Structure and Mio cene evo lu tion of the Gdów tectonic ...the Outer Carpathians in bore hole...

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Geological Quarterly, 2012, 56 (4): 907–920 DOI: http://dx.doi.org/10.7306/gq.1067 Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep) – a new model based on reinterpreted seismic data Piotr KRZYWIEC, Krzysztof BUKOWSKI, Nestor OSZCZYPKO and Aleksander GARLICKI Krzywiec P., Bukowski K., Oszczypko N. and Garlicki A. (2012) – Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep) – a new model based on reinterpreted seismic data. Geol. Quart., 56 (4): 907–920, doi: 10.7306/gq.1067 Analysis of previously available stratigraphic data coupled with the re-interpretation of seismic profiles calibrated by boreholes has allowed the construction of a new tectonic model of evolution of the Gdów “embayment” – a tectonic re-entrant located along the Carpathian front east of Kraków (southern Poland). This model shows that the main phase of localized fault-controlled subsidence took place in the Early Badenian and was associated with deposition of the locally overthickened Skawina Formation. Also, deposi- tion of evaporites of the Wieliczka Formation seems to have been tectonically controlled by local basement faulting. Supra-evaporitic siliciclastic deposits have developed as a result of overall north-directed sediment progradation from the eroded Carpathian belt to- wards the Carpathian Foredeep. During the final stages of development of the Carpathian fold-and-thrust wedge the previously sub- siding Gdów “embayment” area was uplifted and basement faults were reactivated either as reverse faults or as low angle thrust faults. Along the leading edge of this inverted structure a triangle zone developed, with backthrusting along the evaporitic level. As a result, overthickened evaporites, formed in local tectonically-controlled depressions within the area of the Gdów “embayment” area have been strongly folded and internally deformed. Piotr Krzywiec, Institute of Geological Sciences, Polish Academy of Sciences, Twarda 51/55, 00-818 Warszawa, Poland, e-mail: [email protected]; Krzysztof Bukowski and Aleksander Garlicki, Faculty of Geology, Geophysics and Environmental Pro- tection, AGH University of Science and Technology, Mickiewicza 30, 30-059 Kraków, Poland, e-mails: [email protected], [email protected]; Nestor Oszczypko, Institute of Geological Sciences, Jagiellonian University, Oleandry 2A, 30-063 Kraków, Poland, e-mail: [email protected] (received: June 28, 2012; accepted: December 12, 2012; first published online: December 19, 2012). Key words: Carpathian Foredeep Basin, Gdów “embayment”, Miocene, wedge tectonics. INTRODUCTION The trailing edge of the Carpathian fold-and-thrust belt be- tween Kraków and Tarnów has been the subject of intense studies because of its control over deposits of rock salt (e.g., Wieliczka and Bochnia salt mines) and hydrocarbon accumu- lations. Frontal thrust sheets of the Outer (flysch) Carpathians south of Kraków include the Silesian and Subsilesian nappes (Fig. 1). Further to the north, a relatively narrow zone of the de- formed Miocene Carpathian Foredeep deposits is located, forming the Zglobice thrust-sheet (Kotlarczyk, 1985; cf. Oszczypko et al., 2006 for extensive review and numerous ref- erences). Miocene foredeep deposits are also present within the so-called Gdów “embayment”, analysed in this paper. Transgression of the Early Miocene sea of the Carpathian Foredeep Basin covered both the foreland platform and in part the Outer Carpathian thrust sheets. During the development of this sedimentary basin (Badenian–Sarmatian; Fig. 2), marine sedimentation prevailed. Only the late Badenian salinity crisis associated with deposition of evaporites (rock salt, gypsum) marked significant shallowing of the basin. In the Early Badenian the axial part of the basin reached up- per bathyal depths, while the northern and southern parts of the basin were within the neritic-littoral zone (cf. Oszczypko et al., 2006). Gradual shallowing of the basin began in Serravalian times (Andreyeva-Grigorovich et al., 2003). Results of recent 40 Ar/ 39 Ar dating indicate that deposition of the evaporites in the Carpathian Foredeep Basin started shortly after 13.81 ± 0.076 Ma (de Leeuw et al., 2010) and thus directly after Mi3b, a major step in the Middle Miocene global cooling, dated in the Mediterranean region at 13.82 ± 0.03 Ma (Abels et al., 2005). Isotopic data of Badenian foraminifera indicate a sharp de- crease in seawater temperature just below the evaporites

Transcript of Structure and Mio cene evo lu tion of the Gdów tectonic ...the Outer Carpathians in bore hole...

Page 1: Structure and Mio cene evo lu tion of the Gdów tectonic ...the Outer Carpathians in bore hole Łapanów 1 in the depth in - terval 1.458–1.765.5 m. In the up per part of the Skawina

Geo log i cal Quar terly, 2012, 56 (4): 907–920DOI: http://dx.doi.org/10.7306/gq.1067

Struc ture and Mio cene evo lu tion of the Gdów tec tonic “embayment” (Pol ish Carpathian Foredeep) – a new model based on re in ter preted seis mic data

Piotr KRZYWIEC, Krzysztof BUKOWSKI, Nestor OSZCZYPKO and Aleksander GARLICKI

Krzywiec P., Bukowski K., Oszczypko N. and Garlicki A. (2012) – Struc ture and Mio cene evo lu tion of the Gdów tec tonic “embayment”(Pol ish Carpathian Foredeep) – a new model based on re in ter preted seis mic data. Geol. Quart., 56 (4): 907–920, doi: 10.7306/gq.1067

Anal y sis of pre vi ously avail able strati graphic data cou pled with the re-in ter pre ta tion of seis mic pro files cal i brated by bore holes hasal lowed the con struc tion of a new tec tonic model of evo lu tion of the Gdów “embayment” – a tec tonic re-en trant lo cated along theCarpathian front east of Kraków (south ern Po land). This model shows that the main phase of lo cal ized fault-con trolled sub si dencetook place in the Early Badenian and was as so ci ated with de po si tion of the lo cally overthickened Skawina For ma tion. Also, de po si -tion of evaporites of the Wieliczka For ma tion seems to have been tec toni cally con trolled by lo cal base ment fault ing. Su pra-evaporiticsiliciclastic de pos its have de vel oped as a re sult of over all north-di rected sed i ment progradation from the eroded Carpathian belt to -wards the Carpathian Foredeep. Dur ing the fi nal stages of de vel op ment of the Carpathian fold-and-thrust wedge the pre vi ously sub -sid ing Gdów “embayment” area was up lifted and base ment faults were re ac ti vated ei ther as re verse faults or as low an gle thrust faults.Along the lead ing edge of this in verted struc ture a tri an gle zone de vel oped, with backthrusting along the evaporitic level. As a re sult,overthickened evaporites, formed in lo cal tec toni cally-con trolled de pres sions within the area of the Gdów “embayment” area havebeen strongly folded and in ter nally de formed.

