Microfacies, foraminifers and carbon and oxygen isotopes in a basinal section of the Zechstein Limestone (Wuchiapingian): Bonikowo 2 borehole, western Poland

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Geo log i cal Quar terly, 2016, 60 (4): 827–839 DOI: http://dx.doi.org/10.7306/gq.1326

Microfacies, foraminifers and car bon and ox y gen iso topes in a basinal sec tion of the Zechstein Lime stone (Wuchiapingian): Bonikowo 2 bore hole, west ern Po land

Danuta PERYT¹, Tadeusz Marek PERYT^2, *, Stanis³aw HA£AS³ and Pawe³ RACZYÑSKI⁴

1 Pol ish Acad emy of Sci ences, In sti tute of Paleobiology, Twarda 51/55, 00-818 Warszawa, 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

3 Maria Cu rie-Sk³odowska Uni ver sity, In sti tute of Phys ics, Mass Spec trom e try Lab o ra tory, 20-031 Lublin, Po land

4 Uni ver sity of Wroc³aw, In sti tute of Geo log i cal Sci ences, Pl. Maksa Borna 9, 50-205 Wroc³aw, Po land

Peryt, D., Peryt, T.M., Ha³as, S., Raczyñski, P., 2016. Microfacies, foraminifers and car bon and ox y gen iso topes in a basinal sec tion of the Zechstein Lime stone (Wuchiapingian): Bonikowo 2 bore hole, west ern Po land. Geo log i cal Quar terly, 60 (4):

827–839, doi: 10.7306/gq.1326

The Zechstein Lime stone of the Bonikowo 2 bore hole is com posed of lime stone with var ied do lo mite con tent and is slightly thicker (3.4 m) than other sup posed con densed sec tions in SW Po land. The microfacies (mostly bioclastic wackestones-packstones) are char ac ter is tic of open ma rine lime stone de pos ited in deep shelf en vi ron ments. The com mon oc cur rence of echinoids in all parts of the sec tion, ex cept close to its top, in di cates no es sen tial change in sea wa ter sa lin ity.

How ever, the palaeotemperature in ter pre ta tion of the high est d¹⁸O value of cal cite re corded in the Bonikowo 2 bore hole (+0.32‰) in di cates a tem per a ture of 22.9°C when a d¹⁸O of wa ter = 2‰ is as sumed. The d¹³C and d¹⁸O curves for cal cite and do lo mite are sim i lar in shape, and their mean val ues are sim i lar: +1.8 ±1.5‰ for cal cite, re spec tively, and +1.9 ±2.3‰ and –4.3 ±2.4‰, –3.8 ±4.3‰ for do lo mite. There is a recrystallized in ter val in the lower part of the sec tion with both d¹³C and d¹⁸O val ues re duced. This may be re garded as an aer o bic equiv a lent of the Kupferschiefer. The di ver sity and abun dance of foraminifers is sig nif i cant through out the sec tion stud ied; lagenids pre vail, and this is con sid ered to be due to the dysaerobic con di tions through out the du ra tion of de po si tion of the Zechstein Lime stone.

Key words: Foraminifers, car bon and ox y gen iso topes, Wuchiapingian, ba sin fa cies, Zechstein Lime stone, Po land.

INTRODUCTION

Late Perm ian reefs in the cen tral part of the Zechstein Lime - stone ba sin in Po land grew on the top most edges of tilted blocks and/or on the top of up lifted horsts of the Brandenburg - -Wolsztyn-Pogorzela palaeo-High (Dyjaczynski et al., 2001;

Kiersnowski et al., 2010; Peryt et al., 2012b). On one such block the Bonikowo Reef was es tab lished. The thick ness of the Zechstein Lime stone in the Bonikowo 1 bore hole, that is lo - cated in the reef cen tre, is 87.5 m. In the Bonikowo 2 bore hole that is ad ja cent to the Bonikowo Reef, the Zechstein Lime stone is 3.4 m thick and oc curs at a depth of 2448.4–2451.8 m. It is un der lain by the Weissliegend and then Rotliegend sand stones and con glom er ates. The ear lier pet ro log i cal (Sylwestrzak, 2001) and petro physical (Ziemianin, 2012) stud ies in di cated that the dark micritic lime stones with mi nor do lo mite are ho mo - ge neous and show po ros ity mostly <1% and per me abil ity

<0.01 mD, with fauna re corded in the up per part of the in ter val that is rep re sented by bryo zoans, ostracods, foraminifers and shells of bi valves or brachi o pods (Ziemianin, 2012). Such a fau - nal as sem blage is sim i lar to that typ i cal of the car bon ate plat - form (e.g., Hara et al., 2013).

