Development of coral–sponge–microbialite reefs in a coated grain-dominated carbonate ramp (Upper Jurassic, eastern Sardinia, Italy)

The Late Jurassic is a peak time of diversification of reefs with corals, stromatoporoids, calcareous and siliceous sponges, and microbialites during the Phanerozoic. This study focuses on the Callovian–Kimmeridgian carbonate succession of eastern Sardinia, deposited at tropical latitudes on the European passive margin that recorded from the late Oxfordian the evolution from a coated grain-dominated to a reef-bearing carbonate ramp. The coated grain-dominated carbonate ramp (phase 1; Callovian–middle Oxfordian) includes inner ramp ooidal shoals and peloidal packstone in the middle-to-outer ramp. The overlying reef-bearing ramp (phase 2; late Oxfordian–late Kimmeridgian) is characterized by three types (1–3) of bioconstructions. The distribution of these build-ups along the middle-to-outer ramp depositional profile reflects bathymetric parameters, related to the interplay of water energy and light penetration. Type 1 build-ups developed in the proximal middle ramp and consist of 45 m thick, 100 m wide, coral-stromatoporoid boundstone associated with coral–stromatoporoid rudstone–grainstone. Type 2 build-ups, colonizing deeper environments in the middle ramp, are lens-shaped coral–calcareous sponge–diceratid boundstone including stromatoporoids and chaetetid sponges, 1–2 m thick and 3–4 m wide, associated with bioclastic packstone–grainstone. Type 3 lens-shaped calcareous and siliceous sponge–coral–microbialite boundstone build-ups (1 m thick and a few metres wide) formed in lower energy, distal middle-to-outer ramp settings. The evolution of the eastern Sardinian carbonate ramp reflects the Oxfordian–Kimmeridgian spread of the coral–sponge-microbialite reefs along the Tethyan European passive margin.

Laterally extensive well-exposed outcrops of carbonate depositional systems with a wide variety of facies and Oxfordian-Kimmeridgian and Tithonian coral-stromatoporoid-sponge-microbialite reefs are present in the Middle Jurassic-to-Berriasian (Lower Cretaceous) stratigraphic succession of eastern Sardinia (Fig. 1). Previous studies focused on the stratigraphic reconstruction of the sedimentary succession (Jadoul et al. 2009(Jadoul et al. , 2010Lanfranchi et al. 2011;Casellato et al. 2012;Muttoni et al. 2018) and on the characterization of Tithonian reefs (Ricci et al. 2018). This study focuses on the Callovian-Kimmeridgian southern portion (Fig. 1) of the carbonate ramp system (Dorgali, Mt. Tului and Baunei formations, sensu Jadoul et al. 2009Jadoul et al. , 2010) that recorded a transition from a carbonate succession characterized by coated grains, echinoderms, and peloids to a carbonate factory dominated by reefs with variable proportions of corals, stromatoporoids, calcareous and siliceous sponges, and microbialites. Thanks to the intermediate palaeogeographic position between the northern Tethys and the intra-Tethys platforms in the south-western portion of the northern margin of the Alpine Tethys ( Fig. 2; Dercourt et al. 2000) and affinities with both provinces, the study of eastern Sardinia carbonate system provides insights on the factors influencing carbonate lithofacies character, reef composition, and architecture during the Late Jurassic.

Geological setting
The up to 650 m-thick Middle Jurassic-Lower Cretaceous carbonate succession of eastern Sardinia (Orosei Gulf; Fig. 1a) accumulated on the southern European passive margin of the Alpine Tethys (Fourcade et al. 1993;Costamagna and Barca 2004;Costamagna 2016). During the Late Jurassic, Sardinia was located at tropical palaeolatitude ( Fig. 2) along the northern coast of the Alpine Tethys (Dercourt et al. 2000;Muttoni et al. 2018) and was characterized by an SW-NE oriented (N-S in present-day coordinates) emerged ridge of Hercynian metamorphic basement and Permian volcanic rocks (Vardabasso 1959;Dieni and Massari 1985) separating the ocean-facing eastern Sardinia succession from a restricted epicontinental sea to the north west (Cherchi and Schroeder 1985;Cherchi et al. 2010).
