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Seismic sequence stratigraphy, structure and subsidence

history of the Romanian Black Sea shelf

Article  in  Geological Society London Special Publications · September 2010

DOI: 10.1144/SP340.9

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doi:10.1144/SP340.9 2010; v. 340; p. 159-180 Geological Society, London, Special Publications

 Catalina Konerding, Corneliu Dinu and How Kin Wong  

of the Romanian Black Sea shelfSeismic sequence stratigraphy, structure and subsidence history 

Geological Society, London, Special Publications

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Seismic sequence stratigraphy, structure and subsidence history of the

Romanian Black Sea shelf

CATALINA KONERDING1,2, CORNELIU DINU3* & HOW KIN WONG1

1Institute of Biogeochemistry and Marine Chemistry, Hamburg University, Germany2Present address: G&W Systems, Corp., Houston, USA

3Faculty of Geology and Geophysics, Bucharest University, Romania

*Corresponding author (e-mail: corneliu.dinu@g.unibuc.ro, codinu@gmail.com)

Abstract: The Black Sea is an extensional back-arc basin developed along the northern activemargin of the Tethys Ocean which was subducted northward from the Triassic to Miocenetimes. The Romanian Black Sea shelf is dominated by mid-Cretaceous extensional structuresand their sedimentary cover, subsequently affected by Cenozoic compression. Here we analysethe post-Oligocene structural and sedimentological evolution of the shelf, based on Romanianoil industry data consisting of (1) 5300 line-km reflection seismic profiles situated on the shelfand continental slope; and (2) depth and lithostratigraphic information from 60 boreholes on theshelf. Our study provides evidence for a changing evolution of the shelf during a relatively shortperiod of time directly related to the pre-Miocene period and the evolution of the Romanianonshore. Mio-Pleistocene subsidence of the Romanian Black Sea shelf is highly variable andis directly dependent on sediment input, tectonic activity as well as water-level fluctuations.Subsidence during the Badenian–Sarmatian and the Dacian–Quaternary was limited. In contrast,during the Pontian, shelf subsidence was progressively faster in the basinward direction. Subsi-dence on the outer shelf was much more significant than elsewhere on the shelf. Tectonically,the most active period during the Mio-Pleistocene was the Pontian. The Badenian–Sarmatianwas largely quiescent and the Dacian–Quaternary saw a decrease in the Pontian tectonic activity,coming possibly even to a halt. From a sequence-stratigraphical point of view, eight systems tractswere identified for the Mio-Pleistocene sedimentary section. The Badenian-Sarmatian unit wasattributed to a HST (highstand systems tract). The Pontian unit was subdivided into P1, P2, P3and P4. Subunit P1, which was laid down on the slope at the time of deposition, is progradationaland attributable to the lowstand wedge of a LST (lowstand systems tract). Subunit P2 is likewisealso attributed to a LST, having the continental slope and the deep basin as palaeo-depositionalenvironments. The reflection terminations and the wedge-shape of P3 suggest that it was depositedin the deep basin during a sea level lowstand. The next transgressive systems tract (TST) andHST developed during the deposition of P4. The boundary between P4 and the Dacian isrepresented by an erosional hiatus, which comprises the LST that follows the formation of thesequence boundary at the end of the Pontian. During the Dacian–Quaternary, the subsequentTST and HST were deposited on the inner and middle shelves. Sedimentation on the Romanianshelf during the Mio-Pleistocene period was thus strongly influenced by sediment input andsubsidence, while sea level fluctuations played a lesser role. As sediment input is related to theevolution of the adjacent land and subsidence is dependent on sediment supply, tectonic activityand sea level fluctuations, these two factors are not totally independent.

The Black Sea is located between Romania, theUkraine, Russia, Georgia, Turkey and Bulgaria. Itis one of the largest enclosed marine seas with anarea of 423 000 km2, a volume of 534 000 km3

and a maximum water depth of 2206 m (Rosset al. 1978). The Black Sea basin came into exist-ence about 200 million years ago and over 15 kmof sediment have accumulated in some parts(Neprochnov & Ross 1978). It is an extensionalback-arc basin developed along the northern activemargin of the Tethys Ocean which was subductednorthward from the Triassic to the Miocene. It con-sists of two parts: the western Black Sea which isunderlain by oceanic to suboceanic crust, and the

eastern Black Sea underlain by continental crust.The two basins are separated by a strike–slip faultsystem along the Mid–Black Sea/AndrusovRidge, which comprises continental crust.

The Romanian sector of the Black Sea lies tothe east and SE of the Danube Delta and has anarea of over 35 000 km2. The Romanian shelfwith an area of 20 000 km2 is up to 140 km wideand usually has a water depth of less than 120–140 m (Popescu et al. 2004). This area is dominatedby mid-Cretaceous extensional structures andtheir sedimentary cover, and was subsequentlyaffected by Cenozoic compression (Robinsonet al. 1996).

From: Sosson, M., Kaymakci, N., Stephenson, R. A., Bergerat, F. & Starostenko, V. (eds) Sedimentary BasinTectonics from the Black Sea and Caucasus to the Arabian Platform. Geological Society, London, Special Publications,340, 159–180. DOI: 10.1144/SP340.9 0305-8719/10/$15.00 # The Geological Society of London 2010.

The present paper is based on reflection seismicprofiles and borehole data made available by theRomanian oil industry (Fig. 1). The seismic profileswere recorded and processed by the Romaniancompany Prospectiuni S.A. between 1980 and1994. About 70 profiles situated on the Romanianshelf and on the continental slope with a totallength of 5300 line-km were used for interpretationof the Mio-Pleistocene sequences. Velocity infor-mation from processing was used for depth con-version. In addition to seismic data, depth andlithostratigraphic information from 60 boreholesdrilled on the Romanian shelf were available.Most of these boreholes are located on the innershelf, particularly on the offshore prolongation ofthe North Dobrogea Orogen.

