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Technika Poszukiwa GeologicznychGeosynoptyka i Geotermia nr 1/99
Zbigniew PERSKIUniversity of SilesiaBdziska 60, 41-200 SosnowiecPoland
POSSIBLE CORRELATIONS BETWEEN THE BASEMENT AND NEAR-SURFACE STRUCTURES: A REASSESSMENT
Two study areas were chosen, each of which containscomplex subsurface structures. Near Wierzchowo,in the Pommeranian Synclinorium, there are twoanticl ines and, near Posk, 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].
Analysis of this area is based on the seismic reflectionstudy performed by the Polish Oil and Gas Companyin 1996 . 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 obszarw badaFig. 1. The location of study areas on the background of theregional geological provinces map
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 Posk 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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Niniejsza praca jest prb znalezienia zwizkw pomidzy lo-kalnymi strukturami tektonicznymi a elementami dynamiki po-wierzchni terenu. Analiza ta oparta zostaa na materiaach kar-tograficznych, opracowaniach sejsmiki refleksyjnej, profilachwiertnice. Do jej przeprowadzenia wytypowano dwa obszary cha-rakteryzujce si wystpowaniem zoonych struktur wgbnychi dug histori rozwoju. S to antykliny w okolicach Wierzchowa(Synklinorium Pomorskie), oraz rowy tektoniczne rejonu Po-ska na obszarze Synklinorium Warszawskiego. Nie stwierdzonozwizku lokalnych struktur wgbnych z powierzchniowymi struk-turami linijnymi co moe to by wyrazem braku wspczesnejaktywnoci tych struktur. Pewien wpyw kierunkw pochodzcychod wau rodkowopolskiego zaznacza si jednak w skali regio-nalnej. Aktywno wzdu odnawianych starych dyslokacji moesi w tym przypadku rozprasza ku grze i przyjmowa odmien-n charakterystyk przestrzenn,. Wystpowanie osadw czwar-torzdu moe skutecznie tumi wpywy dynamiki podoa i na-rzuca plan strukturalny zwizany z aktywnoci plejstoceskie-go ldolodu.
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MOLIWOCI KORELACJI STRUKTUR PRZYPOWIERZCHNIOWYCHZE STRUKTURAMI PODOA: ANALIZA KRYTYCZNA
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Ryc. 2. Przekrj sejsmiczny (wg.  zmodyfikowany) z lokalizacj otworw wiertniczych przedstawiajcystruktury tektoniczne obszaru WierzchowaFig. 2. Seismic section (after , re-interpreted) with boreholes locations showing the tectonic structures inWierzchowo area
Ryc. 3. Profil sejsmiczny (wg. [26, zmodyfikowany) przedstawiajcy inwersyjne struktury listryczne obszaruWierzchowaFig. 3. Seismic section (after , 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  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 . 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.
Within the Posk 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 .
The grabens 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
Ryc. 4. Analiza porwnawcza w systemie GIS lokalnych trendww/g mapy stropu utworw dolnej kredy oraz utworwczwartorzdowychFig.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. Przekrj sejsmiczny obszaru Poska przedstawiajcy struktur rowu tektonicznegoFig. 5. Seismic section; Posk area showing tectonic structure of the graben
of the Jurassic deposits  and the Recent terraindrainage pattern in the studied area shows norelationship. The drainage network is lacking in linearstructures in the Posk 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 .
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 strat