The carbonate succession of eastern Sardinia accumulated above a Hercynian basement peneplain, with shallowwater carbonates transitional eastward, northward, and southward to deeper basinal areas (Fig. 1a, b;Jadoul et al. 2009;Casellato et al. 2012). The succession consists of different lithostratigraphic units (Amadesi et al. 1961;Dieni and Massari 1985;revised by Jadoul et al. 2009;Fig. 1c) that represent superimposed carbonate units reflecting different depositional regimes. The carbonate succession started with marine transgressive carbonates covering the Palaeozoic Hercynian basement, Permian volcanic rocks ( Fig. 1a) or Bajocian-lower Bathonian discontinuous lenses (up to 50 m thick) of polygenic fluvial conglomerates, lacustrine and marginal marine shales, and coastal sandstone, labelled as Genna Selole Fm ( Fig. 1c; Dieni and Massari 1985;Costamagna et al. 2007;Costamagna 2016). The first marine carbonate deposits are referred to as Dorgali Fm (sensu Jadoul et al. 2009Jadoul et al. , 2010, which consists of dolomitized cross-laminated ooidal grainstone-to-packstone with detrital quartz associated with oo-crinoidal grainstoneto-packstone of Bathonian-Callovian age. The overlying Mt. Tului Fm, Baunei Fm and S'Adde Limestone record the evolution through time of a carbonate ramp ( Fig. 1c; Jadoul et al. 2009Jadoul et al. , 2010. In its lower part (approximately until middle Oxfordian), this carbonate ramp was characterized by the deposition of ooidal grainstone in the central NW area (Urzulei Supramonte, US in Fig. 1b) interfingering basinward with oo-crinoidal grainstone-packstone (lower Mt. Tului Fm), laterally passing to packstone-wackestone with chert nodules (lower Baunei Fm, Jadoul et al. 2009) in the southern basinal area (Baunei Supramonte, BS in Fig. 1B). The depositional system evolved (probably during late Oxfordian-Kimmeridgian) to a carbonate ramp with coral-stromatoporoid reefs in the central NW area (upper Mt. Tului Fm;Fig. 1b,c) passing distally to peloidal packstone-wackestone in the northern (S'Adde Limestone) and southern (upper Baunei Fm) basinal areas. The top of Mt. Tului Fm is characterized by a major regressive event marked by the subaerial exposure of the proximal area of the carbonate system (Jadoul et al. 2009) and by conglomerates with black pebbles and peritidal facies with fenestrae, tepees, mud-cracks, and stromatolites (Urzulei Fm, Jadoul et al. 2009). The upper part of the succession (approximately lower Tithonian-Berriasian) represents the recovery of the carbonate system after the subaerial exposure, with the development of coral-stromatoporoid reefs with high coral diversity (lower Mt. Bardia Fm, Ricci et al. 2018). The Fig. 1 a Simplified geological map of Sardinia, with the distribution of the Middle Jurassic-to-Berriasian carbonate succession in eastern Sardinia. The study area is marked by a green box (modified after Carmignani et al. 2008). b Distribution of the outcrops of Jurassic shallow water and basinal carbonates in the Orosei Gulf. The investigated areas are marked by the orange squares: Urzulei Supramonte (US) and Baunei Supramonte (BS); modified after . c Stratigraphic scheme of the Middle-to-Upper Jurassic carbonate succession of eastern Sardinia. The red square marks the stratigraphic succession investigated in this study; the red dashed line marks the separation between lower and upper Mt. Tului Fm; redrafted after Jadoul et al. (2009Jadoul et al. ( , 2010, Lanfranchi et al. (2011) andCasellato et al. (2012) 6 Page 4 of 32 transgression is recorded in the southern basin by the deposition of thin-bedded marly limestone (Pedra Longa Fm), followed by prograding clinostratified sigmoidal bioclastic deposits (lower Mt. Bardia Fm, Lanfranchi et al. 2011).

Materials and methods
This study investigates the Middle-Upper Jurassic (Callovian-Kimmeridgian) carbonate succession, including the upper part of the Dorgali Fm and the Mt. Tului and Baunei formations (sensu Jadoul et al. 2009Jadoul et al. , 2010 in the southern portion of the Orosei Gulf, in terms of facies character and distribution through 11 stratigraphic logs in the Baunei Supramonte (BS) and Urzulei Supramonte (US) areas (Fig. 1b). The correlation of stratigraphic logs was performed based on stratigraphic key surfaces and facies changes integrated with previous stratigraphic studies (cf. Jadoul et al. 2009;. Facies analysis was based on outcrop descriptions and petrographic analysis of about 270 polished slabs and 170 thin sections. The semi-quantitative proportion of skeletal and non-skeletal grains in thin sections was determined based on visual estimates using the comparison charts of percentage of constituents proposed for limestones by Baccelle and Bosellini (1965). For the description of coated grains, the nomenclature by Strasser (1986) was adopted. Strasser (1986) defined 6 ooid types: (1) type 1 ooids are well rounded and display fine micritic concentric laminae and possible sparite replacement, (2) type 2 ooids are irregularly shaped with fine micritic laminae (often evolving into oncoids), (3) type 3 ooids display cortices composed of several fine laminae forming a radial structure; they can appear patchily micritized and evolve into irregularly shaped coated grains, (4) type 4 ooids have 1-4 laminae and display fine  court et al. (2000). The distribution of coeval reefal carbonate systems is marked as: 1 Iberian Basin coral-microbial build-ups and stromatoporoid carpets (Bádenas and Aurell 2010); 2 France Lorraine and Jura carbonate platforms with coral-microbialite reefs (Dupraz and Strasser 2002;Lathuilière et al. 2005;Olivier et al. 2008Olivier et al. , 2011; 3 La Rochelle coral-microbialite reefs (Taylor and Palmer 1994;Olivier et al. 2003Olivier et al. , 2008; 4 Germany siliceous sponge mounds (Swabian and Franconian Alb; Olivier et al. 2004); 5 Adriatic platform stromatoporoid, coral, chaetetid reefs ; 6 Apennines carbonate platform with coral stromatoporoid reefs (Rusciadelli et al. 2011); 7 Apulian carbonate platform stromatoporoidcoral reef (Santantonio et al. 2013); 8 Northern Calcareous Alps coral-stromatoporoid reefs (Plassen carbonate platform; Gawlick 2007, 2008); 9 Portugal carbonate platforms in Atlantic realm with reefs with corals, stromatoporoids, siliceous sponges, and microbialites (Nose and Leinfelder 1997) radial structure, (5) type 5 have one lamina with a coarse radial structure, and (6) type 6 ooids display mixed cortices. Microfacies analysis and nomenclature of sponges are based on Flügel (2004) that defined: calcareous sponges as a nonsystematic term for calcisponges exhibiting calcite spicules or a calcareous skeleton; chaetetids as fossils composed of densely packed, thin calcareous tubes with horizontal partitions; stromatoporoids as fossil sponges (regarded as being related to demosponges or as a separate class of sponges) represented by calcified skeletons of domical to columnar forms, with internal structures parallel and perpendicular to the surface and distinct canal systems (astrorhizae) at the surface; siliceous sponges as an informal term for sponges with a skeleton composed of siliceous spicules, including both demosponges and hexactinellids.
