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Pyritized Radiolarians from the Mid-Cretaceous Deposits of the Pieniny Klippen Belt — a Model of Pyritization in an Anoxic Environment

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Pyritized Radiolarians from the Mid-Cretaceous Deposits of the Pieniny Klippen Belt — a Model of Pyritization in an Anoxic Environment GEOLOGICA CARPATHICA, 51, 2, BRATISLAVA, APRIL 2000 9 1 -9 9 PYRITIZED RADIOLARIANS FROM THE MID-CRETACEOUS DEPOSITS OF THE PIENINY KLIPPEN BELT — A MODEL OF PYRITIZATION IN AN ANOXIC ENVIRONMENT MARTA BĄK and ZBIGNIEW SAWŁOWICZ Institute of Geological Sciences, Jagiellonian University, Oleandry 2A, 30-063 Kraków, Poland; [email protected] (Manuscript received August 24, 1999; accepted in revised form March 15, 2000) Abstract: Excellently preserved, pyritized radiolarian skeletons have been found within the Upper Cenomanian de­ posits in the Pieniny Klippen Belt (PKB—Carpathians, Poland). On the basis of a study of their chemical composi­ tion, structure of replacing skeletons and exceptional preservation of all morphological details, we propose a new model where the pyritization process took place not in sediment but while the radiolarian skeletons were suspended in the anoxic water column. The radiolarians rich in organic matter, sinking through the upper (iron-rich) part of an anoxic water column, became the sites of organic matter decomposition and enhanced bacterial sulphate reduction. Dissolved iron in this zone diffused into the radiolarians and precipitated as iron sulphides replacing the opaline skeletons. This process was controlled by the rates of opal dissolution and of bacterial sulphate reduction, and the availability of dissolved iron. The preservation of radiolarians in the Upper Cenomanian deposits from different depth sub-basins of the PKB was compared. We found that the extent of pyritization and preservation of radiolarian skel­ etons may be dependent on the depth of the basin and the position of the oxic-anoxic interface. Key words: Carpathians, Pieniny Klippen Belt, anoxic event, pyritization, Radiolaria. Introduction which separates two major structural units of the Carpathians: the Inner and the Outer Carpathians (Fig. 1). During the Creta­ Pyritized organic remains are common in the sedimentary ceous, the Pieniny Klippen Basin consisted of several sub-ba­ record. Pyrite may form moulds of diatoms (Geroch 1978; sins representing realms from the outer shelf (the Czorsztyn McNeil 1990), fill empty spaces and/or replace carbonates in Succession), to the lower and middle bathyal zones, (the echinoderms (Jensen & Thomsen 1987), ammonites (Hudson Branisko and Pieniny successions) (Birkenmajer 1977; 1982), gastropods and bivalves (Fisher 1986) or whale bones Birkenmajer & Gasiński 1992; K. Bąk 1993). The Upper Cen- (Bang 1994), delineate tubes and burrows of polychetes omanian deposits in the Pieniny Klippen Basin, which belong (Thomsen & Vorren 1984) and replace soft-bodied trilobites to the foraminiferal Rotalipora cushmani Zone are represented (Briggs et al. 1991). Pyrite has also been found in recent fora- by two facies. These are black marly shales with radiolarians minifers (Seiglie 1973). Pyrite can adopt various forms, from (Birkenmajer 1977; Birkenmajer & Jednorowska 1987), which massive to aggregated, euhedra and framboids. Generally py­ can be correlated with an anoxic event, well developed in the rite replaces the organic matrix (“soft parts”) and carbonate Pieniny and Czorsztyn successions, and grey-green shales skeletons during all stages of sediment burial history. Excel­ with thin sandstone intercalations represented by the shaly fly- lent descriptions of mechanisms of fossil pyritization (but not sch deposits belonging to the Branisko Succession (Birkenma- of silica skeletons) were given by Canfield & Raiswell (1991), jer 1977; K. Bąk 1993). All these deposits contain pyritized ra- Briggs et al. (1996) and Raiswell (1997). diolarians (M. Bąk 1995, 1996a,b, 1999). For the purpose of While pyritized radiolarian skeletons are relatively com­ this study the rocks belonging to the Czorsztyn, Branisko and mon (e.g. Pessagno 1977; Thurow 1988; M. Bąk 1995, Pieniny successions were sampled and the pyritized radiolari- 1996b), this phenomenon has only been recorded in the taxo­ an skeletons compared. nomic literature. In this paper we present the results of the first non-taxonomic study and propose an original model of radiolarian pyritization in the water column. Perfectly and Method poorly preserved pyritized radiolarian skeletons from the up­ per Cenomanian deposits of different successions in the Pol­ The material studied includes 33 samples, from four profiles ish part of the Pieniny Klippen Belt were also compared. in the Polish part in the Pieniny Klippen Belt. Eleven samples have been taken from black shales of the Magierowa Member in the Magierowa Skała section of the Pieniny Succession (M. Geology Bąk 1999). Ten samples