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Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 456–468
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Palaeogeography, Palaeoclimatology, Palaeoecology
j ourna l homepage: www.e lsev ie r.com/ locate /pa laeo
Late Triassic to Early Jurassic palaeogeography and eustatic history in the NWTethyan realm: New insights from sedimentary and organic facies of the CsővárBasin (Hungary)
János Haas a, Annette E. Götz b, József Pálfy c,⁎a Geological, Geophysical and Space Science Research Group of the Hungarian Academy of Sciences, Eötvös Loránd University, 1117 Budapest, Pázmány sétány 1/c, Hungaryb Darmstadt University of Technology, Institute of Applied Geosciences, Schnittspahnstr. 9, D-64287 Darmstadt, Germanyc Hungarian Academy of Sciences, Hungarian Natural History Museum, Research Group for Palaeontology, POB 137, H-1431 Budapest, Hungary
⁎ Corresponding author.E-mail addresses: [email protected] (J. Haas), goe
(A.E. Götz), [email protected] (J. Pálfy).
0031-0182/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.palaeo.2010.03.014
a b s t r a c t
a r t i c l e i n f oArticle history:Received 31 July 2009Received in revised form 6 January 2010Accepted 8 March 2010Available online 15 March 2010
Keywords:NW TethysRhaetianHettangianSedimentary cyclesPalynofaciesCarbon isotopesCsővár BasinHungary
Sedimentary and organic facies of a continuous Late Triassic–Early Jurassic toe-of-slope to basin succession ofthe NE Transdanubian Range (N Hungary) was studied in order to reconstruct the palaeogeographical andeustatic evolution of the Csővár Basin, an intraplatform basin of the NW Neotethys margin. Characteristicfacies successions point to sea-level changes of different hierarchies. Cyclic patterns, inferred to result fromorbital eccentricity forcing, are also reflected in the stratigraphical distribution of sedimentary organicmatter. Furthermore, both palaeontological and isotope data document drastic climatic changes around theTriassic–Jurassic boundary. Detecting sea-level changes leads to a more accurate interpretation of the LateTriassic palaeogeographic setting and evolution of the Transdanubian Range's carbonate platform. Ourintegrated sedimentological and palynological data suggest a complex topography and dynamic sea-levelhistory, which contradicts a previous model of broad, uniform platform setting and lack of any majordrowning and emersion events during the Late Triassic.
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© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Subsequent to initial rifting during Middle Triassic times, oceanfloor spreading and development of large, continent-encroachingcarbonate platforms characterized the evolution of the NWNeotethysin the Late Triassic. The opening of the ocean led to the thinning of thecontinental crust in the marginal zone and gave rise to tectonicbackstepping of the shelf margin and development of narrowextensional basins on the shelf, near to the margin. One of them,the Csővár Basin in north-central Hungary, developed as an intraplat-form basin during the Carnian and persisted till the Early Jurassic.During this long time interval, cherty carbonates of slope, toe-of-slopeand hemipelagic basin facies were deposited in the neighborhood ofcoeval platform carbonates. In the north-easternmost part of theTransdanubian Range, east of the Danube, a small Mesozoic horstpreserved the record of this palaeogeographic setting. Its sedimentsand facies successions display sea-level changes of different hierar-chies and reflect the Triassic–Jurassic boundary events.
This paper aims at presenting an integrated analysis of changes inthe sedimentary and organic facies and at interpreting these changes
in terms of relative sea-level fluctuations and climatic changes withrespect to the specific local palaeogeographic setting. This detailedanalysis may also contribute to a better understanding of globalevents that took place at the Triassic–Jurassic boundary and that led toone of the most prominent mass extinctions in the Phanerozoic.
2. Geologic setting
In north-central Hungary, east of the river Danube, small outcropsof Mesozoic rocks occur in fault-bounded, uplifted blocks, represent-ing the north-easternmost part of the Transdanubian Range Unit. Thistectonostratigraphic unit forms part of the Alcapa terrane, andpreserves an early Mesozoic stratigraphic succession deposited inthe passive margin of the Neotethys and displaced during the Alpineorogeny (e.g. Kovács et al., 2000). The fault blocks are generally madeup of Upper Triassic Dachstein-type platform carbonates. However,the Csővár block also contains coeval slope and basin facies along withplatform facies (Fig. 1). Therefore, this part of the TransdanubianRange Unit is a particularly well-suited area to investigate a platform-to-basin transect with respect to the palaeogeographic and eustaticevolution of the NW Tethyan realm.
The north-western part of the Csővár block is characterized byshallow-water oncoidal facies of the Upper Carnian–Norian DachsteinLimestone (Fig. 1). South-eastwards, an about 1 km wide zone of
Fig. 1. Geology of the study area and location of the Csővár sections: Vár-hegy (Castle Hill), marked by a triangle, Pokol-völgy quarry (Pk. q.) and the borehole Csv-1 (from Haas andTardy-Filácz, 2004).
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patch-reef and proximal foreslope facies of the Dachstein Formation,most likely of Late Carnian age (Kovács, 2004), documents the lateraldevelopment of the platform-to-basin setting. Progradation offoreslope facies onto cherty limestones of the Csővár Formation wasalso recognized (Benkő and Fodor, 2002; Kovács, 2004).
