A new perspective to model subsurface stratigraphy in alluvial hydrogeological basins, introducing geological hierarchy and relative chronology

This paper presents a novel perspective for modelling alluvial stratigraphy. It integrates the spatial geological information, geological maps and well-log descriptions, with the rules describing the hierarchy and relative chronology of the geological entities. As geological modelling tools are moving fast forward, the urgent need for expert geological input, codified as modelling rules, persists. Concerning subsurface alluvial architectures, the concepts of \u201cstratigraphic hierarchy\u201d and \u201crelative chronology\u201d provide the most relevant rules which permit to link the modelling procedure to the geo-history of a region. The paper shows how to formalize this knowledge into modelling rules. This is illustrated and implemented in a Python\u2122 module named HIEGEO which is applied on a 2-D cross-section from the Po Basin (N-Italy). The stratigraphic correlation yields 2-D pictures of the hierarchic stratigraphy and relative chronology of the units. The input are: an attribute table of stratigraphic boundaries expressing their hierarchy and chronology; contact points where these boundaries cross the control logs. Since the aim of HIEGEO is to illustrate the principle of the method but not to replace existing 3-D geological modelling tools, it implements a linear interpolation algorithm which creates joins between contact points. It plots linear joins framing polygons based on their hierarchy, at any user\u2019s desired detail. HIEGEO highlights potential inconsistencies of the input dataset, helping to re-evaluate the geological interpretation. The proposed workflow allows to: i) translate geological knowledge into modelling rules; ii) compute stratigraphic models constrained by the hierarchy of stratigraphic entities and the relative chronology of geological events; iii) represent internal geometries of the stratigraphic units, accounting for their composite nature; iv) reduce uncertainty in modelling alluvial architectures. It represents a starting point for multi-scale applications and could be easily integrated into 3-D modelling packages, to couple the hierarchical concept proposed here with existing advanced interpolation methods

Geology of the San Colombano hill, a Quaternary isolated tectonic relief in the Po Plain of Lombardy (Italy)

The 1:10,000 geological map of the San Colombano hill covers 60 km 2 in the Po Plain, south of Milan. The new and the historical surface geological data-sets are managed by a GeoDB aiming to contribute to re-interpret the Quaternary evolution at the Po Plain-Northern Apennine border. On the hill, the Calabrian shallow marine San Colombano Fm. unconformably overlies the truncated deeper-marine Miocene formations, up-thrusted by the external fronts of the Apennine Emilian Arc during Mio-Pliocene. Late Pleistocene alluvial units rest in unconformity above the marine succession both on the uplifted hilltop and on the surrounding plain. Fault-related offset of Late Pleistocene units, stratigraphic and morpho- structural evidences (facets, relic surfaces and drainage patterns), document the Quaternary tectonic history. Early to Middle Pleistocene ongoing thrust-folding at the northernmost buried reaches of the Emilian Arc was followed by Latest Pleistocene-Holocene transtension, possibly relating to the NNE striking Pavia-Casteggio lateral ramp

From cylindrical to non‐cylindrical foreland basin: Pliocene–Pleistocene evolution of the Po Plain–Northern Adriatic basin (Italy)

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CHARACTERIZATION AND MODELLING OF COMPLEX GEOLOGICAL ARCHITECTURES: THE QUATERNARY FILL OF THE PO BASIN AT THE PO PLAIN-APENNINES BORDER (LOMBARDY, ITALY)