The purpose of this paper is to map the hithertounknown distribution and structure of the Mio-Pleistocene succession on the Romanian Black Seashelf in time slices, as well as to reconstruct (1)the structural evolution of the shelf; and (2) thesequence stratigraphic framework for the Mioceneand the post-Miocene in relation to the evolutionof the transition zone from the Carpathians to theBlack Sea basin.

Structural characteristics of the

Romanian shelf

Pre-Miocene structures

Previous seismic studies show a continuation ofthe main structural features and major faults inthe Dobrogea region (onshore Romania) onto theRomanian Black Sea shelf (Dinu et al. 2002,2005). This includes the Sf. Gheorghe Fault, whichseparates the Pre-Dobrogea Depression from theNorth Dobrogea Orogen, the Peceneaga-CamenaFault, which separates the North Dobrogea Orogenfrom the Moesian Platform (identified on the shelffor over 60 km) and the Capidava-Ovidiu Fault sep-arating central and south Dobrogea from theMoesian Platform (Fig. 2). The seaward extensionof these land structures dips to the east and iscovered by Eocene, Oligocene and Neogene depos-its. Beginning with the Oligocene, the geologicalevolution of the Romanian shelf was decoupledfrom that of the land (Dinu et al. 2002, 2005).

According to Morosanu (2002), three main tec-tonic stages can be distinguished on the Romanianshelf.

Fig. 1. Location of the seismic lines and boreholes used in the present paper. Lines 1 and 2 show the idealized sectionsalong which the subsidence history was reconstructed.

C. KONERDING ET AL.160

(1) During the first stage, which began in the Jur-assic and continued until the Albian, exten-sional tectonics took place with the openingof a NW–SE oriented rift. This rift developedin direct connection with the opening of thewestern Black Sea Basin. In the Albian, theHistria Depression and its landward continu-ation, the Babadag Syncline, were formed(Dinu et al. 2002, 2005; Fig. 2). Extensionwas very active in the Albian and continueduntil the Upper Cretaceous with reducedactivity.

(2) During the second stage that took place fromthe Upper Cretaceous to the Eocene, riftingceased and the depositional rate increased. Thedepression was filled with Upper Cretaceousdeposits, Eocene carbonates covering onlythe highest areas.

(3) The third stage developed from the LateEocene to the Lower Oligocene. It is charac-terized by inversion tectonics, going from anextensional to a compressional regime.

The Oligocene succession was deposited in a deep,subsiding basin and Mio-Pliocene sediments filledthe entire depositional area. Subsidence decreased

in the Miocene and the Pliocene and very thickterrigenous deposits accumulated on the easternpart of the Romanian shelf.

Pre-Miocene faulting on the shelf led to: (i)pre-Albian structures, which represent the offshorecontinuation of the tectonic units that existed inthe Dobrogea region before the opening of theWestern Black Sea Basin during the Lower Cretac-eous; and (ii) extensional structures, directly associ-ated with the opening of the western Black SeaBasin (Dinu et al. 2005).

Eocene and Oligocene sedimentation in theHistria Depression was controlled by the Laramianand Pyrenean orogenic phases when invertedstructures came into being. The reverse faults onthe northern flank of the Histria Depression werepresumably also formed during this period (Dinuet al. 2002).

An important tectonic element on the RomanianBlack Sea shelf is the Histria Depression, the off-shore prolongation of the Babadag Syncline,which represents the southern part of the NorthDobrogea Orogen (Fig. 2). This depression isbounded by a major structural feature, the ‘EuxinianThreshold’, a zone in which Palaeogene depositssubsided strongly (Fig. 2; Ionescu 2000; Patrut

Fig. 2. The major Romanian onshore tectonic structures and their offshore prolongations (from Dinu et al. 2002).

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 161

1975). The Euxinian Threshold was a continentalpalaeo-slope developed during the Upper Eocene,but may also mark the limit of the shelf depositsduring the Upper Cretaceous (Ionescu 2000).The Histria Depression is the subsided block thatdeveloped between the central and the northern seg-ments of the Euxinian Threshold. Mio-Pleistocenesedimentation and tectonic processes in the studyarea are closely related to the development ofthis depression.

Mio-Pleistocene structures

Our study complements previous knowledge onthe pre-Miocene structures with new informationon the post-Miocene evolution of the Romanianshelf. The Mio-Pleistocene units have been ident-ified on seismic data and chronostratigraphicallycalibrated using borehole data. From the beginningof the Miocene to the present, these units are:Badenian–Sarmatian, Pontian (subdivided into P1,P2, P3 and P4), Dacian and Romanian–Quaternary(Fig. 3).

The Badenian and Sarmatian are undifferen-tiated on the Romanian shelf because of their limitedthickness. Our seismic data suggest that they arepresent only in two distinct areas: in the southernpart of the middle shelf in the area surroundingthe Delfin wells, and on the northwestern innerand middle shelf as well as on the palaeo-slopearound the Histria Depression (Fig. 4). Boreholedata suggest their presence also at the 1 Ovidiuand 12 Midia wells, but the reduced vertical resol-ution makes their identification on adjacentseismic lines difficult. The thickness distributionof these deposits varies from 0 m at the periphery

to 245 m in the internal part of the northwesternarea. In the southern area, the distribution patternis similar but thickness values of only about 100 mare reached.

