Stratigraphic significance and resolution of spectral ... · Devonian carbonates of the Barrandian area, Czech Republic; a correlation with magnetic susceptibility and gamma-ray logs

Sedimentary Geology 225 (2010) 83–98

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Sedimentary Geology

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Stratigraphic significance and resolution of spectral reflectance logs in LowerDevonian carbonates of the Barrandian area, Czech Republic; a correlation withmagnetic susceptibility and gamma-ray logs

Leona Koptíková a,d, Ondřej Bábek b,c,⁎, Jindřich Hladil a,b, Jiří Kalvoda b, Ladislav Slavík a

a Institute of Geology AS CR, v.v.i., Rozvojová 269, 16500 Praha 6, Czech Republicb Department of Geological Sciences, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republicc Department of Geology, Palacký University, Tř. Svobody 26, 77146 Olomouc, Czech Republicd Institute of Geology, Faculty of Science, Charles University in Prague, Albertov 6, 12843 Praha 2, Czech Republic

⁎ Corresponding author. Department of GeologicalKotlářská 2, 61137 Brno, Czech Republic. Tel.: +420 54

E-mail address: [email protected] (O. Bábek).

0037-0738/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2010.01.004

a b s t r a c t
a r t i c l e i n f o
Article history:Received 26 March 2009Received in revised form 19 January 2010Accepted 26 January 2010Available online 1 February 2010

Communicated by B. Jones

Keywords:VIS spectral reflectanceCyclostratigraphySea-level changesLower DevonianRed pelagic carbonatesDiagenesis

Spectral reflectance (SR) is a relatively novel technique which is used as a proxy for organic carbon andsiliciclastic input in carbonate sediments, in particular in core research. This study summarizes results fromhigh-resolution outcrop SR logging of a Lower Devonian distal ramp carbonate section of the Barrandianarea, Czech Republic. The SR data were correlated with parallel petrophysical and geochemical data with anaim to improving their cyclostratigraphic and event stratigraphic interpretation potential and the gaps SRdata can fill. Transgressive–regressive trends, indicated by facies stacking patterns, CaCO3 data and spectralgamma-ray (GRS) logs, are well correlated in the regional context and point to third-order eustatic sea-levelfluctuations. Peak regressions of the TR cycles are sensitively marked by CIE L*a*b* parameters and percentreflectance in colour bands. The SR and magnetic susceptibility (MS) data are markedly cyclic on a moredetailed scale. Their inferred periods vary between 49.2 and 205 kyr indicating that they may have beenforced by orbital cycles in the Milankovitch band. The SR and MS cycles are either in phase or out of phase.The latter cycles point to a strong diagenetic forcing of the colour cyclicity. Hematite concentration cycles,inferred from SR signal in the Pragian red hemipelagic carbonates, are interpreted as reflecting changingbottom oxygenation and subsurface redox gradients during early diagenesis. The CIE L* parameter was foundto be related to the presence of diagenetic hematite, which indicates that cyclostratigraphic analysis ofsimple greyscale data can be affected by considerable error due to chromacity. Although the CIE L* parameteris partly correlated with CaCO3 concentrations, the MS data are not, which suggests that the MS signal can besignificantly influenced by diagenetic minerals and its cyclostratigraphic interpretation can be potentiallymisleading. The SR data, backed up by GRS, MS and CaCO3 data indicate that the red colour in the Pragianhemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of anoxygenated ocean floor. This suggests that deep-sea bottom oxygentation may have been in operation duringthe early Devonian, which is traditionally perceived as a typical greenhouse period. The SR data can beobscured in several m thick zones of elevated CIE L* and other colour changes (“leaching zones”) related topostdepositional alteration along subvertical faults, however, this fault-related telogenic alteration has littleinfluence on the MS signal. The combined use of SR and MS has a synergic effect and can significantlyimprove the interpretation of their respective cyclic signals, particularly in relation to the diageneticoverprint and possible carriers of the MS.

Sciences, Masaryk University,9494756.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Numerous recent studies in cyclostratigraphy and event stratigra-phy rely on petrophysical techniques which are traditionally used toestablish a basic stratigraphic framework in subsurface wire-linelogging. Gamma-ray spectrometry (GRS) and magnetic susceptibility

(MS), in particular, have the advantage of quick acquisition of largedata sets with a corresponding focus on very high stratigraphicresolution both in outcrop and core research (Rothwell and Rack,2006). In carbonate sediments, MS is used as a concentration indicatorof mostly paramagnetic Fe-bearing terrigenous minerals whosevariation is often interpreted in terms of climatic and/or sea-levelfluctuation (Robinson, 1993; Mayer and Appel, 1999; Crick et al.,2001; Hladil, 2002; Ellwood et al., 2006; da Silva and Boulvain, 2006;Hladil et al., 2006). GRS logs often serve the same purpose. Gamma-ray spectrometric concentrations of Th and K are thought to be related

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Fig. 1. Location of the Prague Synform and the Požáry-3 section.

84 L. Koptíková et al. / Sedimentary Geology 225 (2010) 83–98

to the concentration of siliciclastic components (illite and other clayminerals, micas, K-feldspars) in carbonates (Rider, 1986; Doveton,1994; Fiet and Gorin, 2000; Ehrenberg and Svana, 2001; Fabriciuset al., 2003) and have proved highly useful in cyclostratigraphy(Postma and ten Veen, 1999; Prokoph and Thurow, 2000). Morerecently, methods focused primarily on sediment colour such asdigital image analysis of core images and visible-light (VIS) spectralreflectance logs are also used as indicators of CaCO3 content andorganic carbon in carbonates which are mostly interpreted in terms ofsediment cyclicity and evolution of palaeoclimate (Warren et al.,1998; Nederbragt et al., 2006; Rogerson et al. 2006). Moreover, bothVIS reflectance spectra and diffuse spectral reflectance (DSR) haveproven to be a highly efficient method to estimate concentration ofspecific chromophores such as hematite, goethite and chloriteminerals (Mandarino, 1967; Deaton and Balsam, 1991; Mix et al.,1995; Scheinost et al., 1998), which are often used as palaeoclimaticproxies (Ji et al., 2001; Debret et al., 2006; Grygar et al., 2006).

Colour in carbonate rocks can be controlled by the presence ofvarious chromophores including organic carbon, CaCO3, dolomite andhematite, all of which can have stratigraphic significance. Nederbragtet al. (2006) have shown that the lightness of a predominantly car-bonate rock expressed in CIE (Commission Internationale d'Eclairage)L* value is sometimes closely related to organic carbon and CaCO3

content. This relationship is particularly strong in two-componentcolour systems composed essentially of these two chromophores. TheL* lightness parameter, however, can fail to quantify colour differencesin coloured carbonates (red, pink, and yellow) whose colour iscontrolled by a multitude of components. Colour in these carbonatescan, nevertheless, carry important information about depositional and/or early diagenetic processes and should be considered in palaeoenvir-onmental, palaeoclimatic and cyclostratigraphic interpretations. Thered colour in carbonates is usually related to a concentration of earlydiagenetic hematite whose formation is thought to indicate either seafloor oxygenation in shallow-water settings (Wang et al., 2005;Wagreich and Krenmayr, 2005) or activity of iron bacteria in oxygendepleted pelagic environments (Préat et al., 1999; Mamet and Préat,2006). Mamet and Préat (2006) provide biological and chemicalevidences (including isotopic δ56Fe values) for the bacterial origin ofsubmicron hematite in red pelagic limestone, which is generally poor iniron.Hematite is precipitatedas a result of theactivity ofmicroorganismsthat oxidize Fe(II) to generate energy for their growth inmicroaerobic oranoxic environments close to the sediment–water interface (Mamet andPréat, 2006; pp.148). The stratigraphic alternation of red and greycarbonates might therefore indicate changes in bottom oxygen levelsrather than the siliciclastic input from the continent. Although redpelagic limestone is usually confined to distinct stratigraphic intervals(Martire, 1992; Wang et al., 2005; Cecca et al. 2005), the genesis of theturning points in their colouration is not well understood.

The Požáry-3 section in Řeporyje near Prague, Czech Republicexposes an almost 125 m thick undisturbed succession of LowerDevonian (Lochkovian to early Emsian) resedimented and pelagicdeep-water carbonates. A distinct band of red nodular carbonate of thePragian age, approximately 20 m thick, is exposed in the upper third ofthe succession within the Praha Formation. The basal and topboundaries of the red interval lie parallel to bedding and thereforerepresent a stratigraphic feature. The aim of this paper is to investigatewhat added information spectral reflectance can give to the otherroutine petrophysical techniques with an emphasis placed on theircyclostratigraphic and sequence stratigraphic interpretation.

