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Sedimentary and diagenetic environments of Middle Ordovician iron-rich limestones (Pobroszyn Formation) in the northern Holy Cross Mountains, Poland

Trela W.

Geological Quarterly

2008

Vol. 52, No 3

199-199

The Middle Ordovician (Dapingian) of the northern Holy Cross Mountains (central Poland) is represented by condensed limestones that make up the Pobroszyn Formation. They reveal a complex stratification reflecting alternating depositional conditions. The basal limestones were deposited in open-marine conditions during the early Middle Ordovician sea level rise (navis Zone) correlated with the Baltoscandian Gärdlosa drowning. Periods of non-deposition associated with this transgression favoured precipitation of authigenic Fe minerals close to the sediment-water interface. The upper part of this succession appears to representa succeeding depositional phase associated with a second transgressive event, which probably involved reworking of the underlying lithified substrate. High energy events were interrupted by periods of non-deposition favouring development of benthic microbial communities contributing to Fe authigenesis. The Pobroszyn Formation reveals features suggesting an early diagenetic alteration of the parent carbonate sediment in a shallow ma rine setting (e.g., beach shoreface) or even was influenced by meteoric diagenesis, which probably took place during a sea level fall preceding the second transgressive event.