the middle triassic magnetostratigraphy from the peri-tethys basin in poland

The Middle Triassic magnetostratigraphy from thePeri-Tethys basin in Poland

Jerzy Nawrocki a;*, Joachim Szulc b

a Paleomagnetic Laboratory, Department of Geophysics, Polish Geological Institute, Rakowiecka 4, 00-975 Warsaw, Polandb Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, 30-063 Krakow, Poland

Received 15 May 2000; received in revised form 19 June 2000; accepted 19 July 2000

Abstract

The paper presents the first complete magnetic polarity scale obtained for the Middle Triassic of the northern Peri-Tethys area. The Roetian and Muschelkalk deposits of southern Poland (Upper Silesia, the Holy Cross Mountains)were studied paleomagnetically. The obtained paleopole `A' (52³N, 143³E) fits well to the Middle Triassic (ca. 240 Ma)segment of the Stable European APWP. A set of various quality characteristic directions isolated in 106 samples wasused for construction of the composite polarity scale. This Peritethyan scale has been tied with conodont zonation andreferred to the scale erected earlier in the Tethyan realm. Comparison of both scales indicates that the whole Roetiansuccession in the southern Polish basin should be correlated with the latest Olenekian. The normal marinesedimentation ceased in the Polish basin as early as in the late Fassanian time. The obtained magnetostratigraphic scalecombined with the sequence stratigraphic framework enables a reliable chronological correlation of the late Olenekian^Ladinian succession over the entire Peri-Tethys Basin. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: magnetostratigraphy; pole positions; Olenekian; Muschelkalk; Poland

1. Introduction

Detailed correlation of the Roetian^Muschel-kalk succession of the northern Peri-Tethys (Ger-manic Basin) with its counterparts from the Te-thys is problematic due to scarcity of index fossilscommon for the both basins. Especially the Roe-tian and Middle Muschelkalk are devoid of theindex macro- and micro-fossils. Therefore themost di¤cult problem to solve is de¢ning of the

chronostratigraphic position of the Roetian^Mu-schelkalk boundary, i.e. the Lower^Middle Trias-sic boundary. Von Pia [1] assumed the lowerboundary of the German Muschelkalk as concur-rent with a beginning of the Anisian stage in theAlpine Triassic. This correlation has been ac-cepted until 1974 when Kozur [2] placed the Ole-nekian^Anisian boundary within the Roetian se-quence, at the base of the Myophoria Beds. In hisrecent work Kozur [3] has included the entireRoetian succession to the Anisian stage. Theyoungest position of Roetian^Muschelkalkboundary has been de¢ned by Visscher et al. [4],which moved it up to the Pelsonian substage.

The magnetostratigraphy appears to be the

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* Corresponding author. Tel. : +48-22-849-5351/452;Fax: +48-22-849-5342; E-mail: [email protected]

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Earth and Planetary Science Letters 182 (2000) 77^92

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most promising method to overcome the correla-tion hindrances. The presented work has beenplanned as continuation of the magnetostrati-graphic examination carried out on the Per-mian^Lower Triassic succession from the Polishpart of the Germanic Basin. This study enabledconstruction of the magnetostratigraphic scale forthe Permian^Buntsandstein interval [5]. Accord-

ing to this scale the Griesbachian^Dienerian ageof the Lower Buntsandstein and the lower part ofMiddle Buntsandstein has been con¢rmed. Theentire Roetian (or at least its majority) has beencorrelated with the lowermost Anisian. However,the latter correlation was assumed as very hypo-thetical and conditional since the biostratigraphicguidelines were vague for this interval and the

Fig. 1. (A) Generalized lithofacies map of the Lower Muschelkalk in Poland (after [38], modi¢ed) and paleomagnetic samplingsites (1: Tarnow Opolski, 2: Gogolin, 3: Strzelce Opolskie, 4: Libia

Lzç, 5: Podstoki, 6: PlIaza, 7: Nietulisko). (B) Paleogeographi-

cal map of the western Tethys area in the Anisian (after [9], modi¢ed). a: Upper Silesia, b: Holy Cross Mountains, FL: Fenno-scandian Land, VB: Vindelician^Bohemian Land, CABM: Central, Armorican and Brabant Massifs, NKA: Northern CalcareusAlps, SA: Southern Alps.

