Geo log i cal Quar terly, 2007, 51 (1): 67–78

Or i gin of siderites from the Lower Ju ras sic Ciechocinek For mation from SW Poland

Paulina LEONOWICZ

Leonowicz P. (2007) — Ori gin of siderites from the Lower Juras sic Ciechocinek Form ation from SW Po land. Geol. Quart., 51 (1): 67–78. Warszawa.

Si der it ic rocks, which are char ac ter is tic con stit u ents of muddy-silty de pos its of the Lower Ju ras sic Ciechocinek For ma tion, oc cur com - monly as lay ers, lenses and small irreg u lar concre tions com posed of sider ite mudstone and siltstone as well as less com mon lenses of si - der itic sand stones. Three types of sid erite cem ent were ob served in thin sec tions: fine-crystal line va riety (SF), coarse-crys tal line rhom bo he dra (SR) and fine-crystal line biogenic aggre gates (SA). In all these types BSE anal y sis re vealed compositional zonation of crys- tals, with inter nal parts enric hed in Mg, Mn and Ca and outer zones al most pure sider ite. d13C val ues and chem i cal com po si tion of siderites com bined with the pres ence of early diagenetic py rite in di cate that sid er ite crys tal lized from brack ish ma rine-de rived so lu tions; only in the case of two sam ples from the lower part of the Ciechocinek Form ation the freshwa ter ori gin cannot be excluded. Mn and Fe were supplied by rivers and releas ed by Fe- and Mn-reduc tion in suboxic zone, whereas Mg and Ca were derived from sea wa ter, which infil trated into the sedi m ent. Crystal zonation resulte d from the diagenetic evolu tion of pore water as the sedi m ent was buried. Sim ilar com po si tion and de vel op ment of sid er ite crys tals from dif fer ent parts of one layer in di cate that sid er ite pre cip i tated si mul ta neously throughout the whole hori zon. Precip i ta tion began from the form ation of num erous nuclei and contin ued by growth of crystals onto them. It could begin alrea dy in the iron reduc tion subzone and contin ued in the sulphate reduc tion and methanogenesis zones.

Paulina Leonowicz, In sti tute of Ge ol ogy, Uni ver sity of War saw, ¯wirki i Wigury 93, PL-02-089 Warszawa, Poland, e-mail: [email protected] (Febru ary 9, 2006; accepte d: Febru ary 10, 2007).

Key words: Cra cow-Silesian Upland, Lower Juras sic, siderites, geochem istry , C and O isotopes, palaeosalinity.

INTRODUCTION not yet been stud ied in de tail. This is the case of the siderites from the Ciechocinek Form ati on, which unti l now have merely been menti oned in general petro graph i cal studies on the Polish Sid er ite is a com mon con stit u ent of epicontinental Lower Li assic (Teofilak-Maliszewska, 1967, 1968; Krystkiewicz, Ju ras sic de pos its, orig i nated in the ex ten sive Cen tral Eu ro pean 1999). In spite of their small econom ical im portanc e, miner al - Basin, which covered in Juras sic time a vast part of Europe. Si - og i cal and geo chem i cal com po si tions of these siderites are der it ic min er al iza tion, which takes form of con cre tions and lay - worth studying, as they can be used in envi ron m ental inter pre- ers in ar gil laceous rocks as well as cem ents and in ter cala tions ta tions and re con struc tion of diagenetic se quences. in oolitic ironstone s, has been re ported from many local iti es in The main pur pose of this pa per is to out line the ori gin of Great Britain, NE France, Luxem bourg and NW Germ any siderites from the Ciechocinek Form ati on, which exposes in (Schellmann, 1969; Raiswell, 1971; Sellwood, 1971; Siehl and the Cra cow-Silesian Upland, betwe en Czêstochowa and Thein, 1989; Spears, 1989; Mücke and Farshad, 2005 and Wieluñ (Fig. 1B, C) as well as ver ify ing whether their chem ical many others). Sider it ic rocks are also com mon in the Pol ish Li- and iso to pic com po si tion re flects the chem is try of wa ter in the assic occur ring in both marine and freshwater depos it s. These sed im en tary ba sin. In the Early Ju ras sic time Po land was situ - are mainly con cre tions and in ter ca la tions of sid er ite ated in a marginal part of the Centra l-Euro pean Basin (Fig. mudstones, in which some dis persed chamosite ooids were ob- 1A), in which dis placem ents of shoreli ne, caused by sea level served (Teofilak-Maliszewska, 1967, 1968). Some of the sider - fluc tu a tions, oc curred quite fre quently, re sult ing in a vari abil ity ite occur rence s, like those from the upper Hettangian of sed i men tary en vi ron ments (Dadlez, 1969; Pieñkowski, Przysucha Ore-Bearing Form ati on in the Holy Cross Mts. re- 2004). Sed i ments de pos ited there are rather mo not o nous and gion, were exploi ted for steel indus try already in the 16th cen- consis t of inter calat ing sands, silts and muds of both marine tury and their pe trol ogy was rec ognize d and descri bed in many (mainly brack ish-ma rine) and fresh wa ter or i gin. Changes of papers. Some others are of no econom ical im portanc e and have sed i men tary en vi ron ment are not al ways dis tinctly marked in 68 Paulina Leonowicz

Fig. 1. Loca tion of the study area A — Pol ish Ba sin as a part of the Central-Eu ro pean Ba sin in the Early Ju ras sic (af ter Pieñkowski, 2004), B — epicontinental Lower Ju ras sic in Po land (af ter Dadlez, 1973 and Pieñkowski, 2004, sim plified), C — loca tion of studied pro files on a geolog i cal sketch-map of the Czêstochowa-Wieluñ region; boreholes: 1 — Praszka 1, 2 — Przystajñ 2, 3 — Wrêczyca 3, 12 — Nowa Wieœ 12, 15 — Gorzów Œl¹ski 15, 16 — Gorzów Œl¹ski 16, 20 — Przystajñ 20, 25 — Przystajñ 25, 36 — Pogorza³ki 36, 40 — Paw³owice 40, 45 — Bór Zajaciñski 45, 46 — D¹browa 46, 49 — Skroñsko 49, 50 — Wichrów 50 the profil es and the lack of indic a ti ve fauna makes it someti mes dif fi cult to dis tin guish fine-grained brack ish and fresh wa ter de- pos its. In such cases geo chemi cal anal ysis of siderites, which re cord chem is try of pore wa ter pres ent dur ing their pre cip i ta- tion, may be helpful.

