Paleoenvironment and vegetational history of a Middle Pennsylvanian intramontane peat swamp: an example from the Lower Radnice Coal, Kladno coalfield (Czech Republic)

Abstract

The depositional environment, hydrology and vegetational history of the Lower Radnice Coal (Duckmantian) in the Kladno coalfield was studied using sedimentary geology, coal petrology and paleobotanical/palynological methods. The peat accumulating wetland of the coal formed in a fluvial paleovalley approximately 15 km long and 2–5 km wide, bordered by basement paleohighs and landlocked in the interior of the central European Variscides. The peat swamp evolved on top of mud-dominated floodplain successions pedogenically modified to a vertic gleyed Protosol. Probably climatically controlled rising ground water table resulted in paludification that from downstream part gradually spread upstream. Most clastic load was deposited in the upper part of the valley, whereas only mud suspension was dispersed downstream throughout the vegetated swamp. The best conditions for peat accumulation were situated in the eastern part of the paleovalley, where up to 1.5 m thick coal with thin bands of impure coal and carbonaceous mudstone formed in an occasionally inundated rheotrophic system with peat accretion controlled by regional ground water table. The peat swamp was vegetated mainly by lepidodendrid lycopsids with Lepidodendron and Paralycopodites being dominant genera. Shrubby to ground cover vegetation was represented by medulosallean pteridosperms, small shrubby lycopsids, sphenopsids, and herbaceous ferns. Tree ferns were locally abundant, especially in mineral-rich substrates. The rheotrophic character of the peat swamp may indicate higher seasonality of the Variscan interior, compared to coastal areas in the North Variscan foreland with contemporaneous ombrotrophic peats. Modern equivalents of the Lower Radnice Coal swamp are inland planar tropical peat swamps in tributary paleovalleys of the Tasek Bera in peninsular Malaysia and central Congo basins.

Graphical abstract

Lower Radnice Coal peat swamp.

Introduction

The Middle Pennsylvanian represents the main coal window in tropical Pangea, during which peat swamp precursors of major economical coal seams in Europe and North America formed and served as important long-term carbon sinks (Cleal and Thomas 2005). Two hydrological types of peat swamps have been identified: ombrotrophic and rheotrophic (e.g., Smith 1962; Diessel 1992). In ombrotrophic swamps, peat accumulates above the regional ground water table (e.g., Smith 1962; Fulton 1987; Littke 1987; Staub 1991; Greb et al. 2002; Jerrett et al. 2011; Zieger and Littke 2019). Hydrologically, ombrotrophic peats form in areas with excessive rainfall, and where annual precipitation exceeds evapotranspiration for most of the year. Plant biomass in ombrotrophic systems is kept permanently saturated by rainwater, which serves to enhance humification and peat formation (Cecil et al. 1993, 2014; Neuzil et al. 1993; Jerrett et al. 2011). Coals formed from ombrotrophic peats are generally low in ash (<5%) and sulfur (<1%) and exhibit an upward dulling trend, which may repeat several times in a single coal bed (Smith 1962; Eble and Grady 1990; Diessel 1992; Jerrett et al. 2011; Opluštil and Sýkorová 2018; Zieger and Littke 2019). In contrast, rheotrophic peats develop under conditions of reduced precipitation, usually due to a more seasonal paleoclimate. The surfaces of rheotrophic swamps coincide with the regional ground water table and are planar in profile. Rheotrophic peats commonly contain elevated amounts of inorganic material, either dispersed or concentrated into clastic bands that were deposited during flood events (Smith 1962; Opluštil et al. 1999, 2013). In addition, plant biomass, kept moist by surficial waters, is more susceptible to degradation in rheotrophic systems.

Coastal lowlands of tropical Pangea, now represented by a belt of paralic coalfields hosted extensive peat swamps some of them continuing up to several hundred kilometers inland suggesting that their formation relied not only on a rising sea level but mainly on rainfall and ground water (Cecil 1990; Cecil et al. 2014; DiMichele et al. 2020). Numerous studies show that the precipitation regimes during Middle Pennsylvanian time promoted some planar (rheotrophic) swamps in these areas to evolve in ombrotrophic peats, which later became low ash, low sulfur coal beds (Smith 1962; Fulton 1987; Littke 1987; Eble and Grady 1990; Jerrett et al. 2011; Zieger and Littke 2019). In contrast, much less information is known about the hydrology of contemporaneous peat swamps in the interior of tropical Pangea. To help fill this gap, we have acquired coal petrographic, geochemical, and palynological/paleobotanical data from analyses of the Middle Pennsylvanian Lower Radnice Coal (Kladno Coalfield). The Lower Radnice Coal developed from an intramontane peat swamp in a small fluvial paleovalley in the landlocked Variscan interior of tropical Pangea. We apply a suite of methods, combined with existing facies and paleogeographic data, to reconstruct the depositional setting, hydrology, and vegetational history of the Lower Radnice Coal. Paleoclimatic implications resulting from comparison with contemporaneous peat swamps of paralic basins are suggested.

Geological background

The Kladno Coalfield is located within the Kladno-Rakovník Basin, which is a subbasin within the much larger Pilsen-Trutnov complex of continental basins. The Pilsen-Trutnov basin complex ranges from western part of the Czech Republic to the Czech/Polish border and into Poland (Fig. 1a). The Kladno-Rakovník Basin is situated in the western sector of this basin complex, and together with the Pilsen, Manětín, Radnice, Žihle and Mšeno-Roudnice basins, constitutes the central and western Bohemian late Paleozoic continental basins (western and central Czech Republic) situated on the Cadomian terrane called the Bohemicum (e.g. Pešek 1994, 2004; Pešek and Holub 2001). These basins are grabens, or half-grabens, that formed from extension that followed a cessation of compression related to the Variscan orogeny (Pašek and Urban 1990; Žák et al. 2018). All Late Paleozoic basins in central and western Bohemia, therefore, have similar tectono-sedimentary histories and lithostratigraphic subdivisions. Deposition started during the Late Duckmantian (~early Moscovian), and with several hiatuses, lasted until early Cisuralian (Fig. 1b). The U-Pb CA-ID TIMS radioisotopic ages of intercalated volcaniclastics suggest an interval of deposition (including hiatuses) between 314.2 and 297.1 Ma (Opluštil et al. 2016a, b). Sedimentary sequences within the basins are up to 1.4 km thick, divided into four formations. Some formations are further subdivided into members (Fig. 1b). The Kladno Formation includes the Lower Radnice, Upper Radnice and Nýřany members separated by hiatuses. The current study is of the oldest lithostratigraphic unit, the Lower Radnice Member at the base of the Kladno Formation.

Fig. 1

Geography and stratigraphy of the study area: a geographic location of the Kladno-Rakovník Basin and Kladno coalfield (abbreviations of basins: P Pilsen, M Manětín, Ž Žihle, R Radnice, KR Kladno-Rakovník, MR Mšeno-Roudnice, ČK Česká Kamenice, Br+A Brandov+Altenberg, B Blanice, MH Mnichovo Hradiště, KP Krkonoše-piedmont, IS Intra-Sudetic); b lithostratigraphic subdivision of sedimentary fill in the Kladno-Rakovník Basin plotted against a time axis with sequence details of the Lower Radnice Member; c paleogeographic position of the continental basins in the Czech Republic during late Duckmantian (Moscovian) time, with the geographic position of the Kladno coalfield. FB Flöha Basin, ISB Intra-Sudetic Basin, SW South Wales Basin, USB Upper Silesian Basin. Topography—R. Blakey, Deep Time Maps, used under license; d position of the study area in the global paleogeographic context

The Lower Radnice Member

The Lower Radnice Member represents the earliest depositional interval in the central and western Bohemian late Paleozoic continental basins (Fig. 1b). A latest Duckmantian to/or earliest Bolsovian (formerly Westphalian B and C) age of the unit was estimated from macroflora (Němejc 1953; Šetlík 1977). In agreement with this age is the recent U-Pb CA-ID TIMS radioisotopic dating of intercalated volcaniclastics (Opluštil et al. 2016a), which constrains deposition to 313.7 and 314.2 Ma. These dates for the Duckmantian/Bolsovian boundary are similar to radioisotopic data from the Ruhr Basin (Pointon et al. 2012). Deposition of the Lower Radnice Member started on an uneven surface of the Bohemicum block, built up by folded Neoproterozoic and to lesser extent by lower Paleozoic rocks, consisting mostly of shales with subordinate intercalations of cherts and volcanites (Hajná et al. 2017). Fluvial erosion of the lithologically and structurally heterogeneous basement, prior to the onset of deposition, resulted in the formation of paleovalleys and paleohighs with elevation differences of 150–200 m (Opluštil 2005a, 2005b). Documentation of fluvial paleovalley axes allowed for reconstruction of the drainage network within the Kladno-Rakovník Basin. The composition of conglomerate pebbles, and of detrital minerals in sandstones of the Lower and Upper Radnice members, suggests the source material to be from the remote Moldanubian zone (terrane), located in southern part of the Bohemian Massif (Žák et al. 2018). The river system continued through the central and western Bohemian basin depocenter, and probably into the Flöha Basin in Saxony, located in the North Variscan Foreland Basin (Gaitzsch et al. 1998; Opluštil 2005a, b) (Fig. 1c). Deposition started by filling of deepest parts of major paleovalleys and gradually spread into smaller and shallower ones. This explains the irregular distribution of Lower Radnice Member deposits with thicknesses varying from 0 m above paleohighs to about 150 m in the deepest paleovalleys (Fig. 2). Deposition was accompanied by volcanic activity with the major center being the Altenberg–Teplice Caldera at the German–Czech border (Tomek et al. 2021).