In the south-eastern part of the block, thin-bedded, chertydolomites and limestones (Csővár Limestone Formation) areexposed in the abandoned Pokol-völgy quarry and in adjacentoutcrops. The Csv-1 borehole drilled in the yard of the quarrypenetrated the lower part of the Csővár Formation, representing theUpper Carnian–Lower Rhaetian interval (Haas et al., 1997). Theupper part of the Csővár Formation is exposed in the Pokol-völgyquarry (Haas et al., 1997; Haas and Tardy-Filácz, 2004) and inoutcrops and trenches on the south-western slope of the Vár-hegy(Castle Hill; Fig. 1). The lower part of the Pokol-völgy quarry yieldedVandaites stuerzenbaumi, whereas Choristoceras marshi is reportedfrom the uppermost part (Haas et al., 1997). These age-diagnosticammonite species indicate the Middle and the Late Rhaetian,respectively (Krystyn, 2008). Based on foraminifera, the Norian–Rhaetian boundary is located at about 20 m depth in the Csv-1 core(Oravecz-Scheffer in Haas et al., 1997). The conodonts Misikellahernsteini and Misikella posthernsteini from the basal part of the Vár-hegy section and Misikella ultima from stratigraphically higher beds(Fig. 2) are reported by Pálfy et al. (2007). Based on ammonites,conodonts, foraminifera, radiolarians and isotope chemostrati-graphic constraints, the Triassic–Jurassic boundary is drawn withinthe Csővár Formation (Pálfy and Dosztály, 2000; Pálfy et al., 2001,2007). Palynological data confirm this stratigraphic level for theTriassic–Jurassic boundary (Götz et al., 2009). Significantly, there isno striking lithological change between the Upper Triassic andHettangian parts of the formation (Fig. 2). Sinemurian chertylimestones of radiolarian basinal facies (Kozur, 1993) occur at the
top of the Vár-hegy section and represent the uppermost preservedpart of the Csővár Formation.
3. Palaeogeographic setting
According to current palaeogeographic reconstructions (Haas et al.,1995;Gawlick et al., 1999;Haas, 2002), during the late Palaeozoic to earlyMesozoic the Transdanubian Range Unit was located at the westerntermination of the Neotethys between the South Alpine and Drauzug–Upper Austroalpine units (Fig. 3). Its present-day position is a result ofmulti-phase dislocations during the Tertiary (Kázmér and Kovács, 1985;Balla, 1988; Csontos et al., 1992; Haas et al., 1995). For the entire LatePermian to Late Triassic interval, the present-day Transdanubian RangeUnit represented a segment of the Tethys margin, showing a strikingfacies polarity. Its south-western part was the landward side of the shelf,whereas its north-eastern part represented the outer shelf.
Opening of the Neotethys Ocean led to segmentation of thepreviously undifferentiated ramp system and formation of extension-al basins and carbonate platforms in the Middle Triassic (Budai andVörös, 1992, 1993; Haas and Budai, 1995). The basins formed in theinner shelf filled upwith fine siliciclastics during the Carnian, on top ofwhich large carbonate platforms started to develop in the latestCarnian. However, in the north-eastern part of the TransdanubianRange Unit (Gerecse and Buda mountains, Danube east-side blocks)new extensional basins developed in the Carnian and some of thempersisted for a long time. Upper Carnian to Rhaetian laminateddolomites and limestones of the BudaMts. were formed in a restrictedbasin (Haas et al., 2000) which was more restricted than the CsővárBasin and records a continuous sedimentation during the Late Triassicand earliest Jurassic. Isolated platforms may have separated the twobasins. Abundant fragments of land plants and a high amount of
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Fig. 3. Palaeogeography of the northwest Tethyan realm during the Late Triassic (Rhaetian) and location of the Transdanubian Range (arrowmarks the Csővár Basin). Abbreviations:TR— Transdanubian Range, Ex. Din.— External Dinarides, LO— Lombardy, DO— Dolomites, CP— Carnian Prealps, SL— Slovenian Trough, JL— Julian Alps, SA— Sava unit, BU— Bükkunit, JA— Jadar block, DR—Drauzug, BR— Briançonnais unit, HE—Helvetic unit, AA— Austroalpine units, BA— Bajuvaricum, TI— Tirolicum, HA—Hallstatt unit, TA— Tatricum, HR—
Hronicum (after Haas and Tardy-Filácz, 2004).
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sporomorphs in the Rhaetian part of the Csővár Formation suggest theexistence of islands on the platforms, implying a complex topography.
Rising relative sea level resulted in drowning of the carbonateplatforms in the NE part of the Transdanubian Range Unit. However,we have no data for this drowning event in the area of the Buda Mts.and Danube east-side blocks where the uppermost part of theDachstein-type carbonate platform sequence has been eroded. Thus,the integrated study of sedimentary and organic facies of the CsővárFormation is important in providing new insights into the earliestJurassic palaeogeographic and eustatic evolution of this region.
4. Sedimentary facies
Based on field observations and detailedmicrofacies investigationsdifferent lithofacies types were defined (Haas and Tardy-Filácz, 2004).Themost important characteristics of these facies types, together withthe interpreted depositional settings, are summarized below.
4.1. Lithoclastic–bioclastic grainstone/packstone (Lb) (Fig. 4a–b)
Fine-grained calcirudite to coarse-grained calcarenite. Fragmentsof echinoderms (mainly crinoids and echinoid spines) are predom-inant, whereas brachiopod and bivalve shell fragments, detritus ofmicroproblematica and microbial crusts, benthic foraminifera, andthick-shelled ostracods are common (Fig. 4a–b).
These beds are grain-flow deposits or basal traction layers of high-density turbidites. Some of the bioclasts (microproblematica, forami-nifera, and thick-shelled ostracods) are most likely of platform origin.The crinoids may have inhabited the foreslope, most likely the
Fig. 2. Lithologic column, key biostratigraphic data, carbon isotope stratigraphy and chronosposition of the Triassic–Jurassic boundary.
foreslope terraces. The lithoclasts are identified as reworked lithifiedbasinal sediments.
