SDGG, Heft 82 149

Late Variscan continental basins in western Bohemia: tectono-

sedimentary, climatic and biotic archives

Stanislav Opluštil1, Richard Lojka2, Josef Pšenička3

1 Institute of Geology and Paleontology, Faculty of Science, Charles University, Prague, Albertov 6, CZ-12843 Prague, oplustil@natur.cuni.cz2 Czech Geological Survey, Klárov 3, CZ-11821 Prague, richard.lojka@geology.cz3 West Bohemian Museum in Pilsen, Kopeckého sady 2, CZ-30100 Pilsen, jpsenicka@zcm.cz

Carboniferous and Permian in the Czech

Republic – an overview

From the geological point of view the territory of the Czech Republic consists predominantly of the Bo-hemian Massif, the Variscan unit resulting from the amalgamation of individual Armorican terrains (Sax-othuringicum, Teplá-Barrandian, and Moldanubicum) during middle Devonian to early Mississippian times and their fi nal collision with the Brunovistulicum dur-ing the Viséan (e.g. Kalvoda et al. 2008, Schulmann et al. 2009). This fi nal stage was accompanied by de-velopment of synorogenic foreland Moravo-Silesian Culm Basin (Tournain-Viséan) and its successor, the Upper Silesian Coal Basin (Serpukhovian-early Kasi-movian) (Fig. 1) along the eastern margin of already amalgamated Moldanubicum, Teplá-Barrandian and Saxothuringicum terranes (Lugodanubicum) as a re-sponse to their collision with the western margin of the Brunovistulicum. This basin, which is located along Czech/Polish border began to develop as fully marine environment during the Devonian. In the Tournaisian

and Viséan it was a deep marine basin with siliciclastic turbidites and gravity fl ow deposits of culm facies (Pešek et al. 1998, Kalvoda et al. 2008, Kalvoda & Bábek 2010). However, since Serpukhovian times it became shallow enough to allow coal-forming paralic deposition on a delta plain that underwent frequent marine incursions recorded in about 80 marine bands (Řehoř & Řehořová 1972). During this episode of ba-sin development, the paralic, highly coal-bearing and fossiliferous strata of the Ostrava Formation (Paralic Series in Polish part of the basin) were deposited in thickness reaching up to 3 km. The Mid-Carbonifer-ous eustatic event coinciding probably with tectonic activity related to the ‘Erzgebirge Phase’ (e.g. Hav-lena 1982, Kotas 1995), interrupted deposition until the middle Namurian (Kinderscoutian), when purely continental coal-bearing sedimentation started. In the Czech part of the basin this deposition continued into the end of the Bashkirian (Langsettian) and consists up to 1 km thick complex of the Karviná Formation. In the Polish part of the basin, further east coal-bearing deposition continued with several interruptions till

Fig. 1: Distribution of Carboniferous and Permian sediments in the Czech Republic and their basic characteristics.

150 SDGG, Heft 82

the end of Moscovian than being replaced by deposi-tion of fl uvial red beds (Kwaczala Arkose) in early Kasimovian times (Kotas 1995).

Another sedimentary basin developed along the Elbe lineament during the late Devonian to Mississipian. Slightly folded and metamorphosed siliciclastic sedi-ments of this age crop out only in the northern Bohe-mia in the Ještěd Hills where trilobite fauna indicates Tournaisian age. Further SE of this area, more than 300 m thick and subhorizontally lying succession was encountered in deep boreholes (e.g. Nepasice near Hradec Králové). The base of the Tournaisian part of succession is dominated by micritic limestones with Siphonodella sulcata conodont biozone followed by bioclastic crinoid limestones. Around the Tournai-sian/Viséan boundary the carbonate deposition started to be gradually replaced by siliciclastics. Sediments of both areas are interpreted as deposits of the same sedimentary basin formed along the Elbe Fault Zone. Palaeogeographically it was probably a narrow SE striking bay of the sea located between Saxothuringi-cum and Teplá-Barrandian zones (Pešek et al. 1998).

In the interior of the Bohemian Massif, the collisional processes resulted in crustal thickening, fast uplift and exhumation of Variscan granites and high-grade meta-morphic rocks (e.g. Kukal 1984, Kotková & Parrish 2000, Schulmann et al. 2009). As a result, the Bohe-

mian Massif became a probably about 2 to 3 km high plateau, with no signifi cant deposition until the early Bashkirian (except the Elbe lineament) when altitude of the Bohemian Massif was lowered substantially as a consequence of erosion and/or gravitation collapse following the phase of crustal thickenning. During Bashkirian and early Moscovian times waning oro-genic processes allowed for development of NW-SE striking normal and/or strike-slip faulting related to Gondwanan rotation. This faulting was responsible for the formation of numerous sedimentary basins in the interior of the Bohemian Massif (e.g. Matte 1986, 2001, Pašek & Urban 1990, Zulauf et al. 2002). Being located far enough from any marine infl uence, they are fi lled purely by non-marine deposits. These basins were es-tablished in two periods. The older period spans the interval from the late Bashkirian to the late Moscov-ian when most of the basins in central and western Bohemia and in the Sudetic area were formed (Fig. 2). The younger interval of basin formation took place between Gzhelian and Cisuralian times and resulted in formation of the NNE – SSW to NNW-SSE striking narrow Blanice, Boskovice and Orlice half-grabens. Deposition in central and western Bohemia lasted till the end of the Carboniferous, however, in Sudetic basins continued until the Permian in and in some of them even to the Triassic (Fig. 2). Sedimentation in the basins was locally accompanied by volcanic activity

Fig. 2: Stratigraphic ranges of deposition in the basins of the Czech Republic and character of their basement.

SDGG, Heft 82 151

(Mašek 1973, Pešek 1994). Carboniferous strata are locally coal-bearing and fossiliferous. In the Permian part of the fi ll, only the ‚Autunian‘ deposits bear richly fossiliferous horizons.