2. Geological setting and stratigraphy

The section Požáry-3 is located in an active quarry at the south-western periphery of Prague, 1 km ESE of the centre of the village ofŘeporyje. The western wall of the quarry provides unique access to a∼130 m long, freshly quarried and tectonically almost undisturbed

Lower Devonian stratal succession, which starts in the lowermostLochkovian and ends in the lower part of the Emsian. The GPScoordinates for the beginning and end of this section are 50° 01′ 44″N,14° 19′ 36″ E and 50° 01′ 38″ N, 14° 19′ 39″ E, respectively (Fig. 1).

Geologically, the study area is located in the Bohemicum (Teplá-Barrandian) terrane approximately at the centre of the BohemianMassif, Czech Republic, which is composed of Neoproterozoic toMiddleDevoniansedimentary andvolcanic rocks. The rocksweredeformedandpartly metamorphosed during Cadomian and Variscan orogeny. Duringthe Variscan orogeny, the Bohemicum terrane, a part of the Armoricanterrane assemblage, was deformed and buried in several steps duringthe Frasnian to late Carboniferous interval (Suchý et al., 2002;Glasmacher et al., 2002; Melichar, 2004; Cháb et al., 2008). Theseprocesses led to the formation of the Prague Synform (Melichar, 2004),an erosional relic of unmetamorphosed Lower Palaeozoic volcano-sedimentary successions. Their facies and faunas show a soundconnection with the Ibero-Maghrebian faunal province of the RheicOcean domain (Chlupáč et al., 1998; Plusquellec and Hladil, 2001).

The Požáry-3 section (Fig. 1) is exposed in a narrow belt of Devonianrocks in the NW limb of the Prague Synform. This NW belt of Devoniansediments has characteristic vertical facies successions, which can betraced for about 25 km approximately along the ENE-WSW strike butonly for a few km or hundreds of metres perpendicular to the strike(Hladil et al., 1996; Hladil and Kalvoda, 1997; Melichar and Hladil,1999).

2.1. Stratigraphy

The biostratigraphy of the Požáry-3 section is mainly based onconodonts and dacryoconarids (Slavík, 2004a,b; Slavík et al., 2007).The carbonate succession at the Požáry-3 section can be subdivided

85L. Koptíková et al. / Sedimentary Geology 225 (2010) 83–98

into the Lochkov, Praha and Zlíchov Formation (Fig. 2). The lowerparts of the section (0 to 77.6 m above the section base) correspond tothe Lochkov Formation (Chlupáč, 1953; Chlupáč et al., 1998). Thesediments comprise mostly of thin bedded and nodular, sometimescherty, calcarenites and calcisiltites alternating with marl laminae.The basal and top parts of the Lochkov Formation comprise thick-bedded, coarse-grained calcarenites and calcirudites. The age of theLochkov Formation is Lochkovian to earliest Pragian. The base of theLochkovian stage (Silurian–Devonian/S/D/boundary) was found inthe parallel Požáry-1 section (Fig. 1), based on FAD of the basalDevonian index Icriodus hesperius (Carls et al., 2007) approximately5 m below the base of the Požáry-3 section. The lower part of theLochkov Formation (0–22 m) is characterized by the presence ofIcriodus ex gr. woschmidti, Zieglerodina remscheidensis, Pedavisbreviramus and the first representatives of Lanea. The middle part(22–72.5 m) starts with Ozarkodina carlsi that co-occur with dacryo-conarid Homoctenowakia senex and contains typical Middle Lochko-vian species Lanea eoeleanorae, Ancyrodelloides transitans andAncyrodelloides trigonicus. The uppermost part of the LochkovFormation is Late Lochkovian (72.5 to ∼76.0 m) and (?) early Pragian(∼76.0 to 77.65 m) in age, according to correlation with the nearbyGSSP at Velká Chuchle. The difference between the thickness-to-timeratio of the lower and middle Lochkov Formation and the upperLochkov Formation may indicate a problem in time spacing of theLochkovian stage (Murphy and Valenzuela-Ríos, 1999) or theextremely low sediment accumulation rate of the upper LochkovFormation.

The overlying Praha Formation (77.6 to ∼118 m) is subdividedinto several stratigraphic members comprising thick-bedded crinoidalcalcarenites and calcirudites at the base overlain by thin-bedded

Fig. 2. Devonian lithostratigraphy of the Prague Synform (after Chlupáč et al. 1998).

crinoidal calcarenites and calcisiltites, red nodular calcisiltites,calcilutites and marls and thin-bedded, dacryoconarid calcilutitesand calcisiltites with Chondrites and Zoophycos trace fossils (Chlupáčet al., 1998). There is an about 0.8 m thick set of 8 sheet-like bedsseparated by black mudstones between 112.8 and 113.6 m; the so-called “graptolite interval” or “basal Emsian anoxic event beds”(Hladil et al., 1996; Chlupáč et al., 1998). This interval is characterizedby increased contents of glauconite, phosphates and the presence ofgraptoloids. The age of the Praha Formation is Pragian to earliestEmsian based on FAD of conodonts from the Icriodus steinachensisstock, Pelekysgnathus ex gr. serratus stock and dacryoconarids insamples close to the “graptolite interval”. The basal Emsian GSSP inKitab, Uzbekistan (Yolkin et al., 1997) can be approximately projectedto strata at 87 to 90 m above the section base, close to the range ofPelekysgnathus ex gr. serratus stock (89.5–91.2 m).

The overlying Zlíchov Formation (above ∼118 m) comprises thin-bedded, normally graded layers of calcisiltite and calcarenite alternatingwith thin blackmarls and shales. The age of the Zlíchov Formation at thePožáry-3 section is early Emsian based on dacryoconarid fauna (FAD ofNowakia zlichovensis and the disappearance of the “typical Pragian”dacryoconarids, P. Lukeš — pers. comm.).

The Praha Formation is dissimilar to the underlying Lochkov andoverlying Zlíchov formations in several aspects. Unlike the grey-to-black background sediments of the Lochkov and Zlíchov Formation,with abundant silica, phosphates, organic matter and dysoxicChondrites trace fossils, most sediments of the Praha Formation arered, pink or light coloured, and contain abundant recycled submarine-weathered skeletal material, hematite and iron oxy-hydroxides,which are suggestive of oxic bottom conditions. In addition, theconodont data indicate that the total production of carbonate in thePraha Formation was considerably lower than in the overlying andunderlying strata. The remarkable bloom of bioeroders, an increaseddiversity of shallow-water faunas in the adjacent shallow-watersettings as well as a number of other indications (Hladil et al., 2008)suggests that the Praha Formation was deposited in a period of arelatively low sea level and a relatively warm climate.

3. Material and methods

Carbonate composition and grain size data were studied by opticalmicroscopy and electronmicroprobe analysis (Cameca SX 100 EMPA).Samples for thin sectioning were taken at 0.5 to 2.0 m interval. Grainsize data were measured from linear scans through a thin section areaprocessed using digital image analysis in object-definition mode(Laboratory Imaging LIM Lucia-S software package). The data weretreated and represented as semiquantitative. Semiquantitative graincomposition data were obtained from digital image analysis andrepresented in two categories: significant (≥20%), accessory (≥5%).Quantities below 0.25% were ignored.

The gamma-ray spectra (GRS) were measured by a GR-320enviSPEC portable spectrometer with a 3×3 in. NaI(Tl) scintillationdetector (Exploranium, Canada). Counts per seconds in selectedenergy windows were directly converted to concentrations of K (%),U (ppm) and Th (ppm). One measurement with a 120-s count timewas performed at each logging point, perpendicular to the sectionwalland at full contact with the rock. The section was logged at a 0.5-minterval in the Lochkov Formation and a 0.25-m interval in the Prahaand Zlíchov Formations. Three hundred and thirty eight measure-ments were obtained. The combined error from conditions, instru-ment and repeated measurements was estimated to be less thanabout ±7.5% for the K, U and Th element concentrations.

Fresh rock samples for bulk magnetic susceptibility (MS) mea-surement, weighing 20–45 g per sample were collected at intervalsmeasuring 0.1-m for the Lochkov and Zlíchov formations, and 0.05 mfor the Praha Formation. The MS was measured using KLY-2 kappa-bridge (Agico, Czech Republic; magnetic field intensity of 300 A m−1,


operating frequency of 920 Hz, and sensitivity of 4.10−8 SI). Raw andnormalized data on mass-specific MS expressed in m3 kg−1×10−9

were used. The error of measurement due to room conditionsstabilizing, shape parameters and orientation of the sample duringthe repeated measurements did not exceed ±2%. Sixteen hundredand ninety three samples were measured in total.

