ABSTRACT

The Arabian Plate has experienced a complex tectonic history while also being widely influenced by eustatic sea-level changes. These diastrophic events either affected changes in the rate and/or location of subsidence that in turn led to the creation of significant new sedimentary accommodation, or caused major erosional hiatuses. As a result, both eustasy and tectonics have played important roles in the development of sedimentary sequences and in determining the locus and characteristics of reservoir, source and seal facies on the Arabian Platform. Here, we present a synthesis (Cycle Chart) of the regional sea-level fluctuations affecting the Platform that is based on Phanerozoic epi- and peri-Platform sequence-stratigraphic data. Information used for the synthesis includes sections from Saudi Arabia, Kuwait, the Greater Gulf area, Oman and Yemen. The regional Cycle Chart incorporates interpreted sedimentary onlap patterns on the margins of the Arabian Platform, as well as models of regional sea-level fluctuations that controlled these patterns. These are compared to eustatic data that represents the ‘global-mean’ models of sea-level changes, largely at second-order cycle level for the Paleozoic and third-order cycle level for the Mesozoic and Cenozoic eras. The comparisons reveal that Phanerozoic sediment accumulation patterns on the Platform were broadly controlled by eustasy, with a strong overprint of tectonics for several long intervals. During periods of tectonic quiescence, however, correlations with the eustatic events improve significantly. Thus, for example, during the Cambrian through early Silurian and mid Jurassic through early Paleogene intervals eustasy may have been the significant controlling factor for sedimentary patterns when long-term trends in both regional and global sea-level curves show similarities.

The use of the Cycle Chart could facilitate exploration efforts on the Arabian Platform, provide better chronostratigraphic estimates and global correlations, and prove a useful accompaniment for sequence-stratigraphic studies. This integrative effort was greatly facilitated by the recent publication of the sequence stratigraphic synthesis of the Arabian Plate. The ages of Maximum Flooding Surfaces, however, have been recalibrated to the new (GTS 2004) time scale. This synthesis also represents a new recalibration of the Mesozoic and Cenozoic eustatic curves of Haq et al. (1988) to an up-to-date numerical time scale (GTS 2004).

INTRODUCTION

Over the years, the eustatic cycle chart of sea-level changes for the Mesozoic and Cenozoic based on global synthesis of sequence-stratigraphic data (Haq et al., 1988) has sustained its usefulness as an exploration and first-order global correlation tool. Although geological time scales against which the cycle chart was calibrated have since been refined several times, the overall validity of this synthesis has been corroborated by many independent studies encompassing discrete stratigraphic intervals (e.g., Mitchum and Uliana, 1988; Barron et al., 1991; Bartek et al., 1991; Plint et al., 1992; Miller et al., 1996; Röhl and Ogg, 1996; Pekar and Miller, 1996; Stoll and Schrag, 2000). These studies represent several different approaches to decode base-level fluctuations, ranging from the study of ancient coral reefs and guyots, to continental margins and interior basin stratigraphies, and the use of oxygen isotopes as a proxy for ice volume in the deep-sea record.

The eustatic curves therefore remain valid working models of the long-term trends of the base level, representing marine transgressions and regressions along the world’s continental margins, and flooding or desiccation of inland seas and wide interior epi-platform basins. The curves can also be used for first-order correlations worldwide where, or when, tectonic influences are relatively less dominant, i.e. during times of relative tectonic dormancy. In addition, the relative magnitude and frequency of sea-level rises and falls and the duration of subaerial exposure, as depicted on the eustatic and regional cycle charts, are valuable exploration criteria when considering the response of reservoir and source facies to base-level changes in any given basin.

In siliciclastic and mixed environments, the facility to anticipate the extent of migration of facies as they follow the movement of the base level, or are point-sourced to the basin floor after shelfal incision, can be an important aid in exploration. In carbonate environments, knowledge of the duration of exposure during each cycle as gleaned from the Cycle Chart could provide valuable clues about diagenesis and reservoir quality. Similarly, information about the relative magnitude of sea-level rise and the duration of the highstand during each cycle can also enhance inferences about the production and preservation of organic-rich source rocks. It is not surprising, therefore, that many major exploration companies have adopted the Cycle Chart, and consider it an important component of their exploration toolboxes, often calibrating it to their preferred geological time scales, or adapting it to focus on special target area.

Rationale for an Arabian Platform Cycle Chart

The Arabian Plate has been subjected to an intricate diastrophic history (see e.g., Grabowski and Norton, 1995; Al-Husseini, 2000) in addition to being widely influenced by eustatic sea-level fluctuations (Figure 1). Though the Arabian Shield has remained relatively stable through much of the Phanerozoic, the stratigraphy of the Arabian Platform and peri-Platform basins is a record of major second- and third-order sequences that are separated by extensive regional unconformities. These erosional and/or non-depositional hiatuses were created by major tectonic events on or at the margins of the Platform. One consequence of these tectonic events was to change the rate and/or locus of subsidence that was responsible for the creation or destruction of new accommodation space for sediment accumulation. Thus, both eustasy and tectonics played major roles in the development of sedimentary sequences and in determining the characteristics of the reservoir, source and seal facies on the Arabian Plate.

A regional Phanerozoic Cycle Chart of relative changes of sea level based on the Arabian Platform and peri-Platform basins (Figure 1) that is tied to known eustatic cycles (global “mean”) could therefore become a useful exploration tool for the entire Arabian Plate. For example, a direct comparison of the regional events with the global cycles can reveal when tectonic influences were predominant on the Platform, or conversely, when eustasy prevailed in controlling sedimentary patterns. The comparison of the two curves could also yield better chronostratigraphic estimates and improved global correlations in regional studies on and around the Platform. In addition, a regional Cycle Chart would be a useful accompaniment to sequence-stratigraphic studies on the Arabian Platform and surrounding basins in all sedimentary environments.

In predominantly carbonate environments, such as those that prevailed for much of the Mesozoic and Cenozoic on the Arabian Platform, the regional onlap and relative sea-level curves will allow the estimation of the relative magnitude, extent, duration and severity of the sea-level fall events. This, in turn, can permit a better grasp of the broad characteristics of carbonate reservoirs and a better understanding of diagenesis and prediction of porosity and permeability. The regional cycles can also enhance our understanding of the reasons for increased or decreased organic productivity, and thus, source rock production and preservation, as well as the location of effective seals in the stratigraphic column.

In predominantly siliciclastic environments in both marine and non-marine settings, such as those that prevailed for much of the Paleozoic of the Arabian Platform, the use of the regional onlap curve will allow better understanding of the reasons for the extent of lateral migration of reservoir facies, magnitude of incision during sea-level falls and better volumetric estimates of reservoir facies during sea-level falls and organic-rich facies during sea-level rises. Such knowledge could, in turn, lead to better tracking of the location of reservoir facies, such as shoreface, basinal, incised-valley fill, and regressive lowstand and highstand sands. As stratigraphic plays and deeper waters become preferred targets for exploration, such “predictive” capability becomes invaluable.

