Multiphase partial and selective dolomitization of Carnian reef limestone (Transdanubian Range,...

Multiphase partial and selective dolomitization of Carnian reeflimestone (Transdanubian Range, Hungary)

J �ANOS HAAS*, TAM �AS BUDAI† , ORSOLYA GY }ORI* and S �ANDOR KELE‡*MTA-ELTE Geological, Geophysical and Space Science Research Group, Hungarian Academy ofSciences, Pazmany P. s�etany 1/c, H-1117 Budapest, Hungary (E-mail: [email protected])†Geological and Geophysical Institute of Hungary, Stefania ut 14., H-1143 Budapest, Hungary‡Institute for Geological and Geochemical Research, Research Centre for Astronomy and EarthSciences, Hungarian Academy of Sciences, Budaorsi ut 45, H-1112 Budapest, Hungary

Associate Editor – Stephen Lokier

ABSTRACT

Partially dolomitized carbonate successions provide a good opportunity to

understand the commonly multistage process of dolomitization. Petrographic

methods, fluid inclusion microthermometry and stable isotope measurements

were applied to reconstruct the diagenetic evolution and dolomitization of a

partially dolomitized Carnian reef limestone from the Transdanubian Range,

Hungary. The diagenetic history began with reef diagenesis and formation of

dolomite micro-aggregates in microbial fabric elements; this was followed by

the development of euhedral porphyrotopic dolomite crystals through over-

growths around the previously formed dolomite micro-aggregates during the

earliest burial stage. Increasing burial resulted in the extension of the dolomite

patches via formation of finely crystalline replacement dolomite. From the

Late Norian, when the Carnian reef carbonates reached the depth of 1�0 to

1�8 km, the diagenetic evolution continued in an intermediate to deep-burial

setting. Contemporaneously, an extensional regime was established, leading to

fracturing. The progressive burial resulted in the recrystallization of the pre-

existing dolomite with increasing temperature, while saddle dolomite cement

was precipitated in fractures. In connection with the Alpine Orogeny, intense

denudation took place during the Late Cretaceous, accompanied by fracturing.

Similar tectonically controlled denudation and fracturing occurred in several

stages during the Cenozoic. As a result of these processes, the studied Carnian

carbonates were raised to a near-surface position or became subaerially

exposed, leading to dedolomitization of the last dolomite phase and precipita-

tion of calcite cement in cavities and fractures. This study revealed that by

investigating partially and selectively dolomitized rock types, it is possible to

document and understand those stages of the multiple dolomitization process

which can barely be detected in the completely dolomitized rock bodies. Rec-

ognition of the dolomitization phases could provide the basis for the analysis

of their relations with the depositional, diagenetic and tectonic processes, and

stages of basin evolution.

Keywords Carbonate diagenesis, dolomitization, fluid inclusions, reef,stable isotopes, Triassic.

© 2013 The Authors Sedimentology © 2013 International Association of Sedimentologists 1

Sedimentology (2014) doi: 10.1111/sed.12088

INTRODUCTION

Dolomite is a very common rock in the upperpart of Earth’s crust. However, in contrast tolimestone, the formation of modern dolomite inmarine and lacustrine environments is rare andthe quantity of recent dolomite is very limited(Fairbridge, 1957; McKenzie, 1991; Purser et al.,1994; Machel, 2004). Applicability of modernanalogues to the genesis of huge dolomite bodiesis also ambiguous. Large dolomite bodies wereformed either from unconsolidated calcareoussediments or from limestone, during the courseof diagenesis. Some formed contemporaneouslyor very shortly after deposition (e.g. Vasconcelos& McKenzie, 1997; Bontognali et al., 2010),whereas other dolomite bodies formed duringthe later stages of shallow-burial (e.g. Budd,1997; Melim et al., 2001; Jones & Luth, 2003;Choquette & Hiatt, 2008) to intermediate anddeep-burial settings (e.g. Wilson et al., 1990;Qing & Mountjoy, 1994; Braithwaite & Rizzi,1997; Machel et al., 2000; Duggan et al., 2001;Chen et al., 2004). In spite of remarkable pro-gress in dolomite research in the last decades(Land, 1985; Sibley et al., 1994; Vasconceloset al., 1995; Budd, 1997; Vasconcelos & McKen-zie, 1997; Melim et al., 2001; Machel, 2004;Whitaker et al., 2004; Jones, 2005; Wright &Wacey, 2005; Choquette & Hiatt, 2008), manyquestions about its origin remain; thus, the gene-sis of dolomite is still one of the hot topics ofcarbonate sedimentology.Dolomite plays a significant role in the geolo-

gical make-up of the Transdanubian Range. Itwas formed over a long period from the LatePermian to the Late Triassic; the total thicknessof the dolomitic rocks may reach 2�5 to 3 km(Haas & Budai, 1995). The stratigraphic settingof the dolomitic units; as well as their temporaland spatial relations and palaeogeographic set-ting is well-documented (Haas & Budai, 1995,1999; Haas et al., 1995; Haas, 2002). Thesefavourable conditions inspired a project for com-prehensive studies of various dolomite forma-tions, based on the re-evaluation of data ofprevious investigations and the results of newstudies. A fundamental problem in the evalua-tion of the dolomite genesis is that the earlystages of the commonly multistage process can-not be recognized due to later diagenetic over-prints. The partially dolomitized Carnian reefrecords consecutive stages of dolomitizationallowing for the study of such a complex diage-netic setting.

Multistage dolomitization akin to that foundin the Ederics Limestone was reported by Hen-rich & Zankl (1986) from Carnian Wetterstein-type platform carbonates of the Bavarian Alps.The striking similarity of the sedimentary faciesand the pattern of dolomitization offered theopportunity to apply new models and conceptsof dolomite genesis to dolomitization of Wetter-stein-type platforms, which are widely deve-loped along the Tethys margin.

GEOLOGICAL SETTING

The study area is located in the KeszthelyMountains at the south-western end of theTransdanubian Range, NW Hungary (Figs 1 and2). The Transdanubian Range forms a mega-syn-cline structure and the investigated area belongsto the south-western limb of this structure.The Keszthely Mountains are made up mainly

by Upper Triassic lithologies (Figs 2 and 3). Asignificant north–south trending fault dividesthe mountain into two parts. The western sideof the mountain comprises Upper Carnian toLower Norian platform dolomite (Hauptdol-omit), Upper Norian dolomite of basinal facies(Rezi Dolomite) and Rhaetian dark grey to blackmarl (Kossen Formation). On the eastern side ofthe mountain, Carnian platform carbonates (Ede-rics Limestone; S�edvolgy Dolomite) and basinalmarl and limestone (Veszpr�em Marl, SandorhegyFormation) crop out, which are in tectonic con-tact with the Norian formations (Csillag et al.,1995; Budai et al., 1999a,b).The evolution of the Keszthely Mountains fits

well into the Alpine evolutionary history of theTransdanubian Range (Haas & Budai, 1999). Fol-lowing a sea-level rise at the Permian/Triassicboundary, a shallow-marine ramp was estab-lished. It was characterized by mixed siliciclasticand carbonate sedimentation during the EarlyTriassic that was followed by carbonate deposi-tion in the Early Anisian. At the beginning of theMiddle Anisian, extensional tectonic activityresulted in disintegration of the carbonate ramp;isolated platforms, and intraplatform basins, wereformed (Budai & Voros, 1992, 2006). Large car-bonate platforms developed during the Ladinian,whereas in the deeper basins pelagic limestoneand volcanic tuff were deposited.Eustatically and climatically controlled terri-

genous influx led to the filling of the basins withfine-grained siliciclastics during the Early toearly Late Carnian (Veszpr�em Marl, Sandorhegy