Object of this work is the 3-D modelling of complex geological architectures in the Quaternary Po Basin (Lombardy, Italy). Reliable surface and subsurface models in Quaternary alluvial basins are important for several applications, including groundwater research and management, geohazard evaluation, exploitation and protection of other natural resources. The study area is the Po Plain-Apennine border in Lombardy (Italy), in a peculiar sector where the structural culminations of the buried Emilian Arc salient of the Northern Apennines determine the location of isolated reliefs in the Po Plain (i.e. San Colombano Hill and Casalpusterlengo \u2013 Zorlesco subtle relic reliefs). This area was selected because it permits to improve the 3-D modelling procedure in a complex tectono-stratigraphic-geomorphological setting, which is of interest for both the still controversial geological reconstructions of the Po Basin and the relevant issues in groundwater management and geothermal energy exploitation. The Quaternary sedimentary fill of the southern margin of the Po Basin in Lombardy records the complex interplay between active Apennine thrusting to the South, rebound and isostatic response to deglaciations at the flexed Alpine margin to the North and the dynamics induced by Quaternary glacial cycles. All of these factors produced the assemblage of nested stratigraphic, structural and geomorphological complexities which are the object of this work. Reliable 3-D models must account for multiple ranks and scales of sedimentary heterogeneity. To obtain such a result, this works attempts to compute 3-D models, constrained not only by the traditional explicit geological \u201chard\u201d surface and subsurface data, but also by the implicit \u201csoft\u201d data represented by the increments of the geological evolution of the basin. At present, none of the available modelling methods incorporates geological evolution, hierarchy of stratigraphic and structural components of geological heterogeneity and uncertainty as formal rules of 3-D model building in a straightforward manner. Aim of the work is to propose an integrated, multidisciplinary methodology to combine both explicit and implicit geological knowledge as constraint for 3-D (4-D) architectural geological modelling of the study area. Specific aims of this work are: i) to reconstruct the surface and subsurface Quaternary geology of the study area at different scales; ii) derive the increments and the autogenic vs. allogenic controlling factors on the geological evolution; iii) develop alternative 3-D (4-D) models of the Quaternary sedimentary infill of the area, honoring the new maps and subsurface reconstructions and accounting for the incremental geological evolution; iv) contribute to improve and implement a method that combines explicit geological data with the implicit hierarchic and evolutionary constraints for 3-D geological modelling. A multidisciplinary methodology has been set-up. It integrates i) classical geological, sedimentological, stratigraphic, geopedological, geomorphological and structural field surveys; ii) subsurface reconstruction based on stratigraphic correlation of borehole logs and geophysical images, along a fence of 2-D cross-sections over an area of 400 km2 and a maximum investigation depth of 150 m b.g.s.; iii) 3-D geological modelling based on integration of the GIS management of the multiple data-sets and the GeoModeller\uae 3-D modelling software. GeoModeller\uae was chosen for the feasibility to deal with the bounding surfaces, which is the key-concept to describe hierarchic frameworks and the key to introduce the genetic interpretation of the basin history (4th dimension) into 3-D representations. To do that, new software routines and novel concepts for the modelling rules were set-up and implemented in the commercial code. Results of the work include: i) a new geological and geomorphological map of the San Colombano hill at 1:10.000 scale over an area of about 60 km2; ii) a hierarchic stratigraphic scheme of the surface-subsurface Quaternary succession of the southern Po Basin, integrated to the iii) incremental tectono-depositional evolution of the Po Basin-Apennine border, that relates the ranking and the significance of the stratigraphic and morphological boundaries to the hierarchy of the Quaternary increments of the geological evolution; iv) the conceptualization of the implicit hierarchic rules to be introduced into 3-D model building, and the procedure to progressively include the explicit and implicit geological rules within multi-scale realizations; v) some new computing routines which let GeoModeller\uae to manage the new rules and vi) alternative 4-D geological models accounting for different interpretations of the geological evolution. Six high-rank increments of the geological evolution (\u201cstages\u201d) punctuated by low-rank steps have been described in this work. During stages 1 and 2, N-ward thrusting along the blind Emilian Arc originated the Zanclean and the Gelasian Unconformities. On the San Colombano hill, the Calabrian shallow marine San Colombano Fm. (PL4 highest-rank succession) unconformably overlies the truncated deep-marine Miocene formations, up-thrusted during Mio-Pliocene. At stage 3, Early to Middle Pleistocene increments of thrust-folding at the northernmost buried reaches of the Emilian Arc induced erosion of the intra-Calabrian unconformity (U1) and separated local depocentres related to the San Colombano and Casalpusterlengo \u2013 Zorlesco structures. These were filled by transitional and alluvial units (PS1 highest\u2013rank succession). These regressive deposits, lap onto the uplifting structures of San Colombano and Casalpusterlengo - Zorlesco, suggesting the onset of their structural separation. At stage 4, these latter two structures were separated from the San Colombano thrust, since the Middle Pleistocene, by means of a newly interpreted dextral lateral ramp (San Colombano lateral ramp), as testified by the