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Abstract

Wadi Naqab, SE of Ras Al Khaimah in the U.A.E., exposes an 800 m thick, shallow-water, Middle Jurassic succession. The base of the Bajocian is placed at a sequence boundary a t ca. 460 m above the top Triassic and 45 m beneath the horizon that yielded the ammonite Poecilomorphus sp. The Bajocian/Bathonian contact is marked by the highest occurrence of Haurania deserta. The questionable Bathonian/Callovian boundary either coincides with the last occurrence of Alzonella cuvillieri or is near the base of the Trocholina palastiniensis zone. The last occurrence of Kilianina bianchiti indicates the top of the Callovian. A new Bajocian foraminifer, Pseudodictyopsella jurassica, has been identified.

In Wadi Naqab, the Middle Jurassic corresponds to a broadly shallowing succession comprising multiple, meter-scale, fifth-order cycles. The Bajocian is predominantly subtidal, and cycles are commonly terminated by thick, massive oncoidal/peloidal packstones or grainstones. Most of the Bathonian and Callovian cycles start with spicular wackestones and end with cross-bedded peloidal/oolithic grainstones and/or stromatoporoid/coral rudstones. A direct comparison is established between the Middle Jurassic of Wadi Naqab and the subsurface of the Emirates. In the subsurface of Abu Dhabi, the Bajocian Izhara and Bathonian–Callovian Araej formations are the rock units that make up the Middle Jurassic. As a result of regional comparisons performed in this study, the Izhara Formation is redefined and a new type section proposed.

Introduction

The Mesozoic section of the Musandam, studied and described by Hudson and Chatton (1959), is a composite of small segments from different wadis of the Ras Al Khaimah area. Since then and especially in the last ten years, the Jurassic of the Arabian Peninsula has been studied in particular detail (Moshrif, 1987; Hassan, 1989; Droste, 1990 and 1993; Manivit et al, 1990; Koepnick, 1993; McGuire et al., 1993; Toland et al., 1993; de Matos, 1997; de Matos and Hulstrand, 1994; de Matos et al, 1994; Alsharhan and Nairn, 1994; Alsharhan and Magara, 1994, 1995; Alsharhan and Whittle, 1995a, 1995b; and de Matos and Walkden, 1995). The Arabian Jurassic picture is therefore now clearer, but there are stratigraphic uncertainties regarding the Jurassic of U.A.E. that still need to be addressed. Published biostratigraphic studies of the Jurassic of onshore and offshore Abu Dhabi are scarce.

Therefore, and in order to understand Jurassic stratigraphy and sedimentation and to evaluate reservoir characteristics of the subsurface of Abu Dhabi, a surface section of ca. 1310 m was measured, sampled (at ca. 1–2 m intervals), and described (on 1/100 scale) in Wadi Naqab, 16 km southeast of Ras Al Khaimah (Figs. 1, 2). This section is located between ca. 25° 42’ N, 56° 03’ E and 25° 44’ N, 56° 09’ E, within the Rub’al-Khali basin. The field work was undertaken between November 1992 and May 1995, and the Middle Jurassic section turns out to be 800 m thick.

Fig. 1.

—Geographical location of Wadi Naqab, SW Musandam Peninsula (U.A.E.) with schematic geological map.

Fig. 1.

—Geographical location of Wadi Naqab, SW Musandam Peninsula (U.A.E.) with schematic geological map.

Fig. 2.

—Schematic lithological log of the Jurassic of Wadi Naqab. Detailed Middle Jurassic section showing the sequences and the formations of the subsurface is included. Also indicated are the intervals illustrated in Figures 5 and 6.

Fig. 2.

—Schematic lithological log of the Jurassic of Wadi Naqab. Detailed Middle Jurassic section showing the sequences and the formations of the subsurface is included. Also indicated are the intervals illustrated in Figures 5 and 6.

This study also incorporates onshore and offshore subsurface data for the Jurassic. A total of 75 wells were included in the study, over 1300 thin sections and more than 150 cores were either reviewed or described for the first time to better define the sedimentology and microfossil content of the Jurassic in the subsurface of Abu Dhabi. In addition, palynological analyses of selected cores, cuttings, and field samples were undertaken to establish chronostratigraphic control and a better definition of sedimentology. Whenever possible, seismic data were used to study stratal patterns and to check and/or complement log correlations.

