Tectonics and sedimentation during convergence of the ALCAPA and ...

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DICKINSON, W. R. 1985. Interpreting provenance relations from detrital modes of sandstones. In: ZUFFA, G. G. (ed.) Provenance of Arenites. NATO. ASI Series.
Tectonics and sedimentation during convergence of the ALCAPA and Tisza – Dacia continental blocks: the Pienide nappe emplacement and its foredeep (N. Romania) ¨ GER1,4, M. TISCHLER1,4, L. MATENCO2, S. FILIPESCU3, H. R. GRO 1 5 ¨ A. WETZEL & B. FUGENSCHUH 1

Geologisch-Pala¨ontologisches Institut, Universita¨t Basel, Bernoullistr. 35, 4056 Basel, Switzerland (e-mail: [email protected])

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Netherlands Centre for Integrated Solid Earth Sciences, Vrije Universiteit, Faculty of Earth and Life Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands 3

Department of Geology, Babes¸ -Bolyai University, Str. Koga˘lniceanu 1, 400084 Cluj-Napoca, Romania

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Present address: Statoil ASA, Statoil Head Office, Forusbeen 50, 4035 Stavanger, Norway

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Geology and Paleontology, Innsbruck University, Innrain 52f, A-6020 Innsbruck, Austria Abstract: The juxtaposition of the ALCAPA and Tisza–Dacia continental blocks, although one of the key issues in the evolution of the Carpathians, is not well known in terms of associated effects on the sedimentary systems during frontal foreland development. Most of the contact between ALCAPA and Tisza– Dacia being covered by post-tectonic deposits, these effects can best be observed in northern Romania. Sedimentological data on facies, palaeocurrents and modal composition of sandstones combined with micropalaeontological data and 2D wellcalibrated seismic lines constrain the tectonic history of the contact zone between ALCAPA and Tisza –Dacia. Pervasive deposition of sand-dominated siliciclastics beginning in late Early Oligocene (Late Rupelian) times is interpreted to reflect the onset of convergence between ALCAPA and Tisza– Dacia in the study area. The depocentre of coarse siliciclastic material migrates southward, finally forming a southeastward-thinning clastic wedge in the Transylvanian Basin. This Burdigalian-age clastic wedge is interpreted as fill of a flexural foreland basin that formed in response to the coeval thrusting of parts of ALCAPA (Pienides) over Tisza –Dacia. A shift from an E –W to SE –NW striking basin axis during Oligocene times towards a WSW– ENE oriented basin axis during Burdigalian times is interpreted as a result of clockwise rotation of Tisza –Dacia during basin formation.

The Miocene geological history of the Carpathians is characterized by the emplacement of continental blocks, ALCAPA and Tisza –Dacia, into the so-called Carpathian embayment, a large-scale bight in the European margin between the Moesian and Bohemian promontories (Fig. 1). Emplacement of these continental blocks and their subsequent extensional collapse was coeval to thin-skinned nappe stacking at the exterior of the thrusted chain (e.g. Sa˘ndulescu 1988; Horvath 1993). Lithospheric slab retreat of the distal parts of the Carpathians foreland (Royden 1993) is considered to represent the principal driving force for the invasion of the two continental blocks (Fig. 1). They were separated by a broad zone of deformation, the Mid-Hungarian fault zone (e.g. Fodor et al. 2005). Along this fault zone, substantial strike-slip movements and block rotations are documented for Palaeogene to Early Miocene

times (e.g. Csontos & Vo¨ro¨s 2004; Horva´th et al. 2006 and references therein). Convergence between the invading continental blocks ultimately led to thrusting of ALCAPA onto Tisza–Dacia (Csontos & Nagymarosy 1998). This process culminated during the Early Miocene (Pienide nappes, Sa˘ndulescu et al. 1981; Fig. 1). However, the onset of thrusting remains rather poorly constrained to Late Oligocene times in the Pannonian Basin (Csontos & Nagymarosy 1998; Fodor et al. 1999) and Transylvanian Basin (Krezsek & Bally 2006), respectively. A very suitable place to study the tectonics and sedimentary processes linked to the convergence of ALCAPA and Tisza– Dacia along the MidHungarian fault zone is located at its ENE-most tip, in northern Romania. This is the only place where the thin-skinned thrust units and associated foredeep sediments are exposed (Fig. 1). An

From: SIEGESMUND , S., FU¨ GENSCHUH , B. & FROITZHEIM , N. (eds) Tectonic Aspects of the Alpine-DinarideCarpathian System. Geological Society, London, Special Publications, 298, 317–334. DOI: 10.1144/SP298.15 0305-8719/08/$15.00 # The Geological Society of London 2008.

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Fig. 1. Major tectonic units of the Alps, Carpathians and Dinarides (simplified after Schmid et al. 2006).

additional advantage is the availability of detailed subsurface data obtained during exploration of the Transylvanian Basin (e.g. De Broucker et al. 1998). The goal of this study is to provide new constraints on the thrusting of ALCAPA onto Tisza –Dacia at the northeasternmost tip of the Mid-Hungarian fault zone (i.e. the Pienide nappe contact) by analysing syntectonic sediments that accumulated on Tisza –Dacia.

