GeolCarp_Vol61_No4_257

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, AUGUST 2010, 61, 4, 257—272 doi: 10.2478/v10096-010-0015-2

Introduction

The  Prague  Synclinorium  in  central  Bohemia  (Czech  Re-
public) provides many instructive sections exposing the Si-
lurian-Devonian  (Pridoli-Lochkovian)  boundary  strata,
including the Global Boundary Stratotype Section and Point
(GSSP)  Klonk  near  Suchomasty  and  its  auxiliary  section  at
Budňanská skála (Budňany Rock) near Karlštejn. Two stan-
dard sections were approved by a decision of the Internation-
al  Commission  on  Stratigraphy  at  the  24

International

Geological  Congress  in  Montreal,  1972  (McLaren  1977).
During more than thirty years of investigation these standard
sections have been studied by various methods. Stratigraphic
correlations  here  were  traditionally  based  mainly  on  bio-
stratigraphic data. In this stratigraphic succession graptolites
and trilobites are practical for biozonation. However, the oc-
currences  of  index  species  depend  on  facies  to  various  de-
grees. Microfossils, namely conodonts and Chitinozoa have
high  resolution  potential,  but  detailed  micropaleontological
research  was  predominantly  concentrated  on  the  standard
sections  (Paris  et  al.  1981;  Jeppsson  1988,  1989;  Brocke  et
al.  2002,  2006;  Carls  et  al.  2007).  In  many  other  sections

Stratigraphic correlation potential of magnetic susceptibility

and gamma-ray spectrometric variations in calciturbiditic

facies (Silurian-Devonian boundary, Prague Synclinorium,

Czech Republic)

FRANTIŠEK VACEK

, JINDŘICH HLADIL

and PETR SCHNABL

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

fvacek@natur.cuni.cz

Institute of Geology AS CR, v.v.i., Rozvojová 269, 165 00 Prague 6, Czech Republic; hladil@gli.cas.cz

(Manuscript received November 9, 2009; accepted in revised form March 11, 2010)

Abstract: Magnetic susceptibility (MS) and gamma-ray spectrometry (GRS) stratigraphy were used for correlation and
characterization of eight Silurian-Devonian (S-D) sections in the Prague Synclinorium (Czech Republic). They repre-
sent two different facies developments: lower subtidal to upper slope deposits and slope-to-basin-floor distal
calciturbidites. Sections from relatively shallow- and deep-water sections are easy to compare and correlate separately,
although the detailed relationship between these two facies is still not entirely clear and correlations between the two
settings are difficult. This may be due to sharp facies transitions and presence of stratigraphic gaps. The MS and GRS
stratigraphic variations combined with sedimentologic data have been also used for reconstruction of the evolution of
the sedimentary environment. The beds close above the S-D boundary show noticeably enhanced MS magnitudes but
weak natural gamma-ray emissions. It may correspond to an increased amount of terrigenous magnetic material occur-
ring with short-term shallowing (sedimentological evidence). In deep-water sections the uppermost Silurian is charac-
terized by high MS and GRS values. It corresponds to a supply of recycled sediment to the lower wedge which occurred
during the late Pridoli regression phase. The basal Devonian beds correspond to gradual deepening, but the overlying
sequences reflect other shallowing episodes which are expressed in increasing MS and gamma ray activity of rocks. The
MS and GRS fluctuations are interpreted as a result of local subsidence of the sea bottom along synsedimentary growth-
faults and/or a biotic event rather than of eustatic sea-level changes.

Key words: Silurian-Devonian boundary, Prague Synclinorium, magnetic susceptibility stratigraphy, gamma-ray
spectrometry, carbonate slope system.

precise biostratigraphic data are incomplete to mostly absent.
In  several  recent  papers  different  stratigraphic  approaches
were applied, including magnetic susceptibility (MS) strati-
graphy (Crick et al. 2001) or chemostratigraphy (Hladíková
et  al.  1997;  Herten  2000;  Kranendonck  2000;  Mann  et  al.
2001;  Frýda  et  al.  2002;  Buggisch  &  Mann  2004).  Crick  et
al.  (2001)  introduced  the  MS  stratigraphic  profile  for  the
GSSP at Klonk and a drilling core situated close to the surface
section.  They  used  the  MS  record  for  establishment  of
magnetosusceptibility  event  and  cyclostratigraphic  (MSEC)
zones as an alternative stratigraphic tool. They also suggest-
ed  possible  interregional  correlations  with  the  area  of  the
Anti-Atlas in Morocco using MSEC.

This study involves eight sections, which have been stud-

ied  before  for  paleontology  and  sedimentology,  but  not  for
MS and GRS (gamma-ray spectrometry) stratigraphic varia-
tions.  Both  methods  will  be  tested  for  the  detailed  strati-
graphic correlations across varying facies and could also be
used  in  combination  with  supplementary  data  for  complex
characteristics of the depositional environment and its evolu-
tion in several Silurian-Devonian (S-D) boundary sections in
the Prague Synclinorium.

258

VACEK, HLADIL and SCHNABL

Geological setting

The Variscan folded Silurian and Devonian formations crop

out  in  the  central  part  of  the  Prague  Synclinorium  between
Prague and vicinity of Beroun (Fig. 1). They consist of marine
sediments (mostly shales and limestones; Chlupáč et al. 1998)
and submarine volcanic rocks (basic volcanics and coeval ba-
sic/ultrabasic  volcaniclastics;  Fiala  1970;  Patočka  &  Štorch
2004). Prague Synclinorium was interpreted as located on the
northern  margins  of  Gondwana  during  the  S-D  interval  with
affinities  to  Armorica  (e.g.  Krs  &  Pruner  1995;  Krs  et  al.
2001), at a paleolatitude of about 17°S (Patočka et al. 2003).

The S-D boundary is situated close to the boundary be-

tween the Požáry (approximately corresponding to the Prido-
li Series) and Lochkov Formations ( ~ the Lochkovian Stage).
Both formations are generally characterized by lateral transi-
tion  from  coarse-grained  bioclastic  limestones  in  the  NW
part  of  the  synclinorium  to  fine-grained  limestones  and
shales  in  the  SE  part  (Chlupáč  et  al.  1998),  defining  a  NW
shallow zone and SE deeper zone. However, the boundaries
of these two lithostratigraphic units are slightly diachronous
over  the  region.  Proximity  of  the  S-D  boundary  is  broadly
characterized  by  blooms  of  pelagic  crinoids  with  plate-type
loboliths,  typically  Scyphocrinites,  which  often  (but  not  al-
ways) occur in the beds of the latest Pridoli and early Loch-
kovian  ages.  They  may  form  several  meters  thick  beds  of
coarse-grained crinoidal limestones at the base of the Loch-
kov Formation, informally called the Scyphocrinites Horizon
(Scyphocrinites H). It may be locally associated with cepha-
lopod limestones and also with beds of flat-pebble conglom-
erates. It is better recognizable in the deeper zone because in
the shallow-water environment it may be concealed by over-
all bioclastic deposition.

