Ammonite biozonation and chronostratigraphy of the Upper Bajocian–Upper Oxfordian of the Dalichai Formation at the Kuh-e-Varvari section (North of Damghan, Alborz and North Iran)

Abstract

ABSTRACT The Middle–Upper Jurassic strata in the eastern part of the Alborz, North of Iran (Dalichai Formation) were studied based on ammonite fauna to correlate with zonation of other West Tethyan areas (Sub-Mediterranean province). In this section, like other studied sections in North and northeast of Iran, the ammonite assemblages such as Parkinsoniidae, Morphoceratidae, Sphaeroceratidae, Macrocephalitidae and Reineckeiidae mainly show the characteristics of etyan groups. Abundant ammonite fauna in some levels has enabled an accurate chronostratigraphy. The presence of this ammonite fauna that is seen in the epicontinental seas, bordering the northern margins of the western Thetys, led to the identification of some zones in accordance with the standard of Sub-Mediterranean province zonation. The Garantiana and Parkinsoni Zones of the Late Bajocian, the Zigzag and Aurigerus Zones of the Early Bathonian, the Retrocostatum and Discus Zones of the Late Bathonian, the Bullatus and Gracilis Zones of the Early Callovian, the Anceps and Coronatum Zones of the Middle Callovian and the Athleta and Lamberti Zones of the Late Callovian have been recognised and characterised. These assemblages show a relation between these regions with the Sub-Mediterranean province during the Jurassic. Low percentage of Phylloceratidae in this section is an indicative of Sub-Mediterranean affinity. Based on the ammonite fauna, the Dalichai Formation is Late Bajocian-Late Oxfordian in age.