Facies (2007) 53: 57–66 DOI 10.1007/s10347-006-0085-x
ORIGINAL ARTICLE
Alessio Checconi · Davide Bassi · Leonsevero Passeri · Roberto Rettori
Coralline red algal assemblage from the Middle Pliocene shallow-water temperate carbonates of the Monte Cetona (Northern Apennines, Italy) Received: 27 April 2006 / Accepted: 12 June 2006 / Published online: 14 November 2006 C Springer-Verlag 2006
Abstract During the Pliocene and Pleistocene, the Monte Cetona (Northern Apennines, central Italy) was part of an elongated island. The Middle Pliocene deposits around the Monte Cetona are represented by shallow-water marine carbonates rich in coralline red algae and bryozoans. These skeletal carbonates, characterising a coralline algaldominated factory, were analysed in terms of microfacies, taxonomy, and growth-forms of coralline red algal assemblage. Three microfacies were distinguished on the basis of component distribution and fabric analysis: coralline algal rudstones, coralline algal floatstones, and bioclastic packstones. Skeletal components are commonly abraded, bioeroded, and encrusted. The shallow-water skeletal carbonates are strongly bioturbated and any primary sedimentary structure is obliterated. The distribution of the coralline growth-forms suggests a decreasing hydrodynamic gradient from the coralline algal rudstone, through the coralline algal floatstone to the bioclastic packstone microfacies. The coralline algal flora consists of eight species representing the subfamilies Lithophylloideae, Mastophoroideae and Melobesioideae. The assemblage is dominated by lithophylloids. Other biogenic components are bryozoans, barnacles, echinoderms, and benthic foraminifera. These coralline algal assemblages were deposited just above the fair-weather wave base and indicate a shallow-marine temperate water setting for the eastern Tyrrhenian Sea during the Mid Pliocene.
A. Checconi · L. Passeri · R. Rettori Universit`a degli Studi di Perugia, Dipartimento di Scienze della Terra, Piazza dell’Universit`a 1, 06100 Perugia, Italy D. Bassi () Universit`a degli Studi di Ferrara, Dipartimento di Scienze della Terra, via Saragat 1, 44100 Ferrara, Italy e-mail:
[email protected]
Keywords Coralline red algae . Temperate water carbonates . Palaeoecology . Palaeoclimate . Pliocene . Italy Introduction The coralline red algae (Corallinales, Rhodophyta) are common to abundant biogenic components of Neogene shallow-water carbonate and mixed siliciclastic-carbonate sediments (e.g., Braga and Mart´ın 1988; Brachert et al. 1998, 2001). The Mid-Miocene to Pleistocene was a period of coralline algal diversity stasis or even slight decrease. In particular, the Lower Pliocene extinction of corallines seems to coincide with one of the periodic MesozoicCenozoic extinction events (Aguirre et al. 2000). Pliocene to Pleistocene coralline red algae have been reported and documented from a number of shallow-water marine carbonate deposits of Mediterranean sedimentary successions (e.g., Lemoine 1919, 1939; Segonzac 1972; D’Atri and Piazza 1988; Vannucci et al. 1994; Aguirre and Jim´enez 1997; Di Geronimo et al. 2000, 2003; Braga and Aguirre 2001). This study deals with coralline algae, which are the most abundant biogenic components of the Middle Pliocene carbonate sediments cropping out in the Monte Cetona (Northern Apennines, central Italy; Fig. 1). During the Pliocene-Pleistocene, the Monte Cetona was part of an elongated island surrounded by coastal sediments with siliciclastic terrigenous conglomerates and sandstones in the lower part of the succession and by carbonates in the upper part. In the carbonates coralline algae, bryozoans and molluscs are the main biogenic components, together with various proportions of barnacles, echinoderms, solitary corals, and terrigenous particles. These are all characterising nontropical carbonate platform facies (Bryomol sensu Hayton et al. 1995; Rhodalgal sensu Carannante et al. 1988), which were common and widespread in the past as they are today (e.g., Carannante et al. 1988; Betzler et al. 1997; Brachert et al. 1998; Halfar et al. 2004). The aims of this study are to describe the Middle Pliocene coralline algal assemblages of the Monte Cetona and to
58 Fig. 1 Geographical map of the Monte Cetona and the location of the studied Pliocene outcrop in the Pian del Giuncheto (Northern Apennines, southern Tuscany, Italy)