Depending on their geological position, tectono-sed-imentary history, and stratigraphic range, the conti-nental basins of the Bohemian Massif are subdivided into basins of the central and western Bohemian area, the Sudetic (Lusatian) area, the Krušné hory (Erzge-birge) Mountains area and the so-called grabens, the late Stephanian to Permian half-grabens located in the central part of the Moldanubian zone (Blanice Graben) and along its eastern margin (Boskovice Graben). This subdivision corresponds approximately with the loca-tion of the initial depocentres that subsequently spread over the entire area. As a result, a system of intercon-nected continental basins was established around the turn of the Carboniferous and Permian (Pešek 2004, Pešek et al. 1998). Besides frequent hiatuses and vol-canic rocks, the sedimentary fi ll of these basins is char-acterized by alternating units of grey coal-bearing, and red coal-barren strata that are hundred of metres in thickness and of basin-wide extent. This alternation is commonly explained in terms of climatic changes (Opluštil & Cleal 2007, Opluštil 2013) characterized by a series of alternating humid and drier periods, with each drier interval becoming more severe than the previous ones, whereas the humid intervals gradually became shorter and less pronounced. This trend fi rst becomes apparent around the Moscovian-Kasimovian boundary, and grades during the Late Pennsylvanian and Permian. At that time the Bohemian Massif was ruptured by N-S grabens that became fi lled with Gzhe-lian to Cisuralian sediments.

Paleogeographically the Bohemian Massif and its ba-sins were located near the eastern margin of the equa-torial Pangea. Paleomagnetic measurements indicate that this part of Pangea underwent a northward drift from 00 paleolatitude during the middle Pennsylva-nian to 20 – 40 N latitude in the early Permian (Krs & Pruner 1995). This northward shift is connected with an aridization trend (Tabor & Poulsen 2008) recorded in equatorial Pangea including the Bohemian Massif. However, the Late Pennsylvanian – Early Permian transition was not gradual. Instead, it was character-ized by climatic fl uctuations between moist sub-humid conditions, with the dominance of hygrophyllous fl ora and hydrologically open lacustrine systems, and dry sub-humid conditions, with the dominance of „dry-type“ fl oral assemblages, hydrologically closed lacus-trine systems and abundant carbonate deposition (e.g. Schneider et al. 2006, Opluštil & Cleal 2007, Martínek et al. 2006). These climatic oscillations had a profound effect on composition of the fl ora in basinal lowlands, although only plant remains from wetter periods are

relativelly well represented in otherwise poorly fos-siliferous strata.

Central and Western Bohemian area

In central and western Bohemia, Pennsylvanian strata cover about 6,000 km2, of which 3,500 km2 are ex-posed; the rest is covered by younger, mostly Creta-ceous sediments. The complex of continental strata is formally subdivided into Plzeň, Manětín, Žihle, Radnice, Kladno-Rakovník and Mšeno-Roudnice ba-sins (Fig. 1), but which originally consisted of a single depositional basin located on the Teplá-Barrandian Zone. The basement consists mostly of late Protero-zoic weakly metamorphosed shales and greywacke with volcanic and chert intercalations, and subordinate medium metamorphic rocks cut by Cadomian and Variscan granitoids or unmetamorphosed but folded Early Palaeozoic sediments (Pešek 1994).

Formation of the Late Paleozoic basins in the

central and western Bohemia

Various opinions have been published to explain for-mation of these basins. Pešek (1994) characterized this basin complex as a simple asymmetric mega-graben formed on the Teplá–Barrandian block, subsiding along the SW–NE-striking Central Bohemian and Litoměřice fault zones (terrane boundaries). Pašek and Urban stressed the role of strike-slip movements on especially NW-SE and also NNE-SSW trending faults. These faults are not only characteristics of the central and western Bohemian basins but of the whole Bohemian Massif and the European Variscides (e.g. Arthaud & Matte 1977, Martínez Catalán 2011, Ziegler 1986). They are generally interpreted as a result of intraplate tectonics related to the continental collision between the southern margin of the Laurussia and north edge of the Gondwana which generated dextral strike-slip movements on a broad intracontinental shear zone (Pašek & Urban 1990, Martínez Catalán 2011). Pašek & Urban (1990) distinguished three pe-riods of tectonic history of these basins correspond-ing to early Moscovian (Bolsovian), Late Moscovian to early Gzhelian (Asturian – Stephanian B) and late Gzhelian (Stephanian C) times. These periods, in turn correlates with the main sedimentary phases separated by basin-wide gaps in deposition (Pešek 1994). During the Bolsovian period Pašek & Urban (1990) interpreted that the main tectonic compression was oriented ap-proximately N-S resulting in dextral strike slip faulting on NW-SE faults and associated normal faulting on about NNE-SSW conjugate faults. The latter gener-ated localised depocentres with maximum thickness of the Radnice Member. During the Asturian – Stepha-nian B period, the tectonic compression rotated to

152 SDGG, Heft 82

NNW-SSE position. As a consequence the activity of NW-SE strike-slip faults and N-S conjugate nor-mal faults decreased while normal faulting on NW-SE conjugate faults becomes dominant. During the hiatus between the Stephanian B and Stephanian C boundary orientation of the principal compression changed into the NW-SE direction. As a result, sinistral strike-slip movements on NNE-SSW faults became dominant accompanied by normal faulting on NW-SE conjugate faults. Some of these lines were reactivated during the Saxonian tectonics in Cenozoic times.

Lithostratigraphy of the Late Paleozoic basins

in the central and western Bohemia

Deposition in the basins of central and western Bo-hemia started around the Duckmantian–Bolsovian boundary, and with several hiatuses lasted until the end of the Carboniferous Period, or even to the begin-ning of Permian times (Fig. 3), although unequivocal palaeontological evidence for Cisuralian (early Per-mian) strata is lacking (Pešek 1994). The basin-wide alternation of coal-bearing predominantly grey and coal-barren red-bed units was used as the basis for the lithostratigraphical subdivision of the basin fi ll into the Kladno, Týnec, Slaný and Líně formations respectively (Weithofer 1896, 1902, Pešek 1994).