Geological setting The arcuate belt of the Carpathians acquired its present shape during the Cenozoic emplacement of continental blocks into the Carpathian embayment. These continental blocks are referred to as ALCAPA (e.g. Csontos 1995) and Tisza –Dacia (e.g. Balla 1986; Internal and Median Dacides of Sa˘ndulescu 1980, 1988), respectively (Fig. 1). They differ in their contrasting Triassic and Jurassic sedimentary facies and fossil assemblages (e.g. Csontos & Vo¨ro¨s 2004 and references therein).

Evolution of the continental blocks ALCAPA and Tisza – Dacia Between the name-giving constituents of the Tisza – Dacia block, Tisza and Dacia, remainders of an oceanic domain (Transylvanides, Sandulescu 1984) occur. After the closure of this oceanic domain starting in Albian times (e.g. Sa˘ndulescu & Visarion 1977; Csontos & Vo¨ro¨s 2004), Tisza – Dacia can be considered as a single block. The Dacia block (or Rhodopian fragment, Burchfiel & Bleahu 1976) was separated from the European platform by a partially oceanic basin,

whose remnants are presently found in the highly deformed Ceahla˘u and Severin nappes (Sa˘ndulescu 1984). Following an initial Albian shortening, this oceanic domain has been completely subducted in latest Cretaceous times (Sa˘ndulescu 1988). Shortening continued eastwards and ended after Late Miocene continental collision (c. 11 Ma, Matenco & Bertotti 2000). Cenozoic slab-retreat (Royden 1988) in the Carpathians, favoured by deep lithospheric processes related to the subducted oceanic slab of the Ceahla˘u–Severin domain, is acting as the principal driving force for the emplacement of the two continental blocks (ALCAPA and Tisza– Dacia) into the Carpathian embayment. A number of recent studies addressed these deep lithospheric processes (e.g. Wortel & Spakman 2000; Cloetingh et al. 2004; Knapp et al. 2005; Weidle et al. 2005). The emplacement of ALCAPA was additionally affected by ‘lateral extrusion’, eastward escape along conjugate strike-slip faults coupled to indentation processes in the eastern Alps (Ratschbacher et al. 1991a, b). The northeastward movement of ALCAPA was guided by strike-slip zones oriented subparallel to the collision suture (Nemcˇok 1993). Corner effects at the Bohemian promontory led to counter-clockwise rotation of the ALCAPA block. The Tisza –Dacia block started its emplacement into the Carpathian embayment during the Eocene (Fu¨genschuh & Schmid 2005). Corner effects at the Moesian promontory (e.g. Ratschbacher et al. 1993; Schmid et al. 1998) led to large clockwise rotations during the emplacement of Tisza–Dacia (e.g. Panaiotu 1998). The convergence of ALCAPA with the European margin (Krzywiec 2001) is terminated by ‘soft-collision’ (sensu Royden 1988) simultaneously with the last

THE PIENIDES AND THEIR FOREDEEP

moments of the emplacement of Tisza –Dacia over the European and Moesian foreland during Middle–Late Miocene (Matenco et al. 2003).

Palaeogene to Lower Miocene sedimentary architecture near the ENE contact between ALCAPA and Tisza – Dacia The study area lies internally in the main Carpathian mountain chain at the northern border of the Transylvanian Basin, at the contact of Tisza –Dacia and units assigned to ALCAPA (Fig. 2, Pienides). The large-scale, thick-skinned nappe emplacement of the basement units outlined above was followed by the deposition of significant post-tectonic covers (e.g. Sa˘ndulescu 1988). Sedimentation started with uppermost Cretaceous to Paleocene continental to shallow marine deposits (Jibou Fm., e.g. Popescu 1984), well exposed on the NW margin of the Transylvanian Basin. The Pienide nappe stack. Top to SE emplacement of the Pienide nappe stack (Fig. 2) commenced during Burdigalian times (Sa˘ndulescu et al. 1981; Tischler et al. 2006). The Pienides mainly consist of Eocene to Oligocene turbiditic siliciclastic units and can be divided into internal (Botiza nappe and Pienniny Klippen type of units, Sa˘ndulescu et al. 1993) and external nappes (Petrova, Leordina and Wildflysch nappes). While the internal nappes of the Pienides are correlated to the Inacovce – Krichevo units, the external Pienides correspond to the Magura flysch of the Western Carpathians (Sa˘ndulescu et al. 1981; Sa˘ndulescu 1994). The Pieninny Klippen belt originally represented the outermost rim of ALCAPA (e.g. Csontos & Vo¨ro¨s 2004). More external units such as the external Pienides have been accreted during the invasion of ALCAPA into the Carpathian embayment (e.g. Fodor et al. 1999). Hence, the Pienides represent the easternmost tip of ALCAPA in Burdigalian times. It is worthwhile to note that during Oligocene times sand-dominated siliciclastics of similar facies and petrography as the ones in the foreland have been deposited in the external thrust sheets of the Pienides (Aroldi 2001). This suggests a genetic connection, i.e. limited post-dating thrusting (Sa˘ndulescu & Micu 1989). Eocene. During the Eocene, epicontinental deposits formed in the NE part of the study area (Sa˘ndulescu et al. 1981). Carbonate platforms in the area of the Rodna Horst indicate relatively shallow marine environments (Lutetian to Priabonian Iza Limestone, 1982). Conglomerates and sandstones prevail in the NE (Prislop Fm., Kra¨utner et al. 1982, 1983; Sa˘ndulescu et al. 1991). They grade westwards into marly deposits (Vaser Fm.,