Spectral reflectance data were measured by a hand held X-RiteSP62 (X-Rite Inc., U.S.A.) visible-light (VIS) spectrophotometer andprocessed with X-Rite Color Master software. The output datarepresent a spectral reflectance curve with readings taken in 10 nmincrement in a 400 to 700 nmwavelength range plus CIE L*a*b* colourdata taken in the D65/10° Illuminant/Observer mode. The spectralreflectance data were measured from Ø 8 mm circular area on dry,fresh and even surfaces of the same rock chips sampled for MS. Thepercent reflectance in the spectral bands, violet (400–445 nm), blue(445–485 nm), green (485–555 nm), yellow (555–585 nm), orange(585–625 nm) and red (625–700 nm), was normalized to the totalreflectance in the visible light (400–700 nm). Peaks on the firstderivative of the spectral reflectance curve were used as a semiquan-titative estimate of goethite and hematite concentration (Barrancoet al., 1989; Deaton and Balsam, 1991; Ji et al., 2001). Goethite andhematite were inferred from the height of peaks on the 1st derivativecorresponding to their absorbance bands (425 to 445 nm and 525 to535 nm for goethite; 565 to 575 nm for hematite). These estimates aresemiquantitative as the height of the peak is a function of both theconcentration of the iron oxide minerals present and the compositionof the matrix. The peak height is substantially altered by changes inmatrix composition (Deaton and Balsam, 1991; Ji et al., 2001).

Concentrations of calcium carbonate and total organic carbon(TOC)were analysed from the same samples used forMS. The sampleswere ground to fine powder to pass the 70-μm size sieve.Homogenized subsamples weighing 0.05 g were dissolved in HCland H3PO4, dried at 105 °C temperature, dry-combusted in stream ofpure oxygen up to a 1000 °C temperature and analysed usingStrohlein C-mat 5500 non-dispersive infrared carbon analysers. Thestatistical error was kept at ±4% TOC.

Table 1Principal facies characteristics of the Požáry-3 section.

Facies Rock type(Dunham)

Grain size/sorting Bed geometry/bed thickness/bedding and sedimentarystructures

F4 Packstone/grainstone

Coarse-grained calcarenite/moderately sorted

Sheet-like/∼50 to ∼150 cmthick/massive

F1 Grainstone(rarely packstone)

Coarse-grained calcarenite/moderately to well sorted

Sheet-like–lens-like/≤20 cm/amalgamation gradedbedding, sharp bed bases

F2 Grainstone/packstone

Coarse-grained calcarenite/moderately to well sorted

Sheet-like/≤20 cm/wavylamination (HCS?)


Fine-grained calcarenite/moderately to well sorted

Obscured by stylolites andnodular fabric/massive,nodular fabric, rare parallellamination

F5 Packstone Moderately to poorlysorted

Sheet-like–lens-like/≤40 cm/graded bedding, sharp bedbases, rare parallellamination, large floatingallochems, geopetal fills,shelter-type porosity

F6 Wackestone/packstone

Calcisiltite, fine-grainedcalcarenite/poorly sorted

Obscured by stylolites andnodular fabric/nodular fabric,geopetal fills


Fine-grained calcarenite/well sorted

Sheet-like/≤10 cm/parallellamination, shell alignment

4. Facies, allochem composition and stacking patterns

Carbonate rocks at the section are mainly composed of fine- tocoarse-grained packstone and grainstone and, to a lower extent,wackestone with scarce nonskeletal grains and abundant skeletalallochemswithmixed shallow-water benthic and deep-water benthicand pelagic origin. Beds are often normally graded and parallellaminated and often have a nodular fabric. Seven facies types, F1 to F7,were recognised (Table 1, Figs. 3, 4).

4.1. Interpretation of facies, facies associations and stacking patterns

The common presence of “deeper-water” benthic skeletal grainssuch as echinoderms, bryozoans, trilobites and ostracods, thepresence of sponge spicules, conodonts and pelagic dacryoconaridsand scarcity of shallow-water corals and stromatoporoids (cf.Armstrong and Mamet, 1977; Playford, 1980; Franke and Walliser,1983; Yochelson and Lindemann, 1986) suggest a relatively deepsubtidal and/or pelagic setting for most of the facies types. The facieswith graded bedding and sharp bed bases (F1 and F5) representdeposits from turbulent density flows (storm or gravity-flowdeposits). Many of the resedimented facies (F3, F4, and F5) containa mix of relatively deep-water allochems (sponge spicules, ostracods,cephalopods, dacryoconarids, and conodonts) and relatively shallow-water benthic allochems (corals, thick-shelled molluscs, brachiopodsand peloids) and. The F7 facies with parallel lamination and alignmentof pelagic dacryoconarids indicates deposition from unidirectionalflows in (hemi) pelagic environment (turbidites, bottom currentdeposits). The fine-grained facies F3 and F6, often with nodular fabric,were presumably deposited from (hemi)pelagic suspension andpossibly partly reworked by bottom currents. Interpretation of theirdeep-water origin is further supported by the presence of Zoophycosand Chondrites trace fossils.

The variation in facies types and allochem composition allow usto subdivide the carbonate succession into four facies associations,A1–A4 (Figs. 3, 4). The first facies association (A1) comprises the basal

Allochemsdominant/subordinate

Remark Interpretation

Echinoderms, bryozoans,brachiopods, trilobites, ostracods,molluscs/dacryoconarids,cephalopods, conodonts

Dedolomitization Amalgamatedturbidites (?),mass-flow deposits

Echinoderms, brachiopods,bryozoans, trilobites/ostracods,molluscs, corals, ooids

Micrite envelopes,dedolomitization,opaque minerals

Turbidites(?storm deposits)

Echinoderms, brachiopods,bryozoans, trilobites/ostracods,molluscs, corals, ooids

Micritic envelopes,dedolomitization

Storm deposits

Echinoderms, brachiopods,bryozoans, trilobites, molluscs/dacryoconarids, sponge spicules,orthoconid nautiloids, conodonts,peloids, intraclasts

Bioturbation,dedolomitization

Deep subtidaldeposits, reworked bystorms or bottomcurrents

Trilobites, echinoderms,ostracods, gastropods, bivalves,brachiopods, dacryoconarids

Dedolomitization Turbidites(?storm deposits)

Sponge spicules, dacryoconarids,ostracods, echinoderms, trilobites,molluscs, bryozoans/cephalopods,conodonts

Fitted fabric, stylolites,little fragmentation oflarge skeletal grains,bioturbation, ±chertsdedolomitization

Pelagic suspensiondeposits, very deepsubtidal

Dacryoconarids, unidentifiableskeletal debris

Shell fragmentationdue to compaction

Turbidites/bottomcurrent deposits

Fig. 3. Stratigraphic framework of the Požáry-3 section: facies and facies associations, lithostratigraphy, chronostratigraphy and basic outcrop logging patterns (spectral gamma-raylogs, magnetic susceptibility log, brightness /CIE L*/ and percentage reflectance in the red band).


parts of the Lochkov Formation (0 to ∼12 m). It consists of layers ofpresumably storm deposits (see below) of the facies F1 and F2. Thebeds are either separated by thin marl laminae in the lower part (0 to2.2 m) or amalgamated to form thicker units (N1 m). The sedimentgrain size corresponds to a medium arenite to fine rudite fraction(Φ=2 to −2) and at times has a multimodal grain size distribution(Fig. 4). Sediments of this association are rich in more shallow-waterallochems such as corals, brachiopods, molluscs, bryozoans and ooidsand only rarely contain sponge spicules, ostracods and trilobites.Wavy lamination, present in the facies F2 (wave ripples 5.5 m abovethe section base, Fig. 3) can represent HCS. This is only exposed in asingle plane, however, making precise interpretation impossible.These two characteristics, when combined, clearly distinguish thebasal facies association from the overlying deposits of the LochkovFormation and may support the interpretation that the depositiontook place above the storm wave base (SWB). However, the width ofthese laterally accreted systems of small waves does not exceedseveral tens of metres and some of the bases are truncated in adjacentoutcrops. Thus interpreting them as the upper-slope channel fills andbioclastic lobes must be considered a possibility (Vecsei 1998). Thelatter interpretation is supported by the fact that this basal faciesassociation quickly passes upward into the overlying one, depositedbelow the SWB, in a zone which only about 2 m thick.

