DOI: http://dx.doi.org/10.7306/gq.1201
U-Pb zir con age of the Krásné Louèky tuffite: the dat ing of Visean flysch in the Moravo-Silesian Pa leo zoic Ba sin (Rhenohercynian Zone, Czech Re pub lic)
Jakub JIRÁSEK1, *, Jiøí WLOSOK1, Mar tin SIVEK1, Dalibor MATÝSEK1, Mark SCHMITZ2, Ivana SÝKOROVÁ3 and Zdenìk VAŠÍÈEK4
1 VŠB – Tech ni cal Uni ver sity of Ostrava, Fac ulty of Min ing and Ge ol ogy, 17 listopadu 15/2172, 708 33 Ostrava-Poruba, Czech Re pub lic
2 Boise State Uni ver sity, De part ment of Geosciences, 1910 Uni ver sity Drive, Boise, ID, U.S.A.
3 Acad emy of Sci ences of the Czech Re pub lic, In sti tute of Rock Struc ture and Me chan ics, V Holešovièkách 41, 182 09 Praha 8, Czech Re pub lic
4 Acad emy of Sci ences of the Czech Re pub lic, In sti tute of Geonics, Studentská 1768, 708 00 Ostrava-Poruba, Czech Re - pub lic
Jirásek, J., Wlosok, J., Sivek, M., Matýsek, D., Schmitz, M., Sýkorová, I., Vašíèek, Z., 2014. U-Pb zir con age of the Krásné Louèky tuffite: the dat ing of Visean flysch in the Moravo-Silesian Pa leo zoic Ba sin (Rhenohercynian Zone, Czech Re pub lic).
Geo log i cal Quar terly, 58 (4): 659–672, doi: 10.7306/gq.1201
The only pre vi ous U-Pb zir con date for the the Early Car bon if er ous flysch se quence of the Moravo-Silesian Pa leo zoic Ba sin was pub lished in 1987 from tuffogenic ma te rial from Kobylí Quarry at Krásné Louèky near the town of Krnov (Silesia, Czech Re pub lic). The mea sured age of 319 Ma did not agree with its strati graphic po si tion, and was used as the ba sis for a hy poth e - sized block of Late Car bon if er ous paralic molasse in cor po rated dur ing a later tec tonic event. Dur ing a sur vey of the still-ac - tive quarry in 2010, volcaniclastic ho ri zons were iden ti fied and sam pled. Di rect cor re spon dence of the tuff units to those sam pled in 1987 can not be proved but is likely. High pre ci sion chem i cal abra sion – ther mal ion iza tion mass spec trom e try (CA-TIMS) U-Pb dat ing of zir con from this new ma te rial has yielded an age of 340.05 ± 0.22 Ma, which cor re lates to the pre vi - ously in ferred strati graphic age of the lo cal ity and the cur rent cal i bra tion of the Early Car bon if er ous geo logic time scale. The newly es tab lished age cor re sponds to the Visean stage and dates the bound ary be tween the Horní Benešov and Moravice for ma tions that can be cor re lated with other foredeep bas ins of the Culm in the Eu ro pean Variscides. A pop u la tion of de tri tal Cambro-Or do vi cian zir cons and a sin gle 2.0 Ga old zir con crys tal from the same volcaniclastic layer dated by la ser ab la tion in duc tively cou pled plasma mass spec trom e try (LA-ICPMS) are con sis tent with the known age of source ma te rial in the Variscan orogenic front.
Key words: volcaniclastic sed i ment, Horní Benešov For ma tion, Variscan fore land, Visean, Car bon if er ous, chronostratigraphy.
INTRODUCTION
The stra tig ra phy of the flysch de pos its of the Culm in the Early Car bon if er ous of the Moravo-Silesian Pa leo zoic Ba sin con tains many un solved prob lems. This is mainly due to its uni - form li thol ogy and lack of biostratigraphically im por tant fos sils.
There is on go ing de bate about the age and re la tion ships of its lithostratigraphical units. This dis cus sion may be solved (at least partly) by ra dio met ric dat ing, but so far, the only avail able ab so lute age is that re ported in an un re viewed pub li ca tion (Pøichystal, 1987). The pub lished iso tope di lu tion – ther mal ion - iza tion mass spec trom e try (ID-TIMS) U-Pb zir con age of
319 Ma for the volcaniclastic ho ri zon ex posed in Kobylí Quarry at Krásné Louèky near the town of Krnov was prob lem atic for var i ous rea sons (not be ing con sis tent with ei ther lo cal li thol ogy or biostratigraphy) doc u mented be low. Nev er the less, this age has been quoted in a num ber of later stud ies deal ing with the Moravian-Silesian Culm (e.g., Pøichystal, 1993; Hladil and Dvoøák, 1994; Cháb, 2010).
“Two decimetre-thick lay ers of weath ered crystalloclastic tuff” from an un spec i fied lo ca tion are men tioned in the un pub - lished ex plan a tory notes that ac com pany the geo log i cal base map at 1:25,000 scale, sheet 15-134 Brantice (Maštera and Otava in Otava, 1984). This date for these rocks was also re - ported by Pøichystal (1987), but fur ther de tails of the lo cal ity were not given. It was men tioned that one of these lay ers had been quar ried out in 1985. Pøichystal (1987) also men tioned spores which should have con firmed a Late Car bon if er ous age, but gave no more in for ma tion. Karkusz (1989) stated that this layer of “crys tal tuff” had been quar ried out, and de scribed at least one other tuffaceous layer that was very sim i lar to the orig -
* Corresponding author, e-mail: jakub.jirasek@vsb.cz
Received: April 15, 2014, accepted: June 25, 2014; first published online: November 14, 2014
i nal ho ri zon. How ever, he found no ev i dence for a Late Car bon - if er ous age of any tuffaceous beds in the quarry. Sev eral at - tempts were made af ter 1990 by ge ol o gists un der the lead er - ship of O. Kumpera to lo cate the lay ers of volcaniclastic ma te - rial in the quarry, but these were not suc cess ful, nor did they find any rock or palaeontological ma te rial with an age cor re - spond ing to the Late Car bon if er ous. Palaeontological re vi sion of flora found in the quarry (Purkyòová, 2003) did not con firm any oc cur rence of Late Car bon if er ous spe cies. Pøichystal, Hladil, Kuovo, Otava and Valterová in Weyer and Menning (2006) again pub lished the orig i nal in for ma tion and ra dio met ric age with out any fur ther de tails of the site. The lat ter au thors are also vague re gard ing the palynological ev i dence for a Namurian A age – no sin gle taxon is iden ti fied and no rel e vant pho to graphs are given. They re fer only to an un pub lished manu script by Otava and Valterová (1992, in Weyer and Menning, 2006).
