Introduction

The timing of final tectonic paroxysms of the Variscan orogeny in Central Europe have been so far poorly constrained since their structural and thermal record in the nearby metamorphic complexes of the Bohemian Massif were relatively weak and/or later erased by erosion. Important information on the late orogenic evolution of the Variscan belt, in particular on its terminal deformation and uplift, can, however, be derived from the thick but now deeply buried Carboniferous sedimentary succession of the adjacent foreland basin. The Variscan foreland basin of Western Europe continues from Germany eastwards into western Poland together with the Variscan foreland fold-and-thrust belt on its southern flank, the latter generally believed to represent an equivalent of the tectonostratigraphic Rhenohercynian zone of Germany (Fig. 1). The deformed Carboniferous strata of the Variscan foreland basin define the topmost layer of the basement to the Permo-Mesozoic Polish basin (Figs. 2, 3), being deeply concealed below its thick sedimentary fill. The provenance studies made so far on the Carboniferous of western Poland (Jaworowski 2002; Krzemiński 2005; Mazur et al. 2006a) were not sufficient to result in a complete and consistent picture and the structural interpretations based on borehole data appear to be mutually incompatible (e.g. Oberc 1978; Karnkowski and Rdzanek 1982; Wierzchowska-Kicułowa 1984; Pożaryski et al. 1992).

Fig. 1
figure 1

Tectonic setting of the Carboniferous basin of Poland at the northern flank of the Variscan orogen. Variscan belt: MGCH Mid-German Crystalline high, MO Moldanubian zone, MS Moravo-Silesian zone, RH Rhenohercynian zone, ST Saxothuringian zone, SU Sudetes, TB Teplá-Barrandien zone, VF Variscan deformation front; TTZ Teisseyre-Tornquist fault zone–SW boundary of the East European craton

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Fig. 2
figure 2

Shallow-level geology of Poland showing the structure of Permo-Mesozoic fill of the Polish basin that overlies the Carboniferous Variscan foreland succession (modified after Dadlez et al. 2000). HCM Holy Cross Mountains

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Fig. 3
figure 3

Areal extent of the Carboniferous succession (shown in grey) on the background of deeply uncovered geological sketch map of Poland (without Permian, Mesozoic and Cenozoic) showing the main units of the basement. CDF Caledonian deformation front, HCM Holy Cross Mountains; STZ Sorgenfrei-Tornquist zone, TTZ Teisseyre-Tornquist zone, USB Upper Silesian block, VDF Variscan deformation front (after Pożaryski et al. 1992); WLH Wolsztyn-Leszno high

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In this paper, we employ structural, geochronological and geochemical data from 34 new and 10 revisited older boreholes (Fig. 4) to constrain the provenance of the material supplied to the Carboniferous basin and the timing of the basin inversion. Our provenance study shed a new light on the uplift and erosional history of the adjacent Variscan orogen prior to the final deformation of its foreland. The problems addressed here have remained unsolved in spite of the fact that since the early 1960s the Carboniferous succession of SW Poland was reached by approximately 250 deep boreholes.

Fig. 4
figure 4

Location of the investigated wells referred to in the paper (except for those used exclusively to collect dipmeter records) and of cross-section from Fig. 5, shown on uncovered tectonic sketch map of SW Poland (without Permian, Mesozoic and Cenozoic). GSM Góry Sowie massif of the Central Sudetes

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Geological setting

The Polish Lowlands, between the mountainous area of the Sudetes at the NE margin of the Bohemian Massif to the south, and the Baltic Sea to the north, are underlain by the extensive Permo-Mesozoic Polish basin, unconformably covered with up to 350-m thick Cenozoic sedimentary succession. The present-day thickness of the Permo-Mesozoic strata consistently increases to the north, from c. 1 km near to the Odra fault zone in the south, to exceed 4 km north of Poznań and c. 8 km in the axial part of the Mid-Polish trough still further to the NE. The latter syndepositional palaeostructure was subsequently inverted into the Mid-Polish anticlinorium by the end Cretaceous (“Laramide”) compressional event, whereas the SW part of the basin was tilted at that time to become the Fore-Sudetic homocline with strata gently dipping to the NE (Fig. 2). The Fore-Sudetic homocline and the adjoining Szczecin-Miechów synclinorium (Fig. 2) are underlain by the thick Carboniferous succession (Fig. 3). The erosional top surface of the Carboniferous plunges northeastward below the Permo-Mesozoic strata and quickly becomes mostly inaccessible by drilling. As a result, the well data presented in this paper are restricted to the part of the Carboniferous succession that underlies the Fore-Sudetic homocline and its direct neighbourhood (Fig. 4).

