Con trols on basal Zechstein (Wuchiapingian) evaporite de po si tion in SW Po land
Kazimierz DYJACZYÑSKI1 and Tadeusz Marek PERYT2, *
1 Pol ish Oil and Gas Com pany, Bohaterów Westerplatte 15, 65-034 Zielona Góra, Po land
2 Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Rakowiecka 4, 00-975 Warszawa, Po land
Dyjaczyñski, K., Peryt, T.M., 2014. Con trols on basal Zechstein (Wuchiapingian) evaporite de po si tion in SW Po land. Geo - log i cal Quar terly, 58 (3): 485–502, doi: 10.7306/gq.1166
The de vel op ment of basal Zechstein (Wuchiapingian) strata in SW Po land in di cates the ex is tence of a var ied re lief in her ited af ter the flood ing of the pre-ex ist ing de pres sion by the trans gress ing Zechstein sea. The deeper parts of the ba sin were the place of de vel op ment of thin basinal Zechstein Lime stone show ing sed i men tary con den sa tion man i fested by bored and en - crusted grains and thick evaporites (mostly ha lite). In shal low parts, Zechstein Lime stone reefs fol lowed by thin ner evaporite se quences (dom i nated by anhydrite) were formed. The anal y sis of 2D sec tions ex tracted from 3D seis mic data showed that in stead of three con ven tion ally rec og nized evaporitic units in the PZ1 cy cle, five units oc cur (from the base to the top: Lower Anhydrite, Lower Old est Ha lite, Mid dle Anhydrite, Up per Old est Ha lite, Up per Anhydrite). In any par tic u lar place their num ber may vary from two (Lower Anhydrite at the base of the PZ1 cy cle and Up per Anhydrite at the top of the PZ1 cy cle) to five. Two units of Lower Anhydrite oc cur through out the plat form and basinal zones, show ing a deep en ing-up wards (transgressive) trend. Ha lite sed i men ta tion, in the deep est parts of salt bas ins, be gan shortly af ter the de po si tion of the up per Lower Anhydrite while on sul phate plat form ar eas, sul phate de po si tion con tin ued. The Lower Old est Ha lite de pos its oc cur in the de - pres sions. Be tween the ha lite bas ins, anhydrite plat forms oc cur, and the thick ness of anhydrite plat form de pos its is smaller than is ob served in salt bas ins. The Up per Old est Ha lite in turn is re corded above the anhydrite plat form. The two ha lite units rep re sent dif fer ent phases of de vel op ment of ha lite bas ins. For ma tion of the Lower Old est Ha lite bas ins was re lated to pre-Zechstein de pres sions, al though in some cases their syndepositional sub si dence was con trolled by re ac ti va tion of for - mer faults within the sub-Zechstein base ment. In turn, the Up per Old est Ha lite bas ins used the ac com mo da tion space cre - ated due to anhydritization of the Lower Anhydrite de pos its, that were com posed orig i nally of selenitic gyp sum, and the as so ci ated vol ume loss. 3D seis mic data, con strained by bore hole data, show that the PZ1 evaporites in SW Po land were de pos ited in a far more com plex and dy namic sys tem than pre vi ously as sumed.
Key words: Zechstein Ba sin, evaporites, 3D seismics, lithofacies, sed i men ta tion, anhydritization.
INTRODUCTION
En vi ron men tal con di tions of large-scale evaporite pre cip i ta - tion, as char ac ter is tic of the Lopingian Zechstein ba sin of NW and Cen tral Eu rope (Fig. 1A), have been dis cussed by Sonnenfeld (1984) who sum ma rized the ba sic pre req ui sites.
One of the most cru cial con di tions is that a gen tle de pres sion must pre-ex ist to ini ti ate brine con cen tra tion (Sonnenfeld, 1984).
The ex is tence of a var ied re lief in her ited af ter the flood ing of the pre-ex ist ing de pres sion by the trans gress ing Zechstein sea at some 258 Ma (see Peryt et al., 2012a with ref er ences therein) is in di cated by the de vel op ment of basal Zechstein strata in SW Po land (D. Peryt et al., 2012; Fig. 1). Dur ing the de po si tion of the Zechstein Lime stone the deeper parts of the ba sin were the place of de vel op ment of thin basinal fa cies and in shal low parts (e.g., up lifted tec tonic blocks lo cally form ing is lands) car bon ate
build ups (reefs) formed (Kiersnowski et al., 2010). The Kupfer - schiefer is lack ing over the el e va tions of the Zechstein sub strate, mostly built of Car bon if er ous and/or Perm ian vol ca nic rocks, where the Zechstein Lime stone reefs de vel oped. In turn, in the de pres sions the Kupferschiefer com monly oc curs. Sub se quen - tly, thin PZ1 evaporites were de pos ited in the reef lo ca tions while much thicker evaporites ac cu mu lated in the for mer de pres sions.
Those ob ser va tions sug gest that the area of SW Po land can serve as a clas sic ex am ple of the fill ing of an te ced ent to pog ra phy by ha lite. If the sub si dence dur ing the de po si tion of the PZ1 evaporites is ig nored, then the dif fer ences be tween the reef el e - va tions and ad ja cent bas ins could have ex ceeded 100 m in places at the on set of Zechstein de po si tion (see Kiersnowski et al., 2010: fig. 6). How ever, Rockel and Ziegenhardt (1979) showed that south of Berlin the Old est Ha lite pre cip i tated on down thrown fault blocks, and their con clu sion was sup ported by sub se quent re ports from var i ous parts of Ger many and the Neth - er lands (see ref er ences in Underhill and Hunter, 2008). Ac cord - ingly, the ac tual re lief dur ing the de po si tion of the low est Zech - stein strata in west ern Po land may have been smaller.
In this pa per we dis cuss the re la tion ship be tween the sul - phate and chlo ride fa cies in the south and west of Grodzisk Wlkp. (Figs. 1B and 2) which is one of the best known ar eas in
* Corresponding author: e-mail: tadeusz.peryt@pgi.gov.pl Received: September 9, 2013; accepted: March 4, 2014; first published online: April 17, 2014
the Fore-Sudetic Monocline (SW Po land) in terms of avail able in for ma tion from deep bore hole and seis mic data, both 2D as well as 3D.
GEOLOGICAL SETTING
Dur ing the Perm ian, the area of west ern Po land be longed to the South ern Perm ian Ba sin, which was formed in the Late Car bon if er ous to Early Perm ian as a re sult of sub si dence caused by rift ing (Gast, 1988; Pha raoh et al., 2010). The be - gin ning of these extensional move ments cre ated a com plex, mul ti di rec tional sys tem of horsts and grabens (Ziegler, 1990).