Pontian deposits have been identified in all wellsdrilled on the Romanian shelf (Fig. 5). Theirthickness varies from 0 m on the inner shelf to4000 m on the outer shelf in the eastern Cobalcescuarea. This variation is due primarily to an increase insubsidence of the base of the Pontian to the east,non-uniform sedimentation rates on the shelf, andcomplex tectonics in the Cobalcescu area. TheDacian unit occurs over the entire study area andincreases in thickness from the inner to the outershelf, from values of 0 m in the vicinity of thecoast to 1150 m in the east and southeast (Fig. 6).The Romanian–Quaternary section occurs overthe entire study area as well. It shows only smallvariations in thickness from 0 m in the coastal areato 575 m on the outer shelf and the continentalslope (Fig. 7). The boundary between the Dacianand the Romanian–Quaternary deposits is conform-able. Therefore, their separation was based solely onborehole information.

These Mio-Pleistocene units are separated by thefollowing erosional unconformities (Fig. 3):

† the BBU, which represents the unconformityat the base of the Badenian–Sarmatiandeposits;

† the BPU, the unconformity at the base of thePontian;

† IPU 1, the unconformity that separates the P1and P2 subunits, occurs only locally on theinner shelf;

† IPU 2, separating the P2 and P3 subunits;† IPU 3, separating the P3 and P4 subunits; and

Fig. 3. Interpreted seismic line showing the distribution of the Mio-Pleistocene seismo-stratigraphic units: Badenian–Sarmatian, Pontian (1, 2, 3 and 4), Dacian and Romanian–Quaternary, and the main erosional unconformities: the BBU(the unconformity at the base of the Badenian), the BPU (the unconformity at the base of the Pontian), the IPU 1, IPU 2,IPU 3 (intra-Pontian unconformities) and the PDU (Pontian–Dacian unconformity) on the Romanian shelf. TWT,two-way travel.

C. KONERDING ET AL.162

† BDU, the unconformity near the top of thePontian as well as near the base of the Dacian.

Faulting activity and its classification

Faulting on the Romanian shelf evolved significa-ntly during the Mio-Pleistocene. It was most activein the Pontian. In contrast, the Badenian–Sarmatianand the Dacian–Quaternary are considerably morequiescent. The Euxinian Threshold separates twodistinct zones on the shelf: the northwestern zonewhich is practically unaffected by tectonic proce-sses, and the southeastern zone which is tectonicallyvery active in connection with the development ofthe Histria Depression.

The Badenian–Sarmatian was a period of tec-tonic quiescence; only local extensional structuresidentifiable on our seismic data developed (Fig. 4).The vertical offset of such structures is typicallyabout 350 m.

The Pontian was a time of intense tectonicactivity on the Romanian shelf. These activitiesdid not affect the entire shelf, but were concentratedin the area basinward of the palaeo-slope (the HistriaDepression) as well as in a small depression around

the Delfin wells (Fig. 5). The structures that devel-oped are mostly NE–SW trending grabens, horstsand flower structures (Figs 8 & 9). Faulting thataffected the Pontian section can be classified intwo ways: temporal and spatial. From the temporalpoint of view, we distinguish between faults that ori-ginated in the pre-Oligocene and continued theiractivities into the Pontian, faults that developedduring the Oligocene–Pontian, and faults thatwere active only during the Pontian. Spatially, fault-ing can be related to a NE–SW depression (whichwe call the ‘Pontian Depression’) with its depocen-tre linking the 1 Ovidiu and 75 Cobalcescu wells(Fig. 5), and gravitational faulting at the shelf-break.

Temporal classification of the Pontian faults. Thepre-Oligocene faults are related to the formationof the Histria Depression (Figs 8 & 9). Presumablythese faults were reactivated during the LowerPontian (Tambrea et al. 2000) in an extensionalregime characterized by NE–SW trending normalfaults and grabens because of large sedimentloading and rapid subsidence (Fig. 8). Verticaloffsets are in the order of a few metres up to about800 m. They are mostly present in the central and

Fig. 4. Isopach map of the Badenian-Sarmatian section showing only local faulting during this period. The location ofthe Histria Depression as well as the positions of the major faults (Peceneaga-Camena and Capidava-Ovidiu) arealso given.

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 163

northern Histria Depression (around and NE of thewell 75 Cobalcescu in Fig. 5), but some of themdeveloped also on the outer shelf (Fig. 9).

The second category of faults that started theiractivity in the Oligocene and continued into thePontian are gravitational faults that developed insimilar sedimentary facies existing both duringOligocene and Pontian times (Figs 8 & 9). Thecentral and northern Histria Depression is an exten-sional zone characterized by NE–SW trendingnormal faults and grabens that extend northeast-ward. The vertical offsets vary from a few metresto almost 1000 m. To the south, extensional faultingwith offsets of less than 100 m occurs. Because ofthe lack of seismic data, however, it is not possibleto follow these faults to the NE. Transfer faultingbetween these faults and the northern HistriaDepression has not been observed. The northeasternpart of the outermost shelf is characterized byNE–SW oriented negative flower structures, ahorst and normal faults over a distance of about20 km (Fig. 9). The horst is developed along theshelf-break and has a lateral extent of 13 km in thenortheast, increasing to 20 km in the SW. Verticaloffsets are in the order of a few metres up to about

700 m. A NW–SE oriented transfer fault separatesthis complex from the southern fault system.On the southeastern and southern outer shelves,NE–SW striking normal faults, grabens and nega-tive flower structures developed locally. They arebounded by transfer normal and thrust faults witha NW–SE trend. The Delfin area is less affectedby faulting (Fig. 5); normal faults with verticaloffsets of only a few metres have been mapped.

The last category consists of gravitational faultsthat developed only during the Pontian. Some ofthem were active only in the Lower Pontian in rela-tion to the extensional regime (Fig. 8), whereasothers continued their activity in the UpperPontian and in the Dacian–Quaternary (Fig. 9). Inthe Lower Pontian, roll-over structures character-istic of an extensional regime formed (Figs 8 & 9).