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J. Nawrocki, J. Szulc / Earth and Planetary Science Letters 182 (2000) 77^9278

polarity pattern in the overlying Muschelkalk se-quence has not been recognized. Paleomagneticinvestigation of the Roetian^Muschelkalk sedi-ments was expected to approach the chronostrati-graphic position of the discussed succession sincein the Tethyan area progress has been made incalibration of the sequence of the Middle Triassicreversed and normal magnetozones against a de-tailed biostratigraphic zonation [6^8]. The pre-sented magnetostratigraphic scale has been re-ferred to the conodont biozonation erected forthe Polish Muschelkalk by Trammer [9] and Za-widzka [10] and updated by Narkiewicz [11].

The next aim of this paper is to present a newMiddle Triassic paleomagnetic pole that might beuseful for de¢nition of the Middle Triassic paleo-position of the European Plate. Until now onlyone good quality pole has been isolated from theMuschelkalk deposits [12].

2. Geology

2.1. General setting

The Triassic Germanic basin called also `north-ern Peri-Tethys' occupied the northern margin ofthe Tethys Ocean (Fig. 1). This basin was partlyopen to the south and closed to the north. It wasseparated from the Tethys by elevated hercynianblocks cut by tectonically controlled depressions(gates). The gates provided communication be-tween the northern Peri-Tethys and the TethysOcean, however they were opened in di¡erenttimes. The paleogeographical setting resulted insigni¢cant lithofacies diachroneity between theeastern (Polish) and the western (German) partsof the basin (see ¢gure in the EPSL Online Back-ground Dataset1). Normal marine and fossilifer-ous carbonates dominate in the Roetian and Low-er Muschelkalk succession of southern Poland(Upper Silesia, Holy Cross Mountains). To thewest these sediments are replaced gradually by

deposits typical for more restricted and shallowerenvironments, i.e. fossil poor, shallow water lime-stones and dolomites, sabkha evaporites andplaya^mud£at siliciclastic deposits [13]. The circu-lation reversed in Ladinian time when the westerncommunication dominated whereas the easternand northern parts of the basin were upliftedand underwent emersion.

The facies diachroneity within the northernPeritethyan area makes it di¤cult to get an un-equivocal basinwide correlation of the Triassiclithostratigraphic units on the one hand and hin-ders their reliable correlation with the Alpine Tri-assic succession on the other hand. The men-tioned isolation and restricted circulation withinthe Peri-Tethys basin resulted in scarcity of indexfossils useful for biostratigraphic correlation ofthe Germanic Triassic.

The ¢rst attempt to get an overregional biostra-tigraphic correlation of the lithostratigraphic unitsin the Germanic Basin has been carried out byKozur [2], and based on integrated biostrati-graphic studies of di¡erent fossil groups includingthe conodonts. The succeeding studies on cono-dont and foraminifera stratigraphy [9^11,14^16]enabled a relatively good biozonal subdivision ofthe Muschelkalk from the Pelsonian to Fassaniansubstages and enhanced correlation of these inter-vals with the Alpine Middle Triassic.

Recently, a sequence stratigraphic method hasbeen employed to approach the problem of theregional lithofacies variation and for correlationof the sequences de¢ned in the Tethys and Peri-Tethys basins [13,17]. A close number and timingof the depositional sequences featuring the bothbasins, argue for a dominant role of the eustaticcontrols upon the sedimentary processes both inthe Tethys and its northern periphery [13].

2.2. The Upper Silesian section

The Roetian and Muschelkalk of Upper Silesiahave been chosen for magnetostratigraphic studysince the succession is complete and the rocks arequite well exposed with exception for the lowerpart of the Upper Muschelkalk. Moreover, theSilesian subbasin was situated within the transi-tional belt between the Tethys and the Germanic

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J. Nawrocki, J. Szulc / Earth and Planetary Science Letters 182 (2000) 77^92 79

areas [17] which makes it easier to correlate bothregions to each other.

The Muschelkalk succession of Upper Silesiahas a relatively well established conodont zona-tion founded by Zawidzka [10]. The Alpine am-monoids have long been reported from the Sile-sian Muschelkalk [18,19], but due to theirsporadic occurrence they play a subordinate rolein biostratigraphic zonation of the Muschelkalk.No age-diagnostic fossils have been so far foundfrom the Roetian, lowermost part of the LowerMuschelkalk, Middle Muschelkalk and from theLower Keuper interval.

