Record of Late Eocene Continental Escape of the Bakony Unit

695

GeologischeRundschau81/3 [ 695-716/ Stuttgart1992

Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary): Record of Late Eocene continental escape of the Bakony unit

By L. FODOR, A. MAGYARI, M. KAZMI~R and A. FOGARASI, Budapest*)

With 15 figures

Zusammenfassung demonstriert den Deformationsstil innerhalb des ab- wandernden Blockes eng an der stidlichen Grenz- Die obereoz~ine Folge der Budaer Berge wird aus zone. Die tektonisch kontrollierte Sedimentation un- fluviatilen und Abtragungskonglomeraten, Sandstei- terstreicht, dass die Ausquetschungstektonik bis in hen, bioklastischen Flachwasserkalken, Mergelge- das sprite Eoz~in hinein aktiv war. steinen sowie pelagischen Globigerina-Mergelnauf- gebaut. Die Abfolge weist eine rapide, im gesamten Abstract Gebiet vonstatten gehende Absenkung von kontinen- taler Entwicklung bis zu bathyalen Tiefen nach. The Upper Eocene sequence of the Buda Hills con- Die Sedimentation tritt auf Pal~ioabhfingen an den sists of fluvial and shallow marine conglomerates, Flanken von synsediment~iren Antiklinalen auf Ge- sandstones, bioclastic shallow-water limestone, marl- steinen des Basements auf. Die Hebung der An- stone and pelagic Globigerina marl. The succession tiklinalen verursachte eine progressive Kippung der illustrates rapid, overall subsidence of the area, from Sedimente eine schichtparallele Dehnung, durch terrestrial environments to bathyal depths. Sedimenta- Boudinage und Verwerfungen und induzierte eine tion occurred on slopes situated on the flanks of Umlagerung dutch Massenstr6me. Die Antiklinalen synsedimentary basement antiforms. Vertical growth sind staffelf6rmig in der rechtsdrehenden Scherzone of antiforms caused progressive tilting of beds, layer- von Buda6rs und in der verschuppten Brandungs- parallel extension by boudinage and faulting, and in- s~iule yon Buda angeordnet, die die Verwerfung be- duced redeposition by mass flow. Antiforms are local- gleiten. Die Zone von Buda korrespondiert zu einem ised in the dextral Buda/3rs shear zone and in the Buda SE-vergenten komplexen Abhang. Es liegt eine imbricate stack, which accommodated the dextral dis- Unterlagerung von antithetischen Blindverwer- placement. The latter is underlain by blind reverse faults fungen vor, die vermutlich in geringer Tiefe in eine probably merging into a detachment tault at shallow Abscherungsverwerfung/dbergeht. Alle Strukturen depths. These structures were formed by WNW-ESE wurden durch WNW-ESE gerichtete Kompressionen oriented compression and NNE-SSW directed tension. und NNE-SSW orientierte Dehnungen geformt. The morphological expression of the imbricate stack is Die Einheit des Bakony war w~ihrend der Ab- the SE-facing Buda slope. trennung von den Alpen umgrenzt dutch eine n6rdli- The Bakony unit, while "escaping" from the Alps, che sinistrale und eine siidliche dextrale Scherzone. was bordered by a northern sinistral and a southern dex- Die synsediment~ire Tektonik in den Budaer Bergen tral shear zone. Synsedimentary tectonics in the Buda

*) Authors' addresses: L. FODOR, A. MAGYARI,A. FOGARASI,Department of Geology,E/Stv/Ss University, Mfizeum krt. 4/A, H-1088 Budapest, Hungary,and M. KAZMt~R,Department of Palaeontology,E{3tv/Ss University, Ludovika tdr 2, H-1083 Budapest, Hungary Present address: L. FODOR,Department of Applied and EngineeringGeology, E6tv6s University,Mfizeum krt. 4/A, H- 1088 Budapest, Hungary. Manuscript received: 19.1.91; accepted: 13.7. 1992 696 L. FODOR, A. MAGYARI, M. Kg,ZMI~R, A. FOGARASI