Piotr Krzywiec, In sti tute of Geo log i cal Sci ences, Pol ish Acad emy of Sci ences, Twarda 51/55, 00-818 Warszawa, Po land, e-mail:[email protected]; Krzysztof Bukowski and Aleksander Garlicki, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro -tec tion, AGH Uni ver sity of Sci ence and Tech nol ogy, Mickiewicza 30, 30-059 Kraków, Po land, e-mails: [email protected],[email protected]; Nestor Oszczypko, In sti tute of Geo log i cal Sci ences, Jagiellonian Uni ver sity, Oleandry 2A, 30-063Kraków, Po land, e-mail: nestor_oszczypko@ya hoo.com (re ceived: June 28, 2012; ac cepted: De cem ber 12, 2012; first pub lished on line: De cem ber 19, 2012).

Key words: Carpathian Foredeep Ba sin, Gdów “embayment”, Mio cene, wedge tec ton ics.

INTRODUCTION

The trail ing edge of the Carpathian fold-and-thrust belt be -tween Kraków and Tarnów has been the sub ject of in tensestud ies be cause of its con trol over de pos its of rock salt (e.g.,Wieliczka and Bochnia salt mines) and hy dro car bon ac cu mu -la tions. Fron tal thrust sheets of the Outer (flysch) Carpathianssouth of Kraków in clude the Silesian and Subsilesian nappes(Fig. 1). Fur ther to the north, a rel a tively nar row zone of the de -formed Mio cene Carpathian Foredeep de pos its is lo cated,form ing the Zgłobice thrust-sheet (Kotlarczyk, 1985; cf.Oszczypko et al., 2006 for ex ten sive re view and nu mer ous ref -er ences). Mio cene foredeep de pos its are also pres ent within the so-called Gdów “embayment”, ana lysed in this pa per.

Trans gres sion of the Early Mio cene sea of the CarpathianForedeep Ba sin cov ered both the fore land plat form and in part

the Outer Carpathian thrust sheets. Dur ing the de vel op ment ofthis sed i men tary ba sin (Badenian–Sarmatian; Fig. 2), ma rinesed i men ta tion pre vailed. Only the late Badenian sa lin ity cri sisas so ci ated with de po si tion of evaporites (rock salt, gyp sum)marked sig nif i cant shallowing of the ba sin.

In the Early Badenian the ax ial part of the ba sin reached up -per bathyal depths, while the north ern and south ern parts of theba sin were within the neritic-lit to ral zone (cf. Oszczypko et al.,2006). Grad ual shallowing of the ba sin be gan in Serravaliantimes (Andreyeva-Grigorovich et al., 2003). Re sults of re cent40Ar/39Ar dat ing in di cate that de po si tion of the evaporites in theCarpathian Foredeep Ba sin started shortly af ter 13.81 ±0.076 Ma (de Leeuw et al., 2010) and thus di rectly af ter Mi3b, a ma jor step in the Mid dle Mio cene global cool ing, dated in theMed i ter ra nean re gion at 13.82 ± 0.03 Ma (Abels et al., 2005).Iso to pic data of Badenian foraminifera in di cate a sharp de -crease in sea wa ter tem per a ture just be low the evaporites

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(Gonera et al., 2000; Bicchi et al., 2003; Peryt and Gedl, 2010;Bukowski, 2011; Gonera and Bukowski, 2012), also in agree -ment with the glob ally rec og nized Mi3b cool ing event (deLeeuw et al., 2010). The drop in global sea level at Mi3bcaused the fi nal clo sure of the ex ist ing gate ways be tween theCen tral Paratethys and the Med i ter ra nean part of the Tethys.Sea level low er ing likely re stricted the deep out flow of densesa line wa ters from the Carpathian Foredeep, trap ping the saltwithin the Paratethys bas ins and con se quently set ting off theBadenian sa lin ity cri sis (de Leeuw et al., 2010; Bukowski,2011).

Shal low parts of the evaporite ba sin were dom i nated by sul -phate and subordinary car bon ate-lit to ral fa cies (e.g., Kasprzyk, 1993; Peryt et al., 1997; Bąbel, 2004; Jasionowski and Peryt,2010). Deeper parts of ba sin, lo cated along the Carpathianfront, were oc cu pied by chlo ride-sul phate fa cies – rock salts ofthe Wieliczka For ma tion (Fig. 2; see Garlicki, 1979; Peryt,2006). Ac cord ing to Bąbel (2004), the gyp sum sub-ba sin wasvery shal low, zero to sev eral metres deep, whilst the depth ofthe ha lite sub-ba sin was es ti mated at not less than 30–40 m.

Af ter the Badenian sa lin ity cri sis, the edge of theCarpathian thrust belt moved by a few kilo metres to the north(see Oszczypko, 1997, 1998; Kováč et al., 1998; Oszczypkoand Oszczypko-Clowes, 2012). At the front of the Carpathianorogen this re sulted in a deep en ing of the ba sin, which reached

depths char ac ter is tic for the outer shelf, dur ing the de po si tionof the clayey sed i ments of the Chodenice Beds (Gonera, 1994;Kovač et al., 1998; Oszczypko, 1999). At the same time thehigh level of the sea caused ex ten sive ma rine transgresssiononto the Outer Carpathian thrust sheets, in clud ing the MaguraNappe. This is marked in the Nowy Sącz Ba sin bytransgressive neritic de pos its (NN6/7 Zone) over the fresh wa -ter Biegonice For ma tion (Oszczypko et al., 1992;Oszczypko-Clowes et al., 2009).

