This field guide describes eleven outcrops of the Natih Formation in the Al Jabal al Akhdar-Jabal Shams and Adam Foothills areas, not far from Nizwa, at the foot of the Oman Mountains. The outcrops have been chosen for their accessibility, as well as for the fairly complete picture of the Natih Formation, which they piece together. To visit all eleven outcrops requires several days and the use of 4-wheel-drive vehicles, but the locations offer no serious physical difficulty, nor long hiking, to gain access.
The outcrop descriptions follow in stratigraphic order from the lower to the upper Natih members, roving back-and-forth across the outcrop area. Much of the detailed account of the sedimentology and stratigraphy of the Natih that has been used as the basis of this field guide, has been given previously by van Buchem et al. (1996, 2002), Grélaud (2005), Schwab et al. (2005) and Grélaud et al. (2006). The observations and interpretations given here come in part from those studies, but this paper is also largely the product of a subsequent project that was carried out for the Fahud Studies Team of Petroleum Development Oman (PDO), to provide detailed sedimentology and high-resolution sequence stratigraphy of the Natih Formation for further development of the Fahud field (Homewood et al. 2006). In this respect, this field guide is not so much intended to be an original contribution in terms of the science concerning the Natih Formation. The intent is to provide the ways-and-means for all to gain a first-hand personal understanding of the rocks we have enjoyed working on.
Following the outcrop descriptions, a general section provides a discussion on facies and facies associations in terms of the constraints of sequence stratigraphy, sea-level change and clay influx on the carbonate factory. With the incorporation of limited subsurface seismic and well data, geobodies and depositional assemblages, the three-dimensional objects that form the stratigraphic packages at outcrop and seismic scales, respectively, are also discussed. Facies are thus interpreted not only in terms of depositional environments, but are also placed within both geometrical (geobody, depositional assemblage) and sequence-stratigraphic frameworks.
In the conclusion, it is argued that a deeper understanding of the Natih Formation has been gained by comparison of outcrop data with subsurface data, and by contrast with modern analogs. The detail required to apply what was learned from outcrop to the nearby subsurface, in a practical manner (but also properly to reconstruct the successive Natih scenarios), requires building several facies models. This is in contrast to giving a single composite picture of Natih facies distribution in space and time, under one single facies model.
This field guide describes and interprets outcrops of the Natih Formation at eleven locations, which have been selected from among the many accessible locations on the southern flank of Al Jabal al Akhdar and in the Adam Foothills (Enclosure I). These outcrops represent a part of the data base for a major study carried out by the JVRCCS Carbonate Research Centre (a Shell-Sultan Qaboos University joint venture) for the Fahud Studies Team of Petroleum Development Oman (PDO). The objectives of the study were to provide detailed sedimentology and a high-resolution sequence-stratigraphic model of the Natih Formation, and to extend it to the subsurface for further development in the giant Fahud field (Homewood et al., 2006). The Discussion and Conclusions sections, which follow the outcrop descriptions, provide a broader overview of the Natih Formation that builds on the first section of this field guide.
Reference to basic work and literature on sedimentology, sedimentary structures, facies interpretation, and so forth, is minimized however much their importance is recognised. This material can be found in basic textbooks as well as from previous studies on the Natih Formation (Philip et al., 1995; van Buchem et al., 1996, 2002; Droste and Van Steenwinkel, 2004; Grélaud, 2005; Schwab et al., 2005; Grélaud et al., 2006). Although the field guide is presented as a stand-alone document, users would be able to make most of their visits by having several of the previous studies at hand, particularly van Buchem et al. (2002) and Grélaud et al. (2006). Useful Geological Map sheets are: 1:100,000 Rustaq, NF 40-3D; 1:250,000 Seeb, 40-03; and 1:250,000 Nazwa, 40-07. They may be obtained from the Directorate General of Minerals, Ministry of Commerce and Industry, Sultanate of Oman.
Lithostratigraphy, Sequence Stratigraphy and Chronostratigraphy
The Natih Formation was originally described in terms of seven members, denoted, from oldest to youngest, by lower case letters “g” to “a” (Hughes-Clarke, 1988); however, we have adopted upper-case letters (“G” to “A”) as has been used in many subsequent publications. The studies cited above have shown that the seven, shallow to moderately deep, but open-marine carbonate members developed on the extremely broad Arabian platform as an apparently simple layer-cake (Enclosure II; Figures 1 and 2). The members are delimited by clay intervals and were first described from subsurface data.
Detailed sedimentology, high-resolution sequence stratigraphy and seismic stratigraphy have been used to show the complexity of changing depositional geometry within the layer-cake and petroleum system (Figure 3 and Enclosure II). The depositional scenarios changed through time, alternating between an extremely broad and flat, submerged-to-emergent platform (G, F, D and C members), and moderately deeper-water intra-shelf basins, with anoxic conditions allowing deposition of source rocks (E, B and A members). The intra-shelf basins of the Natih E and A-B members narrowed as they were filled in by high-energy, shallow prograding banks or intra-shelf platform margins (Enclosure II and Figure 4). Eustacy, climate and tectonics have been cited as the main controls of the successive scenarios (van Buchem et al., 2002).
Obviously, the need to diminish, to the greatest extent possible, the risk in prediction of the subsurface 3-D distribution of sedimentary facies can only be answered by the best high-resolution sequence stratigraphic understanding. The results of that aspect of the study are summarised very briefly here, building on the models already proposed by van Buchem et al. (1996, 2002), Droste and Van Steenwinkel (2004) and Grélaud et al. (2006).
Within the Natih Formation, for which a well-substantiated regional stratigraphic picture has been given by Droste and Van Steenwinkel (2004; Figure 4), four third-order sequences have been distinguished (Natih Sequences I to IV, Figure 5), within which 34 higher-frequency cycles have been established (fourth- and fifth-order; Enclosure II; Figure 6 for Sequence I, Grélaud et al., 2006).
(1) Sequence I: Natih G, F and E members (Enclosures IIA-1 and IIB-1);
(2) Sequence II: Natih D and C members (Enclosures IIA-2 and II-B2);
(3) Sequence III: Natih B and lower part of the Natih A members (Enclosures IIA-3 and IIB-2);
(4) Sequence IV: upper part of Natih A member (Enclosures IIA-3 and IIB-2).
Sequences I and III recorded a repetition in the evolution of depositional systems, with a mixed carbonate-clay ramp system at the base, passing upward and in time, to a carbonate ramp system, with a more pronounced inclined geometry. Intra-shelf basins developed during the main transgressive phases recorded within Sequences I and III. This type of intra-shelf basin did not develop during Sequence II, presumably due to a lower creation of accommodation during this mid-Cenomanian time interval. The top of each third-order sequence corresponds to a period of platform emersion.
Deposits belonging to Sequence IV had a lower preservation potential due to synsedimentary deformation, linked with the onset of the obduction of the Semail Ophiolite and the Hawasina Nappes. Facies evolution and sequence-stratigraphic architecture are therefore more difficult to constrain in this upper unit. A Sequence V has also been interpreted from the record at the very top of the Natih A on the southwest of Al Jabal al Akhdar (Enclosure IIB-2). However, this sequence is not discussed in the context of this field guide.
The Natih sequence stratigraphic organisation is rather poorly constrained in terms of age interpretation due to the scarcity of biostratigraphic zone fossils that have been found so far (Figure 5). A revision of the ammonites from the Wasia Formation by Bulot et al. (in preparation, 2007) does however give a more solid basis to the bio- and chronostratigraphy of the Natih with a much tighter constraint on the Albian-Cenomanian boundary (lower Natih E), but still leaves the Cenomanian-Turonian boundary uncertain (somewhere above the Natih A/B limit). Note that the carbon stable-isotope stratigraphy newly proposed for the Natih Formation by Vahrenkamp (in preparation) is in fairly good agreement with the revised ammonite stratigraphy.
The Natih outcrop study for PDO, which this field guide is largely based on, was part of a broader outcrop and subsurface-based study with special focus on building 3-D models of the Fahud field. The rocks in outcrop were first described and interpreted from a sedimentological point of view (Homewood et al., 2006). Sections were measured and logged in the field at a 1:100 scale, commonly using as a starting point earlier data comprising measured sections made by some of the present authors (e.g. van Buchem et al., 1996, 2002; Grélaud, 2005). Sedimentary facies were analysed, for the most part, directly on the outcrops (macroscopic features and magnification up to 20 times with hand lens for microfacies) with some light microscope sedimentary petrography on stained or non-stained thin sections. Microfacies descriptions had been made on thin sections of outcrop samples from the Natih during an earlier, seismic modelling study. The samples had been taken for analysis of seismic velocities and density attributes when building the impedance model for synthetic seismograms of the Natih (Schwab et al., 2005). Short microfacies descriptions and thin section photomicrograph illustrations from that study are given here in Enclosure III.
Observational data is provided here, for the most part, as measured sections and other figures. The commentaries give our interpretations as much as (or more than) they present purely descriptive observations, since the outcrops themselves provide the unadulterated data to the field-guide user. The user will be in a position to make relevant observations directly on the outcrop, and thus be able to debate and assess the interpretations that are presented here.
The sedimentological study provided the identification, description and analysis of the facies at the outcrops, and when combined with distinctive faunal associations, should lead to their interpretation in terms of depositional processes and sedimentary environments. This was obviously the premise to much of the work in many of the studies leading to this field guide.
With this in mind, outcrop facies and facies associations were compared with subsurface facies and their groupings from cored wells. Seismic stratigraphic analysis of the Kauther 3-D survey, contiguous to the outcrop area, was carried out in parallel with the outcrop study, and this analysis was extended across several 3-D surveys to reach the Fahud field area (Enclosure II; Figures 6 and 7). Lateral and vertical facies relationships from outcrop and the subsurface were constrained by the stratigraphic geometry interpreted from seismic data. In order to ensure continuity in the recognition, understanding and merging of outcrop and subsurface data, joint fieldwork and core-description work sessions were held between JVR Centre for Carbonate Studies and the Badley Ashton and PDO Studies Centre teams working on data from seismic and well data in the Fahud field (Grélaud, 2005; Grélaud et al., 2006; Homewood et al., 2006).
The Natih B, F and G members were not included in the study, and so they are given briefer comments in this field guide. Discontinuous and scattered outcrops of Natih F and G may be found at the base of the Natih E, but there are no representative outcrops of these lower members in the areas that are described here. Good exposures of the Natih B member, characterised by a kerogenous chalk source-rock facies, may be seen, particularly on Jabals Qusaybah and Nahdah.
Outcrop Selection, Locations and Nameplaces
Throughout the field guide the spelling of locality names follows those most commonly seen on signposts and maps. Several wadis that do not have formal names are frequently referred to in the field guide. During field trips they have been informally referred to by the first name of several authors (Carine, Henk and Philippe) and these are here denoted as wadi C, H and P.
The importance of the different outcrops that are commented upon here, when building a picture of the Natih Formation, is severely weighted in favour of two locations: Jabal Madmar (Location 1, Wadi H in particular), and the higher area lying around and near the “Canyon Overlook” of Jabal Shams (Location 3). Obviously the other outcrops that are indicated, as well as all those not presented here for a variety of reasons, provide important data and insights to complete the picture. The selection that has been made, and the different features that have been emphasised, are the outcome of working visits (too numerous to count), several correlations between outcrop and subsurface data, and the feedback from many field trips to the Natih, guided over a period of more than 12 years.
In order to follow the logic of the Natih stratigraphic succession and to avoid unnecessary repetition, the descriptions below follow from lower part of the Natih E to Natih A in ascending stratigraphic order. The descriptions are then grouped according to outcrop locations. The successive stratigraphic units, at the selected locations, are described in the following order (Enclosure I):
(1) Lower to upper parts of the Natih E member in Wadi H, Jabal Madmar (22°26.9′N, 57°35.21′E).
(2) Upper part of the Natih E member, in Wadi C, Jabal Madmar (22°26.8′N, 57°37.6′E).
(3) Upper part of the Natih E member, around the “Canyon Overlook” on Jabal Shams (22°26.8′N. 57°12.1′E).
(4) Natih D and C members in Wadi P, Jabal Madmar (22°26.7′N, 57°32.9′E).
(5) Natih D and C, in Wadi Mi’Aidin, Al Jabal al Akhdar (22°58.2′N, 57°39.9′E).
(6) Upper part of the Natih C member on Jabal Qusaybah (22°32.6′N, 57°04.2′E).
(7) Upper part of the Natih C member on Jabal Shams (23°12.1′N, 57°10.7′E).
(8) Natih A and B members in Wadi Nakhr (23°09.1′N, 57°12.3′E).
(9) Natih A and B members in Wadi Al Hamrah (23°07.7′N, 57°17.3′E).
(10) Natih A and B members in Wadi Kahmah (23°01.1′N, 57°32.4′E).
(11) Natih A and B members, western Adam Foothills, Jabal Qusaybah (22°32.6′N, 57°04.2′E)
Location 1: Wadi H, near the Adam Foothills, is one of the best places to start, with nearly the whole Natih Formation exposed. This location has the advantage of easy access, and excellent exposure of the different facies of the Natih E member. The wadi cuts perpendicular to the more-or-less 15° structural dip, and shows lateral continuity of stratigraphic units over several hundred metres, basically an inter-well scale for a fairly mature oilfield. Notions of reservoir heterogeneity in the Natih E are therefore easily demonstrated here at scales from that of the medium- and high-frequency stratigraphic-cycle layering, to the cm- and dm-scale of the contrasting porosity-permeability networks created by the intense Thalassinoides burrowing. These insights have been important for geoscientists and engineers involved in developing production scenarios for nearby oilfields.
Location 2: Wadi C, on the southern flank of Jabal Madmar, adds significantly to the picture of the upper part of the Natih E member gained in Location 1. It has excellent exposures of 1-m to 10-m-scale geometrical attributes and geobodies of the shallow, high-energy platform-top to platform-margin depositional environment.