Spatial classification of the Pontian faults. The firstcategory consists of faults related to the develop-ment of the Pontian Depression. They strike bothNW–SE (parallel to the Histria Depression),and NE–SW (crossing the wells 1 Ovidiu and 75Cobalcescu; Fig. 5). The NE–SW striking faultswere developed during the Pontian extension.

Fig. 5. Isopach map of the Pontian deposits showing the general NE–SW pattern of faulting during this period of time.The wells used for calibration are also shown.

C. KONERDING ET AL.164

The second class of faults formed along theshelf-break under a gravitational regime. Some ofthem originated in the Oligocene while othersdeveloped during the Pontian. These faults wereactive till the Quaternary. Their vertical offsets arein the order of a few metres. Basinward of the shelf-break, the Pontian sediments were deposited in atectonically quiescent regime.

The Dacian–Quaternary section is less faultedthan the Pontian sequences. Most of the faultsobserved here are Oligocene in age or older, andwere reactivated during the Pontian (Fig. 9). Verti-cal offsets are in the order of a few metres to tensof metres. The fault systems trend NE–SW andare separated by NW–SE oriented transfer faults.Only in the west does the fault trend change toeast–west.

Mio-Pleistocene subsidence history of the

Romanian shelf

The subsidence of a passive continental margin iscontrolled by a number of factors, of which sedi-ment loading and thermal cooling are the mostimportant. Our subsidence analysis is based on

lithostratigraphic information from 11 wells locatedon the Romanian shelf (Figs 10 & 11). The wellswere projected along two lines across the shelf,one striking WNW–ESE and the other NE–SW(Fig. 1). The present-day stratigraphic thicknessvalues were corrected for compaction, palaeo-bathymetry and absolute water-level fluctuations.The palaeo-water depth was estimated from pala-eontological information (Tambrea et al. 2000).The occurrence of a relatively rich association ofostracods indicates a brackish-to-fresh water envi-ronment with a palaeo-water depth estimated atabout 20 m. Information on water-level fluctuationswas taken from a water-level curve estimatedfor the Romanian shelf (Konerding 2006). Theunconformities at the base of the Badenian(c. 16–15.8 Ma) and at the base of the Pontian(c. 9–7.3 Ma) were taken into account (Fig. 9).We assume that during the 16–15.8 Ma and9–7.3 Ma time periods only erosion took place onthe Romanian Black Sea shelf. The subsidencewas computed over the time interval 16 Ma (begin-ning of the Miocene) to the present.

The subsidence history of lines 1 and 2 is shownin Figures 12 and 13 respectively. The history of tec-tonic subsidence and the history of total subsidence

Fig. 6. Isopach map of the Dacian section showing the faults that were active during this period.

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 165

are similar along lines 1 and 2. From the beginningof the Miocene to the end of the Sarmatian, subsi-dence was slow. It increased abruptly during thePontian, slowed down during the Dacian, and

almost came to a halt in the Romanian–Quaternary.Along line 1, the rate of total subsidence during theBadenian–Sarmatian was small (Fig. 12). Subsi-dence increased from 27 m in the well 814 Lebada

Fig. 8. Seismic line crossing the Romanian shelf in a WNW–ESE direction. Note the intense Pontian faulting activityin the Histria Depression. TWT, two way travel time.

Fig. 7. Isopach map of the Romanian–Quaternary section. Note that faulting is predominantly NE–SW.

C. KONERDING ET AL.166

(on the inner shelf) to 125 m in the well 1 Ovidiu (onthe outer shelf). By the end of the Pontian, the totalsubsidence amounted to 584 m on the inner shelfand 2803 m on the outer shelf, implying an increaseof 557 m and 2678 m in these two areas respect-ively. At the end of the Dacian, the total subsidencereached 733 m on the inner shelf and 3428 m on theouter shelf. This suggests a much smaller increase insubsidence during the Dacian, namely 149 m on theinner shelf and 625 m on the outer shelf. The corre-sponding values for the Romanian–Quaternary are175 m (inner shelf) and 242 m (outer shelf) respect-ively. Thus, the Romanian shelf was largely stableduring the Badenian–Sarmatian and the Dacian–Quaternary, but was strongly subsiding during thePontian. This large increase in subsidence, both inthe vertical (for each borehole) and lateral (basin-ward) directions was a result of intense tectonicactivities and high sediment supply during thistime period.

Subsidence along line 2 (NE–SW; Fig. 13) issignificantly lower than along line 1 (WNW–ESE;Fig. 12). Line 2 was stable during the Badenian–Sarmatian, with a total subsidence of 9 m exceptfor the well 18 Lotus, where 11 m of uplift tookplace. During the Pontian, the total subsidenceincreased considerably in the southern area ofwells 10 Tomis and 6 Delfin. In the Lebada andLotus boreholes, the total subsidence was onaverage about 400 m higher than during theBadenian–Sarmatian, while for the Tomis andDelfin boreholes, this value is over 1100 m. This

difference is attributed to the different positions ofthe boreholes on the shelf. Lebada and 18 Lotuswells are located on the inner shelf, whereas 10Tomis and 6 Delfin lie farther basinward where thebase of the Pontian began to subside strongly. TheDacian and the Romanian–Quaternary depositswere less affected by subsidence. During theDacian, the total subsidence decreased from 208 m(at the end of the Pontian) in the well 25 Lebadato 150 m in the well 18 Lotus and increased againto 223 m in the well 6 Delfin. The Romanian–Quaternary is marked by very small changes insubsidence, from 148 m in the north to 142 m inthe south.