To identify the presence of dolomite and ferroan calcite cement, some of the thin sections were stained with alizarin red and potassium ferricyanide (cf. Dickson 1966). Thin sections were examined under cathodoluminescence microscopy with a luminoscope Cambridge Image Technology Limited (CITL), Cambridge, UK (model MK 5-2 operating system at 10-16 kV with a beam current between 200 and 400 μA, and vacuum gauge 50-70 millitor).

Dolomitized facies
As displayed in the stratigraphic logs (Fig. 7), facies characterization suffered the widespread presence of replacive dolomitization in the lower part of the succession, close to the Hercynian basement comprising the Dorgali Fm and lower part of Mt. Tului Fm (e.g., the basal 20 m at Mt. (2) type 2 dolomite ( Fig. 8b) with coarser planar-e mosaic of zoned crystals (200-300 µm) with a turbid non-luminescent to dull nucleus and an external limpid bright luminescent rim (Fig. 8c, d).

Depositional model
The facies distribution observed in the studied Callovian-upper Kimmeridgian carbonate succession of eastern Sardinia allowed recognizing two superimposed depositional phases (phase 1 and 2) characterized by different carbonate Fig. 3 a Outcrop photograph showing herringbone cross-lamination in F1 cross-bedded ooidal-coated grain grainstone-to-packstone. b Photomicrograph of F1 cross-bedded ooidal-coated grain grainstone showing type 3 ooids and type 1 micritic ooid laminae (which form also aggregate grains) and crinoids, partially dolomitized. Arrows point to euhedral dolomite crystals partially replacing the ooid. c Photomicrograph of facies F1 showing grainstone with type 1 micritic ooids alternated with irregular wackestone lenses with sponge spicule wackestone. d Outcrop photograph showing F2 echinoderm packstone-to-grainstone-rudstone. e Photomicrograph of F4-peloidal packstone with echinoderms and thin-shelled bivalves. f Photomicrograph of F5 ooidal grainstone with intraclasts and bioclasts showing type 1 and type 3 ooids, intraclasts (I) and fragments of coral (C) and crinoids, well-cemented by equant blocky calcite cement, with oomouldic porosity. g Ooidal grainstone with intraclasts and bioclasts (F5) showing a compacted fabric with type 1 ooids and intraclasts of clotted peloidal micrite and an isopachous cement rim around grains, precipitated before compaction. h F7 bioturbated peloidal packstoneto-wackestone with sponge spicules and Lenticulina foraminifers (L) coral, stromatoporoid, calcareous, and siliceous sponge and microbialite build-ups (facies from F5 to F13). The phase 1 carbonate ramp dominated by coated grains was characterized by the gradual lateral facies variability common in a low-angle carbonate system ( Fig. 9b) with laterally adjacent depositional environments and facies belts (from facies F1 to F4; Fig. 10). The detailed reconstruction of facies architecture in the lower part of phase 1 succession (Dorgali Fm) is frequently prevented by irregularly distributed replacive dolomitization; nevertheless, the local presence of nondolomitized or partially dolomitized strata confirms facies composition from F1 to F4. The petrographic and cathodoluminescence features of the replacive dolomite mosaics suggest that dolomitization took place during burial diagenesis in contact with fluid rich in Mn 2+ , in reducing environments (cf. Hiatt and Pufahl 2014). The irregular distribution of dolomitization, the thickness variability of the dolomitized portions, the irregular shape of the dolomitization front, and the evidence of dolomite formation post-dating mechanical compaction, suggest that this burial dolomitization partially overprinted the sedimentary succession in proximity of the contact with the Hercynian basement.
