GEOLOGICA CARPATHICA, DECEMBER 2007, 58, 6, 497—510
www.geologicacarpathica.sk
Carbonate microfacies and depositional environments of the
Silurian—Devonian boundary strata in the Barrandian area
(Czech Republic)
FRANTIŠEK VACEK
Institute of Geology and Paleontology, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague, Czech Republic;
f.vacek@seznam.cz
(Manuscript received February 15, 2007; accepted in revised form June 13, 2007)
Abstract: The sedimentary record of the Silurian-Devonian (Pridoli—Lochkovian) boundary interval was assessed
using eight sections, which essentially represent two different environments and facies types. The local predominance
of bioclastic packstones to grainstones indicates the presence of relatively shallow marine environments, between the
upper carbonate slope and lower subtidal. The thickest limestone beds show locally wave ripple-bedding structures,
erosional surfaces, and hardgrounds with accumulated angular lithoclasts; these features may have been markers of
deposition above the storm wave base level. Slightly beneath these environments, the irregular rhythmites of the upper
slope contain erosional channels, which are also frequently associated with hardgrounds. Here, the debris-flow
material fills braided-channel systems and contains various clasts from the slope. Rhythmites deposited in relatively
deeper water are dominated by platy limestones: bioclastic and peloidal wackestones, mudstones. They represent
typical lower fan calciturbidites interbedded with basin-floor mudstones and shales. Very thin-bedded distal calciturbidites
occur here frequently. Normal-graded calciturbidite beds with horizontal lamination and current-ripple bedding have
large proportions of Tc and Td units of the Bouma sequence, the indication of lower flow-regime deposition.
Calcareous shale interbeds represent background hemipelagic sediments (Tep-Tet). Occurrences of several meters
thick layers composed of coarse-grained proximal calciturbidites with submarine mass-flow conglomerates are typical
of the lowermost part of the Lochkov Formation (Lochkovian). These deposits were accumulated near the carbonate
shelfbreak and on adjacent parts of the slope.
Key words: Silurian, Devonian, Barrandian area, carbonate microfacies, calciturbidites.
Introduction
The Barrandian area (particularly the Prague Synform,
central Bohemia) has been known as a classical region
for the study of Silurian and Devonian sequences. The
Global Boundary Stratotype Section and Point (GSSP) of
the Silurian-Devonian (Pridoli—Lochkovian) boundary
with auxiliary reference sections were established here
(sections at Klonk near Suchomasty and Karlštejn;
McLaren 1977). The detailed biostratigraphic, chemos-
tratigraphic, magnetostratigraphic and sedimentological
studies were concentrated primarily on these type sec-
tions (e.g. Chlupáč & Kukal 1977; Davies & MacQueen
1977; Paris et al. 1981; Kříž et al. 1986; Jeppson 1988,
1989; Hladil 1991, 1992; Hladil & Beroušek 1992;
Hladíková et al. 1997; Crick et al. 2001; Mann et al.
2001; Frýda et al. 2002; Brocke et al. 2002, 2006; Bug-
gisch & Mann 2004), whereas a number of other sections
have been investigated to a lesser extent. The present
study has been, therefore, focused on different sections
in different parts of the Prague Synform (Fig. 1) to assess
the basic facies variability of sediments and their ap-
proximate proportions (Figs. 2 and 3). From the strati-
graphical viewpoint, these eight outcrops were already
described in previous papers (e.g. Chlupáč et al. 1972),
but both the sedimentological characteristics and their
depositional context were sporadically reported (Čáp et
al. 2003) or unnoticed in the past.
Some preliminary results of microfacies and sedimen-
tological studies in this stratigraphic level were pub-
lished in a paper by Čáp et al. (2003). Nevertheless, only
three sections have been described (Požáry Quarry near
Praha-Řeporyje, Praha-Podolí, and Praha-Radotín – near
the Cement Plant). This paper contributed to more de-
tailed biostratigraphy (as cited in the following text).
Standard microfacies (SMF) sensu Wilson (1975) and
Flügel (1982) have been used for description of carbon-
ate rocks. Depositional settings have been vaguely inter-
preted according to association of SMF types and
idealized succession of facies belts.
In the presented paper these three sections have been
sampled in more detail for microfacies study and a new
sedimentological interpretation and model are intro-
duced.
Geographical and geological setting
The Prague Synform, principally an eo-Variscan defor-
mation structure, is situated in the central part of the Bar-
randian area between the cities of Prague and Beroun
(Fig. 1). Long stripes of folded limestone units dissected
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by brook valleys give good access to the Silurian-Devo-
nian (S-D) sequences. Eight S-D sections were selected to
exemplify the diversity of carbonate slope facies deposit-
ed both in shallower and in deeper environments (Fig. 1).
The S-D boundary is principally situated close to the
boundary of the Požáry (
~Pridoli) and Lochkov Forma-
tions (Fm). The boundaries of these two lithostratigraphic
units are slightly diachronous over the region, however.
The explanation of the extent and details of the diachro-
neity is directly related to particular aims of this study.
Two different facies developments may be distin-
guished. The shallower of these two contrastive deposi-
tion types, in the Lochkov Fm (
~Lochkovian), is the
Kotýs Limestone facies (Chlupáč et al. 1972). It occurs
mainly along the NW flank of the Prague Synform (e.g.
in areas between villages of Praha-Nová Ves and Tetín
near Beroun). Bioclastic, mostly crinoidal limestones
predominate (studied sections at the sites of Srbsko, in
the Opatřilka Quarry near Praha-Holyně, and Požáry
Quarry near Praha-Řeporyje; Fig. 1).
The second facies, the Radotín Limestone (Lochkov
Fm, Lochkovian), is distributed mainly along the SE
flank of the area. It corresponds to relatively deeper-water
depositional conditions. Blackish-grey coloured wacke-
stones to packstones alternate with calcareous shale inter-
beds as the rhythmites. They are often marked by the
presence of disseminated, laminated or nodular cherts. The
successional structure of rhythmites was locally disturbed
by the deposition of several meters thick accumulations of
crinoidal and cephalopod limestone beds located just be-
neath and above the S-D boundary, but principally in the
lowermost part of the Lochkov Fm. They belong to the
Scyphocrinites Horizon (Bouček 1936). In its typical
form, it is related mainly to stratigraphically condensed
but coarse-grained bioclastic sediments (e.g. sections at
Karlštejn, Praha-Radotín, and Praha-Podolí; Fig. 1).
