The Permian deposits show remarkable variations in carbon isotopic composition, and carbon-isotope excursions, recognized in a wide range of marine and continental deposits, are often global events (e.g. Metcalfe et al., 2009; Bond et al., 2010; Cao et al., 2010; Korte and Kozur, 2010; Richoz et al., 2010; Liu et al., 2013; Mii et al., 2013; Shen et al., 2013). They offer, thus, a correlation potential of the global C-isotope profile with the sedimentary sequences that lack reliable biostratigraphic resolution, such as the Upper Permian Zechstein sequence of NW and Central Europe. At its base, the Zechstein contains one of the prime correlation markers in NW European stratigraphy, the Kupferschiefer. The basal Zechstein strata are characterized by a marked increase in the δ13C values, from generally low values in the lower part of the Kupferschiefer (often 0 to +2‰) at the base of the Kupferschiefer, to much higher values (+5 to +6‰) in the middle part of the Kupferschiefer and then in the succeeding Zechstein Limestone (e.g. Marowsky, 1969; Magaritz et al., 1981; Turner and Magaritz, 1986; Peryt et al., 2012a). Magaritz et al. (1981) correlated the sharp change of the carbon isotopic composition found in the Zechstein, that was regarded to occur within only a few thousand years, with a similar record from the Delaware Basin, and suggested that the event was world-wide. In fact, the δ13C curve from the basal Zechstein from northern Poland shows a similarity to the δ13C record from sections in South China (Bond et al., 2010; Detian et al., 2013) that represent the middle Capitanian, however, the negative excursion reported by Bond et al. (2010) is not present in the equivalent horizon at the Guadalupian/Lopingian boundary GSSP section (Chen et al., 2011), possibly because of a local signal or diagenesis (Shen et al., 2013). Therefore, although being similar, the recorded change in the Delaware and Zechstein basins occurred in different times. Rare conodonts recorded in the basal Zechstein indicate its early (but not the earliest) Lopingian age (Korte et al., 2005, with references therein). An even younger age was recently concluded by Szurlies (2013) on the basis of the combination of magnetic polarity records, suggesting that the basal Zechstein is most probably equivalent to the uppermost lower–upper Wuchiapingian (cf. Kozur, 1989, 1994; Denison and Peryt, 2009). When compared to the generalized isotopic curve for the Late Permian proposed by Richoz et al. (2010) and Shen et al. (2013), the increase of the δ13C values observed after the Isotope Event 0 (some 258 Ma) fits well the recorded increase (although clearly more eminent) starting from the Kupferschiefer (Peryt et al., 2012).
There are two aims of this paper that is based on the study of basal Zechstein deposits occurring below the PZ1 (=Werra) evaporites in three selected borehole sections located in the basinal setting in western Poland. The first aim is the facies and isotopic characterization of sections coming from the palaeo-lows, with well-developed Kupferschiefer deposits, of depths being several tens of metres deeper-located than the sections coming from palaeo-highs within the basin (both represented by reef sections and condensed sections). The second aim is to use eventual isotopic variation of calcite to verify the concept that the abundance of hemispheroid aragonitic cement recorded in the Zechstein Limestone reefs (e.g. Peryt, 1984; Tucker and Hollingworth, 1986; Weidlich, 2002), including also the isolated reefs in western Poland (Peryt et al., 2012b), is the result of prolific carbonate precipitation due to occasional upwelling of saline waters on shelf environments in the stratified Zechstein Basin (Weidlich, 2002). If this interpretation is correct, then the basinal deposits of the Zechstein Limestone should reveal features (including the isotopic composition) indicating their origin in an environment characterized by increased salinity and/or temperature.
2 Geological Setting
The Zechstein inland sea (Fig. 1) was established due to the rapid flooding of a land-locked topographic depression, developed as a result of subsidence exceeding sedimentation rates (Pharaoh et al., 2010, with references therein), by Arctic seas, possibly through a combination of rifting and eustatic sea-level rise (Smith, 1979; Glennie and Buller, 1983). During the initial Zechstein transgression, the topmost parts of the Rotliegend dune sands were reworked and form the so-called Weissliegend. The Weissliegend sandstone locally grades into limestone in Germany and Poland. This Basal Limestone, known as Mutterflöz or Border Dolomite in Germany, is only present in shallow-water environments. All these lowermost Zechstein units are overlain by the Kupferschiefer, followed by the Zechstein Limestone (Ca1), Lower Anhydrite, Oldest Halite and Upper Anhydrite (Fig. 2; Peryt et al., 2010). The Zechstein Limestone (Ca1) is actually often dolomite and hence the Ca1 nomenclature is used further in this paper for the carbonates of this lithostratigraphic unit.