The purpose of this study was to describe in detail the biostratigraphy, cyclicity, and sequence stratigraphy, to analyze and to establish the relationship between the stratigraphic units of the Middle Jurassic section of the subsurface of Abu Dhabi with the section of the Musandam Peninsula. The Middle Jurassic formations of the subsurface are the Izhara and Araej formations, but only the Izhara Formation is discussed in this study.

The Oman Mountains have a complex geology. Studies by Glennie et al. (1974), Glennie (1992, 1995), Hughes Clarke (1988), and those included in Robertson et al. (1990) have contributed substantially to the understanding of it. Hudson and Chatton (1959) were the first to compile a comprehensive stratigraphy of the Mesozoic of the Musandam Peninsula, and Middle Jurassic to Lower Cretaceous source rock–reservoir–seal relationships in the Middle East were outlined by Murris (1980). Alsharhan and Kendall (1986) reviewed and interpreted the occurrence of oil and gas in the Precambrian to Jurassic rocks from the Arabian Peninsula, Iraq, and Iran. Middle Jurassic stratigraphy and hydrocarbon occurrences in the central and western parts of offshore Abu Dhabi were further discussed by Hassan (1989).

Until 1990 little attention had been paid to the Middle Jurassic of the Emirates in general and Musandam Peninsula in particular. Recently, however, the Middle Jurassic Araej Formation of U.A.E. and Qatar has been the subject of more focused research (Al-Saad et al, 1992; Alsharhan and Magara, 1994, 1995; Alsharhan and Whittle, 1995a, 1995b).

Biostratigraphy

The present study provides new biostratigraphic data on Middle Jurassic foraminifera, algae, and stromatoporoids. The stratigraphic ranges of most genera and species of algae, foraminifera, and stromatoporoids recognized in the Jurassic of Wadi Naqab, are illustrated in de Matos (1997, fig. 4.1). These data confirm the previous stratigraphie ranges of most of the foraminifera (Septfontaine et al., 1991, fig. 2) and algae (Granier and Deloffre, 1993) of the southern margin of Tethys. However, the local stratigraphic ranges of some stromatoporoids (de Matos, 1997) do not conform with Wood (1987) or Toland (1994).

Bajocian

In Wadi Naqab, the limits of the Bajocian are placed on the basis of a combination of sequence stratigraphic and biostratigraphic data. The concentration of omission surfaces (cf. Walkden and de Matos, this volume) at ca. 460 m above top Triassic and ca. 45 m below the horizon yielding the sole Lower Bajocian ammonite Poecilomorphus sp. is interpreted as the regional hiatus of a large part of the Toarcian and all the Aalenian in the Arabian Peninsula (Manivit et al., 1990). Therefore, we place the base of the first sequence of the Bajocian (Bj1) coincident with the uppermost omission surface of that interval (de Matos, 1997, fig. 4.15).

The top Bajocian is marked by the highest occurrence of Haurania deserta Henson, the first certain occurrence of Protopeneroplis striata Weynschenk, and the inceptions of Pseudomarssonella sp. and Pfenderella sp. at approximately 1025 m in the section. The joint appearance of these foraminifera indicates early Bathonian.

The microfossil biozonation of the Bajocian is not as clear as the rest of the Middle Jurassic (Bathonian and Callovian stages) because of several intervals with a sparse biota. Four biozones were nevertheless defined as: zone 1: Dasycladacean BjA; zone 2: Thaumatoporella parvovesiculifera; zone 3: Amijiella amiji; and zone 4: Pseudodictyopsella jurassica. These are separated by four intervals with a nondiagnostic fossil content (designated A, B, C, and D, Fig. 3).

Fig. 3.

—Micropaleontological biozonation of the Middle Jurassic of Wadi Naqab.

Fig. 3.

—Micropaleontological biozonation of the Middle Jurassic of Wadi Naqab.

A new foraminifer, Pseudodictyopsella jurassica (Septfontaine and de Matos, 1998), of biozone 4 occurs higher than the Early Middle Jurassic Timidonella-rich storm layers. The new genus, in association with Spiraloconulus sp. and Selliporella donzellii, is dated as Bajocian (Septfontaine and de Matos, 1998).

