Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine...

Deep-burial alteration of early-diagenetic carbonateconcretions formed in Palaeozoic deep-marine greywackes andmudstones (Bardo Unit, Sudetes Mountains, Poland)

MACIEJ J . BOJANOWSKI*, ANDRZEJ BARCZUK* and ANDREAS WETZEL†*Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, _Zwirki i Wigury 93,Warszawa 02-089, Poland (E-mail: [email protected])†Geologisch-Pal€aontologisches Institut, Universit€at Basel, Bernoullistrasse 32, Basel CH-4056,Switzerland

Associate Editor – Stephen Lokier

ABSTRACT

Carbonate concretions formed in bathyal and deeper settings have been stu-

died less frequently than those formed in shallow-marine deposits. Similarly,

concretions affected by catagenetic conditions have rarely been reported. Cal-

cite concretions in deep-marine mudstones and greywackes of the Bardo Unit

(Sudetes Mountains, Poland) formed during early diagenesis and were buried

to significant depths. Petrographic and geochemical (elemental and stable C

and O isotopic) analyses document their formation close to the sediment–

water interface, prior to mechanical compaction within the sulphate reduction

zone and their later burial below the oil window. Although the concretions

were fully formed during early diagenesis, the effects of increased temperature

and interaction with late-diagenetic interstitial fluids can be discerned. During

maximum burial, the concretions underwent thorough recrystallization that

caused alteration of fabric and elemental and O isotope composition. The ini-

tial finely crystalline cement was replaced by more coarsely crystalline, sheaf-

like, poikilotopic calcite in the concretions. These large calcite crystals engulf

and partially replace unstable detrital constituents. The extremely low d18Ovalues (down to �21�2& Vienna Pee Dee Belemnite) in the concretions are the

result of the increased temperature in combination with alteration of volcanic

glass, both causing a significant 18O-depletion of bicarbonate dissolved in the

interstitial fluids. Recrystallization led to uniform O isotope ratios in the con-

cretions, but did not affect the C isotope signature. The d13C values of the late-

diagenetic cements precipitated in the greywacke and in cracks cutting

through concretions imply crystallization in the catagenetic zone and decar-

boxylation as a source of the bicarbonate. These late-diagenetic processes took

place in a supposedly overpressured setting, as suggested by clastic dykes and

hydrofractures that cut through both concretions and host rock. All of these

features show how the effects of early and late diagenesis can be distinguished

in such rocks.

Keywords Burial diagenesis, concretions, recrystallization, stable C and Oisotopes, Sudetes.

INTRODUCTION

Carbonate concretions are common in clasticrocks, especially in mudstones (Raiswell, 1987;

McBride, 1988; Mozley & Burns, 1993). Concre-tions occur more frequently in shallow-marinethan in deep-marine deposits (cf. Dietrich, 1999)if studies of concretions formed in connection

© 2013 The Authors Sedimentology © 2013 International Association of Sedimentologists 1211

Sedimentology (2014) 61, 1211–1239 doi: 10.1111/sed.12098

with methane seepage and gas hydrates are nottaken into account (e.g. Campbell, 2006;Wheeler & Stadnitskaia, 2011, and referencestherein). However, this could be an artefact ofdata availability because deep-marine depositsare far less common on the continents. This arti-cle addresses such seldom-studied concretionshosted in deep-marine deposits. As with mostconcretions, they started to form during earlydiagenesis at burial depths ranging from justbelow the sediment–water interface to severaltens of metres (Sell�es-Mart�ınez, 1996).Cement can precipitate in a concretion over a

significant time span as long as it resides in geo-chemical conditions favourable for concretiongrowth (Hennessy & Knauth, 1985; Klein et al.,1999; Raiswell & Fisher, 2000). There are notmany records of concretions formed in the cata-genetic zone where thermal decarboxylation isthe main source of bicarbonate (Hennessy &Knauth, 1985; Scotchman, 1991), perhapsbecause not all concretions are buried so deeply.If deeply buried, secondary alteration may occurdue to increased temperature, the significantlymodified stress field and the chemical composi-tion of the interstitial fluid (Dix & Mullins,1987; Morad & Eshete, 1990). Nonetheless, manyconcretions preserve some original features,despite having been buried to significant depth(Mozley & Burns, 1993). Therefore, as concre-tions grow during burial, it is still a matter ofinvestigation as to which burial conditions arerecorded, namely that of initial formation orcomplete lithification, that of maximum burial,an intermediate stage, or all of these processes(Mozley, 1996). This question is addressed inthe present study. Deciphering the genesis ofsuch deeply buried concretions and reconstruct-ing the sources of the parent fluids are compli-cated by possible secondary alterations. Thus, arecognition of the effects of secondary alterationrelated to deep burial is crucial in analysingconcretions and interpreting their formation.This study focuses on concretions in adjacent

beds of different host-rock lithologies, greywackesand mudstones of the Bardo Unit (SudetesMountains, Poland), that were later buried tosignificant, several-kilometre depths in the cata-genetic zone. To unravel the role of pore fluidsin late diagenesis, concretions in the mudstoneswere compared to those in the adjacent greywac-kes and to the fill of cracks cutting through boththe host rocks and the concretions. The geo-chemical and petrographic data for the hostrocks, concretions and crack fill provide an

opportunity to reconstruct the origin of the con-cretions, to track their diagenetic path and toanalyse the influence of deep burial in the cata-genetic zone.

GEOLOGICAL SETTING

The Bardo Unit in south-western Poland is astrongly deformed and partly eroded remnant ofa Palaeozoic deep-marine basin that was foldedduring the Variscan Orogeny at the turn of theVis�ean to the Namurian stages. The Bardo Unitoccurs among crystalline units of the LowerSilesia block, which represents a northern partof the Bohemian Massif. It comprises two majorlithostratigraphic units (Wajsprych, 1995). Theautochthonous platform unit consists of UpperFammenian to Vis�ean strata deposited on apassive continental margin capped by LowerCarboniferous flysch deposits. The allochtho-nous unit is a chaotic complex that is composedof olistoliths and slide-sheets formed by UpperOrdovician to Upper Devonian siliceous andturbiditic rocks that are embedded in a flyschsuccession (Haydukiewicz, 1990, 1998). Theallochthonous unit was incorporated into anaccretionary prism and thrust over the autoch-thonous unit during the late Vis�ean. The extre-mely complicated tectonic setting causes severedifficulties in establishing a lucid stratigraphicframework for the different parts of the BardoUnit (Oberc, 1987; Haydukiewicz & Muszer,2002; Kryza et al., 2008). In general, however, itis clear that the Bardo Unit experienced subduc-tion, incorporation into an accretionary prism,flysch sedimentation and eventual collision.The rocks examined in this study are Lower

Carboniferous clastics that are very common inthe eastern part of the Bardo Unit (Fig. 1). Theseclastics belong to the Opolnica Formation (Oberc,1987; Oberc et al., 1994) which is part of the fly-sch succession represented by turbidites, fluxo-turbidites or debris flows that accumulated infront of the accretionary wedge (Wajsprych,1995). Two main lithologies are distinguished; upto 3 m thick, massive greywacke beds (Fig. 2) andstrongly deformed siliceous mudstones (Fig. 3).These lithologies were investigated at three

localities: in a quarry in Młyn�ow, on the banksof Nysa Kłodzka River in Podtynie and near thetown of Bardo. The Bardo section was examinedin particular detail and sampled for geochemicalanalyses. It is located on the northern banks ofthe Nysa Kłodzka River in scarps along the road

© 2013 The Authors Sedimentology © 2013 International Association of Sedimentologists, Sedimentology, 61, 1211–1239

1212 M. J. Bojanowski et al.

1 km north of Bardo (Fig. 1). The strata dip 30to 60� to the north-east in this area. The out-crops are very steep, locally requiring the use ofclimbing equipment to facilitate access.