Fig. 2

Cross-section approximately perpendicular to the Kladno paleovalley axis and showing paleorelief buried by sediments of the Lower and Upper Radnice members

The Lower Radnice Member records a suite of facies (Fig. 3) deposited in fluvial, lacustrine, wetland and colluvial (slope to piedmont) settings (Pešek 1994; Opluštil 2005a). The colluvial facies are represented typically by breccia composed of angular to subangular clasts derived from surrounding paleohighs located within the basin depocenter. Breccias to conglomerates are matrix to clast supported suggesting their transport occurred as cohesive to non-cohesive gravity flows (Fig. 3c, f). They form sharply based beds, a few decimeters thick, that are often stacked into composite bodies, meters to even several tens of meters thick. They typically accumulated at the foothill where they form fan to tongue like bodies (Pešek 1994; Opluštil 2005a). The proportion of breccia decreases up-section as the area covered by sediments of the Lower Radnice Member expanded.

Fig. 3

Typical lithofacies, and their associations, of the Lower Radnice Member: a floodplain find-grained sandstone—mudstone heteroliths with abundant roots (mostly Appendices) underlying the Lower Radnice Coal, Mine Mayrau, thickness of the succession is ~60 cm, photo V. Žáček; b mudstone laminated with fine grained sandstone penetrated by stigmarian appendices, MVDD 3, depth ~315 m; c fining up succession of poorly stratified clast-supported breccia-conglomerate of local provenance interpreted as colluvium deposits, MVDD 4, depth 222.1–222.6 m, core diameter 80 mm; d centimeter-scale alternation of coal (C), clayey coal (Ccl) and carbonaceous mudstone (Mc) in lower part of the Lower Radnice Coal in the borehole MVDD 1, depth 54.35–54.55 m core diameter 80 mm; e floodplain sandstone-mudstone heteroliths with roots overlain by the Lower Radnice Coal with mudstone band and terminated by the kaolinized Bělka tuff bed, borehole MVDD 3; f colluvial deposits represented by matrix supported breccia overlain by the Lower Radnice Coal followed by the Bělka tuff bed in the borehole MVDD 1

The fluvial facies are mostly floodplain mudstone (Fig. 3b, e), some with pedogenic overprint. Also common are fine-grained, usually ripple bedded sandstone, as well as an intimate mixture of both lithologies in the form of sand-mud heteroliths, which are often rooted (Fig. 3a). Fluvial channel sandstones to conglomerates are subordinate, although some basal conglomerates in the axis of the major paleovalleys may contain pebbles up to several decimeters in diameter (Opluštil 2005a). The lacustrine facies are uncommon and are typically represented by laminated mudstones with drifted plant remains without roots. Wetland (swamp) deposits are associated with floodplain strata and consist of carbonaceous shales to banded humic coal; sapropelic coal is very rare. Coal bed thicknesses vary from a few centimeters to over 10 m and consist of two groups. The stratigraphically older Pilsen coal group is usually represented by several thin and laterally impersistent coal seams, occasionally reaching economical parameters. In contrast, the Radnice Coal group in the upper part of the member is the most important coal group of the continental basins in central and western Bohemia (Pešek 1994). The Radnice Coal Group consists of the Lower and Upper Radnice coals, separated by the Whetstone Horizon (Fig. 1b), which varies in thickness from a few tens of centimeters to more than ten meters (Pešek 1994; Opluštil et al. 2014). The base of the horizon is marked by a 40–60 cm thick bed of volcanic ash called the “Bělka”. The volcanic ash layer occurs in roof strata overlying the Lower Radnice Coal (Figs. 1, 3e, f) and contains in situ buried peat-forming flora (e.g. Libertín et al. 2009a; Opluštil et al. 2009a, b). Overlying the Bělka are laminated mudstones, composed of volcaniclastics washed down from surrounding basement paleohighs, that were deposited in a shallow lake initiated by compaction of peat of the Lower Radnice Coal (Opluštil et al. 2014). These lacustrine sediments are called the “Whetstone” (Fig. 3e) and contain drifted plant fragments and upright stems up to several meters tall rooted in the roof of the Lower Radnice Coal. The presence of these stems in their original growth position suggests very rapid burial. Collectively, the Lower and Upper Radnice coals represent a single “coal seam”, the formation of which was interrupted by a volcanic ash fall event. The volcanic center generating this laterally very widespread volcaniclastic horizon is the Altenberg-Teplice Caldera situated on Czech-German border (Tomek et al. 2021).

The Lower Radnice Coal in the Kladno coalfield

The Kladno coalfield is a mining district along the southeast margin of the Kladno-Rakovník Basin (Fig. 1a), abandoned since 2002. The coalfield coincides with the Kladno paleovalley (Opluštil 2005a, b), in which the Radnice coal group, the main target of mining for more than two centuries, formed. The coal-bearing part of this east–west striking valley is about 15 km long and its width at the end of deposition of the Lower Radnice Member varies from 2.5 km to more than 5 km (Fig. 4). The Kladno paleovalley is bordered by the Bílichov and Smečno paleohighs on the north and by the Křivoklát paleohigh along its southern margin. On their slopes, the coal seams of the Radnice coal group wedge out (Fig. 2). Protruding from these paleohighs into the Kladno paleovalley are numerous smaller ridges, which dissect the area covered by the Radnice coal group into a very complex outline (Fig. 4a, b). The Whetstone Horizon separating both coals of the group is between 1 and 5 m thick. The Upper Radnice Coal is typically between 7 and 8 m thick (maximum ~12 m) and covers most of the paleovalley. The Lower Radnice Coal, or its carbonaceous shale equivalent, also covers most of the paleovalley, though its area is smaller because of its formation in a narrower paleovalley. The coal is thickest in the eastern part of the Kladno paleovalley, where it was mined until 1990. Here, its average thickness is about 1 to 1.5 m, but locally may reach 3 to 5 m. Bands of carbonaceous shale or mudstones, centimeters to decimeters thick, are especially abundant in the lower part of the coal (Fig. 3d–f). Further west, in central and western parts of the Kladno paleovalley, the coal thickness decreases to <1.2 m whereas the ash content increases. Locally the coal is represented by carbonaceous shale, several decimeters thick.

Fig. 4

Paleogeographic models of the Lower Radnice Coal in the Kladno coalfield: a 2D paleogeographic map of the coal with distribution of principal paleoenvironments. LRC Lower Radnice Coal, URC Upper Radnice Coal; b 3D block diagram showing principal topographic features in SE part of the Kladno-Rakovník Basin including the Kladno paleovalley with the swamp of the Lower Radnice Coal and surroundings basement paleohighs. Top of the Bílichov and Smečno paleohighs is between 150 and 180 above the paleovalley with the Lower Radnice Coal peat swamp

Data and methods

Although coal mining ceased 2 decades ago four of seven boreholes, recently drilled in the eastern part of the Kladno coalfield, provided complete sections of the Lower Radnice Coal (the Upper Radnice coal was mined out in all boreholes), suitable for detailed study by multiple methods to reconstruct the vegetational history and hydrologic evolution of the peat swamp precursor. In addition, clastic strata below and above the coal provided important information regarding characteristic facies and depositional processes which, together with the previous studies of Opluštil (2003, 2005b), provided a base for comprehensive paleoenvironmental and paleogeographic reconstruction.

Coal petrography and geochemistry

Coal petrographic (lithotype and maceral) and proximate analyses were performed on 69 samples of Lower Radnice Coal and carbonaceous mudstone collected from boreholes MVDD 1, MVDD 3, MVDD 5, and MVDD 7 (Figs. 5–8; Electronic Appendix = EA 1–5). All analyses, except lithotype identification, were performed in the fuels laboratory of the Kentucky Geological Survey in Lexington, Kentucky, USA. Sample preparation followed procedures outlined by ASTM D2013/D2013M-18 (ASTM 2018a). Collected samples were first crushed to −8 mesh (particle size < 2.36 mm) using a hammermill crusher and split with a sample riffler. One split was further reduced in size to −20 mesh (particle size < 850 µm), using a plate-grinding mill, for the construction of coal petrographic pellets. The other split was further reduced to −60 mesh (particle size < 250 µm), using a pulverizer, for geochemical analyses and for the chemical isolation of palynomorphs.

