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RESEARCH PRODUCT

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

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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. A paramount quantity of information characterizes the formation and distribution of shell concentrations and reefs in modern and Cenozoic strata, but the knowledge of Phanerozoic shell concentrations and reefs is decreasing when increasing in age. The Palaeozoic is a key time span in the history of life owing to the wide occurrence of skeletonized metazoans (the so-called ‘Cambrian explosion’: Zhuravlev & Riding 2000) and the successive biodiversifications related to: (i) major extinctions, such as the end-Ordovician (Cooper 2004), the Frasnian–Famenian boundary (Buggisch 1991; Copper 2002; Racki & House 2002) and the endPermian extinction (Wignall & Hallam 1992); and (ii) major community replacements, such as the Lower–Middle Cambrian transition 2 J. J. ALVARO ET AL. remains (Webb 1996). Following Wood’s (1998) definition, reefs ‘develop due to the aggregation of sessile epibenthic marine organisms, with the resultant higher rate of in-situ carbonate production than in surrounding sites’. Although the relative capacity of some gregarious epibenthic organisms bearing mineralized skeletons to be preserved in living position is a common characteristic of reef organisms, this character is also shared with other non-reef metazoans adapted to soft substrates and forming bundles-like clusters. One group of these organisms, named ‘secondary soft-bottom dwellers’ by Seilacher (1984), differs from mud stickers (e.g. non-encrusting pelmatozoans and sponges) in the heavy weight of their shells. These metazoans, mainly brachiopods and molluscs, develop common cup-, boulderand fan-shaped convergent morphologies favouring recliner strategies (Seilacher 1984; Savazzi 1999). Skeletal concentrations can be subdivided, according to their biostratinomic features, into biogenic, sedimentary, diagenetic and mixed concentrations (Kidwell et al. 1986). Biogenic concentrations can be intrinsic or extrinsic in character, the former generated by the organisms that produce the hard parts resulting from intrinsic gregarious behaviours of autochthonous and parautochthonous skeletal organisms (e.g. preferential colonization by larvae, and single colonization events of opportunistic species), and the latter produced by other organisms that interact with skeletonized organisms on their discarded hard parts leading to the formation of parautochthonous and allochthonous concentrations (e.g. hard-part-rich fecal masses and shell-filled pits). Sedimentary concentrations result from physical (usually hydraulic) processes of concentration, in which hard parts behave as sediment particles and non-bioclastic matrix is either reworked or fails to accumulate (e.g. shelly storm lags, aeolian beach pavements, channel lags in fluvial, intertidal and subtidal environments or shell-paved turbidities). Diagenetic concentrations form or are significantly enhanced by processes acting after burial, such as compaction or selective dissolution of matrix. And, finally, mixed concentrations form by the interplay of two or more kinds of the aforementioned processes, and can display both autochthonous, parautochthonous and allochthonous shell