Foraminiferal colonization related to the Zechstein (Lopingian) transgression in the western part of the Wolsztyn Palaeo-Ridge area, Western Poland

Abstract

The basal Zechstein succession in SW Poland is dominated by breccias and/or conglomerates or extraclast-bearing bioclastic limestones, which were deposited during rapid flooding of the pre-existing intracontinental basin in the early Lopingian (Late Permian). Of these, the boulder-cobble breccias and conglomerates are interpreted as deposited in a rocky shore-zone where density flows and upwelling prevailed. The breccias gradually pass up into bryozoan (or other bioclastics) grainstones. The matrix-supported breccias were deposited as large extraclasts and blocks of Carboniferous rock were rolled down or detached from a cliff and were then either embedded into a carbonate sand or formed a framework supplying voids that could be colonized by tubular encrusting foraminifers. These foraminifers abound in all basal Zechstein facies (except in the debris-flow deposits) and are attributed to Palaeonubecularia . The associated faunas include other foraminifers (uniserial and hemigordiopsids), bryozoans, brachiopods, bivalves, gastropods, and microbial deposits. The prolific growth of tubular encrusting foraminifers has resulted from nutrient supply from the basin by upwelling. Botryoidal aragonite cements (also interpreted as due to upwelling) also characterize the basal Zechstein strata, although they were previously reported only from the upper Zechstein Limestone. The d13C values of the basal Zechstein deposits show small variation and oscillated around 4.0‰, suggesting that these deposits are younger than the Kupferschiefer. The basal Zechstein strata in SW Poland consisting of breccias and/or conglomerates or bioclastic limestones (often containing extraclasts of Carboniferous rocks) originated following the rapid flood of the pre-existing intracontinental basin; this general transgression was followed by subsequent increases of sea level. Boulder-cobble breccias and conglomerates occurring at the base of the Zechstein are interpreted as a rocky-shore deposit; d epositional processes of the basal Zechstein deposits were dominated by density flows. The breccias are gradually passing into bioclastic (mostly bryozoan) grainstones, and m atrix-supported breccias originated when usually large extraclasts and blocks of Carboniferous rocks were rolled down or detached from cliff and were either embedded into the carbonate sandy material or formed a framework supplying voids that could be colonized by tubular encrusting foraminifers. These foraminifers abound in all basal Zechstein facies and are included into Palaeonubecularia . They are accompanied by other foraminifers (uniserial, Agathammina , spiral) as well as bryozoans, brachiopods, bivalves, gastropods and microbial formations. The prolific growth of tubular encrusting foraminifers has resulted from supply of the nutrients from the basin by the upwelling. The botryoidal aragonite cements (interpreted as due to upwelling as well) occur in the basal Zechstein strata; previously they were reported only from the upper part of the Zechstein Limestone. The δ 13 C values of basal Zechstein deposits show small variation and oscillated around 4.0‰; they strongly suggest that these deposits are younger than the Kupferschiefer.