Hills demonstrates the style of deformation inside the qbyH~IaMeHTa. HO/IH~ITHe aHTHKJIHHaaefl BbI3BaJIO escaping block, close to the southern border zone. nporpeccrmHoe HaKJIOHeHHe C0IOeB, paCT~t>KeHHe, Tectonically controlled sedimentation suggests that es- OWMe,-IaeMoe 6y)IHHa>KeM H pa3:IOMaMI4, H nepe- cape tectonics was active as early as Late Eocene time. OT0m:,Ir ~3emecTBa c HOTOKaMH MaccbI. AHTg- Ir HpOCTHpaloTC~ B HpaBocTOpOHHefI ByaaepmcKoh 3OHe CaBHrOB ByalaficI~ofI ~Iemyfe4a- R6sum6 TOfI 06JmCTH, B KOTOpO~I HaKOnHJIC~t MeOlaH:,K. ByjIa/acKa~ 30Ha COOTBeTCTByeT CJIO>KHOMy La s6quence sddimentaire d'fige 6oc~ne sup6rieur cKJIOHy, o6pameHHOMy Ha loPO-BOCTOK. OHa rIO~I- des collines de Buda est constitude de conglomdrats CTmiaeTc~ CKpMTbIMH B36pocaMH, BepOgTHO HpH- fluviatiles et matins, de gr&, de calcaires na~HHbIMI4 K 3OHe cpbIBa Ha ue6oJIbnlOfI rJIy6HHe. bioclastiques et de mames n6ritiques ou pdlagiques h Bce aTH cTpyKTyp~,I o6paaoBaamcl, npH C*aTHH U Globigerina. Cette succession correspond h une npocwHpanm~ WNW-OSO u paCT~:~eHHH B aa- subsidence rapide, r6gionale, passant d'un environ- npaBJIemm NNO-SSW. nement terrestre fi un environnement bathial. La BaKOHBCKI~Ie cTpyKTypHble aJIeMeHTbI, BbI)KH- s6dimentation s'est effectu6e sur les pentes situ6es MaeMble H3 AOIBII, @blJIH orpaHHqeHbI JIeBBIM sur les fiancs des anticlinaux synsddimentaires. Le C~BHFOM Ha ceBepe vt IlpaBHM Ha lore. CHHCellI/I- souRvement de ces anticlinaux a provoqu6 le MeHTaKHOHHa~I TaKTOHHKa B ByJIaflcK~x ropax basculement progressif des couches, une extension JIeMOHCTpHpyeT CT~IaI~ ~IeqbopMalmH BWTpU paralRle h la stratification avec boudinage et fractu- BbDKFIMaeMoFO 6aIOKa, pacIloJIO)KeHHOFO B~JIH3H res, ainsi que le remaniement de sddiments par divers ~O>KHOfI rpaHgKbI. OcatlKOHaKOII.neHHe, KOHTpOOIH- 6coulements gravitaires. Les anticlinaux se situent pyeMoe TeKTOHHKOfI, CBHJIeTeJIbCTByeTO TOM, qTO dans la zone d6crochante de Buda6rs dont le ddplace- BbI:,KI/IMaHHe gMealo MeCTO yme B IIO3~IHeM aoKeHe. ment dextre a 6t6 accompagn6 par un syst~me d'dcailles: la zone de Buda. Cette zone correspond une pente complexe sddimentaire h vergence SE. Elle Introduction est caract6ris6e par des failles inverses aveugles qui se raccordent probablement h faible profondeur h une Continental escape is a common process in orogenic surface de ddcollement. Toutes ces structures ont dt6 belts (TAPPONNIER, 1977; BURKE & SENGOR, 1986). engendrdes par une compression WNW-ESE et une Brittle deformation is often concentrated at the bound- extension NNE-SSW. ary of the escaping wedge, but faults formed in the same Durant l'dchappement continental, l'unit6 de kinematic system cross-cut the internal part, too. Bakony 6tait bord6e au nord par une zone cisaillante Tectonically controlled sedimentation, if connected to s6nestre et au sud par une zone cisaillante dextre. La the internal or border fault zones, may offer excellent tectonique synsddimentaire dans les collines de Buda temporal constraints to the motion history of the faults montre le style de la ddformation h l'intdrieur du bloc (e.g. HEMPTON & DUNNE, 1984; TAPPONNIER et al., s6par6, pros de sa limite m6ridionale. La sddi- 1982; SENGOR et al., 1985). mentation, rdgie par la tectonique, indique que The Bakony Unit, the earliest recognized of the es- l'6chappement continental avait ddjh commenc6 h caping wedges of the Alpine-Pannonian region l'Eoc~ne supdrieur. (KOv~,cS, 1983) was forced out between the Eastern and Southern Alps during Tertiary time. Detailed structural analysis and kinematic modelling describes the KpaTKOe coRep~Kaune Neogene escape history and related fault pattern (RATSCHBACHER et al., 1989; CSONTOS et al., 1991). BepxHeaolaeHoBas To:itaa By~IaflcKHx top npeJI- The idea of Paleogene motion is based mainly on dis- cTaBaleHa a6pa3HOHHbIMg KOHrJIoMepaTaMH, placed Permian-Oligocene facies boundaries and kin- HecqaHHKaMH, 6HOO~YlOMOqHBIMH FI3BeCTI-I,qKaMH~ ematic analysis (Fig. 1; KAZMt~R, 1984; KAZMI~R & MepreJIgMH H neJIaFHqeCKHMH FJIO6eFepI/IHOBblMH KOVr 1985; and BALLA, 1988a, FODOR et al., MepYeJI~lMH. ~Ta HOC01eTIOBaTe0IBHOCTb ocaJIKOB 1990a). In fact, kinematic models demonstrate that the OTo6pamaeT 6blCTpOe Bceo6tlIee npocelIaHHe Bakony Unit is only a part of the escaping North pafioHa OT BepxHero ypOBH~ KOHTFIHeHTa JIO 6a- Pannonian block (Fig. lb; BALLA, 1988a, b). It is gen- THaJIbHblX rJIy6ldH. HaKoIIaeHHe oCa~KOB npogcxo- erally accepted that during the Oligocene, dextral dis- ~HoIO Ha JIpeBHHX CKOIOHaX, paclIO0~O>KeHHblX Ha placement was already going on along the southern qbJIaHFaX CHHCe/IHMeHTaLIHOHHblX aHTHKalHHaJIeI4 border fault zone (Periadriatic and Mid-Hungarian Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 697

O lOOkm I I

/ ",~

PAL"""\") O lOOkm t

Fig. 1. Continental escape of the Bakony unit (a) Triassic palaeogeography: the facies boundary (dashed line) between Main Dolomite (oblique haching) and Dachstein Limestone (vertical haching) is displaced to the east (after KAZMI~R&; KOVA,CS, 1985). (b) Kinematic analysis: the Bakony unit is part of the escaping North-Pannonian Block (NPB) which is bordered by sinistral and dextral strike-slip zones (vertical and oblique haching, respectively; after BALLA, 1988b), Tectonic units are shown in actual positions. PAL: Periadriatic Lineament; PK: Pieniny Klippen Belt; M-H Z: Mid-Hungarian Zone. Solid triangles indicate the external boundary of the Carpathian fold belt.

zone, BALLA, 1988a; RATSCHBACHER et al., 1989). eroded, partly covered by younger sediments and were The possibility of Eocene movement is generally ne- deformed by subsequent tectonics. glected, or can only be suggested without relevant tec- In the present study we discuss the sedimentology tonic and/or sedimentological data. and tectonics of the eastern part of the Transdanubian After an extended period of erosion and bauxite Central Range (referred to as Bakony, after the largest deposition during the Early Paleogene, marine sedimen- topographic unit), the Buda Hills. Here original Late tation started in the Middle to Late Eocene in the Ba- Eocene palaeogeography and structures can be recog- kony Unit (BALDI-BEKE, 1984; MINDSZENTY, 1984). nized and separated from the effects of younger tectonic Sediments were deposited in newly formed, separate events. We describe the kinematic and dynamic char- basins. Abrupt facies changes in depressions, occur- acter of tectonic structures and their effect on sedimen- rence of gravity-displaced deposits, rapid subsidence tation and provide evidences for Late Eocene escape and a rhombohedral outline of the sedimentation area movement of the Bakony Unit. led B~LDI & BALDI-BEKE (1985) to suggest a strike- slip-related origin for the basin system. The assumed right-lateral shear zone was attributed to continental Geological settings escape (ROYDEN & B~d~DI, 1988). Nevertheless, nei- ther original paleogeography, nor Eocene fault patterns Part of Buda Hills is made of Middle to Upper have been described, therefore, even the right-lateral Triassic dolomites and limestones. These rocks were sense remains doubtful. In fact, these features can slightly deformed during the Cretaceous, the main struc- hardly be recognized, since the basins have been partly ture of this phase being a NW-SE trending syncline 698 L. FODOR, A. MAGYARI, M. KAZMt~R, A. FOGARAS1

Fig. 2. Geological map of the Buda Hills (after WEIN, 1977, simplified). Location is in the black quadrangle of Budapest on Fig. 1. Stars and numbers correspond to figures, a-a', b-b" and c-c" are sections in Figs. 11 and 12, respectively. Hills: R. Rdka, K.: Kevdly, NSZ: Nagy-Szdnfis, H.: Htirmashat~_r,J.: Jfinos, O.: Odvas, M.: M~ity~s,G,: Gelldrt Hill. NA.: Nagykovficsi Basin. Gravity-flow dominatedsedimentation on the Buda paleoslope (Hungary) 699