We assume that the amount of sediment deliv-ered to the Black Sea during Mio-Pleistocene timewas related to uplift of the Carpathians as well asto a possible Messinian drop of the Black Sealevel and rapid erosion of the exposed areas (corre-lated with the intra-Pontian IPU2; Gillet et al. 2007).The distribution pattern of Mio-Pleistocene sedi-ments in the Romanian foreland basin (the DacicBasin; Jipa 1997; Gillet et al. 2007) correlates nega-tively with that on the Black Sea shelf. Duringthe Badenian–Sarmatian when uplift of the Car-pathians started, a large amount of sediment wasdelivered to and deposited in the foreland (.5 km;Tarapoanca 2004). In contrast, on the Black Seashelf, Badenian–Sarmatian sediments are presentonly locally and are very thin (,250 m). UpperMiocene (Meotian) deposits reach a thickness ofup to 1.6 km in the foreland basin, while on the

Fig. 9. Seismic line crossing the Romanian outer shelf from NW to SE. TWT, two way travel time.

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 167

Fig. 10. Lithological description of the wells projected on line 1, the WNW–ESE profile.

C. KONERDING ET AL.168

Fig. 11. Lithological description of the wells projected onto line 2, the NE–SW profile.

ST

RU

CT

UR

EA

ND

SU

BS

IDE

NC

EH

IST

OR

YO

FT

HE

RO

MA

NIA

NB

LA

CK

SE

AS

HE

LF

169

Black Sea shelf they are absent. In the uppermostMiocene (Pontian), a sediment layer of 1.6 kmmaximum thickness was laid down in the DacicBasin. A major part of the sediment supplied fromthe Carpathians, together with erosional productsfrom the subaerial parts of the shelf and slopeexposed by a possible Messinian draw-down of thesea level, was transported into the Black Sea,where the Pontian reached a thickness of 4 km inthe central Histria Depression. During the Dacian–Quaternary, up to 4.5 km of sediment were depositedin the Dacic Basin, while on the Black Sea shelf,thicknesses of only about 1.5 km were reached.

The tectonic subsidence curves, obtained byremoving the effect of sediment loading from thetotal subsidence, show a trend similar to that ofthe total subsidence curves and suggest that thetotal subsidence is controlled more by sedimentloading than by vertical tectonics. Tectonic subsi-dence of the Badenian–Sarmatian deposits isobserved only in the well 13 Heraclea, where itreached 19 m at the beginning of the Badenian and

28 m at the end of the Sarmatian. In all of theother wells, uplift of a few metres took place.During the Pontian, tectonic subsidence along line1 (WNW–ESE) increased basinward from anaverage of 190 m on the inner shelf to 325 m atthe well 13 Heraclea and 751 m on the outer shelf.Thus, tectonics contributed significantly to totalsubsidence on the outer shelf, where intense tectonicactivity took place during the Pontian. In the north-eastern segment of line 2 (NE–SW), the averagetectonic subsidence was 130 m except for the well18 Lotus, where a value of only 67 m wasdeduced. In the SW, it increased to 300 m. Duringthe Dacian, the tectonic subsidence increasedabout 90 m along line 1 as compared to the end ofthe Pontian, except for the well 1 Ovidiu, where theincrease was 205 m. Along line 2 the correspondingincrease was 90–100 m. For the Romanian–Quaternary, the tectonic subsidence amounted to80–90 m.

To summarize, vertical tectonics had lessinfluence on the Mio-Pleistocene subsidence than

Fig. 12. Subsidence history along line 1 from the Miocene to the present. Zero denotes present-day water level of theBlack Sea.

C. KONERDING ET AL.170

sediment loading. Only on the outer shelf whereintense tectonic activities occurred during thePontian and in part during the Dacian did verticaltectonics play an important role.

Mio-Pleistocene seismic sequence

stratigraphy of the northwestern Black

Sea shelf

A sea level curve for the Black Sea shelf during theMio-Pleistocene time span was estimated (Fig. 14).This was done using the global sea level curve (Haqet al. 1987), an unpublished sea-level curve basedon seismic data from the northwestern Black Seashelf (Tambrea et al. 2000; Konerding 2006;Tambrea, pers. comm.), and salinity information(Jones & Simmons 1997). Because the unpublishedsea-level curve for the northwestern Black Seashelf gives only relative sea levels, it was correlatedwith the sea-level curve of Haq et al. (1987), toconvert it into a curve with absolute values for the

present study. This correlation was carried out bychoosing four points on the curve of Haq et al.(1987), during a period when the Black Sea wasconnected to the global oceans (Fig. 14). To identifythese points, information on palaeo-salinity wasused, assuming that salinities of 20–30‰ indicatemarine conditions. At the four points chosen, theabsolute sea levels on the two curves were identical.This provides the necessary calibration of the absol-ute sea-level scale.

In order to estimate the depth of the Badenian–Sarmatian sequence at the time of deposition, theinfluence of subsidence from the Badenian to thepresent was taken into account. In the northwesternarea, this total subsidence is 38 m, suggesting thatthe Badenian–Sarmatian section was located 38 mhigher than the present-day seafloor at the time ofdeposition. Borehole information points to a palaeo-water depth of 50 m in this area (Konerding 2006;Tambrea, pers. comm.), implying that the sectionwas located 12 m below the present-day sea levelon the inner shelf at the time of deposition. In the

Fig. 13. Subsidence history along line 2 from the Miocene to the present. Zero denotes present-day water level of theBlack Sea.

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 171

south, the total subsidence is 35 m and the palaeo-water depth was about 65 m, yielding a location30 m below the present-day sea level on the innershelf at the time of deposition. The internal con-figuration of the unit (Fig. 15) and its inner shelfposition suggest that it was deposited during a latehighstand when the sea level began to fall slowlyafter reaching a maximum. At that time, the rateof deposition was higher than that of sea level fall;the parasequences prograded basinward and down-lapped onto the lower sequence boundary.