The Kladno Formation comprises the Radnice and Nýřany members. The Radnice Member (Bolsovian ~ middle Moscovian) is interpreted as the fi ll of an incised or tectonically formed system of river valleys (Fig. 4), with a palaeotopography at the depocentre of up to 200 m (Opluštil 2005a, b). Therefore their thick-ness which does not exceeds 300 m and distribution are strongly irregular. Opluštil (2005a) reconstructed the early Moscovian palaeotopography and drainage system before the onset of deposition in the Kladno-Rakovník Basin and inferred that this area was not higher than 1000 m above sea level, perhaps even lower, based on the gradient of river courses. Depo-sition of the unit was driven by tectonically induced base-level changes which generated the accommo-dation and together with paleotopography controlled amount of clastics delivered into particular valleys and their dispersal within depocentre. Fill of the valleys consists of fl uvial, lacustrine to lacustrine delta and colluvial sediments. Fluvial sediments were deposited in meandering to braided fl uvial systems. Channel fi lls mostly occupy valley axes, whereas fl oodplain sediments with coal seams are located usually along valley margins, and colluvial sediments (mostly brec-cias) are occupy areas along the foothills of basement outcrops. Coal seams are concentrated in the Pilsen, Radnice and Lubná groups. The Radnice group is the most important, and consists of the Lower and Up-per Radnice Coals separated by volcanoclastics of the

Fig. 3: Lithostratigraphic subdivision of the basin fi ll in the continental basins of the central and western Bohemia.

Whetstone Horizon. The Upper Radnice Coal, which is locally more than 10 m thick, is economically the most important seam of the Radnice Member. Some red-beds occur along the basin margin, probably de-

SDGG, Heft 82 153

rived from reworking of laterite weathering crusts (Skoček & Holub 1968). Widespread volcanoclas-tic horizons are in the northern part of the Kladno-Rakovník Basin accompanied by rhyolite/ignimbrite effusions of a cumulative thickness of over 100 m. Lacustrine and lacustrine delta sediments are much less common and quite thin, being best developed in the roof of some coal seams. The macrofl oras of the Radnice Member, which are mainly concentrated in roof shales and fossiliferous tuff beds, are very rich and diverse; Opluštil & Cleal (2007) noted that 139 whole plant species have been recorded.

The Nýřany Member (Asturian – Cantabrian, i.e. late Moscovian – early Kasimovian) is separated from the Radnice Member by a basin-wide hiatus, related to the Leonian phase (Opluštil & Pešek 1998) and spread out quickly over most of the basin complex (Fig. 5). The member is of fl uvial origin, and is characterised by a predominance of coarse- to medium-grained arkoses with pebble admixture over fi ne-grained sandstone and mudstone intercalations. Sediments are arranged

into 10 to 20 m thick upward fi ning cycles, termi-nated occasionally with thin and only locally mine-able coal seams. These seams are divided into the Touškov, Nýřany, Chotíkov and the youngest Nevřeň groups (Pešek 1994). Sediments of the Nýřany Mem-ber represent deposits of a large braidplain supplied with clastics mainly from a source area located south of the basin, in the Central Bohemian Pluton (Pešek 1994).

The fossil record of the Nýřany Member is concen-trated in the roof shales of local coals and frequent mudstone layers alternating with fl uvial channel de-posits. Rich macrofl oras indicate a late Asturian age for the lower part of the Nýřany Member (up to lower part of the Chotíkov group of coals) whereas in the upper part of the unit some early Stephanian species appear (Wagner 1977, Pešek 2004). Famous vertebrate and invertebrate faunas have been described from a sapropelitic coal forming a few decimetres thick bench in the Main Nýřany Coal in the Nýřany and Třemošná coalfi elds, in the Pilsen Basin.

Fig. 4: Paleogeography of the Radnice Member depocentre during the deposition of the Upper Radnice Coal (Modifi ed from Opluštil 2005b). 1 – Current extend of the basins, 2 - Source area, 3 – Extent of the Radnice Member depocentre during the deposion of the Upper Radnice Coal, 4 – Dominantly channel fi ll sediments, 5 – Upper Radnice Coal – proved extent, 6 – Upper Radnice Coal – as-sumed extent, 7 – Acid volcanic to subvolcanic bodies, 8 – Presumed extent of the Radnice Member, 9 – Town, 10 – State border.

154 SDGG, Heft 82

The character of deposition typical for the Nýřany Member continued in the overlying Týnec (Lower Red) Formation (Barruelian, i.e Kasimovian), as in-dicated by similar facies and architecture. The main difference between the two units is the red colour of the fi ne-grained sediments and absence of coal seams in the Týnec Formation, interpreted as a response to climatic change from humid to drier and more seasonal conditions (Pešek 1994, Opluštil & Cleal 2007). Plant remains were found mostly in local grey mudstones and indicate a Barruelian age (Wagner 1977, Pešek 2004).

The following Slaný (Upper Grey) Formation (Stepha-nian B, i.e. late Kasimovian) consists of fl uvial, lacus-trine and deltaic strata. The lower part of the unit is represented by sediments deposited in similar fl uvial systems to those represented in the Týnec Formation but under more humid climatic conditions that favoured mire formation (Mělník group of coals). These basal and locally coal-bearing fl uvial strata of the Jelenice Member are sharply overlain by a more than 100 m thick, upward-coarsening lacustrine to lacustrine delta sequence (Skoček 1990). It starts with a horizon of dark laminated claystone traceable over the whole basin complex up to the Polish part of the Intra-Sudetic Basin, and represents deposits of one of largest Stephanian lakes in Euramerica. The lamination is interpreted in a term of seasonal climate (Skoček 1990, Lojka et al. 2009, 2010). Laminites are overlain by prodeltaic het-erolites, passing upward into ripple-bedded sandstones, and then to medium- and coarse-grained arkosic sand-

stones occasionally with scattered pebbles, inter-preted as fl uvial sediments. The upper part of the Slaný Formation consists of fl u-vial strata containing the laterally persistent fl uvio-lacustrine horizon of the Kounov group of coals. A very rich but more or less uniform macroflora throughout the formation indicates a Stephanian B age (Wagner 1977, Pešek 2004).

