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Biostratigraphy and palaeoecology of the Dalichai Formation (Lower Cretaceous) based on calcareous nannofossils from the western Alborz Mountains (north-west Iran)

Shiri Rana, Hadavi Fatemeh

Geological Quarterly

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2021

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Vol. 65, No. 4

art. no. 53

EN

We describe the biostratigraphy and palaeoecology of the Dalichai Formation based on calcareous nannofossils from the Guydagh section in the western Alborz Mountains, north-west Iran. The nannofossils are moderately to well-preserved, and 25 species belonging to 8 genera have been distinguished, representing the Early Berriasian to Late Hauterivian ages according to the CC1-CC4b biozones of Sissingh (1977). The recorded nannofossils are generally Tethyan and cosmopolitan, while the record of Nannoconus inornatus, a Boreal species, at the top of the section reflects the water mass connection between the Boreal and Tethyan domains during the Late Hauterivian. The records of Nannoconus spp., Watznaueria spp., and Rhagodiscus asper show that the sedi mentary basin of the Dalichai Formation was located at low lati tudes of the Tethyan realm with warm surface waters. Palaeoecologically, the findings represent a marine transgression and increasing depth, and the palaeoenvironmental conditions change from low-high mesotrophic (shown by the dominance of W. britannica) to oligotrophic (from the dominance of nannoconids), r- to K-strategy of life (from the dominance of nannoconids), and became increasingly stable (shown by the reduction of W. britannica) towards the top of the section.

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Biostratigraphy and palaeoenvironmental interpretation of the Dalichai Formation (Lower Cretaceous) in the eastern and central Alborz Mountains (North Iran) based on calcareous nannofossils

Shiri Rana, Hadavi Fatemeh, Hazaveh Fereshteh Sajadi

Geological Quarterly

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2020

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Vol. 64, No. 3

641--657

EN

OpracCalcareous nannofossils are described from 4 sections of the Lower Cretaceous Dalichai Formation from the Eastern (Tepal, Talu and Lavan sections) and central Alborz Mountains (Yonjezar section), Iran. Forty-two species belonging to 18 genera have been identified from the 4 sections studied, including 13 species belonging to 8 genera of ascidian didemnids only from the Tepal section. These taxa indicate the CC1-CC5 biozones of Sissingh's (1977) biozonation and an age of Berriasian to Early Barremian. The nannofossils recorded from the Dalichai Formation are generally Tethyan and cosmopolitan, while the presence of some Boreal taxa (Nannoconus abundans and N. borealis) may reflect connection between the Boreal and Tethyan realms in the Early Barremian. The sedimentary basin of the Dalichai Formation of the Tepal, Talu, and Yonjezar sections was located at low latitudes of the Tethyan realm with warm surface water. Also, trends of increasing depth and productivity, of a shift from r-selected to k-selected strategies and of a change from low-to-high mesotrophic to oligotrophic conditions, were deduced. A change from unstable to stable conditions towards the tops of sections is inferred for these localities. Laterally in the Dalichai Basin from the eastern to central Alborz, there was an increase in depth and productivity, and also a trend towards oligotrophic conditions and a dominance of low-stress conditions.owanie rekordu ze środków MNiSW, umowa Nr 461252 w ramach programu "Społeczna odpowiedzialność nauki" - moduł: Popularyzacja nauki i promocja sportu (2021).

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Identification of the Oligocene-Miocene boundary in the Central Iran Basin (Qom Formation) : Calcareous nannofossil evidences

Parandavar Mohammad, Hadavi Fatemeh

Geological Quarterly

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2019

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Vol. 63, No. 2

215--229

EN

A diverse Late Oligocene to Early Miocene calcareous nannofossil assemblage was examined from the Qom Formation in the Central Iran Basin, and the Oligocene-Miocene boundary was identified based on the quantitative analysis of the assemblages in 303 smear slides. Eleven well-established calcareous nannofossil bio-events are delineated in the Upper Oligocene through Lower Miocene. The results clearly show that the Highest Occurrence (HO) of Sphenolithus delphix is the closest bio-event to the boundary as traditionally delineated on the lithostratigraphic criteria, and provides a distinct biohorizon below it. The Lowest Occurrence (LO) of the species Discoaster druggii is the oldest Miocene bio-event that is observed shortly after the HO of S. delphix, showing that calcareous nannofossils are well suited for approximating the Oligocene-Miocene boundary in the Qom Formation. The Oligocene-Miocene boundary is placed in the upper part of Sub-member “c1” in all three sections studied here and it is traceable throughout the Central Iran Basin, which makes a potentially reliable marker horizon for sequence stratigraphic and hydrocarbon studies in the area.