Bathonian

In Wadi Naqab the thickness of the Bathonian is between 175 and 200 m, depending of the definitions used for both boundaries (de Matos, 1997, appendix 4.3) (Fig. 2). The base is constrained by the highest occurrence of Haurania deserta (Fig. 3). Normally, the base of the Bathonian is defined by the highest occurrence of the association Haurania deserta–Amijiella amiji (Athersuch et al., 1992). In Wadi Naqab the two forms have different ranges. The highest record of Haurania deserta is at the base of the Bathonian but the highest occurrence of Amijiella amiji is in the late Bathonian because it coincides with the inception of Trochamijiella gollesstanehi Athersuch, Banner and Simmons, a late Bathonian foraminifer (Athersuch et al., 1992). The Bathonian/Callovian boundary could be at either the last questionable occurrence of the foraminifer Alzonella cuvillieri (Bajocian to Bathonian) or near the base of the Trocholina palastiniensis zone (Fig. 3). According to Gollesstaneh (1965) in South Iran Trocholina palastiniensis Henson occurs sporadically near the Bathonian–Callovian boundary.

A regionalbreak of sedimentation between the early Bathonian and the middle Callovian was recognized in Saudi Arabia (Manivit et al., 1990). Comparable hiatuses undoubtedly occurred in Wadi Naqab and are probably present in other areas of the Arabian Peninsula.

Callovian

The Callovian is approximately 45 m thick (de Matos, 1997, appendix 4.3). Its top (top Middle Jurassic) is placed directly above the grainstones containing the last occurrence of Kilianina blancheti Pfender in association with Kurnubia palastiniensis Henson (late Callovian to Kimmeridgian), Trocholina palastiniensis (?Bathonian to lower Kimmeridgian), Trocholina elongata (Callovian to Kimmeridgian) and Praekurnubia crusei (late Bajocian to middle Oxfordian).

Cyclicity, Sedimentation, and Sequence Stratigraphy

The Middle Jurassic of the Musandam Peninsula is a succession of shallowing-upward cycles arranged in sequences of different orders, meters to decameters thick, in a lagoonal to locally intertidal environment. Meter-scale shallowing-upward, fifth-order cycles are the building blocks of larger depositional sequences.

The Middle Jurassic of Wadi Naqab displays a remarkably consistent shallowing-up trend. The lower part of the Bajocian is dominated by lime mudstones and the middle Bajocian by wackestones. Packstones predominate higher up in the upper Bajocian, and the grainstones with subordinate rudstones are the major constituents of the remainder.

Cycle definition, identification of parasequence sets, and analyses of the omission surfaces of the Middle Jurassic indicate the existence of eight sequences in the Bajocian, three in the Bathonian and two in the Callovian (Fig. 2; de Matos, 1997, appendix 4.3).

Bajocian

A total of at least 146 small shallowing-upward cycles were identified in the ca. 565 m of the Bajocian of Wadi Naqab. The characteristics of Bajocian cycles vary throughout the section, with the subtidal cycles predominant. A typical subtidal cycle starts with thin-bedded nodular lime mudstones to wackestones and commonly ends with thicker (decimeter-thick) massive oncoidal/peloidal wackestones, packstones, or grainstones (Figs. 4, 5). Rarely, the cycle is capped by intertidal/tidal-flat limestones with birdseye structures and algal laminites.

Fig. 4.

—Legend for Figures 5, 6, 9, and 10.

Fig. 4.

—Legend for Figures 5, 6, 9, and 10.

Fig. 5.

—Bajocian of Wadi Naqab. Lithology, relative abundance of major groups of organisms, and cycle composition of the HST of sequence Bj7 and the TST of sequence Bj8. The column designated “thickness (m)” indicates the thickness of the section above top Triassic.

Fig. 5.

—Bajocian of Wadi Naqab. Lithology, relative abundance of major groups of organisms, and cycle composition of the HST of sequence Bj7 and the TST of sequence Bj8. The column designated “thickness (m)” indicates the thickness of the section above top Triassic.

The pattern of sedimentation indicates a stable balance between subsidence and sediment accumulation, which maintained broadly open shelf conditions with increasing energy towards the top of the Bajocian. Assuming a period of 7.3 million years for the Bajocian (Hardenbol et al., 1998), then each small cycle took ca. 50,000 years to be formed, which means that the Bajocian would be composed of fifth-order cycles and the average rate of subsidence would have been 7.5 cm/1000 years.

More than half of the sequences of the Bajocian (sequences Bj2 to Bj5 and Bj8) end with irregular, burrowed iron-stained omission surfaces representing either firmgrounds or hardgrounds. Only a few exhibit limoninc crusts and iron-coated molds of burrows suggesting the removal of the mineralized crust by mechanical erosion. These cemented surfaces are more resistant to weathering than the enclosing sediments and therefore stand out clearly in outcrop. A high degree of biorurbation or hardground development may indicate a decrease of sedimentation at the top of a regressive sequence. These omission surfaces probably indicate a rapid deepening of the sea after the break of sedimentation at the top of the Bajocian sequences.