Several localities exposing the S-D boundary strata repre-

senting the two different facies were selected for study (see
Fig. 1, Table 1). These facies were deposited on the margin of
an open-sea carbonate shelf with adjacent carbonate slope en-
vironment (Vacek 2007). Generally a deepening trend can be
traced from the NW to the SE of the basin.

Methods

Limestone “beds” are traditionally numbered 1, 2, 3, etc.,

designations such as 1/2, 2/3 are used for the shale “inter-
beds”.

Magnetic susceptibility study

In the last ten years, the number of studies on stratigraphic

MS variations in the Devonian marine carbonate or mixed se-
quences  has  increased  significantly  (Crick  et  al.  1997,  2000,
2001,  2002;  Ellwood  et  al.  2000,  2001,  2006;  da  Silva  &
Boulvain  2006;  Hladil  et  al.  2006;  da  Silva  et  al.  2009a,b,
2010; Koptíková et al. 2010).

The outcrop sections were sampled for the MS study at

0.05 m intervals. Small cubic or slice rock samples were col-
lected (20—50 g). Only fresh samples were taken (i.e. avoiding
the veins, visible pyrite or limonite aggregates, various spots

related to late diagenetic alterations and weathering, epigenet-
ic dolomitization and also shear-deformed parts of the rock).
The thickness of the MS profiles ranges from 4 to 11 meters
depending  on  geological  conditions.  The  complete  sample
collection  includes  more  than  1,000  samples.  Measurements
were  carried  out  in  the  Laboratory  of  Paleomagnetism  (Inst.
Geol.  AS  CR,  Prague)  on  Kappabridges  KLY-2  and  3  (pro-
duced  by  Agico  Ltd.  Brno;  for  technical  details  we  refer  to
www.agico.com).  The  values  of  magnetic  susceptibility  in
this  paper  are  expressed  as  mass-related  magnetic  suscepti-
bility (10

—9

· kg

—1

). These are further referred as MS values

which are used for plotting the curves and assessment of
their possible stratigraphic importance.

Magnetic susceptibility is the intrinsic property that deter-

mines the amount of magnetism, which a rock can have in a
given magnetic field. It is related to bulk chemistry and mag-
netic  mineralogy  and  particularly  to  the  amounts  of  easily
magnetizable  minerals  in  a  rock  sample.  The  increased  MS
signal in limestones is induced by presence of various ferro-
magnetic  (s.l.)  minerals  (magnetite,  maghemite,  hematite,
monoclinic pyrrhotite), and also weakly magnetic but much
more abundant paramagnetic minerals (clay minerals, pyrox-
ene, amphibole, biotite, chlorite, pyrite, chalcopyrite, a.o.).

In contrast, diamagnetic minerals such as calcite, quartz,

and  others  have  very  weak  negative  MS  magnitudes  and  re-
duce mass susceptibility of a rock sample. However, the MS
of  detrital  ferromagnetic  and  paramagnetic  minerals  is  much
greater  than  the  MS  of  diamagnetic  minerals.  Therefore,  a
small  amount  of  even  weakly  paramagnetic  mineral  can  sig-
nificantly outweigh the MS of volumetrically more abundant
diamagnetic minerals (Ellwood et al. 2000).

The amount of magnetic particles mainly depends on terrig-

enous influx, which is mostly controlled by fluctuations in sea
level.  Generally,  the  maximum  input  of  terrigenous  detritus
corresponds  to  intensive  erosion  during  the  lowstand  of  sea
level.  This  is  considered  to  be  recognizable  on  both  the  re-
gional and global scale because of synchronous variations in
global  erosion  controlled  by  eustasy  (Ellwood  et  al.  2000,
2001). The large-scale redistribution of sub-silt and silt-sized
particles  (< 63 µm)  often  comes  about  through  eolian  trans-
port, and the deep parts of carbonate slopes can also be affect-
ed by distant riverine flux (Hladil 2002; Hladil et al. 2006).

Magnetite can also be produced by magnetotactic bacteria

or algae. However, it is mostly formed in shallow-water con-
ditions  with  restricted  circulation,  which  is  not  the  case  of
the studied sections. The other magnetically important min-
eral  components  related  to  deep-water  carbonate  or  mixed
carbonate-siliciclastic  sediments  are  authigenic  carbonates
with  iron  in  lattices  or  iron-oxide  inclusions  (siderite  and
rarely other minerals; e.g. Ellwood et al. 1988; Frederichs et
al. 2003), and these are also tentatively related to bacterially-
mediated  precipitates.  For  more  discussion  on  the  primary
and  secondary  magnetic  minerals  in  carbonates  we  refer  to
da Silva et al. (2009a).

Rock magnetic methods

Several methods have been used for identification of possi-

ble carriers of the MS. They have been applied both on miner-

259

MAGNETIC SUSCEPTIBILITY AND GAMMA-RAY IN CALCITURBIDITIC FACIES (PRAGUE SYNCLINORIUM)

al concentrates obtained by dissolution in acids (10 samples;
X-ray diffraction – XRD; temperature dependence of the
MS; magnetic hysteresis) and the whole-rock samples (5 sam-
ples; isothermal remanent magnetization – IRM).

Mineral concentrates have been obtained by leaching in

10% hydrochloric and acetic acids, separately. However,
some important magnetic minerals such as iron oxides may
be dissolved in these acids, therefore we had to also use the
IRM method applied to whole rock (see above).

The method of temperature dependent MS identifies mag-

netic minerals and mineralogical phase changes during heat-
ing. It was measured using KLY-4S Kappabridge (produced
by  Agico  Ltd.  Brno;  Jelínek  &  Pokorný  1997)  combined
with  a  temperature  control  unit  CS3  (Parma  &  Zapletal
1991) in the temperature range of 20—700 °C in an argon at-
mosphere.  Paramagnetic  minerals  exhibit  parabolic-shaped
MS  decay  curves  at  relatively  low  temperatures  (up  to
~

200 °C) because the MS of these minerals is inversely pro-

portional  to  the  temperature  (Hrouda  1994).  On  the  other
hand,  ferromagnetic  minerals  usually  show  increasing  MS
up to the point where it decays to the Curie temperature. For
magnetite the Curie temperature is  ~ 580 °C and for hematite
it is  ~ 680 °C.

The method of magnetic hysteresis is based on response of a

magnetic  material  to  magnetic  field.  Hysteretic  behaviour  is
highly  dependent  on  mineralogy  and  grain  size  (Tauxe  et  al.
1996).  The  sample  is  placed  in  an  intensive  magnetic  field
(+ 1 T) and magnetization is examined as the applied intensi-
ty drops to zero and then increases to the negative maximum
(—1 T).  Changes  in  magnetization  during  regaining  of  the
original  intensity  (+ 1 T)  are  significant  for  interpretation  of
magnetic components. These measurements were performed
on a vibrating sample magnetometer Model 3900 VSM (pro-
duced by Princeton Measurement Corporation).