In 2010, dur ing a sur vey of the still-ac tive quarry, we dis cov - ered lay ers of volcaniclastic sed i ment (tuffite) which are mac ro - scop i cally and mi cro scop i cally very sim i lar to the orig i nally de - scribed ex po sures. Here we re port a newly de ter mined high-res o lu tion CA-TIMS U-Pb zir con-based age of de po si tion for one of these lay ers which is com pat i ble with the geo log i cal set ting and struc ture of the quarry and its sur round ings. In this study, we pres ent a num ber of ar gu ments in sup port of the ac - cu racy of this new age (e.g., geo log i cal set ting and de gree of meta mor phic trans for ma tion).
GEOLOGICAL SETTING
MORAVO-SILESIAN PALEOZOIC BASIN
Ac cord ing to Unrug (1966) and Unrug and Dembowski (1971), the Moravo-Silesian Pa leo zoic Ba sin was formed in the east ern do main of the Cen tral Eu ro pean Variscides. It was de - vel oped in the fore land of the Variscan orogen and be came part of its outer zones, the so-called Rhenohercynicum and Subvariscicum (Schulmann and Gayer, 2000; Schulmann et al., 2009).
The ba sin is filled by some of the youn ger de pos its over ly - ing Brunovistulicum (Bu³a and ¯aba, 2005; Kalvoda et al., 2008). De vo nian sed i men ta tion started with pre-flysch car bon - ates (Bábek et al., 2007) and con tin ued into the ma rine clastic sed i ments of the Car bon if er ous flysch (Kumpera, 1983;
Dvoøák, 1994). This so-called Culm fa cies pro vides in di rect ev i - dence of syn-orogenic sed i men ta tion dur ing the Variscan orog - eny from pos si bly as early as the Tournaisian through the whole of the Visean to the low er most Serpukhovian (Bábek et al., 2007; Jirásek et al., 2013a). The fi nal stages are marked by de - po si tion of the coal-bear ing paralic and terrigenous molasse of the Up per Silesian Ba sin. The ba sin fill is over lain mostly by Neo gene and Qua ter nary sed i men tary se quences and, in the east ern part of the ba sin, also by Neo gene de pos its of the Carpathian Foredeep and, fur ther to the south-east, by the nappes of the Outer West ern Carpathians (Late Ju ras sic to Paleogene).
In the Czech Re pub lic, the Early Car bon if er ous flysch se - quence is ex posed in the Drahany Up land, the Oderské vrchy Mts., and the Nízký Jeseník Mts. (e.g., Patteisky, 1929;
Kumpera, 1983; Dvoøák, 1994), and in Po land west of the Up per Silesian Ba sin and east of the town of G³ucho³azy (Trzepier -
czyñska, 2003). It is com posed of four ma jor lithostratigraphic units: the Andìlská Hora Fm., the Horní Benešov Fm., the Moravice Fm., and the Hradec-Kyjovice Fm. (Fig. 1). Their to tal thick ness prob a bly reaches 7.5 to 12 km (Kumpera and Martinec, 1995; Mazur et al., 2006), but pre cise thick nesses can - not be given be cause of strong tec tonic re work ing in the Variscan accretionary wedge (Grygar and Vavro, 1995; Kumpera and Martinec, 1995), as well as in tense post-Car bon if er ous and prob - a bly also Car bon if er ous ero sion in those parts close to the orogenic front (Francu et al., 2002). Only lim ited in for ma tion has been ob tained from a few bore holes.
The types of strata fill ing the ba sin, to gether with their thick - ness and ex tent, vary both stratigraphically and spa tially. Gen - er ally, in the Andìlská Hora Fm., the coarse-grained de pos its (con glom er ates, greywackes) are as abun dant as are siltstones and clayey shales. In the Horní Benešov Fm. greywackes pre - dom i nate. In the Moravice Fm. and Hradec-Kyjovice Fm. (with the ex cep tion of the Hradec Mem ber) the pro por tion of siltstones and clayey shales is greater and the de pos its are more ma ture so sand stones are more abun dant than greywackes (Kumpera, 1974, 1976; Kukal, 1980; Dvoøák, 1994). Kumpera and Martinec (1995) con cluded that sed i men - ta tion was driven by mul ti ple geotectonic events in the fore land ba sin. Mostly deep-wa ter siliciclastic turbidites were de pos ited on an elon gate sub ma rine fan, al though shal low-wa ter strata are also pres ent. Cyclicity in the sed i men tary se quence was also rec og nized by Skoèek (1989) and Bábek et al. (2004). The in ten sity of the diagenetic over print de creases up wards in the strati graphic se quence i.e., from the west to the east. In the Hradec-Kyjovice For ma tion the vitrinite reflectance Ro is mostly be tween 5.5 to 2.0% (au thors’ unpubl. data); in the Ostrava For - ma tion val ues of Ro in the range 2.0 to 0.5% were re ported by Sivek et al. (2003) and Kandarachevová et al. (2009).
Biostratigraphic in for ma tion on the Culm strata of the Moravian-Silesian Ba sin var ies con sid er ably. The stratigraphi - cally higher units (the Moravice and Hradec-Kyjovice for ma tions) are known to con tain a lo cally rich fauna, es pe cially bi valves and goniatites, and flora in clud ing both macroflora and spores. Ac - cord ing to Kumpera (1996), fos sil re mains in the stratigraphically lower units (the Andìlská Hora and Horní Benešov for ma tions) are very rare and be cause of the higher grade of meta mor phism they are not very well-pre served, so they are of lit tle use for biostratigraphic cor re la tion. The lack of biostratigraphic in for ma - tion does not pre clude the idea that the flysch zone in the Nízký Jeseník Mts. con sists of two in ter nally imbricated nappes and that the Andìlská Hora and Horní Benešov for ma tions are strati - graphic equiv a lents of the Moravice and Hradec-Kyjovice for ma - tions (e.g., Cháb, 2010; and coun ter-ar gu ments by Dvoøák, 1986). In this study, how ever, we adopt the lithostratigraphic di vi - sion of the Culm in the Nízký Jeseník Mts. pro posed by Zapletal et al. (1989). This is the re sult of sys tem atic work by three most dis tin guished post-World War II in ves ti ga tors of this area and is widely ac cepted. The zonation of heavy min er als in the flysch strata de scribed by Hartley and Otava (2001) also sup ports this di vi sion of the stra tig ra phy. The Moravice For ma tion is clas si fied as be long ing to goniatite zones Goa2–3 to Gobmu (Kumpera, 1983). In terms of the con tent of heavy min er als, the lower part of the Moravice For ma tion be longs to the Lower Heavy Min eral Zone, and the mid dle be longs to the Mid dle Heavy Min eral Zone, while the up per sec tion ranges up to the bot tom of the Up per Heavy Min eral Zone (Hartley and Otava, 2001).