(WEIN, 1977) (Figs. 2, 3). This Triassic basement and particularly, bryozoans suggest deep neritic depo- syncline is cut by Late Cretaceous lamprophyre dykes sitional environment (KAZMER, 1985; SZTRAKOS, (KUBOVICS et al., 1989). 1987). The bryozoan marl is followed by thin-bedded A long interval of erosion was terminated by the marl and laminated argillaceous marl (Buda Marl). It deposition of (Cretaceous?)-Palaeogene bauxites, contains abundant planktonic foraminifers, nanno- Middle Eocene coal measures and overlying limestone- plankton and ostracods of Late Eocene-earliest marl sequence (SZOTS, 1956), occurring mainly in the Oligocene age (B~LDI-BEKE, 1972; BALDI et al., 1984; northwestern Nagykovficsi depression. The Late MONOSTORI, 1987; SZTRAKOS, 1987). These fossils Eocene sequence starts with sandstones, conglomerates suggest a bathyal origin for the sediments (BODA 8z and breccias of alluvial and shallow marine origin (Fig. MONOSTORI, 1972; BALDI, 1986). The Buda Marl is 3). Clasts were derived from local basement rocks, or overlain by the Lower Oligocene laminated Tard Clay, have been transported as much as 30 km by rivers or formed in an anoxic environment. It is followed by the longshore currents (HORVATH & TARI, 1987). pelagic Kiscell Clay of Middle Oligocene age. The This series is overlain by shallow marine limestones shallow marine quartzose Hfirshegy Sandstone and con- containing red algae, large foraminifers (Nummulites, glomerate is synchronous with the uppermost Tard Clay Discocyclina), echinoid fragments, molluscs and corals and the lower Kiscell Clay (BALDI, 1986). The Lower (MONOSTORI, 1965). These represent variable shallow Oligocene sequence is capped by neritic Upper water carbonate environments associated with moder- Oligocene sandstones, followed by Early Miocene ate subsidence (KAZMER, 1985). The limestone gradu- coarse clastics, Middle Miocene carbonates, Upper ally passes into a bioclastic calcareous marl, the Miocene sandstone, Pliocene fresh-water limestone and "bryozoan marl". Its fossil content: echinoids, molluscs Quaternary deposits (WEIN, 1977).

Ill

| L. III Kiscel[ Clay I ~. ~.'~ o0 -- -- pl ' / H~rshegy Sandsfone ~~~ Tard (:lay I ~ ~ ~/~ ~"~~ ---- ~ ~ Buda Mart .~_ _-"Bryozoo Mart" W __ __

U m0 ~ k~_lw1413~\A%~o,o'o.o.'o3

sand • cLcLy sandstone ~ bauxite co[coreous ~ (cherty) I.imesfone, sondsfone (cherty) do[omife Z~ shallow wafer cong[omerofe [imesfone with reefs breccia ~'~--] [amprophyres Fig. 3. Stratigraphiccolumn of the Buda Hills. (a) after Wein (1977) and BALDI(1986), (b) detailedcolumn of the Upper Eocene sequence. 700 L. FODOR, A. MAGYARI, M. KAZMI~R, A. FOGARASI

(~~ O. l m

, o o ~176Oo~ -'~~176:~ ~ co: ~", ,- -,. ~-.,.,, j.,,?-~:_%..?, -.,t~ ,\ ,.' d "-~ \x so ~. = I" -~~K-.,- ."~.'~ ~ . t,,,, x. ~ e-.\x, -' c~o%'-.. "-...~,:\%

/33. ~ t',-~" / J ~~-K-"~--~=-~-. "':~-~.~,,~t~l"-.-~ ----,',',',~--_--=="------'\-:'-', ,, ~ \ ", "- I ",

55 --- M-, >o. o'-~ /7~, -~ NNW 55E

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I1 .~" o ,5..,:..~ .... ;'g~:po~W, o ..... 38 o.o . o~.~ ;%. c~ o " "":" :::':"::':':'::::" >":' }~:::.

oo..,:~-~ .... ,...!:!:77!:,.:~};;i;,)ij}7~i~?izT~i:.~:)?~.;:i)ii)i.;;:i,!,i::!:ii~;!.7~;U.. :::~:

#5 c i

(m.) ar~lstor~ r'l').

#0 N S

fp~eccia

(m ~,::~;::;~::,: ) , , , sandslone wedge ctey send 3 m silt pebble

Fig. 4. Redeposition and soft-sediment deformation in the Upper Eocene sequence. (a) Redeposited conglomerates and calcareous turbidites on the Martinovics Hill (FOGARASI, 1991). m: matrix-supported, c: clast-supported. (b) Parallel alignment of Discocyclina tests due to soft-state shearing and boudinage in semi-lithified lime mud (Martinovics Hill). Arrows indicate the initial stage of the necking of boudins, (c) Soft-sediment deformation in conglomerate and calcareous sandstone (Rdka Hill). Note that faults are dying out below the top of the thick conglomerate layer. Faults in the conglomerate are truncated. Thin coal seams embedded in the sandstone were disrupted during redeposition or soft-sediment deformation. (d) Synsedimentary slump fold on the Odvas Hill (after MAGYARI, 1991b). Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 701