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

2 J. Haas et al.

Formation), whereas in relatively elevated areasgrowth of the carbonate platforms (Ederics Lime-stone and S�edvolgy Dolomite) continued (Haas& Budai, 1999). A huge platform was establishedin the latest Carnian, producing a more than1 km thick carbonate succession by the end ofthe Middle Norian. In the Late Norian, exten-sional basins developed in the western part ofthe Transdanubian Range where carbonates andorganic-rich marls of restricted basin facies wereformed (Haas, 2002).Due to multistage denudation from the Late

Cretaceous onward, Mesozoic formations youn-ger than Rhaetian are not known in the area ofthe Keszthely Mountains. However, the youngerMesozoic to Cenozoic record has been pre-served in other parts of the TransdanubianRange, east to the Keszthely Mountains, allow-ing the reconstruction of the evolutionaryhistory for the missing parts (Haas et al., 1985;Horvath & Cloetingh, 1996; Voros & Galacz,1998; Budai et al., 1999b; Magyar et al., 1999;Haas, 2001).

METHODS

From the preserved cores of the BalatonedericsBet-1 borehole, 18 samples were taken fordetailed petrographic and geochemical studies.More than 140 thin sections (prepared for earlierstudies) were investigated and the results of

calcite/dolomite ratio determinations (made bythe laboratories of the Hungarian Institute ofGeology) were used in this study (Fig. 4).Alizarin red-S and potassium ferricyanide

stains were used to determine the carbonatephases in the samples (Dickson, 1966). Fordescription of the dolomite texture, the classifi-cation proposed by Machel (2004) was applied.It is a supplemented version of the textural clas-sification of Sibley & Gregg (1987), which ismore appropriate to the description of partiallydolomitized rock types.Ultraviolet epifluorescence was acquired on a

Zeiss Axioskop 40 (Carl Zeiss GmbH, Jena, Ger-many), equipped with Filter Set 09 (ExcitationFilter BP 450–490, Beam Splitter FT 510, Emis-sion LP 515) using a Hg light illuminator. Ca-thodoluminescence (CL) studies were performedusing a MAAS-Nuclide ELM-3 cold-cathode Lu-minoscope (Measurement and Analysis Systems,Inc., Lowell, MA, USA) at the Department ofPhysical and Applied Geology, Eotvos LorandUniversity. Doubly polished thin sections,100 lm thick, were carefully prepared for fluidinclusion studies. Microthermometric measure-ments were performed on a Linkam FT-IR heat-ing-freezing stage (Linkam, Guildford, UK) at theDepartment of Mineralogy, Eotvos Lorand Uni-versity. Standardization was carried outat�56�6°C, 0�0°C and 385�0°C on synthetic quartz-hosted H2O and H2O–CO2 fluid inclusions.Accuracy of the measurements was 0�1°C during

A

B

Fig. 1. (A) Position of theTransdanubian Range (TR) inHungary (H) (A, Austria; SK,Slovakia; UA, Ukraine; RO,Romania; SRB, Serbia; CR, Croatia;SLO, Slovenia). (B) Simplified mapof the TR (after Haas & Budai,1999) showing the surfaceoccurrences of the Triassicformations and the position of theKeszthely Mountains (see Fig. 2).


Multiphase partial dolomitization of Carnian reef limestone 3

heating experiments and 1�0°C during freezing.Scanning electron microscopic investigationswere carried out on an Amray 1830i instrument(Amray, Bedford, MA, USA) equipped with anINCA energy-dispersive X-ray spectrometer (EDS)at the Department of Petrology, Eotvos LorandUniversity.Stable isotope measurements were performed

on micro-drilled powders of calcite and dolomitesamples, at the Institute for Geological and Geo-chemical Research. The analyses were carried outusing the continuous flow technique (Rosenbaum& Sheppard, 1986; Spotl & Vennemann, 2003).13C/12C and 18O/16O ratios were determined inCO2 gases liberated by phosphoric acid using aFinnigan delta plus XP mass spectrometer(Thermo Fisher Scientific, Bath, UK). Standardi-zation was conducted using laboratory calcitestandards calibrated against the NBS 18 and NBS19 standards. During the measurement of thedolomite samples, a laboratory dolomite standard(DST) was used. All samples were measured atleast in duplicate and the mean values are in thetraditional d notation in parts per thousand (&)relative to Vienna Pee Dee Belemnite (V-PDB).Reproducibilities are better than �0�1& for d13Cand �0�15& for d18O.

STRATIGRAPHIC SETTING,LITHOLOGICAL AND FACIESCHARACTERISTICS OF THE STUDIEDSECTION

A 130 m thick continuous core section (CoreBalatonederics Bet-1) of the transitional intervalbetween the typical non-dolomitized EdericsLimestone and its upper, dolomitized member(S�edvolgy Dolomite) was the subject of this study.The core was obtained on the south-eastern mar-gin of the Keszthely Mountains (Figs 2 and 3;Csillag et al., 1995; Budai et al., 1999b). The litho-logical column and the mineralogical compositiondiagram of core Bet-1 are shown in Fig. 4, wherethe sampling points are also indicated.The lower part of the core (41 to 130 m) is

predominantly comprised of only slightlydolomitized limestone. It is commonly lightgrey, locally with slightly darker patches(Fig. 5A). Calcareous sponge and coral frame-stone occurs, which contains growth-frameworkpores with isopachous cement fill. However,rudstone that consists of millimetre-sized to cen-timetre-sized reef-derived bioclasts and inter-granular pores with isopachous cement is morecommon. The strongly dolomitized intervals are

Fig. 2. Simplified geological mapof the Keszthely Mountainsshowing the location of theinvestigated core, Bet-1 (after Budaiet al., 1999a; Budai & Gyalog,2009).


4 J. Haas et al.

characterized by a mottled appearance withirregular light and dark grey patches (Fig. 5B);millimetre-sized to centimetre-sized empty poresand pores filled by coarsely crystalline dolomitecement are common. Sucrose, porous dolomitealso occurs locally.The upper part of the core (4 to 41 m) consists

predominantly of dolomite; accordingly it canbe assigned to the S�edvolgy Dolomite. It is usu-ally light grey, or mottled with brownish-greyand darker patches locally; centimetre-sizedpores filled with white coarsely crystalline dolo-mite cement fill are common.The chronostratigraphic assignment of the

section is based on foraminifera. Lamelliconusmultispirus (Oberhauser), Autrocolomia marsch-alli Oberhauser, Kollmannita multilocolataFuchs and Schmidita inflata Fuchs were deter-mined in the samples by Oravecz-Scheffer whoassigned the succession to the Carnian (Gyaloget al., 1986; Oravecz-Scheffer, 1987).

LITHOFACIES TYPES

Based on observations of cut cores, polishedslabs and detailed microfacies investigations ofselected samples, the following lithofacies typescould be distinguished.