An additional rationale for the construction of an Arabian Platform-wide Cycle Chart at this time was the publication of Sharland et al. (2001) synthesis of sequence stratigraphy of the Arabian Plate, which greatly facilitated such an effort. This synthesis has attempted to integrate a vast quantity of Phanerozoic data not only from the Arabian Platform but also from the surrounding basins. Thus, the construction of the Cycle Chart was opportune and was made considerably less time-consuming than it would have been otherwise.

A BRIEF SYNOPSIS OF TECTONICS OF THE ARABIAN PLATE

The Arabian Plate, a tectonic unit that includes the Arabian Shield, the adjoining Arabian Platform and the surrounding sedimentary basins has been subjected to a complex tectonic history since the Proterozoic (Figure 1). Especially obvious in the stratigraphic record is the evidence of major diastrophic events that caused widespread erosion, peneplanation and/or incision on the Platform. It may be instructive to review the broad Plate-wide tectonic events that governed these patterns so as to discriminate those intervals when the tectonic signal is expected to dominate eustasy. A brief summary of the major events that affected the Plate is given in Table 1.

Through much of the Paleozoic and Mesozoic the Arabian Plate was a part of Gondwana. As evidenced in the Arabian Shield and elsewhere, the Arabian Plate came together as a result of late Precambrian suturing of several terranes of largely basic and dense volcanic and plutonic materials. Once sutured, the composite was prone to long-term subsidence. Variations in the subsequent sedimentary cover reflect the orientation of the mostly northeast and north-northwest trending basement sutures (Al-Husseini, 2000; Ziegler, 2001). The late Precambrian also saw the onset of the northwest-southeast Najd strike-slip movement, which is evidenced from Egypt to Oman, with possible continuation into the then contiguous Indian Plate (Husseini, 1988; Al-Husseini, 2000). This was accompanied by the development of rift basins within which the Hormuz and Ara salt were deposited. Early Cambrian uplift led to widespread erosion and the subsequent Cambrian-Devonian sequences were mostly deposited on a peneplaned platform (Konert et al., 2001). The stratigraphic record on the northeastern shelf indicates gentle subsidence in the Late Cambrian and Early Ordovician, followed by increased subsidence in the mid Ordovician (Sharland et al., 2001) that led to transgressions in this area.

During Late Ordovician times (while the Arabian Plate occupied relatively high southern latitudinal location), a glacial episode occurred which affected the western part of the Arabian Plate, leaving behind evidence of glaciation on the Arabian Shield and broad and deeply cut sub-glacial valleys. The Plate continued to reside in relatively high latitudes until the Early Devonian and persisted as a promontory of Gondwana with passive margins. Thereafter, it started to drift northwards into lower latitudes, reaching tropical environments by Permian times (Konert et al., 2001). In Oman the Late Silurian saw an uplift, which may have been a precursor related to the eventual breakup of Gondwana. This led to a broad regression and stratigraphic gaps on the Platform (Sharland et al., 2001).

The next major deformational event that widely influenced the Arabian Plate was the so-called Hercynian Orogeny. The Hercynian is a term commonly used for Late Devonian to Permian diastrophic movements in Europe and North America. The timing of this orogeny on the Arabian Plate is more restricted (Late Devonian through Late Carboniferous), when the Plate suffered multiple phases of compression and block faulting. The northern edge of the Plate saw initiation of back-arc rifting and basaltic eruption. The compression, uplift of central Arabia and the clockwise rotation of the Plate produced widespread inversion and erosion, leading to the removal of several kilometers of sediment from the uplifted areas (Konert et al., 2001). The Plate then tilted gently towards the northeast and fluvial to alluvial clastics dominated over much of the peneplaned Hercynian relief (Ziegler, 2001).

During the Late Carboniferous and Permian the Arabian Plate moved from the relative high latitudes to lower latitudes, but not before another episode of glaciation near the Carboniferous-Permian boundary that affected the southernmost part of the Plate. The early Permian saw another phase of major crustal extension that weakened the crust enough to allow sediment load alone to drive subsidence and aid in the accumulation of thick carbonate sediments in subtropical latitudes. In the Late Permian further rifting and block faulting along the northeastern front of the Plate, as well in Syria, Iraq and India initiated the splitting of this part of Gondwana. It may also have reactivated the Precambrian Najd Rift system creating north northeast-trending ridges separated by broad valleys (Wender et al., 1998; Sharland et al., 2001). Thus, the Late Permian not only saw the initiation of the continental breakup of this region, but also the creation of a passive margin along most of the northeast boundary of the Plate fronting the newly opened Neo-Tethys Ocean after the Iranian terranes had split and drifted away. This was also the beginning of the dominantly carbonate sedimentation on the Platform over a breakup unconformity. The subsidence at the northeastern passive margin was initially largely post-rift thermal, to be replaced by dominantly due to sediment loading (Bishop, 1995).

The next major tectonic event was initiated in the Early Jurassic when the Indian Plate first began to split from Gondwana (Grabowski and Norton, 1995), completing its separation from Arabia in the Late Jurassic. Rifting in the central Mediterranean also began in the Early Jurassic, affecting the northern part of the Arabian Plate. Late Early Jurassic rifting at the northwestern boundary of the Plate led to later development of a new passive margin and new accommodation space along this subsiding shelf (Sharland et al., 2001).

The Mediterranean rifting continued into the Early Cretaceous, which may have been partially responsible for uplifting in western Arabia (Grabowski and Norton, 1995). Also by the Early Cretaceous, separation of India from Arabia had been accomplished and a new ocean had begun to open between the two Plates. The southern Oman margin was now also a subsiding passive margin, open to transgression. The far-field opening of the central and south Atlantic at this time may have also contributed to the uplift of the western part of the Arabian Plate and changed the gentle northeastward tilt of the Plate to the east. This shifted the dominant sediment transport direction, i.e. eastward from the uplifted area that supplied large volumes of sediments to the Platform (Al-Fares et al., 1998; Sharland et al., 2001). In the mid Cretaceous a new short-lived spreading ridge system formed off the northeastern margin of the Arabian Plate, most likely related to changes in the motion of the Indian Plate. In the early Late Cretaceous, accelerating convergence between the Arabian and Asian Plates led to the obduction of this new Neo-Tethyan oceanic crust as ophiolites along the northeastern margin of the Arabian Plate. This convergence also exerted compressive stresses over the Plate and caused uplift and erosion over many of the re-activated arches and anticlines.

