An oxygen and sulfur isotopic study of gypsumfrom the Wapno Salt Dome cap-rock (Poland)

Geo log i cal Quar terly, 2010, 54 (1): 25–32

An ox y gen and sul fur iso to pic study of gyp sum from the Wapno Salt Dome cap-rock (Po land)

Joanna JAWORSKA

Jaworska J. (2010) – An ox y gen and sul fur iso to pic study of gyp sum from the Wapno Salt Dome cap-rock (Po land). Geol. Quart., 54 (1):

25–32. Warszawa.

The sta ble iso tope com po si tions of ox y gen and sul fur from gyp sum sam ples de rived from bore hole cores of the Wapno Salt Dome cap-rock have been de ter mined. The d¹⁸O val ues were mea sured (for both the SO4 and the H2O) in bulk sam ples of gyp sum (CaSO4·2H2O) and in its de hy drated equiv a lents (CaSO4). The SO4 is en riched in the heavy ox y gen iso tope, which un equiv o cally in di - cates the Zechstein sea as the place of or i gin of the sul fate. The vari able d¹⁸O val ues of the H2O (wa ter of crys tal li za tion) in di cate dif fer - ent con di tions (and stages) of diagenetic pro cesses of these sam ples, pri mar ily recrystallization (hydration) ep i sodes. Two groups of gyp sum were dis tin guished: (1) a group in which the d¹⁸O val ues of H2O range be tween –9.0 and –10.2‰ (in ter preted as gyp sum in equi - lib rium with re cent or subrecent me te oric wa ter), (2) a group in which the d¹⁸O val ues of the H2O were less than –10.2‰ (this gyp sum has recrystallized in the pres ence of wa ter en riched in the light iso topes of ox y gen). In ad di tion, in one sin gle anal y sis the d¹⁸O value of the H2O amounts to –5.6‰ (gyp sum with mixed iso to pic com po si tion).

Joanna Jaworska, In sti tute of Ge ol ogy, Adam Mickiewicz Uni ver sity, Maków Polnych 16, PL-61-606 Poznań, Po land, e-mail:

veronika@amu.edu.pl (re ceived: Sep tem ber 04, 2009; ac cepted: Jan u ary 12, 2010).

Key words: Wapno Salt Dome, cap-rock, gyp sum, ox y gen and sul fur iso topes.

INTRODUCTION

Cap-rocks are in te gral and im por tant parts of salt bod ies.

They over lie salt domes, re sult from mul ti ple pro cesses and have a long diagenetic his tory (e.g., Posey and Kyle, 1988;

Prikryl et al., 1988; Werner et al., 1988; Jaworska and Ratajczak, 2008). On top of the salt diapirs (salt mir ror area), salt rock co mes into con tact with wa ter that is fresh or NaCl

-undersaturated, and the rock salt is dis solved; this is the salt dis so lu tion zone. Rock salt is com monly im pure, con tain - ing anhydrite in par tic u lar, but also clay min er als and car bon - ates. On top of salt domes there fore anhydrite (in the form of anhydrite sand, later anhydrite sand stone) is left with the rest of the less sol u ble min er als. Diagenetic pro cesses trans form the an hyd rites by hydration into gyp sum, and com pac tion re sults thus in a rocky cover, the so-called cap-rock. The youn gest lev - els of the cap-rock are al ways at its base, and the old est ones are found at its top, in an in verted stra tig ra phy.

Since the end of the 1970s, salt domes have been thor - oughly in ves ti gated and ex plored as po ten tial re pos i tory sites for ra dio ac tive waste and for un der ground stor age of oil and gas by the pe tro leum in dus try (Jack son and Seni, 1984; Kreitler

et al., 1985; Posey and Kyle, 1988; Werner et al., 1988;

Bornemann and Bräuer, 1999; Klinge et al., 2002; War ren, 2006). Cap-rocks are nat u ral cov ers and usu ally pro tect salt bod ies quite well against ero sion (ground wa ter dis so lu tion) al - though not al ways (Zuber et al., 2000).