The zone of max i mum sub si dence within the CarpathianForedeep Ba sin mi grated in Sarmatian times ca. 40–50 km to -wards the NE, to wards the Teisseyre-Tornquist Zone that wasre ac ti vated due to fore land flex ural ex ten sion (cf. Krzywiec,2001; Oszczypko et al., 2006; Gągała et al., 2012).

The fron tal thrust of the Outer (flysch) Carpathian orogenicfront in the area south-east of Kraków re cedes to the south,form ing a “bay” or “embayment”, termed by Niedźwiedzki(1883–1886) the Gdów embayment, af ter the town of Gdów(Fig. 3). The sur face and shal low subsurface ge om e try of thisarea prompted Niedźwiedzki (1883–1886) to pro pose that inMio cene times the fron tal Carpathian thrust did in deed form abay filled by the Mio cene sea. Sub se quent stud ies (see be low)have clearly shown that this orig i nal con cept was in er ror andthat this re treat of the fron tal Carpathian thrust was due to com -bi na tion of tec ton ics, up lift and ero sion. In or der, how ever, to

908 Piotr Krzywiec, Krzysztof Bukowski, Nestor Oszczypko and Aleksander Garlicki

Fig. 1. Sim pli fied geo log i cal map of the Outer Carpathians and their fore land be tween Kraków and Przemyśl

Red rect an gle – area shown on Fig ure 3

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main tain com pat i bil ity with the pre vi ous no men cla ture, theterm “embayment” will be used through this pa per, al though itshould not be un der stood in its orig i nal strict mean ing, coinedby Niedźwiedzki (1883–1886).

The Mio cene de pos its of the Gdów “embayment” con sistmostly of clays and shales, with sub or di nate sands. They are allin cluded in the Lower Badenian sub-evaporitic de pos its (Połto -wicz, 1962; Alexandrowicz, 1965; Moryc, 1970a, b). Alexan -drowicz (1965) di vided this suc ces sion into four foraminiferallev els IIA, IIB, IIC, IID, and pro posed the name Skawina Beds(Alexandrowicz, 1963). Later, the term “Skawina Beds” hasbeen ex tended and de fined as the “Skawina For ma tion” (Ale -

xandrowicz et al., 1982). The be gin ning of Skawina For ma tionde po si tion is dated to the Lower Badenian Orbulina suturalisZone (see Oszczypko et al., 2006). In the south ern part of theGdów “embayment” the Skawina Fm. has been drilled be neaththe Outer Carpathians in bore hole Łapanów 1 in the depth in -ter val 1.458–1.765.5 m. In the up per part of the Skawina For -ma tion, late Badenian cal car e ous nannoplankton be long ing tothe lower part of NN6 Zone have been de ter mined (Peryt,1997; Oszczypko and Oszczypko-Clowes, 2012). The sameNN6 nanoplankton as sem blages have been de scribed from thesub-evaporitic Skawina For ma tion in the Bochnia and KalushSalt Mine (Andreyeva-Grigorovich et al., 2003; Fig. 2).

In the south ern part of the Gdów “embayment” (i.e. Gdów 1 and Gdów 2 bore holes; Fig. 3) a sig nif i cant in crease in theamount of sands and grav els is ob served. The same coarseclastic de pos its are also known from out crops in the SypkaGóra vil lage, near Gdów, where grav els with flysch ma te rialoc cur (cf. Jasionowski, 1997). Their thick ness reaches at least40 metres; they have been in ter preted as hav ing formed ongravel del tas (Doktor, 1983). The Lower Badenian foramini -feral as sem blages IIA–IIC have been iden ti fied in these de pos -its (Alexandrowicz, 1965). Sim i lar flysch grav els, oc cur ring di -rectly be low the evaporites, were de scribed from the mar ginalzone of the Carpathian Foredeep, in Wrząsowice (Gonera et al., 1990) and Bacharowice (Doktor, 1983), south of Kraków.

This pa per con sists of two main parts. The first is de voted to crit i cal anal y sis of main pre vi ously pub lished tec tonic mod elsof the Gdów “embayment” area in or der to pro vide a his tor i calper spec tive for geo log i cal stud ies of the fron tal Outer Carpa -thians east of Kraków, dat ing back at least to the XVIIIth cen -tury. The sec ond part de scribes a new tectono-strati graphicmodel of the evo lu tion of the Gdów “embayment” based on re -in ter preted seis mic data cal i brated by boreholes.

PREVIOUS TECTONIC MODELS OF THE GDÓW“EMBAYMENT”

The ear li est geo log i cal stud ies of the fron tal Carpathianorogenic wedge in Po land have fo cused on the area lo cated im -me di ately east of Kraków, where rock salt has been mined forcen tu ries in Wieliczka and Bochnia salt mines (e.g., Schober;1750; Townson, 1797; Pusch, 1824; Hrdina and Hrdina, 1842;Niedźwiedzki, 1883–1886; Uhlig, 1903; cf. Poborski, 1965;d’Obyrn and Przybyło, 2010). In the 20th cen tury, ex plo ra tionfor oil and gas greatly in ten si fied and re sulted in the ac qui si tionof a large amount of subsurface geo phys i cal data that sig nif i -cantly im proved the un der stand ing of the geo log i cal struc tureof the fron tal Carpathian fold-and-thrust belt in, among oth ers,Kraków–Tarnów area. Fol low ing de cades of in tense geo log i -cal and geo phys i cal study, many struc tural mod els have beenpro posed for the fron tal Carpathians (see Oszczypko et al.,2006 for de tailed over view and fur ther ref er ences).