CHARACTERISTICS OF THE CIECHOCINEK FORMATION

The Ciechocinek Form ati on is com posed of grey, ol ive and wil low-green, poorly con sol i dated mudstones and siltstones with lenses and sub or di nate in ter ca la tions of fine-grained sands and sandstone s (Teofilak-Maliszewska, 1967, 1968; Leonowicz, 2005). The sedi m ents often contai n diagenetic sid- er ite in ter ca la tions and con cre tions as well as py rite con cre - tions. Sedi m enta ti on of these de posit s was linked with the ma- rine transgres sion, which did not result in fully-marine condi - tions in the Pol ish Basin. Based on the presence of Diplocraterion burrows, fauna of Estheria, some ag glu ti nat ing foraminifera and dinoflagellate cysts as well as the lack of typ i- cal marine fauna, the en vi ron ment was defined as a brack - ish-ma rine (Pieñkowski, 2004). Pieñkowski (op. cit.) de scribed the sed im en tary ba sin as a large, shal low embayment with some deltaic fa cies in marginal parts. Dadlez (1969) and

Fig. 2. Lithostratigraphy of the Lower Ju ras sic in the Cra cow-Silesian re gion (af ter Deczkowski and Daniec, 1981; Kopik, 1998; Pieñkowski, 2004) and the repre sen ta tive lithological pro file of borehole Wrêczyca 3; lacking part of core sup plem ented after Pieñkowski (2004) Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland 69

Pieñkowski (op. cit.) link this trans gres sion with the early MATERIAL AND METHODS Toarcian time, when the transgressional trend was pronounced throughout the whole Central-Euro pean Basin. The most pre - Succes sions exposed in two clay-pits, belong ing to the cise dating of the Ciechocinek Form ati on, based on “Cerpol-Koz³owice” En ter prise and “Boroszów” Brickyard, as dinoflagellate cysts, pointed to the late Pliensbachian–early well as 14 boreholes , drilled by the Polish Geolog i cal Insti tut e Toarcian age (margaritatus–tenuicostatum zones) (Barski and (Fig. 1C), were de scribed and sampled. Sedimentological de- Leonowicz, 2002). However, taking into ac count sequence scripti ons of the depos it s as well as obser va ti ons of lithol ogy, stra tig ra phy cor re la tion and result s of macrospore analy sis dim ension and shape of sider it ic bodies were made. Samples of (Marcinkiewicz 1957, 1960, 1964, 1971), Pieñkowski (2004) sid er ite mudstones, siltstones and sand stones were col lected proposed to place it within the lower Toarcian. from differ ent parts of the Ciechocinek For ma tion pro file for In the Cra cow-Silesian Upland the Ciechocinek For ma tion pet ro log i cal in ves ti ga tions. over lies sand stones and sub or di nate mudstones of the Around 30 samples were selected for de tailed ex am ina tion Blanowice For mation and is over lain by the lithologically simi - un der po lar iz ing mi cro scope, which in cluded gen eral de scrip- lar Borucice For ma tion (Fig. 2). Both of them were depos it ed tion of the rock texture , analy sis of devel op m ent and volum e of mainly in al lu vial and lac us trine en vi ron ments (Pieñkowski op. sid er ite cem ent as well as its re lation to the grain framewor k and cit.). The lower and upper boundari es of Ciechocinek For ma- other ce ments. Se lected samples of sand stones and siltstones tion are disti nct in these profil es where it is confine d by ero- were point-counted to esti mate the cem ent vol ume and poros it y. sional surface s. Loca ti on of these boundari es is less precis e and From the set of samples exam ined under polar iz ing micro - someti mes approx i m ate when the change of depositional envi - scope, 6 were selec ted for carbon and oxy gen isotope analy sis ron ment was gradual. (Fig. 3). Grain size of detri tal framewor k, tex ture of sid er ite ce-