Bathonian

A total of 34 cycles was identified in the 199 m of the Bathonian of Wadi Naqab. Most of the cycles of the Bathonian consist of spicular wackestones, locally filament-rich at the base and cross-bedded peloidal/oolithic grainstones and/or stromatoporoid/coral rudstones at the top (Fig. 6). Inclusion of a rudstone as the start of a cycle, in the middle or at the top, depends on the cycle constituents and the cycle stacked facies.

Fig. 6.

—Bathonian of Wadi Naqab. Lithology, relative abundance of major groups of organisms, and cycle composition of the upper part of the TST of sequence Bt1 and part of the TST of sequence Bt2. The column designated “thickness (m)” indicates the thickness of the section above top Triassic.

Fig. 6.

—Bathonian of Wadi Naqab. Lithology, relative abundance of major groups of organisms, and cycle composition of the upper part of the TST of sequence Bt1 and part of the TST of sequence Bt2. The column designated “thickness (m)” indicates the thickness of the section above top Triassic.

Rudstones occur in tidal flats and channels. They also occur as condensed horizons. Similar to the reworked fossils of the lag deposits, the bioclasts of the rudstones of Wadi Naqab also show breakage, coatings, borings, and locally geopetal structures. The intraclasts may consist either of rip-up clasts and/or of fragments of rocks not actually represented in the local stratigraphic column because of total erosion. Consequently, a large part of the time of the Bathonian and Callovian (i.e., time of the development of the stacked coral/stromatoporoid rudstones, floatstones, and boundstones) was consumed during the deposition of those rocks. This may explain why the thickness of the Bathonian and Callovian of Wadi Naqab is so small in comparison with the thickness of the Bajocian. The differences in the time spans of each stage would not justify such a great variation in thickness.

Assuming a period of 4.8 million years for the Bathonian (Hardenbol et al., 1998), then the average rate of subsidence would be ca. 4 cm/1000 years and each cycle would have taken ca. 141,000 years to be formed. There is, however, a strong possibility that each cycle was deposited in a much shorter period. If the hiatus in sedimentation defined in Saudi Arabia between the early Bathonian and the base of the middle Callovian (Manivit et al., 1990; Le Nindre et al., 1990) also exists in the Musandam Peninsula, then the 34 cycles of the Bathonian of Wadi Naqab needed only ca. 1.3 million years (the time span of the Lower Bathonian) to be deposited. The presence of this hiatus is not evident in the studied section. The concentration of the rudstone lag beds might, however, explain the discrepancies.

The first horizon with silica replacement occurs in the transgressive systems tract (TST) of the first sequence of the Bathonian (de Matos, 1997, fig. 4.45). Silicification (replacement by beekite) of the stromatoporoids, corals, and other bioclasts is common throughout the Bathonian, Callovian, and Upper Jurassic. Similar large silicified stromatoporoid colonies also occur in the Early Oxfordian of Ethiopia, Tigre Province (Bosellini et al., 1997) and in the Jurassic of Yemen (Bosellini and Al Thour, personal communication). The presence of beekite suggests a low rate of subsidence with discontinuous deposition and major time breaks in sedimentation (Kazanci and Varol, 1993). In addition, chert nodules are abundant and concentrated along certain horizons. The existence of silicification can be used in the field as a guide to distinguish the Middle Jurassic and Upper Jurassic from the Lower Jurassic.

There is a clear upward thinning of the sequences and TST (i.e., from the oldest to the youngest sequence) of the Bathonian (de Matos, 1997, appendix 4.3). This, together with the occurrence of beekite, suggests the decrease of accommodation space favoring erosion and/or nondeposition towards the top of the Bathonian succession.

Callovian

A total of 11 cycles, distributed over two sequences, were recognized in the 45 m of Callovian sediments of Wadi Naqab (de Matos, 1997, figs. 4.65 and 4.80). Composition and characteristics of the Callovian cycles are very similar to those of the Bathonian.