IRM was measured on Pulse Magnetizer MMPM 10 (pro-

duced  by  Magnetic  Measurements  Ltd.)  and  magnetometer
JR6a (produced by Agico Ltd. Brno) in order to identify co-
ercivity  spectra.  The  used  field  range  was  10  to  2000 mT.
Contribution  of  particular  magnetic  components  ferromag-
netic to the total remanent magnetization has been tested by
the  IRM  component  analysis  (Kruiver  et  al.  2001).  Various
magnetic minerals can be identified by B

1/2

values, which is

the  magnetic  field  at  which  a  half  of  Saturated  Isothermal
Remanent  Magnetization  (SIRM)  is  reached.  For  magnetite
it  is  20—63 mT,  hematite  63—200 mT,  and  goethite  > 1 T
(Grygar et al. 2003).

Gamma ray spectrometry study

The spectral gamma-ray approach is a significant parallel to

MS-detected concentrations of background sediment impurity
in  limestone  (Hladil  et  al.  2006).  The  MS-GRS  combination
has  an  overall  potential  to  improve  the  quality  of  MS  based
stratigraphic  correlation,  with  the  background  reasoning  in
magnetomineralogy.

The gamma-ray spectrometric (GRS) based correlations of

outcrop logs have been frequently used in the last decade in
the Devonian of the Czech Republic on the platform to basin
formations of Moravia (Hladil et al. 2000, 2003a,b; Hladil

2002; Geršl & Hladil 2004; Bábek et al. 2007, a.o.) or Pra-
gue Synclinorium (Slavík et al. 2000; Koptíková et al. 2007,
2008, 2010).

For this study, a gamma-ray spectrometer Geofyzika-Satis-

Geo GS-512 with NaI(Tl) scintillation detector 3”

×3”

(7.62

×7.62 cm) and 3” photomultiplier was used (SatisGeo

2009). This instrument was used in the mode that the whole ele-
ment concentrations of K (%), U (mg/kg=ppm) and Th (ppm)
were automatically calculated. The instrument was calibrated
at the regional reference centre of Bratkovice near Příbram
(parameters frequently quoted, e.g. Lis et al. 1997). Using this
technique and instrument, the gamma rays registered for this
purpose correspond to isotopes

214

Bi and

208

Tl, uranium and

thorium decay series isotopes in naturally occurring materials,
respectively. The data on potassium is obtained using the
spectra for

K isotope. The total natural gamma-ray variation

has  been  inferred  from  selected  energy  windows,  all  above
720 keV.  With  this  instrument,  this  additional  parameter  is
set  to  display  automatically  a  notional  uranium  equivalent
(eU) that is routinely expressed in mg/kg (ppm) of U-equiv-
alent  contents,  but  for  imagination  or  rough  comparison
only.  In  addition,  the  recalculation  to  API  units  or  radioac-
tive  doses  cannot  be  accomplished  in  general  terms,  for  its
relationships to techniques, conditions and details of probes
or  instruments  (Geršl  &  Hladil  2004).  These  approximate
data on the totals of natural gamma ray (NGR or GR) emis-
sion from measured sedimentary rocks often differs accord-
ing  to  apparatuses  and  has,  therefore,  only  relative  and  not
absolute information value.

The thicknesses of the GRS logs are identical with the MS

ones, except the lower part of the Praha-Podolí section, which
could not be measured due to its intensive weathering.

The GRS measurement was performed with 0.25 m step at a

time of 240 seconds, perpendicular to the rock face at the full
contact.  This  regular  spacing  strategy  was  preferred  over  the
irregular (rock-type selective) one. This choice was based on
the preliminary-test findings that gamma-ray signal of differ-
ent  magnitudes  and  structure  was  obtained  from  the  beds  of
comparable  lithology  (e.g.  great  variation  within  the  class  of
coarse-grain  calciturbidites,  or  the  same  for  the  very  fine-
grained shale interbeds). The size of this 0.25 m step was se-
lected  heuristically  but  with  respect  to  the  fact  that
approximately 95% signal at the front of the probe (with crys-
tal)  originates  from  a  slightly  deformed  hemisphere  of  mea-
sured rocks that corresponds to a target of 0.25 m radius at an
ideal planar surface (L

vborg et al. 1971). Hence, this empiri-

cally  tested  precondition  for  overlapping  of  measurements
with these sections makes possible to keep the overlap below
15 %  of  the  signal,  even  for  irregular  arrangements  of  beds
and  rock  materials.  The  combined  error  from  conditions,  in-
strument  and  repeated  measurements  was  established  to  be
less than about  ± 7.5 % for the whole element U, Th, K auto-
matically calculated results.

Detected concentrations of K, U, and Th are mostly related

to amount of feldspars, micas, and clay minerals, among oth-
ers. Uranium is also known to be remarkably trapped in or-
ganic matter (e.g. Durrance 1986). Higher concentrations of
these elements should again reflect increased amount of non-
carbonate impurities in limestones that are caused by detrital

260

VACEK, HLADIL and SCHNABL

influx from a supposed land surface in both the regional and
interregional contexts.

Sedimentology and studied sections

The lithology, sedimentology, and biostratigraphy of the se-

lected sections have been described in many previous papers
(for more details we refer to Chlupáč et al. 1972; Hladil 1991,
1992; Čáp et al. 2003; Vacek 2007).

Shallow facies

The relatively shallow-water carbonate facies with predomi-

nance of bioclastic, mainly crinoidal packstones to grainstones
is distributed in the NW flank of the synclinorium with several
other  finger-like  projections  in  its  western  part  (studied  sec-
tions  at  Požáry  Quarry  near  Praha-Řeporyje,  Srbsko,  and
Opatřilka  Quarry  near  Praha-Holyně;  Fig. 1,  Table 1).  These
deposits  locally  show  reworking  by  storms,  which  indicates
the  conditions  above  the  storm  wave  base.  It  corresponds  to
the lower subtidal to upper slope environment.

The shallow-water carbonate facies possess rich benthic

fauna, including crinoids and trilobites, and brachiopods. The
uppermost Silurian is characterized by abundant occurrence of
the index trilobite Tetinia minuta. The first appearance of
trilobite Warburgella rugulosa rugosa indicates the base of
Devonian (Chlupáč et al. 1972).