During the Kupferschiefer deposition, deep- and shallow-water facies have been distinguished (Oszczepalski and Rydzewski, 1987; Paul, 1987). The deep water facies is characterized by a consistent thickness of 20–60 cm and is composed of alternating organic-rich shale containing fine planar laminae of clay and planar- and wavy-laminated dolomitic-calcareous clays. The floor of the deep-water facies was mostly located within the anaerobic zone or at its transition into the dysaerobic zone. The shallow-water facies is characterized by a varied thickness of sediment (up to several m thick), consisting mainly of planar- and wavy-laminated dolomitic-calcareous marl. It was deposited mostly in dysaerobic (in Poland) and aerobic (in Germany) conditions (Oszczepalski and Rydzewski, 1987; Paul, 2006). The coeval deposits are developed in a calcareous facies formed in well-aerated waters above the chemocline. There is a consensus that reducing conditions were established shortly after the Zechstein transgression, whereby the nutrient-rich waters promoted a high organic productivity in surface waters (Brongersma-Sanders, 1971; McCann et al., 2008), and together with the high evaporation rates, the development of permanent stagnant bottom-water conditions. Three carbonate-clay cycles occur in Germany; most likely, they resulted from fluctuations of productivity which partly depended on climatic variation (Paul, 2006).
The Ca1 carbonates show a fairly consistent development throughout the basin, with up to >120 m of carbonates and marls in marginal areas (carbonate platforms and their slopes), in contrast to 5 to 10 m of carbonate in the sediment-starved basinal setting (Peryt et al., 2010, with references therein). A notable exception is western Poland where due to occurrence of inherited intra-basinal ridge (Brandenburg–Wolsztyn High) thick sequences (termed reefs – Dyjaczynski et al., 2001; Kiersnowski et al., 2010a) are associated with uplifted tectonic blocks built of Carboniferous rocks (Fig. 1); these blocks are lacking the Kupferschiefer. The reef biota is dominated by bryozoans, brachiopods and echinoderms – a typical bryonoderm association indicating cool-water and cold-water environments (cf. Peryt et al., 2012b).
Seismic data augmented by core data indicated that the transitions between the thick, shallow-water and thin, basinal sequences are abrupt (Górski et al., 2000; Dyjaczynski et al., 2001). The Kupferschiefer occurs in the basinal setting. There are thin (often <1 m thick) sequences in shallow parts of the Brandenburg–Wolsztyn High that are typically devoid of the Kupferschiefer and are characterized by grainy deposits (oncolites, oolites, bioclastic deposits) (Peryt and Ważny, 1980; Hammes et al., 2013). For the adjacent deeper parts smaller thicknesses of the Ca1 carbonates (usually less than 4 m) are characteristic, and argillaceous micrites have been deposited (Peryt and Ważny, 1980). This facies pattern suggests that in the region of the Wolsztyn High existed a mechanism promoting the preferential carbonate precipitation in some areas, whereas in other parts starved basin conditions existed.
In the depressions between reef bodies the PZ1 section is complete and it includes a relatively thick Lower Anhydrite and thin Upper Anhydrite. In addition, the Oldest Halite occurs between both anhydrite units in some areas outside the reefs (Fig. 2). The PZ1 evaporites constitute a sulphate platform up to 300 m thick except for the elevations of the Brandenburg–Wolsztyn High where the PZ1 evaporites are thinner (Kiersnowski et al., 2010a, tables 1 and 2).
3 Material and Methods
Three cored sections have been studied: one of them, Kościan 21, is located at the SE margin of the Kościan reef complex (Peryt et al., 2010), about 0.6 km from the reef margin as indicated by 3D seismic data, and Paproć 28 and Czarna Wieś 4 sections are located 4–8 km north of Jabłonna and Elżbieciny reefs (Fig. 3); the Ca1 carbonates in the latter section are condensed.
The basal Zechstein deposits in those three boreholes were studied in standard thin sections; in a total of 53 thin sections. The slabbed specimens (with other slabs used to produce standard thin sections) have been sampled selectively with a 1.5 mm diameter stainless steel drill with tungsten carbide coating which was used for material extraction from the surfaces of the specimens. Considering the diameter of sampling (1.5 mm) and the petrographic variability shown by studied rocks, the isotopic sampling has to be regarded as it was for whole rock samples. Consequently, each resulting isotopic measurement would reflect both depositional and diagenetic fluids.
