Foraminiferal micro-buildups (“reefs”) in the Wuchiapingian basin facies of the basal Zechstein carbonates in western Poland

Abstract

Encrusting foraminifers locally can play an essential reef-forming role such as in the Carnian fossiliferous Hallstatt Limestones of Austria where a dense succession of hardgrounds occurs with numerous small buildups (up to 2 cm high) composed of sessile foraminifers. Similar foraminiferal micro-buildups occur in a 10-cm-thick bed in the basinal facies in the basal Zechstein (Upper Permian) strata in the Radlin 60 borehole, in western Poland. Foraminiferal micro-buildups of presumably columnar habit account for up to a half of the volume. The growth of columns was interrupted by hiatal surfaces, although usually the growth of columns was restored afterwards. In the upper part of the bed with foraminiferal micro-buildups, the foraminiferal encrustations become visibly less abundant, and in most cases, they have a shape of less regular masses. The bed with foraminiferal micro-buildups abounds in closely spaced discontinuity (hiatal) surfaces, occasionally encrusted by foraminifers. Tubular foraminifers in the bed with foraminiferal micro-buildups are accompanied by various encrusting organisms, possibly microbes; they are grouped under the name Palaeonubecularia. In the intercolumnar spaces, common Midiella sp. occur. During deposition, the conditions were mostly suboxic, and various types of ferruginous morphs, common in the lower part of the bed, suggest that dysoxic–anoxic interfaces produced various microaerophilic environments for iron-bacteria. The environmental conditions during the deposition of the upper part of the bed were very similar to those characterizing the starved basin environment in which the bioclastic-peloidal and oncoidal sediments in the Zechstein Limestone have been formed. In turn, the lower part of the bed registers the transition from normal (= not starved) marine conditions, with a clear chemocline, to the starved basin conditions. The deposition of the bed was coeval with the Kupferschiefer deposition based on the correlation of δ13C curves, which implies that the previous stratigraphical concept of basal Zechstein strata has to be re-evaluated.