Deep facies

Deep-water facies are distributed in the SE flank of the

Prague  Synclinorium  (sections  at  Karlštejn,  Klonk,  Praha-
Radotín  and  Praha-Podolí;  Fig. 1,  Table 1).  These  facies  are
characterized  as  dark  bioclastic  and  peloidal  wackestones/
packstones  to  mudstones  alternating  with  calcareous  shales,
locally with several meters thick Scyphocrinites H. They yield
common  pelagic  fauna,  including  graptolites,  cephalopods
and ostracods. The S-D boundary interval is characterized by
abundant occurrence of crinoids of Scyphocrinites sp. The up-
permost  Silurian  corresponds  to  the  graptolite  Monograptus
transgrediens  Zone.  The  base  of  Devonian  is  marked  by  the
first appearance of the index graptolite Monograptus unifor-
mis  (Chlupáč  et  al.  1972).  Other  fossil  groups  (conodonts,

Fig. 1. Position of the studied localities in the Prague Synclinorium area: 1 – Požáry Quarry near Praha-Řeporyje; 2 – Opatřilka Quarry
near Praha-Holyně; 3 – Srbsko; 4 – Karlštejn (Budňany Rock); 5 – Praha-Radotín (U topolů); 6 – Praha-Radotín (near Cement Plant);
7 – Praha-Podolí; 8 – Klonk near Suchomasty. GSSP – Global Boundary Stratotype Section and Point. The geological sketch map of the
Prague Synclinorium benefits partly from the working materials provided by R. Melichar.

261

MAGNETIC SUSCEPTIBILITY AND GAMMA-RAY IN CALCITURBIDITIC FACIES (PRAGUE SYNCLINORIUM)

No. Section

Location

Sampling interval

GRS Environment

1 Požáry Quarry

50° 1' 42.3" N; 14° 19' 28.4" E beds 155–163 (9 m)

177

Shallow

2 Opatřilka Quarry

50° 2' 8.1" N; 14° 21' 2.7" E

beds 1–8 (10 m)

200

Shallow

3 Srbsko

49° 56' 29.6" N; 14° 7' 57.2" E beds 1–3 (6 m)

120

Shallow

4 Karlštejn

49° 56' 4.5" N; 14° 10' 51.4" E beds 1–42 (11 m)

224

Deep

5 Praha-Radotín (U topolů)

49° 59' 51.2" N; 14° 20' 2.6" E beds 1–31 (7 m)

141

Deep

6 Praha-Radotín (near Cement Plant) 49° 59' 33.9" N; 14° 20' 46.4" E beds 9–14 (3.5 m)

Deep

7 Praha-Podolí

50° 3' 6.9" N; 14° 25' 7.6" E

beds 1–11 (3.5 m)

Deep

8 Klonk near Suchomasty

49° 54' 1.3" N; 14° 3' 46.3" E

1–44 (12.75 m)

adopted from Crick et al. (2001) 52

Deep

Table 1: List of studied sections with their geographical position, measured intervals and numbers of analysed MS samples and GRS mea-
surements. * The GRS measurements were performed in the bed interval 10—12 only due to poor state of the lower part of the section.

Chitinozoa) can be used as auxiliary indicators (Paris et al.
1981; Brocke et al. 2002, 2006; Carls et al. 2007).

This facies is interpreted as rhythmical distal calciturbidites

deposited on carbonate slope and its toe (often with the Bou-
ma Tc and Td units). These turbidite beds alternate with layers
of  the  “background”  hemipelagic  sediments  (Te),  which  are
preserved mostly in the form of highly compacted calcareous
shales.  The  occurrences  of  channelized  calciturbidite  grain-
stones  and  rudstones  with  several  layers  of  flat  pebble  con-
glomerates  are  interpreted  as  debris  flow  deposits  or  dense
turbidite flows. The input of the coarse-grained detrital materi-
al of shallow-water origin was interpreted as the result of rela-
tive sea-level drop in the S-D boundary interval and possible
subsidence along synsedimentary growth faults (Vacek 2007).

The MS and GRS stratigraphy of the studied

sections

Main characteristics of the MS and GRS records of the
shallow facies

Generally, the carbonate rocks in the studied sections have

relatively low MS signal in the order of 10

—9

· kg

—1

(further

referred as 10

—9

SI Units). Shallow-water bioclastic pack-

stones/grainstones exhibit relatively low differences of the av-
erage MS values between the Požáry and Lochkov Formations
(see Table 2, Fig. 2). The MS curves mostly show only low to
moderate  oscillations  (see  Fig. 2).  The  critical  S-D  boundary
interval in the shallow-water deposits (especially the Požáry Q
and Opatřilka sections) is marked by enhanced MS values. In
the  Požáry  Q  this  increase  is  observable  directly  above  the
boundary in the lowermost part of bed No. 159 (see Fig. 2A).
In  the  Opatřilka  section,  the  same  pattern  characterized  by
high  oscillation  is  recognizable  in  bed  No. 8  approximately
1 m above the first appearance of W. rugulosa rugosa, which
determines the S-D boundary (Fig. 2B). However, this pattern
is less distinctive in the Srbsko section (Fig. 2C).

This facies is characterized by relatively low concentrations

and variations of potassium in the Požáry and Srbsko sections
(0.3—0.5 %; Table 2). The concentrations of K show a consid-
erably weak covariance with those of Th (R

= 0.37 and 0.47).

On the contrary, at Opatřilka this covariance is very high
(R

= 0.91). Correlation between K and U and Th and U is also

generally weak, with the concentration of U changing quite in-
dependently  of  K  and  Th.  Trends  of  the  eU  curves  visually
correspond  mostly  to  variations  of  U,  less  to  Th  concentra-
tions (Fig. 2A—C). It corresponds well to the fact that the Th/U
ratio is generally very low, with an average of 0.17—0.39 (i.e.
the  GRS-based  concentrations  for  U  are  much  higher  than

Table 2: Average magnitudes of the MS and GRS-based concentrations in the studied sections or their distinguished segments. S – Silurian;
D – Devonian; Po – Požáry Fm; Sc – Scyphocrinites H; Lo – Lochkov Fm. The uppermost part of the Požáry Fm in the Podolí section
was not GRS measured due to weathering. The “raw” MS data for the Klonk section were not available.

Sections/their segments

MSχ [10

–9

·kg

–1

]

eU [ppm]

K [%]

U [ppm]

Th [ppm]

Požáry Q. Po (0.0–4.40 m)

9.8

7.5

0.5

5.3

1.5

Lo (4.45–8.7 m)

11.0

5.8

0.5

3.8

1.5

Srbsko Po (0.0–4.75 m)

7.3

4.5

0.3

3.6

0.9

Lo (4.8–6.0 m)

5.3

4.6

0.4

2.7

0.8

Opatřilka Q. Po (0.0–6.5 m)

9.0

14.9

1.0

10.3

2.1

Lo (6.55–9.9 m)

11.3 11.7

0.3

10.6

1.0

Klonk Po (0.0–5.25 m)

–

13.6

1.7

5.2

5.1

Lo (5.3–12.75 m)

–

8.8

1.2

3.1

3.2

Karlštejn Po (0.0–2.45 m)

30.2

22.4

1.7

13.6

5.6

Sc (2.5–7.45 m)

1.8

8.4

0.5

6.1

1.6

Lo (7.5–11.1 m)

10.2

7.2

0.8

3.6

2.3

U topolů Po (0.0–1.95 m)

28.9

14.8

1.9

6.0

4.6

Sc (2.0–3.35 m)

