Rhodolith assemblages from the lower Tortonian carbonate ramp of Menorca (Spain): Environmental and paleoclimatic implications

Palaeogeography, Palaeoclimatology, Palaeoecology 226 (2005) 307 – 323 www.elsevier.com/locate/palaeo

Rhodolith assemblages from the lower Tortonian carbonate ramp of Menorca (Spain): Environmental and paleoclimatic implications Marco Brandano a,*, Grazia Vannucci b, Luis Pomar c, Antonio Obrador d b

a Dipartimento di Scienze della Terra, Universita` di Roma bLa SapienzaQ, P. Aldo Moro 5, 00185 Rome, Italy Dip.Te.Ris.-Dipartimento per lo Studio del Territorio e delle sue Risorse, Corso Europa 26, 16132 Genoa, Italy c Department de Cie`nces de la Terra, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain d Department de Geologia, Universitat Autono`ma de Barcelona, 08193 Bellaterra (Barcelona), Spain

Received 4 May 2004; received in revised form 13 April 2005; accepted 14 April 2005

Abstract Lower Tortonian distally steepened carbonate ramp of Menorca mostly consists of foramol and rhodalgal facies deposited in inner-middle ramp, ramp slope and outer ramp settings. Red algae are abundant from the middle ramp to the lower part of the slope and their taxonomic assemblages are clearly related to the bathymetric position. Melobesioids percentage increases basinward, passing from 55.8% in the middle ramp to 97% in the toe of slope. Mastophoroids are more abundant in the middle ramp (43.1%) and decrease toward deeper paleoenvironments (1.2%). Lithophylloids and sporolithaceans appear as accessory components from the middle ramp to slope settings. The percentage of melobesioids and mastophoroids observed in the middle ramp suggest that growth of the rhodoliths started in a water depth below 10–20 m. The deepest occurrence of the rhodoliths is in the ramp slope environment, where the dominance of melobesioids and the low percentage of shallower-water subfamily suggests a water depth range of 70 to 100 m. Shape and structure of rhodoliths are indicative of high-energy conditions in all ramp settings and they do not reflect a decrease in hydrodynamic energy related to water depth increase. These conditions are related to the presence of unidirectional currents that produced cross-bedded grainstones existing in the middle ramp, ramp slope and at the toe of the ramp slope settings. The high percentage of mastophoroids in the shallowest environments and the presence of Lithoporella and Sporolithon in the coralline assemblages suggests that carbonate production took place in tropical waters. D 2005 Elsevier B.V. All rights reserved. Keywords: Tortonian; Coralline algal assemblages; Carbonate ramp; Paleoclimate

1. Introduction * Corresponding author. Fax: +39 064454729. E-mail address: [email protected] (M. Brandano). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.04.034

Rhodoliths, a common growth form of coralline algae (Bosellini and Ginsburg, 1971; Adey and

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MacIntyre, 1973; Bosence, 1983a,b), are important sediment contributors in most rhodalgal carbonate platforms. In modern shelves, red algae thrive from the shallowest euphotic to the deepest oligophotic environments. They are widespread in the forereef and on shelves and banks in tropical regions (Bosellini and Ginsburg, 1971; Bosence, 1983b; Steneck, 1986; Bourrouilh-Le Jan and Hottinger, 1988; Hallock et al., 1988; Iryu et al., 1995; Testa and Bosence, 1999), as well as on temperate shelves (such as the Mediterranean: Pe´re`s and Picard, 1964; Blanc, 1968; Forno´s and Ahr, 1997; Basso, 1998). Rhodoliths grow, associated with larger foraminifera, in tropical forereef areas at depths between 60 and 150 m in Japan (Tsuji, 1993), and associated to currents have been reported from the Nicaraguan Rise (Hallock et al., 1988), off the Miyako Islands, Japan (Tsuji, 1993) associated to bryozoan sands. In the temperate Mediterranean, they thrive between 20 and 180 m depending on water turbidity (Bressan, 1974; Basso, 1998), being the most prolific between 40 and 90 m (Forno´s and Ahr, 1997). South of Menorca, rhodoliths also occur in the Modern shelf, associated to subaqueous dunes at 40–60 m water depth (Forno´s and Ahr, 1997). Moreover, coralline algae, together with bryozoans, molluscs, echinoids and foraminifers are the main skeletal components of tropical sediments where the chlorozoan skeletal association is limited by environmental conditions (Carannante et al., 1988; James, 1997; Pomar, 2001a; Mutti and Hallock, 2003; Pomar et al., 2004). As paleoclimatology has become a central topic of carbonate sedimentology, paleoecology is particularly significant for climate research because the carbonate secreting biotas are very sensitive archives of climate. In this context, the taxonomic composition and internal structure of the rhodoliths provides a powerful tool for paleoecological reconstructions. Recent papers have evidenced the possibility to use fossil coralline algal assemblages for paleoclimatic reconstruction (Braga and Aguirre, 2001; Piller, 2003). The aim of this paper is to document the distribution of the red algal assemblages along the depositional profile in the lower Tortonian carbonate ramp of Menorca. Rhodoliths are abundant on the middle ramp and dominate the ramp slope. The taxonomic analysis of the rhodoliths provides paleobathymetric

