GeolCarp_Vol68_No3_269

GEOLOGICA CARPATHICA

, JUNE 2017, 68, 3, 269 – 281

doi: 10.1515/geoca-2017-0019

www.geologicacarpathica.com

Significant hiatuses in the terrestrial Late Variscan Central

and Western Bohemian basins (Late Pennsylvanian–Early

Cisuralian) and their possible tectonic and climatic links

KAREL MARTÍNEK, JIŘÍ PEŠEK and STANISLAV OPLUŠTIL

Institute of Geology and Palaeontology, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic;

karel.martinek@natur.cuni.cz, jiri.pesek@natur.cuni.cz

(Manuscript received September 3, 2016; accepted in revised form March 15, 2017)

Abstract: Significant changes in the stratigraphy of the Central and Western Bohemian Upper Palaeozoic basins occur

during or shortly after hiatuses. The different extent and changes in the depocentres of the Radnice and Nýřany members

(Moscovian) in the Plzeň Basin clearly indicate changes in the structure of this basin taking place during a break in

sedimentation between these two units (311.9–308.3 Ma). Thick weathered rocks that occur in boreholes in the Mšeno–

Roudnice Basin indicate another sedimentation break (305.9–304.1 Ma) between the Nýřany Member and the Týnec

Formation (Kasimovian). Another possible hitherto undiscovered hiatus occurred between the Týnec and Slaný formations

(Kasimovian–Gzhelian, about 304–303 Ma). The most significant changes in the configuration of the basins occurred

between the Slaný and Líně formations (Gzhelian–Asselian, 301.6–300.6 Ma). This is indicated by deeply cut river

valleys at the top of the Slaný Formation, by high thickness of weathered deposits occurring immediately beneath the

Líně Formation, and mainly by the shift of depocentres from the southern to the northern part of the Central Bohemian

basins. The hiatuses between the Radnice and Nýřany members and between the Slaný and Líně formations are

accompanied by significant changes in the depocentres, and they are therefore interpreted primarily as tectonic events

related to the extensional collapse of the Variscan orogenic belt. By contrast, the hiatuses beneath and above the Týnec

Formation are interpreted as being the products of lower sedimentation rates during drier climates, which is consistent

with the characteristics of the sediments, correlation with other Central European basins, and with climate models for this

period. Due to the characteristics of the Líně Formation, in which the effects of climate aridization are clearly seen, the

authors presume that tectonic as well as climatic changes occurred at the hiatus between the Slaný and Líně formations.

Keywords: Pennsylvanian, Cisuralian, terrestrial basins, hiatuses, climate changes, tectonics, weathering.

Introduction

The Bohemian Massif is the easternmost and also the largest

block of the eastern branch of the Variscides. This area was

formed during the Carboniferous as a result of the collision of

Baltica (the eastern branch of Laurasia) and Gondwana, when,

during simultaneous rotation, the new supercontinent Pangea

was formed (Kearey et al. 2009). Variscan processes of con-

siderable intensity occurred in the territory of present-day

Central Europe because of its location in the collision zone.

The emerging Variscan orogeny also incorporated older, par-

ticularly Cadomian consolidated blocks such as the Vosges,

the Black Forest and the Bohemian Massif (Kroner et al.

2008). The regressive nature of Middle Devonian sediments

and subsequent termination of sedimentation in the Prague

Basin reflect the onset of the Variscan Orogeny, which culmi-

nated in the Bohemian Massif in the Late Visean. At that time,

folding of Cambrian to Early Namurian (Serpukhovian) sedi-

ments occurred in this area (McCann et al. 2008). The youn-

gest deposits that were folded in the Bohemian Massif are the

sediments, tuffs and tuffites of the Ostrava Formation in the

Czech part of the Upper Silesian Basin (folded as early as

between the Lower and Middle Namurian, comp. Dopita et al.

1997), which is a foreland basin partially filled with paralic

deposits. The other Late Palaeozoic basins in the Czech

Republic are terrestrial intermontane basins, which formed

primarily in an extensional or transtensional regime.

Arthaud & Matte (1977) deduced that these types of basins

formed as a result of the extensional collapse of the Variscan

orogenic belt after the collision of Gondwana and Baltica,

which reactivated older NW–SE oriented fault structures

transforming them into normal faults and predominantly

right-lateral strike-slip faults. According to Ziegler (1990),

the formation of the Late Palaeozoic terrestrial basins of the

Bohemian Massif are probably also related to the movements

along these structures and their conjugate equivalents. The fill

of these basins never underwent significant folding. Initially,

the thickness of deposits was usually more or less influenced

by the unevenness of the basin basement. Later, interruptions

in sedimentation of variable duration may have occurred in

response to the Variscan post-orogenic tectonics and climatic

changes.

The terrestrial basins of the Czech Republic include the

Western Bohemian, Central Bohemian, Sudetic (Lusatian) and

Brandov basins, Blanice and Boskovice grabens, and Late

Palaeozoic relics preserved in the surroundings of nearly all

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basins (Fig. 1). The Western Bohemian basins include the

Plzeň Basin, Manětín and Žihle basins and the small Radnice

Basin (see Fig. 2). The first three basins, now more or less

independent, were undoubtedly connected at least from the

Asturian (Moscovian), and a connection to the Radnice Basin

cannot be ruled out (Pešek et al. 1998). The Central Bohemian

basins include the Kladno–Rakovník and Mšeno–Roudnice

basins, which pass into one another. In the west, the fill of the

Kladno–Rakovník Basin is separated only formally from the

Žihle Basin, whereas in the east, the fill of the Mšeno–

Roudnice Basin passes into the Mnichovo Hradiště Basin in

the Sudetic (Lusatian) region. Individual basins are thus deli-

mited and named merely historically. The use of the term

“basin“ is purely formal and not based on basin structure, as

the all basins mentioned above represent a single gradually

developing sedimentation basin.

