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Abstract

The Tertiary Piedmont Basin (TPB) is bounded by the Western Alps on the south and west, and northward by the northwestern end of the Apennines and is filled with a succession of siliciclastic sediments over 4,000 m thick. It develops internally to a south-dipping suture zone, on a basement consisting of allochthonous Alpine and Apenninic units. The TPB's history began at the end of the Eocene and continued through Oligocene time under mainly extensional tectonic conditions; whereas in the Miocene time the tectonic regime is dominated by compression.

In the "Langhe" region, we recognized three groups of depositional sequences. Group A consists of continental to coastal conglomerates, shallow-marine sandstones and hemipelagic mudstones with thickness ranging from a few tens of meters to 600 m, and two depositional sequences (Early Oligocene; locally Late Eocene?) characterize this group. Group B is represented by six depositional sequences (B1-B6) of Late Oligocene to Burdigalian age. Each sequence consists of turbidite sandstones and subordinate resedimented conglomerates in the lower part and hemipelagic mudstones with intercalated thin-bedded turbidites in the upper part. Total thickness of group B may be greater than 1,000 m. Group C is represented by six depositional sequences (C1-C6) of Late Burdigalian-Early Tortonian age. It consists of turbidite systems with sandstone/mudstone ratios ranging from »1 to 1 at the depocentres and of mudstones on the slopes bounding the basin. Total thickness may be more than 2,000 m.

Synsedimentary tectonic activity is indicated by the following: (a) angular unconformities (at the B1-B2 transition and at the lower boundary of B6) (b) onlap of turbidite sandstones on slightly folded mudstones (B1-B2 and B2-B3 boundaries) and (c) vertical and lateral evolution of facies which differs from that of the models proposed for eustatically-controlled sequences (Cl, C2 and C5).

On the basis of biostratigraphic data (planktonic foraminifers and calcareous nannoplankton), the recognized sequences show the same frequency as 3rd-order global cycles (Haq et al., 1988). Biostratigraphic data provides a tool in correlating the sequences B5, B6, Cl, C2 and probably Bl to 3rd-order cycles 1.4 +ss 1.5 (supercycle TBI), 2.1, 2.2, 2.3 (supercycle TB2) and 1.1 (supercycle TBI) respectively.

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