analyse the present microfacies in order to assess their palaeoenvironmental setting and the palaeoclimatic context of sedimentation. Microfacies are distinguished in terms of biogenic components with particular regard to the coralline algae (coralline algal taxonomic diversity, coralline growthforms and taphonomic signatures). Geological and stratigraphical setting The Monte Cetona is the southern part of a tectonic structure stretching from northern (Val di Lima, Montecatini) to southern Tuscany (Rapolano, Monte Cetona). The Monte Cetona consists of the Tuscany and the Subligurian tectonic units. The Tuscany unit, with a Mesozoic succession spanning in age from the Late Triassic to the Late Cretaceous, is represented by Upper Triassic sulphate evaporites, limestones, and marls (Ciarapica et al. 1982, 1987), Lower Jurassic shallow-water limestones, and Lower Jurassic to Upper Cretaceous deep water cherty limestones, marls, shales and radiolarites (Passerini 1964). The Subligurian unit is characterised by shales, marls, limestones and turbiditic sandstones spanning in age from the Cretaceous to the Eocene (Passerini 1964). The Tuscany unit of the Monte Cetona preserves the inverted limb of an eastward recumbent anticline with the western side dissected by normal faults. This structure was generated after the thrusting of the Subligurian unit during the Mid Miocene; in the western side of the Monte Cetona faulting took place after the folding (Passerini 1964). Extensional tectonics, related to the opening of the Tyrrhenian Sea, caused a marine transgression during the Pliocene. During this time, the Monte Cetona area was the southern part of a N–S elongated island and a deep sea
separated the Monte Cetona from the eastern continental area of the modern Umbria region (Figs. 1 and 2). Plio-Pleistocene shallow-water carbonates outcrop all around the Monte Cetona. The most continuous and best exposed section is located in the Pian del Giuncheto area, in the north-eastern part of the Monte Cetona (Fig. 1). The studied marine coastal deposits are carbonates lying on the Hettangian massive limestones of the Calcare Massiccio Formation through an angular unconformity in the western part, and on Pliocene conglomerates and sands in the eastern part of the Monte Cetona. In the studied area, the Pliocene succession consists of five units, from the bottom to the top (Amici 1985; Fig. 3): (a) conglomerates with highly rounded pebbles in a sandy to clay matrix (3.5 m); (b) fining-upward bioturbated sandstones (2.5 m); (c) massive siltstones, interbedded with calcarenite horizons, rich in larger foraminifera (amphisteginids) and brachiopods (terebratulids), with abundant burrows in the upper part (14.5 m); (d) strongly bioturbated calcarenites with abundant corallines and bivalves (pectinids, ostreids); and (e) alternations of bioturbated bioclastic calcarenites and laminated sandstones that grade upward into bioturbated calcarenites with corallines, bryozoans and Amphistegina (ca. 34 m). The units (d) and (e) were deposited above the wave base level as evidenced by broken shells accumulations (coquinas) and by erosional surfaces. This paper focuses on the uppermost part of the unit (e) which was measured and sampled in the wellexposed Pian del Giuncheto outcrop (Figs. 1 and 3). This unit is constituted of carbonate wedges thinning toward the W–NW and showing a crude, decimetre- to metre-scale bedding. The Pliocene is overlain by Pleistocene deep-sea marlyclay deposits (“Argille Azzurre”; Fig. 3), which crop out at Radicofani and Fabro.
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was used allowing for a general component and matrix description. A qualitative assessment of the coralline algal distribution in thin sections was made (Table 2). Taxonomic uncertainties concerning fossil coralline taxonomy have been discussed by Braga and Aguirre (1995), Rasser and Piller (1999), and Bassi and Nebelsick (2000). Species circumscriptions used herein follow Braga et al. (1993), Braga and Aguirre (1995) and Aguirre and Braga (1998). Family and subfamily circumscriptions follow Woelkerling (1988), Braga et al. (1993), Aguirre and Braga (1998) and Braga (2003). Coralline algal growth-form terminology follows Woelkerling et al. (1993). All the samples studied are stored in the Dipartimento di Scienze della Terra, University of Perugia, under the acronym BL. Results Coralline red algae
Fig. 2 Schematic geological map showing the marine PliocenePleistocene deposits around the elongated Monte Cetona island (modified from Passerini 1964)
Materials and methods The presence of the planktonic foraminifer Globorotalia aemiliana allows the studied unit to be dated as early MidPliocene in age (Amici 1985). This unit occurs at the top of a calcareous massive bundle of beds, ca. 34 m thick and about 300 m long, which outcrops above the road that lies from S. Casciano dei Bagni to Sarteano. The top of the bank was sampled along a W–NW/E–SE transect line sub-perpendicular to the Middle Pliocene palaeocoast-line. The studied transect is 102 m in length and 4.5 m in thickness (Fig. 3). Samples were collected in order to carry out a detailed assessment of coralline algal distribution and to perform microfacies analysis. Twenty samples were taken along the transect, spaced 4–6 m apart (Fig. 3). Where possible, different strata were sampled in the same vertical sample position. A total of 66 thin sections were analysed. The textural classification of Embry and Klovan (1972)
