Sequence Stratigraphy of the “Langhe” Oligo-Miocene Succession, Tertiary Piedmont Basin, Northern Italy

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 k1 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 (C1, 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, C1, C2 and probably B1 to 3rd-order cycles 1.4 � 1.5 (supercycle TB1), 2.1, 2.2, 2.3 (supercycle TB2) and 1.1 (supercycle TB1) respectively.