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

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.