3.8

13.0

0.6

10.2

1.6

Lo (3.4–7.0 m)

5.6

9.3

0.9

5.4

2.2

Radotín Sc (0.0–1.15 m)

6.9

10.4

0.7

7.4

2.5

Lo (1.2–3.45 m)

5.9

6.5

0.6

3.8

1.9

Podolí Po (0.0–1.65 m)

29.5

–

Sc (1.7–3.75 m)

3.3

16.1

0.7

12.7

2.5

262

VACEK, HLADIL and SCHNABL

Fig. 2. Magnetosusceptibility and gamma-ray spectrometric logs of
sections  representing  the  shallow-water  facies  (bioclastic  pack-
stones  and  grainstones  predominate).  The  S-D  boundary  is  deter-
mined  by  the  first  occurrence  of  W.  rugulosa  rugosa  and
I. hesperius. Notice remarkable increase of the MS at or immediate-
ly above the boundary.

for Th) and shows only slight variations. The eU curves of all
sections possess more or less conspicuous wave-like cyclic
patterns.

Main characteristics of the MS and GRS records of the deep
facies

Three different segments can be distinguished in the deep-

water  sections  (mudstones/wackestones  alternating  with  cal-
careous  shales).  The  first  one  corresponds  to  the  uppermost
part of the Požáry Formation. It is characterized by high oscil-
lations and the highest MS mean values in the studied sections
(28.9—30.2  10

—9

SI Units, maximum up to 95; Table 2). The

overlying coarse-grained crinoidal limestones of the Scypho-
crinites H (the lowermost part of the Lochkov Formation)
have much lower average magnitudes (1.1—7.6 10

—9

SI Units;

see Fig. 3B—E). Amplitudes of the MS curves are also much
lower. The upper segment corresponds to recovery of distal

calciturbidite deposition higher in the sections. It is character-
ized by a slight increase in the MS (mean 5.6—10.2 10

—9

Units), but not as high as in the uppermost part of the Požáry
Formation.

The broader S-D interval is characterized by a remarkable

decrease of the MS magnitudes associated with facies change
(Fig. 3).

This facies shows much higher variations in the K content.

The K concentrations are highest in the distal calciturbidite fa-
cies  of  the  uppermost  part  of  the  Požáry  Formation  (average
concentrations  1.7—1.9 %;  Table 1).  The  K  contents  tend  to
decrease upwards and reach their minima within the Scypho-
crinites H (average contents 0.5—0.7 %). The recovery of platy
limestone/shale  deposition  is  marked  again  by  a  slight  in-
crease in K concentrations (Fig. 3B—E). Generally, the amount
of  K  shows  excellent  covariance  with  Th  (R

= 0.87—0.98),

while correlation between K and U and Th and U remains
weak or has even slightly negative values (U topolů, Radotín,

264

VACEK, HLADIL and SCHNABL

and Podolí sections). Exceptionally, the Klonk and Karlštejn
sections are characterized by good correlation between U and
Th+ K concentrations (R

> 0.8). This is expressed in similar

trends and variations of K, U, Th and total gamma activity
curves (see Fig. 3A and B). In the above mentioned three sec-
tions (U topolů, Radotín, Podolí), the eU variation is related
mostly to changing U content (Fig. 3C—E).

The Th/U ratio is slightly higher than in the lower subtidal

deposits, but only exceptionally exceeds 1 (average 0.21—1.02).
In some sections this ratio tends to increase upwards (Karlštejn,
Praha-Radotín sections, Fig. 3B—D).

The MS and GRS correlations of the studied sections

A crucial attempt to understand the high-resolution MS

and GRS stratigraphy in the studied area is based on formal-
izing the MS and GRS patterns and their successions. Here,
from technical viewpoint, it must be repeatedly stressed that
all these MS sections were characterized using the 5 cm sam-
pling  but  variable  stratigraphic  thicknesses  were  involved
due  to  geological  and  geographical  conditions.  The  “raw”
MS  data  are  plotted  in  Figs. 2  and  3,  with  exception  of  the
GSSP Klonk section (Fig. 3A), which was adopted from the
paper by Crick et al. (2001). The latter authors published the
MS values normalized by their mean, so that the shape and
patterns of the curve are not changed. This section is consid-
ered  to  be  the  standard  for  proposed  stratigraphic  correla-
tions (see below).

In sections with similar facies development, several correl-

ative MS patterns and their successions can be distinguished.
The boundaries of these segments are mostly placed at local
minima.  These  patterns  are  numbered  by  Roman  numbers I,
II, III, etc. Each pattern is characterized by a trend, magnitudes
and  amplitudes  of  the  MS  curve  and  number  of  main  peaks.
Each  MS  pattern  usually  contains  several  limestone  beds
(designated 1,  2,  etc.)  and  shale  interbeds  (designated 1/2,
etc.). Twelve patterns can be recognized in the deeper-water
facies, while only 7 patterns are distinguished in the shallow-
water (see Figs. 4 and 5). However, preliminary comparison
of  our  sections  showed  that  there  are  probably  numerous
stratigraphic  gaps  in  the  Praha-Podolí  and  Praha-Radotín
sections (near the Cement plant). This assumption was also
supported  by  biostratigraphic  data  (Chlupáč  et  al.  1972;
Brocke  et  al.  2002;  L.  Slavík  –  pers.  comm.  2007).  Thus,
we excluded these two sections from our further correlations
as they might make our work rather speculative.

In spite of the scarcity of biostratigraphic indicators in

some sections, they can be used at least for approximate con-
trol of the proposed more detailed MS-GRS correlations. As
presented on Figs. 6 and 7, the MS record appears to be suit-
able  tool  for  comparison  of  sections  with  roughly  similar
lithologies,  namely  1)  shallow-water  bioclastic  packstones/
grainstones,  and  2) deep-water  distal  calciturbidite  mud-
stones/wackestones.  However,  the  detailed  correlation  be-
tween  these  two  contrasting  facies  is  not  fully  clear.  It  is
especially due to the sharp facies transition between carbon-
ate lobes in proximal environments and flat calciturbidite fans
in distal areas (Vacek 2007), but the possibility of eo-Variscan
and younger tectonic obliteration of appropriate “transitional”

facies developments must also be considered (Melichar &
Hladil 1999; Melichar 2004 vs. Röhlich 2007).

As visible in Figs. 4 and 5, the thicknesses of distin-

guished patterns vary from section to section. This can indi-
cate  a  fluctuation  of  depositional  rates  at  the  studied
localities  and/or  insertion  of  erosional  hiatuses.  The  partial
erosion of older sediments was documented in both the rela-
tively  shallow-  and  deep-water  conditions  by  eroded  hard-
grounds, erosional bases of distal calciturbidite beds or their
common amalgamation (Vacek 2007).