The studied section starts with marls and ooliticlimestones comprising the Roetian-indicative fos-sils : Costatoria costata and Beneckeia tenuis(Lower Roetian), and marly limestones with Myo-phoria vulgaris (Upper Roetian). The succeedingbioclastic limestones (Lower Gogolin Beds) com-prising stenohaline Dadocrinus crinoids begin theMuschelkalk succession in the Upper Silesian Ba-sin and may be correlated with the Scythian^Ani-sian transitional zone in the Alpine Triassic [20].

The ¢rst conodont taxa belonging to the ho-meri^regale composite zone (N. germanicus, N.

cf. homeri) of the Aegean age occur ca. 30 mabove the Roetian^Muschelkalk boundary, withinthe Upper Gogolin Beds [10]. The index cono-dont, Neospathodus kockeli, de¢ning the begin-ning of the Pelsonian substage appears some 5 mbelow the Gogolin^Gorazdze Beds boundary andcoincides with the appearance of the Balatonitesoltonis ammonite, typical of the Pelsonian in theAlpine domain [21]. The next index conodont,Paragondolella excelsa, occurs ¢rst in the Karch-owice Beds [10] and de¢nes the lower limit of theexcelsa assemblage zone. This limit is assumed byZawidzka also as the Pelsonian^Illyrian bound-ary. Unfortunately the subsequent conodont-dis-advantageous sedimentary environments resultedin stratigraphically barren zone encompassing theDiplopora Dolomite and the Lower TarnowiceBeds (Table 1). Therefore the boundary positionis not de¢nite. According to the occurrence ofdasycladales algae Kotanski [22] has placed theboundary higher, within the Diplopora Dolomite.

Stratigraphic zonation of the Upper Muschel-kalk constructed by Kozur [23] for the GermanMuschelkalk, is based on phylogenetic stages ofthe genus Gondolella and consists of seven cono-

Table 1Roetian^Muschelkalk lithostratigraphy and selected fossil extents in the Holy Cross Mountains and Upper Silesia

Paleomagnetic sampling intervals are marked by circles.

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dont zones. The ¢rst, after the mentioned inter-mission, index conodont assemblage (includingNeogondolella mombergensis) appears only in theupper Tarnowice Beds and de¢nes the `¢rst con-odont zone' of the Upper Muschelkalk. The sub-sequent `second conodont zone' is de¢ned by theoccurrence of Neogondolella cornuta, Neogondolel-la constricta and Neogondolella excentrica taxa.Zawidzka [10] assumed the ¢rst appearance ofthe latter taxon as an indicator of the Anisian^Ladinian boundary which lies within the Wilko-wice Beds. The third and fourth conodont zonesare determined by ¢rst occurrences of Neogondo-lella media and Paragondonella haslachensis, re-spectively. The last-named zone, indicative forthe Late Fassanian and earliest Longobardian,encompasses the entire Boruszowice Beds [10].The subsequent conodont zones 5^7 are absentin the Silesian Muschelkalk since the normal ma-rine conditions were replaced by brackish, clastic-diluted sedimentation, typical of the LowerKeuper environments.

2.3. The northern Holy Cross Mountains section

As indicated by paleontological data [24,25] theMuschelkalk succession in the northern HolyCross Mountains commences only with the Pelso-nian sediments overlapping the Lower Roetianclastics. Since the succeeding deposits representalmost complete late Pelsonian^Fassanian succes-sion, this pro¢le has been chosen for constructionof the composed magnetostratigraphic scale of thePeritethyan Middle Triassic.

The generally accepted lithostratigraphic divi-sion of the Triassic succession of the Holy CrossMountains has been erected by Senkowiczowa[26] and this scheme is applied also in our work(Table 1). The ¢rst age-diagnostic conodonts,Gondolella aegea ( = Gondolella regale) have beenfound by Trammer [9] within the WellenkalkBeds. The occurrence range of this taxon de¢nesthe Aegean age of the Wellenkalk Beds. The sub-sequent taxon Paragondolella bulgarica which oc-curs in the lower part of the Lukowa Beds de¢nesthe Bithynian age of this interval. The next indexconodont taxon, N. kockeli, accompanied by Pa-ragondolella navicula, P. excelsa and Paragondo-