KRÁSNÉ LOUÈKY – KOBYLÍ QUARRY
The lo cal ity Krásné Louèky – Kobylí, where the dated volcaniclastic ho ri zon is ex posed, is the op er at ing quarry owned by KAMENOLOMY ÈR s.r.o./Ltd. It lies on the north - west ern slope of Kobylí Hill, ap prox i mately 1 km south-west of the vil lage of Krásné Louèky, a sub urb of the town of Krnov, about 50 km north-west of the city of Ostrava (co or di nates Lat.
N 50°7’13.08’’ Long. E 017°37’40.30‘’).
The flysch se quence, con sist ing mainly of coarse flysch with lesser amounts of rhyth mic flysch and shale, is per ma - nently quar ried (Novák and Horák, 1981). These strata be long to the Brantice Mem ber of the Horní Benešov For ma tion near the tran si tion to the pelite-dom i nated Moravice For ma tion (Kumpera, 1961; Kumpera and Vašíèek, 1961). The whole
area has been strongly af fected by tectonism (Kumpera, 1966).
In de tail, the com plex struc ture of the Kobylí greywacke de posit re sults in sig nif i cant vari a tions in fa cies in both hor i zon tal and ver ti cal di rec tions (Karkusz, 1989).
The first fos sils from Kobylí Quarry were de scribed by Kumpera (1961), who found a layer rich in frag ments of Asterocalamites cf. scrobiculatus (Schlotheim). The same spe - cies was also found at sev eral sites nearby (Patteisky, 1929;
Kumpera, 1966). Karkusz (1989) re fers to the dis cov ery of fos - sil-bear ing, grey-black, mi cro-biodetrital lime stone con tain ing the foraminifera Globoendothyra sp., Eostaffella sp., Archaediscus krestovnikovi Rauser, Archaediscus sp., Endothyra sp., Palaeotextulariidae indet., and the al gae Calcisphaera sp. and Stacheiinae indet. This layer is de scribed as a slipped block with an age in the range from the up per Mid - Fig. 1. Simplified geological map of the Nízký Jeseník part of the Moravo-Silesian Paleozoic Basin (modified from Dvoøák, 1994) with stratigraphic column and goniatite zonation (after Kumpera, 1983, modified) and Heavy Mineral zones (Hartley and Otava, 2001)
dle to Late Visean. Purkyòová (2003) pub lished a new as sess - ment of palaeobotanical ma te rial col lected from the quarry. In ad di tion to finds of Asterocalamites scrobiculatus (Schloth.), abun dant frag ments of the tree-like lycopodiophyta Lepido - dendron lossenii Weiss, to gether with rarer Lepido dendron volkmannianum Stern berg and Knorria sp. and also prob a bly the pteridosperms Sphenopteris (?Rhodeopteridium – Sphenopteridium) sp. ind. and ?Fryopsis sp. were iden ti fied.
Purkyòová (2003) there fore in ter preted the plant thanato - coenosis as be ing of Early Car bon if er ous age with out youn ger flo ral el e ments. In terms of the oc cur rence of heavy min er als (Karkusz, 1989) and their uti li za tion for lo cal stra tig ra phy (Hartley and Otava, 2001), the quarry lies in the Lower and at the tran si tion to the Mid dle Heavy Min eral Zone (Fig. 1).
Low-tem per a ture hy dro ther mal min er ali sa tion oc curs in fis - sures and tec toni cally dis turbed zones in the quarry and was de scribed by Zimák et al. (2002). The min eral as sem blage con - sists of cal cite, quartz, li mo nite, py rite (Kruïa, 1954–1955), bar - ite, ga lena (Kruïa, 1963), chal co py rite, mal a chite, stilpno - siderite (Kruïa, 1973) and chlorite (Zimák et al., 2002).
CONTROVERSIAL DATING AND SUPPORTING EVIDENCE
Tuffaceous ho ri zons in the quarry at Krásné Louèky – Kobylí were first de scribed by Otava (1984). Two ap prox i mately decimetre-thick lay ers were iden ti fied, but no fur ther de tails were given. Zir cons were sep a rated from these tuffaceous lay - ers and sub se quently dated in the Iso tope Ge ol ogy De part ment at the Geo log i cal Sur vey of Fin land and the re sults were pub - lished by Pøichystal (1987). The ID-TIMS method (Krogh, 1973) was em ployed us ing the de cay con stants pub lished by Steiger and Jäger (1977). Ac cord ing to the au thor, three en tirely con - cor dant ages were de ter mined: 238U/206Pb: 319 ± 2 Ma,
235U/207Pb: 319 ± 3 Ma and 207Pb/206Pb: 319 ± 21 Ma. Fur ther - more, it was stated that “in the vi cin ity of the in ves ti gated tuff, J. Otava col lected sam ples of coal ma te rial from which spores of Namurian age were ex tracted (pers. comm. from J. Otava, iden ti fi ca tion by P. Valterová)”. Karkusz (1989 stated that the site of the orig i nal dated tuff had been quar ried out. He also men tioned the oc cur rence of an un spec i fied num ber of other tuffites, of ten con tain ing plant de tri tus. Sub se quently, an iden ti - cal re port on the dat ing of the tuff at 319 (±2, 3, 21) Ma was pub - lished by Pøichystal et al. (in Weyer and Menning, 2006).
MATERIALS AND METHODS
Doz ens of sam ples of tuffite and nearby or ganic-rich shale were ob tained from the quarry. They were used for zir con sep a - ra tion and dat ing, min er al og i cal and pet ro log i cal anal y sis (tuffite) and for the search for palynological ma te rial and or ganic mat ter anal y sis (shale).
Zir con crys tals were sep a rated from the tuffite by con ven - tional den sity and mag netic meth ods. The en tire zir con sep a - rate was placed in a muf fle fur nace at 900°C for 60 hours in quartz beak ers to an neal mi nor ra di a tion dam age; an neal ing en hances cathodoluminescence (CL) emis sion, pro motes more re pro duc ible interelement frac tion ation dur ing la ser ab la - tion in duc tively cou pled plasma mass spec trom e try (LA-ICPMS), and pre pares the crys tals for sub se quent chem i - cal abra sion (Mattinson, 2005). Fol low ing an neal ing, in di vid ual grains were hand-picked and mounted, pol ished and im aged by cathodoluminence (CL) on a scan ning elec tron mi cro scope.
From these com piled im ages, the lo ca tions of spot anal y ses for LA-ICPMS were se lected.
