Microsoft Word - 00_indice_BM03 Available online http:/amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 25 (1), 2012, 35-40 U/TH DATING OF A CLADOCORA CAESPITOSA FROM CAPO SAN MARCO MARINE QUATERNARY DEPOSITS (SARDINIA, ITALY) Maurizio D’Orefice1, Roberto Graciotti1, Sergio Lo Mastro2, Cristina Muraro1, Marco Pantaloni1, Michele Soligo2 & Paola Tuccimei2 1 ISPRA, Servizio Geologico d’Italia, Rome, Italy 2 Dipartimento di Scienze Geologiche, Università Roma Tre, Rome, Italy Corresponding author: R. Graciotti ABSTRACT: A whole specimen, not reworked and well preserved of Cladocora caespitosa has been found within the marine Quaternary deposits, outcropping along the eastern coast of the Capo San Marco Promontory. The U/Th dating of this sample has provided a mini- mum age of 70 ± 4 ka B.P. This dating allows to state that these marine deposits, containing the coral, are not Holocene in age. Keywords: marine Quaternary deposits, Cladocora caespitosa, U/Th dating, Tyrrhenian, Sardinia. 1. INTRODUCTION The Geological Survey of Italy, concerning the insti- tutional activities related to the realization of the Geologi- cal Map of Italy at 1: 50.000 scale (CARG Project), has done quick geological surveys to check and monitoring some sample areas of the 528 Oristano Geological Sheet (middle-western Sardinia), still in progress. In particular, marine and continental Quaternary deposits were taken into account. In the middle-western Sardin- ia, Sinis Peninsula and eastern coast of the Capo San Marco Promontory, these depos- its occurred in peculiar outcroppings which have been studied in detail by several Au- thors (Maxia & Pecorini, 1968; Ulzega et al., 1982; Carboni & Lecca, 1985; Davaud et al., 1991; Kindler et al., 1997; Melis et al., 2001; Lecca & Carboni, 2007; Andreucci et al., 2009; Coltorti et al., 2010; Thiel et al., 2010). The research has especially ad- dressed marine Quaternary deposits, in relation with a scientific debate which has recently risen about the chronological at- tribution of these deposits (AIQUA, 2007; Bartolini et al., 2008; Catto, 2010). In the 528 Oristano Geological Sheet “the beach and beach ridge deposits” have been chronologically attributed to Holocene age, instead of the Upper Pleistocene age (Tyr- rhenian) reported in literature and in the official Geological Map of Italy (Regione Autonoma della Sardegna, 1989). During the field activity many fossils have been collected including a specimen of Cladocora caespitosa which has been dated with the U-series method. The aim of this note is to give a additional element to dating the Quaternary deposits outcropping in this area of the Sardinia Region, and to provide a useful contribution to the scientific debate in progress. 2. GEOLOGICAL SETTING The Capo San Marco Promontory, which from a Fig. 1 - Location map of the Sinis Peninsula (Melis et al., 2001; modified). D’Orefice M. et al. 36 geomophological point of view extends a few kilometers offshore, represents the end of the Sinis Peninsula (Fig. 1). The western coast consists of an high and steep Mi- ocene-Pliocene cliff, while the eastern coast, gently dip- ping seaward, is mainly made of marine and continental Quaternary deposits. In this area, in fact, the pre-Qua- ternary bedrock crops out only in a few sites along of the coast where the Quaternary deposits were disman- tled by the intense wave action. The basal portion of the substrate is represented by the Capo San Marco Formation (Cherchi et al., 1978), divided into two facies: the lower one consists of “dark grey clays” and the upper one is made of “marly - calcareous deposits”. The outcropping thickness is about 50 m, the age is Upper Tortonian – Lower Mes- sinian. The Nuraghe Baboe Formation (Spano, 1989) overlies the marly - calcareous deposits of the Capo San Marco Formation, with a transgressive unconformity surface. This unit consists of a marine succession made of breccias, conglomerates, sandstones, calcareous sandstones, silty-marly clays, calcarenites and marly Fig. 2 - Satellite view (from Google Earth) with location of outcrops and sampling point of the Cladocora caespitosa. Table 1 - Uranium content, uranium and thorium activity ratios and age of the coral. U/Th dating of a Cladocora caespitosa from Capo San Marco … 37 mudstone. The thickness is about 35 m, the age is Low- er Pliocene. The Golfo di Oristano basalt Formation (Costa Randata facies), unconformably overlying on the previ- ous formations, represents the top and the southern cliff of the promontory. These basalts, dated 3.12 Ma (Bec- caluva et al., 1985), are made of several superimposed flows and their maximum thickness is about 30 m; the occurrence of columnar structures suggests a slow cool- ing of the lavas. 