CO2 gas for isotope analysis was extracted from the carbonate samples (their location is indicated on the right side of the lithological column in Fig. 4)) using the selective chemical separation technique described by Al-Aasm et al. (1990). CO2 was obtained at 25°C in 2 h of reaction with 100% phosphoric acid for analysis of calcite. Then the sample kept on reacting until the next day and thereafter all CO2 was pumped away. After that, the reaction was continued at 50°C over 2 days and CO2 was collected for the isotopic analysis of dolomite. The isotopic analysis was performed on a dual inlet and triple collector mass spectrometer with standard uncertainty of δ13C and δ18O values of 0.07‰. The rough delta values were normalized to the VPDB scale by analysis of CO2 extracted from the NBS-19 standard at 25 °C.
Foraminifers were studied in thin sections taken from the Ca1 carbonates (see Fig. 4 for their location in the Czarna Wieś 4 borehole where the thin sections were made from the samples analysed for carbon and oxygen isotopes; in the case of two other boreholes, in addition to such samples, some other thin sections have been prepared). There are several classifications of Permian foraminifers, including higher taxa and genera, with no universally accepted interpretation (Korchagin, 2011). We use the classification by Loeblich and Tappan (1988) supplemented by Groves et al. (2003, 2004) in this paper.
4.1 Facies and foraminifers
Carboniferous mica shales and thin volcanic rocks occur in the Kościan 21 borehole below the Zechstein. The Kupferschiefer (25 cm thick), developed as typical mineralized intercalated clay and dolomitic shales, with TOC values varying between 4.6 and 15.5% (Kotarba et al., 2006, their samples from depths of 2300.37 to 2300.56 m), is followed by the Ca1 carbonates (dolomites 1.75 m thick). In the Paproć 28 borehole, the Zechstein is underlain by fine- to medium-grained, strongly calcareous grey sandstones of the Weissliegend showing bioturbation, rare gravel grains (up to 4 mm across) and possible outlines of dissolved shells (probably bivalves), that gradually pass into thin (3 cm) lamina of bioclastic limestone (this is the Basal Limestone), followed by 40-cm-thick mineralized Kupferschiefer (with three cycles of carbonate-clay) and then by the Ca1 carbonates (calcareous dolomites 70 cm thick). In both the Kościan 21 and Paproć 28 boreholes nodular anhydrites of the Lower Anhydrite occur above the Ca1 carbonates. The PZ1 evaporites consist of the Lower Anhydrite (95.5 m in Kościan 21 and 18.1 m in Paproć 28), Oldest Halite (22.0 and 109.5, respectively) and Upper Anhydrite (17.0 m and 78.0 m, respectively).
The Czarna Wieś 4 borehole is located within the belt of alluvial fan sandstones and conglomerates that frame the Brandenburg–Wolsztyn High (Kiersnowski et al., 2010b). The Ca1 carbonates are 95 cm thick; there are limestones in the lower and upper parts of the unit and dolomitic limestones and calcareous dolomites in the middle part of the unit. In the section the Lower Anhydrite (22.6 m thick) is followed by the Oldest Halite (127 m thick) that contains an anhydrite intercalation (8 m thick) in its upper part and then the Upper Anhydrite (33 m thick) occurs (cf. Fig. 2B).
The results of the petrological study are summarized in Table 1. The principal types of rocks in the sections are shown in Figures 4-7.