Most of the Central and Western Bohemian basins are filled

with the sediments comprising all four formations — the

Kladno, Týnec, Slaný and Líně formations of Duckmantian to

Autunian age (Moscovian–Asselian) (Table 1). The Kladno

and Slaný formations are subdivided into several lithostrati-

graphic units. The formation of these basins was influenced

mainly by two deep-seated faults — the Central Bohemian

Fault to the south of the Central and Western Bohemian basins,

which has been active since the beginning of Langsettian/

Westphalian sedimentation, and, with a slight delay, the

gradually increasingly active Litoměřice Fault to the north of

all these basins (Havlena & Pešek 1980). The works by Pešek

(1994, 2004) provide a detailed description of the lithostrati-

graphic units, their age and the evolution of the above-

mentioned basins. Due to the relatively small area of the

Manětín and Žihle basins, and to the lower level of knowledge

regarding these basins, this study deals only with the Plzeň

Basin and also with the Kladno–Rakovník and Mšeno–

Roudnice basins.

Methods and data

The information presented in this paper is based mostly on

boreholes drilled from the surface into the basement of the

Pennsylvanian after 1945 mainly during the prospecting for

and exploration of coal deposits — specifically around 400

bore holes in the Plzeň Basin, about 200 boreholes in the

Kladno–Rakovník Basin, and roughly 60 boreholes in the

Mšeno–Roudnice Basin. The boreholes in the Plzeň and

Kladno– Rakovník basins were drilled primarily by the em-

ployees of the former companies Uhelný průzkum (Coal

Exploration), later known as Geologický průzkum, n. p. (Geo-

logical Exploration, national enterprise) and Geoindustria, n. p.

Fig. 1. Upper Palaeozoic continental basins. After Chlupáč & Štorch (1992). Upper right: Simplified geological map of the western part of

the Czech Republic without Quaternary cover. Modified after Chlupáč et al. (2002). 1 — Neogene, 2 — Palaeogene, 3 — neovolcanic rocks,

4 — Upper Cretaceous, 5 — Upper Palaeozoic, 6 — Lower Palaeozoic, 7 — Neoproterozoic, 8 — granitoids, 9 — orthogneisses, 10 — different

types of basic rocks, 11 — granulites, 12 — Moldanubian Crystalline Complex, 13 — overthrust and normal faults.

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The boreholes in the Mšeno–Roudnice Basin, whose fill is

completely covered by Upper Cretaceous sediments, were

drilled in the 1960s by the former Central Geological Institute

(known today as the Czech Geological Survey). These were

followed-up in the 1970s and 1980s by the boreholes of

Geoindustria, n. p. The second author of this paper, in

parti cular, has had an opportunity to study most of the bore-

holes as they were being drilled in the Plzeň Basin in the late

1950s and in all of the above-mentioned basins in the 1960s

and later.

The boreholes in the Plzeň and Mšeno–Roudnice basins are

distributed unevenly but cover a greater part of these basins.

This provided detailed information regarding the characteris-

tics and development of the deposits of the Plzeň Basin, pri-

marily in the Radnice and Nýřany members.

Due to tectonic development thicker sections

of the deposits of the remaining units, namely

the Týnec, Slaný and Líně formations, were

preserved only in a much smaller area of this

basin (see App. II in Pešek 1994). The Radnice

Member is absent in the Mšeno–Roudnice

Basin except for a small area in the southwest

because the deposition of sediments in this basin

began mainly in the Nýřany Member. In the

Kladno–Rakovník Basin, which began to be

filled around the same time as the Plzeň Basin,

most of the boreholes were drilled in the

southern part of the basin. The Radnice Mem-

ber occurs only rarely in the remai ning part of

the Kladno–Rakovník Basin. That is why the

data on its thickness and characteristics are suf-

ficient but less detailed when compared with

the Plzeň Basin. By contrast, the deposits of the

Týnec, Slaný and Líně formations in the

Kladno–Rakovník as well as Mšeno–Roudnice

basins provide far better evidence regarding

the characteristics and significant changes in the evolution of

these basins. The data from all the above-mentioned boreholes

and even older boreholes were used to compile isopach maps

as well as other maps, for example, in the papers of Havlena &

Pešek (1980), Pešek (1994) and Pešek et al. (1998).

Hiatuses and their depositional aspects

This study follows the publication of Opluštil et al. (2016)

and focuses primarily on the significant changes in sedimenta-

tion that occurred during hiatuses in individual basins. It also

documents some other possible interruptions in sedimentation,

which are more likely of local significance.

Two significant interruptions in sedimentation are thus far

known in the Central and Western Bohemian basins: the hiatus

between the Radnice and Nýřany members, namely between

the Bolsovian and Asturian, as indicated by a macrofloral

studies, for example, by Němejc (1937), recently by Opluštil

et al. (2016) and the hiatus between the Slaný and Líně forma-

tions, namely between Stephanian B and C as reflected, for

example, in drill cores by the presence of weathered rocks at

the contact of these two units (e.g., Havlena & Pešek 1980;

Bosák 1991). It can also be inferred from a significant shift of

the depocentres from the southern part of the basin to the north

(Pešek 1994). A study of detrital zircons of the volcaniclastic

rocks of all the units of the Central and Western Bohemian

basins was conducted by Opluštil et al. (2016). In addition to

confirming the existence of hiatuses between the above-men-

tioned units, they identified or verified other thus far mostly

unknown or only presumed but not reliably documented

hiatuses and their duration (Table 1). These authors revealed

a previously unknown hiatuses between the Lower and

Upper Radnice members, between the Nýřany Member and

Fig. 2. Relationship of the oldest unit, the Radnice Member, to the

geology of the basement. After Pešek (1994). Basins: PB — Plzeň,

RB — Radnice, MB — Manětín, ŽB — Žihle, KRB — Kladno–

Rakovník, MRB — Mšeno–Roudnice.