Lithofacies I: Reef limestone; non-dolomitizedor slightly dolomitized

Non-dolomitized reef limestone is characterizedby framestone or rudstone-bindstone structure.The framestone is made up mostly of calcareoussponges and colonial corals, which are micro-bially encrusted and bounded (Fig. 6A);Tubiphytes nodules and Rivularia-type calci-microbes are common. The rudstone-bindstonetype rocks consist of detritus of the reefbuilders listed above, remnants of variousproblematic organisms (Tubiphytes, Baccanella,Poriteritubus and Panormidella), lithoclastsbounded by laminated, undulate micritic crust;and fragments of micritic crusts. The clasticcomponents are surrounded by clotted micrite(Fig. 6B). The pores within lithoclasts (Fig. 6C)and some of the vug pores (Fig. 6D) are linedwith a thin (20 to 100 lm) acicular isopachouscement layer. Later generation of vug pores,filled by fibrous calcite (Fig. 6A and C) cutthrough these structures. The remnant porespace is commonly filled with 10 to 250 lmsized transparent mosaic calcite that oftenexhibits twinning (Fig. 6A and D). A smallamount of microcrystalline or very finely crys-talline dolomite aggregates and fine floatingdolomite rhombs, i.e. porphyrotopic (planar-p)

Fig. 3. Triassic stratigraphic chartof the western part of theTransdanubian Range (modifiedafter Haas & Budai, 1995) showingthe stratigraphic position ofcore-sections referred in the text(S, S�andorhegy Formation; SD,S�edv€olgy Member of EdericsFormation; KL, Kardosr�etLimestone).



dolomite were encountered in laminated andundulated micritic crusts (Fig. 6E).

Lithofacies II: Selectively dolomitized reeflimestone

The Lithofacies II reef limestone (its sedimento-logical features are similar to that describedabove) is characterized by partial dolomitization.Dolomitization here was clearly fabric-selective.Clusters of finely crystalline planar-s dolomite(20 to 90 lm) and porphyrotopic (planar-p)dolomite crystals (100 to 200 lm) with cloudy

cores occur preferentially in a microbial fabric(filamental or peloidal micritic material; Fig. 7A)while the primarily calcitic skeletal grains, cal-cite-filled biomoulds and the isopachous fibrouspore-filling calcite cement do not contain thiskind of dolomite (Fig. 7B).Finely crystalline to medium-crystalline

planar-s dolomite occurs in fractures and alongstylolites (Fig. 7C). The wider fractures and theremnant space of larger pores are commonlyoccluded by medium-crystalline to coarselycrystalline saddle dolomite and limpid blockycalcite cement of similar crystal size. (Fig. 7D).

Fig 4. Lithological column,lithofacies types and mineralogicalcomposition diagram of the coreBet-1. Samples collected in therecent project are marked bycrosses. Lithofacies (LF) I: Reeflimestone, LF II: Selectivelydolomitized reef limestone, LF III:Strongly dolomitized limestonewith remnants of precursor fabricelements, LF IV: Dolomite withmimic fabric elements, LF V:Dolomite of fabric-destructivetexture.


6 J. Haas et al.

Saddle dolomite is often calcitized both inpatches and along growth zones (Fig. 7D).

Lithofacies III: Heavily dolomitized limestonewith remnants of precursor fabric elements

The dolomite replacement of the reef limestone ishighly advanced in rocks classified to LithofaciesIII. Totally dolomitized and partially dolomitizedpatches are visible in the samples. There aremillimetre-sized moulds of corals, calcareoussponges and other bioclasts filled by xenotopiccalcite cement (Fig. 8A). A few brachiopod skele-tons and isopachous fibrous pore-filling calcitecements were unaffected by dolomite replace-ment. In the partially dolomitized patches, 20 to100 lm sized planar-s to non-planar-a dolomitecrystals are typical. The individual crystals mayform clusters or may amalgamate, forming dolo-microsparite–finely crystalline dolomite patches.In 0�1 to 1 cm wide fractures and in the remnantpore space of the voids saddle dolomite andblocky calcite occur.

Lithofacies IV: Dolomite with mimic fabricelements

In dolomite with mimic fabric elements theoutlines of the precursor limestone fabric arestill visible, at least locally (Fig. 8B). In one ofthe representatives of this lithofacies type, peloi-dal dolomicrosparite and finely crystalline dolo-mite (planar-s to non-planar-a) were observed(Fig. 8B). The peloids are probably preservedelements of the precursor limestone. The struc-

ture of fibrous pore-filling cement is preserved(Fig. 8C). In other samples, ghosts of bioclasts(Fig. 8D) could be recognized. Sharp contact ofthe mimic and non-mimic dolomite fabrics wasobserved in one of the samples. In some of theremnant voids of larger pores, saddle dolomite,dedolomite and coarsely crystalline calciteoccur.

Lithofacies V: Dolomite of fabric-destructivetexture

Dolomites with fabric-destructive texture arecharacterized by irregular patches of dolomicro-spar and finely crystalline dolomite (Fig. 8E).The larger pores are filled (or partly filled) bysaddle dolomite, dedolomite and coarselycrystalline calcite. In another group of samples,patches of medium to coarsely crystalline pla-nar-s dolomite occur in dolomicrosparite – finelycrystalline planar-s dolomite matrix.Dolomite that is characterized by unimodal

medium-sized planar-s crystals with cloudycores and clear rims (cement overgrowths) isclassed as a third group. It may contain patchesof dolomicrosparite. Some of the vugs are filledby limpid blocky calcite. Millimetre-sized emptypores typically occur.

Types of matrix and cement dolomites

Porphyrotopic dolomite (pd)Porphyrotopic dolomite is present as 30 to200 lm sized crystals with cloudy nuclei and aclear rim (Fig. 9A). The dark, cloudy core of the

A B

Fig. 5. (A) Typical macroscopic image of LF III (strongly dolomitized limestone) – irregular patches of the originalreef limestone (yellow), vugs filled by white fibrous and transparent calcite (white, marked by an arrow) and thedolomitized limestone (grey) (82�5 m). (B) Typical macroscopic image of LF V (fabric-destructive dolomite) – yel-low and grey irregular patches are all dolomite, fractures are filled by white saddle dolomite (marked by arrows)(60�3 m).



individual crystals is rich in organic matter(Fig. 9B). However, in some cases only 20 to100 lm sized irregular patches of organic-richmicrocrystalline dolomite, similar to those ofnuclei of the floating rhombs occur (Fig. 9C). Thecalcimicrobial structure usually remained well-preserved all around these tiny dolomite patches

(Fig. 9D). Larger patches of small planar-e or pla-nar-s crystals are also locally present (Fig. 9D).The scattered dolomite crystals occur predomi-nantly in calcimicrobial structures, micriticcrusts or clotted micritic fabric elements, and aremissing in the mould-fill, interparticle andgrowth-framework calcite cement (Fig. 7B).

A B

C

E

D

Fig. 6. Characteristic texture of the reef limestonelithofacies (LF I): (A) Framestone with bioclast (coral),microbial crust and vug pore filled by fibrous calcite(fc) and mosaic calcite (mc) (129�7 m). (B) Clottedcalcite cement (cc) surrounding microbial crust(125�3 m). (C) Thin, acicular calcite cement rim(marked by an arrow) around bioclasts and peloids inthe reef limestone, cross-cut by vugs, filled withfibrous calcite (fc) (126�5 m). (D) Vug pore in reeflimestone with acicular calcite cement-fill (marked byan arrow) and transparent twinned mosaic calcite(mc) (129�7 m). (E) Scattered porphyrotopic (planar-p)dolomite crystals (marked by arrows) in filamentalcalcimicrobe structure (126 m).