The Cenozoic tectonic history of the Arabian Plate is largely that of continued rapid convergence between the Plate and Eurasia and the closure of the eastern part of the Neo-Tethys seaway. Much of this convergence was accommodated by subduction along the northern margins of the shrinking Tethys seaway, under the Iranian terranes and the Makran subduction zone of Pakistan. In the late Eocene the Plate began to collide with Asia and the closure of the Neo-Tethys was essentially complete by the late Oligocene. The Plate once again was tilted to the northeast due to rifting along the western margin and structural loading in the northeast. Rifting in the Gulf of Aden and Red Sea may have been initiated in the late Oligocene (Hughes et al., 1991), but it manifested itself fully in the mid to late Miocene, most likely aided by loading of the eastern part and flexing of the Plate (Sharland et al., 2001).

It is apparent from the above summary that the Arabian Plate suffered several long-term periods of tectonic deformation that alternated with periods of relative quiescence. The largely cyclic record of sedimentary patterns will reflect the influence of tectonism by displaying major gaps and less concurrence with the eustatic curves, which represent a mean global model, during the periods of upheaval, while intervals of relative stability will show greater correlation with eustasy.

CONSTRUCTION OF THE CYCLE CHART FOR THE ARABIAN PLATFORM

The Arabian Platform Cycle Chart was initially constructed based on existing plate-wide data from the public domain. It was later revised and refined and with the assistance of explorationists from Saudi Aramco, who offered invaluable information based on their experiences from the sedimentary sections they were familiar with. Following is a brief description of the various sections of the Cycle Chart and the data upon which they are based.

Geological Time Scales

A careful selection of Paleozoic and younger time scales for the calibration of cycles of sedimentation and attendant tectonic events is a crucially important first step in the construction of a cycle chart. The inherent problems with building time scales and the array of sources of error in various dating criteria (e.g., radiometry, biostratigraphy, magneto-stratigraphy, isotopic-stratigraphy) have been discussed in some detail in Haq et al. (1988) and will not be repeated here. However, these authors suggested that a meaningful time scale must make no unnecessary assumptions about constancy of rates of various events used to calibrate time scales (e.g., biological evolution, sea-floor spreading, etc.). It must also incorporate a large body of good radiometric dates to discern significant trends in the data, rather than relying on a few chosen dates as “tie points” to pin down segments of the time scale. And above all, it must use multiple, overlapping correlative criteria to reduce the noise to signal ratio (see Haq et al., 1988, for details). Though we still have a long way to go in producing a truly acceptable time scale that overcomes all of these caveats, especially for the older Periods, it should be noted that in recent years the new time scales are converging towards this philosophy. Thus, while good bio- and chrono-stratigraphic resolution is now available for the younger Periods (i.e. the Late Cretaceous through Cenozoic interval) that allows meaningful wider correlations, the older part of the stratigraphic column still suffers from lack of such resolution and true wider correlatibility.

This is exemplified by parts of the Paleozoic time scale, especially the Precambrian/Cambrian boundary and the earlier Paleozoic, which has been in a state of considerable flux over the last two decades. New numerical dates have shown the Cambrian/Precambrian boundary to be some 48 million years (my) younger than previous estimates (i.e. 542 Ma vs 590 Ma). Many of the younger boundaries of the Paleozoic have also been modified significantly (e.g., Ordovician/Cambrian, Devonian/Silurian, Carboniferous/Devonian and Permian/Carboniferous boundaries have all been modified by several million years). Similarly, there have also been refinements of up to several my in the placement of the Mesozoic Epoch (and sub-epoch) boundaries (especially in the Triassic), and, to a lesser extent, in the Cenozoic.

After considering the nuances of all the available geological time scales, we had originally adopted the time scale summarized by Gradstein and Ogg (1996) for the construction of the Arabian Platform Cycle Chart. These authors had presented a composite time scale of the three Eras of the Phanerozoic based on three different time scales. They essentially used the Harland et al. (1990) time scale for the Paleozoic, the Gradstein et al. (1994) time scale for the Mesozoic, and Berggren et al. (1995) time scale for the Cenozoic. One of the added advantages of using this composite was that it was in wide use and had also been adopted by Sharland et al. (2001) for their synthesis of the Arabian Plate stratigraphy and its use would have facilitated direct comparisons, especially in assignment of ages of the major maximum flooding surfaces (MFSs) identified by these authors. However, since the submission of this paper for publication a new composite time scale (Gradstein et al., 2004) known as the Geological Time Scale 2004 (GTS 2004) was presented at the International Geological Congress in Florence, Italy, that has the concurrence of the International Commission on Stratigraphy. Much of the Paleozoic and Mesozoic (with the exception of Late Cretaceous) has been modified in the GTS 2004 over the old (Gradstein and Ogg, 1996) version. We have adopted GTS 2004 and have revised the Arabian Platform Cycle Chart accordingly. This entailed recalibration of the major MFSs of Sharland et al. (2001, 2004) to the new numerical time scale. This recalibration is presented in Table 2. As an added consequence, the Haq et al. (1988) eustatic curves have also been recalibrated to GTS 2004 for the first time, and presented on the accompanying Cycle Chart.

Arabian Platform stratigraphy and chronostratigraphy

Since a Cycle Chart with an abundance of stratigraphic names from the Platform, often different in different countries for the same formation, would have been too cumbersome, the formation and member names have been largely adopted from the existing stratigraphic nomenclature from Eastern Saudi Arabia. Regional equivalents of these names in other countries are well established, and can be found in a number of key country-specific papers, as well as platform-wide comparisons (e.g. Powers, 1968; Sugden and Standring, 1975; Hughes Clark, 1988; Hassan, 1989; Chaube and Al-Samahiji, 1995; Yousif and Nouman, 1997; Al-Husseini, 1997; Christian, 1997; Sharland et al., 2001, and the references therein). As an initial step the chronostratigraphic extent of the formation and members was plotted following Sharland et al.’s (2001) synthesis of the ages of the Formation boundaries. These boundary placements were further refined following discussions with specialists from Saudi Aramco based on existing data. Thus, the stratigraphic extent of the formations/members shown is a “consensus” and does not necessarily conform to any individual opinion. Similarly, the general i.e. gross lithology of the formations also follows Sharland et al. synthesis, but was modified after discussions with specialists. For simplicity, the lithological units are distinguished only into four major types: sandstone, shale, carbonate and evaporite.

Regional onlap and sea-level curves

On the Arabian Plate much of the ocean’s transgressive activity that inundated the Platform was from the present north and the northeastern directions, while sediment supply was mostly from the western highs toward the north and northeast. Thus, the dominant sediment onlap patterns also trend from the north and northeast and as is to be expected, sections are in general more completely preserved basinward i.e. toward the northeast.

