ABSTRACT

Early Tertiary sediments are widely exposed in Qatar and only in coastal areas and a few places inland do Holocene deposits mask them. The variable resistance to weathering of the Tertiary sediments is responsible for the general low-relief landscape interspersed with flat-topped hills, the most prominent of which are in western Qatar where they are the surface expression of the Dukhan anticline. The early Tertiary was a time of shallow-marine sedimentation and several transgressive and regressive cycles occurred. Sedimentation began on a dolomitic carbonate shelf, which gave way to a mixed carbonate-siliciclastic shelf that became increasingly calcitic. Coarsening-upward sequences with fine-grained muddy sediments at the base and grainstones and boundstones on top attested to a cyclic change in the energy regime. Small-scale cycles and major unconformities resulted from eustatic sea-level changes. In the Dukhan study area, the most prominent sequence boundary was near the Ypresian-Lutetian boundary. Following this hiatus, a Lutetian transgressive system tract evolved that was terminated by a maximum flooding surface correlated with the named MFS Pg20. Minor unconformities and NE-trending faults of post-Miocene age resulted from the interplay of local diapiric salt movements in the Dukhan anticline and a regionally changing stress field. Extensional faulting was succeeded by a compressional phase that caused the reactivation of some normal faults as steeply dipping reverse faults.

Diagenetic processes and the pervasive etching of the landscape formed ferrous, siliceous, and gypsiferous duricrusts. Phosphate was reworked and concentrated above the Ypresian-Lutetian unconformity. Ferricretes were derived from iron-bearing phyllosilicates and disulfides. Ferric iron-oxide hydrates were the result of oxidizing conditions but their parent material furnished evidence of less-oxygenated conditions and a rising sea level during the Lutetian transgression. The studies provided information on the economic potential of aggregates, clay, hydraulic binders, and building stones, and the prediction of subsidence-prone areas.

INTRODUCTION

The Qatar Peninsula projects northward into the Arabian Gulf as the surface expression of the Qatar Arch (Figure 1). The Peninsula is of low relief and is characterized by flat-topped hills or mesas that rarely exceed 100 m in altitude. The low relief and the few field-based investigations of the structural geology and lithology led previous workers (e.g. Sugden, 1962; Noweir and El-Kassas, 1988; Al-Hinai et al., 1997) into thinking that there had been little structural disturbance in Qatar during the recent geological past (Cavelier, 1970). The Cenozoic structural geology is, however, by no means simple and studies of the more recent structural evolution of the Arabian Gulf Region make reference to hydrocarbon migration and accumulation (Vaslet et al., 1991; Weijermars, 1998, 1999a,b;Stenger et al., 2003).

Our study was the lithological and structural evolution of the Dukhan anticline, a major hydrocarbon-bearing structure in western Qatar (Figure 1). The work provided an insight into the interaction between lithology and structural evolution during the early Tertiary. Close attention was paid to the nature of the bounding surfaces of depositional sequences and their distribution in the sedimentary record, together with marker lithologies and geometry. Such bounding surfaces denote chronological events that sometimes tell more about the evolution of the geological successions than the beds themselves (Ollier and Pain, 1997; McNeil et al., 2001).

Based upon these lithological and structural analyses, we correlated the early Tertiary strata of the anticline with charts of worldwide eustatic sea-level changes. The principles for these eustatic and tectonic charts were elaborated by Vail et al. (1977, 1991) and van Wagoner et al. (1988), who defined various criteria for the recognition of the depositional sequences and their sequence boundaries and correlative conformities. On a regional scale, chronostratigraphic and lithostratigraphic sections in Sharland et al. (2001) were checked for the correlative value and used for stratigraphic fine tuning in the study area.

The early Tertiary sedimentary series have been marginalized as not worthy of in-depth studies. However, although they do not contain hydrocarbons they may be of interest to exploration geologists in search of industrial minerals (see Qatar General Petroleum Corporation and Amoco Qatar Petroleum Company, 1991). In this respect, our detailed geological mapping and litho-sequence stratigraphic interpretation created a framework for the geologic setting of non-metallic deposits, such as cement stones and drilling mud materials, and the detection of karst-induced sinkholes. This sort of information is essential in the economic development of a country (Harben and Kuzvart, 1996). The survey may also help tackle some lithological and structural problems related to the uplift of the Dukhan anticline, which is host to one of the major oil fields in the Gulf.

Geological Setting

The Dukhan anticline on the west coast of Qatar (Figure 1) is host to the most important on-shore oil field in Qatar. It has been the subject of numerous investigations by engineers, stratigraphers, and paleontologists (Qatar General Petroleum Corporation and Amoco Qatar Petroleum Company, 1991; El Beialy and Al-Hitmi, 1994; Hewaidy and Al-Hitmi, 1993a,b,c, 1994, 1999; Al-Saad and Ibrahim, 2002; and other literature cited).