Phase 1 cross-bedded ooidal-coated grain grainstone-topackstone (F1) represents ooidal shoals or strand plain in a shallow subtidal environment of the inner ramp, above the effective fair-weather wave base, as demonstrated by the grain-supported texture, well-sorting and sedimentary structures. The presence of herringbone cross-lamination ( Fig. 3a) suggests, at least locally, tidal influence on these deposits. The predominance of high-energy type 3 ooids reflects the continuous agitation of the seafloor. Episodes of low-energy sedimentation are demonstrated by thin irregular wackestone beds, suggesting that coated grain shoals were temporarily abandoned, enabling the deposition of mud in protected areas (cf. Samankassou et al. 2003). The cross-laminated echinoderm packstone-to-grainstone-rudstone (F2) was deposited in an open-marine, moderateto-high-energy environment, probably in a deeper setting with respect to F1, under currents or storms action in a proximal middle-ramp environment. This is suggested by the packstone-to-grainstone texture, good sorting, and cross-lamination. The accumulation of crinoid ossicles can form carbonate sand bars in open-marine, moderate-energy environment on carbonate ramps and at platform margins (cf. Della Porta et al. 2004 and references therein). Facies F3-peloidal packstone with ooids and echinoderms suggests deposition in lower energy environments with respect to F1 and F2. The presence of sparse ooids and echinoderm fragments, frequently broken, suggests that these grains were reworked from adjacent shallower environments. Facies F3 was probably deposited in the middle ramp, not far from ooidal shoals, between fair-weather wave base and storm wave base, where low-energy conditions were occasionally interrupted by storms. Phase 1 more distal deposits consist of peloidal packstone with chert nodules and thin-shelled bivalves (F4). The abundance of micrite matrix and fine grain size that characterize facies F4 suggests deposition in a low-energy environment below the storm wave base. The skeletal biota association suggests open-marine conditions, in particular the presence of thin-shelled bivalves (probably Bositra-like; Molina et al. 2018). Due to the frequent interfingering with middle-ramp facies F3, this environment could be interpreted as an outer ramp setting.
During phase 2, the inner ramp was characterized by the deposition of ooidal grainstone-to-packstone with intraclasts and bioclasts (F5). Phase 2 ooidal grainstone (F5) differs from phase 1 ooidal-coated grain grainstone (F1) for the abundance of bioclasts and intraclasts of clotted peloidal micrite. Facies F5 was deposited in a shallow subtidal high-energy environment, likely above the effective fair-weather wave base in the inner ramp. The common occurrence of micritic (type 1) and micritized radial ooids (type 3), formed in the high-energy environment under the continuous agitation of the sea floor, suggests it represents an ooidal shoal. The presence of fragments of The middle ramp was characterized by the deposition of peloidal packstone with reworked bioclasts and coated grains (F6), the stromatoporoid rudstone-to-packstone with coated grains (F13), and by the development of the coral, stromatoporoid, calcareous, and siliceous sponge build-ups (from F8 to F12). F6 peloidal packstone with reworked bioclasts and coated grains indicates deposition in a subtidal open-marine environment below the fair-weather wave base in the middle ramp. The occurrence of fragmented skeletal and non-skeletal grains indicates reworking of inner ramp sediment and deposition above the storm wave base in a middle-ramp environment. According to Flügel (2004), the benthic foraminifer Trocholina, common in facies F6, is typical of high-energy environments.
In type 1 build-ups (F8, F9 and F10), facies F8 bioclastic packstone-to-grainstone-rudstone with clotted peloidal micrite fragments suggests sedimentation in a relative highenergy environment where most of the grains derive from F10 coral-stromatoporoid boundstone, producing skeletal fragment of reef builders and intraclasts of clotted peloidal and dense micrite. In a similar way, the coral-stromatoporoid rudstone-grainstone (F9) is interpreted as the result of deposition of reefal debris in the higher energy area deriving from the erosion of the build-ups. Indeed, the composition of F9 is similar to F8, with some differences in the size of corals, stromatoporoids, and other calcareous sponges. In the coral-stromatoporoid boundstone (F10), the dimension of coral colonies (reaching 2 m in width) and the common presence of stromatoporoids suggest a high-energy depositional environment. According to Leinfelder et al. (2005), stromatoporoids were well-adapted to live in high-energy abrasive environments. Type 1 build-ups formed in the proximal middle-ramp environment seaward with respect to the F5 ooidal shoals of the inner ramp (Fig. 11). The widespread presence of debris deriving from the bioconstructions in facies F8 and F9 testifies that they were periodically exposed to storms and currents. San Miguel et al. (2013) interpreted deposits similar to F8 in the Kimmeridgian of the Iberian Basin as derived from storm-induced flows probably below the fair-weather wave base. Debris sheets rich in reefal material were frequently deposited following storm erosion and disintegration of the reefs (Bertling and Insalaco 1998).
