The Triassic stratigraphic succession of Nakhlak (Central Iran), a record from an active margin

Abstract

An important, 2.4 km-thick Triassic succession is exposed at Nakhlak (central Iran). This succession was deformed during the Cimmerian orogeny and truncated by an angular unconformity with undeformed Upper Cretaceous sediments. This integrated stratigraphic study of the Triassic included bed-by-bed sampling for ammonoids, conodonts and bivalves, as well as limestone and sandstone petrographic analyses. The Nakhlak Group succession consists of three formations: Alam (Olenekian–Anisian), Bāqoroq (?Upper Anisian–Ladinian) and Ashin (Upper Ladinian). The Alam Formation records several shifts from carbonate to siliciclastic deposition, the Bāqoroq Formation consists of continental conglomerates and the Ashin Formation documents the transition to deep-sea turbiditic sedimentation.Petrographic composition has been studied for sandstones and conglomerates. Provenance analysis for Alam and most of the Ashin samples suggests a volcanic arc setting, whereas the samples from the Bāqoroq Formation are related to exhumation of a metamorphic basement. The provenance data, together with the great thickness, the sudden change of facies, the abundance of volcaniclastic supply, the relatively common occurrence of tuffitic layers and the orogenic calc-alkaline affinity of the volcanism, point to sedimentation along an active margin in a forearc setting.A comparison between the Triassic of Nakhlak and the Triassic succession exposed in the erosional window of Aghdarband (Koppeh Dag, NE Iran) indicates that both were deposited along active margins. However, they do not show the same type of evolution. Nakhlak and Aghdarband have quite different ammonoid faunal affinities during the Early Triassic, but similar faunal composition from the Bithynian to Late Ladinian. These results argue against the location of Nakhlak close to Aghdarband.

Biostratigraphy of Triassic Ammonoids

“…Other areas such as South Primorye (Zakharov 1997;Zakharov et al 2004bZakharov et al , 2005, South China (Wang 1978;Wang and He 1980;Stiller and Bucher 2008) and Nepal (Waterhouse 1999(Waterhouse , 2002a have received attention, but on a less frequent basis. Faunas have also been described intermittently from other scattered areas (e.g., Germanic Basin: Kaim and Niedźwiedzki 1999;Spiti: Balini and Krystyn 1997;Iran: Krystyn and Tatzreiter 1991;Balini et al 2009;Vaziri 2011;Mexico: Estep et al 1997;Greece: Jacobshagen et al 1993;Malaysia: Ahmad and Mohd 2010;Vietnam: Shigeta et al 2010). Correlation of these various faunas and biostratigraphic schemes has been discussed to a certain extent by several authors (Bucher 1989;Weitschat 1989, 1993;Mietto and Manfrin 1995a;Zakharov et al 1997;Monnet and Bucher 2005b;Waterhouse 2002b).…”

The record of the Late Palaeozoic active margin of the Palaeotethys in NE Iran: Constraints on the Cimmerian orogeny

“…Repeated unconformities, olistholithes and strong lateral facies variations suggest an important tectonic activity, possibly related to the reactivation of Palaeozoic tectonic lineaments. An extensional to transtensional tectonic setting has been envisaged during the Early to Middle Triassic (Baud et al, 1991;Balini et al, 2009). Volcanoclastic products were deposited in the basin especially during the Ladinian and possibly part of the Carnian (Sina Fm.…”

The Cimmerian evolution of the Nakhlak–Anarak area, Central Iran, and its bearing for the reconstruction of the history of the Eurasian margin

“…This supposed correlation has been used as the most persuasive argument to state that these localities were part of the same Palaeotethyan convergent margin, and that the Nakhlak succession was originally close to Aghdarband in the same trench-slope interval (Alavi et al 1997). Balini et al (2009) question the possible proximity of the two localities based on comparison among ammonoid faunas. Olenekian ammonoids from Aghdarband show a Periscaspian afinity, whereas the ones from Nakhlak show a typical Tethyan affinity: such an important palaeobiogeographic difference is not consistent with a proximity of the two areas.…”

“…3), in contrast with the carbonate-platform dominated sedimentation (Elikā Formation, Shotori Formation, etc.) that characterizes most of the Iranian region during the EarlyMiddle Triassic (Alavi et al 1997;Seyed-Emami 2003;Balini et al 2009). The approximately 2400 m-thick succession (Olenekian -Ladinian) of Nakhlak includes three main stratigraphic units.…”

“…A detailed study of the petrography of the Nakhlak succession is reported in Balini et al (2009) and is here summarized in its main results. Sandstones of the lower part of the Alam Formation are volcanic arenites dominated by microlithic-felsitic lithic fragments and plagioclase, and containing quartz, granophyric-granitoid rock fragments and chessboard albite.…”