U-Pb iso tope sys tem at ics and trace el e ment com po si tions were ana lysed by LA-ICPMS us ing a ThermoElectron X-Se ries II quadrupole ICPMS and New Wave Re search UP-213 Nd:
YAG UV (213 nm) la ser ab la tion sys tem. In-house an a lyt i cal pro to cols, stan dard ma te ri als, and data re duc tion soft ware were used for si mul ta neous ac qui si tion and real-time cal i bra tion of U-Pb dates and a suite of high field strength el e ments (HFSE) and rare earth el e ments (REE). Zir cons were ab lated with a la - ser di am e ter of 25 or 40 micrometres us ing fluence and pulse rates of ~5 J/cm2 and 10 Hz, re spec tively, dur ing a 45 sec ond anal y sis (15 sec gas blank, 30 sec ab la tion) that ex ca vated a pit
~25 µm deep. Ab lated ma te rial was car ried by a 1.2 L/min He gas stream to the 0.8 L/min nebulizer flow of the plasma. Spe - cific analyte dwell times, back ground sub trac tion, el e men tal and iso to pic cal i bra tion meth ods, and er ror prop a ga tion al go - rithms are de scribed in Rivera et al. (2013). Quoted un cer tain - ties on the iso to pic ra tios and dates in the data ta ble are based upon an a lyt i cal count ing sta tis tics for the pur pose of intercomparison of dates in our screen ing study. While not the pur pose of our work here, the cal cu la tion of ab so lute age un cer - tain ties on in di vid ual data points or group sta tis tics should in - clude prop a ga tion in quad ra ture of the cal i bra tion un cer tain ties noted in the foot note to the data ta ble.
The pro ce dure for U-Pb geo chron ol ogy us ing iso tope di lu - tion ther mal ion iza tion mass spec trom e try fol lows that given by Davydov et al. (2010). Zir con crys tals were sub jected to a mod i - fied ver sion of the chem i cal abra sion (CA-TIMS) method of Mattinson (2005), en abling care fully se lected sin gle crys tal frag ments to be pre pared and ana lysed. Many anal y ses were made on crys tals pre vi ously mounted, pol ished and im aged by cathodoluminence (CL), and se lected on the ba sis of zon ing pat terns and spot la ser ab la tion in duc tively cou pled plasma (LA-ICPMS) anal y ses. U-Pb dates and un cer tain ties for each anal y sis were cal cu lated us ing the al go rithms of Schmitz and Schoene (2007) and the U de cay con stants of Jaffey et al.
(1971). Other de tails of the an a lyt i cal con di tions are given in the notes that ac com pany Ta ble 1. The anal y ses were car ried out in the Iso tope Ge ol ogy Lab o ra tory of the De part ment of Geosciences at the Col lege of Arts and Sci ences, Boise State Uni ver sity, Idaho, USA. Un cer tain ties are due to non-sys tem - atic an a lyt i cal er rors, in clud ing count ing sta tis tics, in stru men tal frac tion ation, tracer sub trac tion, and blank sub trac tion. These er ror es ti mates should be con sid ered when com par ing our
206Pb/238U dates with those from other lab o ra to ries that have used tracer so lu tions cal i brated against the EARTHTIME gravimetric stan dards. When com par ing our dates with those de rived us ing other de cay schemes (e.g., 40Ar/39Ar,
187Re/187Os), the un cer tain ties in tracer cal i bra tion (0.05%; Con - don et al., 2007) and U de cay con stants (0.108%; Jaffey et al., 1971) should be added to the in ter nal er ror in quad ra ture.
Quoted er rors for cal cu lated weighted means are thus of the form ± X(Y)[Z], where X is solely an a lyt i cal un cer tainty, Y is the com bined an a lyt i cal and tracer un cer tainty, and Z is the com - bined an a lyt i cal, tracer and 238U de cay con stant un cer tainty.
The min eral com po si tion of the clay frac tion of the volcaniclastic ho ri zon was de ter mined by X-ray pow der dif frac - tion anal y sis at the In sti tute of Geo log i cal En gi neer ing (VŠB – Tech ni cal Uni ver sity in Ostrava). Rep re sen ta tive sam ples were mea sured af ter crush ing the ma te rial to a grain size be low ca.
20 mm and also ana lys ing the sedimented clay frac tion of grain size be low 1 mm. These clay frac tions were pre pared by sed i - men ta tion in de min er al ised wa ter af ter dis in te gra tion and dis - per sion of the sam ples us ing ul tra sound. Sam ples of the clay frac tion were mea sured in a nat u ral state, and af ter sat u ra tion with eth yl ene gly col (sat u ra tion in vapour at 50°C for 12 hours) and af ter heat ing to 250°C for one hour. Car bon ates and Fe ox -
ides and hy drox ides were not re moved from the sam ples. Mea - sure ments were car ried out us ing a Bruker-AXS D8 Ad vance in stru ment with 2Q/Q ge om e try mea sured us ing a LynxEye po - si tion-sen si tive de tec tor un der the fol low ing con di tions: ra di a - tion CoKa/Fe fil ter, ac cel er at ing volt age 40 kV, cur rent 40 mA, step by step mode with 0.014 2Q with an in ter val of 1 sec per step. The data were dig i tally pro cessed us ing Bruker Diffrac Suite soft ware. Semi-quan ti ta tive anal y ses of the phase com - po si tion were car ried out on sam ples of grain size <20 mm by means of the Rietveld method us ing the Bruker To paz ver sion 4.2 pro gram.
The mor phol ogy of zir con grains on the frac ture sur faces of sam ples was ex am ined in the In sti tute of Geo log i cal En gi neer - ing (VŠB – Tech ni cal Uni ver sity in Ostrava) us ing an FEI Quanta 650 FEG scan ning elec tron mi cro scope equipped with EDX, WDA, EBSD and CL de tec tors. The ob ser va tion was car - ried out un der low vac uum (50 Pa) on spec i mens which were not coated by con duc tive layer. Only semi-quan ti ta tive anal y - ses of the com po si tions of the grains were made us ing EDX. In ad di tion to the zir con grains, other phases con tain ing con cen - tra tions of heavier el e ments were also lo cated us ing back scat - tered elec tron im ag ery and analysed us ing EDX.
The pe trol ogy of thin sec tions of rep re sen ta tive sam ples was stud ied un der the mi cro scope us ing trans mit ted light.
Semiquantitative es ti mates of each com po nent were made.
A petrographic study of sam ples from two lay ers of the sed i - men tary rock rich in or ganic mat ter from the vi cin ity of the volcaniclastic rock was made to de ter mine the char ac ter of the or ganic mat ter. Both thin sec tions and pol ished grain mounts were pre pared. Frac tions of these sam ples were ground to less than 0.2 mm for anal y sis of ash, sul phur, or ganic and in or ganic car bon con tents.