Type 2 build-ups (F8 and F11) are similar in composition to type 1 build-ups but are thinner and with reduced lateral continuity. The bioclastic packstone-to-grainstone-rudstone with clotted peloidal micrite fragments (F8) represents the debris deriving from the erosion of F11. In the coral-calcareous sponge-diceratid boundstone (F11), microbialites might have coexisted with the coral growth encrusting intercorallite space and cryptic cavities, as suggested also by Olivier et al. (2003) for Kimmeridgian coral-microbial reefs in the Aquitanian Basin (France) and by Della Porta et al. (2013) for Lower Jurassic coral-calcareous sponge-microbial reefs in the High Atlas (Morocco). The association Lithocodium aggregatum-Bacinella irregularis suggests deposition in the photic zone (Dupraz and Strasser 1999). The presence of stromatoporoids associated with other calcareous sponges and the dominance of phaceloid coral colonies suggest moderate-energy environments. Build-ups type 2 are interpreted as deposited in middle-ramp settings under the action of storm waves associated with peloidal packstone with reworked bioclasts and coated grains (F6; Fig. 11).
In type 3 build-ups (F8 and F12), the presence of siliceous sponges suggests relatively deep environments, because typically during the Jurassic siliceous sponge mounds developed in deep shelves, middle-to-outer ramps or deep slopes, below storm wave base (cf. Pittet and Mattioli 2002;Bartolini et al. 2003;Olivier et al. 2004;Della Porta et al. 2013). This is confirmed by the presence of the microencruster Crescentiella morronensis and relatively lack of the light-dependent association Bacinella-Lithocodium (cf. Dupraz and Strasser 1999) with respect to type 1 and 2 build-ups. Type 3 build-ups lack high-energy stromatoporoids and accumulated in association with peloidal packstone with reworked bioclasts and coated grains (F6) and peloidal packstone with Lenticulina (F7) in distal middle rampto-outer ramp settings. The abundance of the microbially mediated clotted peloidal micrite precipitates in the studied build-ups increases from type 1 to 3, i.e., towards more distal domains (Mt. Oro). This trend could be observed in other similar depositional systems, such as in the Kimmeridgian carbonate ramps of Spain, where the relative abundance of microbial crusts increases towards deeper settings (San Miguel et al. 2017). The stromatoporoid rudstone-to-packstone with coated grains (F13) was probably deposited in the middle ramp during storm events. Stromatoporoids and ooids were reworked respectively from build-ups type 1 and type 2 and shoals in higher energy environments.
The F7 peloidal packstone with Lenticulina accumulated in a low-energy distal middle-to-outer ramp environment based on the common occurrence of these foraminifers in open-marine distal settings (cf. Hughes 2004).

Global controls on Jurassic reefs
The evolution of Jurassic carbonate platforms was influenced by climatic and oceanographic changes, eustacy, as well as syn-sedimentary tectonics (cf., Merino-Tomé et al. 2012;Brigaud et al. 2014). The extension of Upper Jurassic coral reefs at palaeolatitudes higher than 30 N and S suggests that global climate and seawater temperature were generally warm, at least in marine and coastal areas (Hallam 1985;Leinfelder et al. 2002;Sellwood and Valdes 2008). The uniformity of climatic conditions during the whole Jurassic is debated and recently questioned (Dromart et al. 2003;Dera et al. 2011;Jenkyns et al. 2011). The early Oxfordian is considered a relatively cool period characterized by a worldwide crisis of coral reef distribution , whereas a climate warming is suggested in the middle Oxfordian on the basis of biogeographical and geochemical data (Cecca et al. 2005;Dera et al. 2011), leading to ice pole melting and eustatic sea-level rise (Dromart et al. 2003). During the warm middle Oxfordian time (Cecca et al. 2005), coral reefs expanded towards the northernmost latitudes in the Jurassic (20-35° N) and were absent at low latitudes close to the Equator (Martin-Garin et al. 2012). The late Oxfordian is considered to be cooler with increased precipitation along the northern Tethys margin, lack of coral reefs at the higher latitudes occupied in the middle Oxfordian, and their shift towards the equatorial belt . Nevertheless, according to Dera et al. (2011Dera et al. ( , 2015, the Late Oxfordian-to-Early Tithonian was a global warming event with acceleration of the hydrological cycle and shifts of the humid belts and locally cooler seawater temperatures might have been related to changes in ocean circulation driven by higher sea-level (Dera et al. 2015). Hence, the global peak of reef distribution and their diversity during middle Oxfordian-to-Kimmeridgian was probably related to optimal environmental conditions favoured by climatic oscillations (Cecca et al. 2005;Martin-Garin et al. 2010;Colombié et al. 2018) and/or by other parameters related to global sea-level and ocean circulation (Kiessling 2009;Dera et al. 2015).