The procedure for constructing an onlap curve for the Arabian Platform comprises a region-wide approach. As far as possible, sections from Saudi Arabia were used to determine the position of the maximum flooding surfaces (MFS) and the intervening sequence boundaries (SB). However, when no sections from Saudi Arabia were available, either due to non-deposition/erosion in those intervals or because none have as yet been studied, sections from neighboring Oman, Yemen, southern Iraq and the Greater Gulf were used to fill the gaps. This is especially true for periods when the sea level withdrew below Platform top or tectonics led to extensive erosion on the Platform. During such times sea-level history is preserved only on the margins of the Platform or in the incised valleys, making it necessary and appropriate to include the peri-Platform sections in a regional sea level synthesis. Consequently, it should be underscored that the resultant onlap curve is for the whole region of the Arabian Platform and its margins and not limited to Saudi Arabia. It should be noted, however, that sections from deformed margins of Turkey and Iran were not included. These areas have undergone dramatic tectonic events, implying that local tectonics may sufficiently complicate the record that it may be at variance with that of the Platform.

The flooding of the Platform surface is represented first by deposition of the transgressive deposits (transgressive systems tract – TST) overlain by deposits of the highstand systems tract (HST). Lowstand (lowstand systems tracts – LST) equivalents are only preserved on the Platform within incised valleys (as incised valley fills). Although Sharland et al. (2001) did not identify sequence boundaries, they attempted to date major maximum flooding surfaces (MFSs) and tie them regionally. Consequently, it was considered practical and advantageous to use the major regional MFSs identified in this synthesis as a starting point for constructing the onlap curve (recalibrated to GTS 2004 for the numerical time scale, see Table 2). However, these authors underscored that the identification and age assignment of many MFS, especially those in the Paleozoic and Triassic, are fraught with numerous uncertainties. These include both the uncertainties of actual physical placement of MFSs within a section and biostratigraphic dating, as well as uncertainties in their calibration to the absolute time scales (see Sharland et al., 2001, p. 52). Thus, sequence boundary ages as identified here suffer even more from the uncertainties ascribed to the dating of MFSs, the latter being normally much better constrained than the hiatuses represented by sequence boundaries.

At present no meaningful approach exists to determine the absolute scale (in meters) of rises and falls of sea level. All of the various methodologies (e.g. coastal aggradation during highstands, extent of marine inundation, depth of shelf incision during lowstands, changes in fossil paleobathymetry, oxygen-isotopic variations reflecting ice-volume fluctuations, etc.) have inherent uncertainties and imply considerable error ranges. Thus, instead of trying to estimate the absolute movement of base level in meters (with significant error bars), a relative scale has been used for the construction of the onlap curve and the sea-level curve that is based on it.

In a series of paleofacies reconstructions of the Arabian Plate for the Permian to Holocene interval, Ziegler (2001) has produced a valuable graphic history of marine inundation (and relative water depth) and subaerial exposure of the Platform. These paleofacies maps have been very helpful in assigning the relative magnitude of the sea-level fluctuations, as was the discussion of the overall paleofacies in Sharland et al. (2001). The approximate position of the Platform surface, i.e. not the entire surface, but its lowest mean elevation, has been identified as a useful marker level for the onlap curves. In this regard it should be noted that even though much of the central Platform may be bare during some intervals, shallow-marine, or even deeper marine facies may continue to be deposited in some peri-Platform basins, e.g. the Middle Cambrian paleofacies map in Konert et al. (2001). In such cases, local relative base-level height would need to be adjusted when consulting the Cycle Chart. Clues to the long-term behavior of sea level are provided by several indicators: (1) the flooding of the Platform surface, indicated by preservation of extensive marine deposits; (2) the thickness of the sections preserved on or off the Platform; (3) the depth of incision and the nature and thickness of sediments preserved in incised valleys and on distal shelf and slope; and (4) paleobathymetric estimates from litho- and biofacies. These data provide the template against which the shorter-term relative sea-level changes can be scaled, i.e. mostly third-order cycles, but occasionally fourth-order cycles as well, when these can be recognized consistently over a wide area.

Once the overall regional onlap patterns were in place, individual sections were consulted to interpret the details. Sequence boundaries and additional maximum flooding surfaces, as well as transgressive surfaces (TSs) and lowstand equivalents, where they could be identified, were demarcated from these sections. The individual sections considered (see some of the examples in the Appendix) are mostly those from the Sharland et al. (2001) synthesis, which has the advantage that they are all in the public domain, have all been treated in a uniform manner, and that attempts to date individual horizons according to internally consistent criteria.

The relative sea-level curve for the Arabian Platform is a derivative of the regional onlap curve. While the onlap curve shows a saw-toothed pattern, because of the continued coastal onlap above the maximum flooding surfaces, represented by the deposition of non-marine fluvial and near-coastal sediments during the regressive part of the highstand, the relative sea-level change curve shows individual events as more sinusoidal (as the sea began to recede after the maximum flooding surface was reached). Another difference is that the onlap curve is scaled against an approximate marker level of the Platform top, while the relative sea-level curve is scored against the modern base level for convenience. Thus, the onlap curve can be thought of as representing the changes in the sedimentary patterns, while the relative sea-level curve represents the actual changes in the base level that drove the stratigraphic changes.

Reservoir, known and potential source-rich intervals

Known reservoir intervals and their nomenclature (as shown on the Cycle Chart) are largely those from Eastern Saudi Arabia, and have been differentiated into oil, non-associated gas, and oil plus gas reservoirs. In addition to the known reservoirs, lowstand incised valley fills may also prove to be good local reservoirs with considerable extent. The known source rock intervals are adopted from Sharland et al. (2001), whilst potential source rock intervals include ‘condensed’ sections associated with maximum flooding surfaces identified by Sharland et al. as well as other intervals interpreted here.

Systems tracts

The column between the regional onlap and relative sea-level curve contains the interpretation of the systems tracts associated with each identifiable sequence. In most cases, lowstand systems tracts (LST) cannot be separated from transgressive systems tracts (TST) and these are lumped together below the MFS. On the Platform itself the lowstand interval would span the time of the erosion associated with the sea-level fall, represented by the SB, overlain directly by transgressive deposits, thus, on the Platform top, the LST does not occur physically but only in the sense of an equivalent time gap. However, this interval may include a lowstand component on the Platform margins, as for example incised valley fill, or in the basin as a lowstand wedge and/or fan. The generally upward-deepening TST sediment packages are overlain by the commonly upward-shoaling deposits of the highstand systems tracts (HST). The sequence numbering (or rather the MFS numbering, e.g. P10, P20, P30, Tr10, Tr20, Tr30, etc.) follows the Sharland et al. (2001, 2004) scheme.