Where Pontian 1 (P1) is developed, the totalsubsidence is 89 m from the beginning of thePontian to the present. This suggests a position ofP1 89 m below the present-day seafloor, whichwas 50 m below the present-day sea level. This inturn implies a location on the slope at the time ofdeposition, since the Black Sea level at P1 timewas about 80 m higher than that of today

(Fig. 14). The onlap terminations against the lowersequence boundary coupled with the progradationalnature of the subunit (Fig. 16) suggests that it wasdeposited during a late lowstand, when sea levelbegan to rise very slowly. The high sedimentsupply and slow sea level rise led to a progradationaldepositional pattern, corresponding to the lowstandwedge of a lowstand systems tract (LST).

Subunit Pontian 2 (P2) exhibits two differentseismic facies patterns. On the western inner shelf,it is thin and is characterized by divergent, discon-tinuous, low-amplitude reflectors that onlap thelower boundary, while the upper boundary is rep-resented by an erosional truncation (Fig. 17). Onthe middle and outer shelves, its thickness increasesrapidly eastwards and it is characterized either bymedium amplitude reflectors parallel to the lowerboundary, or occasionally by chaotic reflections(Fig. 17). Both the lower and the upper boundaries

Fig. 14. Chronostratigraphic calibration of the Mio-Pleistocene seismic units from the Romanian Black Sea shelf. PDU,Pontian-Dacian unconformity; IPU 1, IPU 2, IPU 3, intra-Pontian unconformities; BPU, erosional unconformity at thebase of the Pontian; BBU, erosional unconformity at the base of the Badenian; IPU 1 occurs only locally in a limitedarea; MFS, maximum flooding surface; TS, transgressive surface; SB, sequence boundary. Sea-level curve is relative tothe present-day sea level (0 m). The interval in black on the water-level curve was estimated for the Romanian shelf,while the grey portion of the curve was taken from Haq et al. (1987). The geologic periods shown in grey illustrate theMio-Pleistocene erosional unconformities.

C. KONERDING ET AL.172

are erosional. The internal configuration and exter-nal geometry of subunit P2 suggests that at thetime of deposition, it straddled the continentalslope and the deep basin. Since basinal depositionis possible only during a lowstand when the shelfis exposed and the sediment bypasses the shelf tobe laid down on the slope and in the basin, thisunit is attributed to a LST.

For the subunit Pontian 3 (P3), only an approxi-mate subsidence correction is possible because ofinsufficient borehole information. On the outer shelf,a difference of .200 m between the present-day

depth of P3 and the total subsidence was estimated.At the time of deposition, P3 was over 300 m belowthe present-day sea level. The reflection termin-ations and the wedge-shape (Fig. 18) suggest thatthis subunit was deposited during a sea level low-stand. The sea level fall that produced the lowersubunit boundary was beyond the shelf-break; theshelf became subaerial and rivers incised into theexposed shelf. The rate of sea level fall exceededthe rate of subsidence, and a large sediment supplyproduced basinward progradation of sedimentsdeposited on the slope.

Fig. 15. Detailed view of the Badenian–Sarmatian section from the northwestern middle shelf; grey arrows markdownlap and toplap terminations respectively.

Fig. 16. Detailed view of the section P1; grey arrows indicate onlap terminations against the lower subunit boundary(also a sequence boundary).

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 173

For the subunit Pontian 4 (P4), a subsidencecorrection is not possible because of the lack ofborehole information for this interval. Based onreflection terminations and the seismic faciescharacteristics (Fig. 19), we speculate that P4 waslocated on the shelf at the time of deposition. Thelower part of the subunit is attributed to a trans-gressive systems tract (TST), which formed as sealevel began to rise and reached the shelf-edge.Thereafter, accommodation space was created ata rate faster than it could be filled with sedimentand a retrogradational pattern marked by onlapterminations developed. At the end of the TST, thesea level rose at a rate faster than the rate of sedi-ment supply to the basin and a maximum flooding

surface formed. Thereafter, sea-level rise sloweddown, while the rate of sediment supply increased;an aggradational pattern typical for the HSTdeveloped. The subsequent sea-level fall led to theformation of a sequence boundary.

The external geometry of the Dacian unit(Fig. 20) suggests that it was located on the shelfat the time of deposition. After the formation ofthe previous sequence boundary, sea level fellbelow the shelf-break. The continental shelf wasexposed and retrogressive subaerial erosion of theshelf-break took place, leading to c. 300 m deep,canyon-like incisions (Gillet et al. 2007). Sub-sequent to the lowstand, the incised canyons werefilled transgressively during Dacian time.

Fig. 17. Details of subunit P2 showing its characteristics on the inner shelf (above) and on the middle and outer shelves(below). In the upper figure grey arrows indicate onlap terminations against the lower subunit boundary. Note theerosional truncations at both the upper and lower boundaries (below).

C. KONERDING ET AL.174

We presume that the Romanian–Quaternarysection (Fig. 20) was deposited after the trans-gressive phase of the Dacian during a sea levelhighstand. As the rate of sea-level rise sloweddown after reaching a maximum, sediment supplyincreased and a maximum flooding surfaceformed. Sediment continued to be delivered at afaster rate than that of sea-level rise and typicalaggradational parasequences developed.

Stratigraphic modelling

The results obtained here were used to create a sedi-mentation model that reproduces the findingsdescribed above. The model was generated in

order to determine how different parameters con-trolled the depositional processes. The position ofthe modelled profile was constructed on the basisof six wells distributed from the inner to the outershelf, following the direction of line 1 from thesubsidence analysis (Fig. 1).

