The Ordovician acritarchs of the Pomeranian Caledonides and their foreland-similarities and differences

Abstract

Ordovician acritarch assemblages of the West Porneranian Caledonides and their platformal foreland in the Peribaltic Syneclise immediately adjoining the T-T Zone were investigated. Material come from the Ordovician section of the Kościerzyna IG 1, Lębork IG 1 and Gdańsk IG 1 boreholes drilled in the marginal part of the East European Craton, though acritarchs were found only in two samples from the Gdańsk IG 1 borehole. In general, microflora was abundant and the investigations supported current ideas on the stratigraphical level of these rocks. The acritarch assemblages from the Gdańsk IG I borehole, found at a depth of 3214.8 m, contain species of Baltisphaeridium, characteristic of the Llanvirn and described by Górka from the eastern part of the Peribaltic Syneclise and the Podlasie Depression. The abundant microflora from West Pomerania contains index taxa of the uppermost Llanvirn (Llandeilo) and Caradoc. Numerous palynomorph associations from the Ordovician rocks of the Koszalin-Chojnice Zone can be divided into three local microflora zones, using quantitative data, and these may be useful for local stratigraphical correlations. Acritarch assemblages on either side of the T-T Zone show many similaritics and suggest that, in the late Llanvirn and Caradoc, there was little palaeogeographic separation between the sedimentary basins of Baltica and West Pomerania; at least, their palaeolatitudinal positions were similar. The thermal maturity of Ordovician organic matter (low and moderate) is also similar in both these areas. The highest heat flow values seem to be characteristic of the western part of the Peribaltic Syneclise, adjoining the T-T Zone. The degree of their thermal alterations in the West Pomeranian Caledonides is surprisingly low when the strong tectonic deformation and considerable depth of the Ordovician rocks are taken into account. The high morphological diversity of the acritarch assemblages, the presence of Baltisphaeridim and Ordovicidium, and especially of abundant Veryhachium, suggest that microflora from the Koszalin-Chojnice Zone represents an open-marine depositional palaeoenvironment, distant from the coast.