The Carboniferous sedimentary succession of western Poland generally exceeds 2,500 m in thickness and is predominantly composed of clastic marine sedimentary rocks, locally tectonised (folded and thrust-faulted) before Permian times (Żelichowski 1964; Grocholski 1975; Wierzchowska-Kicułowa 1984; Pożaryski et al. 1992, Fig. 5). It consists of monotonous Upper Viséan to Westphalian turbidites (all the Carboniferous chronostratigraphic ages used here follow the European classification scheme of Harland et al. 1990, Table 1) that continue downward to the terminal depths of all the studied wells and are locally capped by upper Westphalian to Stephanian shallow water sediments (Żelichowski 1980; Witkowski and Żelichowski 1981). The drilled Carboniferous sections of all the wells range from a few to a few hundred metres in length, with sections in two wells exceeding 1,000 m. Exceptionally, the Carboniferous turbidites resting on top of the popped-up metamorphic basement of the Wolsztyn-Leszno high (Fig. 5) are characterized by highly reduced thicknesses, usually of the order of a few tens of metres.

Fig. 5
figure 5

Vertically exaggerated geological section across the Variscan foreland in SW Poland (modified after Grocholski 1975). Additionally supported by seismic refraction evidence (Grad et al. 2002). DFZ Dolsk fault zone

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Table 1 A comparison of international (Gradstein and Ogg 2004) and European (Harland et al. 1990) stratigraphic subdivisions of the Carboniferous period
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Further to the north, beyond the Fore-Sudetic homocline, the Carboniferous basin clastics extend into the Upper Palaeozoic platform cover of the Pomeranian Caledonides, and to the east and southeast they reach the Holy Cross Mts, the Lublin trough and the Upper Silesian block, respectively (Fig. 3). In these areas, the Lower Carboniferous is represented by shallow marine, mostly carbonate and clayey rocks succeeded by clastic sediments deposited in Namurian and/or Westphalian times (Żelichowski 1995).

Rock complexes underlying the Carboniferous succession in western Poland remain mostly inaccessible by drilling, except at two WNW–ESE-elongated basement highs (Fig. 4), in which phyllites were drilled at a depth of c. 2,000 m in several wells (e.g. Oberc 1972). The larger, Wolsztyn-Leszno high, consists of phyllites derived from an Upper Devonian protolith (Haydukiewicz et al. 1999) and affected by multi-stage low-grade metamorphism, which terminated at 340.1 ± 2.6 Ma (Żelaźniewicz et al. 2003). The cooling signature of the phyllites, recorded by micas that define the dominant fabric of these rocks, is estimated at 358.6 ± 1.8 Ma (Mazur et al. 2006a). This timing falls in the range of 370–355 Ma Late Devonian cooling ages yielded by an important population of detrital muscovites from upper Viséan/Namurian A turbidites in the vicinities of the Wolsztyn-Leszno high (wells Marcinki IG-1, Września IG-1; Mazur et al. 2006a). Another population of detrital muscovites in those rocks cooled at c. 335 Ma (Mazur et al. 2006a), which correlates with the cessation of metamorphism in the phyllites (c.f. Żelaźniewicz et al. 2003). Along the NE border of the Wolsztyn-Leszno high is the Dolsk fault (Figs. 4, 5), which, on deep seismic refraction profiles (Guterch et al. 1994; Grad et al. 2002) represents a major structural discontinuity between a low-velocity crust to the SW, resembling that of the Variscan basement, and a three-layer, “transitional” crust of the Trans-European suture zone (Dadlez 1997; Grad et al. 2002).

To the southwest, the aeral extent of the Carboniferous succession is limited by the Middle Odra fault zone that defines the northern boundary of the Variscan crystalline basement of the Sudetes (Fig. 4). The non-metamorphosed Carboniferous strata are in contact, across this fault, with medium-grade gneisses and schists intruded by the several granite plutons of Early Carboniferous age (Oberc-Dziedzic et al. 1999; Dörr et al. 2006, Fig. 5).

The age of deformation of the Carboniferous succession beneath the Fore-Sudetic homocline is hitherto poorly constrained. Some authors assumed the main folding event to have occurred in the late Namurian (Oberc 1978, 1991; Karnkowski and Rdzanek 1982), whereas others argued for its middle Westphalian age (Wierzchowska-Kicułowa 1984; Pożaryski and Dembowski 1984; Pożaryski et al. 1992). None of those hypotheses was supported by firm structural and/or stratigraphic evidence. Even more uncertainty is related to the position of the Variscan deformation front in the Carboniferous basin, which is generally supposed to extend from the frontal part of the Rhenohercynian zone in Germany (Fig. 1), eastward into the territory of Poland. Due to the thick Permo-Mesozoic cover (Fig. 5), the location of this front is inferred on the basis of sparse and, often, equivocal well data. The hitherto adopted interpretations located the Variscan deformation front line roughly in the middle of the Carboniferous subcrop area in Poland (e.g. Jubitz et al. 1986; Pożaryski et al. 1992), either in an attempt to divide it into a deformed and little deformed/not deformed parts or, more often, with the front line understood as the trace of a frontal thrust of a foreland fold-and-thrust belt of the Variscan orogen.