4.2. Oncoid grapestone grainstone/packstone/wackestone (On) (Fig. 4c)
Coarse-grained calcarenite that is made up predominantly ofcoated grains (oncoids and grain aggregates). 1–2 cm-sized lithoclastsof radiolarian wackestone (Fig. 4d) and mm-sized fragments ofcemented sediments are also common. Bioclasts are generally scarcebut abraded shell fragments of mollusks, brachiopods, and ostracodsare found sporadically. However, bivalve and brachiopod coquinasoccur in distinct horizons.
The oncoids and the grapestones may have formed on the top of acarbonate platform or a shallow ramp in moderately agitated environ-ments. However, the intercalation of the oncoid–grapestone facieswithin the deeper-water succession, and the common occurrence oflithoclasts of pelagite and coquina horizons indicate redeposition of thecoated grains from a shallow environment to the basin.
4.3. Medium-grained calciturbidite (Mt) (Fig. 4e)
Gradedpackstone, coarse- tofine-grained calcarenite. Skeletal debrisof crinoids is the predominant component. Fragments of mollusks,debris of microbial crusts and benthic foraminifera are common. Sand-sized lithoclasts may also occur in the basal part of the layers.
Top-absent, amalgamated Bouma sequences that were formed byhigh-density turbidity currents and were deposited relatively far fromthe source area on the distal part of the lower slope or on the toe-of-slope. The platformmargin and the foreslopemay have been the source
tratigraphy of the Csővár (Vár-hegy) section. Grey-shaded bar marks the most probable
Fig. 4. Photomicrographs illustrating microfacies types distinguished in the Csővár (Vár-hegy) section. a: Lithoclastic–bioclastic grainstone, Bed 29b; b: lithoclastic–bioclasticpackstone, Bed 11b; c: oncoid–grapestone grainstone, Bed 80a; d: radiolarian wackestone lithoclast in oncoid–grapestone grainstone, Bed 80b; e: basal part of a medium-grainedcalciturbidite layer that overlies radiolarian–sponge spicule wackestone pelagite, Bed 7; f: basal part of a fine-grained calciturbidite layer that overlies radiolarian wackestonepelagite, Bed 23b. Scale bar is 1 mm.
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of the redeposited bioclasts. In the foraminiferal assemblage, alongwithbasinal elements, forms of carbonate platform origin are recognized.
4.4. Fine-grained calciturbidite (Ft) (Fig. 4f)
Laminated beds that are made up of an alternation of graded, fine-grained calcarenite packstones and wackestones. The calcarenitelaminae are peloidal and contain fragments of echinoderms (mainlycrinoids), mollusks, and ostracods. In the wackestone laminae spongespiculae and radiolarians are common.
Incomplete, base-absent calciturbidites that were formed by low-density turbidity currents andwere deposited far from the source areaon the toe-of-slope or in the basin.
4.5. Calcisiltite–calcilutite laminite (La) (Fig. 5a–b)
Thin-bedded, platy or laminated fine-grained limestone. Finecalcarenite–calcisiltite laminae (peloidal biomicrite) alternate withcalcilutite (peloidal microsparite and micrite) laminae. This facies ispoor in recognizable bioclasts (radiolarians, sponge spiculae and thin-shelled ostracods) (Fig. 5a–b).
Fig. 5. Photomicrographs illustrating microfacies types distinguished in the Csővár (Vár-hegy) section. a: Calcisiltite–calcilutite laminite, Bed 20; b: calcisiltite–calcilutite laminite,Bed 27; c: sponge spicule wackestone, Bed 8; d: thin-shelled bivalve wackestone, Bed 16; e: radiolarian wackestone, Bed 3; f: radiolarian packstone, Bed 14a. Scale bar is 1 mm for aand b, 400 µm for c, d, e and f.
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These laminites are interpreted as very distal, low-densityturbidites that were deposited on the basin floor. The calcisiltitelaminae settled out from diluted low-density turbidity currents. Thepeloids are most likely of platform origin. The finest material of theturbidity currents settled out together with the background pelagitesforming the calcilutite laminae.
4.6. Sponge spicule wackestone (S) (Fig. 5c)
Thin-bedded limestone, locally with chert nodules. Spongespiculae are the predominant bioclasts. Radiolarians (calcite moulds),
fragments of echinoderms and thin-shelled bivalves are common incalcisiltite–calcilutite matrix. These sediments represent deep-waterbasin facies.
4.7. Thin-shelled bivalve wackestone (B) (Fig. 5d)
Thin-bedded limestone with a high amount of fragments of thin-shelled bivalves (“filaments”). In some layers, ostracods are quitecommon and occur along with radiolarians, echinoderm fragments,and foraminifera. These deposits represent deep-water basin facies.
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4.8. Radiolarian wackestone (Rw) (Fig. 5e)
Thin-bedded limestone, locally with chert nodules. Radiolariansare predominant; sponge spiculae, thin-shelled bivalves, echinodermfragments are common. These limestones represent deep-water basinfacies.
4.9. Radiolarian packstone (Rp) (Fig. 5f)
Siliceous, cherty limestone. The radiolarians are present in rock-forming quantity. Sponge spiculae and fragments of echinoderms alsooccur. This condensed radiolarian facies most likely formed in thedeep basin near to the carbonate compensation depth.