Location 3: Jabal Shams “Canyon Overlook” is located in the Oman Mountains proper, where the rocks have undergone considerable burial compaction. In spite of this, facies, sedimentary structures, and stratigraphic cycles are well-preserved and this location provides unmatched access to both seismic-scale and postage stamp-scale features of the Natih Formation. Again, the whole Natih is well-exposed here, although for this field guide the focus is on specific parts of the section. This location is easily accessible even with a sedan car. However, from a practical point of view, for the whole visit of this location presented below, a sedan would not suffice. The exposures of the incised valley in the upper part of the Natih E member, the sequence boundary between Sequences I and II, both in panorama and as a surface to walk-around on, are exceptional, as are the exposures of (and the access to) the infill of the incised valley. The trip to this location alone, with magnificent views on the “Exotics” (atoll-size slabs of allochthonous Triassic platform and platform-margin carbonates such as Jabal Misht) justifies the ride.
Location 4: Wadi P is easily reached by a short ride from Wadi H, Location 1, and complements the first stop. Exposures here of the Natih C and D members are excellent, whereas in Wadi H the same units are mostly covered and hidden under rubble and weathered boulders.
Location 5: Of all the locations mentioned here, Wadi Mi’Aidin is probably the most visited by geologists, but more so for the whole section than just for the Natih Formation. The wadi may be accessed by car or even by coach, but a 4x4-wheel vehicle is mandatory for the ride up to the Saiq Plateau for those who wish to visit its beautiful views and scenery. The flanks of the wadi offer reasonable outcrops of the whole Natih Formation, albeit rather weathered or encrusted in places. The Natih C and D members here do provide good outcrops however, and their shallow littoral facies are better-exposed than elsewhere.
Location 6: Jabal Qusaybah is easily reached from the blacktop road to Fahud, and the Natih B facies, hardly commented-upon in this field guide, are well-exposed, both here and close by on Jabal Nahdah. The Natih C member of Jabal Qusaybah shows a very particular development of tidal sandwaves at the top of Sequence II, and this detail is worth adding to the other exposures of the Adam Foothills area.
Location 7: Jabal Shams, adjacent to the outcrops indicated at Location 3 for the upper Natih E member, provides the subject for a debate on synsedimentary tectonics as well as for a spectacular development of tidal sandwaves and other littoral facies in the upper Natih C member. Arguments have raged among the coauthors as to the relative importance here of an intra-Natih palaeohorst, as opposed to a low-angle fault that locally reduced the stratigraphic section.
Locations 8, 9 and 10: Although rather closely spaced on the southern flank of Al Jabal al Akhdar, these locations complement each other for different aspects of the upper Natih A and B members, or Sequences III, IV and V. Location 8, Wadi Nakhr, provides easy access to the B member source-rock facies, and spectacular views of steep clinoforms of a late-stage prograding shelf-margin wedge in the A member. Location 9, Wadi Al Hamrah, shows synsedimentary deformation of the Natih A member. Location 10, Wadi Kahmah, shows the multiple incisions, channels or incised valleys of the upper Natih A in Sequence V.
Location 11: At Jabal Qusaybah, the basinal facies of the Natih B directly overlie the C member of Location 6. The B member is overlain by a full section of A, capped by conglomerates below the Fiqa shales. This location in the western Adam Foothills provides access to the closest analogs to the source rocks in the oil-producing subsurface. The Jabal Salakh section further east, described in van Buchem et al. (2002), is located a little closer to the eastern limit of the basinal facies.
Choice of Itineraries
Obviously it might be more practical in terms of an itinerary to visit locations near to each other first (in an order such as: 5; 1, 4, 2; 6, 11; 3, 7; 8, 9, 10) rather than to follow in the stratigraphic order going back-and-forth from Locations 1 to 11. However, the text follows the constraints of describing the rocks according to stratigraphic organisation. This avoids unnecessary and inevitable repetition that would occur if the descriptions followed an order dictated by the easiest itinerary rather than the stratigraphy.
For those who have very little time, there are several alternatives depending on focus and personal preference. Wadi Mi’Aidin (Location 5 for the Natih C and D in this field guide) provides a good overview of the whole Natih Formation in a northern, carbonate-rich and shallow setting (no intra-shelf basin facies with source rocks may be seen here for example). The lithostratigraphic organisation into reservoir-seal pairs of carbonate formations and members, with clays in between, is very well-exposed. Other descriptions to consult for this site are given by Philip et al. (1995) and Hanna (1995).
The rocks in the Adam Foothills have not suffered the same burial and compaction as have those in the Oman Mountains. For visitors with little time, Location 1 provides an excellent view of the whole Natih but with best exposures in the Natih E. Together, the Adam Foothills locations (1, 2, 4, 6 and 11) also cover the whole Natih sequence, and the locations described in this field guide from Jabal Madmar (1, 2 and 4) give a very good idea of reservoir facies in the subsurface and of the complexity of facies associations and geometry at the reservoir scale.
Jabal Shams (Locations 3 and 7), although a bit further to reach and located back in the Oman Mountains, has the advantage of superb views, the sight of the whole Cretaceous stratigraphic package, and very good access to the whole Natih sequence (here with source-rock facies of the Natih B in Wadi Nakhr, Location 10). These locations also provide considerable information on heterogeneity at the reservoir scale.
The foot of Al Jabal al Akhdar between Nizwa and Nakhr (Locations 8, 9 and 10) gives by far the best overview of lateral stratigraphic and facies changes in the upper Natih A and B members. Together with Wadi Mi’Aidin (Location 5) the southern flank of Al Jabal al Akhdar provides an excellent view of the whole Natih, although somewhat squashed from the considerable overburden of the Semail Ophiolite and associated units.
In addition to the outcrop locations for the Natih Formation described here, stops in the Semail area, along the Muscat to Nizwa road that follows the impressive lateral ramp of the Al Jabal al Akhdar Anticline, are convenient to give a general introduction to the tectono-stratigraphy of Oman, the geology of the Oman Mountains and the Semail Ophiolite. The stratigraphic section in Wadi Mi’Aidin, commented on below for part of the Natih Formation (Location 5), is spectacular and includes the Proterozoic, Permian, Triassic, Jurassic, and Cretaceous of the autochthonous Arabian Platform. It also includes the whole Natih Formation and the overthrust deep-water deposits of the Jurassic base-of-slope environment, at the entrance to the wadi beyond the restored fort (Hanna, 1995).
Outcrop Access and Guidelines
Outcrop access to the locations described here is not difficult but does require 4x4-wheel vehicles and off-road driving in some places. Mobile GSM telephone coverage is available in most places, but reception can be patchy; satellite phones ensure communication. The locations normally are not dangerous, but all due care and attention should be taken for all the trip participants. Codes of good conduct and behaviour are to be followed, as expected both by the traditions of the Sultanate of Oman and by all the geoscience groups in Oman. Excessive hammering of rock surfaces is unnecessary and should be avoided, since this commonly obliterates features that are otherwise useful to observe and therefore should be preserved. Clothing, hats and shoes, and availability of drinking water should be appropriate for fieldwork under strong hot sun and in very dry conditions. Dangerous flash floods may occur after thunderstorms and rain, particularly in the narrower wadis, and roads are generally dangerous during and shortly after rain. Plastic bottles, waste and litter should be taken away to be disposed of. Sampling should only be carried out after obtaining appropriate permits.
It is not feasible to visit all the locations in a single day, so overnight accommodation should be planned for trips to more than one location. Camping at the field locations is pleasant during the cooler weather from November to March and does not require any special permit. Several hotels are located in the Nizwa area, but advanced booking for these is recommended since many are fully-booked during the high season, given the strong growth of tourism in Oman.
Location 1: Lower to Upper Part of the Natih E, Northern Jabal Madmar, Wadi H (22°26.9′N, 57°35.21′E)
Wadi H is reached from the Nizwa to Adam road by blacktop (turning off to the east some five km north of Adam, signposted to the village of Al Bashaer Al Ganoobi), a short stretch of graded road, and then turning south on fairly good tracks leading to the northern flank of the Jabal (Enclosures I-1 to I-3). The track ends at a clearly defined parking area, 50 m from a group of trees that provide welcome year-round shade. The outcrops are along both the wadi flanks, on either side and to the south of the track’s end, as well as on the surrounding flanks. The base of the section here, a little above the base of the Natih E, is reached after a short hike for 200 to 300 m up the wadi (Figure 8).
In order to stay in the shade take the path up the foot of eastern flank in the morning and up the foot of western flank in the afternoon. The incisions and the sequence boundary between Sequences I and II are well-exposed around the foot of the flanks of the wadi at the parking area. The infill above the first incision, with laterally accreting tidal channels, is very well-exposed on the slopes above the western side of the wadi, just a short 50 to 100 m hike up the slope to the little overhang that is clearly seen when looking southwest from the parking area. The peritidal facies can be accessed by looking around above the little overhang.
The section provided by Wadi H shows excellent, continuous exposures not only of the full Natih sequence above the F member to the base of the Natih A, but of Natih E in particular (Figures 8 and 9). Natih E features of note in this locality are: (1) the source-rock intervals of the lower Natih E, (2) Thalassinoides bioturbated wackestones and mudstones above the kerogen-rich chalks, (3) the wackestone-packstone-grainstone coarsening-up prograding clinoform sequence, and (4) the incision IS1 (higher-frequency cycle sequence boundary) with lateral accretion in aggrading tidal channels at the base of the infill above IS1. The major sequence boundary IS2, separating Sequences I and II (Grélaud et al., 2006), is clearly seen as a well-pronounced, highly altered pink-to-reddish pedogenic breccia.
Intra-shelf Basin Facies
Near the base of the measured section (Figure 9), two intermediate-scale cycles of development and expansion of the oxygen-starved, intra-shelf basin (kerogenous “chalks” and lime mudstones) are clearly exposed. The two cycles are illustrated in Figure 10a. They are, in turn, composed of metre- to several-metre-thick, very-high-frequency cycles of three facies: (1) plane-bedded, finely laminated kerogenous pelagic chalks and lime-mudstone (Figures 10b and c); (2) irregular to wavy bedded-mudstones with scoured bases and tops (Figure 10d); and (3) Thalassinoides bioturbated lime-mudstones and wackestones with remnant bedding sometimes encrusted by oyster colonies (Figure 10e).
The cycles illustrated in Figure 10c combine physical, chemical and biological indications of the progressive development to (and progressive release from) oxygen-poor or anoxic conditions. Under normal conditions, with mixed aerated water in the basin, bioturbation generally destroyed the primary bedding and textures (facies “c”). With decreasing oxygen but not full anoxia, bioturbation decreased and erosive bed-surfaces, created by storm scouring, were preserved (facies “b”). With stratification of the water column, storm currents did not reach nor reworked the sediment surface, such that anoxic conditions developed and organic carbon was preserved within the sediment (facies “a”). The facies successions clearly show very-high-frequency cycles within the medium-term ones (Figure 10a).
In both of the two medium-term cycles, the basinal deposits are underlain and overlain by more-or-less bioturbated heterolithic sediments. These are interpreted by some of the co-authors to be storm deposits with gutters and furrows along the erosive basal bed surfaces. Mounds or brioches occur along the polyphased erosive upper bedding surfaces (Figures 10c and d). The scoured brioche features on bedding surfaces are clearly a primary feature, as shown by monospecific colonies of oysters that encrust the scoured surfaces (indicated by “S” in Figure 10e).
However, in many instances, the irregular surfaces bounding the more cemented mudstones indeed have been enhanced by compaction and pressure solution. Although the storm origin of the beds is preferred by some of the co-authors, others among us consider that there is little convincing evidence for storm deposits or processes, and that there are none of the essential ingredients, in terms of sedimentary structures such as Hummocky Cross Stratification etc., to found here. The same co-authors point to diagenetic features of compaction and nodular concretions or cementation as being the characteristic features of these beds and their surfaces. So the debate continues as to the depositional or diagenetic origins of these characteristic, irregular wavy beds.
Beds of mixed, redeposited material, with dispersed textures of coarse bioclastic shelly mudstones in the upper of the two medium-term cycles, show for some co-authors that sediment gravity flows brought down a variety of shallower-water fossils and intra-clasts to the oxygen-poor basin floor in this second cycle (Figures 10f and g). The shallower-water initial sites for the accumulation of these bioclasts is argued from the sediment fill types inside whole fossil clasts, as well as the diversity of fauna. This suggests to some co-authors that the slope around the basin floor became steeper as the margins aggraded or even started to prograde at the turn-around from increasing to decreasing accommodation space within the longer-term Sequence I cycle. The oxygen-poor environment is clearly attested-to by the development of the previously mentioned monospecific oyster bivalve colonies.
However, the sediment-gravity-flow origin of the beds is contested by others among the co-authors, who consider that there is little convincing evidence for shallow-water fossils in these beds, that they are not comparable to well-established sediment gravity flow deposits of older “Habshan” facies (Figure 2), and that the slopes around the basin were not sufficient for sediment gravity flows to have taken place.
Three independent observations provided estimates for the depth of the intra-shelf basin of between 50 and 100 m. (1) Seismic images of clinoforms of the Natih E, converted from time to depth, gave an estimate of 80 m. (2) The measured section of coarsening and shallowing upward facies from the intra-shelf basin-facies to the shallow, high-energy platform-top (non-decompacted) is between 40 and 50 m. (3) The oysters encrusting bedding surfaces, a little above the intra-shelf basin facies, are interpreted to have lived in water depths of up to 40 m.