Fig. 5

Section of the Lower Radnice Coal in the borehole MVDD 1. Note that Spencerisporites is included in the dispersed miospores (specimens < 250 µm) but also in the megaspores (specimens > 180 µm = 80 mesh). Assignment of the genus Spencerisporites to miospores or megaspores remains still questionable (see Drábková et al. 2004)

Proximate analyses (moisture, volatile matter, fixed carbon and ash) were performed according to ASTM D7582-15, using a Leco 701 thermogravimetric analyzer (ASTM 2015a). Total carbon and sulfur analyses were performed according to ASTM D4239–17, using a Leco SC-432 carbon/sulfur analyzer (ASTM 2017). Weight percent mineral matter values were calculated from ash and sulfur data using the Parr formula (Parr 1928; Thiessen et al. 1934; ASTM, 2018b), where:

$${\text{Mineral matter}}_{\text{wt\%}} = ({\text{Ash}}_{\text{wt\%}} * 1.08) + ({\text{Sulfur}}_{\text{wt\%}} * 0.55).$$

Volume percent mineral matter was calculated according to ASTM D2799-13 (ASTM 2013a), where:

$${\text{Mineral matter}}_{\text{vol\%}} = \left[100*\left({\text{MM}}_{\text{wt\%}} / 2.8\right)\right]\div \left[\left(\left(100 - {\text{MM}}_{\text{wt\%}}\right)/1.2\right)+ \left({\text{MM}}_{\text{wt\%}}/2.8\right)\right]$$

Coal petrographic pellets were constructed by mixing 2 to 3 g of −20 mesh coal with epoxy resin in 3.2 cm diameter phenolic ring molds and allowing them to cure. Once cured, the molds were ground using 320, 400 and 600 grit papers, and polished using 1.0 and 0.3 µm alumina slurries. Final polishes were obtained using 0.04 µm colloidal silica. All polished petrographic mounts are housed at the Kentucky Geological Survey, University of Kentucky, Lexington, Kentucky, USA.

Reflected light analyses, following procedures outlined in ASTM D2799 (ASTM 2013a), were performed on a Zeiss Universal microscope, using a Zeiss epi 40X oil immersion objective coupled with a 1.6× magnification changer (final magnification 640X). Cargille Type FF (fluorescence free) immersion oil was used. Maceral analysis (EA 2–5) involved the use of both white and fluorescent light, the latter for positive identification of liptinite macerals. White light was supplied by an Osram Xenophot HLX 12V, 100W bulb. Fluorescent light was provided by a Lumen Dynamics 120W, high-pressure metal halide arc lamp, used in conjunction with a Zeiss 09 filter set (450–490 nm excitation, 510 nm beam splitter, and 515 nm emission filters). Vitrinite and inertinite maceral identification followed recommendations outlined by the International Commission for Coal and Organic Petrography (ICCP) for vitrinite (ICCP 1998), inertinite (ICCP 2001) and liptinite (Pickel et al. 2017) macerals.

Vitrinite reflectance analyses were performed by first calibrating a Hamamatsu 928A photomultiplier with a Schott glass standards SF13-714-276, with a certified reflectance of 0.496, and LaF12-836-423, with a certified reflectance of 0.921, were used for the acquisition of reflectance values. Following this, 50 random reflectance measurements were collected using non-polarized light for each sample in accordance with methods outlined in ASTM D2798-11a (ASTM 2011) for coal samples and D7708-14 (ASTM 2014) for rock samples. Maximum vitrinite reflectance (VR_{o maximum}) values were calculated from random vitrinite reflectance (VR_{o random}) values using the conversion formula outlined in ASTM D2798-11a (ASTM 2011), where:

$$\text{Calculated }{\text{R}}_{\text{o maximum}} = ({\text{R}}_{\text{o random}}*1.09) - 0.034.$$

Miospores

Miospore data were obtained from the same samples from boreholes MVDD 1, MVDD 3, MVDD 5 and MVDD 7 (Figs. 5–8) that were used for proximate and maceral analyses and were also macerated from clastic sediments below the Lower Radnice Coal.

Miospores were liberated from coal and rock matrices by first immersing 2–3 g of −60 mesh coal (particle size < 250 µm) in a mixture of hydrofluoric, nitric and hydrochloric acids to remove mineral matter, and then oxidized with Schulze’s Solution (concentrated nitric acid saturated with potassium chlorate). 42 Baumé (67.2%) nitric acid was used to make the Schulze solution at a ratio of 15 ml HNO₃/1 g KClO₃. Samples were left to oxidize in the Schulze solution for 12 to 24 h. Following demineralization and oxidation, sample residues were digested with 5% potassium hydroxide and repeatedly washed with distilled water. Organic residues were concentrated with zinc chloride (specific gravity 1.9) after digestion. Amorphous organic matter (AOM) was removed from the residues using ethylene glycol monobutyl ether (2-ethoxybutanol), ultrasonic vibration, and short centrifugation (Eble 2017). 250 µm (=60 mesh) coal and rock were used to eliminate the need for screening and to help record the presence of larger spores and pollen (e.g., Schopfipollenites). Samples were strew-mounted onto 25 mm square cover glasses with polyvinyl acetate (PVAc) and fixed to 75 × 25 mm microscope slides with Acrytol, a synthetic, acrylic resin. All maceration residues and microscope slides are housed at the Kentucky Geological Survey, University of Kentucky, Lexington, Kentucky, USA.

Miospore percentages are based on a count of 250 palynomorphs/sample (EA 6–9). Palynomorph data are listed according to natural affinity for the following plant groups: lycopod trees, small lycopods, tree ferns, small ferns, seed ferns (pteridosperms), calamites, cordaitaleans and gymnosperms. Parent plant affinities of dispersed Carboniferous miospore taxa were derived from extensive summaries provided by Ravn (1986), Traverse (1988), and Balme (1995) and numerous later publications.

Megaspores

Megaspores were macerated from 19 samples of coal, carbonaceous mudstone and mudstone associated with the coal (EA 10) in boreholes MVDD 1, 3, and 7 (Figs. 5, 6, 8). Coal samples were macerated using a modified method of Šimůnek and Bureš (2015). About 5 g of coal were macerated in 35 ml of 65% nitric acid (HNO₃) with the addition of one-gram potassium chlorate (KClO₃) for 2 days and 16 h to nearly 3 days. Macerated coal was washed in water and treated in 10% potassium hydroxide (KOH) for 0.5 to 1 h and again washed in water. An 80 mesh (180 μm) sieve was used for separating cuticles and megaspores from microspores. Megaspores were selected from the organic residue remaining after cuticle selection. The >180 μm fraction was used to capture large pteridosperm pollen (e.g., Monoletes).

Fig. 6

Section of the Lower Radnice Coal and accompanying clastic sediments in the borehole MVDD 3

Samples of clastic rocks in volume ~50 g were broken along their bedding planes into smaller pieces and demineralized using standard procedures with HCl, HF and HCl. Oxidization of organic matter, using nitric acid (65%), was applied if necessary, following the same procedures as for the coal samples. An Olympus Stereomicroscope, Nikon Eclipse 80-i microscope and scanning electron (SEM) microscope were used to study and document the taxonomic identification and abundance of megaspores (EA 10). Parent plant affinities of megaspore taxa are based on summarizing papers of Balme (1995) and Bek (2017, 2021) as well as on number of other papers (Drábková et al. 2004; Bek et al. 2023). Studied specimens are stored in the Laboratory of Micropaleontology and Stratigraphy, Czech Geological Survey, Prague.

Dispersed cuticles

Samples of the Lower Radnice Coal used to extract and identify dispersed cuticles include two samples from the borehole MVDD 1, two samples from the borehole MVDD 3 and seven samples from the borehole MVDD 7 (EA 11). Of these 11 samples, only 7 samples from the boreholes MVDD 3 and MVDD 7 provided a sufficient number of cuticles suitable for interpretation (EA 11). A total of 695 dispersed cuticles were recovered, from which 143 slides were prepared. Cuticular slides (labelled: 684/1–31, 685/1–20, 714/1, 717/1–2, 726/1–24, 727/1–3, 728/1–18, 729/1–17, 731/1–15, 732/1–5, 733/1–6) are stored in the archive of Czech Geological Survey in Prague. Some cuticles were stained with Safranin, Bismarck Brown, Malachite Green or Neutral Red to increase contrast and enhance morphological structures like stomata, trichome bases, or resin cavities (Krings 1999). The cuticles were mounted on the glycerine jelly slides for observing in the light microscope.

Macroflora

Although the absence of coal balls excludes study of the botanical composition of the original peat, the Bělka tuff bed in the roof of the coal bears in situ preserved plant remains of the peat swamp flora just prior to the volcanic ash fall (Opluštil et al. 2007, 2016c). A revision of flora from various collections (the National Museum in Prague, Chlupáč Museum of Earth History, Faculty of Science, Sládeček Museum in Kladno as well as the private collection of J. Haldovský) provided insight into the composition of peat-forming vegetation of the Lower Radnice Coal in the latest stage of formation. In addition, three of the authors (SO, ZŠ and JD) have personal experience from collecting plant remains preserved in the Bělka from coal mine discard areas. Relative abundance of identified plant taxa was estimated based on the collection of plant fossils in unsorted rocks from these areas.