The MS records can particularly be used for the more pre-

cise  correlation  between  two  standard  sections,  Klonk
(Fig. 3A) and Karlštejn (Fig. 3B), which lithologically differ
in  the  critical  boundary  interval.  The  boundary  strata  at
Klonk are developed as platy mudstone/wackestone and cal-
careous shale interbeds, while at Karlštejn it consists of mas-
sive  coarse-grained  crinoidal  and  cephalopod  packstones/
grainstones  with  a  bed  of  flat-pebble  limestone  conglomer-
ates. In both sections, the S-D boundary is indicated by the
first occurrence of the index graptolite M. uniformis (in the
upper part of bed No. 20 at Klonk and in thin shale interbed
No. 19/20  at  Karlštejn,  cf.  Chlupáč  et  al.  1972).  Using  the
MS  curves,  the  S-D  boundary  at  Klonk  corresponds  to  the
level  within  pattern  V,  which  is  characterized  by  solitary
peaks at the base and the top and a group of several peaks in
between.  The  boundary  is  located  in  the  lower  part  of  this
pattern with increasing magnitudes (see Figs. 3A and 7). The
same pattern (although with lower MS magnitudes) and level
indicating  the  S-D  boundary  can  be  traced  in  the  Karlštejn
sequence  (Figs. 3B  and  7),  but  approximately  0.5 m  below
the first appearance of M. uniformis. Thus, we have to con-
sider a diachronous first occurrence (or preservation) of this
index  species  at  Karlštejn  and  take  it  into  account  in  local
biostratigraphy of this area.

The combination of the MS and GRS (eU) data certainly

decreases the risk of miscorrelation in this mosaic of facies
(Figs. 6 and 7). The GRS curves also possess several features,
which can be recognized in most of the sections and approxi-
mately fit the above suggested MS correlation of the studied
sections. Here, it must be emphasized again that it also has at
least approximate biostratigraphic control.

It is remarkable that even the uppermost Silurian part of

the Klonk section is characterized by upwards decreasing of
the eU values, which is followed by a distinctive peak (related
to enhanced concentrations of K, U, and Th) just above the
S-D  boundary.  This  peak  is  clearly  recognized  in  the  other
sections (e.g. Karlštejn, U topolů; Fig. 7), where it should in-
dicate  precisely  the  correlative  point  for  the  S-D  boundary.
However,  this  is  not  in  agreement  with  the  biostratigraphi-
cally determined boundaries and could signify a diachronous
onset of index fossils over the region; either it is the case of
their real occurrences, or it is a consequence of the always lim-
ited  depth  of  sampling  and  investigation.  For  example,  the
comparison of two standard sections shows that GRS-based
S-D boundary at Karlštejn is approximately 0.6 m lower than
the  first  occurrence  of  M.  uniformis,  and  this  also  fits  well
with  the  MS  correlation  of  these  stratotypes  (see  above;
Fig. 7).  Actually,  it  is  not  astonishing  because  we  have  to
take  into  account  the  low  preservation  potential  of  pelagic

267

MAGNETIC SUSCEPTIBILITY AND GAMMA-RAY IN CALCITURBIDITIC FACIES (PRAGUE SYNCLINORIUM)

Fig. 6. Thermomagnetic and hysteresis behaviour of three mineral concentrates obtained by dissolution in acetic acid and isothermal rema-
nent magnetization (IRM) curves measured on whole-rock samples. Sample O8 (for description see the text): The hysteresis curve
a) shows both paramagnetic and ferromagnetic behaviour. It is confirmed by the IRM acquisition curve b) with B

1/2

= 1071 mT typical for

highly coercive goethite. The temperature variations of magnetic susceptibility c) indicate formation of magnetite between 400 and 500 °C.
The Curie point of this magnetite is at 560 °C. Sample K10: The hysteresis curve d) indicates only paramagnetic behaviour. Nevertheless, the
IRM acquisition e) proves that a small amount of low to medium coercivity mineral such as magnetite or hematite is present (B

1/2

= 72 mT).

The temperature variations of susceptibility f) show formation of magnetite between the temperatures of 450 and 500 °C. The Curie point
of this magnetite is at 570 °C. During progressive heating pyrrhotite is formed, its Curie is point at 320 °C. Sample R10: The hysteresis
curve g) demonstrates paramagnetic behaviour and only subordinate indications of a ferromagnetic material. However, the IRM acquisition
h) shows the presence of two different magnetic minerals: magnetite/hematite (B

1/2

= 70 mT) and goethite (B

1/2

= 2041 mT). The increase in

magnetic susceptibility during heating above 400 °C i) is caused by newly-formed magnetite with the Curie point at 560 °C. Consequent
heating creates pure magnetite with its Curie point at 580 °C.

Fig. 7. Plot showing relationship between percentage magnetite
contribution to the total remanent magnetization (IRM component
analysis) and the MS

χ. It is obviously weak (R

= —0.34) so that we

can exclude possible effect of secondary diagenetic magnetite on
the MS variations.

The major effects on rock magnetic susceptibility must be as-
cribed to varying amounts of paramagnetic detrital minerals
(e.g. iron-bearing muscovite, chlorite) and only subordinately
to oxides and sulphides (hematite/maghemite, pyrite/pyrrho-
tite). This fact justifies our following interpretations of the MS
stratigraphic variations with respect to changing input of erod-
ed detrital material related to sea-level fluctuations.

It is surprising that illite, which was often reported as an

abundant  component  of  the  insoluble  residues  in  the  Požáry
Formation  on  many  places  of  the  Prague  Synclinorium
(Suchý & Rozkošný 1996; Suchý et al. 1996) was not found.
On  the  other  hand,  the  indicated  amounts  of  white  mica  are
considerably higher than normally expected. It also belies the
infrared-absorption  and  chemically  based  detections  of  up  to
several per cent of illite in the S-D sediments at Klonk (Hladil
1992) where the XRD evidence was also unclear. In this case,
it was tentatively explained that due to the extensive damage
to  illite  structures  in  ultrafine  subcrystalline  mixtures  with
quartz, organic matter, and carbonates. The absence of typical
illite spectra in XRD diagrams can be explained by its low rel-
ative concentrations at ~ 1 % or less, but the possible presence

268

VACEK, HLADIL and SCHNABL

of both very high and very low crystalline forms related to il-
lite remains unsolved.

The greatly increased amounts of albite and microcline are

interesting in comparison with the proportions of plagioclases,
pyroxenes  and  amphiboles,  which  were  detected  in  these
rocks together with small, basalt related volcaniclastic grains
by  direct  observation  and  Energy  Dispersive  X-ray  Spectro-
scopy (EDX; e.g. Hladil 1992), but which have no significant
record in XRD. At least some of these albites and microclines
can be considered authigenic, but the differentiation between
the detrital and authigenic populations according to their crys-
tal  shapes  and  compositions  (cf.  Kastner  1971;  Kastner  &
Siever 1979; Mišík 1994) does not yet provide unambiguous
evidence  in  favour  of  this  origin.  Of  course,  quartz  and  also
kaolinite (to lesser extent) are probably not only of purely de-
trital origin (Hladil 1992).