Fig. 3. Cor rela tion of se lected mea sured sec tions with lo ca tion of stud ied samples Sim pli fied lithostratigraphic divi sion, in which Ciechocinek For ma tion in cludes de pos its mark ing tran si tion be- tween conti nen tal and marine envi ron m ents, is marked; refer to Figure 3 for sec tion loca tions; lacking parts of cores sup ple mented af ter Pieñkowski (2004) and Kie¿el (1990, unpubl.); other expla na tions as in Figure 2 70 Paulina Leonowicz ment and posi ti on in the profil e as well as the lack of other car- ence of diagenetic py rite crys tals, un de tect able us ing the op ti cal bonate cem ents was taken into ac count. Addi ti onall y, 5 mi cro scope. Ob ser va tions were car ried out on a Tesla BS 301 sub-samples were collected from the K-9 sample, rep re sent ing scan ning elec tron mi cro scope. sid er ite mudstone layer from Koz³owice outcrop, to deter mine From the same 6 sam ples, 2 (K-4 and NW 12/8) were se - whether there was any vari ati on in isoto pic com po siti on of sid- lected for back scat tered elec tron (BSE) and wave length er ite ce ment in dif fer ent parts of the layer. To iden tify the min- dispersive system (WDS) anal yses. Grain size of detri tal par ti- eral com po si tion of siderites cho sen for iso to pic anal y sis, cles and the tex ture of sid er ite cem ent were record ed. Samples bulk-rock XRD anal yses were car ried out. Samples were ana - were ex am ined to de ter mine the quan ti ta tive el e men tal com po - lysed in air-dried condi ti ons, with a step size of 0.04° 2q and 4 s si tion of sid er ite crys tals. Pol ished thin sec tions were an a lyzed counting time on a DRON 2.0 diffractometer. Co-Ka ra di a tion us ing Cameca SX 100 electron microprobe equipped with a and a Fe-filter were used. Then, siderites were crushed to a fine back scat tered electron de tec tor, at 15 kV with a 10–40 nA dry powder and left to re act with anhy drous phospho ric acid: beam cur rent. Re sults of chem ical anal yses are ex pressed in sam ples K-4, K-9, K-26, P40/9 for 4 hours at 100°C and sam - mol per cent ages. ples NW12/8, P2/7 for 72 hours at 50°C. The re sul tant CO2 was pu ri fied by cryo genic dis til la tion and an a lyzed on a Finnigan MAT Deltaplus spec trom e ter. Re sults are re ported as OBSERVATIONS AND RESULTS d13C de vi a tion rel a tive to VPDB stan dard and d18O rel a tive to SMOW standard. DESCRIPTION OF SIDERITE BODIES Sam ples an a lyzed for isoto pic com posi ti on were also stud- ied by scanning electron micro scope (SEM) in order to recog - Si der it ic min er al iza tion oc curs in all types of Ciechocinek nize the tex ture of sid er ite cem ent and to de ter mine the pres - Form ati on depos it s. The most com mon are 1 to 20 cm thick layers and lenses of sid er ite mudstones and siltstones, which oc- cur within muddy-silty suc ces sions (Fig. 4A). The grain size of de trital par ticles in these lay ers and lenses is usuall y the same as in the host rock. Some times min er al iza tion oc curs also in coarser-grained in ter cala - tions, how ever, these cases are less com mon. More over, where sand and mud in ter ca la tions ap pear, it is usu- ally mud which under went sideritization. Sid er ite lay ers usu ally show lateral per sis tence for sev eral hundreds of metres, which is possi - ble to observe in a single out crop. Cor re la tion of bore hole pro files showed, however, that the layers were in fact ex ten sive, flat lenses random ly distri buted in the profil e. In some parts of the succes sion, sid- erit e is so abundant that it does not con cen trate in sep a rate lay ers but oc - curs throughout the whole sedi m ent, which becomes heavy and grey-brown in colour. Such accu m u - la tions of si der it ic mudstones, up to 16 m thick, were observe d in the lower part of Nowa Wieœ 12 and Wrêczyca 3 cores. In the case of sands, al most ex - clusive ly thin, up to 5 cm thick, Fig. 4. Siderites from the Ciechocinek For ma tion lenses un der went sideritization. Most of them show well pre served A — layer of sid erite mudstone in muddy-silty depos its; B — sand layers with thin lenses and small oval cross-lam i na tion, how ever, there are and ir reg u lar con cre tions, com posed of sid er ite mudstone; C — strongly deform ed shrinkage crack and small, flat tened bur row (black ar row) in cross-section; D — slightly flat tened burrow of Planolites isp.; also load-type sandy lenses with E — oval and discoidal concre tions of sider ite mudstone, col lected from the sand layer, scale in cm; A, B, strongly de formed lam i na tion. Rel a- D, E — Koz³owice outcrop, C — Wrêczyca 3 borehole, depth 111.5 m tively thick and more exten sive sandy Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland 71 units are usuall y poorly consol i date d and sid er ite min er al iza tion is poorly devel oped, taking the form of small aggre gate s growing on sand grains. How ever, in such de pos its, in ter ca la- tions and lenses as well as small oval, discoidal and ir reg u lar con cre tions, com posed of sid er ite mudstones and siltstones, com monly occur (Fig. 4B, E). The latter struc tures, 1 to 10 cm long, are often arranged in hori zons which lat er ally pass into the con tin u - ous sid er ite lay ers. Si der it ic rocks dif fer from sur- rounding depos it s in their greater densit y and hardness as well as in the brown, rusty or cherry-red col our of weath ered sur faces. Small bur rows, mainly of the Planolites isp., occur - ring in these rocks are usuall y only slightly flat tened (Fig. 4D), however, there are also burrows which show len tic u lar, strongly flat tened cross-sec tions (Fig. 4C), re sult ing from the sig nif i cant com pac tion. Also un com mon shrink age cracks, which were observe d in siderites from Wrêczyca 3 and Wichrów 50 cores, show strong defor m ati ons vis - i ble in cross-sec tion (Fig. 4C).