Table 1. Characterization of studied sections from the basinal facies of the Zechstein Limestone
Czarna Wieś 4
Occurs (25 cm thick) (Fig. 5A); bituminous shale with bioclasts (shells, ostracods)
Occurs (0.4 m thick) (Fig. 5B, C); bituminous shale (with quartz grains) followed by clay-bituminous shale
1.75 m thick
0.7 m thick
0.9 m thick
Principal rock textures
Peloid wackestones–packstones (accompanied by bioclast–peloid packstone) (Fig. 5I, K, L) in the lower and middle parts of the section followed by peloid–oncoid packstones (Figs. 5M, 6B) in the upper part, at the top recrystallized oncoid packstones (Figs. 5J, N-O, 6A)
Peloid wackestones–packstones with bioclasts and common extraclasts (quartz, feldspar, and siliceous rocks) (that disappear upsection) (Fig. 5E, G, H) followed by peloid–oncoid packstones (Fig. 5D, F)
Peloid packstones with bioclasts and, in the lowermost part, quartz grains and glauconite (in some cases abundant) (Fig. 6D), followed by peloid packstones and bioclastic wackestones (Fig. 6C, E), then oncoid–peloid packstones and, in the upper part, oncoid packstones (Fig. 6F, G)
Mostly dolomitic in the middle part of the section; calcitic in its lower and upper parts
Dolomitic, some larger grains (mostly bioclasts and oncoids) replaced by anhydrite
Common: encrusting (Fig. 7A–E, H, K, L), hemigordiopsid (Fig. 7F, G, J, M, N, W–Z), uniserial (Fig. 7I, J, O–V)
The Basal Limestone (Ca0) was recorded only in the Paproć 28 borehole where it is sandy; the sand content decreases upsection and this is accompanied by the common appearance of horizontally arranged shells (often both valves occur) of bivalves and brachiopods accompanied by encrusting foraminifers. The Kupferschiefer shows a similar development in the Kościan 21 and Paproć 28 sections (Fig. 5A–C); it is calcitic (Paproć 28) or dolomitic (Kościan 21). Only dolomite occurs in the Ca1 carbonates of the Kościan 21 borehole; in the Paproć 28 borehole the matrix is dolomitic and grains (or their sparitic filling) are calcitic (or dolomitic). In the Czarna Wieś 4 borehole dolomite appears in the matrix of the middle part of the Ca1 carbonates. The carbonates of the Zechstein Limestone contain various, usually small (a few percent) admixtures in studied samples which is the smallest in the case of Czarna Wieś 4 borehole.
In the Czarna Wieś 4 borehole the Weissliegend sandstone is overlain by thin (5 mm) clay lamina followed by peloidal packstones with quartz grains and glauconite and, in the lowermost part, lithoclasts (quartzite, limestone) (Fig. 6D). There occur bivalve shell fragments and hemigordiopsid foraminifers. Higher up in the section, quartz and glauconite disappear and brachiopods and bryozoans occur. In the upper part of the Ca1 carbonates oncoids occur – first they are small, and then larger and more frequent; they are accompanied by radial ooids and a rich faunal assemblage comprising bivalves, brachiopods, bryozoans, gastropods, corals, and foraminifers (including abundant sessile forms) (Figs. 6C, E, and 7). In the uppermost part recrystallized oncoid rock occurs with bioclasts (shells and bryozoan zoaria) coated by sessile foraminifers which form thin irregular crusts (Fig. 6F, G). In addition, sessile foraminifers occur within oncoids (0.7–1.2 mm across) (Fig. 7E, L). In the latter case, they clearly record phases of non-deposition and microbial corrosion of the earlier-formed grains, and then the restoration of microbial growth (cf. Peryt and Peryt, 1975).
The samples from Czarna Wieś 4 borehole showing common foraminiferal encrustations abound in hemigordiopsids that are also accompanied by common uniserial foraminifers (Fig. 7). Foraminifers in Kościan 21 and Paproć 28 boreholes are often dwarfed; in a few cases they are large (Fig. 5), and foraminifers (in particular encrusting forms) are much rarer compared to the Czarna Wieś 4 borehole.
The results of isotopic analyses are shown in Figure 4 and are summarized in Table 2. They are also shown in plots (Fig. 8 for the Kupferschiefer and Fig. 9 for the Ca1 carbonates).
Table 2. Statistical data on the stable isotopic composition in the studied boreholes (Kościan 21, Paproć 28 and CzarnaWieś 4); for comparison, data on six boreholes located within the Wolsztyn reefs are given, and the data on the basinal section of basal Zechstein in northern Poland, Zdrada IG8 (Peryt and Peryt, 2012; Peryt et al., 2012a). Ca0 – Basal Limestone, Ca1 – Zechstein Limestone, T1 – Kupferschiefer
The average δ13C values for the Kupferschiefer of the Kościan 21 and Paproć 28 boreholes are 1.6 ± 1.4‰ and −3.0 ± 1.0‰, accordingly. In the Kościan 21 section, δ13C values increase from −0.4‰ in the Kupferschiefer at its base to +2.8‰ at its top (Fig. 4, Table 2). In the Paproć 28 borehole the δ13C values change from −1.9‰ at the base to −3.9‰, then to −1.9‰, next to −4.5‰ in the uppermost Kupferschiefer, and then rapidly increase to 1.1‰ in the lowest part of the Ca1 carbonates. The δ18O values of the Kupferschiefer show a similar range as the δ13C values, from −4.8‰ to +2.5‰ in the Kościan 21 borehole and from −4.1‰ to −2.1‰ in the Paproć 28 borehole (Fig. 4, Table 2), and the average values are −1.6 ± 2.2‰ and −2.8 ± 0.8‰, accordingly. The range of δ13C values of the Ca1 carbonates is from −2.2‰ to +7.2‰, and the average δ13C values are +3.8 ± 0.5‰ (Kościan 21, dolomite), 1.6 ± 0.7‰ (Paproć 28, calcite), 3.3 ± 2.0‰ (Paproć 28, dolomite), 1.4 ± 1.4‰ (Czarna Wieś 4, calcite) and 4.1 ± 0.9‰ (Czarna Wieś 4, dolomite) (Table 2). The δ18O values show a range from −10.6‰ to +7.1‰, with a considerable range of average values for particular boreholes and mineralogies (Table 2). In the section, a clearly upward increase of δ13C values is noticed in the Paproć 28 and Czarna Wieś 4 boreholes and remains quite stable in the Kościan 21 borehole (Fig. 4).
