Polyphase dolomitisation of the Wuchiapingian Zechstein Limestone (Ca1) isolated reefs (Wolsztyn Palaeo-Ridge, Fore-Sudetic Monocline, SW Poland)

Abstract

Dolomitisation was the main diagenetic process in the Upper Permian Zechstein Limestone of the Wolsztyn High -dolomite cementation (“over-dolomitisation”) also occurred. The rocks studied usually have a mixed mineralogy and represent a continuous spectrum from pure limestone to pure dolomite. This is due to varying degrees of dolomitisation, dolomite cementation and dedolomitisation. There are two main types of dolomite: replacement dolomite (mostly planar unimodal dolosparite mosaics that are mainly fabric-destructive) and cement dolomite (planar isopachous rims and pore-filling non-planar sad-dle-dolomite crystals). The timing of dolomitisation and dolomite cementation is difficult to ascertain, but comparing petro-graphical and geochemical data indicates that the reef carbonates were dolomitised shortly after deposition in a near-surface sabkha/seepage-reflux and then in burial systems. It seems that many of the dolomites gain their present isotopic composition when buried in relatively high-temperature conditions, as shown by low oxygen isotopic ratios (δ 18 O as low as -9%o PDB) and the presence of saddle dolomite. No isotopic support for a water-mixing mechanism is documented.