The R1 coral reefs are only present in the Cala di Labra–Ciappili–Sperone area, where they onlap the granitic basement (Fig. 1). They consist in three- to five-metres-thick bodies, lying upon a granitic conglomerate with decimetric to metric elements surrounded by carbonate packstone. The packstone/grainstone matrix comprises corals (Porites, Thegioastraea and Tarbellastraea), red algae, bryozoans, bivalves (oysters, pectens), larger Foraminifers (miogypsinids, amphisteginids) and 10 to 15% of coarse-grained siliciclastics. The reefs contain the highest diversity of coral assemblage of the Bonifacio area (Fig. 3), with 16 species belonging to the genera Porites, Thegioastraea, Tarbellastraea, Favia, Favites, Palaeoplesiastraea, and Stylophora. At Sperone (Fig. 1), the assemblage is completed with Heliastraea s.l. sp.1, Heliastraea sp. 2, Favia valceppii, and Favites detecta. From base to top, the reefs reveal a vertical zonation: in their lower part, they display lamellar and binding colonies with few genera; in their uppermost part, they display highly diversified lamellar, massive and cauliflower-like colonies (Fig. 4) bored by lithophagid organisms. At Cala di Ciappili, several coral thickets alternate with wavy bedding bioclastic packstone/grainstone limestones. The R1 reefs are overlain by the coarse-grained sandstones of the F2 unit, which were deposited in shoreface environments, without erosion at the top of R1.

The R2 coral reefs are often comprised of two subunits, R2A and R2B, reaching ten metres in thickness. R2A and R2B are generally separated by a sharp surface or by bioclastic beds.

R2A reefs are massive and stratified. They are well developed in the Pointe Sperone and Maora areas, and localized outcrops are found along the western edge of the Bonifacio basin (Fig. 1B). R2A reefs are present only in the vicinity of granitic highs (Sperone, Paraguano, Ricetti, Balistra, Capo di U Ficu). Between Ciappili and Sperone, littoral channels infilled with bioclastic grainstones with current ripples and thickets of lamellar coral colonies are interbedded in the coral complex. Except at Cala di Paraguano, R2A reefs display thin to thick lamellar colonies dominated by poritids and Tarbellastraea (Fig. 4). The coral assemblage comprises Tarbellastraea reussiana, Thegioastraea haimei, Th. burdigalensis, Porites arenosa, and Porites sp. (Fig. 3). The matrix of the reefs is comprised of packstones/grainstones/floatstones, with rounded bioclasts of red algae and benthic foraminifers (amphisteginids, miogypsinids, operculinids, rare miliolids, Sphaerogysina globula, Rupertina and rare Martinotiella communis). Coarse siliciclastic grains are present and can represent up to 5–10% of the matrix. At Cala di Paraguano, lamellar colonies of Porites and Tarbellastraea are locally associated with columnar and branching colonies (length 20–50 cm, 2 cm in diameter) upward changing into thick tabular colonies. The packstone/grainstone matrix is dominated by red algae, rotaliids, bivalves, echinoids and gastropods, and siliciclastic grains reach 15 to 20% in abundance (Fig. 4).

R2B reefs are stacked above R2A reefs. Porites, Tarbellastraea and Thegioastraea are the main framebuilders (Fig. 3). At Sperone, the coral growth forms are dominantly thin to thick lamellar, associated with red algae, bryozoans and bivalves (Fig. 4). The matrix is a foraminifer-rich grainstone (20%: amphisteginids, operculinids, rotaliids, arenaceous) with red algae debris (15%). The reefs grade towards the northeast (in the direction of granitic substrate) into muddy and red algae/benthic foraminifer-enriched packstones to floatstones. At Balistra, the R2B reefs are made of peri-reefal bioclastic limestones with sparse coral colonies, together with some Miogypsinidae, Operculina gr. complanata, Amphisteginidae, bivalves and echinoids. From north to south, this facies laterally changes into operculinids and red algae-enriched argillaceous limestones, then into bryozoan enriched siltstones and finally into sandy/silty marls with planktonic foraminifers of Unit 4 (Fig. 2).

R3 reefs are present in the Rocchi Bianchi-Maora area and at Cala di Paraguano (Fig. 1B). They are massive and bedded build-ups resting on both the granitic basement and the R2B reefs. They are widely dominated by Thegioastraea and Porites genera organized into lamellar to massive colonies (Fig. 4). The matrix displays frequent bryozoans, larger Foraminifers (Miogypsinids and Amphisteginids), encrusting foraminifers, miliolids, some planktonic foraminifers, bivalves, echinoids, and ostracods. R3 reefs laterally change into bedded red algae boundstone (Maora) or into red algae-rich wackstone to floatstone with a micritic, pelloidal matrix. At Rocchi Bianchi, only R3 reef-derived debris of tabular Poritids are found in a conglomerate dominated by granite-derived elements.