δ13C values for the Kupferschiefer in the studied boreholes are generally lower than, and δ18O values are similar as, in the coeval basinal deposits from other parts of the basin (Fig. 8). As far as the Ca1 carbonates are concerned, δ13C values of the three boreholes are clearly comparable to both basinal sections from other parts of the basin (Fig. 9) as well as more shallow-water settings (Magaritz and Peryt, 1994; Becker, 2002), including also the Wolsztyn reef sections (Figs. 10 and 11). The range of δ18O values of limestones is similar to the range recorded in the basin facies (Fig. 9) and in the Wolsztyn reefs (Fig. 10A), and the dolomites show, in turn, higher δ18O values compared to the basin sections (Fig. 9) and the Wolsztyn reef sections (Fig. 10B).
5 Interpretation and Discussion
5.1 Carbon isotopes
The studied sections show clear shifts of δ13C values (Fig. 4) that are recorded despite the different mineralogy (calcite versus dolomite). We concur with the conclusion by Scholle (1995, p. 140) that the large-scale relative carbon isotope shifts probably reflect real shifts in oceanic chemistry through time, but the absolute values of those shifts and the smaller-scale fluctuations of the data are probably artifacts of environmental, lithological, and diagenetic changes. Scholle (1995, p. 144) also pointed out that isotopic shifts coincide possibly with formational boundaries, and thus the reality of the trend should be considered first. However, the same trend – a quite rapid increase from relatively low δ13C values to higher values – is observed despite the lithology (it was recorded both in the Kupferschiefer as well as in the sections lacking the Kupferschiefer, although in the latter case the fluctuations in δ13C values are less pronounced) and despite the nature of the underlying strata (it is observed both in the case when the Zechstein is underlain by the Rotliegend deposits, as well as when various Carboniferous lithologies occur below). In addition, in the reef sections there is no such shift in the lowermost Ca1 carbonates (Peryt et al., 2012b, fig. 12 and the discussion in the text). The δ13C values are clearly different in the reef sections (e.g. Dyjaczynski et al., 2001, fig. 10) and the studied sections due to different contribution of organic carbon. In the Kupferschiefer of the Paproć 28 borehole fluctuations in δ13C (as well as δ18O) values is observed; they are similar to those reported from northern Poland (Peryt et al., 2012).
The carbon isotope signature of almost all dolomite samples coming from the Wolsztyn reefs (δ13C from +2 to +8‰, average ca. +4‰ VPDB) suggests no significant shift in carbon isotopic composition due to diagenesis; the source of the carbonate ion was the Zechstein Limestone itself (with or without a contribution from Zechstein marine water) (Jasionowski, 2010). The same is concluded for the studied sections, and this implies a possibility to use the recorded changes of δ13C values for carbon isotope stratigraphy.
The carbon isotope curve for the Capitanian and Wuchiapingian carbonates developed on an ancient seamount in mid-Panthalassa (Kamura area, Kyushu, Japan) shows that the high positive δ13C values (+5 to +6‰) (Kamura event) until the late Capitanian when after three negative shits the δ13C value fell to +2‰ (Isozaki et al., 2007). The δ13C record from sections in South China, calibrated against a high-resolution conodont biostratigraphy, revealed that after the topmost Capitanian showing values up to 5.5‰, a fall of ca. 1.5‰ occurs in the lowest Wuchiapingian (Wang et al., 2004; Bond et al., 2010; Isozaki et al., 2011) that is followed by a general increase in δ13C values (Korte et al., 2004; Bond et al., 2010), to the previous high value of ca. 4–5‰. This relatively high value remained steady for several Ma until a consistent, gradual and progressive decline in δ13C began in the early Changhsingian, ca. 255 Ma (Richoz et al., 2010).