delayed migration of the depocentres of the Middle Pleistocene glacio-fluvial units and by the time-shift of the onlaps onto the different structures. After folding of U1, at the base of these units, the Early-Middle Pleistocene unconformity U2 was carved, bounding the base of the PS2 alluvial and glacio-fluvial high-rank succession. During stage 5, Late Pleistocene alluvial and glacio-fluvial units (PS3 highest-rank succession, correlative to late Besnate and Cant\uf9 Alpine glaciations) covered, through the Late Pleistocene unconformity (U3), the older glacio-fluvial succession in the subsurface of Casalpusterlengo and Zorlesco areas, while they terraced the deformed marine succession in the San Colombano area, both on the uplifted hilltop and on the surrounding \u201cPlain Main Level\u201d (Castiglioni and Pellegrini, 2001). Syndepositional normal faulting, related to dextral wrenching regime, occurred during this stage. Fault-related offset of Late Pleistocene units, stratigraphic and morpho- structural evidences (facets, relic surfaces and drainage patterns), document ongoing transtension, at stage 6 (Latest Pleistocene \u2013 Holocene; U4 unconformity), plausibly relating to the NNW-wards thrusting and related wrenching along the Pavia-Casteggio lateral ramp (Benedetti et al., 2003). Field evidences suggest to propose a link between the entrenchment and the anomalies of the post-glacial river network at the southern margin of the Po Plain to this tectonic stage. This reconstruction links the origin of the highest-rank unconformable stratigraphic boundaries to the Quaternary tectonic stages of Apennine thrusting, wrenching and extension. The intermediate- and low-rank unconformities relate to both minor tectonic increments and to the climatic-driven glacial cycles, because the bases of the glacio-fluvial units are nested within the highest-rank tectonic-induced unconformities. On the isolated reliefs, in situ paleosols testify the preservation of non-erosional surfaces, i.e. morphological surfaces, related to sites of morphological stability. These became the sites for loess aggradation during the Late Pleistocene, that means when the isolated reliefs had been already uplifted and the main controlling factor on deposition was climatic. The recognition of unconformable stratigraphic boundaries vs. conformable \u201cmorphological\u201d boundaries permits to unravel the different chronostratigraphic significance of these two surface types (respectively time-transgressive and almost isochronous) and to use them to constrain the reconstruction of the chronological evolution of the basin and the 4-D model to be computed. A novel approach in the use of GeoModeller\uae is proposed by implementing a model building procedure based on coded \u2018hierarchic rules\u2019, at present not encompassed in the modelling suite. A rigorous routine is proposed to apply these rules to obtain at least three ranks of visualization of the 3-D geological architecture of the study area. The ordering of the geological units in the stratigraphic pile, combined with the set of the reference surface (top/bottom) and the nature of the interpolation for each surface (erode/onlap) conceptualized the hierarchic rules valid to represent complex stratigraphic architectures at each scale. 1) The isopotentials of GeoModeller\uae (i.e. the lowest rank surfaces which can be computed and represented by this software) describe well the morphological surfaces, i.e. surfaces stable through time. Using the orientation of the morphological surfaces as reference top boundary for model computation means to constrain the isopotentials to the deformation history of the area. This concept strongly impacts on the 3-D model application to the simulation of internal facies, as it would be necessary to simulate the distribution of hydrostratigraphic parameters. 2) Since crossing the isopotential, the erode stratigraphic boundaries bring the significance of the time-transgressive unconformable surfaces, in accordance with the geological evolution. 3) By attributing erode nature to the high-rank surfaces, and onlap rules and reverse ordering in the stratigraphic pile to the intermediate-rank ones, the resulting 3-D model displays the high-rank surfaces as composite stratigraphic unconformities, like they have been described by the geological model, since they collect the minor increments of deformation, deposition and erosion through the geological time. As a result, the proposed 3-D models are multiscale and honour the explicit geological observations and the implicit geological evolution at each scale of observation. The intermediate-rank boundaries and sediment volumes represent the result of the intermediate-rank evolutionary increments. On larger spatial and temporal scales, they can be grouped and visualized into higher-rank boundaries (\u2018U\u2019 unconformities) and volumes, related to the major tectono-depositional stages. The relationship between geological history and geometrical features, with the possibility to upscale and downscale the model according to its hierarchic configuration in view of any specific application, is one novelty of the modelling results here presented. The uncertainties derived from the interpretation of the geological evolution gave rise to two alternative geological models of the San Colombano hill area. Both honour the input explicit data and differ on the interpretation of the extent of the conjugate fault systems that involved the Late Quaternary stratigraphy. The final visualization of the 3-D, ranked stratigraphic units and surfaces highlights the basic role of consistent 4-D geological models as the best synthesis of heterogeneous and multi-scale datasets, that represent the base for several applications at different scale. The adopted approach yields a model that can be easily updated, as soon as new knowledge gets available and modified, and permits to test different hypotheses accounting for any new implicit geological constraints

