Technika Poszukiwañ GeologicznychGeosynoptyka i Geotermia nr 1/99

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Zbigniew PERSKIUniversity of SilesiaBêdziñska 60, 41-200 SosnowiecPoland

POSSIBLE CORRELATIONS BETWEEN THE BASEMENT AND NEAR-SURFACE STRUCTURES: A REASSESSMENT

INTRODUCTION

Two study areas were chosen, each of which containscomplex subsurface structures. Near Wierzchowo,in the Pommeranian Synclinorium, there are twoanticl ines and, near P³oñsk, in the WarsawSynclinorium, grabens are present. Both of thestudied areas are located in the vicinity of the contactbetween the East European Craton and the PaleozoicPlatform in the southwest (Fig.1). No halokineticstructures are known from either area [10,21,25], sothe structures here may be regarded as createdduring typical tectonic processes. They have had along-lasting development during the evolution of theMid-Polish Swell [9,10].

WIERZCHOWO AREA

Analysis of this area is based on the seismic reflectionstudy performed by the Polish Oil and Gas Companyin 1996 [26]. In the Wierzchowo area, two localanticlines are developed in Mesozoic rocks. They are

4 and 16 km long and 4 and 6 km wide respectively,and both have an amplitude of 800 m (Figs 2 to 4).The thickness of Permian and Triassic sediments ishere variable, and depends on the activity within the

Ryc. 1. Lokalizacja obszarów badañFig. 1. The location of study areas on the background of theregional geological provinces map

ABSTRACT

This work attempts to assess the extent of the relationships be-tween subsurface and surface dynamics in local tectonic struc-tures. Based on cartographic, seismic reflection and boreholedata two study areas containing complex subsurface structureswere chosen. Near Wierzchowo (Pommeranian Synclinorium)there are two anticlines and, near P³oñsk � in the Warsaw Sync-linorium, grabens are present. The structures have had a longterm development during the evolution of the Mid-Polish Swell.Any positive relationship between local subsurface structures andsurface lineations has yet to be demonstrated, a conclusion ba-sed on the lack of any obvious Recent tectonic activity. Howe-ver, it seems possible that some of the tectonic features of thearea might be identified in regional scale. In the local structuresrenewed movement on old dislocations might be dissipatedupwards and may be assumed to have different spatial proper-ties in near-surface conditions. The Recent structural patternseems more likely to be related to the nature of the Pleistoceneice sheets which have covered that area.

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STRESZCZENIE

Niniejsza praca jest prób¹ znalezienia zwi¹zków pomiêdzy lo-kalnymi strukturami tektonicznymi a elementami dynamiki po-wierzchni terenu. Analiza ta oparta zosta³a na materia³ach kar-tograficznych, opracowaniach sejsmiki refleksyjnej, profilachwiertniceñ. Do jej przeprowadzenia wytypowano dwa obszary cha-rakteryzuj¹ce siê wystêpowaniem z³o¿onych struktur wg³êbnychi d³ug¹ histori¹ rozwoju. S¹ to antykliny w okolicach Wierzchowa(Synklinorium Pomorskie), oraz rowy tektoniczne rejonu P³oñ-ska na obszarze Synklinorium Warszawskiego. Nie stwierdzonozwi¹zku lokalnych struktur wg³êbnych z powierzchniowymi struk-turami linijnymi co mo¿e to byæ wyrazem braku wspó³czesnejaktywno�ci tych struktur. Pewien wp³yw kierunków pochodz¹cychod wa³u �rodkowopolskiego zaznacza siê jednak w skali regio-nalnej. Aktywno�æ wzd³u¿ odnawianych starych dyslokacji mo¿esiê w tym przypadku rozpraszaæ ku górze i przyjmowaæ odmien-n¹ charakterystykê przestrzenn¹,. Wystêpowanie osadów czwar-torzêdu mo¿e skutecznie t³umiæ wp³ywy dynamiki pod³o¿a i na-rzucaæ plan strukturalny zwi¹zany z aktywno�ci¹ plejstoceñskie-go l¹dolodu.

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MO¯LIWO�CI KORELACJI STRUKTUR PRZYPOWIERZCHNIOWYCHZE STRUKTURAMI POD£O¯A: ANALIZA KRYTYCZNA

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Ryc. 2. Przekrój sejsmiczny (wg. [26] zmodyfikowany) z lokalizacj¹ otworów wiertniczych przedstawiaj¹cystruktury tektoniczne obszaru WierzchowaFig. 2. Seismic section (after [26], re-interpreted) with boreholes locations showing the tectonic structures inWierzchowo area

Ryc. 3. Profil sejsmiczny (wg. [26, zmodyfikowany) przedstawiaj¹cy inwersyjne struktury listryczne obszaruWierzchowaFig. 3. Seismic section (after [26], re-interpreted) showing the inverted listric structures in Wierzchowo area

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base of Paleozoic basement rocks [1,2]. Geologicalinterpretation of seismic sections (Figs 2 and 3)involved the application of the model of listric faultingand inverted extensional basins [3,17,27]. Thebasement in the Wierzchowo area is cut by series oflistric faults which were active during Permian andTriassic deposition (Figs 2 and 3). In Jurassic andCretaceous times, synsedimentary activity of theselistric faults decreased, but influenced the thicknessof the Jurassic and Cretaceous deposits (Fig 2).