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Sa˘ndulescu et al. 1991) developed in a littoral– neritic facies (Dicea et al. 1980). A further deepening towards the west is indicated by basal-slope turbidites (‘hieroglyphic flysch’), outcropping north of the Bogdan Voda fault (Pıˆrıˆul Mocilnei Fm., Sa˘ndulescu et al. 1991). In the southern part of the study area, two sedimentary cycles formed during the Eocene (Popescu 1984). Lower Eocene red-bed continental deposits were followed by two Middle to Upper Eocene shallow-marine highstand deposits that are separated by a continental red-bed and lacustrine lowstand interval (De Broucker et al. 1998). During the second, Late Eocene highstand, a carbonate platform developed during Priabonian times (Culmea Cozlei Fm., Cluj limestone, De Broucker et al. 1998; Rusu et al. 1983). Several local cycles of subsidence and deposition in the N –NW part of the Transylvanian Basin are related to ratherreduced Eocene tectonic events such as the Puini thrust (e.g. De Broucker et al. 1998; Krezsek & Bally 2006). Oligocene to Lower Miocene. A phase of regional subsidence is recorded near the Eocene– Oligocene boundary. Following successive drowning of the carbonate platform from north (earliest Oligocene, Kra¨utner et al. 1982) to south (Early Oligocene, Gyo¨rfi et al. 1999), muds became the prevailing deposits. In the north (Fig. 3), these Lower Oligocene strata are associated with slumps and olistoliths and have large variations in thicknesses (25 to 1200 m, Dicea et al. 1980). In the south black shales dominate. The shales are progressively overlain by sandy turbidites, deposited within a basin having an initially W– E to NW–SE trending basin axis. The base of these sandy turbidites becomes increasingly younger towards the south (Fig. 3). This coarse sedimentation started during the Late Rupelian (Sa˘ndulescu et al. 1991) in the north (Fig. 3, Domains 1, 2), Early Miocene in the central autochthonous domain (Fig. 3, Domain 3) and Burdigalian in the south (Fig. 3, Domain 4). In the southern part of the study area, the siliciclastic deposits are grading into molasse-type deposits (Hida Fm., Ciupagea et al. 1970) with a southwestwards thinning, wedge-shaped geometry (Fig. 2, see also Ciulavu et al. 2002). The axis of the Burdigalian Basin is oriented WSW– ENE. Within the Hida Fm., a general shallowing-up trend can be observed, starting with deep marine outer fan turbidites and ending with coarse sands and conglomerates deposited by fandeltas (Koch 1900; Popescu 1975; Kre´zsek & Bally 2006). The eustatic sea-level variations are bound to affect the study area only in limited time intervals due to intermittent changes between open marine and restricted conditions. Open marine conditions

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Fig. 2. Map of the study area, based on published geological maps (1 : 50 000 and 1 : 200 000) of the Geological Survey of Romania (Dicea et al. 1980; Sa˘ndulescu 1980; Sa˘ndulescu et al. 1981; Aroldi 2001). In the southern part of the study area, isopachs of the Burdigalian molasse deposits, based on well-calibrated 2D seismic lines, are shown.

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Fig. 3. Correlation scheme of Oligocene to Lower Miocene deposits in the study area, illustrating progressive younging of sand-dominated flysch towards the south. Approximate locations of sedimentary logs are indicated; width of the brackets corresponds to the approximate percentage of the interval covered. The number above the columns refers to Domain number given in Figure 2. The ages of the respective units are compiled from official 1 : 50 000 map sheets (Geological Survey of Romania; Domains 1 and 3), Sa˘ndulescu pers. comm. (2002, Domain 2). Ages for Domain 4 are compiled from Rusu (1989), Popescu (1984) and Moiescu (1981) in Gyo¨rfi et al. (1999) as well as De Broucker et al. (1998). Unfortunately, the age of the Minget Fm. is rather poorly constrained to the Early Miocene. However, the base of the Minget Fm. is considered to be older (around the Aquitanian–Burdigalian boundary) than the base of the Hida Fm. (mid-Burdigalian), since all available geological maps show the Hida Fm. to overlie the Minget Fm.

might have prevailed in the Paratethys (i.e. including the study area) from mid-Oligocene to Burdigalian times (Ro¨gl 1999). A significant shortterm sea-level drop occurred during the Late Oligocene (Haq et al. 1987). It probably coincided with the deposition of the Bors¸a Sandstone Fm. (Fig. 3). However, this apparent compatibility is put into perspective by the continuing progradation during the Burdigalian (Hida Fm.), a time of shortterm sea-level rise (Haq et al. 1987).

