Understanding the causes and mechanisms of land
subsidence is crucial, especially in densely populated coastal plains. In
this work, we calculated subsidence rates (SR) in the Po coastal plain,
averaged over the last 5.6 and 120 kyr, providing information about land
movements on intermediate (103–105 years) time scales. The calculation
of SR relied upon core-based correlation of two lagoon horizons over tens of
km. Subsidence in the last 120 kyr appears to be controlled mainly by the
location of buried tectonic structures, which in turn controlled
sedimentation rates and location of highly compressible depositional facies.
Numerical modelling shows that subsidence in the last 5.6 kyr is mainly due
to compaction of the Late Pleistocene and Holocene deposits (uppermost 30 m).
Introduction
The effects of land subsidence could be devastating on heavily settled,
low-gradient, coastal plains, in a scenario of 0.9 m of relative sea-level
rise by 2100 (Rohling et al., 2013). Increasingly high costs are expected to
protect coastal cities and touristic hotspots and to keep drained reclaimed
lands (Syvitski et al., 2009; Erkens et al., 2016).
In the Po coastal plain, extending more than 100 km south of the Venice
lagoon, decadal to centennial land movements in the order of several cm yr-1 (Teatini et al., 2011; Cenni et al., 2013), were strongly driven by
human activities (mostly water and gas withdrawal, Teatini et al., 2006). To
separate anthropogenic from natural subsidence, understanding the role of
subsidence through time before human intervention is required. Long-term
(106 years) natural subsidence has been calculated based on deep seismic
analysis (Carminati and di Donato, 1999). On the contrary, spatial
distribution of SR on intermediate (103–105 years) time scales is
poorly known.
In this work, we measured SR in the last 120 kyr along a 40 km-long transect
in the Po coastal plain. We used as reference marker beds two lagoon
horizons identified in sediment cores, from the Last and Present
Interglacials. To discuss the relations between subsidence and structural
setting, we reconstructed stratal architecture down to 3 km through seismic
analysis. We focused on the stratigraphic significance of marker beds and on
their use for SR calculation. The contribution of sediment compaction to
land subsidence was assessed through geotechnical analysis and decompaction
modelling.
Geological setting
The Po Plain is a ∼ 40 000 km2-wide alluvial plain
bounded by the Alps to the north and by the Apennines to the south. Frontal
thrusts of both chains are sealed beneath the Po Plain (Amadori et al.,
2019). The Apennine thrusts are north-verging and exhibit arcuate shapes.
Thrusts SE of Ferrara (Fig. 1) were active since the Early Pliocene. The Po
Basin fill shows a shallowing-upward trend from deep-marine turbidites to
coastal and continental units (Ghielmi et al., 2013). The rhythmical
alternation of Middle-Late Pleistocene coastal and alluvial deposits
reflects glacio-eustatic fluctuations at the Milankovitch scale (Amorosi et
al., 2004). Two transgressive-regressive coastal wedges in the uppermost 130 m were assigned to the Last (MIS 5e) and to the Present (MIS 1)
Interglacials based on pollen data, 14C dates and
electron-spin-resonance age determinations (Ferranti et al., 2006).
Landwards, the maximum marine ingression is marked by a thin brackish-lagoon
horizon, sandwiched between inner estuary (below) and upper delta plain
freshwater deposits. Laterally extensive brackish water bodies settled in
the Po coastal Plain around 7–5 cal kyr BP, after sea-level stabilization
(Vacchi et al., 2016), in areas sheltered by fluvial sedimentary input
(Amorosi et al., 2017).
Study area with location of cores, exploration wells and cross
sections of Figs. 2 and 3. The projection of north-verging buried thrusts is
depicted.
MethodsStratigraphy
Calculation of Late Pleistocene and Holocene SR relies on the elevation
(meters above modern sea level – m a.m.s.l.) of two stratigraphic markers. The
lower one is a lagoon horizon marking the top of the MIS 5e coastal wedge.
The upper one is a thin lagoon horizon, dated to 5.6±0.5 cal kyr BP,
encountered at depths <20 m. Chronological constraints for core
correlation are provided by 76 14C ages (see Supplement).
