The Contessa megabed is a basin-wide turbidite bed in the Marnoso-arenacea Formation (Miocene) of the northern Apennines (Italy). It has a mixed siliciclastic/carbonatoclastic composition and was derived from the southern end of the elongate, NW-SE-trending Periadriatic foreland basin. Heavy-mineral contents of fifteen Contessa samples taken across the whole outcrop area vary systematically. Zircon, tourmaline, rutile, garnet, chloritoid, picotite and monazite are ubiquitous. Staurolite and, to a lesser degree, titanite are absent along the southwestern margin of the basin and become increasingly present toward the foreland to the northeast, where the basin fill becomes progressively thinner. Other petrologic parameters of the Contessa light-mineral composition depict a similar geographic trend. Intrabasinal carbonate clasts are progressively more abundant northeastward toward the shallowest part of the basin, whereas the relative percentage of sparite cement increases toward the southwest, where the basin fill is thickest. Among the clay minerals, mixed-layer illite/smectite is more abundant in the shallowest part of the basin and decreases progressively toward the deepest part of the basin. Petrologic parameters of mostly siliciclastic sandstone beds stratigraphically close to the Contessa megabed and derived from the Alps to the northwest ("non-Contessa" samples) depict similar trends, in spite of their different provenance. Assuming that the respective compositions of the Contessa and non-Contessa samples were initially homogeneous across the entire basin, these areal differences in mineralogy are interpreted to result from varying degrees of diagenesis due to differential sedimentary/tectonic load in the asymmetrical Periadriatic basin. Provenance studies must consider this possibility to avoid erroneous reconstructions of sediment paleodispersal systems.