Data In order to constrain the depositional setting of the Oligocene to Burdigalian siliciclastic units and evaluate their relationship to the Pienide nappe emplacement, sedimentological studies have been combined with seismic data. Facies analysis based on detailed lithostratigraphic logs, assessment of the detrital modes of the siliciclastic units and palaeocurrent analysis are combined with new biostratigraphical data. These data will be related to the insights gained by the interpretation of well-calibrated 2D seismic records from the Transylvanian Basin.

Facies analysis Based on the lithofacies classification of Pickering et al. (1989, 1995), the depositional trends have been defined by using the hierarchical scheme of Mutti & Normark (1987). First order features are at the scale of basin fills (turbidite complex), while second order features (turbidite system, c. 400 m thickness) include depositional sequences, commonly bounded by highstand mud facies. Third order features (turbidite stage, c. 250 m thickness) are facies associations reflecting different development phases of the system. Fourth order features comprise bed packages (turbidite sub-stage, c. 15 m) and characterize sub-stages of system development. The smallest, fifth division, is defined by individual lithofacies. Continuous, long sections, even when sparse, reflect the development of the depositional setting (Figs 4 and 5). Correlation in between the outcrops is based on the corresponding units shown on geological maps. As an example, the Birt¸u Sandstone Fm. overlies the Valea Carelor Fm. and the sections are correlated accordingly. In the case of the Hida Fm., the relationship of the three sections covering the basal part has been established in the field, while

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Fig. 4. Simplified overview of logs 1 and 2. Following the gradual progradation from basin plain deposits towards a distal lobe setting (log 1a, 1b), a complete 2nd-order depositional sequence (progradation and retreat) is documented in log 2 (Lower Oligocene Birt¸u Sandstone).

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Fig. 5. Simplified overview of logs 3– 11. A progradational pulse is documented in log 4 (Aquitanian). Further to the south (logs 5 –7), coarse clastic input is documented later, in Burdigalian times. Logs 8– 11 are situated at the top of the Hida Fm., featuring coarse intercalations of reduced thickness in a silty matrix.

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the relationship of the four logs constituting the top part has been established by projection of the outcrop locations using the dip of the recorded strata. The characterization of the used facies classes and the detailed lithostratigraphic logs and their description are published separately and these are available online at http://www.geolsoc.org.uk/ SUP18312. A hard copy can be obtained from the Society Library. Facies development. The earliest coarse clastic input is Late Rupelian in age; it occurs in the northern part of the study area (Fig. 4). The lithofacies associations documented in Log 1 suggest a gradual change of the depositional setting from a basin plain towards a distal lobe (Fig. 4). The trend towards more and coarser sediment input continues with the deposition of the overlying Birt¸u Sandstone Fm. Log 2, covering the Birt¸u Sandstone Fm., shows a gradual progradation from distal, intermediate and proximal lobe towards a channel-lobe transitional setting (Fig. 4). After an interval possibly indicating channel facies, the depositional setting steps towards a distal lobe setting. Although this retreat occurs on a larger scale (it corresponds to the strata mapped as Valea Morii Fm.), it is not documented in the central parautochthonous realm (Domain 2, Fig. 3). Due to insufficient temporal resolution, it remains unclear if this retreat reflects a basinwide decrease in clastic input due to a moment of decreased tectonic activity (Mutti et al. 1999) or represents a localized event and is not related to regional tectonics. Approximately 30 km further to the south (Domain 3), coarse clastics arrive around the Aquitanian–Burdigalian boundary, documented by prograding lobe fringes (Fig. 5, log 3). Further progradation of the system is recorded by deposits suggesting a channel-lobe transitional setting followed by channelized deposits typical for a middle-fan setting (Fig. 5, log 4). When comparing the northern and central area, the lobe-dominated sedimentation shifted more than 30 km towards the south between Late Rupelian and Late Aquitanian to Early Burdigalian times. About 10 km still further to the south (Domain 4), Burdigalian channel-fill deposits (Fig. 5, logs 5/6) and intermediate, sand-rich lobes (Fig. 5, log 7) confirm the southward migration of proximal depositional settings. In the upper part of the coarse-grained siliciclastics of Domain 4 (Fig. 5), the transition from channel-fill deposits (Fig. 5, log 8) to silt-rich basin plain deposits (Fig. 5, logs 9 and 10) suggests a decrease in clastic input. A renewed progradational pulse is documented by the development from an intermediate lobe to a channel-lobe