One of the intra-ba sin horsts is the Brandenburg-Wolsztyn - -Pogorzela High (WPH) that sep a rated the Zielona Góra Ba sin from the Variscan Fore land (Pol ish Ba sin) (Kiersnowski et al., 2010). The WPH has a com plex geo log i cal his tory and struc - tural frame work, and its struc tural seg men ta tion sug gests that the WPH is an up lifted el e ment of a pos si ble post-Variscan broad, mega-dextral strike-slip zone trending NW–SE with a se ries of pull-apart bas ins lo cated oblique to the gen eral re - gional tec tonic frame work (see Kiersnowski et al., 2010, for de tailed dis cus sion). Kiersnowski et al. (2010) doc u mented a tec tonic dif fer en ti a tion of par tic u lar tec tonic blocks of the WPH and a rapid, strong tec tonic re ju ve na tion dur ing lat est Rotlie - gend and Early Zechstein times, ac com pa nied by var ied tec - tonic sub si dence dur ing the Zechstein; how ever, by the be gin - ning of the PZ2 cy cle, tec tonic ac tiv ity was weak or even ab - sent in the area stud ied (Kiersnowski et al., 2010). As the Zechstein flood ing was rapid (Smith, 1979), the Rotliegend land scape was prac ti cally un touched or, as de scribed by D. Peryt et al. (2012), only slightly changed by pro cesses ac - com pa ny ing the trans gres sion.
In the ma jor part of the study area (Fig. 2), out side the reef area, re lated to the Wolsztyn High that was char ac ter ized by Dyja czynski et al. (2001), Kiersnowski et al. (2010) and Peryt et al. (2012b) , Ca1 is thin and its thick ness usu ally ranges be - tween 0.5–3.0 m. It mostly shows a wackestone-packstone tex - ture (Peryt et al., 2014). The PZ1 evaporites which fol low are ca. 140–ca. 215 m thick. Char ac ter is tic sec tions of the PZ1 evaporites are shown in Fig ure 3. A1d is a lowstand de posit with re spect to sea level out side the iso lated ba sin (Wag ner and Peryt, 1997), but it is a highstand sys tems tract with re spect to the sub se quent drawdown hydrogeology (cf. War ren, 1999).
Wag ner and Peryt (1997) re garded Na1 as a LST, but Peryt (2010) in di cated that in the cen tral part of the ba sin Na1 should be re garded as a TST (cf. Kend all, 2010). Strohmenger et al.
(1996) con cluded that on the evaporite plat form Na1 is HST.
BrA1g is re garded as a TST, and the rest of the A1g sec tion as a HST (Fig. 1C; Peryt et al., 1996).
The study area is lo cated within the mar ginal evaporite plat - form that was wide in the south ern mar gin of the Zechstein PZ1 (Werra) ba sin (e.g., Rich ter-Bernburg, 1985; Peryt et al., 2010a).
In west ern Po land, the PZ1 de pos its are mostly >200 m thick over much of the evaporite plat form area; thin ner (<200 m thick) se quences are re lated to the Wolsztyn High. The Old est Ha lite (Na1) ba sin, stretch ing NW–SE and lo cated just be yond the northeast ern mar gin of the Wolsztyn High, is ca. 35 km long and 1–6 km wide. This ba sin is sur rounded by Lower Anhydrite (A1d) plat forms within which lo cal, smaller ha lite bas ins de vel oped, in par tic u lar at the north ern side of the main ba sin. In the ha lite bas - ins, and es pe cially in the main ba sin, the thick ness of Na1 reaches about 120 m while that of A1d is about 10–20 m. The PZ1 de pos its are cov ered by the youn ger Zechstein cy cles (PZ2–PZ4, usu ally 260–280 m thick), fol lowed by up to 1650 m Fig. 1A – Zechstein ba sin show ing the lo ca tion of the Branden -
burg- Wolsztyn-Pogorzela High (af ter Kiersnowski et al., 2010;
ar row shows lo ca tion of Grodzisk Wlkp.); B – ba sin fa cies of the Zechstein Lime stone in Po land show ing the area stud ied (grey poly gon cor re spond ing to Fig ure 2); C – stra tig ra phy of the basal Zechstein (af ter Wag ner and Peryt, 1997, with some changes re sult ing from the cur rent study)
Fig. 2. Lower Old est Ha lite bas ins in the south and west of Grodzisk Wlkp., show ing the lo ca tions of seis mic sec - tions (solid red lines; Figs. 10–20) (af ter K. Dyjaczyñski and K. Kucharczyk, 2008, un pub lished) and faults cut ting the bound ary of the Zechstein with the older strata (af ter Kiersnowski et al., 2010, up dated by H. Kiersnowski)
Fig. 3. Lower Anhydrite-Main Do lo mite in ter val in the Boruja 3, Paproæ 28 and Cicha Góra 5 bore holes (for ex pla na tion of ab bre vi a tions of strati graphi cal units see Fig. 1C)
thick of Tri as sic, up to 437 m of Ju ras sic and lo cally re corded Cre ta ceous de pos its up to 229 m thick, which are in turn cov ered by thin Ce no zoic de pos its (Kotarba et al., 2006).
METHODOLOGY
The ba sis of the pa per is a seismo-strati graphic anal y sis of basal Zechstein strata along se lected 2D lines ex tracted from a 3D seis mic vol ume, cal i brated by well data, in par tic u lar by re - sults of sedimentological and geo phys i cal log ging data of basal Zechstein strata in bore holes lo cated within the study area (Fig.
3). The ba sis of strati graphic sub di vi sion of evaporite strata of the PZ1 cy cle was the lo ca tion, in the sec tion, of the char ac ter - is tic marker unit – the Anhydrite Brec cia (BrA1g), that oc curs through out the en tire Pol ish Zechstein ba sin at the base of the Up per Anhydrite (A1g) (e.g., Tomaszewski, 1962; K³apciñski, 1966; Podemski, 1973; Lorenc, 1975; Wag ner et al., 1978;
Wag ner, 1994; Peryt et al., 1996). It is eas ily dis tin guish able in par tic u lar bore holes based on geo phys i cal log ging data (see Fig. 3). The brec cia is the transgressive sys tems tract de posit al though the ma te rial com pos ing it was a lag de posit of the preceding exposure period (Peryt et al., 1996).
The ver ti cal res o lu tion of the seis mic data is 20 m. The lo ca - tions of the seis mic sec tions and bore holes dis cussed in this pa per are shown in Fig ure 2; the ac tual num ber of sec tions stud ied was con sid er ably greater and they were used for map - ping the out line of the main salt ba sin and sat el lite bas ins. All the data was ac quired by Geofizyka Toruñ on be half of the Pol - ish Oil and Gas Com pany, al though for the pur pose of this pa - per many mod i fi ca tions have been made. In many places the in ter pre ta tion of those sec tions, as far as the PZ1 evaporites are con cerned, was mod i fied af ter the in ter pre ta tion done at Geofizyka Toruñ; in 1995, salt bas ins had al ready been mapped by A. Klecan (unpublished) based on 2D seismic data.
