hypothetical paleo-water depth of 30 to 40 m in northern Jabal Akhdar and an average basinward gradient of 0.01° (which again is comparable to the Persian Gulf), 50 to 60 m water depth results for the Foothills (100 km vertical distance). This result is compatible with our assumption that wave base was lowered down to the basin floor during sea-level lowstands. The bathymetrie changes recorded by each marker bed were in the same range of 30 to 50 m. None of the Upper Albian beds in Oman recorded a “dramatic” fall of sea level prior to a rise of even higher amplitude in the Rotalipora appenninica zone (Grötsch et al., 1993).

Conclusions

The Nahr Umr Formation in northern Oman is of uppermost Aptian to Upper Albian age. In a 100 km transect measured from the margin of the Neo-Tethys (Jabal Akhdar) south into the fringes of the intracratonic Bab Basin (Foothills), formation boundaries are quasi-isochronous. Three marker-bed successions were recognized and correlated throughout all sections. The marker beds are pure, thick limestones deposited during periods of sea-level lowstand.

Disconformity surfaces were found in all sections and are of composite submarine-subaerial origin. In Jabal Akhdar, hardgrounds formed during sea-level drop and subsequently became exposed to subaerial conditions during the peak of regression, and were again overprinted as marine hardgrounds during transgression. Hardgrounds in the more basinward setting of the Foothills remained submerged or underwent only short-lived exposure. Several of the surfaces can be traced and correlated between Jabal Akhdar and the Foothills.

Systematic paleontology and discussion of Knemiceras uhligi found in Wadi Bani Kharus (I)

  • Superfamily Hoplitaceae Douvillé, 1890

  • Family Engonoceratidae Hyatt, 1900

  • Genus Knemiceras Böhm, 1898

  • Knemiceras uhligi (Choffat 1886)

  • 1995 Knemiceras uhligi uhligi (Choffat 1886); Geyer, p. 12, figs. 3a & b, 4a-d (for further synonymy).

  • 1995 Knemiceras uhligi subcompressum (Hyatt) n. subsp.; Geyer, p. 12 (and synonymy).

  • 1995 Knemiceras uhligi choffati n. subsp.; Geyer, p. 12, fig. 3 c & 5 a-f (and synonymy).

  • 1997 Knemiceras uhligi subcompressum (Hyatt) Geyer 1995; Geyer et al. (1997), p. 226, fig. 2f-h, 3f-g.

Material.—

One incomplete specimen is figured and described here (inventory number GPIT 1823/1 at the University of Tübingen, Germany; Fig. 10). A badly preserved fragment of Knemiceras sp. without diagnostic species features is not considered (inventory number GPIT 1823/2; University of Tübingen, Germany).

Description.—

The new specimen is flattened and has about half a whorl of the body chamber preserved. Because of distortion it is difficul t to estimate the original shape of the whorl section and its ventral tuberculation, but it appears to be highly compressed with a narrow venter. The greatest breadth is at mid flank in intercostal section, and has distinctly clavate ventrolateral tubercles. Coiling is evolute in comparison with other representatives of the genus, with the umbilicus constituting about 21% of the diameter. Umbilical tubercles are very prominent, especially on the phragmocone, where they are situated directly at the umbilical margin. On the body chamber the umbilical wall is smooth. Tubercles are situated more ventrally on the inner flank, up to one-third of the flank height. The distinct prosiradiate ribs arise from the umbilical inner-flank tubercles; there are two to three intercalated secondary ribs.

Comparison and Discussion.—

The prominent umbilical, inner-lateral tuberculation and fairly coarse ribs of the specimen described above resemble Knemiceras compressum var. subcompressum Hyatt (1903; Hyatt, pl. 16, fig. 14) which is, however, more involute and depressed. Geyer (1995) placed Hyatt’s subspecies in the species, K. uhligi subcompressum, referring to figure 4 on pl. 16 of Hyatt (1903) as lectotype. This is obviously a misprint, fig. 4 is K. syriacum von Buch, and, as can be supposed from his synonymy, Geyer assigned fig. 14 of Hyatt (1903) as lectotype. We do not follow Geyer’s (1995, 1997) subspecific divison of K. uhligi, however, because his subspecies are not stratigraphically or geographically separated. Although the stratigraphic ranges are not well known, this is at least true for the typical subspecies and K. uhligi choffati (sensu Geyer). Furthermore, it is more likely that these subspecies fall into the variation of a single morphotype, as indicated by figures 6789 of Geyer (1995). This is confirmed by a reinvestigation of Knemiceras from the Albian of Iran by Reyment and Kennedy (1991), who showed that there is a great variability in inflation and ornamentation of specimens. These ammonites are probably referable to a single species, but have been referred to different taxa before. This great variability can be explained by the habitation of a shallow, labile (epicontinental) environment (Reyment and Kennedy, 1991).

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Acknowledgments

We gratefully acknowledge financial support from the Swiss National Foundation (Immenhauser; No. 83EU-051870) and the Vrije Universiteit Industrial Associates in sedimentology research grants held by Wolfgang Schlager. Fieldwork was possible due to the help of the Ministry of Commerce and Industry of Oman, especially Mohammed Bin Hussein Bin Kassim, Director General of Minerals, and Hilal Al Azri, Director of the Geological Survey, who provided logistical help and support. We would like to thank A. Matter, University of Bern, for support of stable-isotope measurements and Tj. Peters, University of Bern, for logistical help in the field. W. Gerber (GPI, Tübingen) took the ammonite photos. W. J. Lustenhouwer operated the electron microprobe. Elemental analyses were provided by the Vrije Universiteit Amsterdam and by NWO, the Netherlands Organization for Scientific Research. Development of the MIDK composite standard data base by R. Scott was initiated by support of W. Schlager’s project at the Vrije Universiteit Amsterdam. This paper benefited greatly from discussions and inputsby J. Erbacher, L.Hottinger, H.Oterdoom, J.-P. Masse, J. Touret, and H. Weissert. Comments by SEPM reviewers R. Loucks and T. Dickson, and by technical editor J. Southard improved an earlier version of the manuscript.

Figures & Tables

Contents

GeoRef

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