The modelled profile has a length of 71 km, andthe simulation started at the base of the Badenian(16 Ma BP). Lithological information from bore-holes suggests that only a marginal amount of car-bonates are present within the basin; almost allsediments are clastic. The direction of the simulateddeposition is from NW to SE, namely from the left.

Modelling was carried out taking into accountthe effect of isostasy during deposition. Hereby the

Fig. 18. Details of the subunit P3. Grey arrows indicate downlap terminations against the lower subunit boundary.

Fig. 19. Details of subunit P4. Grey arrows indicate onlap terminations against the lower subunit boundary.

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 175

lithosphere was assumed to behave as an elasticbeam with a flexural rigidity of 1023. Informationon the water-level curve and subsidence history isbased on this study. The compaction parametersfor this area are from Ionescu (2000).

The best-fit model of the simulation is shown inFigure 21 (upper panel), while the correspondingWheeler diagram is shown in the lower panel. Forcomparison, a seismic profile that crosses the shelfin a NW–SE direction close to the modelled lineis shown in Figure 22. Simulation began with theerosional unconformity that marks the base of theBadenian–Sarmatian unit (BBU). The final modelshows that the Badenian–Sarmatian formationextends to the SE on the outer shelf, while this exte-nsion is not obvious in the seismic data because oftheir low vertical resolution. The second unconfor-mity at the base of Pontian (BPU) could also besimulated over the entire profile. However, onlytwo of the intra-Pontian unconformities, namelyIPU 2 and IPU 3, could be simulated. IPU 1 occursonly locally in the seismic data and did not appearin the simulation. The Pontian deposits could besimulated satisfactorily, but this is not the case forthe position of IPU 2 and IPU 3 because of insuffi-cient data. While the shapes of these unconformitiesare well modelled on the inner shelf (left part ofFig. 21), they are less precise basinward, wherethe dip of IPU 2 is too large and the position ofIPU 3 is too high. We attribute this to a lack oflithological information between the wells 13 Hera-cleea and 40 Albatros as well as between 40 Alba-tros and 1 Ovidiu. A second problem concerns theuncertainties in the age of the formations boundingIPU 2 and IPU 3 since these intra-Pontian uncon-formities are not seen in the borehole data. In addi-tion, the strong influence of tectonics on Pontian

sedimentation in this area could not be taken intoaccount during the simulation. The internalseismic characteristics of the intra-Pontian unitsP2 and P3 could be well modelled. The LowerPontian comprises two different seismic facies:thin divergent reflectors on the inner shelf and par-allel reflecting packages that thicken basinward onthe middle and outer shelves. P3 (the intra-Pontianunit between IPU 2 and IPU 3) has a modeled thick-ness larger than that deduced seismically but has therequired internal pattern, namely wedge-shapedreflectors that downlap at the lower boundary andare erosionally truncated at the top. They dipsteeply in the west but are almost horizontal in theeast. Above IPU 3, parallel reflectors characteristicfor the Upper Pontian–Quaternary are modelled.

The Wheeler diagram in the lower panel ofFigure 21 shows the corresponding chronostrati-graphic development. During the first 7 Ma ofsimulation, the Badenian–Sarmatian unit, whichhas a non-depositional hiatus in the middle part ofthe profile, was deposited. Two periods of non-deposition (3–3.4 Ma and 4–5 Ma simulationtime, ST) occurred in the proximal section. TheBadenian–Sarmatian deposits are thin, reachingonly a maximum of 250 m on the inner shelf.This low sedimentation rate was controlled largelyby low sediment input (most of the sedimentseroded from the Carpathians were deposited in theDacic Basin) and slow subsidence, with sea levelfluctuations playing a minor role. At the end of theBadenian–Sarmatian (c. 8.5 Ma ST), the Wheelerdiagram shows another hiatus on the inner shelf(left side of the diagram) which corresponds to thesequence boundary (BPU) that formed in responseto sea level fall. Basinward the hiatus pinches outand the water depth increases abruptly from about

Fig. 20. Details of the Dacian, Romanian and Quaternary section. Incised valley of about 300 m depth occurs at the baseof the Dacian incised into the Pontian.

C. KONERDING ET AL.176

Fig. 21. Modelled profile for the simulation with isostatic compensation; the flexural rigidity was chosen to be 1023. Upper panel: the simulated model with colour-coded waterdepths at the time of deposition of the different units. Lower panel: the corresponding Wheeler diagram. For this panel, the vertical scale is simulation time in years, 0 marks the startof simulation. The Black Sea water-level curve is plotted in light blue on the right, the vertical violet line represents present-day sea level. The light blue line to the left of the violetline gives the water-level scale. The water depth at the time of deposition is colour-coded. Light and dark grey fields mark the areas of zero accumulation: the former corresponds tolocations where sediment was deposited but was subsequently eroded, and the latter locations where deposition never occurred (hiatuses).

ST

RU

CT

UR

EA

ND

SU

BS

IDE

NC

EH

IST

OR

YO

FT

HE

RO

MA

NIA

NB

LA

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SE

AS

HE

LF

177

Fig. 22. Seismic profile crossing the shelf in a NW–SE direction, chosen for comparison (see Fig. 1 for profile location).

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200 to 800 m. Above the BPU, the palaeo-waterdepths varies from c. 50 m on the present-dayinner shelf to .1000 m on the present-day outershelf. The unconformity IPU 2 (c. 10–10.5 MaST) could be simulated only in three areas, else-where there is an abrupt increase in water depth.IPU 2 corresponds to a transgression and the sub-sequent highstand, as indicated by the sea levelcurve. Above IPU 2, subunit P3 developed atwater depths of c. 50 m on the present-day innershelf to 1700 m on the present-day outer shelf.That is, the depositional environment was a shelfproximally and a basin distally. IPU 3 (c. 11.5–11.75 Ma ST) is represented on the Wheelerdiagram only in areas where borehole data areavailable. The thick, wedge-shaped Pontian sectiondeposited only in about 2 Ma marks a time when theDacic Basin was filled and sediments were laiddown on the continental shelf and beyond. Theyoungest (Romanian, c. 1.3 Ma ST) unconformityvisible in the Wheeler diagram could not be ident-ified in the seismic data. Our simulation showsthat Romanian–Quaternary sedimentation after theformation of this unconformity was quiet.