Provenance

Geochemistry

Geochemical investigation on major and trace elements has been carried out by analysing 34 samples of turbiditic sandstones from 4 wells located on both sides of the Dolsk fault (Fig. 6; Table 2). The analysed sandstones represent fine- to medium-grained, texturally and compositionally immature arcosic wackes, lithic and feldspathic greywackes and rarely lithic arenites (mainly in well Siciny IG1). They usually show relatively narrow ranges of major and immobile trace-element contents: SiO2 66.4–81.5 wt%, Al2O3 9.6–15.6 wt%, TiO2 0.19–1.02 wt%, Fe2O3T 2.46–7.38 wt%, Na2O 1.16–4.03 wt%, K2O 1.24–3.13 wt%, Zr 109–417 ppm, La 7–38 ppm, Ce 40–73 ppm, Sc 4.4–13 ppm, Th 6–20 ppm, Ni 11–61 ppm, Co 6–21 ppm. The concentration of mobile trace elements such as Ba, Sr, Zn, and Pb is more scattered. Chondrite-normalized REE patterns (not shown) display moderate enrichment of light rare earth elements [(La/Yb)N = 6.8–10.5; (La/Gd)N = 4.0–7.1], moderate negative Eu anomaly (Eu/Eu* = 0.62–0.88), and relatively flat heavy rare earth element patterns with (Gd/Yb)N values between 1.15 and 1.88. In general, the chemical composition of sandstones and inter-element ratios for selected pairs of elements [e.g. La/Sc, Th/Sc, (La/Yb)N] correspond to those characteristic of average upper continental crust and an “average greywacke” (Table 2, Wedepohl 1995). These features emphasize a remarkably homogeneous nature of the Carboniferous succession in the studied area, and suggest its derivation from a common source.

Fig. 6
figure 6

Tectonic discrimination diagrams for Carboniferous turbidite sandstones from SW Poland. Discrimination fields defined by Bhatia and Crook (1986): PM passive margin, ACM active continental margin, CIA continental island arc, OIA oceanic island arc

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Table 2 Representative chemical analyses of Carboniferous sandstones from Variscan foreland basin of SW Poland
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All the analysed sandstones, plotted in the La–Th–Sc, Th–Co–Zr/10, and Ti/Zr versus La/Sc tectonic-discrimination diagrams of Bhatia and Crook (1986), occupy the same areas, mostly within the field of continental island arc and, subordinately, active continental margin (Fig. 6). Regardless of the particular tectonic settings indicated on discrimination diagrams, the latter confirm the relative homogeneity of the detritus supplied to the Carboniferous basin and point to orogenic derivation of the Carboniferous clastic material.

Detrital zircon geochronology

Zircons for geochronological studies were separated from 1 to 2 kg rock samples at the mineral separation laboratory of Wroclaw University, using standard crushing, sieving and density techniques with final manual separation under a binocular microscope. At the Research School of Earth Sciences, Australian National University (ANU), zircon grains were mounted in an epoxy resin disc together with chips of FC1 and SL13 reference zircons, sectioned approximately in half and polished. Prior to analysis, transmitted light photographs and cathodoluminescence (CL) SEM images of the zircons were prepared. The CL images were used to decipher the internal structures of the sectioned grains and to target specific areas within the zircons. The U–Th–Pb analyses were carried out using SHRIMP II ion-microprobe at ANU, following the analytical procedure described by Williams (1998, and references therein). A primary O− ion beam was used to extract positive secondary ions from areas ~25 μm in diameter. Each analysis consisted of six scans through the mass range. Normalization of Pb/U ratios was carried out by reference to analyses of the FC1 reference zircon (1,099.1 Ma; Paces and Miller 1993). U and Th concentrations were determined relative to those measured in the SL13 standard (Claoué-Long et al. 1995). The analytical data were processed using the SQUID and ISOPLOT software of Ludwig (1999, 2000).

Two concentrates of detrital zircons from wells Siciny IG-1 and Września IG-1 were analysed (Figs. 7, 8). They are both from turbiditic sandstones with palynologically established stratigraphic positions. A medium-grained Namurian A sandstone sample was taken from well Siciny IG-1 at a depth interval of 2,727.4–2,731.6 m, situated below the deformed part of the sedimentary succession. The zircons from this sandstone are represented by a population of colourless, transparent and clear crystals, 70–380 μm long, showing generally euhedral to subhedral shapes. They are mostly prismatic with {100} prism dominating over {110} and usually {101} bypiramid better developed than {211} form. Cathodoluminescence imaging shows well-preserved primary (igneous) oscillatory compositional zoning and ovoid cores inside some grains (Fig. 7).

