Geological Society Special Publications

The breakup of the former Pangea supercontinent culminated in the modern drifting continents. Increased rifting caused the establishment of the Atlantic Ocean in the middle Jurassic and significant widening in Cretaceous. An explosion of calcareous nannoplankton and foraminifers in the warm seas created massive chalk deposits. A surge in submarine volcanic activity enhanced supergreenhouse conditions in the middle Cretaceous with high CO2 concentrations. Angiosperm plants bloomed on the dinosaur-dominated land during late Cretaceous. The Cretaceous dramatically ended with an asteroid impact, which resulted in a mass extinction.

The Cretaceous Period

\n To better constrain the spatial and stratigraphic distribution of the depositional facies, a synthesis of outcrop and subsurface data for the depositional system of the Upper Dalan Member and Kangan Formation in the Zagros to the offshore Fars area was carried out. The areas that were studied in detail are the Kuh-e Surmeh and Kuh-e Dena sections of the Zagros Mountains, Iran, and their equivalent in the offshore Fars subsurface. The observations and interpretations based on these sections were then integrated with the regional subsurface descriptions, interpretations and models, and related to the Upper Khuff system across the region.\n The synthesis of the core descriptions and the Zagros outcrop facies data, together with integration of published data resulted in the definition and characterisation of 16 principal facies associations that were used to interpret the depositional environment. Qualitative comparisons of Upper Khuff sections and subsurface cores across the Zagros area, offshore Fars and Middle East Gulf region, showed that this classification of depositional facies is applicable at a larger regional scale and useful in rapid regional comparisons and correlations of the Upper Khuff depositional systems. The large range in documented facies types reflects the great variety in depositional systems and sub-systems that were present across the Khuff platform. The range also shows the temporal evolution of the Khuff environments and palaeoecological conditions from the Permian to the Triassic. The general importance of microbial facies is highlighted and a variety of microbial facies are defined. These microbial events provide reservoir and regional scale isochronous marker horizons that are correlatable over large distances. These microbial facies are associated with periods of poor oxygenation and restriction, but nevertheless can occupy a range of environments from intertidal to mid-to outer-ramp settings.\n Several significant stratigraphic surfaces were picked and correlated based on the detailed core descriptions, the bio- and ecostratigraphic analysis, wireline logs, stratigraphic stacking patterns and the regional understanding of other Upper Dalan-Kangan/Upper Khuff sections in the region. The correlations in cored wells for the Upper Dalan cycles are supported by a well-constrained biostratigraphic framework. Four large third-order stacking cycles (Cycle IV to Cycle I) were defined on the basis of cycles bounded by surfaces representing baselevel and accommodation potential minima. The correlations and stratigraphic analysis suggest that the major stratigraphic trends and large-scale stratigraphic architecture are relatively isopachous (“layer-cake”) at the production scales, a function of the almost flat platform geometry. At a larger scale, significant changes in thickness occur: either thickening towards palaeodepocentres or thinning with onlap towards palaeohighs. At this large-scale, progradation of the oolite shoals occurred during the late highstands in the large accommodation areas. However, on the topographic palaeohighs and platform tops, the main stratigraphic locations of the oolite shoal are in the trangressive and maximum accommodation zones of the cycles. Integrating the facies and stratigraphic interpretations, conceptual depositional models have been constructed for the main stratigraphic intervals. From these interpretations and models it is evident that there were significant changes in platform type/geometry, facies organisation and climate from Cycle VI through to Cycle I.\n At a large scale the Late Permian depositional setting of the Upper Khuff was organised into a platform profile that gently deepened from the south with a platform-top interior zone, a platform-top edge zone, an intrashelf low, and then rose again in the north with palaeohighs around Kuh-e Surmeh and Kuh-e Dena (structurally-controlled basement highs). There was however a major change in the platform profile in the Early Triassic which had a monoclinal ramp platform geometry which opened to the north to deeper-marine conditions with the absence of effective palaeohigh barriers. These two large-scale palaeogeographic profiles controlled the overall distribution of facies belts across the platform. This change in platform profile was coincident with other events within the lowest part of the Kangan Formation (Triassic Khuff Formation of the Arabian Plate) at the Permian-Triassic Boundary, including: (1) major facies changes on the platform tops with the appearance of thrombolites and associated microbial grainstones; (2) major facies changes in the northern shelf edge areas where there is a change from shallow-water high-energy grainy facies to deeper-water mid-ramp muddy facies; (3) change in pattern of relative stratigraphic thickness; and (4) appearance of high gamma-ray shales in the eastern Zagros subsurface area. These events are all consistent with a major flooding across the Permian-Triassic Boundary causing: (1) drowning of palaeohighs; (2) encroachment of anoxic waters into the intrashelf lows; (3) termination of bioaccumulations at the shelf edges; (4) flooding the platform tops with more grainy facies, and developing microbial facies across the shelf; and (5) the quasi-synchronous end-Permian mass extinction.\n Based on the stratigraphic distributions of the biostratigraphically significant fauna and flora, age determinations are interpreted for the main stratigraphic intervals between the Lower Dalan to top Dalan (Lower Khuff to Permian Upper Khuff). Palaeoecologically, five biofacies types have been defined based on the faunal and algal content, the foraminiferal diversity, their sedimentological context and palaeoenvironmental interpretation. This generalised classification is applied to the depositional models developed from the sedimentological analysis and has enabled a validation of the depositional schemes by identifying palaeoenvironmental trends which are not always clear from the sedimentological analysis alone. The analysis of the biofacies distribution has allowed the subdivision of the Upper Dalan Member (Permian Upper Khuff) into six different ‘palaeoecological systems’ that correspond to characteristic faunal assemblages and biofacies sets. The main characteristics of the six palaeoecological systems, and their lateral variability, have been documented. The limits of the defined intervals correspond to important sequence stratigraphic events and markers at various stratigraphic scales. This relationship allowed the integration of ecostratigraphic events to the previously defined sequence stratigraphical framework based on the sedimentological and stratigraphic analysis, and hence confirms and refines the stratigraphic correlations.\n A synthesis of stratigraphic, depositional and diagenetic facies, lithological, isotopic, spectral gamma-ray wireline logs and palaeoecological data suggests that there is no major stratigraphic gap between the KS3 and the KS2 stratigraphic intervals, and hence between the Permian and Triassic periods. In the numerous subsurface sections, and outcrop investigations in the Zagros, no evidence for a major unconformity/disconformity or stratigraphic surface is associated with the Permian-Triassic Boundary; furthermore the extinction of Permian fauna occurs within a grainstone body. The faunistic analysis shows that the Permian Fauna Extinction (PFE) event generally occurs within a strongly calcite-cemented and microbially mediated ooid grainstone rich in intraclasts in the lower part of the KS2 sequence. Above the PFE event is a thin Permian azoic interval, followed by the Triassic faunal recovery and associated with the Early Triassic thrombolitic microbial event. In the Zagros area the PFE occurs within pyrite-bearing muds under poorly oxygenated conditions. The outcrop data also show a similar pattern with a thin azoic interval occurring between the last Permian taxa and the first Triassic taxa. In the Zagros outcrops there is a general muddying (deepening-upwards) from the Upper Permian to the Lower Triassic. The analysis suggests there is a low (third) order transgression between upper KS3 stratigraphic interval (Upper Permian) and the KS2 stratigraphic interval (Lower Triassic), and that the ‘Permian-Triassic oceanic event’ is located in the late third-order TST.