Table 1: Stratigraphic units of the Central and Western Bohemian basins.

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formations, which were verified by acoustic logging

(Svrčinová in Žbánek et al. 1990 a, b; Bosák 1991) in the bore-

hole Sč

1 (Semčice) in the Mšeno–Roudnice Basin (Fig. 4a).

The acoustic logging was unfortunately not used in other bore-

holes due to technical reasons, so the authors of this study used

the data on the dip of the mudstone beds that were obtained in

the boreholes drilled in the Mšeno–Roudnice Basin only

(Fig. 4b). The existence of these hiatuses is also indicated by

the weathered rocks (i.e primary grey sediment colour is highly

altered by motting) at the top of the Nýřany Member and

primarily by those of the Slaný Formation (Fig. 5) and also by

numerous erosional features in the upper part of the Slaný

Formation (Fig. 6) that were previously discovered by Havlena

& Pešek (1980) and specified, for example, by Skopec et al.

(1990 and 2001). All the data shown in Figures 4 a, b and 6

were gathered mainly from boreholes drilled in the 1960s to

1980s in the Mšeno–Roudnice Basin.

Relationship between the basement and the basal unit —

the Radnice Member

The Radnice Member, of Duckmantian to Bolsovian

(Moscovian) age, occurs in the area of the Plzeň, Kladno–

Rakovník and Radnice basins (Figs. 1 and 2). It is also known

from small occurrences in the Žihle and Mšeno–Roudnice

basins and also from a series of small relics that lie in a NE–SW

band south of the Central and Western Bohemian basins.

The Radnice Member is the most explored unit of this Upper

Palaeozoic complex due to a number of boreholes and coal

mines that verified its coal–bearing capacity. The influence of

the basement on the extent and characteristics of the deposits

of this unit can only be studied in the areas of the Plzeň and

Kladno–Rakovník basins. Opluštil (2005) made a detailed

reconstruction of the river network, of the extent of peatlands

and of other specific environments in the Kladno–Rakovník

Basin. The lithological characteristics of the deposits of this

unit are relatively well-known and also its spatial and tem-

poral changes. It is presumed that the present extent of the

Radnice Member is only a relic of the larger area originally

covered (see App. 35 in Pešek et al. 1998; Opluštil 2005).

Plzeň Basin

According to Pašek & Urban (1990), the Plzeň Basin is

an extensional basin. The so-called central depocentre (Pešek

1968), in which by contrast to its surroundings all four forma-

tions were largely preserved, runs approximately through the

basin centre in NNE–SSW direction. The central depocentre

also predisposes the NNE–SSW elongation of the whole basin.

The Radnice Member (Fig. 3) occurs in approximately one

half of the basin area (e.g., Dvořák 1960; Havlena & Pešek

1980; Pešek 1994). Folded rocks of the Teplá–Barrandian Neo-

proterozoic with NE–SW oriented axes occur in the basement

of the predominantly volcaniclastic fill of the Plzeň Basin.

Bands of variously resistant weakly metamorphosed shales

and greywackes alternate with isolated intercalations of

palaeobasalts and palaeobasaltic tuffs. Erosion-resistant grey-

wackes and palaeobasaltic rocks form ridges mostly only

a few tens of metres high, which protected peatlands evolving

in depressions from the input of coarse-grained clastic material.

As a result a sediment shadow developed (Pešek 1968), in

which several (and rarely up to 14) metres thick seams were

formed and mostly grey claystones and siltstones as well as

tuffs and tuffites were deposited. The fill of these mainly ero-

sional palaeo-valleys is generally only a few tens of metres

Fig. 4. a — Dip directions and dips of mudstones in the borehole Sš 1

based on acoustic logging. After Svrčinová in Žbánek et al. 1990a.

b — Bedding dips of mudstones in the Líně, Slaný and Týnec

formations and Nýřany Member in drill cores from the Mšeno–

Roudnice Basin. Based on numerous unpublished drilling reports.

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thick. These depressions are divided into two parts by the

NNE–SSW oriented central depocentre. It may have origi-

nated either as a deep river valley of unknown age formed by

erosion or as a tectonically formed synsedimentary structure.

In the Radnice Member, it is filled mainly with up to more

than 250 m thick coarse clastic rocks — subarkoses and

conglomerates. Coal seams are either entirely absent or only

a few tens of centimetres thick.

Kladno–Rakovník Basin

The Kladno–Rakovník Basin is elongated in a roughly ENE

direction and tectonically bounded to the north. By contrast,

its boundary with the surrounding units to the south and west

is mostly erosional. In the east, it passes into the siliciclastic

fill of the Mšeno–Roudnice Basin. The Radnice Member fills

only about one-third of the area of the Kladno–Rakovník

Basin. Even though it is, as in the Plzeň Basin, underlain

mostly by folded weakly metamorphosed Neoproterozoic

shales and greywackes of the Teplá–Barrandian area, it was

verified that it contains several larger bodies of palaeobasalts

and palaeobasaltic tuffs as well. The basement also comprises

the granites of the Čistá Massif, which continues to the north-

east. At the northwest of the basin (Fig. 1), the basement is

formed by phyllites, schists and schistose gneisses of the

Krušné hory (Erzgebirge) Crystalline Complex. A narrow

body of amphibolites of the Mariánské Lázně Complex

extends into the basement in the west.