transitional setting (Fig. 5). Log 11 covering the top of the Burdigalian strata is composed of silty distal lobe deposits. Note, however, that the Hida Fm. in general is characterized by a shallowing-up trend, which results in very coarse fan-delta deposits being found at the top of the Hida Fm. in other places. Interpretation of the facies analysis. Since submarine fans may have a very complex architecture, local observations derived from discontinuous outcrop data may not be characteristic for the entire system. Since the lithostratigraphic logs only cover a part of the complete Oligocene to Lower Miocene succession, these rather local observations have to be interpreted as merely indicative for the regional development, and their extension to a regional scale has to be done very carefully. The depositional environments match those known in foreland basins. Significant amounts of sand, a generally high sand : mud ratio and the development of well-defined 4th and 3rd order coarsening-up trends typical for lobes, fit to an active margin setting (Shanmugam & Moiola 1988). Hence, the general development of the depositional environment is interpreted to be dominated by tectonic activity.

Palaeocurrents Palaeocurrent indicators (such as flute casts and scour marks, subordinate tool marks, groove casts and parting lineations) have been measured focusing on Oligocene to Burdigalian strata (Fig. 6). In the case of conglomeratic layers, long axes of imbricated pebbles have been measured, and are displayed in the form of a rose diagram (Fig. 6). Palaeocurrent indicators derived from beds with a dip of more than 308 have been backtilted. A pronounced change in the palaeocurrent directions occurs between W-directed transport in Eocene times and to NE- to ESE-directed transport in Oligocene to Burdigalian times (Fig. 6). This latter distribution is evident in individual outcrops as well as on the regional scale (Fig. 6, locations 1–3). The observed directions are in good accordance with existing data from Oligocene deposits (Jipa 1962; Mihailescu & Panin 1962). A log in the Oligocene sandy flysch of the most external Pienide nappe (Wildflysch nappe) yielded compatible palaeocurrent directions. The persistence of the observed palaeocurrent directions suggests a similar basin geometry and depositional setting for Oligocene through Burdigalian strata. The two dominant directions are longitudinal, respectively transversal, to the maximum visible thickness within the clastic wedge (Fig. 2).

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Fig. 6. Map showing palaeocurrent trends in the study area. Palaeocurrent directions change from Eocene westward (inset) to Oligocene southeastward transport. During Oligocene to Burdigalian times, northeastward and east– southeastward directions are dominant. Locations 1– 4 comprise indicators from logs covering 100– 400 m of sediment thickness. For legend and location of the map, compare Figure 2.

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Petrography of the studied units Thirty-two thin sections from Lower Oligocene to Burdigalian sandstones have been analysed and reveal only minor changes in sandstone composition. The sandstones are moderately to well-sorted litharenites with a dominantly pseudosparitic matrix (classification of Pettijohn et al. 1987). Grains are subangular to subrounded and have a low sphericity. Most samples only show weak signs of compaction, and point contacts between grains dominate. Compaction features like bent micas and solution of carbonate grains are common in the Lower Oligocene to Lower Miocene strata, while rarer in Burdigalian samples. The lithic clasts are mainly limestone (10–20%) and chert (c. 5%), subordinately volcanic clasts. Micaschists/ gneiss and mud-/siltstones occur rarely. A rough estimate of the modal composition of the samples has been obtained by using visual estimation charts (Flu¨gel 1978; Tucker 1981). An

increase in the overall percentage of lithics implies decreasing compositional maturity for the Burdigalian samples. While the average composition of the Oligocene and Early Miocene sands is roughly Qt50F20L30 (Total Quartz – Feldspar –Lithics), the average composition of the Burdigalian samples is estimated to about Qt50F15L35. The modal compositions of the individual samples are compared to the compositional fields indicative for different provenance types using standard triangular diagrams (Dickinson 1985; Fig. 7). The analysed samples plot in the QtFL and QmFLt: diagrams in the ‘recycled orogenic’ and ‘continental block’ provenance category fields. A trend towards the ‘recycled orogenic’ field with decreasing rock age can be inferred, reflecting the decrease in compositional maturity. The abundance of sedimentary lithic grains (Ls, QpLvLs –plot) reflects a relatively short transport suggestive of a fold-and-thrust-belt setting, where

Fig. 7. Modal compositions of the analysed samples. The compositions and low maturity are suggestive for a foreland basin setting. The Burdigalian samples show a decrease in compositional maturity and an affinity towards fold-and-thrust belt sources, well comparable to their interpretation as molasse deposits.

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identified lagenids are typical for the Chechis¸ Formation (Early Burdigalian). The Burdigalian ages of the Hida Fm. clearly show that the onset of coarse clastic input in the southern part of the study area commences later than in the north. The microfossil assemblages indicate a general shallowing-up trend for the Hida Fm., while showing a transition from bathyal to offshore and marginal assemblages.

