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1 Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain
2 Laboratoire LP3, UMR 8014 du CNRS, Université de Lille I, 59655 Villeneuve d'Ascq, France(e-mail: Jose-Javier.Alvaro{at}univ-lille1.fr)
3 Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49a, 50674 Köln, Germany
4 Pétrologie sédimentaire, B20, Sart Tilman, Université de Liège, 4000 Liège, Belgium
5 Institut für Paläontologie, Universität Erlangen-Nürnberg, Loewenichstrasse 28, 91054 Erlangen, Germany
6 UMR 5561 CNRS, Biogéosciences, Université de Bourgogne, 6 bd. Gabriel, 21000 Dijon, France
One unresolved conceptual problem in some Palaeozoic sedimentary strata is the boundary between the concepts of ‘shell concentration’ and ‘reef’. In fact, numerous bioclastic strata are transitional coquina-reef deposits, because either distinct frame-building skeletons are not commonly preserved in growth position, or skeletal remains are episodically encrusted by ‘stabilizer’ (reef-like) organisms, such as calcareous and problematic algae, encrusting microbes, bryozoans, foraminifers and sponges. The term ‘parabiostrome’, coined by Kershaw, can be used to describe some stratiform bioclastic deposits formed through the growth and destruction, by fair-weather wave and storm wave action, of meadows and carpets bearing frame-building (archaeocyaths, bryozoans, corals, stromatoporoids, etc.) and/or epibenthic, non-frame-building (e.g. pelmatozoan echinoderms, spiculate sponges and many brachiopods) organisms.
This paper documents six Palaeozoic examples of stabilized coquinas leading to (pseudo)reef frameworks. Some of them formed by storm processes (generating reef soles, aborted reefs or being part of mounds) on ramps and shelves and were consolidated by either encrusting organisms or early diagenesic processes, whereas others, bioclastic-dominated shoals in barrier shelves, were episodically stabilized by encrusting organisms, indicating distinct episodes in which shoals ceased their lateral migration.
