Sedimentological evidence for bottom-water oxygenation during deposition of the Natih-B Member intrashelf-basinal sediments: Upper Cretaceous carbonate source rock, Natih Formation, North Sultanate of Oman

The fine-grained, carbonate-dominated, Middle-Late Cenomanian succession of the Natih-B Member (Natih Formation, North Oman; Figures 1 and 2) was deposited in a sediment-starved intrashelf basin (40 to 60 m deep) during a marine transgression, surrounded by a shallow-water carbonate-ramp system (Murris, 1980; Harris and Frost, 1984; Philip et al., 1995; van Buchem et al., 1996, 2002; Immenhauser et al., 2000; Homewood et al., 2008; Figure 1b). Its deposition was intimately associated with abundant accumulation of organic carbon (average 3.2%, range 0.3 to 13.7% total organic carbon [TOC], based on samples from a well in the Natih Field). This interval is likely to be a regional prolific source rock (e.g. Grantham et al., 1987; Terken, 1999; van Buchem et al., 2005). Here, organic-matter accumulation is commonly interpreted as having occurred within a “silled basin” (sensuDemaison and Moore, 1980), within an overall intrashelf setting. As a consequence, this interpretation has led many authors (e.g. Murris, 1980; Scott, 1990; Burchette, 1993; Alsharhan, 1995; Philip et al., 1995; Droste and Van Steenwinkel, 2004; Homewood et al., 2008) to argue that bottom-water “anoxia” was the fundamental factor controlling enhanced organic-matter preservation in the Natih Formation.

Sedimentary successions deposited on continental shelves and within intrashelf basins commonly exhibit cyclical and repeated lithofacies motifs. For instance, Mesozoic-aged sediments deposited in these settings, preserved over much of the globe, comprise alternating lithofacies of clay- and organic-matter-rich and carbonate-rich facies (e.g. Arthur et al., 1984; Weedon, 1986; Hallam, 1987; Droste, 1990; Jenkyns, 1991; Burchette, 1993; Macquaker and Taylor, 1996; Kuhnt et al., 1997; Damholt and Surlyk, 2004; van Buchem et al., 2005; Macquaker et al., 2007; Varban and Plint, 2008). These patterns are commonly interpreted as being either products of: (1) varying clastic dilution of primary production-derived components in hemipelagic settings, (2) varying accommodation availability and resulting energy available at the sediment/water interface during periods of shallow water (usually manifested by the presence of storm layers and changes in grain size), (3) development of anoxia when the basin was deeper, or (4) preferential cementation of particular strata, particularly during early burial.

The overarching control on these factors is typically interpreted to be long-term changes in climate, usually driven by a Milankovitch mechanism (e.g. Weedon et al., 2004). In spite of the apparent simplicity and elegance of this controlling factor, the variable microfacies in these systems suggest that the specific controls on lithofacies variability and organic-matter preservation in intrashelf-basinal settings are likely to be very complicated. In particular, facies variability was probably linked to subtle changes in primary production (both organic and inorganic), rates of sediment accumulation and burial, porewater oxygen concentrations, and early diagenesis (Pedersen and Calvert, 1990; Macquaker and Gawthorpe, 1993; Macquaker, 1994; Bohacs et al., 2005; Katz, 2005; Tyson, 2005), all of which are ultimately controlled by climate.

In the Natih-B Member, the carbonate content (derived from both primary production and diagenetic processes) varies from 80 to 100%, clay content (both detrital and authigenic) from 0 to 10% (van Buchem et al., 2005), TOC content from 0.3 to 13.7%, quartz content from 0 to 10%, with pyrite and calcium phosphate (fish debris) each varying from 0 to 5%. The Natih-B facies variability has traditionally been interpreted in terms of variations in primary production or alternating oxic/anoxic cycles (e.g. Demaison and Moore, 1980; Pedersen and Calvert, 1990; van Buchem et al., 2005; Homewood et al., 2008).

Existing models used to explain organic-matter preservation in the distal parts of the Natih-B intrashelf basin are generally simple and mainly rely on the development of regional “anoxic” bottom water, associated with a major transgression. Bottom-water anoxia is important in this context as its presence likely reduced the rate of organic-carbon decay, thereby maximising the preservation of organic carbon. In these models, however, the roles of variations in primary organic production, coupled with episodic delivery and relatively rapid burial of organic matter may have been underestimated. Overall, existing published sedimentological and stratigraphical work on the Natih Formation (e.g. Philip et al., 1995; van Buchem et al., 2002, 2005; Droste and Van Steenwinkel, 2004; Schwab et al., 2005; Grélaud et al., 2006; Homewood et al., 2008) seems fairly robust, although it lacks process-detail analyses on the more distal facies (source-rock intervals) and their associated units.

The main aim of this paper, based on more in-depth study of process-detail analyses, is to illustrate that persistent bottom-water “anoxia” is unlikely to have been the key factor controlling organic-carbon preservation during deposition of the Natih-B intrashelf-basinal sediments, and to discuss some of the alternative mechanisms that might have controlled: (a) organic-matter enrichment, and (b) lithofacies variability. A detailed description of the methods used in this study is provided in the Appendix.

During the time from Early Jurassic to Late Cretaceous, Oman was located on the northeastern margin of the Arabian Plate and was the site for an extensive carbonate-platform deposition, which also covered most of the Arabian Peninsula and Gulf (Murris, 1980; Droste and Van Steenwinkel, 2004). This major Mesozoic deposition took place following the breakup of Gondwana and opening of the Neo-Tethys Ocean (Loosveld et al., 1996). In the interior of North Oman, a peripheral foreland bulge formed as a result of the building of the Oman Mountains during the Late Cretaceous (Robertson, 1987; Boote et al., 1990; Warburton et al., 1990; Terken, 1999).