lella bifurcata which occur in the upper part of theLukowa Beds and in the Lima striata Beds de¢nethe Pelsonian age of this interval. The last namedthree taxa occur in the northern Holy CrossMountains some 1^1.5 m above the Roetian^Muschelkalk boundary [24]. This indicates thatthe Muschelkalk transgression reached the north-ern Holy Cross Mountains area as late as in Pel-sonian time, i.e. during the maximum £oodingphase of the Lower Muschelkalk. This inferenceis con¢rmed by the coexisting crinoid fauna of theSilesiacrinus silesiacus zone (Hagdorn in [25]),which comprises in Upper Silesia the Karchowiceand Diplopora Beds, representing the Mid^LatePelsonian^Illyrian interval [20]. The Middle Mu-schelkalk facies featured by evaporites and emer-sion events are devoid of index fossils. They ap-pear only in the Upper Muschelkalk and comprisespecies typical for its lower part (Pecten discitesBeds), i.e. Gondolella excentrica, Gondolella cornu-ta and Gondolella mombergensis [24]. These taxarepresent the 1st and 2nd conodont zones (seeabove). The uppermost part of the Muschelkalkin the studied section comprises Ceratites spinosuscephalopod remnants (Table 1; Hagdorn, in [25])and G. mombergensis media conodonts de¢ningthe third conodont zone [24]. Since Gondolellahaslachensis has not been found in the Muschel-kalk deposits of the northern Holy Cross Moun-tains section, one may assume that the normalmarine deposition terminated here one conodontzone earlier as in the southern Holy Cross Moun-tains area and in Upper Silesia, and four zonesearlier as in the German part of the MuschelkalkBasin.

3. Sampling localities and paleomagnetic procedure

A total of 171 drill samples for paleomagneticstudies were collected from 132 beds exposed insix quarries and two ravines situated in the Silesiaand Holy Cross Mountains (Fig. 1). Up to threespecimens were cut from each sample. A set ofsamples taken from the same bed was consideredas one magnetostratigraphic sample. The group ofspecimens obtained from one independently ori-ented sample was regarded as one sample used for

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paleopole calculation. In all localities the studiedrocks were characterized by a very small dip thatdid not exceed 7³.

The natural remanent magnetization (NRM) ofthe specimens was measured using a JR-5 spinnermagnetometer with a noise level about 0.3U1035

A/m. Some pilot specimens were subjected to al-ternating ¢eld (AF) and thermal demagnetizationexperiments. Since the AF method was not e¡ec-tive the whole sample set has been subjected tothe stepwise thermal demagnetization in a W-metalshielded oven, which reduced the ambient ¢eldclose to a few nT. After each thermal demagneti-zation level a magnetic susceptibility signal wasmonitored. Least-square line ¢t methods, as pre-sented by Kirschvink [27], were used to calculate

the components of NRM and their unblockingtemperature spectra. Thermomagnetic analysisand analysis of isothermal remanent magnetiza-tion (IRM) acquisition were used to determinethe nature of magnetic carriers.

4. Results of paleomagnetic analysis

About 50% of the specimens revealed NRMintensities lower than 0.5U1034 A/m. The NRMintensities in 38% of the specimen collectiondropped down below 0.7U1034 A/m at demagne-tization temperatures higher than 200³C. 28 sam-ples taken from 26 beds yielded well-de¢ned, anti-podal directions of presumed Middle Triassic age,

Fig. 2. Orthogonal demagnetograms and stereographic projections of NRM demagnetization directional tracks prepared for rep-resentative samples of Roetian^Muschelkalk rocks from the Holy Cross Mountains and Upper Silesia. (a) Samples containingwell-de¢ned (see text) component A. (b) Samples with component A oscillating near the expected Middle Triassic position.(c) Partly remagnetized sample that showed the expected Middle Triassic direction A at two last demagnetizations levels.(d) Sample containing component A that is strongly overlapped by a steep component of presumed Cenozoic age. The expectedMiddle Triassic position of paleodirection has been attained at the last demagnetization level only. (e) Sample with well-de¢nedsecondary component B of Late Triassic age. (f) Totally remagnetized sample containing Cenozoic component only. In the lastgroup all data are presented after bedding tilt correction. In steroeplots, open (closed) symbols denote upward (downward) point-ing inclinations.