Two significant depocentres were active in the Radnice

Member. The first is the Rakovník depocentre in the vicinity

of the town of Rakovník, which is narrow and elongated in

a NNE direction. It is bounded to the west by the granites of

the Čistá Massif (Fig. 1). This is probably the fill of a pre-Late

Westphalian river valley that cut not only into the Neoprotero-

zoic shales and greywackes but also into the palaeobasalts and

palaeobasaltic tuffs. It comprises a complex of up to more than

260 m thick mostly grey clastic rocks, in which coal seams are

either entirely absent or which contain coal seams only a few

tens of centimetres thick. Unlike the central depocentre of the

Plzeň Basin, this structure was probably not active during the

deposition of other lithostratigraphic units. By contrast,

the second depocentre, which lies northeast of the Rakovník

depo centre and which is elongated in a NW direction and

referred to by Havlena & Pešek (1980) as the Zlonice–Peruc

depocentre (and by Opluštil 2005 as the Třtěno–Zlonice

depocentre), is a structure that is apparently bounded by

NW–SE trending faults. In addition to the relatively thick

Radnice Member, the more than 500 m thick deposits

of the Nýřany Member (see below) were preserved in this

structure as well. The extensive, nearly E–W oriented valley,

which was filled with coal- bearing deposits, begins near

the village of Petrovice west of Rakovník and extends as far as

the surroundings of Otvovice near Kladno in the east of this

basin (Opluštil 2005). This author rightly presumes the exis-

tence of a number of E–W to NW–SE oriented tributary

valleys.

Hiatus between the Radnice and Nýřany members

The existence of the hiatus between the Radnice and Nýřany

members was already known at the end of the 19

century, as

is evident from the palaeobotanical research of Frič (1879) or

Purkyně (1899) and from a number of geological and palaeon-

tological papers published in the past and in this century (e.g.,

Němejc 1937; Pešek 2004, 2005). Until now, the duration of

this hiatus has only been the subject of speculation. However,

it was probably a longer break in sedimentation as indicated

by, among other things, the fact that, with the exception of

local areas, nearly the entire upper part of the Radnice Member,

including the Lubná seams, is absent in the Plzeň Basin or by

the fact that, for example, in the former mines Důl Obránců

míru and Důl v Týnci in the Plzeň region, the basal clastic

rocks of the Nýřany Member lie directly on the Upper Radnice

Seam, which was locally partially or completely eroded

(Fig. 7). In addition, the logs of some boreholes in the Kladno

area (e.g., Bř

1 and 2 near Beřovice, Br

5 near Třebusice) sug-

gest that a larger section or even the entire upper part of the

Radnice Member was eroded during the hiatus. The study of

the palaeosol in this unit indicates several stages in the deve-

lopment of vertisols, which had to take at least several tens of

thousands of years to form (Opluštil et al. 2015). When Opluštil

et al. (2016) dated the detrital zircons in the volcanogenic inter-

layers of all formations, they also documented, among other

things, the age of the Kladno Formation and determined the

dura tion of the hiatus between the Radnice and Nýřany mem-

bers (about 3.6 Ma), which reflects the Leonian Phase of the

Variscan Orogeny. Their research suggests that as many as seve-

ral hundred metres of deposits of the Radnice Member may

have been eroded prior to the deposition of the Nýřany Member.

Fig. 7. Erosion of the Upper Radnice Coal Seam. Former Mine

Obránců míru, Plzeň Basin. From Pešek (1978).

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Significant tectonic reorganization of the Central and

Western Bohemian basins occurred during the break in sedi-

mentation between the Radnice and Nýřany members. For

example, in the Plzeň Basin, the depocentres changed signifi-

cantly (Fig. 3). They were elongated in a NNE–SSW direction

during the deposition of the sediments of the Radnice Member,

but they are oriented in a NW–SE direction in the Nýřany

Member. Few boreholes have explored the sediments of these

units in the Kladno–Rakovník Basin, so that definite conclu-

sions cannot be made here. In the major part of the Mšeno–

Roudnice Basin, sedimentation began in the Nýřany Member.

Nýřany Member

The sediments of the Nýřany Member, of Asturian to Canta-

brian (Late Moscovian to Early Kasimovian) age, occur in all

Central and Western Bohemian basins and also in some Penn-

sylvanian relics, which flank these basins. In this unit, subar-

koses and mostly grey siltstones and claystones dominate over

tuffs, tuffites and seams occasionally up to 2.5 m thick.

A northward fining of sediments is noticeable in the Central

Bohemian basins (see App. VIII in Pešek 1994). The deposits

of the Nýřany Member cover a substantially larger area than

the preceding unit. They occur in the entire area of the Plzeň

Basin, cover more than half of the Manětín and Žihle basins,

and most of the area of the Kladno–Rakovník and Mšeno–

Roudnice basins. They occur locally in the Radnice area as

well. They are also known from a number of Pennsylvanian

relics, particularly west of the Plzeň and Manětín basins.

The varying age of the basal sediments of this unit, for

example, in the Plzeň area and the Mšeno–Roudnice Basin

indicates a gradual onset of their deposition.

Plzeň Basin

During the break in sedimentation between the Radnice and

Nýřany members, significant changes occurred in the struc-

ture of the Central and Western Bohemian basins (see above).

Due to the relatively detailed exploration of the Plzeň Basin,

it is possible to reliably clarify its tectonic reconstruction.

The central depocentre was still active in the basin and, in

addition, four NW–SE oriented tectonically predisposed indi-

vidual depo centres separated by narrow horsts were formed.

While the ave rage thickness of the Nýřany Member is less

than 290 m in the Plzeň Basin, it is a several tens of metres

greater in the synsedi mentary central depocentre. The sedi-

ments of this unit are also thicker in the NW–SE oriented

structures (Fig. 3), the most prominent of which is the depo-

centre in the south of the basin.