The correlation of the higher-frequency stratigraphic units of the lower Natih E between Jabals Madmar and Madar, lying further to the east (van Buchem et al. 1996, 2002; Grélaud, 2005; Grélaud et al., 2006), illustrates a stratigraphic paradox (Enclosure IIB-1). In Jabal Madar, the Natih E shows a vertical stacking to seaward-stepping stacking pattern of higher-frequency units during the transgressive phase of Sequence I. If considered alone this pattern might suggest decreasing accommodation and therefore a highstand systems tract just above the base of the Natih E. Increased sediment supply at Jabal Madar, is certainly shown by the shallower more skeletal-rich facies, coinciding with the paradoxical stacking pattern.
However, regional correlation places the maximum flooding interval above the seaward stepping/vertically stacked package and not below it. The Jabal Madar location must have been the site of much greater carbonate production than elsewhere, possibly due to a slight difference in bathymetry caused by uplift related to the growing Madar salt diapir. This apparent contradiction to the regional stacking pattern (simultaneous basinward and transgressive patterns in different places) points to the necessity of regional verification of local stacking patterns when they are used to determine variations in regional accommodation space. This outcrop section led van Buchem et al. (1996, 2002) to place the basin margin to the west of Jabal Madar. Note that more recent studies (Grélaud, 2005; Homewood et al., 2006; Grélaud et al., 2006) placed the maximum flooding of Sequence I a little higher in the same measured section than did van Buchem et al (2002).
Platform-margin Prograding Clinoform Facies
Thalassinoides burrowed mudstones, wackestones and packstones, above the kerogenous chalks, indicate increasingly oxygenated conditions (Figure 10e). A clear coarsening succession, from wackestones to packstones and grainstones, recorded a regular shallowing upward sequence through highstand prograding sets. Finally, very coarse cross-bedded grainstones and rudstones, which terminate with rudist rudstones or grainy floatstone biostromes, indicate deposition on the shallow high-energy top of the platform (Figure 11).
From a regional perspective, the margins to this successively expanding and then contracting intra-shelf basin have been mapped-out by observing the pattern of clinoforms on seismic data (Figures 6 and 7; Droste and Van Steenwinkel, 2004; Grélaud, 2005; Grélaud et al., 2006). Although the Natih E platform may be considered to compose a single unit at the broader scale of the Arabian margin, at the scale considered here the directions of progradation distinguish a western, a northern and an eastern platform (Enclosure IIA-4).
Closer to the outcrop, from nearby 3-D seismic data, the angles of the slopes at the intra-shelf basin margin were measured (Figures 6 and 7; Grélaud, 2005; Grélaud et al., 2006). The platform itself has a regional slope of less than 0.1°, whereas the expanding basin gently rose toward the margins at angles of less than 0.1° to 0.5°. During progradation and infill of the basin, the sediments were deposited on slopes of less than 0.5° up to a little more than 1.0°; not sufficient to be seen at the outcrop but clearly imaged by seismic data as clinoforms. When sea-level fell below the platform margin, towards the sequence boundary, shelf-margin wedges were deposited as clinoforms at angles from a little more than 1° up to nearly 4°. However, at the outcrop itself, the basin-scale clinoforms, with slopes as low as all these, are very rarely observed as such. The apparently flat bedding, lying at the structural dip, is in fact the indicator of the palaeoslope, and is imaged by the seismic-scale data as clinoform surfaces.
Platform-top Incision and Fill Facies
The upper part of the Natih E member varies considerably from one location to the next. The high-energy shallow-water conditions of the platform edge led to the accumulation of many different geobody types, such as channels, shoals, banks and patches colonised by macrofauna. The incised valleys led to a variable degree of incision by the two bedrock incisions IS1 and then IS2 (Figure 12) that cut-out parts of the older deposits, and the different fill types of the valleys also cause changes in the detail of stratigraphy from place-to-place. In this field guide, descriptions of the incisions and of their infills are made at three localities: two are on Jabal Madmar (Wadi H in this section, and Wadi C at Location 2, Jabal Madmar); and one is on Jabal Shams (Location 3, “Canyon Overlook”).
In Wadi H, at the top of cycle I-6, a first incision surface IS1 is well-exposed running along the top of the Natih E scarp of the western flank (Figures 8 and 13d). This incision cuts-out the rudist biostrome and rudstones (seen on the eastern flank at wadi level) by further eroding two metres or more into the platform-margin facies near where the western flank reaches the wadi. The incision can be seen to rise gently through the stratigraphy when looking back up the western lip of the wadi cut (Figure 13d).
Dolomitisation (Figure 12 and Enclosure III), together with nodular cherts, tends to occur close to the bedrock incisions and major channel features. Either the incised deposits close to the incision surface or the deposits above the incision or channel floor, or the sediments both above and below, may be dolomitised. This distribution of dolomite suggests a link with depositional or early diagenetic environments. However, ongoing studies involving digital mapping, geochemistry and petrography point to late diagenetic, thermobaric alteration of host rock (Erwin Adams and Anita Csoma, personal communication, 2007). Further study will show how much the primary depositional attributes were involved in dolomitisation, compared to later stage dolomitising fluids being constrained to flow along these significant heterogeneities.
Above this IS1 incision, a 20-m-thick aggradational sequence of tidal mudflat and channel deposits, is made-up of several high-frequency cycles or genetic units of intertidal to peritidal facies with laterally accreting tidal channels (Figure 13). The second surface (IS2, Figure 14) is a rubble-like layer here, heavily altered by pedogenesis on an interfluve between deeper incisions along the sequence boundary (IS2, Grélaud et al., 2006). Grélaud (2005) has mapped-out these incisions and recorded the successive infills (Figure 15).
The peritidal facies is found below the IS2 surface, in the areas where no deep IS2 incision occurs. Figure 14 shows the nodular highly bioturbated wackestones of cycle I-7 of Figure 12, with bivalves and gastropods, and a mudstone bed, 10–15-cm-thick, with an open network of burrows or roots that was subsequently filled with sparry calcite cement.
The east–west transect illustrated in Figures 12 and 15 shows the lateral extent of facies between the first incision surface (IS1) and the top of the fill of the second incision (IS2). In Jabal Madmar, the IS1 surface is conspicuous from the abundance of associated chert. The IS1 fill consists of a coarse lag interval at the base, forming the resistant bed that protrudes at the lip of the scarp on the west flank of the wadi here. The lag is overlain by more-or-less dolomitised mudstone with orbitolinids. Above, lateral accretion bedsets of a tidal point bar form the top of IS1 incision fill (Figure 13c).
The major sequence boundary IS2, separating Sequences I and II, is a well-pronounced, highly altered, pink-to-reddish, pedogenic breccia (Figure 14). The deposits here recorded an interfluve setting, standing prominently between the deeper incisions that may be traced laterally. This IS2 surface comes to the wadi level and is easily recognised at the main parking lot. In places, the IS2 incision erodes down to the lower part of Unit 1 deposits, and is filled with a coarse lag interval that may be overlain by up to 13-m-thick marls and green clays. The last carbonate bed at the top of the IS2 fill is capped by a hardground (Figure 12a).
The main sequence boundary between Sequences I and II thus lies within the upper part of the Natih E member (Figure 12). The lithostratigraphic contact between the Natih D and E members is determined by the “D shale”, a thicker layer of greenish clay that is poorly visible, if found at all, on outcrop, but which does provide an easily recognised recessive bench in the topography. In the subsurface, these clays and shales are green or reddish and contain marine fauna. Regionally, the longer-term sequence boundary is indicated by the first occurrence of marine clay draping over the major incision towards the top of Natih E (Grélaud et al., 2006). These clays line the incision surface IS2, at the base of the infill and below several high-frequency cycles, which occur below the thicker clay forming the “D shale”. In terms of sequence stratigraphy, the succession of shallow-marine deposits above a deep incision indicates rising sea-level after a major sea-level fall. The incision fill and the draping units below the “D shale” were clearly deposited during a transgressive phase at the base of Sequence II, but they still occur in the upper beds of the Natih E member.
The influx of terrigenous material was derived from the Arabian Plate hinterland. The first transgressive deposits not only filled the topographic depressions, such as the incised valleys (see Location 3), but also formed land surfaces such as those of nearby outcrops on the northern flank of Jabal Madmar, Figure 15). During this early phase, emergent relief continued to be weathered or eroded. At this location in Wadi H the interfluve setting only resulted in the severely weathered and altered layer discussed above (Figure 14).
Location 2: Upper Part of the Natih E Member, Southern Jabal Madmar, Wadi C (22°26.8′N, 57°37.6′E)
Wadi C is reached from the Adam to Sanaw black-top road (drive through Adam and then to the east to Sanaw as signposted (Enclosures I-1 to I-3). A stretch of graded road runs at a right angle north from the black-top, towards the eastern end of Jabal Madmar; turn west towards the Jabal from this graded track onto the old well-site track, and then leave this poorly maintained track at the foot of the Jabal to drive a short distance over rough terrain, along the southern flank of the Jabal. The outcrops are at the confluence of several minor wadis, joining more-or-less at the foot of the Jabal.
This location provides excellent exposure of the platform-margin grainy rudist and stromatoporoid rudstones and floatstones, in the upper part of the Natih E member. Also exposed are clear geobody geometries, as well as the very shallow to intertidal and supratidal facies between the two incisions IS1 and IS2.
Platform-top High-Energy Grainstone and Floatstone Facies
Bedding patterns on the eastern flank of Wadi C (Figure 16, panorama and line drawing) may be traced over several 100 m (Figure 17). They show how the coarse foraminiferal grainstones to gravelly rudstones and floatstones build prograding and aggrading banks and bars about eight metres high. These high-energy deposits accumulated near the platform margin to the east of the intra-shelf basin. No genuine carbonate build-ups, such as rudist bioherms and rudist mounds, have been observed within the Natih. Sessile macrofauna, comprising rudists, corals and stromatoporoids, seem to have formed floatstone patches within both high-energy grainy facies and low-energy muddy facies, occasionally building biostromes that have km-scale lateral continuity.
Platform-top Incision, Infill and Overlying Facies
As in Wadi H, the high-energy, platform-top facies are incised here by a major truncation surface that is overlain by dolomitised sediment with abundant nodular chert (Figure 17); this is the IS1 incision. Above the dolomitised chert-rich incision fill, come subtidal to supratidal cycles comprising bioturbated packstones with rudists, stromatoporoids and corals (Figure 18). Above these cycles, mottled, bioturbated and brecciated grey mudstones and wackestones with gastropods and other marine macrofauna, show an orange dolomitic matrix (Figure 19a). The matrix sometimes lies in burrows (burrows show regular rounded tubular features, Figure 19b), but more often it fills cracks and wedges that have angular or horsetail terminations. The cracks and wedges, which created a brecciated fabric, are interpreted as pedogenetic features of water-logged soil development (hydromorphic soils). No clear evidence of soil deposits has been found in the study area. However, coaly shales, possibly from organic soil deposits have been found near the base of the incision fill at the Jabal Shams site. Coals were reported from the Mishrif deposits in the United Arab Emirates, and fossilised wood was reported from rocks of similar age in Saudi Arabia.
Location 3: Upper Part of the Natih E Member, Jabal Shams, “Canyon Overlook” (23°12.6′N 57°12.1′E)
This site is known as the “Canyon Overlook” (Enclosures I-1, I-2, I-8 and I-9). Follow signposts to Jabal Shams. Turn northwest off the main Nizwa-Bahla road to Al Hamrah, and then along Wadi Ghul, to Ghul. Follow the blacktop road on up towards Jabal Shams as signposted. Note that this is a very steep road with tight “hairpin” bends (make sure to engage very low gear when going down). Continue straight-on where the blacktop becomes a graded track. Turn-off southeast, to the right as signposted, before the military check-point that limits access to the mountain top. Follow the graded track past the 2006-constructed tourist camp and then past the cliff-top village (leaving both to the left), to the canyon panorama overview park.
The visit to this location requires several hours and focuses on three areas. First the “Canyon Overlook” itself from the cliff top promontory about 100 m southeast of the “Canyon Overlook” parking area (site 3-1 in Enclosure I-8); second the view of the little hill to the west of the track about one kilometre south of the main “Canyon Overlook” parking area (site 3-2 in Enclosure I-8); and third the broad exposures of the incision surface to the southwest of the main “Canyon Overlook” parking area (site 3-3 in Enclosure I-8). The easiest way to conduct the visit is to hike from sites 1 to 2 and then, either by 4x4-wheel vehicle or on foot, take the rough track leading 2 km southwest off the main graded track to site 3, the junction being a few hundred metres south of the older permanent tourist camp.
The panoramic “Canyon Overlook” view, with the kilometre-deep incision down through the Cretaceous to reach the Jurassic, is one of the major scenic attractions of north Oman. Looking back across and down from the panorama vantage point, roughly in the direction of the present-day village on the top of the Natih Plateau (site 3-1 in Enclosure I-8), one can see walled terraces and abandoned dwellings (Figure 20a), all tiny at this distance. Some were built from stone while others were carved into tufa. These are located on either side of the ephemeral stream that is ponded under a small overhang at the base of the Natih cliffs (Figure 20). The abandoned village can be easily reached by a track that runs along the recessive ledge created by weathering-away of the Nahr Umr shale above the Shu’aiba limestones. The trail is signposted and takes a bit more than two hours hiking each way. Relocation of families from this secluded spot to the new village on the Natih Plateau certainly facilitates the ride to school for the children! The village people make and sell to passers-by a variety of brightly coloured hand-woven carpets (Figure 20b), plaited key-ring tassels and various other handicrafts.
The kilometre-wide palaeo-incision that cuts into the cliff near the top of the Natih E can be observed from the “Canyon Overlook” (Figure 21). This synsedimentary feature, an “incised valley”, may be studied in detail both on the canyon-wall cliff face, and on the plateau and low hills to the west and south of the “Canyon Overlook” (Figure 22). This is the IS2 incision that marks the sequence boundary between Natih Sequences I and II (see further discussion at Location 1, Wadi H, Jabal Madmar). The bedrock incision surface shows dessication cracks and marine sediment karst fills, as well as geopetal cavity sediments and cement fills (see Figure 23). The infill overlying the incision surface comprises a number of channel features and both the incision and the overlying infill have been studied and mapped out in detail by Grélaud (2005), and reported in Grélaud et al. (2006).