Early Oligocene palaeogeography is characterized Soft-sediment deformation by two different regions separated by the NNE-SSW trending "Buda Line", marked in the field as a zone of Deformation phenomena affecting unconsolidated, intense silicification of the HArshegy Sandstone (BA.LDI or semi-consolidated sediments are here termed "soft- & NAGYMAROSI, 1976). West of the Buda Line, earli- sediment deformation". The unlithified state of est Oligocene erosion was followed by shallow marine sediments is suggested by: (1) plastic extension and deposition of the pure quartzite Hfirshegy Sandstone. thinning of layers; (2) internal reorientation of grains To the east of the line, the laminated, pelagic Tard Clay, due to loading and plastic shearing. then the similarly pelagic Kiscell Clay accumulated in a Features produced by layer-parallel extension are rapidly subsiding basin. Recognizing the lack of similar in appearance to tectonic boudinage: one of the inteffingering formations BALDI (1982) suggested that layers thins and pinches out while the overlying and the sharp facies boundary corresponds to a single strike- underlying ones preserve their thickness and continu- slip fault. BALLA & DUDKO (1989, p. 25), while ac- ity. Sometimes the lenses formed in this way are con- cepting the existence of this facies boundary, empha- nected to each other by narrow necks. If the displace- sized that a tectonic line of this trend cannot be observed ment was large enough, the high-viscosity layer disin- on the surface. tegrated into boudin-like lenses or angular clasts. The The westward termination of the pelagic Buda Marl latter resembles the features described by WALDRON et during Late Eocene lies at the Buda Line; therefore, the al. (1988) as "chocolate tablet" boudins formed by ex- palaeogeographic pattern recognized for Oligocene tension fractured boudinage (FOGARASI, 1991). Lenses sediments was probably established as early as the Late consist of massive bioclastic limestone, while the ma- Eocene. Describing Late Eocene facies pattern and tec- trix is clayey limestone. There is no distinct surface, tonics, we intend to determine the nature of "Buda but a thin (0.1-1 cm) transition zone between the lenses Line", the most important facies boundary in the Buda and their matrix, suggesting a rather limited mixing of Hills. highly cohesive masses. Around the lenses and thin sandy intercalations of conglomerate, closely spaced anastomosing planes oc- Sedimentary features cur parallel to bedding, or slightly oblique to it. Foraminifers are oriented parallel to these planes, in Resedimentation contrast to massive lenses, where forams are not oriented and more abundant (Fig. 4b). Fossils were prob- Most beds were deposited by a variety of sediment ably rotated parallel with each other during the defor- gravity flows. Conglomerates at the bottom of the Up- mation of the soft sediment. per Eocene sequence are usually massive, and rarely The geometry of anastomosing planes resembles poorly bedded; pebble orientation is usually random, extensional crenulation cleavage fractures (PLATT & rarely subhorizontal, or weekly imbricated. Some lay- VISSERS, 1980). In spite of the different physical cir- ers show inverse-to-normal grading. All features sug- cumstances, we interpret this structure as a result of gest deposition by high viscosity debris flows. Con- shearing of highly viscous, almost lithified sediments. glomerate and sandstone strata embedded in the Buda Lenses contoured by these shear planes represent shear Marl are poorly to well-stratified, well-sorted, and rarely boudins (WALDRON et al., 1988). Bothtype ofboudins show weak normal grading; probably these were depos- are products of progressive bed attenuation and reflect ited by grain flows (Fig. 4a). Limestone shows distinct layer-parallel extension due to slow downslope creep- soft sediment deformation caused by slow downslope ing of the highly viscous material. creep (Fig. 4b) (FOOARASI, 1991). The Buda Marl con- In some outcrops we observed small-scale faults, tains alternating turbidites with A-B-(C) and (C)-D-E which are clearly restricted to one single layer. Other divisions of the Bouma sequence. Beds consist of structures can hardly be called faults, since there are no bioclasts derived from shallow-water carbonate distinct surfaces but only a thin, plastically deformed depositional environments (BODA & MONOSTORI, zone (Fig. 4c). Both features formed by synsedimentary 1972; VARGA, 1985; BALDI, 1986). or soft-sediment deformation, probably related to This short description shows that high viscosity redeposition of the sediment. flows were deposited in the lower part of the series, Close to steep erosional surfaces layers are deformed while less viscous flows containing more water and dis- to a steeply inclined position. Soft-sediment deforma- integrated grains characterize the upper part. This evo- tion frequently occurs in the buckled segments of lay- lution reflects growing transport distance of gravity ers, indicating that bending partly took place before flows in a gradually subsiding and enlarging basin. lithification. 702 L. FODOR, A. MAGYARI, M. K,dZMgR, A. FOGARASI

Slump folds have been described in sediments of embedded in fluviatile sandstone/siltstone (Fig. 5a). different grain size, ranging from marls to coarse The scene might have been similar to the rockfall pho- breccias (FODOR & KAZMt~R, 1989). At Odvas Hill one tographed by MARTINIS (1977) after the Friuli earth- particular fold occurs in a sandstone layer sandwiched quake in Italy: large boulders fallen from adjoining hill- between a lower conglomerate and an upper breccia bed sides are scattered in the alluvial plain of the (Fig. 4d). The lamination of the pebbly, silty sandstone Tagliamento river. shows deposition by traction currents. Both thickness At Odvas Hill, a 5 m thick, massive, clast-supported and dip of laminae gradually increase downslope and breccia layer covers Eocene conglomerate beds. The from top to bottom, respectively. Curvature of the fold closely spaced angular clasts were only slightly disin the lower conglomerate is larger than in the upper placed from each other; obviously, an originally frac- breccia. We suggest that the deposition of the sandy tured rock body slid into the sea, while its components wedge and gradual steepening of the laminae were con- suffered only minor rearrangement. The fall of these temporaneous with the growth of the fold below. bodies was probably initiated by seismic shocks, which In the upper part of the conglomerate the otherwise fractured elevated basement rocks (TARI, 1989, pers. randomly oriented pebbles have been stated parallel to comm.; MAGYARI, 1991 b). bedding (Fig. 4d). Both this plane and the sandstone !ayer undulate. The formation of this "pebble train" and the undulation took place in cohesive sediment due to Decrease in dip uneven loading of the overlying bed (SURLYK, 1989, pets. comm.). "The loading of the overlying breccia One of the most striking features of the sediments is could induce the escape of water, indicated by the the remarkable range of dips from 5 to 65 ~ Part of this subvertically oriented clasts (Fig. 4d). In other locali- variation must be synsedimentary because the dip of ties pebble trains foiTn asymmetrical rarely overturned Eocene strata is often steeper than the dip of the under- folds; there, the sediment was not only loaded, but lying Triassic basement. As a general rule the dip de- dragged by the slowly creeping overlying layer. We creases from the bottom to the top of the sequence. suggest the name "pebble fold", or "load fold" for these Upward decrease is clearly visible in the field: in a small features. fan on Odvas Hill the dip gradually decreases from 45 ~ to 15 ~ (Fig. 5b), while on Ut Hill limestone beds dip- Tectono-sedimentary observations ping at 45 ~ have been eroded and covered by marl dipping at 20 ~ (BALDJ et al., 1983; FODOR & KAZMER, Rockfall 1989). Each case indicates continuous, or episodic tilting of strata during sedimentation. In other cases tilting Near J•nos Hill, a single layer of subrounded (deformation) of beds seems to be partly post-sedimen- Triassic dolomite boulders, up to 2 m in diameter are tary.