The second facies association (A2) comprises the remainder of theoverlying Lochkov Formation and the basal part of the PrahaFormation (∼12 to ∼80 m) and consists of cyclic alternation ofturbidites (F1, F4), deep-subtidal deposits (F3) and (hemi)pelagic

suspension deposits (F6). The common presence of relatively shallow-water benthic allochems and the relatively deep-water allochemsmay indicate downslope mixing via turbidite sedimentation. Pelagicand deep-water allochems are present even in the thickest and mostcoarse-grained layers of the facies F1 and F4, which may support thisgravity-flow origin interpretation. This facies association shows ageneral upward coarsening and thickening trend from the lower partwith a predominance of facies F3 and occasional beds of F6 towardsthe top parts with a predominance of facies F1. Facies are furtherorganised into cycles ∼5 to ∼11 m thick, which are superimposed onthis trend. The cycles are mostly asymmetric, fining upward, startingwith thicker, coarse-grained facies F1 (F2) and passing upward intomore fine-grained facies F3 or F6. The coarse-grained parts of thecycles correspond to medium arenite to a very fine rudite fraction(Φ=2 to −2) while the most of the fine-grained parts correspond toa silt to fine-grained arenite fraction (Φ=6 to 2). Cycle bases partlycorrelate with allochem composition changes, which also show acyclic pattern. Intervals enriched in more shallow-water skeletalgrains such as corals, brachiopods, bryozoans and molluscs alternatewith those enriched in pelagic or “deep-water” sponge spicules,dacryoconarids, trilobites and ostracods (Fig. 4). This facies associa-tion traverses both the Lochkovian/Pragian stage boundary at a heightof 76 m and the Lochkov/Praha Formation boundary at a height of76.6 m without significant facies- and compositional change. How-ever, its topmost part is very coarse grained (Φ=2 to −2) anddistinguished from the underlying parts by the presence of theamalgamated facies F4.

Fig. 4. Facies associations at the Požáry-3 section: semiquantitative representation of main skeletal grains and grain size data.


The overlying facies association (A3) comprises the entire PrahaFormation and the basal part of the Zlíchov Formation (∼80 to∼118 m) and can be subdivided into two segments. The basalsegment (∼80 to ∼85 m) comprises a thinning- and fining-upwardsuccession of beds of the facies (F5), which passes gradually into the(hemi)pelagic sediments (F6), which are typical of the overlyingsegment (∼85 to ∼118 m). The graded beds of facies F5 of the basalsegment typically contain a mix of shallow-water benthic allochemsand pelagic or deep-water benthic allochems, which supports aninterpretation of their turbidite origin. The overlying segment is com-

posed entirely of the hemipelagic nodular deposits of facies F6, whichshow amultimodal grain-size distribution and predominance of pelagicdacryoconarids. The “graptolite interval” (∼112.8–113.6 m) comprisesseveral sheet-like layers about 8 to 10 cm thick, which correspond tothe facies F5 and contain more shallow-water allochems than theunderlying and overlying background deposits and probably representturbidites.

The fourth facies association (A4) (∼118 m to the upper end of thesection) represents a coarsening-upward succession, in which hemi-pelagic calcilutite (F6) passes upward into fine-grained (Φ=4 to 2)


calciturbidites of facies F5 and finally to medium- to coarse-grained(Φ=2 to 0) calciturbidites of facies F1 with an elevated proportion ofshallow-water allochems (Fig. 4).

5. Gamma-ray and magnetic susceptibility log

5.1. Gamma-ray log

The GRS logs show a considerable degree of vertical organisation(Fig. 3). Carbonates of the facies association A1 (20 measurements)have low average concentrations of K (0.4%) and Th (2.4 ppm) andrelatively high concentrations of U (4.1 ppm). The concentrationsshow a relatively high variation for K (standard deviation, SD 0.4) andU (SD 2.4) and a low variation for Th (SD 0.6). The A1/A2 boundaryis indicated by a marked drop in U concentrations and a rise inK/Th ratios at a height of 10–12 m above the section base. Carbonatesof the association A2 (143 measurements) have comparable concen-trations of K (Ø 0.5%), Th (Ø 1.8 ppm) and U (Ø 4.2 ppm) but theyshow a relatively lower variation for K (SD 0.2), Th (SD 0.7) and, inparticular U (SD 1.0). Both A1 and A2 associations have relatively lowTh/U ratios (Ø 0.8 and SD 0.4 for A1; Ø 0.5 and SD 0.2 for A2). Whilethe U concentrations show relatively random vertical distribution, theK and Th are organised into distinct coincident cycles in the A2association. There are six such cycles with the wavelength rangingfrom 7 to 11 m. The cycles tend to be slightly asymmetrical,particularly in the upper part of the A2 (Fig. 3). The K and Th cyclesshare a similar wavelength with the facies cycles referred to inSection 4.1. Minima in the K and Th concentrations tend to correlatewith the packets of coarse-grained turbidites (facies F1) although anunequivocal correlation cannot be inferred from the available data.Concentrations of K statistically correlate with Th concentrationsthroughout the section (R2=0.639). Th and K concentrations usuallyrelate to the presence of aluminosilicates (illite and other clayminerals, potassium feldspars, micas) in carbonates while a goodcorrelation between K and Th is considered to reflect a fine-grainedsiliciclastic admixture in carbonate rocks (Rider, 1986; Fiet and Gorin,2000; Ehrenberg and Svana, 2001; Fabricius et al., 2003). In our logs,the maxima in K and Th concentrations coincide with minima inCaCO3 contents (Fig. 3), which supports the previous interpretation.U concentrations, by contrast, usually relate to the presence of organiccarbon and calcium phosphates, and uranium has a very heteroge-neous distribution in sediments (Rider, 1986; Lüning et al., 2004). Weconsequently interpret the K and Th cycles as reflecting cyclic dilutionof fine-grained terrigeneous material in carbonate due to a variationin carbonate productivity and/or siliciclastic input into the basin. Asindicated by the facies, these cycles are presumably related to landwardand basinward facies shifts (transgressive–regressive, TR cycles). Thefunnel-type shapes (Rider, 1986) of the cycles suggest that abruptbasinward facies shifts occur at cycle bases, which are followed up-section by gradual transgressions, see Section 7.1 below. If we take intoaccount the total thickness of the Lochkov Formation (82 m) and theestimated duration of the Lochkovian stage (∼4.8 Myr, Gradstein et al.,2004), the thickness of the K and Th cycles implies cycle periodsbetween ∼0.4 and ∼0.64 Myr.

The GRS logs show no clear indication for the Lochkovian/Pragianstage boundary at a height of 76 m or the Lochkov/Praha Formationboundary at a height of 77.6 m above the section base. The faciesassociation A3 demonstrates different GRS patterns. Relative to theunderlying strata, the A3 (151measurements) has significantly higheraverage concentrations of K (0.9; SD 0.2), Th (3.8; SD 0.9) and mark-edly lower average concentrations of U (1.2; SD 0.6). This overturn isthe most prominent feature in the entire GRS log. Th/U ratios are high(Ø 4.2) throughout the A3 association. The Th/U ratio is considered anindicator of terrigenous-to-marine influence (Doveton, 1994) due tothe terrigeneous affinity of Th (Schmoker and Hester, 1983) and theaffinity of U, mostly in the form of soluble U6+, to absorb to organic

matter and/or co-precipitate in calcium phosphates in marine anoxic/dysoxic environments (Lüning et al., 2004). High Th/U ratios (N7)are regarded as reflecting oxidizing depositional conditions whilelow Th/U ratios (b2) are regarded as indicating reducing conditions(Adams and Weaver, 1958; Doveton, 1994, pp. 38). The considerablyhigher Th/U ratios in A3 may therefore indicate more significantterrigeneous influence andbetter-oxygenated environments than in theunderlying facies associations A1 and A2. The upward thinning andfining succession in the basal segment of the A3 is associated with amajor increase in K and Th concentrations (Fig. 3) betweenheights of 80and 87m, which is followed by a sudden but less marked drop in boththe values. This drop correlates with the Pragian/Emsian boundaryaccording to the GSSP in Kitab, Uzbekistan. The overlying hemipelagicnodular carbonates of A3 have relatively uniform concentrations ofK and Th and uniform and very high Th/U ratios between ∼89 and∼113 m. Between ∼113 m and ∼118 m in height, the K andTh concentrations gradually decrease while the Th/U ratios slowlydecrease. This trend is bound by two rapid excursions towards positivevalues for K, Th and inparticular U andhereto associated Th/U ratios, the“classical” Pragian/Emsian boundary (“graptolite interval” at a heightof ∼113 m) at the base and the Praha/Zlíchov Formation boundary(∼118 m) at the top. From this point upward, carbonates of theA4 association (22 measurements) again have relatively steady andlow average concentrations of K (0.6; SD 0.2) and Th (2.5; SD 0.7)and relatively high average concentrations of U (2.3; SD 0.5) and highTh/U ratios (Ø 1.1).

