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Origin of Middle Jurassic siderite rocks from Central Poland

Maliszewska A., Kozłowska A., Kuberska M.

Volumina Jurassica

2006

Vol. 4, no. 1

95-96

Siderite rocks in the Middle Jurassic marine deposits in Central Poland occur from Aalenian to Upper Bathonian in form of intercalations and concretions. Most frequently, they are clayey siderites composed of siderite and sideroplesite, being accompanied by illite, kaolinite, pyrite, quartz, sometimes also chamosite. Some layers contain also the chamositic and chamosite-sideritic ooids and fine bioclasts. Selected samples of the clayey siderites were isotopically analyzed in the Institute of Physics of UMCS in Lublin. It appears that δ18O values fall into the interval from -2.66 to 0.58‰ PDB. It was calculated, adopting a water temperature in the Middle Jurassic basin Central Poland between 18 and 24°C (Coleman et al. 1997), that the siderites crystallized mainly from the marine mixed with meteoric water of δ18O in the limits between -7 and -1‰ SMOW. The δ13C values oscillate between +11.78 and +5.67‰ PDB. That points to the origin of the carbon present in the crystallization waters from the organic matter altered in the zone of microbiological metanogenesis (Mc Kay et al. 1995). The abundance of pyrite is an evidence for strong reduction conditions of sedimentation and diagenesis of the siderite formation. Sulphide minerals and micas accumulated in the Aalenian and Bajocian dark bottom muds, further altered in the marine waters, were the most probable iron source. Layers of siderite coquinas also occur in the deposits under description. They contain numerous bioclasts built of calcite or ankerite, sideritic, sideroplesitic and ankeritic spars, less frequently - euhedral pistomesite crystals and pyrite. The chamosite, kaolinite, phosphates and detrital quartz are subordinate. According to present knowledge, the formation of the coquinas resulted from the intraformational alteration of the deposits connected with the Middle Jurassic salt tectonics and mobilization of the Zechstein saline waters rich in ions of metals. The results of chemical analyses of zonal siderites and sideroplesites point to the progressive increase of magnesium content in the pore solutions in the Jurassic deposits.