Subsurface data

Fig. 8. Sample locations and ages of micropalaeontological dating samples.

detritus from a recycled orogenic source is deposited (Dickinson 1985). The relative proximity to the source area is also reflected by the low degree of rounding of the constituent grains. The modal compositions of the analysed sandstones show an affinity to recycled orogenic source areas and a decreasing compositional maturity with decreasing rock age. These modal compositions as well as the abundance of calcareous lithoclasts (not included into the Ls pole, 10 –25%) are well compatible with a foreland basin setting (Dickinson 1985).

Micropalaeontological data Only few data are available regarding the age of the Hida Formation, therefore samples have been collected for microfossil dating. The preparation of samples followed standard procedures (Wissing & Herrig 1999). Sample locations are indicated in Figure 8, the results are shown in Table 1. All analysed samples yielded Burdigalian (in a few cases Early Miocene in a broad sense only) ages, based on foraminifera assemblages consisting mainly of benthic agglutinated and planktonic taxa. In two cases, a more exact age could be inferred. Sample 1119, collected below the base of the Hida Fm. (Fig. 8), yielded an age closer to the beginning of the Early Miocene, while sample 1125, collected from the basal Hida Fm., yielded a probable mid-Burdigalian age. The probable mid-Burdigalian age of the basal parts of the Hida Fm. (sample 1125) confirms the generally accepted Burdigalian age of the Hida Fm. The micropalaeontological assemblage found in this sample, together with the lithofacies associations at this locality (basal part log 7, Fig. 5), suggest a deep marine turbiditic environment. Sample 1149 yielded a Burdigalian age. The

In the southern part of the study area, the Upper Oligocene to Lower Miocene foredeep sediments are buried below the Middle to Upper Miocene sediments (Fig. 9) related to the evolution of Transylvania as a backarc basin (e.g. Ciulavu et al. 2002; Krezsek & Bally 2006). Here, these sediments rest on the Jurassic to Eocene deposits linked to the continental break-up, passive margin phase, subduction/obduction and continental collision of the Transylvanides ocean. In the north of the Transylvanian Basin, small patches of obducted ophiolites and/or Jurassic limestones are embedded in Cretaceous sediments deposited as pre-, syn- or post-tectonic sediments in relationship to the Albian continental collision (see also Sa˘ndulescu & Visarion 1977). The last pulses related to the active margin evolution were recorded during the Senonian and at the end of the Eocene, when the Puini Basin was inverted and finally thrusted (Fig. 9, De Broucker et al. 1998). Depth interpretations (well-calibrated 2D seismic data) in north Transylvania indicate an overall unconformity between the syn-Puini Eocene thrusting and subsequent (Upper) Oligocene to Lower Miocene strata. The most apparent feature is the Lower Miocene wedge onlapping over older strata, subsequently tilted during the Middle to Late Miocene events (Krezsek & Bally 2006). Although the overall wedge appears unitary, small internal reflector terminations (onlaps) suggest at least two pulses of loading in the foredeep (Fig. 9) prior to the deposition of the Middle Miocene Dej Fm. and Ocna Dejului Fm (salt). In map view, the Lower Miocene wedge strikes WSW –ENE. The deposits are up to 2200 m thick in the NE (Fig. 2). The orientation of onlaps inside the wedge indicates a sediment transport towards the SE, a direction compatible with the SE thinning geometry of the clastic deposits. The SE-directed sediment transport within the wedge, as well as the WSW– ENE oriented basin axis (Fig. 2), are in good agreement with the SE-directed emplacement direction of the Pienides (Tischler et al. 2006). The internal unconformities in the wedge are interpreted to reflect successive tilting of the basin floor.

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Table 1. Microfossil assemblages of the analysed samples and their probable environment. Compare Figs 8 and 10 for location of samples. Coordinates are in lat/long WGS 84 Sample No.

Y

Micropalaeontological assemblage

0554 Hida Fm.

24.055

47.4374

1119 Vima Fm.

24.0749

47.4571

1125 Hida Fm.

24.0582

47.4408

1126 Hida Fm.

24.0558

47.4362

1142 Hida Fm.

23.9497

47.4434

1149 Hida Fm.

24.0303

47.4145

1152 Hida Fm.

23.8648

47.197

Agglutinated foraminifera (Nothia robusta—very frequent, Bathisyphon filiformis, Rhabdammina discreta, Reticulophragmium rotundidorsatum, Reticulophragmium acutidorsatum). Flattened planktoni and other poorly preserved foraminifera (Catapsydrax unicavus, Globigerina eurapertura, Globigerina officinalis, Globigerina praebulloides, Globigerina wagner). Agglutinated foraminifera (Retigulophragmium acutidorsatum, Cyclammina bradyi, Rhizammina algaeformis, Karrerulina horrida) and rare, poorly preserved, planktonic foraminifera (Globigerinoides trilobus, and other indeterminable species). Flattened and poorly preserved planktonic foraminifera (Globigerina cf. anguliofficinalis, Globigerina cf. falconensis, Globigerinoides cf. trilobus, and other indeterminable species). Agglutinated foraminifera (Nothia excelsa, Reophax scorpiurus, Haplophragmoides cf. fragilis) together with fish bones and teeth. Agglutinated foraminifera (Rhizammina algaeformis, Ammodiscus miocenicus, Glomospira charoides) and calcareous foraminifera (Pyramidulina latejugata, Lenticulina arcuatostriata—probably reworked). Very rare calcareous foraminifera tolerant to salinity fluctuations (Ammonia div. sp.), reworkings from Cretaceous, fish bones and teeth.