In conclusion, the durations of the time span of each stage of the Middle Jurassic (Bajocian, 7.3 million years; Bathonian, 4.8 million years; and Callovian, 5 million years) are not proportional to the respective thickness differences (Bajocian, 565 m; Bathonian, 199 m; and Callovian, 45 m), so the preserved sediments must therefore represent just a small proportion of total elapsed time. Large parts of the Bathonian and Callovian must have been taken up by condensed deposition and reworking leading to the development of the stacked lag deposits (coral/stromatoporoid rudstones, floatstones, and boundstones). One should also bear in mind the late Bathonian–late Callovian unconformity identified in the Middle East by Lewy (1981) and the break in sedimentation recognized in Saudi Arabia (Le Nindre et al., 1990) of at least 7 million years between the early Bathonian and the middle Callovian. There are also hiatuses in the late Lias, Bajocian, and Bathonian in Jebel Akhdar in Oman (Le Nindre et al, 1990). It is clear, therefore, that during parts of the Middle Jurassic there were regional breaks of sedimentation in the Arabian Peninsula, and similar important hiatuses clearly occurred in Wadi Naqab.

The Middle Jurassic of the Abu Dhabi Subsurface

The Middle Jurassic of the subsurface of the Emirates comprises the Izhara and Araej formations (Fig. 7). In Abu Dhabi, the section between the Izhara and Marrat formations, previously correlated with the Hamlah Formation, is not equivalent to the Hamlah as defined in Qatar. The Hamlah Formation of Qatar, which has been considered to be Lower Jurassic since 1975 (Sugden and Strandring, 1975), was recently dated as upper Triassic (Hassan, 1989; de Matos et al., 1994; de Matos, 1997). Log correlation has demonstrated that the Abu Dhabi equivalent of the Hamlah Formation lies beneath the Marrat and Minjur formations (Fig. 8). It was necessary, therefore, to decide how to allocate that part of the subsurface sequence formerly designated Hamlah Formation by ADCO geologists.

Fig. 7.

—Middle Jurassic stratigraphy of the subsurface of Abu Dhabi and equivalent formations in Saudi Arabia.

Fig. 7.

—Middle Jurassic stratigraphy of the subsurface of Abu Dhabi and equivalent formations in Saudi Arabia.

Fig. 8.

—East-west gamma-ray log correlation, Late Triassic to the Middle Jurassic Izhara Formation is highlighted.

Fig. 8.

—East-west gamma-ray log correlation, Late Triassic to the Middle Jurassic Izhara Formation is highlighted.

Palynologic analyses from onshore and offshore Abu Dhabi wells have indicated a Bajocian age for that part of the section between the Marrat and Izhara formations. In addition, sedimentation seems to have been continuous between the Marrat and the top of the Araej Formation (the formation directly overlying the Izhara Formation, Fig. 7). These factors favored the inclusion of that sequence in the Izhara Formation.

Here we offer a detailed description of the redefined Izhara Formation, and its regional setting and distribution in the Arabian Peninsula in general and in the Emirates in particular.

Stratigraphic and Sedimentological Revision of the Izhara Formation

New Type Section.—

The section of the ADCO well Jam Yaphour-7 (Jy-7), previously interpreted as Izhara plus Hamlah formations (362 m thick), was chosen as the type section of the newly redefined Izhara Formation (Fig. 9). The well Jy-7 of onshore Abu Dhabi is located at 24° 15’ 50.6” N, 54° 38’ 58.9” E. The previous type section was in Qatar Petroleum Company well Kharaib-1, where this formation is ca. 137 m thick (Sugden and Strandring, 1975). The offshore Abu Dhabi reference section is ca. 208 m thick in the ADMA well Ghasha-7 (Gh-7) (Hassan, 1989).

Fig. 9.

—New type section for Izhara Formation (WeU-Jarn Yaphour-7). Comparison between previous and present interpretation.

Fig. 9.

—New type section for Izhara Formation (WeU-Jarn Yaphour-7). Comparison between previous and present interpretation.

Lithology.—

In Abu Dhabi, the Izhara Formation can be divided into three members, lower, middle and upper (Fig. 9) based on both lithology and gamma-ray log characteristics. The Lower and Middle Members are equivalent to what has been considered the Hamlah Formation. The Upper Member corresponds to the former definition of the Izhara Formation in the “Lexique Stratigraphique of Qatar” (Sugden and Strandring, 1975).

The top of the Lower Izhara Member is located at the top of a clean low-gamma-ray section composed mainly of lime mud-stones and wackestones (Figs. 8, 9). The boundary between the Middle and Upper Izhara Members correspond to a clear log break between a clean low-gamma-ray interval, composed of peloidal wackestones and packstones, and the overlying siltstones and shales with high gamma-ray response (Fig. 9).