Quaternary hydrostratigraphy in the Po Plain: multiscale geology and forward modelling

Hydrogeological modelling at different scales is best performed if the hydrostratigraphic architecture is well described, accounting for the hierarchic arrangement of heterogeneities and the multiple scales of the geological features. To this purpose, an efficient combination of surface geology data (high detail and accuracy, 1D - 3D data, small to large-scale, low-rank) with subsurface borehole/geophysical data (1D - 2D, very small to very large scale, low detail and accuracy, undistinguished rank) is mandatory. The management of such a multi-scale and heterogeneous dataset is the starting point of this work. The aim is to define a 3-D stratigraphic framework at different scale, constrained by all the available data, as a mean to validate the different evolutionary and geometrical hypotheses formulated in the 1 - and 2-D data analysis carried out in a GIS environment. The study area is a 25x50 km, N-S transect of the Central Po Plain, the widest alluvial basin of Italy hosting multi-layered aquifers of Quaternary age. A reliable geological reconstruction in this area contributes to better define the 3-D geometries of the stratigraphic units, verifying the existent 2-D geological maps and sections. It also contributes to validate the evolutionary history of the Basin, as the result of the interaction of Quaternary Alpine glacial cycles (North), tectonic evolution driven by the Apennine thrusts (South), and changes in the accommodation and sedimentation rates. The method relies on the integration of geological, field-based \u201chard\u201d data, interpretative cross-sections and maps, geo-history and evolutionary hypotheses, with GIS and GeoModeller software. GIS is dedicated to data management and pre-processing for the 3-D analysis; GeoModeller permits to simulate and visualize the stratigraphic/tectonic relations rapidly, honouring the geological constraints, in order to elaborate several forward models comparing contrasting architectural end evolutionary hypotheses. The dynamic approach I want to discuss is multi-scale and hierarchical. It refers to i) the hierarchic order of the different \u201cgeological entities\u201d and ii) the nature/number of constraints that have to be taken into account at each scale of reconstruction, by the integration of both surface and subsurface geological data

Effects of Late Pleistocene synsedimentary tectonics on alluvial architecture at the Po Plain-Apennines border (N-Italy)