The Cretaceous carbonate-platform succession of Oman is up to 1.2 km thick and 1,000 km wide (Droste and Van Steenwinkel, 2004). This large-scale succession started to grow in Central Oman during the Late Berriasian, after a major marine transgression that had facilitated the deposition of shallow-water carbonates over tilted, uplifted, and eroded Jurassic and older strata, following the rifting between the African-Arabian Plate and Greater India (Droste and Van Steenwinkel, 2004; Razin et al., 2005). Droste and Van Steenwinkel (2004) pointed out that the carbonate deposition during Berriasian to Turonian times was associated with regular subaerial exposures and influxes of terrigenous material from the Arabian Plate hinterland. The Late Aptian, large-scale, relative sea-level fall recorded a major regional unconformity, and is related to extensive karstification and erosion in Oman. This major tectonic and eustatic event (Sharief et al., 1989) terminated the deposition of the Shu’aiba Formation platform carbonates, enabling the prevalent distribution of siliciclastics of the latest Aptian – Late Albian Nahr Umr Formation. During the Late Albian, Cenomanian and Early Turonian times, platform carbonates of the Natih Formation were deposited over the Nahr Umr mudstones (e.g. van Buchem et al., 2002; Figure 2). In the Turonian, a regional phase of uplift and erosion concluded the progression of the Cretaceous platform-carbonate succession at the end of the Natih Formation deposition (e.g. Hughes Clarke, 1988; Droste and Van Steenwinkel, 2004; Figure 2). This emergent period was then followed by the influx of the Fiqa siliciclastic mudstones, which unconformably overlie the Natih Formation carbonates (Figure 2).

The Natih-B Member occurs within the Upper Albian – Lower Turonian Natih Formation, which forms the upper part of the Wasia Group (Smith et al., 1990; Philip et al., 1995; van Buchem et al., 2002; Figure 2). Lateral, age-equivalent units to the Natih in the Arabian Gulf region are the shallow-water carbonates of the Mauddud and Mishrif formations, and intrashelf-basinal carbonates of the Shilaif/Khatiyah Formation (also known as Rumaila Formation in Kuwait and Iraq) (e.g. Harris and Frost, 1984; Alsharhan and Nairn, 1988; Burchette, 1993; Aqrawi et al., 1998; Ehrenberg et al., 2008; Figure 2). The Natih Formation includes intrashelf-basinal carbonate source rocks and adjacent, time-equivalent shallow-water carbonate reservoirs (e.g. Figure 1b). The carbonate petroleum system of the Natih Formation is well known in the subsurface of interior Oman (e.g. Harris and Frost, 1984; Grantham et al., 1987; Terken, 1999; Droste and Van Steenwinkel, 2004; Morettini et al., 2005) and in the excellent outcrops in the Oman Mountains (Al Jabal Al Akhdar) and Adam Foothills (e.g. van Buchem et al., 1996, 2002; Schwab et al., 2005; Homewood et al., 2008).

The Natih Formation is approximately 400 m thick, and it is informally subdivided in the subsurface into seven members: Natih-A to Natih-G, top to bottom (Hughes Clarke, 1988; Figure 2). It mostly comprises mud-supported and some grain-supported limestones, with local rudist growth, alternating with calcareous mudstones. The shallow-water, rudist-bearing platform carbonates are prolific hydrocarbon reservoirs for the Cretaceous petroleum system in the Middle East (e.g. Harris and Frost, 1984; Burchette and Britton, 1985; Alsharhan, 1995; van Buchem et al., 1996). The Natih Formation is also considered a significant carbonate source rock because it includes at least two organic-carbon-rich intervals deposited in intrashelf basins (lower Natih-E and upper Natih-B; Figure 2) during transgressive phases (Grantham et al., 1987; Scott, 1990; Terken, 1999; van Buchem et al., 2002; Droste and Van Steenwinkel, 2004). A thick shale succession of the overlying Fiqa Formation (Aruma Group) provides a major regional seal for the Natih reservoirs.

In order to be more predictive with regard to the distribution of reservoir and source-rock facies, and to the geometrical and genetic relationships between intrashelf basins and adjacent carbonate platforms of the Natih Formation, high-resolution sequence-stratigraphic studies have been carried out in exposures in the Adam Foothills and Al Jabal Al Akhdar (see van Buchem et al., 1996, 2002; Schwab et al., 2005; Grélaud et al., 2006; Homewood et al., 2008). These researchers typically subdivide the Natih Formation into three major depositional units: Sequences I, II and III, from base to top (Figure 3). These sequences are regarded as larger-scale accommodation cycles that were subject to third-order (0.5 to 3.0 My) eustatic sea-level variations, each recording scenarios of transgressive-regressive patterns.

Van Buchem et al. (2002) interpret the deposition of Sequence I (Late Albian – earliest Cenomanian, Natih-G, F and E members; Figures 2 and 3) as being mainly controlled by eustatic sea-level fluctuations, and that it developed intrashelf basins at lower Natih-E. A sequence boundary at the top of Natih-E is recognised, which was associated with the development of hardgrounds and incisions along emergence surfaces (Grélaud et al., 2006). Sequence II (Mid Cenomanian, Natih-D and C members; Figures 2 and 3) did not develop an intrashelf basin and was dominated by clay deposition, which – as Homewood et al. (2008) have stated – inhibited the carbonate factory. Sequence III (Mid-Late Cenomanian – Early Turonian, Natih-B and A members; Figures 2 and 3) is very similar to Sequence I in that it records another development of an intrashelf basin during a major transgression, and that it experienced extensive subaerial exposures at its top (Homewood et al., 2008; Figures 1b and 3) during subsequent relative sea-level fall. The timing of this subaerial exposure is not definitely constrained, but the first overlying sediments are of Senonian age. However, van Buchem et al. (2002) argued that facies variability in the lower part of Sequence III was controlled by a more rapid relative sea-level rise, enhanced by a slight differential subsidence, which both enabled the creation of the relatively broader and more organic-carbon-rich Natih-B intraplatform basin.

Many studies (e.g. Murris, 1980; Scott, 1990; Philip et al., 1995; Homewood et al., 2008) have linked the development of bottom-water “anoxia” in the Natih-B intrashelf basin to this rapid rise in relative sea level. More specifically, Homewood et al. (2008) argue that stratification of the water column developed during this transgression, which also increased the clay and nutrient flux to the intrashelf basin, causing organic bloom and “oceanic anoxia”. Considering the biostratigraphically-constrained age (Mid-Late Cenomanian; van Buchem et al., 2005) of the Natih-B Member, Homewood et al. (2008) and Vahrenkamp (2010), however, found it difficult to tie the deposition of the excellent Natih-B source rock (characterised by relatively light δ¹³C values; Vahrenkamp, 2008) to a specific global Oceanic Anoxic Event (cf. Jenkyns et al., 1994; Paul et al., 1994; Rodriguez-Lázaro et al., 1998; Jarvis et al., 2006), and concluded that their occurrence might represent a unique local “expanded anoxia”.