Interpretation of the MS and GRS records

The MS and GRS variations can be used not only for

stratigraphic correlations of the studied sections but also for
interpretation of sedimentary environments and their evolu-
tion (especially in combination with sedimentological data).
It is based on methods and principles described in chap-
ter Methods.

The lowermost Lochkovian (and approximately the basal

part of the Lochkov Formation) in the shallow-water sections
is characterized by an abrupt increase of the MS (Fig. 2). On
the other hand, the eU curves mostly exhibit decreasing trend
in the proximity of the S-D boundary (Fig. 2). It is mostly re-
lated to decline of U content. It does not need to respond to the
decreasing content of clay, however. Very low covariance of
K  and  U  contents  indicates  different  natures  and  sources  of
these  two  components.  Potassium  is  related  to  clay  minerals
and  K-feldspar,  while  U  is  also  known  to  be  significantly
trapped  in  organic  matter.  A  slight  increase  of  K  concentra-
tions  immediately  above  this  pattern  is  indicative  of  higher
amount of clay minerals.

Thus, both magnitudes indicate enhanced amount of non-

carbonate  impurities  (magnetic  components  and  clay)  and
may be interpreted as a result of sea-level fall, which caused
increased  erosion  and  terrigenous  influx  to  marine  environ-
ments (Ellwood et al. 2000). This is in accordance with sedi-
mentological  data,  which  also  suggest  a  shallowing  trend  in
the lowest parts of the Lochkov Formation (approximately the
base of Lochkovian). It is expressed in partial sorting and re-
working/rounding  of  bioclasts  and  washing  out  of  fine-
grained matrix in grainstone deposits in contrast to underlying
strata (Vacek 2007).

In deep-water facies the high oscillation of the MS curve in

the upper part of the Požáry Formation (generally the upper-
most  Pridoli)  is  related  to  alternation  of  limestone  and  shale
interbeds (Figs. 3 and 8). It is noticeable that there are a num-
ber of analysed limestone beds, which have higher MS values
than the background hemipelagic shale interbeds, which usu-
ally  possess  a  higher  amount  of  insoluble  residue.  It  may  be
indicative  of  larger  amount  of  detrital  magnetic  particles  de-
livered to the basin with calciturbidites. The short-term facies

change  occurring  in  the  slope  environment  as  the  Scypho-
crinites H at the base of the Lochkov Formation (and close to
the S-D boundary) is usually interpreted as a result of a rela-
tive sea-level fall, which caused increased erosion in shallow-
water  areas  (e.g.  Kříž  et  al.  1986;  Chlupáč  &  Kukal  1988;
Crick et al. 2001; Vacek 2007). Such facies changes indicat-
ing  relative  shallowing  of  sedimentary  environments  have
been  described  from  other  regions  of  Europe  (e.g.  Carnic
Alps–Schönlaub et al. 1994), and North America (e.g. cen-
tral  Nevada–Klapper  &  Murphy  1975;  Matti  &  McKee
1977; Appalachian Basin–Denkler & Harris 1988). Howev-
er, this event should be accompanied by increased magnitudes
of  the  MS  with  enhanced  supply  with  terrigenous  detrital
magnetic particles. It is interesting that the MS of these rocks
is  much  lower  than  of  the  underlying  limestone/shale  se-
quence (Fig. 3). It might be explainable either by dispersion of
fine-grained magnetic particles in the bulk of calcium carbon-
ate  (carbonate  dilution  effect)  or  by  significant  washing-out
before  re-deposition  to  slope  and  toe-of-slope  environments
(e.g. da Silva & Boulvain 2006). More properly, the observed
effects of irregular washing of fine-grained matrix (often com-
bined with current-driven orientation of cephalopod shells in
these beds) suggest condensed deposition affected by bottom
currents. Another explanation of deposition of Scyphocrinites H
occurring  in  the  described  facies  mosaics  may  be  increased
local  subsidence  at  synsedimentary  growth  faults  (namely
“the precursor” Koda Fault, as presumed e.g. by Kříž 1992
or  Vacek  2007)  and  a  large  amount  of  carbonate  material
with primary low concentrations of magnetic minerals derived
from the upper part of the slope (as documented by the pres-
ence of carbonate lithoclasts derived from slope areas). Thus,
this locally developed rapid carbonate sedimentation alternat-
ing with periods of sedimentary starvation is not expressed in
enhanced MS values. Another explanation of the decline of
the MS can be proposed as a restriction of terrigenous input
during transgression. According to Schlager et al. (1994), the
maximum thickness of calciturbidites corresponds to periods
of  increased  carbonate  production  during  sea-level  rise
(highstand shedding).

An evident decreasing eU tendency from the upper Požáry

Formation  to  the  lowermost  Lochkov  Formation  (related  to
concurrently decreasing K, U, and Th concentrations) was doc-
umented  in  records  from  the  deep-water  sections  (especially
Klonk and Karlštejn; Fig. 3). In the latter, it culminates within
the Scyphocrinites H. The overlying limestone/shale sequence
of  the  Lochkov  Formation  is  again  characterized  by  a  slight
increase  of  detected  GRS  values  (Fig. 3).  Here,  the  main  eU
peaks  partly  correspond  to  background  shale  sediments  with
greater proportion of insoluble residue (namely clay minerals).
However,  there  are  also  peaks  situated  within  the  seemingly
massive  bedding  sets  of  proximal,  often  amalgamated  calci-
turbidites (Karlštejn or Radotín-U topolů; Fig. 3B and C). At
the  U  topolů  section  two  distinctive  U  peaks  (related  to  the
GRS-based concentrations of 15.1 and 16.4 ppm) are situated
within  and  slightly  above  the  Scyphocrinites  H  (bed 11,  sec-
tion  2.75  and  3.75 m  –  Fig. 3C),  which  do  not  match  en-
hanced  K  and  Th  values.  Uranium  is  known  to  be  highly
mobile during diagenesis, so these enormous peaks may corre-
spond to post-sedimentary concentration or indicate consider-

269

MAGNETIC SUSCEPTIBILITY AND GAMMA-RAY IN CALCITURBIDITIC FACIES (PRAGUE SYNCLINORIUM)

able dissolution (in some beds stylolites or extensive dissolu-
tions can be observed). However, similar peaks can be traced at
least in two other sections (Klonk section 5.5 m and Karlštejn
section 4.0 m; Fig. 3A and B) and may therefore correspond to
some widespread basinal events such as hiatuses or periods of
sedimentary starvation, or delivery and concentration of exotic
U-rich material.

At least three models must be considered for prograding of

basinal  carbonate  deposits  with  reduced  shale  intercalations:
1) the increased input of eroded material from shallow-marine
areas during the falling stage and lowstand system tracts; 2) the
opposite situation of a period of enhanced carbonate production
during the transgressive pulse, accompanied by highstand shed-
ding  effect  (Schlager  et  al.  1994),  and  3)  other  environmental
effects  influencing  the  shallow-water  carbonate  factories  or
pelagic  carbonate  productivity  would  be  employed  (e.g.  in-
creased abundance of pelagic crinoids, cephalopods).