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based on at least three subtracted vectors and a¢tted-line with an angular standard deviation nothigher than 5³ (Figs. 2a and 3). They were usedfor Middle Triassic paleopole calculations (Table2, component A). During thermal experiment 38samples coming from 31 beds yielded directionsthat oscillated near the expected Middle Triassicposition (Fig. 2b). These small oscillations wereconnected with measurement errors caused by ex-tremely low intensities. They did not allow to dis-tinguish adequate ¢tted-line directions. In the oth-er 41 samples from 35 beds the expected MiddleTriassic component A was reached at the two lastdemagnetization levels (Fig. 2c). It was partlycovered by a component with a steep inclinationthat corresponds to the local present-day ¢eld di-rection. 23 samples from 14 beds showed the ex-pected Triassic position of paleodirection only atthe last demagnetization level (Fig. 2d) that usu-ally preceded an abrupt increase of magnetic sus-ceptibility. These samples contain component Astrongly overlapped by a steep component refer-ring to the local present-day ¢eld direction. Theexpected Middle Triassic position of paleodirec-

tion has been attained at the last demagnetizationlevel only. The above presented subdivision of de-magnetization resulting into four paleomagneticbehaviors was used for determination of four cat-egories of reliability of magnetic polarity. Obvi-ously, the most reliable (¢rst category) polaritieswere determined on the base of well-de¢ned ¢tted-line directions. The last (fourth) category of po-larity determination was based on the samplesshowing expected paleodirections at the last de-magnetization level only. It should be stressedthat the reliability grading of magnetostrati-graphic data from weakly magnetized beds hasbeen widely applied by other authors (e.g. [28]).

Ten samples from the upper part of the LowerGogolin Beds (Plaza quarry) revealed the presenceof well-de¢ned component with characteristic in-clination of about 15³ higher than expected forthe Middle Triassic (Fig. 2e). Because of thisand because of di¡erent magnetic mineralogy(see Section 5) this remanence was regarded assecondary in origin (Table 2, component B).Nineteen samples taken from 11 beds were total-ly remagnetized in the present ¢eld direction(Fig. 2f).

5. Magnetic carriers

Magnetic minerals from the Muschelkalk car-bonates of the Silesia region have been alreadyanalyzed by Symons et al. [29]. According to theresults of that study Triassic remanence resides insingle^pseudosingle domain magnetite in lime-stone and is probably primary, and in single^pseudosingle domain magnetite and hematite inearly dolomite and is probably diagenetic [29]. In-deed, the IRM acquisition and thermomagneticcurves prepared for the Silesian samples with theMuschelkalk remanence (component `A') showthe presence of low coercivity mineral with max-imum unblocking temperatures not higher than580³C (Fig. 4; sample PL5e and SO24). Di¡erentpetromagnetic behavior displays samples from theHoly Cross Mountains that also contain the Mu-schelkalk component. Apart from magnetite it re-veals the presence of goethite and hematite (Fig.4; sample NT25). Samples with secondary com-

Fig. 3. Stereographic projection of the primary Muschelkalk(circles) and secondary Late Triassic (squares) line ¢tted di-rections from the Upper Silesia and Holy Cross Mountains(for summary of statistics see Table 2). Cross shows the localpresent-day ¢eld direction. All directions are presented afterbedding tilt correction. Open (closed) symbols denote upward(downward) pointing inclinations.

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ponent `B' isolated in the Plaza section (GogolinBeds) contain a high coercivity mineral that wasdemagnetized at temperatures higher than 600³C(Fig. 4; sample PL12). These properties indicatethe presence of hematite. Samples totally remag-netized in the present ¢eld direction containmainly goethite (Fig. 4; sample SO27).

6. Paleomagnetic poles

Paleopoles were computed only for localitiescontaining more than ¢ve independently orientedsamples with well-de¢ned ¢tted-line directions.These paleopoles and the mean Muschelkalk pa-leopole are listed in Table 2. The mean Muschel-kalk paleopole (labeled À') and the paleopolefrom the remagnetized zone detected in the Plazasection (labeled `B') have been compared with theapparent polar wander path (APWP) character-istic for Stable Europe. This APWP was con-structed using only good quality (Qs 4) poles.