5. Distribution of sedimentary organic matter
An interval of 30 m spanning Beds 21 to 86 of the CsővárFormation (Fig. 6) exposed on the south slope of the Vár-hegy wassampled for palynofacies analysis of the Upper Rhaetian and LowerHettangian sediment series. All samples were prepared usingstandard palynological processing techniques, including HCl (33%)and HF (73%) treatment for dissolution of carbonates and silicates,and saturated ZnCl2 solution (D≈2.2 g/ml) for density separation.Residues were sieved at 15 µm mesh size. Slides have been mountedin Eukitt, a commercial, resin-based mounting medium. The relativepercentages of sedimentary organic constituents are based oncounting at least 400 particles per slide.
For palynofacies analysis, the sedimentary organic matter of thestudied Csővár Formation is grouped into a continental fractionincluding phytoclasts, pollen grains and spores, and a marine fractioncomposed of acritarchs, prasinophytes and foraminiferal test linings.Three palynofacies parameters were calculated to detect stratigraphicchanges in the composition of sedimentary organic matter reflectingeustatic signals (Fig. 6): (1) The proportion of marine plankton. Thisparameter quantifies the percentage rate of acritarchs and prasino-phytes in the sedimentary organic matter. It is linked to the marineconditions of the water column, depending on distance to coastline,water depth, temperature, salinity, and nutrient availability; (2) Theratio of opaque to translucent phytoclasts (OP/TR ratio). Opaquephytoclasts (OP) partly consist of charcoal originating from forestfires, but mainly develop by oxidation of translucent phytoclasts (TR).Another source for opaque phytoclasts might be resedimentation ofrefractory particles. Generally, the ratio of opaque to translucentphytoclasts increases basin-ward due to fractionation processes andthe higher preservation potential of opaque particles (Summerhayes,1987; Tyson, 1993; Pittet and Gorin, 1997; Bombardière and Gorin,1998). Most of the oxidation is of subaerial, continental origin (Tyson,1995). However, in proximal high-energy shelf areas this trend maybe reversed by in-situ (bio)oxidation at the seafloor (Batten, 1982;Boulter and Riddick, 1986; Bustin, 1988; Tyson, 1993), enhanced bythe high porosity and permeability of coarse-grained sediments(Tyson, 1993); and (3) The size and shape of plant debris (ED/BS ratio)are additionally used to decipher proximal–distal and transgressive–regressive trends. Small, equidimensional (ED) woody fragments arecharacteristic of distal deposits, whereas in proximal settings, largeblade-shaped (BS) particles are quite abundant (Steffen and Gorin,1993). In addition, proximal assemblages reveal a greater variety ofparticle sizes (Tyson, 1993; Tyson and Follows, 2000).
Generally, the preservation of sedimentary organic matter of thecarbonates studied is very poor. However, distinct horizons showwellpreserved organic particles. The most striking feature is the very highcontent of terrestrial particles, mainly phytoclasts (Fig. 7), throughoutthe studied section. The size and shape of plant debris shows a highvariety with a high amount of large, needle-shaped fragments (Beds30, 52, and 74). Translucent particles are abundant, whereas mostphytoclasts are opaque. Three intervals are characterized by maxi-
mum abundance of equidimensional, opaque phytoclasts (Beds 26,44, and 67) corresponding to peak abundance of marine plankton(Fig. 6). Upsection, palynofacies of the carbonates studied isdominated by degraded organic matter and small, equidimensionalphytoclasts. Spores reach maximum abundance in Bed 47.
Another striking feature is the high amount of prasinophyteswithin the marine fraction. Acritarchs are very rare (Beds 26, 44, and67), dinoflagellate cysts are absent. A sudden increase in theabundance of prasinophytes is recognized in Bed 47 (Götz et al.,2009). Foraminiferal test linings become more common in the upperpart of the interval studied (Beds 61 to 86) and are a dominant marineconstituent in Beds 67 and 69.
6. Stable isotope stratigraphy
Carbon and oxygen stable isotope ratios were measured through-out the lower 55 m of the Csővár Vár-hegy section, across the Triassic–Jurassic boundary (Fig. 2). A prominent end-Triassic negative carbonisotope anomaly (CIE) was first documented in both bulk organicmatter and bulk carbonate (Pálfy et al., 2001). Additional, high-resolution δ13Ccarb data from the same section was reported by Pálfyet al. (2007). Oxygen isotopic ratios in bulk carbonate are more proneto diagenetic overprint. Thus, inferences for palaeotemperaturechanges are somewhat tenuous but δ18O values are useful to detectand exclude samples with significant diagenetic alteration (Pálfy et al.,2007).
In the past few years, the negative CIE at the Triassic–Jurassicboundary was observed at numerous localities world-wide (seeHesselbo et al., 2007, for a review). It is important as both astratigraphic correlation tool and as a proxy record for palaeoenvir-onmental and biotic changes, reflected in the global carbon cycle. Thedetailed anatomy of the Triassic–Jurassic boundary CIE appears todiffer between individual sections. Some sections show a pattern of aninitial negative CIE followed by a main negative CIE (e.g. Hesselboet al., 2002; Ward et al., 2004; Ruhl et al., 2009), the initial negativeCIE may be followed by a positive CIE (Williford et al., 2007), yetothers are similar to the Csővár section in showing only onepronounced anomaly (Galli et al., 2007). The simultaneous negativeCIE and prasinophyte spike at Csővár suggests correlation with themain CIE and broadly coeval algal bloom in the Northern CalcareousAlps (Kuerschner et al., 2007; Bonis et al., 2009). The CIE is thought toreflect a major perturbation in the global carbon cycle at the close ofthe Triassic, which is probably causally related to the biotic extinction.