The in ter pre ta tion of seis mic data fol lowed the ap proach dis cussed by Antonowicz and Knieszner (1984) and Górski et al. (2000) of cor re la tion of five re flec tors re lated to the bound - aries of rocks with high acous tic impendence and rocks with lower acous tic impendence: Z4 (bot tom of A4 or bot tom of Na4/top of A4), Z3 (bot tom of Na3/top of A3), Z2 (bot tom of Na2/top of A2), Z1 (bot tom of Na1/top of A1d) and Z1’ (bot tom of A1d or bot tom of Ca1/top of T1 or of the Zechstein sub strate) (cf. Figs. 10–20).
The cores avail able in the study area in cluded the Zech - stein Lime stone and the lower part of the Lower Anhydrite and they were sub ject to stan dard sedimentological and micro facies anal y sis aimed at pro vid ing an ad di tional in for ma tion on the geo log i cal back ground of the on set of evaporite de po si tion. A col lec tion of 48 thin sec tions, made from the Zechstein Lime - stone rocks by the Zielona Góra Branch of the Pol ish Oil and Gas Com pany and the PGI-NRI, was ana lysed us ing stan dard pet ro log i cal meth ods in this study; the thin sec tions came from the Boruja 3, 4, Cicha Góra 2, 4, 5, 6, 9, and Czarna Wieœ 2, 5, 6 bore holes. Re sults of the study of two Zechstein Lime stone sec tions lo cated in the study area (Czarna Wieœ 4 and Paproæ 28) have re cently been published by Peryt et al. (2014).
RESULTS AND INTERPRETATION
PETROLOGY OF THE ZECHSTEIN LIMESTONE
The Zechstein se quence in the study area starts with thin, usu ally <1 m thick car bon ate rocks of vari able min er al ogy. In
some bore holes (e.g., Paproæ 28 – see Peryt et al., 2014: fig. 4, and Cicha Góra 5 – Fig. 4) the car bon ate rocks are un der lain by T1 up to 0.7 m thick. Be low it thin Ca0 (usu ally bioclastic wackestones-packstones) oc curs oc ca sion ally (e.g., Figs. 4 and 5J; Peryt et al., 2014: fig. 4). In its up per most part in the Cicha Góra 5 bore hole abun dant, prob a bly monospecific ostracods oc cur (Fig. 4E) that in di cate an ex treme, most likely brack ish en vi ron ment char ac ter ized by con di tions very dif fer ent from those which pre vailed ear lier. The T1 de pos its show microfacies typ i cal of this unit (see Oszczepalski and Rydze - wski, 1987). When those two units are lack ing (e.g., Czarna Wieœ 6), bioclastic wackestones-packstones of Ca1 over lie the Rotliegend siliciclastic deposits (Fig. 5G, H).
The Ca1 de pos its con sist mostly of peloid and bioclastic wackestones and packstones with foraminifers (uniserial, “spi - ral”, Earlandia, en crust ing), bi valves, ostracods, gas tro pods, and a few brachi o pods, bryo zoans and cor als (Figs. 4B–D, 5A–E and 6; Peryt et al., 2014). In ad di tion, peloid-oncoid (oc - ca sion ally bioclastic) packstones, recystallised oncoid packsto - nes (Peryt et al., 2014) and other microbially-me di ated car bon - ates oc cur (Figs. 4F and 5C–E). The Ca1 sec tions in the study area are typ i cally much thin ner (<1 m) than is usual in basinal sec tions; they show many fa cies cri te ria for deep-ma rine car - bon ates (see Flügel, 1982: ta ble 53) and were called “con - densed se quences” by Peryt and Wa¿ny (1980). The con den - sa tion re flects dif fer ent fac tors (Flügel, 2010) of which a de - crease in the ac cu mu la tion rate (sed i men tary con den sa tion) was im por tant for Ca1.
Al though many con densed sec tions ex hibit a three-stage se quence in clud ing a re duced sed i men ta tion stage, an omis - sion stage, and an Fe and Fe/Mn crust stage (Flügel, 2010), only the first stage was re corded so far for the study area. The man i fes ta tion of the re duced sed i men ta tion stage is bored and en crusted grains (Fig. 4B; Peryt et al., 2014: figs. 5–7); other com mon man i fes ta tions such as shell con cen tra tions and re - worked iron crusts are lacking.
DEVELOPMENT OF THE LOWER ANHYDRITE
The bound ary be tween Ca1 and A1d is abrupt (Fig. 4A). In many cases the A1d sec tions show a nod u lar de vel op ment (Figs. 7 and 8) al though clear pseudo morphs af ter sel e nite crys tals can be ob served in the sec tions from the reef area (Fig.
7). This strongly sug gests that at least some nod u lar an hyd rites orig i nated in a shal low sub aque ous en vi ron ment char ac ter ized by con cen trated brines as has been re corded in mod ern ma rine sel e nite fa cies: coastal salt works – evap o ra tive sa linas (e.g., Ortí Cabo et al., 1984) and in some nat u ral coastal lakes (e.g., War ren, 1982). An cient oc cur rences of sel e nite fa cies cover a wide range of set tings that re quire spe cific en vi ron men tal con - fig u ra tions, and in par tic u lar, the ex is tence of an out flow of heavy brines as well as the ac cu mu la tion of chlorides on the platforms and in the basins (see Ortí, 2010).
In the core from Koœcian 21 bore hole, nod u lar anhydrite with do lo mite streaks oc curs in the lower part of A1d, which is fol lowed by recrystallised bed ded anhydrite and, in the up per part, by bed ded nod u lar anhydrite with streaks of do lo mite and rare clay-rich interbeds (Peryt et al., 2010b). A geo log i cal-geo - phys i cal cor re la tion of the two low est units of A1d from the plat - form and basinal zones is shown in Fig ure 8. The lower A unit is com posed of nod u lar anhydrite that is char ac ter ized by low gamma val ues and very high re sis tiv ity. The higher B unit shows anhydrite with car bon ate laminae, of ten of sup posed mi - cro bial na ture, with com mon man i fes ta tions of hy dro car bons;
due to an in creased clay min er als con tent that ac com pa nies the
higher car bon ate con tent the unit is char ac ter ized by in creased GR val ues and de creased re sis tiv ity. It is up per most unit in the A1d ba sin sec tions (e.g., in Paproæ 42 and Paproæ 28) and thus it is sup posed that ha lite sed i men ta tion in the deep est parts of the salt bas ins be gan shortly af ter the de po si tion of the B unit, whereas on the A1d plat form area sul phate de po si tion still con - tin ued (cf. Fig. 22).
INTERPRETATION OF SEISMIC DATA
The seis mic sec tion along the Boruja 3–Paproæ 28–Paproæ 50 line (Fig. 9) shows the ap proach ap plied in this pa per. As a ref er ence level the flat top of Na4 was cho sen. If seis mic data were not avail able bore hole data-based sec tions should look like those shown in Fig ure 9A, B. Fig ure 9A rep re sents an in ter - pre ta tion that ap plies if only the ma jor lithologies are con cerned, whereas in Fig ure 9 the base of A1g in the Paproæ 28 bore hole is put at the base of BrA1g. How ever, if we con sider the avail - able seis mic sec tion pass ing through all three sec tions (Fig.
