Stratigraphic hiatuses at the Frasnian/Famennian boundary in South Ural’s sections

Tagarieva R. Ch.

Institute of Geology USC RAS, Ufa, Russia

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Conodonts play an important role in revealing gaps (hiatuses) within stratigraphic sections. Only detailed conodont-based zonal subdivision makes it possible to determine their duration with an accuracy of one conodont subzone. In Upper Devonian carbonate sections of the South Urals (Inzer and Askyn types) the stratigraphic hiatuses have been defined at the Frasnian/Famennian boundary [4, 9, 10, 1, 2, 11]. In both types of the sections the magnitude of the hiatuses is not constant and varies within the triangularis – rhomboideа zonal interval.

In the Inzer type sections (Lemezinsky, Gabdyukovo, Mendym) Upper Devonian deposits are represented by goniatite biofacies (Fig.) [5, 1, 2]. The base of the Famennian Stage is absent in the Gabdyukovo and Mendym sections. The depth of the hiatus equals the Early-Middle triangularis subzones in the Gabdyukovo section and the triangularis – rhomboideа zones in the Mendym section (see Fig.) [1]. The minimum magnitude of the hiatus that equals the Late triangularis zonal interval has been recognized in the Lemezinsky section [2] (see Fig.).

In the Askyn type sections (Bol’shaya Barma, Akkyr, Kuktash, Ryauzyak, Kuk-Karauk), with brachiopod shell stones developed at the Frasnian/Famennian boundary, the stratigraphic hiatus has been established above the Barma Horizon (see Fig.) [1, 2, 8]. In the Akkyr, Kuktash and Ryauzyak sections the hiatus corresponds to the Late triangularis zonal interval. In the Kuk-Karauk section the hiatus is long and encompasses the Late triangularis – rhomboidea zonal interval (see Fig.).

Many researchers explain the stratigraphic hiatuses recorded in the South Urals on the basis of conodonts by the occurrence of global regression falling on the Frasnian/Famennian boundary [9, 10, 11]. The shoaling process is corroborated by the new data resulted from investigations on biodiversity of conodont assemblages and changes in the δ¹³С variational curve [3, 6, 7, 8].

Fig. Correlation of the Frasnian/Famennian boundary deposits in the South Ural’s sections and the distribution of conodonts: A - the Lemezinsky section; the Akkyr, Kuktash and Ryauzyak sections (summary column); B – the Gabdyukovo section; C – the Mendym and Kuk-Karauk sections.

Abbreviations: Ic. – Icriodus; Pa. – Palmatolepis; Pol. – Polygnathus; a – alternatus; d – delicatula; i – iowaensis; m – marginifera; p – perlobata; q – quadratinodosalobata.

In the Askyn type sections the top of the Frasnian Stage (the linguiformis Zone) records a sudden dominance of the Icriodus taxa as representatives of the shallow-water conodont biofacies [3, 8]. Changes in the icriodid content curve at the Frasnian/Famennian boundary correlates with those in the δ¹³С curve [7]. This can be indicative of the reduction in the basin depth during the linguiformis – triangularis conodont zonal interval.

The investigation was financially supported by the Russian Foundation for Basic Research (RFBR), grants 11-05-01105-а and 11-05-00737-а.

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