The iso to pic com po si tion of the sul fates and wa ter of crys - tal li za tion that build gyp sum may pro vide in for ma tion about the con di tions and stages of hydration of the sul fates, and help un der stand the pro cesses that oc cur near the salt mir ror, par tic - u larly the wa ter/sul fates in ter ac tions that are re spon si ble for the cre ation and evo lu tion of the cap-rock. Re sults of cap-rock re - search may there fore help to de cide whether a spe cific cap-rock is sta ble and dry enough to act as a “pro tec tor” of the salt body beneath. A good ex am ple of such an in ves ti ga tion is Kreitler and Dutton’s (1983) study of the Gyp Hill and Oakwood Domes in Texas.

GEOLOGICAL SETTING

The Wapno Salt Dome is one of the small est diapirs in the

Pol ish Low land; it is lo cated about 70 km to the north-east of

Poznań, be tween the Pom er a nian Swell (which forms part of the

Mid-Pol ish Swell; Fig. 1) and the Mogilno Trough. It con sists of two el e ments: a salt stock and a gyp sum cap-rock. The salt dome is 1000 m long and approx. 600 m wide and its sur face area is 0.5 km

; the salt mir ror is lo cated at a depth of 160–180 m be low the sur face (Jaworska and Ratajczak, 2008). The salt form ing the Wapno Salt Dome co mes from a depth of approx. 6000 m (Marek and Raczyńska, 1974) and pierces through the en tire Me so zoic and al most all the Ce no zoic. The cen tral part of the diapir is built by Zechstein (late Perm ian) evaporites: the older salt rocks, be long ing to PZ2, rep re sent 90% of all salt rock in the diapir (Jaworska and Ratajczak, 2008).

The or i gin and de vel op ment of the Wapno Salt Dome is re - lated to the sub si dence and in ver sion of the Mid-Pol ish Trough (Dadlez et al., 2005; Guterch and Grad, 2006; Krzywiec et al., 2006). The trough was in verted dur ing the Late Cre ta ceous and Paleocene. This re gional antiform struc ture, re ferred to as the Mid-Pol ish Swell, is out lined by the Ce no zoic subcrop of Cre - ta ceous and older rocks (Dadlez et al., 2005; Guterch and Grad, 2006; Krzywiec, 2006).

The Wapno Salt Dome is cov ered by a cap-rock; the thick - ness of the cap-rock ranges from 20 to 160 m (Jaworska and Ratajczak, 2008). The cap-rock, which crops out at two places, is cut by a deep trough/fur row (the C1 bore hole area; Figs. 2 and 3) reach ing a depth of approx. 130 m (Jaworska and Ratajczak, 2008).

The main (al most ex clu sive) com po nent of the cap-rock is gyp sum, which oc curs in vari able shape and size. It forms auto-, hypidio- and xenomorphic crys tals, and has fine-, me - dium and coarse-crys tal line tex tures (Fig. 4A and B). Two spe - cific forms of gyp sum, i.e. len tic u lar and sel e nite gyp sum (Fig. 4C and D), were also ob served. Auto- and hypidio-, coarse- to me dium-crys tal line gyp sum dom i nates near the bot - tom of the cap-rock. The cap-rock changes up wards grad u ally to be come fine and xenomorphic. The Wapno cap-rock still con tains some small amounts of anhydrite. These are com -

monly rem nants of larger anhydrite crys tals, now form ing in - clu sions within large gyp sum grains. In one case, how ever, anhydrite is the main com po nent of the cap-rock: it con sti tutes a 20-cm thick layer of anhydrite sand stone at the bot tom of bore hole C1, at a depth of 157.5 m be low the sur face, just above the salt mir ror. Car bon ates, automorphic quartz crys tals and iron sul fides oc cur also in small amounts (Jaworska and Ratajczak, 2008).

Fig. 1. Sim pli fied geo log i cal map of NW and cen tral part of Po land with out Ce no zoic and lo ca tion of Wapno Salt Dome (af ter Marek and Leszczyński in Narkiewicz, 1997)

Fig. 2. Subsurface range of the Wapno Salt Dome (af ter Jaworska and Ratajczak, 2008; changed by au thor) show ing the lo ca tion

of bore holes and trough

MATERIAL

Eight rep re sen ta tive gyp sum sam ples (cho sen from the 46-sam ple col lec tion) from 5 bore hole cores: C1, 270’, 271, 269, 278’ (Figs. 2 and 3) were an a lyzed for their d

O and d

S val ues. The sam ples re flect the full va ri ety of gyp sum tex tures (het ero-, fine-, me dium-, coarse-crys tal line, len tic u lar and sel e - nite gyp sum; Fig. 4) and come from dif fer ent depths of the cap-rock – from 18.5 to 154–157 m.