10), the geo log i cal cross-sec tion shown in Figure 9C is most likely.
Be cause of its neg li gi ble thick ness the Zechstein Lime - stone is not shown in Fig ure 9 as well as in many seis mic sec -
tions shown in Fig ures 10–20, ex cept for reef lo ca tions (Figs.
15 and 16).
The cross-sec tion along the Boruja 3–Paproæ 28–Paproæ 50 line (Fig. 9) shows that there are more evaporite units of strati graphic po ten tial within the Z1 cy cle than the three con ven - tion ally rec og nized, i.e. A1d, Na1 and A1g (Fig. 1). In stead of those three units, five units oc cur in the area stud ied, al though their num ber may vary from two to five in par tic u lar places (e.g., in a bore hole). Two of these, the Lower Anhydrite (A1d) at the base of the PZ1 cy cle and Up per Anhydrite (A1g) at the top of the PZ1 cy cle oc cur in ev ery lo ca tion. A1g is de lim ited by BrA1g at its base and the Main Do lo mite (Ca2) at its top, and hence it is very well con strained.
In ad di tion to those two anhydrite units, two ha lite units: the Lower Old est Ha lite (Na1d) and the Up per Old est Ha lite (Na1g) oc cur, sep a rated by the Mid dle Anhydrite (A1s). The BrA1g lies upon the Up per Old est Ha lite (Na1g) or Mid dle Anhydrite (A1s) (Figs. 3 and 9); the A1s is dis tin guished only in the case where anhydrite strata oc cur ring above Na1d are over lain by BrA1g, and thus in some cases it is clearly co eval with the up per part of A1d. It may also over lie the Lower Anhydrite (e.g., Fig. 18).
The in ter pre ta tion of seis mic sec tions (with the ref er ence level be ing ei ther the Na4 top or the top of the Mid dle Buntsandstein Tp2; Figs. 10–20) dem on strate the com plex re - Fig. 4. Microfacies of the Cicha Góra 5 (A–E) and Boruja 3 (F–I) sec tions; A – Zechstein Lime stone-Lower Anhydrite
bound ary; B, C, F–I – Zechstein Lime stone; D – above the Kupferschiefer-Zechstein Lime stone bound ary, E – Kupferschiefer
A – nod u lar anhydrite over ly ing recrystallised oncolitic packstone (depth 2653.94–2654.0 m); B – recrystallised oncolite (on) with sparitic por tions (s) con tain ing shells (sh) with foraminiferal encrustations (fe) (depth 2653.98 m); C – peloidal- bioclastic wackestone-packstone with foraminifera (of ten filled by glauconite and more rare py rite; depth 2654.85 m); D – bioclastic-peloidal wackestone with min ute quartz grains, com mon thin shells and foraminifera (left cen tre; depth 2655.03 m); E – microsparite with ostracods (depth 2655.7 m); F – recrystallised mi cro bial do lo mite (depth 2526.1 m); G – Earlandia sp. (depth 2526.3 m); H – shell pseudomorph with foraminiferal encrustation (depth 2526.5 m); I – recrystallised do lo mite with bioclasts – shell filled by peloids, with mi cro bial (ar rows) and foraminiferal (fe) encrustations (depth 2526.7 m)
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la tions of the basal Zechstein strata within the study area. The seis mic data show that the Na1d de pos its are re lated to the de - pres sions (Figs. 12–14 and 17–20). The A1d plat forms are lo - cated be tween the Na1d bas ins, and as the rule their oc cur - rence co in cides with the pres ence of el e vated ar eas dur ing the on set of A1d de po si tion. The thick ness of the Na1d ba sin de - pos its is greater than the thick ness of the A1d plat form de pos its (Figs. 10, 11, 13, 14 and 17–19) as gen er ally for other ar eas of the Zechstein Ba sin (e.g., Peryt, 1994) ow ing to de po si tion of ha lite de pos its in the depocentres (Rich ter-Bernburg, 1955).
Na1g in turn is re corded above the A1d plat forms and com - monly it does not oc cur within the cen tral parts of the Na1d ba - sin area (Fig. 21). Ac cord ingly, its dis tri bu tion in di cates a shift of depocentres at that time, the rea son for which will be dis cussed be low. Usu ally, Na1g pinches out to wards the cen tral part of Na1d bod ies (Figs. 10, 11, 13, 14, 17, 19 and 20). The thick - ness of the Na1d unit is greater than that of Na1g. In the study area the area of Na1g de po si tion is greater than the area of Na1d oc cur rence (Fig. 21). In the Czarna Wieœ 7 bore hole the thick ness of Na1g is rel a tively large (51 m) and the ha lite unit con tains nu mer ous anhydrite in ter ca la tions.
The in ter pre ta tion of seis mic depth sec tions pass ing through a rel a tively thin Ca1 reef re corded in the El¿bieciny 1 bore hole (12 m) shows the oc cur rence of Na1g and a part of the Fig. 6. Do lo mite – bioclastic wackestone with
fora minifers (uniserial, hemigordiopsid, en crust ing), shells (gas tro pods, bi valves, ostracods)
Boruja 3 (depth 2526.9 m)
Fig. 7. Cored in ter vals of the Lower Anhydrite of the Ruchocice 2k, Ruchocice 4 and El¿bieciny 1 bore holes
Out lines of the V-shaped selenitic gyp sum pseudomorph in the up per left photo shown by dots
Fig. 8. Geo phys i cal cor re la tion of the Lower Anhydrite units in the Paproæ-Ujazd re gion PG – gamma log, PNG – neu tron-gamma log, PR – re sis tiv ity log; the thick col umn (lower part of Paproæ 12, 14
and Ujazd 5) shows the cored in ter val; the thin col umn shows li thol ogy ac cord ing to geo phys i cal logs
Fig. 9. In ter pre ta tive sec tions along the Boruja 3–Paproæ 28–Paproæ 50 line A – con ven tional, with out con sid er ing seis mic data; B – tak ing into ac count the oc cur rence of anhy drite brec cias at the base of Up per Anhydrite; C – with use of seis mic data; A1d – Lower Anhydrite, A1g – Up per Anhydrite, A1s – Mid dle Anhydrite, A2 – Basal Anhydrite, A2r – Screen - ing Anhydrite, A3 – Main Anhydrite, BrA1g – Anhydrite Brec cia, C – Car bon if er ous, Ca1 – Zechstein Lime stone, Ca2 – Main Do lo mite, Na1 – Old est Ha lite, Na1d – Lower Old est Ha lite, Na1g – Up per Old est Ha lite, Na2 – Older Ha lite, Na3 – Youn ger Ha lite, Na4 – Youn gest Ha lite, Pr-sed – sed i men tary Rotliegend, Pr-vol – vol ca nic Rotliegend, T3 – Grey Salt Clay, T4 – Red Salt Clay; no - tice that be cause of its neg li gi ble thick ness the Zechstein Lime stone is not shown in this fig ure nor in many seis mic sec tions shown in Fig ures 10–20
Na1d ba sin be tween the El¿bieciny and Ruchocice reefs (cf.