The area of the Gdów “embayment” has been ex ten sivelystud ied: in to tal, sev eral tens of deep bore holes have beendrilled, nu mer ous geo phys i cal stud ies (grav ity, mag netic, seis -mic) have been made, mostly dur ing ex plo ra tion for rock salt

Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep)... 909

Fig. 2. The lo cal Carpathian Foredeep Mid dle Mio cene stra tig ra phyin com par i son with the stan dard global chronostratigraphy

(GSSP af ter Hilgen et al., 2009)

Cal car e ous nannoplankton NN zones af ter Peryt (1997) and Raffi et al.(2006), lo cal foraminiferal zones af ter Alexandrowicz (1963); po si tion ofthe Chodenice Beds af ter de Leeuw et al. (2010) and Śliwiński et al.(2012); no tice that de Leeuw et al. (2010) put the base of evaporites at13.81 ± 0.076 Ma; colours used to de pict lo cal lithostratigraphic unitsmatch those used on Fig ures 4–6 and 8

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and, in more re cent years, for hy dro car bons. All the sur face and subsurface data have al lowed many re search ers to come upwith dif fer ent mod els of the pres ent-day struc ture and evo lu -tion of this part of the Carpathian front. Be low, a brief sum -mary of these mod els is given, that will be con se quently used as a ref er ence point for dis cus sion of new re sults of seis mic datain ter pre ta tion. Pre vi ous mod els of the Gdów “embayment” arepre sented in chro no log i cal or der, to il lus trate the evo lu tion ofthe ideas trig gered by the in creased amount of strati graphic and tec tonic ev i dence. Se lected tectono-strati graphic mod els of theGdów “embayment” that il lus trate the evo lu tion of con cepts ofits geo log i cal struc ture are shown on Fig ure 4.

The first model that pre sented, in mod ern style, the struc -ture of the Gdów “embayment” was pub lished by Garlicki, fol -low ing in tense ex plo ra tion for rock salt in the Łężkowice –Siedlec area (Garlicki, 1960 and his fig. 2). In this model, theGdów “embayment” is filled by the gently folded Chodeniceand Grabowiec beds (cf. Fig. 2) en train ing also Mio ceneevaporites (rock salt). The north ern edge of the “embayment”co in cides with salt de pos its that are de formed, up lifted to thesur face and over-thick ened. In this model, there is no tec tonicde for ma tion within the pre-Mio cene base ment al though thisbase ment is shown by hatched lines in di cat ing un cer tain ge om -e try (Garlicki, 1960) – a re sult of the scar city of re li able good

910 Piotr Krzywiec, Krzysztof Bukowski, Nestor Oszczypko and Aleksander Garlicki

Fig. 3. Lo ca tion of the in ter preted seis mic pro files (Figs. 5 and 6), geo log i cal cross-sec tion (Fig. 7)and bore holes on a back ground of a sim pli fied geo log i cal map of the Outer Carpathians

and their fore land with out Qua ter nary for ma tions (af ter Żytko et al., 1989)

For other ex pla na tions see Żytko et al. (1989)

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qual ity subsurface data, as this model pre-dates the ac qui si tionof mod ern dig i tal seis mic re flec tion data.

In 1962, Połtowicz pub lished the next model of the Gdów“embayment”, us ing not only well and sur face geo log i cal databut also geo phys i cal subsurface in for ma tion: re flec tion seis mic and geoelectric data (Połtowicz, 1962 and his fig. 4). His model is shown on Fig ure 4A. Połtowicz, hav ing ac cess to the sub -surface geo phys i cal data, first sug gested the pres ence of lowan gle re verse faults (thrusts) de tached within the Me so zoicbase ment, cut ting also the Mio cene infill of the Gdów “embay -ment”. Its Mio cene sed i men tary infill con sists of the Chode -nice Beds (con tain ing also rock salt de pos its), pres ent withinthe en tire “embayment” and over ly ing the Me so zoic base ment.The Grabowiec Beds are pres ent only to the north of the north -ern edge of the “embayment”.

Sig nif i cant im prove ment in the geo log i cal in ter pre ta tion ofthe Gdów “embayment” was fu elled by the de tailed biostra -tigraphic stud ies of the Mio cene foredeep suc ces sion in this

area. Alexandrowicz (1965; cf. also Doktor, 1983) dis tin gui -shed the sub-evaporitic suc ces sion pres ent within the en tire“embayment” as the Skawina Beds (IIA–D foraminiferal zo -nes), with the Chodenic and Grabowiec beds (IIIA, B foramini -feral zones) lo cated en tirely above the evaporitic level (Fig. 2;cf. also Łuczkowska, 1958; Olewicz, 1973a, b). This more pre -cise tri par tite strati graphic zonation pro vided a new ref er encepoint for the tec tonic anal y ses of this area, with the evaporiticlevel de fined as a marker ho ri zon lo cated at the bound ary be -tween the Skawina Beds and Chodenice Beds. It is also worthmen tion ing that Mio cene foredeep evaporites form an ex cel lent marker in in ter pre ta tion of the seis mic data

In early 1970s Garlicki sum ma rized a de cade of geo log i caland geo phys i cal stud ies of the Gdów “embayment” and pre -sented his new re fined tectono-strati graphic model of the pres -ent-day struc ture of this area and also of its Mio cene evo lu tion(Garlicki, 1971; Fig. 4B). He adopted the tri par tite strati graphic model of the Mio cene infill es tab lished by Alexandrowicz

Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep)... 911

Fig. 4. Geo log i cal mod els of the Gdów “embayment”

A – ac cord ing to Połtowicz (1962); B – ac cord ing to Garlicki (1971); C – ac cord ing to Połtowicz (2004); geo log i cal cross-sec tions are shown at ap prox i mately the same scale and are ver ti cally aligned along the ap prox i mate lo ca tion of the Kłaj 1 bore hole (cf. Fig. 3); all the key ex pla na tions

are shown on this fig ure; for ex pla na tions of other sym bols see orig i nal pa pers of Połtowicz (1962), Garlicki (1971) and Połtowicz (2004)