Kladno–Rakovník and Mšeno–Roudnice basins

Despite the fact that a large number of boreholes were

drilled into the basement in both basins (several hundred in the

Kladno–Rakovník Basin and about 60 in the Mšeno–Roudnice

Basin), the level of knowledge regarding the tectonic evolu-

tion of these basins during the deposition of the Nýřany Mem-

ber is slightly lower in comparison with the Plzeň Basin. How-

ever, it is very likely that similar changes occurred in the

Central Bohemian basins as in the Plzeň Basin. The Zlonice–

Peruc depocentre (see above) undoubtedly remained active

even though the trend of the axis of this depocentre differs

somewhat from that of the Radnice Member (Fig. 8). Its acti-

vity is indicated by a significant increase in the sediment

thickness of this unit. The average thickness of the mostly grey

sediments of the Nýřany Member is about 335 m in the

Kladno–Rakovník area, whereas their verified thickness in

this structure is greater than 500 m. A further increase in thick-

ness in a NW–SE direction, albeit considerably smaller in

comparison with the Zlonice–Peruc depocentre, is evident in

an area northwest of Rakovník. It cannot be excluded that this

is a narrow tectonically predisposed depocentre.

A relatively considerable decline in the thickness of the

sedi ments of the Nýřany Member is noticeable to the east in

the Mšeno–Roudnice Basin. While the average thickness of

the Nýřany Member in the Roudnice (western) part of this

basin is about 170 m, it decreases below 100 m in the Mšeno

(eastern) part of the basin. This decline is probably related to

the subsequent onset of sedimentation in this part of the basin

(see above). In both of these basins, the axes of the depocen-

tres of the Nýřany Member are clearly oriented in a roughly

E–W or ENE–WSW direction, which may be considered as

the oldest reflection of terrestrial sedimentation of the reacti-

vation of main basin boundary faults governing the sedimen-

tation along the structure of the Variscan Arc of the Bohemian

Massif (Havlena & Pešek 1980).

Hiatus between the Nýřany Member and the Týnec Formation

The notion that a hiatus occurred between these two units

was first expressed by Wagner (1977) based on a study of

flora. Its existence was also considered by Havlena & Pešek

(1980) based on further spreading of the sediments of the

Týnec Formation over the crystalline basement, and the

Fig. 8. Major depocentre axes showing shifts of maximum subsidence

areas.

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“... formation of a single Central Bohemian – Sudetic sedimen-

tation area ..., and the connection of the thus far independently

developing sedimentary basins ...”, namely the Central and

Western Bohemian basins on one side and the Sudetic (Lusa-

tian) basins on the other. The break in sedimentation between

the Nýřany Member and the Týnec Formation is also indicated

by occurrences of relatively massive conglomerate beds at the

base of the Týnec Formation in the Plzeň area. During this

hiatus, or prior to the deposition of the sediments of the Týnec

Formation, the tectonic movements that occurred may not

have been strong or they occurred rather locally because the

average dip of the siltstones of the Nýřany Member and Týnec

Formation differs by only 2 degrees (Fig. 4b). However, the

weathered rocks at the top of the Nýřany Member that were

identified from the descriptions of drill cores, for example,

from boreholes Je

1, MB

20, ŘP

1, Vs

1 and Vůj

1 from

the Mšeno-Roudnice Basin (Fig. 5) may provide evidence of

the break in sedimentation. In borehole Je

1, the weathered

rocks at the top of the Nýřany Member were up to about 15 m

thick. The existence of this hiatus and its duration of 1.8 Ma

was clearly documented by Opluštil et al. (2016).

Týnec Formation

The sediments of the Týnec Formation, of Barruelian to

Lower Saberian (Kasimovian) age, occur in all the Central and

Western Bohemian basins with the exception of the Radnice

Basin. Their thickness varies in individual basins due to the

varying subsidence rate of the basement. The deposits of this

unit are occasionally preserved in the Pennsylvanian relics

near several basins. However, the sediments of this unit do not

differ much petrographically from the preceding unit. By con-

trast, the usually pronounced red colours of mudstones are

distinctive for this unit. Beds of whitish conglomerates and

dark green mottled siltstones and claystones occur relatively

frequently in the Plzeň Basin. In comparison to the Nýřany

Member, the volcanic products and coals in the Central and

Western Bohemian basins occur rarely in the Týnec Forma-

tion. Carbonate concretions occur more or less frequently,

with abundant occurrences mainly at the top of this unit.

Plzeň Basin

The Týnec Formation was preserved in this basin only in the

central depocentre and in transverse NW–SE oriented grabens

that originated and were apparently still weakly subsiding

(Fig. 3). This is indicated by a slight increase in the thickness

of the deposits of the Týnec Formation in these structures, in

which they are up to 125 m thick, while their average thick-

ness slightly exceeds only 110 m in the entire basin.

Kladno–Rakovník and Mšeno–Roudnice basins

The sediments of the Týnec Formation fill roughly the same

area as the preceding unit, since they are absent particularly at

the western and eastern margins of the Rakovník part of the

Kladno–Rakovník Basin. Locally, however, they slightly

extend beyond the preceding unit and lie on the Neoprote-

rozoic basement. They are, however, absent along the entire

southern margin of the Central Bohemian basins, where this

unit was eroded. The average thickness of the sediments of the

Týnec Formation is about 175 m in the area of Kladno–

Rakovník Basin and probably 125 m in the Mšeno–Roudnice

Basin. However, mudstones occur more often in the

Mšeno– Roudnice Basin than in the Kladno-Rakovník Basin.

The Třtěno–Zlonice depocentre was active in the Týnec For-

mation in the east of the Kladno–Rakovník Basin, where the

deposits of this unit were up to 245 m thick, and a NW–SE

oriented depocentre was also active in an area north of

Rakovník with a roughly 200 m thick fill. In both of these

basins, the E–W axis of maximum subsidence more or less

follows a similar structure of the Nýřany Member (Fig. 8).