Platform-top Emersion, Incised Valley and Infilling Beds
The “Canyon Overlook” location provides unparalleled access to seismic-scale outcrops of the upper Natih E incision IS2 (Figure 21), as well as to three dimensional exposures of the infilling beds. The Transect “T” (Figure 21) has been constructed from numerous measured sections that are indicated by vertical lines on the correlation panels. Note that the correlation has not been projected onto a single vertical plane but follows around the curvature of the cliff face. It is not recommended to venture along the narrow benches nor to attempt to reach the locations of the measured sections. This site is best appreciated safely with binoculars from the promontory.
The incision fill can be divided into three sub-units (Figure 21). Where the incision is the deepest, a first phase of fill is characterised by a sigmoid-shaped grainstone body, overlain by marly mudstones. These are overlain by a channel-shaped grainstone bed that passes laterally (where the incision was shallower towards the north and south) to wackestone and packstones with palaeosoil features. This bed drapes the incision surface. Above this first fill, the second sub-unit is a marly mudstone, up to 10 m thick and that is the main part of the overall incision fill. Within these marls, several thin dolomitised beds are found, especially near the edges of the incision. The last sub-unit is a packstone to grainstone bed of irregular thickness, showing internal lateral accretion features. This last bed is capped by a hard ground.
Lateral accretion geometries may be found at the top of the incision fill here. Close-up view of the upper sub-unit of the incision fill (Figures 21b to d) shows lateral accretion geometries towards the south-southwest, and towards the east. These lateral accretion geometries show that the channels were deposited in a context of increasing accommodation, during which sandy bars could develop, migrate and be preserved in the stratigraphy. On top of the migrating bars, palaeosoils and bioturbation intensely modified the original sediments and may have contributed to stabilizing the bars during deposition.
Platform-top Emersion Features, Incision and Fill Facies
The second focus of the location is reached by hiking a few hundred metres to the south along the cliff top. This vantage point provides a closer view of the incision fill (Figure 22). To the left of the graded track, the “top Natih E” IS2-incision surface is visible as a smooth surface. The base of the small hill to the right of the track is a recessive unit, within which irregular and discontinuous beds are visible. These beds are laterally accreted grainstone bodies within the incision fill. The channel-shaped plug body probably corresponds to the last fill of the incision. Surface weathering has emphasized certain features of the incision surface to the left (east) of the graded track. Cherts emphasize micro-karst features on the Natih E emersion and incision surface at this place.
Platform-top Emersion Features
The third spot of this general location is also the site of Location 7 where several interesting features of the Natih C and D members may be visited. The location is reached by a moderate 2-km-hike along the rough track starting about 100 m from the tourist camp, indicated under Access for Location 3, or by driving the same path by 4x4-wheel vehicle.
Palaeokarst features are well-developed here on the smooth incision surface that can best be identified by tracking first the “base Natih D shale” that forms a recessive bench at the base of the well-bedded Natih D member. The rough track runs along this bench, which provides a sufficiently wide and smooth pathway for vehicles. The incision fill and/or the more regional drape of beds that overlie the infill proper, then form a bench between the “D shale track” and the smooth incision surface below. A number of features that characterise the emersion of this surface are illustrated in Figures 23c to e. Curvilinear and star-shaped cracks, with several generations of cement, are interpreted as dessication features that were later flooded. These were then eroded by a karst network that was filled by marine mudstones and wackestones (Figures 23c to e). Open-karst networks show several phases of cementation with geopetal sediment fills (Figures 23a and b). Similar features may be seen at the top of the Natih E in Wadi Nakhr (Figures 23c and d; Location 8)
Location 4: Natih D and C Members, Northern Jabal Madmar, Wadi P (22°26.7′N, 57 °32.9′E)
Wadi P lies on the northern flank of Jabal Madmar, a little to the west of Location 1 (Enclosures I-1 to I-3). The easiest way to reach the outcrop is to follow the track to Location 1 and then to drive along the rough track that runs at the foot of the Jabal to the west.
Although the Natih D and C members may be seen in Wadi H (Location 1), the exposure of these units here is not only much better, but the outcrops also show high-energy facies as a channel deposit at the top of Natih C member. As mentioned in the Introduction, the morphology of the depositional environment was quite different during the deposition of the D and C members, compared to that of the Natih E. There is no evidence, either from outcrop or from seismic data, of intra-shelf basin development with thick prograding shallowing-up sequences, such as there are in the Natih E. The relatively thinner sequences, the very low relief, and the lack of clinoform development, “basin infill” or progradation all indicate a rather shallow lagoonal environment, although open-marine planktonic fossils paradoxically dominate the microfauna and nannoflora of this vast platform-top environment.
Extreme Lateral Continuity of Facies in Lower Natih D and C, Sequence II
In strong contrast to Natih E (Sequence I) and Natih A-B members (Sequence III) where lateral variations of facies are well-marked in the shallower, higher-energy deposits, the Natih D and C members (approximately Sequence II) show extremely constant facies across much of the area, particularly in their lower parts. Certain beds and groups of beds (carbonate beds with terrigenous clay interbeds) may be traced for up to 100s of kilometres with very little change, thus providing valuable markers for correlating the outcrops with the subsurface.
Each of the Natih D and C member (Figure 24) forms a separate cycle with a lower clay-dominated or clay-carbonate alternation, and an upper, medium- to thick-bedded carbonate succession. These are identified as two medium-frequency accommodation cycles (II-1 and II-2 inGrélaud et al., 2006; II-2 and II-3 invan Buchem et al., 2002; see Figure 5).
In the beds above the IS2 incision (boundary between Sequences I and II, Figure 12) but directly below the thick shale marking the base of Natih D, alternations of green clays and rudist-bearing lime wackestones are overlain by alternations of clays with thin rusty-coloured beds that are burrowed, dolomitised and show ferruginous hardgrounds. Two of these are marker beds that may be traced all along the Adam Foothills and are also found in Wadi Mi’Aidin (Location 5). Fully-marine fauna in both the clays and the limestones, as well as evidence of emersion in these thin alternations, may be due to sea-level fluctuations that were of the same order of magnitude, or greater, than the full relief of the flat depositional profile of the lagoon. The upper part of the Natih D member is a clay-free, medium- to thick-bedded, commonly bioturbated carbonate similar to the Natih E Thalassinoides bioturbated facies with fully-marine fauna such as corals, stromatoporoids, echinoids etc. However, it is still interpreted to have been a platform-top “lagoonal” environment, as opposed to a basinal depositional system.
In the lower part of the Natih C member (Figure 24), a bundle of eight or so more open-marine, lime-wackestone beds form a conspicuous feature that may be correlated over most of the Adam Foothills area (Figures 25 and 26a; as in Wadi H, Location 1), and this group of beds is clearly recorded on logs from the subsurface (van Buchem et al., 1996, 2002). The correlation of this group of beds, from the outcrops into the Fahud field (Enclosure IIA-2), shows a clear lens-shape, wedging-up onto the northern and eastern outcrop areas. The Jabal Shams area was probably undergoing structural uplift during deposition of the Natih C as suggested by the correlation panel (Figure 26a and Enclosure IIA-2).
The wedge-shape of the lower part of the Natih C, in conjunction with the pelagic fauna (planktonic foraminifera and nannofossils), the low-energy environment, and the very flat depositional profile suggests that this unit is some sort of a drift deposit (a deeper lagoonal “contourite” package or a washover system?), draping fine-grained sediment up against the shallower edge of the lagoon at that time (see discussion in following section). There is certainly a clear difference between these platform-top deposits laid down in a lagoonal context (be they from shallower or deeper water) and the intra-shelf basinal facies of kerogen-rich chalks in the Natih E and Natih B (Sequences I and III), for which both a clearly different depositional profile and a moderate basin depth have been established with confidence.
High-energy Facies at the Top of Natih C, Sequence II
The upper part of the Natih C member is formed by a thick, rusty orange-weathering bioclastic grainstone to rudstone (Figures 24 and 26b). The bed or beds lie on an erosion surface, here or there with a conglomerate lag (Figure 27). This is clearly a broad low-relief channel feature indicating much higher energy than in the deposits below. Coarse-grained, rudist bearing shoals, sandwaves and low-relief, tidal-channel fills make a striking appearance regionally towards the top of the Natih C, when compared to the other facies associations both of the Natih D and C (Enclosure IIA-2). These facies are well-exposed here in Wadi P of Jabal Madmar, but also on Jabal Shams (Location 7), as well as on Jabal Qusaybah (Location 6). The channel environment preserving the coarse facies here contrasts with the sandwaves of rudstone preserved at the same stratigraphic level on Jabal Qusaybah (see commentary Location 6) and the rudstone sandwaves of Jabal Shams (see commentary Location 7). This suggests a more proximal depositional environment, higher-up the depositional profile in this more easterly position. The high-energy facies, more in connection with the open-marine environment at the maximum of accommodation, was accumulated in local incisions at this location, as opposed to aggrading as sandwaves on the more open platform to the west at Qusaybah and northwest at Jabal Shams.
Location 5: Natih D and C Members, Southern Al Jabal al Akhdar, Wadi Mi’Aidin (22°58.2′N, 57°9.9′E)
Wadi Mi’Aidin is clearly signposted on the road from Muscat between Izki and Nizwa (Enclosures I-2, I-6 and I-7); turn off the main Muscat-Nizwa highway for Birkat Al Mawz, follow signs for Wadi Mi’Aidin (there are several different spellings) that lead to the centre of the village and then past the restored fort at the entrance to the wadi.
After several kilometres, the blacktop road leaves the wadi to the right and climbs on up to the Saiq Plateau, providing another spectacular trip with excellent outcrops of Proterozoic glacial deposits, “cap carbonates” of Snowball Earth fame, Permian and Triassic carbonates with well-exposed Permian-Triassic boundary section.
For Location 5, leave the blacktop at the junction to the left, before the road climbs towards Saiq, and follow the graded track along the wadi bed. The junction with the blacktop is just at the thrust contact of the Jurassic Hamrat Duru turbidite sequence, which overlies the autochthonous carbonates. The Natih is the first cliff-forming unit of the autochthon, and is clearly defined between the overthrust units and the next major recessive unit formed by the Nahr Umr shale (Figure 28a).
The Natih D member, clearly defined above the Natih E cliff-forming unit, is easily reached by a short walk from the wadi, on either flank (Figure 28b). A fault has cut-out some of the base of the D member on the western flank of the wadi, and this led to a shorter section being recorded in van Buchem et al. (1996). However, good exposures of the upper part of the D member can be seen here. The C member can be easily inspected by walking along the wadi floor at the foot of the cliff.
This locality has long been the most popular section to visit, on account of both its accessibility and of its stratigraphic completeness. This is true not only for the Natih (see Philip et al., 2005) but even more so for the complete Cretaceous section (Figure 29; not to mention the rest). Perhaps this is a little ironic, knowing that in the 1950’s, the IPC geologists working on the Fahud prospect and making the first detailed observations in the area were not allowed access to the wadi where they had hoped to establish a reference section (Sheridan, 2000; Heward et al., 2006).
The whole Natih section clearly shows the packaging of carbonate units between shale- or marl-rich layers, an illustration of reservoir-seal pairs. However, the Natih here was close (from a regional point of view) to the oceanic Cretaceous platform margin, which itself was near to the northern flank of Al Jabal al Akhdar. As a result, the sequence is carbonate-dominated, lacks the intra-shelf basin facies, and is mainly composed of shallow, fairly high-energy deposits. The trend of the oceanic margin must have been oblique to the present mountain range, running more to the north or northwest, since the outcrops to the west along the southern flank of the mountain show intra-shelf basinal facies of the Natih B (Locations 8, 9 and 10).
Measured sections at close intervals along the southern flank of Al Jabal al Akhdar (Enclosure IIB-2) provide the detail for the correlation of high-frequency stratigraphic cycles in Sequences II and III. The contrast in depositional profile between the very flat systems of the C and D members and the platform-slope-basin profile of the A and B members (partly time equivalent) is particularly well-illustrated by this correlation panel, even though this is not a true-dip section.
Stacked-beach and Swash-zone Facies in Sequence II
In this platform-top, proximal section of Al Jabal al Akhdar, numerous high-frequency cycles may be identified in both the Natih C and D (Enclosure IIB-2), and these high-frequency cycles are superimposed on the medium-frequency II-1 and II-2 cycles. The upper D member in Wadi Mi’Aidin shows several units of grainy swash zone or beach facies (Figures 28c and d) that are anomalously thick (nearly 10 m thick, much thicker than a single beach sequence). The anomalous thickness implies that the shoreline must have stayed close to this location, aggrading for a considerable period of time during a number of very-high-frequency cycles. This can be attributed to a vertical stack of landward-stepping to seaward-stepping turn-arounds, within a pile of genetic units (Enclosure IIB-2).
The Natih C member also contains closely similar beach or swash-zone facies here in Wadi Mi’Aidin. These beach facies, lying within thin laterally extensive high-frequency cycles (Enclosure IIB-2) indicate that littoral-zone deposits are well represented in the C and D members, as well as both fully-marine and emersive associations. The whole range of supratidal to subtidal lagoonal facies associations is therefore preserved, thus allowing the reconstruction of a very flat depositional profile. This is confirmed by the lack of clinoforms on corresponding seismic reflections. This precludes the interpretation of the lower parts of the D and C members in terms of deeper-platform-to-basin deposits. Instead it suggests that sea-level fluctuations were greater than the height of the depositional profile, which would explain the paradox of the dominant planktonic micro- and nannofossils in the lower parts of the two members. This conundrum will be further considered in the Discussion.