Coralline algae are the frequent to dominant components of the carbonates and are represented by different growthforms and species. Coralline algal growth-forms are mainly encrusting, warty, and lumpy. The term “debris” refers to small and abraded coralline fragments where the growthforms cannot be identified (Tables 2 and 3). Eight species representing the subfamilies Lithophylloideae, Mastophoroideae, and Melobesioideae (Corallinales, Rhodophyta) were identified. Lithophylloids are represented by Lithophyllum pustulatum (Lamouroux) Foslie, Lithophyllum dentatum (K¨utzing) Foslie, and Lithophyllum cf. incrustans Philippi. Mastophoroids are present with three species: Spongites fruticulosus K¨utzing (Foslie), Spongites sp. 1, and Neogoniolithon notarisii (Doufour) Hamel and Lemoine. Lithothamnion philippi Foslie and Mesophyllum lichenoides (Ellis) Lemoine are the members of the subfamily Melobesioideae (Fig. 4). The studied coralline algal species show particular growth-forms that result in different types of detrital thalli (Table 3). All the species growth as encrusting thalli. Lithophyllum dentatum, Spongites fruticulosus, Spongites sp. 1 and Lithothamnion philippi show lumpy and more rarely warty morphologies. No unattached or detrital occurrences are reported for most of the species. Spongites fruticulosus and Lithothamnion philippi reveal unattached/detrital occurrences dominated by branched growth-forms. Lithophyllum pustulatum, L. cf. incrustans, Neogoniolithon notarisii and Mesophyllum lichenoides only reveal encrusting growth-forms, and therefore only crustose unattached/detrital habits were recognised. Microfacies Three microfacies were distinguished on the base of component distribution and fabric analysis: coralline algal rudstone, coralline algal floatstone, and bioclastic packstone (Fig. 6).
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Fig. 3 Pliocene lithostratigraphic sequence outcropping in the eastern side of the Monte Cetona. See text for the explanation of the units (a–e). The top of the succession was sampled along a W–NW/E–SE transect line 102 m long (samples 1–20) that is sub-perpendicular
to the Middle Pliocene palaeocoast-line; note the deep-sea marlyclay sediments of the Pleistocene “Argille Azzurre” formation in the distance. The microfacies distribution is also shown
Coralline algal rudstone
Lithophyllum are common and show respectively warty and lumpy, and encrusting and lumpy growth-forms; Spongites develops warty to lumpy protuberances (Table 1). Fragments of undetermined encrusting and branched thalli are present in the matrix. The skeletal surfaces of the bioclasts show heavy abrasion evident in fragmentation and rounded edges. Borings and pits are common and sometimes extensive. Encrustation by coralline algae and foraminifera is frequent. This microfacies alternates and grades into the coralline algal floatstone.
Biogenic components are dominated by corallines, bryozoans, and echinoderms. Gastropods are abundant; rare serpulids, balanids, and bivalves are also present. Common erect, arborescent branching, and encrusting, sheet-like, unilaminar bryozoans were recognised. Benthic foraminifera are represented by abundant textularids, rare miliolids, and common Amphistegina, Asterigerina, Haddonia, and Sphaerogypsina. Planktonic foraminifera are rare. Small colonies of the coral Cladocora caespitosa locally occur. These small colonies, about 1 m2 in width, present within the Pliocene transgressive carbonates, seem to lie directly on the hard substrates of the Mesozoic basement at the most proximal part of the studied transect (W–NW). Three coralline algal genera were recognised: Lithothamnion, Spongites and Lithophyllum. Lithothamnion and
Coralline algal floatstone Dominant biogenic components are coralline algae, bryozoans (Fig. 5A) and echinoderms. Balanids, abundant gastropods, common brachiopods, serpulids, and bivalves are
61 Neogoniolithon; undet., undetermined; e encrusting; w warty; l lumpy; d debris. See Fig. 2 for sample location
Table 1 Coralline red algal genera and growth-forms along the W–NW/E–SE study transect. Lithoth., Lithothamnion; Mesoph., Mesophyllum; Lithoph., Lithophyllum; Spong., Spongites; Neogon., Genus
W–NW 1a 1b 2
Lithoth. Mesoph. Lithoph. e Spong. l Neogon. undet l
w
3a 3b l
4a 4b 5a 5b
w-l l e
l l w-l e-w
l
d
d
l
l
Samples E–SE 6 7 8 9 10 11a 11b 12 13a 13b 13c 14 15 16a 16b 17 18 19 20 l
l
e-l l w-l d d d d d
also present. Foraminifera consist of textularids, miliolids, common Amphistegina (Fig. 5C), Asterigerina, Sphaerogypsina (Fig. 5D), and Haddonia. Planktonic foraminifera are rare. Coralline algae are represented by Lithothamnion, Mesophyllum, Spongites, Neogoniolithon, Lithophyllum. Corallines show encrusting and warty growth-forms. Rare sub-spheroidal rhodoliths, approximately 2 cm in diameter, are dispersed in the matrix and do not form any particular accumulation. Fragments of fruticose thalli of undetermined coralline algae are also present. Encrustation by
l
l
l
l
d
l
l
l
l
l
e l e
d
e-d
e
e
d
d
d
l
l l l
d
l
d
d
coralline algae and foraminifera is frequent. This microfacies grades into the bioclastic packstone microfacies. Bioclastic packstone Serpulids dominate the benthic assemblage. Erect, branching and unilaminar bryozoans, echinoderms, bivalves and balanids (Fig. 5B) are common. Benthic foraminifera are represented by common Amphistegina and Asterigerina. Planktonic foraminifera are rare.