In order to assess the impact of buried faults and folds on lateral changes
in SR, correlations were carried out perpendicular to these structures. A
similarly oriented seismic profile was interpreted and time-depth converted
through calibration with exploration-well logs (depth of 300–5000 m).
Seismic interpretation relied on the identification and tracking of the main
discontinuities (i.e. reverse faults) and unconformities.
Subsidence rates calculation
The calculation of SR for a selected time interval (Δt) between the
time of deposition (td) and the Present (t0=0) was based on Eq. (1).
SR(Δt)=Ztd-Zt0Δt,
where Ztd is the elevation of the stratigraphic marker at the time of
deposition and Zt0 is the present elevation. An error of 0.15 m
associated to Zt0 due to core stretchening/shortening was included in
the calculation (Hijma et al., 2015). Sedimentological data (Amorosi et al.,
2017) indicate that the 5.6 kyr BP lagoon horizon accumulated in a lower
delta plain environment (∼0 m above coeval sea level).
Estimation of sea level at 5.6 cal kyr BP was based on the prediction curve
of Vacchi et al. (2016). As sediment deposition presumably took place in a
water body <1 m deep, an error of ±0.5 m was taken into
account in SR calculation.
The MIS 5e lagoon horizon likely accumulated in water depths <2 m
(associated error of ±1 m; Amorosi et al., 2004). An additional error
of ±1 m was added to compensate possible inaccuracy in correlation
due to low chronologic resolution. The lagoon horizon was generically
assigned to the MIS 5e highstand (120–116 cal kyr BP, Rovere et al., 2016),
7±2 m a.m.s.l.
Subsidence rates between 120–116 and 5.6–0 cal kyr BP were compared with SR
averaged over the last 1.5 Myr. The 1.5 Myr unconformity (Amadori et al.,
2019) is marked in exploration-well logs by the first occurrence of
Hyalinea Baltica (Carminati and di Donato, 1999).
Sediment compaction
In order to assess the contribution of sediment compaction to natural
subsidence, a finite-element 1-D decompaction model (Gambolati et al., 1998;
Zoccarato and Teatini, 2017) was applied to core B4 (see Supplement). The mechanical characterization of the main lithofacies
associations was based on oedometer tests, bulk-density and loss on ignition
tests (van Asselen et al., 2009).
Interpreted seismic profile (see Fig. 1 for location)
perpendicular to the main buried tectonic structures.
Results and discussionDeep stratal architecture
The subsurface of the study area, down to 3 km depth, consists of imbricated
thrusts, locally associated with backthrusts (Fig. 2). Two major
unconformities, dated to 5.5 and 1.5 Myr (Ghielmi et al., 2013),
respectively, allow identification of three tectono-stratigraphic units
(Fig. 2): (i) an intensely deformed and faulted pre-tectonic unit; (ii) a
syn-tectonic wedge, made up of marine turbidites (Ghielmi et al., 2013); and
(iii) a nearly undeformed, post-tectonic unit, sealing the main structures.
The 1.5 Myr unconformity depicts a major fold, with a wavelength of
∼30 km resulting from faults propagation and imbrication.
Lower-rank folds (wavelength of ∼ 3–5 km) are associated with
single-fault propagation. The thickness of the post-tectonic unit ranges
between 300 m at the culmination of the main anticline and 2000 in the
backlimb and forelimb depocentres. Post-1.5 Myr turbidites accumulated in
synclinal areas (Ghielmi et al., 2013), whereas truncation of Pliocene
strata is observed above the anticline. Deformation of Pleistocene strata
was observed close to the anticline culmination, suggesting local post-1.5 Myr tectonic activity.
