Abundant trace fossil Polykampton in Palaeogene deep-sea flysch deposits of the Lesser Caucasus in Georgia: Palaeoecological and palaeoenvironmental implications

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Highlights

  • Polykampton georgianum is a new deep-sea ichnospecies in Paleocene of Georgia.

  • P. georgianum is composed of a median tunnel and lateral, petal-like spreite lobes.

  • P. georgianum occupies the middle tier and belongs to the category sequestrichnia.

  • The trace fossil assemblage is atypical but similar to the Nereites ichnosubfacies.

  • Low ichnodiversity suggests dysoxia and periodical anoxia in the Ardigani 1 section.

Abstract

The new ichnospecies of Polykampton georgianum occurs abundantly in a part of the Borjomi Flysch (Upper Paleocene – Lower Eocene) in Georgia, in particular in the 45.3 m thick Ardagani-1 section that is dominated by grey silt- and mudstones having sandstones intercalated. The trace fossil occurs in the middle and upper part of the section. The trace fossil is composed of a median tunnel and side lobes that are filled with material differing from the host sediment. It is interpreted as a sequestrichnion, wherein the tracemaker stored organic-rich mud in the lobes as a food resource for times of food deficiency. The today ferruginization of the median tunnel resulting from the former presence of pyrite suggests strong chemical gradient between its lumen and the surrounding. Chemichnial behaviour is not excluded as nutritional strategy of the worm-like tracemaker (a polychaete?). It occupies a middle tier and is commonly crosscut by Trichichnus, Chondrites and Scolicia. Rare or absent other trace fossils typical of deep-sea fine-grained deposits suggest slight dysoxia. Significant impoverishment of the tiering pattern (only Trichichnus present) and absence of trace fossils in the lower part of the section point to strong dysoxia or episodic anoxia. The trace fossil assemblage is atypical, but in general it resembles the Nereites ichnosubfacies of the Nereites ichnofacies, which characterizes distal parts of the turbiditic depositional systems. The contrast between the dark grey to grey silt- and mudstones deposited during the waning phase of gravitational flows and greenish pelagic mud suggests that the former originated from an organic-rich depositional setting, presumably within the oxygen minimum zone in the upper-slope to outer-shelf domain. Sequestration of the organic-rich mud was a successful adaptation of the Polykampton georgianum producers in this specific environmental setting.

Introduction

Some otherwise rare, deep-sea trace fossils may occur abundantly in some formations during a limited time span and area for reasons that cannot be easily explained with respect to the environmental conditions. It seems that some biological factors conditioned their occurrence and disappearance (e.g. Strzeboński and Uchman, 2015). Nevertheless, such trace fossils can be important components of usually atypical ichnofabrics and their tracemakers could have played a considerable role in ecospace utilization.

Such a trace fossil, in this study distinguished as a new ichnospecies of Polykampton Ooster, 1869 occurs in a part of the upper Paleocene – lower Eocene Borjomi Flysch of the Achara-Trialeti Fold-Thrust Belt (ATFTB) in the Lesser Caucasus, Georgia. Polykampton represents a rare deep-sea trace fossil known only from a small number of occurrences so far (see Uchman and Rattazzi, 2018; Uchman et al., 2019). The ichnospecies discovered in Georgia is abundant in a 30 m-thick interval of flysch exposed in the Ardagani-1 section. It also occurs in a few nearby sections, in a similar stratigraphic position, but has not been observed in other parts of similar deep-sea deposits of the ATFTB.

The aim of this paper is to describe the new ichnospecies of Polykampton in the Ardagani-1 section and to unravel its 3D morphology, ethology, tiering position, and clarify its taxonomy. Based on that and on co-occurring trace fossils and sedimentological data, the environmental conditions and their changes are interpreted.

Occurrences of trace fossils in the Borjomi Flysch were first mentioned in conference abstracts (Lebanidze et al., 2016, Lebanidze et al., 2019). The aforementioned Polykampton was classified as Dreginozoum (Lebanidze et al., 2019).

Section snippets

Geological setting

Disposed between the Great Caucasus and the Lesser Caucasus, Georgia belongs to the Northern Tethyan geological province (Zakariadze et al., 2007, Zakariadze et al., 2012). The studied section is located in the ATFTB, south of the northern Transcaucasian (Georgian and Azerbaijanian) blocks and north of the Lower Palaeogene andesitic belt of the Middle East. The ATFTB is partly covered by Oligocene–Neogene sediments of the river Mtkvari (Kura) intermontane depression and young volcanic rocks (

Systematic description

Ichnogenus Polykampton Ooster, 1869.

Type ichnospecies. Polykampton alpinum Ooster, 1869.

Diagnosis. Horizontal, ribbon structure composed of a median cylindrical tunnel and complex side leaf-like lobes (modified from Uchman and Rattazzi, 2018, and by Uchman et al., 2019).

Polykampton georgianum isp. nov.

Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10B, C.

Derivation of name. From Georgia, country of finding.

Type horizon and locality. The uppermost part of the lower Tusrebi unit of the Tusrebi

Ethological interpretation

The morphology of Polykampton georgianum shows that the median tunnel was open and occasionally true-branching constructed, which was for multiple use by the tracemaker. The spreite structure of the lateral lobes points to a systematic, long-term reworking according to a certain, repetitive programme. As the trace can be several tens of centimetres long, even a few hundred lobes were produced. The lobes are intentionally filled with sediments of different, usually darker colour. It is not clear

Conclusions

Polykampton georgianum isp. nov. is a horizontal structure composed of a winding median tunnel and lateral, petal-like spreite lobes. It is distinguished by more or less perpendicular, mostly separated, symmetrically arranged lobes with respect to the median tunnel. The trace occurs abundantly in the middle part of the Tusrebi Subsuite (Thanetian) of the Borjomi Flysch (upper Paleocene – lower Eocene) in Georgia, especially in the middle and upper parts of the 45.3 m-thick Ardagani-1 section.

Declaration of Competing Interest

None.

Acknowledgements

This works was supported by the Shota Rustaveli National Science Foundation of Georgia (SRNSFG) [Grant number FR-18-3765 – Ichnology and Sedimentology of the Paleocene–Lower Eocene Sediments in the Achara-Trialeti Fold-Thrust Belt, Georgia]. Moreover, A.U. was supported by the Jagiellonian University (DS funds) and by the Department of Geology, Faculty of Exact and Natural Sciences and Alexander Janelidze Institute of Geology, Iv. Javakhishvili Tbilisi State University. Photograph of

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