Fig. 15). The El¿bieciny reef it self orig i nated at the max i mum el - e va tion of the top of the Zechstein sub strate (Fig. 15). Fig ure 18 shows that be sides the main Na1d ba sin there is also a con sid - er ably smaller Na1d ba sin lo cated south of the Ruchocice 4 bore hole (Fig. 2). Those two salt bas ins are sep a rated by a part of the A1d plat form lo cated above a 24 m thick reef lime stone in the Ruchocice 4 bore hole. In the Ruchocice 4 bore hole Na1g is lack ing, while this salt unit of very small thick ness may prob a bly oc cur south and north of the bore hole; it is eas ily vis i ble in the Ujazd 5 bore hole (23 m of Na1g), and poorly vis i ble in the Czarna Wieœ 5 borehole (10 m of Na1g; Fig. 18).
Fig ure 19 shows parts of the main Na1d ba sin sep a rated by a nar row, 600 m wide A1d plat form. Above it and in the ad join - ing Na1d bas ins Na1g is mod er ately eas ily vis i ble as doc u - mented by the Czarna Wieœ 6 (14 m) and Zielêcin 1 (27 m) bore holes. In the Parzêczewo 2 bore hole only 7 m of this salt and 14 m of the Mid dle Anhydrite (A1s) were re corded. Be - tween the Zielêcin 1 and Parzêczewo 2 bore holes its pres ence is enig matic as the thick ness of a pos si ble salt body is below seismic resolution.
In a part of the seis mic sec tion shown in Fig ure 16 two salt bod ies (Na1d and Na1g) iden ti fied in the Paproæ 4 bore hole pass into an anhydrite plat form, which is at tached to and su per - Fig. 10. Seis mic sec tion along the Boruja 3–Paproæ 28–Paproæ 50 line (ref er ence level: Na4 top)
Z1’, Z1, Z2, Z3, Z4 re flec tors: Z4 (bot tom of A4 or bot tom of Na4/top of A4), Z3 (bot tom of Na3/top of A3), Z2 (bot tom of Na2/top of A2), Z1 (bot tom of Na1/top of A1d) and Z1’ (bot tom of A1d or bot tom of Ca1/top of T1 or of the Zechstein sub strate;
for other ex pla na tions see Fig ure 9
Fig. 11. Seis mic sec tion through bore holes Paproæ 28, 48, 49, 46, 42, 12 (ref er ence level: Na4 top) For ex pla na tions see Fig ures 9 and 10
im posed on the Nowy Tomyœl and Paproæ West reefs.
The ha lite body oc cur ring be tween those two reefs is in - ter preted to represent Na1g.
The in ter pre ta tion of the depositional his tory of the basal Zechstein de pos its along a real sec tion (shown in Fig ure 14) is given in Fig ure 22. It is based on the as - sump tion that (1) the re lief of the Zechstein sea bot tom was var ied at the on set of Zechstein de po si tion (see dis cus sion in Kiersnowski et al., 2010) but it was sub ject to fur ther dif fer en ti a tion by sub tle block tec ton ics dur ing basal Zechstein de po si tion; and (2) the depositional sur faces dur ing the ter mi na tion of ha lite de po si tion (i.e., the base of the BrA1g unit) was roughly pla nar. The in - ter pre ta tion im plies that gyp sum de pos its in the sul - phate plat form ar eas un der went mul ti stage de hy dra - tion. Pre vi ously a very early, al most syndepositional anhydri tization of the orig i nal gyp sum de posit on the plat form was in ferred for the mar ginal sul phate plat form sys tem in north ern Po land (Peryt et al., 1998; cf. Hryniv and Peryt, 2003) as well as in other parts of the Zechstein ba sin (see Langbein, 1987; Peryt et al., 1993). The pre ferred oc cur rence of Na1g de pos its on or close to the sul phate plat forms strongly sug gests that the space in which the de pos its of the Na1g could have orig i nated was cre ated due to gyp sum de hy dra tion as Fig. 12. Seis mic sec tion (part of line 750) through the Cicha Góra 5 bore hole (ref er - ence level: top of Mid dle Buntsandstein Tp2)
Fig. 13. Seis mic sec tion along the Paproæ 28–Cicha Góra 7 bore holes (ref er ence level: Na4 top)
For ex pla na tions see Figures 9 and 10
Fig. 14. Seis mic sec tion (line 680) through the Cicha Góra 7 bore - hole (ref er ence level: Na4 top)
For ex pla na tions see Fig ures 9 and 10
shown in Fig ure 22. The shift of depocentres out side the Na1d ba sin and to ward the A1d plat forms is a sim ple con se quence of dif fer ent im pact of gyp sum de hy dra tion that was con trolled by var i ous thick ness of gyp sum strata re lated to the sed i men tary fa cies dif fer en ti a tion dur ing later phases of A1d de po si tion (cf. Peryt, 1994).
As shown in Figure 8, the development of the lowest sulphate strata is similar throughout the entire area study.
DISCUSSION
The PZ1 evaporite ba sin in the Wolsztyn High area is a clas sic ex am ple of the fill ing of an te ced ent to pog ra - phy by ha lite. Becker and Bechstädt (2000, 2006) pro - posed a self-or ga ni za tion model which com bines the shal low-wa ter–shal low-ba sin and deep-wa ter–deep-ba - sin mod els of Kend all (1992) in which ha lite fills newly es tab lished basinal re lief in the more dis tal ar eas. The en vi ron men tal con di tions to be ful filled for large-scale evaporite pre cip i ta tion have been dis cussed by Sonnenfeld (1984) who re ca pit u lated the ba sic pre req ui - Fig. 15. Depth seis mic sec tion through the El¿bieciny 1 bore hole
(line 540)
The Na1g po si tion of the ha lite body ad ja cent to the reef is de ter mined based on geo met ri cal re la tions; for ex pla na tions see Figures 9 and 10
Fig. 16. Seis mic sec tion (route 915) through the Paproæ 4 bore hole The area of oc cur rence of ha lite bod ies is con trolled by the fault; west of the fault it is not pos si ble to dis tin guish A1d and A1g; the Na1g po si tion of the ha lite body ad ja cent to the reef is de ter mined based on geo met ri cal re la tions; for ex pla na tions see Fig ures 9 and 10
Fig. 17. Seis mic sec tion through the Cicha Góra 9 and Cicha Góra 1 bore holes (ref er ence level: seis mic level Z4, Na4 top)
For ex pla na tions see Fig ures 9 and 10
Fig. 18. Seis mic sec tion through the Ruchocice 4, Czarna Wieœ 5 and Ujazd 5 bore holes (ref er ence level: top of Mid dle Buntsandstein Tp2)
For ex pla na tions see Fig ures 9 and 10
Fig. 19. Depth seis mic sec tion through the Czarna Wieœ 6, Zielêcin 1 and Parzêczewo 2 bore holes For ex pla na tions see Fig ures 9 and 10
Fig. 20. Seis mic sec tion through the Czarna Wieœ 7, Czarna Wieœ 3 and Czarna Wieœ 2 bore holes (ref er ence level: seis mic level Z4, Na4 top)
For ex pla na tions see Figures 9 and 10
sites that must be met. These in clude: an orig i nal de pres sion, a def i cit in the wa ter bud get, a starved ba sin, ba sin shape, an en - trance strait, wa ter sup ply, sur face area, brine volume, an adequate time interval, and continued sub si dence.