METHODS

The gyp sum sam ples were an a lyzed by Dr. S. Poulson at the Uni ver sity of Ne vada (De part ment of Geo log i cal Sci ences) for their d

O and d

S val ues. The iso tope anal y ses were per formed in a con tin u ous flow mode, us ing an Eurovector model 3000 el e - men tal an a lyzer to a Micromass IsoPrime sta ble-iso tope ra tio mass spec trom e ter, fol low ing two meth ods. The first was used to

de ter mine the d

S value (cf. Giesemann et al., 1994); V

O

was added to the sul fate sam ples as a com bus tion aid; the anal y ses are re ported as per mil rel a tive to the Vi enna-Can yon Diablo Troilite stan dard (V-CDT) and have un cer tain ties of ±0.2‰. The sec ond method (cf. Kornexl et al., 1999) was used for the d

O anal y sis;

nickelized graph ite was added to each sam ple to pro mote CO for ma tion; the anal y ses are re ported as per mil rel a tive to the Vi - enna-Stan dard Mean Ocean Wa ter (V-SMOW) and have un cer - tain ties of ±0.4‰.

The d

O val ues were mea sured for both the bulk sam ple (CaSO

2 · H

O) and for its de hy drated form (CaSO

res i due af - ter de hy dra tion); the gyp sum pow der was slowly heated un der vac uum to 450°C for 30 min utes (see Dowuona et al., 1992).

The iso to pic com po si tion of the ox y gen in the gyp sum is a mix - ture of d

O com ing from SO

and H

O (wa ter of crys tal li za - tion). The sul fates stud ied (gyp sum sam ples) were there fore also an a lyzed twice, in or der to in ves ti gate the dif fer ences of the d

O con tent in both com po nents. The d

O val ues in the SO

were de tected di rectly by mass spec trom e try, whereas the iso to pic com po si tion was de ter mined in di rectly in the case of

An oxygen and sulfur isotopic study of gypsum from the Wapno Salt Dome cap-rock (Poland) 27

Fig. 3. Ana lysed core-pro files (af ter Jaworska and Ratajczak, 2008; changed by au thor)

the wa ter of crys tal li za tion), by cal cu lat ing the mean d

O value of the bulk gyp sum sam ple (CaSO

· 2H

O) and of its SO

com - po nent (from the res i due af ter de hy dra tion).

RESULTS

OF THE ISOTOPE ANALYSES

Sul fur. The iso to pic anal y ses of the sul fur in the gyp sum sam ples in ves ti gated show a range from 11.2 to 12.5‰ and thus they in di cate that these sul fates are dis tinctly en riched – with re spect to V-CDT – in the heavy iso tope (mean d

S value is 11.7‰; see Ta ble 1, Figs. 5 and 6).

Ox y gen. The d

O value in SO

from the Wapno gyp sum sam ples ranges be tween 11.7 and 14.0‰ (mean 12.8‰; see Ta ble 1, Fig. 5). The d

O val ues in the wa ter of crys tal li za tion from the Wapno gyp sum sam ples are com pletely dif fer ent and range be tween –5.6 to –13.1‰ (Fig. 6).

Fig. 4. Pho tos from thin sec tions of gyp sum sam ples

A – coarse-crys tal line gyp sum (271-5); B – fine- to midium-crys tal line gyp sum (278’-4);

C – len tic u lar gyp sum (C1-12); D – sel e nite gyp sum (270’-3); sected nicols

Fig. 5. Val ues of d³⁴S vs. d¹⁸O of re sid ual sul fate For sam ple de scrip tion and lo ca tion see Ta ble 1

DISCUSSION

SULFUR

Sul fur in sul fates very well pre serves its pri mary iso to pic com po si tion, in clud ing dur ing diagenesis as the chem i cal evo - lu tion of sul fates (hydration/de hy dra tion) does not in volve sig - nif i cant iso to pic frac tion ation; nor does chem i cal sul fate re duc - tion (Worden et al., 1997). Weath er ing does not lead to con sid -

er able frac tion ation of sul fur iso topes ei ther (Hoefs, 2004).