Figure 2 outlines the sedimentary units exposed on the Qatar Peninsula and Figure 3 shows the geological setting of the study area in the northern part of the Dukhan anticline. A more detailed picture of previous stratigraphic studies carried out in the Dukhan area is given in Table 1. The table illustrates the stratigraphic subdivision of the early Tertiary rocks of the Dukhan anticline and provides an overview of the nomenclature applied to the Eocene successions in Qatar. An Eocene age has been assigned to the sedimentary rocks of the Rus and Dammam formations that are exposed in the uppermost parts of the anticline (Cavelier, 1970; Abu-Zeid and Boukary, 1984). The Middle Eocene Dammam Formation is succeeded by the Miocene Dam Formation and they are separated by an Oligocene hiatus in the sedimentary record (Al-Saad and Ibrahim, 2002). Our present level of knowledge of the early Tertiary sediments that roof the Dukhan anticline was provided mainly by a transect through Jebel Dukhan (Cavelier, 1970; Abu-Zeid and Boukary, 1984; Boukary, 1985).

Methodology

Our study takes an approach that is different from that of previous workers, for example Cavelier (1970), Abu-Zeid and Boukary (1984), and Boukary (1985), by emphasizing both lithology and structural geology. Much of our work was field based. The objective was not to go through the Jebel Dukhan section again and refine the stratigraphy, but to provide easy-to-read and easily accessible maps and cross-sections that may be helpful in the search and exploitation of industrial minerals.

Fieldwork was based on 1:10,000 scale mapping, measurements at more than 160 stations, and various traverses across the Dukhan anticline. The geological and contour maps (Figure 3) were created on a digital elevation model that was converted into a grid file by the Centre of GIS, Doha. The sedimentary succession was divided into four stratigraphic units (Figure 4) that are traceable throughout the study area and allow for a comparison of facies, genetic interpretations, and correlation by means of sequence stratigraphy (Vail et al., 1977; Haq et al., 1988; van Wagoner et al., 1988). The result of bulk chemical and mineralogical analyses are given in tables 2 and 3 (see Results, Chemistry and Mineralogy), and photomicrographs illustrating the microfacies are shown in Plate 1.

RESULTS

Lithostratigraphy

The early Tertiary sedimentary rocks in the study area were subdivided into four major units based on lithological and structural characteristics (Figures 3 and 4 and Table 1). Figure 4 is the reference log for the Eocene succession. The field units were coded with Roman numerals and subunits were designated with additional lower case letters. Figure 5 is a S-N transect along the Dukhan anticline using the top limestone of unit IIb as a datum to show the thickness variations in the various units (for more details of the lithology, see reference section Figure 4).

Figure 6 shows the geomorphological response to variations in lithology for units I to III. ‘Staircase morphology’ reflects the coarsening-upward sequences of soft marls and chalky limestones overlain by hard, resistant limestones. Strong silicification of unit III formed the cap to the mesa. In unit I, marl- and chalky limestones are recessive, whereas the more resistant limestones form ledges.

Unit I (Lower Chalk Member)

Unit I consists of two subunits. The lowermost subunit is yellow-brown soft dolomitic marl (unit Ia). The marl forms the gently dipping slopes at the base of the mesas and underlies the depressions between isolated mesas. Soft sediments of unit I crop out in the central part of the study area. They are disconformably overlain by cross-bedded Quaternary calcarenites to the west, and conformably overlain by well-bedded limestones and marls of unit II to the north and east (Figure 7). Nodular chalky limestone up to 1 m thick was used as a marker horizon on the western flank of the anticline. A further subdivision of unit Ia was made into subunit Ia1 beneath the nodular limestone and subunit Ia2 above it (Plate 1a; Figures 8a,b). The dolomitic rocks of unit I were originally biomicrites and pelmicrites (Plate 1b). Their granular to saccharoidal texture resulted from strong dolomitization but this did not totally extinguish the original texture. Greenish argillaceous interbeds occur within the marls of subunit Ia1.

In unit Ib, coarser-grained calcareous rocks are more widespread than in the underlying unit. Unit Ib limestone stands out as a ledge, sheltering the underlying less resistant rocks from erosion (Figure 9). Common laminoid fenestral porosity gives the limestone an outward stromatolitic appearance. However, the structural and textural characteristics of unit Ib are less well pronounced than in equivalent limestones higher in the stratigraphic column (Figure 4; see Unit IIb). Concretions of chalcedony and clusters of low-quartz/beta-quartz crystals that resulted from recrystalization of chalcedony are present in the upper half of the unit (Plate 1c).

Unit II (Upper Chalk Member)

Chalky limestones and soft marl that overlie unit Ib belong to unit IIa of the Upper Chalk Member and are very similar in outward appearance to unit Ia. In contrast to the lithologically equivalent Lower Chalk Member, the Upper Chalk is devoid of greenish argillaceous interbeds. On the geological map it has not been shown as a separate unit for reasons of scale. It becomes covered with debris from rock falls as the angle of slope increases.

Overlying unit IIa is thick, hard limestone of unit IIb. The well-bedded limestone constitutes a large step that is easily recognized across the study area and can be used as a datum in stratigraphic sections (Figure 10). Apart from unit IIIb, it is the most prominent stratigraphic unit. Its competent limestones form steps on the slopes of the mesas and escarpments because of their high bearing capacity and resistance to weathering. In places, the limestones of unit IIb are thinly bedded in laminae of less than 10 mm. Taking into consideration bed thicknesses and the uneven wavy bedding, the calcareous rocks may be designated a stromatolitic limestone in the field.