The youngest Pennsylvani-an sediments in central and western Bohemia belong to the Líně (Upper Red) Formation (Stephanian C – ?Autunian, i.e. Gzhelian-?Cisuralian). It is separated from the underlying Slaný Formation by a basin-wide erosional surface with a

prominent topography (Skopec et al. 2000) and also by a fl oristic break (Šetlík 1977), which indicates the existence of a hiatus between these two units. Depo-sition of the Líně Formation took place in generally drier, probably seasonally-wet climate comparing to more humid one of the previous unit (Opluštil & Cleal 2007). That is why sediment of the Líně Formation, especially the mudstones, is predominantly red col-oured with frequent horizons of pedogenic carbonate nodules or calcretes. The presence of horizons of grey or mottled mudstones a few tens of metres thick, lo-cally with cherts, volcanoclastics, lacustrine carbon-ates or even thin coal seams may indicate periods of temporarily humid climate. Scarce biostratigraphical data have proved a Stephanian C age for most the for-mation, although a Cisuralian (early Permian) age is assumed for the upper part based on the correlation of lithological markers with basins of the adjacent Sudetic area (Pešek 1994). The fossil record of the formation is very poor, probably due to unfavourable conditions for the fossilisation of plant remains. Most fossils were found in grey lacustrine mudstones of the Klobuky and Zdětín horizons, with the macrofl oras indicating a late Stephanian C age (Wagner 1977, Pešek 2004).

3. Descriptions of stops

The main goals of the fi eld trip are to:• show the main lithology of the main stratigraphic

units in basins of western Bohemia;

Fig. 5: Paleogeography of the depocentre of the Nýřany Member (Opluštil et al. 2005).

SDGG, Heft 82 155

• examine sedimentary facies and sedimentary en-vironments during the deposition;

• examine climatic indicators throughout the basin fi ll to demonstrate changes in climate throughout the basin development

The location of stops is depicted on Fig. 6.

Stop 1 RadčiceRoad cut near Radčice 800 m SE of Radčice, east-ern part of the Pilsen Basin (WGS84 coordinates: 49°45’29.719”N, 13°20’19.648”E)

This locality consists of a series of discontinuous and approximately 1 km long series of outcrops along the road cut between the village of Radčice and the city of Pilsen. Stratigraphically the exposed strata dipping gently ENE, record transition between upper part of the Nýřany Member (Kladno Formation) and the Týnec Formation (Cantabrian/ Barruelian). The exposed strata are dominated by coarse-grained fl uvial deposits (Fig. 7, 8). The top of the Kladno Formation (Nýřany Member) at the lower part of the section comprise lenticular medium to coarse grained sandstones of

possibly crevasse channels intebedded by initially thin layers of siltstones (Fig. 8b) that form up to 3 m thick unit of fi ne grained fl oodplain deposits embedded in crevasse channel sandstones. Laterally continous unit of fl oodplain fi nes consists of laminated and rooted siltstones, thinly developed coal seams, tuff layers and fi ne grained sandstones representing variety of fl ood-plain depositional environments (Fig. 8a,c). However, most of the outcrops belong to the coarse-grained fl u-vial deposits of the Týnec Formation. Its basal surface is highlighted by sheet-like cobble conglomerate rich in grey siltstone intraclasts corresponding to chan-nel lag. The fl uvial deposits of the Týnec Formation consist mainly of clast-supported conglomerate and cross-bedded pebbly sandstone. Width/depth ratios range approximately between 5 and 20. Finer grained sandy bedforms (architectural element SB) occur less frequently. They form the top of the channel fi ll, and are often eroded by channels (CH). Low-angle, cross bedded gravel and sand bars of the GB architectural element occur rarely. Paleocurrent indicators show a low spread of vectors of axes of trough cross bedding and foresets of planar cross bedding between 550 and 1550 (Opluštil et al. 2005).

Fig. 6: Distribution of the Carboniferous sediments in the western Bohemia.

156 SDGG, Heft 82

Fig. 7: Eastern part of the Radčice cliff near Pilsen. Týnec Formation, Cantabrian-Barrualian (After Opluštil et al., 2005).

Fig. 8: Radnice section; A) measured section of the Kladno/Týnec formation boundary interval showing position of fi ne grained units (detailed log of fl oodplain fi nes) and cyclic arrangement of fl uvial deposits; B) basal part of the outcrop comprise series of lenticular crevasse channel sandstone; C) detail of fl ooplain lacustrine facies with tuff horizon and organic rich clay.

SDGG, Heft 82 157

The features mentioned above lead to the interpretation of the strata record gravel bars and channel fi lls depos-ited in low-sinuosity gravel-bed braided rivers with multistory and multilateral arrangement of channels (Opluštil et al. 2005). Considerable lateral channel mi-gration indicate unstable banks. No substantial changes in grain size were observed in vertical sections.

Subordinate and thinly developed tabular to lenticu-lar units of laminated siltstones to fi ne-grained sand-stones usually rich in fl oral detritus represent deposits of abandoned channels. Facies arrangement into 4 – 7 m thick fi nning upwards successions resemble to that observed at kaoline opencast mine at Kaznějov (Stop 2), where fl uvial deposits of similar age are ex-posed. Subhorizontal surfaces bounding fi ning upward successions of similar thickness were interpreted in Kaznějov open cast mine to bound individual channel belts developed in response to climatic fl uctuations.

Fine grained fl oodplain deposits provided quite rich palynological assemblage, while the fi ne grained de-posits of abandoned channels from the overlying Týnec Formation display only poorly preserved palynomorfs (Drábková et al. 2005). The rich assemblage of the monolete spores of the genus Punctatisporites, Laevi-gatosporites and Speciososporites occuring in the thin coal seam at the lower part of the fl oodplain unit (Fig. 8) represent maratialean ferns. Calamite spores are also common. Crassispora kosankei microspores and Tuberculatisporites sp. macrospores corresponding to Sigillaria prevail in the upper part of the fl oodplain series dominated by creavasse splay deposits. Index fossil for Bohemian Westphalian D Vestispora fenes-trata belonging to the species Sphenophyllum emar-ginatum occur in the whole section. Stratigraphically younger species such as Punctatosporites speciosus, Wilsonites kasankei, Spinosporites sp., Gillespieis-porites discoideus and Potonieisporites novicus are also present (Fig. 9). Abandoned channel fi lls contain poorly preserved spore assemblages with dominance of monosacate pollen (Fig. 9). The most of index fos-sils for Wesphalian D present in the alluvial plain are absent, while the Kosankeisporites elegans and An-gulisporites splendidus have their fi rst occurence here. Observed palynological assemblages represent either

two diferent habitats of Cantabrian age or older-low-land assemblage of Cantabrian age found in fl oodplain unit and younger-upland assemblage of Barruelian age preserved in abandoned channel-fi lls. Stop 2 Kaznějov Stop 2 Kaoline opencast mine 2500 m W of Kaznějov (WGS84 coordinates: 49°53.702’N, 13°20.222’E)

The Kaznějov kaoline open cast mine exposes about ~60 m thick series of coarse-grained fl uvial deposits of uncertain stratigraphic level spanning from late Moscovian to early Kasimovian. There are two coarse-grained fl uvial units developed at the northern part of the Pilsen Basin each of which is up to 100 m thick and correspond to the mid part of the Nýřany Mem-ber of the Kladno Formation and to overlying Týnec Formation respectively. Its stratigraphic position is problematic due to absence of fossils and similarity in facies composition of both formation in a probably marginal setting at the northern part of the basin.