The Early Triassic poles were taken from the listof Van der Voo [30]. They were obtained in thecratonic area of Europe (i.e. East European Plat-form). The six best quality Middle TriassicÊarlyJurassic poles were determined in NW France andSW Germany [12,31]. The calculation andsmoothing of the apparent wander path was per-formed using the GMAP plotting package [32].The smoothing procedure involves the sphericalspline method and the following splining param-eters have been applied: tension factor = 200, timeresolution = 2.5 Ma. It is clearly visible that theMuschelkalk paleopole À' ¢ts well to the MiddleTriassic (ca. 240 Ma) segment of the Stable Euro-pean APWP (Fig. 5). Paleopole `B' from Plaza isshifted towards the Late Triassic part of thisAPWP, similarly as the paleopole obtained earlierfrom the Silesian Muschelkalk by Symons et al.[29].

The paleolatitudinal data are important in pa-leoecological and paleofacial models. Accordingto paleoenvironmental maps of Dercourt et al.

Table 2Summary of statistics of paleodirections isolated in the Muschelkalk of the Polish basin and selected Late Permian^Middle Trias-sic poles used for construction of the Stable European APWP (Fig. 5).

Locality Group Comp. N D I K95 K Lat. Long. dp dmPol. Plat.

PIaza Lower A 9 210 337 5.3 95.8 52³N 149³E 4 6 Mix. 21³N(19.5³E, 50.1³N) Muschelkalk B 10 217 354 5.1 90.7 59³N 126³E 5 7 R 34³N

(55³/3³)*Strzelce Lower/Middle A 12 217 341 4.6 91.1 51³N 138³E 3 6 Mix. 23³NOpolskie Muschelkalk(18.2³E, 50.5³N) (42³/4³)*Nietulisko Middle/Late A 7 216 342 7.0 74.9 51³N 142³E 5 9 Mix. 24³N(21.1³E, 50.9³N) Muschelkalk

(223³/3³)a

Polish basin Muschelkalk A 28 214 340 3.0 84.2 51³N 143³E 2 4 43% N, 57% R 23³N(summary) 3a 214 340 6.0 421.5 52³N 143³E 4 71. France, Illyrian/Fassanian carbonates 3 53³N 141³E

Ukraine Serebryansk, Donbas (Early Triassic) 12 56³N 146³ERussia Varieg. Suite, Urals (Early Triassic) 4 51³N 151³E

2. Russia Romasska Redbeds (Early Triassic) 4 52³N 165³ERussia Veltugian Red Clays 4 53³N 158³ERussia Upper Tatarian Redbeds 4 51³N 166³E

Comp. = name of component; N = number of independently oriented samples (anumber of localities); D = declination; I = inclina-tion; K95, K = radius of the cone of 95% con¢dence and precision parameter [43]; Lat. = latitude of north paleomagnetic pole;Long. = longitude of north paleomagnetic pole; dp = error of the distance between site and paleopole; dm = paleolongitude error;Pol. = polarity (Mix. = mixed, N = normal, R = reversed); Plat. = paleolatitude calculated for Plaza locality. *Mean azimuth andmagnitude of dip. All data are presented after bedding correction. 1 = Middle Triassic pole of Theveniaut et al. [12], 2 = Late Per-mianÊarly Triassic poles listed by Van der Voo [30].

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[33] in the Middle Triassic time Silesia was locatedaround 32³N. The Muschelkalk inclinations ob-tained in this paper were derived mainly fromthe Anisian samples. These inclinations corre-spond to 21^24³ of paleolatitude (Table 2).

7. Magnetic stratigraphy and correlation

7.1. Upper Silesia

A very good (bed to bed) internal stratigraphiccorrelation of Silesian outcrops allows to presentthe paleomagnetic data in the one diagram com-prising the Roetian^Middle Muschelkalk strata(Fig. 6). Magnetostratigraphic subdivision ofthese rocks is based on the results of various reli-ability. In the case where well-de¢ned ¢tted-linedirections were not obtained the declination andinclination diagrams were constructed by themeans of directions obtained at the last demagne-tization level. However, no magnetozone was dis-tinguished using poor quality (third and fourthcategory) samples only. The magnetozones werenumbered if the same polarity record was notedin more than one sample. In this way seven nor-mal and seven reversed magnetozones were de-

¢ned in the Silesian pro¢le. The normal polarityrecord seems to be predominant in the lower partof the Roetian and lowermost Muschelkalk. Re-versed directions of magnetization occur espe-cially in the Pelsonian segment of the section.

