Geology, soil and seismic risk

The Emilia-Romagna alluvial plane

The Emilia-Romagna alluvial plain is the south-eastern sector of the Po alluvial plain which, in turn, is that crossed by the River Po and its tributaries.

Alluvial Plane

The Emilia-Romagna alluvial plain is the south-eastern sector of the Po alluvial plain which, in turn, is that crossed by the River Po and its tributaries. It is bounded to the north and west by the Alps and to the south by the Northern Apennines, while to the east it opens up to meet the Adriatic Sea.Extending over approximately 30,000 square kilometres, the Po Plain is the largest alluvial plain in Western Europe.

Geologically speaking, the Adriatic Sea and the Po Plain are two parts of the same sedimentary basin: the Po Basin which is the last foredeep basin that developed in front of the Apennine chain from the upper-middle Pliocene, about 3.5 million years ago. At that time, the area where the Po Plain now lies was submerged by a sea several hundred metres deep, in which sand and clay were deposited. The sea bed gradually sank, becoming increasingly lower due to tectonic subsidence and this allowed the continuous accumulation of new sediment over geological time, while the oldest sediment gradually sank to increasingly greater depths. During the Quaternary period (1.8 million years ago) the gradual filling of the Po Basin, in the area we now know as Emilia-Romagna, brought about a transition from an increasingly shallow marine environment to the alluvial plain we know today. Since  the middle-upper Pleistocene (around 800,000 years ago), sedimentation was strongly influenced by the glacio-eustatic cycle, i.e. fluctuations in sea level linked to the melting and formation of ice caps. The sea-level rise in concurrence with warmer periods during which the ice caps partially melted (interglacial phases) and sea flooding, followed by long periods of withdrawal during glacial periods. Stratigraphic architecture of Late Quaternary deposits was strongly controlled by this cyclicity and a regular alternation of alluvial deposits, related to major sea level falls, with coastal/shallow marine sediments, which accumulated during subsequent episodes of sea level rise.

The last glacial-interglacial cycle (post 125.000 years ago) is well documented in the subsoil of Emilia-Romagna alluvial plain. In the subsoil of the modern coastal area this cycle consists of alluvial plain deposits of Pleistocene age overlain by littoral to shallow marine deposits of Holocene age; in the inland areas of the plain this cyclicity is recorded by an  alternations of coarse-grained bodies separated by thick silt and clay bodies.

At the Last Glacial Maximum, around 20,000 years ago, the sea-level was approximately 120 metres lower than it is today. The coastline was around 250 kilometres further south than its current position and the northern part of the Adriatic Sea was an alluvial plain crossed by rivers. With the melting of the glaciers, starting some 18,000 years ago, the sea-level began to rise rapidly. Around 5,000-6,000 years ago, with maximum sea flooding, the coastline lay approximately 25 km further west of its current position. In the last 5,000 years, thanks to the sediment load carried by rivers and the relatively stationary sea-level, alluvial and deltaic plain has reclaimed space once covered by the sea, giving us the geography we know today.

The landscapes of the Emilia-Romagna plain

The present-day landscape of the plain is the result of interaction between various physical processes and the dogged, pervasive intervention of Man.The landscape of the Emilia-Romagna plain should be described taking into account three different landscape unit:

What are alluvial plains?

Alluvial plains are relatively flat areas composed of alluvium. Rivers transport water and sediments from higher to lower ground, eventually reaching the sea. There they form a delta or an estuary, and alluvial deposits merge imperceptibly with adjacent lagoonal, estuarine, tidal flat and beach deposits. Alluvial plains accompany streams; they may start out as very narrow, expanding to tens of kilometers wide nearer the sea, where the alluvial plain merges with the coastal plain.

During floods, alluvial plains receive new alluvium: coarse-grained sediment is deposited in and close to the stream; fine-grained sediment may be deposited at a greater distance. Alluvial soils are recent deposits of alluvium; they may be very fertile depending on their chemical and mineralogical composition, which, in turn, depends on the nature of the rocks outcropping in the drainage basin.

The basin which is needed to hold the sediment load carried downstream is created through a natural process that geologists call subsidence. As a result of this process, a large part of the Earth’s crust downwarps with respect to the surroundings. Natural subsidence rates in coastal plain areas with active deposition are typically in the region of 0.2-0.5 mm/year

History of the Apennines below the Po Plain

The Adriatic Sea and the Po Plain are two parts of the same sedimentary basin: the Po Basin which is the last foredeep basin that developed in front of the Apennine chain from the upper-middle Pliocene (about 3.5 million years ago). The development of foredeep basins is typical of the continental phase of the Apennine orogenesis. The Apennine chain, like the Alps, is the result of the collision between the Eurasian plate and a promontory of the African plate, called Adria micro-plate, which started in the Late Cretaceous (90 million years ago). Initially, the convergence between the plates occurred in a context of subduction of oceanic crust which led to the gradual closing of the Ligurian-Piedmont Ocean located between the two plates. Once the oceanic crust had been “worn away”, the two continental plates collided, effectively forming a very misshapen embryon of the Apennine chain. During the Oligocene, around 29 million years ago, foredeeps began to form along the margin of the uplifted Apennine chain: in these deep asymmetric depressions, thick layers of sediment were deposited above all by turbidity currents. With the continuing orogeny, these initial foredeeps were absorbed, becoming an integral part of the chain, while others still formed in more external positions, in an almost uninterrupted process through to the last Apennine foredeep which coincides with the Po Basin, the superficial expression of which is the Po Plain and the Adriatic Sea.

The interpretation of seismic lines carried out for the purposes of oil and gas exploration documented how the most ancient soils forming the substrate of the Po Basin are greatly deformed. This demonstrates that, from a geological point of view, the Apennine chain is not limited to what is visible on the surface, but continues underground, deep below the surface of the Po Plain. The earthquakes which tragically struck Emilia-Romagna in 2012 in an area of the plain between Modena, Ferrara and Reggio-Emilia, testify to the activity of tectonic structures buried deep below the plain.


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last modified 2024-05-22T13:11:33+02:00
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