In gen eral terms, such de vel op ment is char ac ter is tic of the spe cific fa cies termed “con densed” (Peryt and Wa¿ny, 1980;

Peryt et al., 2015). As a rule, the con densed se quences are

<2.5 m thick, of ten <1 m thick (Peryt and Wa¿ny, 1980; Dyja - czyñski and Peryt, 2014; Peryt et al., 2015) in con trast to typ i - cal basinal Zechstein Lime stone se quences which are 5–10 m thick (Peryt et al., 1978; cf. Kotarba et al., 2006). The Zechstein Lime stone of the Bonikowo 2 bore hole is thicker than most such con densed se quences and one pos si ble ex - pla na tion of this slight thick ness in crease is the con tri bu tion of ma te rial de rived from the Bonikowo Reef; thus one of the aims of this pa per is to con sider the im pact of the reef on de po si tion in the ad ja cent area. Ear lier study of typ i cal con densed se - quences (Peryt et al., 2015) sug gested con tin ual dysaerobic con di tions, and pos si bly el e vated sa lin ity of sea wa ter, dur ing de po si tion of thin basinal Zechstein Lime stone de pos its. The aim of this pa per is to es tab lish the en vi ron men tal con di tions in close prox im ity to the basinward side of the Bonikowo Reef. In con trast to the thick reef se quences which lack the char ac ter - is tic d¹³C curve of the low er most part of the Zechstein, the thin

* Corresponding author, e-mail: tadeusz.peryt@pgi.gov.pl Received: August 31, 2016; accepted: October 21, 2016; first published online: November 4, 2016

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con densed se quences prob a bly re cord the en tire in ter val of the Zechstein prior to the on set of evaporite de po si tion (Peryt et al., 2015); hence the Bonikowo 2 sec tion of fers an ad di - tional pos si bil ity to test the va lid ity of iso to pic cor re la tion of reef and basinal pro files.

GEOLOGICAL SETTING

The Bonikowo 2 bore hole is lo cated NE of the Bonikowo 1 bore hole (Fig. 1). The Zechstein de pos its of the Bonikowo 2 bore hole are un der lain by an 187 m thick se quence of Rotlie - gend (in clud ing Weissliegend) con glom er ates and sand stones de pos ited in the Poznañ Ba sin (Dyjaczyñski and Kucharczyk, 2000). The Bonikowo Reef is cov ered by the PZ1 an hyd rites, 37.0 m thick; the to tal Zechstein thick ness is 414.5 m (Kiersnowski et al., 2010: ta ble 1). In the Bonikowo 2 bore hole, the thick ness of the PZ1 an hyd rites is 93.1 m (Peryt et al., 2010b: fig. 4), and the to tal Zechstein thick ness is 405.5 m (Kiersnowski et al., 2010: ta ble 2). As sum ing a roughly pla nar sur face of the Zechstein top in both bore holes and the dis tinct el e va tion of the Bonikowo Reef block com pared to the Boniko - wo 2 bore hole at the on set of the Zechstein Lime stone de po si - tion, the sub si dence of the Reef block had to ex ceed that of the block on which the Bonikowo 2 bore hole is lo cated.

The Zechstein Lime stone is over lain by Lower Anhydrite (73 m thick), com posed of bed ded anhydrite with pseudo - morphs af ter up right-growth gyp sum crys tals, then clastic anhydrite and thick mas sive anhydrite with pseudo morphs af ter up right -growth gyp sum crys tals form ing the ma jor part of the Lower Anhydrite. This mas sive anhydrite con tains, in its up per part, an in ter ca la tion of bed ded anhydrite with pseudo morphs af ter up right-growth gyp sum crys tals. The up per part of the Lower Anhydrite is com posed of clastic and then bed ded anhydrite (Denison and Peryt, 2009). These de pos its orig i nated in shal low sa lina en vi ron ments (cf. Peryt, 1994).

METHODS

Mac ro scopic ob ser va tions on bore hole were com ple men - ted by study of 25 thin-sec tions (in clud ing 7 thin-sec tions be - long ing to the Pol ish Oil and Gas Com pany which were pre vi - ously stud ied by Sylwestrzak, 2000).

The Zechstein Lime stone sec tion was sam pled for car bon and ox y gen iso to pic stud ies; al to gether 13 sam ples were ana - lysed. A 1.5 mm di am e ter stain less steel drill was used for the ex trac tion of ma te rial from the sur faces of the sam ples al - though the di am e ter of holes drilled was about 4 mm. Iso to pic anal y sis was car ried out at the UMCS Lublin Mass Spec trom e - try Lab o ra tory us ing a dual in let and tri ple col lec tor mass spec - trom e ter (a mod i fied and mod ern ized MI1305 model). In this anal y sis we have ap plied se lec tive acid ex trac tion (Al-Aasm et al., 1990). Cal cite frac tions of car bon ates were ana lysed on CO2 pro duced by re ac tion with 100% H3PO4 in a glass vac uum line con nected to the in let sys tem of the mass spec trom e ter.