The relatively pure carbonate facies contains a rich
benthic fauna, including crinoids, brachiopods and trilo-
bites. According to the occurrences of macropaleonto-
logical index species, the uppermost Silurian was
characterized by short-term increase in abundance and
rapid cessation of the trilobite Tetinia minuta. The first
appearance of the index trilobite Warburgella rugulosa
rugosa indicates the base of the Devonian, and concur-
Fig. 1. Position of studied localities in the Barrandian area: 1 – Požáry Quarry near Praha-Řeporyje, 2 – Opatřilka Quarry near Praha-
Holyně, 3 – Srbsko, 4 – Karlštejn, 5 – Radotín Valley – U topolů, 6 – Radotín Valley – near the Cement Plant, 7 – Praha-Podolí,
8 – Klonk near Suchomasty. The geological sketch map of the Prague Synform benefits partly (not completely) from the working
materials provided by R. Melichar and J. Hladil.
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CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
rently the base of the Lochkovian Stage (Chlupáč et al.
1972). The increased proportion of pelagic material in
deeper-water facies also corresponds to common occur-
rence of pelagic fauna, such as graptolites, cephalopods
and ostracods. The uppermost Silurian beds belong to
graptolite Zone Monograptus transgrediens, and the
base of the Devonian is marked by the first appearance of
the index graptolite Monograptus uniformis. In general,
graptolites and trilobites have very practical, although
approximate, use in the local S-D biostratigraphy, be-
cause the occurrences of chitinozoans and conodonts of
high-resolution potentials are also dependent to various
degrees on the facies, recycling of sediments and diage-
netic conditions. The onset of the index chitinozoa An-
gochitina chlupaci indicates the base of the Lochkovian,
while Urnochitina urna and Linochitina klonkensis ter-
minate at the S-D boundary (Paris et al. 1981; Brocke et
al. 2002). Recent studies of conodonts show problematic
establishment of biozonation in the uppermost Pridoli
and the lowermost Lochkovian, which causes problems
when the conodont-based boundary is correlated with
the S-D GSSP (e.g. Carls et al. 2007).
Methods
Although the prevailing part of the selected strati-
graphic sections have been published in classical pa-
pers where the intervals of beds (“beds”) were
arbitrarily numbered, a general re-numbering of these
“beds” was implemented. Nevertheless, the new num-
bers were introduced to prevent the confusion where
photographic documentation of the original state was
insufficient as original numbers are rarely preserved
(e.g. in the old Požáry Quarry or Praha-Podolí). There-
fore, new numbers were introduced for visible limestone
beds (some of them can consist of more beds with ob-
scure contacts; 1, 2, 3 etc.) and for shale interbeds (1/2,
2/3 etc.) in the S-D sections.
The study of sedimentary structures and microfacies
was based on the method of visible facies contrast.
Therefore, samples for microfacies analysis were collect-
ed selectively in order to represent all the main lithologi-
cal types and sedimentary structures. More than seventy
thin-sections were studied. Dunham’s (1962) classifica-
tion is used for description of macro- and microfacies.
Fig. 2. Lithological logs of the sections with predominance of bioclastic limestones. The Silurian-Devonian boundary is marked by the
first occurrence of index trilobite Warburgella rugulosa rugosa. The left columns show distribution of microfacies: MF1 – crinoidal
grainstones, MF2 – crinoidal packstones, MF3 – homogeneous packstones with sponge spicules, MF4 – cephalopod wackestones
with peloids, MF5 – peloidal wackestones/packstones. Dominating microfacies are indicated by title letters, microfacies designated by
small letters are present subordinately. For a detailed description of microfacies see the text.
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The orientation of conical cephalopod shells was
measured in several S-D sections to find the directions
of paleocurrents. For interpretation, it was generally
agreed that conical shells oriented their apices facing
mostly the current, also following the data related to dep-
ositional environments of limestones in the Barrandian
area (e.g. Petránek & Komárková 1953; Turek 1983;
Hladil 1992 or Ferretti & Kříž 1995).
Sections
Požáry Quarry near Praha-Řeporyje (Fig. 1.1)
The old abandoned quarry is located approximately
1 km to the east of Řeporyje village. The S-D boundary
strata are in the eastern face of the quarry close to the
southern end of the tunnel. The biostratigraphically de-
termined boundary lies within the sequence of bioclastic,
mostly crinoidal grainstones to packstones at the base of
bed 159 (Fig. 2, according to Čáp et al. 2003), where the
first Lochkovian index conodont Icriodus hesperius was
found in the lowermost part (Carls et al. 2007). The first
Warburgella rugulosa rugosa appears in bed 162. The
thickness of individual and visually discernible beds on
the quarry face ranges from 0.4 to 2.5 meters. Wave-rip-
ple bedding was found in beds 159, 161, and 162. Erod-
ed hardground covered by angular lithoclasts occurs in
the upper part of bed 160. Crinoidal debris with brachio-
pods and trilobites are the main components. Remains of
terrestrial plant Cooksonia sp. were reported from the up-
per part of bed 158 (Čáp et al. 2003). The interval of
beds 155—157 holds possible (diagenetically altered)
bioturbations visible both on bedding planes and in ver-
tical sections (Fig. 4F).
Opatřilka Quarry near Praha-Holyně (Fig. 1.2)
The boundary beds are exposed in abandoned quarries
on the left bank of the Dalejský Brook, in the distance of
about 300 m NNE of Praha—Holyně railway station.
The lower part of the section consists of thick-bedded
crinoidal, brachiopod, cephalopod packstones/grain-
stones (the interval of beds 1—4; Fig. 2). The overlying
part is characterized by thin-bedded fine-grained bioclas-
tic limestones (packstones). The upper part (beds 6—8) is
composed of thick-bedded, medium- to coarse-grained
crinoidal limestones. The S-D boundary is indicated by
the first occurrence of Warburgella rugulosa rugosa,
which enters the section at ca. 1 m above the base of
bed 6 (first reported by Chlupáč et al. 1972).