Fluvial series exposed in the Kaznějov kaoline open cas mine consists mainly of cross-stratifi ed clast-supported conglomerates with clast size ranging from granules to cobbles up to 40 cm in diameter and cross-stratifi ed coarse pebbly sandstones that together rep-resent about 85% of exposed section (Fig. 10). Rare tabular to lenticular units of medium clayey sand-stones contain up to 18 m long silicifi ed tree trunks, mostly belonging to conifers. Laminated mudstones with occasional macrofl ora impressions (cordaites) are subordinate but if well developed the up to 1.5 m thick paleosol can be obdserved. It consists of fi ve Bhss and Bss horizons showing fi ne to coarse wedge shape aggregate structure; large to fi ne propogating slickensides; common fi ne-medium circular, tubular to diffuse purple, orange, and yellow mottles; and many diffuse-vermicular light reddish brown mottles and concentrations of iron-rich materials documented also by X-ray diffraction (Fig. 11). All of these features are typical for vertisol indicative of highly seasonal climate. Clay mineral assemblage is dominated by kaolinite with only traces of illite/smectite mixed-layer mineral, which is typical for rather arid vertisols. The

Fig. 9: Palynomorfs common to both alluvial plain and abandoned channel fi ll facies, the scale is 20 µm. A) Potonieisporites novicus; B) Potonieisporites sp.; C) Florinites mediapudens; D) Guthörlysporites magnifi cus.

158 SDGG, Heft 82

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bulk chemical composition of B horizons allows cal-culation of mean annual precipitation that give values between 1200–1300 mm/y with deviation +/- 200 mm (Sheldon & Tabor 2009). Kaolinite dominance and prevalent oxic environment of vertisol under relatively high precipitation indicate well-drained fl oodplain with probably no prolonged arid periods.

Sedimentary lithofacies represent gravel bars, sandy/gravelly channel-fi lls and fl oodplain deposits bounded by hierarchy of bounding surfaces and organized into fi ning upward cycles of the two scales corresponding to channel-fi lls (2-4 m, usually incomplete) and chan-nel belt complexes (3-10 m) that encompass several laterally and vertically stacked channels. Facies com-

SDGG, Heft 82 159

position and architecture correpsonds to deposition in gravel-bed braided rivers with low-sinuosity channels. Conglomeratic braid bars formed by amalgamation of gravell sheets and dunes under peak discharge con-ditions remained stable morphological elevations in channel belts during low discharge periods when sur-rounding channels became abandoned, as indicated by lenses of claystone embedded in thick conglomerate units (Fig. 10). Braid bars grade laterally to continuous cobble size channel lag horizons, defi ning base of the channels and channel belts where conglomeratic braid bars are absent. Channel fi lls consist mostly of unit

and compound sand dominated bars with number of erosional surfaces resembling to erosion dominated channel bodies with multistory and multilateral ar-rangement. Lateral migration of highly mobile chan-nels produced the 1st order bounding surfaces defi ning internal structure of channels by bounding of partial channel-fi ll “stories”. It represents both, accretionary and erosional in-channel surfaces. Vertical stacking of channels into multistory pattern resulted from repeti-tive avulsions and re-occupation of previously aban-doned channels within channel belt producing the 2nd order surfaces that defi ne partial channels. These sur-

Fig. 11: Paleosol section developed in the 4 m thick laterally continuous fl oodplain deposits (> 500 m). Chemical alteration indexes indicate 1200 mm of annual precipitation and pronounced oxidation leading to Fe-oxides formation well apparent also in XRD of clay fraction of the three lower horihzons.

160 SDGG, Heft 82

faces are usually erosional, concave-up with erosional relief reaching up to 5 m. The topmost part of the chan-nels, where preserved, are formed by medium clayey sandstones bearing large silicifi ed tree trunks indicat-ing low-energy river fl ows. However, this facies can occur also immediately above the channel lag-avulsion surfaces recording high, posssibly seasonal discharge variability. The highest order surfaces bound individ-ual channel belts including fl oodplain fi nes at the top (Fig. 12). Subhorizontal, laterally continuous surfaces (> 500 m) bound channel belt complexes forming fi n-ing upwards cycles that can be correlated across much of the lateral profi le and display similar lateral and vertical facies pattern. As indicated from photomosaic (Fig. 10), the surface hierarchy is derived from natural continuum of surfaces where each surface represents composite surface of lower order surfaces. It follows that 1st order surface bounding channel belt originate from repetitive erosion and lateral migration of chan-nels bounded by 2nd order surface at the base.

The paleofl ow measurements indicate dominantly southern transport direction with south-western to south-eastern lateral migration of channels. Mature clastic material of conglomerates consists dominant-ly of subrounded to well rounded quartz, laminated quartzite probably of Ordovician age and locally neoproterozoic chert; minor components represent kaoline-altered rhyolitic volcanics and granite. Sand-

stones are slightly arcos-ic formed by subrounded to subangular grains dominated by quartz and accesorial feldspars that are partly or completely altered to kaoline. Heavy mineral associations of sandstones are relative-ly stable throught the section consisting of leukoxene (30-50%), staurolite (10-25%), tourmaline (10-20%), ru-tile (10-20%), zircon (3-10%) and kyanite (~3%) indicating the mixture of metamorphic and gra-nitic rocks in the source area situated north-ward with signifi cant propor-tion of medium-grade metamorphic rocks in the drainage basin.

Observed fl uvial cycles refl ect most probably cli-matically induced, grad-

ually decreasing transport capacity of river fl ows with maximum discharge at the base producing extensive gravel accumulation. Decreasing discharge and fl ow energy culminated during deposition of fl oodplain fi nes and subsequent exposition and paleosol forma-tion under seasonal precipitation.

