Palynostratigraphy, paleoclimate, and paleoenvironment of the Dalichai Formation in the Bashm stratigraphic section (Eastern Alborz)

The Middle Jurassic strata (Dalichai Formation) is widespread across the Alborz Range. It is chiefly characterized by alternating shale beds, bluish-grey thin to medium bedded limestone, and marl including ammonites, belemnites, and sponge (Steiger, 1966). Various mega- and microfossil faunas are recorded in the formation, including fairly rich ammonites, spores, pollen grains, dinoflagellate cysts, belemnites, bivalves, brachiopods, echinoderms, sponges, bryozoans, and foraminifera. The 212-m-thick Dalichai Formation in the Bashm stratigraphic section, Eastern Alborz, mainly consists of marl, argillaceous limestone, and a few intercalations of cherty limestone. With distinct disconformity, it succeeds the Upper Triassic-Lower Middle Jurassic Shemshak Formation and gradationally underlies the thick-bedded limestone of the Upper Jurassic Lar Formation. This study aimed to make and correlate a palynostratigraphic framework for the studied interval and interpret the palynofacies and paleoclimate of the Dalichai Formation based on palynological data at the Bashm stratigraphic section, Eastern Alborz, northern Iran.

In the studied section, twenty-four samples were collected for palynological analysis. All samples were prepared following standard palynological processing procedures (e.g., Phipps & Playford, 1984), comprising treatment with hydrochloric (10-50%) and hydrofluoric (ca. 40%) acids for dissolution of carbonates and silicates, respectively. Thereafter, hot 50% HCl was used to dissolve silica-gel formed during the HF-silicate reaction. Then the solution was neutralized and centrifuged in ZnCl2 (specific gravity 1.9) for density separation. Neither oxidative nor alkali treatments were applied. Residues were sieved through a 20μm mesh and strew mounted on glass slides using liquid Canada balsam. All the samples, slides, and figured specimens were stored at the Laboratory of Paleontology of the Department of Geology, Faculty of Science, Imam Khomeini International University, Qazvin, Iran.

The 24 samples examined yield well-preserved, diverse palynofloras consisting predominantly of dinoflagellate cysts and miospores (spores and pollen grains). Fungal spores and acritarchs occur as minor constituents. The presence of some index species of dinoflagellate cysts leads to the identification of three of the North-western European biozones (Woollam & Riding, 1983; Riding & Thomas, 1992; Poulsen & Riding, 2003), i.e., Cribroperidinium crispum Total Range Biozone (late Bajocian), Dichadogonyaulax sellwoodii Interval Biozone (Bathonian to early Callovian) and Ctenidodinium continuum Interval Biozone (early to middle Callovian). Moreover, the presence of such dinoflagellate species as Mendicodinium groenlandicum and Systematophora areolata has been used to attribute a late Callovian age to the upper part of this studied section. Therefore, the late Bajocian - late Callovian age is collectively attributed to the studied section. This age assignment based on palynological data is consistent with the age presented by the ammonites (Hossein Zadeh, 2011) in this stratigraphic section. The erected palynozones correspond largely to those established in Northwestern Europe and reveal the marine connection between North of Iran with Northwest Europe and the Northwestern Tethys during the late Bajocian to late Callovian ages. Quantitative analyses of the palynological data indicate the dominance of a relatively shallow, low-oxygenated marine environment with a relative increase in depth at some intervals upward in the studied section. Such spore species as Klukisporites, Cyathidites, Dictyophyllidites, and Gleicheniidites known to have been produced by Pterophyta dominate the palynofloras. Thus, the contemporaneous coastal vegetation was characterized by the predominance of Pterophyta and minor components of Ginkgophyta, Pteridospermophyta, Coniferophyta, Lycophyta, and Cycadophyta. Moreover, this reconstruction accompanied by the dominance of warm and intermediate dinoflagellate cysts collectively indicates a mild to warm climate for the Dalichai Formation in the studied section during the Middle Jurassic (late Bajocian - late Callovian). Indeed, according to the paleogeographic maps, in the Middle Jurassic (Middle Callovian), the Alborz Mountains of northern Iran occupied lower latitudes between 20 to 30 degrees north (Thierry, 2000; Seyed-Emami et al., 2008; Wilmsen et al., 2009, 2010). This position confirms this paleoecology for the Dalichai Formation.

All samples examined yield well-preserved, diverse dinoflagellate cyst assemblages. 26 identified dinoflagellate cyst species led to the identification of three biozones: Cribroperidinium crispum (late Bajocian), the Dichadogonyaulax sellwoodii (Bathonian to early Callovian), and Ctenidodinium continuum (early to late Callovian). Due to such dinoflagellate species as Mendicodinium groenlandicum and Systematophora areolata, a late Callovian age is attributed to the upper part of the studied section. This age assignment is consistent with the age presented by the ammonites in this stratigraphic section. Palynological data designate the dominance of a relatively shallow marine environment with a relative increase in depth at some intervals upward the studied section. The abundance of intermediate and warm water dinoflagellate cysts and ferns in the surrounding area of the Dalichai Formation depositional setting and also fungal spores collectively indicate a mild to warm climate for the Dalichai Formation in the studies section.