The Zechstein carbonates show 13C-enriched signals relative to their Tethyan counterparts (Korte et al., 2005; Peryt et al., 2012a, with references therein) which fits well with the interval of relatively high δ13C values related to the upper Wuchiapingian and the lower Changhsingian. Peryt et al. (2012a) compared the trend observed in the basal Zechstein of northern Poland to the generalized isotopic curve for the Late Permian proposed by Richoz et al. (2010) and concluded that the increase of the δ13C values observed after the Isotope Event 0 (ca. 258 Ma) fits well with the increase (although clearly more eminent) from the Kupferschiefer until the Main Dolomite.
Therefore, the observed trend towards heavier δ13C values in time is considered as the expression of a major change in ocean water chemistry (Peryt et al., 2012). However, clearly lower δ13C values of condensed sections compared to the reef sequences (as well as other sections from the marginal carbonate platform – Magaritz and Peryt, 1994) and the more basinal sections (Fig. 11; Peryt and Peryt, 2012) possibly indicate exchange with an organic carbon reservoir (Hudson, 1977; cf. Ehrenberg et al., 2002: p. 109–110). Other possible explanations of the origin of carbonate depleted in 13C, such as increasing water depth, freshwater input, and meteoric diagenesis (Patterson and Walter, 1994) are excluded because of the geological situation.
In particular, a significant burial alteration of the δ13C of the micrites seems unlikely, because of the large mass of carbon in the limestones relative to the small mass of carbon in any potential diagenetic fluid (Immenhauser et al., 2002, p. 89).
In the basinal sections of basal Zechstein, low or negative δ13C values typical for the Kupferschiefer increase in the lowermost part of the Ca1 carbonates to 4-5‰ and remain stable, with small deviations, throughout the Zechstein Limestone (Peryt et al., 2012a). In the studied boreholes, δ13C values of calcite samples are distinctly lower than in the case of the Wolsztyn reef sections (Fig. 10), and δ13C values of dolomite samples are statistically lower as well (Fig. 10). The Wolsztyn reef sections show those high values from the very beginning of the Zechstein (Peryt et al., 2012b) and thus such a record can be related to the lack of counterparts of the Kupferschiefer in those sections. Whether those deposits were eroded in the highly-energetic environments of the basal Zechstein (cannibalism) or never came into existence, is uncertain. The basinal sections of the Zechstein Limestone occurring near the reefs, described in this paper, and characterized by the occurrence of the Kupferschiefer show a well expressed isotopic trend typical for the basinal sections (cf. Fig. 4 and Peryt et al., 2012a, fig. 3). This in turn implies that the upper portions of the Zechstein Limestone in the studied boreholes show great condensation coeval with prolific carbonate precipitation in adjacent reefs that evidently trapped the bulk of carbonate ions available in the basin water reservoir. As the thickness and facies of the Kupferschiefer in those condensed sections do not differ compared to the typical Kupferschiefer sections, the condensation came into existence afterwards.
5.2 Oxygen isotopes
The geological setting of the Kościan 21 borehole suggests that the dolomites have originated due to the reflux or eventually a mixing mechanism. A precise estimation of oxygen isotopic composition of brines or the dolomite precipitation temperature is difficult because of the uncertainty in the fractionation factor between dolomite and the parental (mother?) fluids (Vasconcelos et al., 2005; Wacey et al., 2007). At low temperatures the water-dolomite fractionation factor is much lower than at high temperatures (Vasconcelos et al., 2005). Figure 12A and B shows the interpretation of crystallization temperatures depending on the equation for low-temperature dolomite (Vasconcelos et al., 2005; Fig. 12A) and high-temperature dolomite (Friedman and O'Neil, 1977; Fig. 12B) and the arbitrary assumption of δ18Owater value as high as +4‰ vs. SMOW; the assumption of a higher δ18Owater value would result in much higher temperatures of dolomitization which seems to be unrealistic (cf. Jasionowski and Peryt, 2010). For dolomites of the Zechstein Limestone of the Kościan 21 borehole the temperature was slightly above 40 °C (within the range of 36–47 °C), with a slightly lower average temperature for the Paproć 28 well, although in this case the variation was much greater (from 14 °C to 67 °C). Higher temperatures are inferred for the Czarna Wieś 4 borehole (average 57 °C, range from 48 °C to 63 °C) and for the Kupferschiefer of the Kościan 21 borehole (ca. 67 °C) which is explained by diagenetic alteration related to higher temperatures in a deeper burial environment (cf. Jasionowski and Peryt, 2010) (average 57 °C), leading to a depletion in the heavier oxygen isotope in the dolomite (Figs. 4, 8, 9 and 12A). Jasionowski (2010) concluded that the dolomites of the Wolsztyn reefs that show a very wide range of δ18O values (from +3 to −9‰, Fig. 10), originated in a deeper, higher temperature environment, and in some cases the lighter oxygen content has been related to an inflow of meteoric water. We concur with such an interpretation.