constraints for the different ramp settings and hints for interpretation of paleoclimatic conditions in Western Mediterranean during late Miocene.

2. Geological setting Menorca Island belongs to the Balearic archipelago, the emergent part of the Balearic Promontory. This promontory is a northeast–southwestern trending block of thick continental crust (30 km) in Western Mediterranean (Fig. 1A), whereas the Valencia trough to the northwest consists of thinned (14–15 km) continental crust (Gallart et al., 1995). The Emile Baudot escarpment to the southeast is the expression of an important northeast–southwestern trending fault separating the Balearic Promontory from the AlgerianBalearic basin, where Neogene deposits are floored by oceanic crust (Sa`bat et al., 1995; Doglioni et al., 1997, 1999). As a whole, the Balearic Promontory experienced extension and thinning during Mesozoic times, whereas during the Cenozoic compressional and extensional structures developed. The major compressional events occurred during middle Miocene, producing NE–SW oriented thrusts and NW–SE oriented strike slip faults (Gelabert, 1998). During the late Miocene, carbonate complexes prograded, preferentially to the south, on the shallow margins of paleoislands that resulted from middle Miocene tectonics (Pomar et al., 1996). These upper Miocene carbonate rocks are fairly flat-lying, having undergone only slight tilting and flexure associated with normal and strike-slip faulting during the late Neogene to middle Pleistocene times (Pomar et al., 1996). On Menorca Island (Fig. 1B), they conform the Migjorn region to the south, whereas in the north, the Tramuntana region is composed of Paleozoic, Mesozoic and lower Cenozoic rocks (Obrador, 1972–1973). The Migjorn region is composed of upper Miocene carbonates onlapping the folded and thrusted basement (Paleozoic to lower Cenozoic) and, locally, overlies two unconformity-bound shallow-marine depositional units: the Basal Unit and the Detrital Unit, which have been tentatively attributed to the lower and the middle Miocene respectively (Obrador et al., 1983a,b, 1992). Like in all Balearic Islands, upper Miocene carbonates of the Migjorn region belong to two stratigraphic sequences (Obrador et al.,

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A PYRENEES

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1992; Pomar et al., 2002). The lower sequence (Lower Bar Unit) is a distally steepened carbonate ramp, attributed to the early Tortonian (N16 foraminiferal biozone of Blow; Bizon et al., 1973). The upper sequence is a Reefal Complex, which has been dated as late Tortonian–early Messinian on Mallorca (N17 foraminiferal biozone of Blow; Bizon et al., 1973; Alvaro et al., 1984). Age estimations from Sr isotopes (Oswald, 1992) gave a late Tortonian age for the reef complex. K–Ar dates are 7.0 F 0.2 Ma for biotite and 6.0 F 0.2 Ma for sanidine phenocrystals indicating early Messinian age (Pomar et al., 1996). 2.1. The lower Tortonian Menorca ramp In the lower, early Tortonian sequence, the ramp profile for the prograding and aggrading highstand systems tract (HST) was reconstructed by Pomar (2001a) and Pomar et al. (2002) using bedding geometries, relative position of facies belt and dependence of skeletal components upon the presence of the light (Fig. 2). It corresponds to a distally steepened ramp (sensu Read, 1985) that resulted from high rates of sediment production/accumulation in the oligophotic zone (Pomar, 2001a,b). The ramp profile is divided in four lithofacies that correspond to different depositional settings. In the inner ramp, fan delta and pebbly beach deposits pass into unsorted, highly bioturbated,