A certain break in the magnetic polarityscheme, related to the Late Triassic remagnetiza-tion, occurs in the middle part of Gogolin Beds(in Fig. 6 marked by gray belts). The origin ofLate Triassic secondary component B is not evi-dent. One possibility is to link remagnetizationwith the £uid^rock interaction. In the GogolinBeds authigenic potassium feldspar was found[34]. It has been ascribed to burial diagenesis pro-ceeded without contact with seawater. The K-feld-spar has been earlier de¢ned as a tracer of paleo-£uid £ow in carbonates [35,36] related to theirremagnetization [37].

Volume magnetic susceptibility of the Muschel-kalk carbonates from Silesia varies between 368and 287U1037 SI units. However these variationsare not accidental. It is clearly visible that in sev-eral places maximum values of magnetic suscepti-bility are in agreement with the boundaries ofdepositional sequences (Fig. 6). Hence, magneticsusceptibility graphs can be used as a tool ofstratigraphic correlation of these rocks.

Fig. 4. (a) Thermal demagnetization of orthogonal-axis IRM curves obtained for selected Roetian^Muschelkalk samples from theUpper Silesia (PL5e, PL12, SO24, SO27) and Holy Cross Mountains (NT25). (b) IRM acquisition curves prepared for the samesample set.

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7.2. The Holy Cross Mountains

The Nietulisko section is composed of Pelso-nianÊarly Fassanian carbonates with a certainbreak near the PelsonianÎllyrian boundary [13].A continuous magnetostratigraphic graph wasprepared only for the upper part of the sectionwhere good quality Middle Triassic ¢tted-line di-rections were obtained (Fig. 7 and Table 2). Itslower part yielded hardly ever six dispersed pointswhere polarities were de¢ned. Three normal andfour reversed magnetozones were established inthe Illyrian^Fassanian part of the studied section.Normal polarity record seems to be predominantin the upper part of the Illyrian and lowermostpart of Fassanian strata.

7.3. The composite polarity scale

The composite polarity scale (Fig. 8) has beenconstructed using a set of characteristic directionsisolated in 106 samples. Magnetic polarity patternin the Roetian and Lower Muschelkalk was de-¢ned on the basis of the results from Silesia. TheUpper Muschelkalk part of the scale was adoptedfrom the Nietulisko section. Magnetostratigraphicdata from Nietulisko and Libia

Lzç sites were used

for construction of the Middle Muschelkalk seg-ment. Correlation of the Muschelkalk Beds fromthe Holy Cross Mountains and Silesia was per-formed according to conodont biozonation [9,10]and the sequence stratigraphic procedure [13].

Within the entire Roetian^Muschelkalk se-

Fig. 5. TriassicÊarly Jurassic APWP for European Plate (for construction details see text) and the Middle Triassic paleopole (la-beled A) from this paper. Paleopole calculated for the secondary component B, and paleopole obtained earlier [29] from the Mu-schelkalk of Silesia (marked by star) are also presented. The study area is indicated by the arrow. The ages were adopted fromthe time calibration of Menning [39] and Mundil et al. [40] (Early^Middle Triassic), Kent and Olsen [41] (Late Triassic), andPalmer [42] (Early Jurassic). They are shown in Ma.

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Fig. 6. Lithostratigraphy and sequence stratigraphy of the Roetian^Muschelkalk deposits from Upper Silesia and the results oftheir magnetostratigraphic investigations. On the right of lithostratigraphic column, the plots of magnetic susceptibility, character-istic declination, characteristic inclination and magnetic polarities are presented (black belt = normal polarity record, white bel-t = reverse polarity record). In the declination and inclination plots the biggest circles represent the ¢rst (best) category of data(see text). The two next smaller size circles are characteristic for second and third category data. Fourth category data yielded in-clinations and declinations marked by the smallest circles. The polarity column is divided into numbered magnetozones.

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quence nine normal and nine reverse polarityzones have been found. The magnetic polaritypattern noted in the Roetian sediments from Si-lesia is the same as in the Roetian section drilledin the Polish Lowland, where a normal polarityrecord was characteristic for the Lower Roetian,in contrast to its upper part that was magnetizedreversely [5].