8 J. Haas et al.

Dolomicrospar (dmi) and finely crystallinedolomite (fds)Dolomicrospar (5 to 20 lm) and finely crystal-line (20 to 60 lm) non-planar-a and planar-sdolomite. The distribution of dolomicrospar andfinely crystalline dolomite shows an irregularpatchy pattern (Fig. 8B). In some patches, theintercrystalline pores are filled with organicmatter, as revealed by fluorescence microscopy.In some cases, only mimic fabric elementscould be recognized in the totally dolomitizedrock, or the sedimentary fabric was destroyedcompletely.

Medium-crystalline fabric-destructive dolomite(mds)Medium-sized planar-e and planar-s crystalscommonly exhibit cloudy nuclei and overgrowthrims around them (Fig. 8E). This type is usuallypresent as patches in finer crystalline dolomite.

Fibrous dolomite (fd)In some totally dolomitized samples, fibrousdolomite with undulose extinction fills thepores (Fig. 8D). Dolomite inclusions, 40 to60 lm in size, occur in the crystals, which areeither equally dispersed or present along growthzones. A 200 lm wide zone at the terminationsof the fibrous cement is very rich in organicmatter.

Saddle dolomite cement (sd)Coarsely crystalline (0�3 to 3 mm) saddledolomite cement was observed in millimetre tocentimetre-sized fractures, in the centre of thefibrous cement-filled growth framework cavitiesor in interparticle and vug pores (Figs 7D and10A). The surface of the preceding fibrouscement is commonly rather irregular below thesaddle dolomite (Fig. 7D). The crystals showtypical features of saddle dolomites, such as

A

C D

B

Fig. 7. Characteristic texture of the non-dolomitized or slightly dolomitized (LF I) and selectively dolomitized(LF II) reef limestone: (A) Euhedral dolomite rhombs and clusters of rhombs (marked by the white arrow) in themicrobial crust (LF II). The black arrow marks a vug, filled by medium-crystalline dolomite (stained thin section,103 m). (B) Neomorphosed calcareous sponge fragment (sp). The vugs are filled with fibrous calcite cement (fc)and saddle dolomite (sd) in the micrite matrix amalgamation of the floating dolomite rhombs is also observable. Afracture filled by medium-crystalline dolomite is marked by the white arrow (LF II) (stained thin section, 64�5 m).(C) Finely to medium-crystalline dolomite along stylolite (marked by an orange arrow) in peloidal limestone (LFII). The peloids are surrounded by acicular calcite. (115 m). (D) Vug in the reef limestone (LF I). It is filled withfibrous calcite (fc), saddle dolomite (sd), dark internal sediment (marked by the white arrow) and transparent cal-cite. The black arrow marks calcitized patches of saddle dolomite (stained thin section, 129�7 m).



curved crystal faces and sweeping extinction(Fig. 10B). In some cases, only patches of coarsedolomite cement are visible within medium-crystalline dolomite; the largest crystals usuallyoccur in the central part of the patches andclearly show undulose extinction and curvedcrystal faces. Under CL excitation, the crystals

are non-luminescent with a mottled non-lumi-nescent to dull-red core (Fig. 10C). Minoramounts of Fe were detected in some saddledolomite crystals by EDS.The vug-filling saddle dolomite crystals are

commonly covered by dark brown to black fine-grained (micritic) internal sediment that is

A B

C

E

D

Fig. 8. (A) Calcite-filled biomould (after fragment ofcoral; marked by an arrow) in strongly dolomitized lime-stone (LF III). Vug filled by radiaxial fibrous calcite (rfc).A predominant part of the matrix was replaced by finelycrystalline dolomite and dolomicrosparite (stained thinsection, 82�5 m). (B) Finely crystalline dolomite (fds)and dolomicrospar (dms) in dolomite, classified to LFIV. Rounded clasts and changes in crystal size in theupper part of the picture are probably remnants of thesedimentary fabric (69 m). (C) Fibrous dolomite exhibitsundulose extinction. Note the 40 to 60 lm sized dolo-mite inclusions along zones, marked by arrows (crossednicols) (69 m). (D) Ghost of bioclasts in completelydolomitized sample from LF IV. The structure of the bio-clast is depicted by changes in the crystal size (34�4 m).(E) Finely to coarsely crystalline planar-s dolomite infabric-destructive dolomite (LF V) (98�4 m).


10 J. Haas et al.

relatively rich in organic matter (Fig. 10D). Itusually does not occlude the entire remainingpore, but rather forms a few hundred micron-thick layer at the bottom of the vugs.

Dedolomite (dd) and coarsely crystallinecalcite cement (ccs)In some samples, the replacement of saddledolomite by calcite along growth zones and inrandom patches is clearly visible (Fig. 7D). Coar-sely crystalline (0�01 to 1 mm) limpid calcitecement occludes the remnant voids of the pores(Figs 7D and 10D); this is clearly the last pore-fill generation. Similar calcite fills some of thefractures. Dedolomite and coarsely crystallinecalcite are weakly stained to pink (Fig. 7D). Thecore of the calcite crystals is non-luminescent,but younger zones commonly exhibit brightorange to red luminescence (Fig. 10C). Neithermanganese nor iron content was measured byEDS, however the crystals show slight enrich-ment in magnesium.

DOLOMITE PARAGENESIS

In the slightly dolomitized reef limestone litho-facies, dolomite occurs exclusively as scatteredtiny patches of organic-rich microcrystallinedolomite or euhedral porphyrotopic dolomite inmicritic fabric elements. These dolomite typesare present predominantly in calcimicrobes, ormicritic crusts, and clotted micrite (Figs 6E, 7A,7B and 9D) exhibiting characteristics of micro-bial carbonates (Riding, 2000; Fl€ugel, 2004) andrarely in peloids or micritic intraclasts. Sincethe calcimicrobial structure usually remainedwell-preserved around the tiny dolomite aggre-gates (Fig. 9D), their origin cannot be explainedby subsequent dissolution of the outer euhedralrim. Therefore, the microcrystalline aggregatesprobably represent the earliest dolomite genera-tion and the euhedral crystals probably formedvia overgrowths around the previously formedtiny dolomite aggregates, which acted as thenuclei for the small dolomite rhombs.

A B

C D

Fig. 9. (A) Euhedral porhyrotopic dolomite rhombohedra characterized by cloudy core (CC) and clear cement rim(CR) (103 m). (B) Picture of (C) under fluorescence microscope – note the organic matter rich core of the euhedraldolomite crystals. (C) Irregular patches of microcrystalline dolomite (porphyrotopic non-planar-a) in the structureof a calcimicrobe (stained thin section, 126�5 m). (D) Clusters of euhedral–subhedral porphyrotopic dolomite crys-tals (marked by arrows) in the structure of calcimicrobe (Rivularia).