7. Depositional cycles
The Csővár Formation represents foreslope and basin deposits ofa broad carbonate platform and a related intraplatform basin. Inthis setting, the morphology of the platform and sea-level changesmay have controlled the amount and composition of sedimentexported to the slope and the adjacent basin. Consequently,changes in the quantity and composition of bioclasts and lithoclastsin the gravity flow deposits can be used as a proxy of relative sea-level changes. However, the palaeogeographic setting and assumedmorphology of the adjacent platform must be taken into consid-eration as well.
High abundance of terrestrial phytoclasts and palynomorphs in thebasal light grey marl layer of the Pokol-völgy quarry suggests asignificant sea-level drop in the late Early to Middle Rhaetian (basalpart of the Vandaites stuerzenbaumi Zone), when large parts of theformer platforms became subaerially exposed (Haas et al., 1997). Thesea-level drop led to increasing restriction of the intraplatform basinand establishment of oxygen depleted conditions near the bottom. Ahigh amount of prasinophytes (Góczán, 1997) points to a stratifiedwater column (cf. Tyson, 1995).
Fig. 6. Stratigraphy, microfacies types, palynofacies patterns, and sequence stratigraphic interpretation of the Csővár (Vár-hegy) section. Three phases of maximum flooding aredocumented by maximum abundance of marine plankton, and equidimensional, opaque phytoclasts (Beds 26, 44, and 67). Transgressive intervals are marked by increasing marinecomponents and equidimensional, opaque phytoclasts. The opposite trend is recognized within highstand deposits. Abbreviations of microfacies types: Lb — lithoclastic–bioclasticgrainstone/packstone, On — oncoidal grapestone, Mt — medium-grained calciturbidite, Ft — fine-grained calciturbidite, La — calcisiltite–calcilutite laminite, S — sponge spiculewackestone, B — thin-shelled bivalve wackestone, Rw — radiolarian wackestone, Rp — radiolarian packstone. Abbreviations in depositional cycles: HST — Highstand Systems Tract,mfz — maximum flooding zone, TST — Transgressive Systems Tract, LST — Lowstand Systems Tract. For key to lithologic symbols, see Fig. 2.
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Above this horizon the facies characteristics of the successionexposedin the Pokol-völgy quarry and the Vár-hegy section suggest a long-termdeepening-upward trend in the Middle Rhaetian to Early Hettangian
interval. It corresponds to the transgressive phase of theRhaetian to EarlySinemurian cycle described by De Graciansky et al. (1998) and Hallam(2001), both in the Tethyan and the Boreal provinces ofWestern Europe.
Fig. 7. Palynofacies of the Csővár (Vár-hegy) section. a — Lowstand deposits, Bed 52: high variety of the size and shape of plant debris with a high amount of large, needle-shapedfragments; b — highstand deposits, Bed 70: high amount of opaque, needle-shaped phytoclasts; c — maximum flooding, Bed 44: high amount of opaque, equidimensionalphytoclasts; d — maximum flooding, Bed 67: high amount of opaque, equidimensional phytoclasts and marine particles (foraminiferal test linings); e — transgressive deposits, Bed21: high amount of translucent phytoclasts and degraded organic matter; f — transgressive deposits, Bed 23: high amount of translucent phytoclasts and marine particles(foraminiferal test linings).
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Superimposed upon the long-term deepening-upward trend, a 5to 10 m cyclicity was encountered in the alternation of turbiditic(hemipelagic) and basinal (eupelagic) facies. The cyclic pattern ofthe succession is most clearly documented in the lower part of theVár-hegy section where medium-grained turbidites are present. Inthis part of the section dm-scale deepening-upward cycles documentedin a turbidite–pelagite alternation were also deciphered. The cyclicityis less pronounced in the upper part where turbidites are generallymissing and calcisiltite–calcilutite laminites (most likely very distalturbidites) and typical fine-grained basinal deposits prevail.
The palynofacies patterns recognized within the sedimentaryseries studied clearly reflect the cyclicity described from sedimento-logical data. Maximum abundance of equidimensional, opaquephytoclasts and marine particles document three phases of maximumflooding. Furthermore, the general increase in marine componentsdisplays the superimposed transgressive trend from Late Rhaetianinto Early Hettangian times.
The first cycle in the Vár-hegy section (Beds 1–15; Fig. 6) startswith proximal and distal turbidites grading upward into radiolarianbasin facies. This trend may be interpreted to reflect retrogradation of
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the platform margin due to rising sea level. Within the turbiditicmember, an alternation of crinoidal turbidites and radiolarianwackestone pelagites points to a higher frequency cyclicity, whicheither reflects sea-level fluctuations, or autocyclic sedimentaryprocesses such as lobe- and channel switching. This pattern is bestobserved in the upper part of this interval (Beds 5b–12) wherecoarser-grained turbidite layers occur.
Bed 16 contains large, redeposited bioclasts along with a highamount of crinoid ossicles of platform margin to upper slope origin(foraminifera, and microproblematica) and may be considered as thebasal layer of the next cycle (Beds 16–29) that is made up mainly ofmedium to fine-grained turbidite facies. Decimetre-scale turbidite–pelagite cycles were also detected within this interval (Fig. 8).Maximum flooding is detected around Bed 26 with maximumabundance of small, equidimensional phytoclasts and marine parti-cles, occurrence of acritarchs and last occurrence of Triassicammonoids, indicating normal marine conditions. Conodonts arerelatively abundant and diverse up to Bed 29 (Pálfy et al., 2007).