5.2. Magnetic susceptibility log

The MS log can be subdivided into several, distinct verticalsegments. Between 0 and ∼14 m (facies association A1 and the baseof A2), the MS values (145 measurements) are relatively high (Ø 6.5)and highly variable (SD 5.23; coefficient of variance 80.4). Thefollowing segment (∼14 to ∼81 m, A2 facies association) is separatedfrom the underlying one by a gradual transition and has a generallylower (Ø 2.7) and a highly variable (SD 2.12; coefficient of variance78.8) MS signal. When smoothed using a five-point running average,the MS curve shows a cyclic arrangement. Five such cycles with awavelength between ∼2 and ∼3.5 m were recognised in the intervalfrom ∼5 to ∼17 m while another thirteen cycles with a wavelengthbetween ∼1 and ∼3 mwere found in the interval between ∼55 m and∼80 m (Figs. 3, 5). The resolution of the facies log makes it impossibleto link the cycles directly to facies, as it was partly possible for thelower-order GRS cycles (see above). The MS peaks, however, tend tostackmore densely in zones that correlate withmaxima on the Th andK log (Fig. 3). Throughout the section, theMS log and the K and Th logslook similar at a glance with, however, only a weak statisticalcorrespondence between them (MS:K, R2=0.46; MS:Th, R2=0.37).Although this may be partly caused by the differences in the samplevolume and the sampling interval between both techniques (seeSection 3), it is inferred that the MS signal can be driven by otherfactors beyond the fine-grained siliciclastic admixture in carbonates.In addition, the MS data are not correlated with CaCO3 and TOCconcentration data and their log shapes are different (Figs. 3, 6).Diagenetic minerals may represent important carriers of at least partof the MS signal (Bloemendal et al., 1992; Schneider et al., 2004). It istherefore inferred that theMS cycles are related to the interplay of thesiliciclastic admixture in carbonates and the concentration ofdiagenetic, iron-bearing minerals. Crick et al. (2001) interpreted theMS signal in the Lower Devonian hemipelagic strata of the Barrandianarea as a proxy for the siliciclastic supply due to relative sea-levelfluctuations. The siliciclastic admixture in carbonates, however, can bealso controlled by water agitation (cf. Mabille et al., 2008) andclimate-related carbonate productivity. As Munnecke and Westphal(2005) have stressed, carbonate production can be favoured by lowterrigenous input and oligotrophic surface water conditions during

Fig. 5. Correlated cyclic patterns on the MS and CIE L* log in the interval from 55 to80 m. Curves are filtered using 3-point moving window. Note the correspondencebetween MS and CIE L* cycles.


phases of arid climate. Such arid–humid climate cycles are knownfrom the late Silurian and S/D boundary (Munnecke et al., 2003).Without isotope geochemistry and mineralogy data (cf. Munneckeand Westphal, 2005), however, any interpretation of the steeringfactor of the carbonate/siliciclastic cycles is difficult.

In the overlying strata (∼81 to ∼112.5 m, facies association A3) theMS values are typically very high (Ø 18.0) and less variable than in theunderlying segments (SD 7.7; coefficient of variance 42.9). The upperboundary of this segment is marked by a return from valuesfluctuating around a steady mean to a generally decreasing trendapproximately at ∼112.5 m, close to the “graptolite interval”, which isconsistent with a similar turning point on the K and Th logs (seeabove). The data are more positively skewed (skewness 2.36) than inthe underlying strata (skewness 1.65), indicating a pronounceddifference in thickness between thinner carbonate-poor layers andthicker carbonate-rich ones. Together with the generally higher MSvalues, this observation may support an idea of slower carbonatesedimentation rates than in the previous facies associations. The MSrecord in this segment shows a cyclic arrangement. Spectral analysisusing Fast Fourier Transformation in the depth domain failed to showany distinct frequency peaks, although a distinct variation is visible onthe MS curve smoothed by a low-pass frequency filter (five-pointrunning average). Positive data excursions from the smoothed curve,comprising at least two successive data points, are regarded as high-frequency cycles (Fig. 10). Due to the low error due to measurement(b2%) these cycles are interpreted as representing a primary strati-graphic signal. The 18-m thick succession between ∼94 to ∼112 mcan be subdivided into 37 such cycles of a relative uniform thickness(Fig. 10). The mean cycle thickness is 48.6 cm. With the thickness ofthe whole Praha Formation being 40.5 m (∼77.5 to ∼118 m) and itsestimated total time of deposition of ∼4 Ma (Chlupáč, 2000), thisaverage cycle thickness implies an average cycle duration of 49.2 ka.This figure roughly matches the results of Crick et al. (2001) whorevealed similar MS cycles with a duration between 38.6 and 39.9 ka

in the S/D hemipelagic sediments of the Barrandian area. Crick et al.(2001) have interpreted these cycles as obliquity cycles within theMilankovitch band.

The fourth interval (∼112.5 to ∼123.5 m, upper parts of faciesassociation A3 and facies association A4) is characterized first by adecreasing trend, followed by an increasing trend in MS values. Thesetrends are similar to those visible in the K and Th log (Fig. 3), andtogether with the facies characteristics (coarsening-upward trend)indicate a return from the condensed hemipelagic sedimentationof the A3 association back to the calciturbidite deposition of theA4 association. The MS values (Ø 9.35) are generally lower than in theunderlying segment, but still much higher than in the faciesassociations A1 and A2. The values are moderately variable (SD 4.73;coefficient of variance 50.66) and positively skewed (skewness 2.1).

6. Spectral reflectance log

Spectral reflectance data show a high degree of stratigraphicorganisation on different scales. The CIE L*a*b* data are organised intovertical segments, which are very well consistent with the faciesassociationsand the segments on theGRS andMS logs. Carbonatesof theA1 facies association (101 measurements) have high values of bright-ness (CIE L*: Ø 51.0 and SD 4.6) and yellow to orange hues (CIE a*:Ø 2.5 and SD 1.1; CIE b*: Ø 7.7 and SD 2.7). The upper parts of theA1 association correlate with a distinct peak on the CIE b* log and thelog of percent reflectance in the yellow band (Fig. 7). These colourcharacteristics are associated with elevated concentrations of goethiteas inferred from the marked peak in the 525–535 nm absorbance bandand a less prominent peak in the 425–445 nm band on the firstderivative curve of the reflectance spectra (Fig. 8).

In a 3-metre thick transition zone between A1/A2 facies associa-tions (10 to 13 m above the section base), the CIE L*a*b* valuesquickly decrease (Fig. 7). In most of the A2 association (13 to ∼76 m),the carbonates are slightly darker but much less coloured with theircolour tending to vary along the greyscale (CIE L*: Ø 42.3, SD 3.9;CIE a*: Ø 0.7, SD 0.2; CIE b*: Ø 3.3, SD 1.0). An overall look at thesection and its colours (Fig. 9) reveals subvertical zones of fadedcolour, about 5 to 15 m wide, which are associated with subverticalfaults. Three of them are located between 37 m and 43 m, 49 and54 m, and 60 and 68 m, respectively, above the section base. These“leaching zones” correlate with positive excursions on the CIE L*,CIE b* and yellow-band reflectance logs that tend to represent a kindof cyclicity although these “cycles” are obviously not related tostratigraphy (Fig. 7). The leaching zones are yellow coloured and areindicated by an increased height of the goethite absorbance bands onthe first derivative curves (Fig. 8). This indicates increased concentra-tions of goethite and possibly other iron oxy-hydroxides with asimilar position of their absorbance bands (cf. Scheinost et al., 1998).Unaltered carbonates from this interval have a low brightness. TheLochkovian/Pragian stage boundary at a height of 75 m is not indi-cated by the spectral reflectance log. From ∼76 m up to the top ofthe A2, the values quickly increase towards positive ones on the CIE L*,a* and b* log (Figs. 3, 7). This shift roughly correlates with the Lochkov/Praha Formation boundary. The peak values occur between 79 and79.5 m just below theA2/A3 facies association boundary (CIE L*: Ø 64.4;CIE a*: Ø 4.3; CIE b*: Ø 6.0; 8 measurements). The carbonates are lightgrey with slightly yellow-to-yellow-orange hues.

The boundary between the facies associations A2 and A3 isassociated with a distinct shift towards a higher percent reflectance inthe blue and, in particular the green band (Fig. 7). The CIE L*a*b*values quickly decrease between ∼80 and ∼87 m and then remainmore or less stable between ∼87 m to ∼92.5 m, which is characterizedby low values of brightness (CIE L*: Ø 44.1) and yellow (CIE b*: Ø 3.9)and extremely low values of red (CIE a*: Ø 0.3; 110 measurements).The carbonates are grey to dark grey. A sudden colour shift to redcarbonates occurs in about the 1 m thick zone between 92 and 93 m.

