Aquifer-building processes : unravelling allogenic controls to constrain 3D models of the Quaternary alluvial architecture in the Central Po Plain (Italy)

The Po Plain is the widest Quaternary alluvial basin of Italy. During the Pleistocene-Holocene, the stratigraphic architecture of its fill records the complex interplay between the tectonic evolution of the active Apennine thrusts to the South, rebound and isostatic response to deglaciations at the flexed Alpine margin to the North, Middle Pleistocene advances and retreats of alpine glaciers, changing local base levels, fluvial discharges, sediment textures, inflow and accommodation rates. All of these factors determined aquifer building processes, controlling geometry and stacking pattern of alluvial and glacio-fluvial bodies, hierarchic arrangement of aquifer vs. aquitard/aquiclude lithosomes, connectivity of the most and least pervious alluvial elements. Hydrostratigraphic models must account for the hierarchic nesting of heterogeneities; hence, the sedimentary history of aquifer building, the influence and hierarchy of the allogenic factors controlling these processes, and the multiple scales of sedimentological heterogeneity must be evaluated and quantified to constrain the 3-D reconstructions. We apply this approach to a 50 km N-S belt in the Po Plain of Lombardy, from the northern alpine glacial amphitheatres to the southern Apennine foothills. The method we adopt combines field-based geological and modelling approaches with the multi-scale GIS management of the geological dataset. In particular, original geological mapping, stratigraphic, sedimentological, pedological, paleontological, geomorphological and structural observations, integrated with OSL and C14 dating were combined to 1-D facies analysis of subsurface data and 2-D correlations. Surface maps and 2-D cross sections at different scales allowed to derive the controlling factors on the interpreted architecture, and to constrain the 3-D architectural reconstruction. GeoModeller software was selected because it permits i) to compare several different realizations in a reasonably short time, ii) to visualize the stratigraphic relations honouring the geological constraints, to compare the effect of the contrasting controlling factors on the sedimentary and morpho-tectonic history. Within the mentioned transect of the Po Plain, at the present state of the research we are focussing on two key-sectors: 1) the glacio-fluvial and alluvial terraced landscape at the northern Alpine margin, directly influenced by the Pleistocene glacial pulses/retreats; 2) the Apennine tectonic reliefs which emerge in the southern Po Plain and involve the Quaternary, Alpinesourced alluvial succession in the Apennine folding and faulting. The first results include: 1) new geological and geomorphological maps and subsurface reconstructions, which display the progressive modifications that affected the landscape of the study area during Quaternary. They resulted from the interplay of Alpine glacial cycles and the advance of Apennine thrusts. In particular i) syn-glacial fans and sandur progradation from the North built gravel-sand bodies with coarsening-upward stacking patterns and southward fining trends to the base-level, related to an unstable network of multiple braided rivers. ii) Non-synchronous, repeated entrenchment of river valleys occurred at different times in different sectors of the Plain. Deglaciation rebounding and tectonically induced forebulging acted in the North, whereas active folding and faulting, above all, controlled erosional cycles and riverine entrenchment in the South. Under- vs over-fitting of streams, recurrent avulsions, river diversions and piracy are some of the detectable modifications of the fluvial style. iii) Tectonic uplift and entrenchment resulted in erosion and reworking of older, coarsegrained sediments that were recurrently redeposited within the palaeo-valleys and the lowermost terraces in the central and southern sectors of the plain, in contrast with the regional southward fining of the syn-glacial bodies. 2) Comparison between depositional styles and architectures in the two most extreme sectors of the Central Po Plain, allows to associate the different allogenic controls to their peculiar hydrostratigraphic heterogeneity. Uplift, progradation, and entrenchment shaped mostly cross-cutting geometries of the downlapping gravel-sand bodies in the North. They form coarse-grained, complexly connected aquifers that develop above confined aquifer bodies within palaeo-valleys entrenched in a basal, gently tilted marine aquiclude. Contemporaneously in the South, tectonics shaped the pinch-out geometries of the alluvial bodies which onlap the folded units. Syntectonic erosional surfaces developed, merging into major composite unconformities. Consequently, aquifers were confined into different depocentres, and subsequent truncations enhanced the connectivity between alluvial entities of different ages. 3) The proposed integrated, multiscale methodology accounts for the geological constraints and the hierarchic arrangement of the stratigraphic units and surfaces; hence, it can be applied in different depositional settings, to constrain architectural and hydrogeological models independently from the scale of the work.