Results

Throughout this section, geochemical results, except for residual moisture, are presented in weight percent on a dry (moisture free) basis. Petrographic (macerals) data are presented on a volume percent, mineral matter free (mmf) basis (EA 2–5). Representatives of macerals of the Lower Radnice Coal are illustrated in EA 12–15). Palynologic (miospores) taxa are expressed in percent based on number of specimens in 250 counts per sample (EA 6–9) and selected representatives are illustrated in EA 16–19.

Geochemical and petrographic composition of the Lower Radnice Coal

Core MVDD-1 consists of 1.42 m of interbedded coal (n = 13) and rock (n = 7) (Figs. 3f, 5; EA 2). Coal samples were high in ash (avg. 29.71%), except for two samples at the base of the coal, and mostly low to moderate in sulfur (avg. 1.5%). Rock samples contained variable amounts of ash (avg. 67.3%) and were low in sulfur (avg. 0.5%). On a mineral matter free (mmf) basis, coal samples contain moderate amounts of vitrinite (avg. 48.2%), which occurs primarily as telovitrinite (avg. TV/DV + GV = 2.4). The telovitrinite (TV)/detrovitrinite (DV) + gelovitrinite (GV) ratio used in this paper represents a relative measure of vitrinite precursor preservation, with higher values indicating better preservation of the original botanical material. Liptinite contents (avg. 13.8%) occurring mainly as sporinite, are also moderate and inertinite contents (avg. 38.0%), occurring mainly as fusinite, semifusinite and inertodetrinite, are high. Rock samples contain much less vitrinite (avg. 18.0%), which occurs primarily as detrovitrinite (avg. TV/DV + GV = 0.4), and higher amounts of liptinite (avg. 29.0%) and inertinite (avg. 53.0%) (EA 2).

Core MVDD-3 consists of 2.85 m of organic rich mudstone and high ash coal (Fig. 6; EA 3). Six organic rich mudstone samples and four impure coal samples (47.2% mineral matter) were collected from this core. Ash yields of the rock samples are high (avg. 67.1%), and sulfur contents are low to moderate (avg. 1.0%), with the highest sulfur contents occurring in samples with the lowest ash yields. Pyrite framboids are common in the samples with higher sulfur contents. Vitrinite contents (avg. 42.6%) vary widely among the samples, with detrovitrinite (collodetrinite + vitrodetrinite) being more common than telovitrinite (avg. TV / DV + GV = 0.3). Liptinite (avg. 20.2%), which occurs primarily as sporinite, and inertinite (avg. 37.2%), the latter mainly as fusinite, semifusinite and inertodetrinite, is elevated in most of the samples.

Core MVDD-5 consists of 1.05 m of carbonaceous shale and organic poor mudstone at the base of the cored interval (Fig. 7; EA 4). Ash yields are high (avg. 78.0%) and sulfur contents are low (avg. 0.2%). All but the basal two organic poor intervals of core MVDD-5 were analyzed petrographically. The interval from 165.95 to 166.45 m was dominated by inertinite (avg. 54.8%), occurring mainly as fusinite and inertodetrinite. Elevated liptinite percentages (avg. 20.9%) were also observed, primarily as sporinite. Vitrinite contents were low (avg. 24.3%), occurring mostly as detrovitrinite (avg. TV / DV + GV = 0.3), although collotelinite occurs as well.

Fig. 7

Section of the Lower Radnice Coal in the borehole MVDD 5

Core MVDD-7 consisted of 5.74 m of coal (n = 21) and carbonaceous mudstone (n = 12) that was sampled in 33 intervals. The core consists of a coal and rock sequence at the top (334.26 to 334.85 m), coal in the middle (334.85 to 336.42 m) and coal and rock at the base (336.55 to 340.00 m) (Fig. 8). Ash yields and sulfur contents ranged widely for both coal and rock lithologies (EA 5). The average ash yield and sulfur content for coal samples was 26.4 and 2.0%, respectively. For rock samples, the average ash yield and sulfur content was 72.2 and 0.6%, respectively. Petrographically, coal samples had moderate amounts of vitrinite (avg. 57.0%) with an average TV/DV + GV ratio of 1.74. Liptinite macerals, mostly as sporinite, were moderate to high in abundance (avg. 16.7%). Inertinite contents, mainly as fusinite, semifusinite and inertodetrinite, were high (avg. 26.3%). Rock samples contained moderate amounts of vitrinite (avg. 47.7%) with low TV/DV + GV ratios (avg. 0.3%). Liptinite macerals, mostly as sporinite, were high in abundance (avg. 26.0%). Inertinite macerals, mainly as inertodetrinite and fusinite, were also high in abundance (avg. 26.3%).

Fig. 8

Section of the Lower Radnice Coal and part of floodplain succession below the coal in the borehole MVDD 7

Samples of the Lower Radnice Coal from boreholes MVDD-3 and MVDD-7 have average maximum vitrinite reflectance values (VR_{o maximum}) of 0.64 and 0.65%, indicating a high volatile C/B rank assignment. In contrast, samples from boreholes MVDD-1 and MVDD-5 had average VR_{o maximum} values of 1.04 and 1.03%, indicating a high volatile A bituminous rank assignment. The influence of hot, hydrothermal fluids affecting cores MVDD-1 and MVDD-5 appears to be the cause of this rank disparity. Samples from these two cores also contained common carbonate mineralization, in addition to having elevated levels of thermal maturity. X-ray fluorescence (XRF) analysis indicates elevated amounts of CaO, MgO and MnO, suggesting the presence of calcite, dolomite and ankerite. The carbonate mineralization is secondary, being emplaced after burial and coalification.

Miospores of the Lower Radnice Coal and associated clastic sediments

All the studied borehole sections provided abundant miospores counting 106 species, which belong to 45 genera (Table 1; EA 16–19).

Table 1 The list of miospore species and their affinity to parent plants

Samples from the MVDD-1 core are dominated by Lycospora pusilla (produced by Lepidodendron), L. micropapillata and L. orbicula (both produced by Paralycopodites) (Fig. 5; EA 6). Lycospora micropapillata and L. orbicula are more abundant in the bottom and middle portions of the core interval, especially from 54.31 to 54.51 m and 54.61 to 54.67 m. Tree fern (mainly Punctatisporites minutus), and calamite (Calamospora, Laevigatosporites minor, and Reticulatisporites) spores and cordaitalean pollen (Florinites) become more abundant near the top of the column. Miospores from other plant groups are present in minor amounts.

Core MVDD-3 samples are dominated by lycopsid tree spores (Fig. 6; EA 7) with Lycospora pusilla (Lepidodendron) being the most prevalent taxon in the top half of the core (312.9–313.7 m), and Lycospora pellucida (Lepidophloios), L. orbicula and L. micropapillata (both produced by Paralycopodites) being more common in the lower half of the core (313.7–315.75 m). Small lycopsid (mainly Endosporites, produced by Chaloneria), tree fern (mainly Punctatisporites minutus with subordinate Apiculatisporites saetiger, Laevigatosporites minimus, L. globosus, L. ovalis and Cyclogranisporites minutus), small fern (Granulatisporites, Lophotriletes, Deltoidospora + other sphaerotriangular, trilete taxa) and calamitalean (mainly Calamospora and Laevigatosporites minor) spores are more abundant in the top half of the core, as is cordaitalean pollen (Florinites). All of the samples from core MVDD—3 contain significant amounts of mineral matter (44.7 to 99.3 wt%, dry basis), and as such, it is possible (likely?) that some portion of the palynoflora is allochthonous in origin, and therefore not indicative of the local paleoflora.

Most samples of the core MVDD-5 are dominated by Lycospora pusilla (produced by Lepidodendron), with minor contributions from other plant groups (Fig. 7; EA 8). Sample MVDD5/2 (166.35 to 166.45 m) differs in that it contains high percentages of Punctatisporites minutus (a tree fern spore), with subdominant small fern (Granulatisporites, Lophotriletes, Deltoidospora and others) and calamitalean (Calamospora, Laevigatosporites minor and L. vulgaris) spores, and cordaitalean pollen (Florinites). The sample at the base of core MVDD—5/6 (166.9 to 167.0 m) was unique in that it contained Thymospora pseudothiessenii and Torispora securis, two tree fern spores that were rarely seen in other samples.

