Depositional processes of the mixed carbonate–siliciclastic rhodolith beds of the Miocene Saint-Florent Basin, northern Corsica Marco Brandano & Sara Ronca
Facies International Journal of Paleontology, Sedimentology and Geology ISSN 0172-9179 Volume 60 Number 1 Facies (2014) 60:73-90 DOI 10.1007/s10347-013-0367-z
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Author's personal copy Facies (2014) 60:73–90 DOI 10.1007/s10347-013-0367-z
ORIGINAL ARTICLE
Depositional processes of the mixed carbonate–siliciclastic rhodolith beds of the Miocene Saint-Florent Basin, northern Corsica Marco Brandano • Sara Ronca
Received: 1 December 2012 / Accepted: 1 March 2013 / Published online: 28 March 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Many sedimentary processes can lead to the formation of mixed carbonate–siliciclastic sediments in shallow shelf environments. The Miocene Saint-Florent Basin (Corsica), and in particular the Monte S. Angelo Formation, offers the possibility to analyze coarse mixed sediments produced by erosion of a rocky coast, ephemeral stream input, and shallow-water carbonate production dominated by red algae. The Monte S. Angelo Formation was deposited during the Burdigalian to Langhian interval. During this interval, the island of Corsica experienced increased subsidence related to the development of the Ligurian-Provenc¸al Basin and associated Sardinia-Corsica block rotation. Four main rhodolith-rich subfacies have been recognized: conglomerate with rhodoliths, massive rhodolith rudstone, well-bedded rhodolith rudstone, and rhodolith floatstone. The four facies have been interpreted as having been deposited in different environments of a gravel-dominated, nearshore to offshore prograding wedge. Deep-water melobesioids dominate the red algal assemblage from shoreface to offshore. Shallow-water subfamilies of lithophylloids and mastophoroids occur in only accessory amounts. Poor illumination is believed to be due to terrigenous input by ephemeral streams and wave- and current-resuspension. Resuspension processes are favored by the limited occurrence of seagrasses. Two types of siliciclastic–carbonate mixing processes characterize the investigated rhodolith-rich deposits: (1) punctuated mixing, produced by the re-deposition of terrigenous sediments by debris-flow processes during flooding events onto
M. Brandano (&) S. Ronca Dipartimento di Scienze della Terra, La Sapienza Universita` di Roma, P. Aldo Moro 5, 00185 Rome, Italy e-mail:
[email protected]
carbonate sediments together with rhodoliths of the shoreface environments, and (2) in situ mixing, produced by growth of coralline algae on siliciclastic pebbles to form the rhodoliths. Keywords Coralline algae Mixed siliciclastic–carbonate Miocene Corsica
Introduction Sediments formed by free-living, non-geniculate coralline algae include both rhodolith pavements and maerls composed of small branching rhodoliths, coralline branches, nodules, and their detritus (Bosellini and Ginsburg 1971; Bosence 1983a; Adey 1986; Freiwald et al. 1991; Freiwald 1994; Bassi 1995, 2005). These sediments are widespread in the forereef, shelves, and banks of tropical carbonate systems (Bosellini and Ginsburg 1971; Bosence 1983b, 1985; Steneck 1986; Bourrouilh-Le Jan and Hottinger 1988; Iryu et al. 1995; Cabioch et al. 1999), as well as on cool water shelves (Freiwald et al. 1991; Freiwald 1994; Henrich et al. 1995). However, coralline algal sediments are also common in mixed siliciclastic–carbonate shelves (Braga et al. 2010), both in modern (Pe´re`s and Picard 1964; Blanc 1968; Basso 1998; Brandano and Civitelli 2007; Nalin et al. 2008; Bracchi and Basso 2012) and fossil (Rasser 2000; Rasser and Piller 2004; Nalin et al. 2008; Benisek et al. 2009) examples. The morphology of the rhodoliths and their algal assemblages have been widely used for paleoecological and paleoenvironment reconstructions (Bosence and Pedley 1982; Bosence 1983b; Braga and Martı´n 1988; Bassi 1995, 2005; Basso 1998; Braga and Aguirre 2001; Rasser and Piller 2004;
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Fig. 1 a Corsica within the framework of the Mediterranean area. b Location of the study area and geological map of Corsica. c Schematic geological map of the St. Florent area, after Fellin et al. (2005), with location of the measured stratigraphic sections
Brandano et al. 2005; Kroeger et al. 2006; Nalin et al. 2008; Quaranta et al. 2012; Aguirre et al. 2012). The present work presents the results of sedimentological and paleoecological analyses of Miocene rhodolith-rich deposits of the Saint Florent Basin in northern Corsica (Fig. 1a). During the Burdigalian to Langhian period, the Island of Corsica was characterized by a eustatic sea-level rise and increased subsidence related to the development of
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the Ligurian-Provenc¸al Basin and related rotation of the Sardinia-Corsica Block (Cavazza et al. 2007). These processes created the accommodation for a shallow-marine, mixed siliciclastic–carbonate succession to form within narrow basins (Fellin et al. 2005; Cavazza et al. 2007). This paper combines the sedimentological and compositional characteristics of the rhodolith-rich deposits to infer the paleoenvironmental controls on their occurrence, geometry,
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and internal architecture and to refine the depositional model for the shallow-water, mixed siliciclastic–carbonate succession. Finally, the siliciclastic–carbonate mixing processes that affected the rhodolith-rich deposits are also discussed. The study shows that deep-water red algal assemblages dominate throughout the shelf. Poor illumination was due to the combination of terrigenous input by ephemeral streams and wave and current resuspension.
Geological setting Two structural domains have been recognized on the island of Corsica: the northeastern Alpine Corsica and the western Hercynian Corsica (Durand-Delga 1984). The Hercynian Corsica consists mainly of Precambrian to middle Paleozoic metamorphic rocks and scattered outcrops of Paleozoic sedimentary rocks cut by Carboniferous to Permian granitoids and acidic volcanic dykes related to the final stages of the Hercynian orogeny (Fig. 1b). The Alpine Corsica consists of a tectonic stack dominated by Jurassic oceanic crustal nappes, with intercalated slices of continental basement and its metamorphic sedimentary cover, which were thrust westwards onto the continental basement of western Corsica during the Alpine orogeny. The main accretion of the nappe has been dated as Cretaceous to middle Eocene (Durand-Delga 1984). The islands of Corsica and Sardinia (Corsica-Sardinia Block) rifted away from Europe at the start of the westdirected subduction which formed the Ligurian-Provenc¸al back-arc basin (Jolivet and Faccenna 2000; Carminati et al. 2010, 2012). This rifting occurred during the Aquitanian and the subsequent drifting is constrained by the 60° counter-clockwise rotation of the Corsica-Sardinia Block, which rotated primarily during the early-middle Burdigalian between 20.3 and 15 Ma (Carminati et al. 2010; Demory et al. 2011). The rotation stopped at the end of the Langhian (Vigliotti and Langenheim 1995; Speranza et al. 2002; Carminati et al. 2010; Demory et al. 2011). Many small basins developed in Corsica during the Early Miocene as a consequence of this phase of extensional tectonics, such as the St. Florent, Bonifacio, and Ale´ria basins. The St. Florent Basin is located in the NW part of Corsica. Its basement consists of the Nebbio unit (Fig. 1c), the uppermost Alpine unit, which has been preserved in the hanging wall of a Neogene graben (Fellin et al. 2005). The oldest sedimentary unit is the Fium Albino Formation, which is Aquitanian in age (Fig. 2). This unit is up to 60 m thick, crops out locally in erosional depressions cut into the underlying Nebbio nappe, and consists of continental pebble conglomerates and very-coarse- to coarse-grained sandstones. This unit is overlain by the Torra Formation (Aquitanian to Burdigalian), which is 50 m thick and made up of massive
Fig. 2 Miocene sedimentary succession of the St. Florent Basin, after Fellin et al. (2005) and Cavazza et al. (2007)
medium- to coarse-grained sandstones and pebble conglomerates with abundant skeletal remains (molluscs, echinoids, and bryozoans). The Monte S. Angelo Formation conformably overlies the Torra Formation, is up to 250 m in thick, and is late Burdigalian to Langhian in age. The skeletal assemblages of this unit are dominated by bivalves, bryozoans, foraminifera, and coralline algae that form rhodoliths. A preliminary study of these rhodoliths (Orszag-Sperber et al. 1977) highlighted the dominance of the red algae genera Mesophyllum and Lithothamnion. The Farinole Formation overlies the Torra Formation, is about 80 m thick, is Serravallian in age (Fellin et al. 2005), and is dominated by pelagic components (Fellin et al. 2005; Cavazza et al. 2007).
Materials and methods Stratigraphic and sedimentological results from three logged sections are discussed in the present study (Fig. 3). The field observations were complemented with the petrographic examination of 250 thin-sections for textural characterization and identification of skeletal components. Eight rhodolith-rich beds were analyzed. Sampling of calcareous red algae in these beds was semi-quantitative. The rhodolith structure was analyzed on polished handsample surfaces and thin-sections. Shape, structure, nucleus, and branching density of rhodoliths were described according to the approach of Bosellini and Ginsburg (1971), Bosence (1983b), and Bassi et al. (2012). The analysis of rhodolith morphology was performed by measuring the maximum, minimum, and intermediate axes of 25 rhodoliths in each bed, as well as long-axis orientation.
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Fig. 3 Measured stratigraphic sections. See Fig. 1c for locations
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Three stratigraphic sections were selected to analyze the depositional processes that produced the rhodolith-rich
beds of the Monte S. Angelo Formation (Fig. 3). The formation consists of alternating cross-bedded bioclastic packstones to grainstones with bryozoan floatstones, bioturbated bioclastic sandstones, siliciclastic conglomerates, and rhodolith-rich beds. The cross-bedded packstones to grainstones form largescale sets that are up to 5 m thick (Fig. 4a) and are made of well-sorted, very-coarse- to coarse-grained bioclastic sediments. The main components are represented by larger benthic foraminifera (Amphistegina, Heterostegina), bryozoan colonies (celleporids), bivalve fragments, echinoid plates and spines, and corallinacean algal debris. Siliciclastic lithoclasts are a subordinate component. The bounding surfaces of the cross-sets are generally planar, but curved bounding surfaces are observed in the Viscinosa stratigraphic section. Foresets vary from angular to tangential, with angles that dip about 20°. The most common components of the bryozoan floatstones are bryozoan colonies (celleporids and adeoniforms), large benthic foraminifera such as Amphistegina and nummulitids (Heterostegina, Operculina), and rare,
Fig. 4 a Cross-bedded packstone to grainstone, the bounding surfaces of the cross-sets are generally planar, with foresets forming angular to tangential basal contacts at angles that dip about 20°. b Bryozoan floatstones are characterized by generally structureless tabular beds, with some layers containing bioturbation and a nodular
texture. c The bioturbated bioclastic sandstone appears crudely stratified without any main sedimentary structures. d The siliciclastic conglomerate is clast-supported with a sandstone matrix. Inverse to normal grading characterizes many beds; clasts consist of Paleozoic granites and shales
Algal growth form, relative species abundance, and taxonomic composition were studied in 150 ultra-thin-sections (4.7 9 2.7 mm in size). Growth-form terminology follows that proposed by Woelkerling et al. (1993). Relative abundance was calculated using the point-counting method of Perrin et al. (1995). Coralline algae were identified at the lowest possible taxonomic level using taxonomic criteria applied to present-day corallines. Based on Le Gall et al. (2010), two orders of corallines were recognized: Sporolithales (including the family Sporolithaceae) and Corallinales (including the families Corallinaceae and Hapalidiaceae). The subfamilies within the last two families are those recognized by Harvey and Woelkerling (2007).
Results Lithofacies of the Monte S. Angelo Formation
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b Fig. 5 a Conglomerate with rhodoliths, organized in 2 to 3-m-thick
massive beds. b Rhodoliths in this subfacies are ellipsoidal to subspherical in shape and show an exclusive laminar structure, rhodolith nuclei consisting of bryozoan colonies, although pebbles are very common. c Massive rhodolith rudstone displaying a flat base and a large-scale, wedge-shaped geometry. d Rhodoliths in this subfacies are sub-spherical and ellipsoidal in shape with a dominant laminar structure. e The beds in the well-bedded rhodolithic rudstone are 20–50 cm thick and stacked in 10 to 15-m-thick sets, containing some thin (10 to 20-cm-thick) grainstone beds. f Ellipsoidal rhodoliths are dominant, with structure varying from laminar to columnar. g The individual rhodolith floatstone beds may be characterized by a crosslamination that forms angles of 5–10° with the bedding planes. h Rhodoliths are ellipsoidal as well as subspherical in shape. The laminar structure is still most abundant, but columnar and branching structures are also common
branching rhodoliths. The sand-sized skeletal assemblage consists of red-algal debris, small benthic foraminifera (rotaliids, Cibicides, Rotalia), worm tubes, balanid fragments, echinoid plates and spines, pectinid fragments, and planktonic foraminifera. The matrix is formed by calcisiltite. The cement consists of micro- and pseudospar that fills primary inter-particle pores and blocky spar that fills the intraskeletal and secondary mouldic pores. Syntaxial cement has developed on echinoid plates. This lithofacies is characterized by tabular beds that are up to 1 m thick. The beds are commonly structureless, although some layers contain bioturbation structures and show a nodular aspect, whereas in other beds ghosts of low-angle cross-stratification can be observed (Fig. 4b). The bioturbated bioclastic sandstone is crudely to well stratified, with subhorizontal to gently seaward-dipping bedding planes. Sediments are structureless, moderately well sorted, fine- to medium-grained mollusc–foraminifera sandstones with some coarse grains, scattered bivalve shells, and irregular echinoids (Fig. 4c). Trough lamination and ghosts of large-scale, seaward-dipping cross-stratification are present. The siliciclastic conglomerates are composed of 1 to 3-m-thick, crudely stratified, subhorizontal to gently basinward-dipping beds. The bedding surfaces are sharp and erosional. Most conglomeratic intervals are clast-supported with a sandstone matrix. Inverse to normal grading
characterizes many beds, but homogeneous beds are also common. Pebbles average 5 cm in diameter, but cobbles with a diameter of 15 cm (some reaching 30 cm) occur in some beds. Most clasts are derived from Paleozoic granites and shales (Fig. 4d). Rhodolith facies Component distribution and textural characteristics were used to define four subfacies in the investigated sections. Note that all four subfacies show compaction features, including pressure solution structures, sutured grain contacts, and grain interpenetration. The four subfacies are described below. Conglomerates with rhodoliths This subfacies is organized in 2 to 3-m-thick massive beds with lenticular geometry (Fig. 5a). The conglomerates are composed of clast-supported, poorly sorted pebble gravel. The gravel clasts do not generally show clear grading, although at the base of the bed it is possible to observe local inverse to normal grading. The clasts generally lack preferred orientation, but some large clasts are aligned parallel to the bedding planes and upward clast imbrication can be observed. The lower contact of the beds is irregular because of erosion and loading. The conglomerate matrix consists of poorly sorted siliciclastic sand (with bioclasts) and shows marked grain-size variations both vertically and laterally. Clasts mainly consist of lithic fragments from meta-volcanic felsic rocks, meta-volcanic/meta-hypabyssal felsic rocks, medium- to fine-grained orthogneisses, and quartzites. Bioclastic sandstone intraclasts are common and abundant in the upper part of the bed. Rhodoliths are subspherical (49 %) and ellipsoidal (51 %) in shape. The major axis of the ellipsoidal rhodoliths varies between 8 and 3 cm, the medium axis between 4 and 1.5 cm, and the short axis between 4 and 1 cm (Table 1). The diameter of subspherical rhodoliths ranges
Table 1 Morphological and structural characteristics of rhodoliths from the conglomerate with rhodoliths Shape
Size
Structure
Thallus growth form
Rhodolith accessory component
Hydrodynamic setting
Ellipsoidal (51 %)
L = 8–3 cm (M = 4.4 cm)
Laminar (100 %)
Encrusting (77 %) warty (23 %)
Bryozoans, serpulids
High
M = 1.5–4 cm (M = 3 cm) S = 1–4 cm (M = 2.2 cm) Subspherical (49 %)
D = 1–5 cm (M = 2.2 cm)
L long axis, M medium axis, S short axis, D diameter
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Fig. 6 a Large pebble of meta-volcanic/meta-hypabyssal felsic rocks forming the rhodolith nucleus (SFVR1). b Laminar rhodolith made up of encrusting melobesioid thalli (SFVR1). c Large pebble of calcareous lithic arenite forming a rhodolith nucleus (SRR1).
d Lumpy/fruticose melobesioid thallus (SFVR2). e Laminar rhodoliths made up of encrusting melobesioid thalli. f Lumpy sporolithacean thalli (SFCR1)
from 5 to 1 cm. Both ellipsoidal and subspherical rhodoliths always show a laminar structure. Melobesioids dominate (89 %), with Mesophyllum being the most common
genus. Sporolithaceans (7.7 %) are less common, while mastophoroids (3.3 %) are rare and represented by Spongites (Fig. 7a). The encrusting and warty growth form
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dominates in the rhodoliths of this facies (Fig. 6b). Rhodolith nuclei are represented by bryozoan colonies (66 %), pebbles (34 %), and subordinately by red algal nodules (10 %). Pebble nuclei consist of lithic fragments (Fig. 5b), most of which are from meta-volcanic felsic rocks. Large lithic fragments consist of meta-volcanic/meta-hypabyssal felsic rocks (Fig. 6a) and medium- to fine-grained orthogneisses. Smaller lithic fragments can also be formed by phyllites, slates, quartzites, chlorite-epidote schists, rare glaucophane schists, metabasites, fine-grained micaschists, and feldspatic arenites.
Melobesioids (94.7 %) dominate the algal assemblages, consisting primarily of Mesophyllum, Lithothamnion, and subordinate Phymatolithon genera. Mastophoroids (mostly Spongites and rare Lithoporella) are accessory (3.5 %), as are lithophylloids (0.5 %) and sporolithaceans (1.3 %) (Fig. 7b). The laminar rhodoliths are characterized by encrusting and warty growth morphology, while columnar and branching rhodoliths display warty and lumpy growth (Fig. 6d).
Massive rhodolith rudstone
This subfacies is characterized by compound, planar crossbedding sets that are 20–50 cm thick and dip up to 10° (Fig. 5e). These bedsets stack in 2 to 3-m-thick co-sets that contain some thin (10 cm) grainstone beds (Figs. 5e, 8). The co-sets are the vertical expression of large-scale linear clinoforms that correspond to the oblique clinoform; sensu Quiquerez and Dromart (2006). Individual clinoforms exhibit about 5 m of relief and extend for up to 300 m laterally. Components of these clinoforms are dominated by rhodoliths and red-algal debris, and contain small benthic foraminifera (rotaliids, textularids), encrusting foraminifera, echinoids, bryozoans, serpulids, and fragmented pectinids. Larger benthic foraminifera are represented by Heterostegina and Amphistegina. The terrigenous component, which can form up to 20 % of the total, consists of quartz grains and metamorphic lithoclasts. Rhodoliths are mostly ellipsoidal (68 %), with the major axis varying between 2 and 4.5 cm, the medium axis between 1 and 3.5 cm, and the short axis between 1 and 2.5 cm (Fig. 5f). Subspherical rhodoliths (32 %) have a diameter ranging between 1 and 3.5 cm (Table 3). The observed structures are laminar (66 %), columnar (20 %), and branching (14 %). Rhodolith nuclei consist of bryozoan colonies (90 %) and rare pebbles (10 %). The pebbles comprise lithic fragments (arenites, orthogneiss, chlorite schists), epidote, alkali-feldspar, and quartz. Volcanic/
These beds are laterally continuous and have generally flat bases, a large-scale, wedge-shaped geometry, and are up to 4 m thick (Fig. 5c). The beds are generally not internally stratified, but some beds show internal planar cross-stratification. Siliciclastic clasts, pebbles, and rare cobbles generally do not exceed 20 % and are concentrated in the lower part of the bed. The matrix is represented by bioclastic grainstones to packstones consisting of red algal debris, larger benthic foraminifera (Amphistegina and Heterostegina), small benthic foraminfera (rotaliids and textularids), bryozoans, serpulids, balanid and bivalve fragments, echinoid plates, and subordinate siliciclastic clasts. The majority of rhodoliths are subspherical (56 %), although ellipsoidal (44 %) forms are also abundant (Fig. 5d). The diameters of subspherical rhodoliths range from 1.5 to 6 cm. The major axis of ellipsoidal rhodoliths varies from 3 to 7.5 cm, the medium axis from 2 to 4 cm, and the short axis from 1 to 4 cm (Table 2). Observed growth patterns are laminar (62 %), columnar (28 %), and branching (10 %). The rhodolith nuclei consist principally of celleporid bryozoan colonies (65 %), as well as subordinate red algal nodules (19 %) and pebbles (10 %) (Fig. 6c). Bryozoans and the encrusting foraminifera Gypsina and Acervulina represent an important percentage of the total biogenic carbonate that forms the rhodoliths.
Well-bedded rhodolith rudstone
Table 2 Morphological and structural characteristic of rhodoliths from the massive rhodolith rudstone Shape
Size
Structure
Thallus growth form
Rhodolith accessory component
Hydrodynamic setting
Ellipsoidal (44 %)
L = 7.5–3 cm (M = 4.3 cm)
Laminar (62 %) columnar (26 %) branching (10 %)
Encrusting (43 %) warty (37.5 %) lumpy (19.5 %)
Encrusting foraminifera (Gypsina, Acervulina) bryozoans, serpulids
Moderately high
M = 2–4 cm (M = 3 cm) S = 1–4 cm (M = 2.6 cm) Subspherical (56 %)
D = 1.5–6 cm (M = 3.2 cm)
L long axis, M medium axis, S short axis, D diameter
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Fig. 7 Type of rhodolith nucleus, relative abundances, and red algal assemblages for each rhodolith subfacies. a Conglomerate with rhodoliths. b Massive rhodolith rudstone. c Well-bedded rhodolith rudstone. d Rhodolith floatstone. Melobes = Melobesioideae; Mastoph = Mastophoroideae; Lithoph = Lithophylloideae; Sporolith = Sporolithaceae
hypabyssal acid rocks having a sphaerulitic-granophyric texture are also present. Gypsina plana, acervulinids, and bryozoans are the non-coralline components that contribute substantially to nodule growth, with Gypsina and
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bryozoans being more abundant than coralline algae in many rhodoliths. Melobesioids (89 %) are the main component in the rhodoliths (Fig. 7c), with Mesophyllum being dominant
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Fig. 8 Line drawing of oblique rhodolith clinoforms at Punta di Saeta
Table 3 Morphological and structural characteristic of rhodoliths from the well-bedded rhodolith rudstone Shape
Size
Structure
Thallus growth form
Rhodolith accessory component
Hydrodynamic setting
Ellipsoidal (68 %)
L = 2–4.5 cm (M = 3.1 cm)
Laminar (66 %) columnar (20 %) branching (14 %)
Encrusting (30 %) warty (50 %) lumpy (20 %)
Encrusting foraminifera (Gypsina, Acervulina) bryozoans, serpulids
Moderately high
M = 1–3.5 cm (M = 2 cm) S = 1–2.5 cm (M = 1.6 cm) Subspherical (32 %)
D = 1–3.5 cm (M = 2.6 cm)
and Lithothamnion subordinate. Mastophoroids (9 %) are subordinate and are mainly represented by Spongites and occasionally by Lithoporella. Sporolithon thalli are accessory (2 %). The laminar rhodoliths display encrusting and warty growth forms, the columnar and branching ones are characterized by a warty and lumpy growth morphology.
Rhodolith floatstone In this subfacies, the gravel-sized sediment is represented not only by rhodoliths but also by bryozoan nodules, fragmented pectinids, and scattered metamorphic pebbles (some of which are encrusted by balanids). These components are dispersed in a coarse bioclastic grainstone to
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packstone matrix consisting of larger benthic foraminifera (Amphistegina, Heterostegina), small benthic foraminifera, echinoid plates and spines, pectinid, balanid fragments, and terrigenous components represented mainly by quartz. The individual beds in this lithofacies are 10–50 cm thick and are limited by planar to undulating bedding surfaces. Lamination forms angles of 5–10° with bedding planes (bedding-plane discordant), thus producing straightplanar first-order sets. These first-order sets stack in 2 to 3-m-thick co-sets that are bounded by subhorizontal surfaces (Fig. 5g). Bioturbation is present and in some cases may obscure the sedimentary structures. Rhodoliths are ellipsoidal (54 %) and subspherical (46 %) in shape. The major axis of the ellipsoidal rhodoliths range from 2 to 7 cm, the medium axis from 4.5 to 2 cm, and the short axis from 4 to 1 cm. The diameter of the subspherical rhodoliths varies between 5 and 1 cm (Table 4). The observed structures are laminar (53 %), columnar (31 %), and branching (16 %) (Fig. 5h). Rhodolith nuclei are mainly bryozoan colonies (85 %), less commonly pebbles (15 %). Algal assemblages consist primarily of melobesioids (89 %), with abundant Mesophyllum, subordinate Lithothamnion and Phymatolithon, accessory lithophylloids (5 %), and trace of mastophoroids and sporolithaceans (Fig. 7d). The laminar structure is mainly made up of melobesioids encrusting thalli (Fig. 6e), the columnar structure by encrusting and warty melobesioids and subordinate lithophylloids, and the branching structure by warty and lumpy thalli of melobesioids and, rarely, sporolithaceans (Fig. 6f).
Discussion Depositional model of the mixed siliciclastic–carbonate deposits of Corsica and Sardinia During the Early Miocene, in the area of present-day southern Corsica and Sardinia, coastal mixed siliciclastic–
carbonate successions and small carbonate platforms developed along a series of syn-rift sedimentary sub-basins (Sowerbutts 2000; Vigorito et al. 2005, 2010; Bassi et al. 2006; Benisek et al. 2009; Brandano et al. 2009). Topography and synsedimentary tectonics can exert a control on facies distribution and on the location of carbonate factories (Bosence 2005; Vigorito et al. 2010). As a general scheme, the coastal mixed siliciclastic–carbonate systems of Sardinia and Corsica are characterized by nearshore to shoreface deposits that have a conspicuous terrigenous content and are represented by siliciclastic conglomerates and coarse bioclastic sandstones. The skeletal fraction is formed by bivalve fragments, small bryozoans, and balanids. These deposits grade seaward into shoreface deposits that consist of crosslaminated hybrid sandstones containing echinoid plates and spines, bivalves, gastropods, balanids, small benthic foraminifera, and encrusting foraminifera. The shoreface deposits pass basinward into seagrass meadow deposits and coral boundstones, and then into rudstones to floatstones rich in red algal and larger benthic foraminifers (Cherchi et al. 2000; Brandano et al. 2009, 2010; Vigorito et al. 2010). The carbonate platforms were also affected by a conspicuous siliciclastic input (Benisek et al. 2009, 2010). Their facies belts are characterized by beach deposits formed by crossbedded foraminiferal grainstones that pass into coral reefs and coralline algal bindstones. The platform slopes are rhodolith-rich clinoforms (Benisek et al. 2009, 2010) that pass distally into bioturbated, echinoid-rich basinal wackestones and packstones. The cross-sectional profile along the depositional dip direction of these coastal and platform systems was wedge-shaped during the early Burdigalian due to significant nearshore siliciclastic input and limited carbonate sedimentation in the deeper parts of the euphotic and oligophotic zones (Pomar et al. 2012). During the late Burdigalian and Langhian, the coastal mixed-carbonate systems evolved into an infra-littoral prograding wedge (sensu Herna´ndez-Molina et al. 2000), while the platform evolved from having a low-angle-ramp geometry into flattopped platform-banks with prograding geometries and
Table 4 Morphological and structural characteristic of rhodoliths from the rhodolith floatstone Shape
Size
Structure
Thallus growth form
Rhodolith accessory component
Hydrodynamic setting
Ellipsoidal (54 %)
L = 2–7 cm (M = 3.9 cm)
Laminar (53 %) columnar (31 %) branching (16 %)
Encrusting (56 %) warty (30 %) lumpy (14 %)
Encrusting foraminifera (Gypsina, Acervulina) bryozoans, serpulids
Moderately high
M = 2–4.5 cm (M = 3.2 cm) S = 1–4 cm (M = 2.9 cm) Subspherical (46 %)
D = 1–5 cm (M = 3 cm)
L long axis, M medium axis, S short axis, D diameter
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steep-angled slopes (Benisek et al. 2009). These changes were induced by an increase of oligophotic carbonate production by coralline algae and larger benthic foraminifers, associated with significant downshelf sweeping of shallowwater sediments (Pomar et al. 2012). During the Burdigalian to Langhian interval, the SaintFlorent Basin records a transgressive phase marked by progressive deepening, from fluvial alluvial deposits of the Fium Albino Formation, through transitional deposits of the Torra Formation, to the initially shallow-marine deposits of the Monte Sant’Angelo Formation. The successive regressive phase is represented by the upper part of this formation and is followed by a new transgressive phase represented by the pelagic sediments of the Farinole Formation (Cavazza et al. 2007). This evolution is represented by the three measured sections. The Viscinosa section represents the basal part of Monte S. Angelo Formation, the Strutta Rau section the middle part and the Punta di Saeta the upper part (Figs. 2, 3). The Monte S. Angelo Formation is a gravel-dominated, nearshore to offshore wedge with interbedded sandwave deposits, bioturbated hybrid sandstones, and rhodolith beds. These beds are interpreted here as being the result of gravity reworking of the rhodoliths and other bioclastic detritus from a shallow nearshore environment into deeper environments (Cavazza et al. 2007). Rhodolith facies interpretation The four recognized subfacies are interpreted here as having been deposited in different environments of a gravel-dominated nearshore to offshore prograding wedge (Fig. 9).
Fig. 9 Depositional processes and environment proposed for the rhodolith-rich deposits of the Monte S. Angelo Formation (adapted from Herna´ndez-Molina et al. 2000). 1 conglomerate with rhodoliths,
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Conglomerates with rhodoliths The conglomerate with rhodoliths subfacies is characterized by poor sorting, an erosional lower surface, clast imbrication, and intraclasts; in addition, the observed lenticular geometry of the conglomerate beds is typical of debris flow deposits (Sohn 2000; Sohn et al. 2002). These deposits can develop in front of the mouth of small, highgradient ephemeral streams. These streams transport a coarse bed load into the marine environment, thereby contributing to the development of a gravelly shoreface where rhodoliths start to form by encrusting the shoreface pebbles. Floods and storm events may have triggered the gravity-driven flow that involved and transported the rhodolith-rich shoreface deposits. The dominance of a concentric laminar structure of rhodoliths and the abundance of pebbles in the rhodolith nuclei are in agreement with the growth of rhodoliths in a high-energy environment. The wide range of long-axis orientations of the ellipsoidal rhodoliths supports the idea that chaotic, gravity-flow processes formed these beds (Fig. 1c). Massive rhodolith rudstone The sedimentological and compositional characteristics of massive rhodolith rudstone beds suggest deposition in the rip head (sensu Herna´ndez-Molina et al. 2000), an area that develops in front of rip current channels in the transition zone between the shoreface and offshore. Rip currents, induced by breaking waves, are able to transport offshore pebbles, cobbles, and even boulders (Hart and Plint 1995) that are concentrated in the lower part of the beds. The rip
2 massive rhodolith rudstone, 3 well-bedded rhodolith rudstone, 4 rhodolith floatstone
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channels also transport skeletal components and rhodoliths formed in the shoreface. Sedimentation in this transition zone is the result of accumulation of sediment transported by rip currents and of in situ carbonate production. In fact, the rhodoliths of this subfacies show more varied structures than the previous conglomerate rhodolith subfacies. The dominant laminar structure indicates growth in a high water-energy setting, although the presence of lower energy columnar and branching structures and the different types of nuclei indicate a different origin for the rhodoliths. The long-axis orientation of the rhodoliths is primarily N–S and NNE–SSW with only minor examples trending WNW–ESE, thus indicating that the main orientation of the rhodoliths was normal to the current flow moving towards the basin (Fig. 1c). Well-bedded rhodolith rudstone The well-bedded rhodolith rudstone represents the combined accumulation in the offshore-transition zone of in situ—produced coralline algae and of sediment (rhodoliths, bioclasts, and subordinate siliciclastics) swept downslope from the shallower environments to form the depositional slope of the prograding wedge. In this upper slope setting, the bottom currents are thought to have removed fine-grained sediment and to have been responsible for rhodolith turnover. The general low relief of the clinoforms in this facies may reflect the hydrodynamic equilibrium profile, in other words the balance between sediment production and transport efficiency by currents, as illustrated by Quiquerez and Dromart (2006). The dominant shape and structure of the rhodoliths are in agreement with high-energy conditions and with the characteristics of rhodoliths from clinoforms, where ellipsoidal shape, laminar structure, encrusting and warty growth forms dominate (Brandano et al. 2005; Benisek et al. 2009). Additionally, the presence of rhodoliths having a pebble nucleus indicates downslope transfer, implying important pulsating movement of sediment from proximal environments onto the slope. The orientation of the rhodolith long axes is coherent with a general rolling and transport towards the basin (Fig. 1c). Rhodolith-rich sedimentary slopes are widely documented in the Mediterranean areas. Densely packed rhodolith beds have been reported by Carannante and Simone (1996) in the middle to outer sectors of distally steepened ramps in the Apennines. In Sardinia, during the Burdigalian, small carbonate platforms characterized by rhodolith clinoforms and lobes have been widely documented (Cherchi et al. 2000; Vigorito et al. 2005, 2010; Bassi et al. 2006; Benisek et al. 2009, 2010). Rhodolith beds from a Tortonian distally steepened carbonate ramp in Menorca
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have been described as having formed large-scale slope clinobeds with a length of 100–200 m (Pomar 2001; Brandano et al. 2005). In Sicily and Malta, the Upper Oligocene and Upper Miocene distally steepened ramp slopes are characterized by rhodolith clinoforms (Grasso et al. 1982; Bosence and Pedley 1982; Pedley 1998). Coralline red algae live in a wide range of carbonate platform and shelf environments (Adey and Macintyre 1973; Bosence 1983b; Braga et al. 2010; Bassi and Nebelsick 2010); however, as these studies underline, particularly favorable conditions should have existed during the Miocene to favor rhodolith growth in the oligophotic zone. This would have consequently promoted slope development because the type of sediment production/ accumulation loci controls the depositional profile, facies belt development, and the slope-break location (Pomar and Kendall 2008). Rhodolith floatstone The rhodolith floatstone subfacies represents accumulation in the bottomset layers at the base of the clinoforms. These layers are enriched in bivalves, mainly pectinids and oysters. The laminar rhodolith structure is still dominant, but there is a relative increase in columnar and branching structures, which suggests alternating contributions of rhodoliths swept from the slope and of rhodoliths that grew in situ in a relatively moderate energy environment. During low-energy periods, the bottomsets suffered intensive bioturbation. Conversely, episodic high-energy periods (storms) are marked by shell transport and accumulation in parallel-bedded bioclastic layers. Bosence and Pedley (1982) described a similar alternation between rudstone beds and low energy (floatstone-wackestone) beds and interpreted this alternation as facies interdigitation at the base of a sand ridge formed in the oligophotic zone, with intermittent storm activity. Paleodepth Coralline algae assemblages have been widely used to infer the paleo-water-depth of sedimentary environments (e.g., Adey 1986; Steneck 1986; Brandano et al. 2005; Braga et al. 2010; Aguirre et al. 2012). Usually, the different taxonomic composition in the different depositional settings of the carbonate platform is related to the proximal/distal (depth) gradient (Bassi et al. 2012). In the modern Mediterranean Sea, the shoreface extends to 10 m water depth, while the transition zone between the shoreface and offshore is characterized by the low-angle slope that extends to the slope break at 20–25 m (at the Atlantic coast of Spain it is 30–35 m). These depths correspond to the mean level of the storm wave-base
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(Herna´ndez-Molina et al. 2000). According to Aguirre et al. (2000), at the present day, mastophoroid and lithophylloid subfamilies dominate in shallow water and melobesioids become more abundant at depths greater than 10–20 m. In the Monte S. Angelo Formation, such a change in the red algal assemblages is not evident. The sciaphilic subfamily melobesioids dominate in all of the recognized facies, from the shoreface to the offshore, indicating that oligophotic conditions were present throughout the shelf. The shallow lithophylloid and mastophoroid subfamilies are generally accessory, a slight increase of mastophoroids (9 %) occurring only in the slope facies. Poor illumination may have been due to terrigenous input by ephemeral streams and wave and current resuspension. The most frequent natural sediments source along rocky coasts is resuspension and transport of sediments from nearby soft-bottom areas (Storlazzi and Field 2000). Periodic inundations of sand by coastal currents or storm action are a very common feature of rocky coasts throughout the world (Airoldi 2003). In this example, resuspension processes might have been favored by reduced seagrass growth, as shown by the rare occurrence of sedimentary features and components typical of seagrass environments (e.g., epiphytic foraminifera, cortoids, gastropods, lucinids, etc.) in the investigated deposits. The effect of seagrass on sedimentation is the result of reduced water flow and the protection of sediments from resuspension due to energy dissipation within the plant canopies (Gacia and Duarte 2001). Recent studies on Posidonia oceanica show that this phanerogame significantly buffers sediment resuspension, which can be reduced more than three-fold compared to an unvegetated bottom (Gacia and Duarte 2001). Coarse siliciclastic–carbonate mixing Mount (1984) proposed several different mixing modes that can lead to the development of mixed siliciclastic– carbonate deposits. Two of those modes are thought to have produced the rhodolith-rich deposits of the Monte S. Angelo Formation. One is the so-called ‘‘punctuated mixing’’, where sediment from contrasting sedimentary environments is mixed during short-term, high-energy events such as river flooding or severe storms. In this way, coarse clastic sediments, ranging from sand to boulder size, may be re-deposited by debris flows during flooding events into areas where carbonates accumulate, thereby producing conglomerates within rhodolith facies. Another mixing mode, defined by Mount (1984), is the ‘‘in situ mixing’’ of autochthonous and parautochthonous carbonates which form on a siliciclastic substrate. In this example, coralline algae grew on siliciclastic pebbles, thus forming the
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rhodoliths. These rhodoliths characterize the conglomerate with rhodolith facies and the massive rhodolith rudstone. The mixing in both rhodolith facies may be considered a ‘coarse’ siliciclastic-carbonate mixing because the carbonate and siliciclastic sediments involved in the processes are in the gravel size.
Conclusions The Monte S. Angelo Formation includes four rhodolithrich subfacies that represent four different environments of a prograding wedge system that developed in the St. Florent Basin during the Burdigalian to Langhian interval. The conglomerate with rhodoliths subfacies developed in front of the mouth of small, high-gradient ephemeral streams. Floods and storms triggered gravity-driven flows involving the rhodolith shoreface deposits. The massive rhodolith rudstone subfacies formed in front of rip current channels in the transition zone between the shoreface and the offshore. The well-bedded rhodolith rudstone subfacies formed oblique clinoforms that developed in the offshoretransition zone. The clinoforms were the product of in situ production of coralline algae and of sediment swept downslope from shallower environments that formed the depositional slope of the prograding wedge. The rhodolith floatstone subfacies represents accumulation at the end of the clinoforms at the bottomset layers. The deep-water melobesioids dominate the red algal assemblage of the four subfacies and, consequently, throughout the shelf. The shallow-water lithophylloid and mastophoroid subfamilies are generally accessory. Poor illumination was probably due to terrigenous input by ephemeral streams and wave and current resuspension. Reduced seagrass colonization may have favored the resuspension process. Two modes of siliciclastic–carbonate mixing are observed in the rhodolith-rich deposits of the Monte S. Angelo Formation. The first is punctuated mixing, represented by the conglomerate with rhodolith subfacies. In this facies, coarse clastic sediments are re-deposited by debris flow processes during flooding events onto carbonate sediments and rhodoliths produced in the shoreface. The second is in situ mixing produced by coralline algal growth on siliciclastic pebbles, thus forming the rhodoliths that are common in the conglomerate with rhodolith and in the massive rhodolith rudstone. The siliciclastic–carbonate mixing of rhodolith-rich deposits of the Monte S. Angelo Formation may be considered a ‘coarse’ siliciclastic–carbonate mixture because the type of sediment involved in the process are in the gravel size.
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Author's personal copy 88 Acknowledgments Financial support was provided by the University of Rome, La Sapienza (progetto di ateneo 2011 resp. M. Lustrino). The manuscript benefited greatly from criticism and suggestions of the reviewers Dan Bosence and Francesca Bosellini and of the editor of Facies, Franz Theodor Fu¨rsich. Many thanks go to Michele (Bike-Volcano) Lustrino who pushed us to attend at Corse-Alp 2010 and for discussion on Mediterranean Geology. We are grateful to Salvatore Milli for useful discussions and comments. Laura Tomassetti and Demetrio Meloni are thanked for assistance in the field. We are thankful to Stan Beaubien for his comments and for improving the English.
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