Palinozonation of Basal Part of the Shemshak Group's Diposits(Upper Triassic)in North Alborz Domain on the basis of Dinoflagellates

Basal part of the Shemshak group in Galandrud area, in north Alborz about 16 km south of Ruian city (Alam- Deh) was investigated palynologicaly. The studied section attains a thickness of 590 meters and consists of limestone, shale, sandstone and siltstone beds. A total of 29 samples were selected and treated in the palynological laboratory of the geological survey of Iran. Eighty one palynomorph taxa were encountered. The recorded dinoflagellate species were arranged in four local bioassemblage biozones and a superbiozone (Rhaetogonyaulax). Biozone I; Heibergella asymmertrica, occurs in a thickness of 60 m, suggesting an Early Norian age. Biozone II; Heibergella aculeata, occurs within 50m, representing a Middle Norian age. Biozone III; Hebecysta balmei, which occurs in a thickness of 40 m, indicating the Middle to Late Norian age. Biozone IV; Rhaetogonyaulax rhaetica, occurs in a thickness of 330 m, indicating a Late Norian to Rhaetian age. Statistical studies on the recorded palynomorph assemblages show that lower parts of the section have been deposited in an open marine environment. In Late Norian a regression of the sea caused sediment of the Rhaetian to be deposited in a deltaic environment. The Early Kimmerian and the following orogenic phases have had the least effected on the sedimentation processes during Late Triassic age in this area and the carbonaceous Elika Formation turned into Shemshak group gradually and without any disruption.