Environment

Age

bathyal

Burdigalian (lower Hida Fm.)

pelagic

Probably base of the Early Miocene (upper Vima Fm.)

bathyal

Burdigalian (lower Hida Fm.)

pelagic

Probably Burdigalian (?base of Hida Fm.) Burdigalian (lower Hida Fm.)

bathyal upper bathyal

Burdigalian (lower Hida Fm.)

deltaic

Probably Burdigalian (mid to upper Hida Fm.)

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Fig. 9. Interpreted 2D seismic line (B– C), showing the wedge-shaped Burdigalian deposits. The shaded area correlates to the Burdigalian wedge. The two major unconformities within the wedge are interpreted as result of progressive loading and resultant flexure of the northern part of the Transylvanian Basin, acting as a foredeep. See Figure 2 for the trace of the section.

Comparable to the Lower Miocene wedge, the Oligocene deposits also show an increase in thickness towards the north, although with a WNW– ESE striking basin axis (De Broucker et al. 1998; Krezsek & Bally 2006). Hence, the following interpretation is put forward: Subsidence and wedging of Oligocene to Lower Miocene sediments coeval with the emplacement of the Pienides nappes implies that the clastic wedge represents the fill of the frontal foredeep of the Pienides (Fig. 10). Thrusting of the Pienide nappes led to progressive loading and flexure of the northern sector of the Transylvanian Basin causing the formation of a foredeep. This took place in successive tilting stages due to the flexural response of the loaded plate during southeastward emplacement of the Pienides (Fig. 9).

Summary It is likely that eustatic sea-level variations accentuated sedimentation during Late Oligocene to Burdigalian times. On the other hand, the study area only intermittently showed open marine conditions, and the progradation phases of sanddominated siliciclastics do not match the eustatic sea-level curve of Haq et al. (1987). Hence, eustatic

sea-level variations cannot exclusively account for the overall southeastward progradation of the sanddominated siliciclastics documented in the study area. An overall southward migration of subsidence followed by the deposition of coarse terrigenous clastics is documented in the study area. The onset of this deposition is of late Early Oligocene (Late Rupelian) age in the northern part of the study area, continuously younging to Burdigalian age at the northern rim of the Transylvanian Basin. The facies associations and their depositional trends are suggestive for an active margin setting as confirmed by compatible sandstone compositions. Bad exposure conditions and insufficient temporal resolution prevented the clear recognition of depositional sequences related to enhanced tectonic activity as apparent for later (Burdigalian) stages from the analysis of seismic lines (Fig. 9). Within the Burdigalian Hida Fm. a shallowing-up trend can be recognized, evidenced by changing micropalaeontological assemblages as well as by the transition from predominantly deep-water turbidites towards a succession locally showing fluvial– deltaic character. The southeastward thinning clastic wedge, as seen in seismic records from the Transylvanian Basin, shows an

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Fig. 10. Schematic geological cross-section A –B–C. The thrust front of the Pienide nappe pile, interpreted to cause the formation of the flexural basin, can be seen in the northernmost part of the section. The reverse faults in the middle of Section A– B (e.g. Ineu fault) are attributed to a later stage of deformation. The cross-section has been compiled by integrating information from available map sheets (Geological Survey of Romania), seismic data (Fig. 9) and unpublished data (C. Kre´zsek pers. comm. 2004).

internal geometry suggestive for a foreland basin fill (Fig. 9). Two important intra-Burdigalian unconformities indicate phases of tectonic activity, forcing the fan system to further prograde towards the SE. The geometry of the clastic deposits is suggestive for an elongate basin (WSW –ENE striking axis) with major clastic input derived from provenances from the NW. This geometry is consistent with the bimodal palaeocurrent distribution measured within the Burdigalian strata, showing NE directed (longitudinal) transport as well as SE directed current directions. The predominance of NE directed transport is interpreted as an effect of deposition within a WSW– ENE striking, elongated basin. Regarding source areas, there are two likely candidates. The first possibility is to derive the siliciclastic material from the SW, i.e. the shallower parts of the Transylvanian Basin. The second possibility is to derive the material from the approaching accretionary wedge of the Pienides situated in the NW. An argument against a derivation of the Oligocene siliciclastic rocks (Domains 1 and 2) from the SW is that Domain 3, situated in the SW, features muddominated deposits during this time. Therefore a source area in the NW is more likely. Regarding Burdigalian times, the SE thinning of the Burdigalian wedge is a powerful argument for a source area located in the NW. The most likely cause for the formation of the flexural basin is the emplacement of the Pienides,

as already suggested by Ciulavu (1998). Their SE-directed emplacement direction is in good agreement with the basin axis of the suspected flexural foreland basin, which is oriented roughly perpendicular. It seems highly likely that the Oligocene to Burdigalian clastic deposits at the northern border of the Transylvanian Basin are related to a continuously south to southeastward migrating flexural foreland basin. The initiation of convergence causing flexural foreland basin formation is indirectly dated by the onset of sand-dominated clastic input to Late Rupelian times.