5 km

m

g re e fe w d e

medium- to fine-grained mollusk-foraminifer dolopackstone. In this euphotic, shallow-water environment subjected to wave agitation, sediment transport and sorting was prevented by baffling, trapping and sheltering in seagrass beds. Basinward, the inner ramp grades into cross-bedded medium- to coarse-grained dolograinstones of the middle ramp, representing twodimensional subaqueous dunes produced by episodic, unidirectional bottom currents, below wave base (Pomar et al., 2002). The main components are rhodoliths and fragments of red algae, echinoids, bryozoans and molluscs and foraminifera (Heterostegina, Amphistegina, rotaliids and textularids). These authors, based on the absence of wave related structures and the presence of larger foraminifera (Heterostegina and Amphistegina) and red algae (mesophotic zone), suggested a water depth ranging between 40 and 70 m. The cross-bedded grainstone lithofacies passes basinward into seaward-dipping (15–208) large scale clinobeds composed mainly by rhodolithic rudstone to floatstone, interbedded with coarse- to medium-grained red-algae grainstones. These lithofacies were interpreted to represent sedimentation on a depositional slope extending basinward, beyond the larger foraminifera-dominated middle ramp. This steepened slope and the progradational character of the clinoform indicate a zone of increased sedimentation rate resulting from combined accumulation of in

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Fig. 2. Depositional model of the lower Tortonian Menorca ramp (from Pomar, 2001a).

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situ production and sediments swept from the shallower inner and middle ramp by waves and currents. Rhodoliths and coralline algae growing in the deepest part of the photic zone (oligophotic sensu Pomar, 2001b) were episodically moved by storm-induced currents but mostly represent in situ accumulation. Rounded red-algal and larger-foraminifera fragments may represent the recognizable fraction of sediment transferred from the middle ramp by currents. At the toe of the slope, the rhodolithic clinobeds thin out and interfinger basinward with fine-grained wackestones and packstones of the outer ramp. Medium- to finegrained packstone to wackestone graded beds (turbidites) and some channeled rhodolithic rudstones (debris-flow deposits) are common. The absence of in situ skeletal components derived from light-dependent biota places the outer ramp below the photic zone. The common skeletal components of the fine-grained wackestone and packstone of the outer ramp lithofacies are planktonic and small benthonic foraminifera. Subordinate whole echinoid tests, echinoid fragments, pectinids and bryozoans are present. The outer ramp fine-grained sediments accumulated in the aphotic zone (Pomar, 2001a).

3. Database and methods Samples of rhodoliths were collected at six localities (Fig. 1B) referred to the stratigraphic position and depositional model of the Menorca ramp as described by Pomar (2001a). Rhodolith samples belong to the middle ramp, ramp slope and toe of the slope. Thirty rhodolith samples were collected at each locality. The size, structure, nucleus type and algal association forming the coating sequence, and the growth morphology of algae thalli have been described for each rhodolith. The rhodolith structure has been analyzed on polished hand-sample surfaces and thin sections. Shape, structure, nucleus, and branching density of rhodoliths are described according to Bosellini and Ginsburg (1971), and Bosence (1983a, 1991). Algal growth-form terminology follows Woelkerling et al. (1993). A total of 115 ultra-thin sections (100 sections 2.8  4.8 cm in size; 15 sections 6.2  4.2 cm in size) have been studied to identify the red algae at the most precise taxonomic level. Although red algae

