Palynostratigraphy of the Dalichai Formation in Darjazin section, North of Semnan

oleh: khadijeh Mohammad-khani, Elahe Zarei

Format:	Article
Diterbitkan:	University of Isfahan 2018-12-01

Deskripsi

Abstract The Dalichai Formation in Darjazin section in NE Semnan with a thickness of 631 meters consisting of an alternation of bulish-gray was studied.This formation overlies the Shemshak formation disconformity with conglomerate bed and it has gradually been covered by the thick-beded limestones of the Lar formation. Based on the presence and stratigraphic distribution of the most dominant miospore, Klukisporites variegatus and Callialasporites dampieri of pollen areidentified in the Dalichai (late Bajocian - Callovian) and Presence of some index dinoflagellate species led to identification of three biozones in the Dalichai Formation. These include in ascending order, Cribroperidinium crispum Total Range Biozone (late Bajocian), Dichadogonyaulax sellwoodii Interval Biozone (Bathonian - early Callovian), Ctenididinium continuum Interval Biozone (early to middle Callovian). This biozonation corresponds largely to those established in Northwest Europe and Northwestern Tethys and reveals the marine connection between North of Iran with Northwest Europe and the Northwestern Tethys during the Middle Jurassic. Keywords: Palynostratigraphy, Dalichai Formation, Dinocyst, Spore and Pollen Introduction The Dalichai Formation in Darjazin section in northeast Semnan with a thickness of 631 meters consisting of an alternation of bulish-gray shale was studied. This formation overlies the Shemshak Group disconformity with conglomerate bed and it has gradually been covered by the thick-bedded limestones of the Lar Formation. Darjazin section is located in the central part of Alborz Mountains (northeast of Semnan). The grid coordinates for the base of the section is 53° 35΄ 14˝ eastern longitude and 35° 46΄10˝ northern latitude. The formation contains a very rich ammonite fauna, sponge spicules, belemnites, bryozoans, bivalves, echinoderm debris, ostracods, benthic foraminifers, and the trace fossil Zoophycos. Material & Methods Sixty one rock samples were collected systematically from the formation under study and prepared in the palynology laboratory. The preparation method of Traverse (2007) was used. Cold hydrochloric (20 %) and hydrofluoric acids were used to dissolve carbonate sand silicates, respectively. The residue was neutralized and centrifuged in ZnCl2 with specific gravity (1.9 g/cm3). The materials were then sieved using a 15-μm nylon mesh and mounted on microscope slides using liquid Canada balsam. Discussion of Results & Conclusions The present study shows that the Dalichai Formation is rich in terrestrial (spores and pollen grains) and marine palynomorphs (dinoflagellate cysts, acritarchs, and scolecodonts). Based on the presence and stratigraphic distribution of the most dominant miospore, Klukisporites variegatus and Callialasporites dampieri of pollen areidentified in the Dalichai Formation (Late Bajocian–Callovian). In addition to spores and pollens, dinoflagellate and fungus bodies form the palynomorphs assemblages in these strata. Presence of some index dinoflagellate species led to identification of three biozones in the Dalichai Formation. These include in ascending order, Cribroperidinium crispum Total Range Biozone, Dichadogonyaulax sellwoodii Interval Biozone, Ctenididinium continuum Interval Biozone. Cribroperidinium crispum total range biozone The interval between the FAD and LAD of Cribroperidinium crispum. This biozone encompasses 115 m of the base of the Dalichai Formation from samples 1 to 17. The zone differentiated here is equivalent to the Cribroperidinium crispum zone of Poulsen and Riding (2003) for sub-boreal دorthwest Europe. There are a few proximate dinoflagellate cysts in this zone. اCarpathodinium predae, Chytroeisphaeridia chtroeides, Cleistosphaeridium sp.,Ctenidodinium cornigera,Endoscrinium luridum,Nannoceratopsis pellucida,Valensiella ovulum,Gonyaulacysta pectinigera,Nannoceratopsis triceras, Perisseiasphaeridium sp.,Sentusidinium sp.,Downiesphaeridium sp.,Cribroperidinium sp.,Meiourogonyaulax valensii,Lisabeldinium sp.,Carpathodinium sp.,Meiourogonyaulax caytonensis,Pareodinia ceratophora,Rhynchodiniopsis cladophora,Dictyotidium sp.,Chytroeisphaeridia pocockii,Gonyalacysta sp.,Meiourogonyaulax sp. Meiourogonyaulax valensii is also relatively common in the Late Bajocian and many important FADs occur within this substage. These are considered to indicate an age of Late Bajocian for the zone. Dichadogonyaulax sellwoodii interval biozone The interval between the LAD Cribroperidinium crispum and the LAD of Ctenidodinium combazii. This biozone encompasses 315m of the Dalichai Formation above the Cribroperidinium crispum zone (from samples 17 to 35). The Dichadogonyaulax sellwoodii zone (Riding and Thomas 1992) is equivalent to the Ctenidodinium sellwoodii zone of Poulsen and Riding (2003). The dinoflagellate cyst assemblages tend to be relatively diverse, with forms having epicystal archaeopyles, especially Ctenidodinium spp. dominant. Riding et al. (1985) recorded levels of Ctenidodinium spp. in excess of 90% of the dinoflagellate cyst assemblages in the Bathonian strata of southwest England. Ctenidodinium sp.,Kalptea stegasta,Tubotuberella sp. cf. T. dangeardii,Endoscrinium sp.,Preodinia sp.,Korystocysta gochtii,Pareodinia prolongata,Pareodinia antennata,Gonyaulacysta jurassica,Rhynchodiniopsis sp.,Turbotuberella sp.,Turbotuberella dangeardi. These are considered to indicate an age of Bathonian to Early Callovian for this zone. Ctenidodinium continuum interval biozone The interval between the LAD of Ctenidodinium combazii, and the FAD of Scriniodinium crystallinum. This zone encompasses 166 m of the Dalichai Formation above the Dichadogonyaulax sellwoodii zone (from samples 35 to 49) and is marked by the LAD of Ctenidodinium combazii. There are many proximate and proximochorate dinoflagellate cysts recorded in this zone. Common taxa include Gonyaulacysta jurassica,Rhynchodiniopsis sp.,Turbotuberella sp.,Turbotuberella dangeardii,Chytroeisphaeridia sp.,Gonyaulacysta centriconnata,Valensiella sp.,Ctenidodinium ornatum,Leptodinium sp., Scriniodinium crystallinum, Atopodinium sp.,Cometodinium jurassicum,Pareodinia halsa,Tubotuberella apatela,Voodooia tabulate,Wallodinium laganum are seen. These are considered to indicate an age of Middle to Late Callovian for this zone. This biozonation corresponds largely to those established in northwest Europe and northwestern Tethys and reveals the marine connection between north of Iran with northwest Europe and the northwestern Tethys during the Middle Jurassic.

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Palynostratigraphy of the Dalichai Formation in Darjazin section, North of Semnan

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