The Lower Izhara Member in Jy-7 is about 98 m thick and is dominated by gray, brown, buff, hard and dense bioclastic lime mudstones and wackestones with a variable degree of dolomitization. It is locally chalky with pyrite and minor shales, and packstone streaks are also present. The biota consist of echinoids, ostracodes, gastropods, benthonic foraminifera, and recrystallized, thin-shelled molluscan fragments. Syntaxial overgrowths are present on echinoid fragments. The uppermost 16 m of the Lower Izhara in Jy-7 contains abundant anhydrite laths and nodules at several horizons. Cryptalgal laminites associated with meter-scale peritidal cycles are evident.

The Middle Izhara Member is 142 m thick in Jy-7 and comprises a lower clean limestone section (91 m thick), with lithologies similar to the Lower Izhara Member, and an upper more argillaceous section, 51 m thick, where limestones predominate but gray platy hard fissile shales and siltstones also occur.

The Upper Izhara Member is ca. 122 m thick in Jy-7 and was divided into two units based on gamma-ray log response (Steele et al., 1983), a lower unit with a high gamma-ray trace (Unit A) and an upper unit (Unit B) with a clean low-gamma-ray pattern (Fig. 9). The lower unit (Unit A = Hassan’s Unit 4 or Unit 2 in QPC well Kharaib-1) is 89 m thick in Jy-7 and consists of inter-bedded shelly, silty (locally sandy) lime mudstones, calcareous siltstones, and shales. The uppermost Izhara Unit B (equivalent to Dhibi Limestone of the Dhruma Formation in Saudi Arabia, or to Unit 1 of Kharaib-1 well of Qatar or Hassan’s Unit 5 in Abu Dhabi) is 33 m thick in Jy-7 and is muddy at the base, changing to packstones and grainstones with some lime mudstones and bioclastic wackestones at the top.

A detailed study of the Upper Izhara Member has been conducted in an offshore well (Fig. 10). The lower unit of the Upper Izhara in this well is mostly of argillaceous lime mudstones to wackestones. Several pyritic, shelly packstone, condensed layers occur locally with subrounded coarse quartz sand grains or silt-size grains. The shales are calcareous and pyritic, and subrounded phosphate particles are abundant in the condensed shelly and packstone layers. The upper unit (equivalent to Dhibi Limestone) consists mainly of lime mudstones to wackestones.

Fig. 10.

—Upper Izhara lithology and fossil distribution for western offshore Abu Dhabi.

Fig. 10.

—Upper Izhara lithology and fossil distribution for western offshore Abu Dhabi.

Biostratigraphy and Age.—

The age of the Izhara Formation in the previous type locality was considered to be Bajocian (Sugden and Strandring, 1975), although Hassan (1989) attributed a Liassic age to the entire Izhara Formation of the offshore Abu Dhabi. More recently, Manivit et al. (1990), in a comprehensive study of the entire Jurassic of Saudi Arabia, consider that the Lower Dhruma Formation (equivalent to the Izhara Formation) is also Bajocian. Because of this controversy and owing to a lack of microfossil content, particularly in the Lower and Middle Members of the Izhara Formation, palynological analyses were undertaken in several onshore and offshore wells (Figure 8 shows some of these). The miospores and palynomorphs support an age not older than late Bajocian for the Middle Izhara in the studied wells. The microfossil association recognized in the upper unit of the Izhara Formation (Fig. 10) also supports a late Bajocian age for the topmost Izhara. Therefore, all the Izhara Formation belongs to the Middle Jurassic-Bajocian.

Environment of Deposition.—

The occurrence of the miospores Classopolis spp. and Araucariacites australis Cookson in the Izhara Formation indicate that arid climatic conditions were dominant. The occurrence of fine-grained and bioturbated limestones in the Lower and Middle Izhara indicates low-energy conditions, while the accompanying fairly diverse biota suggests deposition in an open marine-shelf environment. The limestones of the uppermost Middle Izhara Member in southeast Abu Dhabi are locally rich in pyrite and contain abundant pelagic bivalves (filaments), sponge spicules, and shell debris, suggesting deposition under deeper open-marine conditions.

The lithology and bioclasts of the lower part of the Upper Izhara (Fig. 10) indicate deposition in a marine euxinic environment with periodic siliciclastic influxes. Phosphate particles are abundant in the condensed shaly and packstone layers closer to the maximum flooding surface (MFS), which is marked by a very high gamma-ray peak (Figure 10). Shallow, low-energy-shelf conditions probably prevailed during deposition of the topmost Izhara.