Eustatic sea-level fluctuations likely affected the accommodation for build-up growth: several studies pointed out  (Haq et al. 1987;Hardenbol et al. 1998;Hallam 2001;Miller et al. 2005;Haq 2018). Weissert and Mohr (1996) and Miller et al. (2005) identified a sea-level maximum in the late Oxfordian and early Kimmeridgian time, followed by a lowstand in the late Kimmeridgian. On the northern Tethys shelf, the sea-level rise is well reflected by the increase of reef sites during late Oxfordian and early Kimmeridgian (Leinfelder et al. 2002). The evolution of eastern Sardinia carbonate system from a coated grain-dominated carbonate ramp (phase 1) to a reef-bearing ramp (phase 2) is likely related to the global spread of reefs with corals, stromatoporoids, calcareous and siliceous sponges, and microbialites driven by the middle Oxfordian-Kimmeridgian climatic fluctuations and sea-level rise.

Comparison of eastern Sardinia build-ups with coeval reefs
The distribution of phase 2 build-up types with corals, diverse sponges, microencrusters, and microbialites in the eastern Sardinia ramp fits well with the model proposed for the Late Jurassic "reef window" by Leinfelder et al. (2002). According to Leinfelder et al. (2002), three basic intergrading reef types existed throughout the Jurassic: siliceous sponge reefs, microbialite reefs, and scleractinian coral and stromatoporoid reefs. The first two types developed  Figure 1c) 6 Page 24 of 32 in distal middle-to-outer ramp settings, whereas the scleractinian coral reefs developed at shelf margins or in shallow middle ramp environments (Crevello and Harris 1984;Leinfelder 1993b;Leinfelder et al. 2002). Coral-stromatoporoid-microbialite reefs and coral-debris reefs developed around or above storm wave base, whereas siliceous sponge mounds and microbialite reefs occurred in deeper, below storm wave base, settings (Leinfelder et al. 2002). ' The Upper Jurassic coral-stromatoporoid, siliceous sponge, and microbialite reefs' distribution was governed by mechanisms different from present-day reefs. The reef window in the Late Jurassic was relatively wide (Leinfelder et al. 2002): corals thrived both in mesotrophic and oligotrophic settings, because the photosymbiontic relationship with zooxanthellae was probably far from being perfect in the Jurassic (Nose and Leinfelder 1997;Leinfelder et al. 2002). Many coral taxa likely required a higher amount of particulate nutrients and reef settings were influenced by fine siliciclastic input that may have reflected the importance of heterotrophic nutrition for corals (Nose and Leinfelder 1997). Stromatoporoids could be  (Leinfelder et al. 2005).
During the Late Jurassic, reefs with corals, stromatoporoids, calcareous or siliceous sponge, and microbialites developed in the Alpine Tethys (Fig. 2) and in the Central Atlantic. Reefs from these palaeogeographic settings show differences in terms of composition, geometries, and distribution along the depositional profile. The build-up types of eastern Sardinia display similarities and differences with coeval reefs from all these palaeogeographic settings.
As for the eastern Sardinia reefs, the northern Tethys reefs mainly developed on carbonate ramps in middle ramp settings exposed to the effect of storm waves, seaward of ooidal shoals representing the inner ramp facies belt. On the western margin of the Iberian Basin during the Kimmeridgian (Aurell et al. 2003;Alnazghah et al. 2013), pinnacle reefs are reported in the middle ramp (San Miguel et al. 2017), with similar composition but different geometry with respect to the type 2 lens-shaped build-ups of eastern Sardinia. Carbonate ramps with ooidal shoals in the inner ramp and coral-microbial reefs in middle-ramp environment are reported from the Oxfordian of the French Jura and Paris Basin (Burgundy, Armorican and Lorraine platforms), where an epicontinental sea flooded the area among scattered emerged lands (Bertling and Insalaco 1998;Lathuilière et al. 2005;Olivier et al. 2011;Brigaud et al. 2014;Olivier 2019).
Carbonate-producing biota associations are different in the northern Tethys coral-microbialite reefs that generally lack stromatoporoids (Leinfelder et al. 2002;Kiani Harchegani and Morsilli 2019) with respect to eastern Sardinia reefs. This is considered to be due to lower water temperatures and scarce adaptation of stromatoporoids to mesotrophic conditions driven by high terrestrial run-off (Leinfelder et al. 2002(Leinfelder et al. , 2005. Exceptions are bioherms of chaetetids and Cladocoropsis mirabilis in lagoon and backshoal environments (e.g., Spain, Sequero et al. 2019) and some coral-stromatoporoid-microbialite mounds in the Kimmeridgian of central Spain (Pomar et al. 2015). The microbial contribution to reef growth appears to be higher during the Oxfordian-Kimmeridgian in French (Olivier et al. 2003) and Spanish (San Miguel et al. 2017) reefs than in the eastern Sardinia build-up types.
