Geo log i cal Quar terly, 2016, 60 (3): 637–649 DOI: http://dx.doi.org/10.7306/gq.1283
Coal-bear ing ca pac ity of the Petøkovice Mem ber (Ostrava For ma tion, Serpukhovian, Mis sis sip pian) of the Up per Silesian Ba sin (Czech Re pub lic and Po land)
Lada HÝLOVÁ1, Jakub JIRÁSEK2, *, Mar tin SIVEK2 and Janusz JURECZKA3
1 Palacký Uni ver sity, Fac ulty of Sci ence, 17 listopadu 12, 771 46 Olomouc, Czech Re pub lic
2 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
3 Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Up per Silesian Branch, Królowej Jadwigi 1, 41-200 Sosnowiec, Po land
Hýlová, L., Jirásek, J., Sivek, M., Jureczka, J., 2016. Coal-bear ing ca pac ity of the Petøkovice Mem ber (Ostrava For ma tion, Serpukhovian, Mis sis sip pian) of the Up per Silesian Ba sin (Czech Re pub lic and Po land). Geo log i cal Quar terly, 60 (3):
637–649, doi: 10.7306/gq.1283
The Petøkovice Mem ber is the basal unit of the paralic suc ces sion of the Ostrava For ma tion of the Up per Silesian Ba sin. This mem ber is a valu able source of in for ma tion about the tran si tion from a ma rine ba sin filled with siliciclastic sed i ments into a paralic ba sin with the be gin ning of coal-bear ing sed i men ta tion. Mod els of: (1) the num ber of coal seams, (2) their to tal thick - ness, and (3) the coal con tent with re spect to the to tal thick ness of the suc ces sion were cre ated to de scribe and as sess the coal-bear ing ca pac ity of the Petøkovice Mem ber. The au thors pres ent mod els for coal seam thick nesses ex ceed ing ei ther 10 or 40 cm. The coal-bear ing ca pac ity of the Petøkovice Mem ber is very low. The av er age share of coal seams thicker than 0.1 m is 3%. The share of coal seams with a thick ness that ex ceeds 0.4 m is only 1.66%. More over, in large ar eas of the ba - sin, in the N and NE parts, the coal-bear ing ca pac ity is close to zero, be cause coal seams of greater thick ness were de tected only lo cally there. Based on these mod els and on other geo log i cal data, it is ob vi ous that the on set of coal sed i men ta tion was grad ual and lim ited to par tic u lar sites show ing the great est sub si dence of the ba sin floor. In places where mi nor sub si dence took place, there were likely un fa vor able con di tions for ac cu mu la tion of or ganic mat ter.
Key words: Up per Silesian Ba sin, Serpukhovian, Mis sis sip pian, Car bon if er ous, coal-bear ing ca pac ity.
INTRODUCTION
The Up per Silesian Ba sin (here in af ter re ferred to as the USB) is cur rently eco nom i cally the most im por tant Eu ro pean bi - tu mi nous coal ba sin. The an nual out put of the USB amounts to about 70.2 mil lion tons of bi tu mi nous cok ing and ther mal coal.
The ma jor ity of the pro duc tion co mes from Po land (61.6 mil.
tons in 2013; Malon and Tymiñski, 2014), where ap prox i mately four-fifths of the area of the ba sin is sit u ated. A to tal of 8.6 mil.
tons (Starý et al., 2014) were ex tracted in 2013 in the Czech Re - pub lic, where the re main ing part of the ba sin is lo cated.
The Petøkovice Mem ber is the old est lithostratigraphical unit of the Ostrava For ma tion, which rep re sents the paralic stage of the de vel op ment of the USB. The re sults of the in ves ti ga tion con trib ute to knowl edge of the na ture of the tran si tion from rel a - tively deep sea flysch sed i men ta tion of the Moravian–Silesian Ba sin into the coal-bear ing sed i men ta tion of the paralic de vel - op ment of the USB. Si mul ta neously, this study con trib utes to
the knowl edge of gen eral fac tors con trol ling the de vel op ment of coal-bear ing bas ins in the fore land po si tion of the Variscan orog eny. Hýlová et al. (2013) rec og nized the geo log i cal sig nif i - cance of anal y sis of the over all thick ness and the con tent of sand in the Petøkovice Mem ber.
This study of coal-bear ing ca pac ity il lus trates the vari able con di tions that pre vailed af ter the tran si tion from ma rine to paralic sed i men ta tion. As ex am ples, au thors pro vide maps of the to tal coal-bear ing ca pac ity (sum of all coal seams thicker than 0.1 m) and maps of eco nomic coal-bear ing ca pac ity (sum of coal seams >0.4 m). The coal-bear ing ca pac ity of the Ostrava For ma tion has been the sub ject of many stud ies, pa - pers and un pub lished re ports (e.g., Folprecht, 1928; Havlena, 1964; Kotas and Malczyk, 1972; Adamusová et al., 1992;
Gabzdyl, 1994; Konstantynowicz, 1994; Jureczka and Kotas, 1995; Kaštovský and Dopita, 1997; Pešek et al., 1998), but they have never fo cused on a sin gle lithostratigraphical unit over its whole ex tent.