PETROLOGY OF SIDERITE

Ex am i na tion of thin sec tions re- vealed three types of si der itic ce- ment: fine-crys tal line va ri ety (SF), coarse-crys tal line rhom bo he dra (SR) and fine-crys tal line ag gre gates im - pregnate d by mixture of unde ter - mined Fe hydrox ides (SA). Fig. 5. Dif fer ent va ri et ies of sid er ite ce ment Fine-crys tal line sid er ite (SF) oc- curs mainly as xenomorphic crys tals, A–C — fine-crys tal line va ri ety (SF): A — gran u lar ag gre gates of xenomorphic crystals, sam ple P40/9, 1–15 mm in di am e ter, of ten com- one nicol; B — microcrystalline rhom bohe dra, sam ple NW12/8, SEM im age; C — well-pre served, coalified plant de bris in the thin in terca la tion of sid erite mudstone, sample K-9, one nicol; D — euhedral bined in gran u lar ag gre gates rhom bo he dra and xenomorphic crys tals of coarse-crys tal line va ri ety (SR), sample K-4, crossed nicols; (Fig. 5A). They look like spheruli tes, E–H — fine-crys tal line ag gre gates (SA) im pregnated by Fe hy droxides: E — differ ent shapes of aggre - however, this is only the im pression gates; de for mation of or ganic laminae around them (white ar rows) as well as preserved structure of plant result ing from the presence of con- debris in some of them is visi ble, sample K-30, crossed nicols; F — structureless or ganic matter preserved in side sid er ite ag gre gate, sam ple K-40, crossed nicols; G — well-preserved structure of plant tis sue cen tric ac cu mu la tions of fer ric hy - within ag gre gate, sam ple K-30, crossed nicols; H — ac cu mu la tion of el lip soi dal ag gre gates; de for ma tion droxides to which sider ite is par tially of organic laminae around them is well vis i ble (white ar rows), sam ple K-26, paral lel nicols altered. SEM and BSE im age anal y- ses re vealed that sid er ite SF could also appear as within other ce ments or clay matrix . This type of sider ite is microcrystalline rhombo he dra, up to 5 mm long (Fig. 5B). In- rarely al tered. Clay min er als (mainly kaolinite) overgr ew sider - ter ca la tions con sist ing of SF sider it e type often contai n some ite crystals in a few samples, how ever, any signs of re place ment well-preserve d plant debris (Fig. 5C). Coarse-crys tal line sid er - by other authigenic miner als were not observe d. The third type ite (SR) is de veloped as euhedral or subhedral rhom bo he dra, the of sid er ite ce ment con sists of fine-crys tal line ag gre gates (SA) of long axis of which varies from 0.01 to 0.1 mm (Fig. 5D). el lip soi dal, elon gated, an gu lar or ir reg u lar shape, the long axis Xenomorphic forms are less comm on. The SR va ri ety oc curs as of which reaches 0.5 mm (Fig. 5E–H). Ag gre gates oc cur usu- pore-fill ing or basal ce ment as well as dis semi nated crys tals ally in fine-crystal line background but someti mes they are also 72 Paulina Leonowicz pres ent be tween grains of de trital framewor k. Mica flakes and BSE IMAGE ANALYSIS organic laminae are usuall y de formed around them (Fig. 5E, H). In spite of the high concen tra tion of Fe hy droxides , it is of- BSE anal ysis of two se lected samples re vealed de tails of ten pos sible to observe the well-preserve d struc ture of plant tis- sid er ite crystals from the dif fer ent types of cem ent de scribed sue in side these ag gre gates (Fig. 5G). Some of them contai n above. Fine-crys tal line sid er ite (SF) was observe d in the sam ple only structureless, un iden ti fied or ganic mat ter (Fig. 5F); some of sid er ite mudstone (NW12/8; Fig. 6A), fine-crys tal line ag - oth ers (usu ally these of el lip tical shape) are completely de void gre gates (SA) and coarse-crys tal line rhom bo he dra (SR) — in of organic rem nants. the sample of sid er ite siltstone (K-4) (Fig. 6B–D). Note wor thy Sid er ite ce ment in mudstones and clayey siltstones is usu - is the inter nal struc ture of aggre gate s (SA), which are simi lar to ally de vel oped as the fine-crys tal line va ri ety (SF) in which SF sider it e but have grown prefer en ti ally in certai n direc ti ons fine-crys tal line ag gre gates (SA) com monly occur. Cem ent vol - (Fig. 6B). ume is usuall y high and reaches up to 65–70% of the whole Crystal s of all ce ment types are zoned, with one to four zones rock vol ume. BSE anal y sis of mudstone from NW12/8 sam ple com monly de vel oped (Fig. 6C, D). This zoning appears as re vealed that ag gre gates of sid er ite crys tals, filling the pore bright-dark-bright se quences in BSE im ages, result ing from dif- space be tween de trital grains, are usu ally larger that de trital fer ent chem i cal com po si tion of par tic u lar zones. WDS anal y sis con stit u ents (Fig. 6A). Quartz siltstones and sandstone s are in di cated that dark, usu ally inter nal parts of crys tals are en riched mostly ce mented with coarse-crys tal line sid er ite (SR); in Mg and Mn (Fig. 7). Mg con tent reaches there 26% of all cat - fine-crys tal line ag gre gates (SA) are less com mon. De trital ions, thus they are clas sified as sideroplesite; they also contai n up framework of sandstone s and siltstones changes from to 7% of Mn and up to 6% of Ca. Dark parts of crystal s have usu- grain-supporte d to this, in which indi vid ual grains are sepa - ally reg u lar, rhombohedric shape. Bright zones are alm ost pure rated by cem ent, result ing in vari ous volum e of sider it e. Ce- sid er ite with small con cen tra tion of Mn and sometimes Mg ment crys tals are usu ally smaller than de tri tal grains; their size (Fig. 7). Mg, Mn-poor sider it e forms the exter nal parts as well as changes depend ing on grain size of de tri tal con stit u ents. sin gle, ho mog e nous crys tals. Com par i son of anal y ses of both

Fig. 6. BSE im ages of sid erite cem ent

A — fine-crystal line sider ite cem ent (SF, light grey fields on a picture ), sam ple NW12/8; B — fine-crys talline ag gre - gate (SA), sam ple K-4; C, D — rhombohedral and xenomorphic crys tals of coarse-crys tal line sid er ite (SR); zonation of crystals is vis ible in both pic tures, sam ple K-4; scale bar — 50 mm Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland 73

Fig. 7. A — ter nary di a gram il lus trat ing el e men tal com po si tion of zoned sid er ite crys tals from sid er ite mudstone (NW12/8) and sid erite siltstone (K-4), de ter mined by WDS anal y sis; B — Mg and Mn con tent in dark and bright zones of sid erite crys tals from sam ples NW12/8 and K-4

sam ples (K-4 and NW12/8) re vealed that they dif fer slightly in chemi cal com posi ti on of dark zones. Those from sam ple NW12/8 are richer in Mg and Mn than sim ilar parts of crys- tal in sample K-4 (Fig. 7B). The com- posi ti on of bright zones in crystal s from both samples is simi lar. This com par i son con cerns mainly crys tals of SF and SR types.