The stratigraphic architecture of the Quaternary Po basin fill records the interplay between local and remote controls on alluvial sedimentation: Apennine thrusting acted at the southern basin margin; Pleistocene dynamics of alpine glaciers controlled the Alpine margin; relative eustasy forced the eastern base-level, the Adriatic Sea, to fluctuate. Multiple base-levels, fluvial discharges, sediment textures, inflow and accommodation rates changed through space and time under these forcing factors, which determined the incremental geo-history and the alluvial architecture of the basin fill. In this frame, the role of Late Pleistocene-Holocene synsedimentary tectonics has been investigated at the Po Plain-Apennines border in Lombardy, where a structural culmination of the Emilia salient (San Colombano Hill ramp anticline) exposes the Mio-Quaternary stratigraphy. Geological mapping, stratigraphic, sedimentological and petrographic analyses, complemented by C14 and OSL age determinations, show the tectonic imprint on alluvial architecture: composite unconformities, pinch-out of alluvial sedimentary bodies, cross-cut relationships among alluvial terraces, uplifted palaeo-valley fills, cannibalism of pre-existing alluvial clastics, fault-related colluvial wedges and soft-sediment deformation structures. A high-rank, Middle-Late Pleistocene angular unconformity truncates the Gelasian regional unconformity and the local intra-Calabrian and Early-Middle Pleistocene unconformities, due to N-ward thrusting increments along the Emilia salient. This composite unconformity bounds alpine-sourced alluvial and glacio-fluvial units: Cascina Parina Synthems 1 and 2 (CPS1 and CPS2, Late Pleistocene, bracketed by OSL data to the MIS5-MIS4 time span and bounded respectively by intermediate-rank unconformities S0 and S1), Invernino Synthem (INS, Latest Pleistocene-LGM, bounded by the intermediate-rank erosional unconformity S2). They terrace the folded Miocene-Calabrian marine succession on the uplifting hill. Progressive wedging and S-ward thinning, recurrent amalgamation, petrographic changes and soft-sediment deformation structures of CPS1, suggest that the system was confined by an uplifting mild relief, ancestor of the present-day Hill. CPS2 glacio-fluvial system, fed from the N-western Verbano-Lario glacial amphitheatres, fringed-out towards the western hill sector, while a N-S flowing, distal braided glacio-fluvial system eroded the structural culmination to the East, originating the planation surface S1 at present uplifted at the hilltop. On the hill, CPS2 sediments fill relicts of syn-tectonic paleo-valleys, i.e. the first drainage pattern of the Late Pleistocene San Colombano relief. S2 unconformity heralds the S-ward progradation of the LGM glacio-fluvial depositional system (INS). A local, lateral ramp-related transtensional regime triggered differential uplift and tilting of the INS terraces bounding the hill, causing the progressive shifting of INS depositional systems on the lowered hangingwall. The increasing energy of the relief enhanced colluviation along the steep fault-slopes. A late-LGM muddy flood plain developed N of the hill, owing to tectonic-induced subsidence during glacial retreat. This was cross-cut by the meandering streams of the paleo-Sillaro Synthem during the Late Glacial, while paleo-Po River large meanders were carved on the SW flank of the hill. Post-glacial-Holocene entrenchment of the river network led to deposition of the Po Synthem, proposing a link among the origin of this lowermost terrace, the Holocene-to-recent river network anomalies and a phase of uplift and transtension related to the ongoing N-wards propagation of the Emilian salient

Late Quaternary sedimentation and tectonics in the Po Basin: field evidences at the Po Plain-Apennines border (Lombardy)

Topographic reliefs and terraced landscapes represent key-sectors to constrain recent geological evolution and subsurface stratigraphy in almost flat landscapes like the Quaternary Po Plain. Aiming to investigate the role of Late Quaternary tectonics on the complexity of stratigraphic and geomorphological features exposed at the Po Plain-Apennines border in Lombardy, we focus on a structural culmination of the Emilia salient, the San Colombano Hill ramp anticline. Geological and geomorphological mapping at 1:10.000 scale, stratigraphic, sedimentological, paleontological, petrographic and morpho-structural analyses, complemented by C14 and OSL age determinations, show the incremental tectonic imprints on the Quaternary stratigraphy originating the present-day palimpsest landscape. Location of unconformable stratigraphic vs. conformable morphological boundaries, pinch-out and cross-cut relationships among alluvial sedimentary bodies, uplifted paleovalley fills, cannibalism of pre-existing alluvial clastics, colluvial wedges and sediment deformation structures highlight how, where and when tectonic-driven processes controlled the evolution of the Hill and the adjacent plain. The S. Colombano ramp anticline underwent Early-Middle Pleistocene thrusting, which uplifted and folded the Gelasian regional unconformity between littoral Calabrian and deep-marine Miocene formations. Late Pleistocene, alpine-sourced alluvial and glacio-fluvial units, terraced the deformed marine succession through the composite Late Pleistocene unconformity. The mapped synthems progressively wedge, thin and amalgamate S-wards, suggesting the syn-sedimentary confinement by an uplifting mild relief ancestor of the present-day Hill. Relicts of syn-tectonic paleo-valley fills testify the first drainage pattern of this proto-hill, where also polycyclic loess-soil aggraded during early Late Pleistocene. The S. Colombano structure underwent dissection since latest Pleistocene along three fault systems, while LGM glacio-fluvial and alluvial units prograded from the NW. Evidences of Latest Pleistocene fault activity are observed as thickness variations of the LGM synthem, offset of the Late Pleistocene unconformity, paleosol reworking in colluvial wedges on the fault-block hangingwalls, marked by triangular facets and abrupt diversions of the river network. A late-LGM muddy flood plain developed N of the hill, owing to tectonic-induced subsidence. This was cross-cut by the meandering tributaries of the paleo-Po River during the Late Glacial. The post-glacial-Holocene entrenchment of the river network and river anomalies suggest an eventual latest phase of uplift and transtension of the San Colombano structure, plausibly related to the ongoing N-wards propagation of the Emilia salient. The progressing research is integrating the surface and subsurface field geological and evolutionary constraints into 3D(4D) geological models, as the basis for hydrogeological and geohazard applications