Fig. 7
figure 7

Cathodoluminescence images (top) and Tera-Wasserburg diagram (bottom) with U–Pb ages of analysed zircons from a sandstone sample collected in well Siciny IG-1. Ellipses mark SHRIMP spots. Data points arrayed along the concordia plot reflect zircon growth and recrystallisation through time, without any evidence of significant alteration. Inset Probability diagram of 238U/206Pb zircon ages

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Fig. 8
figure 8

Cathodoluminescence images (top) and Tera-Wasserburg diagram (bottom) with U–Pb ages of analysed zircons from a sandstone sample collected in well Września IG-1. Ellipses mark SHRIMP spots. Data points arrayed along the concordia plot reflect zircon growth and recrystallisation through time, without any evidence of significant alteration. Inset Probability diagram of 238U/206Pb zircon ages (the oldest grain is not included)

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The U–Th–Pb SHRIMP results are listed in Table 3 and plotted on the Tera-Wasserburg diagrams (Figs. 7, 8). All the analysed grains from the Siciny sandstone yield Palaeozoic age spectra with well-defined two maxima. Eleven spots gave weighted mean average (WMA) of 238U/206Pb ages at 345.4 ± 3.2 Ma (95% conf., MSWD = 1.3; probability = 0.21). This age is interpreted as the maximum depositional age for the sandstone. Zircons from this group contain 590–1,179 ppm U, 178–463 ppm Th with Th/U, typical of igneous zircons, ranging from 0.28 to 0.40. The second major detrital zircon component yielded 238U/206Pb WMA at 363.8 ± 3.3 Ma (10 spots; 95% conf.; MSWD = 1.2; probability = 0.29). The U content in this age group is between 64 and 1,271 ppm, whereas the Th content falls in the range of 49–520 ppm. The Th/U ratio varies from 0.32 to 0.76. Two analyses produced older 238U/206Pb ages at ca. 400 Ma. They represent zircons with cores and therefore the ages could be interpreted as “mixed” and, therefore, geologically meaningless.

Table 3 Summary of SHRIMP U–Pb zircon results for turbidite sandstone samples from wells Siciny IG-1 and Września IG-1
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In well Września IG-1 a medium-grained sandstone was sampled at the depth interval of 4,921.4–5,196.2 m, situated in the upper part of the drilled succession. The sandstone is palynologially dated as not older than Westhphalian D. The zircon population derived from this sandstone is more diverse than that collected from the Siciny well. The largest age group of detrital zircons reveals ages scattered in the time span representing almost the entire Carboniferous (Fig. 8; Table 3). Zircons from this group are heterogeneous in terms of size, shape, colour and internal structures. Some of them are clear, euhedral to subhedral, prismatic with well-preserved oscillatory zoning (2, 4, 10, 12, 14, 17, 20, 25). Some others are transparent, rounded or anhedral with truncated oscillatory zoning (1, 11, 16, 18, 26, 28). There are also anhedral or ovoid zircons with lack of, or complicated, internal structures (9, 13, 15, 19, 26, 21). Analysed zircons contain 58–2,146 ppm U, 17–1,283 ppm Th with Th/U ratio ranging from 0.06 to 0.63. The broad scatter of results clearly shows that analysed Carboniferous zircons represent several successive stages of prolonged igneous activity. The obtained dataset is not sufficient to discriminate between individual events, but proves that the maximum depositional age of sampled sandstone is not older than Westphalian D. Six analyses from the Września sample yielded older ages. The oldest analysed zircon reveals almost concordant age at 2,081 ± 18 Ma (1σ). This is a slightly rounded, normal-prismatic, colourless, transparent zircon which can be defined as S12 subtype in classification Pupin and Turco (1972). The remaining grains—3, 5, 6, 22 and 23 gave 238U/206Pb ages at c.a. 566, 544, 380, 516 and 426 Ma, respectively. The youngest U–Pb ages from the sample are represented by zircons 27 and 29. Grain 27 (clear, transparent, colourless, well-rounded, spherical) is free of inclusions and homogenous in composition. It yielded a discordant 238U/206Pb age of 255 ± 3 Ma (1σ). Zircon 29 (angular fragment, transparent with strongly disturbed primary zoning) also gave a discordant 238U/206Pb age of 227 ± 3 Ma (1σ). The internal features of both the analysed grains puts into question the geological value of the obtained ages.

Structural data

Data from drill cores

In the area south of the Wolsztyn-Leszno High, thrusting/reverse faulting phenomena have been proved by repetitions of stratigraphic intervals in cores from a number of wells. The tectonically repeated sections are usually a few hundred metres long and fault-bounded at the bottom and top. They mostly occur only in the upper sections of the Carboniferous profiles. The strata in such intervals commonly show moderate to steep dips, with local signs of an overturned orientation. They are usually underlain by shallow dipping to horizontal strata down to the terminal depths of the studied boreholes.

Well Marcinki IG-1

The tectonic deformation concentrates in the upper part of the Carboniferous profile at the depth interval of 2,323–2,154 m (Fig. 9). Drill core reveals moderate to steep dips of bedding with a common occurrence of overturned strata and recumbent mesofolds of variable size with subhorizontal axial planes. The domains of steeply dipping beds are bounded by subhorizontal zones of localized high deformation at depths of 2,323 and 2,154 m, probably representing thrust planes. They contain tectonic breccias and separate rocks of contrasting sedimentary facies. Effects of the intense deformation are concentrated in narrow intervals up to several metres in thickness, situated immediately beneath the inferred thrust planes. Above the deformed intervals, sandstone and mudstone strata are subhorizontal with only rare steeper intervals that dip up to 50° (Fig. 9).