Location 6: Upper Part of the Natih C Member, Jabal Qusaybah (22°32.6′N, 57°04.2′E)
Jabal Qusaybah lies to the north of the road leading to Fahud from the Nizwa-Adam highway (Enclosures I-1, I-2, I-4 and I-5). Turn west off the main Nizwa-Adam highway about 25 km south of the Muscat-Nizwa/Nizwa-Adam junction, following signs for Fahud or Fuhud (there are several different spellings). This is still a blacktop road. The road makes a right-hand bend just before an oil pipeline booster station. After passing the booster station, Jabal Nahdah lies to the left, on the south side of the road, and rounding a broad left hand curve of the road, Jabal Qusaybah lies to the north, on the right. Turn off the blacktop to the north, towards the eastern end of Jabal Qusaybah. Drive directly across the terrain or on a rough track to the Jabal (depending on the departure point from the blacktop) and follow the foot of the hill round to the northern side. The outcrop is a short hike up a conspicuously broad wadi cutting into the northern flank of the Jabal (Enclosures I-4 and I-5). The location is easy to see from a distance on account of the orange weathering-colour of the rudist rudstone facies that compose the tall sandwaves.
An alternative itinerary, after leaving the blacktop, is to take the old well-site track up the southeastern flank of the Jabal to the well-site where the track ends. It is then a short hike to the northwest, over the crest and down to the stop location that may be seen from a distance thanks to the weathering colour mentioned above.
Jabal Qusaybah is the westernmost Jabal of the Adam Foothills. Good outcrops expose the Natih section from the Natih C to A members, with more distal facies compared to those of the northern or eastern locations that were closer to shallow platform or platform-margin environments.
High-energy Rudstones at the Top of Sequence II
Coarse rudist debris, weathering to a rusty orange, build up to form steep, 3-m-high, steep-angle of repose foresets accreting northwards. A 100-m-long sandwave feature (Figure 30) more-or-less preserves the original depositional topography. Mudstones and wackestones onlap the northern termination of this geobody, and together they form the upper high-frequency cycle of the Natih C.
The high-energy rudstones that characterise the upper Natih C here differ from those of Jabal Madmar (Wadi P, Location 4) in that they form sandwaves building up to 3 m or more in height, whereas the rudstones at the top of the Natih C in Wadi P form a low-relief, broad channel-fill. The commonality between them, together with the sandwaves in the upper part of the Natih C of Jabal Shams (Location 7), is the much higher energy of the depositional environment that seems to have affected much of the basin at the top of Sequence II.
The widespread occurrence of high-energy, coarse-grained rudstones and grainstones, both as channel-fills but particularly as 3-m-high sandwaves, with preserved depositional topography, indicates greater accommodation at this higher position in the Natih C member, than before. This is therefore interpreted to be the maximum accommodation of Sequence II (see van Buchem et al. 1996, 2002 as well as Figures 26a, 27 and Enclosure IIB-2).
Location 7: Upper Part of the Natih C Member, Jabal Shams (23°12.1′N, 57°10.7′E)
This site is reached by the same itinerary as the “Canyon Overlook” (Location 3) and lies adjacent to Location 3.3 (Enclosure I-8). Follow signposts to Jabal Shams then turn northwest off the main Nizwa-Bahla road to Al Hamrah, and then along Wadi Ghul, to Ghul. Follow the blacktop road and then the graded track on up towards Jabal Shams as signposted. Turn off to the right (southeast), as signposted, before the military check-point that limits access to the mountain top. Follow the graded track past the 2006-constructed tourist camp and then past the cliff-top village (both to the left of the track), past the “Canyon Overlook” car park to the track leading off to the right just past the older tourist camp (Enclosure I-8). To the west and south of the “Canyon Overlook”, low hills and gullies stretch over a broad area, offering numerous camping sites as well as the shelter of the more permanent tourist camp. Follow the track running up the low slope to the south from the tourist camp, wind around two broad recesses to the terminus, about two kilometres away (this again is a pleasant camping site). Hike on for several hundred metres, along the track that is now more of a footpath, to the location (Enclosure I-8).
Synsedimentary uplift appears to have occurred between deposition of the Natih E and C members in the Jabal Shams location. The outcrops to the west of the “Canyon Overlook” on Jabal Shams (Figures 31a and b) are interpreted to preserve an intra-Natih E to D horst with thinning and onlap of the lower Natih D. Detailed study of this location confirms the onlap of the lower D onto a Natih E structure, but also indicates that more recent deformation has overprinted the primary stratigraphic relationships (low-angle faults linked either to emplacement of the allochthon or to later formation of the Al Jabal al Akhdar structure).
The correlation between the outcrop areas and the subsurface suggests progressive shifts, both in time and space, of the subsidence on the platform towards the more open-lagoonal areas. Correlation of the Natih C and D (Sequence II) from outcrop to Fahud field (Enclosure IIA-2) shows the shifting of the minima and maxima of accommodation in the sequence due to local synsedimentary structural control. This is the first evidence for incipient deformation propagated from the obduction that was taking place further north.
Coarse rudist debris, deposited in high-energy near-shore or littoral environments, is a common feature of the upper part of the Natih C member. These deposits, in the Jabal Shams-Wadi Nakhr area, show beach and swash-zone features, and also form major sandwave fields. The inference is that high energy was now able to affect the broader shallow platform and lagoon, perhaps due to a less effective platform margin barrier at the time of maximum accommodation.
Rudist Sandwaves and Littoral Rudstones at the Top of Sequence II
The correlation panel between Wadi Nakhr and Jabal Shams (Figure 32, not to horizontal scale) illustrates the variation in the coarse rudstones over some 5 km. At the Location 7, a sandwave field is built by 2-m-high foresets rising up to about 20° in slope (“Sandwaves” in Figure 32). The sandwave field is some 200 m long (north-westerly accretion direction), more than 100 m wide and appears to fill a nearshore depression or swale as a tidal deposit (Figures 31 and 32). Closer to the tourist camp, thin sheets of gravel-sized rudist debris show features of beach runnels and swash-zone lags that overlie mudstones and rudist wackestones to floatstones (“Camp” in Figure 32).
Location 8: Natih A and B Members, southern flank of Al Jabal al Akhdar, Wadi Nakhr (23°09.1′N, 57°12.3′E)
Wadi Nakhr feeds the wadi stream-water runoff into Wadi Ghul from the magnificent, 1,000-m-deep Jabal Shams Canyon erosional feature (Enclosures I-1, I-2, I-8 and I-9). On the road from Nizwa to Bahla, turn left (west) just before reaching Bahla and follow signposts to Jabal Shams. Turn northwest off the main Nizwa-Bahla road to Al Hamrah, and then along Wadi Ghul, to Ghul. Turn in to Wadi Nakhr at Ghul (and having admired the ruins of successive villages on the western flank of the entrance to the narrow wadi, Figure 33a), drive along the wadi bed and then up the steep incline to the village of Nakhr for a splendid view of the whole Natih sequence when looking back down the wadi (Figure 33b).
The small parking area at the entrance to the village provides a very good view of the Natih E cliff, where clinoform sets (c. 3–5 m high) are separated by flat-lying beds, some of which are dolomitised and weathered as darker-brown compared to the rest. These are excellent examples of stacked progradational-aggradational cycles (high-frequency accommodation cycles) and they show the complex internal geometry of this apparently simple thick carbonate member (Figure 34; see Discussion below).
The Natih A and B members are the focus of this location however. These members, forming Natih Sequences III, IV and V, are in stark contrast with the C and D members of Sequence II. The correlation panel of the southern flank of Al Jabal al Akhdar illustrates how the very laterally extensive, thin-bedded, flat-lying deposits of both the Natih C and D are overlain by geometrically more complex units Natih A and B (Enclosure IIB-2). The Natih A shows high-frequency units that stack basinward in a shingle pattern from east to west, composing wedges first of Upper Cenomanian and then of Turonian deposits. The Natih B undergoes major facies changes laterally between Wadi Mi’Aidin in the east (outer ramp wackestones and mudstones) and where the road starts the steep climb towards Jabal Shams at the west (basinal anoxic facies).
Village children and their parents enthusiastically sell home-made plaited key-rings and small hand-woven rugs of sheep and goat wool to passers by at the small parking area at the entrance to the village of Nakhr.
Natih B Member Kerogen-rich Chalk Source-rock Facies
The Natih B kerogen-rich chalk facies (compacted and cemented here) may be seen at the track level on the eastern wadi flank (Figures 35b and c). Ten-cm-thick beds of darker weathering carbonate mudstones to whole-fossils wackestones contain a monospecific fauna of small bivalves. The lighter-weathering, finely-laminated beds that alternate with the darker-weathering beds are kerogenous chalks, rich in organic matter (up to 3% TOC of type II marine organic matter in outcrops). Equivalents of this source rock in the subsurface have contributed to the oil in several fields in northern Oman, including the Natih and Fahud fields. Van Buchem et al. (2005) described these source rocks and compared them with those of Carboniferous and Devonian examples. Their analysis and comparison with other source rocks of Devonian and Carboniferous ages is briefly commented in the discussion section below.
Top and bottom surfaces of the darker beds are commonly clearly erosional even if, in places, compaction has enhanced these sharp scours, gutters, furrows and mounds (“brioches”). Other contacts between the dark- and light-coloured beds are gradational. The depositional environment is therefore thought to have been close to the storm wave-base of this intra-shelf basin; above storm wave-base where the scoured bed surfaces occur, but below this energy threshold where the contacts between kerogenous chalk and whole-fossil wackestones are gradual. The kerogenous chalks accumulated during periods of expanded anoxia in the intra-shelf basin (see discussion on the development of this facies under Location 1, Wadi H).
Regional correlation of the Natih B shows that the B sequence observed here is contemporaneous with part of the upper cliff-forming Natih member in Wadi Mi’Aidin (Enclosure IIB-2). This is explained by the kerogen-rich facies that characterises the B member being deposited only in the intra-shelf basin, whereas laterally equivalent shallower-water facies are closer to the prograding platform-margin facies, in their weathering pattern at least.
Twenty-metre-tall Clinoforms in the Natih A Member
The Natih A forms a massive cliff above the thinly-bedded Natih B source rock (Figure 35a). The cliff is clearly separated into an upper part with steeply dipping clinoforms (20° or more once the structural dip is accounted for) and a lower part with the same dip as the B member below. The steeper, rudist rudstone clinoforms, together with the chert-rich part of the section below, is a somewhat younger “Turonian” package characterised by Hippurites rudists, shifted basinward above the lower part of the cliff-forming mudstone to packstone facies. The strongly progradational nature of the A member here is well-illustrated by the correlation panel (Enclosure IIB-2). Quiquerez and Dromart (2006) comment on the highly contrasted vertical facies differentiation in these clinoforms, when compared with examples from numerous other carbonate outcrops of similar or different age around the world. They interpret them to have been deposited in a fairly distal position of the intra-shelf platform.
Location 9: Natih A and B members, southern flank of Al Jabal al Akhdar, Wadi Al Hamrah (23°07.7′N, 57°17.3′E)
Al Hamrah is the large village at the eastern entrance to Wadi Ghul (Enclosures I-1, I-2, I-8 and I-9). Take the road from Nizwa to Bahla; follow the signs to Al Hamrah and Jabal Shams at the junction branching to the west just before Bahla. The Wadi Al Hamrah is the western gully cut into the dip slope of Al Jabal al Akhdar, behind the village itself. Turn left off the road before it climbs from Al Hamrah to Misfah and wind left around the village into the broad gravel and cobble sheet at the wadi mouth. The outcrops of Natih A are well-exposed for a 100 m or so along both flanks of the wadi, at the foot of the dip-slope. Together with the lower part of Wadi Misfah (just to the east of Location 9) the whole section between base Natih D and Natih A is very well-exposed, and quite accessible by hiking-up the wadi and climbing over giant boulders.
Although the whole Natih D to A section may be studied here, this is one of the few locations where larger scale synsedimentary deformation may be observed in the A member. Synsedimentary deformation of the Natih has been suggested by previous authors (Boote et al., 1990; Rabu, 1988; Hanna and Smewing, 1996; van Buchem et al., 2002), and the onset of deformation has commonly been proposed to have led to the demise of the carbonate platform in the Turonian. Van Buchem et al. (2002) have attributed the basinward shift of facies and the forced-regressive wedge of Sequence III (here the higher-frequency sequences III-8 to III-12, Enclosure IIB-2) to the propagation of deformation from the collision to the north.
Synsedimentary Deformation in the Natih A Member
The western flank of the wadi, where the stream bed becomes more covered with boulders, shows gentle warping of the strata. The overlying beds compensate for this by a change in thickness, increasing by a metre or more from north to south over 10 to 20 m along the wadi. This is best observed by looking across from the eastern flank (Figure 36).
Location 10: Natih A and B members, southern flank of Al Jabal al Akhdar, Wadi Kahmah (23°01.1′N, 57°32.4′E)
Wadi Kahmah is close to Nizwa, and is reached from the Kahmah village that is signposted to the right from the blacktop, just having left Nizwa on the road to Bahla (Enclosure I-2, I-6 and I-7). Park at the village and follow the footpath through the palm-grove and along the wadi for 100 to 200 m. The Natih A is well-exposed across the wadi, and the Natih B may be observed a little further along the wadi.
The less subsident, but also probably uplifted portions of the upper Natih A were affected by numerous phases of erosion and incision. Incisions at top Natih A are well-exposed over the whole of the eastern portion of the transect (Enclosure IIB-2). Infillings are commonly composed of bioclastic tidal channel complexes, and may be easily observed at the top of the section in Wadi Mi’Aidin, Wadi Kahmah and Wadi Sumayt.