Microscopic textural analysis substantiates the designation as stromatolitic limestone. According to Cole and Picard (1975) uneven algal-type stratification may be recognized and the laminar fenestral fabric known from shallow-water limestones is widespread. The rocks are transitional from biomicrosparite to biolithite (Folk, 1959), or packstone to boundstone (Dunham, 1962). Stromatolitic layers (algal mats) alternate with layers of shell debris composed of abundant small foraminifers, mollusks, and ostracodes (Plate 1d). Unit IIb is thickest in the central part of the anticline (Figure 5).

Unit III

Unit III consists of two subunits. Unit IIIa is composed of fine-grained limestone and argillaceous marl (Figure 11). It is thickest in the southern part of the anticline (Figure 5). In the north and center it is concealed by debris from the overlying unit IIIb limestones, or its thickness is drastically reduced to a few meters or so. Gypsum and hematite and goethite concretions are widespread. Gypsum forms a network of crystals or occurs as isolated platy encrustations.

Unit IIIa passes vertically upward into whitish nodular limestone rich in gypsum that was attributed to unit IIIb for genetic and mapping purposes (Figures 12a,b). Due to its hardness, unit IIIb limestone forms a large platform that dips gently eastward away from the center of the anticline (Figure 3). Gypsum is common in unit IIIb calcareous rocks filling voids and pores in limestone that underwent strong karstification (Figure 13).

Unit III is noticeably less dolomitized than the underlying units (Table 2) and its lowest dolomite/calcite ratio is in unit IIIa. Calcareous rocks of unit III cover the full spectrum of lithofacies from oosparite to pelmicrite. Tests of large foraminifers and bivalves are well preserved and abundant in limestones of unit III (Plate 1e,f).

Unit IV

Unit IV is exposed in one isolated mesa in the northeast of the study area where dolomitic marls of unit IVa are overlain by well-bedded limestone of unit IVb (Figure 14) that dips at a low angle toward the northeast. The limestone contains abundant gastropod and bivalve molds. The calcareous rocks of unit IV have the lowest dolomite content of any unit in the series (Table 2).

Chemistry and Mineralogy

The bulk mineralogical and chemical compositions of the Early Eocene sedimentary rocks studied are presented in Tables 2 and 3, respectively. In order to study the non-carbonate mineral assemblage in the calcareous rocks, the samples were ground and leached with HCl. The mineral content of the insoluble residue is fairly constant, consisting of quartz, gypsum, anhydrite, feldspar, illite, and palygorskite. Of particular interest is palygorskite, a phyllosilicate that has a fibrous or lath-like crystal structure (Weaver, 1989). It had previously been recorded only from Middle Eocene beds in northern Qatar (Holail and Al-Hajari, 1997). The present study has shown that palygorskite is a common phyllosilicate in the Early Eocene units I and II, and abundant in the Middle Eocene unit IIIa up to the boundary with the overlying unit IIIb (Table 2). It has not been found in units IIIb and IV. In unit I, it is associated with illite. In unit IIIa, palygorskite is the most common mineral in the HCl-insoluble residue and may be as much as 20 percent by volume.

Structural Geology

Lithostratigraphic fieldwork involved measurements at more than 160 stations. The mapping refined the structural evolution of the upper part of the Dukhan anticline. The contour map for the top of unit II (Upper Chalk Member) shows an approximate north-northwesterly strike of the Dukhan anticline in the northern part of the study area (Figure 3). Farther south, NE-striking faults offset the axis in a southerly direction. The rocks dip at angles of between 2° and 12°. The dips of the anticlinal limbs at the surface do not change significantly with variations in the dip azimuth. Only in the southwest is there a slight increase of the dip angle to as much as 15°.

A set of shear joints is well developed in the mechanically brittle unit Ib limestone but they die out in the underlying mechanically ductile chalky marl of unit Ia2 (Figure 15). The argillaceous rocks of unit IIIa are also mechanically more ductile and are therefore less prone to fracturing. The shear joints cause the blocky structure of the unit Ib limestone. The spacing of joints averages 0.3 to 0.5 m and they are aligned parallel to the NE-striking fault system. Fractures and joints that are parallel to the bedding planes are host to fibrous gypsum (selenite), and quartz and chalcedony is concentrated near the NE-striking faults.

The orientation and apparent displacement along the major NE-striking faults shown in Figure 3 were deduced from the surface mapping. They were inferred to have caused the topographic depressions between mesas. Vertical displacement on the fault planes is generally less than 10 m but, in the southwest, up to 20 m of vertical displacement was observed on one fault (Figure 3b). The interpretation of satellite images showed that a few NE-striking faults in the central part of the study area may be traced northeastward into the overlying Dam Formation (Noweir and El-Kassas, 1988). This set of faults was therefore active in or until the post-Early Miocene. Some other less-prominent NE-striking faults in the southern and northern parts of the area cannot be traced into the stratigraphic younger Dam Formation. A single NW-trending fault may be present in the northwest of the study area (Figure 3). In some localities, the thinly bedded limestones of unit IIb are cut by widely spaced reverse faults. The reverse dip displacement along the faults ranges from a few centimeters to several decimeters (Figure 16). The faults cannot be traced far into the overlying and underlying calcareous rocks.

