Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples

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