Arborescent lycopsid spores dominate most of the samples (Fig. 8; EA 9) in core MVDD-7. The bottom coal/rock interval is co-dominated by Lycospora pellucida (produced by Lepidophloios), L. pusilla (produced by Lepidodendron), L. micropapillata and L. orbicula (both produced by Paralycopodites). Fern (tree and small) and calamite spores and cordaitalean pollen are sporadically abundant (e.g., 336.50–337.65 m) in the bottom coal/rock interval. The basal part of the overlying coal interval (336.00–336.42 m) is dominated by Lycospora micropapillata and L. orbicula (both produced by Paralycopodites), with a return to a co-dominance of Lycospora pellucida, L. pusilla, L. micropapillata and L. orbicula immediately above this from 335.65 to 336.00 m. The top part of the coal interval (334.85–335.63 m) is dominated initially by Lycospora micropapillata and L. orbicula, with Lycospora pusilla expanding towards the top of the coal interval. Granasporites medius (produced by Diaphorodendron and Synchysidendron—e.g. Bateman et al. 1992) increases in abundance in the top part of the coal interval as well. Palynotaxa of other plant groups are consistently present in minor amounts in the coal interval. The coal and rock sequence at the top of the core (334.26–334.85 m) continues to be dominated by arborescent lycopsid spores with Lycospora pusilla, L. orbicula and L. micropapillata being the dominant palynotaxa. The sample at the top of the core (334.26–334.33 m) contains increased percentages of tree fern spores (mainly Punctatisporites minutus) and cordaitalean pollen (Florinites).

Megaspores of the Lower Radnice Coal

Thirteen megaspore genera were identified in the Lower Radnice Coal (EA 10, 20, 21). Megaspores produced by lycopsids (nine genera), especially by those of arborescent stature, were dominant (~90%) in most samples. The arborescent forms are represented mainly by the genus Lagenoisporites (principally by L. rugosus) and rarely (MVDD-7) present is Lagenicula, which both were produced by Flemingites-type cones born on Paralycopodites (Bateman et al. 1992; DiMichele and Phillips 1994) (EA 10, 20j–o). Cystosporites, a seed-like megaspore produced by the Lepidodendraceae and Diaphorodendraceae, occurs in subordinate amounts (EA 10, 20a, b, d–f, g). The most common representatives of these two families are Lepidodendron, Lepidophloios, Diaphorodendron and Synchysidendron (DiMichele and Phillips 1994). Tuberculatisporites, produced by arborescent genus Sigillaria, is locally abundant (EA 10, 20p–s) whereas seed-like megaspores of the genus Caudatosporites were identified rarely (EA 10, 20c, h, i). Collectively, megaspores of arborescent lycopsids varied between 0 and 97.5% in relative abundance, with mean value 47.6%. Triletisporites, a subarborescent lycopsid megaspore produced by Polysporia, is present as is Spencerisporites, which was produced by Spencerites (EA 10, 21). Collectively, their content varied between 0 and 92% with a mean value of 23.5%. All these genera represent small-statured plants, a few meters tall, that developed in an undergrowth beneath larger arborescent lycopsids and calamites. Bentzisporites and Triangulatisporites are also present, ranging from 0 to 35.3% (avg. 8.2%). Both taxa were produced by plants of Selaginellales affinity, and represent herbaceous lycopsid groundcover plants (EA 10, 21d, e, g–m).

Megaspores assigned to Calamospora are locally abundant, their abundance ranging from 0 and 99.4% with a mean value of 15.9% (EA 10, 21n–p). They were produced by the arborescent sphenopsid Calamites and by progymnosperms of noeggerathialean affinity (Šimůnek and Bek 2003).

Schopfipollenites, which is a monolete, medullosan pollen, is irregularly present, ranging in abundance from 0 to 32% with mean value 3.8%. (EA 10, 21q–s). Triletes, Palespora and Fragilipollenites were identified in two samples, with a maximum occurrence of 17% in the upper part of the MVDD-7 core section (EA 10, 21t–v). The affinities of these genera are uncertain.

Dispersed cuticles of the Lower Radnice Coal

Of 11 coal and carbonaceous mudstone samples from the Lower Radnice Coal only 2 samples from the borehole MVDD-3 and 6 samples from the MVDD-7 provided diverse, dispersed cuticle associations representing all the major Carboniferous plant groups (EA 11, 22–25). Confidence of their assignment to parent plant groups or genera depends on morphological features preserved in the cuticles and knowledge of in situ cuticles (Thomas 1966, 1970, 1977; Thomas and Masarati 1982; Šimůnek 2007, 2020; Šimůnek and Cleal 2011, 2018; Hübers et al. 2013; Šimůnek and Lojka 2021). The shape and orientation of stomata were used for classification and botanical assignment (EA 11).

Dispersed cuticles, characterized by simple and sunken stomata and cells, straight anticlinal walls and lacking specialized cells, such as hairs or glands, are assigned to lycopsids (EA 11, 22). Unfortunately, cuticles of Lepidodendron, Lepidophloios, Ulodendron and Sigillaria are all fundamentally similar (Thomas and Masarati 1982). Moreover, different parts of a leaf cushion may have different cuticular structure, which makes classification of dispersed lycopsid cuticles to the generic level nearly impossible. Šimůnek (2020) and Šimůnek and Lojka (2021) classified lycopsid cuticles into nine morphotypes. These are applied to the current work and six newly defined morphotypes are also recognized (EA 11).

Cordaitalean cuticles of the genus Cordaabaxicutis were identified by their typical tetragonal cell imprints and stomatal complexes formed by two guard cells, two to four oblong to reniform lateral, and two small circular subsidiary cell imprints. Cuticles of this type were rarely present (EA 11, 23). A new form, having stomata regularly dispersed within a row—100 or more μm distant between two stomatal complexes and density of about 80 stomata per mm² was also recognized. Previously described cordaitaleans from the Radnice Member have stomata arranged in stomatal rows and stomatiferous bands (Šimůnek 2007; Šimůnek and Haldovský 2015).

The bulk of cuticles liberated from our samples do not preserve stomata, making classification difficult. Reliable identification is possible only where pinnule shape is preserved (EA 23a). Otherwise, these cuticles can belong to any plant group, but most of them belong to ferns or pteridosperms. Others may represent lycopsids, sporangium walls of lycopsids and integuments of pteridosperm or cordaitalean seeds. Cuticles of this type are here classified into six categories based on cell shape and orientation (EA 11): (1) Cuticles with polygonal, randomly oriented cell imprints (EA 23c, h, i; morphotype 14). (2) Cuticles with elongated polygonal cells, parallel oriented (EA 23b, g; morphotype 15). (3) Cuticles with nearly square, randomly oriented cells (EA 23d, e; morphotype 16). (4) Cuticles with tetragonal, parallel oriented cell imprints (EA 23j, l, m; 24a; morphotype 17). (5) Cuticles with usually fusiform, parallel oriented cell imprints (EA 24d, e; morphotype 18). (6) Cuticles with elongated cell imprints with strongly sinuous anticlinal walls (EA 24f; morphotype 19), possibly belonging to medullosalean taxon Lonchopteris. The separate type is Silesicutis (EA 24g, h) originally described by Roselt and Schneider (1969) from the Langsettian of the Upper Silesian Basin with only one named species, Silesicutis prosenchymatica. The cell imprints are elongated tetragonal to fusiform with trichome bases of different dimensions, which are typical for midribs and rachides of pteridosperms, mainly the Lyginopteridales and Callistophytales.

Some cuticles from our samples show only stomata with no discernable anticlinal cell walls (EA 24i, j). This is typical for the medullosan pteridosperm Laveineopteris, which is very common in the Radnice Member flora (Opluštil et al. 2007, 2016c). Within this group, two cuticles with anomocytic stomata, typical of Alethopteris, were identified (EA 24k). One cuticle of unknown affinity showed stomata with wide guard cells (EA 24l, m) dispersed among tetragonal, parallel oriented cell imprints. Morphotype number 24 (EA 11), which represents cuticles with cyclocytic stomata (EA 25a, b), probably belongs to Cyclopteris of Laveineopteris, indicated by the common occurrence of these cuticles in the same samples as abaxial cuticles with Laveineopteris stomata. Morphologically identical cuticles without stomata were classified separately (EA 24b, c; morphotype 25 in EA 11). They probably represent adaxial hypostatic cuticle of the cyclopterid aphlebia belonging to Laveineopteris.

Uncommon multicellular bodies (EA 25f), composed of polygonal cells 200–300 μm in diameter with unknown affinity and function, were also identified. They were previously reported from the Radnice Member coals by Šimůnek (2020) and Šimůnek and Lojka (2021). In addition, common fragments of arthropod cuticles were identified (EA 25g–k). EA 25g represents a nearly 3 mm long cuticle fragment with seven appendages. Six appendages are attached to a similar cuticle not figured here. These appendages were very small with holes on one side (EA 25h, i). They are probably sensorial organs. Their affinity is unknown but resemble those of trigonotarbid origin described from other European coalfields (Reichel and Barthel 1964; Roselt and Schneider 1969; Haug et al. 2014).