Stop 3 VelečínAbandoned clay pit 600 m SW of Velečín (WGS84 coordinates: 50°04.166’N, 13°23.297’E)The upper part of lacustrine fi ne grained series of the Malesice Member of the Slaný Formation (stephanian B, lowermost Gzhelian) is exposed in the abandoned clay pit near Velečín at the central part of the Žihle Basin (Fig. 13). The Malesice Member represents up to 100 m thick coarsening upwards fi ne-grained lacus-trine successsion with organic rich claystones at the base and fi ne-grained sandstones at the top. The basal claystones form up to 20 m thick unit recording the deepest lacustrine environment with almost continu-ous sedimentation as indicated by thinly developed varve-like lamination consisting of organic-clay/silt-siderite reverse-graded couplets that refl ect seasonal variation in clastic input (Fig. 13e,d). Fine-grained clastic material was transported to basinal areas by quasi permanent low density fl ows fed by seasonal fl oods (Skoček 1968, 1990, Lojka et al. 2010).

Fig. 12 : Complete fi ning-upward cycle recording channel-belt evolution developed between A3 and A4 bounding surfaces.

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Fig. 13: Lacustrine series of the Malesice Member (Slaný Formation) exposed near Velečín. A) measured section showing upward coarsening transgressive/regressive cycles; B) sharp base of distributary channel rich in large charcoals; C) organic-rich lamina represents maximum fl ooding surface, orange laminae refl ect increased microcrystalline siderite admixture; D) coarsenning upward regular couplet lamination, basal part of the Malesice Member, Tr-1 core near Slaný; E) microlaminated organic-rich claystone at the base of the Malesice Member represent the deepest lake environment, Tr-1 core near Slaný.

162 SDGG, Heft 82

The mid and upper part of the Malesice Member con-sist of alternating claystones, siltstones and very fi ne grained sandstones that are arranged into 2 – 4 m thick coarsening upward cycles. Basal organic-rich lamianted clayey siltstones sharply overlay very fi ne to fi ne grained sandstones at top of the undelying cycle. Thin laminae of very fi ne grained sandstones gradu-ally interbed siltstones, their thickness and frequen-cy increase upward to form a series of sharp-based, graded layers that amalgamate in a thick unit of very fi ne sandstones usually with parallel lamination at the top of the cycles. Sharp bases of cycles with organic rich siltstones (Fig. 13c) refl ect maximum fl ooding surfaces, while very fi ne sandstones at the tops record delta lobe progradation forced by lowering of lake level (Dam & Surlyk 1993). It follows that periodic repetion of coarsening upward sections records trangressive-regressive cycles forced by fl uctuating lake-level and driven by regional variation in precipitation/evapora-tion ratio.

In the section exposed at Velečín about 3 cycles can be observed (Fig. 13a). The top of the fi rst complete cycle is formed by up to 1 m thick unit of sharp-based fi ne grained sandstones that display number of erosional surfaces, large plant axes accumulation and number of large fragments of charcoal (Fig. 13b). Repetitive erosion together with coarse grain size and fl oral frag-ments accumulation indicate relatively proximal sedi-mentation probably in a distributary channel. The rest of exposed cycles are topped by amalgamated series of prodelta turbidites or prodelta lobes.

Exposed section provided rich macrofl oral and paly-nological assemblage, that are partly facies dependent. Siltstones from the lower part of the cycles contain rare specimens of relatively hydrophilous ferns, while very fi ne grained sandstones are dominated by coni-fers (Walchia) prefering rather well-drained substrates (Fig. 14). Diversify palynological assemblages are dominated by monosaccate and bisaccate pollen grains (up to 92 %) representing advanced permian-like el-

ements refered as upland/dryland fl ora that occuped well-drained sub-strates. The most frequent are mon-osacate Potonieisporites (up to 56 %) and bisacate Illinites unicus Kos. (up to 40 %) belonging to conifers and pteridosperms (Peltaspermales, Callistophytales, Medullosales and Gnetales), subordinate pollens be-long to cordaites and Cycadaceae. Proportion of spores representing coal-forming floral assemblage is only marginal that can be explained by prevalent wind transport and condensed sedimentation in basinal lake areas. Palynological elements typical for the mělník and kounov group of seams directly underlying and overlying lacustrine succesion of the Malesice Member were not even identifi ed, which may indicate dy-namic changes in a vegetation cover.

Stop 4 Kryry

Brick pit 1100 m S of Kryry (WGS84 coordinates: 50°09.553’ N, 13°25.348’ E)

The red coloured mudstones exposed in the brick pit represents the mid-dle part of the Líně Formation at the westernmost part of the Kladno-Rakovník Basin. This part is also called the Žatec sub-basin due to its different tectono-stratigraphic devel-

Fig. 14: Paleobotany and paleozoology of the Malesice Member at the Velečín locality (Šimůnek et al., 2010); A) seed Cardiocarpus sp. (C), fern Pecopteris polymorpha (P), shark egg capsules Palaeoxyris sp. 1,5× (Pa); B) Cordaites sp, 1,5×; C) Poa-Cordaites sp. (P), Walchia cf. goeppertiana (W), 2×; D) Annularia stellata, 2×; E) Culmitzschia frondosa f. zeilleri, 3×.

SDGG, Heft 82 163

opment. It is a NE-oriented, fault-bounded, graben-like structure adjacent to the main depocentre at the central Bohemia. Initially isolated depocentre with poorly developed lower formations became more ac-tive during Stephanian B/C boundary (lower Gzhelian) and deposited up to 1 km thick alluvial plain to fl uvial sediments and volcanoclastics of the Líně Formation (Gzhelian).In its lower part, it constists of mainly sandy channel fi ll deposits with subordinate preservation of aluvi-all plain facies, locally associated with vertisols and calcic vertisols. Proportion of fi ne-grained alluvial plain deposits increase upward to the mid of the for-mation, where thick series of distal creavasse splay and ephemeral lake deposits are rarely intercalated by several meters thick units of coarse-grained ribbon sandstones. Fine grained alluvial plain deposits can

preserve primary sedimentary lamination, which is often overprinted by bioturbation, or in case of long-term exposition by pedogenesis producing usually thinly developed vertisols and calcisols. Interplay of these three features indicates relative acomodation/condensation of alluvial plain deposits and proximity of the river channels. Vertical and lateral variation in intensity of bioturbation and pedogenesis display cyclic arrangement (Fig. 15a, b) starting by laminated mudstones to fi ne-grained sandstones, that became upward increasingly massive and disturbed by biotur-bation containing rhizoliths, calcic-dolomitic nodules and greenish mottling at the top. These regular 1 – 2 m thick cycles probably refl ect variation in water-table that controls the amount of accomodation space on al-luvial plain. Condensed horizons at the tops comprise variety of paleosols like calcic protosols, calcisols,