Pres ent-day sul fates in oce anic wa ter are char ac ter ized by a fairly con stant

S/

S ra tio, with val ues that reach +20 ±0.5‰

with re spect to V-CDT (Pi erre, 1988); the frac tion ation be - tween dis solved sul fates in oce anic wa ter and gyp sum or anhydrite is neg li gi ble (Thode and Mon ster, 1965; Raab and Spiro, 1991). In the past, this re la tion ship changed con tin u - ously, but the gen eral trends of the d

S value in the geo log i cal past (S-iso tope age curve) is known, so that it is pos si ble ap - prox i mately to de fine the time of evaporite crys tal li za tion (e.g., gyp sum; see Claypool et al., 1980; Strauss, 1999).

The value d

S (11.7‰) in the gyp sum sam ples in ves ti - gated from Wapno is char ac ter is tic of Perm ian evaporites; for in stance, the sul fur iso tope data for Zechstein evaporites from N Ger many range from 9.8 to 12.0‰ (Kampschulte et al., 1998) and those for Zechstein evaporites from SW Po land range from 9.4 to 13.3‰ (Vovnyuk and Czapowski, 2007).

This im plies that the iso to pic com po si tion of the sul fur in these sam ples, tak ing into con sid er ation the frac tion ation fac tor, amount ing to 1.65‰ (Thode and Mon ster, 1965; Pi erre, 1988), re flects the iso to pic com po si tion of the Zechstein brines.

OXYGEN

Anal y sis of the ox y gen-iso tope com po si tion of gyp sum is more com pli cated, be cause ox y gen is pres ent in the form of SO

and of wa ter of crys tal li za tion. Ini tially, the gyp sum and its wa ter of crys tal li za tion were formed in iso to pic equi lib rium with the mother brine (Sofer, 1978), but the gyp sum later eas ily loses its orig i nal wa ter of crys tal li za tion (pri mary com po si tion of H

O) dur ing con tact with sea or ground wa ter. Changes in the d

O of wa ter of crys tal li za tion may thus arise from re peated phases of de hy dra tion and hydration (some times many times), by ab sorp tion of “fresh” wa ter.

Pres ent-day oce anic wa ter con tains sul fates of which the d

O val ues amount to 9.5 ±0.5‰ with re spect to V-SMOW (Longinelli and Craig, 1967). Dur ing recrystallization of the oce anic sul fates, the d

O is raised by 3.5‰ (the fac tor of frac -

An oxygen and sulfur isotopic study of gypsum from the Wapno Salt Dome cap-rock (Poland) 29

Sam ple num ber

and depth Type of gyp sum rock d¹⁸O (‰, V-SMOW)

in CaSO4 · 2H2O d¹⁸O (‰, V-SMOW) in CaSO4

d¹⁸O (‰, V-SMOW)

in H2O d³⁴S (‰, V-CDT)

1 C1-3

140 m het ero-crys tal line 6.0 14.0 –10.0 12.5

2 C1-12

154–157 m len tic u lar

(hypautomorphic) 6.2 12.1 –5.6 11.8

3 269-1

40 m coarse-crys tal line 4.5 13.3 –13.1 11.5

4 270’-3

45.5 m sel e nite 4.8 13.1 –11.8 11.7

5 271-1

18.5 m me dium-crys tal line 4.3 12.0 –11.1 11.8

6 271-5

34.5 m coarse-crys tal line 5.4 12.6 –9.0 11.2

7 271-7

47.5 m coarse-crys tal line 4.7 11.7 –9.3 11.6

8 278’-4 46.5 m

fine-crys tal line

and me dium-crys tal line 5.5 13.3 –10.1 11.5

T a b l e 1 Re sults of iso tope anal y sis

Fig. 6. Val ues of d³⁴S vs. d¹⁸O of wa ter of crys tal li za tion For sam ple de scrip tion and lo ca tion see Ta ble 1

gyp sum sam ples (dis tinctly en riched in the heavy iso tope of ox y gen, mean d

O value is 12.8‰) in di cates the Zechstein evaporite ba sin as the en vi ron ment of the sul fate crys tal li za tion and is con sis tent with the “ma rine” or i gin of these sul fates.