XRD ANALYSIS

Bulk-rock XRD anal y sis, car ried out on sam ples selec ted for isoto pic anal y sis, in di cated that sid er ite is the only carbon ate in the rocks analy sed (Fig. 8). Other con stit u ents in clude: quartz, kaolinite, illite and/or mica, Fig. 8. Min eral compo si tion of siderites from the Ciechocinek For ma tion ex em pli fied by X-ray diffrac tion pat terns of selected samples; air-dried whole-rock sam ples small quan ti ties of chlorite and — in sample K-9 — ap atite. The latter is S — sid erite, Q — quartz, Kln — kaolinite, M — mica and illite, Ch — chlorite, A — apa tite authigenic, possi bly of biogenic or i- Ta ble 1 gin, as no ac cu mu la tions of de tri tal ap a tite were ob served dur ing thin Ox y gen and car bon iso to pic com po si tion of siderites from the Ciechocinek For ma tion sec tion ex am i na tion. It is worth men- tioning that the pres ence of d13C d18O diagenetic phosphate s in carbon ate Sam ple VPDB SMOW Framboidal pyrite Sid er ite type Li thol ogy [‰] [‰] concre ti ons was also reporte d by Pearson (1974, 1977) and Fisher et K-4 –13.47 24.89 ++ SR siltstone al. (1998) in siderites from K-9(1) –8.83 27.31 + SF mudstone Westphalian shales of Yorkshire in K-9 –8.61 27.04 + S mudstone (2) F England, thus it may be more com - K-9(3) –8.59 27.29 + SF mudstone mon phe nom e non, worth fur ther in- K-9(4) –8.77 27.42 + SF mudstone ves ti ga tion. K-9(5) –8.54 27.24 + SF mudstone

K-26 –10.91 27.59 +++ SA clayey siltstone CARBON AND OXYGEN ISOTOPIC P2/7 –13.27 27.52 + SF mudstone COMPOSITION OF SIDERITE NW12/8 –10.46 24.67 – SF silty mudstone

P40/9 –10.11 27.92 – SF sand stone Iso to pic val ues for dif fer ent types of sid er ite ce ment are only slightly d13 Framboidal py rite: + oc casional, ++ com mon, +++ abun dant; sid erite types: SF — fine-crys tal line, SR — vari able (Ta ble 1). C in all sam- coarse-crys tal line rhom bo he dra, SA — fine-crys tal line ag gre gates ples is nega ti ve and ranges from 74 Paulina Leonowicz