Macroflora of the Lower Radnice Coal

In the Kladno coalfield, the Bělka flora was studied by Opluštil et al. (2007, 2016c) who identified remains of 16 biological species. This relatively low diversity, compared with some other localities in the Radnice and Pilsen basins, seems to reflect a sampling bias related to limited exploitation of the Lower Radnice Coal in the Kladno coalfield. Among the taxa identified from coal mine waste areas, or identified in the museum collections, the most common fossils are remains of a medullosalean pteridosperm with foliage assignable to Laveineopteris loshii. Arborescent lycopsids are represented by Lepidodendron mannebachense (=L. obovatum) and Lepidophloios laricinus. The herbaceous sphenopsids Sphenophyllum myriophyllum, S. cuneifolium and S. pseudoaquense and the arborescent genus Calamites (e.g., C. goeppertii) and its foliage (e.g., Asterophyllites longifolius) were also identified. Less common are remains of the small lycopsid Polysporia robusta (=Chaloneria in anatomical preservation, e.g., DiMichele et al. 1979), medullosalean foliage (Alethopteris distantinervosa and A. lonchitica), the lyginopteridalean pteridosperm Eusphenopteris nummularia, progymnosperm Rhacopteris bipinnata and small gymnosperm of possible conifer affinity Dicranophyllum dominii.

Discussion and interpretation

Depositional setting

The boreholes MVDD 1, 3, 5, and 7 provide an excellent opportunity to examine the transition from a clastic succession to the peat swamp that became the Lower Radnice Coal (Figs. 3e, f; 5–8). In areas distant from the paleovalley margin, the Lower Radnice Coal begins as a thick succession of rooted mudstone with fine grained ripple bedded sandstone heteroliths, and massive or laminated mudstones alternating together in a complex pattern typical for floodplain deposits (Opluštil 2005a, b). Although no channel fill facies were identified in the MVDD boreholes, they are probably represented by sandstone bodies, up to few tens of meters thick, encountered in some coal mine shaft sections located close to the paleovalley axis (Fig. 2). Floodplain strata below the coal show variable intensity of pedogenic alteration resulting in poorly developed paleosols. Pedogenic slickensides are locally present, and are interpreted to represent shrink–swell structures, possibly related to alternating wet/dry seasonality (Mack et al. 1993). However, low chroma gray colors, indicative of superimposed gleying, suggest polygenetic development of paleosol profiles from better drained (oxidizing) to poorly drained (reducing) stages (Driese and Ober 2005; Rosenau et al. 2013; Opluštil et al. 2015). These pedogenically overprinted floodplain sediments are, therefore, classified as gleyed Protosols or vertic gleyed Protosols. Together with the Lower Radnice Coal (Histosol) above, they represent a succession of pedogenic processes indicative of a rising ground water table. They may also record a transition from a more seasonal to more humid climate in a continental setting (Driese and Ober 2005; Rosenau et al. 2013; Opluštil et al. 2019). The floodplain succession below the Lower Radnice Coal thins to the paleovalley margin, where it is dominated by colluvial deposits. These consist of clast- or matrix-supported and unsorted to poorly sorted breccia derived from eluvium redeposited from slopes of the Smečno and Křivoklát paleohighs as debris flows and mudflows (Figs. 2, 3c, f). They commonly alternate with floodplain sediments. However, in proximity to valley margins, floodplain deposits are often absent, and vegetation is rooted in matrix-rich colluvium.

The Lower Radnice Coal covers most of the Kladno paleovalley, though its quality and thickness vary significantly. Although there is a general increase in thickness, and decrease in ash content, from the western to eastern part of the paleovalley (coalfield), this trend has many deviations. In the western part of the coalfield and along the paleovalley margin (Fig. 4), the coal seam mostly occurs as a $\le$1 m thick carbonaceous shale with intercalations of impure coal and/or mudstone. The elevated amounts of mineral matter indicate consistently high, but variable, clastic input into the mixed peat—clastic swamp, reaching maxima during flooding events (Mach et al. 2013). In eastern part of the Kladno paleovalley (except the valley margin), the coal seam is composed of 1 to 3 m (locally up to 5 m) of alternating coal, impure coal, carbonaceous shale and mudstone intervals, the latter predominating in lower 2 to 3 m of the coal section (Figs. 3d, f, 5–8). This indicates that paludification possibly related to intensification of rainfall started in local, short-lived, mixed peat-clastic swamps on a predominantly clastic floodplain. In modern tropics, increased precipitation intensifies chemical weathering and density of vegetation cover, which, in turn reduces up-stream-derived clastic load (e.g., Noorbergen et al. 2018). Rivers transport only little clastics (black-water rivers, e.g., Cecil and Dulong 2003), which supports peat accretion on floodplain. Abundant thick vitrite (colotellinite) bands point to presence of forest vegetation, dominated by arborescent lycopsid genera Lepidodendron, Lepidophloios and Paralycopodites (Fig. 8, intervals MVDD7/a–g). This initial interval of short-lived local peat swamps is best developed in the eastern part of the Kladno coalfield and is followed by an onset of widespread and long-term paludification. It gave rise to a 1 to 1.5 m thick coal seam composed mainly of dull and dull/bright banded coal lithotypes interbedded with only few decimeters to centimeters-thick partings of impure coal, carbonaceous shale and organic-poor mudstone (Fig. 3d, f). A smaller proportion of clastic-rich intervals in the coal seam profiles suggests reduced clastic input and lateral accretion (paludification) of the peat swamp over most of the paleovalley. Thick intervals of high ash coal and carbonaceous shale most likely resulted from prolonged periods of time when the regional water table was raised above peat surface, enabling the distribution of suspended sediments across the swamp (e.g., Smith 1962; Littke 1987; Strehlau 1990; Zieger and Littke 2019). However, thin partings of organic-poor mudstone, intimately alternating with coal, probably represent short-term flooding events (McCarthy et al. 1989; Diessel 1992; Calder 1993; Mach et al. 2013; Opluštil et al. 2013).

No contemporaneous fluvial channels have been identified from historic Lower Radnice Coal mine operations. Only fine mud suspension spread down the valley and into the swamp during flooding events, indicating that the fluvial systems entering the western part of the paleovalley were low in energy and competency. A similar scenario was suggested for the overlying Upper Radnice Coal (Opluštil 2005b). This commonly 6 to 8 m-thick coal covers the Kladno paleovalley from the northern to southern margin without interruption by siliciclastics from contemporaneous fluvial channels. A transition of coal to carbonaceous shale and mudstone, without change in thickness, is observed in the western end of the coalfield (=upper part of the paleovalley). The carbonaceous shale and mudstone occur in a meander-like belt, tens to about hundred meters wide, and gradually disappear down the paleovalley/coalfield (Čepek et al. 1936; Opluštil 2005b). A modern analogue to this system may be the tropical “black water” rivers draining peat swamps in SE Asia and carrying very little sediment (Anderson 1983; Cecil and Dulong 2003; Wüst and Bustin 2004; Latrubesse et al. 2005; Page et al. 2006).

Peat accretion of the Lower Radnice Coal was terminated abruptly across the Kladno coalfield by a volcanic ash fall, existing in the rock record as the approximately 0.6 m thick Bělka tuff bed in the roof strata of the Lower Radnice Coal (Figs. 1b; 3e, f). This tuff bed also covers the Lower Radnice Coal in other coalfields in the central and western parts of the Czech Republic (Čepek et al. 1936; Pešek 1994; Opluštil et al. 2014; Tomek et al. 2021).

Hydrology of the Lower Radnice Coal swamp

The generally high ash content in the Lower Radnice Coal, either dispersed or concentrated as clastic partings, indicates that the Lower Radnice peat swamp was topogenous and rheotrophic (Smith 1962; Diessel 1992; Calder 1993; Jerrett et al. 2011; Zieger and Littke 2019). Water tables in topogenous/rheotrophic peat swamps correspond to regional ground-water tables and are fed by both rainfall and surficial waters, the latter potentially containing sediment. Distribution of clastic-rich intervals in the Lower Radnice Coal vary spatially and temporally, as indicated by different patterns of alternation of coal and clastic intervals in the studied sections. Correlation of these coal and clastic intervals, across even short distances, is problematic (Figs. 5–8).