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from some samples cannot be identified due to pervasive dolomitization, a minimum of 10 thin sections have been studied for each locality. The percentage of the non-geniculate coralline algae is reported at family, subfamily and generic levels. Geniculate algae are rare and have not been considered in this paper. The taxonomic analysis at family and subfamily levels follows Harvey et al. (2003). The taxonomic analysis at genus level is based on the diagnostic characters defined by botanists (Womersley, 1996) and paleobotanists (e.g. Braga et al., 1993; Aguirre and Braga, 1998; Basso et al., 1998; Rasser and Piller, 1999; Bassi and Nebelsick, 2000; Stockar, 2000; Vannucci et al., 2000). According to Chamberlain and Irvine (1994) Titanoderma and Lithophyllum are considered different genera. The relative abundance of different taxa has been estimated from thin sections according to the method proposed by Perrin et al. (1995). In the analyzed rhodoliths, melobesioids include thalli of Mesophyllum and thalli with multiporate conceptacles and non-coaxial core, in which Lithothamnion cannot be discriminated from Phymatolithon because the epithallial cells and the immediate inward derivatives of the subepithallial initials are not observable or are not preserved. We use Lithothamnion/Phymatolithon to differentiate these thalli from Mesophyllum thalli.

4. Rhodolith analysis 4.1. Middle ramp The middle ramp lithofacies are mostly composed of cross-bedded, locally bioturbated, skeletal grainstones, but terrigenous dolomite is locally present. The main components are red algae fragments, mollusc, echinoid and bryozoan fragments, larger foraminifera (Amphistegina, Heterostegina), and benthonic foraminifera (rotaliids, textularids). According to Pomar et al. (2002), the cross-bedded grainstone is interpreted as medium two-dimensional subaqueous dunes with compound cross bedding produced by the migration of superposed small bedforms. The rhodoliths are commonly concentrated in pockets between the subaqueous dunes (Fig. 3). Rhodoliths from the middle ramp were sampled at two localities: Torre de Binisaida and Es Barracons.

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Fig. 3. Rhodoliths concentrated in pockets between the dunes; locality: Es Barracons.

4.1.1. Torre de Binisaida The rhodoliths are ellipsoidal in shape with major axis ranging from 3 cm to 8 cm, medium axis from 3 cm to 7 cm and short axis from 2.5 cm to 6 cm. The nucleus consists of medium-grained skeletal grains. The inner structure is generally (82%) laminar passing outward to columnar structure. Rhodoliths show an asymmetric overgrowth of the columnar structure

(Fig. 4). Alternation of columnar and branching structure is less common (18%) with branching density ascribed to group III sensu Bosence (1983a). Melobesioids (55,8%) and mastophoroids (43.1%) dominate the algal assemblages, with occasional sporolithaceans (1.1%). Melobesioids mostly include Lithothamnion/Phymatolithon thalli. Mastophoroids are represented almost exclusively by Spongites.

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Fig. 4. Rhodoliths with asymmetric overgrowth of the columnar structure; locality: Torre de Binisaida.

The laminar to columnar rhodoliths consist of alternating melobesioids and mastophoroids, but some rhodoliths are constructed by mastophoroids only. These rhodoliths are characterized by encrusting/ warty thalli evolving to lumpy in the outermost part. The columnar to branching rhodoliths are formed only

by melobesioids with lumpy/fruticose growth morphology (Fig. 5). 4.1.2. Es Barracons Es Barracons outcrop is located basinward of Torre de Binisaida. Coralline algae mainly occur as rhodo-

Fig. 5. Lumpy/fruticose growth morphology in melobesioid thallus with numerous large buried multiporate conceptacles; locality: Torre de Binisaida, thin section.