have been taken from grey-green shales of the Jaworki Formation (Trawne Member and The Pieniny Klippen Belt (PKB) represents a zone of Sneżnica Siltstone Member) in the Kietowy stream section of strongly deformed Mesozoic and Paleogene sedimentary rocks the Branisko Succession (M. Bąk 1995, 1996a), and twelve 92 BĄK and SAWŁOWICZ Radiolarian skeletons from the black marly shales of the Pieniny Succession (the deepest part of the PKB) (Fig. 2A) are usually moderate to poorly preserved. Pyritized skeletons with poorly preserved outer structures (due to dissolution) dominate in the radiolarian association which consists of nu­ merous Nassellaria (mostly cryptocephalic and cryptothorac- ic forms), belonging to genera such as Holocryptocanium, Hemicryptocapsa, Squinabollum) with only a few forms of Spumellaria (genera Archaeocenosphaera, Orbiculiforma). Siliceous skeletons are rare here. Pyrite microconcretions, possibly pseudomorphs after radiolarian skeletons, are also present. Radiolarian skeletons from the black shales of the Czorsztyn Succession (the shallowest part of the PKB ba­ sin) (Fig. 2C) are usually poorly preserved (although often sufficiently for taxonomic determinations), and seem to be formed exclusively of pyrite. However, the examination of cross-sections shows that irregular masses of pyrite grains cover various parts of the majority of the siliceous skeletons of radiolarians. This radiolarian association comprises both Nassellaria (genera Holocryptocanium, Hemicryptocapsa, Fig. 1. The geological position of the Polish part of the Pieniny Dictyomitra, Stichomitra) and Spumellaria (genera Patellu- Klippen Belt (P.K.B.) within the Carpathians. Tectonic elements la, Crucella, Cavaspongia) (forming about 60 and 40 per mainly after M. Książkiewicz, simplified by K. Birkenmajer (1985); cent of the association, respectively). A: Location of the investigated sections in the Pieniny Klippen Belt The majority of the radiolarians in one sample (Kietowy (geology after Birkenmajer (1977) — simplified). Designation of section — sample Ki-14 — see M. Bąk 1995) from the grey- the sections: Ki — Kietowy, Lor — Lorencowe Klippes, Sz — green shales of the Branisko Succession (Fig. 2B) (lower to Szaflary quarry, Mag — Magierowa Skałka. middle bathyal, K. Bąk 1993) consist of pyrite, with only a few siliceous skeletons. The radiolarian assemblage consists samples have been collected from black shales of the Altana predominantly of Nassellaria. The pyritized forms belong Shale Bed in two profiles of the Czorsztyn Succession mostly to the cryptothoracic Nassellaria (Pl. I) such as Holo­ (Szaflary quarry and Lorencowe Klippes sections — see M. cryptocanium barbui, Hemicryptocapsa tuberosa and Hemi­ Bąk 1996b). The samples were collected in the sections ev­ cryptocapsa prepolyhedra. Xitus mclaughlini and Thanarla ery 10 to 80 cm (depending on the changes of lithology and pulchra are less abundant. The rare siliceous specimens, all the quality of exposure), especially near lithological bound­ corroded, also include the Nassellaria (Cryptamphorella co­ aries. Marls and marly shales were the dominant lithotypes. nara, Sethocapsa sp.) (Pl. II: Fig. 2), with only one specimen A few samples were taken from mudstones. of Spumellaria (Haliommura sp.). The above mentioned radi- Samples of about 1 kg were taken for preparation. Each olarian taxa do not occur in both siliceous and pyritized sample was broken into pieces 1-2 cm across and dried out forms. Pyrite very faithfully replaces all the original silica under a temperature of 105 °C. Next the samples were skeletons in the studied sample, even the finest details of or­ soaked in a hot solution of glauberic salt and boiled, usually namentation (Pl. II: Figs. 3, 4). Even in cryptothoracic forms for several days. Hard cemented parts of samples were with a thick abdomen wall (e.g. H. barbui, H. tuberculatum) soaked in hot acetic acid for 7-8 hours. Then the residue of four layers, the internal layers are perfectly preserved (Pl. was washed through a 63 |im sieve. II: Figs. 7, 8). SEM-EDS study showed that pyrite is actually Radiolarians were first examined under a binocular mi­ the only sulphide mineral present, without the presence of sil­ croscope. They were picked out manually from the residue ica or silicates. At lower magnifications (below 1000x), SEM (maximum 300 specimens per sample). The best preserved images reveal very even surfaces of pyritized skeleton ele­ specimens were mounted on Scanning Electron Microscope ments. However, higher magnifications (5000-10000x) show (SEM) stubs for photography. that these skeletons are built of masses of small irregular grains of pyrite (size about 0.5 |im), intergrown or closely packed, sometimes with pores (Pl. II: Fig. 1). Secondary dis­ Radiolarian association and description solution of the pyritized skeletons caused corrosion, resulting of pyritized skeletons in fine to coarse-granulated
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