Fig. 15: Deposits of the Líně Formation exposed in brickpit south of Kryry; A) simplifi ed sketch of the outcrop with position of measured section and prominent surfaces, note the coexistence of mature paleosol horizon and fl oodplain channel/lake; B) measured section shows repetitive occurence of condensed surfaces with paleosols and associated cyclic facies pattern, element distribution obtained from XRF reveals particularly horizons with volcanic admixture; C) horizonation and clay mineralogy of P1 paleosol with red tuff horizon at the top, large vertical rhizoliths are very often in the overlying P2 paleosol

164 SDGG, Heft 82

vertisols or calcic vertisols, depending probably on local groundwater regime and paleogeography rather than different climatic conditions durign periods of low water-table. Periods of condensed deposition were also favourable for volcanic tuff deposition and preser-vation as partial paleosol horizons that usually differ in mineral composition and structure (Fig. 15c). The most mature paleosol P1 at the base of the section displays angular blocky structure, green mottles and contains dolomitic nodules and rhizoliths typical for calcisols. The topmost horizon is formed by tuff layer with well developed slickensides and fi ne-medium wedge shape aggregate structure. It is dominated by Fe-smectite with accessorial kaolinite and illite while the lower three horizons consist mainly of illite/smectite mixed-layer mineral and illite. Kaolinite proportion decreases up-profi le with increasing illite and illite/smectite. Described features indicate exposition under highly seasonal climate with initially prolonged periods of evaporation leading to precipitation of carbonates in shalow subsurface. The topmost tuffi tic paleosol ho-rizon was exposed to a number of wetting and drying cycles but possibly with less intense evaporation.

Stop 5 Ovčín opencast mine Former opencast mine located next to the road from Radnice to Přívětice (WGS84 coordinates: 49°50’40.086”N, 13°36’32.750”E). The Ovčín opencast mine is located along southern margin of the Radnice Basin, which is a ca. 20 km2 large erosional relict of Carboniferous sediments once connected with much larger the Kladno-Rakovník Basin further north and the Pilsen Basin further west (Fig. 6). Only the Radnice Member (early Moscovian = Bolsovian) in lower part of the Kladno Formation is present in this basin. The variable thickness and ir-regular distribution of this member refl ect deposition in a system of fl uvial palaeovalleys (Opluštil 2005a). Long-lasting mires developed during periods of in-creased subsidence rate in areas protected against high clastic input, either in minor palaeovalleys or along the margins of the main palaeovalley between the active fl uvial channel and fl anking ridges (Opluštil 2005b). The Ovčín opencast mine worked the Lower and Up-per Radnice coals from 1979 to 1985 (Fig. 16). The basal part of the Radnice Member beneath the coals

is over 15 m thick, and consists here mostly of colluvial and floodplain deposits; the former be-ing more common at the base, and the latter over-lie them up to the base of the Radnice coal group (Fig. 17). Each of coals is between 4 and 5 m thick, being separated by the 8 to 11 m thick Whetstone Horizon composed of in situ and redeposited vol-caniclastics. This horizon is known from all of the coalfi elds in central and western Bohemia where the Radnice group of coals was mined, spread over a distance of about 90 km. In its typical development, as in case of the Ovčín opencast mine, the Whet-stone Horizon consists of ca. 40 to 60 cm of mas-sive, pale yellow to whit-ish tuff with sand-sized grains at the base, over-lain by a laminated tuffi te to tuffaceous mudstone up to 10 m thick (Fig. 18). The tuff bed at the base, called

Fig. 16: A: Exploitation of the Radnice group of coals in the Ovčín opencast mine in early eighties (Photo: Drábek). B: Current state of the former opencast mine Ovčín.

SDGG, Heft 82 165

“Bělka” by miners (translating as “whitish rock”), is dominated by a kaolinite matrix with dispersed entire or fragmentary sanidine and quartz crystals. Vermicular kaolinite crystals are also common, pos-sibly representing biotite pseudomorphs (Mašek 1973). Radiometric dating of sanidines from the tuff using the 40Ar/39Ar method yielded results of 309.0±3.7 Ma (plateau) and 306.9±1.5 Ma (total gas) (Hess et al. 1985) whereas new, yet unpublished data on sin-gle zircon crystals using ID-TIMS U-Pb geochronol-

ogy indicate ages about 314.5 ± 0.4 Ma (Opluštil & Schmitz, in prep.). The rocks above the Bělka are called “Brousek” (translating as “whetstone”) due to the quarrying of these rocks in the past for use as an abrasive. They consist of parallel laminated and locally even ripple cross-laminated tuffi tes and tuffaceous mudstones with minor silty sandstone. From a genetic point of view, the basal Bělka bed is interpreted as a single volcanic ash fall that buried the entire landscape over an extensive area (Mašek1973).

Fig. 17: A: Geological map of the area of the Ovčín coal deposit. B: Section of the Radnice Member in an area of the Ovčín opencast mine. For location see fi g. 9a. C: Cross-section of the Ovčín coal deposit.

166 SDGG, Heft 82

Fig. 18: Architecture of the Whetstone Horizon in the opencast Ovčín Mine (Opluštil et al. 2009a).

The Upper Radnice Coal above the Whetstone Hori-zon is capped in most parts of this locality by fl uvial channel facies, mainly by coarse-grained arkosic sand-stone or conglomerates, and only locally by lacustrine mudstones. This coal splits to the north of the former opencast mine into two benches. The thickness of the upper bench has been reduced by post-sedimentary erosion. This effect coincides with the overal increase in the thickness of the channel facies to the north in nearly the whole section of the Radnice Member. In the same direction, in both coals of the Radnice group, the number and thickness of sedimentary partings increas-es, and the seams grade into carbonaceous shale and then into clastic sediments of fl oodplain and fl uvial channel origin. A gradual transition of coal-bearing fl oodplain deposits to coal-barren fl uvial channel sedi-ments to the north indicates the simultaneous existence of the mire and a river channel fl owing north to the centre of the Radnice Basin (Fig. 17c).