MATERIALS AND METHODS

From the section near Bardo (N50°30′44, E16°43′47), 10 concretions and eight host-rock samplesdirectly surrounding some of the concretionswere collected for further investigation. Spheri-cal to lenticular concretions (type A) are embed-ded in the greywackes (Fig. 2), while discoidalconcretions (type B) occur in the mudstones(Fig. 3). Six type B concretions were collectedfrom the top (B1) to the base (B6) of a 20 mthick mudstone series (Fig. 3A). Four type Aconcretions were taken from the greywackes(Fig. 2): A1 and A2 from greywackes overlyingthe mudstone interval, and A3 and A4 fromgreywackes beneath the mudstone. Host-rocksamples taken adjacent to a concretion aremarked by the letter ‘h’: A1h, A3h, A4h, B1h,B2h, B3h, B4h and B6h.Standard petrographic microscopy was per-

formed on thin sections using a Nikon EclipseE600POL polarizing microscope (Nikon Corpora-tion, Tokyo, Japan). NIS Elements software wasused to operate a digital camera, to take photo-micrographs and to perform counting of micro-fossils and measuring their dimensions in

concretion B6 and the surrounding mudstoneB6h. High-resolution observations and analysesof mineral and elemental composition weremade by electron microscopy. Uncoveredpolished thin sections coated with carbon wereexamined using a Cameca SX–100 electronmicroprobe (EMP) equipped with wavelengthdispersive X-ray spectrometer (WDS) (CamecaInstruments, Grennevilliers, France). Observa-tions were undertaken in the back-scatteredelectron mode (BSE). Operating conditions were15 kV accelerating potential and 10 to 20 mAbeam current. Rock chips coated with gold anduncovered thin sections without coating wereexamined using a JEOL JSM–6380LA scanningelectron microscope (SEM) equipped withenergy dispersive X-ray spectrometer (EDS) (JeolLtd., Tokyo, Japan). Cathodoluminoscopy wasconducted on uncovered, polished thin sectionsusing the Technosyn cold cathode Model 8200Mk5-1 (Cambridge Image Technology, Cam-bridge, UK) combined with Nikon Optiphotpolarizing microscope (Nikon Corporation;housed in the Geologisch-Pal€aontologisches Insti-tut, Universit€at Basel, Basel, Switzerland). Oper-ating conditions were ca 20 kV beam energy andca 0�3 mA beam current at ca 1�2 mBar vacuum.Thermal analyses were performed on eight

powdered samples: A derivatograph MOM Q-1500D (MOM, Budapest, Hungary) was used torecord thermal effects (DTA), weight changes(TG), and derivative of the weight changes

Fig. 1. Location map of the study area [based on Haydukiewicz & Muszer (2002) and Kryza et al. (2008), modi-fied]. Samples for geochemical analyses were collected from the section near Bardo.


Deep-burial alteration of calcite concretions 1213

A

C

D E

F G

B



(DTG) during heating of the samples. Analysisparameters were sample starting net weight(400 mg), sensitivity (200 mg), corundum cruci-bles and heating rate (10° min�1). X-ray diffrac-tion was performed on powdered samples withthe use of a PANalytical X’Pert PRO diffractome-ter (PANalytical, Almelo, the Netherlands) withCo Ka (k = 1�78896 �A) radiation and a high effi-ciency one-dimensional solid-state detector.Major element whole-rock analyses were car-

ried out by inductively coupled plasma opticalemission spectrometry (ICP-OES). The sampleswere reacted with Aqua Regia. Concentration ofK, Mg, Ca, Mn, Fe and Al was measured in theresultant solution. The undissolved residue wasweighed. All of these methods were performedin the laboratories of the Faculty of Geology,University of Warsaw, Poland.Stable carbon and oxygen isotope measure-

ments were performed on 58 powdered bulk sam-ples reacted with 100% phosphoric acid (density>1�9; Wachter & Hayes, 1985) at 75°C using a KielIII online carbonate preparation line connected toa ThermoFinnigan 252 mass spectrometer (Ther-moFinnigan MAT GmbH, Bremen, Germany;housed in the University of Erlangen-N€urnberg,Germany). All values are reported and discussedin per mil relative to Vienna Pee Dee belemnite(VPDB) by assigning a d13C value of +1�95& and ad18O value of �2�20& to NBS19. Reproducibility(1r) was verified by replicate analyses oflaboratory standards and is better than �0�02&for d13C and �0�03& for d18O. Samples weretaken along vertical transects across theconcretions by using sintered diamond micro-drills (diameter 2 or 3 mm).

RESULTS

Field observations

The greywackes are greyish, very thick-bedded(up to 3 m), massive and poorly sorted clastic

rocks. These rocks are mostly typicalfluxoturbidites, with an erosional base cuttinginto the underlying mudstone or greywacke;sometimes they show fining upwards at the topof a bed (Oberc, 1987) and large-scale cross-bedding. Sub-vertical clastic sandy dykes cutthrough some beds (Fig. 2B). Arrays of anasto-mosing vertical fractures filled with quartz andcalcite cross-cut the greywackes and theenclosed concretions (Fig. 2C; see below).In the greywackes, the type A concretions

occur preferentially at certain levels (Fig. 2A) orare randomly dispersed. The shape of the con-cretions varies from lenticular to spherical, theratio between the shortest and the longest axisranging from 0�6 to 1. The concretions usuallyrange from ca 10 to 50 cm in diameter, butsometimes they are very large, up to 1 m indiameter (Fig. 2D). Some are coalesced one ontop of the other forming ‘snow man’ morpho-logies (Fig. 2F). The concretions are sometimesrotated and their maximum diameter deviatesfrom the bedding by up to 60° (Fig. 2A and E).Within the greywackes, the concretions showvery little relief against the host-rock surfacebecause they have a similar resistance to weath-ering; sometimes, however, a distinct partingfrom the greywackes clearly defines concretionboundaries (Fig. 2A and E). In some cases, Mnoxides are present in the space between the con-cretion and the greywacke host rock. Sphericalforms also occur and, although they have aslightly higher resistance to weathering than thesurrounding greywacke, they are not easily sepa-rated from the host rock (Fig. 2G). These formshave been recognized by Barczuk (1974) torecord the initial stages of concretion formation.The mudstones are dark grey, usually sili-

ceous and form up to a few centimetre-thickbeds. Very commonly, individual beds arestrongly folded and disrupted (Fig. 3D and E).These structures are suggestive of soft-sedimentdeformation because they do not exhibit anycommon arrangement and the thickness of the

Fig. 2. Greywacke and the type A concretions. (A) Massive, 3 m thick greywacke bed with three concretions inthe upper part. Note that the concretion on the right is slightly rotated relative to the bedding plane. (B) Clasticdyke (between the solid lines) cutting through the greywackes (bedding planes marked with dashed lines). (C)Array of subvertical fractures filled with quartz and calcite cutting through a 40 cm large concretion (below thedashed line) and the host greywackes. (D) Large spherical concretion placed in front of the municipal building inBardo. (E) Lenticular concretion with distinctive parting from the host greywacke. The concretion is rotated rela-tive to the bedding plane (marked by the knife). The top part of the greywacke above the knife exhibits bedding-parallel parting which is related to grain-size fining upwards. (F) Two vertically coalesced concretions forming‘snow man’ morphology. (G) Spherical structures without parting from the host greywacke. These probably repre-sent the initial stages of concretion formation.



A B

C

D E

Fig. 3. Outcrops showing the mudstones and type B concretions. The dashed lines represent bedding planes. (A)Mudstones dipping 50° to the north-east (left) with five concretions in different horizons: from the top (concretionB1) towards the base (concretion B5). (B) Bread loaf-shaped concretion B2 with mudstone bedding undeformedunderneath, but bending above the concretion. (C) The largest concretion (B3) with mudstone bedding unde-formed underneath and bending above the concretion. (D) and (E) Folded and disrupted bedding of the mudstoneindicating its plastic rheology during deformation and a soft sediment consistency as the thickness of the bedsincreases towards the axis of the fold.



beds increases towards the axis of the folds(Fig. 3D). The mudstones sometimes showcross-lamination and some beds are erosionallytruncated at the top.Type B concretions are unevenly distributed.

Sometimes they are rather rare, one specimenper ca 100 m2 of outcrop, and normally occur asisolated specimens dispersed within the mud-stones. However, at some places they are foundin large numbers, 10 specimens per ca 100 m2

of outcrop, as in the examined outcrop (Fig. 3A).These concretions are discoidal in shape, with aratio between the shortest and the longest axisbetween 0�15 and 0�5. Some of these concretionsare bread loaf-shaped, having a flat lower sur-face, and a convex upper profile (Fig. 3B). Thebeds of the host mudstone are not deformedbeneath a concretion, whereas they are bentaround its top (Fig. 3B and C). The concretionsare more resistant to weathering than the hostmudstones; they range from a few to 80 cm indiameter.