The re ac tion pro ceeded at an elec tron i cally con trolled tem per - a ture of 25 ±0.2°C to achieve d¹⁸O in the PDB scale. For nor - mal iza tion of both d¹³C and d¹⁸O val ues the in ter na tional stan - dard NBS-19 was ana lysed in each se ries of sam ples. Af ter iso tope anal y sis of the cal cite frac tion the tem per a ture was switched to 50 ±0.2°C for anal y sis of the do lo mite frac tion.

CO2 evolved dur ing tem per a ture in crease from 25 to 50° was evac u ated. There af ter, the re ac tion tubes were iso lated from vac uum and re mained closed for 24 hours. Af ter this time, CO2

gas of the do lo mite frac tion was ana lysed sim i larly to that of the cal cite frac tion. The an a lyt i cal un cer tainty of both d val ues in terms of stan dard de vi a tion was better than 0.08‰ for the cal cite frac tion and about 0.15‰ for the do lo mite frac tion. For the last frac tion d¹⁸O was cor rected for acid frac tion ation ac - cord ing to Rosenbaum and Sheppard (1986).

RESULTS

Li thol ogy. As pre vi ously noted by Sylwestrzak (2001) and Ziemianin (2012), the Zechstein Lime stone mostly com prises lime stone with var ied do lo mite con tents and small amounts of in sol u ble res i due, mainly quartz (3–7% ac cord ing to Sylwe - strzak, 2001; de creas ing up wards). Late diagenetic anhydrite ce ments are rare and oc cur as mould fill ings and tab u lar crys - tals im preg nat ing the ma trix. On the other hand, synde po - sitional anhydrite nod ules are mod er ately com mon (Fig. 2);

their tex tures dif fer from that of the anhydrite ce ment by very fine, elon gated crys tals that are cha ot i cally ar ranged (Sylwe - strzak, 2001).

Where pres ent, do lo mite oc curs mostly in the ma trix.

Sylwestrzak (2001) noted a no ta ble ex cep tion is the low er most part of the sec tion that is com posed of recrystallized car bon ate 828 Danuta Peryt, Tadeusz Marek Peryt, Stanis³aw Ha³as and Pawe³ Raczyñski

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Zdrada IG 8

Warszawa

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10 km Czarna Wieœ 4 Paproæ 28

A

B

carbonate platform basin

Wolsztyn reefs

Fig. 1A – the lo ca tion of the Bonikowo 2 bore hole on a map show ing the gen eral palaeo geo graphi cal zones of the Zech - stein Lime stone (the isopach 10 m in the ba sin zone af ter Peryt et al., 2010a); the quad ran gle shows the area of Fig ure 1B; B – the dis tri bu tion of the Wolsztyn reefs (from >10 to 90.5 m thick) and the lo ca tions of the bore holes re ferred to in the text

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(peloidal grainstone ac cord ing to her in ter pre ta tion), and cal cite (late diagenetic ce ments), that is a very mi nor con stit u ent there.

Sylwestrzak (2001) con cluded that a basal sam ple (2451.5 m) which shows a uni form dark red col our in CL and non -stoichio - metric com po si tion, rep re sents late diagenetic dolomitisation.

Microfacies. The sec tion is micritic ex cept for its top most part that is sparitic and its low er most part that is microsparitic (Figs. 2–6).

The low est in ter val is com posed of recrystallized car bon ate, orig i nally prob a bly gran u lar, with lithoclasts (mostly millimetric in size) and rare moulds af ter shells, ei ther whole ones or their frag ments (Figs. 4I, J and 6C). The micritic part is com posed of bioclastic (with some peloidal ad mix ture) wackestones and packstones (Figs. 2, 3, 4D–H and 6A, B). The bioclasts are usu - ally <1 mm in size and they in clude frag ments of cri noids (Figs.

2, 4E, 5L and 6A), bryo zoans (Figs. 5G–I, L and 6A, B), bi - valves and brachi o pods (Figs. 3, 4D–F and 5G–K), echinoids (Fig. 5G, I), ostracods (Figs. 4D and 6A, B) and foraminifers (Figs. 5G–J, L, 6A, B and 7–9). The pat tern of dis tri bu tion of foraminifers is char ac ter ized be low. Other fauna does not show clear strati graphic changes (see Fig. 2).