Fig. 9
figure 9

Simplified stratigraphic sections of Carboniferous from wells Marcinki IG-1, Siciny IG-1, and Dankowice IG-1 showing position of investigated samples. Hachure shows dips and deformation of strata

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The intensity of deformation declines considerably downwards, starting from the depth of 2,323 m. Strata in drill cores show shallow dips (0°–15°), only locally increasing up to 45°–90° in short limbs of asymmetric folds. The latter are in places intersected by fracture cleavage dipping at an angle of 60°–65°. In the lower part of the borehole, steep to subvertical carbonate veins commonly occur, bearing slickensides with subhorizontal striae.

The palynological documentation of Namurian A above the depth of 2,100 m (Górecka-Nowak 2007) comes from the weakly deformed interval (Fig. 9). This is underlain by the Westphalian B rocks at depths from 2,135.1 through 2,141.5 m to 2,348.4 m through 2,352.8 m. All the deeper collected samples reveal late Viséan or Namurian A ages (Górecka-Nowak 2007).

Well Siciny IG-1

Conspicuous tectonic deformations are concentrated here in the middle part of the penetrated part of the Carboniferous succession (Fig. 9). Steeply dipping and partly overturned strata with abundant slickensides were encountered at the depth of 2,450–2,660 m. This interval is bound at the base by a tens of centimetres thick subhorizontal zone of tectonic breccia (Fig. 9) presumably corresponding to a thrust fault. Another similarly oriented zone of localized deformation occurs at the depth of 2,542 m. Inverted strata are recognized at depths intervals of 2,526–2,521 and 2,643–2,638 m, directly above the inferred thrust planes. The overturned orientation of strata in the hanging wall of these thrusts can be related to the shorter limbs of drag mesofolds. The upper boundary of the deformed interval has not been observed and must occur between 2,450 and 2,418 m, not documented by drill core. The upper part of the Carboniferous succession (above 2,418 m) is represented by distal turbidites dipping at 0°–15° (locally up to 25°). Similarly, the lower part of the profile, at 2,660–3,000 m, also reveal subhorizontal to shallow (<20°) structural dips of Carboniferous strata.

Palynological data document Upper Viséan-Namurian A ages below 2,660 m underneath the highly deformed interval of the profile (Górecka-Nowak 2007; Fig. 9). In contrast, the sandstones and conglomerates with volcanogenic fragments that occur in the upper part of the deformed interval, between 2,542 and 2,450 m, were dated at Westphalian C times (Górecka-Nowak 2007). The overlying weakly deformed rock package from the depth interval of 2,310–2,319 m yielded Upper Viséan-Namurian A age. These are capped without any sign of a tectonic contact by Westphalian B–C rocks between the depth of 2,255.5 m and the base of Permian strata at 2,004.5 m (Fig. 9).

Well Dankowice IG-1

The tectonic deformation in this well is similar to those observed in wells Marcinki IG-1 and Siciny IG-1 (Fig. 9). Steep to overturned strata accompanied by abundant calcite-filled vertical fractures were recorded in the upper part of the Carboniferous section in the depth interval of 1,240–1,462 m. This interval is separated from the underlying horizontal strata by a several decimetre-thick horizon of tectonic breccia. The latter is interpreted as a record of a thrust fault, but the lack of palynological data does not allow verification of a possible tectonic repetition of strata. The upper boundary of the tectonized interval has not been observed and must be located between the depths of 1,240 and 1,197 m, where no drill core was extracted. The uppermost part and the lower half of the Carboniferous profile in this well (above 1,197 m and below 1,462 m, respectively) are nearly devoid of any signs of tectonic deformation.

Other wells in the southern area

Not all of the wells situated in the southern part of the investigated area reveal effects of thrust-faulting and folding. The profiles of wells Wołczyn IG-1 and Rzeki IG-1 are only weakly affected by deformation. Although, in places, they show moderate to steep orientation of bedding, no evidence for thrust faults or/and overturned bedding was found in these wells. Subhorizontal attitudes of the strata and scarce effects of tectonic deformation, characterize most parts of the drilled Carboniferous profiles.

Northern slope of Wolsztyn-Leszno high

Drill cores were investigated from seven hydrocarbon wells of the Brońsko gas field immediately north of the Wolsztyn-Leszno high (Fig. 4). They all reveal a roughly uniform average bedding dip of ca. 30°, except in relatively frequent domains of decimetre- to metre-scale asymmetric recumbent tight folds. The longer limbs of the folds are parallel to the mean attitude of the strata, whereas the shorter limbs are steep with frequent occurrence of inverted bedding. The hinges and shorter limbs of the folds bear well-developed zonal cleavage inclined at an angle of 30°–35°. A comparison of drill core observations with dipmeter record (see below) points to a fairly uniform, NNE-directed dip of the Carboniferous succession over the Brońsko field and a stable NNE asymmetry of the folds. The Carboniferous strata are, thus, striking parallel to the elongation of the Wolsztyn-Leszno high and dipping away from this basement elevation. Palynological data from the Brońsko area (T. Górecka, personal communication) show that tectonic deformation took place during the late Viséan to Westphalian B.

