Boletín de la Sociedad Geológica Mexicana Volumen 71, núm. 3, 2019, p. 773-804 http://dx.doi.org/10.18268/BSGM2019v71n3a9

Systematic paleontology and taphonomic studies of Ypresian mollusks at the Kopet-Dagh Basin, NE Iran

Amir Salahi¹, Abbas Ghaderi¹, Ali Reza Ashouri¹, Alejandro Cristín², Saeedeh Senemari³

¹Department of Geology, Faculty of Sciences, Ferdowsi University of Mashhad, Azadi Square, 9177948974, Mashhad, Iran.

²Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán, 04510, CDMX, Mexico.

³Faculty of Technical & Engineering, Imam Khomeini International University, 3414896818 Qazvin, Iran.

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Abstract

Marine invertebrate fossils have long been considered important tools for age dating and stratigraphic interpretation of the Paleogene deposits of Central Asia, however information has not yet been provided from the Kopet-Dagh Basin (NE Iran). In this research, fossiliferous horizons of the Chehelkaman Formation at the Sheikh and Ghaleh-Zou sections (which have never been recognized previously), both in the Sheikh Syncline area, are discussed. These sedimentary beds overlay the terrestrial siliciclastic Pesteligh Formation and are the first evidence of marine flooding of the Paleogene transgression in the central Kopet-Dagh. Systematic paleontological studies of molluscan fossils in this succession led to the identification of ten genera, twelve species and two subspecies of bivalves and five genera and five species of gastropods. The assemblage belongs to four different fossiliferous horizons: (1) Turritellidae Dominated Assemblage (TDA), (2) Pycnodonte – Turkostrea horizon, (3) Cordiopsis - Cardium horizon and (4) Globularia shell beds in the Sheikh section and only TDA horizon in the Ghaleh-Zou section. The age of the whole succession, based on Cordiopsis subathooensis – Turritella subathooensis Zone co-occurrence with nannofossil zones NP12 and NP13, is Late Ypresian (early middle Cuisian). The TDA in both sections confirms the abundance of nutrients and their bimodal orientation demonstrates effects of oscillatory waves rather than unidirectional paleo-currents. High rates of bioerosion, encrustation, fragmentation, disarticulation and corrosion of the shells in the Pycnodonte – Turkostrea horizon reveals high volumes of nutrients and energy-rich conditions. This interval, overlain by the Cordiopsis – Cardium horizon in a pavement arrangement, represents the stable conditions after a storm. The Globularia shell beds interbedded with marly beds indicate stability and a deepening trend of the environment, terminating to deeper marine shales and marls of the Khangiran Formation.

Keywords: Taphonomy, paleoenvironment, Early Eocene, Kopet-Dagh Basin, Chehelkaman Formation.