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Chapter 27 – Cretaceous

The publication of Arabian Plate Sequence Stratigraphy (Sharland et al., 2001), commonly referred to as SP2, provided a unifying stratigraphic interpretation of the Arabian Plate within a modern sequence stratigraphic framework. In 2002 revisions to the stratigraphic positions of some SP2 Cretaceous maximum flooding surfaces (MFS), and some resulting new interpretations, were presented by Davies et al. (2002). New interpretations of mixed carbonate-clastic systems presented by these authors have application to other parts of Middle East stratigraphy.

Arabian Plate Sequence Stratigraphy – revisions to SP2

The ability to recognise and correlate third-order depositional sequences across Arabia and between Arabia and other plates indicates that these sequences are driven by synchronous eustatic sea-level change. This is of value in providing guidance for the definitions of stages, which are the fundamental units of chronostratigraphy. Each Phanerozoic stage requires a Global Stratotype Section and Point (GSSP), which is a location and specific bedding plane where the base of each stage is defined. This definition is tied to an event in the rock record useful for correlation. Progress in defining GSSPs has been delayed because of difficulties in choosing the most appropriate event and section to relate to a definition. It is recommended here that stage boundaries be related to correlative conformities of sequence boundaries. This closely links chronostratigraphy with sequence stratigraphy and honours the original concepts upon which many stages were first described in the 19 th Century.

