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Space-time geological model of the Quaternary syntectonic fill of a foreland basin (Po basin, Northern Italy)
Full text linkQuantitative models of large and widely populated Quaternary foreland basins, which host relevant resourceslike groundwater, heat, and fertile soils, may be computed if space–time models of the basinfills are available.The paper presents the combination of geometrical (space) and evolutionary (time) constraints to modelQuaternary 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 subsur-face equivalents obtained from borehole logs and published geophysical images, to draw the bestfitting model,which was constrained to the Quaternary geological evolution.Discrete, long-term thrusting, wrenching, folding, and extensional collapsing stages are inferred to have shapedthe Gelasian, intra-Calabrian, Early–Middle Pleistocene, Middle–Late Pleistocene and Latest Pleistocene–Holocene high-rank composite unconformities. These surfaces are cut into synsedimentary, polyphasic, buriedanticlines 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 regressivefill of the basin. These surfaces might correspond to steady tectonic uplift/subsidence periods when depositionwas mostly controlled by Pleistocene climate changes since the Gelasian.The hierarchic space/time structure, translated into hard geometrical constraints (a fence diagram of cross-sec-tions) and relative chronology of the stratigraphic relationships (intersect/erode, onlap and offset) allows com-puting the geological quantitative model using a potentialfield interpolation method. The model buildingphase and subsequent inspection permit critically analysing the current interpretations on the evolution of thePo 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 exportedfor hydrogeological, geothermal and environmental modelling
Effects of Late Pleistocene synsedimentary tectonics on alluvial architecture at the Po Plain-Apennines border (N-Italy)
No full textThe 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
Deciphering Quaternary palimpsest landscapes to constrain 3D (4D) geological models: an example from the Po Plain-Apennines border (N-Italy)
No full textPalimpsest 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)
No full textThe 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
Turbidite channel-levée transitions: insights from the Tachrift system (Complex 6, Taza-Guercif Basin, NE Morocco)
No full textThis contribution reports on the outcrop of one (Complex 6, hereafter) of multiple turbidite channel-levée complexes making the Tachrift system (Late Tortonian- early Messinian), beautifully exposed in NE Morocco as part of the clastic fill of the Taza-Guercif Basin. Geological mapping was integrated with facies analysis on 46 logs and physical stratigraphic correlations. Complex 6 consists of three sandstone-rich units labelled A, B, and C that progressively increase in grain-size and are laterally stacked in a SE-ward shifting fashion. Particularly, channel- fills and correlative levées of Units B and C have contrasting architectural styles which likely reflect changes in flow parameters (e.g., grain size, volume, density stratification etc.), channel sinuosity, and morphodynamics. Owing to magnificent 3D exposures, the ongoing study of Complex 6 outcrops will provide sedimentological characterization of channel-fills and correlative levée deposits, insights into evolution of their parent channel from inception trough abandonment, and sub-seismic lithological calibration of subsurface analogues
Soil evolution and origin of landscape in a late Quaternary tectonically mobile setting : the Po Plain-Northern Apennines border in Lombardy (Italy)
Full text linkThe interactions among Quaternary (paleo-) soil-forming, erosion, re-deposition and geomorphic processes at the origin of the present-day landscape are discussed at three isolated tectonic reliefs of the Southern Po Foredeep Basin of Lombardy (San Colombano, Casalpusterlengo and Zorlesco reliefs, Italy). These sites offer the possibility to combine different scales of geopedological, geomorphological, stratigraphic and structural ob- servations, to unravel the contribution of climate and tectonics to the Quaternary geological evolution and to the origin of the landscape. Field surveys, laboratory analyses and soil micromorphology were used to characterize the Late Quaternary pedogenic processes acting on different (paleo)-surfaces, in combination with the detailed study of seven se- lected soil profiles. Pedo-stratigraphic correlations revealed that two alluvial/loess-paleosol sequences, which originated from the Late Pleistocene to Holocene glacial/interglacial fluctuations, are differently preserved throughout the present-day hill landscape. In situ paleosols allowed to localize the paleo-topographic surfaces of geomorphic, tectonic and sedimentary stabilization; redeposited paleosols, occurring systematically close to the main faults and/or at the erosional bottom of paleo-valleys, allowed to mark the stages of tectonic instability. The integrated approach contributed to unravel the composite nature and rank of the stratigraphic boundaries. Polycyclic loess-soil aggradation characterized the stable paleo-highs, uplifted during different phases of active thrusting of the northernmost Emilian arcs of the Apennine, while valley incision, fluvial terracing, soil truncation and redeposition occurred in the intervening structural lows. In the San Colombano hill area, Late Pleistocene transtensional faulting induced changes of the drainage network and enhanced redeposition of pa- leosols in colluvial wedges on the hangingwalls, along the fault scarps. LGM loess was preserved above different dissected remnants of the paleo-topography composing the hilltops. These new constraints permit to refine the Late Quaternary tectono-depositional history and landscape evolution at the southern margin of the Po Basin
A new perspective to model subsurface stratigraphy in alluvial hydrogeological basins, introducing geological hierarchy and relative chronology
Full text linkThis 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 “stratigraphic hierarchy” and “relative chronology” 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 PythonTM 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’s 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
Quaternary evolution of the Central Po Plain: surface constraints from the San Colombano relief (Lombardy, Italy)
No full textThe Quaternary evolution of the Po Basin reflects the complex interaction between the tectonic evolution of the active Apennine thrusts (S), the rebound at the flexured alpine margin (N), the Middle Pleistocene advances and retreats of the Lario-Verbano glaciers, the changes in sediment and accommodation rates. In such almost flat areas, topographic reliefs and terraced landscapes represent the key-sectors to obtain geological constraints to the reconstruction of both evolution and subsurface stratigraphy.