In the selectively dolomitized and stronglydolomitized lithofacies calcite-filled moulds,calcitic bioclasts and vugs with isopachous car-bonate cement are common. Some of the vugpores cut through partially dolomitized calcimi-crobes and microbial crusts (Figs 6E and 7B). Noplanar-p dolomite was found, either in isopac-hous cement or in calcite bioclasts or calcite-filledbiomoulds. Accordingly the mould-creating andvug-creating dissolution, the precipitation of themould-filling and the isopachous carbonatecement post-date the earliest dolomite generation.In some cases, the neomorphosed bioclasts or

calcite-filled biomoulds are totally or partiallyreplaced by finely crystalline to medium-crystal-line planar-s and/or non-planar-a dolomite.Therefore, this dolomite replacement post-datesthe mould-filling and vug-filling calcite cementand can be linked to the next dolomitizationstage. Completely dolomitized segments withoutlines of original sedimentary fabric elements(Fig. 8B and D) and replaced fibrous cements(Fig. 8C), as well as totally fabric-destructive

dolomite (Fig. 8E), may have formed eitherduring this stage or subsequently but prior to theprecipitation of medium-crystalline dolomite andsaddle dolomite cement representing the lastdolomite generation. These phases are usuallypresent in fractures or in the internal, remainingpart of growth-framework pores and in intergran-ular or vug pores in every lithofacies type, fromreef limestone to totally dolomitized rock types(Figs 7D and 10D). Limpid coarsely crystallinecalcite is the last cement stage in the pores(Figs 7D and 10D) and also in the fractures thatcut through all of the above-named structures.

FLUID INCLUSION STUDIES

Fluid inclusion studies were carried out on pri-mary fluid inclusions of the saddle dolomite (sd)and coarsely crystalline calcite cement (ccs). Pri-mary fluid inclusions in saddle dolomite crystalsare 1 to 10 lm in size and their shape is irregular(Fig. 11A). Primary inclusions are commonly

A B

C D

Fig. 10. (A) Saddle dolomite crystals (11�5 m). (B) Undulose extinction pattern of saddle dolomite crystals (previ-ous picture, crossed nicols, 11�5 m). (C) CL picture of saddle dolomite (sd) and coarsely crystalline calcite (ccs)filling vug in reef limestone (95 m). (D) Vug pore filled by saddle dolomite (sd), dark internal sediment and coar-sely crystalline calcite (ccs) (95 m).


12 J. Haas et al.

present along growth zones (Fig. 11B); they aretwo-phase (liquid–vapour) inclusions with con-stant phase ratios (L:V = 90:10 to 95:5; Fig. 11A).Several secondary inclusions were also observed,along cleavage planes (Fig. 11B). Both the pri-mary and the secondary fluid inclusions in thesaddle dolomite crystals are commonly filled bydark brown to black material that can also befound in the intercrystalline pore space andalong stylolites (Fig. 11C). This material is prob-ably hydrocarbon (most probably bitumen), asindicated by its intense fluorescence (Fig. 11D).Microthermometry was carried out on the pri-

mary two-phase aqueous inclusions. The inclu-sions homogenized into liquid phase. Measuredhomogenization temperatures fall between 60°Cand 98°C (Table 1 and Fig. 12). The vapourphase of the inclusions usually did not reappearduring cooling to room temperature or below;therefore, it was not possible to measure thefinal melting temperature.Coarsely crystalline calcite cement contains

single-phase (all-liquid), 1 to 5 lm sized primaryfluid inclusions (Fig. 11E) which are sparselypresent in the core of the crystals (Fig. 11F).Elongated and irregularly shaped inclusions werealso observed. All-liquid inclusions imply thatthe precipitation of the mineral occurred below50 °C (Goldstein & Reynolds, 1994). Artificialstretching was applied to the inclusions (sensuGoldstein & Reynolds, 1994) in order to generatea bubble by increasing the inclusion cavity. Thevapour phase is required to observe phasechanges during freezing experiments. In thisway, it was possible to detect the final meltingtemperature of the ice in the primary, originallyall-liquid inclusions. Final melting temperaturesare between �0�7°C and 0°C. Assuming a NaCl–H2O system (based on the freezing point depres-sions), the salinity of the inclusions variesbetween 0 and 1�22 NaCl eq. wt%.

STABLE ISOTOPES

Stable isotope (O and C) analyses were per-formed on limestone, partially dolomitizedlimestone and dolomite samples. Separate mea-surement of the petrographically distinguishedfabric elements was attempted; sample pointswere selected from thin sections and the sam-ples were taken via microdrilling from the corre-sponding slabs. Selective measurement ofporphyrotopic dolomite was also attempted onone sample. The results of the analyses are pre-

sented in Fig. 13 and listed in Table 2. InFig. 13, the C and O isotope range of theCarnian sea water is also displayed, using dataof Korte et al. (2005).The d13C and d18O values of the samples taken

from various fabric elements (micrite, microbialcrust and calcite cements) of the non-dolomi-tized or slightly dolomitized reef limestonelithofacies (d13C 3�0 to 3�2&; d18O �3�8 to�2�7&) are within the range expected for calciteprecipitated in equilibrium with Carnian seawater. For separated porphyrotopic dolomited13C 3�7&; d18O �3�5& values were determined.Both d13C and d18O values of the finely crystal-line dolomite measured in strongly dolomitizedrock types are similar to those of the reeflimestone but show a slightly wider range (d13C2�7 to 4�1&; d18O �4�3 to �2�0&). Values of thefibrous dolomite cement (d13C 3�3 to 4�0&; d18O�3�4 to �2�9&) are within the field of finelycrystalline dolomite. The three samples takenfrom fibrous calcite cement yielded a distinctgroup of values with a narrow d13C range (1�6 to2�0&) but a rather wide d18O range (�5�7 to�4�1&). The values representing medium-crys-talline dolomite and coarsely crystalline saddledolomite can be classified into another distinctgroup that is typified by a narrow d13C range(2�4 to 3�1&) but a wide d18O range (�9�1 to�5�3&). Values of coarsely crystalline calcitecement (coexisting with dedolomite calcite)differ significantly from previously presentedresults; they are significantly depleted both ind13C and d18O (d13C �8�1 to �6�4&; d18O �8�6 to�7�0&).

DOLOMITE GENESIS AND EVOLUTION

The reconstruction of the complex diageneticevolution and multistage dolomitization of theCarnian reef carbonates in the Keszthely Moun-tains is based on the evaluation of the resultsof the presented investigations, also taking intoaccount the available regional geological andstratigraphic data. A paragenetic sequence forthe Ederics Limestone is presented in Fig. 14,while a schematic model of the dolomitizationof the reef limestone is in Fig. 15.

Reef diagenesis with syngeneticdolomitization

The history begins with the development of bio-genic build-ups (reefs) at the margin of an iso-



lated platform during Carnian time (Fig. 15A). Inreefs, depositional and diagenetic processes oper-ate together (Tucker, 1990). The reef developmentincludes various preburial diagenetic processesthat are classified as reef diagenesis (Halley,1984; MacIntyre, 1984; Schroeder & Purser,1986). Biological processes (encrustation, bioero-

sion and soft-tissue destruction), biologicallyinduced and abiotic cementation, as well asmechanical destruction, are the most importantagents of reef diagenesis. Biogenic disintegrationof the reef framework is ubiquitous; extensiveboring and biologically induced dissolution ledto the formation of pores and cavities at micron to

A

E

B

C D

Fig. 11. (A) Primary L-V fluid inclusion in saddledolomite (68�2 m). (B) Primary fluid inclusions alonggrowth zone (marked by yellow dashed line) and sec-ondary fluid inclusions along cleavage planes (markedby arrows) in saddle dolomite (68�2 m). (C) Darkpatches in the core and along growth zones of saddledolomite crystals (87�5 m). (D) Picture of C under flu-orescence microscope – note intense fluorescence ofpreviously dark zones (marked by arrow) (87�5 m). (E)Elongated and spike-shaped all-liquid fluid inclusionsalong growth zone (marked by red dashed line) incoarsely crystalline calcite (103 m).