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Arabian Plate sequence stratigraphy: Potential implications for global chronostratigraphy


 Maximum Flooding Surfaces (MFS) in the Early to mid-Cretaceous mixed carbonate-clastic shelfal systems of the Arabian Plate have been incorporated into a new sequence stratigraphic model that links Kuwait, Iran, Saudi Arabia, Qatar, and the United Arab Emirates, to Oman and Yemen. It is based on regional sequence stratigraphic concepts supported by biostratigraphic, sedimentological and mineralogical data. The model has amended the positions of some existing MFS. The diachronous interplay between large-scale, proximal clastic systems and outboard (down-systems-tract) carbonate platforms was emphasized by concentrating on the depositional history of prodelta areas during delta advance and retreat. The prodelta area of relatively deep water separating the depositional systems has been termed the ‘Migratory Carbonate Suppressed Belt’ (MCSB).
 The model proposes that platform limestones expanded back over preceding prodelta areas during transgressions. The most extensive transgressions ultimately led to the demise of MCSBs. The maximum landward retreat of the shoreline coincided with the cessation of clastic input in the most up-systems-tract localities. Thus, the model has predicted that in many places MFS are located in the basal parts of clean carbonates even though these are not the deepest-water sediments. Examples are the Zubair-Shu’aiba (K70 MFS) and the upper Burgan-Maddud (K100) sections of the northern Gulf. Where carbonate platforms did not expand completely across the MCSBs, perhaps because of fault-control, the MCSBs survived and MFS are present within deeper-water, prodelta shales deposited below the most efficient window for carbonate production. Examples are the K40 to K60 MFS in intraformational shales of the Zubair, Biyadh, and Qishn formations of Kuwait, Saudi Arabia, and Yemen, and K100 in the Burgan-Wasia formations of Kuwait and Saudi Arabia. Even in these cases, the MFS are present within limestones deposited further down-systems-tract, notably in Iran (K60—Khalij Member, Gadvan Formation; K100—Dair Limestone Member, Burgan-Kazhdumi formations). Deeper-water dense limestones and shales with accompanying MFS were deposited along the northeastern passive margin of the Arabian Plate, or within intrashelf basins with some limited connection to the open ocean. From a regional perspective it can be seen that eustatic or tectonically forced MFS do not necessarily occur within the deepest-water facies.
 A regional understanding is needed for a more precise sequence stratigraphic interpretation of the Early to mid-Cretaceous succession of the Arabian Plate. The identification of the stratigraphic architecture is of major economic importance at the reservoir scale, for instance in recognizing vertical permeability and transmissibility barriers, as well as at the regional-play fairway scale in the distribution of seals and their potential influence on migration pathways. Our interpretations are also relevant to the prediction of source-rock distributions and, in the longer term, may help identify stratigraphic trap potential related to the interplay between clastic and carbonate depositional systems. Although the model proposed relates to the Arabian Plate, general conclusions may be applicable to other regions where mixed carbonate-clastic systems are well developed, for example in many basins of Tertiary age in South East Asia.

Early to mid-Cretaceous mixed carbonate-clastic shelfal systems: examples, issues and models from the Arabian Plate

Almost 10 years have passed since Sharland et al. (2001) published Arabian Plate Sequence Stratigraphy. This work correlated 63 maximum flooding surfaces across Arabia and, for the first time, placed the complex lithostratigraphy of the Middle East into a sequence stratigraphic framework. We will illustrate that: - Since its publication we have developed the sequence stratigraphic model by further investigations in the Middle East coupled with an analysis of the validity of the sequence stratigraphic model worldwide. We now know that with some minor modifications, the Arabian Plate Sequences can be seen globally and are thus eustatically driven. We have also been able to further develop the model with additional surfaces. - It is interesting to speculate on the causes this eustasy throughout the Phanerozoic. The pace and amplitude of the changes points strongly to a glacio-eustatic origin for our sequences this is supported by a growing body of direct and proxy evidence that points to a coincidence of climatic fluctuation and eustasy, suggesting that melting and creation of ephemeral polar ice may be a causal mechanism, even in what is commonly regarded as a “greenhouse” times. - A robust 3rd order sequence stratigraphic model for the Arabian Plate is a valuable tool for region correlation and mapping, and the recognition of exploration analogues, as well as placing existing reservoirs and source rocks in regional context.

David M. Casey

Papers

Arabian Plate Sequence Stratigraphy – revisions to SP2

Arabian Plate sequence stratigraphy: Potential implications for global chronostratigraphy

Early to mid-Cretaceous mixed carbonate-clastic shelfal systems: examples, issues and models from the Arabian Plate

Arabian Plate Sequence Stratigraphy: 10 Years On