The light reflectance of dis persed or ganic mat ter (DOM) was de ter mined in three sam ples of rock which were cut and pol ished in sec tion per pen dic u lar to bed ding. Mea sure ment of ran dom reflectance of the pre vail ing types of or ganic par ti cles was made in nor mal light at l = 542 nm us ing an Opton-Zeiss mi cro scope-microphotometer with oil ob jec tives (mag ni fi ca tion 40´ and 100´), and oil im mer sion (n = 1.518). The max i mum reflectance and op ti cal ani so tropy were de ter mined un der the po lar ized light at the same equip ment and mea sure ment con di - tions. Zir con (R = 3.12%) and stron tium ti tan ate (R = 5.41%) were used as op ti cal stan dards to cal i brate mea sure ments.
The size, shape, and op ti cal anisotropic tex ture of the or ganic Fig. 2. Sketch-map of Kobylí Quarry
The position of the volcaniclastic layers is marked by a black asterisk; the direction of strike and dip are also indicated 1el baT SMIT-ACeht morf sno criz rof atad ci po tosi bP-hT-U énsárKykèuoLetiffut a re tfa dedarba ylla c imehc dna delae nna stne mgarf ro sniarg no criz el gnis era .cte 2z ,1z –nosnittaM)5002( ;bcine goi dareht morfdeta lu clac ylevitaretioi tarU/hT ledom –802 /bP602 tn era ppa dna oi tarbP 602 /bP832 ;ega Ucdna *bP –bPc% lom ;ylevi tcep ser ,bP no mmoc dna cine goi dar tne se rper602 ;bP no mmoc lai tini dna knalb ,cine goi dar ot tcep ser htiw *bPdnoit anoi tcarf dna ekips rof detce rroc oi tar deru saem – ± 61.0 ta deta mi tse noit anoi tcarf ;ylno ;289-SBN dna 189-SBN fo si s ylana no desab .u.m.a/%30.0 e:knalb laru de corp eb ot demu ssa saw bP no mmoc lla ;bP no mmoc dna ,ekips ,noit anoi tcarf rof detce rroc – 602 /bP402 ;%16.0 ± 240.81 = bP702 /bP402 ;%25.0 ± 735.51 = bP802 /bP402 eht gn isu ,bP no mmoc lai tini ot dengi ssa saw knalb revo sse cxe ,)amgis-1 sei tnia tre cnu lla( %36.0 ± 686.73 = bPdna yecatSsremarK )5791( ;ega el pmas la n imon eht ta ledom noi tu love epo tosi bP egats-owt f fo smhti ro gla eht gn isu deta g aporp ,amgis-2 era sro rre –ztimhcSdna eneohcS)7002( ;gnoc ya ced eht no desab era snoi ta lu clac –- fo stnatsyeffaJ)1791( .la te ,602 /bP832 dna U702 /bP602 lai tini rof detce rroc sega bPmuirbilini 032 /hT832 ;3 = ]amgam[ U/hT gn isu Uh–DWSM2 = .tni .fnoc %59 * ;snoi t ai ved dethgiew fo erauqs naem = ss’tne dutS * ( * TDWSM5.0^)
mat ter were clas si fied ac cord ing to Diessel et al. (1978), Suchy et al. (1997, 2007), Kwieciñska and Petersen (2004), and Køíbek et al. (2008).
The con tent of or ganic mat ter was de ter mined by el e men tal anal y sis af ter dis solv ing the in or ganic car bon ates us ing 1N HCl and heat ing to 80°C. The el e men tal anal y sis was car ried out us ing a Flash FA 1112 Thermo Finnigan CHNS/O mi - cro-analyser in the In sti tute of Rock Struc ture and Me chan ics at the Acad emy of Sci ences of the Czech Re pub lic.
RESULTS
Two to three lay ers of pale grey volcaniclastic ma te rial were iden ti fied in the cen tral part of the east ern face of the sec ond level of the quarry (Figs. 2 and 3, the co or di nates N 50°7’12.79"
E 017°37’42.89"). They are con cor dant to the bed ding and the sep a ra tion be tween them is ap prox i mately 1.5 m. Their av er - age strike and dip de rived from ten mea sure ments is 48/57 NW.
Fig. 4. Layer of volcaniclastic material (tuffite) within the greywacke sequence The layer shows evidence of tectonic disturbance; laminae of grey-black organic
matter alternate with the pale tuffaceous detritus (photo J. Jirásek, 2012) Fig. 3. Layer of the light grey volcaniclastic material exposed in the central part
of the eastern face of the second level of the quarry (GPS coordinates N 50°7’12.79" E 017°37’42.89"), state of June 2012 (photo J. Jirásek)
The thick ness of the ho ri zons ranges from 1 to 10 cm, and they are trace able for a min i mum dis tance of 8 m. In ter vals of graded bed ding and dark laminae richer in or ganic mat ter can be seen by na ked eye within these volcaniclastic lay ers (Fig. 4).
COMPOSITION OF THE NEWLY EXPOSED TUFFITE
Thin sec tions of the sam ples col lected from the volcaniclastic ho ri zon show that it is fine-grained psam mit ic sed i ment with a high pro por tion of clay and can be clas si fied as an arkosic greywacke. Quartz grains are abun dant. In ad di tion
to the more rounded grains, there are an gu lar frag ments which are pre sum ably of vol ca nic or i gin. Feld spar, tab u lar ag gre gates of mus co vite and bi o tite, in places chloritised, are less abun dant (Fig. 5). Ac ces sory min er als were not in ves ti gated in de tail.
Dif frac tion pat terns of rep re sen ta tive sam ples of the volcaniclastic rock show that the main con stit u ents are quartz, illite-mus co vite, 1.4 nm min er als (chlorite, illite-smectite), plagioclase and po tas sium feld spar. The re sults of semi-quan ti - ta tive anal y ses show that the quartz con tent is rel a tively low (ca. 40 wt.%). The con tent of illite mica (illite, illite-mus co vite) is es ti mated to be 35 wt.%. The con tents of plagioclase are rel a - tively high, vary ing be tween 15 and 20 wt.%. The 1.4 nm min - Fig. 5. Photomicrograph of a thin section of the volcaniclastic material
in plane polarized light (A) and between crossed polars (B)
Angular and subangular quartz grains (Qz), the former most likely of volcaniclastic origin, together with plates of biotite (Bt), partly or completely chloritized (Chl) in clay
matrix with disseminated organic matter
eral (chlorite, illite-smectite) is char ac ter ized by a dis tinctly broad ened (001) dif frac tion peak with a half-height width of up to 1.5° 2Q. Ev i dently sev eral min eral phases con trib ute to this dif frac tion peak. For the pur pose of semi-quan ti ta tive anal y sis this pat tern can be ap prox i mated by us ing the dis or dered chlorite struc ture. Es ti ma tion of chlorite con tent then cor re - sponds to 4–8 wt.%. Only traces of po tas sium feld spar were de - tected, though one sam ple con tained up to 2 wt.%. Lat tice pa - ram e ters of the plagioclase up dated by the Rietveld method match those of al bite based on the pub lished nomograms (e.g., Kroll, 1983). Re fined lat tice pa ram e ters for plagioclase are as fol lows: a0 = 0.814143(75) to 0.8147445(74) nm, b0 = 1.27968(13) to 1.28039(12) nm, c0 = 0.715747(49) to 0.716052(66) nm, a = 94.220(13)° to 94.3457(98)°, b = 116.5739(51)° to 116.6019(60)°, g = 87.7870(68)° to 87.90499(80)°. The Rwp er ror pa ram e ter of Rietveld anal y ses ranged from 9.7 to 10.8%. A higher con tent of plagioclase (about 8 wt.%) was found in a sam ple of shale from the same lo - cal ity. This sam ple of shale also con tains a 1.4 nm min eral, match ing well with chlorite.