Shallow stratigraphy
The stratigraphy of the uppermost 140 m is depicted in the correlation panel
of Fig. 3 (see Supplement for facies description). The MIS 5e
sediment wedge, at 60–100 m depth, consists of coastal sands, replaced by
lagoon deposits at inland locations (core 204S4). A thin (0.5–3 m) lagoon
horizon drapes the coastal sands and is overlain by a shallowing upward
succession of coastal plain and alluvial deposits, 35–65 m thick, deposited
between ∼110 and 10 kyr BP. Delta plain estuarine and swamp
deposits, pinch out toward the anticline culmination. Fluvial-channel
deposits are abundant around MIS 4 and MIS 2, whereas closely spaced
paleosols mark MIS 3 floodplain deposits. A prominent paleosol marks the
Late Pleistocene-Holocene boundary and is overlain by estuarine deposits
(Bruno et al., 2017). A thin lagoon horizon, mostly sandwiched between
freshwater deposits and dated to ∼5.6 cal kyr BP, is clearly
recognizable along the whole cross-section.
(a) Subsidence rates averaged over the last 5.6, 120 and 1500 kyr;
(b) core correlation of Late Pleistocene-Holocene deposits (see Fig. 1 for
core location); (c) elevation of the 1.5 Myr BP unconformity.
Factors controlling subsidence rates
Subsidence rates in the Po coastal plain appear to be strongly influenced by
the buried tectonic structures. Particularly, SR averaged over the last 5.6
(SR5.6), 120 (SR120), and 1500 (SR1500) kyr decrease towards
the anticline culmination (Fig. 3). In syncline areas, subsidence may have
been enhanced by high sedimentation rates and by the preferential
accumulation of highly compressible prodelta, delta plain and swamp muds
(Fig. 3).
Subsidence rates in the last 5.6 kyr are in the range of 0.8–2.0 mm yr-1,
about twice the values measured for the last 120 kyr (0.4–0.8 mm yr-1).
Superimposed to the structurally controlled trend, Holocene stratigraphic
architecture influenced lateral changes in SR5.6. For example,
SR5.6 are higher in EM1, where the 5.6 kyr BP horizon overlies soft
estuarine muds, and lower in EM8, where the same marker bed lies onto the
less compressible, 7 m-thick coastal sand body. The 5.6 kyr lagoon horizon,
encountered at ∼-9 m a.m.s.l. in core B4, was placed at -3.5 m
after decompaction, ∼1 m below sea-level predictions (Vacchi
et al., 2016). Therefore, subsidence at this location is almost entirely
accommodated by the compaction of the uppermost 30 m-thick sediment package.
Particularly, the Late Pleistocene succession did not act as an
“incompressible substratum” but compacted of ∼23 % (see
Supplement), likely due to soft swamp peaty clays.
Conclusion
Subsidence rates for the last 5.6 and 120 kyr were assessed in the Po coastal
plain within a chronologically constrained stratigraphic framework. Two
laterally extensive lagoon horizons were used as reference marker beds.
Subsidence rates in the last 120 kyr appear to be mainly controlled by the
location of buried Apennines thrust-related folds, which in turn influenced
sedimentation rates and distribution of highly compressible depositional
facies. Compaction of Late Pleistocene and Holocene sediments strongly
impacted SR in the last 5.6 kyr.
Data availability
Exploration-well logs used in this work are available at https://www.videpi.com/videpi/pozzi/pozzi.asp (Ministry for Economic Development DGRME – Italian Geological Society – Assomineraria, 2019). Oedometer tests were downloaded from the database
of the Geological Seismic and Soil Survey of Regione Emilia Romagna (https://ambiente.regione.emilia-romagna.it/en/geologia/cartografia/webgis-banchedati/webgis-e-banche-dati?set_language=en, Geological, seismic and soil survey, Regione Emilia Romagna, 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/piahs-382-285-2020-supplement.
Author contributions
This work relies on a stratigraphic study carried out by LB, BCa
and BCo and coordinated by AA. Laboratory analyses were
carried out by BCo under the supervision of ES.
Decompaction modelling was carried out by CZ and PT. LB prepared manuscript and figures, with the contribution of all
co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
Financial support
This research has been supported by the RFO grant to Alessandro Amorosi. This paper
is also a contribution to the International Geoscience Programme (IGCP)
Project 663 “Impact, Mechanism, Monitoring of Land Subsidence in Coastal
Cities”.
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