14 J. Haas et al.

centimetre-scale. Internal sedimentation of fine-grained detritus and precipitation of aragoniteand/or high Mg-calcite (HMC) and microbial

peloidal cement (Marshall, 1983, 1986; MacIn-tyre, 1985; Chafetz, 1986) in the cavities are alsotypical processes of reef diagenesis (Tucker,1990). Alteration of original aragonite and HMCduring reef development can also be considered areef-diagenetic process (Schroeder & Purser,1986).In the case of the Carnian reef carbonates,

microbial encrusters play a crucial role assecondary frame-builders, binders and precipita-tors. Internal sediments, microbial peloidalcement (Fig. 6B), abiotic acicular and fibrous(originally aragonite and HMC) cements(Fig. 6A, C and D) commonly occur in thegrowth framework or dissolution-derived milli-metre-sized to centimetre-sized cavities. A Mg/Ca ratio >2 in the Triassic sea water implies thataragonite or HMC was the equilibrium abioticprecipitate (‘aragonite sea’; Stanley & Hardie,1998; Stanley, 2008).In the slightly dolomitized reef limestone sam-

ples, a definite relation between the distributionof the porphyrotopic dolomite and the microbialfabric elements was found (Figs 7A, 7B, 9A, 9C,

Table 1. Microthermometric data measured on pri-mary fluid inclusions occurring in saddle dolomite(sd) and coarsely crystalline calcite (ccs).

Mineralphase Th (°C) Tm (°C)

Salinity(NaCl eq. wt%)

sd 80 – –sd? 98 – –sd? 92 – –sd? 90 – –sd 60 – –sd 62 – –sd 66 – –sd? 67 – –sd? 63 – –sd 60 – –sd 66 – –sd 66 – –sd 85 – –sd 81 – –sd 60 – –sd 91 – –ccs – �0�1 0�17ccs – �0�7 1�22ccs – �0�1 0�17ccs – 0�0 0�00ccs – 0�0 0�00ccs – 0�0 0�00ccs – �0�1 0�17ccs – �0�2 0�35

Th refers to homogenization temperature; Tm to finalmelting temperature of ice.

Fig. 12. Histogram of homogenization temperaturesmeasured in primary two-phase aqueous inclusions ofsaddle dolomite.

Fig. 13. Relation between d18O (V-PDB) and d13C(V-PDB) for reef limestone, calcite and dolomitecements.



9D and 15B). This unambiguous relation might beexplained by HMC mineralogy of the microbialmicrite (Wright, 1990). The small HMC crystalsprovide a higher surface area for dolomite nucle-ation. In dolomitized Holocene peritidal deposits,matrix-replacive and selective dolomitization ofHMC micrite were observed (Mazzullo et al.,1987). Studies in various modern natural envi-ronments (mostly in peritidal settings) and labo-ratory experiments demonstrate the possibility ofdolomite nucleation in microbe-secreted extracel-lular polymeric substance (EPS) and the dolomiteprecipitation may have continued during theearliest burial stage due to the mineral-templateproperties of EPS (Vasconcelos et al., 1995;Sanchez-Roman et al., 2008; Bontognali et al.,2008). Accordingly, the possibility of microbialnucleation may also be taken into consideration forinterpretation of the genesis of the earliest dolomitephase. No direct evidence for microbially mediatednucleationwas encountered. However, the presenceof organic matter in the tiny microcrystalline aggre-gates and core of the planar-p crystals (Fig. 9B)seems to support this interpretation.Some of the vug pores cut through partially

dolomitized calcimicrobes and microbial crusts(Fig. 7B); this implies remarkable dissolutionsubsequent to the first stage of dolomite forma-tion. These vugs are usually lined by isopachousacicular (originally aragonite) and/or fibrous(originally HMC) cement precipitated undermarine phreatic conditions (Figs 6A, 6C, 6D and7B) (e.g. James & Ginsburg, 1979; Land & Moore,1980; Marshall, 1986; Aissaoui, 1988; Tucker,1990; Kimbell & Humphrey, 1994). Transforma-tion of aragonite and HMC cements to calciteprobably took place in the early diageneticstages (Tucker, 1990). Stabilization of aragoniteor HMC bioclasts (fragments of corals andcalcareous sponges) via dissolution and calciteprecipitation or volume per volume replacementprobably took place under meteoric conditionsduring the low sea-level intervals (James & Cho-quette, 1983; Land, 1986; Melim et al., 2001),although there is evidence for aragonite dissolu-tion at shallow sea floors (Palmer et al., 1988) orin deeper water settings (Melim et al., 2001).

Shallow-burial sea water dolomitization

Examples of partial to total dolomitization ofshallow-marine Cenozoic carbonates with a well-constrained history of tens to hundreds of metresof burial are reported from the Bahamas and fromthe Cayman Islands. Two cores cut at the western

Table 2. d18O (V-PDB) and d13C (V-PDB) values ofreef limestone; calcite and dolomite cements.

Sample descriptionDepth(m)

d13C(V-PDB)

d18O(V-PDB)

Reef limestone 126�5 3�2 �2�7Reef limestone 126�5 3�2 �3�2Reef limestone 48�8 3�0 �3�8Xenotopic calcite 64�5 1�5 �6�5Fibrous calcitecement

82�5 1�6 �5�7

Fibrous calcitecement

82�5 2�0 �4�7

Fibrous calcitecement

48�8 2�0 �4�1

Transparent mosaiccalcite

126�5 3�4 �5�8

Porphyrotopic dolomite 103�0 3�7 �3�5Finely crystallinedolomite

95�0 4�1 �2�0

Finely crystallinedolomite

87�5 3�6 �2�9


87�5 3�7 �2�8


82�5 3�1 �2�8


82�5 2�7 �3�6


60�3 3�5 �4�0


60�3 3�7 �4�3


34�4 3�5 �3�2


27�0 3�2 �3�5


21�0 3�2 �2�8


69�0 3�3 �3�9

Medium crystallinedolomite

77�1 2�9 �7�5


68�2 2�6 �7�1


36�0 2�9 �7�0

Fibrous dolomite 69�0 3�8 �3�1Fibrous dolomite 57�6 4�0 �3�4Fibrous dolomite 48�8 3�5 �3�0Fibrous dolomite 27�0 3�3 �2�9Saddle dolomite 87�5 3�1 �5�5Saddle dolomite 87�5 2�7 �6�4Saddle dolomite 68�2 2�4 �5�3Saddle dolomite 68�2 2�6 �7�0Saddle dolomite 57�6 2�7 �9�1Saddle dolomite 36�0 2�5 �8�6Coarsely crystallinecalcite

95�0 �6�4 �8�5

Coarsely crystallinecalcite

69�0 �8�1 �8�6

Coarsely crystalline calcite 68�2 �7�2 �7�0


16 J. Haas et al.

margin of the Great Bahama Bank penetratedQuaternary to Late Neogene platform and peri-platform carbonate sediments. Dolomite occurredin both cores except in the uppermost 100 to150 m. In platform facies fabric-preserving,microsucrosic and micritic dolomite texture typeswere distinguished. Reef facies encounteredbetween 300 m and 360 m below the present-daytop of the platform was affected by pervasivemarine-burial dolomitization (Melim et al., 2001).In the cores cut on the Cayman Islands, a ca

150 m thick Cenozoic dolomitized carbonate suc-cession was penetrated (Budd, 1997; Jones &Luth, 2003). The Oligocene is represented bypartly dolomitized shallow-marine limestoneand fabric-destructive sucrosic dolomite. TheMiocene interval is made up of fabric-retentivedolomite with relatively well-preserved shallow-marine bioclasts (corals, bivalves, green algae, redalgae and foraminifera). Shallow-marine lime-stone and fabric-retentive dolomite occur in thePliocene section. Multiple dolomitization of Mio-cene carbonates is reflected by an increased popu-lation of dolomite crystals and zoned crystalshaving cores and cortices that formed during dif-ferent episodes of dolomitization (Jones & Luth,2003; Jones, 2005).