The up per in ter val (Figs. 3, 4B, C and 5B–E) is com posed of sparitic bod ies, usu ally >1 mm in size, that oc cur in a bioclastic wackestone-packstone ma trix con tain ing a slightly poorer fauna

Microfacies, foraminifers and car bon and ox y gen iso topes in a basinal sec tion of the Zechstein Lime stone (Wuchiapingian)... 829

Fig. 2. Char ac ter is tics of the Bonikowo 2 sec tions show ing the lo ca tion of some sam ples stud ied (rock sam ples – this fig ure and Fig. 3; thin-sec tions – Fig. 4; frag ments of thin-sec tions – Fig. 5)

Red star shows the com mon oc cur rence of uniserial foraminifers and/or Agathammina

Fig. 3. Patchily recrystallized car bon ate with bioclasts

tb – terebratulid brachi o pod; sh – shell (bi valve or brachi o pod); see also Fig ures 4B, C and 5B, C

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830 Danuta Peryt, Tadeusz Marek Peryt, Stanis³aw Ha³as and Pawe³ Raczyñski

Fig. 4A – recrystallized do lo mite from the up per most part of the Zechstein Lime stone show ing gran u lar fab ric; B, C – recrystallized oncoids (the nu clei some of them con sist of bioclasts en crusted by foraminifers – lower part of the photo B) em bed ded in bioclastic and peloidal ma trix with com mon foraminiferal encrustations (white ar rows in B in di cate the places shown in Fig. 5B, C);

D–F, H – bioclastic wackestone-packstone with peloids [D – mostly shell frag ments; E – com mon cri noids (the larg est frag ments are marked by “c”) and foraminifers; F, H – mostly shell frag ments and foraminifers]; G – flaser-lam i nated wackestone with quartz silt (see Fig. 5K); I, J – microsparite with lithoclasts and recrystallized bioclasts (of ten moulds af ter shells; ar row in J in di cates dis - solved brachi o pod shell); thin-sec tion pho tos of sam ples the lo ca tion of which is given in Fig ure 2; scale bars = 1 cm

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as sem blage com pared to the pre ced ing in ter val though, in its lower part, cri noids still oc cur. Very char ac ter is tic of this in ter val are com mon encrustations of tu bu lar foraminifers and fora - miniferal -mi cro bial con sor tia de vel oped on bioclasts (Figs. 4B and 5B, D, E) and other, pos si bly mi cro bial, sub strates that at pres ent are sparitic bod ies (Figs. 4B, C and 5A, B).

The top most part (Figs. 2, 4A and 5A) is recrystallized gran - u lar car bon ate con tain ing rare shells and foraminifers (uniserial and Agathammina) and com mon anhydrite nod ules. The con - tact with the Lower Anhydrite is sharp (Fig. 2).

Foraminifers. Foraminifers were re corded in 18 thin-sec - tions; they are lack ing in the low est recrystallized part of the Microfacies, foraminifers and car bon and ox y gen iso topes in a basinal sec tion of the Zechstein Lime stone (Wuchiapingian)... 831

Fig. 5A – sparite show ing gran u lar fab ric; ar rowed are ostracod shells; B, C – foraminifers en crust ing shell (B) and pre vi ously ex - ist ing sur face, prob a bly mi cro bial (C); D – gas tro pod (Straparollus or Euomphalus?; ar rowed) and bryo zoan zoarium frag ment (above it) en crusted by foraminifers, E – strophomenid? brachi o pod (sb) en crusted by foraminifer-mi cro bial con sor tium; F – microsparite – bioclastic wackestone; G–I – packed bioclastic packstone; some bioclasts are micritized and en crusted by foraminifers; no tice echinoid spines (G – up per left cor ner, I – cen tre) and bryo zoan zoaria (G – lower left cor ner, H – cen tre); J – recrystallized bioclastic (mostly shell frag ments) wackestone; K – bioclastic wackestone with quartz silt; L – bioclastic packstone;

scale bars = 1 mm

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Zechstein Lime stone and they oc cur in all but one thin-sec tion from the over ly ing part of the sec tion. Uniserial foraminifers oc - cur in 13 thin sec tions, Agathammina in 11, en crust ing in 7, spi - ral (other than Agathammina) in 6, and Hemigordius in 2.

Agathammina and uniserial and spi ral foraminifers oc cur through out this over ly ing part of the sec tion, and en crust ing foraminifers oc cur mainly in its up per part (al though they are lack ing in the top most thin sec tion); an ex cep tion is the thin-sec tion from the depth 2450.5 m. Foraminifers (ex cept for en crust ing ones) are not usu ally com mon, but there are in - stances where uniserial foraminifers and/or Agathammina are quite nu mer ous (e.g., Figs. 4H, 7A–C, J and 9H), and these are aste risked in Fig ure 2.

CARBON AND OXYGEN ISOTOPES

The re sults are shown in Fig ure 10 and are sum ma rized in Ta ble 1.

The d¹³C and d¹⁸O curves for cal cite and do lo mite are sim i - lar in shape (Fig. 10A). In the lower part of the sec tion, a neg a - tive ex cur sion is ob served in the case of both d val ues. The in - ter val with neg a tive d val ues cor re sponds to the recrystallized level with lithoclasts and the low est por tion of the bioclastic wackestone-packstone suc ces sion. Then, d val ues in crease to about +2‰ (d¹³C) and about 0 to –1‰ (d¹⁸O) for the sam ple

shown in Fig ure 4G. Start ing from that sam ple, d¹³C curves for cal cite and do lo mite show a slight up wards-in creas ing trend, al - though with some fluc tu a tion, and as a re sult the top most sam - ple lo cated close to the Zechstein Lime stone-Lower Anhydrite bound ary gave the high est d¹³C val ues for that sec tion (Ta ble 1). In turn, the d¹⁸O curve for cal cite shows a de crease to ca.