Duricrusts

In the southern and eastern parts of the Dukhan anticline, platforms and terraces developed on units IIb and IIIb have a dense covering of cobbles and pebbles of iron-bearing siliceous material (Figure 17). The relative abundance of silicification throughout most of the units is demonstrated in Figure 4, but no silicification is present in Unit I. Some irregular siliceous concretions as much as 0.5 m in size have developed in the upper chalk of unit IIa from above. In areas where the stratabound silicification has been intense, chalcedony (cryptocrystalline silica) may fill fissures and joints as much as 5 cm wide (see Figure 3b).

Phosphatic concretions are scarce and confined to the unit II/III boundary (Abu-Zeid and Boukhary, 1984) (see P2O5, Table 3). Iron in silcretes gives a dark-brown luster to the siliceous duricrusts. Where iron is present in abundance it forms ferricretes, fragments of which litter the slopes and outcrops of unit IIIb. Gypsum is abundant in unit III (see SO3, Table 3) near the contact between unit IIIa and IIIb. It occurs principally in a massive duricrust that is composed of a network of gypsum crystals or lustrous isolated crystals.

DISCUSSION

Lithostratigraphic Correlation

Units I to II may be stratigraphically correlated with the upper part of the Rus Formation (Cavelier, 1970; Abu-Zeid and Boukary, 1984) (Table 1). Unit IIb is assumed to be equivalent to the Khor Limestone and the overlying Fhaihil Velatea Limestone. To place the Rus/Dammam boundary between the two lithologically similarly limestones is not constructive from the point of view of geological mapping. The Midra (or Saila) Shale is equivalent to unit IIIa in the stratigraphic sections of Cavalier (1970). The term ‘shale’ is, however, misleading in the area under consideration as unit IIIa contains a significant proportion of calcareous sediments and could be designated as well-bedded marl. The Dukhan Alveolina Limestone of Cavelier (1970) may be correlated with the nodular limestone of unit IIIb and the Simsima Dolomite/Limestone with the ridge- and plateau-forming karstic limestone lying directly above. Unit IV is stratigraphically equivalent to the uppermost part of the Dammam Formation—the Abaug Member of Cavelier (1970).

Lithological Evolution and Environment of Deposition

The Early Eocene Rus Formation consists of three distinct coarsening-upward cycles. Their common lithological character and the overall variation in grain size toward younger members indicate a shallowing-upward trend in the basin throughout the time of deposition of the Rus Formation (Figure 4). Toward the top of each sequence, coarse-grained limestones (grainstone) become more dominant at the expense of the calcareous mudstones and argillaceous marls of the lower half.

The coarsening-upward sequences in units I and II are monotonous shelf limestones laid down on a shallow dolomitic carbonate shelf. The predominant carbonate mineral is dolomite that replaced calcite in the various limestones studied (Table 2). A muddy basal facies with predominantly micrite/mudstones that is indicative of a lower energy regime is gradually replaced upward by cleaner and coarser-grained high-energy limestones. The high-energy lithofacies is consistent with accumulation under relatively shallow conditions. The stromatolitic character (or laminoid fenestral texture) at the top of each sequence tends to increase within units I and II (Plate 1d). They were best developed during the deposition of the well-bedded stromatolitic limestone of unit IIb in an intertidal to lagoonal environment (Szulc, 1997; Seong-joo et al., 2000; Braga and Martin, 2000).

The water depth varied markedly during the deposition of unit Ib and unit IIb limestones. Layers rich in small benthic foraminifers alternate with those in which algal mats and ostracode shell debris are abundant. The algal mats and shell debris reflects very shallow-water conditions, whereas the benthic foraminifers are indicative of deeper water resulting from marine incursions that closely resemble tempestites (Plate 1d). Together with the change in the lithofacies type of carbonate rocks, silica became more abundant in the uppermost limestones of each cycle from unit Ib to unit IIb (Figure 4).

Locally in the coarsening-upward sequences of unit I, stacked patterns made up of cosets of argillaceous and marly rocks occur. These minor cycles pass upward into clean yellow marls (Figure 8a). The sharp basal contact of the lowermost argillaceous horizon marks the sudden onset of a new phase of deposition. These breaks in sedimentation have only local significance and cannot be traced throughout the area under consideration. The crinkled upper argillaceous band seen in some outcrops is thought to have resulted from differential compaction rather than disruption caused by drying out or emergence (Figure 8b). Argillaceous limestone interbeds are commonly affected by sedimentary boudinage that may have contributed to the formation of ball-and-flow structures and the unevenly bedded nodular limestones (Figures 11, 12). This was the result of differential compaction with the more argillaceous beds undergoing compaction much more easily than the cleaner calcareous beds.