Northern area

A common feature for most of the wells studied in the northern part of the investigated area is a relatively weaker record of tectonic deformation in the Carboniferous succession than those in the southern part. In three wells: Września IG-1, Kalisz IG-1 (Fig. 10) and Zakrzyn IG-1, the dip of the strata is between 0° and 15°, only locally increasing to 20°–30°. In well Kalisz IG-1, subhorizontal bedding is cut by vertical, up to 3-cm thick fractures, filled with calcite and fine-grained breccia. The fractures bear subhorizontal slickenlines. However, the Objezierze IG-1 well may have an important tectonic imprint. The entire Carboniferous section (4,595–5,094.5 m, Fig. 10) in this well reveals a uniform dip of strata at an angle of 40°–45° accompanied by locally developed subvertical fracture zones with calcite infill.

Fig. 10
figure 10

Simplified stratigraphic sections of Carboniferous from wells Września IG-1, Kalisz IG-1 1 and Objezierze IG-1 showing position of investigated samples. Hachure shows dips and deformation of strata

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Dipmeter record

Directional data acquired with a six-arm dipmeter Halliburton SED from 34 gas wells were applied to resolve structural problems of the Carboniferous strata underlying the Fore-Sudetic homocline. The dipmeter data, provided by the Polish Oil and Gas Company (POGC), came from a c.150 km long and WNW–ESE elongated belt straddling the Dolsk fault (Fig. 11). The raw data were previously numerically processed by POGC Geofizyka Kraków and Geofizyka Toruń, using SHIVA (Halliburton™) software and applying standard input parameters appropriate in cases of little deformed, well-stratified rocks, but rather not suitable for the studied Carboniferous complex, strained and lithologically fairly homogeneous. The results proved to be, in general, noise-dominated and of little help in interpreting Carboniferous turbidite rocks. Therefore, an attempt to re-process the data, using more appropriate computing parameters was made by Geofizyka Kraków on data sets from several wells.

Fig. 11
figure 11

Structural attitude of Carboniferous strata in SW Poland, partly generalized, based on dipmeter records from 34 gas wells. Bedding orientation symbols: bold structural dips stable over long depth intervals in wells (several tens of metres and more), thin local structural dips shown by short intervals; arrows axes of folds or gentle bedding deflections

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The reprocessed data were used to study the Brońsko gas field near Kościan (SSW of Poznań), located on the NE slope of the Wolsztyn-Leszno high (Fig. 12). The Carboniferous clastic rocks in that area show, in general, a homoclinal attitude and dip to the NE at an angle of 30°–50°. Such a steep dip over a distance of several kilometres should have resulted in subcropping older members of the Carboniferous complex in the SW part of the area. This, however, is not the case and according to unpublished palynological data (in POGC archives) Late Viséan to Westphalian strata subcrop beneath the Permian all over the entire Brońsko field. One of the wells (BR-9), studied in detail due to a relatively good quality of re-processed data (Fig. 13), revealed that the generally homoclinal attitude of strata (average dip 40°–50° NE to ENE) was overprinted by folds sized up to 50 m (amplitude/wavelength), showing NW–SE axes and shallow-dipping axial planes with NE polarity. The fold style interpreted from the dipmeter data was confirmed in drill cores, containing numerous hinges of metre-scale recumbent folds, with local, shallow dipping axial-plane or fan cleavage, particularly well developed in the overturned limbs of the folds (Fig. 14). The cleavage was also recorded by the dipmeter tool (Fig. 13). The core and stratigraphic data from the studied gas field show that the Carboniferous succession occurs in a normal position there, except in the lower limbs of the recumbent folds. The borehole geophysical data, locally supported by core observations, detected several trachyandesitic dykes, SW-dipping at moderate to steep angles. In the regional context, the identified recumbent NE-verging folds can be explained as either related to a major positive flower structure near to the Dolsk strike-slip fault zone or as originally upright folds later reoriented due to intense uplift and tilting on the northern slope of the NW–SE trending Wolsztyn-Leszno high; the latter representing a Late Carboniferous strike-slip-related pop-up or horst of the metamorphic basement. This tilting may have also been related to a kind of a gravitational collapse of the Carboniferous rocks, accomplished through domino-style faulting (which can explain why approximately the same stratigraphic level subcrops on the base Permian surface all over the Brońsko field). Still, it is possible that regional-scale thrust tectonics are manifested in the structure of the Brońsko area, however, evidence is lacking due to insufficient quality of the studied dipmeter data and the lack of appropriate reflection seismics.