–7‰ and then an in crease, with some fluc tu a tion, to about –4‰. Most prob a bly, a sim i lar trend is char ac ter is tic of the d¹⁸O curve for do lo mite (the res er va tion is due to the smaller num ber of anal y ses in a crit i cal in ter val – see Fig. 10A) and for the up - per most part of the sec tion a dis tinct in crease is char ac ter is tic, up to the max i mum value of +1.4‰ in the top most sam ple stud - ied (Fig. 10A).

The data shown in Fig ure 10B in di cate that for the cal cite the sam ples stud ied do not dif fer from other sam ples in the re - gion and for the do lo mite the sam ples from Bonikowo 2 bore - hole show more neg a tive d¹⁸O val ues.

INTERPRETATION AND DISCUSSION

The microfacies of most of the Bonikowo 2 sec tion, ex cept for its top part, are char ac ter is tic of open ma rine lime stone de - pos ited in deep shelf en vi ron ments (see Flügel, 1982: ta ble 53). They orig i nated fol low ing the rapid in un da tion of the wide -

Fig. 6. Pho to graphs of pol ished slabs

A, B – bioclastic packstone with frag ments of branched bryo zoans (white ar rows), Agathammina (yel low ar row) and cri noids (A, cen tre); C – lam i nated wackestone with quartz silt (lower part of the sam ple) and bioclastic packstone (up per part) with

strophomenid (white ar rows) and terebratulid (yel low ar rows) brachi o pods

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Fig. 7. Foraminifers in thin-sec tions from the Zechstein Lime stone of the Bonikowo 2 bore hole

A – Hemigordiopsid; B – Glomospira sp. and Nodosinelloides sp.; C – show ing the lo ca tion of Figure 8C, F; D–I – Palaeonubecularia spp.; J – Pseudoglomospira spp.; K–M, O – Agathammina pusilla; N – Hemigordius? sp.;

scale bar = 1 mm ex cept for I where scale bar = 200 mm

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spread de pres sion lo cated well be low the con tem po ra ne ous sea level and thus rep re sent the for ma tion of a ma rine ba sin in which the first Zechstein de pos its started to ac cu mu late (see D. Peryt et al., 2012).

North of the Wolsztyn palaeo-High, the Zechstein se quence be gins with the Kupferschiefer (T1) that is spo rad i cally un der - lain by the Basal Lime stone (Ca0), and when those two units are lack ing, bioclastic wackestones-packstones of the Zech -

Fig. 8. Foraminifers in thin-sec tions from the Zechstein Lime stone of the Bonikowo 2 bore hole A – Nodosaria permiana; B – Langella? sp.; C – Nodosaria thuringica; D – Pseudonodosaria lata; E, R – Langella sp.;

F–H – Nodosinelloides spp.; I, K, L, O, Q, T, U – Geinitzina postcarbonica; J, P – Geinitzina spp.; L, N – Geinitzina richteri;

M – Geinitzina cf. flabellata; S – Geinitzina? sp.; V, W – Earlandia spp.; X – Polarisella woodwardi; scale bar = 200 mm

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stein Lime stone (Ca1) over lie the Rotliegend siliciclastic de pos - its (Dyjaczyñski and Peryt, 2014). In the Bonikowo 2 bore hole, bioclastic wackestones-packstones are un der lain by recrystalli - zed car bon ate with lithoclasts that oth er wise abound in the basal Zechstein de pos its de vel oped in the ar eas where there are reefs but they are of ten en crusted by tu bu lar foraminifers (D. Peryt et al., 2012), and this is not the case in the Bonikowo 2 bore hole. This recrystallized car bon ate can be re garded as an aer o bic equiv a lent of the Kupferschiefer, based on the ex cur - sion of d¹³C val ues to wards neg a tive val ues (cf. Peryt et al., 2012a, 2015).

The depth at which the recrystallized car bon ate de pos its and bioclastic wackestones-packstones formed did not dif fer

much. Both have suf fered in ten sive bioturbation and hence they are mas sive in ap pear ance ex cept for two in stances in the lower part of bioclastic wackestone-packstone unit where centimetric in ter ca la tions of flaser-lam i nated de posit (such as that shown in Fig. 4G) oc cur. A sim i lar com po si tion of bioclasts through out the bioclastic wackestone-packstone unit, and in par tic u lar the com mon oc cur rence of cri noids in its up per part (e.g., Fig. 2) strongly sug gests no es sen tial change in sea wa ter sa lin ity at least up to the de po si tion of the lower part of the over - ly ing unit (Fig. 4B, C) as the last cri noids were re corded there.

