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Petrological studies of the Early Jurassic siderites from Western Pomerania (Poland)

Maliszewska A., Kozłowska A., Kuberska M.

Volumina Jurassica

2006

Vol. 4, no. 1

96-97

Petrological studies of siderite layers and concretions occurring in the Early Jurassic periodically marine deposits, described as the brackish shelf facies (Feldman-Olszewska 1997), were conducted. The analyzed rocks are mainly built of the sideroplesite, while siderite and pistomesite occur sporadically. Two generations of the sideroplesite were distinguished, that essentially differ due to the size of the rhombohedra. The older generation forms micritic concretions and micritic layers of the clayey siderites. The younger generation either represents a groundmass of the sparry siderites or cements of the sideritic sandstones. Some layers of siderites contain ooids and aggregates of the chamosite and phosphates, chamosite intraclasts, pyrite, detrital quartz and micas. Numerous shells of foraminifers and occasionally molluscs are present, too. Nine samples of the clayish siderites from the Mechowo IG1 borehole were isotopically analyzed aiming at oxygen and carbon ratios. The recognition of δ18 O of crystallization waters responsible for the siderite formation was the final aim of the studies. A formula given by Carothers et al. (1998) as well as the results of studies on the siderites of Baker et al. (1995) were applied. It results from the calculations that the δ18 O of the crystallization water oscillated between -12 and -3‰ SMOW in the Late Sinemurian, while in the Early Pliensbachian and the Early Toarcian it changed from -10 to +2‰ SMOW. That points to the meteoric waters gradually enriched in the 18 O isotope. The δ13C values in the siderite samples oscillate from -0.85 to -10.57‰ PDB. That suggests that the pore waters were enriched in carbon formed in the microbiological zone of the metanogenesis due to the alteration of the organic matter (Longstaffe & Ayalon 1997). The whole petrological analysis of the siderites points to their origin as the product of the diagenetic processes which operated in the bottom mud of the shallow brackish basins in the anoxic conditions with the influence of the meteoritic waters.