Al Jabal al-Akhdar, Oman Mountains

Forke, H.C., Pöppelreiter, M., Aigner, T., Koehrer, B., Walz, L., Bendias, D., Haase, M. (2012). Integrated biostratigraphy of the Saiq Formation (Al Jabal al-Akhdar, Oman Mountains) and its implication for the regional correlation of Khuff time-equivalent deposits. In: Third Arabian Plate Geology Workshop, Part I The Permo–Triassic Sequence of the Arabian Plate, GeoArabia, v. 17, no.1, p. 230-234.

11720 Integrated biostratigraphy of the Saiq Formation (Al Jabal al-Akhdar, Oman Mountains) and its implication for the regional correlation of Khuff time-equivalent deposits Holger Forke (Museum of Natural History Berlin, ), Michael Pöppelreiter (Shell), Thomas Aigner (University of Tübingen), Bastian Koehrer (University of Tübingen), Lisa Walz (University of Tübingen), Daniel Bendias (University of Tübingen) and Marlene Haase (University of Tübingen) We present a multi-disciplinary approach (sequence-biostratigraphy, palaeoenvironmental analysis, gamma-ray, and carbon isotopes) for the stratigraphic subdivision of Khuff timeequivalent deposits in Oman (Saiq and lower Mahil Formation, Al Jabal al-Akhdar). The Khuff is herein subdivided into six third-order depositional sequences. According to the proposed biostratigraphic zonation, the KS6 to KS5 is Middle Permian (Roadian?–Wordian– Capitanian), the KS4 to basal KS2 is Late Permian and the remaining part of KS2 and KS1 is Early Triassic. Major biotic crises occurred during the upper KS5 (“end-Guadalupian faunal crisis”) and in the basal KS2 (“end-Permian faunal extinction”). The transition from Middle to Late Permian is further accompanied by a palaeoenvironmental shift from a differentiated bioclastic ramp to more uniform microbial-mediated platform deposits. The biozonation and biotic events have been applied to correlate outcrop sections and subsurface on a regional scale across the Arabian Platform. The Middle Permian to Lower Triassic Khuff Formation and its time-equivalent deposits represent a major target for hydrocarbon exploration in the Middle East. In order to enhance reservoir models, integration of data from outcrop sections and subsurface wells and from multiple disciplines (sedimentology, biostratigraphy, geochemistry and geophysics) are needed. The Khuff Formation covers most of the Arabian Platform and represents over large areas a flat epeiric mixed carbonate-evaporitic ramp. Bio- and ecostratigraphic analysis of benthic foraminifera (e.g. Gaillot and Vachard, 2007; Hughes, 2009) has been applied for stratigraphic subdivision of the Khuff Formation due to the predominantly shallow water, restricted to open-marine environments. Within the framework of a multidisciplinary outcrop to subsurface study, five sections on the Saiq Plateau and in the Wadis of the Al Jabal al-Akhdar area (Oman Mountains) have been investigated spanning the Khuff time-equivalent interval (Saiq and lower Mahil formations) in Oman. Stratigraphic subdivision herein is mainly based on the FOD of foraminiferal species, but biotic marker beds, palaeoenvironmental shifts as well as geochemical (δ13C) and geophysical (GR) data are integrated to correlate the sections on a local scale (10–50 km) (Figure 1). Biostratigraphy and Biofacies in Oman Mountains The basal part of the Saiq Formation displays open-marine, bioclastic ramp deposits with a diversified macro- and microfauna. Larger benthic foraminifera are present showing a much higher biodiversity than described elsewhere from the Arabian Platform with several species previously unrecognised. Verbeekinid, schwagerinid, schubertellid, and staffellid species are particularily common during the initial transgression, right above the eroded Pre-Khuff basement. The foraminiferal assemblage with Afghanella tereshkovae, Verbeekina grabaui etc. point to a Murgabian (Roadian?–Wordian) age. Diversity decreases rapidly in the overlying muddy interval interpreted as the maximum flooding zone of the KS6. Fauna and flora recovers on top of the KS6 and becomes more diversified during the lower KS5, which is characterised by large bivalves (Alatoconchidae), cerioid corals and poorly preserved larger benthic foraminifera. Local variations of facies and fossil assemblages indicate a topographic relief from shallower to deeper palaeoenvironments

