An electrostratigraphic cross-section across the central Po plain: bearings on subsurface geology and hydrostratigraphy

Abstract

A 72-kilometers-long N-S cross-section was surveyed with Direct Current resistivity soundings through the Quaternary clastic infill of the Apennine foredeep in the Po plain of Lombardy (Northern Italy). The aims are: 1) to test the potential of DC resistivity methods at the regional scale; 2) to draw the electrostratigraphic picture of the basin fill, down to the pre-Quaternary substratum; 3) to refine and test the methodology and concepts of hierarchic electrostratigraphy at the basin scale; 4) to contribute to characterize the regional hydrostratigraphy of a relevant hydrogeological basin. 163 resistivity-versus-depth profiles with a maximum investigation depth of 450 m were obtained from Vertical Electrical Soundings. To draw the electrostratigraphic cross-section, the polarity of the resistivity contrast across resistivity interfaces (i.e., resistive over conductive layers or vice-versa) was considered and the discontinuities were correlated according to the persisting pattern form North to South. Doing so, the hierarchic assemblage of four major electrostratigraphic boundaries and the five intervening Groups of Electrostratigraphic Units (EsUs) was established. The resulting picture was compared to the available subsurface reconstructions from 2 literature, using the 1-D borehole data only to check the geophysical image at some random sample sites. The results show that the electrostratigraphic picture captures the geological and hydrostratigraphic complexity of the subsurface of the basin comparing well with the available direct subsurface geological and hydrostratigraphic reconstructions. At the regional scale of the cross-section, two hierarchic orders of EsUs and correlative boundaries could be drawn (1 st order Groups and 2 nd order individual EsUs), while the lowermost 3 rd order rank, the electrolayer, is recognised at the single VES sites only.

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

“…3), subsurface correlations suggest that a WNW-ESE-trending anticline folds the High-Rank Stratigraphic Unit 0, re-deforming the intra-Zanclean and the Gelasian unconformities (Ghielmi et al, 2013). At stratigraphic depths corresponding to the position of the High-Rank Stratigraphic Unit 0, Vertical electrical soundings show electrical resistivities between 2 and 50 Ωm, in keeping with the expected electrical response of the succession of fine-grained, saltwater-logged sediments covering the basal conglomerate-sandstone facies, which was found in the nearest boreholes (Mele et al, 2018).…”

“…The High-Rank Stratigraphic Unit 0 correlates to the informal subsurface unit GU0 of Bersezio et al (2010) and its features compare to the conductive electrostratigraphic Group 4 of Mele et al (2018). Regionally, the Unit corresponds to part of the generic marine Quaternary as it is defined in logs from the hydrocarbon wells drilled in the central Po Plain (MiSE et al, 2009).…”

“…3,8). The electrostratigraphy of the High-Rank Stratigraphic Unit 3 displays electrical resistivity higher than 150 Ωm, typical of the freshwater-logged, coarse-grained clastic sediments of the Po basin (Mele et al, 2018). The high-rank unconformity U3 gets gently folded and offset by wrench and extensional faults at the San Colombano hill, together with the overlying High-Rank Stratigraphic Unit 3 (Fig.…”

“…Vertical electrical soundings (VES, Fig. 2A) from Mele et al (2013Mele et al ( , 2018Mele et al ( , 2019 were considered; older data from Alfano and Mancuso (1996) were re-interpreted taking notes from Mele et al (2018). Ten N-S cross-sections, parallel to the regional trend of the stratigraphic bodies and orthogonal to the regional tectonic structures, and five W-E crosssections oriented to depict the sedimentary variations between the tectonic sectors of the area (SCS and CZS, Fig.…”

“…Moreover, the rare occurrence of Carya and Pterocarya permits to exclude attribution to the upper part of the Middle Pleistocene that is barren of these pollens (Moore et al, 1991). The deposits of the High-Rank Stratigraphic Unit 2 are associated with electrical resistivity values ranging between 130-170 Ωm, which are calibrated in boreholes to freshwater logged, coarse-grained clastic sediments in the specific setting of the study area (Mele et al, 2018(Mele et al, , 2019.…”