Regional setting and role of meteoric water in dolomite formation and diagenesis in an evaporite basin: studies in the Zechstein (Permian) deposits of Poland

Abstract

ABSTRACTThe Zechstein Basin of Poland was an area of widespread cyclical deposition of carbonates and evaporites during Late Permian time. The Zechstein shelves, along both the northern and the southern margins of the basin, were sites of shallow‐water sedimentation during the formation of the Main Dolomite and Platy Dolomite, two widespread carbonate units. These units consist of oolitic, peloidal, skeletal, micritic and evaporitic carbonates formed in depositional settings ranging from open marine to coastal (lagoonal, sabkha and salina). Although originally deposited as limestones, the Main Dolomite and Platy Dolomite are inferred to have been completely replaced by dolomite through very early stage (essentially penecontemporaneous) reflux of hypersaline brines.The dolomites of the two basin margins, however, have very different petrographic and isotopic characteristics. Many northern shelf dolomites show early stage calcitization (dedolomitization) and even, in some cases, evidence of a subsequent redolomitization event. These northern shelf samples also have a broad range of carbon and oxygen isotopic ratios (up to 12%0 for oxygen). Samples from the southern shelf, on the other hand, are petrographically much simpler; they do not show complex calcitization and redolomitization patterns. Likewise, their isotopic values are much more tightly clustered, with only about a 5%0 range of oxygen isotopic ratios.The differences between dolomites of the same age from the northern and southern margins are best explained by regional variations in river water influx during episodic exposure events associated with regional or global sea‐level fluctuations. The distribution of clastic terrigenous materials and palaeokarstic features indicate that areas of the northern shelf had extensive river input, an influx largely lacking on the southern shelf. Early formed dolomites appear to have been calcitized during sea‐level lowstands through the infiltration of meteoric fluids into the evaporitic dolomites created during the previous highstand. In some cases, redolomitization occurred when meteoric fluids were again replaced by hypersaline brines during subsequent sea‐level highstands.Although repeated sea‐level fluctuations are clearly evident in these strata, it is likely that associated climatic changes (rainfall variations) also played a role in forming these complex diagenetic patterns. Age‐equivalent strata from Texas and New Mexico (from sites at much lower palaeolatitudes) show no such alteration patterns; samples from Greenland (slightly higher palaeolatitudes) show even more intense diagenetic alteration during depositional cycles. Thus, the examination of patterns of diagenesis may be useful in interpreting ancient, palaeolatitudinally sensitive climate patterns.