GEOLOGICAL SETTING
The USB was formed dur ing the fi nal stages of the evo lu tion of the Moravo-Silesian Pa leo zoic Ba sin (Unrug, 1966), sit u ated in the east ern do main of the Cen tral Eu ro pean Variscides (Kro -
* Corresponding author, e-mail: jakub.jirasek@vsb.cz
Received: September 1, 2015; accepted: December 17, 2015; first published online: March 1, 2016
ner et al., 2008). The USB is a typ i cal ex am ple of the fore land ba - sin of the Variscan Orogen, later en closed in the outer zones of the orogenic belt, termed Rhenohercynicum and Subvariscicum (Kumpera and Martinec, 1995; Narkiewicz, 2007).
The post-ero sional bound aries of the USB have an ap prox i - mately tri an gu lar shape, shared be tween Po land and the Czech Re pub lic. The area of coal ba sin ex ceeds 7,000 km2. The ma jor part (~78%) lies in Po land (Dembowski, 1972;
Dopita et al., 1997).
The base ment of the ba sin is formed by the sed i men tary cover of the Brunovistulicum Terrane, rep re sent ing a microcontinent sit u ated at the south ern mar gin of Baltica in the Early Pa leo zoic. Cam brian, Or do vi cian, De vo nian and Mis sis - sip pian strata have been iden ti fied in the sed i men tary en ve lope of the Brunovistulicum (Kalvoda et al., 2008; Bu³a et al., 2015).
Car bon if er ous sed i men ta tion be gan with pre-orogenic car bon - ates. The ef fect of the Variscan orog eny is rec og niz able in a se - quence of ma rine siliciclastic flysch de pos its (Culm fa cies) with pre-ero sional thick nesses of up to 11 km (Francu et al., 2002).
Sed i men ta tion con tin ued through the coal-bear ing paralic and later terrigenous molasse be fore and also af ter the fi nal col - lapse of the orogen and ter mi na tion of the as so ci ated tec tonic events (Dopita et al., 1997).
The coal seams in the USB, con tain ing mostly bi tu mi nous coal, were formed dur ing the Late Car bon if er ous (Namurian to Westphalian in the west ern Eu ro pean sub di vi sion). Two ma jor sed i men tary units have been dis tin guished in the Car bon if er ous coal-bear ing suc ces sion: paralic and terrigenous se quences (Figs. 1 and 2). An older, paralic type of sed i men ta tion, with a wide spec trum of sed i men tary en vi ron ments, has been doc u - mented (e.g., Kêdzior et al., 2007). Se quences con sist of ter res - trial clastic de pos its with in ter ca la tions of ca. 200 fau nal ho ri zons, of which about 80 con tain a spo radic ma rine fauna. The de pos its are ar ranged into cy clic pat terns, stud ied by sev eral au thors, who iden ti fied three or ders of cy clic ar range ment; mega cy cles, mesocycles and ba sic cy cles. This sub di vi sion de pends mainly on the in ten sity of ma rine in cur sions (Jansa and Tomšík, 1960;
Zeman, 1960; Gastaldo et al., 2009).
This type of sed i men ta tion is char ac ter ized by the cy clic al - ter na tion of clastic sed i ments with coal seams. Fre quent in ter - ca la tions of volcanoclastic ma te ri als (coal tonsteins [e.g., tuffs]
and whet stones [e.g., tuffites]) have been doc u mented. The en - tire paralic suc ces sion is known as the Ostrava For ma tion in the Czech Re pub lic or the Paralic Se ries in Po land. These units con sist of the Petøkovice, Hrušov, Jaklovec and Poruba mem - bers (in Po land, in for mal lithostratigraphic units) and were de - pos ited in the pe riod from ~330 to ~324 Ma, cov er ing al most the whole Serpukhovian Stage (Jirásek et al., 2013b and un pub - lished data of au thors). The youn ger, con ti nen tal de pos its, char ac ter ized by lack of ma rine or brack ish ho ri zons, de pos ited af ter a sig nif i cant strati graphic gap oc cur across the whole ba - sin. The coal seams here are usu ally thicker than in the older part, oc ca sion ally up to 24 m. Volcanoclastic ma te rial is also pres ent here but is less fre quent. Cy clic pat terns of sed i men ta - tion are re cog nis able, but dif fer from those of the paralic part in the av er age thick ness of ba sic cy cles and in other pa ram e ters (e.g., Havlena, 1982). In the Czech part of the ba sin this suc - ces sion, called the Karviná For ma tion, be longs to the Bashkirian and con sists of the Sad dle, Suchá and Doubrava mem bers (Dopita et al., 1997). In the Pol ish part of the ba sin, sed i men ta tion con tin ued up to the Gzhelian and con sists of the Up per Silesia Sand stone, Siltstone and Kraków Sand stone units, com posed of 7 sub units (Fig. 1; Dembowski, 1972; Kotas et al., 1988). The ex act strati graphic po si tion of the Jejkowice Mem ber, known only lo cally as an interbed be tween the
Ostrava and the Karviná for ma tions (resp. Paralic and Up per Silesia Sand stone units), is un known (Jureczka, 1988).