7.4. Magnetostratigraphic correlation with theTethyan realm

The index conodonts common for the HolyCross Mountains, Upper Silesia and the Alpinearea were fundamental for construction of theMiddle Triassic biostratigraphic framework insouthern Poland (Table 1) and for its correlationwith the Tethys section (Fig. 8). Other fossils

Fig. 7. Lithostratigraphy of Muschelkalk deposits from the Holy Cross Mountains (Nietulisko section) and the results of theirmagnetostratigraphic investigations. On the right of the lithostratigraphic column, the plots of magnetic susceptibility, characteris-tic declination, characteristic inclination and magnetic polarities are presented.

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(crinoids, cephalopods, dasycladales) appeared asuseful aid-tools of the chronostratigraphic corre-lation between the both basins. The biostrati-graphic tools contrived an adequate backgroundfor correlation of the Peritethyan Middle Triassic

magnetostratigraphic scale with the Tethyan scaleby Muttoni et al. [8]. The resulted magnetostrati-graphic correlation indicates that the entire Roe-tian succession from the southern Poland repre-sents the Olenekian age. Also the Muschelkalk

Fig. 8. Composite Roetian^Muschelkalk polarity scale for the Peri-Tethys basin in Poland and its correlation with polarity scalecomposed [8] for western Tethys. Broken lines demonstrate magnetostratigraphic correlation. Solid lines shows boundaries of sub-stages. Dotted line with a question mark presents the hypothetical correlation (rejected in this paper) that would include thewhole Roetian into the Anisian. Double line signals the Pelsonian^Illyrian boundary placed according to the occurrence of dasy-cladales algae [22].

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facies onset is coincidental with the latest Olene-kian. As already mentioned, Kozur [3] has as-cribed the entire Roetian succession to the Ani-sian stage based mainly on a holothurian faunathat can not be indexed with fossils since its realstratigraphic range is unknown. Regarding Ko-zur's concept as correct one should assume theAegean age of the whole Roetian (cf. the correla-tion line marked with `?' in Fig. 8). However, itwould be very di¤cult to interpret the presence ofonly one reverse zone in the Aegean and Bithy-nian substages within the Tethys pro¢le and asmuch as three reversed zones in the Roetian^Low-er Muschelkalk of the Peritethys basin. The pre-sented magnetostratigraphic data support theolder concepts of the correlation of the Triassicfrom the Germanic and Alpine areas, assumingthe beginning of the Muschelkalk as nearly con-current with the Scythian^Anisian boundary (e.g.[1]).

The end of the marine sedimentation of theMuschelkalk facies in the Polish basin took placein Fassanian time, i.e. much earlier than in theGerman basin. The earlier constructed sequencestratigraphic framework throughout the entirePeri-Tethys Basin [13,17] coupled with the con-structed magnetostratigraphic scheme gives away for chronostratigraphic correlation of themost important events in the basin history.

8. Conclusions

Eighteen biostratigraphically calibrated magne-tozones were recognized in the Roetian^Muschel-kalk sequence from the Peri-Tethys basin in Po-land. The correlation of established magneticpolarity scale with the scale erected earlier in theTethyan realm indicates that the whole Roetiansuccession in the southern Polish basin shouldbe included to the Olenekian stage. The normalmarine sedimentation ceased in the Polish basinas early as in the Late Fassanian time. Our studiesshow that the studied Muschelkalk sequence wasdeposited during the latest Olenekian^Early Fas-sanian time. The primary paleomagnetic direc-tions from examined rocks gave a pole that ¢tswell to the Middle Triassic segment of the Stable

European APWP. The Muschelkalk inclinationsobtained in this paper corresponds to 21^24³ ofthe paleolatitude of the studied area.

Acknowledgements

This research was ¢nancially supported bythe Polish Geological Institute (Project6.20.9411.00.0), the Jagiellonian University (Proj-ect DS. V/ING/5/97) and the State Committee forScienti¢c Research (Project 9T12B02415). Wethank Jarek Zacharski for assistance during ¢eldwork. Special thanks are expressed to Kasia Nar-kiewicz for fruitful discussion concerning cono-dont biostratigraphy of the Muschelkalk. Specialthanks are expressed to Jim Ogg and MaurizioGaetani for critical review of the manu-script.[RV]

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the middle triassic magnetostratigraphy from the peri-tethys basin in poland

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