This unit – per haps most prop erly de scribed by Smith (1986) as a het er o ge neous de posit mainly of ill-sorted oncoids (many of these show ing ev i dence of com plex growth his to ries) Microfacies, foraminifers and car bon and ox y gen iso topes in a basinal sec tion of the Zechstein Lime stone (Wuchiapingian)... 835

Fig. 9. Foraminifers in thin-sec tions from the Zechstein Lime stone of the Bonikowo 2 bore hole

A – Geinitzina? sp. and Polarisella woodwardi; B–D – Geinitzina flabellata; E – Langella sp.; F, M – Langella sp. 2; G – Pseudonodosaria lata; H – uniserial foraminifers; I, Q – Palaeonubecularia spp.; J – Midiella? sp.; K – Pseudoglomospira sp.;

L – Geinitzina? sp.; N – Nodosinelloides sp.; O – hemigordiopsid; P – Hemigordius sp.; scale bar = 200 mm ex cept for J, O, P, where scale bar = 1 mm

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and peloids with stromatolites – char ac ter izes the basinal sec - tions in the en tire Zechstein Ba sin (e.g., Szaniawski, 1966;

Füchtbauer, 1968; Tay lor and Colter, 1975; Peryt and Peryt, 1975; Smith, 1986; Becker, 2002; Hammes et al., 2013). The es ti mates of the depth at which this de posit was formed dif fer, though most au thors ad vo cate deeper wa ter con di tions (see dis cus sion in Peryt and Peryt, 2012).

The thin (8 cm) top unit in di cates an over all shallowing and in crease in sa lin ity (cf. Peryt and Pi¹tkowski, 1977), and thus ex cept for this unit, no ev i dence of ma jor dif fer ence in depth of depositional en vi ron ments and in sea wa ter sa lin ity ex ists through out the en tire Zechstein Lime stone se quence of the Bonikowo 2 bore hole. The bound ary with the Lower Anhydrite is sharp as in other con densed se quences in the re gion (e.g., Dyjaczyñski and Peryt, 2014: fig. 4A).

The Bonikowo 2 sec tion is thicker (3.4 m) than the much thin ner con densed se quence in west ern Po land (Dyjaczyñski and Peryt, 2014; Peryt et al., 2015) but this is the only char ac - ter is tic that dif fer en ti ates the Bonikowo 2 sec tion from the other se quences. As re cently com mented by Föllmi (2016), the term “con den sa tion” (and “con densed sed i ment”) are ap - plied in var i ous ways, and pro posed a set of sedimento logical, min er al og i cal and palaeo ntological cri te ria to de fine the “con - densed” beds. Both the Bonikowo 2 bore hole sec tion as well

as very thin basinal se quences of the Zechstein Lime stone, such as those de - scribed by Peryt et al. (2015), meet all cri te ria listed by Föllmi (2016) ex cept for authi - genesis be ing prac ti cally re stricted to the mi - nor pre cip i ta tion of glaucony and py rite. An im por tant fac tor that re sults in the con den sa - tion is the ac cu mu la tion rate (sed i men tary con den sa tion), and the man i fes ta tion of re - duced sed i men ta tion in the Bonikowo 2 bore - hole is the com mon oc cur rence of bored and en crusted grains in the up per part of the sec - tion (Figs. 4B, C and 5B–E).

The d¹³C val ues for cal cite and do lo mite show a slight in crease upsection (from about +3.2‰ at the base of the unit to ca. +4‰ at its top – Fig. 10A). In the case of do lo mite the in crease is more dis tinct (from about +2.4‰ at the base of the unit to ca. +4.8‰ at its top – Fig. 10A). A sim i lar upse ction in crease is char ac ter is tic of basinal sec tions both in west ern Po land (Peryt et al., 2015) and in north ern Po - land (Peryt and Peryt, 2012) and was in ter preted as due to a global in crease dur ing the time span of ca. 1 My when the Zechstein Lime stone was de pos ited. In the recrystallized car - bon ate with lithoclasts a dis tinct de crease in both d¹³C val ues is re lated to diagenesis that in gen eral leads to de crease of both val ues (Hud son, 1977).

If the high est d¹⁸O value of cal cite is re garded as re flect ing the tem per a ture of the am bi ent fluid, then the cal cu lated palaeotemperature us ing the equa tion of Ep stein et al. (1953) would yield val ues of ca. 14°C for cal cite if the d¹⁸O of wa ter = 0‰ is as sumed, 18.3°C when the d¹⁸O of wa ter = +1‰, and 22.9°C when the d¹⁸O of wa ter = +2‰ (cf. Fig. 10). The Zechstein Lime stone sea was sur rounded by a wide spread con ti nen tal area and be cause of the arid sub trop i cal con di tions (cf. Fluteau et al., 2001; Peryt et al., 2012b), the first value is clearly far be low the range es ti mated for con tem po ra ne ous sea wa ter and thus the as sumed sea wa ter d¹⁸O would have to be en riched in ¹⁸O by about +2‰.