On the other hand, the d

O val ues in the wa ter of crys tal li - za tion from the Wapno gyp sum sam ples (ranges be tween –5.6 to –13.1‰) means that the wa ter of crys tal li za tion is not in iso - to pic equi lib rium with the mother brine. The vari able d

O val - ues of the H

O in di cate that the sul fates have had con tact with me te oric and/or ground wa ter (en riched in light iso topes) and in di cate dif fer ent stages of diagenetic pro cesses, primarily recrystallization (hydration) of the sul fate. These anal y ses are com pa ra ble to the anal y ses of wa ter of crys tal li za tion from the cap-rock of the Gorleben Salt Dome (Her bert et al., 1990), where the d

O value in the H

O of the gyp sum sam ples ranges be tween –5.2 and –12.4‰.

In the Pol ish Low land area, the mean iso tope com po si tion of ox y gen in me te oric wa ter and in re cent in fil tra tion wa ter ranges be tween –9.2 and –10.2‰ (D’Obryn et al., 1997;

Duliński et al., 1997). The wa ter sup plied dur ing cold pe ri ods (such as Pleis to cene melt wa ter) is en riched in the light iso tope of ox y gen and its d

O val ues are more neg a tive than in the wa - ter from warm pe ri ods such as the Paleogene and Neo gene, which is en riched in the heavy iso tope of ox y gen (Duliński et al., 1997). The sta ble-iso tope com po si tion of wa ter col lected from the bore holes in the Wapno area is slightly richer in the heavy iso tope of ox y gen than ex pected for in fil tra tion wa ter in this part of Po land (Górski and Rasała, 2008). The d

O val ues of wa ter from the C1 bore hole range be tween –8.5 and –8.7‰

with re spect to V-SMOW. This dis crep ancy is prob a bly caused by a small ad mix ture of wa ter com ing from the in clu sions of dis solved rock-salts or al ter na tively re sults from the pres ence of crys tal li za tion wa ter re leased from dis solved evaporite min - er als (Duliński et al., 2001).

The anal y ses of the d

O of wa ter of crys tal li za tion of the Wapno gyp sum sam ples sug gest the pres ence of two groups (Fig. 6): (1) a group in which the d

O val ues of the H

O lie be - tween –9.0 and –10.2 ‰ (this is the gyp sum which came into

of wa ter sup plied dur ing cooler time spans such as – pos si bly – the Pleis to cene). In ad di tion, one anal y sis shows a d

O value of H

O of –5.6‰ which is prob a bly the ef fect of par tial mix ing of iso to pic “ma te ri als”, pos si bly of Zechstein age with more re - cent ones. The high con tent of heavy ox y gen iso topes sug gests the be gin ning of the recrystallization pro cesses.

CONCLUSIONS

Sum ming up the re sults, the fol low ing con clu sions can be drawn.

1. The ox y gen-iso tope com po si tion (in both sul fate and wa - ter of crys tal li za tion) of the gyp sum is a highly use ful di ag nos - tic tool for iden ti fi ca tion of the source of the sul fate and of its diagenetic his tory.

2. The d

O and d

S mea sured in the SO

pre served its pri - mary Zechstein iso tope com po si tion de spite diagenesis.

3. The iso tope com po si tion of the crys tal li za tion wa ter in di - cates a prob a bly Pleis to cene char ac ter of the diagenetic pro - cesses.

4. Me te oric (prob a bly re cent/postglacial) wa ter from near the sur face and out side the cap-rock quite eas ily pen e trates the cap-rock, which does not form a tight, her metic cover, but is cut by fis sures. Con se quently, the cap-rock of the Wapno Salt Dome does not ful fill the re quire ments for a seal which pro tects the rock salt and salt mir ror against in flows of fresh wa ter. This find ing may have con se quences for salt struc tures which are pre pared for un der ground dis posal of ra dio ac tive waste or for the stor age of hy dro car bons.

Ac knowl edg ments. The study was sup ported by grant no.

0385/PO4D 008421 from the Pol ish State Com mit tee for Sci en - tific Re search (KBN). The au thor ac knowl edges the late Prof. J. Głazek for help and in spi ra tion in pre par ing this con tri - bu tion and prof. UAM dr. A. van Loon for re vi sion of the Eng - lish manuscript.

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An oxygen and sulfur isotopic study of gypsum from the Wapno Salt Dome cap-rock (Poland) 31