MATERIALS AND METHODS
Logs of ex plo ra tion bore holes were the prin ci pal data source for cre at ing the data files used for mod el ling the de vel - op ment of the Petøkovice Mem ber. On the Czech side, the da ta - base of ex plo ra tion bore holes from the De part ment of Min eral Re sources and Mod el ling at the Fac ulty of Min ing and Ge ol ogy, VŠB-Tech ni cal Uni ver sity of Ostrava, were used, sup ple - mented with in for ma tion from the Czech Geo log i cal Sur vey. On the Pol ish side, the da ta base of ex plo ra tion bore holes from the Up per Silesian Branch in Sosnowiec of the Pol ish Geo log i cal In sti tute, National Research Institute, were used.
Ex plo ra tion bore holes were drilled from the be gin ning of the 20th cen tury, al though most were drilled in the sec ond half of the last cen tury. Thus, be cause of de vel op ments in drill ing tech - nol ogy and data in ter pre ta tion, the data file is di verse and it was nec es sary to in di vid u ally con sider the in clu sion of bore holes into this file.
A to tal of 404 drill ing logs of ex plo ra tion bore holes which pen e trated the Petøkovice Mem ber were avail able from the whole of the ba sin. How ever the ma jor ity did not have a com - plete drill ing log and, there fore, could not be used for mod el ling of the coal-bear ing ca pac ity of the Petøkovice Mem ber. 206 bore holes did not reach the bot tom of this unit, while for 75 bore holes the roof of the unit was miss ing due to ero sion. A to tal of 65 ver i fied com plete logs and two in com plete drill ing logs were used for mod el ling. For the two in com plete logs, the miss - ing pa ram e ters were cal cu lated us ing cor re la tion with the clos - est datapoints. The ap par ent thick nesses of coal seams were con verted into the true thick nesses us ing in cli na tion val ues spec i fied in the logs. The fi nal val ues in cluded all types of coal with var i ous ash con tents. Only car bo na ceous shales and shaly coals were ex cluded from the cal cu la tions.
The method of bi sect ing the dis tances be tween the pos i tive and the neg a tive bore holes was used to con struct the mod els of coal-bear ing ca pac ity of the Petøkovice Mem ber in the ba sin.
This means that for the mod eled pa ram e ter the zero isoline is given in the half dis tance be tween the last point with known value and the first (clos est) point with zero value. Out put maps were cre ated us ing the In Roads and MicroStation 8.5 pro grams (Bentley Sys tems, Inc.). A method of in ter po la tion be tween tri - an gles formed by the known doc u men ta tion points was used in the in ner part of the model, while an ex trap o la tion method was used be tween the end points and the bound ary. Mod els in ar - eas with out known val ues ad ja cent to the ex trap o lated val ues were based on ex pert es ti ma tion.
Re gional chronostratigraphic units were used in the text, in ad di tion to in ter na tional no men cla ture of the Late Car bon if er - ous sub di vi sion in West ern and Cen tral Eu rope (cf. Menning et al., 2006; Davydov et al., 2012).
THE PETØKOVICE MEMBER IN THE USB
The Petøkovice Mem ber is the old est lithostratigraphic unit of the USB. Sed i men ta tion started at the end of the purely ma - rine siliciclastic flysch sedimention of the Hradec-Kyjovice For - ma tion in the Czech Re pub lic or the Malinowice Mem ber in Po - land. Zir con age dat ing of the top of the Petøkovice Mem ber gives 327.25 ± 0.15 Ma (Jirásek et al., 2013a), while the base of the unit is ap prox i mately 329.7 Ma (Jirásek et al., 2013b). The
Coal-bearing capacity of the Petøkovice Member (Ostrava Formation, Serpukhovian, Mississippian)... 639
Fig. 1. Lithostratigraphic division of the Czech and Polish parts of the Upper Silesian Basin According to: Dembowski (1972), Jureczka (1988), Kotas et al. (1988) for the Polish part
and Dopita et al. (1997) for the Czech part, modified
change in sed i men ta tion from ma rine to paralic con di tions seems likely to be linked with global glacioeustatic events, so that the base of the Petøkovice Mem ber can be roughly cor re - lated with the Visean/Serpukhovian bound ary. The Visean/Serpukhovian bound ary it self lies in the up per part of the Hradec/Malinowice Mem ber, ac cord ing to the goniatite fauna. How ever, the use of biostratigraphic meth ods is lim ited due to the lack of in dex fos sils in all for ma tions. For this study, we used the def i ni tion of the Petøkovice Mem ber bound ary spec i fied by Hýlová et al. (2013). That is to say, the Petøkovice Mem ber is sep a rated from the older Kyjovice Mem ber (or equiv a lent Malinowice or Zalas Beds) by the roof of the Štúr Fau nal Ho ri zons Group and its up per bound ary is de ter mined by the roof of the Main Ostrava Whet stone ho ri zon. In places where the Whet stone ho ri zon is not de vel oped, the up per bound ary of the Petøkovice Mem ber is placed at the top of the ma rine (lo cally even fresh wa ter and Lingula-bear ing, i.e. brack - ish) ho ri zons of the Naneta Group.