X-ray pow der dif frac tion anal y sis (XRD) of the sedimented clay frac tion with a grain size be low 1 mm showed that sam ples of the psam mit ic rock with an as sumed ad mix ture of tuffitic ma - te rial con tain a sig nif i cant pro por tion of polycomponent clay ma - te rial. The main com po nent of the clay frac tion con sists of mica min er als (illite-mus co vite). The dif frac tion pat terns show a ra tio - nal se quence of peaks with interlayer spac ings of 1.0 nm, 0.5 nm and 0.25 nm. The peaks of the mica min eral do not shift af ter sat u ra tion with eth yl ene gly col or with heat ing. More over, the dif frac tion pat terns also show peaks for a min eral in di cat ing an interlayer spac ing of 1.35 to 1.45 nm. Af ter sat u ra tion with eth yl ene gly col the peaks shift to val ues from 1.60 to 1.66 nm and dis ap pear af ter heat ing or the interlayer dis tance be comes the same as for illite. This be hav ior is typ i cal of the mixed struc - tures of illite-smectite type with a high pro por tion of the smectite com po nent. The diffractograms also con tain rel a tively broad dif frac tion peaks cor re spond ing to an interlayer dis tance of 0.7 nm, which match that of min er als of the kaolinite group. The width of these peaks is af fected to some ex tent by peaks of a mixed struc ture. Af ter heat ing to 250°C, the sam ples show a peak for the 0.7 nm spac ing that is also quite wide, which sug - gests the pres ence of a mixed struc ture of three com po nents con tain ing a pro por tion of kaolinite. The small pro por tion of the mixed struc ture com po nent in the sam ples did not al low more pre cise con clu sions to be drawn. The re sults of XRD anal y sis of the clay frac tion in a sam ple are shown in Fig ure 6.
The mor phol ogy of the zir con grains in sam ples of the psam mit ic rock was stud ied us ing scan ning elec tron mi cros - copy and en ergy dispersive microanalysis. It was found that zir - con is a rel a tively rare ac ces sory and shows marked vari a tions in grain size and mor phol ogy (Fig. 7). It is ob vi ous that the sam - ples con tain sev eral pop u la tions of zir cons. It is pos si ble to find co lum nar, thin, per fectly de vel oped crys tals with a length un der 85 mm, also semi-rounded zir con grains show ing signs of crys - tal edges about 10–40 mm in length, and al most per fectly rounded zir con grains 20–40 mm in di am e ter. Other ac ces sory min er als in clude framboidal py rite up to 10 mm in size, microcrystals of bar ite con fined to tiny frac tures, grains and also ag gre gates of im per fectly de vel oped co lum nar microcrystals of rare earth phos phate with pre dom i nance of Ce (prob a bly rhabdophane) and rare grains of a phos phate rich in Th. Coat - ings and very thin ag gre gates of fluorocarbonate of Ca and Ce were found on the fo li a tion planes of the shales. The min eral is likely to be a mem ber of the bastnäsite or synchysite group.
Tiny amounts of dis persed sec ond ary lead min er als were found in mi cro-frac tures in the psam mit ic rock. Microcrystalline pseudo morphs af ter ga lena crys tals up to 50 mm across con - sist ing of a mix ture of angle site and cerussite were iden ti fied. In the vi cin ity of these pseudo morphs there is a zone of microcrystalline pyromorphite-vanadinite oc cu py ing frac tures.
In di vid ual acicular crys tals and ag gre gates range be tween 5 and 10 mm in size. Grains in which the va na dium con tent is higher than that of phos pho rus tend to form at a greater dis - tance from the ga lena pseudo morphs and are usu ally of elon - gate co lum nar form. Grains in which phos pho rus is more abun - dant than va na dium oc cur closer to the pseudo morphs and take the form of small spher i cal bod ies show ing only traces of crys tal planes on their sur face.
U-PB GEOCHRONOLOGY
Sam ple GP-8 from the Krásné Louèky tuffite yielded a mixed pop u la tion of silt to fine sand-sized zir con grains, rang ing from skel e tal, highly elon gate, pris matic crys tals to rounded equant grains (Fig. 7). From the min eral sep a rate, the most Fig. 6. XRD powder diffractograms of the sedimented
fine-grained fraction from the volcaniclastic layer The peaks of the nat u ral sam ple are com pared with those from the sam ple sat u rated with ethyene gly col and af ter heat ing to 250°C; IM – illite-smectite (mont mo ril lo nite), IMK – illite-smectite (mont mo ril lo - nite)-kaolinite
sharply fac eted crys tals were se lected and mounted for CL im - ag ing, which re vealed a di ver sity of lu mi nes cence in ten si ties and zon ing pat terns (Fig. 8). LA-ICPMS spot ages con firmed this di ver sity, in clud ing one crys tal dated at 2.0 Ga, a pop u la tion of Cambro-Or do vi cian grains (544 to 448 Ma), and a dom i nant pop u la tion of grains with ages grouped around ca. 340 Ma (see Ap pen dix 1*). This lat ter group of ages was ob tained from both CL-dark and CL-bright crys tals.
Seven sin gle zir con grains se lected on the ba sis of CL-dark pat terns and young LA-ICPMS spot ages yielded, by CA-TIMS, a range of 206Pb/238U dates from 343.2 to 339.8 Ma (Fig. 9 and Ta ble 1). The two older grains are in ter preted as re worked de - tri tus or in her ited xeno crysts. The five youn gest grains clus ter around a weighted mean 206Pb/238U date of 340.05 ± 0.22(0.27)[0.45] Ma (MSWD = 2.76), which, given their reproducibility is as sumed to cor re spond to the age of erup tion and de po si tion of the tuff.
