Abstract

The Manciano Sandstone, consisting of medium-coarse arenites with intercalations of conglomerates and very rare pelitic beds, forms several discontinuous outcrops in different zones of north western Latium and southern Tuscany. This succession shows a homogeneous composition and is transgressive above the ligurid and austroalpine units. The stratigraphic position of the Manciano Sandstone within the Miocene (Langhian) is very uncertain owing to the absence of significant microfaunas and is still based on echinids (in particular Scutella striatula DE SERRES) described by Giannini (1957). Because of this uncertainty a method of chronostratigraphic determination based on measurement of the strontium isotopic relationship expressed as <sup>87</sup> Sr/ <sup>86</sup> Sr ratio was used. Outcrops of the Manciano Sandstone at Colle Diana, Pescia Fiorentina, Monteti, Casa Lascone and Manciano were examined (fig. 1) and shells of ostreids, Scutella striatula DE SERRES (species currently referred to the Langhian age) and pectinids were sampled for the measurement of strontium isotopic ratios. The chronostratigraphic data suggest an age between 5.47 and 5.77 Ma, corresponding, in the chronostratigraphic standard scales (Haq et alii, 1988; Servizio Geologico Nazionale, 1992; Berggren et alii, 1995), to the upper part of Messinian. The new data based on the <sup>87</sup> Sr/ <sup>86</sup> Sr isotopic ratio determined on different types of macrofauna allows the Manciano Sandstone to be referred to Late Messinian, suggesting the need of a re-examination of the chronostratigraphic evolutionary range of the echinid Scutella striatula.

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