Unit III is built up of three coarsening-upward cosets that closely resemble others of the Dukhan anticline. However, unit III strongly contrasts with the underlying coarsening-upward successions in terms of their mineralogical and chemical compositions (Tables 2 and 3). For example, unit IIIa contains more argillaceous beds than the underlying equivalent basal subunits. Consequently, this part of the stratigraphic section can be called a mixed calcitic to dolomitic siliciclastic-carbonate shelf.

The shelf’s lithology and biodata reflect the strongly fluctuating physicochemical conditions that occurred during deposition. The calcite content increases at the expense of dolomite. Iron oxide pseudomorphs after pyrite in unit IIIa are clear evidence of reducing conditions in the basin and also show that the water level was much higher than in subtidal/dysaerobic units I and II. On the other hand, layers near the top of unit IIIa that contain abundant gypsum are witness to strong evaporation and oxidizing conditions in shallow-marine to possibly supratidal zones. However, the alternating successions of evaporites, dolomites and fine-grained calcareous sediments that are typical of a sabkha environment (Chan Min Yoo and Yong Il Lee, 1998; Duane and Al-Zamel, 1999; and Al-Sulaimi and Mukhopadhyay, 2000) did not develop in this area during the early Tertiary. No indications of a true sabkha facies were seen in drill cores or outcrop (Boukary, 1985). Further deepening of the basin may be inferred from the biodata and lithofacies in unit IIIa (Plate 1e). However, by the end of unit IIIb time, the increased sulfate concentration suggested another lowering of sea level. It was especially during the initial stages of deposition of unit III that the basin received detrital matter in excess of the amount of carbonate produced by organic and inorganic processes.

The small amount of gypsum in unit IV shows that the marine conditions were consistent with a carbonate-shelf environment. Changes in the types of phyllosilicates through the vertical succession (as compiled in Table 2) support the environment analysis. Mineralogical and geological studies on palygorskite elsewhere have indicated that this chain-structured phyllosilicate is a valuable tool in identifying the climatic and bathymetric conditions of the depositional environment (Weaver, 1989; Colson et al., 1998; Pletsch, 1998; Pletsch et al., 1996, 2000). It is reported to be a common constituent of deep ocean sediments. On the other hand, the fibrous variety of palygorskite is also known to form in peri-marine environments under semi-arid climatic conditions. In the studied area, palygorskite is assumed to have been formed during the Early Eocene in a lagoonal to tidal environment under schizohaline conditions on a shallow dolomitic carbonate shelf, as was the case during deposition of sediments of units I and II. The phyllosilicate was reworked during the Middle Eocene after a significant break in sedimentation and concentrated in deeper parts of the sea on a mixed calcitic to dolomitic siliciclastic-carbonate shelf that is represented by unit IIIa (Table 2). Following this major re-distribution of palygorskite at the Ypresian-Lutetian boundary, the climatic and bathymetric conditions on the carbonate shelf during the deposition of unit IV were no longer favorable for the development of authigenic palygorskite. Moreover, the source area had already been so depleted of palygorskite that no allogenic concentrations of the mineral could be formed.

Updoming and Sequence Stratigraphic Correlation

Sequence breaks

The coarsening-upward successions are assigned member status and in terms of sequence stratigraphy they correspond to system tracts (Figure 4). Several unconformities of different magnitudes occur in the early Tertiary succession in the upper part of the Dukhan anticline. In the southernmost sector, the beds of unit Ib limestone are, in places, inclined to the plane of the unconformity at about 25° and the dip azimuth is approximately N160°E. It is ranked as a minor unconformity of local importance and points to an Eocene or intra-Rus tilting of unit Ib limestones that was possibly related to tectonic activity along the NE-trending faults. Regionally the Rus contains evaporites but no occurrences are known from the study area, so dissolution and collapse may be ruled out as a cause for these structural disturbances. In the northern part of the study area such an unconformity cannot be established between units I and II.

Another gap in the stratigraphic record is an angular unconformity (Figure 18) of about 5° between units IIa and IIIb in the central part of the anticline and about 2° elsewhere. Unit IIIa has a maximum thickness of 6 m in the southern half of the study area but, to the north, its thickness cannot be determined precisely because of detritus from the overlying Simsima Limestone. According to the reference lithologs in Figure 4, a reduced rate of sedimentation or even non-deposition may be suspected in some places. The upper surface of top unit IIIb (Simsima Limestone) has been recognized as another disconformity. Both units IIb and IIIb form prominent topographic platforms or steps. The platform of unit IIIb is littered with silcrete nodules and is strongly karstified with many examples of rillenkarren. Efflorescence of gypsum is widespread on this disconformity surface.

The vertical grain-size profile of early Tertiary units of the Dukhan anticline shows a pronounced cyclicity. Each cycle consists of soft, fine-grained rocks at the base that give way to compact limestones on top (Figure 4). The Eocene cycles are interpreted as being genetic increments, each reflecting a shallowing-upward trend in sedimentation on the northeastern margin of the Arabian Plate.

Eustatic sea-level changes

In this section we will discuss the reasons for local Eocene sea-level changes in Qatar; and how they are correlated with variations in sea level on a worldwide and regional scale.