Type 3 build-ups lack stromatoporoids and contain calcareous and siliceous sponges; the latter typically formed mounds in the northern Tethys in middle-to-outer ramp settings (Leinfelder 1993b;Leinfelder et al. 2002). Among the most important occurrences of Upper Jurassic siliceous sponge build-ups, there were the Swabian and Franconian Alb (Oxfordian-Kimmeridgian) of southern Germany (Pittet and Mattioli 2002;Bartolini et al. 2003;Olivier et al. 2004), where siliceous sponge reefs formed isolated mounds on a homoclinal ramp. In the Oxfordian of Southern Spain, siliceous sponge-microbial bioconstructions developed in lowenergy, nutrient-rich conditions (Olóriz et al. 2003a(Olóriz et al. , b, 2006(Olóriz et al. , 2012Reolid et al. 2005Reolid et al. , 2007. Other coeval examples in similar depositional conditions were described in Poland (Trammer 1989;Matyszkiewicz et al. 2012) and Romania (Franţescu 2011). In eastern Sardinia, type 3 build-ups were probably formed in an ecological window overlapping between coral-stromatoporoid and siliceous sponge and microbialite environmental demands.
The Oxfordian-Kimmeridgian intra-Tethys reefs present similarities with eastern Sardinia reefs type 1 and 2 and show a different distribution pattern with respect to northern Tethys (Leinfelder et al. 2002). The characteristics of the intra-Tethys reefs are related to a structural setting deriving from extensional tectonics and lack of terrigenous input reaching the isolated platforms, favouring oligotrophic conditions (Leinfelder et al. 2002). The Upper Jurassic intra-Tethys reefs are characterized by abundant stromatoporoids and various calcareous sponges and reef zonation (Kiani Harchegani and Morsilli 2019). For instance, in the Oxfordian-Kimmeridgian of Croatia (Turnšek et al. 1981), the reef complex consists of a back-reef lagoon with Cladocoropsis patch reefs, a high-energy zone dominated by stromatoporoids, and a lower energy one dominated by coral-  Schlagintweit and Gawlick 2008). The contribution of stromatoporoids, chaetetids, and undetermined calcareous sponges in the eastern Sardinia type 1 and type 2 build-ups is important, even though corals dominate. Nevertheless, the geometry of isolated platforms of the Alpine Tethys differs from eastern Sardinia carbonate ramp because of the different structural setting. The intra-Tethys platforms are characterized by tectonically controlled high-relief steep margins, with stromatoporoid-rich reefs developing at high-energy platform margin and coral-stromatoporoid patch reefs and microencruster-cement crust boundstone characterizing the fore-reef slope (Schlagintweit and Gawlick 2008).
Differently from eastern Sardinia and other settings, in the southern Tethys extensive epeiric platform of the Arabian Peninsula, the bioconstructions were dominated by stromatoporoids and corals were not important reef builders (Hughes et al. 2008). This was likely controlled by high water temperature and oligotrophy in arid climate. Among stromatoporoids, the most widespread was Cladocoropsis mirabilis in shallow protected lagoons (Hughes et al. 2008). The Oxfordian reefs of the Hanifa Fm represent an exception, because mixed coral-stromatoporoid bioconstructions are similar in composition to the eastern Sardinia type 1 and type 2 reefs, developed as well in middle-ramp environment (Fallatah and Kerans 2018).
The eastern Sardinia build-up types 1-3 with corals, stromatoporoids, calcareous and siliceous sponges, and microbialites show some similar characters with reefs developed on carbonate systems in the Atlantic province in the Gulf Coast and Portugal.
Subsurface data from the Oxfordian Smackover Fm (U.S. Gulf Coast) show ooidal shoals in the inner ramp facing siliceous and calcareous sponge, coral, and microbialite reefs in the middle ramp (Crevello and Harris 1984) and coral-stromatoporoid reefs in higher energy settings. The depositional model suggested for the Smackover Fm resembles that proposed for phase 2 of eastern Sardinia. The siliceous and calcareous sponge, coral, and microbialite build-ups formed in moderately agitated environment and show variability in composition related to bathymetry, with increasing abundance of siliceous sponges with depth and increase in coral content in shallower environments (Baria et al. 1982;Crevello and Harris 1984). Transitional reefs (coral-siliceous sponge reefs) formed at intermediate depths between the pure siliceous sponge mounds and the coral-microbial reef environments, as observed in type 3 reefs of eastern Sardinia. Coral-stromatoporoid reefs in high-energy settings are similar in composition to type 1 build-ups. The eastern part of the North Atlantic province (Portugal) is represented by a wide variety of reefs developed in diverse structural settings. The most similar to the eastern Sardinia reefs developed in carbonate ramps (e.g., Algarve Basin, Oxfordian-Kimmeridgian; Leinfelder 1993a) where coral reefs with debris deposits developed in the distal part of the inner ramp, mixed coral-siliceous sponge reefs accumulated in the middle ramp and microbial or siliceous sponge mounds occurred mostly in the outer ramp (Leinfelder 1993a). In the Atlantic province, there are also reefs developed in structural settings different from eastern Sardinia showing less similarities with eastern Sardinia type reefs. Examples are the coral-calcareous sponge reefs with stromatoporoids and chaetetids developed at the margin of high-relief platforms in the Oxfordian-Kimmeridgian of the Abenaki Fm along the shelf of Eastern Canada (Nova Scotia; Jansa et al. 1988;Jansa 1988), in Ota (Portugal, Kimmeridgian, Leinfelder 1992;Nose 1995), in the Oxfordian-Kimmeridgian coral-microbialite reefs developed at the margin of a tilted block in the eastern margin of the North Atlantic (Morocco, Ourribane et al. 2000;Martin-Garin et al. 2007;Olivier et al. 2012) or coral patch reefs occurring on coastal siliciclastic shelves (Portugal, Leinfelder 1993a).