The δ18O value of calcite is 2.6‰ lower than those of a coexisting dolomite, if both minerals precipitate in isotopic equilibrium at the same temperature from the same solution (Vasconcelos et al., 2005); the difference between average δ18O values for calcite and dolomite in the Paproć 28 and Czarna Wieś 4 boreholes is 4.2‰ and 4.1‰ (Table 2), respectively, and thus both minerals precipitated either at different temperatures and/or from different solutions. The calcite samples of the Zechstein Limestone of both boreholes show considerable ranges of δ18O values (Fig. 4, Table 2), similar to the reef sections from the Wolsztyn High (Fig. 10A). Although there is no doubt that the diagenetic alterations have affected the oxygen isotope values, available data sets for different palaeocontinents show large differences in δ18O values (Chen et al., 2013) that are attributed to local hydrological conditions associated with regional aridity and differences in the evaporation/precipitation ratio. It is remarkable that even in well-preserved shells of Permian brachiopods, Mii et al. (2012) demonstrated the range of δ18O values reaching 5‰. If the maximum δ18O values are considered to be the closest to the pristine original ones and if δ18Owater value = 0 is assumed, then the calculated temperature in the Czarna Wieś 4 and Paproć 28 boreholes is 24 °C and 19 °C, respectively, and if δ18Owater value = 2‰ is assumed, the temperature rises to 34 °C and 27 °C, respectively (Fig. 12C). The palaeotemperatures calculated for the Kupferschiefer of the Paproć 28 borehole range from 23.5 to 34 °C (if δ18Owater value = 0 is assumed). All these values are close to the range of calculated palaeotemperature of seawater during the Zechstein Limestone deposition in northern Poland which was 23 to 33 °C (or more) (Fig. 12C; Peryt and Peryt, 2012), and are within the range of the palaeotemperature of Wuchiapingian sea water (Chen et al., 2013).
5.3 Environmental conditions
The faunal restriction and the predominance of lagenids in the foraminiferal assemblage of the Ca1 carbonates indicate continual dysaerobic conditions and possibly elevated salinity of seawater (cf. Peryt and Peryt, 2012). Because of a small tidal range in the intracontinental Zechstein Basin that was connected via a long narrow strait to the Boreal Ocean (e.g. Sørensen et al., 2007), a limited vertical mixing existed. There were persistent anoxic conditions during some intervals of the Kupferschiefer, otherwise suboxic conditions prevailed that were also typical during the Zechstein Limestone deposition in the basin centre. The Lopingian was a time of widespread marine anoxia (Şengör and Atayman, 2009, with references therein); for example, anoxia-euxinic conditions existed during some intervals of the Changhsingian at Meishan D section (Cao et al., 2009).
Considering the sudden flood of the intracontinental depression of the Southern Permian Basin by Arctic waters that initiated the Zechstein deposition, the water temperature was temperate at the beginning, but soon increased because of the location of the basin within the tropical zone. It is remarkable that Chen et al. (2013) recorded the increase of sea water temperature in the early Wuchiapingian and then, starting at ca. 258 Ma, its gradual decrease. Peryt and Peryt (2012) concluded a slight (ca. 1.5 °C) decrease in temperature in the basinal facies of northern Poland. Also, a steady increase of δ18O values upsection in the Czarna Wieś 4 borehole (Fig. 4) strongly suggests the decrease of sea water temperature with time, although undoubtedly this increase was affected by the increase of sea water salinity. The changes of δ18O values in two other boreholes studied also indicate a decrease of sea water temperature and/or increase of sea water salinity during the deposition of the Kupferschiefer and the lowermost Ca1 strata, while the interpretation of the data from higher Ca1 strata is enigmatic for Kościan 21 and Paproć 28 boreholes because of the important diagenetic imprint. It is highly probable that there was a distinct temperature stratification of seawater. Peryt and Peryt (2012) concluded that the faunal restriction and the dwarfed forms of foraminifers suggest elevated salinity of seawater in the basinal zone of the Zechstein Limestone in northern Poland, although geochemical data suggest that the salinity increased insignificantly (about 1‰) since the beginning of Zechstein Limestone deposition.