The small-scale cyclicity and unconformities within the succession have resulted largely from eustatic processes. Sequence boundaries reflect major falls in sea level that were probably enhanced by tectonism causing some minor local unconformities. Close to the boundary between the Rus Formation and the overlying Dammam Formation, eustatic processes played an important role and led to the formation of major sequence boundaries SB1 to SB3 (Figure 4). Of lesser significance were processes of this kind that occurred during the deposition of the Rus Formation. The geometry of sedimentary facies and the vertical sequence of grain sizes allowed precise boundaries to be drawn between some of the units but did not allow the nature of the boundaries to be determined with the same degree of certainty (Figure 4).

Concretions

What does the stratabound non-calcareous mineralization tell us about the nature of the sequence breaks and the depositional environment in the immediate surroundings of the bounding surfaces/sequence boundaries? Sarg (1988) discussed examples of carbonate dissolution and various types of cementation, together with the development of sequence boundaries in carbonate rocks. Silicification is ubiquitous in calcareous rocks of unit IIb/IIIb, with two peaks in the distribution curve (Figure 4). It is presumed that silica was derived from the decay of sponge spicules and other siliceous organic debris (Plate 1b) that accumulated with calcareous bioclasts in a shallow-marine depositional environment.

The complex physicochemical processes that led to the formation of the distinctive concretions are difficult to constrain (Seilacher, 2001). Siliceous concentrations may have occurred when the sediments underwent diagenesis and epigenesis. One may, however, also invoke the model of an etched landscape and consider at least part of the silica concentrations to be silcretes in the sense of Thiry and Simon-Coincon (1996), Nash and Shaw (1998), and Bustillo and Bustillo (2000). These models include silcretes that are the result of pedogenic processes and those that formed in zones of groundwater outflow and water table fluctuations. The silica pavements on the benches of unit IIb and unit IIIb limestones may also be explained by an erosional landscape. This model conforms to the sequence boundaries established in this part of the stratigraphic section (Figure 4). The silica concentration is a multistage process including diagenesis and groundwater movements, as well as remobilization in fault zones to form vein-type mineralization. Chert pods or nodules up to boulder size have also been recorded from the upper part of the Rus Formation in the Dammam Dome of Saudi Arabia, and have been shown to be the result of climatic processes involving strong evaporation together with regional uplift on the northeastern edge of the Arabian Platform (Weijermars, 1999a).

The model applied to the concretionary phosphorites in the area is only a small part of the complex phosphate cycle that was described by Nriagu and Moore (1984), Notholt and Jarvis (1990), and Dill and Kantor (1997), among others. Lithological and chemical data collected during the present study supported the idea that the phosphate-bearing strata were produced by storm-driven(?) gradient flows (Table 3); see also the tempestite-like shell debris (Plate 1d). The gradient flows introduced water from an oxygen-minimum zone near to the shelf edge outside the study area. Phosphorite was pre-concentrated in limestones of unit II (Table 3, mean 0.55 wt.% P2O5). Subsequently, the multiple reworking of phosphate grains caused another accumulation of phosphorite above the Ypresian-Lutetian unconformity/SB1 in a shallow-marine environment (Figure 3; Table 2 mean 0.39 wt.% P2O5). The decomposition of iron-bearing clay minerals in the Midra Shale Member of the Dammam Formation caused a build-up of ferrous concretions and the red and brown staining of the rocks. Toward the top of unit IIIa, the number of concretions diminishes but the carbonate content increases, and the red/brown staining becomes gray and white (Figure 11). The presence of trivalent iron in the various minerals closely reflects the distribution of argillaceous layers and parasequences in unit IIIb, and their bounding surfaces. Ferricretes are common on unconformity surfaces and are frequently found in coastal areas where hydrological and pedogenic processes may have contributed to the precipitation of such encrustations (Ollier and Galloway, 1990; Phillips et al., 1997).

Gypsum is widespread on the limestone bench within unit IIIb that coincides with sequence boundary SB3. It is also found in and around the ‘collapse breccia’ at the unit IIIa/IIIb contact that marks sequence boundary SB2 (Figure 12). Primary gypsiferous sediments are scarce and not well expressed in the sedimentary record. When exposed at the surface they tend to be altered by meteoric waters and pedogenic processes into secondary gypcrete that may give the appearance of primary gypsiferous material and cause an overestimation of primary gypsum in the sedimentary record (Heine and Walter, 1996).

Correlations

Correlation with the Cenozoic chronostratigraphic and eustatic cycle charts allows the stratigraphic succession of the Dukhan anticline to be seen in a worldwide perspective (Haq et al., 1988). In the Haq model, emphasis is placed on sequence boundaries. Thus the major unconformity between unit IIIa/IIIa is given the rank of a sequence boundary (SB 1) due to its wide extent (Figure 4). It may be correlated with a significant shift of sea level reflected in the eustatic curves near the Ypresian-Lutetian boundary at about 49.5 Ma (Haq et al., 1988). The same is true for sequence boundary SB 3 at the unit IIIb/IVa disconformity. SB 3 correlates with the base of the Bartonian (Figure 4) in the Cenozoic chronostratigraphic-eustatic-cycle chart of Haq et al. (1988). SB 2, albeit less well expressed in the field, represents an upper intra-Lutetian hiatus (Figure 4). Boundaries marked with a question mark in Figure 3 are bounding surfaces of parasequences rather than true sequence boundaries. For example, the unit Ib/IIa unconformity ranks low in the hierarchical classification scheme of systems tracts and sequence boundaries and is less clearly correlated with the eustatic curves. Breaks in sedimentation and sediment disturbances such as those depicted in Figure 8b are of inferior rank and were not taken into account in terms of sequence stratigraphy.