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(1965) com pris ing (1) the sub-evaporitic siliciclastic suc ces -sion (Skawina Beds), (2) evaporites (rock salt), and (3) the su -pra-evaporitic siliciclastic suc ces sion (Chodenice and Grabo -wiec Beds). In his model (Garlicki, 1971 and his fig. 2) pro -posed a sys tem of re verse faults rooted in the Meso-Pa leo zoicbase ment of the Gdów “embayment”. In this model the sed i -men tary infill, as in the model of Alexandrowicz (1965), con -sists en tirely of the folded sub-evaporitic Skawina Beds char -ac ter ized by sig nif i cant thick ness, up to sev eral hun dreds ofmetres. Overthickened and strongly de formed (folded) salt de -pos its are lo cated along the north ern edge of the “embayment”,where they had been rec og nized by nu mer ous bore holes in theŁężkowice area (Fig. 3). The su pra-evaporitic suc ces sion(Chodenice and Grabowiec beds), con form ably de pos itedabove the evaporites, is pres ent only to the north of the north ern edge of the “embayment”. In this area, the sub-evaporitic suc -ces sion is rather thin, of the or der of tens of metres, in con trastto the thick infill of the Gdów “embayment” at the same strati -graphic po si tion. Us ing his struc tural model, Garlicki (1971)pro posed also a model for the tectono-strati graphic de vel op -ment of this area (Garlicki, 1971 and his fig. 3). He as so ci atedin creased thick ness of the Skawina Beds with syn-depositionalac tiv ity of base ment faults and re lated in creased lo cal sub si -dence. Garlicki (1971) pos tu lated also that salt de pos its of theŁężkowice area, thicker than in the north ern part of the fore -deep ba sin and pres ently strongly de formed, were de pos iteddur ing in creased tec toni cally-con trolled lo cal sub si dence wi -thin the fu ture Gdów “embayment” area. De po si tion of evapo -rites was fol lowed by de po si tion of siliciclastic de pos its of thesu pra-evaporitic suc ces sion (Chodenice and Grabowiec beds).Ac cord ing to Garlicki (1971), dur ing later phases of the tec -tonic evo lu tion of this area, due to the in crease orogenic pushre lated to the fi nal phase of de vel op ment of the Carpa thianfold-and-thrust belt, base ment nor mal faults were re ac ti vatedas re verse faults, and the en tire Mio cene foredeep infill of theGdów “embayment” was folded and thrust to wards the north.

Mod els of the Gdów “embayment” by Alexandrowicz(1965) and Garlicki (1971) were sub se quently chal lenged. Ac -cord ing to Poborski and Jawor (1987), al most the en tire infill of the Gdów “embayment” con sists of the su pra-evaporitic Cho -denice Beds, with only a thin and in com plete cover of thesub-evaporitic Skawina Beds rest ing di rectly above the Me so -zoic base ment (see Poborski and Jawor, 1987 and their fig. 2).Poborski and Jawor (1987) did not, how ever, pres ent anystrati graphic ev i dence in sup port of their new model al thoughthey sug gested sig nif i cant mod i fi ca tions of the pre vi ous strati -graphic find ings of Łuczkowska (1958) and Alexandrowicz(1965).

In their next pa per, Poborski and Jawor (1989) pro posed aneven more gen er al ized struc ture of the Gdów “embayment” asthey used only the gen eral term “Mio cene for ma tion” to de -scribe the en tire Mio cene infill of the “embayment”. Withinthis for ma tion, the evaporitic level was dis tin guished as amarker ho ri zon, but no in for ma tion was given on the pre cisestra tig ra phy of the sub- and su pra-evaporitic suc ces sions.

Ac cord ing to newer mod els of the Gdów “embayment” byPołtowicz (1991, 2004; Fig. 4C), only the up per most part (upto 200 m) of its sed i men tary cover con sists of the sub-evapo -

ritic Skawina Beds. These sed i ments form an allochthonousthrust slice tec toni cally trans ported above a very flat (al mosthor i zon tal) fault (de tach ment level) to the north. Be neath thisslice, the autochthonous cover con sists of the thick Chodeniceand Grabowiec beds and thin Skawina Beds, rest ing above theMe so zoic base ment. This base ment is dis sected by sev eral nor -mal and re verse/thrust faults. Połtowicz (1991, 2004), sim i larly to Poborski and Jawor (1987), did not, how ever, pro vide anyde tailed strati graphic data that would sup port his struc turalmodel. He only quoted ar chive un pub lished strati graphic re -sults of Z. Kirchner from un iden ti fied in dus try wells, with outpre sent ing any solid biostratigraphic data or even ref er ences tothe ar chive well re ports. Tak ing this into ac count one can con -clude that the struc tural mod els of Poborski and Jawor (1987)and Połtowicz (1991, 2004), sig nif i cantly dif fer ent from thepre vi ous mod els based on de tailed strati graphic stud ies (Gar -licki, 1971), are not sup ported by re li able and ver i fi able strati -graphic data and have to be treated with the great cau tion.

The Carpathian front in the area lo cated east of Kraków was first de scribed as as so ci ated with a tri an gle zone by Jones(1997). This model was, how ever, men tioned only in a con fer -ence ab stract and was not sup ported by any more com pre hen -sive pub li ca tion. Re cently, a model of wedge tec ton ics and tri -an gle zones was used in or der to ex plain the struc ture of theCarpathian front im aged on seis mic data from the Woj -nicz–Tarnów area, lo cated to the east of the Gdów “embay -ment” (Krzywiec et al., 2004). Fur ther more, it has been alsosug gested that in the Wieliczka part of the Carpathian frontwedge, lo cated to the west of the Gdów “embayment”, wedgetec ton ics also played an im por tant role, and that the en tireWieliczka Salt Mine is lo cated in the core of the tri an gle zoneformed dur ing the last stages of com pres sion within the Carpa -thian orogenic wedge (Krzywiec and Vergés, 2007). Fi nally,Bukowski et al. (2010) pre sented a seis mic pro file from theGdów “embayment”, re in ter preted us ing ver i fied strati graphicdata from bore holes and out crops (in clud ing new ra dio met ricdates of tuffites). This seis mic pro file (Bukowski et al., 2010and their fig. 9) shows low-an gle thrust faults and steep re versefaults de tached within the Meso-Pa leo zoic base ment of theGdów “embayment”, and tri an gle zone and as so ci ated back -thrusting along its north ern edge, sim i larly to that re ported from the Wojnicz–Tarnów area by Krzywiec et al. (2004).

RESULTS OF SEISMIC DATA INTERPRETATIONFROM THE GDÓW “EMBAYMENT”

In or der to con struct a re vised tectono-strati graphic modelof evo lu tion of the Gdów “embayment”, seis mic data ac quiredin 1992 by Geofizyka Kraków was used. This seis mic datacom prised post-stack time mi grated pro files, with the ref er ence level (0 sec TWT) equal to 170 m a.s.l. Ap prox i mately the first200 mil li sec onds of seis mic data has been muted due to verylow qual ity and could not have been used for in ter pre ta tion.