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liths and occasionally as single free-living branches. The majority of rhodoliths displays ellipsoidal shape (92%), although discoidal (8%) forms also occur. Ellipsoidal rhodolith major axis varies between 5 cm and 2.5 cm, medium axis between 5 cm to 2.5 cm and short axis between 3 cm to 1.5 cm, and they can be grouped in two assemblages. A first assemblage is characterized by laminar structure in the core passing outward to columnar (with an asymmetric overgrowth of the columnar structure). The second assemblage exhibits columnar to branching structure. The branching density may be ascribed to group II sensu Bosence (1983a). The nuclei are skeletal grains. Discoidal rhodolith mean diameter ranges from 4.5 cm to 4 cm, and short axis ranging between 1 cm and 0.5 cm. Internal structure is laminar passing outward to columnar structure. Skeletal grains acted as nuclei. The melobesioids are dominant (54.4%), being Lithothamnion/Phymatolithon the most frequent. Spongites is the most common genus of mastophoroids (36.6%). Lithoporella is a minor component in the framework. The lithophylloids (6%) are represented by Titanoderma (Fig. 6) and subordinately by Lithophyllum. Minor components are sporolithaceans (3%). The laminar to columnar rhodoliths are composed of alternating melobesioids and mastophoroids. The encrusting-warty growth form evolves to lumpy outward. The columnar to branching rhodoliths are

totally formed of mastophoroids with a lumpy/fruticose growth morphology. The unattached thalli and/or branches of red algae have a maximum length of 3 mm and are constituted by mastophoroids and/or lithophylloids. 4.2. Slope Ramp slope lithofacies is characterized by seaward dipping (15–208), large-scale clinobeds composed of rhodolithic rudstone to floatstone, interbedded with bioclastic grainstone. On the outcrops, the length of clinobeds ranges from 100 m to 200 m, and they prograde for a visible distance of about 2.5 km (Pomar et al., 2002). The rhodolithic rudstone beds are 20–50 cm thick and stack in 10- to 15-m thick sets. The rudstone bed sets alternate with grainstone intervals, of about 15-m thick, composed of 10- to 60cm-thick beds. Skeletal components are mostly red algae, but larger foraminifera (Heterostegina and Amphistegina), encrusting foraminifera, benthic foraminifera (textulariids, rotaliids), and fragments of bryozoan, mollusk, echinoid and coral (Porites) are also present. The analyzed rhodoliths come from the rudstone intervals of the upper part (S’Algar, Cala en Blanes) (Fig. 7) and the middle part of the ramp slope (Binidalı´) (Fig. 8).

Fig. 6. Titanoderma thallus with the palisade cells of the primigenous filament and tetra-bisporangial conceptacle with cylindrical pore canal; locality: Es Barrancons, thin section.

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Fig. 7. Rhodoliths displaying a laminar structure from the upper ramp slope; locality: Cala en Blanes.

4.2.1. S’Algar Ellipsoidal rhodoliths are characteristic of this locality. Major axis varies from 8 cm to 5.5 cm, medium axis from 8 cm to 5 cm and short axis from 5.5 cm to 3 cm and they belong to two types of internal structures. Columnar/branching inner structure passing outward to laminar structure (55%) and laminar-columnar inner

structure passing outward to columnar (45%). The nuclei are mostly made by bryozoans but also mollusc fragments occur. Melobesioids (71,2%) are the main component in the rhodoliths, being Lithothamnion/Phymatolithon the most frequent and Mesophyllum common. The mastophoroids (24.4%) are dominated by Spongites

Fig. 8. Rhodolithic rudstone, interbedded with bioclastic grainstone from the steep part of the slope, hammer for scale; locality: Binidalı`.

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Fig. 9. Spongites thallus with several uniporate conceptacles; locality: S’Algar, thin section.

(Fig. 9) and subordinate Lithoporella (Fig. 10). Lithophyllum is the only genus recognized among lithophylloids (4.4%). Lumpy-fruticose thalli of Lithothamnion/Phymatolithon constitute the columnar/branching to laminar

rhodoliths, whereas laminar/columnar rhodoliths have a core of encrusting-warty to lumpy thalli of Lithothamnion/Phymatolithon or Spongites and an outer part made warty-lumpy thalli of Spongites, Mesophyllum, and rare Lithophyllum.

Fig. 10. Thin crusts of Lithoporella composed of primigenous filaments with squarish cells: S’Algar, thin section.