The Wolsztyn reefs are surrounded by areas characterized by the presence of condensed sequences which are evidence of a crisis in carbonate production and thus the vast basin area is incompatible with the concept of CO2-rich seawater. Mixing of shallow and deeper waters in the stratified Zechstein Basin would result in prolific carbonate precipitation, as suggested by Weidlich (2002), however, there is no isotopic record of eventual upwelling in the reef sections. On the other hand, deeper water masses have lower δ13C values, since nearly all of the organic matter that is produced by photosynthesis is subsequently remineralized in the water column (Bickert et al., 1997), and thus the isotopic composition of condensed sections of the Ca1 carbonates do not show an advection mechanism.
Peryt et al. (2012b) recorded, in the reef sections, the common occurrence of tubular foraminifers and various accompanying encrusting organisms grouped under the name Palaeonubecularia (e.g. Kabanov, 2003). In places, encrusting foraminifers are accompanied by other microencrusters, possibly microbes. Peryt et al. (2012b) concluded that the abundance of encrusting foraminifers in all facies of the basal Zechstein of the Wolsztyn area indicates that they were highly invasive with relatively rapid colonization rates, in contrast to the previous reports of the occurrence of encrusting foraminifers in oncoids and stromatolites of the basinal zone (e.g. Peryt and Peryt, 1975) that were interpreted to have occurred in association with very low sedimentation rates. In the studied sections, encrusting foraminifers are common (or even occur at all) only in some intervals; in the Czarna Wieś 4 section they abound in the upper part of the Zechstein Limestone (Figs. 4, 6E–G and 7) what is otherwise a rule for the basinal zone (Peryt and Peryt, 2012, fig. 2), and in any case they register important environmental perturbations.
Thin (<1 m) dolomite and/or limestone sections of the Ca1 carbonates in SW Poland occurring in the major part of the basin facies, are underlain by the Kupferschiefer and/or Weissliegend, and show peloid wackestone to packstone depositional texture with rare to common bioclasts (including encrusting, hemigordiopsid and uniserial foraminifers).
The range of δ13C values for the Kupferschiefer of the Kościan 21 and Paproć 28 boreholes is from −4.5‰ to +3.2‰, and the range of δ18O values is from −4.8‰ to +2.5‰. δ13C values for the Kupferschiefer in the studied boreholes are generally lower than, and δ18O values are similar as, in the coeval deposits from other parts of the basin.
The range of δ13C values of the Ca1 carbonates is from −2.2‰ to +7.2‰, and the δ18O values show a range from −10.6‰ to +7.1‰. In the section, a clearly upward increase of δ13C values is observed in the Paproć 28 and Czarna Wieś 4 boreholes and remains quite stable in the Kościan 21 borehole. The δ13C and δ18O values of all three studied boreholes show similarity to both basinal sections from other parts of the basin as well as more shallow-water settings, including also the Wolsztyn reef sections, although δ13C values of condensed sections are clearly lower compared to the reef sequences. The Wolsztyn reef sections show high δ13C values from the very beginning of the Zechstein and, thus, such a record can be related to the lack of counterparts of the Kupferschiefer in those sections. The basinal sections of the Zechstein Limestone occurring near the reefs, described in this paper, are characterized by the condensation coeval with prolific carbonate precipitation in adjacent reefs.
The faunal restriction and the predominance of lagenids in the foraminiferal assemblage of the Zechstein Limestone indicate continual dysaerobic conditions and possibly elevated salinity of seawater.
The presence of condensed sequences indicating a crisis in carbonate production is not compatible with the concept of CO2-rich seawater. Mixing of shallow and deeper waters in the stratified Zechstein Basin could result in prolific carbonate precipitation in local reef areas spreading into the basinal facies.
The research was funded by the State Committee for Scientific Research (grant no. 9 T12B 028 15) and by statutory funds of the Polish Geological Institute-National Research Institute (grant no. 61.5101.1101.00.0). We thank Polish Oil and Gas Company for the access to core and seismic data and P. Raczyński for his remarks on the fauna recorded in the studied sections. We appreciate helpful comments by the journal referees U. Hammes and S.-Z. Shen and the editor I. D. Somerville.