Srbsko (Fig. 1.3)
The S-D boundary beds crop out in the rock-wall Na
bříči on the right bank of the Berounka River north of
Srbsko village. The relevant part of the sequence is com-
posed of thick-bedded poorly-stratified crinoidal grain-
stones to packstones (Fig. 2) containing common benthic
fauna (brachiopods, crinoids, and trilobites). The S-D
boundary was indicated in the upper part of bed 3 by the
first occurrence of Warburgella rugulosa rugosa
(Chlupáč et al. 1972).
Karlštejn (Fig. 1.4)
The Budňany Rock section (Budňanská skála) near
Karlštejn village serves as the auxiliary stratotype of the
S-D boundary. The boundary is marked and labelled
within the rock face, at an exposed bedding plane on the
left side of the exposure. Its position corresponds to the
first appearance of Monograptus uniformis in the thin
shale interbed 19/20 (cf. Chlupáč et al. 1972). The first
Angochitina chlupaci occurs in bed 20 (first by Paris et
al. 1981), nevertheless Brocke et al. (2002) reported only
moderately preserved specimens not allowing detailed
biozonation and correlation. The more continuous sec-
tion on the right side of the exposure was selected for the
study. The upper part of the Požáry Fm (Upper Silurian,
Přídolí Series) is developed as alternation of dark mud-
stones to wackestones with calcareous shale interbeds
(Fig. 3). The thicknesses of limestone beds range from
0.05 to 0.25 m, and the thicknesses of shale interbeds
vary from 0.05 to 0.3 m. The overlying, more than 4 m-
thick accumulations of crinoidal and cephalopod pack-
stones/grainstones (beds 14—23), with horizons of
intraformational conglomerates (bed 16) belong to the
lower part of the Lochkov Fm. Amalgamation of coarse-
grained crinoidal packstone beds is common in the upper
part of this horizon (beds 20—23).
The following sequence consists of laminated wacke-
stone to packstone beds and calcareous shale interbeds
(beds 24 to 43, Radotín Limestone facies). Limestone
beds show normal grading, parallel horizontal lamina-
tion, and current ripple-bedding.
The orientation of shells of orthocone nautiloids was
measured in several beds. The preferred directions of
shell apices were as follows: bed 7 – S to SW, minor to
NE to E (40 shells have been measured); bed 11 – S to
SW, minor to N (34 shells have been measured); and
bed 19 – SW, minor to E (43 shells have been mea-
sured). It is in accordance with Kříž (1998), who reported
apparent current-driven orientation of cephalopod shells
in a SSW-NNE direction in beds 14—19 (or 40 and 41
sensu Chlupáč et al. 1972). The evident imbrication of
carbonate lithoclasts in the bed 16, however, cannot be
unequivocally interpreted due to unfavourable exposure
and strong protection of this outcrop as a Natural Monu-
ment (only one section is available).
Radotín Valley – U topolů (Fig. 1.5)
The boundary beds are exposed in an old railway
trench on the left bank of the Mlýnský Brook to the NW
of the Lochkov Cement Plant. The upper part of the
Požáry Fm is mostly covered. It consists of dark platy
limestones (mudstones to wackestones) alternating with
calcareous shales (beds 1—10; Fig. 3). The thickness of
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CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
limestone beds ranges from 0.05 to 0.2 m, shale interbeds
reach from 0.05 to 0.4 m. The first occurrence of
Monograptus uniformis angustidens in bed 9 fixes the S-
D boundary (Chlupáč et al. 1972). Study of Chitinozoa
indicates a considerable condensation of stratal thick-
nesses in the lower part of the section (Brocke et al.
2002). The overlying 1.8 m thick bank of crinoidal pack-
stones to grainstones (bed 11) corresponds to the base of
the Lochkov Fm. It is overlain by platy limestones
(wackestones to packstones) and shale interbeds
(beds 12—31) with a single bed of intraformational con-
glomerates (bed 18). Nodular cherts occur in bed 28.
Radotín Valley – near the Cement Plant (Fig. 1.6)
The section is situated in the road trench to the S of the
Lochkov Cement Plant. The upper part of the Požáry Fm
is covered (beds 1—8; Fig. 3). The thickness of individual
beds ranges from 0.15 to 0.25 m, and the thickness of
shale interbeds from 0.25 to 0.5 m. Limestone beds con-
tain poorly represented pelagic fauna (cephalopods and
ostracods). The lowest part of the Lochkov Fm consists
of a 1.2 m thick bank of crinoidal and cephalopod pack-
stones (bed 10) with abundant and more diversified fau-
na (e.g. crinoids Scyphocrinites with preserved large lob-
oliths, cephalopods, and bivalves; for detailed
description of bivalve communities see Kříž 1999). The
first occurrence of Monograptus uniformis in bed 9 indi-
cates the position of the S-D boundary (Čáp et al. 2003).
The apices of nautiloid shells in bed 10 are oriented
mainly to the NW with secondary N to NE maxima (55
specimens have been measured).
The overlying part of this stratal sequence developed as
fine-grained platy limestones (packstones) with cherts and
subordinate shale intercalations (beds 11—14; Fig. 3).
Praha-Podolí (Fig. 1.7)
The S-D boundary beds crop out in the northern face of
the former quarry of the Podolí Cement Plant. The
boundary is situated in shale interbed 6/7 according to
the first occurrence of Monograptus uniformis angus-
tidens (Chlupáč et al. 1972). The uppermost part of the
Požáry Fm (Pridoli and the lowermost levels of the Loch-
kovian Stage) consists of platy wackestones with shale
interbeds (beds 1—8; Fig. 3). The thicknesses of the lime-
Fig. 3. Lithological logs of the sections with predominance of platy limestones with shale interbeds. The Silurian-Devonian boundary is
indicated by the first occurrence of index graptolite Monograptus uniformis. The left columns show distribution of microfacies: MF2 –
crinoidal packstones, MF3 – homogeneous packstones with sponge spicules, MF4 – cephalopod wackestones with peloids, MF5 –
peloidal wackestones/packstones, MF6 –laminated wackestones/packstones, MF7 – homogeneous mudstones, MF8 – calcareous
shales with graptolites. Dominating microfacies are indicated by title letters, microfacies designated by small letters are present subordi-
nately. For a detailed description of microfacies see the text.