Mineralogy and petrography

Thermal and X-ray diffraction analyses wereperformed on eight powdered samples: four con-cretions (B2, B3, B6 and A3) and four adjacenthost-rock samples (B2h, B3h, B6h and A3h). Forall samples, the X-ray spectra are very similar;they indicate that the crystalline material com-prises mainly chlorites, mica, feldspars (bothpotassium feldspar and plagioclase) and quartz.However, the samples show differences in theircarbonate content. Calcite is the dominant com-ponent of the concretions, while the greywackescontain only minor amounts of calcite, and themudstones contain no carbonate.The derivatograms reveal thermal effects char-

acteristic of calcite, pyrite, organic matter andphyllosilicates (Fig. 4). Maximum dehydroxyla-tion of phyllosilicates occurs between 550�C and600°C, which indicates that they belong to themixed layer-type or chlorite-type minerals(indistinguishable at such low concentrations).The type B concretions contain 60 to 70 wt% ofcalcite, up to 10 wt% phyllosilicates, and lessthan 1 wt% organic matter (Fig. 4B). The analy-sed type A concretion (A3) shows an analogousderivatogram, but it contains less calcite (45 wt%) and much more pyrite (2�5 wt%) relative tothe type B concretions (Fig. 4A). Both hostrocks, greywackes and mudstones, are rich inphyllosilicates that can constitute up to 30 wt%of these rocks (Fig. 4C and D); they also contain

pyrite. With respect to the carbonate content,the greywackes are strikingly different from thecarbonate-free mudstones (Fig. 4C and D). Thepresence of organic matter is recorded as a flatexothermic effect related to a little weight lossdue to oxidation between 200°C and 600°C inthe type B concretion and in the mudstone(Fig. 4B and D).All of the constituents determined by thermal

analyses of the host rocks comprise only 20 to31 wt% of the material. The remainder is com-ponents which do not give any thermal signalsduring heating up to 1000°C. Surprisingly, thereis no evidence, either in the concretions or inthe mudstones, for a quartz content higher than40 wt% because this would produce an endo-thermic effect on the DTA curve close to 575°C(Wyrwicki, 1996). Only the greywacke shows avery delicate endothermic deflection on theDTA curve at 575°C, visible only after someprocessing of the curve. Therefore, the quartzcontent may exceed 40 wt% only in the grey-wackes, but not in the mudstones or concre-tions.Microscopic examination of the greywackes

revealed that they are typical lithic wackes richin matrix with some carbonate and chloritecement (Fig. 5). The detrital material is very het-erogeneous and almost identical to that withinthe type A concretions; quartz and lithic frag-ments are the dominant constituents, but potas-sium feldspar and plagioclase are also abundant(Figs 5 and 6). Quartz very often reveals a volca-nic origin, as it exhibits very sharp-edged out-lines (Fig. 6A to D), thermal shrinkage cracks(Fig. 6E and F), and sometimes even subhedraldevelopment and corrosion embayments filledwith devitrified glass (Fig. 6E). K-feldspar andplagioclase are commonly slightly weatheredand may also be derived from a volcanic source.Lithic fragments are represented mainly byvarious volcanics or subvolcanics, such asaltered basaltoid, diabase (Fig. 5C and D) and vi-trophyric rhyolite clasts; they exhibit varioustextures, including porphyritic, vitrophyric,ophitic and intersertal. Scattered grains ofpyroclastic rocks are also present; althoughstrongly altered, they can easily be recognizedby their texture and major mineral componentsand mostly represent silica-rich tuffs (Fig. 5Eand F). Chlorite often appears to be the productof glass alteration (Fig. 5C and D). Authigenicpyrite is present between, and also within, thegrains as aggregates or euhedral crystals(Fig. 6E). Calcite cement occurs mainly in



intergranular pores as isolated anhedral sparitecrystals (Fig. 5A and B) probably replacingfeldspars and as disseminated microspar. Insome places, microspar cement is particularlyabundant and intergranular contacts may beabsent there, probably due to matrixreplacement as some grains are strongly cor-roded (Fig. 6F). Calcite also fills residual poresand fissures. The texture of the greywackes ischaracterized by very poor sorting; althoughsandy material dominates, there is aconsiderable silt and clay content and numerouspebbles occur. Overall grain roundness is lowand there is no sign of any directional arrange-ment. Therefore, the greywackes examined arecompositionally and texturally immature.The texture and composition of the detrital

material of the type A concretions are almostidentical to the surrounding greywackes. The

detrital material is cemented by abundant calcite.This cement forms microspar or subhedral, poi-kilotopic spar crystals that are larger than theframework grains (Fig. 7A to D). Observations incathodoluminescence revealed that some frame-work grains have been replaced by bright orangecalcite cement (Fig. 7E and F). In places, frame-work grains are separated and do not show inter-granular contacts. These areas are mostly filledwith calcite cement enclosing minor fine-graineddetrital material which is corroded and floatingin the cement (Figs 7F and 8). Isolated spheres(ca 0�2 mm in diameter), comprising blocky cal-cite and rimmed with framboidal pyrite werefound in the concretions (Fig. 9A and B). Anasto-mosing veinlets, which cut through and disruptthe framework grains and all generations ofcements lithifying the concretions, enclosemosaic blocky calcite (Fig. 7G and H).

A B

C D

Fig. 4. Derivatograms representative of the four rock types: Type A concretions (A), type B concretions (B), grey-wackes (C) and mudstones (D). The curves are records of thermal effects (DTA), weight changes (TG) and deriva-tive of the weight changes (DTG) during heating of the samples. The asymmetrical weight loss between 600�C and900°C recorded on the TG curve in (A), (B) and (C) is related to the dissociation of calcite. The prominent exother-mic effect on the DTA curve with a maximum close to 500°C seen in (A) and (C) is related to the dissociation ofiron sulphides. Dehydration (maximum in 100°C or slightly more seen on DTG curve) and dehydroxylation (maxi-mum close to 600°C seen on DTG curve) of phyllosilicates (mixed-layer-type or chlorite-type) give endothermiceffects on DTA [seen in (C) and (D)]. The insignificant and flat exothermic effect between 200°C and 600°C isprobably related to a small contribution of organic matter seen in (B) and (D). The calculated contents of theseconstituents are given in the text and Table 1.



A B

C D

E F

Fig. 5. Paired photomicrographs of the greywackes under plane polarized light (A), (C) and (E), and crossed ni-cols (B), (D) and (F). (A) and (B) Massive texture, poor sorting and very different roundness, from angular torounded, can be observed. Felsic grains ‘F’ are very frequent. Calcite cement ‘Cal’ forms large spar crystals. (C)and (D) Diabase-type lithoclast consisting of euhedral plagioclase ‘Pl’ and chloritized glass ‘Chl’. (E) and (F) Twolarge felsic grains, quartzite lithoclasts (upper left, marked with a dashed line) and angular quartz grain (lowerright, marked with an arrow).



A B

C D

E F

Fig. 6. Photomicrographs of quartz (Qtz) pointing to volcanic affinity under plane polarized light (A) and (C), andcrossed nicols (B), (D), (E) and (F). (A) and (B) Paired photomicrographs of angular quartz grains in the type Aconcretion. Partially chloritized biotite (Bt), feldspar (Fsp) and zircon (Zrn, encircled) grains are present. (C) and(D) Paired photomicrographs of angular quartz grains in the type A concretion, feldspar and orange coloured glassgrains (arrow). (E) Subhedral quartz with corrosion embayments (greywacke sample). Opaque minerals are euhe-dral pyrite crystals (Py). (F) Quartz with shrinkage cracks (greywacke sample) surrounded by microspar cement(Cal) which appears to be replacing detrital material as some grains are corroded (arrows).



The mudstones are mainly pelitic (Figs 9C,9D, 10A and 10B), thus restricting microscopicexamination mainly to textural observations.Only some samples exhibit normal gradingwhile having some silty material enriched at thebase (Fig. 10C and D). The mudstones often dis-play a distinct lamination of variable thicknessand laminae that sometimes wedge-out or arebent. These structures are typical of soft-sedi-ment deformation. Lamination is accentuated bythe presence of brownish-red organic materialthat is arranged locally in oval patterns ca20 μm in size (Fig. 9C). These oval organicstructures are probably the remains of benthiccyanobacteria, as demonstrated by Kremer &Ka�zmierczak (2005) and Kremer (2006). It is notclear whether they are in situ or have been rede-posited from a shallow-water setting, but the lat-ter possibility is more likely in the studiedturbiditic setting. Large (up to 300 lm in diame-ter) oval components rimmed with framboidalpyrite (Fig. 9D) may represent flattened shells ofradiolarians. Within a thin section of sampleB6h, 86 microfossils were found; their majoraxis is 126 lm long on average (standard devia-tion = 29 lm) and their aspect ratio is 0�60 onaverage (standard deviation = 0�13; Fig. 11).These microfossils appear to be oriented as theyhave the longest axes parallel to each other andto the lamination.Scanning electron microscope observations

combined with EDS in situ chemical analyseswere used to determine the mineralcomposition of the mudstones (Fig. 10). Themain constituents of the mudstones are silica,chlorite, plagioclase and K-feldspar. Silicagrains are usually angular showing corrodedoutlines and contain Al, K and Fe (as deter-mined by chemical EMP-WDS analyses); bothfindings together are suggestive of glass shards.Chlorite usually replaces large, up to 0�2 mm,biotite plates containing rutile (Fig. 10C), but isalso present as very fine authigenic plates in thepore space (Fig. 10A and B) or within litho-clasts, usually pyroclasts or volcanics(Fig. 10D). The latter suggest that chlorite wasformed by the transformation of volcanic glass,probably during burial diagenesis (Milodowski& Zalasiewicz, 1991). Lithic fragments exhibit-ing a porphyritic texture are also found; theyconsist of subhedral K-feldspar phenocrysts in asilicic groundmass which is probably glass(Fig. 10D). Occasionally, biogenic phosphatesare observed (Fig. 10B). Organic matter is pre-sent as clasts.