The post-ero sional area of the occurence of the Petøkovice Mem ber is al most iden ti cal to the pres ent-day ex tent of the USB. Ero sion of the Car bon if er ous mas sif and the later nearby de vel op ment of the Carpathian orogenic belt had a de ci sive in -
flu ence on the cur rent ar eal ex tent of the Petøkovice Mem ber and the depth of its burial be low the pres ent-time sur face. This is re flected in the ir reg u lar de gree of ex plo ra tion of the unit ex - pressed by bore hole dis tri bu tion over the USB area (Fig. 3).
The unit is best rec og nized along the west ern and north east ern bound ary of the USB, where the strata are ex posed on the post-ero sional sur face of the Car bon if er ous or very close to it. In these parts of the ba sin, some coal seams have been ex ploited (the Heømanice, Mariánské Hory, Paskov, Petøkovice, Pøívoz, Staøíè, and Svinov min ing fields in the Czech Re pub lic; the Gliwice min ing field in Po land). In the re main ing ar eas of the ba - sin, there has been less geo log i cal ex plo ra tion of the Petøkovice Mem ber, thus its rec og ni tion and anal y sis is less re li able. To - wards the east, an in creas ing num ber of youn ger stra tal units is pre served in the roof of the Petøkovice Mem ber. In some ar eas, par tic u larly in the Pol ish part of the ba sin, de pos its of the Petøkovice Mem ber and its coun ter part Sarnów Beds oc cur at depths be yond the tech ni cal and eco nomic pos si bil i ties of ex - ploi ta tion. The coal-bear ing ca pac ity of the Petøkovice Mem ber de creases ap prox i mately in an east ern di rec tion. The poor level of ex plo ra tion in the east ern part of the ba sin in Po land is mainly a re sult of dif fer ent fa cies de vel op ment (see Fig. 1). By con trast Fig. 2. Geological map of the sedimentary infill of the Upper Silesian Basin indicating the main lithostratigraphic units
According to Aust et al. (1997) and Jureczka et al. (2005), modified
with the Petøovice Mem ber, the Sarnów Beds con sist of a thin lithosome of coarser-grained de pos its. These are coal-bar ren ter res trial de pos its in the roof of an al most purely ma rine suc - ces sion that is poorly ex plored, where only a few thin coal seams are lo cally known. Their pri mary de scrip tion given by Doktorowicz-Hrebnicki (1935) is rather vague, and de tailed in - for ma tion on the tran si tion of the Petøkovice Mem ber to the the Sarnów Beds is not avail able. For the study pur pose, the bor der be tween those two units was es tab lished in places where the unit’s sand con tent ex ceeds 75% and at the same time its thick - ness is <300 m. Such a di vi sion es sen tially cor re sponds to the first de scrip tion of the Sarnów Beds (Doktorowicz-Hrebnicki, 1935). To wards the SSE, the thick ness of the over bur den grows as coal-bear ing de pos its of the USB be come cov ered by the Outer West ern Carpathians nappes.
The coal seams of the Petøkovice Mem ber ac cord ing Martinec et al. (2005) show a gen er ally sim ple de vel op ment of coal, which mostly con sists of coal macrotypes typ i cal of a higher de gree of coalification, (i.e., banded bright coal and banded coal). The pre vail ing microlithotype of coal is vitrinite.
Macerals of the liptinite group are pre served only in parts with a lesser de gree of coalification (Jurczak-Drabek, 1996; Sivek et al., 2003). The de gree of coalification cal cu lated from the Vdaf (Sivek et al., 2008) ranges be tween a mean vitrinite reflectance Ro 0.5 to 2.6% (Kandarachevová et al., 2009) in the west ern
part of the ba sin, while in the north east ern and east ern parts the Ro value is <0.8% (Jurczak-Drabek, 1996). Such vari abil ity in the de gree of coalification prob a bly re flects largely sec ond ary rea sons. The most prom i nent one is the dif fer ence in the thick - ness of over ly ing strata in the past.