The Pienide nappe emplacement and its foredeep in the regional context Convergence between the continental blocks ALCAPA and Tisza–Dacia during their emplacement into the Carpathian embayment finally resulted in the thrusting of parts of ALCAPA onto Tisza–Dacia (Csontos & Nagymarosy 1998). While the Pieninny Klippen belt originally represents the outermost rim of ALCAPA (e.g. Csontos & Vo¨ro¨s 2004), more external units such as the external Pienides have been accreted during the invasion of ALCAPA into the Carpathian embayment (e.g. Fodor et al. 1999). In the final stages of the juxtaposition of these continental blocks, the easternmost tip of ALCAPA (i.e. the

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Fig 10. Continued. Pienides) has been thrust onto Tisza –Dacia (e.g. Csontos & Nagymarosy 1998). The formation of the Burdigalian flexural foredeep and its wedge-shaped clastic infill are highly likely to be related to the coeval last stages of Pienide nappe emplacement (e.g. Ciulavu et al. 2002). At the northern rim of the Transylvanian Basin, the Oligocene-age onset of turbidite deposition at a high rate suggests an earlier stage of thrusting of ALCAPA onto Tisza –Dacia as already proposed by Gyo¨rfi et al. (1999). Hence, the interpretation is put forward, that the Late Rupelian to Burdigalian flexural foredeep development and subsequent deposition of coarse-grained siliciclastics reflects a continuous process resultant of the convergence of ALCAPA and Tisza –Dacia. The Late Rupelian onset of extensive sanddominated sedimentation in the study area suggests a coeval timing for the convergence of ALCAPA and Tisza –Dacia across the northeasternmost segment of the Oligocene Mid-Hungarian fault zone (i.e. the Pienide nappe contact). The change in strike of the respective basin axes (Oligocene: E –W to SE –NW, De Broucker et al. 1998; Burdigalian: WSW– ENE, Fig. 2) is likewise interpreted as a continuous development. Palaeomagnetic studies indicate (e.g. Panaiotu 1998; Ma´rton et al. 2007) that Tisza –Dacia underwent significant clockwise rotation after the Oligocene, i.e. during emplacement into the Carpathian embayment. Such a clockwise rotation of Tisza –Dacia during ongoing flexural foreland basin formation could result in a passive turning of older basin axes (Fig. 11), while the younger basin axis remains in its original orientation.

Figure 11 summarizes the proposed model for Oligocene to Early Miocene times. Initial convergence between ALCAPA and Tisza– Dacia in Late Rupelian times results in the formation of a flexural foredeep on Tisza–Dacia being in the lower plate position. The overriding plate, ALCAPA, constitutes the source area for sanddominated siliciclastic deposits, which are shed into an initially W –E to NW–SE (present-day coordinates) trending foredeep. During further convergence, Tisza– Dacia rotates in a clockwise sense, which results in a progressive migration of deformation southwards (present-day coordinates). By this migration of deformation, the loci of topography and foredeep formation also move towards the south, promoting longitudinal transport in the foredeep basin. The rotation of the lower plate results in a passive rotation of the foredeep sediments. As convergence continues, intervening units are accreted to ALCAPA. The last increment of convergence between ALCAPA and Tisza– Dacia results in the formation of the Burdigalian Basin striking WSW–ENE in present-day coordinates. Successive filling of this basin is demonstrated by the general shallowing-up trend of the sedimentary record. The authors are indebted to M. Sa˘ndulescu for his support and fruitful discussions during the project. M. T. is particularly grateful to S. M. Schmid and L. Csontos for the careful and constructive review of a first version of the manuscript. C. Kre´zsek is thanked for very stimulating discussions and his contagious enthusiasm. C. Kre´zsek and D. Ciulavu are thanked for their constructive and careful reviews.

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Fig. 11. Schematic block diagram of the proposed tectonic setting causing the development from an E–W to NW–SE trending Oligocene basin axis towards a WSW–ENE trending Burdigalian Basin axis. Rotation of Tisza–Dacia during convergence with ALCAPA results in a migration of shortening as well as a passive rotation of the older basin axes. Financial support by the Swiss National Science Foundation (NF-project Nr. 21-64979.01, granted to B.F.) is gratefully acknowledged.

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