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

Abstract

Abstract: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.

The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid

“…These are therefore the oldest echinoderms from Gondwana and the second oldest worldwide-only the Laurentian Poleta Formation of California with its fauna of helicoplacoids appears to be older. The lower part of the Issafen Formation at Lemdad Valley has been dated to 517 Ma by Maloof et al 21 , but is highly diachronous throughout the Atlas Mountains, commencing in the Sectigena Zone of the late Banian in the southern High Atlas (Lemdad valley 27,30 ).…”

Skeletal carbonate productivity and phosphogenesis at the lower–middle Cambrian transition of Scania, southern Sweden

“…The hydrodynamic regime is reflected by the episodic presence of intra-bed erosive surfaces, overlain by grading structures, and not encrusted by microbial veneers or mats. The absence of these microbial crusts is a key difference between the Gislöv and Forsemölla units and another coeval and mixed, condensed succession, namely, the Brècheà Micmacca in the Atlas Mountains, Morocco (Álvaro & Clausen, 2006(Álvaro & Clausen, , 2008Alvaro et al 2007). The limestone interbeds of the latter also display the episodic development of chancelloriidechinoderm-sponge meadows, affected by intensive bottom current abrasion and winnowing (responsible for the onset of intra-bed erosive diastems), and an abundance of volcanogenic, phosphate and glauconite clasts.…”

Arquitectura estratigráfica, ambientes de depósito y geocronología de la Formación Olinalá (Pérmico tardío), noreste de Guerrero, México

“…Esta sucesión sedimentaria ha recibido dos nombres; por un lado Corona-Esquivel (1982) describe estas rocas denominándolas Formación Los Arcos y le asignó una edad de Pérmico superior. No obstante, el término Formación Olinalá, introducido por Flores de Dios y Buitrón (1982), es el más ampliamente utilizado en la literatura (e.g., Vachard et al, 1993Vachard et al, , 2004Buitrón et al, 1997Buitrón et al, , 2005Silva-Pineda et al, 2003;Álvaro et al, 2007).…”