Plant Macrofossils of Nayband Formation (Late Triassic), NE of Isfahan, Central Iran

The extent of the Shemshak group with Late Triassic-Middle Jurassic age in Iran can be divided into two structural-sedimentary zones of Iran and Zagros, which are separated by the main thrust of Zagros. The first zone is divided into two areas of Alborz and Central Iran (Moin-o-Sadat & Zade-Kabir, 1991). After the Early Cimmerian orogeny, a sedimentary sequence consisting of marl, sandstone, and reef limestone was deposited in Central Iran, called Nayband Formation. The Nayband Formation type section is located near the Naybandan village, 220 km south of Tabas city (Douglas, 1929). Its thickness reaches 3000 m in type locality (Kluyver et al., 1983; Seyed-Emami, 2003; Fürsich et al., 2005). The study area of the current research is located 74 km northeast of Isfahan, 15 km northwest of Chahriseh village (33°04’10’’N, 52°01’40’’E). This region is located in the western margin of the central Iran zone and south of Kashan. The thickness of the Nayband Formation in the north and northeast of Isfahan reaches a maximum of 660 meters, divided into five members (Mannani & Yazdi, 2009).

Upper Triassic succession in the northeast of Isfahan (Chahriseh section) was studied using Kashan's quadrangular geological map (Zahedi et al., 1991). Characteristics of sedimentary sequences such as stratification, lateral expansion and sedimentary structures were recorded. Sampling was taken systematically and regularly at one-meter intervals. Twenty-two plant macrofossils of Nayband Formation from the Qadir member have been collected.

A key bed of gray sandstone with plant macrofossils, up to 15 m thick, is identified in the Qadir member. Two genera and four species have been found in the section, include Clathropteris meniscoides (Brongniart 1825) Brongniart 1828 (Age: Carnian- Early Lias), Clathropteris obovata Oishi 1932, emend. Harris 1961 (Age: Norian- Middle Jurassic), Nilssonia pseudobrevis (Barnard 1967b) Corsin & Stampfli 1977 (Age: Norian- Rhaetian) and Nilssonia sp.Based on the Clathropteris meniscoides, Clathropteris obovata, and Nilssonia pseudobrevis in Qadir member (Nayband Formation) and their in situ stratigraphic position, these strata are deposited in Rhaetian. In this study, Clathropteris obovata is reported from Late Triassic age in Iran for the first time. Before this, Clathropteris obovata in Iran has been reported from only Jurassic (Late Liassic-Early Dugger); however, this species has a wider range from Late Triassic to Late Dugger succession in the other parts of the world. So, its record in the Nayband Formation (Late Triassic) is not unexpected. Moreover, the record of Clathropteris in the northeast of Isfahan is its first occurrence in an area outside of Kerman in Central Iran. Due to the low presence of plant macrofossils and the abundant presence of Clathropteris in the northeastern region of Isfahan, high humidity and warm weather of the Late Triassic may be suggested.