Fig. 10A – car bon and ox y gen iso tope curves for the Zechstein Lime stone of the Bonikowo 2 bore hole; B – the plot of d¹³C and d¹⁸O val ues for the Bonikowo 2 bore hole, three other basinal sec tions in SW Po land (Czarna Wieœ 4, Koœcian 21 and Paproæ 28 bore holes, af ter Peryt et al., 2015) quad ran gle shows the field of plot ted data for the Zdrada IG 8 bore hole, north ern Po land (af ter Peryt and Peryt, 2012)

T a b l e 1 Sta tis ti cal data on sta ble iso to pic com po si tions in the Zechstein Lime stone

of the Bonikowo 2 bore hole

Num ber

of sam ples Min i mum

[‰] Max i mum

[‰] Mean

[‰] Me dian [‰]

Stan dard de vi a tion

[‰]

d¹³C cal cite 13 –1.5 3.98 1.85 2 1.5

d¹³C do lo mite 10 –2.72 4.78 1.86 2.16 2.3

d¹⁸O cal cite 13 –8.06 0.32

–

^4.51 ^–4.26 ^2.4

d¹⁸O do lo mite 10 –10.81 2.66 –3.75 –3.87 4.3

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The d¹⁸O value of cal cite is 2.6‰ lower than those of co ex - ist ing do lo mite, if both min er als pre cip i tate in iso to pic equi lib - rium at the same tem per a ture from the same so lu tion (Vasconcelos et al., 2005). The dif fer ence be tween av er age d¹⁸O val ues for cal cite and do lo mite is small (ca. 0.4‰; Ta ble 1), and the dif fer ence be tween max i mum d¹⁸O val ues for cal cite

and do lo mite (2.3‰) is close to the the o ret i cal equi lib rium value, yet the ob served dif fer ences im ply that cal cite and do lo - mite pre cip i tated ei ther at dif fer ent tem per a tures and/or from dif fer ent so lu tions. The same con clu sion was pre vi ously reached for the Paproæ 28 and Czarna Wieœ 4 bore holes which show a dif fer ence slightly above 4‰ (Peryt et al., 2015: fig. 10).

Fig. 11. Re la tion ship be tween pre cip i ta tion tem per a ture and ox y gen iso to pic com po si tion of wa - ter for dolomites (A) and cal cites (C), and its in ter pre ta tion for av er age val ues for the Bonikowo 2 bore hole (red) as well as, for com par i son, three other bore holes (af ter Peryt et al., 2015) from the basinal sec tions: Koœcian 21 (pur ple), Paproæ 28 (green) and Czarna Wieœ 4 (blue)

Col oured (red, pur ple, green or blue) ver ti cal lines re fer to max i mum d¹⁸O val ues in do lo mite and cal - cite (solid line for av er age value, and dashed line for max i mum value), ar rows point to pos si ble pre cip - i ta tion tem per a tures for as sumed ox y gen iso to pic com po si tion of wa ter (in A, dashed ar rows in di cate cal cu lated tem per a ture for max i mum d¹⁸O value as sum ing ox y gen iso to pic com po si tion of wa ter of +4‰ SMOW; in B, solid ar rows in di cate ox y gen iso to pic com po si tion of wa ter of 0‰ SMOW and dashed ar rows of +2‰ SMOW)

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The max i mum d¹⁸O value (+2.66‰) for do lo mite re flects the tem per a ture of the dolomitising fluid equal to 35°C if a d¹⁸O of wa ter = +4‰ is as sumed. The tem per a ture would in crease if the d¹⁸O of wa ter is higher: for ex am ple, 46°C when d¹⁸O of wa - ter = +6‰ (Fig. 11). These val ues can be ex pected if the early re flux model of dolomitisation is ap plied.

A com par i son of the Bonikowo 2 and Koœcian 21 bore holes shows that, in both, the do lo mite shows sim i lar max i mum d¹³C val ues (+4.78‰ in Bonikowo 2 and +4.57‰ in Koœcian 21) and d¹⁸O val ues (+2.66‰ in Bonikowo 2 and +2.5‰ in Koœcian 21), but the min i mum (and as the re sult, the mean) val ues are dis - tinctly lower in Bonikowo 2. This is due to the de pleted in ter vals that oc cur be low and above the max i mum d¹⁸O val ues, these iso to pic sig na tures re flect ing late diagenesis (cf. Sylwestrzak, 2001). They can re flect ei ther a high-tem per a ture event, which is the pre ferred in ter pre ta tion, or me te oric wa ter in flu ence, or both. Sylwestrzak (2000) con sid ered that in gen eral dolomites re lated to the Wolsztyn reefs orig i nated through mix ing as in di - cated by neg a tive d¹⁸O val ues and a par tial, ma trix-se lec tive dolomitisation, but sub se quently Jasionowski et al. (2014) con - cluded, based on low ox y gen iso to pic ra tios (d¹⁸O as low as –9‰ VPDB) and the pres ence of sad dle do lo mite, that many of the dolomites gained their pres ent iso to pic com po si tion when bur ied in rel a tively high-tem per a ture con di tions. On the other hand, no iso to pic sup port for a wa ter-mix ing mech a nism is yet doc u mented (Jasionowski et al., 2014).

