revue roumaine de geologie geophvsioue.

A C AD EMf A REP U BLl C II S

o-C J A L I S T E

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REVUE ROUMAINE DE GEOLOGIE GEOPHVSIOUE. ET GEOGRAPHIE

TIRAGE

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TOME 28

ED ITU RA

198'4

AC A 0 EM IE I

REPUBLIC 11

S 0 CI A L'l'S TE

R 0 MAN I A

JURASSIC ROCKS OF NORTH DOBROGEA. A DEPOSITIONAL-TECTONIC APPROACH

EUGEN GRAD IN ARU The Jurassic rocks are now preserved only in the Tulcea Unit and along the Cirjelari-Camena outcrop belt. In the Tulcea Unit, mainly terrigencius turbidite deposits were accumulated during the Lower and Middle Jurassic while in the early Upper Jurassic basinal carbonate deposits were laid down. The Middle and Upper Jurassic sedimentary rocks and bimodal volcanites of the Cirjelari-Camena outcrop belt were formed in an early dextral transtensional phase and were deformed during the pre-Cenomanian Mid-Cretaceous tectogenesis in a later sinistral transpressive phase, both phases being related to oblique-mobile strike-slip tectonic regime of the Pecencaga-Camena Fault. The sedimentary and tectonic history of the North Dobrogean Jurassic rocks were controlled by the variously reversed strike-slip tectonics of the Pcceneaga-Camena Fault, a major pre-Cenomanian boundary continental transform fault.

INTRODUCTION

Detailed litho- and biostratigraphic studies, carried out by the author in the last ten years, have provided a great deal of data showing that the North Dobrogean Jurassic rocks have a more important development that was known hitherto (Mutihac 1963, 1964 a/b ; Miri'iiuta. & Mirauta 1964), with great implications for the Jurassic geology of this area. The a_im of the present paper is to summarize ~uthor's knowledge to date on the stratigraphy of the North Dobrogean JurassiC rocks, todefine their depositional-tectonic settings, in order to outline the paleogeodynamics ·of the North Dobrogea from the viewpoint of its Jurassic geology. The Jurassic rocks are now preserved only in the Tulcea Unit and along the northern side of the Peceneaga-Camena Fault in the CirjelariCamena outcrop belt, their distinctive lithologies and structural patterns being the expression of a specific geodynamic evolution of their corresponding development areas (Fig. 1). JURASSIC ROCKS OF THE TULCEA UNIT

In contrast with the previous data (l\Iutihac 1963, 1964 a/b) o'ur studies clearly demonstrate not only the presence of the almost entirely Lower Jurassic but also the presence of the Middle and Upper Jurassic in the Tulcea Unit (Figs 2 and 3 ). The overall distribution of the Jurassic rocks in this unit exhibit a clear-cut asymmetry, due to the evolutionary trend of the Tulcea 8ubmarine fan system made up of the westerly-developRev. Roum. Geol., Geophys. et Geogr., GEOLOGIE, Tome 28, p. 61-72, 1984, Bucarest

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EUGEN GRADINARU

cont mem in T with the float age-

ed Lm.ver Jurassic N albant Fan and the easterly-developed Middle .Jurassic Dit'navafu Fan. ' · Detailed descriptions of the new named Jurassic lithostratigraphic units will be done in other forthcoming papers (Gradinaru, in press) .. LOWER JURASSIC ROCKS