Some of the bore holes drilled in the study area (cf. Fig. 3)had sonic logs and check-shot data avail able, and this data wasused to con struct time-depth ta bles, nec es sary for pre cisewell-to- seis mic tie.

912 Piotr Krzywiec, Krzysztof Bukowski, Nestor Oszczypko and Aleksander Garlicki

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The gen eral struc ture of the study area is shown on a re -gional seis mic transect, con structed us ing sev eral seis mic pro -files re pro cessed by Geofizyka Kraków us ing orig i nal fielddata (Fig. 5). It shows three main struc tural do mains, im por tantfor the anal y sis de scribed in this pa per:

– fron tal part of the Outer (flysch) Carpathians,– Gdów “embayment”,– north ern part of the Carpathian Foredeep. The fron tal Outer Carpathian thrust sheet is strongly in ter -

nally de formed, but its struc ture and evo lu tion has not been afo cus of this pa per, and con se quently, it was treated en bloc as a tec tonic block that was thrust to the north dur ing the fi nal stages of the Carpathian col li sion. Within the Gdów “embayment”,res o lu tion of the seis mic data is sig nif i cantly higher and onecan ob serve there nu mer ous fairly con tin u ous seis mic re flec -tors that pre cisely de pict the in ter nal ge om e try of this part ofthe Carpa thian thrust belt. Strati graphic in for ma tion from nu -mer ous bore holes as well as pub lished re sults of ear lier strati -graphic stud ies show that all the Mio cene de pos its that coverthe Meso-Pa leo zoic base ment of the Gdów “embayment”should be re garded as sub-evaporitic Skawina Beds (cf. Buko -wski et al., 2010). Over all thick ness changes in the sub-evapo -ritic Mio cene suc ces sion within the Gdów “embayment” showa grad ual thick ness in crease gen er ally to wards the south. There are also clearly vis i ble faults within the Meso-Pa leo zoic base -ment (Fig. 5). One fault, lo cated in the north ern part of theGdów “embayment”, may be clas si fied as a low-an gle thrustfault that car ries Me so zoic (Ju ras sic) rocks to wards the north,above the Skawina Beds (Figs. 5 and 6). Along the trail ingedge of the Gdów “embayment” (vi cin ity of the Łysokanie 1and 2 bore holes) foredeep evaporites to gether with over ly ingChodenice and Grabowiec beds are up lifted, and the over allstruc ture may be de scribed as a tri an gle zone, filled by de -formed Skawina Beds, with the backthrust de vel oped along the duc tile evaporitic level. Far ther to the north, sub-evaporitic de -pos its are rather thin and form only a thin ve neer above theMeso-Pa leo zoic base ment. The su pra-evaporitic Chodeniceand Grabowiec beds make up the bulk of the foredeep de pos itsin this part of the foredeep ba sin. They have not un der gone anysig nif i cant compressional de for ma tion dur ing the fi nal Carpa -thian thrust ing move ments. They are char ac ter ized by an ap -prox i mately in ter nal con fig u ra tion par al lel to the evaporitesand deeper base ment, and, far ther to the north, by an in clinedseis mic pat tern, with a clearly vis i ble an gu lar un con formityabove the evaporites. Such in ter nal ge om e try could be in ter -preted as an ef fect of over all sed i ment progradation from thesouth to wards the north, sim i larly to the depositional modelpro posed for the part of the foredeep lo cated more to the east, in the Wojnicz–Tarnów area (Krzywiec, 2001; cf. Oszczypko etal., 2006).

Two other seis mic pro files have been se lected to more pre -cisely il lus trate the in ter nal struc ture of the Gdów “embay -ment” (Fig. 6). The first of them (Fig. 6A) is lo cated in its morecen tral part and is cal i brated by sev eral bore holes (cf. Fig. 3).The Książnice 2 bore hole pro vides cru cial strati graphic in for -ma tion on the pre cise lo ca tion of the low-an gle thrust fault de -

Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep)... 913

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Page 9: Structure and Mio cene evo lu tion of the Gdów tectonic ...the Outer Carpathians in bore hole Łapanów 1 in the depth in - terval 1.458–1.765.5 m. In the up per part of the Skawina

tached with the deeper (sub-Mio cene) base ment. Bore holesfrom the Łężkowice–Grodkowice area pro vided in for ma tionon de formed thick evaporites lo cated at shal low depths (cf.Garlicki, 1971). It should be stressed, how ever, that due to thelow qual ity of seis mic data at shal lower lev els, the ex act struc -tural in ter pre ta tion of the evaporites in the Łężkowice–Grod -kowice area should be re garded as a first-or der ap prox i ma tiononly, partly based on pub lished mod els (Garlicki, 1971). TheSkawina Beds within the Gdów “embayment” are gen er allyonly gently de formed and pre serve pre-deformational in ter naldepositional ge om e try. The only ex cep tion is the most north ern tip of the “embayment”, where all the Mio cene de pos its arestrongly de formed. Evaporites with their siliciclastic cover ofthe Chodenice and Grabowiec beds are con cor dantly up liftedalong the trail ing edge of the Gdów “embayment”, sim i larly towhat has been ob served on the re gional transect de scribedabove (Fig. 5). Far ther to the north, evaporites are un der lainonly by a thin ve neer of the sub-evaporitic de pos its. A verysim i lar struc ture of the Mio cene foredeep de pos its could be ob -served on the sec ond pro file, lo cated within the east ern part ofthe Gdów “embayment” (Fig. 6B). On this pro file one can alsoob serve thick Skawina Beds, stratigraphicaly doc u mented bythe Siedlec 3 bore hole (Łuczkowska, 1958), that along thetrail ing edge of the Gdów “embayment” are compressionallyde formed. Above this zone of rel a tively strong de for ma tion,evaporites to gether with the su pra-evaporitic Chodenice andGrabowiec beds are lo cally up lifted. The main dif fer ence be -tween these two pro files is the pres ence (Fig. 6A) or lack (Fig.6B) of the thick-skinned thrust faults rooted within the Meso- Pa leo zoic base ment. The tri an gu lar ge om e try of the zone of the de formed Skawina Beds be neath the up lifted evaporites andsu pra-evaporitic siliciclastics may be in ter preted, sim i larly towhat has been de scribed above for the re gional seis mictransect, as a tri an gle zone re lated to the wedge tec ton ics ac tivedur ing last stages of the Carpathian thrust ing (cf. Krzywiec etal., 2004, Krzywiec and Vergés, 2007). Su pra-evaporiticsiliciclastic de pos its are backthrust to wards the south along thede tach ment re lated to the foredeep evaporites. Seis mic im ag ing of the tri an gle zone it self is not very good, al though one can ob -serve traces of fold struc tures pres ent in the north ern cor ner ofthe tri an gle zone.