Galloway (1989) introduced the alternative concept of Genetic Stratigraphic Sequences (GSS) with respect to sequence stratigraphy. He considered the maximum flooding surface (MFS) as being more suitable for stratigraphic subdivision than sequence boundaries. However, breaks in deposition (of interest in the present study) may be less easily recognized by the Galloway (1989) approach. In the compilation of sequence stratigraphic data of the Arabian Plate by Sharland et al. (2001), the MFS is the key element of stratigraphic subdivision and correlation. MFS Pg20 is the key element for the Eocene. It was defined based on a strong influx of an open-marine fauna in the Midra Shale and was assigned an early Eocene (latest Ypresian) age by Sharland et al. (2001) as a result of studies by Weijermars (1999a) at the reference section in the Dammam Dome (Figure 1). In the study area in Qatar, MFS Pg20 is located in the middle of unit IIIa, where the argillaceous rocks are rich in marine fossils such as shark’s teeth and allogenic palygorskite. This suggests that MFS Pg20 should have been assigned an early Lutetian age rather than Ypresian.

Updoming of the Dukhan Anticline and Faulting of Early Tertiary Sediments

Based on the structural data obtained during our fieldwork, it is difficult to create an all-embracing model of the paleo-stress field for this part of the Arabian Gulf. The observations can, however, be used to shed light on the structural evolution of the Dukhan anticline during the recent geological past.

The N-trending Dukhan anticline in western Qatar is the result of deep-seated salt doming. The neighboring, and similarly N-trending, Awali anticline of Bahrain (Figure 1) has the same halokinetic origin, and both structures are believed to be located over sub-salt, N-trending basement faults. The multiple activation of these basement faults since the Paleozoic triggered doming of infra-Cambrian salt and the deformation of the overlying sedimentary cover. The growth pulses of the Bahrain anticline are well documented and Samahiji and Chaube (1987) described and analyzed 10 such events from the late Paleozoic to the Holocene. Similar growth throughout the Mesozoic is documented for the Dukhan anticline (Qatar General Petroleum Corporation and Amoco Qatar Petroleum Company, 1991). About one-third of the vertical growth of the Awali anticline occurred during the Cenozoic (post-Cretaceous growth of 365 m).

The NE-trending faults mapped in the Tertiary rocks cut the axis of the Dukhan anticline at high angles and are interpreted as being part of a regional fault system that is unrelated to the growth of the salt domes. Hustedt et al. (1998) made similar observations for NE-trending structures in the Awali anticline (Figure 19). Evidence is as follows:

  • The missing regular geometrical relationship of the faults with respect to the anticline structure (e.g. radial, parallel, or perpendicular to the anticline trend).

  • The occurrence of the NE-trending faults is not restricted to anticlinal structures in the region (Figure 20).

At first sight, the mapped displacement along the NE-striking faults appears to indicate a simple horst-and-graben system on the eastern limb of the anticline (Figure 3b). Also, the NE-trending conjugate joint systems seen in some of the outcrops indicate extension perpendicular to the faults (Figure 15). However, two observations appear to indicate a subsequent compressional strike-slip movement along the NE-striking faults:

  1. The southward change in strike of the axis of the Dukhan anticline (Figure 3b) implies some combination of strike-slip faulting and vertical displacement on the fault planes; and

  2. Compressional deformation on at least some of the NE-trending faults is shown by steeply dipping reverse faults (Figure 16).

The compressional tectonics are probably related to the Zagros mountain-building processes that started in the Late Eocene to Early Oligocene and is continuing today. Stratabound silica accumulations were mobilized during movement on the faults in the area to form vein-type quartz mineralization.

CONCLUSIONS

In the northern part of the Dukhan anticline, the early Tertiary sedimentary rocks developed in a shelf environment with a gradual change in composition from dolomitic through mixed carbonate-siliciclastic to a calcitic carbonate shelf. Coarsening-upward sequences of various types indicate a cyclic change in the energy regime. The small-scale cyclothems and major unconformities within the sedimentary record resulted from eustatic sea level changes. The most prominent one was recognized in the sedimentary record near the Ypresian-Lutetian boundary.

The mineralogical responses to the changes in the sedimentary environment are instrumental in the classification of the bounding surfaces. A drastic lowering of sea level with subaerial exposure in the more landward locations and the concentration of various silica, iron, and phosphate concretions, took place at the Ypresian-Lutetian boundary and triggered a major redistribution of phyllosilicates. The results are characteristic of a large-scale type-1 sequence boundary (SB 1). Similar conditions at the Lutetian-Bartonian boundary resulted in SB 3, although the effects were less dramatic than for SB 1. Sea-level changes were also less apparent at the intra-Lutetian SB 2 boundary than at SB 1 and SB 3 as strong oxidation did not take place and long-term exposure can be ruled out. SB 2 is a type-2 sequence boundary. The most prominent surface for correlation is MFS Pg20 that occurs within the marly and argillaceous rocks of unit IIIa.