Multiple Incisions Infilled by Tidal Deposits, Upper Part of the Natih A Member
The southern flank of Wadi Kahmah (Figure 37) shows good exposures of the upper part of the Natih A with several broad, low-angle incisions. The coarse, cross-bedded grainstones to rudstones, interbedded with wackestones, are commonly iron-stained, and hard grounds are developed in places. Laterally accreting beds and heterolithic facies suggest that these incision-fills were deposited in tidal environments. This is the uppermost sequence of the Natih (Sequence V) and outcrops of this unit appear to be restricted to this area.
Location 11: Natih A and B members, western Adam Foothills, Jabal Qusaybah (22°32.6′N, 57°04.2′E)
This is the same route as for Location 6. Jabal Qusaybah lies to the north of the road leading to Fahud from the Nizwa-Adam highway (Enclosures I-2, I-4 and I-5). Turn west off the main Nizwa-Adam highway about 25 km south of the Muscat-Nizwa/Nizwa-Adam junction, following signs for Fahud or Fuhud (there are several different spellings). This is still a blacktop road. The road makes a right-hand bend just before an oil pipeline booster station. After passing the booster station, Jabal Nahdah lies to the left, on the south side of the road, and rounding a broad left-hand curve of the road, Jabal Qusaybah lies to the north some 5 km further, on the right.
For Jabal Qusaybah (Enclosures I-1, I-2, I-4 and I-5) turn off the blacktop to the north, towards the eastern end of the Jabal. Drive directly across the terrain or on a rough track to the Jabal (depending on the departure point from the blacktop). At this point, to gain access to the Natih A and B members, it is better to take the old well-site track up the south-eastern flank of the Jabal (Enclosure I-4) to the well-site where the track ends. It is then a short hike to the northwest, up to the crest, where the Natih A can be examined, and on down through the Natih B towards the orange-weathering Natih C. The hike down the western-facing slope runs across excellent outcrops of the Natih B facies. The two other spots to visit here and that are indicated on Enclosure I-4 are the Natih A grainstones and the top Natih hardground and overlying conglomerates.
As already written for Location 6, Jabal Qusaybah is the westernmost Jabal of the Adam Foothills, some 5–10 km or so away from Jabal Nahdah. Good outcrops expose the Natih section from the Natih C to A members on Jabal Qusaybah (Figures 38 and 39), and from B to A on Jabal Nahdah nearby. These locations show more distal facies compared to those of the northern or eastern locations that lay closer to shallow-platform or platform-margin environments. The Natih A and B members are the focus of this second commentary for this general location (after Location 6), and these are the closest analogs to the nearby subsurface oilfields. Correlations between Jabal Qusaybah outcrops and Fahud field for Natih Sequences III, IV and V are shown on Enclosure IIA-3. The closest correlated well of the Fahud field, F 401, is only 55 km away.
Ammonites from the base of and within the Natih A and B members confirm the Cenomanian-Turonian boundary to lie within the A (Bulot et al., in preparation). The general stratigraphic organisation of the Natih B/A (Sequences III, IV and V; Enclosure IIB-2) is very similar to the organization of Natih E (Sequence I), with a first phase of differential aggradation leading to the development of an intra-shelf basin, where organic-rich sediments are preserved, and a second phase of progradation of a low-angle rimmed carbonate ramp, progressively filling in the intra-shelf basin. However, unlike for the Natih E (Sequence I), the Natih A (Sequence III) records the tectonic deformation of the platform margin, interpreted to be linked with the onset of the obduction of the Semail Ophiolite. Post-Natih uplift led to incisions of up to 150 m in depth (Droste and Van Steenwinkel, 2004), but it is interesting to note that the Natih A, in outcrop, shows none of the karst features that were developed in equivalent deposits of the Fahud oil field.
Basinal deposits of the Upper Natih: Natih B source rocks
The outcrop shows the very regular, well-bedded aspect of facies in both B and A members (Figures 38 and 39). The facies that characterise this intra-shelf basinal environment are mudstones and wackestones, with organic-rich, oyster-bearing beds that developed under anoxic conditions to form the organic-rich - organic-poor couplets that compose the Natih B. The organic-rich beds are thinner, cm-to-dm scale, and are commonly laminated. The organic-poor beds may reach up to m-scale thickness (Figure 40).
The lower part of the basinal section (Natih B) correlates with shallower muddy ramp facies in Wadi Nakhr (Location 8). The whole basinal section, with its source rock facies of Jabals Qusaybah and Nahdah, correlates with shallow ramp deposits in Wadi Mi’Aidin (Location 5). Differential aggradation of carbonates occurred in the marginal areas causing local vertical stacking to seaward-stepping stacking patterns, while less sediment was deposited in the future basinal areas in between. This depositional system progressively created inclined depositional profiles and the differentiation of an intra-shelf basin approximately 50–80 m deep, characterised by anoxic conditions where organic-rich sediments could be preserved. The maximum flooding surface of Sequence III is placed at the level of maximum lateral extent of the intra-shelf basin. In the outcrops, the maximum flooding surface is picked a little higher than in the subsurface, and therefore above the maximum flooding of the Fahud area (see below and Enclosure IIA-3).
The organic matter has been shown by palynology to be composed of a marine type II (van Buchem et al., 2002). Van Buchem et al. (2005) provide an analysis of the Natih B, focusing on stratigraphic cycle hierarchy and global climate. This is commented on in the discussion section below. Rock-Eval analyses reported by van Buchem et al. (2002) give results from outcrop samples reaching 3% total organic carbon in an overmature organic facies. In comparison, subsurface TOC values of up to 14% are recorded from immature source rocks in the Natih field (well number 68). Pelagic fossils cited by van Buchem et al. comprise planktonic foraminifera, planktonic echinoderms (Saccocoma), planktonic bivalves (filaments), sometimes solitary corals, and calcispheres. Similarly as for the Natih E source-rock deposits, water depth for the Natih B facies is estimated from fossils and from clinoform geometry to have been below storm wave base (50–80 m). Van Buchem et al. (2002) have estimated that individual organic-rich beds with their characteristic gamma-ray peaks can be followed by subsurface correlation over more than 150 km.
A synsedimentary eastward shift of the intra-shelf basin axis may have led to the deposition of more organic-rich sediments in the Jabal Qusaybah area than in the Fahud area. In the Fahud area, alternating wackestone/packstone decimetre-thick beds with oysters, corals and gastropods were deposited on the western margin of the intra-shelf basin at the same time as the upper part of the organic-rich interval was deposited in Jabal Qusaybah. The maximum flooding surface of Sequence III is therefore difficult to pick when established according to the concentration of organic matter. On the correlation transects, it has been placed at the time of maximum extent of the intra-shelf basin in the Al Jabal al Akhdar area (MFS of sequence III-4, Enclosure IIB-2). It does not correspond to the time of maximum extent of the basin when established from the sequence in the Fahud area, which explains the lower position of the MFS in Fahud.
Natih A Grainstones
On the western flank of the Jabal Qusaybah (Enclosure I-4), an interval of coarse grainstones to rudstones with cross bedding in the Natih A provides a conspicuous difference in facies from the commonplace mudstones and wackestones (Figure 41). These high-energy deposits may correlate with the basinward shift illustrated by the steeper clinoforms of Wadi Nakhr (Enclosure IIB-2, Location 8). The lack of data on the geometry of these high-energy grainstone deposits in a basinal setting leaves their depositional system in doubt. They may have been deposited as storm lobes, or as the infill of a broad scour or channel.
Conglomerates overlying the Natih A
At the north-western foot of the Jabal, coarse cobble-to-boulder conglomerates cap the Natih sequence, overlying the hardground that caps the Natih A (Figure 42 and Enclosure I-4). Similar facies have been taken to indicate upper Cretaceous structural deformation and erosion (Hannah and Smewing, 1996).
Two Natih Paradoxes
The interpretations of facies and their associations in terms of depositional environments given in this field guide are the result not only of observations on facies at the outcrops, but also from the iteration of that understanding with other observations, as much with those of seismic analysis and seismic stratigraphy as with those of palaeontology (macro-, micro- and nanno- of course), and with those of sequence stratigraphy. As an end result, there are at least two features of the interpretation of the Natih given here that are out of the ordinary and are the subject of debate.
First, the unexpectedly high-amplitude of sea-level variations at a fairly high-frequency (fourth-order), for which there is solid evidence at the top of the Natih E member. Such high-amplitude variations in sea-level, if they were a regular occurrence, may have led to numerous emersions during deposition of the C and D members. However, the accumulated sediments appear to have been deposited under several tens of metres of water depth.
The second paradoxical feature is the interpretation of water depth for deposition of the lower parts of the Natih C and D members. In opposition to the shallow-water depth interpreted from the low-relief geometry of the depositional system, a much greater bathymetry is suggested by the dominantly pelagic micro- and nannofossils content. If taken out of context, the fossils would suggest a deeper intra-shelf basin environment, such as that deduced for the lower parts of the E and B members. The paradox between faunal associations and interpretation of palaeobathymetry in the lower parts of the Natih D and C is perhaps the most striking issue to arise from the comparative and iterative procedure outlined above.
Natih Facies and Facies Associations: Their Interpretation in the Light of Rapidly Fluctuating Sea Level and Inhibition of the
Carbonate Factory by Clay
The outcrop observations described and commented-upon above provided a fairly simple and clear picture of depositional environments for most of the Natih Formation. The facies of the formation are mostly marine carbonates, laid down upon an extremely broad platform between the Arabian coastline to the west or southwest, and the Neo-Tethys Ocean to the east or north. The Natih Formation contains several well-identified layers of terrigenous clays brought in from the Arabian hinterland during major lowstands but deposited during subsequent transgressions. These clays subdivide the formation into successive members (Hughes-Clarke, 1988; Droste and Van Steenwinkel, 2004). Each member represents a transgressive-regressive cycle over the platform, with shallow- to moderate-depth, intra-shelf basins becoming filled by the progradation and expansion of mini-platforms nucleating on the broader shelf (Droste and Van Steenwinkel, 2004). In general, the Natih carbonates comprise a wide range of non-cohesive, low-energy, muddy or shelly sediments, ranging through intermediate-energy mixed, fine-to-coarse carbonate lithologies to high-energy, grainy to gravelly carbonates. Various build-up deposits were formed locally with binding to floating textures, and bioturbation (sometimes spectacular), occurs more or less throughout the succession.
Within a single facies association, such as those described by van Buchem et al. (1996, 2002; Enclosure III), there may be numerous different microfacies. In the case of the Natih, many facies associations are seen to be dominated by micrite-rich or “muddy” sediments. The diagenetic history of the Natih lies outside the scope of this field guide, but it is interesting to note that the paradoxically high porosities of the subsurface reservoir rocks (typically between 20% and 30% but with very low values of permeability) are commonly due to micro-porosity from later diagenetic leaching (M. Esteban and C. Taberner, personal communication, 2005). Note that a given facies association comprises a range of microfacies with coarser and finer grain sizes that may be better or more poorly sorted (Enclosure III). Apart from a few typical end-members, such as planktonic foraminiferal mudstones to wackestones or coarse rudist debris grainstones or rudstones, many wackestone to packstone microfacies are to be found in a fairly wide spectrum of shallower to deeper environments.
Consequently, it is generally not feasible to infer a particular depositional environment just from a single microfacies. The interpretation of depositional environments is better made from a succession or an association of microfacies, together with their position in regard to depositional geometries (such as clinoforms) and sequences.
Facies and microfacies are customarily interpreted in terms of a unique setting for each different faunal assemblage. The energy of the depositional environment is commonly interpreted as a function of water depth, or to result from the geographical exposure or sheltering by depositional environments. Facies substitutions (lateral changes of facies across or down a depositional profile) and facies successions (vertical stacks of different facies) are then used as the basis for reconstructing the depositional profile (using “Walther’s Law”); iteration with seismic geometries has confirmed the validity of this general approach in this study.
However, in the case of the Natih, higher-frequency fluctuations of sea-level (up to 20 or 30 m in magnitude) are now shown to have affected the system (see descriptions and commentaries above of IS1 and IS2 incisions, Locations 1 to 3). Moreover, some facies (lime-mudstones in particular) are not indicative of a well-established bathymetry. In conjunction with the considerable changes in sea-level during higher-frequency cycles, clay influx at sequence boundaries or in response to global climate constraints may well, at times, have inhibited production of shallow-marine carbonates where this would otherwise normally have occurred.
The detailed study of the incisions situated towards the top of the Natih E member and their infill (Locations 1 to 3) has established that up to about 30-m of sea-level change constrained the I-6, I-7 and cycles that are subordinate to the third-order Natih Sequences I and II (Grélaud, 2005; Grélaud et al., 2006). These fluctuations in sea level are well-documented for the upper part of the Natih E, but the amplitudes of the fluctuations controlling other cycles of the Natih have not been directly established. Nevertheless, it seems reasonable to extrapolate the well-established 20–30-m-magnitude of sea-level change to the other cycles of the same fourth-order. In this case, such variations may well have led to changes in bathymetry, during Sequence II, as great as the vertical range of the depositional profile, which has been deduced from seismic geometry together with the presence of the full-range of open-marine to shoreline facies associations (see Location 5), over much of the area in question.
In the Natih D and C members (Sequence II; Locations 4 and 5), numerous single-metre-scale beds or groups of beds composed of lime-mudstones and wackestones, with strong Thalassinoides bioturbation, may be traced laterally for 10s to 100s of kilometres. Some beds contain fossil assemblages indicating fully-marine waters (e.g. well-developed echinoderm populations, or abundant nannoplankton and planktonic foraminifera). However these thin, flat-lying and regionally continuous muddy deposits, although grading into moderate-energy grainy beach facies in Wadi Mi’Aidin, lack any correlative slope deposits and grade directly into higher-energy, shallow-platform facies. This suggests that they were laid down in a shallow-water, low-energy and platform-top lagoonal environment.