This interval is overlain by Bed 30 containing small lithoclasts(cemented fragments of crinoids) and a high amount of shallow-marine skeletal elements in a pelagic wackestone matrix. Coarse-
Fig. 8. Decimetre-scale cycles in the uppermost Rhaetian part of the Csővár (Vár-hegy)section, inferred from sedimentological observation and microfacies types.
grained detritus of corals and mollusks occur in the topmost part ofthe bed. Consequently, it may be interpreted as a lowstand deposit.Numerous large, needle-shaped opaque phytoclasts also point to alowering of sea level, which forced the progradation of river deltas.Elevated amount of silt-sized terrigenous siliciclasts is found in cm-sized intraclasts in the topmost part of this bed and also in theoverlying clay-rich Bed 31.
Above Bed 31, laminitic deposits and an increase of small,equidimensional phytoclasts as well as marine particles indicate aclear transgressive trend. An almost total lack of any recognizablebioclasts in the laminitic layers might be attributed to this facieschange. Maximum flooding occurred around Bed 44 characterized bymaximum abundance of small, equidimensional phytoclasts andacritarchs. Highstand deposits of radiolarian basin facies are markedby maximum abundance of prasinophytes (Beds 47 and 49).Additionally, a significant negative shift in the δ13Ccarb and a markeddecrease in the TOC were encountered (Pálfy et al., 2001) within thisinterval. A drastic decline of the foraminiferan and conodontassemblages is observed in Bed 30, indicating a significant bioticchange that may be related to the end-Triassic mass extinction event(Pálfy et al., 2007).
The next sedimentary cycle starts with a thin lithoclastic andbioclastic proximal turbidite layer (Bed 52), characterized by a highvariety in size and shape of phytoclasts, both opaque and translucentand may also reflect an extraordinary event (“boundary-eventhorizon”). Upsection, various basinal facies prevail and an increaseof foraminiferal test linings is recognized, indicating continuation ofthe large-scale transgressive trend superimposed by a medium-scaletransgressive trend with maximum flooding around Bed 67. This bedis characterized by maximum abundance of small, equidimensionalphytoclasts and acritarchs, accompanied by the first occurrence ofpsiloceratid ammonoids. The lack of platform-derived bioclasts ismost likely the consequence of the drowning of the adjacentplatforms during maximum flooding and the early highstand phase.However, fragments of crinoids commonly occur both in the laminitic,distal turbidites and in the deposits of the typical basin facies. The topand the slope of the drowned platforms may have been the habitat ofcrinoid colonies.
The most striking lithological change in the Hettangian part of thesection is the appearance of redeposited oncoids and grapestones in a2 m thick, thickening-upward bundle (Bed 75–80), which mayindicate the end of the Rhaetian to earliest Hettangian third-ordersequence (Hardenbol et al., 1998). The coated grainsmay have formedon the top of the drowned platforms during the late highstand tolowstand period and redeposited at the toe of the slopes during thelowstand (Fig. 6). Sedimentary organic particles are not preserved inthese sediments.
The oncoidal beds are overlain by distal turbidites. In the upperpart of the studied section (Beds 75–80), various basinal facies typesprevail including biogenic components such as radiolarians, thin-shelled bivalves and sponge spicules which document a large-scale,deepening-upward trend.
8. Orbital signal
Estimation of the time represented by the above describeddepositional cycles remains ambiguous as any calculation mustdepend on the controversial duration of the Rhaetian Stage. Thelatest Geological Time Scale (Ogg et al., 2008) estimates the durationof the Rhaetian as 4 my (between 199.6 and 203.6 Ma). However, thisscale was compiled prior to agreement on the definition of theboundaries of the stage and before the latest radiometric ages(Schaltegger et al., 2008; Friedman et al., 2008) and new magnetos-tratigraphic constraints (Gallet et al., 2007 and references therein)became available for the latest Triassic and the Triassic–Jurassicboundary. A preliminary decision of the Subcommission on Triassic
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Stratigraphy puts the base of the Rhaetian at the base of the Misikellaposthersteini Zone, whereas the International Commission on Stratig-raphy agreed on defining the base of the Jurassic by the base of thePsiloceras spelae Zone. The new developments suggest a significantlyshorter duration of the Rhaetian, probably not exceeding 2 my.Thickness ratios within the Upper Triassic platform carbonates mayalso suggest a relatively short duration of the Rhaetian, although asimple conversion of stratigraphic thickness to time of sedimentationmust be treated with caution. In the NE part of the TransdanubianRange, the Norian Dachstein-type platform carbonates (DachsteinLimestone and Main Dolomite) are about 2000 m thick whereas thethickness of the Rhaetian Dachstein Limestone reaches only up to200 m (Haas, 1995). Cyclostratigraphic analysis of the Dachstein-typeplatform carbonates in the Transdanubian Range led to the sameconclusion, suggesting 2 my duration (Balog et al., 1997).
In the study area, the total thickness of the Rhaetian succession isabout 55 m (Haas and Tardy-Filácz, 2004). Assuming that it wasdeposited in 2 my, the uppermost 20 m of the Rhaetian, exposed inthe Vár-hegy section, may represent about 0.8 my. Two and a halfdepositional cycles are recognized within this interval, placingthe duration of these cycles within the Milankovitch frequency band(20–400 ky), matching either the short (100 ky) or the long (400 ky)orbital eccentricity signal. Taking into account that the entire Rhaetianis built up by five fourth-order depositional cycles (Haas and Tardy-Filácz, 2004), the long eccentricity signal seems to be plausible.However, in coeval cyclic platform carbonates of the TransdanubianRange the 100 ky bundling is much more pronounced (Balog et al.,1997). The dm-scale turbidite successions (Fig. 8), recognized in thefirst and second depositional cycles, may reflect precessional cycles,comparable to those found in the Upper Norian to Lower Rhaetiancalciturbiditic Pedata Beds of the Gosau Lake section, NorthernCalcareous Alps (Reijmer and Everaars, 1991; Reijmer, 1991; Reijmeret al., 1994).