The Lower Jurassic rocks outcropping in the western part of the Tulcea Unit are mainly resedimented terrigenous rocks, being grouped in the Telita Formation (pro parte), the Denis Tepe Formation and the Nalbant Beds (Fig. 2). Telifa . Formation (Rhaetian-Lower Pliensbachian), conformably overlying the Cataloi Formation (Ladinian-Sevatian) displays two members. The lower member, Frecatei Sandstone (RhaetianHettangian) has a lower part with fine-grained calcareous to quartzose sandstones and siltstones, interbedded nodular limestones and coquinoid calcareous siltstones, and an upper part with quartz-rich calcareous sandstones and interbedded mudstones. The upper member, Po~ta Sandstone (SinemurianLower Pliensbachian) is mostly made up of amalgamated, medium- to thick-bedded, massive, argillaceous quartzose sandstones and siltstones and rare T(ae) Bouma sequences with thick-bedded, graded argillaceous quartzose sandstones and very thin' shaly divisions. Den·is Tepe Formation {Rhaetian-Toarcian), made up of proximal turbidites, shows two mflmbers . The lower member exhibits a :iriixture of T(a), T(ae) and various basemissing Bouma sequences with very thin shaly divisions; the basal divisions being made up mainly of medium- to thick-bedded, niassive, argillaceous . quartzo-feldspathic sandstones, containing mudstone . rip-up clasts, or g·raded, micaceous 'quartzose sandstones, subordinately of hard, glassy, thin-bedded, graded quartzose sandstones and siltstones of Nalbanttype, and occasionally of thick-bedded, graded conglomeratic sandstones -0r granule conglomerates. The upper member is made up of amalgamated, thick-bedded; massive or graded, · · well-winnowed quartzo-feldspathic ~andstones and thick ·packets of trailing suspension cloud-related thinly · parted · micaceous siltstones and shales, recurrently intercalated in its lower part, being interpreted here ·as storm-triggered turbidites. Nal bant Beds (Sinemurian-Lower Pliensbachian), made up of distal turbidites, show a mixture of complete and base-missing Bouma sequences capped by well-developed shaly divisions, the basal divisions containing hard, glassy, graded or laminated, quartzose sandstones and siltstones. Lower Jurassic rocks may be also inferred to develop in the subsurface of the Dunavatu de Jos-Valea Nucarilor area, underlying the Dunavatu ~ormation (Bajocian-Bathonian), but with a non-turbidite lithology by comparison with the coeval rocks from the -western part of the Tulcea Unit.

light lime ago, "Pl

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la,r· MIDDLE AND UPPER JURASSIC ROCKS

These rocks occur only in the eastern part of the Tulcea Unit, being grouped in the Dunavatu Formation and the Carabair Limestone (Fig. 3).

nor

JURASSIC ROCKS OF NORTH DOBROGElA

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Dunavat'u Forrnatfon (Bajocian-Bathonian) displays a lower member contaiping oolitic lime sand-rich terrigenous . turbidites and · an upper member made up exclusively of teriigenous turbidites, both organized in T(a), T(ae) and rare T(a-e) Bouma sequences, subordinately intermixed with packets of various base-missing Bomna sequences. Occasionally, the massive quartzose sandsfones of the T(a) Bouma sequences contain floating, large~sized mudstone and siltstone rip-up clasts which yielded age-diagnostic faunas for the Bajocian-Bathonian interv\11. Carabair Lirnestone (Middle Oxfordian), partially exposed, shows light grey crinoidal calcarenites grading upwards into dark grey marly limestones with odd, dark black chert nodules and lens~s. Some years ago, I have narrated the odissey of the fortunate finding of Peters' (1867} "Planula.t en-Kalkstein vom Kara-Bair" (Gradinaru 1972) 1 . A large covered stratigraphic interval (Callovian-Lower Oxfordian) 1 for which. hemipelagic deposits similar to those developing i:µ the northerly adjoining :f>re-Dobrogea depression (Patrut et al., 1983) may b e inferred, connects the two outcropping fo;rmations in a remarkable rock sequenc,e with a composite thickness of more than 600 m., bracketing a large stratigraphic interval from Bajocian to Middle Oxfordian, to which presumed underlying buried Lower Jurassic rocks may be also added. A reference drill hole in thil'l stra.t igraphically well-controlled ,Jurassic succession from the Carabair Hill area would .be of the greatest interest for a better correlation with the Jurassic rocks of the offshore North Dobrogea and the Pre-Dobrogea depression.

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DEPOSITIONAL-TECTONIC SETTING

The quartz-rich hemipelagites and interbedded swell lag-type coquinoid siltstones of the Frecatei Sandstone were . laid down below the wave base either on the other shelf or an incipiently developing basinal slope, driven by storms and highly supplied with fine-grained sediments by turbid-layer flows. Further on, the development of a mature river deltasiliciclastic shelf-basinal slope system has favoured the massive downslope transfer of terrigenous materials by broadly nonchannelized sand flows ·and high-density turbidity sheet flows in the case of the Pof}ta Sandstone (B 2 and C turbidite facies) and channelized turbidity flows. building up a small-sized Lower Jurassic deep-water submarine · fant the Nalbant J!,an. From this submarine fan only the proximal midfan. depositional lobes (sandy suprafan bulge), represented by the progradationally-deposited, coarsening- and thickening-upward sequence of the proximal turbidite-facies association of the Denis Tepe Forrnation (B 2, C and E turbidite facies), together with the distal midfan and outer fan apron, represented by the distal turbidite-facies association of the Nalbant B eds (C and D turbidite facies), are now preserved. Depositional and time-rock correlations (Fig. 2), petrofacies similarities, turbidite-facies associations and sequences, bed geometry and 1 Gradinaru E., .Asupra prezenfei Jurasicului supuior la Dunavafu de Jos ( Dobrogea de nord), Com. Ses. ~t., 16 - 17 dee. 1972, Univ. Bucure~ti.