This conclusion is paradoxical when compared with the depth estimated for the depositional environment from the faunal association. As the lower Natih D and Natih C associations of nannoplankton and planktonic foraminifera are analogous to those of the intra-shelf basin chalks of the Natih E and B members, it would be reasonable to ascribe similar water depths to such pelagic facies throughout the entire formation. In short, for the lower parts of the Natih D and C, the occurrence of lime-mudstones (“chalks”) containing pelagic micro- and nanno-fossils, tempered by the information on the shallow, low-angle depositional profile, is not considered to indicate a bathymetry as deep as that attained during the development of Natih E and B intra-shelf basins. Further debate may focus on the very “old” species determinations of nannoplankton (Figure 5, but see below), perhaps suggesting development of an endemic fauna in this particular environment.
The very low-gradient depositional profile (reconstructed both from facies, outcrop geometries and seismic data), the inhibition of the carbonate factory by clay influx, and the considerable fluctuation of sea level compared to the vertical range of the depositional profile during higher-frequency cycles may have led to creation of bedsets with a complex, composite history. Stages of flooding, deposition, starvation, perhaps even emersion and then overprinting by bioturbation, may have “smeared-out” through time, a succession of somewhat more varied “snapshot” pictures of muddy subtidal to intertidal flats, shoals and ponds. Comparatively short times of deeper bathymetry, during flooding when the carbonate factory was generally non-productive, could have allowed the influx of planktonic foraminifera to be incorporated in these lagoonal facies. The debate over an endemic, lagoonal planktonic production or a detrital influx of pelagic material from the ocean (“washover”) remains unresolved. This hypothesis could perhaps be tested by a study focused on the early diagenesis of the Natih deposits.
Natih Pelagic Facies, Source Rocks, and Global Oceanic Anoxic Events
The anoxia developed in the lower parts of the Natih E and B members, with their typical source-rock development, is commonly thought to tie with global anoxic events. There are several global anoxic events during the Albian to Turonian time period considered, some of which could be candidates to correlate with the Natih E or B anoxic facies. These are the Oceanic Anoxic Events 1b, c, and d in the Albian, a possible mid-Cenomanian event, and OAE 2, at the Cenomanian-Turonian boundary (Leckie et al., 2002); or the Albian-Cenomanian Boundary Event, the Mid-Cenomanian Event 1, and the Cenomanian-Turonian Boundary Event (Jarvis et al., 2006). Without wishing to trespass into the debate on Oceanic Anoxic Events, it is reasonable to seek a correlation between the anoxic developments of the Natih and global oceanic signatures. In spite of this, the Natih platform-top setting with its excessively broad but more-or-less closed-off lagoon can always be used to explain away unusual local features and the absence of any link to global signatures.
Obviously, global oceanic anoxia and its impact on depositional environments mostly affected deeper oceanic waters, but either global anoxia itself or the underlying causes may also seriously have affected the fauna and flora in the Natih platform-top environment. The clay influx that characterises the C and D members could coincide with a global increase of weathering that led to an increased nutrient flux to the oceans, contributing in turn to development of oceanic anoxia. This would also allow for another explanation of the conundrum opposing fossil content and depositional profile of the Natih C and D.
The complication in the correlation of Natih features with global events comes from the uncertainty in age attribution of the Natih (Figure 5). There is general agreement that the Natih spans the Albian to Early Turonian. However, compared to the scheme proposed by van Buchem et al. (2002), and used in the study by the present authors for PDO (Homewood et al., 2006), the regional correlation of flooding surfaces proposed by Sharland et al. (2001) tends to give considerably younger ages with Turonian from the B member upwards. On the other hand, nannoplankton studied by Jacovides and Varol (unpublished report by Millennia, 2000, for PDO; Droste, personal communication, 2006) tends to give older ages than those proposed by van Buchem et al., with the Albian-Cenomanian boundary in the Natih C member.
The time subdivision based on carbon stable isotope stratigraphy from subsurface samples from cored wells, recently completed by Vahrenkamp (in preparation, 2007) basically establishes a similar chronostratigraphy. A rise in δ13C over the A interval in Fahud would correlate with the initiation of the OAE2 and put the Cenomanian/Turonian boundary near to the top of the A section. Peak values corresponding to the OAE2 maximum have not been observed and may have been eroded at Fahud.
However, more recently, a revision of the ammonites from the Wasia Formation by Bulot et al. (in preparation, 2007) does give a more solid basis to the bio- and chronostratigraphy of the Natih, with a much tighter constraint on the Albian-Cenomanian boundary (Figure 5). The Albian to Cenomanian limit is placed by them within the lower part of the Natih E, since an uppermost Albian ammonite was found in the lower beds of the Natih E. However the Cenomanian to Turonian boundary remains a little vague, put above the transition between the Natih B and Natih A (a little below where it was placed by van Buchem et al.). Bulot et al. placed the Early to Middle Cenomanian limit at the base of the Natih D, and the middle to Late Cenomanian limit is bracketed by a late Middle Cenomanian ammonite assemblage at the top of the Natih C, and by early Late Cenomanian index fossils in the base of the Natih B.
To summarise, the source-rock facies in the lower part of the Natih E would correspond well with the Albian-Cenomanian boundary event, and the paradoxical pelagic deposits of the C and D with the mid-Cenomanian event or its causal nutrient influx. The major Natih B source rock is less easy to tie in to the global record, but may also correlate with the end of the mid-Cenomanian event, since the Cenomanian-Turonian boundary would seem to lie higher, within the Natih A. Van Buchem et al. (2005), through comparison of the Natih B source rocks with those of the Devonian Duvernay Formation in Western Canada, and of the Upper Carboniferous Paradox Formation in the United States, point to the “greenhouse” controls on the Natih environment compared to the “icehouse” Paradox and “intermediate-house” Devonian. They argue that the variable carbonate production observed through a hierarchy of stratigraphic cycles contrasts with a more stable clay-organic matter ratio, and that the stratigraphic organization of third and fourth order cycles controls the distribution of organic matter.
Geobodies and Depositional Assemblages
As already emphasised, in a study where the aim is to build 3-D subsurface models, once the facies and their associations have been established and given a detailed interpretation, there still remains a considerable gap between this sedimentological understanding and construction of 3-D spatial grid-cell models tied to seismic images or to correlations between wells. In order to bridge this gap, it is necessary to solve a problem compounded by different scales of observation, by different data types and by the different concepts used when making interpretations from the disparate data-sets. In this process, it is fundamental to establish the link between facies sedimentology and the resulting stratigraphic architecture. The approach adopted in the study summarised here was to establish the geometry, scale and hierarchy of architectural elements called geobodies and depositional assemblages.
Geobodies and depositional assemblages both are stratigraphic, geometrically distinctive packages of facies (a given facies association) but at two different scales. At a smaller scale, sedimentary geobodies are the direct product of geomorphic units (e.g. reef patch, tidal channel, intertidal shoal, supratidal flat) that have been preserved “in full”, “smeared out”, or “expanded” in response to higher or lower accommodation. Depositional assemblages are broader-scale mosaics of geobodies, with a particular seismic geometry, and are generally associated with a specific sequence stratigraphic context (“transgressive”, “regressive”, etc.) and position. Although depositional assemblages are smaller in scale than systems tracts (highstand, lowstand, transgressive, regressive), their stratigraphic context is similarly identifiable from the stacking pattern, as are their geometrical relationships with regard to platform, slope and shelf break. Conversely, a systems tract generally comprises several smaller-scale depositional assemblages. Examples of geobodies and depositional assemblages in the Natih are listed in Tables 1 and 2.
The difference between geobodies and depositional assemblages, as defined here, is essentially one of scale, but also to some extent as to how (i.e. from which data type) they are identified. Geobodies typically measure up to about 100 m in lateral extent and 1 to 5 m in vertical thickness; a good example would be a laterally accreting, meandering tidal channel deposit such as described in the upper Natih E at Location 1. They are therefore close to the scale of a grid cell in an upscaled 3-D reservoir model. Depositional assemblages reach several km to 10s of km laterally and are several 10s of m thick; an obvious example would be a prograding and down-stepping clinoform package, accumulated during forced regression, such as the smaller packages together forming the shelf margin wedge that corresponds to the incision IS1 of the Natih E (Location 1).
Whereas geobodies may commonly be described on outcrop (e.g. a rudist rudstone sandwave field such as at Location 7, or a tidal channel body such as at Location 1) they are generally at the limit of (or below) the resolution of seismic images. Depositional assemblages (e.g. a forced-regressive wedge or a low-angle sigmoidal clinoform package) may be identified on seismic data, but are rarely perceptible at outcrop scale. The features shared between geobodies and depositional assemblages (geometrical and facies attributes) are essential to know when correlating between sections on outcrop or between wells in the subsurface. These features also allow prediction of facies from the geometrical attributes identified on seismic data. The iterative loop comprising observation and interpretation of facies, geobodies and depositional systems is completed at this stage by seismic modelling, using geometrical and lithological attributes established from outcrop observations. This has been carried out at the regional scale of the Natih (Schwab et al., 2005) and at the scale of the Natih E channels and incisions (Grélaud, 2005; Grélaud et al., 2006).
Since geobodies and depositional assemblages are commonly defined by clinoform shapes and angles, it is important to distinguish between those features seen on outcrop (usually of fairly limited size), and those seen on seismic images (usually with considerable vertical exaggeration). Outcrop features are too small, and sometimes too steep (exceeding 25°), to be resolved by seismic data. Seismic features are normally much too low-angle (much less than 5°) to be recognised as clinoforms on outcrop. Clinoform angles derived from the study of 3-D seismic near the Adam Foothills outcrops (Figure 7) have been summarised in Figure 6.
The Natih E cliff in Wadi Nakhr (see above: Location 8, General; Figure 34a) shows how a thick, apparently homogeneous, massive unit can be composed of a stack of prograding-aggrading cycles or very-high-frequency stratigraphic units. These units can be described in terms of geobodies (built by migration of 3–5-m-high sandwaves) in order to qualify their heterogeneity at the reservoir scale. Although the Natih E cliffs in Wadi Nakhr look quite homogeneous at first glance, in detail they are seen to be made of several cycles separated by dolomitised layers (standing out as brown beds on the weathered outcrop surface, dotted lines in Figure 34a). Between and below these dolomitised beds that show the structural dip, more steeply inclined geometries are visible. These clinoform sets form geobodies as defined in this field guide.
The superposition of flat-lying and clinoform bedsets shows that the Natih E member (Sequence I) is made of a stack of several prograding-aggrading cycles. The apparently flat-lying dolomitised beds separating composite prograding-aggrading packages (dotted lines in Figure 34b), if resolved by seismic data, would be seen as low-angle to very low-angle clinoforms, bounding surfaces within a depositional assemblage. The higher-angle clinoforms (dashed lines in Figure 34b), resulting from the migration of composite bars or sandwaves (geobodies), are much too small to be resolved by seismic data.
The depositional assemblages of the Natih E from the Fahud field provide a clear illustration of architectural elements at this scale (Figure 43 and Enclosure IIA-1). In terms of physical stratigraphy the Natih E of the Fahud area is composed of eight successive time units or packages of strata (Figure 43). These depositional assemblages are set in a given mode of stacking, as opposed to “depositional systems” which are the pattern of environments that one would observe at any given instant in time. Each package of the Fahud area Natih E has a typical depositional profile, bathymetry, and set of geomorphic elements, and the different combinations of these attributes, are numbered 1 through 5. During sedimentation, the combination of attributes (or a given “assemblage”) determined the depositional system, and therefore the resulting sedimentary facies. The successive packages define, in sequence from bottom-to-top and laterally, the following assemblages: 1, 2, 3, 4, 5, 3, 4, and 1, where packages with a same number are built by a similar depositional system (Figure 43).
Four of the stratal packages described here (1-4, Figure 43) were based largely on outcrop data, the largest database at the onset of this study. Iteration of this with the growing set of reinterpreted subsurface data led to the identification of a fifth, complementary but distinctive, transgressive to aggradational assemblage, and a different depositional system. This assemblage (5 in Figure 43), with specific facies recognised on core, was created by the onlap of the steeper depositional profile following a major sea-level fall (Upper/Middle E, Sequence I-7). The facies correlations between wells, tied to the outcrop area, are illustrated in Enclosure IIA-1.
From these examples, it is clear that to some extent these architectural elements merge with systems tracts at the scale of fourth-order cycles. In rare cases, the scales of geobodies and depositional assemblages converge. The upper, cliff-forming unit of the Natih in the entrance to Wadi Nakhr (Figure 35a) corresponding to the Natih A member (Sequence III-9 and III-10) is a case in point. Note the steeply dipping clinoforms of III-10 that reach angles of 20° or more. Sequences III-9 and III-10 correspond to a prograding lowstand wedge, deposited during a forced-regression period at the beginning of the Turonian. Whereas the clinoform package seen at outcrop scale in Figure 35a is a geobody, the whole Sequence III-7 to III-12 prograding lowstand wedge is at a scale that could be seen on seismic data. The whole wedge is therefore a depositional assemblage as defined in this field guide.
CONCLUSIONS: CONSEQUENCES FOR FACIES MODELS FROM MODERN ANALOGS AND THE CHANGING SCENE OF THE NATIH THROUGH TIME
Facies models are made by putting together the bits and pieces of relevant information on lithologies, sedimentary processes, fossils, bathymetric profile, depositional environments and modern analogs for a rock sequence, all in one representation. Most facies models are presented as cross-sections of a single depositional profile or as perspective views (“block diagrams”), with features of sedimentary processes, palaeogeomorphology and orientation with regard to major elements such as slopes, channel orientation, shorelines, shoals, build-ups and so forth. Facies models for a given rock unit may therefore vary considerably depending upon the scale of enquiry (regional, local) and the focus within which the model is to be used.