High-resolution sequence stratigraphic and cyclostratigraphicanalyses of Upper Jurassic and Lower Cretaceous sections suggestthat the 400 ky eccentricity cycle contributed to the genesis of majordepositional sequences (i.e. third-order sequences in the charts ofHardenbol et al., 1998), whereas beds and bedsets formed in tunewith the 20 ky precession and 100 ky eccentricity cycles (Strasseret al., 2000). Our observations permit a similar interpretation ofstratigraphic intervals in the Csővár section. It is important to notethat the duration of the long eccentricity cycle has remained relativelystable over geological time. For the latest Triassic–earliest Jurassicperiod, the precession cycles are presumed to be somewhat shorterbut nevertheless close to 20 ky in duration, whereas the obliquitycycles were significantly shorter in the Mesozoic than at present(Strasser et al., 2006).
Calculations using sedimentation rate estimates from analogousdeposits provide independent constraints on duration. Middle Triassicintraplatform basinal calcareous turbidites with precise radiometricage control yielded an average sedimentation rate of 13.5 m/my(Maurer, 2003). At Csővár, the thickness of cycles (excluding theslumps) varies from 6 to 8 m. Assuming that the cycles represent400 ky long eccentricity forcing, 15 to 20 m/my average sedimenta-tion range is estimated. This compares favorably with the estimates ofMaurer (2003).
Furthermore, this tentative temporal framework is useful for theinterpretation of the observed local sequence of events at the Triassic–Jurassic boundary. The following events took place within a single,presumably 400 ky cycle: 1) disappearance of ephemerally abundantplatform-derived biota, possibly related to a drowning event; 2) declineof bothplanktonic and benthic biota (reduction in biogenic sedimentarycomponents, and conodont and foraminiferan diversity); 3) significantperturbation of the biosphere as documented by marine and terrestrialpalynomorph assemblages; 4) geochemical anomalies in C and Oisotopes; and 5) concurrent reduction of TOC (Pálfy et al., 2001, 2007).
The first appearance of a new, Jurassic radiolarian fauna, the finalextinction of decimated conodont populations, and the recovery ofprimary production occurred in the following cycle.
9. Discussion
In the Csővár Basin, the entire Rhaetian succession is characterizedby a high amount of land-derived plant debris. Since the Pangaeacontinent did not serve as a proximal coastline to the Csővár Basin, itis supposed that several islands must have been situated between therestricted Buda Basin and the Csővár Basin (Haas, 2002). Theredeposited shallow marine bioclasts and lithoclasts of platformmargin and upper slope origin, occurring in some beds of the Rhaetianpart of the Csővár Limestone Formation imply the establishment ofsmall fringing platforms on the upper slope next to an island (Haaset al., 1997). Crinoid ossicles are the predominant components in theobserved turbidites, i.e. crinoid meadows of the slope terracesproduced a predominant part of the redeposited bioclasts; theplatform-derived clasts are subordinate. For the evaluation of thesedimentary cycles this palaeogeographic setting must be taken intoaccount. Since the source of the redeposited carbonates was mostly aslope (and moreover a slope of an island) and not a large carbonateplatform, the concept of the highstand shedding (Droxler et al., 1983;Droxler and Schlager, 1985; Reijmer et al., 1992; Schlager, 2005)cannot be applied to this case. In contrast, it is very probable thatinstability of the slope increased during the lowstand and earlytransgressive stages, due to a decrease of hydrostatic pressure andforced progradation of deltas at low sea level. Recurring slope failuresinduced the gravity-driven redeposition.
Sedimentological features suggest a sea-level dropduring the latestRhaetian and another one coevalwith themain isotope excursion nearto the Triassic–Jurassic boundary (Haas and Tardy-Filácz, 2004; Pálfyet al., 2007). During these periods of low sea level the subaeriallyexposed areas may have extended over large parts of the innerplatform in the NE part of the Transdanubian Range Unit, resulting inthe erosion of the uppermost part of the Dachstein Limestone. Thisarea may have been flooded again during the earliest Hettangian sea-level rise. However, sedimentation began somewhat later, probably asa consequence of the end-Triassic ecological crisis, and the tropicalplatform conditions did not resume.
The here presented palynofacies data suggest the existence ofislands probably located between the Buda and Csővár basins alsoduring the earliest Hettangian, although the fining-upward trend ofredeposited carbonates implies transgression and accordingly areduction of the subaerially exposed areas. A topographic high mayhave been the source of shallow marine coated and composite grainswhich were accumulated in the Csővár Basin during the next sea-leveldrop. It was followed by continuous deepening which terminated thehigh input of land-derived phytoclasts.
The detailed analysis of the high-frequency Lofer cycles in thecentral and NE part of the Transdanubian Range Unit (Bakony andGerecse mountains) suggests a sequence boundary near to theNorian–Rhaetian boundary (Balog et al., 1997). In the Pokol-völgyquarry section, the horizon in the basal part of the Rhaetiansuccession, showing a high amount of land-derived plant remains,was correlated to this sequence boundary (Haas and Budai, 1999).This interval of major subaerial exposure was followed by continu-ation of the evolution of Dachstein-type platforms in the central andNE part of the Transdanubian Ranges that resulted in the formation ofan about 200 m thick Lofer-cyclic succession. In the NE part of theTransdanubian Range Unit (Gerecse Mts., Tata) the uppermost part ofthe Dachstein Limestone is missing and the earliest overlyingformation is the shallow-water Pisznice Limestone of Middle–UpperHettangian age (Fülöp, 1976; Pálfy et al., 2007). The top of theDachstein Limestone is truncated, and most probably a few metres ofthe formation were already eroded prior to the deposition of the
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marine oncoidal lag deposits in the basal part of the PiszniceLimestone (Fülöp, 1976; Haas, 1995). A similar setting was reportedfrom the Northern Calcareous Alps, where Upper Rhaetian reefallimestones are overlain by the Adnet Limestone with a hiatus ofvariable duration (Böhm et al., 1999). However, the history of this gapis not clear and is discussed controversially, both for the Transdanu-bian Range (Haas, 1995) and the Northern Calcareous Alps (Mazulloet al., 1990; Satterley et al., 1994; Flügel and Koch, 1995).