The de vel op ment of basal Zechstein strata in di cates the ex - is tence of var ied re lief af ter the flood ing of the pre-ex ist ing de - pres sion by the trans gress ing Zechstein sea (D. Peryt et al., 2012). This re lief re sulted partly from tec tonic re ju ve na tion dur - ing lat est Rotliegend and early Zechstein times (see Kiersno - wski et al., 2010), and seis mic sec tions clearly show tec tonic con trol via in creased sub si dence dur ing the de po si tion of Na1d (e.g., Figs. 16–18).
T1 is ab sent over the el e va tions of the Zechstein sub strate, mostly Car bon if er ous and/or Perm ian vol ca nic rocks, where the Ca1 reefs de vel oped. In turn, in the in ter ven ing de pres sions, T1 com monly oc curs. Ca1 is thin and shows a peloidal wackestone to packstone depositional tex ture with rare to com mon bio - clasts. The re stricted fauna, com mon dwarf foraminifers, with a pre dom i nance of lagenids, in di cates con sis tently dysaerobic con di tions and pos si bly el e vated sa lin ity in the basin (Peryt et al., 2014).
The Ca1 de pos its orig i nated in a deep-wa ter en vi ron ment and rep re sent a con den sa tion ho ri zon. Across most of the Ca1 ba sin, in clud ing its basinal zone, there are three, or at least two transgressive-re gres sive cy cles (e.g., Peryt, 1984, 1986; Paul, 1986, 2011; Peryt et al., 2010a; Hammes et al., 2013). The lower Zechstein ba sin in SW Po land was clearly in sen si tive to fluc tu a tions in sea level re corded else where, pre sum ably be - cause it was too deep. Bio- and magnetostratigraphical data in - di cate a pos si ble du ra tion of the Zechstein of only 2.8–3.5 Myr (Szurlies, 2013 with ref er ences therein; cf. Denison and Peryt, 2009), and Ca1 lasted ca. 0.4 Myr (see Peryt, 1984). Very thin
rock units, com pa ra ble to those oc cur ring in the Ca1 de pos its of the study area, may rep re sent large time spans last ing up to sev eral mil lions of years, such as in the Ju ras sic of Spain (Flügel, 2010: p. 214 with ref er ences therein). Most of that time is sup posed to be hid den within the goethite crusts, hard - grounds and the ero sional dis con ti nu ities char ac ter is tic of most con densed se quences, but lack ing in the sec tions stud ied. At the top of Ca1, recrystallised oncoid packstones and other microbially-me di ated car bon ates oc cur lo cally (Peryt et al., 2014) that are rem i nis cent of much thicker stromatolitic - -oncolitic strata. This is char ac ter is tic of the top most part of the basinal Ca1 fa cies through out the ba sin (Smith, 1986 with ref er - ences therein; Peryt and Peryt, 2012). These strata are con sid - ered to have been formed in re sponse to in creased sa lin ity dur - ing de po si tion (Smith, 1995; Pope et al., 2000). Eventually, gypsum started to be deposited across the entire study area.
Pre vi ous stud ies in di cated that, within the wide belt of the PZ1 evaporite plat form sys tem at tached to the car bon ate plat - form, dur ing sul phate de po si tion on lo cal plat forms, con cur rent chlo ride de po si tion oc curred in the ad ja cent bas ins (e.g., Hollingworth, 1948; Czapowski, 1987; Peryt and Kovalevich, 1996), per haps due to den sity strat i fi ca tion of a ba sin of con sid - er able bathymetry. Those bas ins fol lowed the pat tern in her ited af ter pre-Zechstein sed i men ta tion, al though in sev eral cases the seis mic sec tions strongly sug gest that vari able sub si dence dur ing early Zechstein times was re lated to re ac ti vated faults as shown in seis mic sec tions (Figs. 12, 15–18 and 20) as well as in the re con struc tion of depositional his tory dur ing PZ1 de po si tion as shown in Fig ure 22.
This re con struc tion as sumes that the re lief ex ist ing at the on set of Zechstein de po si tion con trolled the dis tri bu tion of the Zechstein Lime stone reefs lo cated on the sea-bot tom el e va - tions (Dyjaczynski et al., 2001). Sub se quently, gyp sum plat form de pos its started to ac cu mu late across the en tire area (I in Fig - ure 22). Af ter the de po si tion of two sul phate units shown in Fig - ure 8, the de po si tion of ha lite be gan in the de pressed, faster sub sid ing part, fault-bounded blocks (II in Fig ure 22; see Figs.
10–12, 14, 16, 18 and 20). Mi nor sed i men tary tec ton ics com - monly re ported from the basal Zechstein on the mar ginal anhydrite plat forms through out the en tire Zechstein Ba sin (e.g., Ziegler, 1990; Geluk, 1999) could have had a de ci sive con trol on the de vel op ment, or at least the ini ti a tion, of the ha lite ba sin (Fig. 22). This sub tle tec tonic con trol on evaporite fa cies dis tri - bu tion is not unique – it is well-dem on strated, for ex am ple, in north ern Po land (e.g., Peryt, 1994), where the interfingering of gyp sum and ha lite fa cies dur ing de po si tion of the PZ1 evapo - rites was also dem on strated (Peryt and Kovalevich, 1996). This interfingering finds its ex pres sion in the bore hole cores as al ter - na tions of ha lite and anhydrite.
In time, the area of ha lite (Na1d) bas ins de creased, and the place of ha lite sed i men ta tion was taken by gyp sum (III in Fig ure 22). In gen eral, the mar ginal evaporite plat form sys tem of the Fore-Sudetic area is dom i nated by anhydrite and in ter ca la tions of ha lite are rare and rel a tively thin (see Zieliñska-Pikulska and Pikulski, 2002, in: Hryniv and Peryt, 2010: fig. 2B).
Then, ow ing to the vol ume change re lated to the phase of gyp sum de hy dra tion (al beit the trig ger of this re mains enig - matic) and re lated cre ation of a var ied re lief (IV in Fig ure 22), Na1g bas ins could be formed. These are mostly lo cated on and ad ja cent to the anhydrite plat forms as the spa tial ef fect of the gyp sum de hy dra tion was the great est there. Even tu ally the Na1g bas ins were filled (V in Fig ure 22), and the Br1A1g de - pos its ac cu mu lated through out most of the study area. How - ever, the gyp sum de hy dra tion was then not com plete and the pro cess im pacted also on the sed i men tary his tory of the PZ2 strata.