Pyrite is mostly euhedral without anyinclusions. It forms large octahedral crystals up to1 cm in diameter; these are now usuallyoxidized. Pyrite is associated with organic matterin some places. Although volumetrically lessprominent than the large euhedral crystals, pyriteis commonly developed as small, several-micrometre, spherical grains that are dispersedrandomly between the framework grains.The type B discoidal concretions are mainly

composed of medium-crystalline equant calcite.These calcite crystals are usually poikilotopicand significantly larger than the detritalmaterial; they usually form very characteristicsheaf-like radial fan structures (Fig. 12). Verycommonly, the sheaf-like structures containspherical domains (up to 300 lm in diameter),constituted by calcite spar (Fig. 9E to H), andsometimes rimmed with pyrite. These sphericaldomains are considerably larger than the frame-work grains and are taken to represent calcifiedmicrofossils. Within thin sections of concretionB6 and the surrounding mudstone (B6h), thesupposed microfossils were found in much lar-ger numbers in the concretion (284 items) thanin the host mudstone (86 items). The averagemajor axis length of the microfossils in the con-cretion is 118 lm (standard deviation = 32 lm)which is very similar to those measured in themudstone (126 lm on average; Fig. 11). Theiraspect ratio in the concretion is considerablyhigher (0�82 on average; standard devia-tion = 0�09) than that of the microfossils in themudstone (0�60 on average). Optical orientationof the surrounding poikilotopic calcite cementappears to be a continuation of that of the sparfilling the spherical domains because twinlamellae cross-cut both (Fig. 9F). The cracks inthe concretions cross-cut all cement generationsand they are filled with mosaic spar (Fig. 12C).The SEM-EDS examination proved that the

fine-grained detrital material within the type Bconcretions is almost identical to that in themudstones (Fig. 13). The only difference is thatfeldspars are in some places substituted bycalcite in the concretions and, hence, only smallrelicts of the parent mineral can be detected(Fig. 13H). Pyrite is usually spatially related toorganic matter and is typically developed aslarge euhedral (up to 120 lm; Fig. 13E) or poi-kilotopic (up to 200 lm; Fig. 13F) crystals thatare less frequently oxidized than in the mud-stones. As in the mudstones, pyrite also formssmall, several-micrometre sized, spherical grainsthat are randomly dispersed between the frame-



work grains. The shape, size and internal struc-ture of these grains suggest that they were origi-nally framboidal aggregates that underwentrecrystallization (Fig. 13G).

Geochemistry

The concretions and host rock differ signifi-cantly in their elemental composition. Thedifference results from the relative proportion ofcalcite cement to detrital material, which is bestdepicted by the amount of CaO, Al2O3 and thecontents of undissolved residue (Table 1).Correlation between the elements reveals thatthey can be divided into two groups,Al + K + Fe + Mg and Ca + Mn (Table 2). More-over, the amount of undissolved residue isclearly related to the first group of elements(Table 1). Based on SEM-EDS and EMP-WDSexamination, the phyllosilicates chlorite andbiotite are present which are rich in Al, K, Fe,and Mg, while Ca and Mn are fixed in the cal-cite cement.The calcite content was calculated from the

concentrations of CaO and MnO, while the con-tent of phyllosilicates was calculated from thesum of Al2O3, K2O, FeO and MgO (Table 1). Theresults of these calculations, compared with thequantitative results of thermal analysis (Table 1),show that the calcite content calculated fromchemical analyses is overestimated by up to 4%(except one sample), while the phyllosilicatecontent is underestimated. The sum of theundissolved residue, calcite and phyllosilicatesis close to 100% (Table 1). This total generallyconfirms the assumed relations between theconcentrations of elements and mineralcompositions.Correlation between samples (Table 3) shows

that the chemical composition is generallyuniform for each lithological type (r2 = 0�96 to1�00). Correlation between samples of differentlithologies reveals that the chemical composi-tions of the mudstones and greywackes arestrikingly similar (r2 = 1�00) but very different

from the composition of the type B concretions(r2 = 0�32 to 0�46). The composition of the typeA concretions is intermediate between thesetwo, but they are most similar in composition tothe greywackes (r2 = 0�81 to 0�94). This matchimplies that the greywackes and mudstones hada similar detrital source.The chemical composition of the calcite

cements is rather constant and the cements aretypically rich in Mn (Fig. 14). The molarconcentrations of Mg and Fe are very similar forall the cements analysed and do not exceed3�1% and 3�8%, respectively (Fig. 14). The onlydifference is the concentration of Mn which ishigher in type B concretions (between 3�3 and9�0 mol%) than in type A concretions andgreywackes (between 0�5 and 4�5 mol%;Fig. 14B). Calcite filling the cracks and the ovaldomains has an elemental composition similarto the surrounding concretionary cement(Fig. 14) as shown also by BSE imaging(Fig. 8A).The isotopic analyses of the mudstone

samples did not provide useful results becausethe carbonate content was too low. The type Aand B concretions show a similar isotopiccomposition. The d13C values range from �17�3to �12�3& (except sample A4: �9�1&) and d18Ovalues range from �20�7 to �16�2& for the typeA concretions, and from �23�7 to �14�0& andfrom �21�2 to �16�1& for the type Bconcretions, respectively (Table 4; Fig. 15). Theonly difference is that the type A concretionshave slightly heavier C isotope ratios. Onecalcite sample collected from a crack in concre-tion B1 (sample B1c) exhibits a similar d18Ovalue (�19�5&) but a significantly higher d13Cvalue (�8�5&) than samples from the concretionbody (Table 4; Fig. 15). The greywackes exhibitsomewhat heavier isotopic compositions com-pared to the concretions; their d13C and d18Ovalues range from �11�1 to �8�1& and from�18�2 to �10�0&, respectively (Table 4; Fig. 15).In profiles across the concretions, the spread

of d18O values in individual type B concretions

Fig. 7. Paired photomicrographs of the type A concretions under plane polarized light (C) and (G), crossed nicols(A), (B), (D), (E), (H) and cathodoluminescence (F). (A) and (B) Two photographs of the same area, but at differentstage positions (rotated by ca 30°) showing the poikilotopic morphology of the cement that goes into extinctionover large areas. (C) and (D) Volcanic lithoclasts (marked with solid lines) rich in glass with hyaloporphyritic(upper one) and felsic (lower one) textures. Note the poikilotopic character of calcite cement (marked with adashed line). (E) and (F) Grains (marked with dashed lines) replaced by calcite. Note the lack of intergranular con-tacts in some places in (F) (arrows) probably caused by calcite replacing matrix. (G) and (H) Calcite-filled (Cal)crack cutting through detrital quartz (Qtz).



A B

C D

E F

G H



is usually very small (up to 3�0&), and consider-ably lower than that of d13C values, which rangesfrom 4�3 to 8�2& (Table 4; Fig. 16). The verysmall spread of d18O values in individual type Aconcretions between 1�0& and 3�1& is slightlylower than, or similar to, that of d13C valueswhich range from 2�0 to 5�0& (Table 4; Fig. 16).The d13C values across type A concretions varyerratically, while type B concretions exhibit aconsistent centre to edge trend towards lighterd13C values.

