Integrating sedimentologic data and stable isotope geochemistry to comprehend the sequence stratigraphic framework of the Middle–Upper Jurassic Sargelu and Najmah formations in NW Kermanshah (Howramanat, Dowdan and Kezi sections)

Abstract

AbstractIn this study, by integrating sedimentologic data with geochemical data, the sequence stratigraphy framework of the Sargelu and Najmah formations in the Hawraman region of NW Kermanshah were studied. Three depositional sequences of the type “T/R facies cycles” including TR1–TR3 were recognized in the Sargelu Formation. The occurrenceof dark, organic-rich shale and bituminous limestone in these sequences together with depleted δ13C values with some striking negative excursionsin intervals with outer shelf/basin facies indicates a rapidly enhancing accommodationspace during the development of these depositional sequences leading to the prevalenceof deep marine deposition, particularly during the transgressive systems tracts (TSTs). In addition, three other T/R facies cycles were recognized in the Najmah Formation. Facies stacking patterns and internal architecture of these depositional sequences along with the existence of peaks in the δ13C to heavier values suggests their deposition during limited accommodationspace. At the contact between the Sargelu and Najmah formations, there occurreda positive peak in δ18O, which most likely is an indication of a local climate change at the Middle-Late Jurassic interface. The correlation of depositional trends of the TST and RST with that of δ13C may show that eustatic acted as the main control on the variations of δ13C.Keywords: Geochemical stable isotope, Sequence stratigraphy, Sargelu Formation, Najmah Formation, Jurassic. Introduction Following the Late Permian rifting, passive margin platforms are formed on the margins of Neotethys during the Jurassic and Cretaceous, which are mainly associated with the contemporaneous Sargelu (or Hawraman), Arabia, Rub AlKhali, Garau and Bab intrashelf basins (Ziegler 2001). Hawraman or Avroman is a mountainous region located within the provinces of Kurdistan and Kermanshah in western Iran and in north-eastern Kurdistan Region of Iraq (Biglari and Shidrang, 2019). The Middle-Upper Jurassic successions of the Hawraman intrashelf basin studied crop out in the present Hawraman region in NW Iran and form part of the NW sector of the Zagros Mountain belt. The Bajocian to Tithonian units in the Hawraman intrashelf basin (Sargelu, Najmah and Gotnia formations) roughly coincide with mega-sequence AP7 of Sharland et al. (2001). Correlations of the Sargelu and Najmah formations to their stratigraphic equivalents in the Arabian Plate based on maximum flooding surfaces indicate that the Sargelu Formation in Iran, Iraq and Kuwait is equivalent to the Dhruma Formation in Saudi Arabia, and the Araj Formation in Qatar and the United Arab Emirates. The Najmah Formation in the western–southwestern and the central part of Iraq changed to the Neocalekan Formation. Also, the formation is correlated to the Arab, Hanifa, and Tuwaiq Limestone formations in Saudi Arabia and Diab Formation in Qatar and United Arab Emirates. Also, the Gotnia Formation is equivalent to the Hith Formation in Saudi Arabia and the outer regions of the Arabian Plate. The hemipelagic nature of the sediments of the Sargelu Formation with massive to laminated black shale layers with thin to thick-bedded Bositra limestones in the Hawraman region, constitute a good context for carbon and oxygen isotope studies. This kind of sediment is suitable for using isotopes in interpreting the depositional history and palaeogeography of sedimentary successions. Studies of the stable oxygen and carbon isotopic compositions of biotic and abiotic components in the Sargelu and Najmah formations have been used to understand changes in seawater isotopic composition, depositional history of cyclic sequences and the Jurassic tectono-depositional processes of the northern passive margin of the Arabian Plate along the western edge of Neotethys. Materials & MethodsThe study area in NW Iran is located in the High Zagros zone according to this classification. Detailed fieldwork was conducted at two well-exposed outcrop sections, including Dudan (35°01'01.6"N; 46°11'14.6"E) and Kezi (34°51'50.6"N; 46°08'25.1"E). In this study, facies determination is based on field observations of sediment texture and structures and petrographic analysis of thin sections (230 sections). In the studied successions, geometry, lateral continuity, sedimentary trend, the vertical stacking of genetically related facies and facies associations and prominent stratigraphic surfaces (representing sharp shifts in sedimentation) were examined to define meter-scale cycles (from a few dm- to a few m-thick) that are used for the identification of depositional sequences, systems tracts, and sequence boundaries. Thirty-nine samples were analyzed for carbon and oxygen stable isotopes. The analyses were performed in the Friedrich-Alexander Universität Erlangen-Nürnberg (FAU). Discussion of Results & ConclusionsThe carbonate successions of the Sargelu are divided into four facies association consisting of the deep marine basin, slope, outer shelf, and middle shelf, which are interpreted as being deposited in an intrashelf basin. Moreover, the sedimentological dataset documented for the Najmah allowed the recognition of three facies associations, which are interpreted as deposited in a carbonate platform with ramp geometry, from deep subtidal, shallow subtidal to intertidal-supratidal facies associations. The Sargelu and Najmah formations consist of six depositional sequences representing T–R cycles: three T/R facies cycles (TR1–TR3) in the Sargelu Formation and three T/R facies cycles (TR4–TR6) in the Najmah Formation. Supplementary studies on stable isotopes of oxygen and carbon (δ13C and δ18O) in order to reconstruct depositional environment conditions, paragenetic sequence and sequence stratigraphic framework for the Sareglu and Najmah formations in the Doudan section indicate the strong effect of meteoric diagenesis or meteoric water influx on the strata of the studied formations. The negative peak of the carbon isotope corresponds to the maximum flooding surface (MFS) in the sequence stratigraphic framework. Also, the negative trend in carbon isotope may correspond to the transgressive systems tract (TST) whereas the positive trend in carbon isotope corresponds to the regressive systems tract (RST). The relationship between the systems tracts and δ13C implies that sea-level fluctuations would have been the main mechanism controlling the fluctuations of carbon isotopes. The major geological structures observed in northwestern Iran with intrashelf Sargelu Formation basin (ISB), indicate the presence of an extensional pulse. According to the evidence of the Hawraman intrashelf basin (Bayet-Goll et al. 2022), the presence of syn-sedimentary faults, unconformities, growth geometries, and facies distribution among Jurassic successions (early to late) supports the presence of this extensional pulse. According to Bayet-Goll et al. (2022), this pulse reaches its maximum in the Toarcian and causes tectonically driven drowning of the long-lived Triassic to Early Jurassic carbonate platform and facies change from shallow-water carbonate platforms (Sehkaniyan Formation) to deep-water basins. It seems that the physiography of the sedimentary basin of NW Iran is related to troughs and swells in the western margins of Neotethys as a result of tectonic subsidence and sea-level change factors. Logically, such changes can cause extensive changes in the marine current dynamic, trophic conditions and oxygenation degree of the Sargelu sedimentary basin, thus such factors can affect environmental conditions, carbonate production and the activity or inactivity of the benthic community in this area. Consequently, it can be expected that the evolution of the depositional environments in the tectonically controlled (Hawraman) intrashelf basin with the Sargelu, Najmah and Gotnia formations are controlled by eustatic and tectonic factors.

Keywords

• geochemical stable isotope
• sequence stratigraphy
• sargelu formation
• najmah formation
• jurassic