Recognition of upper Triassic temperate foraminiferal assemblages: Insights from the Khodz Group (NW Caucasus, Russia)
Introduction
The “reef-rush” has considerably impacted the study of Triassic carbonate platforms, leaving a grey area outside the tropical belt (palaeolatitudes higher than 35°). Most studies have focused on the Tethyan Realm, particularly the warm, tropical Peri-Tethys area. The study of Triassic foraminiferal assemblages is also clearly unbalanced. Panthalassan and non‑carbonate occurrences of Triassic foraminiferal assemblages were only poorly known until recently (e.g., Apthorpe, 2003; Chablais et al., 2010a, Chablais et al., 2010b, Chablais et al., 2011; Haig and McCartain, 2010; Rigaud et al., 2012, Rigaud et al., 2013a, Rigaud et al., 2013b; Rigaud et al., 2015a, Rigaud et al., 2015b, Rigaud et al., 2016; Rigaud and Martini, 2016), and foraminifers from temperate regions were found (e.g. Efimova, 1975; He, 1980, He, 1982; He and Norling, 1991; Vuks, 2000; Korchagin et al., 2003; Senowbari-Daryan et al., 2010), but never studied as such. The palaeogeographic potential of Triassic foraminifers has, as a result, remained tremendously undeveloped.
The Upper Triassic (Norian – Rhaetian) Khodz Group of the Northern Caucasus was deposited in a marine basin at approximate 40–50° northern palaeolatitude (Stampfli and Kozur, 2006). There, a succession of pinkish and light grey thick-bedded limestone more than 300 m thick contains a rich and diverse assemblage of brachiopods (Dagys, 1963, Dagys, 1974), ammonoids (Shevyrev, 1995), corals and sphinctozoan sponges (Boiko et al., 1991; Belyaeva and Punina, 1994), echinoids, thin-shelled bivalves, sponge spicules, and bryozoans. Previous determinations of foraminifers were performed by Efimova, 1975, Efimova, 1991 and Vuks, 1996, Vuks, 2000, Vuks, 2004. Gaetani et al. (2005) provided new stratigraphic data for the Upper Permian to Jurassic sequences of the Northern Caucasus and identified a rich assemblage of foraminifers in the Mt. Tkhach – Sakhray – Kamennomostsky area, but did not describe the assemblage in detail.
This study provides a thorough account of temperate foraminifers from the Khodz Group and adds important data for the palaeobiogeographic and palaeolatitudinal distribution of Late Triassic foraminifers that support the existence of the Eastern Laurasia palaeobioprovince of Chablais et al. (2011). Compared to tropical-belt shallow marine foraminiferal assemblages, the recorded assemblage contains a relatively high number of diverse trochospiral involutinids, which are known to have remarkably survived the end-Triassic extinction event and characterize more open marine settings in the Early Jurassic (e.g. Gušić, 1975; Blau, 1987a, Blau, 1987b; Blau and Haas, 1991; Ebli, 1993; Blau and Grün, 1997). Their prevalence in high-latitude Triassic shallow seas adds important information in determining the driving factors behind the end-Triassic biotic crisis.
Section snippets
Regional setting
The wide collisional belt of the Caucasus is the result of a long-term convergence between the Eurasian continental margin, Gondwana-derived terranes of Palaeozoic and Mesozoic age (Cimmerian continents), and also the Arabian plate (Nikishin et al., 2001; Golonka, 2004, Golonka, 2007; Adamia et al., 2011). Today, the Caucasus consists of numerous tectonic units (Fig. 1), which comprise, from north to south: (i) the Scythian (pre-Caucasus) platform; (ii) the fold-thrust mountain belt of the
Stratigraphic succession
The Permian – Triassic stratigraphic succession of the Fore Range overlies the Variscan basement that was uplifted and partly dissected during the Late Palaeozoic to form a horst and graben topography. In most areas, Permian clastic sediments filled the grabens, but in the studied Mt. Tkhach – Sakhray – Kamennomostsky area, the metamorphic basement is unconformably overlain by Lower Triassic carbonates of the Tkhach Group (Fig. 4).
An unconformity on the top of the Tkhach Group marks a second
Previous studies of foraminifers in the northern Caucasus
Norian – Rhaetian assemblages of the Khodz Group were previously investigated by Efimova, 1975, Efimova, 1991, Vuks, 1996, Vuks, 2000, Vuks, 2004, and Gaetani et al. (2005). Efimova (1991) and Vuks, 1996, Vuks, 2000, Vuks, 2004 distinguished two local foraminiferal zones for the Triassic of the Caucasian Fore Range. The “Aulotortus friedli” (now Parvalamella friedli) zone in the lower part of the Khodz Group indicates the late Norian, as suggested by the concomitant presence of the bivalve
Material and methods
Rocks belonging to the Khodz Group mainly crop out at the upper reaches of the Tkhach River, forming the highest part of the Bol'shoy Tkhach and of Mt. Kolokolnia, and at Mt. Yatyrgvarta (see Gaetani et al., 2005). Five samples from four levels were collected from a section exposed on the west side of Mt. Kolokolnia (Fig. 4). A total of 27 thin sections of sizes 47 mm × 28 mm, and one thin section of size 15 cm × 10 cm were made. No obvious change in foraminiferal assemblage was detected among
Lithology of the Khodz Group
The Khodz Group consists of thick-bedded pinkish to light grey limestone. The entire group is more than 300 m thick. The Kolokolnia section (Fig. 4) spans only about one-third of the group. Pink bioclastic limestone predominates in the lower part of the section and pink muddy limestone (calcilutite) characterizes the upper part. The limestone displays “dissolution/precipitation and hematite enrichment”, interpreted by Gaetani et al. (2005) as episodes of emersion. Bioclastic limestone and
Depositional environment
The foraminiferal assemblage from the Kolokolnia section corresponds to the Rhaetian (according to great thickness, also late Norian?) Involutina liassica local zone (Efimova, 1991; Vuks, 2000), and its taxonomic composition is remarkably similar to the assemblage described from the Nayband Formation near Esfahan in Central Iran (Senowbari-Daryan et al., 2010). The Norian to Lower Jurassic Nayband Formation is a succession of clastic sediments and, to a minor extent, carbonates, which filled a
Conclusions
The Rhaetian foraminiferal assemblage from the Khodz Group differs from most Late Triassic assemblages from the western Tethys and Panthalassa domains in the abundance of trochospirally-coiled involutinids (Trocholinidae) as well as the paucity and limited diversity of porcelaneous “reef-dwellers”. In view of existing palaeolatitudinal data (40–50° northern palaeolatitude: Stampfli and Kozur, 2006), the heterozoan nature of the carbonate platform and the absence of ooids, we interpret the
Data availability
Any data used in this study and not presented is available on request to the corresponding author.
Declaration of Competing Interest
None.
Acknowledgements
The material investigated in this research was gathered by the late Maurizio Gaetani in the scope of the Peri-Tethys Programme and the Italian MURST–Cofin 1999 programme. The research is financially supported by the Slovenian Research Agency (Programme No. P1-0011 by L.G.). We thank Galina Nestell, one anonymous reviewer, and the editor for their constructive remarks and the time they invested in reading the manuscript.
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Dr Gaetani is deceased.