The studies cited above demonstrate theimportance of shallow-burial dolomitizationwith normal sea water as a dolomitizing agentin isolated platform settings. Based upon theinterpretation from this study, more or less simi-lar processes may have taken place during theearly shallow-burial stage of the investigatedCarnian reef carbonates. This process may haveresulted first in the overgrowths of the previ-ously formed nucleus; the typical tiny rhombiccrystals may have formed in this way (Figs 9Aand 15C). Zoned crystals implying multiple-cement precipitation also occur. The enlarge-ment of the individual crystals may have led toamalgamation of the crystals as clusters (Figs 7Aand 9D). Stable isotope data of the slightlydolomitized limestone and the porphyrotopicdolomite are within the range of the Carnian seawater (Fig. 13 and Table 2), suggesting sea wateras the principle Mg supply.According to Machel (2004), early dolomites

might act as nuclei for later, more pervasive dolo-mitization during burial. This concept can alsobe applied in this case. Increasing burial duringthe Late Triassic (Fig. 16) may have resulted inthe extension of the dolomitized patches and theformation of finely crystalline replacement dolo-

Fig. 14. Paragenetic sequence of the Ederics Limestone/S�edv€olgy Dolomite. Dashed line marks the uncertainrange of the process.



mite (Fig. 15D). Oxygen isotope values of thisdolomite type (�3�8 to 2�7&) are ca 2& heavierthan those values measured on fibrous calcitecement (�5�7 to �4�1&) which are still withinthe range determined for the Carnian sea water ofthe Tethyan realm on the basis of well-preservedbrachiopods shells (Korte et al., 2005; Fig. 13and Table 2). This difference may reflect isotopefractionation between calcite and dolomite pre-cipitated from the same sea water (Land, 1986).This means that sea water may have been theprincipal agent of dolomitization and no

significant temperature elevation can be impliedfor this stage of diagenetic evolution. Circulationwas probably driven by thermal convection(Whitaker et al., 1994, 2004; Sanford et al.,1998).

Medium to deep-burial dolomite formation

The earliest notable fracturing of the Carniancarbonates was probably related to the exten-sional tectonic regime, developed due to inci-pient rifting of the Alpine Tethys, and led to

A B C

D E F

Fig. 15. Schematic model for the dolomitization of the Ederics Limestone: (A) Reef diagenesis (encrustation,micritisation, aragonite and HMC precipitation around the peloids and bioclasts). (B) Precipitation of the inclu-sion-rich cores of the porphyrotopic dolomite crystals in the microbial crusts, dissolution, precipitation of HMCin the vugs. (C) Stabilization of HMC and aragonite to calcite, precipitation of clear cement overgrowth on the por-phyrotopic dolomite cores, precipitation of calcite in biomoulds. (D) Progressive dolomitization around porphyro-topic dolomite clusters (finely crystalline dolomite). (E) Progressive dolomitization around already dolomitizedpatches (medium-crystalline to coarsely crystalline dolomite), dolomitization of vug-filling fibrous calcite, fractur-ing, precipitation of saddle dolomite in the fractures, hydrocarbon migration. (F) Fracturing, dedolomitization ofsaddle dolomite, precipitation of coarsely crystalline calcite in the fractures.


18 J. Haas et al.

development of the Kossen Basin during theLate Norian (Haas, 2002). As a result of theextension and continuous thermal subsidence ofthe Tethys margin, the Carnian reef carbonatesreached 1 to 1�5 km burial depth (see Figs 3 and16), i.e. the intermediate burial zone (sensuMachel, 1999), by the Late Norian (Fig. 16).According to Machel (2004), dolomitization ismost favoured in the intermediate burial setting(0�5 to 2 km) where the ambient temperature is50 to 60°C. At greater depth, it is usually limitedby very low permeabilities.Fractures filled with medium-crystalline to

coarsely crystalline and saddle dolomites weremost probably formed during this evolutionarystage. Saddle dolomite filling the remainingspace in the vugs is also attributed to this event.The similarities in stable isotope data suggestthat the medium-crystalline dolomite (d13C: 2�58to 2�92&, d18O: �7�45 to �7�04&) precipitatedfrom the same parent fluid as the saddle dolo-mite (d13C: 2�43 to 3�11&, d18O: �9�13 to�5�33&); however, the medium-crystalline dolo-mite and the base of the saddle dolomites aremore likely to be replacement of the host rockalong fractures than primary precipitates(Fig. 15E) (cf. Lonnee & Machel, 2006). Thisinterpretation is supported by the mottled lumi-

nescence pattern of these dolomites (Fig. 10C).Depleted d18O values measured for these dolo-mite types (Fig. 13) and 60°C as the minimumvalue of the homogenization temperatures mea-sured on primary inclusions in saddle dolomite(Fig. 12) indicate crystallization in the interme-diate burial realm (Fig. 15E). This is in goodaccordance with the common observation thatthe minimum temperature for the formation ofsaddle dolomite is ca 60°C (Radke & Mathis,1980; Spotl & Pitman, 1998; Machel & Lonnee,2002; Machel, 2004). Based on these conside-rations, the saddle dolomite can be interpretedas geothermal (sensu Machel & Lonnee, 2002) inorigin, formed at or near the same temperatureas the host rock, most probably by advection(Machel & Lonnee, 2002). Nevertheless, itshould be mentioned that knowing the exactcomposition of the inclusion fluid would enablea pressure correction to be applied, which prob-ably would indicate higher temperatures for theentrapment of the inclusions (Goldstein & Rey-nolds, 1994). Stable carbon isotope values of thesaddle dolomites are very similar to that of themarine precipitates; suggesting that the parentfluid was sea water at elevated temperature.After the Triassic, an extensional regime was

entered and the differential subsidence conti-

Eocene marine formations

Oligocene continental formations

Tectonic eventsWater

Latest Triassic - Early Cretaceous formations

Hauptdolomit (T3)

Carnian reefal carbonates

Fig. 16. Burial history curve of the Ederics Limestone/S�edv€olgy Dolomite (thicknesses of the formations wereobtained from Haas, 2001).



nued during the Jurassic into the Early Creta-ceous interval (Voros & Galacz, 1998; Fig. 16),when the studied succession reached the deep-burial zone (2 to 3 km; sensu Machel, 1999).Organic maturity data available for the Triassicsuccession of the Keszthely Mountains mayserve as a basis for the estimation of the temper-ature conditions during the deepest burial stage.From the H�ev�ız Hv-6 core (for its stratigraphicsetting see Fig. 3), based on colour ranking ofspores and pollens Thermal Alteration Index(TAI) values were reported (G�oczan et al.,1983). From below, the Ederics Limestone inthe lower member of the Veszpr�em Marl TAIvalues of 3�00 to 3�05 (corresponding with ca1�2 Ro% of Barker & Pawlewicz, 1994) weredetermined, and of 2�66 to 3�02 (correspondingwith ca 0�6 to 1�2 Ro%) from the upper memberoccurring above the Ederics Limestone. Thesedata suggest a temperature range of 90 to 130°C(Barker & Pawlewicz, 1994) for the EdericsLimestone during maximum burial. Precipita-tion of saddle dolomites in fractures may havecontinued during this evolutionary stage. Thisinterpretation is supported by the stable isotopedata and the relatively wide homogenizationtemperature interval up to 98°C, measured inprimary inclusions of saddle dolomite. The dualclusters in the homogenization temperaturesmight correspond to two stages of precipitation/growth, i.e. in the intermediate and deep-burialrealm. Migration of hydrocarbons, entrappedalong growth zones in the saddle dolomite crys-tals (Fig. 11C and D) and in the intercrystallinepore space of finely crystalline planar-s to non-planar-a dolomites, probably corresponds withthis stage. At this time, potential source rocksof Rhaetian age may have reached the oilwindow and therefore were able to generatehydrocarbons.

