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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.

The Ciechocinek Formation (Lower Jurassic) of SW Poland : petrology of green clastic rocks

Leonowicz P.

Geological Quarterly

2005

Vol. 49, No. 3

317--330

The Lower Jurassic Ciechocinek Formation from the Czestochowa-Wielun region of SW Poland comprises greenish-grey muds and silts as well as poorly consolidated mudstones and siltstones with lenticular intercalations of fine-grained sands, sandstones and siderites. Analysis of a mineral composition indicates that the detrital material was derived mainly from the weathering of metamorphic and sedimentary rocks of the eastern Sudetes with their foreland and of the Upper Silesia area, and that this material underwent repeated redeposition. The Fe-rich chlorites which give the green colour to the mudstones of the Ciechocinek Formation are most probably early diagenetic minerals, genetically linked with the deposition in a brackish sedimentary basin.