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4.2.2. Cala en Blanes At this locality, the upper edge of the ramp slope crops out. Rhodoliths are mostly ellipsoidal, with major axis varying between 6 cm and 2.5 cm, medium axis between 5 cm to 2.5 cm and short axis between 3.5 cm to 1.5 cm. The structure varies from laminar (60%), to laminar/columnar (30%) and columnar passing outward to laminar (10%). Rhodolithic nuclei are skeletal grains only. Algal assemblages mostly comprise melobesioids (95.4%, being Lithothamnion/Phymatolithon abundant and Mesophyllum accessory), but lithophylloids (only Titanoderma) are accessory (3.1%), and mastophoroids (mostly Spongites and rare Lithoporella) occasional (1.5%). The laminar rhodolithic structure is mainly made of melobesioids encrusting thalli (Fig. 11) and rarely by mastophoroids and lithophylloids. The laminar/columnar structure is formed by encrusting-warty to lumpy melobesioids and rarely by Titanoderma and the columnar to laminar rhodolithic structure by lumpy-fruticose thalli of Lithothamnion/Phymatolithon and, externally, by encrusting thalli of Mesophyllum and/or Titanoderma.

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2.5 cm. Skeletal fragments form the nuclei. Four different rhodolith structures have been recognized: laminar to columnar with an asymmetric overgrowth of the columnar structure (73%), columnar passing outward to laminar (13%), laminar (7%), and columnar/branching (7%). Melobesioids (77.1%) are dominant in the coralline assemblages in which Lithothamnion/Phymatolithon is the most frequent, and Mesophyllum rare. The mastophoroid Spongites (21.8%) and few (1.1%) Sporolithon thalli (Fig. 12) account for the rest of the coralline assemblages. The core of the laminar/columnar rhodoliths is made by encrusting/warty thalli of Spongites and occasionally by melobesioids, and the outer layers by lumpy thalli of melobesioids and rarely sporolithaceans. The columnar to laminar rhodoliths are made by melobesioids or mastophoroids, with lumpy core structure passing outward to encrusting-warty growth forms. Encrusting thalli of Mesophyllum are the only constituent of the laminar rhodoliths. The columnar/ branching rhodoliths consist of lumpy to fruticose thalli of melobesioids. 4.3. Toe of slope

4.2.3. Binidalı` The rhodoliths are ellipsoidal shaped, with major axis varying between 8 cm and 4.5 cm, medium axis between 7 cm to 4 cm and short axis between 5 cm to

At the toe of the ramp slope, 0.5 to 2 m thick graded beds and some channeled, rhodolithic rudstones are interbedded with laminated fine-grained

Fig. 11. Laminar rhodolith made up of encrusting thalli of melobesioids; locality: Cala en Blanes, thin section.

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Fig. 12. Section through a protuberance of Sporolithon thallus of rhodoliths from steep part of the slope; locality: Binidalı`, thin section.

wackestone and packstones. Graded beds (turbidites) consist of medium- to fine-grained packstones with fragments of red algae, echinoids, mollusks and bryozoans, as well as planktonic and benthic foraminifera. Channeled rhodolithic rudstones (debris-flow deposits) may locally exhibit an erosive base and pass laterally into fine-grained dolopackstone–wackestone (Pomar et al., 2002). According to these authors, the lower slope lithofacies represent accumulation of sediments transported downslope by gravity flows and reworked by bottom currents. These currents are recorded by three-dimensional subaqueous dunes migrating parallel to the slope. Benthic foraminifera (rotaliids, textulariids), echinoid spines, pectinids, and bryozoan colonies are common (Pomar, 2001a). The analyzed rhodoliths are sampled from the rudstone/floatstone lithofacies at Cala en Porter. 4.3.1. Cala en Porter The shape of the rhodoliths is dominantly ellipsoidal, with major axis ranging from 6 cm to 2 cm, medium axis from 4 cm to 2 cm and short axis from 4 cm to 1.5 cm. Rhodolith nuclei consist of bryozoan colonies or indeterminate bioclastic fragments. Up to 55% of the rhodoliths displays an internal laminar structure, passing outward to columnar structure, whereas 45% of the rhodoliths show a columnar/ branching structure.