Dedolomitization and calcite cementation

In connection with the Alpine orogeny, a crucialcompressional deformation event occurred inthe mid-Cretaceous that resulted in the forma-tion of the large synclinal structure of the Trans-danubian Range (Haas, 2001). This was followedby the establishment of an extensional tectonicregime, uplifting and intense erosion during theTuronian to Coniacian interval, which resultedin the denudation of the entire Jurassic–LowerCretaceous succession and even a large part ofthe Triassic sequence on the limbs of the syn-cline (Haas, 2001). Consequently, the Carnian

reef carbonates of the Keszthely Mountainsreached a near-surface position at this time(Fig. 16).Depleted d13C values of coarsely crystalline cal-

cite cement (Fig. 13 and Table 2) are consistentwith precipitation from meteoric water (Allen &Matthews, 1982). Salinity values of 0 to 1�22 NaCleq. wt% for the primary fluid inclusions in thiscement (Table 1), indicate that the parent fluidwas meteoric to slightly brackish water. Precipita-tion of the calcite from this water below 50°C issupported by the presence of all-liquid inclusionsin the crystals (Goldstein & Reynolds, 1994). Sim-ilar petrographic features of the above-mentionedcalcite and calcitized dolomite (i.e. dedolomite;Fig. 7D) suggest that they had a common parentfluid capable of dissolving dolomite and precipi-tating calcite. Dissolution of dolomite might haveresulted in slight enrichment of the parent fluidin Mg, which is reflected by the elevated Mg-con-tent of the calcite. In some places, the flux of themeteoric fluids was strong enough to brecciate thesaddle dolomites in the voids (Fig. 7D). The pinkcolour after staining, together with the non-lumi-nescent behaviour of dedolomite and the core ofcoarsely crystalline calcite (Fig. 10C), impliescrystallization under oxidizing conditions. Thisis further supported by the fact that neither man-ganese nor iron was found during EDS analysis inthe calcite.Since dedolomitization mainly affected saddle

dolomite crystals, and there was no evidence ofdedolomitization in the porphyrotopic or finelycrystalline to coarsely crystalline dolomite, theprocess is probably late diagenetic, i.e. telogene-tic (Fig. 15F). The present observations are ingood accordance with the common descriptionthat dedolomitization requires high Ca/Mg ratio,high water flux, relatively low pCO2 and tempera-tures less than 50°C (de Groot, 1967); therefore itis usually a near-surface phenomenon related tosubaerial unconformities (Evamy, 1967; Folkman,1969; Torok, 2000; Nader et al., 2003; Ronchiet al., 2004). In this case, whether the Late Creta-ceous to Cenozoic exposures or subrecent torecent exposure may have been responsible forthe dedolomitization is still an open question(Fig. 15F).The change in the luminescence pattern of the

calcite to bright orange suggests that the prima-rily oxidizing conditions changed to reducingduring precipitation and this may indicateincreasing burial. However, the incorporatedmanganese and/or iron are supposedly not suffi-cient to be measured by EDS.


20 J. Haas et al.

CONCLUSIONS

Based on petrographic investigations, geochemi-cal analyses and evaluation of the stratigraphicand regional geological data, the main stages ofthe diagenetic evolution and dolomitization ofthe Carnian reefal carbonates can be summa-rized as follows:

1 Reef diagenesis (encrustation, biologicallyinduced and abiotic cementation, mechanicaldestruction) and synsedimetary dolomite micro-aggregates were selectively formed in microbialfabric elements (microbial crusts, calcimicrobesand microbial cement).2 Early shallow-burial stage when porphyro-

topic dolomite crystals were formed viaovergrowths around early diagenetic dolomitemicro-aggregates.3 Increasing burial resulted in the extension of

the dolomite patches via the formation of finelycrystalline replacement dolomite. Pervasivedolomitization may have affected those parts ofthe reef limestone where more pervasive earlydolomite precipitation had taken place previ-ously, whereas those parts where no or negligi-ble amounts of dolomite had been formedretained their limestone fabric.4 The Carnian reef carbonates reached 1 to

1�5 km burial depth (intermediate burial setting)by the Late Norian. Depleted d18O values mea-sured on medium-crystalline to coarsely crystal-line dolomite indicate increasing dolomitizationtemperature and the appearance of saddle dolo-mite (usually as cement in larger pores) excludestemperatures lower than 60°C.5 The extensional regime established in con-

nection with the opening of the Kossen Basin inthe Late Norian caused the first fracturing. Pro-longation of the extensional regime may haveresulted in the formation of further fracture gen-erations during the Jurassic to Early Cretaceousperiod. These fractures were cemented by saddledolomite.6 In connection with the Alpine orogeny,

elevation and intense denudation, accompaniedby fracturing, took place in the Late Cretaceous.Similar tectonically controlled uplifting, denu-dation and fracturing occurred in several stagesduring the Cenozoic as well. Due to these pro-cesses, the studied Carnian carbonates wereemplaced at a near-surface position, or becamesubaerially exposed, leading to dedolomitizationof the last dolomite phase and precipitation ofcalcite in fractures and cavities.

This study revealed that investigation of par-tially and selectively dolomitized rock types hasthe potential to detect the early stages of multipledolomitization processes that, due to overprint ofthe subsequent diagenetic stages, can barely beobserved in completely dolomitized rock bodies.This approach can be applied to the study of thedolomitization history of any other platformcarbonate where these kinds of transitional rocktypes occur. This is important because recogni-tion of the dolomitization phases can provide thebasis for the analysis of their relations with depo-sitional, diagenetic and tectonic processes, andstages of basin evolution, with significant conse-quences for various aspects of applied geology,mostly in the fields of hydrocarbon geology andhydrogeology.

ACKNOWLEDGEMENTS

Kinga Hips, Zs�ofia Poros, Andrea Mindszentyand Magdolna Het�enyi are acknowledged for theconsultations; Csaba P�er�o for the core photo-graphs and Zsolt Bend}o for his assistance inSEM investigations. The authors are indebted toHenry Lieberman (Houston) for the linguisticcorrection of the paper. The editorial guidanceof Prof. Peter Swart, Dr Tracy Frank and Dr Ste-phen Lokier, furthermore the detailed commentsand suggestions of Prof. Jay M. Gregg and theanonymous reviewers improved the manuscriptconsiderably. This work was supported by theHungarian National Science Fund (OTKA) grantK 81 296.

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Manuscript received 5 November 2012; revision 11September 2013; revision accepted 29 October 2013


24 J. Haas et al.

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