Palynostratigraphy and Reconsideration of the Shemshak Group Based on Dinoflagellate Cysts

The Nayband Formation was sampled at Chal-Sefid and Zard mountains in central Iran، for palynology and palynostratigraphy in order to take the advantage of dinoflagellate cysts to locate the Triassic and so called Jurassic boundary. The Chal-Sefid section is located about 45 km southwest of Kashan city and Zard Mountain some 70 km northeast of Esfahan. The studied strata attain a thickness of 1750 meters in Chal-Sefid and 1820 meters in Zard Mountain. They consists sandstone and shale beds. Totally 75 samples from Chal-Sefid and 22 samples from Zard Mountain were collected and treated in the Palynology laboratory of the Geological Survey of Iran. The recorded dinoflagellate cyst species were differentiated in four palynozones in Chal-Sefid and two palynozones in Zard Mountain as follows: Chal-Sefid section: Palynozone 1: Rhaetogonyaulax wigginsii، encompasses 160 meters of the section، suggesting an early to middle Norian age. Palynozone 2: Suessia listeri with a thickness of 140 meters، suggesting a middle Norian age. Palynozone 3: Hebecysta balmei encompasses 550 meters of the section، suggesting middle to late Norian age. Palynozone 4: Rhaetogonyaulax rhaetica with a thickneses of 900 meters suggests an early to middle Rhaetian age. Zard Mountain section: Palynozone 1: Hebecysta balmei، encompasses 442 meters of the section، suggesting a middle to late Norian age. Palynozone 2: Raetogonyaulax rhaetica، encompasses 491 meters of the section، suggesting an early to middle Rhaetian age. It is revealed that all the rock units investigated here are of late Triassic age and no evidence of Jurassic ages was identified.