Melobesioids (97%) are the main component in the rhodoliths. Lithothamnion/Phymatolithon is the most frequent and, although rare, Mesophyllum is present. Mastophoroids (1.2%) are represented by Spongites. Lithophylloid genera are exclusively represented by Titanoderma which account for 1% of the algae assemblages. Sporolithon thalli are rare (0.8%). The laminar to columnar rhodoliths are characterized by encrusting and encrusting-warty growth morphology. The Columnar/branching rhodoliths display lumpyfruticose melobesioids.

5. Discussion 5.1. Bathymetry Different taxonomic composition of algal assemblages in the different depositional settings of the Menorca ramp is clearly related to the proximal/distal (shallow/deep) gradient existing from the middle ramp to the toe of the ramp slope (Fig. 13). On the lower Tortonian ramp of Menorca, the inner ramp lithofacies is composed of subhorizontally bedded, bioturbated, poorly sorted, mollusc-foraminifer (foramol) packstones interpreted as deposited in shallow-water baffled, trapped and sheltered by seagrass meadows (Pomar et al., 2002). Chlorozoan and/or

4 Cala en Blanes

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Lithoph. Sporolith.

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6 Cala en Porter

Fig. 13. Distribution of the red algal assemblages along the depositional profile in the lower Tortonian carbonate ramp of Menorca.

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chloralgal sediment associations are not documented to occur in this ramp (Obrador et al., 1992; Pomar, 2001a). Nevertheless, few and small fragments of Porites coral have been found recently to occur in the ramp slope lithofacies. Red algae (rhodolith) rich deposits do not occur in the inner ramp and start to arise in the middle ramp. The high percentage of melobesioids and mastophoroids observed in the middle ramp suggest that growth (or formation) of the rhodoliths started in a water depth below 10–20 m, which is consistent with the bathymetries inferred from sedimentological data. The coralline flora from the modern New Caledonian reefs (b5m), is dominated by mastophoroids and lithophylloids, whereas the cryptic and deeper (N 10m) parts of the reefs, melobesioids and sporolithacean become more frequent (Cabioch et al., 1999; Payri and Cabioch, 2004). Also Adey (1979, 1986), Aguirre et al. (2000) and Bosence (1991) refer, from modern tropical environments, that mastophoroids are the most abundant in shallow water and melobesioids become more abundant below 10–20 m. Melobesioids increase in abundance with water paleo-depth, passing from 55.8% in the middle ramp to 97% in the toe of slope. By the contrary, mastophoroids are more abundant in the middle ramp (43.3%) and decrease towards deeper paleoenvironments (1.2%). Nevertheless, in the steeper part of the slope (Binidalı`), mastophoroids are locally abundant (21.8%), in the nucleus of the rhodoliths. Lithophylloids and sporolithaceans appear as accessory component from the middle ramp to slope settings. Additionally, the high percentage of mastophoroids in the lower part of the ramp slope with respect to the upper part indicates downslope transference of rhodoliths, indicating important pulsating contributions of sediment swept from the middle ramp into the ramp slope. Similarly, the presence of the shallow-water genus Spongites in the nucleus of the rhodoliths indicates an initial growth in shallower setting and final growth in a deeper environment. The dominance of melobesioids and the low percentage of shallower-water subfamily (mastophoroids) in the ramp slope lithofacies suggest a water-depth range of 70 m to 100 m. On the late Miocene of Malta, Bosence (1983c) suggests 60 to 80 m of paleo-water depth for sediments dominated by Mesophyllum genus and subor-