COAL-BEARING CAPACITY
The coal-bear ing ca pac ity is a com monly used pa ram e ter for the anal y sis of coal bas ins, which can be de fined in sev eral ways:
(1) the to tal thick ness of coal seams in the ob served suc ces sion;
(2) the num ber of coal seams within the ob served sec tion of the stra tal se quence; and (3) the net con tent (%) of coals within the suc ces sion. The term “coal-bear ing ca pac ity” is usu ally used for the de ter mi na tion of coal con tent in strata but is also used to es ti - mate the amount of coal re serves in ar eas of low level of rec og - ni tion. The first two meth ods for the de ter mi na tion of coal con tent are mainly used in sedimentological stud ies. The ab so lute coal-bear ing ca pac ity is a pa ram e ter used for all coal seams, re - gard less of their thick ness. Other coal-bear ing ca pac i ties are re - ferred to as rel a tive. How ever, the ab so lute coal-bear ing ca pac ity takes into ac count the min i mum thick ness of the coal seams that are in cluded in the cal cu la tion of the coal-bear ing ca pac ity (usu - ally 0.1 m). With out thick ness lim its, in for ma tion on coal-bear ing Coal-bearing capacity of the Petøkovice Member (Ostrava Formation, Serpukhovian, Mississippian)... 641
Fig. 3. Distribution of exploration boreholes in the Upper Silesian Basin that penetrate strata of the Petøkovice Member An approximate scheme of the base of this stratal unit in areas with low density of boreholes is according to Jureczka et al. (2005)
ca pac ity is not in for ma tive and is, there fore, not use ful for fur ther in ter pre ta tions or other ap pli ca tions.
For this anal y sis of the sed i men tary en vi ron ment of the Petøkovice Mem ber de pos its and the ac com pa ny ing palaeo - geo graphic re con struc tion, the au thors used all of the above-men tioned meth ods to cre ate their mod els. Be cause of the gen er ally smal thick nesses of coal seams of the Petøkovice Mem ber, we have com piled mod els of coal-bear ing ca pac ity for lim it ing thick nesses of 0.1 and 0.4 m, re spec tively. The coal-bear ing ca pac ity, usu ally ex pressed by the to tal thick ness of coal seams (Fig. 4), is the ba sis for as sess ment of the spa tial dis tri bu tion of peat sed i men ta tion in the coal bas ins, while mod - els of coal-bear ing ca pac ity, which are usu ally based on the con tent of the coal seams (Fig. 5) or ex pressed by the num ber of coal seams (Fig. 6), are usu ally con sid ered com ple men tary.
TOTAL THICKNESS OF COAL SEAMS
The coal-bear ing ca pac ity ex pressed by the to tal thick ness of the coal seams is a pa ram e ter tra di tion ally used to study the his tory of the de vel op ment of coal bas ins. The au thors cal cu - lated the coal-bear ing ca pac ity un der two al ter na tive sce nar ios;
for lim it ing thick nesses of ei ther 0.1 or 0.4 m (Fig. 4). Both mod - els were cre ated us ing con tours with an in ter val of 5 m.
For the coal-bear ing ca pac ity of the Petøkovice Mem ber (Fig. 5), the N–S po lar ity pre vails over the E–W po lar ity. Based on the au thors‘ mod els of coal-bear ing ca pac ity, this pat tern is quite ev i dent, so that the higher coal-bear ing ca pac ity was re - corded only in the Czech part of the ba sin (Figs. 4 and 5). In this part, the coal-bear ing ca pac ity is ac tu ally char ac ter ized by great vari abil ity and a ran dom spa tial dis tri bu tion. In the Pol ish part of the ba sin, the coal-bear ing ca pac ity is not zero, but var - ies over very low val ues.
The coal-bear ing sed i men ta tion of the Petøkovice Mem ber does not rep li cate the char ac ter is tic NNE–SSW di rec tion, which is mostly in volved in the de vel op ment of the paralic molasse of the USB and which was pri mar ily con nected with the dif fer ences in thick ness of the de pos its and (partly) their sand con tent. As is ev i dent from this study, the im por tance ot this di rec tion is not re stricted to the de vel op ment of the Petøkovice Mem ber thick ness and sand con tent, but is also re - flected in their coal-bear ing ca pac i ties.
The model of coal-bear ing sed i men ta tion at the lim it ing thick ness of 0.4 m sup ports the ex is tence of ar eas with greater to tal thick nesses of coal seams, as was ob served in the model with the lim it ing thick ness of 0.1 m. There are three such ar eas, all of which are lo cated in the Czech (SW) part of the USB. The two larger ones oc cur in the west ern part of the posterosional oc cur rence of the Petøkovice Mem ber, whereas the third, the small est, lies in the Karviná part, south-west of the town of Tìšín (Fig. 4).