The Mesozoic tectonic activity can be related toprevailing extension along the edge of the EastEuropean Craton [10,11,27]. The sedimentation in themarginal part of the Mid-Polish Trough was probablycontrolled by the development of listric faults in thebasement [1,21].

Inversion of the basin during the Alpine orogenyreversed movements along the listric faults (Fig. 3).The unconformity between deformed units andoverlying sequences can be recognised in seismicreflection profiles (Fig. 2); its age can be determinedfrom the borehole data [20,4,5]. In the Wierzchowoarea these phenomena took place at the end of theSantonian or the Cenomanian and before theMaastrichtian.

The thickness of the Upper Cretaceous and Tertiaryrocks overlying inverted units is variable and dependson the shape of anticlines. Depth to the base of theTertiary deposits is 50 m greater over a syncline thanover an anticline. This variable thickness can berelated to the erosion of the deformed units (which

took place during of after deformation) or to the finalstages of inversion.

The GIS analyses (Fig. 4) of local trends from thestructural map of the top of the Lower Cretaceousdeposits [26] and paleovalley trends from the map ofthe base of Quaternary deposits [7,14] show theprobable impact of the basement activity on theTertiary surface morphology. The relief of the top ofthe Tertiary reveals that the paleovalleys follow thetectonic trends of the top of the Lower Cretaceous.However this observation must be regarded in thelight of the differing quality of the materials used. Theaccuracy of the map of the base of the Quaternarydeposits is lower than that of the structural map ofthe top of the Lower Cretaceous deposits. Theisolines showing the morphology of the top of theTertiary deposits have been digitised from thegeological map at 1:200 000 scale based only onrare borehole data. The structural map of the top ofthe Lower Cretaceous is at 1:50 000 scale and isbased on data from seismic reflection and boreholes.

Generally, taking into consideration the scale of theseismic work, the reflections observed in the Tertiaryare of poor quality, difficult to interpret, and thereforecannot be used for structural correlation [24]. Small-scale reflective elements found within some of theseismic sections can be recognised as tectonicfeatures, especially those located exactly under thedistinct linear structures observed on the terrainsurface. The results of tracing these elements on theneighbouring seismic sections did not confirm theircontinuity.

The comparison analysis of the local structural trendsat the top of the Lower Cretaceous and the surfaceterrain does not show a clear relationship. Lineartrends on the geological map (spatial distribution ofthe Quaternary units, morphological and drainagepatterns especially) do not agree with the structuresof the sub-Quaternary units.

P£OÑSK AREA

Within the P³oñsk area, located several kilometresfrom the southwest edge of the East European Craton(Fig. 1), a characteristic graben system 60-80 km longand 2-4 km wide (Fig. 5) occurs in Mesozoic rocks.This graben system was well recognised on theseismic profiles and confirmed by borehole data [22].

The graben�s tectonics is shown on a migratedseismic reflection section (Fig. 5). One can recognisethat the geological development of this structure isvery similar to that of the Wierzchowo area. The

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Ryc. 4. Analiza porównawcza w systemie GIS lokalnych trendóww/g mapy stropu utworów dolnej kredy oraz utworówczwartorzêdowychFig.4. The GIS comparison analysis of local trends from the mapof top of Lower Cretaceous deposits and trends from the map ofthe base of Quaternary sediments

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Ryc. 5. Przekrój sejsmiczny obszaru P³oñska przedstawiaj¹cy strukturê rowu tektonicznegoFig. 5. Seismic section; P³oñsk area showing tectonic structure of the graben

of the Jurassic deposits [15] and the Recent terraindrainage pattern in the studied area shows norelationship. The drainage network is lacking in linearstructures in the P³oñsk area.

DISCUSSION OF RESULTS

In both areas, a relationship between the localsubsurface structures and the l inear surfacestructures cannot be detected. However, it seemspossible that in a regional context some of the tectonicfeatures of the area might be identified on the map ofthe sub-Quaternary surface which has been preparedfor the Neogeodynamica Baltica Project [23].

Reactivated old dislocations can be dissipated upwardand assume different spatial characteristics. In this

graben was created along extensive listric faults,probably in the Lower Cretaceous [22,21]. During acompressive inversion of the basin, rocks inside thegraben and on its margin were deformed. The upwardconvexity of generally flat strata show an amplitudeof about 120 m on the 2000 m width of the grabenand can be traced until the upper layers of the UpperCretaceous complex. The amplitude of a convexityin the Cretaceous strata is about 80 m where thegraben is 4000 m wide. The calculation of the tectonicshortening indicates 3-4% change of the width of thegraben during the inversion.

The relationship between the top of Tertiary rocks andthe top of deformed rocks inside the graben cannotbe recognised due to the poor quality of reflections.Comparison analysis of the structural map of the top

TERTIARY

CRETACEOUS

CRETACEOUS

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instance the local structures are unaffected by suchprocesses. This concept can explain the absence ofcorrelation between the subsurface structures andsurface dynamics in the P³oñsk area. The Quaternarysediments in the Wierzchowo region are unusuallythick (up to 200 m) and it seems possible that thiscover has masked the bedrock dynamics.

CONCLUSIONS

The Recent morpho-structural pattern of both areasseems more likely to be related to the nature andmechanical structure of the Pleistocene ice sheetswhich have covered this area several times.

The present-day stress field results in the origin ofsurface stress directions [12] which are quite differentfrom those of Mesozoic times, and no subsurfacedislocations have been reactivated by its influence.(*)

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