The Ovčín opencast mine is a famous paleontological locality. Rich plant fossils have been collected since open-ing the mine in 1979. Fossils occur in some of sedimentary partings within the coals, however, the most interest-ing remains have been discovered in the Whetstone Horizon, both in the ‚Bělka‘ and the ‚Brousek‘. The Bělka bed bears T0 peat-forming vegetation preserved in growth position by sin-gle a volcanic ash fall preserving the taxonomic composition, spatial distri-bution, vertical stratifi cation, and syn-ecology of this peat-forming ecosys-tem in extraordinary detail (Opluštil et al. 2009a, b). Plant fossil remains represent the pre-eruption vegetation of the precursory peat swamp of the Middle Pennsylvanian (Bolsovian) Lower Radnice Coal (Fig. 19). From a palaeoecological point of view, the fl ora preserved in the Bělka tuff bed represents an instantaneous single-generation snapshot (Fig. 20). The fl ora of the overlying Brousek mud-stone complex was contemporaneous with that of the Bělka, but in contrast these plant remains were transported prior to deposition (Fig. 21). Plant as-sociations from the Brousek were de-rived from vegetation that colonised the mire and adjacent areas of clastic deposition, as indicated by the similar species composition of the Bělka and Brousek fl oras. However, a few pteri-dosperm and rare progymnosperm

species found only in the Brousek unit indicate that some plants might also have been transported from the adjacent slopes of basement palaeohighs along the palaeovalley margins (Opluštil 2005b).

Localized documentation of fossil record in six excava-tions covering a total area of about 200 m2 provided a base for high fi delity reconstructions of local peat-form-ing phytocoenoses of a mixed lepidodendrid–cordaites forest structured into well-developed storeys (Fig. 22). It consists of about 33 biological species which togeth-er colonized occasionally fl ooded planar peat swamp precursor of the Lower Radnice Coal of the Ovčín coal deposit. Canopy storey was dominated by Lepi-dodendron (Paralycopodites) simile, Lepidodendron lycopodioides, Lepidophloios acerosus and Cordaites borassifolius. They formed relatively dense canopy locally interrupted with signifi cant gaps allowing for development of rich herbaceous groundcover which

SDGG, Heft 82 167

Fig. 19: Palaeogeographic map of the Radnice Basin at the end of deposition of the Lower Radnice Coal. 1. Basement palaeohighs; 2–6. Radnice Member strata: 2. Mostly fl uvial channel fi ll, 3. mostly clastic fl oodplain sediments, 4. fl oodplain with occasional mire development, 5. proven extent of long-lived mires, 6. assumed extent of long-lived mires; 7. Current basin margins; 8. Urbanised area; 9. Abandoned underground and opencast mines; 10. Boreholes to the basement; 11. Excavation loca-tion. (After Opluštil et al. 2009a).

Fig. 20: Characteristics of the Bělka tuff bed at the base of the Whetstone Horizon.

168 SDGG, Heft 82

Fig. 21: Characteristics of the Brousek, a redeposited volcaniclastics (tuffi te) in middle and upper parts of the Whetstone Horizon.

Fig. 22: Reconstruction of the peat-forming forest of the Lower Radnice Coal exposed in an excava-tion situated at the east-ern margin of the Ovčín opencast mine (Opluštil et al. 2009b).

SDGG, Heft 82 169

together with liana-like plants represents the most diverse part of the forest. Less diverse un-derstorey composed of calamites, medullosan pteridosperms and Psa-ronius tree fern displays patchy distribution pat-tern presumably related to density of the canopy. The highest species di-versity, however, dis-play liana-like plants and small herbaceous plants growing as ground cover and represented mostly by sphenophylls and small ferns (e.g. Corynepteris angustis-sima).

Stop 5 Bašta

Former opencast mine located in the Břasy coalfield in western margin of the Radnice Basin (WGS84 coordi-nates: 49°49’56.125”N, 13°34’7.558”E)

In this protected outcrop one can see a nearly a com-plete section of the Upper Radnice Coal preserved in a pillar left in a part of the former opencast mine to support a local road. The coal section exposed at this locality is about 4 m thick and contains the lower and middle part of the coal (Fig. 23). Not exposed in this outcrop is the basal part of the coal and about 1.5 m thick up-per part of the seam sec-tion missing due to denu-dation. The coal contains few partings of grey mudstone , however, the most apparent are two pale grey, volcanic ash bands (tonsteins), only a few centimeters thick, in a middle part of the coal section. These tonsteins can locally contain coal-bearing fl ora similar to

that one in the Bělka at the base of the Whetstone Horizon. These tonsteins are known from the entire Radnice Basin and correlate with similar volcanoclas-

Fig. 23: A: Distribution and thickness of the Upper Radnice Coal in the Břasy coalfi eld, Radnice Basin. B: The Upper Radnice Coal exposed the Bašta opencast mine. Two thin light bands represent tonsteins. C: Cross-section of the Břasy coalfi eld.

Fig. 24: Former Klementina opencast mine operating in the Břasy coalfi eld in the second half of the 19th century.

170 SDGG, Heft 82

tic beds in the Kladno coalfi eld located some 60 km NE of the Břasy coalfi eld. This indicates isochronous paludifi cation and onset of peat formation of the Upper Radnice Coal peat swamp across the whole Radnice Member depocentre, which consisted of a system of river valleys arranged into a dendritic pattern (Fig. 4).

The Upper Radnice Coal in the western part of the Radnice Basin (Břasy coalfi eld) commonly reached thickness of 7 – 9 m, with local thicknesses up to 14 m. The coal was exposed in outcrops around the western edge of the coalfi eld thus providing ideal conditions for opencast mining (Fig. 23, 24). Mining started here at the end of the 18th century, but reached its maximum extent in the 19th century and gradually ceased dur-ing the fi rst half of the 20th century. Ovčín opencast mine operating in the second half of the 20th century was the last episode of opencast mining activity in the Radnice Basin (Fig. 16).

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