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Fig. 4. Sedimentary structures of the Silurian-Devonian boundary strata, details of rock faces and slabs: A – Typical example of current rip-
ple-bedding that usually marks the fills in systems of flat and braided channels on the carbonate slope. Praha-Radotín (U topolů), just above
the documented boundary section (Devonian, Lochkovian). B – Normally graded bed with horizontal lamination, little erosion and
fracturing the substrate and slight signs of current ripple-bedding; the position up is marked by the scale bar. Karlštejn, above the depicted
section (Devonian, Lochkovian). C – An example of subangular to partly rounded limestone lithoclast floating in crinoidal debris, from the
sample, which was taken close beneath the Scyphocrinites Horizon. Small parts of broken loboliths are present. Karlštejn, bed 16 (Silurian,
Pridoli). D – Deeply dissected hardground was covered by poorly sorted packstones to grainstones which contain a mixture of “floating”
angular and rounded lithoclasts. Rocks in the fissured, corroded and highly rugged hardground have stylolitic sutures, and the surface has thin
rims of dark colour, locally with pyrite. Požáry Quarry near Praha-Řeporyje, the upper part of the bed 160 (Devonian, Lochkovian). E – An
example of subaquaeous slump structure. Not all deformation structures observed in or around the S-D sections are of tectonic origin, al-
though the section Praha-Radotín (U topolů) lies in the area with many bedding-parallel faults and strong folding of the rocks. Photographed
above the selected S-D section (Devonian, Lochkovian). F – Completely altered parts in a thin-bedded, laminated and rippled succession of
beds (dyed). Strong alteration of these parts, accompanied by brownish coloured dolomitization with sharp fronts enlarged and covered the pri-
mary bioturbation (and bioerosion?) structures. Požáry Quarry near Praha-Řeporyje, the upper part of bed 156 (Silurian, Pridoli).
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CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
stone beds range from 0.1 to 0.25 m and the thicknesses
of the shale interbeds are slightly thinner (0.1 to 0.15 m).
The lower part of the Lochkov Fm is developed as massive
crinoidal and cephalopod packstones to grainstones
(beds 9—12). The fauna of the platy limestone beds is rep-
resented by cephalopods, ostracods, and graptolites, while
the massive bioclastic limestone beds contain crinoids,
cephalopods, trilobites, etc.
Klonk near Suchomasty (Fig. 1.8)
This section was not included to the detailed study,
but it is referred for comparison. The section at Klonk
was approved as the GSSP of the S-D boundary at the ses-
sion of the 24th International Geological Congress in
Montreal in 1972 (McLaren 1977). The S-D boundary is
marked by the first occurrence of Monograptus uniformis
in the upper part of bed 20 (Chlupáč et al. 1972; Fig. 3),
other index fossils such as trilobites, Chitinozoa, con-
odonts are used as auxiliary biostratigraphical indicators
(see the summaries about the stratigraphy of this section
– e.g. Chlupáč & Hladil 2000; Chlupáč & Vacek 2003).
Surface exposure consists of rhythmical alternation of
limestone and shale beds. The first sedimentological
study of Klonk interpreted the limestone—shale rhythms
as the result of cyclic changes in bioproductivity and
carbonate accumulation in a mostly pelagic depositional
environment (Chlupáč & Kukal 1977). Davies & Mac-
Queen (1977) regarded the entire stratal sequence as dis-
tal calciturbidites, but their interpretation was based
only on the observation of one sample taken from the
boundary bed 20. According to the latter authors, this
bed consists of a single turbidite layer with Tb and Tc
units of the Bouma sequence.
Results published by Hladil (1991, 1992) show that an
unequivocal turbidite interpretation is more complex
and partly incorrect; particularly the lower part of this
GSSP section is dominated by hemipelagic laminites, but
divided into segments by some beds of calciturbidites or
re-deposited, washed and drifted turbidite material. The
dominant direction of currents was from the NE, but oth-
er directions from the NNE, SSE and SW were represent-
ed as well. The rate of sedimentation was estimated to be
as high as
~20 meters per million years, and this rate al-
most certainly indicates a significant influx of turbidite
and drifted sedimentary material.
In 1999, a full-cored 57 m deep borehole was drilled
close to the surface outcrop. The core was compared with
the GSSP section on the surface and analysed by a num-
ber of methods: e.g. magnetostratigraphy (Crick et al.
2001), chemostratigraphy (Herten 2000; Kranendonck
2000; Frýda et al. 2002; Buggisch & Mann 2004), bios-
tratigraphy (Brocke et al. 2006).
Sedimentary and post-sedimentary structures
Parallel horizontal lamination is the most common
structure in the platy limestones. It is characterized by the
lack of bioturbation. Lamination can be recognized both
in weathered surfaces and thin or polished sections. The
thicknesses of laminae usually vary from 0.1 to 2 mm.
Current ripple-bedding appears both in the coarse- and
fine-grained limestones (Fig. 4A). It is usually small-
scale (1—1.5 cm in height, 3—5 cm in length, with a few
examples of decimeter sizes), and the upper limits are
erosional in some places.
Wave ripple-bedding is present in the Požáry Quarry
section (namely in beds 159, 161, and 162; Fig. 2), and
sporadically also in the Srbsko (bed 2) and Opatřilka sec-
tions (beds 1 and 4). The wave length is usually about
10 cm, the height does not exceed 3—4 cm (Fig. 5), al-
though some of the observed structures can indicate larg-
er ripple forms.
The normal grading occurs commonly in fine-grained,
platy-shaped limestone beds with smooth bedding sur-
faces (Fig. 4B). The beds have erosional bases, and in
many cases, the coarser-grained detritus deposited at the
base is followed by fine-grained limestones with a gradu-
al transition.
Two types of erosional surfaces can be described main-
ly within platy limestone beds. The first one corresponds
to the bases of coarser-grained portions of calciturbidite
sediment. The second one is identical with the sharp con-
tacts of limestone beds with underlying hemipelagic
shales. Diagenetically sharpened contacts between these
differently compacted materials (e.g. Hladil 1991), or
Fig. 5. Diagram of the upper part of bed 159 in the old Požáry
Quarry section. The structure and succession of beds, including
the wave ripple-bedding and eroded surfaces, are interpreted as
the evidence of storm-influenced deposition.