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EUGEN GR!AD1NARU

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assemblages of sedimentary structures, clearly demonstrate that .the Lower .Jurassic resedii;nented deposits from the western part of the Tulcea Unit, showing complex gradational relationships both vertically and laterally, represent coeval, genetically-related lithofacies into a well-organized facial system. The overall d istribution pattern of the various Lower Jurassic turbidite-facies associations, the general west,vard decrease in division thickness, coarseness, modal completeness of the Bouma sequences, sandstone/shale and . proximal/distal ratios, all clearly indicate the westward outbuilding of the Nalbant Fan. The close juxtaposition on a limited area of the various Lower .Jurassic resedimented deposits, showing a rapid westward proximal to distal gradation from the Denis Tepe Formation to its downfan counterpart Nalbant Beds, indicates a restricted depositional-tectonic setting for the Nalbant Fan. · During the Middle .Jurassic, due to tectonic tilting, a recessional :phase followed in the growth of the Nalbant Fan, attributable to lateral shifting of the main distributary system towards the eastern part of the Tulcea Unit to build up a new submarine fan, the Dunavafu Fan. We have now preserved in the Dunavatu de .Jos-Valea Nucarilor area only the turbidite-facies association (B 2 , C and E turbidite facies) of the midfan depositional lobes (sandy suprafan bulge), progradationally developed at debouching terminus of the switching midfan braided distributary channels from high-density turbidity bed-load currents transitional to liquefied and/or grain flows, containing thick-bedded, coarse-grained, massive, structureless or graded sandy turbidites, without or. with very thin shaly :partings. These alternate at random with extensively bioturbated, thinly bedded, finer grained turbidites, -aggradationally-deposited either by -0verbank spilling or · from trailing suspension clouds. The orientation of the occasionally-developed, small-scale, sh~llow, nonleveed, abandoned sandy distributary channels, fits well with the main vector (N70°vV) -0f the sediment transport direction determined from the orientation of the lineary sole markings, indicating consistent paleoflow directions from SE to NW, the same as for the Nalbant Fan. No reliable source area may be adYocated southeasterly in the actua,l tectonic framework. The occurrence of the fossiliferous mudstone and siltstone rip-up dasts, reworked from the penecontemporaneous upslope or upper fan semilithified hemipelagites by the high-density, high-velocity, channelized turbidity currents, denotes a more productive hemipelagic background during the growth of the Dunavatu Fan than in the case of the Nalbant Fan, as a result of a less restrictive ponding effect or a less massive riverdelta discharge. However, no interturbidite hemipelagites, the so-called (e) Hesse divisions, may be recognized in the Dunavatu Formation, a QJ (/)

Co/umbi.

Alaunian

ower Jurassic rocks fl

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5

JURASSIC ROCKS OF NORTH bOBROGEA

must be reserved only for those turbidites associated with compressive and transcurrent deformations during mountain building. The North Dobrogean J.Jower and Middle Jurassic turbidites were laid down in transient, restricted ensialic ba.sins, in a tectonic framework dominated by strike-slip faulting unrelated to orogenic deformations, similarly to the ancient and modern small-enclosed submarine fans in the Californian continental borderland basins (Nilsen & Clarke, 1975). After the waning of the turbidity--current activity in the 'l'ulcea Unit, the overall change in sedimentary regime during the late Middle JtU·assic was heralded by the deposition of the Carabair Limestone in a fully-developed pelagic basin, the foundering of the sedimentary basin being related to the generalized deepening processes of the Tethyan seas dming the late Middle and Upper Jurassic times . •JURASSIC HOC:KS OF THE (;JRJELARl-CAMENA OUTCBOP BELT

The Jurassic terrigenous and carbonate rocks.of the Cirjelari-Camena outcrop belt (CCOB ) (Pigs 4 and 6) were grouped by Gradinaru (1981) in several lithostratigraphic units, namely the ·JYiiddle Jurassic Aiorrnan Formation and t he Upper Jma,ssic (Oxfordian-Lower Kirnmeridgia:ri) Bai?pttnar Fot·mation and Girjelari Formation, to which are now added the (late?) Middle Jurassic JVIovila Goaza Fo1·mation and_ ,the Upper Jurassic Amara Breccia. Also a tectonostratigraphic unit, the Ba~pu1utr Melange,· iS now introduced. The Jmassic volcanic rocks were grouped into the Oamenct Rhyolite and Ba;9pimar Srilite. DEPOSITIO NAL-TECTONIC SETTING

' .