Our interpretation based on evaluation of the GRS and MS

records of the slope facies and comparison with published
data is as follows: the uppermost part of the Požáry Formation
has a regressive character, which is expressed in high MS and
eU values (Figs. 3 and 8). Decreased carbonate productivity
and low depositional rates have been accompanied by lithifi-

cation of the sea-bottom. The lowermost part of the Lochkov
Formation reflects a transgressive pulse, which resulted in de-
creased input of terrigenous material and pronounced decline
of both magnitudes (Figs. 3 and 8). It was followed by slight
MS and eU rise, which responded to gradual regression. How-
ever, it is possible that deepening during the S-D interval was
caused  by  local  sea-bottom  subsidence  and  delivery  of  lithi-
fied deposits from underlying strata. The following regression
might have corresponded to a eustatic sea-level fall well docu-
mented  in  shallow-marine  areas.  The  deposition  of  the
Scyphocrinites  H  also  did  not  have  to  result  only  from  in-
creased supply of eroded material, which was formerly accu-
mulated on appropriate shallower-water parts of the slope but
rather was related to the mass development of floating echino-
derms in general, as their distribution is widespread across the
area and in many regions worldwide. Although it is certainly
less conspicuous in the shallow-water deposits composed most-
ly  of  crinoidal  limestones,  thicker  accumulations  of  Scypho-
crinites  debris  are  known,  for  example  in  the  Daleje  Valley
(between the Požáry Q and Opatřilka sections).

This interpretation is partly in agreement with Crick et al.

(2001), who presumed pronounced regression during the late
Pridoli followed by a moderate transgressive/regressive pulse

Fig. 8. Interpretation of the sea-level changes based on the MS records of the deep-water sections. Interpretation of the Klonk section was adopted
from Crick et al. (2001), our results correspond in its lower part, but differs in the upper. Po – the upper part of the Požáry Fm, Sc – Scypho-
crinites H., Lo – the lower part of the Lochkov Fm.

270

VACEK, HLADIL and SCHNABL

in  the  critical  S-D  interval.  According  to  results  of  the  last
mentioned study, the earliest Lochkovian has a clearly trans-
gressive  trend  (focused  on  Klonk),  and  this  is  in  contrast  to
our present results, which are based on several juxtaposed sec-
tions. The locally protracted high MS values with slowly de-
creasing GRS values in the combination with the presence of
coarse-grained  crinoidal  beds  up  to  the  lower  Lochkovian
(magnetic intervals VII—VIII) are unexpected or even counter-
intuitive with the first lower Lochkovian transgressive episode
(compare Fig. 8 herein to figs. 3, 4 in Crick et al. 2001).

However, we are aware that there are also alternative inter-

pretations based on the MS and GRS variations and other data
(e.g. carbon and oxygen isotopes) described in numerous pa-
pers from the Silurian and Devonian of the Prague Synclinori-
um and other regions. Due to limited space we briefly refer for
discussion to Hladíková et al. (1997), Slavík et al. (2000),
Mann et al. (2001), Saltzman (2002), Buggisch & Mann
(2004), Buggisch & Joachimski (2006), Bábek et al. (2007),
and Malkowski et al. (2009).

Conclusions

The combined MS-and-GRS stratigraphic assessment and

regional  comparison  of  the  carbonate  facies  around  the  S-D
boundary  in  the  Prague  Synclinorium  showed  a  significantly
good  correlative  value  between  sections  with  similar  facies
development  (i.e.  lower  subtidal  to  upper  slope  bioclastic
grainstones/packstones  and  lower  slope  to  toe-of-the-slope
calciturbidites  with  predominance  of  bioclastic  and  peloidal
wackestones/mudstones and calcareous shales). This compari-
son shows that the onset of the index species and the biostrati-
graphically  determined  S-D  boundary  may  be  diachronous
and  highly  depend  on  facies.  This  fact  makes  the  MS-and-
GRS  stratigraphy  a  powerful  tool  for  precise  correlation
within the region. It also proved remarkable condensation and
gaps in sedimentary record, especially in the lower slope con-
ditions where distal calciturbidites predominate.

A major effect on the MS is ascribed to paramagnetic min-

erals, which have been delivered to the basin from land.
Therefore, we can relate the changing amount of this terrige-
nous material detected by the MS and GRS to fluctuating
erosion and sea-level changes.

The critical S-D interval is characterized in relatively shal-

low marine areas by increased values of MS, which are inter-
preted as related to a higher influx of terrigenous material
during a regressive pulse. This interpretation is supported by
contemporaneous increasing concentrations of GRS-detected
potassium (clay) and is also supported by sedimentological
evidence.

On the contrary, a broader S-D interval in the deep-water

facies is characterized by visible facies change and decreasing
of the MS values. Maxima of MS and GRS in the uppermost
part of the Požáry Formation (generally upper Pridoli) are in-
terpreted as a response to a regressive phase associated with a
low depositional rate and sea-bottom lithification. The very
lowermost part of the Lochkov Formation (generally the base
of the Lochkovian) reflects a transgressive pulse leading to de-
creased input of terrigenous material and distinctive decline of

both  magnitudes.  The  overlying  sequence  characterized  by
slightly rising MS and eU corresponds to gradual regression.
The deepening trend during the S-D interval was probably ac-
companied by local subsidence and influx of eroded lithoclas-
tic  material.  The  following  regression  may  reflect  a  eustatic
sea-level drop well supported by evidence from the shallowest
marine  areas.  The  facies  change  close  to  the  S-D  boundary
and  deposition  of  the  Scyphocrinites  H  might  predominantly
result  from  a  biotic  event  unrelated  to  sea-level  changes  and
local subsidence, rather than from sea-level rise/drop.

Acknowledgments:  We  are  grateful  to  A.C.  da  Silva,  F.
Hrouda and one anonymous reviewer for valuable comments
and  suggestions,  which  helped  improve  the  original  manu-
script. Special thanks are due to colleagues who provided the
complementary  analyses  used  for  the  interpretation  (V.
Goliáš,  XRD,  Charles  University,  Prague;  A.  Langrová,
EDX, WDS; P. Pruner, Academy of Sciences, Prague, rock
magnetism),  and  those  who  discussed  environmental  con-
straints (J.E. Barrick, Texas Tech University, Lubbock, M.A.
Murphy, University of California, Riverside and L. Slavík ,
Academy  of  Sciences,  Prague)  and  the  importance  of  the
Klonk MS section and observed periodicities (B.B. Ellwood,
Louisiana  State  University,  Baton  Rouge).  The  role  of
framework  grants  is  appreciated  (MSM  0021620855,
AV0Z30130516, IAA300130702, IGCP 580).

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