Hiatus between the Týnec and Slaný formations

A short-term, local rather than basin-wide break in sedimen-

tation between the two formations (Pešek 1994) cannot be

ruled out due to local differences in the thickness and other

characteristics of the Jelenice Member (the basal unit of the

Slaný Formation). The unit reaches a thickness of more than

100 m of predominantly coarse conglomerates, in borehole

Kbl

2 in the Mšeno–Roudnice Basin southeast of Mělník. By

contrast, the sediments of this member are only about 10 m

thick in the Plzeň Basin. This notion is also supported, among

other things, by the occurrence of numerous carbonate concre-

tions and signs of weathering (Fig. 5) at the top of the sedi-

ments of the Týnec Formation (e.g., boreholes KV

1 and MJ

in the Mšeno–Roudnice Basin) and also by slight differences

in the average dip of the siltstones of both units, for example,

in the boreholes in the Mšeno–Roudnice Basin (only two

degrees, Fig. 4 b). A hiatus duration of less than 1 Ma may be

inferred from the dating of volcaniclastic horizons (Opluštil et

al. 2016) assuming that the sedimentation rate for the Týnec

Formation is similar to that of the other units.

Slaný Formation

The deposits of the Slaný Formation, of Saberian (Gzhelian)

age, are known from all Central and Western Bohemian basins

with the exception of the Radnice Basin. The petrographic

characteristics of the basal unit, the Jelenice Member, do not

differ much from those of the sediments of the underlying

formation. There is a difference mainly in the colour of the

mudstones, which are mostly grey and in which a locally up to

more than 4

m thick bituminous coal seam occurs in the

majority of boreholes primarily in the Mšeno–Roudnice Basin.

One to two coal seams mostly less than 1 m thick were dis-

covered at the top of this unit in the Plzeň Basin and in the

Central Bohemian basins. Numerous volcaniclastic layers

mostly only a few cm thick, which suggest the resumption of

volcanic activity in source areas or which by contrast to the

preceding unit were better preserved due to a suitable environ-

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MARTÍNEK, PEŠEK and OPLUŠTIL

GEOLOGICA CARPATHICA

, 2017, 68, 3, 269 – 281

ment, occur in the Slaný Formation mainly in the Central

Bohemian basins.

A specific feature of this unit is the formation of an exten-

sive intrabasinal lake during the deposition of the sediments of

the Malesice Member, which probably extended well beyond

its present southern margin to the south in the Plzeň Basin.

In the east, the lake extended into the Intra-Sudetic Basin, or

even further northeastward into Poland through the Mnichovo

Hradiště and Krkonoše Piedmont basins. Previous notions that

the sediments of this unit were deposited in a relatively tran-

quil environment are probably not entirely correct. It cannot be

ruled out that the deposition of the Malesice Member may have

been, as indicated by geological and geophysical studies in the

Mšeno–Roudnice Basin, interrupted locally or across a wider

area several times (e.g., Bosák 1989; Žbánek et al. 1990 a, b;

Pešek 1994 and Lojka et al. 2014). An angular unconformity

was also reported in this unit in a man-made exposure of the

sediments of the Hředle Member east of the village of Pas-

tuchovice in the Kladno–Rakovník Basin (Lojka et al. 2014).

Plzeň Basin

The Slaný Formation was preserved in this basin only in the

central depocentre and in transverse grabens. However, it can-

not be clearly demonstrated that these structures were still

active during the deposition of the sediments of this formation.

The average thickness of the deposits of the Slaný Formation

is nearly 180 m. Despite the fact that only the remains of the

unit were preserved in this basin, a different, coarser, peri-

pheral development of the sediments of the Slaný Formation

was confirmed by several boreholes (e.g., near the villages of

Chotíkov and Tlučná) on the western as well as the eastern

basin margin (Havlena & Pešek 1980).

Kladno–Rakovník and Mšeno–Roudnice basins

As in the case of the preceding Kladno and Týnec forma-

tions, it is appropriate to characterize the deposits of the Slaný

Formation regarding both basins together. The sediments of

this formation fill roughly the same area as the preceding unit,

but they slightly exceed its extent to the north and lie on the

Neoproterozoic basement. However, in comparison to the

Týnec Formation, they are absent over a much larger area

along the entire southern margin of the Central Bohemian

basins, where they were eroded during the Permian or even

later. The average thickness of these sediments is around

175 m in the Kladno–Rakovník area and about 125 m in the

Mšeno–Roudnice Basin. As in the Týnec Formation, the sub-

sidence axis in this unit is also oriented approximately in

an E–W direction. The trend of isopachs clearly shows that the

thickness of the sediments of the Slaný Formation decreases

northward. By contrast, a significant increase in the thickness

of the deposits of this unit was documented in the Central

Bohemian basins in the Slaný area north of Kladno, and also

the Mšeno–Roudnice Basin in the wider surroundings of

Mělník, near Všetaty and Benátky nad Jizerou (Pešek 1994).

Figures 4 a and b clearly show the different dips and dip

directions of the mudstone beds of the Líně Formation, dipping

gently (ca. 6 degrees) northeast. While the sediments of Slaný

and Kladno formations dip steeply (ca. 10 degrees) mostly to

the southwest (Svrčinová in Žbánek et al.1990 a, b; Bosák 1991).

Hiatus between the Slaný and Líně formations

The break in sedimentation between these two units is asso-

ciated with the Intra-Stephanian phase of the Variscan Oro-

geny, which took place in most of the terrestrial basins of the

Czech Republic. Significant changes in source areas, for

example, the formation of new basins — the Blanice and

Boskovice grabens are also associated with this phase.