Variations in the phyllosilicate assemblage and in the concentrations of silica, phosphate, iron, and gypsum may furnish supplementary evidence for breaks in sedimentation. Of these, only silica was involved in a younger fault-induced remobilization during the late Tertiary.

Structural and lithological studies may furnish distinctive criteria for an evaluation of the economic potential of particular rock units, such as marls or limestones (Harben and Kuzvart, 1996). This approach has been a successful tool in assessing the quality and quantity of industrial minerals elsewhere (Tiercelin et al., 1992; Dill, 1994; Dill et al., 2001).

Coarse-grained calcareous rocks in the upper part of each coarsening-upward sequence, which in places underwent dolomitization, are potential aggregates. Silicification that has occurred at various stratigraphic boundaries and sequence breaks and in some faults, may enhance the hardness of the potential aggregates. However, the silicification may be an operational disadvantage as it increases wear on plant and equipment.

Marls in the lower parts of the section and argillaceous interbeds in unit IIIa may be useful as a cement additive. Dimension stones of reasonable quality are present in unit IV but they are of exploitable quantity only to the northeast outside the study area. More mineralogical investigations of the argillaceous rocks at MFS Pg20 are needed to test their suitability for drilling mud or ceramics. Palygorskite (also known as the attapulgite) is obtainable from the marly sediments of unit IIIa. Although it is less frequently used in drilling mud than bentonite, its widespread occurrence close to oil fields makes it a potential target for exploitation.

Sinkholes and dolines are widespread in the central part of Qatar where limestone of the Dam Formation crops out over most of the area and is underlain by a subfacies of the Rus Formation that contains abundant and easily dissolved gypsum/anhydrite. The distribution of the sinkholes and dolines is environmentally controlled and sequence boundaries may be useful in plotting their locations. In western Qatar, the calcareous shelf facies are devoid of evaporites and are less prone to dissolution. Where dolomitization has occurred or siliceous hardpans are present, the durability of the calcareous rocks has increased and they are more resistant to dissolution by meteoric waters.

ACKNOWLEDGMENTS

The authors are grateful for the financial support provided by the College of Science, University of Qatar and we thank the Scientific and Applied Research Centre for providing transport in the field. Mr. Asem of the University of Qatar made the thin sections. We are indebted to the staff members of the Qatar Geographic Information Systems Centre who provided topographic data and other assistance. The senior author is indebted to the staff members of the University of Qatar for the support provided during his visit to Qatar. Discussions with W. Weiss of the German Federal Institute for Geosciences and Natural Resources on the biodata were very helpful to our study. We thank the two anonymous GeoArabia reviewers for their comments, and J. Mattner (GeoTech) for his structural geology contributions. Text editing and the design and drafting of the final figures were by Gulf PetroLink.

ABOUT THE AUTHORS

Harald Dill is involved in international technical training with the German Federal Institute for Geosciences and Natural Resources (BGR) and is an Associate Professor at Hannover University. He studied geology and mineralogy at Würzburg, Aachen, and Erlangen universities and received a diploma in geology in 1975 and a doctorate in mineralogy in 1978. Harald did research at Bayreuth University before joining BGR in 1979. In 1982, he became Lecturer in Applied Geology at Mainz University where he was awarded his Dr. rer. nat. habil in 1985. From 1986 to 1991, he was assigned to the Continental Deep Drilling Program of the Federal Republic of Germany. In 1991, he was appointed Associate Professor in Economic Geology at Hannover University. In the same year he rejoined BGR in the Department of Economic Geology and International Cooperation where he trains geologists in sedimentology and the geology of non-metallic mineral deposits through international cooperation schemes. Harold lectures in economic geology and sedimentology at Hannover University and elsewhere in Germany and abroad. He has published over 200 papers and abstracts on the sedimentology and economic geology of metallic and non-metallic deposits in South America, Asia, and Central Europe. He is interested in the capture of digital data in the field.

dill@bgr.de

Sobhi Nasir is a Professor at the University of Qatar. He was awarded a PhD from the University of Würzburg, Germany, in 1986. Sobhi taught mineralogy at Yarmouk University in Jordan from 1987 to 1992 and at the United Arab Emirates University from 1992 to 1996. His research interests includes the Arabian lithosphere, the Semail ophiolite, applied mineralogy, and geochemistry.

sobhi@bigfoot.com

Hamad Al-Saad is Chairman of the Geology Department at the University of Qatar. He has an Msc in Geology from the University of South Carolina, USA, and a PhD from Ain Shams University, Egypt, 1997. His publications have concentrated on the Middle Jurassic of Qatar. Hamad’s current research interests include the sequence stratigraphy of the Jurassic of eastern Arabia. He is a member of SPE and BMS.

hamadsaad@qu.edu.qu