Abstract
The Tabas Block is part of the Central Iran microcontinent, located between the Lut Block in the East and the Yazd Block in the West. The Baghamshah Formation is the second lithostratigraphic unit from the sedimentary cycle of the Magu Group and the Baghamshah Subgroup in the Jurassic of Tabas Block. This formation is conformably underlaid with the grey pisoidal limestones of the Parvadeh Formation and overlaid with the Pectinid limestones of the Kamar-e-Mehdi Formation (Esfandiar Subgroup). In this research, the biostratigraphy of the Baghamshah Formation in the Rizu and Kamar-e-Mehdi sections, based on calcareous nannofossils, is examined. The thickness of the Baghamshah Formation in the Rizu section is 270 m (mostly including marl and green shales with intercalation of limestones and calcareous sandstones), and in the Kamar-e- Mehdi section is 236 m (composed of gypsiferous marly shales, marl, marly shales and alternation of marl-shale with limestones and calcareous sandstones). According to the taxonomic studies in the Rizu section, 52 species belong to 24 genera, and in the Kamar-e-Mehdi section, 45 species belong to 23 genera of calcareous nannofossils. Based on index calcareous nannofossils, the CC1, CC2, CC3, and CC4 biozones established by Sissinghh in both sections were determined. It is mentioned that CC5 biozone only occur in Kamar-e-Mehdi section. According to the identified biozones, the suggested age of the Baghamshah Formation is early Berriasian–early Hauterivian in the Rizu section, and early Berriasian–late Hauterivian in the Kamar-e-Mehdi section. Keywords: biostratigraphy, Baghamshah, calcareous nannofossils,Tabas, Iran.
Highlights
Central Iran occupies a key position for unravelling the post-Eo-Cimmerian Mesozoic (Jurassic–Cretaceous) history of the Iran Plate
Calcareous nannofossils In the studied samples, 52 species belonging to 24 genera in the Rizu section and 45 species belonging to 23 genera in the Kamar-e-Mehdi section were found (Figures 4, 5)
Nannoconus stainmanii, Nannoconus quadratus, Nannoconus globulus, Nannoconus dolomiticus, Kokia borealis and Diazomatolithus lehmanii are present at the beginning of both sections, and species such as Nannoconus cicularis, Nannoconus bucheri, Nannoconus quadricanalis, Calcicalatina oblongata and Lithraphidites bollii were more abundant in the upper parts of both sections
Summary
Central Iran occupies a key position for unravelling the post-Eo-Cimmerian Mesozoic (Jurassic–Cretaceous) history of the Iran Plate. From the numerous structural units of central Iran, the Tabas Block of east-central Iran shows the thickest, most complete, and best-exposed sequence of Jurassic rocks of the region, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate (Wilmsen et al, 2009). Geological setting and geographical position In Central Iran, the history of Jurassic sedimentary basins was largely shaped by differential subsidence and rotational movements among three blocks, called Lut, Tabas, and Yazd. These blocks are part of the Cimmerian microplate assemblage that collided with the Eurasian (Turan) Plate towards the end of the Triassic (Sengör, 1990). The facies pattern is complex and may change drastically, especially at the boundaries of the block (Figure 1)
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