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smoothing due to small bedding-parallel shifts can also
be observed.
Lithoclasts of centimeter to decimeter dimensions and
intraformational conglomerates and/or breccias occur es-
pecially in the Karlštejn, Praha-Podolí, Praha-Radotín (U
topolů), and Požáry Quarry sections. In the Karlštejn
(bed 16) and Praha-Radotín (U topolů, bed 18) sections,
a number of elongated and considerably rounded lime-
stone clasts were also found “floating” in crinoidal de-
bris material (Fig. 4C). Beds of matrix-supported
carbonate conglomerates are massive without any visible
grading. The lateral extent of such beds is not more than
a few tens of meters, and then they are wedging or miss-
ing from the wall of the outcrop. Large lithoclasts are of-
ten imbricated. Many of them consist of dark grey
coloured, fine-grained crinoidal wackestones to pack-
stones. Clast sizes vary from 2 to 10 cm, in most cases. In
relatively shallow-water deposits, lithoclasts are mainly
angular, consisting of fine-grained, and strongly dolo-
mitized limestones (Požáry Quarry, the upper part of
bed 160; Fig. 4D).
Hardgrounds commonly occur in relatively shallow-
water bioclastic deposits as can be exemplified by the
old Požáry Quarry (in beds 158, 159, 160, and 162) or
the Srbsko section. Hardgrounds are often marked by
several millimeters thick limonitized laminas. Their lat-
eral extent does not exceed several meters in the respec-
tive bed. They might have been originally developed as
microbial pyrite crusts formed during periods of sedi-
mentary starvation. Hardgrounds developed in exempla-
ry forms occur in the old Požáry Quarry section in
bed 160; they are covered by accumulations of angular
lithoclasts, which show nearly identical lithologies if
compared with the dissected bottom of this bed.
Subaqueous slumps (Fig. 4E) occur mainly above the
measured and depicted part of the section in the Radotín
Valley, U topolů site. They were formed on a sea-floor
dipping in a low-angle due to sub-continuous soft-sedi-
ment downslope movements. The direction of transport
is approximately from the NW to SE.
The bioturbations were very rare. Their absence seems
to be typical for the platy limestones facies. In spite of
this scarcity, the ichnogenus Zoophycos sp. was reported
from the GSSP at Klonk near Suchomasty (Mikuláš &
Vacek 2003). In the old Požáry Quarry, bioturbations
were observed in the lower part of the S-D section (in the
beds 155, 156, and 157). Their depth is usually several
centimeters below the upper bedding plane. Burrow fills
are mostly dolomitized. Therefore, primary structures are
obscured: only the shapes or axes of bioturbations can be
distinguished (Fig. 4F).
Microfacies
According to thin and polished sections, eight carbon-
ate microfacies were distinguished, avoiding undue lev-
els of detail. The microfacies were mainly characterized
according to the content of skeletal (e.g. crinoids, cepha-
lopods, sponge spicules) and non-skeletal grains (e.g. pe-
loids), amounts of fine-grained sediment matrices and
sedimentary structures. These microfacies signatures
were used to verify the accuracy of the overall sedimen-
tological evidence related to facies zones.
MF1: crinoidal grainstones and grainstone-to-rudstone
are composed of slightly abraded or corroded crinoidal
columnals, branchials, and other ossicles (
~80 % of bio-
clasts), fragments of brachiopod and mollusc shells
(
~10 %), and trilobite carapaces (5—10 %; Fig. 6A). Other
bioclasts were less important, although fragmented bryo-
zoans, ostracods, and sponges are often involved in mi-
nor bioclastic components. They are locally abundant in
some thin laminas or lenses within the sediment. Micritic
to calcisiltitic fractions were commonly washed out. Syn-
taxial cements and dolomitization are common. The siz-
es of bioclasts range between 0.2 and 2 mm. Such
materials were found mainly in poorly-stratified bodies
with bed thickness of 25 to 35 cm. The typical examples
of this microfacies occur in the old Požáry Quarry section
(beds 160 and 162) and in the Srbsko section (e.g. the
lower part of bed 1).
MF2: crinoidal packstones consist of angular crinoidal
debris (
~80 %), often with sutured grain contacts
(Fig. 6B). Other bioclasts (e.g. ostracods and molluscs)
represent a minor constituent (10—20 %). Micritized bio-
clasts are common. The diameter of grains varies between
0.5 and 5 mm. The microfacies is particularly related to
poorly-stratified bodies with bed thicknesses up to 2.5 m.
However, sediments of this microfacies were also found
as thin intercalations in relatively finer-grained sedi-
ments. Such intercalations have commonly sharp ero-
sional bases and they tend to fine upwards.
MF3: homogeneous packstones with sponge spicules
(Fig. 6C) represent a fine-grained mixture of monoaxone
sponge spicules (
~30—40 %), ostracod shells (~30 %),
and crinoidal detritus (
~20 %). The diameter of bioclasts
is between 0.1 and 0.5 mm. The microfacies forms several
centimeters thick layers without marked stratification
separated by thin shale intercalations.
MF4: cephalopod wackestones with peloids (Fig. 6D)
contain large complete or fragmented cephalopod shells
(
~50 %) scattered in slightly recrystallized matrix with
peloids. Other components are represented by crinoidal
hash (
~40 %) and fragments of ostracod shells (~10 %).
Bioclasts are often micritized. Geopetal fillings are com-
mon. Preserved fragments of shells inside micritic pellets
suggest that many of the peloids were formerly bioclasts
transformed due to bacterially induced micritization. The
populations of mixed, 0.1—0.6 mm peloids are present
both in the matrix and inside the chambers of biomorphs,
such as the orthocone cephalopod shells.
MF5: peloidal wackestones/packstones are character-
ized by a fine-grained mixture of crinoidal debris (40—
50 %), molluscs (
~30 %), together with ostracod shells
(10—20 %), trilobite carapaces and peloids.