Stratigraphic sections used for detailed interpretations

Information on the Cambrian sequences (Upper Hormuz Salt, Siq, Burj and lower part of Saq formations) comes mostly from Oman. These sections include the upper part of the Huqf Supergroup, the Ara Group, the Haima Supergroup and Ghaba Salt Basin, southeastern Oman (after Gorin et al., 1982; Hughes Clark, 1988; Droste, 1997; and modifications in Sharland et al., 2001). (See also the Appendix for examples).

For the Ordovician and Silurian sequences (the upper part of Saq, Qasim, Sarah, Qalibah and lowermost Tawil formations in Saudi Arabia and their chronostratigraphic equivalents elsewhere) several sections are exposed in central Saudi Arabia (after Vaslet, 1990; Mahmoud et al., 1992; Senalp and Al-Duaiji, 2001b). For Devonian sequences (upper Tawil, Jauf and much of Jubah formations and their equivalents) sections are encountered in the Ghawar area and in northwestern Saudi Arabia (after Wender et al., 1998; Boucot et al., 1985; and modifications in Sharland et al., 2001). The Carboniferous (upper part of Jubah and the Berwath formations and their equivalents) stratigraphic information largely comes from the Greater Gulf area (Clayton et al., 2000). For the Permian (Unayzah and Lower Khuff formations and their equivalents) information is drawn from central Saudi Arabia (McGillivray and Husseini, 1992; Senalp and Al-Duaiji, 1995 and 2001a; Evans et al., 1997; and Al-Aswad, 1997), the Ghawar area (Wender et al., 1998) and the Greater Gulf area (El-Bishlawy, 1985) (See also the Appendix for examples).

Details of the Triassic sequences (Upper Khuff, Sudair, Jilh and Minjur formations and their equivalents) were gleaned from sections in the Greater Gulf area and Oman (after Loutfi and Sattar, 1987; Hughes Clarke, 1988) (see Appendix). Note that the Minjur Formation is placed completely within the Late Triassic. The Jurassic sequences (Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila, Arab, Hith and Sulaiy formations and their equivalents) have been identified in sections from central Saudi Arabia, Oman, Yemen, Kuwait and the Greater Gulf area (after Enay et al., 1987; Hughes Clarke, 1988; Droste, 1990; Yousif and Nouman, 1997; Howarth and Morris, 1998; Azer and Peebles, 1998; and modifications in Sharland et al., 2001). Information about the Lower Cretaceous (Yamama, Buwaib, Biyadh and Shu’aiba formations and their equivalents) and the Upper Cretaceous sequences (Wasia and Aruma formations and their equivalents) is drawn from outcrop sections in Oman and largely subsurface data from Yemen, Kuwait and the Greater Gulf area (after Hughes Clark, 1988; Simmons, 1990; Holden and Kerr, 1997; Brannan et al., 1997; Hughes, 2000; Philip et al., 1995; Brennan, 1990; Al-Fares et al., 1998; Aziz and Sattar, 1997; Alsharhan, 1987; Pascoe et al., 1995; Smith et al., 1995; and modifications in Sharland et al., 2001).

The Paleogene sequences (Umm er Raduma, Rus and Dammam formations and their equivalents) are based on sections in eastern Saudi Arabia, Yemen and Oman (Hasson, 1985; Hughes Clark, 1988; Nolan et al., 1990; Brannan et al., 1997; Weijermars, 1999). The Neogene information (for Hadrukh, Dam, Hofuf and Dibdibba formations and their equivalents) comes largely from Oman (Hughes Clark, 1998).

Global onlap and sea-level curves

The global onlap and its derivative sea-level curves are plotted next to regional curves for comparison. For the Paleozoic the onlap curve is largely a curve of second-order events. It is a composite that uses the long-term trends of Vail et al. (1977) and Hallam (1992) Paleozoic curves (calibrated to the new GTS 2004 time scale used here) as a template and builds on it for details with additional information from other areas. To exclude purely local events, only those events are depicted on the global ‘mean’ curve that are common to several different basins. Additional information is based on: (1) the published curves of Ross and Ross (1988), a synthesis that includes widely distributed data (though mostly from the United States interior basins); (2) the Devonian synthesis of Euramerica (Johnson et al., 1985); (3) Ordovician to Devonian of western and northern Canadian basins (Lenz, 1982); (4) late Paleozoic of northwestern China (Chen and Shi, 2003), and published information from North Africa (Keeley, 1989; Schandelmeier and Reynolds, 1997); recent work on the Algerian Paleozoic by Yahi et al., 2001); as well as (5) unpublished data from US interior basins. It must be underscored that the Paleozoic global curve is chronostratigraphically much less well constrained than those for younger Eras and should be considered approximate at best. Nevertheless, it represents an adequate working model, especially for the long-term trend of the base-level fluctuations, i.e. at the level of second-order cycles, and for some intervals at third-order level. As better-constrained chronostratigraphic data accumulates, refinements of the Paleozoic curves are to be expected, especially in identifying more third-order events.

The Mesozoic and Cenozoic part of the global curve (with identified third-order cyclic eustatic events) is based on Haq et al. (1988), rescaled and tied to the time scale adopted here. This global “mean” working model also includes only those sea-level change events that were documented in several widely distributed basins to establish that these fluctuations were of broad extent. The cycles were also tied directly to the standard European chronostratigraphy (i.e. stage stratotypes) to facilitate worldwide correlations. The recalibration of eustatic curves to GTS 2004 (presented on the accompanying Cycle Chart) in itself provides an expedient update of this widely used working model for exploration and paleoceanographic studies.

Major tectonic, paleoceanographic and climatic events

The major tectonic events column comprises both the regional (Arabian Plate) and the global events. These are based on various sources, including Grabowski and Norton (1995), Haq and van Eysinga (1998), Konert et al. (2001), and Sharland et al. (2001), among others.

The paleoceanographic and climatic events’ column also includes those events that occurred specifically on or around the Arabian Platform, as well as global events. These are largely based on Sharland et al. (2001) for local glaciations, and Haq and van Eysinga (1998) for all other global events. The experience and views of Saudi Aramco specialists were also taken into account in completing these columns.

Tectonostratigraphic megasequences

For comparison, in the right-hand column, the chart also includes the extent of the tectonostratigraphic megasequences as defined by the Sharland et al. (2001) synthesis (their boundaries have been recalibrated to GTS 2004). These authors have identified eleven Arabian Plate megasequences (AP1-AP11) that are separated by major unconformities caused by tectonic events.