Fig. 12
figure 12

Structural attitude of Carboniferous (Late Viséan to Westphalian) strata over Brońsko gas field near Kościan (SSW of Poznań), located on the NE slope of the Wolsztyn-Leszno High, based on dipmeter data from 13 gas wells: generally homoclinal dip at an angle of 30°–50°. Dip records generalized over at least several tens of metres-long borehole intervals. Broken line dip symbols—local attitude of strata recorded in short intervals

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Fig. 13
figure 13

Structural profile of Carboniferous section in BR-9 exploration well from Brońsko gas field, studied in detail thanks to relatively good quality of dipmeter data. Generally homoclinal attitude of strata (average dip 40°–50°NE to ENE) is overprinted by “recumbent” folds with NW–SE axes, shallow-dipping axial planes and the NE polarity. The dipmeter record includes orientation of local cleavage and of trachyandesitic dykes. Sparse cored intervals are indicated

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Fig. 14
figure 14

Cut drill cores from Carboniferous sections in wells from Brońsko gas field confirm the structural style recognised in BR-9 well from dipmeter record (Fig. 12) in showing numerous hinges of metric-scale recumbent folds in sandstone–siltstone lithology, with local, shallow dipping axial-planes and fan cleavage, particularly well developed in the overturned limbs of the folds

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Original or reprocessed dipmeter data from a number of wells from other areas of the basement to the Fore-Sudetic homocline (Fig. 11) revealed that NW–SE to W–E structural trend of fold structures and of strike of the tilted bedding is predominant there. Such an orientation is analogous to that which defines the structural grain of the Variscan West Sudetes. It is also compatible with transpressional tectonics along NW–SE to WNW–ESE major fault zones, which can be, at least in part, responsible for the origin of these structures. Local incompatible strikes of strata (e.g. NE–SW) visible in Fig. 11 seem, in turn, to reflect local drag on transverse faults.

No dipmeter data exist for the area located to the north of the East Sudetes, where NE–SW to N–S fold trends and strikes of bedding are a possibility. If these features did, actually, exist in this part of the study area they would reflect a boundary of major basement blocks. Therefore, no reliable orientation can be assigned to important tectonic structures revealed by drill cores from wells Marcinki IG-1, Kalisz IG-1, Wojciechów IG-1, Wołczyn IG-1, Dankowice IG-1, Rzeki IG-1 (Fig. 4).

Discussion

Our geochemical data suggest that the detritus supplied to the Carboniferous basin was entirely derived from a uniform group of sources most probably located within the Variscan orogen. Notably, they consistently indicate the location of a source area within a continental magmatic arc, similarly to the earlier published data (Krzemiński 2005). Remnants of this hypothetical arc are currently missing from the Variscan basement of the Sudetes and, thus, must have been entirely removed by erosion. As shown by detrital zircon geochronology, two main crustal components of Late Devonian and Early Carboniferous ages, respectively, were present in the source area of the Namurian sediments from the Siciny well, located near a potential source area within the Sudetes (Fig. 4). The detritus from well Września IG 1 is much more diversified, probably due to the younger Westphalian D age of its deposition, and roughly reflects the present-day lithological inventory of the Sudetes. The admixture of Late Carboniferous zircons, absent from the Siciny well, suggests continuous unroofing of Variscan granites in the Sudetic hinterland between the Namurian A and the end Westphalian. Whereas the Carboniferous detrital zircons represent the main metamorphic and magmatic events documented in the Sudetes (e.g. Marheine et al. 2002), the Late Devonian zircons have age equivalents only in the Góry Sowie massif (e.g. Bröcker et al. 1998; Fig. 4). This means that the source rocks with Late Devonian cooling signature, which supplied detritus to the Carboniferous basin, have been mostly eroded by now. Such a situation suggests that the currently exposed Sudetic rock complexes that underwent Carboniferous thermal overprint during Carboniferous times were tectonically overlain by a Late Devonian (early Variscan) nappe complex. Likely remnants of such nappes are still preserved below the SW part of the Carboniferous basin of SW Poland as the phyllites from the Wolsztyn-Leszno high, whose Ar–Ar cooling ages are close to the Ar–Ar detrital mica ages from the Namurian turbidites (Mazur et al. 2006a) and to the SHRIMP detrital zircon ages from similar rocks reported in this paper. The scarcity of Late Devonian zircons in the Westphalian sandstone from the Września well, suggests additionally that erosion of the early Variscan nappes supplying the material to the Namurian foreland basin was nearly completed by late Westphalian times. These presumed eroded allochthonous units may have comprised fragments of the magmatic arc that is reflected in the geochemistry and igneous habit of the Devonian detrital zircons. The scatter of the detrital zircon ages shows that the erosional level of the Sudetes at the end Westhphalian times was comparable to that of the present day. This conclusion is in accord with results of the fission track study conducted in the Góry Sowie massif by Aramowicz et al. (2006), which suggest preservation of the Carboniferous topographic surface beneath an at least 4-km thick cover of younger (Late Carboniferous–Permian) sedimentary rocks.