THE PROPORTION OF TOTAL THICKNESS OF COAL SEAMS AND NUMBER OF COAL SEAMS IN THE OBSERVED SECTION
OF THE STRATAL SEQUENCE
Coal-bear ing ca pac i ties ex pressed as net con tent of the coal and the num ber of coal seams in the suc ces sion are, in sedimentological anal y ses of the evo lu tion of coal bas ins, con - sid ered as com ple men tary to coal-bear ing ca pac ity ex pressed as the to tal thick ness of the coal seams. Both types of coal-bear ing ca pac ity pre sented in this study were pre pared un - der two al ter na tive sce nar ios (i.e., for lim it ing thick nesses of 0.1 and 0.4 m). Mod els of coal-bear ing ca pac ity were de vel oped in the form of con tour lines with an in ter val of 2% (Fig. 5). Mod els
pre sented as the num ber coal seams in the ob served sec tion of the stra tal se quence are shown in Fig ure 6. These mod els have been de vel oped in the form of con tour lines, with an in ter val of 20 for the type ex press ing the num ber of coal seams with a thick ness of >10 cm, and an in ter val of 10 for the dis tri bu tion of coal seams with a thick ness of >40 cm.
All mod els con firm the north-south trending po lar ity (NNE–SSW di rec tion) of the coal-bear ing ca pac ity of the Petøkovice Mem ber pre vail ing over the E–W trending po lar ity.
The mod els cre ated for the lim it ing thick ness of 40 cm doc u - ment an ab sence of coal seams of greater thick ness in the whole of the north ern and east ern parts of the USB, where coal seams >40 cm are only lo cally iden ti fied. It should be noted that all of those parts are poorly rec og nized by both bore holes and mine work ings. The mod els of spa tial de vel op ment based on net coal con tent and num ber of the coal seams main tain (ex - cept for small dif fer ences) the ma jor fea tures of the de vel op - ment ob served in mod els of the to tal thick ness of coal seams in the Petøkovice Mem ber.
DISCUSSION
As noted above, the coal-bear ing ca pac ity of the Petøkovice Mem ber is very low. The av er age value of the coal seams con - tent >0.1 m has been es tab lished at 3%, how ever, the value of the coal seam con tents >0.4 m is only 1.66%. More over, this coal-bear ing ca pac ity in the N and NE parts is in the range of 0 to 2%, with coal seams of greater thick ness be ing mostly ab sent.
All mod els con firm the pre vail ing N–S trending po lar ity (or elon ga tion of the main zones of coal-bear ing ca pac ity along ap - prox i mately a NW–SE di rec tion). This di rec tion is strik ingly rem - i nis cent of the di rec tion of ma jor tec tonic zones in the broader area (Kraków–Lubliniec Fault Zone – e.g., Bu³a, 2000; Haná Fault Zone – Dudek, 1980), de fin ing the north ern and south ern bound aries of the Up per Silesia Block of Brunovistulicum in the base ment of the USB (cf. Bu³a and ¯aba, 2005). These trends are likely to have played an im por tant role in the de vel op ment of the un der ly ing sed i men tary ac cu mu la tion of the USB and of sed i ments of the Petøkovice Mem ber, es pe cially in its lower part (Hýlová et al., 2013), where they rep re sent a man i fes ta tion of sub si dence-re lated tran si tion of the Visean sed i men tary ba sin into the paralic molasse of the USB.
For the new emerg ing stage of de vel op ment of the depositional area of the Moravo-Silesian Ba sin, cor re spond ing to the USB (or to its paralic molasse), it is clear that tran si tion to the coal-bear ing sed i men ta tion was not syn chro nous across the whole ba sin, but un doubt edly started on its south ern edge and pro ceeded to the north. There fore, the south ern part of to day‘s post-ero sional ex tent of the ba sin was af fected first by tran si tion to the ter res trial sed i men tary en vi ron ment, whereas the north ern part of the ba sin was af fected later and with less in ten sity. This fact is re flected in the in creased coal-bear ing ca pac ity in the Czech part of the ba sin, par tic u larly in its south ern part. Coal-bear ing ca pac i - ties close to zero were of ten found in the north ern and south east - ern parts of the Pol ish part of the ba sin, where re gions with out coal-bear ing sed i men ta tion are also found.