Van Buchem et al. (2002) and Droste and Van Steenwinkel (2004) have generally characterised the development of intrashelf basins on the extensive Cretaceous carbonate platform of North Oman as being mostly driven by relative changes in sedimentation rates, as a result of high-frequency fluctuations in relative sea level. They suggest that tectonism only played a minor role in the formation of basin topography, probably through the development of slight initial relief (local depression) during differential drowning of platform-interior areas. Moreover, the lack of obvious clinoform geometries in the Natih-B unit also implies a shallow and broad intrashelf basin.

The carbonate-source-rock intervals of the lower Natih-E and upper Natih-B are identified in outcrops in the Adam Foothills and Al Jabal Al Akhdar, and on well logs and core slabs from nearby oilfields in North Oman (Figure 1). It is likely that these intervals were the sources of the hydrocarbons in the Natih reservoirs of many fields, including the giant Fahud and Natih fields (Grantham et al., 1987; van Buchem et al., 1996; Terken, 1999; Homewood et al., 2008). Grantham et al. (1987) and Terken (1999) have described both source-rock levels as being geochemically similar. However, the upper Natih-B unit is the more important source-rock interval in the Natih Formation, simply because it is thicker (up to 40 m thick, about 20 m of which is actual source rock – beds with ≥ 2.0% TOC) and excellent in quality, up to 13.7% TOC, based on our analysis (Table 2, see Appendix). According to Terken (1999), the TOC content of the lower Natih-E source rock rarely exceeds five percent.

The transgressive, dm- to m-thick-bedded, greyish Natih-B sediments overlie an iron-enriched (brownish/reddish coloured), extensively-bioturbated and bored hardground at the top of the Natih-C Member, and conformably underlie relatively thicker, shallower-marine, whitish carbonate beds of the Natih-A Member (Figure 4). In the exposures of Jabal Qusaybah, Jabal Nahdah and Jabal Salakh, the Natih-B Member is informally subdivided into five units, based on their weathering and lithological characteristics: Natih-B1 (NB1) to Natih-B5 (NB5), base to top (Figures 4 to 6). The lower units observed in the exposures (i.e. NB1 and NB2) are equivalent to unit B4 of Natih-B in the subsurface; NB3 is the equivalent to B3; NB4 is the equivalent to the lower 4 to 3 m of B2; and NB5 is the equivalent to the rest of B2 plus B1 (cf. Homewood et al., 2008).

The deposition of the inner-platform facies – abundant evidence of mixed benthic foraminifera (including calcareous forms of miliolids, miliolinid alveolinids, [Praealveolina spp. and Cisalveolina spp.], Aeolisaccus spp., Nezzazata conica, N. simplex, Idalina spp. and Nummolofallotia apula, and agglutinated forms of Chrysalidina spp., Dicyclina spp., Nautiloculina spp. and Pseudolituonella reicheli) – of NB1 over the Natih-C hardground (Figure 7a) marks a significant deepening and transgressive shift of facies. This dramatic change in the depositional environment happened as a response to a major Mid-Cenomanian transgression, the culmination of the longer-term cycle recorded by deposition of the predominantly shallow-marine carbonates of the Natih-G, F, E, D, and C members of the Natih Formation. In exposures, the NB1 and NB2 units are poorly exposed and composed of 0.3- to 0.9-m-thick, nodular, extensively-bioturbated strata (Figures 4, 5 and 7a). The NB2 unit is different from the NB1 unit because it is poorly exposed, structurally deformed, and brecciated (Figure 7a).

Natih-B3 (NB3) and Natih-B4 (NB4) contain slightly darker-grey strata that are well exposed and laterally continuous (Figures 4 and 7b). Both units are bioturbated and bioclastic, but NB4 appears more nodular, bioclast-rich, and with even darker-grey beds than NB3 (Figure 7b).

The NB5 unit, on which this study is focused, covers the upper half of the Natih-B Member (c. 40 m thick; Figures 4 to 6), comprising the intrashelf-basinal organic-carbon-rich sediment, interbedded with (relatively) shallower calcite-cement-rich sediment. In these broad terms, the NB5 lithofacies can be divided into two main lithofacies types alternating with one another (Figure 7c): (a) organic-carbon-rich carbonate mudstone, and (b) calcite-cement-rich wackestone, both are described below (see Table 1 for a summary of both lithofacies).

The organic-carbon-rich carbonate mudstone lithofacies varies in thickness from about 1.5 to 165.0 cm (average about 17.5 cm; Figure 7c). It is typically fine-grained, grey to very dark grey in fresh surface, and in the exposures exhibits a well-developed fissility (Figure 8a and b).

These mudstone units commonly contain well-preserved, in-place bivalves (including thick-shelled oysters (Figure 8a to c), as well as thin-walled pectens (Figures 8f and 9a to f) and brachiopods. The oysters (Exogyra and Amphidonte) are mostly articulated, frequently concentrated on bedding planes and forming shell pavements (Figure 8a, c and e). Ammonites, including Acanthoceras rhotomagense (sensuPhilip et al., 1995), have also been found in association with some of these oyster-shell pavements (Figure 8e).

The organic-carbon-rich mudstone is partially bioturbated. The burrows present are mainly horizontal and have been partially compacted (Figures 8d and 9a, d). They are predominantly filled with detrital carbonate mud (including coccoliths, planktonic and benthic foraminifera, and shell debris; Figure 9c and d). A variety of small and intermediate trace-fossil taxa, including Planolites isp., Phycosiphon isp., and Thalassinoides isp., have been recognised. Burrowing commonly becomes increasingly intense towards the tops of the individual units (Figures 8d and 9a, b).

The partial bioturbation has disrupted most of the primary sedimentary structures in the rock. Detailed observations of the hand specimens and thin sections, however, have enabled us to observe some relict bedding structures, including the presence of remnant parallel laminae, shell pavements, and foraminiferal lags (e.g. Figures 8a to c and 9a to b). Where recognisable, the individual beds (sensuIngram, 1954; Campbell, 1967; Macquaker et al., 2007) here are very thin (3 to 30 mm thick).