3. OBSERVATIONS ON QUATERNARY DEPOSITS The Quaternary deposits widely crop out in the area of the 528 Oristano Geological Sheet. The best and more complete stratigraphic sections occur near the San Gio- vanni di Sinis area, within which some Authors recognize different facies (Maxia & Pecorini, 1968; Ulzega et al., 1982; Carboni & Lecca, 1985; Davaud et al., 1991; Kin- dler et al., 1997; Melis et al., 2001; Lecca & Carboni, 2007; Andreucci et al., 2009; Coltorti et al., 2010; Thiel et al., 2010). Quaternary deposits have been also recog- nized along the fossil beach ridge that closes the Cabras lagoon. These deposits are formed by Thyrrenian bio- clastic calcarenites (Forti & Orrù, 1995). The Quaternary deposits crop out without interrup- tion all over the eastern coast of the Capo San Marco Promontory, from the southernmost end up to 1 km to the north of Tharros archeological site. These deposits are often fractured, disrupted and prone to fall and topple landslides due to the intense marine erosive processes acting along the coasts of the promontory. At the tip of the promontory the basalts are over- lapped by aeolian deposits, made of well cemented Fig. 3 - La Caletta Site. Geometrical relationships between aeolian and marine deposits. a) Front view of the outcrop and present- day tidal notch in the aeolian deposits. b) Detail of the marine deposit. c) Lateral view of the outcrop. d) Detail of the marine deposit clearly capped by aeolian deposit. D’Orefice M. et al. 38 Fig. 5 - The specimen of Cladocora caespitosa which has been dated using U/Th method. quartz-sandstone, characterized by high angle cross stratification with foresets landward dipping. The aeolian deposits continue below sea level and are characterized by a recently formed tidal notch. These deposits are modelled by a flat marine ero- sional surface, gently sloping seaward, located at about 4 m above sea level, delimited upslope by a notch of about 50 cm of height. This surface is capped by 1 m-thick marine deposit. The lower portion consists on huge prevailing basalt blocks, well rounded, with a diameter up to 50 cm, in a well cemented, gravelly-sandy matrix, with abundant fos- sils. The upper part consists of a well cemented level of coarse to medium fossiliferous sandstone. The fossils are mostly bivalves (Anomia ephipphium, Arca noe, Fissurella italica, Glycimeris sp., Car- dium sp., ostreids) and gastropods (Murex taurinensis, Trochocochlea turbinata, Pur- pura haemastoma, Patella ferruginea). The age of the aeolian deposits, as- signed using OSL dating on a sample col- lected just above the notch, is 174 ± 13 ka, thus referable to the Middle Pleisto- cene (Thiel et al., 2010). At La Caletta Site (Fig. 2) a marine deposit crops out at present-day sea lev- el. This deposit is composed by huge meter-sized, well rounded basaltic boul- ders, in a gravelly-sandy supporting ma- trix, rich in fossils (Ostrea sp., Arca noe, Glycimeris sp., Patella ferruginea, etc.). This sedimentary body, dipping 10° sea- ward, rise upward for a few meters along the slope. It is completely capped by Up- per Pleistocene aeolian quartz-sand- stones which deep below sea level. In this site the geometrical relationships between these deposits can be observed in a three-dimensional view. In fact, it is possible to observe the marine deposits underlying the aeolian sandstones through several joints expanded by marine erosion (Fig. 3). Northward, in the direction of Tharros (Figs.1-2), a peculiar beach deposit has been found. This 50 cm-thick layer is made of medium to coarse-grained, well cemented sandstone, rich in Mytilus galloprovincialis and other bivalves (Fig. 4). This marine deposit is very similar to that described by Lecca et al. (2007) and by Andreucci et al. (2009) in the Sinis Peninsula, about 1 km away. In this site a planar bedded sandstone overlying the Mytilus galloprovincialis layer has been dated using OSL method, providing an age of 120 ± 10 ka (An- dreucci et al., 2009). During the field survey it was noteworthy the find- ing of one whole specimen of Cladocora caespitosa at about 2.5 m above sea level. This fossil has been col- lected from marine Quaternary deposits unconformably lying on the bedrock with an articulated erosional sur- face. The specimen not reworked and well preserved has been considered suitable for an isotopic dating ac- cording to the U/Th method. 