DISCUSSION

The preceding sections outlined the procedure followed for integrating regional cycles of sediment deposition and erosion, driven by base-level changes, on and around the Arabian Platform. The regional cycles are plotted against the ‘global’ model of sedimentary cyclicity. In this regard it needs to be underscored that the few, if any, ‘global’ events are ever worldwide or their effect ever instantaneous, and the magnitude of change at various places may differ depending on local conditions. Eustasy is no exception. Hence, the global curve can only be seen as an “average” (or a most likely scenario) during times of relative tectonic quiescence. If a comparison of the regional patterns with the global patterns reveals that during a discrete time interval the trends in regional cyclicity mimics global cyclicity, it indicates that eustasy was the dominant control on sedimentary patterns. Other intervals that show little or no correspondence between regional and global patterns imply an overriding control by tectonic and other factors.

Even a cursory look at the Cycle Chart reveals that there were several conspicuous and long periods of hiatus in sedimentation on the Arabian Platform. The older of such enduring (c. 25 my in duration) period of hiatus lasted through much of the Carboniferous and was linked to multiple phases of compression and deformation and/or inversion during the Hercynian Orogeny that led to the erosion of several kilometers of sediments on the Platform. The next younger major period of hiatus (c. 20 my in duration) lasted from latest Triassic through the Early Jurassic and was associated with a phase of the breakup of Gondawana that led to the rifting of India from Arabia/Africa and tilting of the Arabian Plate towards the north. The youngest major period of hiatus (c. 10 my in duration) occurred in the Oligocene, which may have been a consequence of the collision of the Arabian Plate with Eurasia and the closure of the eastern limb of Neo-Tethys Ocean. There was also a major draw-down of sea level starting in latest Eocene due to initiation of major buildup of ice cap on Antarctica.

A comparison of the regional and the global cycles reveals that while the Platform was gently subsiding during the Cambrian through early Silurian interval, eustasy may have been an important if not the principal controlling factor that determined sedimentary patterns. The two curves (on the Cycle Chart) show similar long-term trends, though there are more relative sea-level change events identified on the Platform. This is not surprising since the global curve shows mostly second-order cycles for much of the Paleozoic, while many of the Arabian Platform sequences may represent third-order or even higher-order cycles. There is a sharp sea-level drop in the Late Ordovician, on both the Arabian Platform and the global curves, which may be attributable to the glacial episode that affected this part of Gondwana at that time.

From late Silurian through Early Carboniferous the regional and global curves show little correspondence. The long-term trend on the Arabian Platform is of a continued regional sea-level rise, while on the global curve much of Devonian through Carboniferous interval saw a gradual sea-level fall. These differences can be ascribed to active tectonics that obscure the underlying eustatic signal. Uplift in Oman in the mid Silurian, which was a precursor to continental breakup, and the beginning of Hercynian compression and the erosional episodes that followed in Late Devonian may have been partially responsible for these differences. Widespread inversion and erosion during the Hercynian Orogeny was responsible for removal of much of the late Carboniferous sediments making it difficult to reconstruct the history of sedimentary cyclicity for this interval on the Platform. The early Permian regional cycles most likely were also influenced by tectonics when the Platform suffered through another phase of major crustal extension with the rifting of the Neo-Tethys margin. However, glacial episodes in the late Carboniferous and early Permian on the Arabian Platform and the Southern Hemisphere (with global repercussions) make the fluvio-glacial record of this interval (preserved as valley fills) of the Platform and elsewhere (e.g. northwestern China – see Chen and Shi, 2003) look similar. Mid Permian sea-level drop is more prominent on the global curve than it is on the Arabian Platform and the late Permian through Triassic record is well developed on the Platform when the Arabian Plate gradually moved into the equatorial latitudes.

The record of latest Triassic through Early Jurassic (Rhaetian through Pliensbachian) is also largely missing from the Platform due to erosion or non-deposition. This break may be related to major tectonic events that initiated the splitting of the Indian Plate from Gondwana and Arabia and rifting in the central Mediterranean in the Early Jurassic. These events affected a good deal of eastern and northern parts of the Arabian Plate. Through much of the remainder of the Mesozoic and early Paleogene sedimentary cycles on the Platform show good agreement with the eustatic patterns, both in the longer-term trends and the frequency of third-order events, though the number of events in some discrete intervals differ. The only exceptions to this general correspondence are in the latest Jurassic through Early Cretaceous (mid Tithonian to mid Valanginian) interval, although the long-term trends are similar. These differences, once again, may be due to the continued rifting in the Mediterranean that lasted into Early Cretaceous, and the beginning of the opening of the Atlantic Ocean, both of which may have led to the uplift of western Arabia. Also, the Turonian sea-level drop seems to occur slightly earlier on the Platform than on the global curve, but this may be an artifact of the placement of the upper limit of the major unconformity within this stage. These overall trends indicate that from Middle Jurassic through the early Eocene eustasy was the dominant control over sedimentary sequences on the Platform. In their study of the Jurassic and Cretaceous stratigraphy of the central Arabian Platform, Le Nindre et al. (1990) also reached the conclusion that in these periods eustatic changes appear to be responsible for sedimentary patterns on the Platform.

The late Paleogene of the Platform and Neogene of the peri-Platform basins (Oman) show longer-term trends similar to the global curve, though the rich number of third-order events seen on the global curve are much less numerous regionally. This is to be anticipated since the tectonic influence was dominant after late Eocene times, as the Tethys Seaway began to close, and especially after the early Miocene as the Zagros Mountains began to emerge along the northeastern margin of the Plate, causing the sea to retreat from much of the Platform.

ACKNOWLEDGEMENTS

The Arabian Platform Cycle Chart was first published at the American Association of Petroleum Geologists’ meeting in Cairo, Egypt in October 2002 (Haq and Al-Qahtani, 2002 and Al-Qahtani et al., 2002). The authors thank Saudi Aramco for the opportunity to construct the Cycle Chart and for the approval for its publication. The Chart was refined with the help of specialists in Saudi Aramco’s various mapping groups: Bassam Bahabri, Roger Price, Kent Norton, Muhittin Senalp, Christian Heine, Geraint Hughes, Sa’id Al-Hajiri, Abdelghayoum Ahmad, Abdel Fattah Bakhiet and Mike Hulver. Their help is gratefully acknowledged. Two anonymous reviewers and Moujahed Al-Husseini are also acknowledged for their very valuable comments that improved the quality of the text and contents of the Cycle Chart.

APPENDIX

The Appendix includes figures and brief descriptions of some of the stratigraphic sections utilized to interpret sequence boundaries and system tracts in various segments of the stratigraphic column. These have been selected (from a larger number considered for this synthesis) to exemplify the process of building the Phanerozoic onlap patterns (and their derivative sea-level curves) as plotted on the Cycle Chart (see also section on “Stratigraphic Sections used for Detailed Interpretations”). Sequence boundaries (SBs) and maximum flooding surfaces (MFSs) are plotted alongside existing interpretations by the authors of the published sections. Occasionally, the existing interpretations are modified and reasons are given for the changes. The methodology of construction of regional onlap and sea-level curves are also discussed in the main body of the text.

