RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES1002, doi:10.2205/2007ES000221, 2007

Characteristics of the Section

[11]  Pechishchi is the Upper Kazanian stratotype that attracted research interest as far back as the late 19th century [Noinsky, 1899, 1924]. In general, the Pechishchi section is not confined to a single outcrop. The uppermost layer, described by Noinsky near the settlement of Pechishchi, is Layer 38 of the Podluzhnik member. The upper layers (up to Layer 52) were described by Noinsky near the Truba ravine and correlated with a section near Krasnovidovo. Thus, the Pechishchi section means a composite section of several outcrops.

[12]  This research was followed up in detail by other geologists [Solodukho and Tikhvinskaya, 1977] who grouped the members identified by Noinsky into four beds (upwards): The Prikazansky strata (Yadreny Kamen and Sloisty Kamen series), the Pechishchi strata (Podboi, Sery Kamen and Shikhany series), the Verkhny Uslon strata (Opoki and Podluzhnik series) and the Morkvashi strata (Perekhodnaya member). The total thickness of the section is 51 m. This paper is based on the published data on the Pechishchi lithostratigraphic section [Burov and Gubarev, 1998]. Recent palaeontological and stratigraphic data provide the less detailed stratification of the section into 31 layers instead of the Noinsky's cyclic scheme of 52 layers, although the both patterns are generally in good agreement. This composite section includes Layers 1 to 7 outcropping on the Volga bank between Pechishchi and Naberezhnye Morkvashi and Layers 8 to 31 at the bottoms and on the slopes of the Kamenny, Cheremushka and Truba ravines.

2007ES000221-fig01
Figure 1
[13]  Samples for isotope studies were collected from Layers 5, 8, 9, 13, 16, 18, 19, 20, 21, 22, 25, 26, 27, 28 and 30 of the Pechishchi section (Figure 1). The Permian trend was identified and described using core samples of the Lower Kazanian (1 sample), Lower Permian (3 samples) and Upper Carboniferous (1 sample) from Well 1/97 near Naberezhnye Morkvashi representing the Lower Permian continuation of the Pechishchi section. These samples were complemented with one core sample from Well 3 drilled through the Artinskian occurring on the southeastern slope of the South Tatarstan Arch. The samples were collected from the most representative, macroscopically least altered carbonate intervals.

[14]  Pechishchi is a unique section that was formed in the Kazanian palaeomarine basin with its axial zone stretching from the lower reaches of the Mezen river through the upper reaches of the Vychegda river, lower reaches of the Kama river and upper reaches of the Sheshma and Sok rivers southwards to Buzuluk. This zone includes the most complete and faunistically richest Kazanian marine sections [Kotlyar and Stepanov, 1984]. Shallow-water and coastal deposits containing gypsums and salts occur west and south of the basin's axis (Samarskaya Luka), and the marine, lagoonal and red terrigenous deposits are replaced by the Belebeevo continental formation east of it.

[15]  The Pechishchi section clearly features three Noinsky's cycles [Noinsky, 1924] associated with the cycles of evaporite formation in the Kazanian palaeosea (Figure 1). Each Noinsky's cycle consists of three components. Lower component - carbonate (rich by marine faunas - on Figure 1 it is signed by letter "F"), middle component - evaporite (carbonates with hypsum and anhydrite - on Figure 1 it is signed by letter "E") and upper component - terrigenous rocks (clays and marls - on Figure 1 it is signed by letter "C"). These components reflect alternation of environments from normal marine ("F") through higher salinity (E) to lake and lagoon conditions ("C").

[16]  These cycles are similar to the Zechstein marine cycles identified in Germany and England. In reduced mode, these are also similar to the Lofer cycles in which fauna-rich carbonate beds are replaced by dolomites of tidal and supratidal zones and by red or green clay rocks [Fischer, 1964]. Fischer relates changes in the basin depth to eustatic sea-level variations. This interpretation seems to be more credible than the concept suggesting a complex spectrum of local epeirogenic movements of the Earth's crust, although their effect on the sedimentation was not excluded by Fischer. Fischer's calculations suggested that eustatic variations of the sea level could be as high as 15 m and its periodicity could be characterised by a cycle duration varying from 20,000 to 100,000 years.

[17]  The origin of the evaporitic component is related to the widely accepted concept that salt basins have to be partially isolated from the open sea by a sill or a bar, supposedly accounting for an increase in water salinity. Otherwise, concentrated brines would flow to the ocean with return currents. Some researchers assume that this isolation could be performed by a physical barrier: for instance, an organic reef, a sand bar or an uplift of sea bedrocks.

[18]  The basin model [Scruton, 1953] suggests the existence of a dynamic barrier that appeared due to the friction between water masses of different densities, similarly to the pattern observed in the Mississippi river. Such a barrier becomes more effective as the channel, which connects the salt basin with the open sea, shrinks. The barrier should be in dynamic equilibrium affected by such factors as temperature, wind pressure and sea level, each in turn affecting water density in the region.

[19]  Some researchers confront the bar hypothesis [Arkhangelskaya and Grigoriev, 1960] assuming that water entering a salt basin could already have increased salinity.

[20]  Some others also believe that the origin of calcium sulphates and halite does not have to be accounted for by the bar hypothesis as large sand bars could impede water circulation in vast, partly isolated, shallow seas, resulting in the deposition of these evaporites [Sugden, 1963]. The above periodicity could, most probably, be controlled by relative changes in sea level and also by the climate. An increase in atmospheric temperature over the whole sedimentation basin enhanced the evaporation that in turn resulted in a higher compensation current from the open sea impeding further increase in salinity. As a result, the evaporitic layer could become thicker. An increase in climatic humidity probably stimulated the entry of fresh waters into the basin by currents from the land reducing the water salinity.

[21]  As it appears from the above, strontium ratios from the Pechishchi section can help studying their relation to the open sea and the effect of evaporisation on the sedimentation, allowing at the same time to tie these data to the Phanerozoic strontium ratio curve and to estimate the age of the studied deposits.


RJES

Citation: Nurgalieva, N. G., V. A. Ponomarchuk, and D. K. Nurgaliev (2007), Strontium isotope stratigraphy: Possible applications for age estimation and global correlation of Late Permian carbonates of the Pechishchi type section, Volga River, Russ. J. Earth Sci., 9, ES1002, doi:10.2205/2007ES000221.

Copyright 2007 by the Russian Journal of Earth Sciences

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