Fig. 6. Binary plots representing linear regression correlation between CIE L* and carbonate content (%) and total organic carbon (TOC, %) from the A1–A2 facies association interval(0–80 m) and A3–A4 interval (80–123.5 m).

Fig. 7. Selected colour parameter logs in the Požáry-3 section (CIE L*a*b*, percent reflectance in colour bands).


Fig. 8. First derivatives of spectral reflectance curves from selected samples of thePožáry-3 section (top); and their transformation in the leaching zones (bottom).

Fig. 9. General overview of the Požáry-3 section with major stratigraphic c


Compared to the immediately underlying and overlying strata, the redcarbonates between ∼92.5 m and ∼112.5 m have distinctly lowerbrightness (CIE L*: Ø 42.8), high average CIE a* and CIE b* values (6.6and 9.7, respectively), a higher percent reflectance in the violet,yellow, orange and red bands and a lower percent reflectance in theblue and green spectral bands (Figs. 3, 7). The red colour is indicatedby a distinct peak in the hematite absorbance band between 565 and575 nm wavelength and, therefore, most of the red signal can beattributed to the concentration of hematite (Fig. 8). The red carbonateinterval is intersected by a fault-related leaching zone between 97 and104 m, which manifests itself with a sharp and marked colour change.The leaching zone is yellow-coloured (CIE b* and yellow-bandreflectance) and has higher values of brightness (CIE L*: Ø 48.9).The leaching is represented by a decreased hematite peak andincreased goethite peaks in the appropriate absorbance bands on thefirst derivative curves (Fig. 8).

The remainder of the facies association A3 (∼112.5 to ∼118 m) islight grey in colour and sharply separated from the underlying redlimestones, with high values of brightness (CIE L*: Ø 53.1) and againlower values of CIE a* (Ø0.4) and CIE b* (Ø5.2). An exception is the dark“graptolite interval” between 113.1 and 113.5 m, which has markedlylower brightness values (CIE L*: 44.2 to 45.6, 3 measurements). Thesharp return from the red to the grey limestones immediately underliesthe base of the “graptolite interval” (the “classical” Pragian/Emsianboundary). The overlying facies association A4 is distinctly darkercompared to the immediately underlying strata (CIE L*: Ø 42.9, SD 3.1;CIE a*: Ø 0.4, SD 0.2; CIE b*: Ø 3.1, SD 0.9). The A3/A4 boundary isassociated with a marked shift towards lower values of percentreflectance in the green and yellow spectral bands (Figs. 3, 7), thebrightness and the CIE b*.

6.1. Possible carriers of the colour signal

The CIE L* data from the greyscale-coloured carbonates of A1and A2 facies association are poorly positively correlated with theCaCO3 concentration data (R=0.41; R2=0.166) and negatively corre-lated with the total organic carbon (TOC) data (R=0.47; R2=0.224,34 measurements) (Fig. 6). In the carbonates of A3 and A4 facies

olour boundaries and leaching zones (see text). Man in circle for scale.


associations (19 measurements), this correlation is slightly better forCaCO3 (R=0.45; R2=0.206) and slightly worse for TOC (R=0.45;R2=0.205). The correlation is low but can be affected by the generallyhigh percentages of CaCO3 (69 to 97%) and very low concentrations ofTOC (0.05 to 0.8). Page et al. (2003) were able to recognise distinctdark beds in periplatform carbonates on slopes of the Great BarrierReef, which were associated with a 40% increase in siliciclasticabundance in carbonates. Statistically, rock brightness is moreinfluenced by TOC than by CaCO3 contents. There is virtually zerostatistical correspondence, however, between magnetic susceptibilityand CaCO3 (Fig. 6). Consequently, CIE L* in our data set represents asuperior proxy of the siliciclastic admixture in carbonates than themagnetic susceptibility.

The brightness andMS signals can also be influenced by diageneticminerals. Koptíková (unpublished data) have suggested that carbo-nates of the Lochkov Fm (A1 and A2) have a higher abundance ofpyrite and pyrrhotite and a lower abundance of iron oxides, mainlyhematite, than the Praha Fm (A3). Consequently, the dark colour ofthe paramagnetic pyrite and ferromagnetic pyrrhotite may contributeto the low brightness and high MS signal regardless of the CaCO3

content. This can partly explain the coincidence of the high MS andlow CIE L* cycles in the Lochkov Fm (Fig. 5) despite the fact that thereis no correlation with CaCO3. Moreover, the lower and upperboundary of the red carbonates are associated with a quick dropand rise, respectively, of brightness indicating the presence ofdiagenetic hematite significantly affecting the CIE L* parameter(Figs. 3, 10). It is suggested, therefore, that stratigraphic interpretationof simple greyscale colour data can be affected by a significant errorrelated to the chromacity (cf. Warren et al., 1998).

Fig. 10. Colour cyclic patterns in the red limestones (94 to 112 m) on the CIE L* log and relativspectral reflectance curve and their correlation with magnetic susceptibility. Cycles are defileast two successive data point. Note the good correlation between brightness and hematit

6.2. Cyclic patterns and correlation with GRS and MS log

The spectral reflectance logs indicate important turning points,trends and even cyclic distribution, most of which can be correlatedwith the GRS and/or MS logs. The prominent gradual drop on theCIE L*a*b* log between 11 and 14 m (Fig. 7) correlates with the grad-ual boundary between the A1/A2 facies association (11 to 13 m), thedecreasing U concentrations between 10 and 12 m and the suddendrop in the MS values at a height of 14 m above the section base. Themost prominent positive excursion on the CIE L*a*b* log between 79and 79.5 m just underlies the A2/A3 facies association boundary at∼80 m, which is associated with a rapid increase in the Th/U ratio anda rise in the MS. The sudden change to the red limestones at ∼92.5 mis accompanied by a sharp increase in the Th/U ratios and a shifttowards highly fluctuating MS values. The upper boundary of the redlimestones (112.5 m) is not accompaniedwith any special excursion onthe K and Th concentration log and the MS log but correlates to a sharpnegative shift on the Th/U ratio andwith the onset of a decreasing trendin theMS and GRS logs, following an interval of relatively steady values.The immediately overlying “graptolite interval” between 113.1 and113.5 is indicated by a sharp negative shift on the CIE L* log, which isaccompanied by a distinct negative peak on the Th/U log. The lastnegative shift on the CIE L* log correlates with the A3/A4 faciesassociation boundary at a height of ∼118 m, a short peak in K andTh concentrations and a distinct peak in U concentrations, with nochange in the MS log.

The brightness data are organised into cycles, which arecorrelatable with MS cycles in the interval from ∼5 to ∼17 and from∼55 to ∼80 m above the section base (Figs. 3, 5). The cycle

e concentration of hematite inferred from hematite peak height on the first derivative ofned as positive excursions from five-point running average (thick line), comprising ate concentration cycles and no correlation between hematite and MS cycles.


wavelength varies between ∼1 and ∼3.5 m thickness. Between ∼55 to∼80 m, these cycles are superimposed on two lower-order cycles witha wavelength of 16 m and 6 m, respectively (the latter cycle isincomplete). Maxima on the brightness curve correlate with minimaon the MS curve and vice versa. This points to a common steeringfactor, which is possibly the interplay between the siliciclasticadmixture in carbonates and the concentration of diagenetic, iron-bearing minerals. Assuming the total thickness of the LochkovFormation (82 m) and the estimated duration of the Lochkovianstage (∼4.8 Myr, Gradstein et al., 2004), the cycle thicknesses implycycle durations between ∼60 and ∼205 kyr, whichmay correspond tothe obliquity and excentricity periods in the Milankovitch band. In thered carbonates of the A3 facies association, however, the CIE L* dataare again organised into cycles. These are not correlated, however,with the cyclic patterns visible on the MS log (Fig. 10). On thecontrary, the CIE L* cycles in the same interval can be correlated withcycles representing hematite peaks on the first derivative of spectralreflectance curve. This suggests that this cyclicity preferentiallyreflects hematite precipitation during early diagenesis and not thesiliciclastic input. This coincidence between the brightness andhematite content is impaired in the leaching zone in the intervalfrom 97 to 104 m above the section base. In contrast, the cyclic patternon the MS log is not by any means affected by the leaching zone,which suggests that telogenetic alteration along faults does not affectthe MS signal. This is in agreement with the findings of Ellwood et al.(2006) who have stated that ferrimagnetic and paramagneticminerals are transformed during diagenesis but in general the totaliron is conserved contributing to the MS signal. Our data suggest thatspectral reflectance data may significantly improve the interpretationof cyclic signals on a parallel MS log.