The Middle and Upper Jurassic -sedilnentary rocks a.n d bimodal acid and basic volcanites (Fig. 6), closely neighbouring the PeceneagaCamena Fault (PCF) along its northern side as a narro\v and discontinuous . NW -SE trending outcrop belt between the Oirjelari and Camena localities, are the witness rocks of a major Jurassic volcanocsedinientary domain that has had an intermediary position between the two for' a long time well-known Dobroge[tn Jurnssic sedimentary domains, namely · the slrallow-watet domain of the Central Dobrogea, and the deep-water domain of the Tulcea Unit. Although 1Iirauta & Mirauta (1964 ) recognized . th~ "Oirjelari-Camena Mesozoic Zone" as a distinct unit, the depositional, and tectonic significance of the Jurassic rocks cropping out in this zone was neglected until recently, when Gradinaru (1981) demonstrated that thesedimentary and tectonic history of these rocks is best explained in the context of an oblique-mobile strike-slip tectonic regime related to the PCF and particularly in terms of the rigid behaviour of the Cen tral . Dobrogean block (CDB). Th.ere are numerous sedimentary .and structural aspects which are consi.stent with looking at both the Middle and 'Upper JUl'assic deposition, and the post-Jurassic deformation in terms of an early tlilar Forma~ tion ancl the submarine out porings of silicic igfiitnbrites and thyolites of the Oamena R hyolite. This acid volcanism started even early in the (late~) Middle J urassic, as it is demonstrated by the sanidine-hearihg calcitized rhyolitic lava fl ows interlayered into the Movila Goala Fotinatioii. Subaerial erupt ions m ay be also documented by the 'Whitish rhyulitic tuffs interlay ered into the Ba~punar Formation and Olrjefari J!'gri:nation (Fig. 6) , The ·.furt h er pronounced stretching ancl thinning of th~ continental crust in the area of the POTB opened the way for sUbrrtM"ine extrusions of pillowed basalts, n ow p artially preserved in the Ea~p'Uha:r' Spiiltl'l. , Exten sive .crustal stretching owing to a continued transtensional growth could h ave led ultimately by continental crustal separation along the P OF to the opening of a narrow ocean in ah embrionic stage, the trans- ,.. tensile gash being isostatically compensated by both partial diapiric intrusion of upper mantle-derived basic rocks and ihassive influx of elastic deposits. The m et abasic rocks found in the Ba$punar :Melange rimming the P eceneaga-Oamena tran sduction suture, interpreted hei'e as sheared remn:mts of an incip iently developing oceanic crust; could eventually sp@ak fol' such an advanced st age in the evolution. of the POTB. · The distribution a.nd lithofacial a spects of the Upper Jurassic rocks (Figs 4 a n d 6) along t h e strike of the OOOB; sho\ving a tapid eastward transition from t he shallow-water carbonate shelf-allodapic basin system developed in' the Oirjelari zone t o t he sub-oceanic do1nain of the Camena zone, suggests t hat the P Ol'B represented a '\vedg'e~shapetl intracontinental arm of t h e Black Sea oceanic basin. This cd1ifigtfration may be assigned to a tran spressiv e t ectonics westwards in the Ofrjelari zone concurren tly with a tran stensile tectonics in the Oamena zone, these varying coeva l t ectonic r egim es being r elated to a clockwise rotational vector superimposed on t he general westward drift of the ODB (Fig. 1, inset B). The them e of sedim entation in oblique-mobile strike-slip belts together with the main aspects of sedim enta tion response as seen in the basin-fill successions were largely debated in a symposium organized by Ballance & Reading (1980). Numerous aspects of the sedimentary domain and lithofaci es of the Middle and Upper Jurassic rocks from the CCOB harmonize well with the transcurrent faulting mod el of oblique-mobile sedi-lherltary zoi1es, namely: - a narrow, elongate fault-angle sedimentary basin, subsiding rapidly and containing thick columns associated with an oblique-mobile transeurrent fault (Figs 4, 5 and 6) ; - a close linear association of extremely localized time-equivaients of various facies ii'1 an elo11gate zone parallel to a major transcurrent fault (Fig~ 4 ilhd 6); . - abrupt facies and thickness changes not only along the strike arid a