dinately by Spongites and by Lithoporella. Modern analogues of red algae associations of slope settings can be found off the Fraser Island in Eastern Australia (Lund et al., 2000) and in the Ryukyu Islands (Iryu et al., 1995). The first example is a subtropical, transitional area between the Tropical Great Barrier Reef to the north and the temperate, cooler waters to the south. In these examples, red algae shallower than 60 m are mainly melobesioids and peyssonneliaceans with minor lithophylloids and mastophoroids in both living and subrecent rhodoliths. Below 68 m, rhodoliths are dominated by melobesioids and sporolithaceans. In the modern reef complex of the Ryukyu Islands (Iryu et al., 1995), mastophoroids and lithophylloids are the only red algae subfamilies from 0 to 15 m of water depth, and Mesophyllum (a melobesioid algae) is present at 15 m. In the 50 to 135 m water-depth interval, rhodoliths are dominated by melobesioids with minor mastophoroids (Spongites and Lithoporella) and lithophylloids. 5.2. Hydrodynamic energy As suggested by Bosence (1991), rhodoliths are sensitive indicators of hydrodynamic conditions existing in shallow-water marine environments, although activity of bottom-dwelling organisms rolling the rhodoliths cannot be excluded (Steneck, 1986; Prager and Ginsburg, 1989; Marrack, 1999; Gischler and Pisera, 1999). In the lower Tortonian ramp of Menorca, the shape and structure of rhodoliths existing from the middle ramp to the toe of the ramp slope are, all of them, characteristics of high-energy conditions. They do not show a decrease of hydrodynamic energy from shallow to deeper depositional settings. This apparent paradox can be explained with the existence of unidirectional currents that produced the ubiquitous cross-bedded grainstones existing in the middle ramp, ramp slope and at the toe of the ramp slope settings (Pomar et al., 2002). Nevertheless, discoid and open branched rhodolith structures and occurrence in pockets between dunes in middle ramp settings suggest they grew in fluctuating hydrodynamic conditions and, consequently, storms were the most plausible mechanism to produce these currents. A modern analogue can be seen in the large field of submarine dunes described from the northeast Brazi-

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lian shelf by Testa and Bosence (1999). Here, the occurrence of these bedforms is determined oceanic and tidal current resulting in a coast-parallel transport and the inter-dune troughs are characterized by freeliving, branching, red algae. 5.3. Paleoclimatic conditions In addition to paleobathymetry and hydrodynamic indicators, coralline algal assemblages are useful to infer paleoclimatic conditions (Bosence, 1983b, 1991; Aguirre et al., 2000; Braga and Aguirre, 2001; Piller, 2003). Braga and Aguirre (2001) analyzed the shallow water coralline algal assemblages of the upper Neogene carbonates from Southern Spain. These authors show that the difference is evident at subfamily level and demonstrate that lithophylloids predominate in temperate conditions whereas mastophoroids are the most abundant in tropical carbonates. The high percentage of mastophoroids (Spongites) in middle ramp settings of the lower Tortonian ramp of Menorca suggest that carbonate sedimentation took place in tropical conditions. Additionally, the presence of tropical genera (Lithoporella and Sporolithon) agrees with this paleoclimatic interpretation (cf. Bosence, 1983b; Piller, 2003).

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The high percentage of mastophoroids in the shallowest environments and the presence of Lithoporella and Sporolithon indicate that carbonate production took place in tropical conditions. Shape, morphology and structure of rhodoliths in all ramp settings from middle ramp to ramp slope are indicative of high-energy conditions and do not reflect a decrease in hydrodynamic energy related to water depth increase. These conditions fully agree with the ubiquitous cross-bedded grainstones existing in the middle ramp, ramp slope and toe of the slope setting that indicate persistent along-slope currents.

Acknowledgements Comments and suggestions from H. Westphal, G. Mateu-Vicens, J. Pignatti, and G. Civitelli are much appreciated. This research has been funded (L.P. and A.O.) by Spanish DGI Project no. BTE 2001-0372(01 and 02). Editors, J. C. Braga and D. Bosence are gratefully acknowledged for their excellent suggestions and detailed constructive criticisms, which improved the manuscript.

References 6. Conclusions The lower Tortonian ramp is an example of a distally steepened ramp resulting from high rates of sediment production and accumulation, distally, in the oligophotic zone. Red algal assemblages are related to the proximal/ distal (shallow/deep) gradient existing from the middle ramp to the toe of slope and can be used to infer paleobathymetry conditions existing along depositional profile. Nevertheless, percentage of red algae assemblages should be cautiously analyzed because downslope transport may affect the red algae associations that form the bulk of the sediments (taxa and growth morphology of algae thalli), rhodolith size and structure, and even taxa and growth shape within single rhodoliths. In our example, inner ramp setting is considered to be shallower than 10/20 m, middle ramp between 20 and 70 m of water depth, and the ramp slope placed in a water depth below 70 m.

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