in a W-E direction. The presence of the enigmatic fossil Sphairionia sikuoides indicates a Midian (Capitanian) age. The upper KS5 witnesses the gradual demise of the latter three groups (“end-Guadalupian faunal crisis”, Isozaki and Aljinovic, 2009; Bond et al., 2010) accompanied by a distinct shift from bioclastic ramp deposits towards more uniform platform deposits dominantly composed of non-skeletal grains with a high amount of microbial-mediated precipitation. The first occurrence of the miliolid foraminifer Shanita amosi and several other genera (e.g. Paraglobivalvulina, Rectostipulina) testify the biotic turnover in benthic foraminiferal assemblages during the latest Capitanian and represent one of the biostratigraphic key markers in the Khuff. Late Permian deposits (KS4-basal KS2) are characterised by thick oolitic/peloidal/cortoidal shoal-related grainstone bodies in the Wuchiapingian with a poorly diversified fauna and flora. Furthermore, the extensive dolomitization hampers a precise taxonomic determination. Most of the occurring species are present already in the upper KS5, but the appearance of Neomillerella mirablis might serve as a potential marker within the KS4. The fauna becomes more diversified during the uppermost KS4 through basal KS2 with several Changhsingian marker species (Glomomidiellopsis uenoi, Paremiratella robusta and Paradagmarita monodi). Additionally, the macrofauna contains abundant fasciculate (Waagenophyllum) and solitary corals (Iranophyllum?) forming distinct marker beds, which can be followed laterally throughout the area. The Permian/Triassic boundary can be traced in all sections by the abrupt loss of biota (“endPermian faunal extinction”), the δ13C and the GR signals. The interval is accompanied by a rapid shift towards muddy offshoal deposits. Fossils remain very scarce throughout the Lower Mahil member (upper KS2 - KS1, Lower Triassic). Regional Framework In order to integrate the biostratigraphic scheme into a regional framework, the data were correlated with existing stratigraphic subdivisions and palaeoenvironmental interpretations from other parts of the Arabian Platfom (Insalaco et al., 2006; Maurer et al., 2009; Kolodka et al., 2011). Biostratigraphic data from the Lower Khuff (KS6–KS5) across the Arabian Platform are rather scarce in the literature. However, existing data from Saudi Arabia, Oman and Iran show considerable variations in facies and fossil assemblages pointing to a stronger differentation of palaeoenvironments during this time, which can be seen even on a local scale. In the inner parts of the Arabian Platform, the lower part of the Khuff is represented by predominantly clastic deposits with reduced thicknesses. In some areas (e.g. Oman, parts of Saudi Arabia), the lower Khuff is dominated by mixed heterozoan-photozoan (bryonoderm-extended) faunal assemblages. Outcrop data from the outer platform in Iran (e.g. Kolodka et al., 2011) show similar faunal assemblages and facies in general, but foraminiferal species composition and stratigraphic range seems to vary locally. The upper KS5 seems to represent a turning point towards more uniform foraminiferal assemblages. The Shanita amosi Zone has been recognized in several sections across the Arabian Platform and first occurs just below the deposition of the Median Anhydrite. Several marker species serve as tie points in the middle/upper Khuff (Late Permian) to correlate the sequence-stratigraphic framework and confirm the uniform, layer-cake structure during this interval (Figure 2). Palaeoenvironmental interpretations of the fossil assemblages further enable to delineate subtle facies variations within the sections and regionally across parts of the Arabian Platform. References Bond, D.P.G., P.B. Wignall, W. Wang, S. Védrine, H.-S. Jiang, X.-L. Lai, Y.-D. Sun, R.J. Newton, H. Cope and G. Izon 2010. The mid-Capitanian (Middle Permian) mass

extinction and carbon isotope record of South China. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 292, p. 282-294. Gaillot, J. and D. Vachard 2007. The Khuff Formation (Middle East) and time-equivalents in Turkey and South China: biostratigraphy from Capitanian to Changhsingian times (Permian), new foraminiferal taxa, and palaeogeographical implications. Coloquios de Paleontología, v. 57, p. 37-223. Hughes, G.W. 2009. Micropaleontology and paleoenvironments of Saudi Arabian Upper Permian carbonates and reservoirs. In Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones. SEPM Special Publication no. 93, p. 111-126. Insalaco, E., A. Virgone, B. Courme, J. Gaillot, M. Kamali, A. Moallemi, M. Lotfpour and S. Monibi 2006. Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: Depositional system, biostratigraphy and stratigraphic architecture. GeoArabia, v. 11, no. 2, p. 75-176. Isozaki, Y. and D. Aljinovic 2009. End-Guadalupian extinction of the Permian gigantic bivalve Alatoconchidae: end of gigantism in tropical seas by cooling. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 284, p. 11–21. Kolodka, C., E. Vennin, D. Vachard, V. Trocme and M.H. Goodarzi 2011. Timing and progression of the end-Guadalupian crisis in the Fars province (Dalan Formation, Kuh-e Gakhum, Iran) constrained by foraminifers and other carbonate microfossils. Facies, DOI 10.1007/s10347-011-0265-1.

Figure 1: Stratigraphic subdivision of the Saiq Formation and lower Mahil member with major key marker species, biotic events and and biofacies characterisation. Al Jabal alAkhdar, Oman Mountains.

Figure 2: Regional correlation of Middle and Upper Khuff time-equivalent strata from offshore Fars and Musandam with sections from Al Jabal al-Akhdar. Facies, biostratigraphic marker and interpreted sequence-stratigraphic frameworks are shown.