All of the Upper Burdigalian coral reefs of the Bonifacio area onlap the granitic basement, indicating that they formed during a general relative sea-level rise culminating during lithofacies F5 deposition (U6A, Fig. 2; Ferrandini et al., 2002, 2003; Galloni, 2003; Reynaud et al., 2012). They are parts of an Upper Burdigalian transgressive system tract (Reynaud et al., 2012). The reefs of the Bonifacio area are dominantly organized into biostromal build-ups emplaced under high-to-medium energy conditions, under fair-weather wave action (domination of binding, lamellar and thicket coral forms, abundance of broken bioclasts, frequent siliciclastic inputs). Current activity is clearly recorded, for example, during deposition of reefs R2A at Sperone, where grainstone deposits are preserved in ENE–WSW-trending littoral channels. The associated larger Foraminifers, amphisteginids, miogypsinids, operculinids, and the absence of heterosteginids are also indicative of shallow-water reefs (BouDagher-Fadel, 2008). As a whole, the corals reefs of the Bonifacio area are reefs fringing a rocky, irregular coastline submitted to wave action (Fig. 5). In some rare cases, protected areas can be found, as for example at Cala di Paraguano. There, branching colonies developed during R2A deposition and branching red algae in muddy sediments during R3 episode. Our results contradict the deep building setting (limit of the upper photic zone) of the R1 reefs at Cala di Labra proposed by Tomassetti and Brandano (2013). Moreover, R1 reefs display a clear vertical zonation and were drowned by shoreface deposits of unit 2, indicating that the R1 reefs are only part of a transgressive–regressive cycle, not a single cycle (Tomassetti and Brandano, 2013).

The R1 coral reefs display the highest coral diversity in the Bonifacio area, with eight genera (Porites, Thegioastraea, Tarbellastraea, Heliastraea, Favia, Favites, Palaeoplesiastraea, Stylophora) related to 16 species (Fig. 3). R2 and R3 coral reefs are more extended than R1 ones, but they display only three genera (Porites, Tarbellastraea and Thegioastraea) related to a total of six species.

The vertical organization also displays major differences from bottom to top. The R1 reefs show a clear vertical zonation (Fig. 5) with at the base few species and at the top a maximum diversification in the coral assemblage. This indicates that the R1 reefs reached a climax stage. In contrast, R2 and R3 reefs do not display significant vertical changes concerning both internal architecture and biodiversity, excepted for the R2 at Cala di Paraguano. R2 reefs did not reach a pronounced diversification stage. The genera Heliastraea, Favia, Favites are no more present. Such an impoverishment in the coral assemblage from R1 to R2–R3, the dominant ubiquitous coral genus as Porites, Tarbellastraea and Thegioastraea in R2–R3 coeval with the development of red algae are significative of a change in seawater parameters (§ 5.3).

The general impoverishment in coral diversity during Cainozoic times in the Mediterranean, culminating with Porites monogeneric reefs during the Late Miocene, was explained by the interplaying effects of a global cooling, the re-organization of the Tethyan realm and the emergence of the Alpine chains, driving the gradual northward movement of the whole region outside the tropical/subtropical belt (Perrin and Bosellini, 2012). During the Burdigalian, Corsica and Sardinia were fully located in the reefal tropical belt (Fig. 6; Perrin and Bosellini, 2012; Popov et al., 2004). The coral diversity often reached 20–25 genera (Harzhauser et al., 2007; Perrin and Bosellini, 2012) and a total of respectively 75 and 40 genera were recorded in the subreefal facies of the Atlantic or the Alps (Fig. 6; Budd et al., 1996; Cahuzac and Chaix, 1996; Chevalier, 1961).

Upper Burdigalian coral reefs are present in northern Sardinia (Arnaud et al., 1992; Assorgia et al., 1988; Brandano et al., 2010; Martini et al., 1992; Mazzei and Oggiano, 1990). At Capo Testa (Fig. 1A), they display rather similar biodiversity and organization than in southern Corsica. They are considered as patch reefs that developed in the 35–45m depth interval dominated by the coral genus Porites (Brandano et al., 2010). Ferrandini et al. (2010) also noticed that the coral diversity decreased upward in the reefal succession, suggesting similarities with R2 and R3 reefs of southern Corsica. By contrast, coral build-ups are poorly known in the Upper Burdigalian deposits of northern, central and eastern Corsica (Fig. 1A; Orszag-Sperber, 1978). Only some coral bioclasts were found in the sediments, suggesting that coral activity extended to Alpine Corsica in the North (Ferrandini et al., 1998, 2003; Löye-Pilot et al., 2004).

The Bonifacio area displayed a favourable area for the installation of coral build-ups, with a shallow-water, granitic shoal between Corsica and Sardinia (Bonifacio Straight; Reynaud et al., 2012). Nevertheless, the presence of 8 then 3 coral genera in South Corsica indicates a low coral diversity regarding other Mediterranean areas, as in North Sardinia (Brandano et al., 2010), reflecting local vanishing conditions through upper Burdigalian times in these areas. Less favourable conditions are also recorded by the changes in internal organization and biodiversity observed between R1 and R2–R3 coral reefs.

The main factors controlling coastal reef growth and diversity are changes in nutrient levels, temperature, salinity, seawater circulations, atmospheric CO₂ concentration and Ca²⁺oceanic concentration (for an overview, see Montaggioni and Braithwaite, 2009). In the southern Corsica–northern Sardinia area, such vanishing conditions can be related to:

• • constant siliciclastic inputs (Fig. 5) issued from the emergent, neighbouring Variscan basement during the reefs building episodes; such inputs probably increased the nutrients level in coastal, protected areas;
• • during the Upper Burdigalian–Early Langhian transition (Units 5 to 6, Fig. 2) the middle part of the Upper Burdigalian Bonifacio straight subsided and water depth increased (Reynaud et al., 2012); such a depth increase was responsible for major changes in current activity: the Upper Burdigalian, a wave-dominated shoal, changed into a high-energy, tide-dominated, bioclastic shoal between the Tyrrhenian shelf and the western Mediterranean (Reynaud et al., 2012; Fig. 6).