The existence of the hiatus between the Slaný and Líně forma-

tions is also indicated by the change of average dip of the mud-

stones between these two units (four degrees difference), by

the sediments of the Slaný Formation that were locally eroded

down to the Hředle Member (Fig. 6), by signs of weathering at

the top of the preserved deposits of the Slaný Formation

(Fig. 5), and also by the different characteristics and colours of

the sediments of these two units (see below). By reinterpreting

the seismic measurements of Kadlečík et al. (1985, 1990),

Skopec et al. (e.g., 1990, 2001) a roughly E–W direction of

a nearly 100 m deep river valley with several smaller tributaries

was discovered. Figure 6 clearly shows that the above-

mentioned stream flowed from the east or east-southeast to the

west, where in the boreholes in the east of the Mšeno–

Roudnice Basin it even eroded down into the Hředle Member

and all its overlying layers (i.e. the Ledec and Kounov mem-

bers). This palaeo-stream probably flowed further westward,

where the original thickness of the sediments of the Hředle

Member is preserved and where only the thickness of the

Kounov Member partially declines in the east of this basin.

The occurrence of weathered rocks of the uppermost part of

the Slaný Formation below the erosional surface of the Líně

and Slaný formations clearly demonstrates that the weathering

of the preserved deposits did not take place until after their

erosion (Fig. 5). The shift of the depocentre from the south to

the north is also very significant (Fig. 8). The duration of this

hiatus can be estimated at about 1 Ma based on the dating of

volcaniclastic horizons (Opluštil et al. 2016).

Líně Formation

The deposits of the Líně Formation, of Stephanian C to

Autunian (Late Gzhelian to Asselian) age (Opluštil et al.

2016), occur in all the Central and Western Bohemian basins

with the exception of the Radnice Basin. However, there are

significant, possibly primary, depositional differences in the

thickness of this unit in individual basins. The average thick-

ness of the Líně Formation is less than 50 m in the Plzeň

Basin, but it reaches about 245 m and nearly 500 m in the

Kladno–Rakovník Basin and Mšeno–Roudnice Basin, respec-

tively. The maximum thickness of the deposits of this unit

(over 1000 m) was verified in the Rakovník part of the

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, 2017, 68, 3, 269 – 281

Kladno–Rakovník Basin in the Žatec area north of Rakovník.

The Líně Formation is mostly dominated by red-coloured

mudstones with green circular spots. One so-called grey hori-

zon occurs in the formation in the Kladno–Rakovník Basin,

whereas up to three such horizons were discovered in the

Mšeno–Roudnice Basin. A bituminous coal seam up to several

tens of centimetres thick occurs in the bottom and middle hori-

zons. Numerous carbonate concretions, in which many mostly

thin volcanogenic intercalations were discovered, formed in

the entire complex.

Plzeň Basin

The sediments of this unit were preserved in the Plzeň Basin

only at the intersection of the NNE–SSW oriented central

depo centre and in some NW–SE oriented transverse grabens.

While the occurrence of the sediments was verified only in

three small relics north of the Mže River (northern basin half),

the largest area that they fill is located in the south of the basin

north of the village of Líně and east of the town of Nýřany (see

Dopita et al. 1985). By contrast to the Central and Western

Bohemian basins, the Plzeň Basin does not contain any grey

horizons or volcanogenic intercalations (see below) because

only a small thickness of the Líně Formation was preserved.

Kladno–Rakovník and Mšeno–Roudnice basins

As in the case of the preceding units, the sediments of the

Líně Formation were characterized regarding both basins

together. The sediments of the formation fill roughly the same

area as the preceding unit. However, they were verified also in

the west and north of the Kladno–Rakovník Basin, where they

locally overlie the Neoproterozoic basement. In the east of the

Mšeno–Roudnice Basin, Zikmundová & Holub (1965) dis-

covered an occurrence of conglomerates with pebbles of the

Barrandian type, which have thus far not been found in any

other lithostratigraphic unit. Breccias with a clayey matrix that

are thus far also not known in the older units of the Central

Bohemian basins occur at the northern margin of this basin.

As in the case of both preceding units, the subsidence axis in

the Líně Formation has an approximately E–W orientation as

well. However, unlike in the preceding units, it has shifted

northward (Fig. 8). In addition, a significant increase in the

thickness of the deposits of this formation is noticeable in

a NNE–SSW oriented depocentre in the west of the Rakovník

part of the Kladno–Rakovník Basin. According to Elznic et al.

(1974), the orientation of this structure is similar to the elonga-

tion of the central depocentre of the Plzeň Basin. An increase

in the thickness of Tertiary clastic rocks in the Žatec area north

of Rakovník suggests that this structure was apparently active

later on as well.

Hiatuses in the Líně Formation

By reinterpreting seismic profiles in the Mšeno–Roudnice

Basin, Skopec et al. (2001) discovered several river valleys at

around 100, 200 and 300 m above the base of this unit. It can

be presumed that, at these levels, a local break in sedimenta-

tion occurred and that hitherto unknown river valley up to

more than 100 m deep were formed. They were oriented in

a roughly E–W direction that is more or less identical to the

direction of the main stream, which erodes the uppermost

lithostratigraphic units of the Slaný Formation. These valleys

were filled with mostly coarse clastic sediments.

Discussion and conclusions

The activity of fault structures and deep fault zones oriented

mainly in a NE–SW and NW–SE directions led to the forma-

tion of the Western and Central Bohemian basins. Our work as

well as a number of previous studies, for example, by Havlena

& Pešek (1980), Pešek (1994) and Pešek et al. (1998) suggest

that, throughout the filling of the Western and Central Bohe-

mian basins, the Central Bohemian Fault Zone was conside-

rably active and that the Litoměřice Fault, or also the Jáchymov

Fault, gradually became more active as well. Their activity is

indicated by the quantity and quality of the material transpor-

ted into these basins, primarily into the Central Bohemian

basins from the south, north and west. The greatest amount of

clastic rocks were undoubtedly transported from the south.

The northward shifting of the depocentres of the units of the

Central Bohemian basins is related to the continuous uplift of

the southern source area, so that the Nýřany Member and all

other subsequent units do not have a peripheral but more or

less mid-basin position.