For consistency, most of the figures (ascribed to various authors mentioned below) included in the Appendix to exemplify our methodology are taken from the versions published by Sharland et al. (2001) who have already to some extent modified some of the original figures. These authors include: Boucot et al. (1985), Loutfi and Abdel Sattar (1987), Wilson (1985), Hughes Clark (1988), Droste (1990), Yousif and Nouman (1997), Droste (1997), Wender et al. (1998), and Al-Fares et al. (1998), Clayton et al. (2000). Five summary chronostratigraphic figures of Sharland et al. (2001), (our Figures 2, 5, 8, 10 and 12) that incorporate their major numbered MFSs, as well as the SBs and MFSs interpreted by us, are also reproduced. The location of various stratigraphic outcrop and well sections selected as examples here are shown in Figure 1.

A schematic chronostratigraphic cross-section that summarizes the Cambrian through early Silurian stratigraphy of the Arabian Platform (adapted from Sharland et al., 2001) is reproduced in Figure 2. This summary is a convenient way to envision the cycles and broad transgressions of the Platform from the northeast and the dominant lithologies associated with these wide transgressive-regressive cycles (indicated by Sharland et al. with large arrows in the retrogradational and progradational directions). For convenience, sequence boundaries (shown as red lines in the SB/MFS columns) are placed in the middle of the identified hiatuses. However, the exact chronological position of the SBs within these intervals is uncertain (and can shift up or down).

It is also obvious from this summary that in addition to hiatuses at the SBs, large gaps also exist in the stratigraphic succession of the Platform where record of additional sequences deposited during these intervals may be missing. In glacial environments (i.e. in the late Ashgill = Hirnantian in GTS 2004) sequences are difficult to interpret, though (as in the case of fluvial incisions) SBs can be drawn at the base of the glacial valleys. In the Silurian, in the regressive successions above the Sarah and within the Qusaiba formations, several higher-order sequences (representing fourth-order cycles of a few 100 ky duration) can be identified. In the Cambrian through early Silurian interval depicted by this summary figure, thirteen major SBs are indicated by the presence of major hiatal surfaces, and nine additional SBs are interpreted (eight of those in the Ashgill are higher-order cycles mentioned above).

The Ara Group of Oman as seen in the Birba-1 well (Figure 3) represents a late Proterozoic through Early Cambrian section (after Amthor et al., 2003) with the lowermost major MFS (Cm10 at 538 Ma according to GTS 2004) that occurs within a carbonate stringer in this largely evaporitic environment. In such environments, SBs can be placed at the top of each evaporitic (desiccation) cycle. Thus, in addition to the major hiatus at 306 Ma, six additional SBs can be identified in this section. Intervening MFSs are normally placed within carbonate or shale intervals, especially where they span relatively high gamma-ray values on the e-logs (though such a relationship may not always hold in pure carbonate environments).

Droste (1997) has summarized the Cambrian through early Silurian lithostratigraphy of the Haima Supergroup of Oman (Figure 4). Here SBs can be placed at the prominent hiatal surfaces or interpreted above regressive units, and intervening MFSs can be placed within shale-rich intervals. Seven major MFSs have been identified by Sharland et al. (2001) in this interval (numbered Cm20–S10 and reassigned ages according to GTS 2004). It should be noted, however, that these cycles represent broad, second-order sequences (each of which may, when seen in more detail, comprise several third-order sequences)

Silurian through early Carboniferous stratigraphy (up to the initiation of the Hercynian Orogeny) is summarized in the chronostratigraphic cross-section of Figure 5 (after Sharland et al., 2001). Sharland et al. identified several broad transgressive-regressive cycles (indicated by large retrogradational and progradational arrows). Prominent hiatuses represent major SBs (where the record of additional sequences may be missing). The record of higher-order cycles (in the Qusaiba above and below the MFS S10) is repeated here.

The next figure (Figure 6) shows the Early Devonian of the Jauf Formation in northwest Saudi Arabia (after Boucot et al., 1985) and the SBs and MFSs interpreted in this section. SBs are placed at the top of the regressive units. In the Pragian through Emsian interval nine SBs have been interpreted that most likely represent third-order and/or higher-order cycles. The MFSs interpreted by Sharland et al’s (D10 and D20 and reassigned ages according to GTS 2004) and those interpreted here (within the marine shale units) are also shown.

The latest Devonian to early Carboniferous interval is exemplified by the Abu Safah-29 well (Figure 7) from the northern Arabian Gulf (after Clayton et al., 2000). Sharland et al. (2001) have identified a single major MFS (C10) in this section which has been moved slightly lower to be centered within the high gamma-ray shale interval. Several higher-order cycles can be identified within the upper Jubah and Berwath formations.

The events during the main Hercynian Orogeny are responsible for the non-deposition and/or removal of over 20 my record of sedimentation on the Arabian Platform (spanning the middle and late Carboniferous) that was followed by a widespread period of glaciation (sea-level lowstand) characterized by glacial and fluvio-glacial incision on the Platform. Figure 8 shows a chronostratigraphic cross-section of the post-Hercynian Permian to Early Triassic interval (after Sharland et al., 2001). Earliest Permian (Unayzah B) interval is characterized by stacked fluvio-glacial valley fills. The incision of each generation of valleys may be seen as a discrete sequence boundary (MFSs can not be interpreted in such environments). The later early Permian (Unayzah A) is also typified by stacked incised valley fills, which may also represent higher-order cycles. Within the Permian eleven SBs can be identified but only a few MFSs can be picked. In the late Permian and Scythian (Khuff A-C) carbonate sedimentation begins to dominate and discrete sedimentary cycles become discernable once again. Major MFSs P10 through Tr30 have been reassigned ages according to GTS 2004 in this summary figure.

A lithostratigraphic cross-section (Figure 9) for the Permian-Early Triassic interval from the Unayzah reservoir (Unayzah and Khuff formations overlying the pre-Unayzah unconformity and the Silurian Qusaiba Member) in the Ghawar area of eastern Saudi Arabia is reproduced here (after Wender et al., 1998). Several third-order cycles can be identified in the Unayzah and are placed near the top of the regressive units. Higher-order cycles are seen in the Khuff Formation where the tops of desiccation events mark the SBs. Ages of the 2 major hiatal events have been reassigned ages according to GTS 2004.