Deciphering Quaternary palimpsest landscapes to constrain 3D (4D) geological models: an example from the Po Plain-Apennines border (N-Italy)

Palimpsest landscapes represent important archives of the geological evolution of a basin. Deciphering the nature, timing and hierarchy of successive episodes of landscape-changing events in dynamic settings helps to constrain reliable evolutionary models and/or simulations, which incorporate the 4th dimension into 3D geological reconstructions. The landscape of the Quaternary Po Plain, in the foreland between the Alps and the Apennines, is the surface expression of active thrusting of the Apennines (South) and Alpine glacial cycles (North). Reliable models of this complex, geomorphological and tectono-stratigraphic setting, are of interest for both the Quaternary reconstructions of the basin and the relevant issues in land and resources management. Aiming to describe the Late Pleistocene-Holocene incremental landscape evolution of the basin, and to develop 3-4D models, we focus on a region close to the Po Plain-Apennines border, where three isolated low-relief hills emerge above the plain as a consequence of N-wards blind thrust-related uplift and wrenching/collapse. The hills expose stratigraphic boundaries and (paleo-) morphological surfaces otherwise buried in the adjacent plain sectors and show the involvement of the Quaternary, mostly alpine-sourced littoral, alluvial and glacio-fluvial succession in Apennine folding and faulting. The adopted multidisciplinary approach combines different scales of geomorphological, geopedological, stratigraphic and structural field observations corroborated by OSL and 14C age determinations to feed GIS-assisted, quantitative morphotectonic and stratigraphic analyses. The 3D forward models were processed by interfacing the GIS management with the 3DGeoModeller software. The results highlight how, where and when climate and tectonic-driven processes fingerprinted the evolution of the hills and surrounding plain landscape. The proto-hills, i.e. paleo-highs uplifted during active Apennine thrusting in the early Late Pleistocene, were the sites for polycyclic loess-soil aggradation. This suite of non-erosional surfaces, characterized by in situ paleosols, relates to sites of morphological stability. Valley incision, fluvial terracing, soil truncation and redeposition occurred along the faulted boundaries of the uplifting reliefs and in the intervening structural lows. Late Pleistocene transtensional faulting induced abrupt changes of the drainage network by shifting the base-levels and enhanced reworking of paleosols. These were redeposited in colluvial wedges on the fault-block hangingwalls, the latter highlighted by peculiar alignments of morphotectonic indicators. A latest uplift stage during the latest Pleistocene-Holocene is recorded by the regional entrenchment of the major drainage network of the southern Po Plain, which is affected by several fault-related diversions, river captures and terrace-slope intersections. These cross-cut relationships and the tectonic increments have been converted into modelling rules, which have been incorporated as new routines into the GeoModeller suite. The work shows that palimpsest landscapes represent the topographic and geomorphological constrains for modelling, where the modelled volumes, assembled by their hierarchy and formative increments, incorporate the evolutionary steps and drivers deciphered from field observations

Significance of the morphological and stratigraphic surfaces in the Quaternary Po Plain : the San Colombano tectonic relief (Lombardy, Italy)

The recent geological, geomorphological, pedological and structural surveys performed at the San Colombano tectonic relief document a Late Quaternary tectonic activity of the structure, located at the Po Plain-Northern Apennine border in Lombardy (Italy). The new maps, corroborated with (palaeo-) geopedological analyses, allow to unravel the composite nature of the morphological and stratigraphic surfaces bounding the observed units and landforms, and to relate them to the climatic and tectonic events that interacted during the Late Quaternary at the Apennine-Po Plain border