INTERPRETATION AND DISCUSSION

The general tectonic and structural setting ofthe Bardo Unit, the dominance of mass-flowdeposits, the composition of the greywackesand the types of microfossils indicate a conti-nental slope to rise environment within anaccretionary wedge (e.g. Wajsprych, 1995).Within such a setting, concretions may experi-ence deep burial to several kilometres and ele-vated temperature.Type A concretions having a lenticular to

spheroidal shape and type B concretions havingdiscoidal morphologies grew in a sediment with

anisotropic permeability. It is an open questionwhether this anisotropy resulted from deposi-tional processes or from very early compaction.The bread loaf shape of some type B concretionsreflects a gradual fabric change. In particular,the spheroidal upper part reflects a highly po-rous uncompacted, isotropic sediment fabric,whereas the flattened lower part implies ananisotropic fabric characterized by particlesbeing oriented (sub-)parallel to bedding(Fig. 17). Such a vertical change in fabric mayhave resulted from either depositional processesor very early compaction. The latter possibilitywas suggested by Seilacher (2001) because asteep compactional gradient may develop veryclose to the sediment–water interface due to sig-nificant mechanical compaction (Bennett et al.,1991). The initial porosity of fine-grained sedi-ments close to the sea floor decreases by ca 20%within a metre of burial (Wetzel, 1990; Burdige,2006). In this case, a strong compactional gradi-ent is quite likely because the mudstone laminaeare bent over the top of the concretion, but arenearly flat below the base (Fig. 3B). In contrast,similar fabric changes have been reported frommuddy (parts of) turbidites (e.g. Stow & Piper,1984; Wetzel, 1987). Preservation of the deposi-

A B

Fig. 8. Scanning electron microscope images of type A concretions. (A) Detrital material, mostly dark grey silicagrains (SiO2), is cemented with light grey calcite (Cal). The crack cutting through the concretion is filled with cal-cite having a similar BSE colour to the concretionary cement. The area in the rectangle is shown in (B). (B) Aclose-up of the area marked with a rectangle in (A). Calcite cement is abundant and intergranular contacts areabsent in some places. Detrital material (mostly dark grey) is often corroded (circled).

Fig. 9. Photomicrographs of microfossils under plane polarized light (A), (C), (D) and (G), and crossed nicols (B),(E), (F) and (H). (A) and (B) Paired photomicrographs of a spherical component filled with calcite spar (Cal) andrimmed with framboidal pyrite (Py) in type A concretions. (C) Organic lamination of mudstone with oval particlescomposed of brownish-red organic matter, probably benthic cyanobacteria (in the square). (D) Slightly flattenedoval components, probably radiolarian shells, rimmed with oxidized framboidal pyrite (mudstone). (E), (F), (G)and (H) Calcified microfossils in the poikilotopic calcite in type B concretions. Note the twin lamellae (arrows)continuing from the microfossils into the surrounding sheaf-like aggregates in (F). (G) and (H) are paired photomi-crographs.



A B

C D

E F

G H



tional fabric requires an early onset of cementa-tion. The same is true for a steep compactionalgradient.

Petrographic evidence of diagenetic processesand conditions

Relatively large, up to 300 lm, oval componentsthat are sometimes rimmed with framboidalpyrite occur in the mudstones and in theconcretions (Fig. 9D to H); they are considerablylarger than the framework grains in the mud-stones and in the type B concretions. Because oftheir shape, relatively large size and outer mar-gin, these components are interpreted to bemicrofossils, probably radiolarian tests. Framboi-

dal pyrite associated with them, although usu-ally oxidized, may be the by-product of postmortem bacterial decomposition of organic mate-rial (Coleman & Raiswell, 1993; Raiswell et al.,1993; Briggs et al., 1996). In the mudstones,these tests are far from spherical (their aspectratio measured in a thin section of sample B6his 0�60 on average; Fig. 11) and their long axis isoriented parallel to the lamination. The originalmaterial appears to be altered in a similar wayto radiolarian tests shown by Scasso & Kiessling(2001, fig. 2I). In the concretions, these particlesare quite spherical, more abundant than in thesurrounding host rocks, and filled with calcite.Although the number and dimensions of theseparticles significantly differ between concretion

A B

C D

Fig. 10. Scanning electron microscope images of mudstones. (A) and (B) Very fine-grained, mainly argillaceousmaterial of the mudstone composed mostly of silica (SiO2), biotite (Bt) and chlorite (Chl). Some chlorite plates arewell-developed and can be authigenic. Biogenic phosphate grains (Ph) occur occasionally. (C) Silt-rich basal partof mudstone lamina exhibiting normal grading with large biotite plates rich in rutile (Rt) intergrowths, silica andauthigenic apatite (Ap). (D) Large volcanic clast (marked with a dashed line) rich in silica (dark grey) and chlorite(intermediate grey) from the basal part of mudstone lamina exhibiting normal grading. In the centre of the grainauthigenic Ti oxides (light grey, marked with arrows) can be observed.



B6 and the host mudstone (sample B6h) in thinsection, both samples were taken from the samehorizon and, hence, they initially contained thesame amount of detrital and biogenic materialand can, therefore, be compared. The range andaverage length of the major axis of the microfos-sils are very similar in both rocks, which sug-gests that they contain the same assemblage ofmicrofossils. However, the microfossils are threetimes more abundant in the concretion than inthe mudstone. This demonstrates that they werepreferentially preserved in the concretions, espe-cially if the dilution effect of the calcite cementis taken into account. In addition, the aspectratio of the microfossils is considerably higherin the concretion than in the mudstone (0�82compared to 0�60; Fig. 11). Therefore, the sup-posed radiolarian shells underwent preferentialpreservation in the concretions in terms ofthree-dimensional morphology and abundance(e.g. Blome & Albert, 1985). Differential preser-vation of microfossils between the carbonateconcretions and the carbonate-free host mud-stones indicates early, precompactional calciteprecipitation and concretion formation (Blome &Albert, 1985; Scasso & Kiessling, 2001). This

early-diagenetic cement probably formed a com-paction-resistant framework (e.g. Wetzel, 1992;Mozley, 1996).The shape and the preservation of the

biogenic remains indicate that the concretionsstarted to form close to the sediment–waterinterface. However, the morphology and thechemical and oxygen isotopic compositions ofthe calcite cement (as outlined below) areindicative of late diagenesis having affected theconcretions during deep burial.The cement engulfs and partially replaces

detrital material. Therefore, concretionsoccurring in sediments having high diageneticpotential; for instance, while being rich involcanic glass or feldspars, may not be suitablefor minus-cement porosity estimates (e.g. Rais-well, 1971, 1988; Hudson, 1978). Furthermore,replacive cementation challenges the commonpremise of original sediment being preferentiallybetter preserved in concretions than in the hostrocks (e.g. Morton, 1984; McBride, 1988).The concretions started to form close to the

sea floor, where they are typically composed ofmicritic or microsparitic calcite cement (e.g.Raiswell & Fisher, 2000). Fine-grainedcarbonates have a high potential to recrystallize(e.g. Bathurst, 1975) and, hence, early-diageneticconcretions can record the phase of recrystalli-zation that probably corresponds to the stage ofmaximum burial. The original micritic ormicrosparitic cements recrystallized at consider-able burial depth to poikilotopic calcite (Saigal& Bjørlykke, 1987; Calvo et al., 2011), asevidenced by the very light d18O values of ca�20& (for details see below). In addition, thehigh concentrations of Mn in the cement canalso be related to the late-diagenetic fluids(Froelich et al., 1979; Morad & Eshete, 1990).Mechanical compaction of fine-grained sedi-

ments usually starts just after deposition of thesediment close to the sea floor (Bennett et al.,1991; Burdige, 2006). As the sedimentation rateof the studied sediments is generally high(Wajsprych, 1995), the alternation of mud-domi-nated intervals and thick-bedded sandy turbi-dites is prone to develop overpressure, whilethe mudstones hinder pore water from flowingupwards (Gretener, 1981; Einsele, 2000). Theclastic dykes originating at a local scale from thegreywacke within the concretion-bearing inter-val probably formed under conditions of porewater overpressure, in particular during rapidlyincreasing depths of burial (e.g. Collinson,1994). Furthermore, pore-fluid overpressure is

Fig. 11. Dimensions of oval microfossils (length ofmajor versus minor axis) found in concretion B6 andthe surrounding mudstone B6h. Projection points rep-resent all microfossils seen in a standard thin section.Dashed lines reflect average aspect ratio of micro-fossils in the mudstone (red line) and in the concre-tion (blue line). Solid black line represents ideallyspherical morphology.