E W 2~,%,. sandstone

NW SE

N~dolomlte~oou: tiers ~ ~_ lom

Fig. 5. (a) Earthquake-induced (?) fall of dolomite blocks into sandy silt near the Jeinos Hill (,drawn by G. TARI). (b) Schematic cross section of a small conglomerate fan at the Odvas Hill. Gradually decreasing dip suggests synsedimentary tilting of layers. For location see Fig. 2 (after MAGYARI, 1991b). Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 703

Fault-bounded talus cones Neptunian dykes

At M~ityfis hill a small, semicircular breccia-con- Many Late Eocene sedimentary dykes cut the under- glomerate talus cone is bordered by a fault (Fig. 6). Its lying rocks. These are 1 to 120 cm wide, tens of metres diameter is less than 120 metres; nearer to the fault the long and are filled by breccia, conglomerate, sandstone, basement is directly overlain by bioclastic limestone or marl; many. of the infillings were later silicified. (JASKO, 1948). The conglomerate body bears no inter- Sediments not only filled some dykes, but were injected nal structure, but contains thin sand/silt intercalations, into the fissures of the host rock by overpressured fluid. one of them capping the cone. These acted as slide The injection might have been induced by the tectonic planes, as shown by a slump body overlying the sand- movement itself which opened the dykes (e.g. BORDET stone. Bioclastic limestone displaying a conspicuous et al., 1982). Dykes are mostly oriented WNW-ESE sheared appearance, with boudins, overlie both the cone (MAGYARI, 1991 a). They represent tension gashes and and the Triassic basement. These observations indicate permit the reconstruction of the stress field. that the boundary fault grew contemporaneously with sedimentation. The bulk of the displacement took place Palaeomorphology prior to the deposition of the limestone. The fault is nearly vertical and shows apparent reverse displace- Stepped erosional surfaces are found below the up- ment. The dip of the overlying limestone is rather ex- per Eocene sequence; a fine example was found on the cessive and can hardly be original, so we suggest that Rdka Hill (for location see Fig. 2). One can differenti- the whole block (containing the fault) has been tilted. ate a nearly horizontal upper "plateau" and a lower steep Restoring the possible tilt (20-30~ the fault apparently slope (Fig. 7). Eocene sediments are generally eroded displays normal displacement. However, the steep dip from the plateau and, if they are present, their thickness and some microfaults in the surrounding Eocene lime- is reduced; the sediments do not show redeposition and stone suggest that it may also have a dextral strike-slip soft sediment deformation. Basal clastic layers are of- component. ten missing; coral patch reefs and algal mounds are directly located on the basement. A relatively elevated

NNW SSE

0 2m

Fig. 6. Cross-section of a fault-bounded breccia talus cone on the Mfityds Hill. The breccia-conglomerate cone is separated by sheared siltstone. The overlying limestone bears features of boudinage and shearing in a semi-lithified state. 704 L. FODOR, A. MAGYARI, M. KAZMI~R,A. FOGARAS[

PLATEAU SSE post-Eocene faults NNW ,, ~, sandstone, conglomerate /I "/ .... ;

5m I ~;erosional co ~,' surface eocene caves SHEARING

limestone ..... ,, f~~eroded eocene fault scarp EXTENSION DRAG

REDEPOSITION

Jj xl FOLDING Fig. 7. Late Eocene palaeoslope on the R6ka Hill. Note that faults controlling the slope propagated through sediments after Eocene time. position of the plateaux is indicated by pre- and Tertiary (BERGERAT, unpublished data; BERGERAT et synsedimentary karstification and unconformities (Fig. al., 1984; and own measurements). The younger one 7) (NADOR, 1992). corresponds to ESE-WNW to SE-NW extension, the Considerably thicker sequences occur on the steep maximum stress axis being vertical or horizontal (Fig. part of the erosion surfaces. On the R6ka Hill, most of 8). In this stress field, inherited fractures trending from the layers pinch out toward the steeply-dipping erosion NNW-SSE to ENE-WSW were reactivated as normal surface. The lower sandstone and conglomerate layers or normal-oblique faults (Fig. 8b). NW-SE to N-S were tilted and deformed into a steep attitude. The dip trending dextral, and ENE-WSW oriented sinistral of the overlying limestone is less significant, indicating strike-slip faults were associated with normal faults the synsedimentary character of the tilting. All obser- (Fig. 8a). Reverse faults or folds occurred locally, e.g. vations suggest that the topographic difference between at the edge of Gell6rt Hill (BALLA & DUDKO, 1991). the plateaux and slopes increased gradually. The soft Synsedimentary tension joints and faults observed on sediment was often redeposited by different types of Tdt6ny Plateau, at the southern vicinity of the Buda Hills gravity mass flows. Redeposition was associated with prove that the deformation is of Middle Miocene age soft sediment deformation during downslope move- (Fig. 8c) (BERGERAT et al., 1983; PALOTAS, 1991). ment (Figs. 4c, 7). Faulting affected lithified Middle Miocene limestone The steep, eroded surfaces represent the upper part and shows the persistence of tectonic activity during of a fault scarp or a folded-tilted part of an originally Late Miocene and Pliocene times. more gentle slope. Some of the steep palaeoslopes may The older stress field was represented by WNW- have formed prior to sedimentation, but their activity ESE to NW-SE compression, and NNE-SSW to NE- during deposition is beyond doubt. SW extension. Dextral, and sinistral strike-slip and Post-Eocene faults often occur along the slopes, ei- normal faults trend E-W, NNW-SSE and NW-SE, re- ther by propagation of buried Eocene faults, or by frac- spectively (Fig. 9). NNE-SSW to ENE-WSW tren- turing along pre-existing discontinuities, for example, ding elevated areas can be considered as compressional basement-cover contacts, or steeply dipping layers (Fig. ridges. Parallel reverse faults and folds are rarely 7). shown on geological maps (e.g. WEIN, 1977), or described by earlier authors (citations in BALLA & Tectonic observations DUDKO, 1991). Microfaults affect Triassic, lithified Eocene, and Microtectonic measurements reveal the existence of Lower Oligocene rocks (Hfirshegy Sandstone and Tard two main stress fields which affected the area during the Clay). While the latter rocks were deformed during the Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 705

Late Oligocene-Early Miocene, microfaults in the (MAGYARI, 199 la, b). Similarly oriented sedimentary Triassic and Eocene may have an Early Oligocene, or dykes are filled with Hfirshegy Sandstone (middle even Eocene age. Synsedimentary structures demon- Lower Oligocene) in the southern part of the Buda Hills. strate the persistence of this stress field during Late WNW-ESE oriented chalcedony dykes (tension Palaeogene. First, we refer to Late Eocene neptunian gashes) fill the Hfirshegy Sandstone (Fig. 9) (B~,LDI & dykes, which are parallel to the direction of compres- NAGYMAROSI, 1976). These dykes are associated with sion (Fig. 10) and can be considered as tension fractures silicifying hydrothermal solutions which did not affect ("tension gashes") formed in this stress field Middle Oligocene Kiscell Clay. Their age is latest Early

| @ N

--~gZ

/ f ,, ..IL ) ~k,,, .~. H ,/ ,',., / . /,' ). "~--A~, J "~ "%" "

normo[ four

'~ ~ s~e~ ~ w~6~6~s strike-slip four

reverse foulf, ~ 6' 4 ~,q[ I=,..- 6"3 monoc[ine "~ 4 m 6"2 onficline

Fig. 8. (a)Stress fie•dandstructura•pattern•ftheBudaHi••sduringMidd•eMi•cene`P•i•cene(•wndata•BERGERATeta•.• 1984, and Bergerat, pers. comm). (b) Stereonet showing normal or normal-oblique faults, reactivating earlier discontinuities on the Hfirs Hill (H~i). (c) Rose diagram of synsedimentary, Middle Miocene faults on the T6t6ny Plateau (PALOTAS, 1991). Except for normal separation, dextral and sinistral displacement were also confirmed along some faults. ~1, c~2 and (~3 are maximal, medium, and minimal stress axes, respectively, obtained by numerical analysis of microfault data using the method of ANGELIER (1984). Stereographic projections are in Schmidt net, lower hemisphere, for topography see Fig. 2. 706 L. FODOR, A. MAGYARI,M. K,{.ZMI~R,A. FOGARASI