MIOCENE EVOLUTION OF THE GDÓW“EMBAYMENT” AREA – A MODEL

A gen eral model for the Mio cene evo lu tion of the Gdów“embayment” was con structed us ing the in ter preted seis mic data de scribed above and is shown on Fig ure 7. It was con structed us -ing the cen tral part of the re gional transect from Fig ure 5.

The model con sists of three main stages, each as so ci atedwith de vel op ment of the three main build ing blocks of the sed i -men tary infill of this part of the Carpathian Foredeep: theSkawina Beds, the evaporitic Wieliczka For ma tion and fi nallythe Chodenice and Grabowiec beds of the Machów For ma tion(cf. Fig. 2).

The first stage of evo lu tion of the Gdów “embayment” wasas so ci ated with de po si tion of the Skawina Beds (Fig. 7A).These are mostly clayey-sandy de pos its, but sig nif i cant in -

crease in coarser ma te rial could be ob served in the south ernpart of the “embayment” (Doktor, 1983). The thick (up to1000 m) Lower Badenian Skawina Beds within the Gdów“embayment” have their strati graphic equiv a lent in the morenorth ern part of the Carpathian Foredeep Ba sin i.e. theBaranów Beds (cf. Oszczypko et al., 2006). They are, how ever, char ac ter ized by much smaller thick ness, ca. 30–40 m or less(Ney et al., 1974) and are sche mat i cally shown on Fig ure 7A asa very thin ve neer of siliciclastic de pos its de vel oped above theMeso-Pa leo zoic base ment. The re con structed ge om e try of theGdów “embayment” dur ing de po si tion of the Skawina Beds(Fig. 7A) sug gests that in creased thick ness of the sed i men taryinfill was con trolled by lo cal ized tec tonic ac tiv ity of this part ofthe foredeep ba sin. Some sub-Mio cene faults might have di -rectly con trolled de po si tion of the Skawina Beds; ad di tion ally,flex ural sub si dence was re spon si ble for a gen eral thick ness in -crease to wards the south, i.e. to wards the Carpathians. Thepres ence of coarser ma te rial in the south ern part of the Skawina Beds within the Gdów “embayment” clearly in di cates rel a -tively short trans port and prox im ity of the source area i.e.emerged Carpathian thrust sheets (Alexandrowicz, 1965;Doktor, 1983).

Dur ing the next stage of de vel op ment of the Gdów“embayment”, up per Badenian evaporites of the WieliczkaFor ma tion were de pos ited (Fig. 7B). Their in ferred thick ness issmaller than the thick ness of the un der ly ing Lower BadenianSkawina Beds; they are, how ever, also char ac ter ized by lat eralthick ness vari a tions, sim i larly to the Skawina Beds. Within theGdów “embayment” evaporites might have at tained largerthick ness, while to wards the north they form a gen er ally thinve neer. It is pos tu lated that such lo cal thick ness in crease of theevaporitic cover within the Gdów “embayment” ar eas was alsore lated to tec tonic ac tiv ity of some base ment faults (cf. Fig.7B). Such model is fully com pat i ble with the ear lier model pro -posed by Garlicki (1971) for the Łężkowice area.

The third stage of the pro posed evo lu tion of the area of thepres ent-day Gdów “embayment” shown on Fig ure 7C was as -so ci ated with de po si tion of the Chodenice and Grabowiec bedsof the Machów For ma tion (cf. Fig. 2). These de pos its havebeen gen er ally shed to wards the Carpathian Foredeep Ba sinfrom the emerged and eroded Carpathians (e.g., Oszczypko etal., 2006). Within the pres ent-day south ern and cen tral parts ofthe ba sin they are char ac ter ized by an over all progradationalpat tern di rected to wards the north (cf. Krzywiec, 2001;Porębski et al., 2002; Porębski and Steel, 2003). Such gen eraldepositional ar chi tec ture of the su pra-evaporitic siliciclasticcover of the Carpathian Foredeep in front of the Carpathianthrust belt is clearly vis i ble on the re gional transect in Fig ure 5.

Within the Gdów “embayment” area, due to the later up liftand ero sion (see be low), only evaporites are lo cally pre servedat the sur face (cf. Bukowski et al., 2010).

Dur ing the fi nal stages of de vel op ment of the Carpathianfold-and-thrust wedge the pre vi ously sub sid ing Gdów“embayment” area was up lifted and base ment faults were re ac -ti vated ei ther as re verse or as low-an gle thrust faults. As a re sult of this up lift, the en tire foredeep suc ces sion de pos ited abovethe Skawina Beds (i.e., evaporites of the Wieliczka For ma tionand siliciclastics of the Machów For ma tion) has been re movedby ero sion. Only evaporites were lo cally pre served and may be

Structure and Miocene evolution of the Gdów tectonic “embayment” (Polish Carpathian Foredeep)... 915

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found at the sur face in parts of the Gdów “embayment” area(see Bukowski et al., 2010 for more de tailed de scrip tion).

Dur ing fi nal thrust ing move ments within the Carpathians, a tri an gle zone de vel oped along the lead ing edge of in vertedstruc ture of the Gdów “embayment”. Its for ma tion was mostprob a bly in flu enced by the lat eral thick ness and fa cies changesin the tran si tion zone be tween the “embayment” area char ac ter -ized by thicker evaporites and the area lo cated to the north,char ac ter ized by much thin ner evaporitic cover. Such lat eralchanges of de tach ment level very of ten trig ger for ma tion of tri -an gle zones (cf. Krzywiec and Aleksandrowski, 2004). Back -thrust of the tri an gle zone pres ent along the north ern edge ofthe Gdów “embayment” formed along the evaporitic level(Figs. 5 and 6). Lo cally, dur ing such backthrusting, over thick -ened evaporites formed in tectoni cally-con trolled de pres sionwithin the Gdów “embayment” area (Fig. 7B) have beenstrongly folded and in ter nally de formed. Such de for ma tionsare doc u mented by nu mer ous bore holes from the Ciężko -wice–Grodkowice area (Garlicki, 1971).