A B

C D

E F

G H



suggested by vertical veins cutting through bothhost rock and concretions including cementsthat recrystallized during maximum burial (forexample, sheaf-like cements; Fig. 12C). At greatburial depth, high formation pressure is crucialto creating low effective confining stress andbrittle rock properties (e.g. Lorenz et al., 1991).Joint initiation starts under overpressured condi-tions at small flaws like fossils, flutes or concre-tions; at joint initiation pore pressure inside theflaws is the same as in the surrounding material(Engelder & Lacazette, 1990). As a joint grows involume through propagation, pore pressuredrops and propagation is arrested. Once a jointhas grown to a length greater than the initialflaw size, crack propagation is re-initiated atinternal fluid pressures that are less thanpore pressure within the surrounding material(Engelder & Lacazette, 1990). The anastomosingveinlets that cut not only through the matrix andcalcite cement of the concretions but alsothrough framework grains are suggestive of recur-rent re-initiation of crack propagation (Fig. 7Gand H). These fractures continue into the grey-wackes (Fig. 2C) and, hence, crack opening andfilling appear to have been the latest diageneticprocesses which affected the concretions and thesurrounding greywackes. It occurred at consider-able depth, as indicated by the deformation ofquartz grains (Fig. 7G and H). However, hydro-fractures most probably opened when pressurestarted to decrease (Gretener, 1981; McConaughy& Engelder, 1999); this might have been relatedto the onset of exhumation (Fig. 18).

Isotopic evidence of diagenetic processes andconditions

The concretions have exceptionally light O iso-tope ratios; they are depleted in 18O to around�20& relative to VPDB. Such low d18O valuesare rarely observed in concretions (Mozley &Burns, 1993). The oxygen isotope compositionof carbonate cement primarily depends on thesource of pore fluid and the temperature of crys-tallization (Hoefs, 2009). Assuming that calcitecement precipitated from sea water, the palaeo-

temperature of crystallization can be calculated.Given that Earth was relatively ice-free duringVis�ean times (Grossman et al., 2008), the seawater d18O value of �1& was predefined inpalaeotemperature calculations using theequation of Anderson & Arthur (1983). The d18Ovalues of down to �21�2& indicate temperaturesof calcite formation of up to 153°C. Such tem-peratures are expected to occur ca 4�6 km belowthe sea floor at an assumed normal geothermalgradient of 33°C km�1, and 4°C on the sea floor.Such deep burial is in agreement with the maxi-mum diagenetic palaeotemperatures (130 to160°C) calculated from illite/smectite propor-tions in other Vis�ean rocks of the Bardo Unit(the Paprotnia series, located 6 km away fromthe study locality; Kryza et al., 2008). Further-more, the results of thermal maturation analysesperformed on Silurian rocks from the allochtho-nous succession of the Bardo Unit indicate deepburial below the oil window (Bauersachs et al.,2009; Grafka & Marynowski, 2011). However,the parental fluids might originally have beensomewhat 18O-depleted (Mozley & Burns, 1993;Raiswell & Fisher, 2000).Several processes are invoked as potential

causes of significant 18O-depletion of porewaters (Mozley & Burns, 1993), such as meteoricwater input, gas-hydrate formation, recrystalliza-tion at deep burial, and water–sediment interac-tion. Meteoric water input can be ruled outbecause it cannot feasibly occur in a deep, fullymarine basin. Moreover, there is no sign ofmeteoric diagenesis or an influence of gashydrates in the host rocks. Recrystallization dur-ing elevated temperatures leads to 18O depletionbecause the O isotope ratio strongly depends ontemperature (Hoefs, 2009). Therefore, recrystalli-zation of mainly low-temperature, early-diage-netic calcite causes re-equilibration of the Oisotope ratios at significantly higher tempera-tures when buried deeply. The studied rockswere buried below the oil window (Bauersachset al., 2009; Grafka & Marynowski, 2011). Thelarge sheaf-like cement textures are very likelyto have formed by recrystallization of a micriticor microsparitic precursor at considerable depth

Fig. 12. Photomicrographs of the type B concretions under plane polarized light (A), (D), (E) and (G), and crossednicols (B), (C), (F) and (H). (A) and (B) Paired photomicrographs of randomly oriented sheaf-like calcite aggregates.(C) Calcite-filled crack. (D) Transition from the body (left of the dashed line), through the rim (between the lines)of the concretion, to the surrounding mudstone (right of the solid line). (E) and (F) Paired photomicrographs ofthe sheaf-like morphology of calcite cement. (G) and (H) Paired photomicrographs of radiaxial fan structure (cir-cled) of the sheaf-like cement.



A B

C D

E F

G H



(Saigal & Bjørlykke, 1987; Calvo et al., 2011).Therefore, recrystallization at elevated tempera-tures is a major reason for the 18O depletion inthe concretions. A further consequence ofrecrystallization is the uniformity of d18O values

across the concretions (Fig. 13; Dix & Mullins,1987). Alteration of volcanogenic material couldhave also contributed to the 18O-depletion ofpore fluids and the cements precipitated there-from (see Lawrence & Gieskes, 1981; Pirrie &

Fig. 13. Scanning electron microscope images of type B concretions. (A) Randomly oriented sheaf-like calciteaggregates. (B) Patches of detrital material (dark grey) isolated in the calcite cement. (C) and (D) The sheaf-likemorphology of calcite cement. (E), (F) and (G) Euhedral (E), poikilotopic (F) and framboidal (G) morphology ofpyrite (Py). (H) Calcite (Cal) replacing plagioclase (Pl). The dashed line represents outline of a former plagioclasegrain. Relicts of plagioclase are dark grey, pores are black.

Table 1. Results of inductively coupled plasma optical emission spectrometry (ICP-OES) analyses and the calcu-lated contents of minerals (in wt.%). Contents of minerals calculated from thermal analyses are given for compari-son on the right.

Al2O3 K2O FeO MgO CaO MnO Residue Phyllosil. Calcite Sum

Thermal analysis:

Phyllosil. Calcite

Type B concretionsB1 2�20 0�08 3�62 1�47 38�92 2�80 22�2 7�4 74�0 103�6B2 2�21 0�27 3�20 1�30 37�33 3�15 23�6 7�0 71�7 102�3 <9 69B3 2�38 0�35 2�09 1�22 37�70 3�13 22�2 6�0 72�4 100�6 <9 68B4 2�25 0�23 2�40 1�27 38�33 3�22 26�0 6�1 73�6 105�8B6 2�04 0�29 2�15 1�17 38�04 2�84 23�6 5�6 72�5 101�7 <20 63

MudstonesB1h 6�16 0�53 7�74 3�74 0�32 0�26 74�8 18�2 1�0 93�9B2h 6�17 0�54 8�01 3�88 0�22 0�21 76�9 18�6 0�7 96�2 <20 0B3h 5�02 0�51 6�92 2�72 0�17 0�15 81�1 15�2 0�6 96�8 <28 0B4h 5�85 0�77 5�35 2�96 0�45 0�23 80�9 14�9 1�2 97�0B6h 7�36 0�75 5�87 3�76 0�23 0�29 77�8 17�7 0�9 96�4 <30 0

Type A concretionsA1 3�01 0�15 4�17 2�39 21�98 0�82 49�0 9�7 40�6 99�3A2 2�67 0�25 3�86 2�35 26�97 1�67 41�1 9�1 50�8 101�1A3 1�94 0�16 3�66 2�35 23�33 1�50 47�3 8�1 44�1 99�5 <10 45A4 0�74 0�18 4�10 4�87 18�70 1�49 49�5 9�9 35�8 95�2

GreywackesA1h 4�52 0�26 6�42 2�85 3�39 0�34 74�6 14�1 6�6 95�3A3h 4�16 0�28 4�64 2�74 3�10 0�36 76�6 11�8 6�1 94�5 <19 6A4h 1�50 0�11 3�69 0�55 0�72 0�13 84�5 5�9 1�5 91�9

Phyllosil, phyllosilicates.

Table 2. Correlation (r2) between the elements and the undissolved residue based on their contents analysedwith the use of inductively coupled plasma optical emission spectrometry (ICP-OES).

Al2O3 K2O FeO MgO CaO MnO Residue

Al2O3 X 0�87 0�81 0�52 �0�71 �0�65 0�67K2O 0�87 X 0�58 0�47 �0�56 �0�45 0�52FeO 0�81 0�58 X 0�70 �0�83 �0�80 0�79MgO 0�52 0�47 0�70 X �0�57 �0�56 0�51CaO �0�71 �0�56 �0�83 �0�57 X 0�97 �0�99MnO �0�65 �0�45 �0�80 �0�56 0�97 X �0�97Residue 0�67 0�52 0�79 0�51 �0�99 �0�97 X



Marshall, 1991). In fact, all the rocks examinedare very similar in terms of the composition ofthe detrital material, being rich in volcaniclasticand subvolcanic clasts. Many of these clasts areat least partly composed of glass whichsometimes devitrified into chalcedony or chlo-rites. A high glass content is also indicated bythe results of thermal analyses.