Controls on eastern Sardinia reef growth
The development of build-ups in the late Oxfordian-late Kimmeridgian (phase 2) carbonate ramp of eastern Sardinia is mostly controlled by light availability and water energy. The distribution of different build-up types reflects the bathymetric position, showing important variations of composition and geometry related to water depth. The interplay of light penetration and hydrodynamic energy controlled the composition of the build-ups. Type 1 coral-stromatoporoid build-ups developed in the proximal middle ramp, whereas lens-shaped coral-calcareous sponge-diceratid type 2 build-ups, including stromatoporoids, formed in distal middle-ramp settings. Type 3 coral-calcareous and siliceous sponge-microbialite buildups lacking stromatoporoids formed in deeper middle-toouter ramp environments, in lower energy and reduced light, where siliceous sponges, indicative of deeper depositional environments (Gaillard 1983;Crevello and Harris 1984;Leinfelder 1993b;Della Porta et al. 2013), were common, associated with microbially influenced clotted peloidal micritic precipitates. The bathymetric control on the distribution of different reef types reflects the control on the coeval reef distribution within carbonate ramps in northern Tethys and some carbonate platforms in the Atlantic region (Crevello and Harris 1984;Leinfelder 1993a;Leinfelder et al. 2002;Della Porta et al. 2013;Kiani Harchegani and Morsilli 2019).
A secondary control on build-up composition could be nutrient level. The absence of stromatoporoids in coral reefs is considered to be indicative of mesotrophic environments and siliciclastic input in the northern Tethys (Leinfelder et al. 2005). Instead, the contemporaneous occurrence of stromatoporoids and chaetetids is suggested as indicative of oligotrophic settings (Leinfelder et al. 2005). The abundance of stromatoporoids in the intra-Tethys platforms is interpreted as a consequence of the oligotrophic conditions related to scarce siliciclastic input in isolated platforms (Leinfelder et al. 2005).
The composition of the eastern Sardinia upper Oxfordian-upper Kimmeridgian reefs is enriched in stromatoporoids, chaetetids, and undetermined calcareous sponges with respect to other northern Tethys coeval reefs. This evidence may suggest for eastern Sardinia reduced nutrient levels and terrestrial run-off with respect to the mesotrophic conditions of the coeval northern Tethys attached carbonate ramps such as those in Spain (Bádenas and Aurell 2010;San Miguel et al. 2017), France (Lathuiliere et al. 2005Olivier et al. 2011), Swiss Jura Strasser 1999, 2002), and Germany (Olivier et al. 2004). 6 Page 28 of 32

Conclusions
The sedimentological study of the Callovian-upper Kimmeridgian portion of the eastern Sardinia Jurassic-Lower Cretaceous carbonate succession (Dorgali, Mt. Tului and Baunei formations) allowed distinguishing 13 depositional facies belonging to two superimposed carbonate ramp phases. The carbonate system evolved from a coated grain-dominated carbonate ramp (phase 1; Callovian-middle Oxfordian) to a reef-bearing carbonate ramp (phase 2; upper Oxfordian-upper Kimmeridgian). The evolution of the carbonate ramp reflects the global middle Oxfordian-early Kimmeridgian increase in reef diversity and distribution, driven by sea-level rise and climatic fluctuations. During phase 2, three different build-up types were identified, largely controlled in terms of composition and spatial distribution by bathymetry that influenced light penetration levels and water energy as in coeval northern Tethys and Atlantic carbonate systems. Coral-stromatoporoid and coral-calcareous sponge-diceratid reefs with stromatoporoids (build-up type 1 and 2) developed in middle ramp, intermittent high-energy environments, exposed to the reworking effect of storm currents, whereas calcareous and siliceous sponge-coral-microbialite reefs lacking stromatoporoids (type 3) developed in lower energy environments in the distal middle-to-outer ramp. The abundance of stromatoporoids, chaetetids, and undetermined calcareous sponges in build-up type 1 and 2 differentiate the eastern Sardinia upper Oxfordian-upper Kimmeridgian reefs from the coeval northern Tethys reefs, likely due to reduced nutrient availability and terrestrial run-off in eastern Sardinia with respect to the northern Tethys shelves. Thus, the eastern Sardinia carbonate ramp represents a transitional reef realm bridging the attached northern Tethys platforms and the isolated intra-Tethys platforms.
Funding Open access funding provided by Università degli Studi di Milano within the CRUI-CARE Agreement. Financial support was provided by PhD grant from University of Milan and MUR Ministry of University.

Compliance with ethical standards
Conflict of interest There are no conflicts of interests and data are available on request from the authors.
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