A com pli cated de vel op ment of depositional con di tions in the south of the Czech part of the USB has pre vi ously been de - scribed by one of the au thors of this study (Hýlová, 2011). This evo lu tion was also pre vi ously the sub ject of in ves ti ga tion of the coal-bear ing ca pac ity and de vel op ment of the thick ness and sand con tent of the Petøkovice Mem ber of the USB (Hýlová et al., 2013). A de vel op ment of fore land ba sin type can be ap plied to the stage of paralic sed i men ta tion of the Petøkovice Mem ber,
Coal-bearing capacity of the Petøkovice Member (Ostrava Formation, Serpukhovian, Mississippian)... 643
Fig. 4 Models of total thickness of coal seams >10 and >40 cm
Fig. 5. Models of coal-bearing capacity (>10 and > 40 cm) expressed as the proportion (%) of coal in strata of the Petøkovice Member
Explanations as in Figure 4
Coal-bearing capacity of the Petøkovice Member (Ostrava Formation, Serpukhovian, Mississippian)... 645
Fig. 6. Models of the number of coal seams (>10 and >40 cm) of the Petøkovice Member Explanations as in Figure 4
which is best seen along an E–W di rec tion. This trend is ev i dent to the north of the Czech part of the ba sin and through out its Pol ish part, both in the de vel op ment of the thick ness of coal seams and in its sand con tent. In the cen tre and south of the Czech part of the ba sin, this sit u a tion is com pli cated and ob - scured by a lo cal NW–SE trending belt with an oma lously low thick nesses and in creased sand con tent (Hýlová et al., 2013), and also low to tal thick ness (Fig. 4) and num ber of coal seams (Fig. 6). We in ter pret this struc ture as a zone of lit to ral drift bar ri - ers (Fig. 7). These are likely to have pre vented fre quent ma rine in cur sions into the south ern parts of the ba sin and also in duced changes in the de vel op ment of fau nal ho ri zons, which is also man i fested by their grad ual tran si tion from ma rine to brack ish, and later to fresh wa ter ho ri zons, in the same di rec tion.
De spite the anom a lous de vel op ment in the south of the Czech part of the ba sin, it is ob vi ous that, in the USB as a whole, seg ments char ac ter is tic of fore land type bas ins (such as foredeep, forebulge and backbulge) can be seen. The best ex - am ple is the north ern part of the ba sin, where such de vel op - ment pre vails. Also dom i nant in this part of the ba sin is the sep - a ra tion of the depositional area into three char ac ter is tic zones of fore land types of ba sin. These in clude the foredeep with max i mum sub si dence, which is lo cated in the west ern part of the post-ero sional area of the Petøkovice Mem ber, the zone of tran sient sub si dence and the zone of min i mum sub si dence in the east of the ba sin. The two ar eas of max i mum coal-bear ing
sed i men ta tion dur ing the de po si tion of the Petøkovice Mem ber are both lo cated in the foredeep seg ment, where the depocentre is lo cated (Fig. 7). The third, the small est of the three, lies in a tran si tional zone, where the coal-bear ing ca pac - ity is at its min i mum in the area of neg li gi ble sub si dence. In the north of the ba sin, where the paralic sed i men ta tion took place only slowly and diachronously, the coal-bear ing ca pac ity is also close to zero.
The rel a tive prox im ity of a sea and nu mer ous ma rine in cur - sions into the ba sin are con sid ered to have played an im por tant role in the dis tri bu tion of the coal-bear ing ca pac ity of the Petøkovice Mem ber. The spa tial vari abil ity of re gions with higher coal-bear ing ca pac i ties may be partly at trib uted to the synsedimentary ero sion of coal seams by river chan nels (Filák, 2006). The anal y ses of youn ger sed i men tary units are not yet com plete, but the pre lim i nary re sults show that the pat tern of the coal-bear ing ca pac ity of the Petøkovice Mem ber likely dif - fers con sid er ably from that of the youn ger mem bers of the Ostrava For ma tion. Al though a di min ish ing ef fect of the change in con di tions at the ma rine/paralic sed i men tary bound ary can be rec og nized dur ing the for ma tion of the Petøkovice Mem ber, newly in com ing trends can also be seen. Based on mod els of the coal-bear ing ca pac ity of the unit as well as on pre vi ously pub lished mod els of thick ness de vel op ment and sand con tent of the unit (Hýlová et al., 2013), the fol low ing con clu sions can be de rived: (1) the coal-bear ing sed i men ta tion of the Petøkovice Fig. 7. Inferred zones of different geological development of the Petøkovice Member in the Upper Silesian Basin
Mem ber is closely re lated to the over all thick ness of the unit and to ba sin base ment ar eas of great est sub si dence; (2) an in - crease in the sand con tent led to the sup pres sion of the cy clic struc ture in the Petøkovice Mem ber and, con se quently, to a re - duc tion in their coal-bear ing ca pac i ties. A larger sup ply of coarse-grained flu vial sed i ments was not fa vor able for the de - vel op ment of peat swamps; (3) the N–S po lar ity pre vailed over the E-W po lar ity dur ing the de vel op ment of the coal-bear ing ca - pac ity of the Petøkovice Mem ber. Ev i dence for this state ment is the higher coal-bear ing ca pac ity area occuring only in the south-west ern part of the ba sin. How ever, this is also af fected by the great vari abil ity in seam num ber and in their to tal thick - ness. In the re main ing parts of the ba sin the coal-bear ing ca - pac ity shows very low val ues.
The coal-bear ing ca pac ity ob vi ously de pends on a num ber of fac tors that cre ated a suit able en vi ron ment for coal-bear ing sed i men ta tion in the ba sin and also af fected the pres er va tion and sub se quent trans for ma tion of the or ganic mat ter. As emerges from our study, these fac tors in clude the sub si dence of the ba sin floor and the over all lithological evo lu tion of the suc - ces sion, as well as the cy clic de vel op ment of the de pos its.
