Reef Constructors of the Wuchiapingian Jablonna Buildup (Western Poland)

Abstract

The Jablonna buildup, one of the system of isolated reefs that was developed in Wuchiapingian times on paleohighs in the basinal zone of the Polish part of the Southern Permian Basin in Europe, is characterized by quite irregular outlines and consists of three separate reef bodies (c. 0.5-1.5 km² each; the thickness of reef complex is usually >60 m). Bryozoans were the main reef-forming organisms. Their relative ecological flexibility combined with lack of essential competition from other organisms such as sponges and corals made it possible to dominate the environment. The ability to fast bioaccumulation combined with a possibility of quick restoring of colonies after the catastrophic events such as strong storms and rapid change of relative sea level, enabled to control the reef growth for a long time in varied environmental conditions. They show two distinct phases of bryozoan reef development during the Zechstein Limestone deposition. The first one was characterized by the dominant branched bryozoans (mostly Acanthocladia) and occurred early in the depositional history. During this phase, botryoidal aragonitic cementation played very important role in the reef formation. Although the early massive aragonitic cementation was of prime importance for the buildup construction, encrusting foraminifers have played an essential role prior to the cementation. They were are equally common in cryptic and light-exposed habitats. The first phase of bryozoan reef development terminated suddenly due to changes in sea level that disturbed the upwelling circulation. The δ13C curves suggest that the boundary of the Acanthocladia biofacies and overlying mollusc-crinoid biofacies is roughly isochronous. Consequently, the bioclastic deposition of mollusc-crinoid and then brachiopod-bryozoan biofacies was prevailing for quite a long time until the second phase of bryozoan reef development occurred, with dominant reticular fenestellids bryozoans (Rectifenestella biofacies). The presence of echinoderms and strophomenid brachiopods indicates that until the lower part of the Rectifenestella biofacies the conditions were clearly stenohaline. Subsequent elimination of stenohaline organisms and progressively poorer taxonomic differentiation of fauna assemblage is characteristic for slight, gradual rise of salinity. Subsequently, microbial reefs developed which abound in the upper part of the Zechstein Limestone sections.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016