–13.5 to –8.5 ‰ VPDB. The highest content of 13C in carbon ter cala ti ons of sands and fine-grained depos it s appear. was noted in sam ple K-9, in other sam ples values of d13C are Grain-size dis tri bu tion in sid er ite beds is usu ally simi lar as in lower than –10‰. d18O val ues range from 24.7 to 27.9‰ muddy-silty host rocks, which gener all y have low perm eabil - SMOW. No sig nif i cant dif fer ences in iso to pic com po si tion be- ity. Thus, it ap pears most likely that pre cip i ta tion of sid er ite be - tween subsamples K-9(1-5) were noted. It shall be re mem bered gan from pore flu ids be tween de trital par ticles in some selected that the val ues ob tained are av er aged, be cause sid er ite crystals hori zons, now cem ented, rather than from solu ti ons expell ed were too small for sam pling parti cu lar zones of differ ent chem - from surround ing sedi m ents. The presence of organic matter i cal com po si tion, which could dif fer also in iso to pic com po si- in side sid er ite con cen tra tions, com mon in fine-crys tal line va ri - tion. Thus, these re sults give only ap prox i mate char ac ter is tics ety (SF) as well as in fine-crys talline ag gre gates (SA), points to of pore water. the close de pend ence of sid er ite crys tal li za tion on or ganic mat- As the inter preta ti on of oxy gen isotopes in siderites is gen - ter de cay. This de pend ence is par ticu larly ev ident in the case of er ally prob lem atic (Raiswell and Fisher, 2000) and, in this par- sid er ite ag gre gates SA, which proba bly formed as a result of ticu lar case, there are too many un cer tain ties, which would sideritization of plant debris, fe cal pellet s and other organic make conclu sions untrust worthy , they will not be discusse d rem nants. Ini tial crys tal li za tion of sid er ite in some pre ferred further in this paper. lay ers could, thus, result from the higher concen tra tion of or- ganic matter in these hori zons. The low perm eabil ity of muds pre vented in fil tra tion of the well-ox i dized ba sin wa ter into the PYRITE sed i ment and ox i da tion of dis persed or ganic rem nants. On the contrary, thicker layers of permeable sands enabled easy Pyrit e is also a com mon consti tu ent of Ciechocinek For ma- penetration of oxidized water and the siderite mineralization tion depos it s. It can be easy traced during out crop exam ina ti on, was poorly developed. where it occurs as round, el lipsoi dal and discoidal concre ti ons, Ini tial crys tal li za tion of sid er ite in or ganic-rich ho ri zons re - few milli metres to sev eral centi metres in di am eter. No de for - sulted in the form ati on of num erous nucle i disperse d in the sed - mati ons of laminae were observe d around concre ti ons, which im ent, which be came pref er en tial sites for the con tin ued points to their late, postcompactional or igin . Py rite was also growth of zoned crystal s. Further growth could procee d by uti- observe d in the vicin it y of wood fragm ents as well as in the li za tion of al ka lin ity and car bon ate ions, dif fus ing from the ad- burrow fillings which are often pyritized. Be side this late, jacent pore water s and was not lim ited by the amount and reac - concretionary py rite, SEM ob ser va tions re vealed also early tiv ity of or ganic matter in the layer, where sid er ite pre cip itated, diagenetic, framboidal and oc ta he dral py rite (Fig. 9), which oc- as most of it could be consum ed for the sider it e nucle i form a- curred in siderites as well as in pure mudstones and siltstones. tion. Simi lar iso to pic compo si tion as well as the same tex ture of m Di am e ter of framboids ranges from 2.5 to 8 m. Octahedra sid er ite crystals in samples col lected from dif fer ent parts of one reach 5.8–10 mm and occur as dissem i nated crys tals or form ir- layer (samples K-9(1-5)) point to the con tem po ra ne ous pre cip i ta- reg u lar con cen tra tions. From the set of sid er ite sam ples des- tion of the sid er ite crys tal lites in the whole ho ri zon. The larger tined for isoto pic analy sis only sam ples NW12/8 and P40/9 size of ce ment crys tals com pared with de tri tal con stit u ents, ob - were de void of fine-crystal line py rite. served in mudstone from sam ple NW12/8, as well as fre quently ob served tex ture of si der it ic sand stones and siltstones, where indi vid ual de tri tal grains are sepa rat ed by cem ent, point to the com mon displacive char ac ter of sid er ite pre cip i ta tion. In some cases, sid er ite may also have pre cip itated pas sively, filling va - cant pore space. This can be inferred from the grain-supporte d de tri tal frame work, of ten ob served in si der it ic sand stones and siltstones, as well as from the size of siderite cement crystals in most of these rocks, which is usually smaller than the size of detrital grains. The de form ati on of mica flakes and organic laminas around fine-crys tal line ag gre gates (SA) and slight flatten ing of some Planolites burrows suggest that cem enta ti on began quite early, Fig. 9. Microcrystalline py rite from the Ciechocinek For ma tion, while the sedi m ent was soft with high poros it y. Cem enta ti on of SEM im age load-type sandy lenses with deform ed lam ina ti on shows that crys tal li za tion of sid er ite con tin ued af ter soft sed i ment de for ma- A — framboidal pyrite; B — oc ta he dral py rite crys tals; tion. It conti nued during deeper burial, post-dating signif i cant Koz³owice outcrop com pacti on of depos it s, which led to lithification of muds with strongly de formed shrinkage cracks and flattene d burrows. INTERPRETATION OF SIDERITE ORIGIN The ori gin of lenses and small oval, discoidal and ir regu lar con cre tions, com posed of sid er ite mudstones and siltstones, MODE AND TIME OF SIDERITE CEMENTATION which occur in sandy lay ers is not certai n. One possi bil ity is that these are muddy intraclasts sideritized af ter ero sion and Sid er ite min er al iza tion from stud ied de pos its oc curs pref er - redeposition, but this is not convinc ing in view of the preva lent a bly in mudstones and siltstones, even in these cases where in - oval, egg-like shape of concre ti ons, differ ing from typi cal Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland 75 shapes of intraclasts. The second possi bil ity is that these con- zonation of sid er ite crys tals. More prob a ble is that in ter nal zonation cre tions formed near the wa ter-sed im ent in ter face in muddy of crys tals result ed from diagenetic evolu ti on of pore wa ter chem is- sedi m ent and were washed-out and transporte d by strong bot- try linked with proces ses oc curring be low the sedi m ent-water inter - tom current s along with the sands that now surround them. This face. In creased con tent of Mg cat ions in the in ter nal zones of crys- ex pla na tion, how ever, is in con sis tent with gen er ally quiet tals points to the marine ori gin of pore water s (Mozley, 1989). depositional en vi ron ment of the Ciechocinek For ma tion de- However, the same parts of crystal s contai n also up to 6.9% of Mn, pos its (Leonowicz, 2002, unpubl.) and the lack of erosiona l linked rather with fresh wa ter en vi ron ments. Such com po si tions struc tures in the flat sid er ite lenses that are as so ci ated with con - could be caused by the mixing of marine and freshwa ter off the river cre tions as well as in con tin u ous sid er ite lay ers, which lat er ally mouths. Thus the start ing com posi ti on of pore wa ter, from which change into hori zons with these concre ti ons. Moreover, accu - the in ter nal parts of sid er ite crys tals pre cip i tated, re sulted from the mu la tions of sid er ite con cre tions oc cur only in sandy ho ri zons; relea sing of land-derive d Mn and Fe ions in Mn- and Fe-reduc ti on concre ti on levels were not observe d in mud-silt depos it s, thus, zones and the presence of Mg ions derive d from brackish-m arine it is less prob a ble that they ever ex isted. The third pos si bil ity is ba sin wa ter. In creased burial re stricted in fil tra tion of ba sin-de rived that these concre ti ons are load-type structure s, derive d from wa ter into the sed i ment, lim it ing sup ply of Mg. Si mul ta neously one, origi nall y conti nu ous layer of mud, which overlai d the manga nese reduc ti on fell and the concen tra tion of Mn ions de- sands. The problem is that it is al ways sand that is loaded into a clined. The only avail able cat ion was Fe+2, which fre quently reaches mud, not vice versa. Maybe, in this par ticu lar case, mud was al - max i mum con cen tra tion deeper than Mn+2 (see Curtis et al., 1986). ready partly cem ented and there fore heavier, but still plastic, Iron could be also derive d to pore water during deeper diagenesis by what enabled loading into underlying sand. Explanation of the re leas ing from de tri tal Fe-bear ing min er als, for ex am ple fer rous these questions requires further studies. chlorites, which occur com monly in Ciechocinek For ma tion muds (Leonowicz, 2005). COMPOSITIONAL ZONATION OF SIDERITE CRYSTALS CARBON ISOTOPIC COMPOSITION AND PYRITE DISTRIBUTION Zonation of sid er ite crystals re cords changes of pore-wa ter chem is try dur ing its pre cip i ta tion. MnMg-rich sid er ite, which Ex ten sive geo chem i cal stud ies of diagenetic pro cesses in - ap pears in the in ner parts of crys tals, had to crys tal lize first; this dicat e that along with progres sive burial sedi m ent passes was fol lowed by pure sider it e, which now forms exter nal zones through sev eral geo chem i cal zones, re sult ing from or ganic-in - and sin gle, ho mog e nous crys tals. This reg u lar ity points to the or ganic in ter ac tions (Fig. 10). In fresh wa ter de pos its, pre cip i ta - decrea se of Mg and Mn content in pore water s during ongo ing tion of sider it e begins in the iron reduc ti on subzone and conti n- diagenesis. Although com posi ti on of pore wa ter was subject to ues — if there is enough of Fe+2 ions — unti l reaching the ther- short-lived fluc tu a tions, re sult ing in for ma tion of mul ti ple mal decarboxylation zone. The sul phate reduc ti on zone is often zonation, general trends occur in all crystals. The reasons of compositional zonation in car bon ate crys tals were ex ten sively dis cussed in many min er al og i cal pa pers (Curtis et al., 1975; Pearson, 1979; Curtis et al., 1986; Mozley, 1989; Macaulay et al., 1993; Fisher et al., 1998; Hugget et al., 2000; Wilkinson et al., 2000 and many others). In general , chem istry of pore water can be influ ence d by two differ ent proces ses: sa- lin ity of wa ter in sed i men tary ba sin and diagenetic al ter ations of un sta ble de tri tal con stit u ents, lead ing to the re lease of chem i cal ele m ents and consum pti on of others. Changes of sea level can mod ify pore wa ter chem is try in more per me able sed i ments, re - sult ing in the chemi cal zonation of con tin u ously grow ing crys- tals (Curtis et al., 1986; Mozley, 1989; Macaulay et al., 1993; Hugget et al., 2000). As the sea water is enric hed in Mg and Ca ions, and Fe and Mn ions are derive d from the fresh wa ter, trans- gression is marked by the increa se of Mg and Ca content toward the margins of sid er ite crystal lites. The same effect can be achieved by diagenetical al ter ations. If sid er ite be gins to crys tal - lize in suboxic zone, where the oxy gen a ti on of organic matter uti lized Mn and Fe oxides , it would be enric hed in these ele - ments (e.g. Curtis et al., 1986; Fisher et al., 1998). At greater depths, Mn and Fe content decli nes as a re sult of sider it e precip i - tati on and decrea sed rate of Fe and Mn re ducti on. Sim ulta - neously Mg and Ca concen tra tions rem ain consta nt with depth or even can in crease, due to al ter ations of de trital par ticles, and Fig. 10. The geochem ical zonation of sed i ment, fol low ing pro gres sive as a re sult Mg/Fe + Mn and Ca/Fe + Mn ra tios in crease. burial; gen er al ized re ac tions, in volv ing re lease of CO2, The low per meabil ity of the Ciechocinek Form ati on makes it are given for each zone (af ter Morad 1998, modi fie d) unli kely that fluc tua ti ons of sea level caused the compositional Zones of sider ite precip i ta tion are marked grey colour 76 Paulina Leonowicz