MF6: laminated wackestones/packstones (Fig. 6F)
consist of alternation of detrital laminas with crinoidal
detritus (
~30 %), ostracod (~40 %) and mollusc (~20—
505
CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
Fig. 6. Main microfacies types of the Silurian-Devonian boundary strata: A – Crinoidal grainstone. Crinoidal debris consists of abraded
clasts, which were altered to various degrees and in different way. Parts of calcite cements were replaced by small dolomite crystals. Požáry
Quarry near Praha-Řeporyje, bed 162 (Devonian, Lochkovian). B – Crinoidal packstone close to Scyphocrinites Horizon consists of
patchy accumulations of unaltered ossicles, which frequently have dissolution contacts. Fine-grained material between accumulated (and
partly also poorly disintegrated) ossicles contains small skeletal chips and fine-grained calcisiltitic material that is also dissolved. Praha-
Podolí, bed 7 (Devonian, Lochkovian). C – Skeletal packstone/grainstone shows a parallel (and imbricated?) arrangement of platelet-
shaped skeletal fragments and ostracod valves. This pattern is supported by common occurrence of sponge spicules. Praha-Radotín (near
the Cement Plant), bed 14 (Devonian, Lochkovian). D – Cephalopod wackestone with peloids contains various assortments of mixed
small grains. Calcisiltite particles are mixed with a variety of completely or partly micritized subangular and well-rounded particles of
0.1—0.3 mm in size. Karlštejn, bed 14 (Silurian, Pridoli). E – Relatively homogeneous mudstone formed from very fine-grained pelagic and
hemipelagic particles. The moderately recrystallized rock contains numerous spherical objects filled by calcite crystals; they usually belong to
mazuelloids (small) and prasinophycean algae (larger). Praha-Radotín (near the Cement Plant), bed 7 (Silurian, Pridoli). F – Slightly
irregular and wedging horizontal lamination of calcisiltites and packstones (both considerably recrystallized and/or dissolved). This section
shows the transition of laminated wackestone/packstone material into overlying calcareous graptolitic shale. Note chains of pyrite crystals in
the lighter bands, which were originally composed of coarser calcisiltite material. Praha-Radotín (U topolů), bed 24 (Devonian, Lochkovian).
506
VACEK
30 %) shells, mazuelloids (Muellerisphaerida), sponge
spicules, and micritic laminas with thickness in millimet-
ric scale. Dissolution seams expressed by accumulated
insoluble residue are common. The microfacies forms
beds several cm to 20 cm thick, commonly with erosion-
al bases. It possesses horizontal lamination or ripple-bed-
ding.
MF7: homogeneous mudstones (Fig. 6E) are character-
ized by slightly recystallized matrix with minor content
of scattered fragments of cephalopod (
~30 % of bio-
clasts) and ostracod (
~50 %) shells, where the presence
of Muellerisphaerida is typical. This microfacies forms
approximately 20 cm thick beds without distinct stratifi-
cation, some of them are really homogeneous, but others
seem to contain poorly recognizable millimetric
rhythms.
MF8: calcareous shale microfacies with graptolites
(Fig. 6F) contain apparent horizontal lamination and
common pelagic faunas (e.g. graptolites, ostracods).
Context and relationships of microfacies and facies
Grainstone-dominated sections
Sections with predominance of bioclastic limestones
show a typical lack of laminated wackestones/pack-
stones, homogeneous mudstones, and calcareous shales
microfacies. The small-scale interfingering of these two
major facies (bioclastic beds and rhythmites with shaly
interbeds) seems to be quite rare, although the first can
continue the second, and vice versa (Figs. 2 and 3).
The lower part of section in the Požáry Quarry (the up-
permost part of the Požáry Fm, beds 155—159; Fig. 2) is
mainly composed of fine-grained wackestones/pack-
stones with peloids and sponge spicules. It is followed
by coarser-grained crinoidal grainstones/packstones of
the lower part of the Lochkov Fm (beds 160—163; Fig. 2).
The lower part of the Srbsko section (approximately
1.5 m above the base of bed 1) is characterized by coars-
er-grained crinoidal grainstones/packstones, but peloidal
wackstones/packstones predominate in the main part of
the section (the upper part of bed 1 up to bed 3; Fig. 2).
On the contrary, the lowest part of the Opatřilka sec-
tion is formed by a bed of peloidal cephalopod wacke-
stone (bed 1; Fig. 2), and the entire middle part of the
section consists of thin bedded peloidal wackestones/
packstones (interval of beds 3 and 5). The overlying mas-
sive beds 6—8 are characterized by the alternation of two
microfacies assemblages: homogeneous, sponge-spicule
packstones and peloidal wackestones/packstones.
Sections dominated by fine-grained limestones and
shales
The number of other sections with alternation of fine-
grained limestones and calcareous shales show similar
vertical trends in change of microfacies types, but poor-
ly-sorted and matrix-washed crinoidal grainstones do not
occur in these deeper depositional settings.
The upper part of the Požáry Fm in Karlštejn is charac-
terized by alternation of laminated wackestones/pack-
stones with thin intercalations of crinoidal packstones
and calcareous shales with graptolites (beds 1—13;
Fig. 3). The Scyphocrinites Horizon with several beds of
cephalopod limestones fills the entire interval of the up-
permost Pridoli and lower Lochkovian, with predomi-
nace of crinoidal packstones and peloidal cephalopod
wackestones microfacies (beds 14—23; Fig. 3). Laminated
wackestones/packstones, peloidal wackestones/pack-
stones, and calcareous shales prevail in the overlying
beds (24—39).
The lower part of the Praha-Podolí section (the upper
part of Požáry Fm, beds 1—8; Fig. 3) consists of laminated
wackestones/packstones with thin intercalations of
crinoidal packstones, and peloidal cephalopod wacke-
stones alternating calcareous shales. Massive beds 9—12
with the alternation of crinoidal packstones, peloidal
cephalopod wackestones, and peloidal wackstones/pack-
stones already belong to the lower part of the Lochkov
Fm.
The uppermost level of the Požáry Fm in the U topolů
section is composed of laminated wackestones/pack-
stones, peloidal wackstones/packstones, crinoidal pack-
stones, and calcareous shales (beds 1—10; Fig. 3). The
overlying massive accumulation of crinoidal packstones
corresponds to the lowest part of the Lochkov Fm
(bed 11). The above-lying rest of the formation is domi-
nated by laminated wackestones/packstones and calcare-
ous shales (beds 12—31).