Planktonic foraminifera frequently dominate the mineralised fossils of this lithofacies. The dominant species present are Heterohelix spp. (some of which show bimodal distribution) with rare Whiteinella spp. and Hedbergella spp. (Figure 9c to h). These species range in size from 50 to 250 μm. The shelter porosity within many of the foraminifer tests is partly infilled with carbonate cements, together with organic matter, clay (kaolinite) and/or pyrite (Figure 9g and h). The sediment enclosing the planktonic foraminifera is typically compacted around the tests. The matrix also contains pressure-solution seams (Figure 9e and f), which mostly display concentrations of organic- and/or clay-rich material.

The rock matrix comprises fragmented foraminifer tests, disarticulated coccolith plates, organic matter (both amorphous and woody), authigenic clay (mostly as kaolinite), fine-grained calcite replacement (‘microspar’), and infrequent pyrite (framboidal and euhedral) (Figure 9g and h; Table 1). The foraminifer tests, coccolith plates, and organic matter are commonly contained in faecal pellets. Other minor components include rare bioclastic debris (bivalve fragments and echinoid spines), thin-walled planktonic bivalves, phosphatic debris (mostly as fish vertebrae), quartz, and rare dolomite (Figure 9c to h; Table 1). This lithofacies is described as being a partially-bioturbated, organic-matter, calcareous-microplankton- (planktonic foraminifera) and calcareous-nannoplankton- (coccoliths) bearing carbonate mudstone.

The TOC content of this lithofacies can reach up to 13.7% but averages 5.4%, based on TOC analyses on samples gathered from a well in the Natih Field (North Oman; Figure 6 and Table 2, see Appendix). In well logs, the organic richness of these carbonate mudstones is reflected by relatively high total gamma-ray readings, which roughly range from 25 to 135, average about 55 API units.

Standard geochemical techniques and maceral observations suggest that the organic matter of the mainly oil-prone Natih-B source rock is hydrogen rich and consists of a mixture of both algal and bacterial, predominantly structureless, mostly load-bearing organic matter (PDO, unpublished data; Terken, 1999). In addition, both maceral observations and pyrolysis flame ionization detector (FID) results (PDO, unpublished data), together with the high hydrogen index (HI) values (range from 400 to 650; van Buchem et al., 2002) indicate that the Natih-B source rock in the well of the Natih Field is immature for any hydrocarbon generation.

The fine-grained texture and common occurrences of planktonic foraminifera and coccoliths in these organic-carbon-rich mudstone lithofacies suggest that a relatively low-energy depositional environment dominated during the deposition of these units (little evidence of erosion and redeposition – the oysters are mostly preserved as living assemblages, rarely winnowed), with their constituents being deposited predominantly from suspension settling (pelagic components) below fair-weather wave base (e.g. Markello and Read, 1982; Burchette and Britton, 1985; Droste, 1990; Philip et al., 1995; van Buchem et al., 1996; Kuhnt et al., 1997).

The lack of well-preserved lamina, presence of burrows and frequent occurrence of in-place benthic fauna suggest that, at the time of deposition, the conditions at the sediment/water interface were oxic-dysoxic, rather than anoxic (cf. Hudson and Martill, 1991; Wetzel and Uchman, 1998; Macquaker et al., 2007). Moreover, the presence of in-place oysters as shell pavements (colonisation surfaces) and moderate bioturbation in these units suggest that the sediment was firm (rather than ‘soupy’), and that there were prolonged breaks between sediment supply events (see Kenig et al., 2004; Macquaker et al., 2007), in order to allow sediment colonisation both by the oysters and burrowing organisms.

The predominance of planktonic foraminifera and coccoliths, together with the abundantly-preserved amorphous and structureless organic matter in this lithofacies also implies that there were episodically high rates of both inorganic and organic primary production during deposition of this sediment. Photosynthesising microorganisms (mainly phytoplankton) living in the upper part of the water column were probably the major contributors to organic-matter production here (e.g. PDO, unpublished data; Killops and Killops, 1993; Arthur and Sageman, 1994). Inevitably, in this setting some of the organic component was diluted by inorganic production, which caused the proportion of organic carbon to be diluted.

Persistent bottom-water anoxia cannot be the main cause of organic-matter preservation. Given: (1) the presence of hiatal surfaces associated with oyster-colonisation events, (2) the preservation of relict thin beds that have had their tops bioturbated, and (3) evidence of high primary production, it is likely that the sediment was supplied episodically to the basin as phytoplankton blooms and colonised during the interbloom periods. High sediment production during the algal blooms meant that large volumes of sediment were delivered rapidly to the seafloor during these events, and that the organic matter was buried rapidly. Rapid burial during these events, coupled with a fairly high reoccurrence frequency of blooms, ensured that not all of the organic matter was mineralised (e.g. Arthur and Sageman, 1994) prior to the occurrence of the next algal bloom. Organic-matter preservation here is, therefore, likely not primarily a function of persistent bottom-water anoxia, but rather seems to have been caused by episodic delivery of large volumes of organic-carbon-rich sediment, coupled with optimising the recurrence frequency of algal blooms. While bottom-water anoxia may have existed for the short period of the bloom, the presence of an infauna between bloom events suggests that it was not persistent. The effects of infaunal colonisation have reduced the overall source-rock quality of this unit.

The carbonate cements infilling the planktonic foraminifer tests in these units may be related to very early diagenetic processes as the tests are uncompacted. The presence of a calcite-pyrite cement assemblage suggests that this early diagenesis was linked to microbial decay of organic matter (cf. Irwin, 1980; Taylor and Macquaker, 2000). Specifically, it is likely that sulphate-reducing bacteria living in the sediment porewaters at a depth of less than 0.2 m below the sediment/water interface were using sulphate as an oxidant to oxidise organic matter, producing hydrogen sulphide (e.g. Berner, 1981; Canfield, 1994) and liberating bicarbonate into the porewaters. It is also possible that associated iron reduction was contributing alkalinity to the porewaters (e.g. Coleman et al., 1993; Macquaker et al., 1997), causing a proportion of the organic matter to be mineralised and converted into carbonate cement (e.g. Sansone et al., 1990; Machent et al., 2007).

The presence of compacted sediment around the foraminifer tests and abundance of pressure-solution seams suggest that both physical and chemical compaction processes dominated part of the diagenetic history of this lithofacies. Pressure solution in these organic-rich mudstones caused the sediment to develop thin partings (fissility). These partings may have been mistaken for genetic laminae (sensuCampbell, 1967) by previous workers.