ANALYSIS OF ORGANIC MATTER
We col lected new sam ples from the lay ers rich in or ganic mat ter that are stratigraphically close to the volcaniclastic ho ri - zons. These lay ers are up to 10 cm thick and mac ro scop i cally re sem ble bi tu mi nous coal. Anal y ses showed that these are pelitic rocks with a mix ture of var i ous types of dis persed or ganic
mat ter (DOM) that in di cate vary ing ther mal trans for ma tion. Val - ues of to tal or ganic car bon (TOC) range be tween 0.14 and 6.57%. Car bo na ceous mat ter con sists of four types of par ti cles which dif fer in size, mor phol ogy and op ti cal prop er ties. The first type of dis persed or ganic mat ter (DOM) is abun dant in all sam - ples stud ied and con sists of small (<10 µm) ir reg u lar (Fig. 10A) or elon gated (Fig. 10B) par ti cles. They are dis persed in in or - ganic mat ter or are ar ranged in planes fol low ing microtectonic struc tures. Reflectance val ues were of ten dif fi cult to ob tain due to small par ti cle size, where mea sured reflectance val ues range from 3.11 to 5.71%, and ran dom reflectance Rr is 4.32%. The sec ond type of DOM con sists of larger (10–50 µm) ir reg u lar or elon gated par ti cles (Fig. 10A, B). Most of the par ti cles are op ti - cally iso tro pic but some ex hibit dis tinct un du la tory ex tinc tion un - der crossed polars (Fig. 10B). Reflectance val ues range from 3.61 to 6.15% and ran dom reflectance is 4.79%. A third type of dis persed or ganic mat ter re sem bles to frag ments of pyrobitumen (Ja cob, 1989). Most of the par ti cles oc cur in the form of rel a tively large (>30 µm across), ir reg u lar, well-de fined frag ments (Fig. 10C) which ex hibit in ter nal pores, cracks and op ti cal ani so tropy that cor re sponds to a fine-grained mo saic tex ture. Max i mum reflectance val ues vary from 6.15 to 8.06%
Rmax. Rel a tively rare, the fourth type of or ganic mat ter rep re - sents dis persed struc tures likely to be of bi o log i cal or i gin (Fig. 10C, D). Their reflectance ranges be tween 4.0 to Fig. 7. BSE images of two different zircon populations from the volcaniclastic horizon
A, B – euhedral volcanic zircon crystals; C, D – rounded detrital zircon grains; white scale bar is 10 mm
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1201
Fig. 8. CL images of Krásné Louèky tuffite zircon crystals shown with LA-ICPMS spot locations Crystals z1 to z7 correspond to Table 1, numbers 205 to 236 correspond to the Appendix 1;
size is indicated by 25 mm spot
Fig. 9. U–Pb concordia diagram for single zircon grain CA-TIMS analyses from the Krásné Louèky tuffite Two empty ellipsoids older than 341 Ma are grains z1 and z7 in Table 1 considered to be reworked detritus or inherited xenocrysts
(they were not counted for the final age)
5.06% Rr. The av er age reflectance Rr = 4.32 to 4.79% of the ma jor ity of car bo na ceous par ti cles sug gests an thra cite to meta-an thra cite rank (ECE-UN, 1998). The rank of the car bo - na ceous par ti cles can be clas si fied ac cord ing to max i mum reflectance Rmax (Kwieciñska and Petersen, 2004): an thra cite (Rmax < 5.0%), meta-an thra cite (Rmax < 6.5%), semi-graph ite (Rmax 6.5 to 9.0%) and graph ite (Rmax > 9.0%).
DISCUSSION
Based on the ob ser va tions made above, we be lieve that, af - ter an in ter val of al most thirty years, we have re dis cov ered the volcaniclastic ho ri zons in the quarry that may have been the sub ject of the orig i nal dat ing re ported by Pøichystal (1987).
Even if these new ex po sures are not iden ti cal, their strati - graphic po si tion is thought to be ap prox i mately the same. The new date of 340.05 ± 0.22 Ma ob tained from zir cons in this volcaniclastic layer is 21 Ma older than the pre vi ously pub lished date. This ends the con tro versy caused by the first date (319 ± 2 Ma) which sug gested that the volcaniclastic ho ri zon be longed to the up per most part of the Ostrava For ma tion of the Up per Silesian Ba sin, i.e. the strati graphic ho ri zon cor re spond - ing to the tonstein con tained in coal seam no. 479 in the Poruba
Mem ber. We sug gest that the ear lier date was likely bi ased by Pb-loss from the bulk anal y sis of rel a tively high-U zir con crys - tals which were not treated via mod ern meth ods (chem i cal abra sion) of open sys tem crys tal do main re moval. It was dif fi - cult to ex plain how rocks equiv a lent to the Ostrava For ma tion were pres ent 50 km, as the crow flies, from the near est out crop of the Poruba Mem ber. More over, this dis tance was prob a bly much greater be fore fold ing of the Culm strata (Jirásek et al., 2013b). Be cause Pøichystal (1987) con sid ered the orig i nal dat - ing to be cor rect, he pro posed two pos si ble ex pla na tions. The first was the pres er va tion of ma rine sed i men ta tion in a sep a rate ba sin in the west ern part of the Nízký Jeseník Mts. that pushes the top of the Horní Benešov For ma tion into the Namurian. The sec ond ex pla na tion of fered was that a frag ment of the Poruba Mem ber was overthrusted to the west from the Up per Silesian Ba sin.
The orig i nal dat ing of tuffite from the Krásné Louèky Quarry cor re sponds ap prox i mately to the bound ary be tween the Ostrava and Karviná for ma tions in the Up per Silesian Ba sin (Jirásek et al., 2013a, b), i.e. in the top of the Poruba Mem ber (ca. 320 Ma). The new dat ing places these rocks in the Visean (Davydov et al., 2012), which is com pat i ble with what is known about their ge ol ogy and biostratigraphic po si tion. Re cords of the oc cur rence of spores of Namurian age at the site Fig. 10. Reflected light photomicrographs showing characteristic types of dispersed organic matter
in the volcaniclastic horizon (oil immersion)
A – mix ture of fine- and coarse-grained car bo na ceous par ti cles in the rock ma trix; B – fine-grained car bo na ceous par ti cles aligned along the cleav age plane in the rock ma trix (crossed polars); C – third type of large, highly re flec tive ir reg u lar par ti - cles DOM re sem bling pyrobitumen and a fourth type of fusinite-like struc ture of bi o log i cal or i gin; D – struc ture of possible biological origin
(Pøichystal, 1987; Pøichystal et al. in Weyer and Menning, 2006) can not be val i dated with out an au thor i ta tive pub li ca tion. How - ever, it would be dif fi cult to ex plain their pres er va tion in an en vi - ron ment where the mean reflectance Rr of vitrinite is 4.32% cor - re spond ing to a rank of meta-an thra cite. If sig nif i cantly youn ger sed i ments had been tec toni cally in cor po rated in the lo cal strati - graphic se quence, then this must have hap pened dur ing the lat est Car bon if er ous, around 310 Ma (Bashkirian, Early Penn - syl va nian) when the lo cal Variscan orogenic belt col lapsed (Schulmann et al., 2009). This would have been the only pos si - ble rea son for lo cal tec tonic de for ma tion. If strata of 319 Ma age re ally were pres ent and sur vived ero sion, then they must have been tec toni cally bur ied al most im me di ately af ter de po si tion, and their grade of meta mor phism should cor re spond to that of the neigh bour ing rocks. It is also dif fi cult to imag ine that dur ing tectonism the sur face ma te rial was al most im me di ately bur ied to depths of 4–10 km, which is the es ti mated thick ness of the eroded de pos its (cf. Francu et al., 2002).