The upper part of the Požáry Fm in the Praha-Radotín
section is poorly exposed and the shale beds are inten-
sively weathered. The limestones are mostly composed
of homogeneous mudstones and laminated wackestones/
packstones (beds 1—8; Fig. 3). Homogeneous mudstones
were not found in other sections. The overlying Scypho-
crinites Horizon (at the base of the Lochkov Fm, beds 9
and 10) consists of crinoidal packstones and cephalopod
wackestones, and beds 11—14 are characterized by the
presence of thin layers of homogeneous sponge-spicule
packstones with thin shale intercalations.
Depositional model
The sedimentary system of the S-D boundary strata
(Fig. 7) is characterized by lateral transition from coarse-
grained bioclastic facies to platy limestones with shale
interbeds (from the NW to the SE). Bioclastic packstones
and grainstones of the Kotýs Limestone (sections in Pra-
ha-Řeporyje, Srbsko, partly Praha-Holyně) were deposit-
ed close to the source of organodetrital material in flat
shallow-water areas and the upper parts of carbonate
slope covered by crinoidal meadows. These deposits
show reworking by storm action resulting in washing out
of fine-grained matrix and erosion of cemented sea floor
(angular limestone lithoclasts are present). This feature
was documented in the Požáry Quarry section in the
beds 159, 161, and 162 (Figs. 3 and 5). However, the
507
CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
deposition was generally located below the fair weather
base level, but still in a well-oxygenated environment
with sea floor colonized by common benthic fauna. Car-
bonate tempestites were reported by Suchý (1991) from
the Choteč Limestone (Middle Devonian, Eifelian). In
spite of these indications, the occurrence of typical hum-
mocky-cross stratification is problematic in both these
places, and some of these sediments can be explained as
braided channels on carbonate slope, at least.
The deeper-water Radotín Limestone facies contains
all the characteristic sedimentary features of calciturbid-
ites (erosional bases, normal grading with presence of in-
dividual units of the Bouma turbidite sequence; Bouma
1962); but later reworking by the traction bottom currents
is quite possible in many cases. This effect may also be ex-
pected for some types of the cephalopod limestones beds
where strong current-driven orientation of cephalopod
shells is associated with irregular wash out of small sedi-
mentary particles. This facies contains numerous distal
turbidite beds with prevailing Tc—Td Bouma units,
which alternate with background hemipelagic deposits
(Te). Platy limestone beds show multiple turbidite
events, in many cases. The transition between shallow-
water and deeper-water environments is still character-
ized by a considerable amount of coarse-grained
proximal calciturbidites (Ta and Tb units are common;
e.g. the Scyphocrinites Horizon in Karlštejn, Praha-Podolí,
and Praha-Radotín). Intraformational matrix-supported
carbonate conglomerates with elongated and rounded
lithoclasts can be classified as deposits from debris flows
to dense turbidite flows, the infill of submarine channels.
Debris flow beds pass upwards into amalgamated coarse-
grained crinoidal turbidites, which typically lack graded
parts (Karlštejn, bed 16). However, the complete Bouma
sequence has not been observed in the studied turbidite
beds. It is one of diagnostic features, which differentiate
calciturbidites from siliciclastic turbidites (for summary
see e.g. Tucker & Wright 1990; Flügel 2004). Possible
diagenetic overprint destroying primary sedimentary
structures must also be taken into account.
According to Ferretti & Kříž (1995) or Kříž (1998) the
deposition of cephalopod limestones took place in rela-
tively shallow-water well-oxygenated conditions with
periodical influence of surface currents. However, the oc-
currences of undeformed completely preserved Scypho-
crinites loboliths in the Karlštejn and Praha-Radotín
sections (up to 20 cm in diameter) suggest relatively rap-
id deposition and durability of calm environments be-
tween the sedimentation events. Neither can the
imbrication of limestone lithoclasts in size between 2
and 10 cm be explained by the activity of surface cur-
rents but by rapid transport and deposition in the upper
flow-regime of gravitational currents.
The orientation of cephalopod shells, therefore, can
also reflect the directions of gravity flows, not only the
subsequent washing and re-sedimentation of this materi-
al. In this context, the dominant paleocurrent directions
in the Karlštejn section are from the S and SW to N and
Fig. 7. Simplified depositional model of the Silurian—Devonian boundary strata in the Prague Synform near Prague. The diagram is
basically not to scale, and the horizontal scale indicates mainly the rapid transitions between the areas covered by proximal and distal
calciturbidites as suggested in this paper. The simplified diagram intentionally neglects the role of partial highs on the seafloor, because
their original positions and the distances between them are not yet sufficiently based on the very detailed data covering all the parts of the
Prague Synform. MF1 – crinoidal grainstones, MF2 – crinoidal packstones, MF3 – homogeneous packstones with sponge spicules,
MF4 – cephalopod wackestones with peloids, MF5 – peloidal wackestones/packstones, MF6 – laminated wackestones/packstones,
MF7 – homogeneous mudstones, MF8 – calcareous shales with graptolites. Microfacies types are interpreted as Ta—Te units of Bouma
turbidite sequence. The position of studied sections in the model is marked in the upper part.
508
VACEK
NE, but the other episodic currents were likely to be ori-
ented from the NE to SW. In Praha-Radotín, the northerly
currents predominated, with the NW to NE variations.
The direction of synsedimentary to early post-sedimenta-
ry slump movements as indicated in the U topolů section
is from the NW to SE, and this and similar data provide
the additional evidence about local inclinations on pa-
leoslopes. Paleocurrent interpretation is in accordance
with Ferretti & Kříž (1995), who presumed the prevailing
currents coming from SW to NE lasting for the whole Sil-
urian (from late Wenlock to latest Pridoli). This direction
seems to be widely spread along the northern Gondwana
shelf (Prague Synform, Carnic Alps, Sardinia, Montagne
Noire, Morocco).