A very good negative correlation of inertinite and vitrinite in the studied sections indicates that both maceral groups originated from essentially the same botanical precursor tissues, but followed different formation pathways (e.g., Diessel 1992; Taylor et al. 1998). Whereas the formation of vitrinite is enhanced by waterlogged anaerobic conditions, which promotes humification of plant tissues as initial step in peat formation, most inertinite macerals result from oxidation of plant tissues, either from intense biological decay or combustion from wildfire (Diessel 1992; Hower et al. 2013). Coal seam sections contain moderate to low amounts of vitrinite (24.3–53.6%) and moderate to high inertinite contents (26.3 and 54.8%) (EA 2–5). The high inertinite contents consist mainly of fusinite and reduced amounts of semifusinite and macrinite. Although fusinite with well-preserved cell structure and high reflectance is believed to be of wildfire origin (Scott 1989), a poor correlation between fusinite and mineral matter content suggesting that fire-derived fusinite is partly of autochthonous and allochthonous origin. A good positive correlation between inertodetrinite and mineral matter content suggests that much of the inertodetrinite was allochthonous, having been transported into the peat swamp with clay minerals as part of a suspended sediment load. Although a high-water table made the Lower Radnice swamp vulnerable to sediment influx it also kept the peat wet and mainly anaerobic, which promoted the humification of plant biomass and formation of vitrinite. In coal with <30% ash most of the vitrinite exists as telovitrinite derived from local peat swamp vegetation, which is mainly represented by arborescent lycopsids (Figs. 5–8; EA 2–9). This is supported by abundant several millimeters thick vitrite (collotelinite) bands and by miospore composition. An autochthonous origin of vitrinite in lower ash coal is further evident from its negative correlation to mineral matter content. However, in carbonaceous shale or clay-rich coal, vitrinite occurs predominantly as detrovitrinite that may be authochthonous or allochtonous in origin. Liptinite, which mainly occurs as sporinite, is abundant in most of the samples (Figs. 5–8).

In general, there are no obvious trends in water table change that accompanied peat accumulation. Instead, fluctuations in water table, recorded as changes of mineral matter content and maceral composition, varied randomly across the peat swamp in space and time. These changes, in turn, were probably driven by local factors including microtopography of swamp, ability of swamp vegetation to filter clastic suspension from local sources to the relatively small Lower Radnice peat swamp, landlocked among paleohighs.

Vegetational history of the Lower Radnice Coal swamp

A combination of data, including miospores, dispersed megaspores and cuticles, were used to reconstruct the vegetational history of the Lower Radnice Coal. Terminal vegetation is further recorded in the Bělka tuff bed, which contains plant remains of an in situ buried flora prior to the volcanic eruption (Opluštil et al. 2007, 2016c).

Interpretations of miospore assemblages assume that miospores are primarily of autochthonous origin and represent local vegetation. This assumption is supported by observations in modern tropical forests, where most pollen and spores are dispersed within a few hundred meters from their parent plants (Anderson and Muller 1975). As such, changes in dispersed miospore assemblages within coal seams are interpreted to represent changes in the composition of local vegetation. However, care must be paid to carbonaceous shale or impure coal where part of miospore assemblage may be allochthonous in origin and therefore not completely indicative of the local paleoflora (Smith 1962; Fulton 1987). Correlation of Pennsylvanian miospore taxa with their parent plants is based on previous studies of in situ spores or pollen from fertile organs (e.g., Ravn 1986; Traverse 1988; Balme 1995; Bek and Opluštil 2004; Bek et al. 2015). Most Pennsylvanian miospores have been linked to their parent flora at the generic or family levels, with spores and pollen of unknown affinity in the Lower Radnice Coal occurring at minimal levels (typically ≤1%, max. 2.8%) (Tab. 1; EA 6–9).

Percentages of dispersed miospore taxa are not directly indicative of the relative abundance of their parent plants in the original vegetation and need to be normalized due to differences in production of miospores (e.g., Willard 1993; DiMichele and Phillips 1994; Opluštil et al. 2009a; Bek et al. 2021). Perhaps the best example of this is medullosalean pteridosperm pollen, which is consistently underrepresented in dispersed miospore assemblages but common in coal balls (Willard 1993; Willard and Phillips 1993) and tuff beds that preserve in situ buried peat swamp floras (Opluštil et al. 2009a, 2014; Bek et al. 2021).

Although dispersed miospores in coal are essentially autochthonous, megaspores are considered to record an even more local assemblage of parent plants because of their larger size (Smith 1962; Bartram 1987). However, interpretations of parent plants and their abundance from megaspore assemblages are not straightforward and have the same caveats that using miospore abundance to estimate parent vegetation have. A more refined estimate of local paleoflora can be obtained using dispersed cuticles, mostly derived from in situ accumulated plant remains, which makes their taphonomy comparable with the coal-ball record (e.g., DiMichele and Phillips 1994; DiMichele et al. 2002). However, their correlation to parent plants is possible only at higher taxonomic ranks representing major plant groups.

All samples of the Lower Radnice Coal and associated clastic floodplain sediments are, with few exceptions, dominated by miospores of arborescent lycopsids, especially lepidodendrids. Their relative abundance varies between 13.2 and 98.8% with mean values ~70 to 75%. Spores produced by the genera Lepidodendron (L. pusilla) and Paralycopodites (Lycospora orbicula and L. micropapillata) show good to very good negative correlation in all sections (r = 0.717 for n = 20 in MVDD1 and r = 0.863 for n = 33 in MVDD 7), except the borehole MVDD 5, which is represented by only a few samples and therefore statistically least representative. This correlation means that the maxima of spore abundance produced by Lepidodendron correspond to a minimum of Paralycopodites and vice versa, as a consequence of different habitats they occupied. However, there is poor (MVDD-1, r = 0.299) or very poor negative (MVDD-7, r = 0.0898) correlation of the relative abundance of Lepidodendron spores and mineral matter and very poor correlation between spores of Paralycopodites and mineral matter (MVDD 1, r = 0.16; MVDD 7, r = 0.156) in these statistically most representative sections. This indicates that mineral matter, represented principally by clay suspension and indicative of water table level and clastic input, is not a sufficient proxy for distinguishing the habitats of the two genera. In the Lower Radnice Member, the genus Lepidodendron is represented mainly by L. aculeatum and L. obovatum (=L. mannebachense sensu Thomas 1970). Both are very abundant in mudstone overlying the Upper Radnice coal but are absent (L. aculeatum) or extremely rare (L. mannebachense) in in situ preserved peat-forming flora of the Velká opuka tuff bed, intercalated in the same coal (Fig. 1; Opluštil et al. 2016c). In contrast, only Lepidodendron mannebachense and slightly more abundant Lepidophloios laricinus were identified from the Bělka tuff bed in roof of the Lower Radnice Coal (Opluštil et al. 2007). Paralycopodites occurs in the Lower Radnice Member as the compression taxa Bergeria dilatata (=Lepidodendron acutum) and Lepidodendron simile (sensu Němejc 1947). Neither species has been found in the Bělka tuff bed roof of the Lower Radnice Coal of the Kladno coalfield, but L. simile is very abundant in the Bělka bed of the Ovčín opencast mine in the Radnice Basin (Opluštil et al. 2009a, b, 2014). In the overlying Upper Radnice coal of the Kladno coalfield, however, both species are common, with B. dilatata being equally abundant in clastic and peat swamps and L. simile being more abundant in peat swamps (Opluštil et al. 2016c). This distribution agrees with the palynological data where Paralycopodites seems to be slightly more abundant in peat substrates (coal) and Lepidodendron in clastics as observed also in the Duckmantian peat swamps of Intra-Sudetic basin (Libertín et al. 2009b) as well as with the coal balls (for overview see DiMichele and Phillips 1994; DiMichele 2014). Bartram (1987) and DiMichele and Phillips (1988, 1994) suggest that Paralycopodites preferred habitats with intermittent flooding and clastic input transitional between purely clastic and peat swamps.

Lepidophloios is the third most abundant lepidodendrid genus in the palynological record of the Lower Radnice Coal. Its distribution in boreholes MVDD-1 and MVDD-7 shows poor negative correlation to Lepidodendron and poor positive correlation to Paralycopodites, suggesting some overlap in habitats dominated by Paralycopodites. There is a poor positive correlation to mineral matter content, indicating some overlap with habitats preferably colonized by Lepidodendron. Coal petrological and palynological data from North American basins suggest that Lepidophloios preferred (were able to tolerate?) consistently flooded peat substrate with standing water (Eble 1990; Calder 1993; Willard 1993; DiMichele and Phillips 1994).

The arborescent lycopsid genera, Diaphorodendron/Synchysidendron and Sigillaria, which produced Granasporites medius (=Cappasporites distortus) and Crassispora, respectively, are always subordinate and their spores are more abundant in coal and impure coal samples (Figs. 5–8). This agrees with coal ball studies, which suggest that Diaphorodendron and Synchysidendron preferred, exposed to partly submerged peat substrates (Phillips and DiMichele 1992; DiMichele and Phillips 1994). In contrast, Crassispora is a typical miospore genus of the Incursion phase of Smith (1962), characterized by increased mineral matter content. Another lycopsid indicative of paleoenvironment in the Lower Radnice Coal is Omphalophloios, a subarborescent lycopsid that produced Densosporites spp. and Cristatisporites indignabundus (Bek et al. 2015). This genus is usually connected with the ombrotrophic phase of peat swamp formation (Smith 1962; Fulton 1987; Diessel 1992), which was not achieved in the case of the Lower Radnice Coal. In the late Middle Pennsylvanian (Asturian) Lower Kittanning coal of western Pennsylvania, USA, Omphalophloios was thought to be a saline tolerant plant that became more abundant in the terminal parts of the coal in response to marine transgression (Habib 1966; Opluštil et al. 2019). However, there is no evidence of marine influence in the Lower Radnice Coal, or in strata that bound it. In the Fire Clay coal of the Central Appalachian Basin, USA, elevated amounts of Densosporites and Cristatisporites were found to be associated with low ash coal, believed to have formed from domed, ombrotrophic peats, as well as high ash coal derived from planar rheotrophic peats (Eble and Grady 1990; Eble et al. 1994). Based on these studies, it appears that Omphalophloios had the ability to exist in several different paleoenvironments.