Figure 10 shows a summary chronostratigraphic cross-section that spans the Triassic interval (after Sharland et al., 2001). Most of the SBs occur at hiatal surfaces where there are prominent gaps in sedimentation. Intervening MFSs of Sharland et al. (Tr10–Tr80) and those interpreted here are also shown. Sharland et al. identified eight broad transgressive-regressive cycles in the Triassic of the Platform (indicated by large retrogradational and progradational arrows). Here we interpret eleven complete (third-order) cycles within the Triassic interval. The close of the Triassic saw a long-term drop of sea-level that lasted nearly 20 my into the Jurassic and is characterized by lack of sedimentation and/or erosion on the Platform. The major MFSs have been reassigned ages according to GTS 2004.

The Triassic succession is exemplified by the stratigraphic section of well Umm Shaif-116 off Abu Dhabi (Figure 11). Sharland et al.’s major MFSs Tr30 and Tr50–Tr70 are recoded here (and shown with reassigned ages according to GTS 2004). In addition, several minor MFSs as well as SBs are also interpreted in this composite section (after Loufti and Abdel Sattar, 1987, fide Sharland et al., 2001).

A composite chronostratigraphic summary (Figure 12) that spans the Jurassic through Cretaceous interval is adapted from Sharland et al. (2001). Major Jurassic and Cretaceous MFSs of Sharland et al. (J10–J110 and K10–K150 with reassigned ages according to GTS 2004) and additional MFSs as well as intervening SBs interpreted here are also shown. As mentioned earlier, a long-term global sea-level fall that began near the end of Triassic (late Norian) continued into the Early Jurassic (Hettangian through Pliensbachian) causing extensive erosion and/or non-deposition on the Platform and is represented by a major sedimentary gap.

A Jurassic lithostratigraphic section from Well Minagish-27 of Kuwait (after Yousif and Nouman, 1997) is reproduced in Figure 13 together with the sequence stratigraphic interpretations. Higher-order cycles are interpreted in the lower Marrat Formation (SBs are picked at the end of the desiccation cycles), topped by the major MFS (J10) of Sharland et al. (2001). Ages of J10 and other MFSs have been modified according to GTS 2004

Figure 14 shows a Middle to Late Jurassic lithostratigraphic section from Qatar (after Droste, 1990) together with the sequence stratigraphic interpretations. These include the Sharland et al.’s major MFSs (J20–J100 with reassigned ages according to GTS 2004) and the intervening SBs interpreted by us. In these carbonate sequences the SBs generally occur very close to the overlying MFSs. The stratigraphic section from the Qatif field of eastern Saudi Arabia (Figure 15) also exemplifies the Middle to Late Jurassic of the region (after Wilson, 1985). Sharland et al.’s J40 through J110 MFSs are recorded here. Three higher-order cycles are interpreted below MFS J80 and five above J100. In the latter, MFSs can not be easily interpreted but most likely occur within the limestone units.

In Well “F” from Kuwait offshore (Figures 16-18) a nearly complete latest Jurassic through Cretaceous interval is represented (after Al-Fares et al., 1998). Figure 16 exemplifies the older part of Early Cretaceous interval. Sharland et al.’s major MFSs (J110 through K40 with reassigned ages according to GTS 2004) are shown together with the intervening SBs and additional cycles identified here. Cycles typically occur in this interval as large limestone-shale couplets and SBs are picked at the top of the regressive units. The middle part of this well (Figure 17) spans the younger Early Cretaceous interval (Sharland’s MFS K30–K40 are repeated, and K50 through K100 with reassigned ages according to GTS 2004 are also shown). Minor cycles are also identified in this dominantly siliciclastic interval. Note that Sharland et al.’s K60 and K70 have been moved slightly (higher and lower, respectively) to center within the organic-rich beds. The upper part of Well F spans the Albian-Cenomanian interval (Figure 18) and once again becomes dominantly carbonate, showing large limestone-shale couplets that represent sequence cycles.

We have selected the section from the Baldi-1 Well from Oman to exemplify the Paleogene of the Platform (Figure 19, after Hughes Clarke, 1988). Although much of the Paleocene-Early Eocene is dominantly limestone and interpreting cycles based on log-response is largely conjectural, the younger part of Early Eocene does show several evaporitic events allowing cyclic interpretations. Sharland et al.’s Pg10 MFS is shown near a gamma-ray peak on the e-log and Pg20 MFS is picked at an organic-rich shale event with reassigned ages according to GTS 2004.

ABOUT THE AUTHORS

Bilal U. Haq received his PhD and DSc in Geology from the University of Stockholm, Sweden. He has conducted research at the Woods Hole Oceanographic Institution, Exxon Production Research Labs in Houston and the National Science Foundation in Washington, DC and has held assignments with the White House and the World Bank. Bilal’s research has included paleoceanography, paleobiogeography, paleoclimatology, sequence stratigraphy and global sea-level change, natural gas hydrates, and global warming and its impact on maritime economies. He has participated in four ocean drilling expeditions to the Pacific, Indian and Southern Oceans, in the last two as the co-chief Scientist. He has published extensively on many topics in marine geosciences and authored or co-authored over 150 papers and books. Bilal’s papers on paleoceanography, sequence stratigraphy and sea level changes of the past have been among some of the most cited publications in Sedimentary Geology for more than a decade. He has been an American Association of Petroleum Geologists’ Distinguished Lecturer and was awarded the Francis P. Shepard Medal for excellence in Marine Geology by the Society for Sedimentary Geology (Society for Economic Paleontologists and Mineralogists). He was conferred honorary professorships by Tongji University in Shanghai and by the Academia Sinica Institute of Oceanology in Qingdao, China. Bilal was elected a Fellow by the Geological Society of America and by the American Association for the Advancement of Science. He also received the National Science Foundation’s Antarctic Medal and was recently awarded the Ocean Sciences Award by the American Geophysical Union for “significant contributions and services to ocean sciences”.

bhaq@netscape.com

Abdul Motaleb Al-Qahtani holds a BSc in Geology from King Saud University (1983), Riyadh, and MSc in Geophysical Engineering from Colorado School of Mines (1988). He has over two decades of exploration experience in subsurface geological/geophysical mapping and evaluating hydrocarbon plays and prospects. Abdul Motaleb has written about and presented several exploration topics concerning Paleozoic and Mesozoic stratigraphy of the Arabian Platform and subsurface mapping in the Gulf region and at AAPG. He held various assignments in geophysical technology, processing, acquisition and petroleum exploration/seismic interpretation (Chevron) prior to joining Regional Mapping and Special Studies of Area Exploration Department at Saudi Aramco and has since held several technical leadership assignments within the organization. He is presently leading a group to evaluate hydrocarbon plays and prospects in the offshore Arabian Gulf. Abdul Motaleb has also represented Aramco on the Stratigraphic Committee of the Arabian Gulf in the Gulf Cooperation Council (GCC). He is a member of the AAPG, SPE and Dhahran Geosciences Society.

abdelmuttaleb.qahtani@aramco.com