4. U/TH DATING Corals are considered excellent samples to be dated with U-series disequilibria methods because in most cases they consist of pure calcium carbonate, free from a detrital component that makes problematic the dating of dirty carbonates. The 230Th/234U method is the most widely used dating technique applied to corals and is based on the extreme fractionation of the parent isotopes 238U and 234U from their long-lived daughter 230Th in the hydro- sphere. Uranium, markedly more soluble than Th in the surface and near-surface environments, is readily mobilised as the highly soluble uranyl ion (UO2 2+) and its complexes, whereas Th is easily hydrolyzed and precipitated or adsorbed on detrital particles. Uranium is co-precipitated with CaCO3 on exsolution of CO2, Fig. 4 - View of the Mytilus galloprovincialis layer. U/Th dating of a Cladocora caespitosa from Capo San Marco … 39 Fig. 6 - Diffractogram showing the aragonitic (ar) nature of the coral with moderate calcite amount (cc) and detrital fraction: quartz , halite gypsum. while Th is generally negligible. In the absence of de- trital Th, 230Th only forms in situ by radioactive decay of co-precipitated U. In a closed system the extent to which the 230Th/234U activity ratio has returned towards unity is a function of time, taking into account also the state of disequilibrium between 234U and 238U (Kauf- man & Broecker, 1965). Moreover, it is important to verify the original aragonitic nature of the coral and check the eventual presence of calcite. The occur- rence of relevant calcite indicate that weathering pro- cesses have affected the coral after its burial, with consequent opening of the chemical system. In this case, semi-quantitative X-ray diffractometry analysis has evidenced the aragonitic nature of the coral with the presence of moderate calcite amount (Fig. 6) that can be estimated around approximately 10%. About 3 g of coral were ultrasonically washed in deionized water and dissolved in nitric acid. Few millili- ters of hydrogen peroxide were added and heated at 100 °C in order to destroy organic matter. Isotopic com- plexes of uranium and thorium were extracted according to the procedure described in Edwards et al. (1986) and alpha-counted using high resolution ion implanted Ortec silicon surface barrier detectors. The age, 70 ± 4 ka, was calculated by means of Isoplot/Ex (version 3·0), a plotting and regression program designed by Ludwig (2003) for radiogenic-isotope data. U-series data are reported in Table I. Errors are quoted as 1σ. 230Th/232Th activity ratio, higher than 50, indicates that the coral does not contain a significant detrital frac- tion. Furthermore, the uranium content, about 3 ppm, approaches the average value of uranium abundance in corals and the initial 234U/238U activity ratio (234U/238Ui) of the carbonate correspond to that of the marine water. These three data are an evidence of the good quality of the obtained age. 5. CONCLUSIONS The U/Th dating on a sample of Cladocora caespi- tosa has provided an age of 70 ± 4 ka B.P. The speci- men of Cladocora caespitosa, not reworked and well preserved, was sampled at about 2.5 m above sea level within the marine Quaternary deposits, overlying the Miocene-Pliocene bedrock, along the eastern coast of the Capo San Marco Promontory. The diffractometric analysis show- ed the sample is mainly composed by aragonite. For the U/Th dating must be taken into account the occurrence of the calcite which could cause the rejuvena- tion of the samples. In this case study, considering the modest quantities of cal- cite, the age of 70 ± 4 ka B.P. represents the minimum age of the sample. Based on these considerations and with reference to the isotopic curve relat- ed to the last 300 ka (Martinson et al., 1987) the examined sample can be lo- cated at the end of the Isotopic Stage 5 even assuming the rejuvenation of the sample for the presence of moderate calcite amount (Fig. 6). This finding agrees with the results reported in Dorale et al. (2010) and Tuccimei et al. (2010), inferred from phreatic overgrowths on speleothems in coastal caves of Mallorca (Balearic Islands - Spain). These Au- thors assert that the western Mediterranean relative sea level stood at about 1m above present sea level during marine Isotope Stage (MIS) 5a, between ~ 82 and 80 ka ago. Furthermore, the data of Mallorca coastal caves, show good coherence with those observed from Tucci- mei et al. (2012) in the Capo Caccia area (north-western Sardinia), located not far from Capo S. Marco. 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