The calcite-cement-rich wackestone lithofacies is slightly coarser grained, lighter coloured, compared to the more organic-rich units (Figures 10 and 11). Additionally, it is both extensively bioturbated (Figures 10a to c and 11a) and pervasively cemented by calcite (Figure 11e to h). Units dominated by this facies vary in thickness from 2.0 to 185.0 cm (average 20.5 cm; Figure 7c). Extensive bioturbation has destroyed most of the primary sedimentary structures and this, coupled with the pervasive cementation, causes these units to exhibit both mottled and recrystallised appearance (Figures 10a to c and 11a). Burrowing in this lithofacies is mostly attributed to Thalassinoides isp. (Figures 10a, c and 11a), and rarely to Planolites isp. and Phycosiphon isp.

In most cases, the bases of these units are sharp and erosive, and internally their grain sizes fine upward (Figures 8a, d and 10b to c). They may exhibit cm-scale scouring features (Figure 10c), and amalgamation of several thin (5 to 50 mm thick), normally-graded individual beds is frequent (e.g. Figure 10d and e). Moreover, these units commonly contain a mix of predominantly uncompacted, disarticulated shelly material, including reworked bivalves, brachiopods, gastropods, patchy echinoderm debris, corals, and occasional rudist fragments (Figures 10c to e and 11b to d).

Benthic foraminifera (mostly as Lenticulina spp.) commonly occur in the background, with some planktonic foraminifera (including Heterohelix spp. and Hedbergella spp.), ostracods, and calcispheres, cemented by fine-grained calcite (Figure 10c to f). The calcite cement and replacement appear to dominate this unit, cementing pore spaces of the bioclasts and replacing depositional micritic mud in the rock matrix (Figure 10e to h). Other minor components include quartz, dolomite (mostly as scattered rhombs) and euhedral pyrite, with scarce clay, calcium phosphate and organic matter, occurring mostly in the matrix of the rock (Figure 10g and h; Table 1). The composition and textures exhibited by this unit cause us to describe it as being an extensively-bioturbated, shell-fragment-bearing, calcite-cement-rich wackestone.

The diagenetic history of this lithofacies is dominated by calcite-cement precipitation. There is little evidence of mechanical compaction or pressure solution; only rare sutured pressure-solution seams (‘stylolites’) are present. Some microfracturing is present, which is often cemented by calcite (Figure 11e and f).

The TOC content in these calcite-cement-rich wackestones can be as low as 0.3% (mostly < 1.5%, average 1.1%). On well logs, these units exhibit relatively low total gamma-ray response, with values ranging roughly from 10 to 25, average approximately 15 API units.

The slightly coarser-grain size, common occurrence of scour surfaces, benthic foraminifera and ostracods, with diverse reworked shell fragments and extensive bioturbation in these units, all indicate that this lithofacies was deposited in a relatively shallower-marine, higher-energy, well-oxygenated environment, close to fair-weather wave base (e.g. Brett and Allison, 1998; Wetzel and Uchman, 1998; Hallam et al., 2000). The reworked shell fragments and the erosive bases of these units, coupled with the presence of normally-graded beds, suggest that sediments in these interbeds were swept by waning flow currents (likely distal storms) from the surrounding shallower carbonate platform down into the basin (cf. Droste, 1990; Hudson and Martill, 1991; Osleger, 1991; Osleger and Read, 1991; Burchette, 1993). In addition, it is likely that these interbeds were formed by the amalgamation and winnowing of individual, thinner storm-generated beds, resulting from the reworking of sediment during major storm events (Myrow and Southard, 1996; Varban and Plint, 2008). Particular offshore-directed and geostrophic currents produced by storms may have been responsible for erosion, sediment mixing, and both shore-parallel and offshore sediment transport (Aigner, 1982; Leckie and Krystinik, 1989; Duke, 1990; Duke et al., 1991).

The abundance of early-marine calcite cement and replacement (predating compaction), low-preserved organic-matter content, and degree of bioturbation in these wackestone lithofacies indicate that there was sufficient time for solutes to be diffused to the sites of precipitation, and for the sediment to be extensively burrowed. These data also imply that overall sediment accumulation rates in these units were rather slower than those of the organic-carbon-rich mudstone units (cf. Fisher and Hudson, 1987; Ekdale and Bromley, 1991; Damholt and Surlyk, 2004). Moreover, the abundance of Thalassinoides isp. here indicates that between storm events the depositional environment was reasonably quiet, and that overall rates of sediment accumulation were relatively slow. This also suggests that the substrate was fairly firm and stable, as these burrows were kept open (unlined) at the seafloor for a prolonged period of time.

The large-scale, spatial context of the Natih-B sediments indicates that deposition occurred within a basin surrounded by a shallow-water carbonate platform (Markello and Read, 1981; Droste, 1990; Burchette, 1993; Philip et al., 1995; van Buchem et al., 2002; Droste and Van Steenwinkel, 2004; Figure 12). The faunal assemblages in both alternating facies overall indicate a relatively shallow-marine environment, up to 60 m maximum water depth (see also Harris and Frost, 1984; Immenhauser et al., 2000; van Buchem et al., 2005; Homewood et al., 2008). This implies that there were significant differences in accommodation availability as a result of relative sea-level change during deposition of the two main alternating facies; with the partially-bioturbated, organic-matter-, microplankton- and nannoplankton-bearing mudstones being deposited during increase of accommodation, compared to the extensively-bioturbated, shell-fragment-bearing, calcite-cement-rich wackestones, which were deposited during decrease of accommodation (see discussion in Mettraux et al., 1999).

In spite of the relatively similar absolute water depths, the faunal assemblages present suggest that suspension settling contributed most of the sediment during deposition of the mudstone lithofacies, whereas a greater proportion of sediment in the wackestone lithofacies was supplied either by advective processes, transporting sediment from the surrounding carbonate platform, or by in-place production of biogenic material (Figure 12). It is possible that the supply of carbonate debris caused some organic-matter autodilution in the Natih-B Member (sensuvan Buchem et al., 2005). Additionally, organic-matter mineralisation by biogenic degradation processes probably also lowered the organic-matter content of this unit.

Taking into consideration the relatively shallow-marine setting and the fact that some of the organic matter is derived from woody material, it is likely that there was some offshore-directed sediment-transport component (e.g. Tyson, 1995). While this woody material may have a land source, it is also possible that it was derived from seagrasses (e.g. Suchanek et al., 1985; Kenig et al., 1990; De Leeuw et al., 1995). Seagrasses are marine-flowering, vascular higher plants that are capable of growing whilst completely submerged in the shallow-water environment, typically at less than 10 m water depth but sometimes down to 30 m because they lack accessory photosynthesising pigments (De Leeuw et al., 1995; Tyson, 1995; Orth et al., 2006; Short et al., 2007). Their evolutionary history might be as long ago as one hundred million years, and there is evidence of their existence during Upper Cretaceous times (Pomar, 2001; Pomar et al., 2005; Short et al., 2007).