The San Colombano hill is one of the topographic expressions of buried Quaternary Apennine-related tectonics scattered in the Southern Po Plain. Nowadays, only regional 1:100.000 scale geological maps exist on this area. Therefore, a detailed cartography and new data are needed to constrain the relative chronologies of the Quaternary tectono-stratigraphic evolution of the Central Southern Po Plain.
The hill exposes the uplifted and folded marine Miocene and Lower Pleistocene units (Sant’Agata Fossili Marls and San Colombano Fm.), unconformably overlain by the Upper Pleistocene alluvial and eolian pedogenized and deformed deposits (Cascina Parina and Invernino Units). The sequence is in turn shaped and covered by the Post-Glacial to Holocene processes and sediments along the slopes and within the valley network of the hill.
We present a new geological and geomorphological map and a revised evolution of the San Colombano structure, which involves the mainly S-Alps-sourced sediments and the N-Apennines thrust-related tectonics.
We applied an integrated approach that combines new geological mapping at 1:10.000 scale with sedimentological, geomorphological and pedological field and laboratory data analyses. In particular, different morphological evidences along the hill, as drainage pattern anomalies, the erosional nature and distinctive pedo- features of sub-planar sectors, nature and recurrent orientations of the escarpments are highlighted. They can be consistently used to derive the complex tectono-stratigraphic evolution of the San Colombano structure during Quaternary as it is suggested below:
1) the main Plio-Early Pleistocene phases of advance of the most external N-Apennine thrusts shaped the erosional unconformity between the transgressive San Colombano Formation (Calabrian) and the folded Sant’Agata Fossili Marls (Upper Miocene). 2) Thrust-folding and uplift persisted until the Upper Pleistocene, involving the Cascina Parina Unit, fed by an alpine glacio-fluvial depositional system. It was terraced by the alluvial deposits and covered by the loess of the Invernino Unit. Evidences of erosion and displacement of the pedologic profiles of Cascina Parina sequence are registered. 3) Transtensional faults dissected the structure close to the end of the Pleistocene. Fine-grained alluvial sediments filled local depocentres in the intervening gentle hangingwall synclines, onlapping the tilted Invernino Unit at the fringing-up of the Alpine fed systems. The river network was reorganized and polycyclic soils developed on gently tilted and partly suspended terraces. 4) Uplift quiescence and development of the post-glacial, NW-SE oriented paleo-drainage. 5) New uplift mainly involves the NNW sector of the structure. Further reworking and entrenchment of the river network, southward shift and entrenchment of the Po river, colluvial deposition and aggradation close to the relief, deposition of the Holocene to historical terraced units of Lambro river.
The dataset used to define the geological evolution has been predisposed to a further 3D reconstruction, that will be a way to compare and validate different geometrical and evolutionary hypotheses carried out in this work. At the present state of the research, the proposed evolution gives new geometrical and chronological tectono-stratigraphic constraints to reconstruct the evolution of the Central Po Plain
Aquifer-building processes : unravelling allogenic controls to constrain 3D models of the Quaternary alluvial architecture in the Central Po Plain (Italy)
No full textThe 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
The channel-levée transition: insights from the Tachrift Turbidite System (Complex T6, Taza-Guercif Basin, NE Morocco)
Full text linkTurbidite channel-levée complexes have been the focus of research over more than forty years. Although high-resolution 3D seismic yielded detailed imaging of these deposits, facies transition from channel to correlative levée remains elusive. To fill this gap, extensive and well-exposed outcrops are important, as they provide information on fine-scale facies heterogeneity. Despite this, documented outcrops where accordant channel fills and overbank sediments can be observed with continuity are relatively few/small. We present the Tachrift Project aimed at the field-based reconstruction of the Tachrift Turbidite System (Tortonian-Early Messinian) in the Taza-Guercif Basin (Rifian Corridor, NE Morocco). Among the 9 superimposed, spectacularly exposed channel-levée complexes (T1 to T9) filling the basin for a total 600 m thickness, here we focus on the T6 complex. Detailed geological mapping was integrated with measurements and facies analysis on 46 logs, and physical stratigraphic correlations. T6 (12 m thick) is bounded above and below by 3 to 10 m-thick mudstone units. It consists of three stacked sandstone-rich units that are separated by erosional surfaces. Each unit comprises peculiar, physically traced facies associations showing different channel-levée transitions. They reveal an eastward channel belt migration and a progressive flow energy/density increase. Benefitting from 3D great exposures, the ongoing project provides i) characterization and correlation of channels and accordant overbank sediments, ii) insights on evolution of a channel-levée complex from inception to its deactivation, iii) sub-seismic scale lithological and architectural calibration for analogue subsurface deposits
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