Recrystallization did not affect the d13C values,which is shown by the large variability in thed13C values across the concretions. In fact, the Cisotopes are much more resistant to resettingwhen calcite recrystallizes (Veizer, 1983; Gautier& Claypool, 1984). Retention of the originald13C signal despite recrystallization is due tothe much weaker temperature dependence of

Table 3. Correlation (r2) between the samples based on their chemical compositions analysed with the use ofinductively coupled plasma optical emission spectrometry (ICP-OES).

Type B concretions Mudstones Type A concretions Greywackes

B1 B2 B3 B4 B6 B2h B1h B3h B4h B6h A1 A2 A3 A4 A1h A3h A4h

Type B concretionsB1 X 1�00 1�00 0�99 1�00 0�32 0�32 0�33 0�33 0�32 0�71 0�83 0�74 0�66 0�37 0�37 0�36B2 1�00 X 1�00 1�00 1�00 0�37 0�37 0�38 0�38 0�37 0�75 0�86 0�78 0�69 0�42 0�42 0�41B3 1�00 1�00 X 1�00 1�00 0�33 0�33 0�34 0�35 0�33 0�72 0�84 0�75 0�67 0�38 0�38 0�37B4 0�99 1�00 1�00 X 1�00 0�40 0�41 0�42 0�42 0�41 0�77 0�88 0�80 0�72 0�45 0�46 0�44B6 1�00 1�00 1�00 1�00 X 0�36 0�36 0�37 0�37 0�36 0�74 0�86 0�77 0�69 0�41 0�41 0�40

MudstonesB2h 0�32 0�37 0�33 0�40 0�36 X 1�00 1�00 1�00 1�00 0�89 0�79 0�87 0�92 1�00 1�00 1�00B1h 0�32 0�37 0�33 0�41 0�36 1�00 X 1�00 1�00 1�00 0�89 0�79 0�87 0�92 1�00 1�00 1�00B3h 0�33 0�38 0�34 0�42 0�37 1�00 1�00 X 1�00 1�00 0�90 0�80 0�88 0�92 1�00 1�00 1�00B4h 0�33 0�38 0�35 0�42 0�37 1�00 1�00 1�00 X 1�00 0�90 0�80 0�88 0�92 1�00 1�00 1�00B6h 0�32 0�37 0�33 0�41 0�36 1�00 1�00 1�00 1�00 X 0�89 0�79 0�87 0�92 1�00 1�00 1�00

Type A concretionsA1 0�71 0�75 0�72 0�77 0�74 0�89 0�89 0�90 0�90 0�89 X 0�98 1�00 0�99 0�92 0�92 0�91A2 0�83 0�86 0�84 0�88 0�86 0�79 0�79 0�80 0�80 0�79 0�98 X 0�99 0�96 0�82 0�82 0�81A3 0�74 0�78 0�75 0�80 0�77 0�87 0�87 0�88 0�88 0�87 1�00 0�99 X 0�99 0�90 0�90 0�89A4 0�66 0�69 0�67 0�72 0�69 0�92 0�92 0�92 0�92 0�92 0�99 0�96 0�99 X 0�94 0�94 0�93

GreywackesA1h 0�37 0�42 0�38 0�45 0�41 1�00 1�00 1�00 1�00 1�00 0�92 0�82 0�90 0�94 X 1�00 1�00A3h 0�37 0�42 0�38 0�46 0�41 1�00 1�00 1�00 1�00 1�00 0�92 0�82 0�90 0�94 1�00 X 1�00A4h 0�36 0�41 0�37 0�44 0�40 1�00 1�00 1�00 1�00 1�00 0�91 0�81 0�89 0�93 1�00 1�00 X

Fig. 14. Ternary diagrams depicting molar proportions of Ca, Mg, Fe+Mn and of Ca, Mn, Fe in calcite cements.



C-isotope fractionation compared to that of O(Veizer, 1983). Therefore, the unchanged originald13C values preserved in the concretion cementhelp to decipher the sources of bicarbonate dis-solved in pore water during concretion formation(see also Dix & Mullins, 1987; Morad & Eshete,1990). The negative d13C values as low as�23�7& for the early-diagenetic type B concre-tions indicate that calcium carbonate oversatura-tion in the pore water was caused by oxidationof organic matter by sulphate reduction. Thisprocess can lead to precipitation of pyrite inframboidal aggregates (Peckmann & Thiel, 2004).Small spherical pyrite grains dispersed in themudstones and the concretions formed by recrys-tallization of originally framboidal pyrite (Rai-swell, 1982) which is indicated by a framboidalarrangement preserved in some grains (Fig. 13G).Sulphate reduction takes place close to the sedi-ment–water interface within the upper fewmetres below the sea floor, which is consistentwith the precompactional initiation of concre-

tionary growth. Therefore, sulphate reduction isthe most likely process leading to the formationof the concretions. While sulphate is reduced,organic matter is oxidized and, in turn, bicarbo-nate with negative d13C values down to around�25& is liberated (Irwin et al., 1977; Raiswell,1987). However, cementation of concretions mayhave continued in deeper diagenetic zoneswhere additional amounts of calcite may havebeen precipitated.The type B concretions embedded in mud-

stones exhibit a centre to edge trend in d13Ctowards higher values (Fig. 16). This C isotopetrend is not interpreted in detail here becausethe primary textures have been obscured andoverwritten by recrystallization, precluding thereconstruction of the concretionary growthprocess. The growth process of a concretion,however, needs to be known for interpretation.The centre to edge trend in d13C, in the case ofan outward growth, would indicate that thecomposition of pore water changed with time;

Table 4. Stable C and O isotope compositions (in &).

Sample d13CPDB d18OPDB Material Sample d13CPDB d18OPDB Material

B1-1 �19�24 �20�66 Type B concretion B6-1 �19�60 �16�08 Type B concretionB1-2 �22�03 �20�94 B6-2 �18�96 �18�80B1-3 �22�32 �21�21 B6-3 �22�42 �18�92B1-4 �22�81 �20�85 B6-4 �23�68 �19�10B1-5 �23�49 �20�83 B6-5 �22�43 �18�80B1-6 �23�52 �20�86 B6-6 �20�90 �18�67B1-7 �23�08 �20�62 B6-7 �16�79 �17�48B1-8 �22�97 �20�84B1-9 �14�94 �20�55 A1-1 �15�75 �20�32 Type A concretionB1-c �8�52 �19�53 Crack A1-2 �13�90 �18�26

A1-3 �14�23 �18�84B2-1 �16�94 �20�34 Type B concretion A1-4 �13�75 �20�70B2-2 �16�69 �19�81 A1-5 �13�91 �19�32B2-3 �19�73 �19�98B2-4 �22�55 �19�97 A1h �10�96 �18�24 GreywackeB2-5 �22�36 �20�01B2-6 �14�31 �19�63 A2-1 �15�63 �20�64 Type A concretionB2-7 �20�29 �19�69 A2-2 �13�06 �20�16

A2-3 �13�49 �19�62B3-1 �14�05 �19�37 Type B concretionB3-2 �16�97 �19�32 A3-1 �13�87 �16�16 Type A concretionB3-3 �19�10 �18�87 A3-2 �12�93 �18�97B3-4 �18�27 �19�36 A3-3 �12�54 �17�29B3-5 �21�40 �19�28 A3-4 �14�66 �17�04B3-6 �19�13 �19�03 A3-5 �17�31 �19�29B3-7 �17�00 �18�18 A3-6 �15�96 �17�98