DISCUSSION AND CONCLUSIONS

The model of evo lu tion of the Gdów “embayment” pre -sented in this pa per was con structed us ing rel a tively good qual -ity seis mic data cal i brated by nu mer ous bore holes and aug -mented by sur face geo log i cal data. To a large de gree, it sup -ports one of the ear lier mod els pos tu lated by Garlicki (1971),and that was later chal lenged (Poborski and Jawor, 1987, 1989; Połtowicz, 1991, 2004). In this model, the sed i men tary infill ofthe Gdów “embayment” con sists of the Skawina Beds, withonly lo cally pre served rem nants of the evaporitic cover at thesur face. The for ma tion of such thick Lower Badenian sili -ciclastic de pos its has been at trib uted to lo cal ized tec toni cally- con trolled sub si dence, ex erted ei ther by lo cal fault ac tiv ity orby flex ure of the lower plate un der the ad vanc ing Carpa thianorogenic wedge. The next stage of the Mio cene evo lu tion ofthis area in cludes a phase of de po si tion of the evaporitic coverthat was also char ac ter ized by larger thick ness within the Gdów “embayment” due to con tin u ing lo cally in creased and tec toni -

916 Piotr Krzywiec, Krzysztof Bukowski, Nestor Oszczypko and Aleksander Garlicki

Fig. 7. Quan ti ta tive model of the three main stages of the Mio cene evo lu tion of the Gdów “embayment”

A – de po si tion of the Skawina Beds. Note that the lat eral north ern equiv a lent of this rel a tively thick infill of the Gdów “embayment”was also de pos ited but is char ac ter ized by rel a tively small thick ness; B – de po si tion of evaporites of the Wieliczka For ma tion.Within the Gdów “embayment” they might have been char ac ter ized by in creased thick ness that later led to de vel op ment of theŁężkowice rock salt de posit char ac ter ized by strong in ter nal compressional de for ma tion (Garlicki, 1960, 1971). Later up lift of theax ial part of the Gdów “embayment” and sub se quent ero sion re moved the south ern part of the evaporitic cover, its ex act south ernedge not be ing known pre cisely (cf. Połtowicz, 1994); C – de po si tion of the Chodenice and Grabowiec beds, with in ferred gen eralsed i ment sup ply from the south (cf. Krzywiec, 2001; Oszczypko et al., 2006)

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cally-con trolled sub si dence. The evaporites were over lain bysiliciclastic de pos its of the Machów For ma tion (Chode nice and Grabowiec beds) dur ing the third stage of de vel op ment of thestudy area. This siliciclastic cover was de rived from theemerged and eroded Carpathian orogenic wedge. It should behow ever stressed that, due to late- and post-orogenic up lift andero sion, the sec ond and third stages should be re garded as more con jec tural than based on qual i ta tive re con struc tions.

The pro posed model for de vel op ment of the Gdów “em -bayment” in cludes also for ma tion of the tri an gle zone along itsnorth ern edge dur ing the last thrust ing move ments of the Carpa -thian orogenic wedge. This part of the model, based on re in ter -preted seis mic data and with ref er ence points pro vided by mod -els for the Wojnicz (Krzywiec et al., 2004) and Wie liczka(Krzywiec and Vergés, 2007) ar eas, is novel and dif fer ent fromall pre vi ously pub lished mod els. How ever, in this con text it isworth men tion ing that the pro posed struc tural in ter pre ta tion ofthe north ern edge of the Gdów “embayment” based on seis micdata is in good agree ment with the much ear lier struc tural modelof this area pro posed by Skoczylas- Ciszewska and Burtan(1954; Fig. 8). On this cross-sec tion one can ob serve a su -pra-evaporitic Mio cene cover con form ably up lifted above theolder strongly de formed foredeep Mio cene de pos its. Due to thelack then of deeper wells in this area, and the lack of re li ablesubsurface geo phys i cal (seis mic) data Skoczylas-Ciszewska and Burtan (1954) did not show the top of the sub-Mio cene Meso-

Pa leo zoic base ment – Fig ure 7 shows its con jec tural con fig u ra -tion. The key el e ment of re in ter pre ta tion of this geo log i calcross-sec tion, shown on Fig ure 7, is the pres ence of a tri an glezone along the lead ing (north ern) edge of the Gdów “embay -ment”, iden ti cal to the model pro posed us ing seis mic data andde scribed in this pa per. It should be also stressed that geo log i calmaps from the mid-XXth cen tury were based on a large amountof sur face geo log i cal data that are pres ently mostly un avail abledue to in creased sur face anthropogenic in fra struc ture. There fore, the ge om e try of the su pra-evaporitic suc ces sion, con cor dantlyup lifted to wards the sur face along the pre sumed backthrust de -vel oped within the fron tal part of the Gdów “embayment”shown by Skoczylas- Ciszewska and Burtan (1954; Fig. 7) ontheir cross-sec tion, should be re garded as gen er ally re li able. This shows, to gether with the re cent find ings of Bukowski et al.(2010), that sur face data sup port the model of the Gdów tri an glezone filled by de formed Skawina Beds pro posed in this pa per.

Ac knowl edge ments. This work has been car ried outwithin the re search pro ject No 6 T12 2005 C/06569 fundedjointly by PGNiG S.A. and the Min is try of Sci ence and HigherEd u ca tion. PGNiG S.A. is thanked for pro vid ing ac cess to seis -mic data. A. Wysocka (Uni ver sity of War saw, War saw) andJ. Verg¾s (In sti tute of Earth Sci ences “Jaume Almera”, Bar ce -lona) are thanked for their thor ough re views that greatly helpedto fi nally shape this pa per.

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Fig. 8. Part of the geo log i cal cross-sec tion from the geo log i cal 1:50 000 sheet “Bochnia”,(Skoczylas-Ciszewska and Burtan, 1954) with added in ferred lo ca tion of top of the lower plate

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