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

Abstract

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 ereas 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. The synthesis of the core descriptions and the Zagros outcrop facies data, together with 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. Several significant stratigraphic surfaces were picked and correlated based on the detailed core descriptions, the bio- and ecostratigraphic analysis, wireline logs, stratigraphic stacking pattern 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 base-level and occommodation 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. Intergrating 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 organisationis and climate from Cycle VI through to Cycle I. At a large scale the Late Permian dep ositional 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 gammaray 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. Based on the stratigraphic distributions of the biostratigr aphically 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. A synthesis of stratigraphic, depositional and diagenetic facies, lithological, isotopic, spectral gammaray 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.