The tectonic evolution of the Plzeň and Central Bohemian

basins was completely different, with the exception of the

Radnice Member. The sediments of this unit were deposited in

the central depocentre in the Plzeň Basin and in the NNE–

SSW oriented Rakovník depocentre as well as in NE–SW

orien ted depressions. The latter are very likely morphological

depressions, which formed due to the differential erosion of

weakly metamorphosed Neoproterozoic shales, greywackes

as well as palaeobasalts and their tuffs. These depressions are

generally very productive. They contain several-metre-thick

bituminous coal seams accompanied mostly by mudstones and

volcaniclastic rocks only a few metres to a few tens of metres

thick. By contrast, the origin of the central depocentre in the

Plzeň Basin remains unclear. It cannot be excluded that it was

originally an old river valley of unknown age. The more than

250 m thickness of the non-coal-bearing mostly coarse clastic

rocks of this unit, the increase in the thickness of other units,

and the postsedimentary recurrence of movements in this

structure clearly demonstrate that this structure was repeatedly

tectonically active.

The geological structure of the Plzeň Basin has two domi-

nant features, the central depocentre with a NNE–SSW direc-

tion, in which also the sediments of the Líně Formation,

among others, were locally preserved and the roughly NW–SE

oriented transverse grabens, separated by narrow horsts.

The sediments of the Slaný Formation occur in these transverse

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GEOLOGICA CARPATHICA

, 2017, 68, 3, 269 – 281

grabens and also locally in the central depocentre in areas

where the deposits of the Líně Formation were not preserved.

In both cases, these are tectonically active synsedimentary and

postsedimentary structures, which is demonstrated by

an increase in the thickness of their fill when compared with

the thickness of deposits of the same age located outside these

depocentres. In view of the fact that only the sediments of

Týnec Formation were preserved in several transverse grabens,

it is evident that the movements in these transverse structures

were undoubtedly younger than in the central depocentre.

The Kladno–Rakovník and Mšeno–Roudnice basins form

a single accommodation space. The Kladno–Rakovník Basin,

for example, shares specific common features with the Plzeň

Basin with regard to the formation of the basal unit, the

Radnice Member. By contrast, the deposits of this unit are

absent in the Mšeno–Roudnice Basin with the exception of

minor areas. In addition, a NNE–SSW oriented depression

filled mainly with coarse-grained clastic sediments more than

260 m thick was discovered north of Rakovník in the Kladno–

Rakovník Basin. However, a significant increase in the thick-

ness of the deposits of younger units was not found. It is there-

fore possible that this is only the fill of an erosional valley,

which was however no longer tectonically active later on.

An erosional valley oriented in a roughly NNE–SSW direction

exists in the Kladno–Rakovník Basin as well. However, given

the extent of the basin, it is considerably longer than the length

of morphological depressions in the Plzeň Basin. The tectoni-

cally predisposed and undoubtedly repeatedly active NW–SE

oriented Zlonice–Peruc depocentre is completely different. In

this structure, the thickness of the Radnice and Nýřany mem-

bers and also that of the Týnec Formation increases conside-

rably. In the Nýřany Member and Týnec Formation, a NW–SE

oriented structure was active in the area north of Rakovník as

well. This is indicated by an increase in the thickness of the

deposits of these units in comparison with the surroundings.

While being more or less identical in the Nýřany Member and

in the Týnec and Slaný formations, the axis of maximum sub-

sidence is shifted northward in the Líně Formation. This

change is probably related to significant changes in the evolu-

tion of the basins and source areas caused by the Intra-

Stephanian phase of the Variscan Orogeny and apparently also

by the uplift of the core of the Bohemian Massif. A NNE–SSW

oriented structure, which lies in the elongation of the central

depocentre of the Plzeň Basin to the north, was active in the

west of the Kladno–Rakovník Basin as well. Unlike the Plzeň

Basin, this structure was still active in the Miocene as indica-

ted by an increase in its sedimentary fill during this period in

the North Bohemian Basin.

Two hiatuses associated with a major shift of the depo-

centres between the Radnice and Nýřany members and between

the Slaný and Líně formations are interpreted here as being the

product of the tectonic reorganization of the basins. By con-

trast, hiatuses below and above the Týnec Formation, where

substantial changes in depocentres did not occur and where

carbonate cementation was discovered in addition to fossil

weathering products, are interpreted as being mainly climati-

cally driven (comp. Roscher & Schneider 2006). It is very

likely that a stronger climate aridization occurred during this

period, which could have slowed down the input of clastic

sediments into the basins and so may have caused the hiatuses.

This is supported by the characteristics of the sediments of the

Týnec Formation (a predominance of red fine-grained clastic

rocks, abundance of carbonate cementation, a poor preserva-

tion of fossils), by correlation with other Central European

basins, and also by climate models for this period (Roscher &

Schneider 2006; Roscher et al. 2008).

In view of the characteristics of the Líně Formation, we pre-

sume that both tectonic and climatic changes occurred during

the formation of the hiatus between the Slaný and Líně forma-

tions, where the severe effects of climate aridization are clearly

evident. This hiatus corresponds to the Early Gzhelian dry phase

of Roscher & Schneider (2006), which could be traced across

the European Late Palaeozoic basins. But there is also evidence

for tectonic activity in the region, for example, in the Saale

and Saar-Nahe basins thick red bed sediments were deposited

during this interval. In the case of the Saale Basin a strong

tectonic event with basin reorganization is indicated between

the Rothenburg and Siebigerode formations (Schneider et al.

2005, 2006; Schneider & Romer 2010), approximately at the

level of the hiatus between the Slaný and Líně formations.

Acknowledgements: This study was supported by the Grant

Agency of the Czech Republic, project No. 16-24062S. Many

thanks to reviewers Pavel Bosák and Joerg Schneider who

helped to improve the manuscript. The authors thank Robert

Alger for English proof reading.

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