A3-7 �12�35 �16�65B4-1 �16�28 �19�29 Type B concretionB4-2 �18�68 �19�31 A3h �11�11 �17�61 GreywackeB4-3 �20�71 �19�62B4-4 �15�91 �19�64



in the case of a pervasive growth, it could berelated to a spatial change in pore water compo-sition (e.g. Bojanowski & Clarkson, 2012).Because of the unknown growth mode of theconcretions, the d13C trend across them remainsa matter of speculation. Nonetheless, even if theconcretions grew in a pervasive way, some late-stage cements were added within their outerzones (Mozley, 1989, 1996; Raiswell & Fisher,2000). Assuming an at least partly concentricgrowth mechanism, the observed trend in d13Cvalues suggests that the outer zones were precip-itated from fluids enriched in isotopically hea-vier bicarbonate produced in the deeperdiagenetic zone of methanogenesis. Generally,the concretions grew as the result of organicmatter oxidation under anoxic conditions. Sur-plus alkalinity and bicarbonate for concretiongrowth was provided mainly by anaerobicallyoxidized organic matter in the sulphate reduc-tion zone (with low d13C values) and by an iso-topically heavier source, possibly pore waterinfluenced by methanogenesis that was admixedin various proportions.The isotopic data of carbonate cements filling

cracks provide direct information about a laterdiagenetic phase and the source of the cement-generating bicarbonate. This information can beconstrained because individual large cementcrystals were analysed. The blocky calcite fromthe crack in concretion B1 differs isotopically(d13C �8�5&) from the concretion body (d13C�23�5 to �14�9&; Table 4). Thus, the blockycalcite precipitated from different pore fluidsthan the concretion cement. The very low d18Ovalue of the cement within the crack (�19�5&),however, is very similar to that of the concretionbody (from �21�2 to �20�5&). Moreover, the

chemical composition of both is very similar,and therefore the blocky calcite cement in thecrack was precipitated during the same late-diagenetic, deep-burial phase when the concre-tion recrystallized at an elevated temperature.Thus, the d13C value of this late-diageneticcement (�8�5&) carries the isotopic signature ofthe late-diagenetic pore fluid that was less 13C-depleted than the early-diagenetic fluid fromwhich the concretions started to form. This shiftof d13C towards higher values may be explainedby an increased role for methanogenesis oversulphate reduction in the deeper setting. How-ever, it is also possible that the late-stagecements were precipitated from pore water car-rying a mixture of bicarbonate liberated by meth-anogenesis (enriched in 13C by ca 15&) anddecarboxylation (depleted in 13C by as much as20&) (Irwin et al., 1977; Curtis & Coleman,1986). As discussed above, precipitation of theblocky calcite and recrystallization of the con-cretions took place at elevated temperaturewhich implies decarboxylation as the most pro-bable source of bicarbonate for the late-stagecements. At this catagenetic stage, still elevatedtemperature favoured resetting and homogeniza-tion of the d18O values. It is possible that over-pressure developed at this deep burial andnumerous fractures opened when the pressuredecreased, probably due to onset of exhumation(Fig. 18). These fractures cross-cut the recrystal-lized cement.The slightly higher d13C values of type A con-

cretions than those of type B concretions suggestthat the pore water trapped during early diagene-sis of the greywackes was enriched in marinebicarbonate relative to the pore water of concre-tion-containing mudstones. This difference can

Fig. 15. d13C VPDB versus d18OVPDB plot. The greywackes(diamonds) exhibit the heaviestisotopic composition. The fields ofpoints for type A (squares) andtype B concretions (circles) largelyoverlap, but the former show ratherhigher d13C VPDB values. However,the highest d13C VPDB value isrecorded in sample B1c, the late-diagenetic calcite cement fillingcracks in type B concretion(triangle).



be ascribed to the different permeability andporosity of the two lithologies, which allowedthe marine water to percolate further into thesand than into the mud. An alternative explana-tion is that type A concretions contain a largeramount of cements produced by methanogenesisthan type B concretions.

The origin of the calcite cement precipitated inthe greywackes, which differs from that of thetype A concretions, has to be deduced from sev-eral indicators. The replacive character points tolate-diagenetic conditions (Sell�es-Mart�ınez,1996). Furthermore, d13C values of this cement(between �11& and �8&) are similar to that of

Fig. 16. d13C VPDB (squares) and d18O VPDB (circles) values of samples taken along vertical transects across theconcretions. Grey areas indicate the shapes of the slabs cut vertically from the concretions. Arrows indicate placeswhere the samples were drilled. Numbers in the arrows correspond to the symbols of the samples used in Table 4.Isotopic trend towards heavier d13C VPDB values from centre to edge can be observed in the type B concretions.All the concretions exhibit distinctively uniform d18O VPDB values.



the crack fill from the type B concretion. Thesetwo facts together indicate decarboxylation as themost probable bicarbonate source under late-dia-genetic conditions. The chemical composition of

this cement is similar to that of the concretionarycements. Therefore, the carbonate cementation ofthe greywackes and recrystallization of the con-cretions took place during the same diageneticstage (catagenesis). Cementation of the greywac-kes may have occurred through precipitationdirectly from interstitial fluids or through recrys-tallization of some carbonate precursor. Theprocesses and the history of the concretionsreconstructed in this work are summarized in aschematic diagenetic sequence (Fig. 18).

CONCLUSIONS

1 The early-diagenetic carbonate concretionsthat formed in a Lower Carboniferous accretio-nary wedge setting record the diagenetic path-way to deep-burial conditions, reaching thecatagenetic zone. Obviously, the effects of deepburial in an overall compressive regime differfrom those encountered in epeiric settings wheremost studies on concretions have been focused.Therefore, the concretions investigated representa rare record that is valuable for reconstructingthe diagenetic history of carbonate-cementedclastic rocks in similar settings.2 The concretions occur in adjacent beds ofdiffering lithology, namely mudstone and grey-wacke. The concretions embedded in both hostrocks started to form during early diagenesisprior to significant compaction, while organic

Fig. 18. Diagenetic sequence of post-depositional processes affecting the rocks in relative time and depth scales.

Fig. 17. Bread loaf-shaped concretions (concretionbody, grey; schematic sediment particles, blackdashes) reflect a gradual fabric change from isotropic(represented by the upper spherical part of the con-cretion) to anisotropic, a mainly bedding parallel-ori-ented fabric (reflected by the lower part and flat base).This fabric change may result from either a compac-tional gradient or depositional processes (for detailsee text).



matter became oxidized under anoxic conditionsin the sulphate reduction zone. This processtook place very close to the sediment–waterinterface. This early cementation facilitated thepreferential preservation of microfossils withinthe concretions. The shape of the concretionsbeing discoidal, sometimes bread loaf-shaped inthe mudstones and spherical to lenticular in thegreywackes, reflects the permeability of the hostrock at the time of initial concretion formation.Cementation of the concretions may have con-tinued during increasing burial into the zone ofmethanogenesis.3 Concretions hosted in both lithologies were

fully formed at a shallow burial depth, as shownby the carbon isotope signature typical of thesulphate reduction and methanogenesis. None-theless, they also record maximum burial condi-tions. The concretions experienced deep burialand temperatures exceeding 150°C. In thissetting, the original typically fine-crystallinetexture of carbonate cements was affected byrecrystallization. During that phase, the fine-grained cement was replaced by the sheaf-like,poikilotopic calcites in the concretions. Theselarge crystals engulf and partially replacedetrital constituents. Therefore, the commonpremise that early-diagenetic concretions pre-serve the initial fabric and petrography of thesurrounding sediments is not always applicable,especially if the sediments have high diageneticpotential and contain material susceptible toalteration. Thus, the often-invoked applicationof concretions for provenance studies and forminus-cement calculations of the original poro-sity is not always valid.4 Recrystallization caused uniform stable O

isotope ratios due to re-equilibration with thepore fluids at high temperatures. Furthermore,the fluids were depleted in 18O due to the alter-ation of significant amounts of volcanic glass.The d13C values remained unchanged because Cisotopes are not subjected to temperature-depen-dent fractionation during recrystallization atthese conditions. Thus, the source of bicarbo-nate for cementation of the concretions could bededuced from their d13C values (see point 2).This recrystallization probably took place in thecatagenetic zone because the late-diagenetic porefluids were heated and enriched in bicarbonateproduced by decarboxylation, as indicated bythe isotopic composition of calcite infillingcracks in the concretions.5 The rocks probably experienced pore-fluid

overpressure when buried deeply, as suggested

by the presence of clastic dykes and late-stagecement-filled fractures which cut through bothhost rock and concretions. These structures wereprobably formed when the overpressure startedto decrease during the onset of exhumation.6 Calcite cement dispersed in the greywackealso formed during late diagenesis becausecalcite crystals grew in a replacive style. Thiscementation took place during catagenesis, asevidenced by the elemental and carbon-isotopiccompositions. The mechanism of this cementa-tion, however, is not exactly known.

ACKNOWLEDGEMENTS

Funding for this research was provided bythe Institute of Geochemistry, Mineralogy andPetrology, University of Warsaw. P. Mozley(Socorro, USA) and S. Lokier (Abu Dhabi,UAE) critically read the manuscript, providedhelpful comments and improved the Englishtext. All of these contributions are gratefullyacknowledged.

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Manuscript received 15 March 2013; revision accepted28 November 2013



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