Oligocene. The parallel nature of Late Eocene and combined sedimentological and tectomc observations Early Oligocene dykes and the direction of compression permitted us to separate Oligocene and Late Eocene shows that the stress field did not change considerably structures (Fig. 9, 10). after Middle Eocene and before Middle Miocene times First we analyzed the faults figured on earlier maps. and that tectonic movements were more or less continu- The kinematics of these structures was determined on ous during this time span. the basis of the direction ofmai~ stress axis and the rela- tive displacement of the rocks. After that we examined Synsedimentary or post-sedimentary Eocene sediments occurring near a post-Eocene struc- structures ture and looked for the erosional surfaces between the Triassic and Eocene rocks. In most of the cases Late Because of the persistency of the stress field, Late Eocene palaeo-slopes were found at locations, where Eocene and Oligocene-Early Miocene structures can- earlier maps only indicate post-Eocene (Oligocene) not be separated by structural methods only. However, faults. Synsedimentary structures in Eocene or in base-

4 i .,

"~-a, _z..-- -F ~r

/~ ....~~ - \ ~. ],'~ sedimentary dykes * >\ / ;.f-~%. ,, "% X/\ o~ ~o,o,,.v.,~ []

-.-_\

g ~

H~rshegy Sandstone ~ ~"'- dykes calcedon

(~ 4km Fig. 9. (a) Stress field and structural pattern during the Oligocene-Early Miocene. (b) Dextral faults between Eocene and Triassic rocks 1 km to the west along strike from the R6ka Hill paleoslope. (c) Microfanltpattern near the Kecske Hill (Ke). For explanations of symbols see Fig. 8. Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 707 ment rocks can be either visible or remain hidden (Fig. Description of Eocene structures 6 versus Figs. 5b, 7). Our tectonical-sedimentological analysis suggests, The Triassic basement rocks in the Buda Hills form that most faults produced by the Eocene-Early Miocene numerous long ridges. The orientation of these paleo- phase were already active in the Late Eocene. Some of ridges (WSW-ENE to SSW-NNE) are almost perpen- the faults turned out to be clearly Late Eocene and not dicular to the orientation of compression (WNW-ESE to be active in Oligocene, in connast to the interpreta- to NW-SE), so they represent true antiforms. tion of earlier maps (Fig. 6). Other structures control- Sedimentological analysis prove that most of these ling the slopes were reactivated after the sedimentation, antiforms were bordered by slopes during Late Eocene during the Oligocene-Early Miocene or even later (Figs. time (Fig. 10). 7-9).

N ~ -- r'nut T ----~~ NA / 3 /

.~. e

-- 4e

/ -./ E3

/ zone

Eocene sedimentary dykes

6 4. km Fig. 10. (a) Stress field and structural pattern during the Late Eocene. (b) Rose diagram shows the strike of 100 synsedimentary dykes measured in the southern Buda Hills (O) by MAGYARI(1991b). For explanations of symboIs see Fig. 8. 708 L. FODOR, A. MAGYARI, M. KAZMt~R, A. FOGARASI | @

SE NW NW SE ~ extension Eocene ~ ~ ~~------.~,,uy~,,~~Oligocene t

brecciated o dolomite - I V. ,

/ t/ dolomite IE froctured dolomite / \ / o 2,m 1 Y 0 ~lkm Fig. 11. (a) A synsedimentary positive flower structure cross-cutting Triassic dolomite, north to Buda6rs. In the top left comer flat- lying Eocene sandstone has been dragged to a steep position. (b) Supposed cross-section through the Buda6rs transpressive shear zone (for location see Fig. 2).

In the vicinity of BudaOrs, synsedimentary (1991) above a transpressive shear zone in the Mecsek antiformal ridges trend ENE-WSW. An exposed cross- Hills, South-Hungary. section of one of the antiforms displays steep, discrete Another dextral fault, the "Nagykovficsi fault" oc- faults with reverse displacement associated with gentle curs in the northem Buda Hills. The southern slope of doming of the Triassic strata (Fig. 1 t). Although dom- R6ka Hill is probably underlain by a dextral fault, sense ing and faulting were contemporaneous with sedimen- is supported by microfaults measured nearby (Fig. 9b) tation, the faults apparently do not continue into the (BERGERAT et al., 1984, and own data). This fault Eocene sediments; the displacement was accommo- seems to extend to the west, to the Nagykovficsi depres- dated by fracturation-brecciation of the dolomite. The sion (Fig. 10). Oligocene-Early Miocene dextral dis- orientation of the faults is oblique to the compression placement along the northern edge has already been and they may also have dextral components. demonstrated by BALLA & DUDKO (1989). Our Similar deformation, in the form of pervasive microtectonic data and the fault pattern are consistent brecciation and steep reverse faults with a strike-slip with dextral slip, but demonstrate a Late Eocene initia- component located at depth (tens to hundreds of metres) tion of movements. are believed to characterize other antiforms. In a cross- Between Buda6rs and R6ka Hill, the NE-SW to section, the antiforms appear to constitute a large posi- NNE-SSW trending ridges represent true antiforms and tive flower structure (Fig. 1 lb). They are arranged in monoclines without strike-slip motion. Their internal an E-W trending zone extending from BudaOrs east- structure can hardly be studied because of lack of out- ward to the Danube, here termed the Buda6rs shear crop. However, on Kecske Hill and Lfitd Hill, antiforms zone. En echelon antiforms oblique to compression are cut by SE vergent reverse faults and NW vergent suggest dextral shear along this transpressive zone (Fig. backthrusts (Fig. 12a). Although these structures are 10). post-Eocene in age, (they affect lithified Eocene units), Sited on the crests of each ridge theie are longitudi- the redeposited character of the Eocene limestone with nal synsedimentary grabens (tens of metres wide). synsedimentary boudinage structures suggest that the These occur along the hinge zones on the external sides displacement along the fault had already started in the of antiforms where extension could take place inde- Late Eocene. These reverse faults disappear along pendent of deep structure and the general stress field. strike; in the section of the Jfinos Hill one can demon- The grabens are similar to those observed by TARI strate synsedimentarydoming, but not a fault cutting the Gravity-flow dominated sedimentationon the Buda paleoslope (Hungary) 709 surface. It is probable that the reverse faults continue was not supported by any structural observations, nappe below the antiforms and monoclines, but did not propa- structures remained questionable. Our study is the first gate up to the actual surface or reach the Late Eocene which has provided structural data to support the exist- sea bottom (Figs. 12b). These Eocene ridges represent ence of a detachment fault. fault-propagation anticlines (SUPPE, 1983) in front of a blind reverse fault tip. The highest antiform extending from Buda6rs in the Discussion south to Rdka Hill in the north is more or less continuous, while others are laterally restricted (Fig. 10). The Tectonics, sedimentation, and latter ones are connected by lateral ramps (BUTLER, palaeogeography in the Buda Hills 1982), e.g. at Mfity~is Hill. The smaller anticlines and monoclines gradually decrease in height and are slightly The occurrence of dextral and reverse faults or folds asymmetric to the SE. These form a SE-vergent imbri- suggest that deposition of Late Eocene sediments took cate stack, called here the Buda imbricate zone. It may place in a dextral transpressive regime. Reverse and represent the accommodation zone for the dextral dis- dextral faults often did not reach the surface during placement along the Buda6rs shear zone. Eocene time, but their propagation induced the growth Uniform vergency suggests that anticlines and ofantiforms during sedimentation. Tectonic movement monoclines may be underlain by a basal detachment initiated the steepening of slopes and beds, slope-paral- surface merging with blind reverse fault and backthrusts lel extension of layers by boudinage, extension shears (Fig. 12). The depth of detachment and amount of and faults. Tectonic instability of slopes led to the for- thrusting still remains unclear. Members of the Lower mation of gravity flows supplying the basin (Figs. 13, Triassic shale-sandstone-dolomite or the Upper Per- 14). Except for these mobile regions, "quiet" carbonate mian evaporite-dolomite sequences could serve as glid- sedimentation persisted on top of some antiforms, on ing surfaces for the subhorizontal motions. narrow plateaux. These limestones directly overlie the Paleogene thrust tectonics in the Buda Hills has al- basement and show no redeposition. Tectonic uplift ready been suggested by HORUSITZKY(1943). His idea resulted locally in submarine, or subaerial erosion with was based on the distribution of Triassic facies. Since karstification. Nevertheless, the general trend is sub- the age of Triassic rocks is poorly known and his view sidence, and during the deposition of the Buda Marl