7. Discussion

7.1. Lochkovian–Zlichovian sea-level fluctuations

The GRS and MS logs can be interpreted in the context of regionalLower Devonian sea-level fluctuations. The carbonates in the Požáry-3section were deposited in a slope to basin setting supplied from anadjacent carbonate platformwith isolated reefs such as the Koněprusyreef complex, located some 20 km SW of the section (cf. Chlupáč et al.,1998; Hladíková et al., 2000). Allochem composition data indicatea general transgressive trend for the Lochkovian to Pragian succes-sion. The A1 facies at the base of the section indicates a high inputof shallow-water skeletal debris, which correlates with relativelylow concentrations of K and high values of brightness. The K andTh concentrations tend to increase to reach local maxima between∼25 and ∼40 m. As indicated by the CaCO3 concentration data (Fig. 3),this trend is due to decreasing carbonate contents in the rock and it isinterpreted as a landward facies shift, which is supported by moredeeper-water facies of the A2 facies association (facies F3, F6) and lowbrightness values. Up-section, the K and Th concentrations decreaseagain to reach local minima between ∼55 and ∼80 m, which, in ourassumption, represents a reverse basinward shift of the carbon-ate facies (progradation). This trend culminates at the base of thePraha Formation, between 78 and 80 m in height above the sectionbase (earliest Pragian), with low Th and in particular especiallyK concentrations, high CaCO3 percentages and peak brightness values(Fig. 3). By analogy, the asymmetrical, 7 to 11 m thick K and Th cycles,which partly coincide with the cyclicity in facies stacking patterns canbe interpreted as TR cycles. Early to middle Lochkovian transgressivetrends as well as a prominent regression event just above theLochkovian–Pragian stage boundary (the Basal Pragian Event) werereported from the Prague Synform and Morocco (Chlupáč et al., 1998;Hladíková et al., 2000; Crick et al., 2001; Vacek, 2007; Lubeseder,2008). The Lochkovian–Pragian boundary is associated with a positiveshift in δ13C values in the Požáry-1 and 2 sections and in other sections

of the Prague Synform, Carnic Alps and Cantabrian Mts., which isinterpreted as the onset of the regression (Hladíková et al., 1997;Buggisch and Mann, 2004). This interval correlates with a prominentsea-level fall in cratonic sequences of Euramerica (just below the Iaeustatic cycle of Johnson et al., 1985).

A principal turning point in facies and logging data occurs in thelower part of the Praha Formation at a height of ∼80 m (earlyPragian). The K and Th concentrations, Th/U ratio and MS valuesrapidly increase and CaCO3 concentrations and brightness valuesdecrease indicating landward facies shift between ∼80 and 87.5 m.This transgression is coeval with the early Pragian gradual sea-levelrise indicated by facies shifts and the aggradation of the Koněprusyreef complex in the Prague Synform (Chlupáč et al., 1998; Hladíkováet al., 2000; Čáp et al., 2003). It can be correlated with the base of theIa eustatic sea-level rise in cratonic Euramerica (Johnson et al., 1985),and a turnover to well-oxygenated facies in Morocco (Lubeseder,2008). The K and Th concentrations, the Th/U ratio and the MS valuesstart to decrease approximately at the Pragian/Emsian boundary(“graptolite horizon” at 112.8–113.6 m), which correlates with asharp increase in brightness values between ∼112 and ∼118 m andsuggests a regressive trend. The Pragian/Emsian boundary is accom-panied by erosion in the nearby Koněprusy reef complex indicatingrelative sea-level fall (Janoušek et al., 2000; Hladíková et al., 2000),and correlates with the top of the Pragian TR cycle Ia of Johnson et al.(1985). The regressive trend in the gamma-ray spectra and MScontinues up to the Zlíchov Formation dominated by carbonateturbidites (A4 facies association). This trend indicates a return fromthe Pragian hemipelagic deposition to the slope sedimentationcontrolled by gravity-flow sedimentation.

The correlation in the regional to continental-scale context and thepublished isotope geochemistry data (Buggisch and Mann, 2004)suggest that the TR trends in the Požáry-3 section were largely forcedby eustatic sea-level fluctuations. Strong eustatic perturbations,including an early Devonian eustatic low, are described from Europe,North America and Australia, from the S/D boundary to the lateDevonian (Saltzman, 2002; Stephens and Sumner, 2003; Haq andSchutter, 2008). Isotope geochemistry data were recently used to linkearly Devonian, third-order, eustatic fluctuations with N35 m ampli-tude and ∼1.7 Myr period to glacioeustasy driven by long-periodexcentricity or obliquity (Elrick et al., 2009). As the early Devonian hasbeen traditionally perceived as a greenhouse period, orbitally-drivengreenhouse climate variations were suggested as the cause of cyclicdeposition of Devonian carbonates by other authors (cf. Elrick, 1996;Crick et al., 2001; Chen and Tucker, 2003). Two major TR cycles in thePragian and at least one in the Lochkovian succession in Požáry-3(Fig. 3) are in accord with this third-order eustatic cyclicity, while theforcing mechanism of the higher-order, K and Th cycles, with ∼0.4 to∼0.64 Myr period (see Section 5.1) remains enigmatic with respect tosea-level fluctuations and can be driven by changes in carbonateproduction.

Despite the fact that the majority of the facies at the Požáry-3section are interpreted as (periplatform) carbonate turbidites, thecoincidence between the increased and reduced shallow-water inputduring lowstands and highstands, respectively, supports an idea thatthe depositional system was not driven by highstand shedding (cf.Droxler and Schlager, 1985; Westphal et al., 1999) and did notconstitute a slope of flat-topped platform. A distally steepened rampgeometry (Read, 1982) is proposed for the Lochkovian to earliestPragian succession at the Požáry-3 section, instead. Examples ofancient outer-ramp successions with turbidites are known from thePhanerozoic (Read, 1982; Burchette and Wright, 1992; Elrick, 1996;Betzler et al., 1999) and their response to relative sea-level changes islargely dependent on the carbonate production and the bathymetricprofile of the shelf (Dolan, 1989). During sea-level rise and highstand,high-production systems show aggradational to progradationalstacking patterns (keep-up systems). In contrast, low-production


ramp systems and carbonate hemipelagic successions tend to developresedimented turbidite and tempestite facies tracts during sea-levellowstands and thick retrogradational successions during periods ofsea-level rise (Sarg, 1988; Elrick, 1996;Molina et al., 1997). Long-termtrends on the K log show a prevalence of regressive successions overtransgressive in the Lochkovian succession and opposite trends in thePragian one (Fig. 3). This change in stacking patterns can beinterpreted as an important drop in the carbonate productivity ofthe Pragian carbonate system compared to the Lochovian one. Notunlikely, the early Pragian facies shift and the accompanyingpetrophysical response were caused by partial drowning or back-stepping due to a glacioeustatic sea-level fall followed by a quick sea-level rise close to the Lochkovian–Pragian boundary.

The high K and Th values, high Th/U ratios (3 to 10), high MSvalues and microfacies data suggest that the Pragian carbonatesuccession (facies association A3) is highly argillaceous compared tothe underlying strata, but it is not particularly enriched in TOC (Ø 0.23in A3 compared to Ø 0.22 in A1 and A2). The prominent positive shiftin the Th/U ratio is therefore regarded as reflecting a turnover towardslower-energy butmuchmore oxidized environments compared to theunderlying Lochkovian–earliest Pragian strata. High terrigenousaffinity of a carbonate succession, which is interpreted as transgres-sive to highstand, would suggest an idea of deposition in a hybrid,mixed carbonate-siliciclastic depositional system (Sarg, 1988; Dolan,1989; Page et al., 2003). In our section, however, as well as in thewhole Lower Devonian of the Barrandian area, there is no hint ofsiliciclastic or mixed carbonate-siliciclastic turbidites (cf. Dolan,1989). The petrophysical response of the A3 facies association istherefore considered to reflect significantly suppressed carbonateproduction in the Pragian on the background of the Lower Devonianfine-grained siliciclastic influx into the basin. The absence ofsiliciclastic turbidites also suggests that the whole area wastectonically extremely calm and not affected by significant tectonicprocesses until the late Middle Devonian (cf. Suchý et al., 2002;Melichar, 2004), which supports the interpretation of the eustaticorigin of the carbonate cyclicity.

The logging data correlate particularly well with local sea-levelfluctuation inferred from facies stacking patterns as well as with thepublished regional and global sea-level trends. The brightness values,in particular, sensitively indicate major regressions/sea-level low-stands in the earliest Pragian (the Basal Pragian Event) and at thePragian/Emsian boundary.