Phytoplankton (and to lesser extent zooplankton and bacteria) were likely the major contributors to the production of the abundant amorphous (structureless) organic matter in the Natih-B Member (e.g. Killops and Killops, 1993; Tyson, 1995). The presence of faecal pellets composed of planktonic foraminifer tests, coccolith plates, and organic matter suggests that this amorphous matter was predominantly produced in the upper part of the water column, before being transported relatively rapidly to the seafloor within faecal pellets (McCave, 1985; Alldredge and Silver, 1988; Tyson, 1995; Kuhnt et al., 1997; Damholt and Surlyk, 2004). It is very likely that these faecal pellets were produced by filter-feeding organisms (including zooplankton and other animal sources), which bundled much of the microscopic components in the water column into organominerallic aggregates (Alldredge and Silver, 1988).

High hydrogen index (HI) values ranging from 400 to 650 of subsurface source rocks, reported by van Buchem et al. (2002), provide further evidence for the presence of the amorphous organic matter in Natih-B. However, interpretation of Type-II organic-matter composition, based on palynofacies analysis (van Buchem et al., 2005), may support the coexistence of both amorphous and woody organic-matter types in the Natih-B Member source-rock interval (see also Killops and Killops, 1993; Tyson, 1995; Terken, 1999).

The very high TOC values in the intrashelf-basinal organic-carbon-rich mudstone units (Figure 6) can be interpreted as a result of increased primary productivity of organic matter, both in the water column (phytoplankton bloom) and in the shallow shelf (woody matter), that was episodically delivered to the sediment/water interface at relatively higher rates and buried rapidly. Evidence for the latter factor is also provided by the abundant preservation of articulated shells of bivalves (cf. Brett and Allison, 1998) in this lithofacies (see above).

Apparently, higher (local) sedimentation or burial rates decrease the exposure time of organic matter at the sediment/water interface (e.g. Arthur and Sageman, 1994; Weedon et al., 2004; Katz, 2005), giving it less chance to be degraded by benthos at the oxic/dysoxic bottom waters where large benthic organisms (abundant evidence of in-place fauna and bioturbation in both lithofacies here) frequently agitate the sediment (see also Damholt and Surlyk, 2004). However, sedimentation rate should not be persistently too high in order to reduce the dilution of organic matter by carbonates (shelly material) and/or clastics (detrital clays) (e.g. Bohacs et al., 2005; Tyson, 2005; van Buchem et al., 2005).

Given this, we suggest that the enhanced organic-matter enrichment in this lithofacies was controlled by high primary organic productivity at the surface waters, and accompanied by rapid but episodic rates of sediment accumulation and burial (cf. Hudson and Martill, 1991). The rapid and episodic input of sediments does, in fact, enhance organic-matter preservation in these units by quickly burying the organic matter and transporting it into the methanogenic zone, out of the zone of substantially and relatively organic-matter degradation (e.g. Tyson, 1995). Moreover, there is no indication of sufficient clastic flux (rare detrital clay and quartz; see Figure 6) into the intrashelf basin (NB5 unit) to significantly dilute the organic-matter constituent during relatively higher sedimentation rates. Burchette (1993) also suggested that changes in sedimentation rates could have controlled the high-frequency cyclic pattern of the carbonate-rich units, interbedded at 200 to 300 mm scale with the organic-rich facies, in the intrashelf basin of the Shilaif/Khatiyah Formation of the United Arab Emirates (age equivalent to the Natih-B Member in Oman).

The reduced organic-matter content in the cement-rich interbeds likely results from a combination of organic-matter autodilution (high carbonate influx during storm events for instance), and more importantly by organic-matter degradation during early aerobic (oxic) and anaerobic (sulphate reduction) diagenesis.

Given the well-oxygenated bottom-water conditions and the relatively lower sedimentation rates during the deposition of these units, it is likely that a high proportion of the original organic matter present at the sediment/water interface was degraded by the abundant benthic fauna living on top or within the sediment (e.g. Tyson, 1995) respiring with oxygen. Once the residual organic component has been overall buried slowly to depths where free oxygen is no longer available (i.e. sulphate-reduction zone), in-place anaerobic bacteria also caused significant amount of organic-matter degradation (e.g. Machent et al., 2007). During their metabolism, these microorganisms converted the organic matter present to carbon dioxide and bicarbonate, which ultimately become incorporated in the sediment as calcite cement/replacement.

This interpretation of organic-matter mineralisation by oxic respiration and anoxic sulphate reduction is also supported by the recent chemostratigraphic work of Vahrenkamp (2010) on the Albian – Turonian Natih Formation sediments. His whole-rock, stable-isotopic analyses of carbonate carbon on cores from the Fahud Field (North Oman) show relatively light δ¹³C values in the Natih-B Member source-rock interval (NB5), in the range from +0.07 to +1.79‰ (average +0.98‰ VPDB [Vienna Pee Dee Belemnite]; cf. Allison et al., 2008). This is compared to a range of δ¹³C values (about +1.5 to about +5.5‰ VPDB) for other Natih Formation members, average around +3.5‰ VPDB (Vahrenkamp, 2010). However, because the δ¹³C values of the Natih-B sediments are still near 0.0‰, and given the relatively shallow setting and oxic-dysoxic depositional conditions of the Natih-B intrashelf basin, it is likely that the majority of the calcite cement in the Natih-B source-rock interval was derived from normal (open) marine porewaters (e.g. Marshall and Ashton, 1980; Dickson, 1985). Moreover, the δ¹³C depletion (negative shift) in the Natih-B Member suggests that the excellent source rock within this succession does not tie to any global Oceanic Anoxic Event (Vahrenkamp, 2010), providing further evidence that “expanded anoxia” probably did not exist during deposition of Natih-B (cf. Jenkyns et al., 1994; Rodriguez-Lázaro et al., 1998; Voigt, 2000; Jarvis et al., 2006).