2- d13 poorly devel oped in such envi ron m ents due to the lack of SO4 compositional zonation of crystals. The in creased of C val - in the overly ing wa ter colum n. In marine sedi m ents, Fe+2 pro- ues, when com pared to those typi cal of organic matter oxi da - duced by iron re ducti on in suboxic and sul phate reduc ti on tion, could result from mixing of CO2 gener ate d in the suboxic zones, is mainly pre cip itated as sulphides, thus sider it e cannot zone with CO2 derive d from basin wa ter and, in later stages, precip i tat e unti l the sul phate ions are consum ed, i.e. in the from mixing of CO2 re leased dur ing sul phate re duc tion and methanogenesis and therm al decarboxylation zones. However, bacte ria l ferm enta ti on. It should be stressed, however, that pre- if there is ex cess of Fe+2 with re spect to HS–, sid er ite can also sented val ues of d13C are aver aged, obtai ned from whole sam - pre cip i tate in the sul phate re duc tion or even suboxic zones. ple analy ses, so they com bine isotope com posi ti on of crystal s Car bon iso tope com po si tion of early diagenetic car bon ates is formed at dif fer ent stages. control led by the d13C of carbon diox ide dissolve d in pore water , which depends in turn on its ori gin (from atm osphere , sea water or gener ate d by proces ses of organic matter degra da ti on) as well CONCLUSIONS as on whether diagenesis took place in a closed or open system . Each source menti oned above suppli es CO of a differ ent carbon 2 Crys tal li za tion of sid er ite in Ciechocinek For ma tion de pos- iso tope com po si tion. CO2 re leased dur ing ox i da tion of or ganic mat ter and bac te rial sul phate re duc tion gen er ally in her its iso to - its began quite early, before noti ceable com pacti on of sedi - pic compo sition from source or ganic matter and is char acter ized ment, and conti nued during deeper burial, post-dating pro- 13 cesses of soft sed i ment de for ma tion and sig nif i cant com pac- by d C val ues about –25‰ PDB (Irwin et al., 1977). CO2 origi - nat ing as a result of bacte rial fer men tation can show vari able val - tion. Anal y ses of car bon iso tope and chem i cal com po si tion in- ues of d13C ranging from –30 to +20‰ PDB (Whiticar et al., di cate that pre cip i ta tion of sid er ite be gan al ready in the iron re - 1986; see also Raiswell and Fisher, 2000), depend ing on the spe - duc tion subzone and conti nued in the sulphate reduction and cific mi cro bial pro cesses in volved. Ther mally-in duced re ac tions methanogenesis zones. The rea son that sid er ite min er al iza tion is com monly con- of decarboxylation supply CO2 of a dif fer ent iso to pic com po si- tion with d13C val ues ranging from –10 to –25‰ PDB (Irwin et centra ted in some preferred lay ers and lenses in monot o nous 13 mud-silt suc ces sion is that they proba bly had a higher content al., op.cit.). CO2 from the sea wa ter has d C val ues about 0‰. River wa ter is of ten slightly de pleted in 13C com paring with sea of organic matter . Decay of abundant or ganic matter result ed in wa ter. Val ues of d13C in car bon ates pre cip i tated in par tic u lar lo cal car bon ate supersaturation and early precip i ta ti on of sider - geochem ical zones can differ from these menti oned above due to ite nucle i, which becam e favoured sites for further growth of cem ent crys tals. Thicker lay ers of sand are not sideritized due the mixing of CO2 derive d by differ ent proces ses of organic mat- ter de cay as well as the in filtra tion of marine and mete oric wa ters to their high per meabil ity, which al lowed organic matter to be into the upper part of depos it s (Irwin et al., op. cit.; McKay et al., com pletely ox i dized soon af ter de po si tion. The or i gin of lenses 1995; Morad, 1998). and small sider it e concre ti ons, occur ring in sandy lay ers is not cer tain. Most likely these are load-type struc tures, derived from Neg a tive val ues of d13C obtai ned for siderites from the a conti nu ous layer of mud, which was partly sideritized. The Ciechocinek For ma tion (Ta ble 1) in di cate that CO was gen er - 2 higher weight and still plastic properties enabled its loading ated either by oxi da ti on of organic matter or by bacte ria l fer- d13 into underlying sand. men ta tion. Slight in crease of C val ues by mixing with CO2 Ini tial crys tal li za tion of sid er ite re sulted in for ma tion of nu - of marine and/or mete oric ori gin is also proba ble. As the depth mer ous MnMg-rich nu clei, which pre cip i tated si mul ta neously of burial of Lower Juras sic depos it s, es ti mated from the origi - through out the whole layer vol ume. Fur ther pre cip i ta tion con- nal thickness of younger rocks in the Cra cow-Silesian Upland tinued by growth of parti cu lar crystal s, zonation of which re- did not excee d 1 km (Soko³owski, 1973; Deczkowski and flects changes in pore water chem istry during progres sive Franczyk, 1988; Dayczak-Calikowska and Moryc, 1988; burial. Tex ture of sid er ite ce ment in di cates that it pre cip i tated Niemczycka and Brochwicz-Lewiñski, 1988; Marek, 1988; mainly by dis plac ing sur round ing de tri tal grains, how ever, pas- Jaskowiak-Schoeneichowa and Krassowska, 1988), CO2 origi - sive filling of va cant pore space in some si derit ic sandstone s nating as a result of abiotic decarboxylation re ac tions, which and siltstones is also pos si ble. Simi lar tex ture and chemi cal occur at tem pera ture s higher than 75°C, can be exclude d compo si tion of crys tals from three sid er ite types (SF, SR and SA) (McKay et al., 1995, Morad, 1998). point to their sim i lar or i gin. Thus, mor pho log i cal dif fer ences As in most sam ples an a lyzed, exclud ing NW12/8 and do not re sult from differ ent diagenetic conditions but from P40/9, the early diagenetic, framboidal and oc ta he dral py rite different grain size of host sediment and accessible space. occurs, so it can be inferred that the precip i ta ti on of sider it e Chem i cal com po si tion of in ter nal crys tal parts, which are took place from brackish or marine -derive d solu ti ons. Only in enric hed in Mg and Mn, result ed from the mixed, marine and the case of sam ples devoid of py rite, freshwa ter ori gin cannot freshwa ter , source of pore solu ti ons. Mg ions were derive d be exclude d. However, microcrystalline py rite is not abundant, from marine ba sin wa ter, whereas Mn and Fe were released in thus its for mation should not limit crystal liza tion of sid er ite. suboxic zone from land-derive d parti cles. Increa sing depth of Taking into ac count this fact as well as result s of carbon isotope burial lim ited the proces s of manga nese reduc ti on and re- anal y sis, it can be in ferred that pre cip i ta tion of sid er ite could stricted ac cess of Mg from ba sin wa ter. As a result ex ter nal began in the iron reduc ti on subzone and conti nued in the sul - parts of crystals are almost pure siderite. phate re duc tion and methanogenesis zones. The state ment that Chem i cal and iso to pic com po si tion of siderites from the the growth of crystal s lasted for a longer time and procee ded in Ciechocinek For mation as well as the pres ence of early dif fer ent geo chem i cal con di tions is sup ported by the Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland 77 diagenetic microcrystalline py rite in sid er ite-bear ing de pos its tions, kind help and discus sions , which enric hed my knowl - points to the brack ish-marine char acter of water in sed im en tary edge of isotopes . I am also grateful to Dr M. ¯ywiecki, Univer - ba sin. In the case of two samples, col lected from the lower part sity of Warsaw, for valuable advices in inter preta ti on of isoto - of the Ciechocinek Form ati on (NW12/8 and P40/9), which are pic de ter mi na tion re sults. The pa per bene fited greatly from devoid of py rite, the freshwa ter ori gin cannot be exclude d, careful reviews by Dr Q. Fisher and Dr B. Laenen, whose cor- how ever, there is no un equiv o cal ev i dence for such in ter pre ta- recti ons and com ments substa nti ally helped me to revise the tion. As the compo si tion of siderites re flects in di rectly sa lin ity manuscri pt. The Manage m ents of the “Cerpol-Koz³owice” En - of basin wa ter it can be an use ful tool in palaeoenvironmental ter prise and the “Boroszów” Brickyard are thanked for provid - con sid er ations. How ever, geo chem i cal anal y ses should be al- ing ac cess to their workings and the Manage m ent of the Polish ways con nected with sedimentological and faunal studies to Geo log i cal In sti tute for al low ing free ac cess to ar chi val de - make interpretation more certain. scripti ons and bore hole cores. Cores were sam pled with the perm ission of Polish Minis try of the Trea sury. This paper is a Ac knowl edge ments. I wish to express my grati tude to Dr part of a Ph.D. thesis finishe d in 2002 and finance d by the Insti - B. £¹cka, Pol ish Acad emy of Sci ence, for iso to pic de ter mi na- tute of Geol ogy, Uni versit y of Warsaw.

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