IMPLICATIONS

When com pared to the se quences char ac ter ized by Peryt et al. (2015), the Bonikowo 2 sec tion shows a greater thick ness, and at the same time more com mon bioclastic packstones. The sec tion is lo cated in close prox im ity to a reef like that of the Koœcian 21 bore hole but the basinward lo ca tion of the Bonikowo 2 reef was the fac tor pro mot ing the flour ish ing of fauna due to in creased sup ply of nu tri ents.

The di ver sity and abun dance of foraminifers is mod er ately high in the Bonikowo 2 bore hole ex cept for the low er most part of the sec tion that is recrystallized car bon ate with lithoclasts re - garded as an aer o bic equiv a lent of the Kupferschiefer. Peryt and Peryt (2012 with ref er ences therein) con cluded that the pre dom i nance of lagenides through out the Zechstein Lime - stone ba sin sec tion stud ied by them (Zdrada IG 8) sug gests dysaerobic con di tions. Ear lier study of typ i cal con densed se - quences (Peryt et al., 2015) sug gested con tin ual dysaerobic con di tions, and pos si bly an el e vated sa lin ity of sea wa ter, dur - ing de po si tion of thin basinal Zechstein Lime stone de pos its.

Our tem per a ture in ter pre ta tion of the high est d¹⁸O value of cal - cite re corded in the Bonikowo 2 sec tion leads to the con clu sion

that the as sumed sea wa ter d¹⁸O would have to be en riched in

18O by about +2‰.

The sharp bound ary with the over ly ing Lower Anhydrite is due to the most com mon fea ture of evaporites: they start to pre - cip i tate im me di ately when the brines reach sat u ra tion (e.g., Caruso et al., 2016). The es sen tial in crease in sea wa ter sa lin ity thus had to oc cur dur ing the de po si tion of the high est (8 cm thick) unit (recrystallized gran u lar car bon ate) and even tu ally dur ing the up per part of the het er o ge neous de posit mainly of ill-sorted oncoids and peloids with stromatolites, above the last oc cur rence of echinoids.

The case of the Bonikowo 2 sec tions gen er ally sup ports the pre vi ous sug ges tion that the thin con densed se quences prob a - bly re cord the en tire in ter val of the Zechstein prior to the on set of evaporite de po si tion (Peryt et al., 2015). The lower part of the Bonikowo 2 sec tion is recrystallized, how ever, and this in ter val, crit i cal for strati graphic cor re la tion based on car bon iso tope stra tig ra phy, shows a de ple tion in both d val ues. Ig nor ing this de vi a tion, d¹³C val ues show a slight up wards de crease that was pre vi ously re corded in other basinal sec tions in Po land (Peryt and Peryt, 2012; Peryt et al., 2015).

CONCLUSIONS

The Zechstein Lime stone of the Bonikowo 2 bore hole sec - tion is thicker (3.4 m) than other con densed se quences in the area ad ja cent to the Wolsztyn reefs but its microfacies de vel op - ment is sim i lar. Most of the sec tion is com posed of bioclastic wackestones-packstones con tain ing com mon cri noids which strongly sug gest no es sen tial change in sea wa ter sa lin ity at least to the de po si tion of the lower part of the over ly ing unit (a het er o ge neous de posit mainly of ill-sorted oncoids and peloids with stromatolites) as the last cri noids were re corded there. The thin (8 cm) top unit in di cates an over all shallowing and in crease in sa lin ity lead ing to the onset of evaporite deposition.

The high est d¹⁸O value of cal cite re corded in the Bonikowo 2 bore hole (+0.32‰) in di cates a tem per a ture of 22.9°C when the d¹⁸O of wa ter = +2‰ is as sumed.

Foraminiferal as sem blages are dom i nated by lagenids sug - gest ing dysaerobic con di tions through out most de po si tion, ex - cept for the low er most, recrystallized part of the sec tion.

Ac knowl edge ments. The study was fi nanced by the Na - tional Sci ence Cen tre (de ci sion No. DEC - -2013/11/B/ST10/04949). We are grate ful to the Pol ish Oil and Gas Com pany for the ac cess to core and thin sec tions, to K. Ch³ódek for lo gis ti cal sup port, and to B. B³a¿ejowski, J. Zalasiewicz and an anon y mous re viewer for help ful re marks.

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