These stable-isotopic analyses were performed on whole-rock samples, rather than individual components of the Natih-B fine-grained sediments. Relying on such data, however, makes it impossible to be specific about the processes controlling either the preservation or degradation of organic matter as these sediments comprise mineral mixtures of biogenic carbonates from bivalve, brachiopod, echinoderm, gastropod, ostracod, foraminifer and coccolith tests, in addition to the crucial authigenic-carbonate components present in the shelter porosity of these mixed tests, as well as in the rock matrix and microfractures.

The observation of shell pavements (organism-colonisation surfaces) within the organic-carbon-rich mudstone lithofacies can be interpreted as bedding planes (or depositional surfaces) that bound individual beds (e.g. Kidwell, 1985; Beckvar and Kidwell, 1988; Banerjee and Kidwell, 1991). These surfaces also represent periods of nondeposition or rapid change in depositional settings (e.g. Campbell, 1967; Brett et al., 2006). These bedding planes are well pronounced in the context of the Natih-B intrashelf basin. They commonly separate the two interbedded lithofacies described here, appearing at the top of the calcite-cement-rich wackestone beds but formed as the depositional surfaces for the overlying organic-carbon-rich mudstone beds (see Campbell, 1967; Figure 13a and b), following a shallowing-up sequence (cf. Macquaker and Howell, 1999; Figure 13c and d).

The early calcite precipitation (predating compaction), which dominates the tops of the extensively-bioturbated, shell-fragment-bearing, calcite-cement-rich units (Figure 11a and d) is interpreted to be explicitly related to breaks in sedimentation (see Macquaker and Howell, 1999; Kenig et al., 2004). Therefore, the in-place oysters that form shell plasters, where present, were probably developed on firm/hardgrounds (e.g. McLaughlin and Brett, 2007). This also suggests that the deposition of the oyster pavements was probably coincident with marine flooding surfaces at the tops of bed sets, formed during periods when sediment supply rates were reduced. These surfaces usually cap thin (average 400 mm thick) upward-coarsening units (parasequences) of the organic-rich mudstone and cement-rich wackestone couplets (cf. Morettini et al., 2005; Figure 13c and d). Genetic beds here are typically less than 30 mm thick (sensuIngram, 1954; Campbell, 1967; see above). These genetic thin beds are commonly exhibited in the organic-carbon-rich mudstone units as a well-developed fissility (described above); whereas in the calcite-cement-rich wackestone interbeds they are frequently displayed as amalgamated, storm-deposited thin beds (mentioned above), resulting in a series of vertically-stacked units.

A parasequence in this context includes both lithofacies as one transgressive-regressive cycle (Figure 13d), with the organic-carbon-rich mudstone (transgressive hemicycle) deposited initially (during marine flooding) and subsequently overlain by the relatively shallower calcite-cement-rich wackestone (regressive hemicycle) that displays colonised and cemented surfaces at the top (see also Mettraux et al., 1999; Morettini et al., 2005). At larger scales, parasequences stack into parasequence sets and systems tracts – sequence boundaries may be marked here by shell beds with robust fauna.

On the basis of detailed analyses, two main lithofacies types are described from the Natih-B Member intrashelf basin, interbedding with one another, namely: (1) partially-bioturbated, organic-matter-, calcareous-microplankton- and nannoplankton-bearing mudstone with in-place thick-shelled oysters and flattened pectens, and (2) extensively-bioturbated, shell-fragment-bearing, calcite-cement-rich wackestone. These observations suggest that the existing models used to explain lithofacies variability and organic-matter preservation in the Natih-B succession, which rely upon changing bottom-water anoxia or styles of carbonate productivity, are incorrect. Instead, these data suggest that the need for bottom-water anoxia to preserve organic matter has been overestimated, and that the mineralisation of organic matter to carbonate precipitates during early diagenesis was probably very important in controlling lithofacies variability. Therefore, the main factors underpinning organic-matter enrichment under the oxic/dysoxic conditions of this depositional setting might have required abundant nutrients to fuel the short-term, high primary organic productivity, rapid delivery of organic components to the sediment/water interface, coupled with rapid and episodic burial of organic material.

Finally, the generally shallow-water depth (40 to 60 m) of the intrashelf basin, rare evidence of clinoform stratification within the Natih-B Member, lacking a shelf-slope break, and the sedimentological and palaeontological data from this study suggest that it is highly unlikely that persistent “anoxia” existed during the deposition of the Natih-B sediments, even in the most organic-carbon-rich facies. Therefore, appreciation of these critical observations will instrumentally improve our understanding of the fundamental controls underpinning source-rock development and distribution in intrashelf depositional basins, and ultimately enhance hydrocarbon exploration and production strategies in such settings.

To meet the aims of this paper, the Natih-B Member has been sampled both from the Adam Foothills where it is best exposed, and from the continuous core slabs of a Natih well in the subsurface. Polished thin sections were prepared from each sample. The constituent components of each lithofacies were then described, utilising optical, electron-optical, mineralogical (X-ray diffraction [XRD]) and geochemical (TOC) methods, in order to obtain textural and mineralogical data at spatial scales ranging from 10⁻⁵ to 10⁴ m, and at temporal scales that likely range from the time taken for individual depositional events to deposit beds to 10⁵ years. New data gathered from these analyses were then integrated with existing information (e.g. Terken, 1999; van Buchem et al., 2002), and the controls on organic-matter preservation and lithofacies variability were investigated.

The excellent outcrop sections and core slabs of the 50- to 60-m-thick Natih-B unit in the Adam Foothills and Al Jabal Al Akhdar regions and in nearby oilfields of North Oman have been used as natural laboratories for this study (Figure 1). Detailed sedimentary logs were measured at the scale of 1:50 (e.g. Figures 5 and 6), and samples were gathered systematically from five locations: Natih Field, Jabal Qusaybah, Jabal Nahdah (distal locations), Jabal Salakh West (distal to intermediate location), and Jabal Salakh East (intermediate to proximal location) (see Figures 1 and 3). Overall, around three hundred samples were obtained from these locations at vertical intervals, 0.5 to 3.0 m apart.