NW SE | Kecske Hill L~t6 Hill soot ..~. o,. oN ,..J/_eQ. cko,. Eocene 0 gocene ~_1 ~~.:+:...:.~...... <~,,.~ " o-_ ~ ,

dip of 7riassm o~ dip of Eocene

Janos Hill Martinovics Hill 500m _~o ~e~_ .,'~..~.TZ-e e.. ~ ~ ~ [Fig. 4/&] ~/~-.. +- ~ .... _-0% -*- ~_ ~!~!!;i!i;i#;:;!!;i;;;i!:::::'>''" ,\L~,- ...... ,.

500_ WNW S Fig. 12. Cross sections through the Buda imbricate zone. (a) shows reverse faults and backthrusts propagated through Eocene sediments; this section reflects the post-Eocene evolution, too. (b) Fault-propagation folds underlain by blind reverse faults. For locations see Fig. 2. 710 L. FODOR, A. MAGYARI, M. KAZMt~R, A. FOGARASI even the highest elevations subsided below sea level (Figs. 13, 14). During the Late Eocene this hinge marks (Figs. 13a, 14c). the extension of the rapidly subsiding eastern bathyal The hinge zone of the highest antiform extending basins, tectonic uplift of the western side of the antiform from Budaars through Jfinos Hill to R6ka Hill closely accommodating the subsidence. approaches the Buda Line. The existence of an actively During earliest Oligocene the uplift of the western deforming antiform provides a meaningful explanation flank of the Jfinos Hill antiform resulted in erosion, for Late Eocene and Early Oligocene palaeogeography while at the eastern side the Tard Clay basin was iso-

N |

~d, tLl" ~ "~ _ "'/ ~5;~_ -

C) ~,km

transport direction Buda Marl

conglomerate, breccia ~ ~ shallow marine cones and submarin fnns carbo|ate platforms Middle Eocene J sediment on po[eo-s[opes sediments Fig. 13. Tectono-sedimentary model of the Buda Hills. (a) Middle Eocene in the Nagykovficsi depression and Upper Eocene in rest of the Buda Hills. (b) Early Oligocene scenario. Gravity-flow dominated sedimentation on the Buda paleoslope (Hungary) 711 lated and became anoxic with a specialized endemic which protected the eastern basin from the masswe m- biota (Figs 13b, 14d) (NAGYMAROSY, 1983; BA~DI, flux of coarse clastics. The existence of a reduced car- 1986). bonate platform is shown by carbonate turbidites and The coeval nearshore Hfirshegy Sandstone was de- olistoliths in the Tard Clay (VARGA, 1982, 1985; posited on the western side of the ~mtiform, while the KECSKEMt~TI & VARGA, 1985). Our observations sug- uppermost Tard Clay and bathyal Kiscell Clay were gest that a similar submarine barrier was situated along deposited to the east. Bf~LDI & NAGYMAROSI (1976) the hinge zone of the actively deforming Jfinos Hill suggested the possibility of reefs along the Buda Line anti form.

..::]:~:. | ....."::.:$':':~:i::':. \ :!~ii~i...,, &.,A

o" ,

~ _:

'~"~ -:--J " .'~:...."i~:~:~:~...,,\ / -

i:: ~. / \ ". " ;:::... ". ~ " :Z= --

-- ..-.. \~ ".. :::::..'.:!~:..'..'~':..,.%...:...:,~:!::..::~.: .... .'.'":.::.:.~,;- "~f-~'~~ =~~_-_~ _, ~.. ~. ..~ .~i~::::"o,~i =- .~:.:.:!:~.::::.-.::. : ...... -: ,x,f/,,,-~:::-,i-,Xf// : 149 : .~ =:,-= _ .

..:'.:.::.':.:.-:.:-~.':.." .. . ..;: ,~ - ; --:~-~--- ==_- .. --:':i:'.:'.:;:..':'.-:i.!:::'~.:.:::';:'::'..:'. o - -"------~;..:.--~ . .:.:j::::-:.::!::::....:=:::..:..-...... ~_ --_ :,:ii:'~:'..':~:!!2:".'. ". ~ . ~i//,-',

,7 , - _ - ~ - - e / " "'~--=----~-~-~ - - - -I. 0=~

s ~,km

~ H~rshegy Sandstone (known/inferred)

Upper Tord C[ay (known/inferred)

.. shol(ow marine carbonate platform 712 L. FODOR, A. MAGYARI,M. K,4,ZMt~R,A. FOGARASI

BUDA LINE NW ~' ~ ~' SE V

Late EOCENE ~ Tard Clay

v b .... redeposition ~ ~ boud inane

~#~,---v~.,"" .~..,~~(,__,._/_:,, _ o ...... ~. ,~.".;.... ~~u,,..,.,._ ~ ~ H~rshegy Sandstone

, Buda Marl J IL~.~."~l I

V c sliding ~ limestone

~..~_~-~. ,,4. "%\ ~ sandstone ., ,.'...