Table 2 Distribution of the coralline red algal genera and their relative abundance along the study transect. Lithoth., Lithothamnion; Mesoph., Mesophyllum; Lithoph., Lithophyllum; Spong., Spongites;
Neogon., Neogoniolithon. See Fig. 2 for sample location. abundant, common, rare
Table 3 Growth features of coralline algae and their occurrences. Table shows the identified coralline species, their growth-forms, the encrusted substrate, the features of detrital occurrences, and the rela-
tive abundance of species in carbonate microfacies. cr coralline algal rudstone; cf coralline algal floatstone; bp bioclastic packstone. abundant, common, rare
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Fig. 4 Thin-section photographs of coralline algal taxa. A Lithophyllum dentatum (K¨utzing) Foslie; sample 11a. B Lithophyllum cf. incrustans Philippi, sample 3b. C Lithophyllum pustulatum (Lamouroux) Foslie, sample 1a. D Lithophyllum pustulatum (Lamouroux)
Foslie, sample 16a. E Lithothamnion philippi Foslie, sample 5a. F Spongites fruticulosus K¨utzing (Foslie), sample 12. Scale bar represents 250 µm
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Fig. 5 Biogenic components. A Erect bryozoan coated by an encrusting coralline thallus, sample 16a; scale bar represents 1 mm. B Fragments of balanids, sample 20; scale bar represents 1 mm.
C Amphistegina sp., sample 17, scale bar represents 250 µm. D Sphaerogypsina sp., sample 16a; scale bar represents 250 µm
Fig. 6 Carbonate microfacies. A Coralline algal rudstone microfacies; rudstone with packstone matrix, coralline algal lumpy protuberances (c), bryozoans (br) and encrusting foraminifera (ef); sample 3a. B Coralline algal floatstone microfacies; floatstone with packstone
matrix, lumpy protuberances of Lithophyllum dentatum along with bryozoans; sample 11a. C Biogenic packstone microfacies; coralline algal fragments and oblique sections of Ditrupa; sample 19a. Scale bar represents 2 mm
Coralline red algae constitute an algal debris that is scarce and poorly preserved. Corallines are represented by Spongites and Lithophyllum with lumpy growth-forms; fragments of encrusting thalli can be common. The scarcity of well-preserved speci-
mens does not allow species identification to be made. Borings and pits on skeletal surfaces are common and extensive; encrusting foraminifera and corallines coat the skeletal components.
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Discussion The attributes of the studied skeletal assemblage can be considered non-tropical in character since the rare recorded corals are not hermatypic, do not form build-ups, and lowdiversity larger foraminiferal assemblage was found (e.g., Betzler et al. 1997; Brachert et al. 2001; Smith and Nelson 2003; Braga et al. 2003). Sediments mainly comprise bioclasts that are locally binded by coralline algal thalli, bryozoans, and serpulids. The three microfacies show a pattern of decreasing water energy from the coralline algal rudstone, through the coralline floatstone to the bioclastic packstone microfacies of the 100-m-long transect studied, which represents a coralline algal-dominated factory. The coralline algal rudstone microfacies characterises the W–NW part of the transect with relatively high turbulence and biogeniccoarse sandy substrate. The high amount of fragmented lithoclasts reflects the relative high hydrodynamic energy. This microfacies is the only one that includes all the coralline algal growth-forms and represents the most diversified substrate with ample presence of large particulate surfaces for encrustation by epibionts (benthic foraminifera and bryozoans). The coralline algal floatstone microfacies is common to dominant towards the middle part of the transect and decreases in importance gradually towards the E–SE part of the transect. This microfacies, which is seen to represent less turbulent conditions than the previous microfacies, is characterised by the rare presence of small sub-spheroidal rhodoliths that are dispersed in the packstone matrix. A further decrease in hydrodynamic energy is indicated by the change from the coralline algal floatstone to the bioclastic packstone microfacies, which indicates a lower-water turbulence setting. The absence of cross-bedding, the high matrix content, and the comparatively large size and low degree of fragmentation of bioclasts (