Conclusion

Structural and sedimentological analyses of theMio-Pleistocene deposits on the Romanian BlackSea shelf using seismic and borehole data show achanging evolution during a relatively short periodof time. This evolution is directly related to thepre-Miocene evolution of the Romanian shelf. Theavailable seismic data, which is relatively uniformlydistributed over the study area, yielded importantinformation on the tectonic evolution of thesedeposits.

Mio-Pleistocene subsidence of the RomanianBlack Sea shelf is directly dependent on sedimentinput, tectonic activity as well as water-level fluctu-ations and is highly variable. Subsidence during theBadenian–Sarmatian and the Dacian–Quaternarywas limited. The amount of subsidence calculatedalong the WNW–ESE profile increases slowlyfrom the inner to the outer shelf. In the NW–SEdirection, subsidence was slow and constant. Incontrast, during the Pontian, shelf subsidence wasprogressively faster in the basinward direction.Subsidence on the outer shelf was much more sig-nificant than elsewhere on the shelf. This is theresult of a large sediment input to the basin andof significant tectonic activity in the HistriaDepression.

The most tectonically active period during theMio-Pleistocene was the Pontian. The Badenian–Sarmatian was largely quiescent and the Dacian-to-Quaternary saw a decrease in the Pontian tectonicactivity, coming possibly even to a halt. We

classified the faulting that affected the Pontian sedi-mentary section both temporally and spatially. Fromthe temporal point of view, we distinguish between(1) faults that originated in the Pre-Oligocene andremained active up to the Pontian; (2) faults thatdeveloped during the Oligocene–Pontian; and (3)faults that were active only during the Pontian.The first are supposedly reactivated faults thatdeveloped during the formation of the HistriaDepression, the second are gravitational faults thatdeveloped in sediments of a similar facies, and thelast are gravitational faults related to the extensionalphase in the Pontian. Spatially, we distinguishbetween (1) extensional faults that accompaniedthe development of the Pontian Depression; and(2) gravitational faults at the shelf-break.

Based on the available seismic data, the recon-structed subsidence history and the water levelcurve, the Mio-Pleistocene sedimentary section wasdivided into eight systems tracts. The Badenian–Sarmatian unit was attributed to a HST (highstandsystems tract), being deposited in a late highstandwhen the sea level began to fall slowly after reach-ing a maximum. The next cycle cannot be observedin the seismic data, because it corresponds to sedi-mentation on the Romanian shelf during Meotiantime, and was subsequently completely eroded.The Pontian subunit P1, which was laid down onthe slope at the time of deposition, is progradationaland attributable to the lowstand wedge of a LST.Subunit P2 is likewise also attributed to a LST,having the continental slope and the deep basin aspalaeo-depositional environments. Because theboundary between units P1 and P2 occurs onlylocally and could not be correlated with our esti-mated sea-level curve, it was presumed to haveformed during a higher order sea-level cycle.Subunit P2 is followed by a non-depositionalhiatus that separates P2 from P3. On the sea levelcurve, it corresponds to the HST and TST thatfollowed the LST of P2 and ends with a sequenceboundary. The reflection terminations and thewedge-shape of P3 suggest that it was deposited inthe deep basin during a sea level lowstand. The suc-ceeding TS (transgressive surface) corresponds onthe seismic data to the erosional unconformitybetween the subunits P3 and P4. This TS is followedby the next TST and HST that developed during thedeposition of P4, but a MFS (maximum floodingsurface) is missing in the seismic data because ofinsufficient vertical resolution. Subunit P4 was dep-osited on the inner and middle shelves, as deducedfrom its reflection terminations and seismic faciescharacteristics. The boundary between P4 and theDacian is represented by another erosional hiatus,which comprises the LST that follows the formationof the sequence boundary at the end of the Pontian.During the Dacian–Quaternary, the subsequent TST

STRUCTURE AND SUBSIDENCE HISTORY OF THE ROMANIAN BLACK SEA SHELF 179

and HST were deposited on the inner and middleshelves. The MFS that separates the TST and theHST could not be observed in the available data.

The stratigraphic model obtained by simulationof the seismic section, in which the subsidencehistory and the Black Sea water-level curve weretaken into account, shows a strong influence of sedi-ment input and subsidence on sedimentation, whilesea level fluctuations played a lesser role. The sedi-ment input is related to the evolution of the adjacentland, while subsidence is dependent on sedimentsupply, tectonic activity and sea level fluctuations.Thus, these two factors are not totally independent.

Gillet et al. (2007) and Hsu & Giovanoli (1979)argued convincingly for a temporary Messinian eva-porative draw-down of the Black Sea from 6.25 to5.2 Ma contemporary with the Messinian salinitycrisis of the Mediterranean. Our proposed sequencestratigraphic framework is consistent with theirscenario. The draw-down event is marked by ourIPU 2, the equivalent of their IPU. However, ourproposed subsidence history as well as our recon-structed palaeo-water-level curve does not have aresolution high enough to verify their hypothesis.

The authors would like to thank the National Agency forMineral Resources of Romania and PETROM OilCompany which gave us access to seismic and well data.Thanks are also due to Dr I. Popescu for her constructivereview of our manuscript. For one of the authors (CD)this work was supported by CNCSIS project numberIDEI 960/2007.

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