Palynological evidence for the Middle Jurassic environmental evolution in the eastern part of the Alborz Mountains (northern Iran)

Abstract

The palynological assemblages of the Dalichai Formation were studied to reconstruct the paleoenvironmental evolution of the Middle Jurassic interval in the eastern part of the Alborz Mountains (N Iran). Dinoflagellate cyst biozones, planktonic foraminifera, and ammonites suggest a Bajocian-Callovian age for the Dalichai Formation. The palynodebris, mostly of terrestrial origin, are dominated by black opaque phytoclasts. Dinocysts, spores, pollen grains, and amorphous organic matters (AOM) are minor components in most samples. Terrestrial palynoflora are mainly represented by coastal gymnosperms (Araucariaceae, Cheirolepidiaceae), lowland simple monosulcate pollen producers (Cycadales/Bennettitales), ferns, upland seed ferns and conifers (Corystospermales and other bisaccate pollen producers). These assemblages indicate a warm climate with seasonally-dry conditions. However, an increase in Cheirolepidiaceae (Classopollis) in Bathonian assemblages reflects higher temperature during this interval. The dominance of the land-derived phytoclasts and presence of brackish water Botryococcus colonies with cells arranged irregularly suggest a proximal setting with high terrestrial and freshwater input during the Bajocian. The palynofacies analysis suggests a sea-level rise during the Bathonian, reflected by higher abundance and diversity of dinocysts as well as the presence of degraded small phytoclasts. This transgression phase is related to the increased subsidence of the Alborz region due to ongoing Neotethys subduction and creation of a large extensional back arc basin in the Eurasian margin. The syn-rift deposits are characterized by ammonitico rosso facies as well as hemipelagic carbonates containing planktonic foraminifera and pelagic bivalves.