Previous studies of the Natih Formation, or equivalents within the Cretaceous stratigraphy of the Middle East such as the Mishrif, have mostly proposed a single comprehensive facies model (Harris and Frost, 1984; Burchette and Britton, 1985; Burchette, 1993; Alsharhan, 1995; van Buchem et al., 1996, 2002). Van Buchem et al. (2002) built on the outcrop studies reported in van Buchem et al. (1996), and gave three main scenarios for the development of the Natih under the varying influence of eustatic, climatic and tectonic controls. More recently the distinction between two contrasting scenarios with different facies associations, under either a transgressive or a regressive regime was made by Droste and Van Steenwinkel (2004). These models may be appropriate at the broader regional scale, and when considering the Natih Formation as a single depositional unit.
This field guide is based on sub-regional scale studies as opposed to encompassing the entire 1,000-km-wide platform from the clastic shoreline to the oceanic margin. Moreover, as already stated, the study for PDO was carried-out in order to provide notions of facies associations and facies substitutions for individual reservoir units of Fahud field. At these scales, it becomes necessary to distinguish between a number of scenarios under which different facies associations were deposited. Given the levels of detail and of the required precision to establish the spatial distribution of facies in static reservoir models for the Fahud field, a number of different facies models were found to be necessary to represent the successive depositional systems that governed the accumulation of the formation. In turn, these different facies models each comprise different facies associations or groups of facies linked to a specific geobody or depositional assemblage. This is in contrast to the single composite depositional profile or facies model commonly presented at the scale of a stratigraphic formation such as the Natih (e.g. van Buchem et al., 1996).
Comparison with Modern Analogs
When building a picture of the Natih Formation, the facies and their associations are compared with modern depositional environments to enhance or to contrast the notions of sedimentary systems involved when composing the facies models. In terms of modern analogs, two main but very different settings are commonly used for the Natih. These are the eastern Australian platform, extending to the Great Barrier Reef, and the shallow, muddy, Florida Bay, barred by the Florida Keys. Obviously, small-scale features such as meandering tidal channels, coarse barrier bars, beaches and swash zones, and deeper-water settings dominated by bioturbation, such as recorded in the Natih, are found to be widely represented in both these two analogs (Figures 44 and 45a).
However, the stark contrast between the pictures of the successive Natih environments and that of any modern eastward-facing, tropical to equatorial setting becomes clear at the broader scales. The first mismatch is that of scale, with the 1,000-km-broad Natih platform dwarfing any modern contender (Figure 45). Secondly, no analog compares usefully with the intra-shelf basins that developed on the broad Natih platform, specifically with the abundant deposits of organic-rich sediments. Thirdly, in terms of the environment, the atmospheric and oceanic chemistry of the Cretaceous greenhouse world was significantly different from today. The higher-frequency sea-level fluctuations in the Natih are found to be of unexpectedly high-amplitude for this greenhouse time, when ice-cap growth is considered to have been negligible. These fluctuations are of course much less than the more than the 100-m glacially-driven variations in sea-level that have punctuated the Quaternary. However, rapid and major fluctuations of climate cannot be ruled out from among the constraints on the Natih system. These specificities gave a particular signature to the Cretaceous environments, colloquially termed the “Cretaceous Mood” by Peter Skelton (personal communication, 2005).
The Changing Scene During Deposition of the Natih Formation: Multiple Facies Models
Both the observation of different facies at similar stratigraphic positions but in different locations (e.g. lower part of the Natih E at Jabal Madmar and at Jabal Madar), and the mapping-out of clinoform units on seismic images show that intra-platform basins developed on the Natih platform and then were progressively more-or-less filled-up (Enclosure IIA-4). As demonstrated by previous studies (van Buchem et al., 1996, 2002) this mode of intra-platform basin development and infill occurred in both the lower part of the Natih, during Sequence I (Natih E) and in the upper part of the Natih during Sequence III (Natih B and A).
Facies differentiation between times of basin development and basin infilling is clear. Expansion of the intra-platform basin led to stratified water bodies at the foot of very low-angle ramps, with accumulation of kerogen-rich chalks. In contrast, basin infill was caused by the progradation of 40–80-m-tall, low-angle to steeper-angle clinoform sets with shallowing- and coarsening-upward, progressively grainier facies. Study of the late Sequence I and early Sequence II (upper Natih E and lower Natih D, Grélaud, 2005; Grélaud et al., 2006) has shown that higher-frequency cycles superimposed on the longer-term cycle of basin development and infilling also show strong facies differentiation. Bedrock incision during emersion of the platform, together with coeval progradation of wedges of higher-angle clinoforms on the platform margin, occurred during times of strongly decreasing or even negative accommodation on the platform-top.
At this higher-frequency scale, aggradation of peritidal deposits, meandering tidal channels and inter-to supra-tidal mudflats on the platform-top, coeval with progradation and aggradation of clinoform sets on the margin, occurred during times of higher accommodation.
In contrast to Sequences I and III, there is no record of intra-platform basin development during Sequence II (Natih D and C). Both outcrop data and the interpretation of seismic images indicate a very flat depositional profile across the whole of the studied area. Mudstones with pelagic foraminifera in both the lower parts of the D and C members attest to repeated open-water and fully-marine conditions, but paradoxically no slope deposits, nor slope geometries, can be located between the deeper-marine and littoral sediments of these units. Terrigenous clays, littoral grainstones and lime-mudstone facies with planktonic micro- and nannofossils record an alternation, at higher-frequency, between shallow to emersive environments and lagoonal conditions albeit open to the ocean influx.
There is no evidence in the study area for any barrier bar, build-up or reef development throughout Sequence II, before the very top of the upper Natih C. However, the low-energy and low-relief characteristics of the Natih C and D cycles suggest that such a barrier may well have existed further away (to the north and the northeast), at the main oceanic margin to the Natih platform. The development of high-energy rudist shoals, banks and channel fills at the top of the Natih C is interpreted to result from breaching this hypothetical marginal barrier at the time of maximum accommodation, allowing high-energy conditions to reach across the shelf. The lack of major development of shallow-marine skeletal deposits, in spite of the record of shallow-marine to littoral environments, is attributed to inhibition of this carbonate factory by the abundance of terrigenous clays in the lower parts of the Natih D and Natih C.
The consequence of this variety of successive scenarios [(1) transgressive and regressive patterns for Sequences I and III; (2) transgressive and regressive patterns for Sequence II; (3) scenarios of platform emersion near the I-II sequence boundary, and (4) tectonically-forced regressive wedges in Sequence III] is that a number of separate sedimentological facies models (at least two for each scenario in order to cover transgressive and regressive facies associations) would be necessary to convey the appropriate information for static model building.
The hundreds of unnamed participants on successive work-sessions, fieldtrips and training courses over some 15 years have contributed significantly and in many ways to the story told here. We are grateful both for their company and for their input, criticism and support. Frans van Buchem and Anne Schwab have been major co-investigators on many of the successive projects on the Natih Formation, contributing significantly to our understanding of the Natih. Anne played the leading role in setting up the seismic modelling effort that has played a major role in constraining the sedimentological debate on interpretations of depositional environments, and Frans has carried out the major editing effort and leading authorship on many previous papers. More recently, at the Carbonate Centre in Muscat, Younis Al Toubi and Rheda Al Lawatia helped to start studies linking outcrops and subsurface data of the Natih. At the Centre, Wim Swinkels was a staunch supporter of the studies on the Natih, and he constantly provided critical but generous advice from “the other point of view”. Asma Al Saidi was most helpful in keeping our computers going during her time as IT Assistant.
The Badley Ashton team as a group, and in particular Quintin Davies and Boris Kostic, freely shared their data and ideas on depositional environments of the Natih from their work on the Fahud cores. The Fahud Studies Group at the Petroleum Development Oman (PDO) Studies Centre was also most supportive, with Foppe Visser at their head, and Rick Singleton, Wadie Mansour and Alia Bahri, in particular. In PDO, Jeroen Peters, Mark Shuster and Henk Rebel were most supportive of our work. Shuram LLC, under the guidance of Salim Al Maskiry, has always provided reliable and highly appreciated logistic support during the work.
The JVRCCS Carbonate Research Centre at Sultan Qaboos University (SQU), a joint venture between Shell and SQU, was an exceptional environment within which to carry out the outcrop analog study for PDO’s Fahud Studies Team. We thank the Executive Committee of the JVRCCS, the Shell Representative Office Oman and SQU, for their approval and support of this study, and we are most grateful to PDO Exploration for providing targets, milestones, focal points and finance. Many companies have supported the studies over the years on the Natih, from both the training and research perspectives, and fortunately both perspectives have often come together. In particular we thank Elf (now Total), PDO, Shell, Schlumberger, ADCO, Oxy, Dubai Petroleum Company and Statoil, both for their research commitment and for their financial support.
We wish to thank the Ministry of Oil and Gas, Sultanate of Oman, and PDO, for permission to publish the subsurface data referred to in this study. The comments by GeoArabia’s editors and two anonymous referees are greatly appreciated. The final design and drafting by GeoArabia Graphic Designers Arnold Egdane and Nestor “Nino” Buhay IV are also appreciated.
ABOUT THE AUTHORS
Peter Homewood is an independent consultant on Geosciences and Reservoir Characterization. He was Director of the Shell-endowed Carbonate Studies Centre at Sultan Qaboos University in Oman, and Professor of Carbonate Geology there, from 2001–2005. Peter was Senior Advisor for Sedimentology, Elf EP and TotalFinaElf (both now Total) between 1988 and 2001. He obtained his PhD (1973) from Lausanne University in Switzerland. Peter was editor of IAS Journal “Sedimentology” (1986–1990), IAS Publications secretary (1990–1994), and AAPG European Distinguished Lecturer (1998–1999). He received the Elf Science Prize (1995), the TotalFinaElf communications award (2000), and the Canadian Society of Petroleum Geologists (2000) Best Paper Award in 2001. Peter was Chairman of the 24th Meeting of the IAS in Muscat (2005).
Philippe Razin is Professor of Geology at the University of Bordeaux, France. He received his Doctoral of Science degree from the University of Bordeaux in 1989 and joined the Bureau de Recherches Géologiques et Minières (BRGM) as an expert in sedimentology and basin synthesis. Philippe was involved in various projects (mapping, water and mineral exploration, geotechnics, 3-D modeling). He moved to the University of Bordeaux in 1997, where he teaches sedimentary and structural geology, geodynamics and field mapping. His research activities concern relations between tectonic and sedimentation, in collaboration with BRGM, IFP, IFREMER and oil companies.
Carine Grélaud is currently an Assistant Professor at the University of Bordeaux. Carine received her PhD in 2005 from the University of Bordeaux, France and obtained an MSc in Petroleum Geology in 2002 from Aberdeen University, UK. Her research concerns the characterization of carbonate reservoirs by the integration of geologic and seismic data. During her PhD, she was working in the Carbonate Research Centre at Sultan Qaboos University, Oman. In this context, she participated particularly in research projects, with Petroleum Development Oman, on the high-resolution sequence stratigraphy of the Natih Formation in the Fahud field, for reservoir modelling purposes. This study was based on the integration of several data sets: outcrop correlations, seismic reflection interpretation, well data, core analysis and production data.
Henk Droste is Senior Geologist with Shell Technology Oman. He was previously employed by the JVR Centre for Carbonate Studies at Sultan Qaboos University, Oman. Henk has an MSc in Geology from the University of Amsterdam, The Netherlands. He joined Shell in 1984 and has been working in The Netherlands, UK and Oman as a carbonate research geologist, regional review geologist, seismic interpreter, production geologist and Head of Geological Services. His main research activities are regional geology in the Middle East and carbonate sequence stratigraphy and reservoir characterization.
Volker Vahrenkamp is a Principle Geologist for the Studies Centre of Petroleum Development Oman (PDO). He holds a MS in Engineering Geology from the University of Michigan (1983), and a PhD in Carbonate Sedimentology and Geochemistry from the University of Miami (1988). Volker specializes in the 3-D modeling of matrix and fractured carbonate reservoirs and geochemical stratigraphy. He has been PDO’s focal point for carbonate technology development, and served in the steering committee of Shell’s carbonate technology development team in The Netherlands. Volker has co-designed and instructed carbonate courses and core workshops for PDO, Shell, AAPG and GEO Conferences. He previously worked for Sarawak Shell Berhad, Malaysia and Shell Research Laboratory, The Netherlands.
Monique Mettraux is an Independent Consultant on sedimentary and chemical geoscience. Recently, Monique has been working with the department of Production Chemistry of Petroleum Development Oman (PDO) on Formation Damage Prevention. She has also been working as a professional training instructor, in particular for PDO, IFP, IAP, Sonatrach and Petrobras. She received her PhD in 1988 and her MSc in 1983 at Fribourg University in Switzerland. Between 1992 and 1998, Monique has worked on a wide range of industrial projects with a focus on carbonates (for Andra, Elf Aquitaine and Gaz de France). Between 1988 and 2007, Monique maintained her academic research activity by working on projects with Universities of Rennes, Strasbourg, and Dijon in France, and with Sultan Qaboos University in Oman. Her research interests focus on the sedimentological and geochemical aspects of sedimentary rocks, mainly carbonates.
Joerg Mattner is a Geoscience Consultant and the Executive Editor for the journal GeoArabia. In 2001 he founded GeoTech in Bahrain, an independent Geoscience Consultancy for Middle East geology and hydrocarbon reservoirs. Joerg received his PhD in 1990 from Clausthal University, Germany. During his studies and subsequent teaching assignment, he worked on geological projects in Europe, South America and Northern Canada. In 1990, he joined the Petrophysical Evaluation Group of Western Atlas in London. Subsequently Joerg moved to Syria, and established a log analysis center. In 1994 he became Chief Geologist for the Middle East and opened Western Atlas’ regional Geoscience Center in Bahrain. Through mergers he joined Baker Hughes, and in 2000 took up an assignment as Director of Marketing and Business Development with Western Geophysical. Joerg’s special interests are structural geology and fractured reservoir characterization. He was member of the GEO 2000, 2002 and 2004 Conference Technical Committees.