Unusually thin (≤ 0.03 mm), polished, large (~ 40 x 60 mm) thin sections were prepared from each sample, in order to acquire information on sediment textures and mineral constituents at different scales of magnification. At first, each thin section was scanned using a flatbed scanner (Epson Perfection 3170) to record textural details at 50 to 10 mm scales. The thin sections were then analysed optically (first under transmitted light [both plane polarised and cross polarised] and then by cold-cathode cathodoluminescence) using a binocular Nikon petrographic microscope, attached to a digital camera (Jenoptik Jena D-07739), to obtain textural, compositional and diagenetic information at 10.0 to 0.1 mm scales. The cold-cathode cathodoluminescence petrography was performed using a CITL Cathodoluminescence Unit (Model CCL 8200 MK3), operated at approximately 20 kv, 300 μA. Finally, at even higher resolution (100 to 1 μm scales), the thin sections were investigated electron-optically using JEOL 6400 scanning electron microscope (SEM), equipped with a backscattered electron detector. With this high-power imaging technique, minerals were identified in the produced images based on their backscatter coefficients (η), and also sometimes with the aid of a semi-quantitative, energy-dispersive spectrometer that is in connection with JEOL 6400. The SEM was operated at 15 mm working distance, and at approximately 2 nA, 20 kV.

XRD analyses were also performed on 91 selected samples to determine bulk-rock mineralogy (e.g. Figures 5 and 6), utilising a Philips PW1730 X-ray diffractometer. Before each analysis, approximately 0.5 g of each powdered sample was treated with a few drops of amyl acetate and left to dry on a flat piece of glass, which is then inserted into the diffractometer. The XRD was operated using copper Ka radiation at 40 kV, 20 ma.

Finally, the TOC contents of the majority of samples (245 in total, both from core and outcrop [see Table 2]) were obtained using a LECO C and N analyser (TruSpec CN). In order to determine the TOC contents, approximately 0.2 g of each powdered sample was reacted with 10.0 mL of HCl solution (1.0 M molar concentration), and left overnight to dissolve all carbonate present. Then, both the acid-treated sample and untreated (natural) sample had the total carbon (TC) contents measured using an induction furnace within the LECO instrument that heated the samples to approximately 950°C. The TOC contents here are considered to be the same as the TC contents obtained from the acid-treated samples, assuming that all the inorganic carbon has been removed from each acid-treated sample, and that the carbon being measured is organic carbon. The total carbonate carbon (TCC) contents were also recorded, determined by calculating the difference between the TC contents of the untreated samples and TC contents of the acid-treated samples (i.e. TOC contents). The reproducibility of these analyses is better than or equal to 0.5%.

The application of the thin-section scanning, optical, and backscattered electron-optical imaging techniques, in addition to the conventional field (logging and photomontages), mineralogical (XRD) and geochemical (TOC) methods, have facilitated making detailed lithofacies descriptions of the predominantly fine-grained, organic-carbon-rich Natih-B sediments. These combined techniques have enabled us to provide details of the various sediment textures, grain size, faunal assemblages, bioturbation processes, mineralogy, organic-matter contents, and transport mechanisms, giving also accounts on how these components change in space and time (see also Macquaker and Howell, 1999; Macquaker and Adams, 2003; Macquaker et al., 2007).

This project was supported by grants from Petroleum Development Oman (PDO), which also provided logistical support in the field, and supplied necessary satellite images, maps, reports, and subsurface core material. We also wish to thank Peter Homewood who initiated and recommended this study; Henk Droste, Cathy Hollis, and Carine Grélaud for useful discussions on the sedimentology and stratigraphy of the Natih Formation; Mohammed Al-Kindy for assistance in locating some of the best outcrop sections of the Natih-B Member; Volker Vahrenkamp for providing his stable-isotopic data on the Natih Formation; Stephen Packer for his help with microfossil analyses; Thomas Heard for useful comments on the trace fossils; Harry Williams and Stephen Stockley for thin-section preparations; Steve Caldwell and David Plant for assistance when using the SEM; Kevin Taylor for suggesting the method to run the TOC analyses; David McKendry for assistance in using the LECO instrument; and John Waters for aid in XRD analyses. Thanks also to both anonymous reviewers, Joachim Amthor (Editor-in-Charge), and Jim Hendry for thorough and thought-provoking comments on the manuscript. The final drafting and design by Arnold Egdane (GeoArabia’s Graphic Designer) and proofreading by Moujahed Al-Husseini (GeoArabia’s Editor-in-Chief), Kathy Breining, Irina Arakelyan and Thomas Heard are also appreciated. Finally, the authors would like to express gratitude to both PDO and the Ministry of Oil and Gas (Sultanate of Oman) for permission to publish this paper.

Said A.K. Al Balushi has recently obtained his PhD in Basin Analysis and Petroleum Geoscience from the University of Manchester (Manchester, UK). His PhD project was sponsored by PDO, where he currently works as a Production Geologist. His PhD research was aimed to understand the fundamental factors controlling lithofacies variability and organic-matter enrichment in the carbonate-dominated, fine-grained sediments of the Upper Cretaceous Natih-B Member, North Oman. Said received his BSc in Resource and Applied Geology in 2002 from the University of Birmingham (Birmingham, UK) and MSc in Earth Sciences in 2005 from Sultan Qaboos University (Muscat, Oman). Said is a member of the AAPG, EAGE, GSO, IAS, and SEPM, and has participated in several international conferences. His particular areas of professional interest include carbonate sedimentology, stratigraphy and petroleum systems, and ancient organic-carbon-rich deposits.

said.balushi@pdo.co.om said_albalushi@yahoo.com

Joe H.S. Macquaker is presently an Associate Professor in Petroleum Geology at the Department of Earth Sciences, Memorial University of Newfoundland (St. John’s, Canada). Previous to that he worked as a Lecturer (1994 to 2000), Senior Lecturer (2001 to 2007), and Reader (2008) in Geology at the University of Manchester (Manchester, UK). Joe is a Chartered Geologist, and has been a member of IAS and SEPM since 1987, and GSA and AAPG since 2006. His main research interest has been focused to investigate the fundamental causes of the variability preserved within fine-grained sedimentary rocks. He has more than 35 papers published in international peer-reviewed journals in this research area, which also has direct economic applications, predicting the location of source rocks and characterising their seal properties, and locating unconventional shale gas reservoirs within shale-dominated successions. Joe is currently a member of the SEPM Research Council and Vice President of the GAC Newfoundland and Labrador Chapter. He has been an Associate Editor for the Journal of Sedimentary Research since 1989, and was an Editor for the Journal of the Geological Society of London from 2000 to 2004. Joe received both his BSc (1980) and PhD (1987) from the University of Bristol (Bristol, UK).

jmacquaker@mun.ca