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Keywords:

  • Bathymetry;
  • bypass;
  • Castagnola;
  • confined basin;
  • contained turbidites;
  • synsedimentary tectonics

ABSTRACT This paper details the influence of syndepositional tectonics in controlling the architecture of a well-exposed confined turbiditic sandbody, which crops out in the eastern part of the Tertiary Piedmont Basin (Castagnola Basin, northern Italy). The Castagnola Basin was tectonically active during sedimentation of the sandbody, and the lateral distribution of turbidity-current deposits has been used to constrain both how the basin subsided and the impact of basin topography on flow behaviour and deposition. The sandbody occurs in the lower member of an Upper Oligocene–Lower Miocene turbidite system (the Castagnola Formation). The sandbody is ≈30 m thick and can be followed laterally for ≈1·8 km; it shows onlap terminations onto both northern and southern basin margins. The outcrop is sufficiently large to allow a detailed analysis of the facies and geometrical heterogeneity, as viewed approximately parallel to the average palaeocurrent trend (SW–NE). Correlation between 41 sedimentological logs reveals the diachronous development of a succession of sandstone packages (subunits). Nine vertically stacked and laterally juxtaposed packages have been recognized (subunits B to I from oldest to youngest), which reflect changes in basin floor accommodation as a result of synsedimentary tectonism. Each package shows the development of different vertical stacking patterns with thinning- and -fining-upward small-scale sequences and variable lateral facies arrangements, as a consequence of the position relative to the basin margins. The geometry, stratigraphic relationships, facies distribution and palaeocurrent directions indicate that turbidite deposition during accumulation of most of the sandbody was controlled by (1) synsedimentary tilting of the basin slopes; (2) the distribution of structural and depositional relief within the basin; (3) the thickness and volume of the turbidite flows; and (4) the angle of impingement of turbidity currents against the basin slopes.