Although miospores of other plant groups are numerically subordinate in the Lower Radnice Coal, they contribute a great deal of taxonomic diversity (EA 6–9). Tree ferns spores (e.g., Punctatisporites minutus) are only locally abundant and tend to be more common in clastic substrates, which agrees with previous observations, where tree ferns were not an abundant component of any wetland landscape until about the beginning of the Asturian/Desmoinesian (e.g., Phillips et al. 1985; DiMichele and Phillips 1994). In contrast, small herbaceous ferns show a good negative correlation with mineral matter and were probably centered in less inundated (? exposed) parts of peat swamp. Sphenopsid (both Calamites and Sphenophyllum) spores are also locally common, especially in clastic substrates. Pteridosperms are typically underrepresented (e.g., DiMichele and Phillips 1994; Bek et al. 2021), as indicated by comparing their minimal occurrence in the Lower Radnice Coal with their abundance in the overlying Bělka tuff bed (Opluštil et al. 2007, 2016c). The rare presence of conifer pollen, Pityosporites westphalensis, in clastics associated with the Lower Radnice Coal is interesting, and probably represents anemophilous (wind-blown) pollen from an upland flora situated on basement paleohighs along the flanks of the Lower Radnice swamp.

Megaspore associations of the Lower Radnice Coal confirm a dominant role of the arborescent lycopsids Paralycopodites, Lepidodendron and Lepidophloios with Paralycopodites being more abundant in impure coal samples and Lepidodendron and Lepidophloios more abundant in carbonaceous shale samples, generally in agreement with the miospore data (Figs. 5–8; EA 10). Surprisingly, the sigillarian megaspore Tuberculatisporites is more abundant in clastic swamps whereas its miospore counterpart is more common in coal. Megaspores of subarborescent (Spencerites and Polysporia) and herbaceous lycopsids (Selaginella-like plants) are locally abundant irrespective of mineral matter content (EA 10). Dispersed cuticles agree with the dominance of lycopsid megaspores and miospores and stress the role of medullosalean pteridoperms, indicated by macroflora preserved in the Bělka tuff bed.

Complex distribution patterns of some arborescent lycopsid genera and other plant groups, which diverge with previous observations based mainly on coal ball data (e.g., DiMichele and Phillips 1994), may be related to various factors. One of them could be the relatively small size of the Lower Radnice Coal swamp, which formed in a paleovalley a few kilometers wide and was land-locked between basement paleohighs. Small wetlands have greater periphery to area ratios than do large wetlands, making them more susceptible to “edge effects”. This could result in a complex mosaic of wetland subenvironments, each occupying relatively small areas separated by narrow ecotones.

Despite the dominance of arborescent lycopsid miospores, megaspore and dispersed cuticles, the swamp vegetation of the Lower Radnice Coal in the Kladno coalfield was probably not monotonous. The high diversity of small herbaceous fern miospores and local abundance of miospores or megaspores of other plant groups may indicate, at least locally, the existence of a swamp flora with relatively diverse and dense shrub and other ground cover plants. The lepidodendrid—cordaite forest preserved in the Bělka tuff bed at the Ovčín opencast mine in the Radnice Basin (Opluštil et al. 2007, 2009a, b, 2014) may be partly comparable in this regard. This forest at this location consists of about 30 biological species discovered on a few hectares of high ash Lower Radnice Coal. Its palynological record (Bek et al. 2021), however, differs from our samples in the Kladno coalfield in having a smaller proportion of arborescent lycopsids (12–23%) and higher content of sphenopsids (especially sphenophylls 36–48%), ferns (7–18%) and locally also cordaitaleans (0–9%).

In many Pennsylvanian coal beds, especially those in paralic basins, the succession of miospore (or megaspore) associations has been attributed to changes in water table position and water source. Most are interpreted as a transition from rheotrophic to ombrotrophic conditions and a succession from Lycospore to Densospore phases (e.g., Smith 1962; Bartram 1987; Fulton 1987; Littke 1987; Eble and Grady 1990, 1993; Greb et al. 2002; Jasper et al. 2010; Jerrett et al. 2011). No such transition was observed in the sections of the Lower Radnice Coal, which is consistent with a planar, rheotrophic swamp origin, as indicated by the geochemical and petrographic analyses. Local changes in the dominance of Lycospora species, produced by Lepidodendron and Paralycopodites, are usually unrelated or weakly related to mineral matter content, and record only “subtle” changes in paleoenvironments within clastic to planar peat swamps (Figs. 5–8). The rheotrophic nature of the Lower Radnice peat swamp and the contemporaneous existence of ombrotrophic peats in the paralic basins of the North Variscan foreland basins may indicate the existence of a precipitation gradient with generally lower humidity in the Variscan interior. A possible modern equivalent of the land-locked Lower Radnice Coal swamp in term of paleoenvironment and habitats are extensive planar tropical peat swamps in the Tasek Bera Basin in western (peninsular) Malaysia and in central Congo Basin in equatorial Africa (Wüst and Bustin 2001, 2004; Crezee et al. 2022). These swamps disconnected from the ocean and surrounded by elevated areas form in a dendritic system of tributary valleys and accumulated up to ~10 m thick rheotrophic peat deposits of variable ash content from low to high ash peats. Resulting hard coal seam would have comparable geometry and quality as the Lower Radnice Coal in the Kladno coalfield.

Summary: generalized model of the Lower Radnice Coal swamp

The Kladno coalfield is situated in southeast part of the Kladno-Rakovník Basin, which is one of several continental basins landlocked in the central European Variscides. The Lower Radnice Coal is a part of the Lower Radnice Member (late Duckmantian), which in the Kladno coalfield was deposited in about 15 km long and <5 km wide west- to east-striking fluvial paleovalley bordered by basement paleohighs (Fig. 4). The Lower Radnice Coal evolved from a floodplain-dominated succession alternating along the valley margin with colluvial deposits. Paludification of floodplain clastics, including gleyed vertic Protosol, started in local short-lived peat swamps in the eastern part of the paleovalley and gradually spread westward. The succession of pedogenic processes from well-drained to poorly drained paleosols suggests rising water table possibly due to increasing and/or less seasonal precipitation. No contemporaneous fluvial channel(s) through the swamp have been identified. While river systems deposited coarser clastics in western part of the paleovalley, only mud suspension was dispersed downstream throughout the Lower Radnice swamp, when and where the water table was above the peat surface. In general, mineral matter content decreases down the paleovalley. In its eastern part, the Lower Radnice Coal attains thicknesses up to 1.5 m, consisting of thick coal with thin bands of impure coal or carbonaceous mudstone. This suggests that the peat swamp was an occasionally inundated rheotrophic system with peat accretion controlled by regional ground water table. Abundant inertinite, mainly fusinite, indicates frequent wildfires, both within and outside of the swamp. The peat swamp was colonized by arborescent lycopsids dominated by Lepidodendron and Paralycopodites, with Lepidophloios being subdominant. Diaphorodendron/Synchysidendron, together with Sigillaria, were numerically subordinate (Fig. 9). Whereas Paralycopodites grew in both peat and clastic substrates, Lepidodendron preferred mixed peat–clastic to clastic swamps. Small herbaceous ferns were diverse and colonized less inundated, more exposed parts of the peat swamp. Tree ferns were locally abundant, mostly in clastic or mixed peat-clastic substrates. Palynological data, including megaspores, also indicate the local abundance of sphenopsids (Calamites and Sphenophyllum), small lycopsids (Spencerites, Polysporia, Selaginella) and cordaitalean plants (Fig. 9). Macrofloral data, based on in situ preservation by volcanic ash as well as dispersed cuticles (EA 11), indicate the presence of abundant medullosalean pteridosperms, especially with Laveineopteris and Alethopteris type of foliage. Pteridosperms are strongly underrepresented in the palynological data (EA 6–10). The rheotrophic nature of the Lower Radnice swamp may indicate that peat accumulated in a less humid climate, compared to coastal areas of paralic basins in the North Variscan foreland where ombrotrophic peats were forming at the same time. Modern equivalents to the Lower Radnice Coal formed in inland fluvial paleovalley are planar tropical peat deposits formed in tributary valleys in the Tasek Bera Basin in western Malaysia and central Congo Basin in the western equatorial Africa.

Fig. 9

Generalized paleoecological transect across the peat−clastic swamp of the Lower Radnice Coal in the Kladno coalfield based on palynological, dispersed cuticles and macrofloral data