Steep Microbial-Dominated Slopes of the Zechstein Limestone Isolated Reefs (Upper Permian, Western Poland)

Abstract

Stromatolite and bryozoan buildups of the Wuchiapingian age in Southern Permian Basin of Europe meet all definitions given for reefs and they commonly occur at the shelf edge. In a few places, including western Poland, they are also recorded in the basinal facies. These isolated reef bodies are usually one to more than 10 km wide and long and several tens to almost 100 m thick. The main part of the reef is formed by fragmented bryozoan zoaria. A characteristic feature is a large amount of fragmented skeletal remains lacking rigidity. The reefs abound in the hemispheroid (botryoidal) aragonitic cement that is otherwise common for the Zechstein reefs. Previous studies indicated the paleogeographic controls on reef location and a very irregular shape of reef bodies and sharp decrease of thickness, corresponding to facies change, at the margins of reefs. Modern botryoidal aragonite cements contribute significantly to a very rapid cementation of steep marginal slopes of reefs and undoubtedly aragonitic cementation (including formation of botryoidal crusts) of Zechstein bryozoan sands has had a similar effect. In addition, crusts showing clotted peloidal, thrombolitic, stromatolitic and fenestral fabrics abound, and they are accompanied by brecciated fabrics. Some crusts show complex history including phases of their fragmentation as indicated by common occurrence of centimetric crust clasts in upper slope boundstones. They are often steeply dipping laminar sheets and the submarine origin of the filled fractures is indicated by the presence of marine deposits and cements; they are regarded to be neptunian dykes. There were several stages of dyke opening in some cases. There is evidence of downslope movement, probably very local, of some blocks (several metres across) of reef-crest boundstones. Narrow columnar stromatolites are characteristic for the uppermost part of the slope, and in some cases they were growing on cement crusts. The Zechstein isolated reefs show thus remarkable similarities to steep high rising carbonate slopes dominated by boundstones such as the Permian Capitan margin of Delaware Basin or Carboniferous steep-sided isolated platforms of the Pricaspian Basin in western Kazakhstan.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015