These fac tors are mainly the re sults of the spa tial de vel op ment of the palaeo ge ogra phy of the ba sin. The au thors con sider they have played the ma jor role in the pro cess of coal-bear ing ca - pac ity for ma tion. The mod els of sand con tent de vel op ment (Hýlová et al., 2013) and of coal-bear ing ca pac ity show that the en vi ron ments suit able for the coal-bear ing sed i men ta tion are char ac ter ized by a cer tain op ti mum sand con tent. Sand-rich or even sand-dom i nated sed i ments are not fa vour able for the for - ma tion of peat swamps, be cause a steady sup ply of clastic sed - i ments sup pressed veg e ta tion growth. As a re sult, coal-bear ing ar eas of the ba sin are gen er ally the same ar eas where the best cy clic struc ture of sed i ments can be ob served. On the other hand, not all fine-grained sed i ments or silts were fa vour able for the de vel op ment of coal-bear ing sed i men ta tion, be cause some of them rep re sent large sea wa ter in cur sions to the peat swamps. These re la tion ships be tween pre vail ing sed i ment type and coal-bear ing ca pac ity un doubt edly re late to the type of en - vi ron ment in which the coal-bear ing sed i men ta tion has the best con di tions for its or i gin and de vel op ment. Fos sil swamps of the Petøkovice Mem ber were mainly con fined to the floodplain en vi - ron ment of streams, but also to near-shore lakes in rel a tive prox im ity to the sea. Al though our find ings on the re la tion ships be tween coal-bear ing sed i men ta tion, sed i men tary en vi ron ment and cyclicity in the ba sin may not have gen eral ap pli ca bil ity, the model of depositional en vi ron ments pre sented and their re la - tion ship to the coal-bear ing sed i men ta tion is very close to that pub lished by Havlena (1982), and also to the views pre sented by Kêdzior et al. (2007) and oth ers.
CONCLUSIONS
Spa tial de vel op ment of the coal-bear ing ca pac ity il lus trates the lat eral vari abil ity of depositional con di tions of coal bas ins. In com bi na tion with track ing the ad di tional pa ram e ters, such as over all thick ness and sand-con tent, this may fa cil i tate better char ac ter iza tion and un der stand ing of the vari abil ity of the sed i - men tary en vi ron ments in the ba sin.
From the view point of the or i gin of the Petøkovice Mem ber, it can be stated that the coal-bear ing sed i men ta tion of this unit was con sid er ably in flu enced by the tran si tion from shal low open ma rine sed i men ta tion of the Kyjovice Mem ber (equiv a lents in Pol ish part of the ba sin are the Malinowice and Zalas beds) to paralic sed i men ta tion of the Ostrava For ma tion (Paralic Se ries in the Pol ish part of the ba sin). Coal-bear ing sed i men ta tion de - vel oped to a greater ex tent, at first, in the south of the ba sin (i.e., to the south in the Czech part). At this point, the north and north - east parts of the ba sin re mained un der sig nif i cant in flu ence of the ma rine in cur sions, where con di tions for the de vel op ment of coal-bear ing sed i men ta tion were gen er ally un fa vour able and time-lim ited. The places of the high est or ganic mat ter ac cu mu - la tion in the Petøkovice Mem ber were lo cated mainly in the ar - eas of max i mum sub si dence of the ba sin floor (foredeep seg - ment), and partly in the tran si tional zone. In the zone of min i - mum sub si dence, or ganic mat ter ac cu mu la tion oc curred only lo cally or was com pletely ab sent. The coal-bear ing ca pac ity of the Petøkovice Mem ber was, there fore, con trolled by the ba sin floor sub si dence and partly also was in flu enced by fluc tu a tions in depositional con di tions at the north ern part of the ba sin, which prob a bly pre vented more fre quent ma rine in cur sions than in the south ern parts of the ba sin, thus al low ing it to par - tially in crease the coal-bear ing ca pac ity in some ar eas.
The con di tions de scribed of the Petøkovice Mem ber that in - flu enced the coal-bear ing ca pac ity are the rea son why this unit, as far as the coal-bear ing ca pac ity is con cerned, is largely dif fer - ent from that of youn ger strata units in the paralic molasse of the USB. Per haps due to such con di tions, the de vel op ment of the coal-bear ing ca pac ity of the Petøkovice Mem ber may serve as an in ter est ing ex am ple of the de vel op ment of coal-bear ing sed i - men ta tion in bas ins where tran si tional pro cesses in sed i men tary evo lu tion from ma rine to paralic en vi ron ments took place.
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 of the Czech Re pub lic (pro ject SGS SP2014/40). The au thors are grate ful to A. Kêdzior and an anon y mous re viewer for their con struc tive re marks and sug ges tions, which sub stan - tially im proved the qual ity of the manu script.
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Coal-bearing capacity of the Petøkovice Member (Ostrava Formation, Serpukhovian, Mississippian)... 649
