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Origin of siderites from the Lower Jurassic Ciechocinek Formation from SW Poland

Leonowicz P.

Geological Quarterly

2007

Vol. 51, No 1

67-78

Sideritic rocks, which are characteristic constituents of muddy-silty deposits of the Lower Jurassic Ciechocinek Formation, occur commonly as layers, lenses and small irregular concretions composed of siderite mudstone and siltstone as well as less common lenses of sideritic sandstones. Three types of siderite cement were observed in thin sections: fine-crystalline variety (SF), coarse-crystalline rhombohedra (SR) and fine-crystalline biogenic aggre gates (SA). In all these types BSE analysis revealed compositional zonation of crystals, with in ternal parts enriched in Mg, Mn and Ca and outer zones almost pure siderite. d13C values and chemical composition of siderites combined with the presence of early diagenetic pyrite indicate that siderite crystallized from brackish marine-derived solutions; only in the case of two samples from the lower part of the Ciechocinek Formation the fresh water origin cannotbe excluded. Mn and Fe were supplied by rivers and released by Fe- and Mn-reduction in suboxic zone, whereas Mg and Ca were derived from sea water, which infiltrated into the sediment. Crystal zonation resulted from the diagenetic evolution of pore water as the sediment was buried. Similar composition and development of siderite crystals from different parts of one layer indicate that siderite precipitated simultaneously throughout the whole horizon. Precipitation began from the formation of numerous nuclei and continued by growth of crystals onto them. It could begin already in the iron reduction subzone and continued in the sulphate reduction and methanogenesis zones.

Holocene environmental instability as recorded in a core from the Słupsk Furrow, Southern Baltic

Zgrundo A.

Oceanological Studies

1999

Vol. 28, No. 3-4

59-73

The aim of the study was to investigate Holocene history as recorded in a single sediment core taken from the Słupsk Furrow, southern Baltic Sea, with the use of diatom analysis. Diatom stratigraphy and abundance of cysts of Chrysophyceae Pascher enabled the changes in salinity and depth of the Baltic during the last 7500 years or so to be followed. Four developmental stages of the Baltic Sea were distinguished on this basis: the decline of the Ancylus Lake (or the Ancylus/Mastogloia transition), the Mastogloia Sea, the Litorina Sea and the Postlitorina Sea.