Space–time geological model of the Quaternary syntectonic fill of a foreland basin (Po basin, Northern Italy)

Abstract

Quantitative models of large and widely populated Quaternary foreland basins, which host relevant resources like groundwater, heat, and fertile soils, may be computed if space–time models of the basin fills are available. The paper presents the combination of geometrical (space) and evolutionary (time) constraints to model Quaternary stratigraphy in the southern Po basin in Lombardy, disentangling the tectonic and climatic controls. Five Quaternary high-rank unconformities and seven intermediate-rank unconformities were recognised, down-traced, and correlated from surface to subsurface. Within this framework, the surface facies associations of high-, intermediate-, and low-rank stratigraphic units were compared to the litho-textural associations of their subsurface equivalents obtained from borehole logs and published geophysical images, to draw the best fitting model, which was constrained to the Quaternary geological evolution.Discrete, long-term thrusting, wrenching, folding, and extensional collapsing stages are inferred to have shaped the Gelasian, intra-Calabrian, Early–Middle Pleistocene, Middle–Late Pleistocene and Latest Pleistocene–Holocene high-rank composite unconformities. These surfaces are cut into synsedimentary, polyphasic, buried anticlines and correspond to conformable boundaries in the adjacent depocentres. The intermediate-rank, low-angle unconformities composing the high-rank ones, bound the stratigraphic units forming the overall regressive fill of the basin. These surfaces might correspond to steady tectonic uplift/subsidence periods when deposition was mostly controlled by Pleistocene climate changes since the Gelasian.The hierarchic space/time structure, translated into hard geometrical constraints (a fence diagram of cross-sections) and relative chronology of the stratigraphic relationships (intersect/erode, onlap and offset) allows computing the geological quantitative model using a potential field interpolation method. The model building phase and subsequent inspection permit critically analysing the current interpretations on the evolution of the Po basin during the Quaternary compared with other foreland settings. The 4-D model integrates the surface–subsurface geometry with the tectono-sedimentary evolution in a multi-scale structure that may be exported for hydrogeological, geothermal and environmental modelling.