Chlupáč (2000) suggested the idea of climatic-driven
cyclicity expressed in bedding couplets (limestone bed
and overlying shale bed). However, the results of concur-
rently developed magnetic susceptibility (MS) studies of
these S-D sections (in progress, to be published soon)
show that the cyclic patterns in the MS record do not re-
spect the lithological characteristics of such successions
of the beds in detail. The MS cycles often include several
limestone beds and shale interbeds or other dispropor-
tions in comparison with the facies and microfacies. This
can be preliminarily explained by the effect of several
processes. First, the orbital driven and other climatic
fluctuations influenced not only the terrigeneous inputs
but also the erosion on the slopes. The medium-long ero-
sional hiatuses (usually several tens of kyr) seem to be
much more frequent in these beds than was previously
assumed. Secondly, also the differential compaction
must distort the possible, orbitally-driven lithological
characteristics (cf. Westphal 2006). In addition, there are
also a number of analysed S-D limestone beds, which
have higher MS values than the interbeds. It is indicative
of a large amount of detrital particles delivered to the ba-
sin with calciturbidites.
The presented model fits well the slope apron of Mul-
lins & Cook (1986). It is characterized by gentle gradi-
ent, redeposited carbonate gravity-flows extend up to the
adjacent shelf/slope break without an upper-slope bypass
zone. In contrast to the submarine siliciclastic fan model,
in which a fan is fed by a point source (submarine can-
yon), carbonate aprons are fed by many small channels
dissecting the upper slope of a carbonate platform where
they act rather as a line source. Any evidence of deeper
channels or canyons was not found in the studied area as
well as megabreccias indicating a steeper slope of the
carbonate platform. Deposits of carbonate aprons form in
general sheets of debris spreading across the slope area.
Organized vertical sequences are not usually developed.
Due to the tectonic shortening from NNW to SSE, the
distance of outcrops with different facies complexes do
not exceed several kilometers. However, the nappe tec-
tonics model presented by Melichar & Hladil (1999) or
Melichar (2004) requires a serious shortening of the ba-
sins in a N—S direction. However, this model does not
need to explain all structural features of the Prague Syn-
form. In this context, particularly the facies research has
a great potential to explain the original dimensions and
distances in the basin.
Sea-level fluctuations
The broader S-D boundary interval in the Prague Syn-
form is characterized by facies change, which is more
pronounced in relatively deeper-water environments
than in their shallower counterparts. The succession of
rhythmites with fine-grained calcilutitic microfacies,
mudstones and wackstones, was interrupted by deposi-
tion of a several meters thick accumulation of coarse-
grained bioclastic limestones (crinoidal and cephalopod
limestones, intraformational conglomerates, and brec-
cias). Such facies changes indicating relative shallowing
of sedimentary environments are also described from oth-
er regions of Europe (e.g. Carnic Alps; Schönlaub et al.
1994), and North America, e.g. central Nevada (Klapper
& Murphy 1975; Matti & McKee 1977), Appalachian
Basin (Denkler & Harris 1988).
The input of coarse-detrital material to a deeper-water
environment is usually connected with increased erosion
in the shallow parts of the shelf during lowstands. The
short-term regression pulse in the uppermost Pridoli in
the Prague Synform was documented in several papers
(e.g. Kříž et al. 1986; Chlupáč & Kukal 1988; Kříž
1991). Hladíková et al. (1997) described gradual increase
in
13
C in Klonk and Karlštejn sections, which has been
interpreted either as a result of increased organic produc-
tivity or relative sea-level drop. A positive excursion in
13
C at/close to the S-D boundary is well documented in
many regions of Europe (Buggisch & Mann 2004), North
America (Saltzman 2002) and seems to reflect a global
event. Saltzman (2002) explains this excursion as a result
of enhanced carbonate weathering during exposure and
erosion of older Silurian platform deposits in combina-
tion with enhanced nutrient delivery to the oceans and
increase in organic carbon burial rates in organic-rich
sediments.
Magnetostratigraphic data (Crick et al. 2001) also sug-
gest a regression event during the latest Pridoli, which
resulted in larger amount of detrital magnetic material
delivered into the basin. This short regression was fol-
lowed by gradual sea-level rise and recovery of hemipe-
lagic deposition alternating with distal calciturbidites.
The maximum thickness of coarse-grained calciturbid-
ites (and debris-flow fills) is known from the Karlštejn-
Budňany Rock (4 m), while small thickness can be
exemplified by outcrops at Praha-Radotín (1—2 m). In the
vicinity of Suchomasty, the stratigraphically condensed
succession of massive, coarse-grained beds is completely
missing, having the analogues in relatively thin and sep-
arated beds with medium increased contents of disarticu-
lated “floating crinoids”. It is interesting, that sections
with maximum accumulation of coarse-grained debris are
usually located close to the Koda Fault (Fig. 1). The in-
put of this material thus might have been triggered by
emergence of unknown paleoheights (a significant eo-
Variscan shortening at this fault), perhaps in combina-
509
CARBONATE MICROFACIES OF THE SILURIAN-DEVONIAN BOUNDARY IN THE BARRANDIAN AREA
tion with a growth-fault system as presumed by Kříž
(1991).
Conclusions
The results of sedimentological and microfacies stud-
ies allow us to characterize the depositional system of
the time interval at the Silurian-Devonian boundary. It
was the margin of an open-sea carbonate shelf with adja-
cent carbonate slope environment (slope apron). The lat-
ter environments are predominantly expressed in the
preserved part of the Prague Synform. If generalized, the
deepening trend can be traced from the NW to SE. The
relatively shalower carbonate facies (upper slope) with
predominance of bioclastic limestones locally show re-
working by storms, which indicates the conditions above
the storm wave base. On the lower carbonate slope and
its toe, rhythmically deposited distal calciturbidites pre-
vail (often with the Bouma Tc and Td units). This quanti-
tatively based observation is highly indicative of distal
calciturbidite deposition. These turbidite beds alternate
with interlayers of the “background” hemipelagic sedi-
ments (Te), which are preserved mostly in the form of
highly compacted calcareous shales. The occurrences of
channelized turbidite grainstones and rudstones with
several layers of intraformational conglomerates are in-
terpreted as debris flow deposits. The input of the coarse-
grained detrital material of shallow-water origin is
interpreted as the result of relative sea-level drop in the
S-D boundary interval.
Acknowledgments: This research was supported by a
grant from the Ministry of Education of the Czech Re-
public, CEZ: J13/98: 113100006. The author is grateful
to Drs. J. Hladil and P. Bosák (Institute of Geology AS
CR) for valuable discussion and critical text review. The
author also greatly benefited from the former supervision
by I. Chlupáč.
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