Boundaries.—

The upper contact with the Araej Formation is conformable (Fig. 8). The upper boundary coincides with a lithologic change and a log break, and is placed at the top of a low-gamma-ray sequence of limestones beneath the argillaceous limestones of the Lower Araej Member (Fig. 8). The underlying contact is predominantly with the Marrat Formation, and is unconformable in offshore and in large areas of onshore Abu Dhabi (Fig. 8). Also, in most areas of the Qatar Arch, the Izhara Formation directly overlies the Triassic Gulailah or Jilh formations (de Matos, 1997, fig. 4.92). The lower contact, however, is conformable in eastern onshore Abu Dhabi, where lowstand deposits occur at the top of the Marrat Formation (Fig. 8; de Matos et al., 1994).

Regional Setting and Distribution.—

In Abu Dhabi, the thickness of the Izhara Formation decreases from east to west, with a maximum of 362 m onshore (type locality) and ca. 120 m in the offshore around Dalma Island (Fig. 11). This thickness variation is mainly the result of: 1) depositional onlap of the Lower Izhara Member to the west, over the irregularly eroded top surface of the Marrat Formation, and 2) the gradual thinning of the Middle and Upper Izhara Members westward (Fig. 8). No Lower Izhara sediments were deposited to the west of the 200 m contour on the Izhara isopach map (compare Figs. 8 and 11).

Fig. 11.

—Izhara Formation isopach map in Abu Dhabi

Fig. 11.

—Izhara Formation isopach map in Abu Dhabi

The Upper Izhara Member maintains similar lithologic characteristics all over Abu Dhabi. The section of the Upper Izhara of onshore and offshore Abu Dhabi, equivalent to the Dhibi Limestone Member, as in Saudi Arabia (Powers, 1968), has a remarkably constant thickness of ca. 30 m. The “Pre-Dhibi” unconformity identified at the base of the Dhibi Limestone in Saudi Arabia (Enay et al., 1987) and noted by Banner et al. (1991), has not been recognized so far in Abu Dhabi. However, the thinning of the lower part of the Upper Izhara westward (Fig. 8) suggests missing section at that level. Future additional data (cores, logs, and seismic) might confirm the existence of that hiatus.

Comparison of the Bajocian Section of Wadi Naqab with the Izhara Formation in the Subsurface of Abu Dhabi

The section of Wadi Naqab equivalent to the Lower Izhara Member of the subsurface of Abu Dhabi comprises sequences Bj2-Bj4 (i.e., approximately between samples MUS-300 and 387, (de Matos, 1997, appendix 4.3). Like the succession of Jy-7, this interval of Wadi Naqab is composed mainly of lime mudstones to wackestones, and includes some horizons with birdseye structures and cryptalgal laminites. If this interpretation is correct, then there is a large change in thickness between the subsurface (98 m in the well Jy-7, Fig. 9) and the outcrop (ca. 230 m in Wadi Naqab, de Matos, 1997, appendix 4.3). The difference in thickness suggests that the increased subsidence rate during the Liassic of Wadi Naqab as compared to the rest of the Arabian Peninsula continued into the Middle Jurassic. The subsidence rates became essentially uniform only during the time of deposition of the Middle Izhara Formation and later.

As far as the Middle Izhara Member is concerned, the equivalent section of Wadi Naqab includes two sequences Bj5 and Bj6 and corresponds to the interval between samples MUS-388 and 442b (de Matos, 1997, appendix 4.3). This section is represented by subtidal cycles. Each cycle starts with nodular argillaceous lime mudstones to wackestones (which can produce a high gamma-ray reading) and ends with oncoidal, peloidal packstones (which would have a cleaner and lower gamma-ray value). There is, therefore, likely correspondence in gamma-ray response between the Middle Izhara in the subsurface and the type section. Although sponge spicules and Saccocoma sp. have been recognized in the uppermost Middle Izhara of Wadi Naqab, it seems that the Middle Izhara of the subsurface is more argillaceous and silty, probably reflecting deposition under deeper conditions. The 140 m thickness of the Middle Izhara in Wadi Naqab (de Matos, 1997, appendix 4.3) matches well with the 142 m of equivalent section in the type locality (Fig. 9).

The Upper Izhara Member of Wadi Naqab and subsurface of Abu Dhabi have similar thickness, and both show a predominance of oncoidal packstones and larger concentration of oncoids towards the top. They differ, however, particularly in the lower part of the Upper Izhara. In the subsurface there is a pyritic, shelly, silty and shaly interval locally with phosphatic particles, characterized by a high gamma-ray response (Fig. 10). Filaments are particularly abundant, indicating deposition under pelagic conditions. Filaments are also abundant locally in Wadi Naqab. There are also some marly layers, and locally shell-rich horizons, but no silt, sand, phosphate, or glauconite were found in this part of the section of Wadi Naqab. A gamma-ray log through this part of the Wadi Naqab succession, which is thin-bedded, might show spiky higher values than would be expected in the sections above and below. Thus, the gamma-ray expression of the Upper Izhara in Wadi Naqab and in the subsurface of Abu Dhabi might be very similar. The upper part of the Upper Izhara (the Dhibi Limestone) shows thicker beds and oncoidrich layers, and is also comparable in both localities.