7.2. Palaeoenvironmental and cyclostratigraphic potential of spectralreflectance data

Two types of cyclicity were inferred from the spectral reflectancedata. The brightness cycles in the grey limestones of the Lochkovian toearliest Pragian age with a period from ∼60 to ∼205 kyr correlatewith theMS cycles. The brightness correlates, to a certain degree, withthe content of CaCO3 and TOC and the cycles, therefore, are assumedto reflect a variation in carbonate/siliciclastic ratios and, hence, theinterplay between carbonate production in the source area andsiliciclastic supply from the continent. The MS and CIE L* signals werealso likely influenced by diagenetic minerals such as pyrite andpyrrhotite although, in cyclic successions, these minerals also tend toconcentrate in layers with low carbonate content (Bellanca et al.,1996). As MS log patterns can be strongly affected by a diageneticoverprint (Bloemendal et al., 1992; Schneider et al., 2004), the CIE L*being in a phase with the MS data, provides an independent proxy fora primary cyclicity. Orbitally-driven climate variations, includingpossibly glacioeustasy (see Section 7.1 above), represent the likelycause for this cyclicity.

There is a prominent cyclic pattern on the MS log in the Pragianhemipelagic limestones (A3 facies association), the frequency ofwhich falls within the obliquity cycles of the Milankovitch band

(∼49 kyr period). The brightness cycles in the red carbonates can becorrelated with hematite cycles inferred from spectral reflectance butdo not correlate with the MS cycles. This suggests that most of the MSsignal is not related to the concentration of hematite but rather to theconcentration of other ferrimagnetic and paramagnetic minerals suchas chlorite, smectite and illite, biotite, iron sulfide and sulphate, etc.(Ellwood et al., 2006). This is supported by our observation thatcomparably high MS values were found in both the red limestonesand grey limestones of the Pragian age (Figs. 3, 10). Therefore, thecolour cyclicity reflects a variation in hematite precipitation presum-ably during early diagenesis, which was not related to siliciclastic/carbonate input. In marine red beds, hematite is considered toprecipitate during early diagenesis in well-oxygenated bottomenvironments (Eren and Kadir, 1999; Wang et al., 2005; Wagreichand Krenmayr, 2005). Precipitation of hematite is related to redoxgradient at the sediment surface in marine environments or capillaryfringe zone in terrestrial environments (Breit, 2001; Staubwasseret al., 2006). Iron necessary for hematite precipitation in the surfaceoxic layer is supplied from the underlying Fe-reduction layer, whichleads to changes in the Fe isotopic ratio and colouration between bothlayers (Staubwasser et al., 2006). Cyclic variation in bottom oxygen-ation and redox gradient might therefore produce such hematitecycles regardless of the sediment supply.

The colour boundaries of the red carbonates (∼92.5 m and∼112.5 m) are extremely sharp and not associated with a particularfacies change. They cannot even be correlated with particular shapeson the K and Th concentration and MS log. The red carbonate interval,however, almost precisely correlates with the prominent positive shifton the Th/U ratio log, which is considered to reflect oxygenatedbottom conditions (see above). This coincidence therefore providesanother indication that the red colouration in pelagic limestones is notrelated to the detrital supply of oxidized iron from the continent butis, instead, a nearly syndepositional or early diagenetic feature thatreflects oxic bottomwater conditions. This conclusion also contradictsthat of Mamet and Préat (2006) who have argued that the red pelagiclimestone originates in depositional settings with low oxygen levels.

The change from dysoxic/anoxic to oxic bottom conditionsrecorded in ancient marine red beds is regarded as an importantpalaeoceanographic process (Hu et al., 2005). Although certainpelagic red beds may be highly diachronous and controlled by localfoctors (Wagreich and Krenmayr, 2005), many can be correlatedhemiglobally or even globally during specific periods such as the lateCretaceous and upper Jurassic (Wang et al., 2005; Li et al., 2005; Ceccaet al., 2005). The Pragian red limestones at the Požáry-3 section can becorrelated with facies deposited in conditions of well-oxygenatedbottom water in Morocco. This may suggest that the Pragian stage,although regarded as a typical greenhouse period with suppressedoceanic circulation, may have been a period of important, regional toglobal, palaeoceanographic changes associated with deep-sea bottomoxygenation.

8. Conclusions

The Lower Devonian (Lochkovian to lower Emsian) succession atthe Požáry-3 section was deposited in a deep-water carbonate rampsetting and bears strong hints of cyclic deposition on different scales.Decametre-scale TR trends, indicated by facies stacking patterns,CaCO3 data and spectral gamma-ray (GRS) logs, are well correlated inthe regional context and point to third-order eustatic sea-levelfluctuations. Confrontationwith literature suggests that glacioeustaticforcing cannot be excluded. On a gross scale, sediment colour does notcorrelate significantly with these TR trends although importantlithologic boundaries such as peak regressions are sensitively markedby CIE L*a*b* parameters and percent reflectance in colour bands. GRSis therefore suggested as the most suitable proxy of facies shifts in


deep-water carbonate successions whereas spectral reflectance hassome added value.

On a more detailed scale, there is a marked cyclicity in themagnetic susceptibility and SR signals. In the greyscale-colouredcarbonates of Lochkovian age, maxima at the CIE L* cycles with cycleduration between ∼60 and ∼205 kyr are in a phase with minima atthe MS cycles suggesting that they were driven by a common steeringfactor, possibly climatic fluctuations in the Milankovitch band. In thered carbonates of the Pragian age, however, the CIE L* cycles correlatewith hematite concentration cycles revealed from SR data but do notcorrelate with a distinct MS cyclicity with the inferred cycle durationof 49.2 kyr. This finding has two implications. First, because CIE L* ispartly correlated with carbonate content and TOC data but the MS isnot, it indicates that theremay be somethingwrongwith the commoninterpretation of MS data as a proxy for the siliciclastic admixture incarbonate successions. The MS signal can be strongly affected by thepresence of diagenetic minerals and the use of MS in high-resolutionstratigraphy without independent proxy data can be misleading.Second, the presence of diagenetic hematite significantly affects theCIE L* parameter, which suggests that the stratigraphic interpretationof simple greyscale data can be affected by a significant error relatedto the chromacity. The hematite cycles, inferred from SR, are inter-preted as diagenetic in origin, reflecting changing bottom oxygenationand subsurface redox gradients during early diagenesis. The asyn-chronous SR and MS cyclicity, low TOC values and high Th/U ratios ofthe Pragian hemipelagic limestone suggest that its red colouroriginated from early diagenetic hematite precipitation under theconditions of an oxygenated ocean floor. The base of the redcarbonates is therefore interpreted as a switchover to oxic bottomconditions. This suggests that deep-sea bottom oxygentation mayhave been in operation during periods traditionally perceived astypical greenhouse, such as the early Devonian.

The SR log is obscured by zones of elevated brightness and yellow-band reflectance, which are associated with subvertical faults. Colourin these “leaching zones” is changed due to fluid flow along the faultsrepresenting a postdepositional overprint. In the red pelagic lime-stones, such a colour change is associated with the transformation ofhematite to goethite. The SR cyclic signal is impaired in the “leachingzones” but the MS cyclic signal remains almost unchanged, whichindicates that fault-related telogenic alteration has little influence onthe MS data.

Our data demonstrate that SR has a value comparable to MS incyclostratigraphy and it can be useful even if visual inspection of thelithology fails to reveal any stratigraphic patterns. The SR scanners areinexpensive, easy to use and the SR logging can be extremely quick,with acquisition of several thousand data points in a single day.Combined use of SR, MS and GRS has synergic effect and cansignificantly improve the interpretation of their respective strati-graphic signals. In particular, SR can contribute to the interpretation ofthe diagenetic overprint of the MS signal and its possible carriers. Fullexploitation of spectral data can lead to better interpretations than theuse of a single colour parameter such as greyscale intensity.

Acknowledgements

This study was supported by the project MSM0021622412 and theCzech Science Foundation project 205/09/1257 (Bábek) and projectsIAAX00130702, IAA300130702, GA205/08/0767 and AV0Z30130516(Hladil, Koptíková, Slavík). This support is gratefully acknowledged.We thank Tomáš Grygar (AV ČR, Praha) for valuable discussionsduring the earlier phase of manuscript preparation and Ján Spišiakand Alžbeta Svitáčová of the Geological Institute SAV, Banská Bystrica,Slovak Republic for CaCO3 and TOC analyses. Comments from twojournal reviewers, Gavin Dunbar and Jeff Lukasik, greatly improvedthe manuscript. Thanks are also due to Milan Trnka from Pikaso Ltd.

(Řeporyje Quarry, crushed stone production) for willing cooperationand technical assistance.

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Stratigraphic significance and resolution of spectral ... · Devonian carbonates of the Barrandian area, Czech Republic; a correlation with magnetic susceptibility and gamma-ray logs