Barolo Zone landscape formation: Building the basement

Discussion of Barolo zone soils requires an understanding of basement and cover rocks from which the soils may be derived. We begin the discussion with a look at the region's basement rocks.

The Hercynian Mountain Belt, stretching from Britain to Eastern Europe, was formed as a result of a continental collision which ended 200 million years ago. This range has been severely eroded over millennia and in many places only exist as "basement" rock, hidden from view by sedimentary deposits. The figures below illustrate the concepts of basement and cover rocks.

Basement and cover rocks of France.
Source:http://www.virtual-geology.info/lozere/lozere.html

Relationship between basement and cover rocks.
Source: http://www.virtual-geology.info/lozere/lozere.html

Formation timeline -- basement and cover rocks
Source: http://www.virtual-geology.info/lozere/lozere.html

According to Martinez Catalán, et al., a crustal basement is "the result of an orogeny that is the consequence of a deep remobilization of the continental crust caused by the convergence of plates." Three major phases of orogeny (mountain building) define the geology of the European continent: Caledonian, Variscan/Hycernian, and Alpine. The timing of these three orogenies are illustrated in the figure below.

Using the Variscan orogeny as an example, Martinez Catalán, et al., see the process as encompassing:

• Shortening and intensely deforming those sediments deposited previously along vast continental margins
• Remobilizing the prior basement on which they laid
• Generating a new crust by partial melting of the former
• Adding fragments of oceanic crust and mantle
• Eroding and re-sedimenting part of the newly formed crust
• Deforming the majority of the new sediments.

The Italian basement recorded a possible subduction of oceanic crust with Ordovician granite during the Caledonian orogeny in the Ordovician to Early Devonian eras (490 - 390 mya). The orogeny was caused by a collision of the continents and terraces of Laurentia, