GeoActa, vol. 5, 2006, pp , Bologna

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1 GeoActa, vol. 5, 2006, pp , Bologna Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) ENRICO CAPEZZUOLI, ANNA GANDIN and FABIO SANDRELLI Dipartimento di Scienze della Terra, Università di Siena, Via Laterina 8, Siena. Abstract The Piacenzian carbonate succession of Villa Buonriposo records the last stages of the Pliocene marine deposition in the Valdelsa Basin. The carbonate succession consists of a Lower Unit (LU) mainly composed of detrital, terrigenous carbonate sediments settled in an alluvial-to-shoreface environment. Pebble conglomerates and chalky carbonate/sulphate arenites are derived from the erosion of the evaporitic-dolomitic Burano Formation and its derivation the Calcare Cavernoso. The Middle Unit (MU) composed of alternated chalky, carbonate/sulphate arenites and irregularly bedded skeletal limestone grading in the upper part to massive limestone with rhodoliths, is interpreted to correspond to the marine carbonate factory developed in a shallow, low-energy marine setting corresponding to a carbonate ramp. Rich but low-diversity heterozoan epiphytic assemblages of the Rodechfor/Rhodalgal type, are composed of branching and crustose coralline algae, foraminifers with dominant Elphidium gr. crispum and sparse Amphystegina sp., associated with echinoids, serpulid worms and bryozoans. The skeletal remains of these organisms which live supported by meadows of seagrass (Posidonia, bushy brown and green algae), were locally accumulated in place at the death of the vegetal support or reworked and redeposited probably by storm currents, as fine-grained sometimes graded bioclastic debris. The Upper Unit (UU), characterized by pebble conglomerates similar to those occurring at the base of the section, attests to the regression from coastal to alluvial conditions and the last stages of the marine deposition in the Valdelsa Basin. The fabric and composition of the carbonate lithofacies suggest that this carbonate succession was deposited on a narrow, low-energy ramp developed on the northern coast of a small island corresponding to the present Poggio del Comune Hill. This shallow setting appears to have represented a shoal connecting the Valdelsa Basin to the adjacent Volterra Basin. The components of the Piacenzian assemblages, similar to those living at present in the western Mediterranean, register depositional conditions in a temperate-warm climate. Keywords: Non-tropical marine carbonates, Palaeoecology, Piacenzian, Valdelsa, Southern Tuscany Introduction Pliocene marine carbonates, exposed in numerous small outcrops in Southern Tuscany at the bottom and/or at the top of the Middle Pliocene marine succession, have been previously reported and described under the generic name of Calcari ad Amphistegina (Lotti, 1910). In recent geological maps these marine carbonates are mapped with different names in the different Neogene Tuscan basins: i.e. Calcari di Volterra, Formazione di San Dalmazio (Costantini et al., 2002a, b; Lazzarotto et al., 2002), Formazione di Guardistallo, Calcareniti e Sabbie ad Amphistegina di Parlascio (Bossio et al., 1981). These deposits, only known for their lithostratigraphic features and fossiliferous content, have been generically referred to as a marginal environment (Bossio et al., 1993a). The aim of this work is to document the fabric, composition and environmental setting of one of these carbonate bodies, the Villa Buonriposo Limestone, deposited in the Middle Pliocene, at the end of the marine Pliocene sedimentation in Valdelsa Basin. It crops out in the San Gimignano area, where it is informally known as Calcareniti di Villa Buonriposo (litofacies SVV c of the Sabbie di San Vivaldo Formation, Costantini et al., 2002b). Geological setting The Valdelsa Basin is one of the NW-SE mor- 97

2 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli Fig. 1 - Distribution of the Neogene-Quaternary basins in the Northern Apennines (after Martini and Sagri, 1993, modified). pho-structural depressions of the Tyrrhenian side of the Northern Apennines, a fold-thrust chain formed during the Tertiary (Carmignani et al., 2001 and references therein). The structural origin of these depressions is still debated. The traditional interpretation envisages extensional tectonics which since Early-Middle Miocene affected the western sectors of Northern Apennines (Martini and Sagri, 1993; Carmignani et al., 2001), but recently other Authors (Boccaletti and Sani, 1998; Bonini and Sani, 2002) interpret them as developed in an overall compressive regime acting until the Quaternary. Despite their tectonic setting, the sedimentary infill of such Neogene- Quaternary basins records a variety of sedimentary environments, ranging from non-marine (alluvial-to-lacustrine) to marine conditions. On the basis of their sedimentary infill and geographical location, Martini and Sagri (1993) differentiated central basins, recording a Late Miocene-Pliocene fluvio-lacustrine and marine sedimentation, from peripheral basins, which during Middle Pliocene-Pleistocene experienced only continental sedimentation. The Valdelsa Basin, considered as a central basin, is bounded to the east by the Chianti Hills and to the west by the Middle Tuscan Ridge (Fig. 1). On the Chianti Hills side, the pre-neogene 98

3 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) Fig. 2 - Simplified geological map of Villa Buonriposo area. A, B 1,...C correspond to the logs showed in Fig. 4. substratum is mostly composed of Late Jurassic- Eocene deposits belonging to the tectonic Ligurid Units and to the upper Tuscan Units, while the Middle Tuscan Ridge, in the study area, is mostly composed of the Calcare Cavernoso breccia (epidiagenetic derivate of the Tuscan Triassic dolostones and anhydrites of the Burano Formation; Gandin et al., 2000) and of lithotypes of the tectonic Ligurid Units. The southern part of the Valdelsa Basin, as well as the other adjacent central basins of Southern Tuscany, is filled up with a thick Neogene-Quaternary succession (up to m; Ghelardoni et al., 1968). This succession is mainly represented by Pliocene marine siliciclastics (Costantini et al., 1988) and, only at the base, by Late Miocene marine and lacustrine deposits now exposed only in the southernmost part of the basin (Casino Basin; Lazzarotto and Sandrelli, 1977, Bossio et al., 2000). Besides some papers concerning Early Pliocene marine deposits exposed near Poggibonsi (representing the first marine sedimentary cycle in Valdelsa: Bossio et al., 1993b) and Quaternary continental carbonates (Capezzuoli and Sandrelli, 2004; Capezzuoli; 2006), most of the studies on the Neogene of the Valdelsa Basin regard the litho-biostratigraphic characterization of the widely exposed Piacenzian marine deposits, (representing the second marine Pliocene sedimentary cycle: Dainelli and Videssot, 1930; Sestini, 1970; Bossio et al, 1993a, 1995b; Dominici et al., 1995; Benvenuti and Degl Innocenti, 2001; Capezzuoli et al., 2005). The Piacenzian marine deposits consist of sands, clays and conglomerates described in literature with different stratigraphic classifications (litho-, bio-, UBSU) and different names ( Sabbie Gialle, Sabbie di San Vivaldo Formation, Ponte a Elsa Synthem, etc.), but all 99

4 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli the Authors agree to consider the various marine deposits to be heteropic. The marginal sectors of the basin are characterized by the occurrence of alluvial-deltaic conglomerates and breccias containing elements deriving from the Chianti Ridge in the eastern sectors (Conglomerato di San Casciano Formation; Canuti et al., 1966; Benvenuti and Degl Innocenti, 2001) and mainly from the Calcare Cavernoso in the western area (pc lithofacies of Dominici et al., 1995; Conglomerato di Bosco delle Volpaie Formation, Costantini et al., 2002b). Fossiliferous siliciclastic sands containing rich malacofaunas deposited in a neritic setting (calcari detritico-organogeni; Calcareniti di San Mariano of Costantini et al., 2002b) are also present in the western sector. These coarse proximal deposits grade basinwards into marine, generally massive medium-fine sands and silts including dispersed shells of large bivalves and gastropods ( Sabbie Gialle Auctorum; p2 e p3 lithofacies of Dominici et al., 1995; Sabbie di San Vivaldo Formation of Costantini et al., 2002b). Generally massive, greyish silty clays (Argille azzurre; Costantini et al., 2002b) fill the distal portion of the basin which, in its deeper part, contains abundant marine mollusc shells locally forming thin coquina beds. The depositional facies distribution in Valdelsa Basin suggests a migration, during Piacenzian, of the depocentral areas towards the western sector, probably connected with the activation of local small normal faults (Capezzuoli et al., 2005). In this period the Valdelsa sector of the Middle Tuscan Ridge was progressively submerged by the sea (with the highest tops emerging as islands; Sestini, 1970; Bossio et al., 1993a; 1995b), while in the eastern sector regression was already in progress (Capezzuoli et al., 2005). In the San Gimignano area, the western sector of the Valdelsa Basin was connected with the adjacent Volterra Basin through a shallow shoal where mixed carbonate/clastic sediments were deposited (Dainelli and Videsott, 1930). In response to the subsequent general uplift of southern Tuscany (Bossio et al., 1995a) the marine regression reached this area, leading to the final emersion of the Valdelsa Basin in the Middle-Late Pliocene. The mixed succession of Villa Buonriposo, studied in this work, was deposited on the shallow shoal and therefore rests on top of the proximal facies of the Pliocene marine succession, or directly on the Triassic evaporitic- Calcare Cavernoso substrate, exposed on the northern part of the Poggio del Comune Hill (628 m a.s.l.) that during the Piacenzian emerged as an island. In this area, the Middle Pliocene siliciclastic succession begins with fossiliferous sands, conglomerates and breccias derived from the Calcare Cavernoso. The coarse deposits generally grade upwards into yellow quartz sands ( Sabbie Gialle ) or rapidly pass, mainly in the western part, to the deeper blue clays ( Argille azzurre ) which correspond to the easternmost clay sediments of the Volterra Basin (Bossio et al., 1994). The foraminiferal assemblage of these clays, consisting of a dominant benthic association with rare planktonic foraminifers, indicates a low infralittoral - upper circalittoral palaeoenvironment and a Piacenzian age (Capezzuoli et al., 2005). The Villa Buonriposo succession The Villa Buonriposo calcareous unit is exposed in a small area of about 6 km 2 to the west of San Gimignano (Figs. 1, 2). It locally rests on the marine Piacenzian Sabbie Gialle or Argille azzurre or on the Ligurian flysch deposits, but mainly overlies the evaporitic/dolomitic rocks of the Triassic Burano Formation and the associated Calcare cavernoso. The succession is composed of three main lithotypes: pebble conglomerate with micritic or arenitic matrix; chalky carbonate/sulphate arenite; skeletal limestone. These lithotypes are assembled in three different units (Fig. 3): Lower Unit (LU) - pebble conglomerates with micritic matrix, followed by chalky carbonate/sulphate arenites; Middle Unit (MU) - alternated chalky, carbonate/sulphate arenites and irregularly bedded skeletal and chalky limestone grading to massive limestone with rhodoliths; Upper Unit (UU) - pebble conglomerates and breccias, with micritic matrix, similar to those occurring at the base of the succession. The lateral and vertical distribution of the lithofacies varies from north to south. The maximum thickness of the carbonate succession at Villa La Ripa area (Section C, Fig. 2; Fig. 3) is about 60 meters, while it decreases towards the SSW to about 50 meters near Villa Buonriposo, 100

5 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) Fig. 3 - Schematic stratigraphic logs of the Villa Buonriposo area and relationships among the different units. 101

6 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli Fig. 4 - Stratigraphic columns with sedimentological features and fossil content. (Section B, Fig 2; Fig. 3) and about 40 meters at Libbiano -Villa Citerna (Section A, Fig. 2; Fig. 3). Poor sediment exposure due to erosion and vegetal cover prevent a proper recognition of the scale and geometry of the depositional bodies. Section A: Libbiano - Villa Citerna area The lower part of the succession (LU) consists of layers of fine-grained, poorly cemented chalky carbonate/sulphate arenites (1-2 m thick) with plane-parallel low-angle lamination, and sub- 102

7 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) ordinate lenticular intercalations (up to 1 m thick) of skeletal, well cemented, limestone. This unit rests on conglomerates or locally on skeletal limestone and is abruptly overlain by the UU thick beds of pebble conglomerates (1-1.5 meters) and breccias (up to 40 m) characterized by channel deposits and basal erosional contacts (Fig. 4). The clasts of medium size (up to 6 cm in diameter) and commonly well sorted, often show a flattened shape; they are mainly composed of crystalline limestone ( Calcare Cavernoso ), fine-grained dolostone (Triassic Burano Formation) and minor quartzose and phyllitic clasts deriving from the Triassic Verrucano or the Ligurian units. They are enclosed in a fine-grained, rusty-brown calcareous matrix that locally becomes dominant. Section B: Villa Buonriposo area - abandoned quarry front and section along S.P. 69 Road The fine-grained chalky carbonate/sulphate arenites (up to 20 meters thick) of the LU grade in to the MU, which in the lower part is composed of irregularly, alternated thin beds (15 to 30 cm thick, up to 1 m) of arenite and porous, coarse skeletal limestone (Fig. 4). The contacts between the two lithologies, marked by the selective erosion of the poorly lithified, chalky arenite, are uneven and commonly sharp. Local concentrations of lithic clasts, sometimes encrusted by coralline algae, are commonly found at the base of the beds. In the upper part of the unit the arenite intercalations decrease in thickness and lateral extension, and the skeletal limestone becomes massive. Remains of organisms apparently in growth position (pelecipods, coralline clusters) are irregularly scattered all over the unit, while small rhodoliths are frequently found in the upper part. Section C: Villa La Ripa area In this area, the fine-grained chalky carbonate/ sulphate arenites resting on the locally fossiliferous quartzose Sabbie Gialle, are poorly exposed for about 60 meters (Fig. 4). Thick layers of skeletal limestone with rhodoliths are interbedded in the upper part of the section. Depositional facies and microfacies The sedimentological features of the main lithofacies have been studied in the three best-exposed outcrops (Fig. 4). Conglomerates Pebble conglomerates form structureless, crudely stratified beds with locally concave up, erosional bases (Figs. 4; 5a) or thin discontinuous layers within the siliciclastic sandstones or the calcareous/sulphate arenites. Rounded to subangular pebbles are a few millimetres in size in the basal layers, while in the upper part of the succession they reach cobble size. Carbonate lithic clasts are the main components of the conglomerates, locally displaying bioerosion/sponge borings (Fig. 5a) in the basal layers. The basal conglomerates are associated, mostly in the northeastern area, to well-sorted quartzose sands with coquina lenses (fossiliferous sands), corresponding to the widespread Sabbie Gialle, filling up the Tuscan Pliocene basins (Sestini, 1970). More commonly they are associated to or are replaced by peculiar, chalky carbonate/sulphate arenites. Petrographic features The fabric and composition of all the pebbles vary from the well-preserved Triassic, fine-grained dolostone, locally containing veins or inclusions of anhydrite, to partially dedolomitized carbonates, to the neomorphic polycrystalline inequigranular calcite mosaics distinctive of the Calcare cavernoso. These pebbles are also found scattered in the limestone facies locally bored or encrusted by thin coatings of red algae. The matrix of the grain-supported conglomerates occurring in the upper part of the succession (UU) is represented by white chalky carbonate/sulphate arenite. At the base of the succession (LU) and in some intercalations of the limestone unit (MU), the pebbles are found floating (Fig. 5b) in a compact micritic matrix containing rare and scattered fragments of bryozoans, red algae, serpulids, ostracods and/or benthic foraminifers (Elphidium, Amphistegina, Textularia). Depositional environment The carbonate grains of all the conglomerates occurring in this succession appear to derive from 103

8 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli Fig. 5 - a) Structureless, crudely stratified conglomerate with bored pebbles near Libbiano (hammer for scale: about 40 cm); b) Small dolomitic pebbles floating in compact micrite at the base of the Lower Unit near Libbiano (marking pen for scale: about 4 cm); c) Poorly lithified chalky carbonate/sulphate arenites with small pebbles scattered or concentrated along erosional surfaces. LU - Abandoned quarry near Villa Buonriposo; (hammer for scale: about 40 cm); d) Faint, parallel or low-angle planar lamination of the LU chalky carbonate/sulphate arenites well exposed on the walls of an abandoned quarry near Villa Buonriposo; (pencil for scale: about 25 cm); e) Microscopic view in plane polarized light of well sorted, fine to medium chalky carbonate/sulphate arenites, composed of calcite grains, partly of biogenic derivation, and variable amounts of clastic gypsum and anhydrite. The fabric is affected by large keystone-like vugs. (bar for scale: 1 mm); f) Monotypic association of skeletal particles: serpulids and echinoids in the chalky arenites of the upper LU; (plain light; bar for scale: 2 mm). the gradual dedolomitization processes that, after exhumation during the Neogene, affected the Triassic, evaporitic/dolomitic Burano Formation, leading to the formation of the neomorphic 104 Calcare cavernoso (Gandin et al., 2000). The occurrence of abundantly preserved anhydrite and gypsum within the clasts and the arenitic matrix suggest that erosion/alteration and prob-

9 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) ably uplift were still in progress during sedimentation. The conglomerates and the scattered, ruditic material included in the micritic matrix, intercalated in the LU and MU deposits, can be interpreted to represent the input of land-derived gravel shed by ephemeral streams on the coast and redistributed by waves and/or longshore currents in shallow, open-marine waters where calcareous muds settled. In the UU the larger size of the grains, their relatively low morphological maturity, the poor selection of the clastic material and the persistence in the arenites of anhydrite and gypsum among the grains, imply a shorter transport from the feeding area and suggest a depositional setting corresponding to the marginal, seaward part of an alluvial complex. On the whole, the upper conglomerates (Section A, Figs. 2; 3; 4) appear to close the marine deposition in the area and, accordingly, may be interpreted as heteropic facies of the seaward sediments (Fig. 4). Chalky carbonate/sulphate arenites These deposits are represented by white, poorly lithified sands with small pebbles scattered or concentrated along erosional surfaces (Fig. 5c). They dominate in the LU, while in the MU they are found alternated with the skeletal limestone (Figs. 4 and 5). In the LU, the arenites, well exposed on the walls of an abandoned quarry, appear sufficiently lithified. They show faint, parallel or low-angle planar lamination and erosional scour surfaces (Figs. 5c and 5d). Intercalations of chalky to poorly lithified limestone are observed in the upper part of the sandy interval. In the MU, the arenite, richer in the fine, chalky fraction, is soft, poorly lithified sediment which locally contains floating lithic gravel. Petrographic features Well sorted, fine to medium sand ( mm in size), showing a grain-supported fabric, is composed of calcite grains, partly of biogenic derivation, and variable amounts of clastic gypsum and anhydrite (Fig. 5e) associated with metamorphic, euhedral quartz and albite. Small pebbles are often scattered in the matrix with the skeletal debris. Cementation is poorly developed; remains of a micritic matrix are locally abundant. The fabric is affected by large keystone-like vugs (Fig. 5e) that can be related in part to air escaping from intergranular pores and/or to diagenetic/ epidiagenetic dissolution processes. Faunal content Skeletal debris is more abundant in the upper part of the LU, commonly represented by monotypic concentrations of abraded, more or less oxidized skeletal remains of benthic foraminifera (Elphidium gr. crispum, Cibicides gr. lobatulus, Ammonia sp.). In association, scattered globigerinas and ostracods; fragments of serpulids (Fig. 5f), coralline algae, barnacles, echinoids, Ostrea and rare cups of solitary corals are observed. Upwards and westwards in the unit the bioclastic content increases and the sandy matrix is gradually replaced by micritic mud. The loose sands alternated to the skeletal limestone layers (MU) are still composed of fine, calcite and sulphate (anhydrite and gypsum) grains (Fig. 5e), but their organic content is richer. It consists of a well preserved and well-developed microfauna dominated by benthic foraminifers and associated ostracods, often with closed valves, siliceous sponge spicules and variable amounts of fragments of skeletal parts of coralline algae and bryozoans. The foraminifer associations are characterized by a common, low diversity assemblage dominated by Elphidium gr. crispum associated with Cibicides gr. lobatulus, Asterigerinata, discorbida, Ammonia gr. beccarii, Amphystegina sp. Deformed specimens of Cibicides or coralline algae have been found encrusted on organic vegetal remains. In the MU upper interval, the same assemblage also contains Gypsina sp. and rare specimens of Bulimina sp., Bolivina sp. and planktonic foraminifers (Globigerina sp. and Globigerinoides sp.). Depositional environment The bedforms observed in the chalky carbonate/sulphate arenites suggest tractive transport and mixing of land-derived and marine biogenic material on the shoreface environment. The skeletal content of the LU arenite lithofacies, which is abraded/corroded and broken, but never coated by micritization rinds, shows evidence of exhumation, winnowing and transport from the emerged beach and/or the adjacent very shallow, marine environments, and rapid 105

10 106 Fig. 6 - Macrofacies of the MU Skeletal limestone exposed along the SP 69 Road:a) Irregularly bedded skeletal limestone alternated with chalky carbonate/sulphate arenites; (hammer for scale: about 40 cm); b) Abrupt uneven contacts between the locally laminated skeletal limestone and chalky carbonate/sulphate arenites; (hammer for scale: about 40 cm); c) Exposed bed surface of the skeletal limestone showing small mounds made of homogeneous lime-mud; (hammer for scale: about 40 cm); d) Clusters of branching coralline algae irregularly scattered in the matrix of the skeletal limestone; (head of the hammer for scale: about 3 cm); e) Poorly bedded limestone with thin lenticular intercalations of chalky carbonate/sulphate arenites in the upper MU; (hammer for scale: about 40 cm).

11 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) burial. The occurrence of lithic ruditic components scattered in the sandy deposits, or forming lags along erosional surfaces, attests to the repeated redistribution by the waves and/or longshore currents of pebbles deriving from alluvialcoastal deposits. The microfaunal content of the lower (LU) arenites consists of selected, evidently transported and/or reworked skeletal remains of forms that were swept from the site where living communities developed. The microfaunal content of the upper (MU) arenites, intercalated to the calcareous beds, is completely different from the assemblages found in the (LU), since it reflects living communities and indicates overall in-place accumulation of skeletal remains (see below). Skeletal Limestone It consists of a porous, mud-supported granular limestone, locally laminated or faintly graded, arranged in uneven beds. In the lower part of the unit, the skeletal limestone beds are alternated with the chalky carbonate/sulphate arenites (Figs. 4 and 6a). Contacts between the two lithologies are commonly abrupt and undulose with positive, small mound-like morphologies irregularly alternated with depressions (Fig. 6b). The positive forms, evident on the exposed bed surface (Fig. 6c), correspond to small mounds made up of homogeneous mud with scattered clusters of coralline algae (Fig. 6d). The irregular bedding of the calcareous surfaces can be at least in part related to the depositional relief of the small mounds. However, subsequent differential compaction of the surrounding soft, granular deposits and local, low relief erosional features and gutter casts cannot be ruled out. In the basal part of some beds, concentrations of subrounded pebbles can be observed floating in a micritic matrix. Large shells of gastropods (Conus), disarticulated valves of pelecipods (mainly Ostrea) and clusters of branching coralline algae (Fig. 6d) are irregularly scattered in the matrix. In the upper part of the Middle Unit, the limestone is poorly bedded and the intercalations of chalky carbonate/sulphate arenites are thin and lenticular (Fig. 6e). Scattered concentrations of rhodoliths, macrofossils (echinoderms, gastropods, and pelecipods often with closed valves) and lithic gravel are locally frequent. Petrographic features Skeletal/lithic packstone made of medium to coarse sand grains ( mm in size) is the dominant fabric in the lower part of the skeletal limestone facies, while rhodolith floatstone is frequently found in the more massive upper part. Unsorted, locally coarsely graded skeletal debris (fragments of coralline red-algae, bryozoans, molluscs, echinoderms, and serpulids) is associated with variable amounts of lithic grains and large, more or less abraded tests of benthic foraminifers. The matrix is generally made of homogeneous, pure micrite but locally, for its high content of calcareous/sulphate sand-silt, is more similar to the carbonate/sulphate arenite facies. Evidence of bioturbation is scarce although the occurrence of commonly structureless packstones suggests the possibility of intensive bioturbation. The rhodolith floatstone consists of small nodules (2-3 cm in size) of laminar coralline algae (melobesiae) embedded in a wackestone/micritic matrix. The laminar coralline algae envelop small colonies of bryozoa, clusters of serpulid aggregates (Fig. 7a), coralline branches or small lithoclasts. Well rounded, sometimes broken lithic grains varying in size from rudstone to coarse sandstone are commonly present in variable amounts, either scattered or concentrated at the base of the beds (Fig. 7b). They are locally enveloped by a thin microbial/coralline coating (Fig. 7c). Rhodoliths, corallinae branches and lithics, are often affected by boring of different sizes. The larger ones, probably produced by clionid sponges, are filled by micritic sediment admixed with very fine bioclastic debris, while the smaller ones possibly due to fungi and/or microbial organisms, are often infilled by calcite cement (Fig. 7d). The rare cements observed, mostly intraskeletal or replacive of aragonitic shells consist of equant calcite spar. Aragonite or fibrous Mg-calcite cements are notably absent. Vug porosity is locally significant owing to the partial-to-total dissolution of the sulphate and carbonate lithics (Fig. 7e) and of the aragonite shells. Micritization rinds around the grains are commonly lacking. Faunal content The calcareous facies are characterized by a 107

12 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli Fig. 7 - Macro- and microfacies in the MU Skeletal limestone exposed along the S.P. 69 Road: a) Clusters of serpulid worm tubes enveloped by laminar coralline algae (melobesiae); (bar for scale: 3 mm); b) Well rounded to angular lithic grains varying from rudstone to coarse sandstone in a graded limestone bed; (head of the hammer for scale: about 3 cm); c) Well rounded to angular lithic grain enveloped by a microbial/coralline coating. Basal bed of the Skeletal limestone (bar for scale: 1 mm); d) Rhodolith affected by borings of different sizes. The larger ones, probably produced by clionid sponges, are filled by micritic sediment admixed with very fine bioclastic debris, while the smaller ones possibly due to fungi and/or microbial organisms, are infilled by calcite cement (bar for scale: 4 mm); e) Vug porosity derived from the partial dissolution of a small pebble of Triassic evaporitic dolostone (bar for scale: 1 mm). skeletal content consisting of the remains of articulate/branching coralline red-algae and benthic, epiphytic foraminifers (Elphidium gr. crispum, Amphistegina sp., Ammonia gr. beccarii, Cibicides gr. lobatulus, discorbida, Asterigerina sp. and rare Miliolida as Quinqueloculina and Triloculina), associated with echinoderms, serpulid worms, bryozoans, molluscs, and barnacles. This low-diversity foraminifer assemblage is the same of that occurring in the intercalated chalky carbonate/ sulphate arenites. In addition, rare specimens of Gypsina sp. Textularia, Globigerina sp., Globorotalia cf. crassaformis, Pyrgo sp. and Bulimina sp., are frequently found together with more frequent rhodoliths, in the upper part of the calcareous, marine MU. Palaeoecologic interpretation The skeletal assemblage of the Buonriposo limestone is composed of dominant coralline algae and/or rhodoliths and benthic, epiphytic foraminifers while echinoids, molluscs, serpulids, barnacles and bryozoans are less well represented. This ensemble can be related to the Rodechfor/ Rhodalgal type-associations respectively defined by Hayton et al. (1995) and Carannante et al. (1988) for the skeletal gravel also known as 108

13 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) maerl that is commonly found in the shallow waters of non-tropical, temperate perioceanic shelves and also on the Mediterranean coasts (Bosence, 1983a, b; Fornos and Ahr, 1997; Pedley and Grasso, 2002). All the components of this assemblage, as well as the Elphidium crispum-dominated foraminiferal association preserved in the MU chalky carbonate/sulphate arenites, are epiphytic organisms which live associated with seagrass meadows in the infra-littoral zone (Blanc-Vernet, 1969; Kitazato, 1988; Lager, 1988). The seagrass meadows provide favourable conditions to the production of carbonate particles giving support to the encrusting/epiphytic calcareous organisms and although they grow in zones of turbulent waters (Blanc-Vernet, 1969; Kitazato, 1988) are able to promote the accumulation of mud by the trapping and binding actions of their rhizomes, stems and leaves (Blanc-Vernet, 1969; Davies, 1970). Moreover, the biogenic granular sediments that, despite having been produced in zones of higher energy, are commonly unsorted, contain an abundant fine-grained fraction together with coarse skeletal and detrital components (Davies, 1970; Brasier, 1975). The coralline algae, some of the bryozoans, the serpulid worms and some of the foraminifers with ventrally flattened shells (discorbides, Cibicides and Asterigerinata) live encrusted to or attached onto the leaves of Posidonia. Mobile (Elphidium gr. crispum) and temporarily mobile forms (Amphistegina) stride in the bushy framework of brown and green algae thalli, others (Ammonia gr. beccarii, Textularia) live sheltered in the loose muddy sediment at the foot of the meadows (Kitazato, 1988; Lager, 1988). Mud, mainly composed of calcite, less frequently of Mg-calcite (Fornos and Ahr, 1997) in temperate shelves is produced by the combined degradation (bioerosion and maceration) of skeletal debris and accumulation of calcareous phytoplankton (Young and Nelson, 1988; Blom and Alsop, 1988). In the deeper (less than 50 m) and more agitated parts of the temperate shelves rhodoliths with thick algal envelopes are formed (Carannante et al., 1988; Aguirre et al., 1993; Hayton et al., 1995). Depositional environment The MU Villa Buonriposo marine succession is composed of Rodechfor/Rhodalgal skeletal limestone, with frequent concentrations of lithic pebbles, alternated with land derived, carbonate/sulphate arenites, locally including a low-diversity foraminiferal assemblage. Facies and skeletal associations reflect overall shallow-marine, temperate conditions with turbulent waters supporting seagrass meadows and related low-diversity communities of epiphytic organisms. Depositional features suggesting a dominant muddy environment colonized by seagrass, on a ramp-like shelf, repeatedly disturbed by storm events, characterize this mixed succession (Fig. 8). The trapping and binding action of the rhizomes seagrass leaves induced the formation of the unsorted skeletal-debris packstone, the carbonate skeletal particles of dead epiphytic organisms, and the accumulation on the seafloor of micritic mud often preserved as small mud mounds on some bed surface (Fig. 6c). Recurring storms could have interrupted the muddy sedimentation and caused the deposition of sandy layers and the dispersion of land-derived gravel on the bottom. The winnowing of the skeletal mud-rich sediments stirred up by the storm currents, resulted in a maerl -like skeletal gravel that eventually was transported into the deeper parts of the ramp by episodic storm events and redeposited as graded debris-packstone (Fig. 7b). In the deeper transition/offshore area, rhodoliths were formed and were embedded in the terrigenous sand and/or in micritic muds. The distribution and fabric of the sediments, the characteristics of the skeletal assemblages in the Buonriposo succession are indicative of a temperate, low-energy carbonate ramp morphology, similar to the Neogene-to-present-day shelves in the northwestern Mediterranean area (Carannante et al., 1988; Fornos and Ahr, 1997; Pedley and Grasso, 2002; Pomar et al., 2004). Temperate conditions of the shelf, inferred from the features of this faunal assemblage are also supported by the exclusive occurrence of equant calcite spar as a cement of the marine sediments, indicative of cold undersaturated marine waters and temperate climatic conditions (Reeckmann, 1988; Fornos and Ahr, 1997). The climatic meaning of the heterozoan associations, widely interpreted as indicative of nontropical conditions, has been recently questioned (Pomar et al., 2004). However, the scarcity among the foraminifers, of Amphistegina and Miliolidae; 109

14 Enrico Capezzuoli, Anna Gandin and Fabio Sandrelli Fig. 8 - Schematic palaeoenvironmental reconstruction of the Villa Buonriposo ramp during the Piacenzian (not in scale). the absence of large porcelaneous forms (Sorites, Peneroplis, Vertebralina) and of Homotrema rubra that are typical of the subtropical assemblages of the warmest zones of the eastern and southern Mediterranean Sea (Glaçon, 1963; Blanc-Vernet, 1969); the absence of non-skeletal grains (ooids, aggregates and peloids) and, last but not least, the joint absence of chlorozoan components (hermatypic corals, Halimeda and dasicladacean algae) and of fine-grained terrigenous materials within the matrix, documenting the transparency of the seawater, let us infer for the Middle Pliocene Villa Buonriposo limestone a non-tropical climate comparable to that of the present-day northwestern Mediterranean Sea. Further palaeoclimatic information supporting non-tropical water temperatures in the western Mediterranean Piacenzian basins have been obtained from the joint study of the oxygen isotope record and the vertical distribution and abundance changes of planktonic foraminifers, in the reference standard section of the Middle and Late Pliocene stages in Sicily (Channel et al., 1992; Becker et al., 2005). In that section, depleted do 18 values of the benthic and planktonic foraminifer tests and the alternated occurrence in the sediments of unkeeled (cool water) and poorly keeled (warmer water) globorotalias such as G. bononiensis and G. crassaformis respectively, are interpreted by Becker et al. (2005) to reflect a glacial trend of the Pliocene palaeotemperatures in the western Mediterranean and in particular pre/ interglacial conditions during Piacenzian. Discussion The Piacenzian carbonate succession of Villa Buonriposo records shallow water sedimentation in a low-energy regime, of detrital terrigenous and marine autochthonous carbonates on the northern coast of a small island of the Valdelsa Basin, corresponding to the present Poggio del Comune Hill. The carbonate deposition developed on a narrow ramp (Fig. 8) localized on the shoal located on the mostly submerged Middle Tuscan Ridge, separating the Volterra from the Valdelsa Basin. The terrigenous carbonate sediments of the LU, conglomerates and chalky calcareous/sulphate arenite, are interpreted to represent the marginal part of an alluvial-coastal complex grading to a foreshore-shoreface setting. The detrital components are mainly derived from the contemporary erosion of the evaporitic/dolomitic Burano Formation (dolomitic pebbles and angular anhydrite/gypsum grains, idiomorphic quartz and albite) and of its epidiagenetic derivate, the 110

15 Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena Italy) Calcare cavernoso (calcite sand grains and polycrystalline calcareous pebbles). This evidence implies the exposure and erosion of the Triassic Burano sulphates and dolostones during the sedimentation of the LU clastic carbonate/sulphate deposits. The MU marine autochthonous carbonates alternated to the clastic chalky calcareous/sulphate arenites appear to be produced in a carbonate factory developed in the transitional zone up to the offshore of the narrow ramp where carbonate communities flourished in association with seagrass meadows. Meadows of Posidonia and bushy, brown and green algae supported oligotypic communities of epiphytic organisms. The trapping and binding action of the rhizome leaves of these macrophytes induced the formation of skeletal packstone, resulting from the accumulation on the seafloor of micritic mud and of the carbonate skeletal particles of dead epiphytic organisms. The winnowing of these muddy sediments resulted in a maerl -like skeletal gravel that eventually was transported by episodic storm events and accumulated as graded debris packstone in the deeper, marginal parts of the ramp where the laminar rhodoliths were forming. The recurrence of lithic gravel forming lags along erosional surfaces or scattered in the muddy skeletal limestones and sandy deposits, attest to the repeated redistribution by storm and/or longshore currents of pebbles deriving from shallower coastal settings. The terminal deposition of the UU conglomerates implies a regressive trend of the Piacenzian sedimentation heading to the emersion of the Buonriposo carbonate ramp. 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