~ conglomerate

Earl OL GOCENE v

~.'-')'~',_~!~' .: -..,,

7"-" ," ,,'.

Fig. 14. Progressive steepening of slopes and related (re)sedimentationdue to reverse fault propagation in a hypothetical section. The Buda Line probably represents a submarine barrier on top of the highest antiform.

Working model of Middle-Late Eocene in the ing of a conglomerate body overlying bauxites near Bakony Unit Halimba was interpreted as being bordered by NE- trending normal faults (MINDSZENTY et al., 1988); our The well-studied example of the Buda Hills leads us alternative interpretation is a sedimentary wedge in to suggest a general tectono-sedimentary working front of an antiform. These examples suggest that a model for the Bakony Unit, despite the scarcity of data uniform structural-sedimentological pattern of the about synsedimentary tectonics (Fig. 15). First, we can Bakony Unit was probably established in the Middle observe a similar micro-fault pattern in quarries every- Eocene, while the Buda Hills are certainly of Late where in the Bakony Unit (BERGERAT et al., 1984 and Eocene age. unpublished data; DUDKO, 1990, pers. comm.). WNW- ESE trending synsedimentary normal faults controlled Role of the structures in the escape tectonics the deposition of an Upper Eocene slope apron con- of the Bakony Unit glomerate at Nyergesdjfalu (FODOR et al., 1990b). The large Middle Eocene breccia wedge in the alluvial fan The dextral shear zone of the Buda6rs represents the of the Mfiny basin (FAY-TATRAY, 1984) was probably south-easternmost Palaeogene outcrop of the Bakony bordered by a strike-slip fault. Northwestward thicken- Unit. The approx. 10 km-wide zone, which extends to Gravity-flow dominated sedimentationon the Buda paleoslope (Hungary) 713

N / , Danub_e ,~ t

a s>

sediments ~ Eocene volcanics Middle Late Eocene (subsurface) coarse clastics Fig. 15. Tentative model for Middle to Late Eocene sedimentation and tectonics of the Bakony unit. Extension of Eocene sediments is after BC,LDI-BEKE & BALDI (1990, 1991). Structural patterns remain the same during Oligocene time: note the dextral displacement of Eocene volcano-sedimentary bodies in the southeast (after DUDKO, 1988). the southern border of the Bakony block is covered by ture of the structures suggest initiation of motions in the Miocene sediments. The stress field and fault pattern in Middle Eocene. the Buda Hills suggest dextral motion along the south- Palaeomagnetic data indicate 35 ~ counterclockwise ern border fault (Fig. 15). The synsedimentary origin rotation, both for the Bakony Unit and Buda Hills just of structures in the Buda Hills suggests that the defor- after the Late Paleogene-Early Miocene tectonic events mation inside the block and consequently along the (1VIARTON,1985, and pers. comm.). Compensating for border zone, started not later than the Late Eocene. this rotation, the original maximum palaeostress axes While similar tectono-sedimentary features seem to pointed NNW-SSE during the Late Eocene-Oligocene. have been already established in the Middle Eocene, This direction is in good agreement with the kinematic- dextral slip of the southern zone of the Bakony Unit may dynamic directions determined for the collision of Eu- have started at this time. rope and the Apulian microplate (BERGERAT, 1987; Similar kinematic conditions persisted during the PLATT et al., 1989). This collision can be related to a Oligocene, as proved by fault patterns in the Buda Hills new cycle of convergence between Africa and Europe and Northern Bakony (MAROS, 1988). The existence starting at the boundary of Early and Middle Eocene of right-laterally displaced Eocene volcanic and sedi- (DEWEY, 1990). The beginning of the rapid shortening mentary bodies close to the southern shear zone repre- conspicuously coincides with the start of sedimentation sent direct evidence (Fig.. 15) (BALLA et al., 1987; and escape tectonics of the Bakony unit. DUDKO, 1988). Dextral displacement of the once con- tiguous depositional areas continued up to the middle Early Miocene (CSONTOSet al., 1992). Conclusions Between the Middle Eocene and Early Miocene, the conferred structural pattern is consistent with dextral Sedimentological analysis demonstrates that the slip along the southern border fault zone of the Bakony Upper Eocene sequence of the Buda Hills was mainly Unit. These structural data support the continental es- deposited on steep palaeoslopes. Sedimentological cape model. While the Bakony seems to be character- structures and sediment deformation suggest that slopes ized only by dextral faults, the escape movement af- were underlain by actively deforming structures in the fected the whole North-Pannonian block, as suggested basement. Tectonic measurements prove that Eocene by BALLA (1984, 1988a, b). The synsedimentary na- and Oligocene-Early Miocene structures were formed 714 L. FODOR, A. MAGYARI, M. KAZMt~R, A. FOGARASI in the same stress field, the latter structures generally response to continental collision between the European reactivating the former ones. The separation of a prob- plate and the Adrian promontory. able Eocene movement is possible only by a combina- tion of sedimentological and tectonic observations. Acknowledgements Deposition of Late Eocene sediments took place in Special thanks are due to Prof. T. Bfildi, who introduced us a dextral transpressive regime. Synsedimentary to the intricacies of sedimentation in the Buda Hills. We are antiforms were situated above dextral reverse faults of grateful for the field comments and advices of F. Surlyk a positive flower structure, or were located on frontal (Copenhagen), H. Reading (Oxford) and M. Kovfie (Bratislava). and lateral culmination walls of fault-propagation anti- We are very grateful for stimulating remarks of A. Nagymarosy, clines. Z. Balla (both Budapest) and Ph. Ott d'Estevou (Paris) during our studies. We thank F. Bergerat for use of her unpublished tectonic The structural pattern and direction of stress axes data and J. Angelier (both Paris) for permitting us to use his stress strongly suggest dextral motion along the southern bor- analysis computer program. der zone of the Bakony unit. The synsedimentary ori- O. Sztan6 and Z. Balla have read an earlier draft of the gin of the structures supports that dextral motion and the manuscript and provided many useful comments. Z. Balla and R. Wiedemann translated the Russian and German abstracts, escape tectonics started in Middle Eocene times. This respectively. L. Csontos, J. Wilson, and A.H.F. Robertson kindly dextral motion and the continental escape were a direct helped with improving the manuscript.

References

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