Due to subsequent erosion, the Upper Rhaetian is missing in theBuda Mts., both in the platform and basin facies (Haas, 2002), and inthe Csővár block the Upper Norian and Rhaetian part of the platformcarbonate succession was eroded prior to the Eocene (Haas, 2002).
The continuous latest Triassic to earliest Jurassic basin successionat Csővár provides valuable information on the palaeogeographicsetting and evolution of the adjacent carbonate platform areas duringthe Triassic–Jurassic boundary interval. Two scenarios are proposedwith respect to the evolution of the platform areas: a) subaerialexposure and erosion in the latest Triassic to earliest Jurassic followedby inundation from the Middle Hettangian onward, or b) solelysubmarine erosion and non-deposition during the period of the gap.The dominance of terrestrial plant debris with a large number of largeblade-shaped particles and a significant variety of particle sizes aswell as a high amount of pollen grains and spores in the latest Triassicto earliest Jurassic sedimentary series at Csővár supports thehypothesis of the existence of small islands in the area of the outerplatform, not only at the very end of the Triassic but also during EarlyHettangian times. The new palynological data confirm the previouspalaeogeographic model of a swell between the HármashatárhegyBasin in the Buda Mts. and the Csővár Basin (Haas et al., 2000) thatwas subaerially exposed for a longer period. Furthermore, our dataindicate subaerial exposure of a broader region during phases of lowsea level.
10. Conclusions
The results of the integrated microfacies and palynofacies analysisof the Csővár section shed new light on the latest Triassic to earliestJurassic evolution of the Transdanubian Range Unit which was part ofthe NW Tethys shelf region. Sedimentary organic matter content ofthe carbonates studied is crucial for the interpretation of similarplatform-to-basin settings along the northern Tethys Ocean. Palyno-facies patterns clearly reflect the cyclicity interpreted from sedimen-tological data. Generally, the high amount of prasinophytes points to astratified water column of a deep basinal setting. On the other hand,the high amount of terrestrial plant debris and sporomorphs impliesclose terrestrial sources and thus a complex topography with smallislands. The high input of terrestrial organic particles from theseislands was continuous from the Late Triassic into the earliest Jurassic.The platform margin setting favored the development of toe-of-slopeaprons characterized by turbidites, which contain distinct layers thatallowed the preservation of sedimentary organic matter.
Both carbonate microfacies and palynofacies patterns clearlyreflect a metre-scale cyclicity in the studied succession. The cyclesshow a deepening-upward trend with lithoclastic–coarse-grainedbioclastic facies at their base overlain by distal or very distal turbiditesand progressing into pelagic facies. Maximum abundance of equidi-mensional, opaque phytoclasts and marine particles document threephases of maximum flooding. According to our approximate calcula-tions, the 400 ky orbital eccentricity signal seems to be recorded.Using this temporal framework, the estimated average sedimentationrate of 15–20 m/my is in good agreement of published estimates ofsimilar deposits. Local manifestation of Triassic–Jurassic boundaryevents is confined to a single 400 ky cycle.
Based on our studies of the Csővár succession, the Late Triassicpalaeogeographic setting and evolution of the Transdanubian Range'scarbonate platform can be more precisely interpreted. Already during
Carnian times, the beginning disruption of the marginal zone of theDachstein-type platform is documented and a distinct topography, i.e.smaller platforms with islands and intraplatform basins among them,developed. The sea-level changes may have significantly modified theextension of the islands. There was a significant sea-level drop in theEarly Rhaetian leading to restriction of the Csővár Basin and thus todevelopment of a stratified water column and coeval extension of thesubaerially exposed areas on the platform.
The palynofacies analysis implies a prolongation of the subaerialconditions also in the earliest part of the Jurassic. The new resultssupport the former hypothesis that the Middle–Late Hettangiandrowning affected a tens of km broad outer belt of the Dachsteinplatform and was preceded by a eustatically controlled emersion.
However, the traces of subaerial exposure (karstification, andpedogenesis) are not recorded because they have been destroyed bythe submarine erosion during the following inundation. High-energyconditions favored the formation of coated grains as lag deposits onthe hard sea floor. In-situ embedded gravel-sized coated grains arecharacteristic of the basal strata of the Middle Hettangian in Tata. Finegravel to sand-size redeposited coated grains appear in the higherpart of the Hettangian sequence in the Csővár section.
Acknowledgements
This study is part of a project on Triassic–Jurassic boundary keysections of the NW Tethyan realm (Hungary, Slovakia) supported bythe German Research Foundation (DFG project GO 761/2-1).Additional financial support was provided by the Hungarian ScientificResearch Fund (K 61872, K 72633) and an EU-funded SYNTHESYSgrant to A.E.G. Constructive reviews by A. Strasser and an anonymousreviewer, and comments from journal editor F. Surlyk helped improvethe manuscript. This is MTA–MTM Palaeo contribution No. 105.
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