Fig. 21. The Na1d and Na1g bas ins in the NW part of the study area
Diachroneity of ap pear ance of the chlo ride fa cies in di cated by cor re la tion of the A1d units (Fig. 8) in var i ous parts of the area stud ied, sug gests that the bound aries be tween many evaporite units are rarely isochronous. Such a (roughly) isochronous bound ary is the base of A1d (= the top of Ca1).
The unit may be over lain by Na1d (e.g., in Paproæ 28 – Fig. 3), Na1g (e.g., in Boruja 3 – Fig. 3) or BrA1g (e.g., in Nowy Tomyœl 2k). The dis tinc tion be tween Na1d and Na1g is pos si ble when both units oc cur in a bore hole (such as in Cicha Góra 5 – Fig. 3) or ow ing to cor re la tion of seis mic re flec tors (e.g., NE of Paproæ 50 – Fig. 10). The lower and up per bound aries of Na1d and A1s
as well as the lower bound ary of Na1g are strongly diachronous as they re flect a lo cal, very dy namic sys tem de spite the lo ca tion of the study area in the centre of the basin.
The seis mic sec tions show that the lithological units above the Old est Ha lite bas ins are lo cated dis tinctly higher than the same units above the A1d plat forms. This phe nom e non is in ter - preted as hav ing been re lated to de hy dra tion of gyp sum. The im pact of the vol ume de crease in volved (ca. 38% – see be low) was greater in the case of thicker units of mostly selenitic gyp - sum com pos ing the sul phate plat forms, lo cated on the struc - tural highs sep a rat ing ha lite ba sin ar eas, than in the case of thin Fig. 22. His tory of de po si tion of the basal Zechstein along
a real seis mic sec tion shown in Fig ure 14
I – the on set of Zechstein de po si tion – the depth of wa ter col umn was sev eral tens to over 100 m deep (sea level shown by dashed line); II – the de po si tion of plat form gyp - sum and then, fol low ing in ten si fi ca tion of fault ac tiv ity, the de vel op ment of a ha lite ba - sin; III – the fill ing of the PZ1 evaporite ba sin (A1d–Na1d–A1s in ter val), the up per sur face of evaporite se quence is roughly pla nar; IV – cre ation of ac com mo da tion space (be tween the dashed line and the top of the A1d–Na1d–A1s suc ces sion) ow ing to the var ied ef fect of gyp sum de hy dra tion (see text for dis cus sion); sub se quently, this space was filled by the Na1g de pos its – V
units of sul phates un der ly ing A1d. Con se quently, ow ing to the gyp sum de hy dra tion, the top sur face of the A1d plat form has been lo cated con sid er ably lower than the Na1d top in the ha lite ba sin, which in gen eral re mained in its pre vi ous lo ca tion, and hence the A1d plat form ar eas be came the loci where the Na1g de pos its could be formed. Con sid er able de flec tion of over ly ing strata – es pe cially clear be tween two salt bas ins – could break unit con ti nu ity. Such a sit u a tion is re corded for ex am ple be - tween the Zielêcin 1 and Czarna Wieœ 6 bore holes (Fig. 19), and be tween Paproæ 46 and Paproæ 49 (Fig. 11). The pres ent lo ca tion of anhydrite be low BrA1g with re gard to the platform top also indicates that these deflections are due to gypsum dehydration.
As al ready men tioned, the anhydrite unit be tween BrA1g and the Na1d top (e.g., in Paproæ 28 – Figs. 3, 10 and 11) or the same unit be low Na1g (e.g., in Cicha Góra 5 – Fig. 3) is a lithofacies equiv a lent of the near-top por tion of the A1d plat - form. This anhydrite unit (so far con sid ered as the lower part of A1g or A1s) is bed ded (as in di cated by dipmeter data in the Paproæ 28 bore hole col lected by Geofizyka Toruñ; un pub lished doc u men ta tion of the Paproæ 28 bore hole) and orig i nated prob - a bly due to resedimentation of gyp sum from the plat form to the ba sin af ter the de po si tion of Na1d. The plat form sur face had to oc cur, at that time, higher than the Na1d top in the salt ba sin and not lower as it is today.
In the study area Na1g of ten oc curs above the struc tural highs that were shal lower ar eas dur ing the de po si tion of A1d.
The de po si tion of ha lite may have led to the elim i na tion of pre-ex ist ing re lief as pre cip i ta tion of salts tends to level out any pre ex ist ing to pog ra phy (Sonnenfeld, 1984). Sub se quent early, but not syndepositional, de hy dra tion of thicker gyp sum formed plat forms that were re lated to the struc tural highs. These sep a - rated in di vid ual ha lite bas ins (with thin anhydrite units) and pro - duced a new palaeo geo graphi cal pat tern that was com monly re versed with the pre vi ous one. The pre vi ous el e va tions (char - ac ter ized by sel e nite gyp sum pre cip i ta tion) be came the lo ca - tions where Na1g salt bas ins oc curred. This ef fect is due to the vol ume de crease of gyp sum of about ca. 38% dur ing its de hy - dra tion to anhydrite. Rough es ti mates of the de hy dra tion-in - duced vol ume de crease in the sul phate plat form ar eas is 21–32% and in the halite basin area it is 12–22% (based on calculations for the section shown in Figure 14).
Pres er va tion of gyp sum pseudo morphs ob served on the sul phate plat form in the study area (e.g., Hryniv and Peryt, 2010; Peryt et al., 2010b; Fig. 7) sug gests that the gyp - sum-anhydrite tran si tion may have been ac com pa nied by in flux of sul phate-rich basinal wa ter and pre cip i ta tion of ad di tional gyp sum very early in the depositional his tory (e.g., Sonnenfeld, 1984; Langbein, 1987). In such a case, a vol ume change would not be seen. On the other hand, the value de rived from the Williston Ba sin Mid dle De vo nian change in vol ume as so ci ated with the gyp sum-anhydrite tran si tion is 37 ± 4% (Kin dle et al., 1987), the same as the the o ret i cal value. Di rect ev i dences of fluid es cape struc tures pro duced by fluidization have been ob - served in the up per part of A1d (Peryt et al., 2010b: p. 390).
They are not com mon be cause nod u lar tex ture (Fig. 7) oblit er - ated many for mer ex am ples. There fore, de spite un cer tain ties re gard ing the mech a nisms of anhydritization of gyp sum de pos - its in the study area, the pat tern of oc cur rence of de pres sions filled by Na1g fa vours the con cept that they were formed shortly af ter the pre vi ous re lief was lev eled by Na1d de po si tion. How - ever, the av er age thick ness of Ca2 that is larger above the A1d plat forms than above the salt bas ins in the Paproæ-Cicha Góra
area (a 5 m-dif fer ence be tween the Paproæ 28 and Paproæ 50 bore holes: Fig. 9C) strongly sug gests that de hy dra tion-re lated de for ma tions con tin ued af ter the de po si tion of A1g. Con se - quently, the prime con trols on the de po si tion of the PZ1 eva - porites in the area stud ied were:
– bathymetrical dif fer en ti a tion of the sed i men tary ba sin, both in her ited and tec toni cally mod i fied dur ing de po si - tion,
– de hy dra tion of orig i nal gyp sum de pos its, as shown in Fig ure 22.