Summary and Conclusions

The defining biostratigraphic events of the Bajocian, Bathonian, and Callovian of Wadi Naqab are as follows:

  1. The Toarcian/Bajocian boundary is marked by an omission surface located ca. 45 m beneath the layer that yielded the Lower Bajocian ammonite Poecilomorphus sp.

  2. The Bajocian/Bathonian contact is defined by the highest occurrence of Haurania deserta and the inceptions of Protopeneroplis striata, Pseudomarssonella sp., and Pfenderella sp.

  3. A new Bajocian foraminifer, Pseudodictyopsella jurassica, has been identified, in association with Spiraloconulus sp. and Selliporella donzelli.

  4. The boundary between the Bathonian and Callovian could be either at the last questionable occurrence of the foraminifer Alzonella cuvillieri or near the base of the Trocholina palastiniensis zone.

  5. Top Callovian (i.e., top of the Middle Jurassic) is placed coincident with the top of the grainstones that contain the last occurrence of Kilianina blancheti in association with Kurnubia palastiniensis, Trocholina palastiniensis, Trocholina elongata, and Praekurnubia crusei.

  6. As far as the dasyclad algae are concerned, although Holosporella siamensis has a stratigraphic range (Bajocian to Callovian) that fits with the worldwide range, the occurrence of Coniporella clavaeformis (Callovian, Fig. 3) is younger than previously recorded (middle Bathonian).

This study confirms the stratigraphic range of most of the foraminifera (Septfontaine et al., 1991) and algae (Granier and Deloffre, 1993) of the South Tethyan realm.

The Middle Jurassic consists of a generally shallowingupward succession of multiple meter-scale fifth-order cycles. Subtidal cycles commonly terminated by thick massive oncoidal and peloidal packstones and grainstones are dominant in the Bajocian. Most Bathonian and Callovian cycles start with spicular wackestones and end with cross-bedded peloidal and oolitic grainstones and/or stromatoporoid and coral rudstones. The lags are more common at the top of the Middle Jurassic, where the section becomes condensed.

A total of 13 sequences were defined in the Middle Jurassic: eight in the Bajocian, three in the Bathonian, and two in the Callovian.

Silicification (replacement by beekite) of the stromatoporoids, corals, and other bioclasts is common throughout the Bathonian, Callovian, and Upper Jurassic. In the Musandam Peninsula, the existence of silicification can be used as a guide to distinguish the Middle and Upper Jurassic from the Lower Jurassic.

The type section for the redefined Izhara Formation is the section between top Marrat and top of former Izhara Formation in well Jy-7. The age of the Izhara Formation is Bajocian, supported by palynomorphs and foraminifera. The Izhara Formation comprises three members. The Upper Member corresponds to the former definition of the Izhara Formation (Sugden and Strandring, 1975). In the subsurface of Abu Dhabi, the Izhara formation typically lies unconformably over the Marrat Formation. The lower contact, however, is conformable in some parts of eastern onshore Abu Dhabi where lowstand deposits exist at the top of the Marrat. Above the Qatar Arch, the Izhara Formation directly overlies the Triassic. The upper contact of the Izhara with the Araej Formation is conformable. In Abu Dhabi, there is a westward thinning of the Izhara as a consequence of the onlap of the Lower Izhara Member over the Marrat, plus the gradual thinning of the Middle and Upper Izhara Members to the west.

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Acknowledgments

This paper resulted from the principal author’s doctoral work at the University of Aberdeen. This research project received joint financial support from PARTEX SERVICES CORPORATION and ADCO, to whom we wish to express our gratitude. ADNOC and ADMA are also thanked for allowing access to study seismic lines, cores, and additional lithological well data of offshore Abu Dhabi. We are most grateful to R. Hulstrand and Maria J. E. de Matos for their assistance during the field work. We are also very grateful to M. Septfontaine for his assistance in the identification of the foraminifera and M. Conrad and B. Granier for their help with the identification of the dasycladaceans. Thanks are also due to Justino Carvalho for his assistance in the preparation of the figures.

Figures & Tables

Contents

GeoRef

References

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