The provenance data of the sediments in the Carboniferous foreland basin of western Poland imply continuous uplift and erosion of the Variscan metamorphic hinterland exposed in the Sudetes that must have lasted until the late Westphalian, synchronously with subsidence of the foreland basin. The deformation of the foreland basin itself took place as late as in the late Westphalian and was associated with the terminal stage of compression exerted on its foreland by the advancing Variscan orogenic wedge. The widespread deformation and uplift of the basin in late Westphalian times is reflected by the regional-extent base Permian unconformity and the scarcity of Stephanian sediments across SW Poland (e.g. Wierzchowska-Kicułowa 1984; Żelichowski 1995). The unconformity is only slightly deformed by Late Cretaceous-Palaeocene compression related to the inversion of the Polish basin. This is explained by a decoupling of deformation between the Mesozoic basin fill and the sub-Zechstein basement along the thick Zechstein evaporite layer (thin-skinned inversion tectonics, e.g. Mazur et al. 2005). Hence, the folding and thrust-faulting recorded in the Carboniferous succession must have been of pre-Permian age.

The late Westphalian is well known throughout entire NW Europe as the time of the final Variscan tectonism. The later resulted in the deformation and partial inversion of the Variscan foreland basin system, including e.g. the foreland basins of the British Isles (e.g. Corfield et al. 1996; Smith 1999) and the Ruhr basin of NW Germany (e.g. Ricken et al. 2000; Werde 2005). The effects of the late Westphalian deformations are widespread across the Carboniferous foreland basin and are reaching far to the east, well beyond the area of the Variscan orogen, to mention only the Lublin trough in eastern Poland (Fig. 3; e.g. Antonowicz et al. 2003) or the Donbas (Don basin; e.g. Saintot et al. 2003; Maystrenko et al. 2003). This may suggest a contribution of a far field compressional stress exerted by the Late Carboniferous Uralian orogeny at the eastern (opposite) margin of the East European Craton (e.g. Berzin et al. 1996; Friberg et al. 2002; Brown et al. 2006). At the same time, the deformation of the eastern Variscan foreland seems to have been related to a dextral transpressional and/or transtensional regime otherwise postulated for the ultimate stage of Variscan tectonism by e.g. Arthaud and Matte (1977) and Ziegler (1990).

Since the work of Arthaud and Matte (1977) major late- to post-Variscan NW–SE trending dextral wrench faults have been inferred for North-Central Europe to facilitate dextral translation of the European with respect to the African plate. This concept, corroborated by reconstructions of Ziegler (1990), finds qualitative support in our structural evidence including the occurrence of horizontal slickensides on meso-scale fault planes and the NW–SE orientation of regional structural grain. Lateral motions along NW–SE striking transpressional strike-slip faults might have accommodated part of the late Westphalian shortening, resulting in a sort of a tectonic escape. The Odra and Dolsk fault zones (Figs. 3, 5) are the structures that probably focused significant amount of dextral displacements. A transpressional regime may have also accounted for the formation of the regional-extent base Permian unconformity, while mostly uninterrupted subsidence continued in some other parts of Variscan foreland e.g. in the Saar-Nahe Basin (Henk 1993; McCann et al. 2006).

Our dipmeter data provide first reliable information on the prevailing NW–SE trend of localized fold structures in the deformed Carboniferous succession of SW Poland. The latter direction is not only consistent with the presumed strike of the advancing Variscan orogenic wedge in SW Poland, but is also parallel to the major strike-slip fault zones in the area (Fig. 3). A similar structural trend is predominant, too, in the Variscan metamorphic complexes of the adjacent West Sudetes (e.g. Aleksandrowski et al. 1997).

Conclusions

The Variscan foreland basin of SW Poland bears evidence for important uplift and unroofing of the neighbouring part of the Bohemian Massif (“Variscan hinterland”) between the Namurian A and the end of Westphalian. At the end of this process, during latest Carboniferous times, the rock complexes cropping out in the Sudetes at the NE margin of the Bohemian Massif were most probably not very different from those that are exposed today there at the topographic surface (see e.g. Aleksandrowski and Mazur 2002; Mazur et al. 2006b). However, at the beginning, during the Namurian A, the Sudetes must have exposed significant amounts of Devonian rocks, including a magmatic suite emplaced in a continental arc setting. These rocks had been probably comprised in the uppermost thrust sheets and/or nappes that subsequently became entirely removed by erosion by the end of the Carboniferous.

An uplift and erosion of the Variscan hinterland were accompanied by subsidence and deposition in the foreland basin, which, subsequently, underwent deformation and tectonic inversion. The tectonic repetitions of tens of metres-thick fault-bounded stratigraphic intervals in several wells located to the SE of the Dolsk fault zone, provide evidence for, at least local, thrusting or high-angle reverse faulting in the SW part of the foreland basin. The stratigraphic position of strata comprised in the tectonically duplicated intervals constrains the age of the deformation as not earlier than the Westphalian C. Consequently, the terminal deformation in the Variscan foreland of SW Poland must have taken place at the same time as that postulated for the final Variscan tectonism in NW Germany and England. The NW–SE trend of folds produced during this deformation event in the Carboniferous of SW Poland is in line with the structural grain (the so called “Sudetic” trend), that prevails in the adjacent part of the Bohemian Massif. It is also compatible with both the strike-slip reactivation of the pre-existing NW–SE trending tectonic discontinuities in the basement of the Carboniferous basin as well as with the frontal thrusting and accretion of the Variscan orogenic wedge.