There is also no doubt that dif fer en tial com pac tion was partly re spon si ble for the or i gin of the anhydrite plat forms, as was indicated by Langbein (1987), and this was closely related to different times of anhydritization in sulphate platforms and adjacent basins.
It was shown pre vi ously that gyp sum diapirs might form within the A1, A2 and A3 units of Ger many (Fulda, 1929;
Hemmann, 1972; Paul, 1987; Wil liams-Stroud and Paul, 1997) very early af ter de po si tion, and pri mary sed i men tary struc tures and gyp sum pseudo morphs are still rec og niz able within the diapirs (Wil liams-Stroud and Paul, 1997); thus the even tual con tri bu tion of very early gyp sum diapirism in ac com mo da tion space and the com pli cated interbedding of ha lite and anhydrite should be con sid ered. Hemmann (1972: figs. 14, 15) showed that diapirs in A3 are up to 60 m high, whereas A3 is only 30 m thick. Paul (1987) in di cated, in A1 in the Harz Mts., very of ten there is a shear ho ri zon within the gyp sum and there fore only the up per part of the gyp sum has moved. The diapir can also in - trude lat er ally into weak ho ri zons thus form ing the A1s. How - ever, al though the pos si bil ity that such mech a nisms op er ated in the case of PZ1 can not be ne glected, the re la tions be tween par tic u lar lithological units ob served for ex am ple in Fig ure 14 SWS of the Cicha Góra 7 bore hole strongly sug gest that they are orig i nal, sed i men tary tran si tions. Their cur rent pattern, how - e ver, was modified by various timings of anhydrite formation.
In ad di tion, the dif fer ent rhe o log i cal prop er ties of ha lite com pared with those of anhydrite could ad di tion ally have im - pacted the orig i nal spa tial pat tern of evaporite oc cur rence as in di cated by an im por tant role played by hor i zon tal or sub-hor i - zon tal move ments within the Old est Ha lite in the south ern part of the Fore-Sudetic area (e.g., Markiewicz and Becker, 2009;
Tobo³a, 2014).
CONCLUSIONS
1. Anal y sis of sec tions from a 3D seis mic vol ume cal i brated by sedimentological and geo phys i cal log ging of basal Zechstein strata in SW Po land shows that in stead of three con ven tion ally rec og nized evaporite units in the PZ1 cy cle (lower and up per anhydrite unit, with ha lite unit be tween them), five units oc cur.
These are, from the base to the top: A1d (Lower Anhydrite), Na1d (Lower Old est Ha lite), A1s (Mid dle Anhydrite), Na1g (Up - per Old est Ha lite), and A1g (Up per Anhydrite). The num ber of these units in var i ous places var ies from two (Lower Anhydrite at the base of the PZ1 cy cle and Up per Anhydrite at the top of the PZ1 cy cle oc cur at ev ery location) to five (see above).
2. The PZ1 evaporites were de pos ited in far more com plex and dy namic sys tem than as sumed pre vi ously.
3. The de vel op ment of basal Zechstein strata was con - trolled by the ex is tence of a var ied re lief af ter the flood ing of the pre-ex ist ing de pres sion by the trans gress ing Zechstein Sea.
The usu ally <1 m thin, deep-ma rine Zechstein Lime stone car -
bon ates con sist of mostly peloidal and bioclastic wackestones and packstones, with some recystallised oncoid packstones and other microbially-me di ated car bon ates. They show sed i - men tary con den sa tion man i fested by bored and encrusted grains.
4. A geo log i cal-geo phys i cal cor re la tion shows that two units of Lower Anhydrite oc cur through out the plat form and basinal zones show ing a deep en ing-up wards (transgressive) trend.
The lower A unit is com posed of nod u lar anhydrite, and the higher B unit con tains anhydrite with car bon ate laminae, of ten sup pos edly of mi cro bial na ture. The lat ter ter mi nates the Lower Anhydrite basinal sec tions and thus ha lite sed i men ta tion in the deep est parts of the salt bas ins be gan shortly af ter de po si tion of the B unit. By con trast, in the Lower Anhydrite plat form area sul phate de po si tion lasted for a long time. The diachroneity of ap pear ance of chlo ride fa cies in var i ous parts of the study area sug gests that the bound aries be tween many evaporite units are rarely (roughly) isochronous.
5. Seis mic sec tions show that the Lower Old est Ha lite de - pos its oc cur in de pres sions. Be tween the ha lite bas ins, anhydrite plat forms of the Lower Anhydrite oc cur, and the thick - ness of anhydrite plat form de pos its is smaller than the thick - ness of ha lite ba sin de pos its. The Up per Old est Ha lite in turn is re corded above the anhydrite plat form and only rarely it oc curs in ar eas where the Lower Old est Ha lite oc curs. Usu ally it pinches out to wards the cen tral part of the Lower Old est Ha lite bod ies al though in a few cases it oc curs above the en tire area of the oc cur rence of the Lower Old est Ha lite. The thick ness of
the Lower Old est Ha lite de pos its is in vari ably greater than that of the Upper Oldest Halite.
6. The lithological units above the Lower Old est Ha lite bas - ins oc cur dis tinctly higher than the same units above the Lower Anhydrite plat forms due to the greater im pact of gyp sum de hy - dra tion in the plat form than in the ba sin ar eas. Ac cord ingly, the ar eas of pre vi ous shal lower zones char ac ter ized by sel e nite gyp sum pre cip i ta tion be came the loci of Up per Oldest Halite deposition.
7. Al though the prime con trol on the de vel op ment of the basal Zechstein strata was in her ited re lief, an ad di tional, im por - tant fac tor in flu enc ing the fa cies ar range ment and depositional his tory was syndepositional sub si dence in duced ei ther by anhydritization of gyp sum (pro duc ing ac com mo da tion space for the de po si tion of the Up per Old est Ha lite) or by lo cal sub si - dence due to the re ac ti va tion of faults that con trolled the spa tial deposition of the Lower Oldest Halite.
Ac knowl edge ments. We thank the Pol ish Oil and Gas Com pany and Geofizyka Toruñ for per mis sion to use rock and seis mic ma te rial for this study, and K. Ch³ódek, H. Kiersnowski, K. Kucharczyk, D. Peryt and P. Raczyñski for help through out this study. The re search was sup ported by stat u tory funds of PGI-NRI (grant No. 61.2901.1301.00.0). Com ments from re - view ers P. Krzywiec, J. Paul and F. Strozyk helped us to im - prove the manu script greatly. We owe spe cial thanks to J. Zalasiewicz for his ed i to rial com ments and im prove ments.
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