The new stud ies of the Up per Silesian Ba sin show that its fill is sim i lar to that of other foredeep bas ins, and its west ern flank prob a bly did not have a length of tens of kilo metres (Hýlová et al., 2013). In other words, the area of de po si tion of the Poruba Mem ber could not have ex tended as far as the town of Krnov.
The con tin u a tion of ma rine sed i men ta tion into the Namurian in a sep a rate ba sin pro posed by Pøichystal (1987) for the Horní Benešov For ma tion has no real jus ti fi ca tion be cause, to the con trary, the up lift of the orogenic belt in the west caused pro - gres sive ero sion of the fill in the ba sin that was can ni bal ized to cre ate youn ger sed i ments. The hy poth e sis of “re verse overthrusting” pro posed by Pøichystal (1987) based on the work of Cháb (1986) has al ready been proved in cor rect by Dvoøák (1986).
In ad di tion to zir con grains of Visean age, older pop u la tions of zir con were iden ti fied. The or i gin of the sin gle grain of Paleoproterozoic age is not clear, but might be de rived from older ma te rial in cor po rated into Brunovistulicum dur ing the Pan-Af ri can (Cadomian) orog eny. More abun dant Cambro-Ordovican grains are con sis tent with the age of granitoids in the Desná Dome (Kröner et al., 2000). These re - sults are in agree ment with the in ter pre ta tion of heavy min eral as so ci a tions de scribed by Hartley and Otava (2001), who iden - ti fied their source in strata and in meta-ig ne ous rocks.
Dif frac tion pat terns of the clay frac tion from the Krásné Louèky sam ples of psam mite with ad mixed tuffitic ma te rial are not iden ti cal to those of clay frac tions from other Late Car bon if - er ous volcaniclastic ho ri zons from the Up per Silesian Ba sin (e.g., Dopita and Králík, 1977; Horák et al., 1992). Tuffitic rocks (the so-called whet stones, tonsteins, tuffites) from the Up per Silesian Ba sin con tain a sig nif i cant pro por tion of dis or dered mixed struc tures such as illite-smectite with a lower con tent of the smectite com po nent. In fact, the grade of meta mor phism is sig nif i cantly lower (Ro 0.5 to 2.0%) in the Up per Silesian Ba sin than in the Culm strata (Ro 2.0 to 5.5%). The dif fer ence in min -
er al ogy may be due to the fact that the volcaniclastic ma te rial in the Up per Silesian Ba sin was de pos ited in a ter res trial en vi ron - ment, while at the Krásné Louèky site, de po si tion took place in a ma rine en vi ron ment.
Hav ing ob tained fur ther dat able ma te rial from the flysch strata of the Culm from the Moravo-Silesian Pa leo zoic Ba sin, it will be pos si ble to re solve the long-stand ing de bate about the rel - a tive age of these for ma tions. It will also be pos si ble to make an ef fec tive cor re la tion with other foredeep Variscan bas ins in Eu - rope that were filled at the be gin ning of the orog eny by Early Car - bon if er ous flysch, e.g. the Rhenohercynian Turbidite Ba sin in Ger many (Ricken et al., 2000), the Culm and South Wales Car - bon if er ous bas ins in Brit ain (Hartley and Warr, 1990) and the bas ins of the Franco-Span ish Pyr e nees (Delvolvé et al., 1998).
CONCLUSIONS
Arkosic greywackes with a sig nif i cant volcaniclastic com po - nent (tuffites) were re dis cov ered in the sec ond level of the ac - tive quarry at Krásné Louèky – Kobylí near the town of Krnov.
We in ter pret them as re de pos ited vol ca nic ash washed down from the ad ja cent main land into a fore land ba sin. In ad di tion to older zir con grains (ex cep tion ally Paleoproterozoic, com monly of Cambro-Ordovican age) grains with ages clus tered around ca. 340 Ma were found to be most abun dant. The CA-TIMS U-Pb age of 340.05 ± 0.22 Ma that was de ter mined is dif fer ent to the pre vi ously pub lished age of 319 (±2, 3, 21) Ma (Pøichystal, 1987) de ter mined us ing sam ples as sumed to be from stratigraphically iden ti cal ma te rial. If this as sump tion is jus - ti fied, then no iso lated oc cur rences of sed i ments of this age are pres ent west of the pres ent bound ary of the Up per Silesian Ba - sin. The new date also de fines an ap prox i mate age for the bound ary be tween the Horní Benešov and Moravice for ma tions of the Culm flysch in the Nízký Jeseník Mts.
Ac knowl edge ments. This study was made pos si ble thanks to a grant from the Min is try of Ed u ca tion, Youth and Sports (pro ject SGS SP2014/40). We also thank KAMENOLOMY ÈR s.r.o., part of the STRABAG SE, par tic u - larly K. Selvek and Ing. A. Maleòáková for ad mis sion to the quarry and per mis sion to col lect the sam ples from which the new dates were ob tained. Part of the an a lyt i cal work was per - formed on equip ment fi nanced by the pro ject “In sti tute of Clean Tech nol o gies for Min ing and Uti li za tion of Raw Ma te ri als for En - ergy Use”, reg. no. CZ.1.05/2.1.00/03.0082, sup ported by the
“Re search and De vel op ment for In no va tions Op er a tional Programme” which is fi nanced by struc tural funds from the Eu - ro pean Un ion and by means of the state bud get of the Czech Re pub lic. We are grate ful for the in spir ing notes of A. Hartley and J. Otava. The de tailed re views of M. Kusiak, M. Paszkowski, and anon y mous re viewer im proved our manu - script con sid er ably.
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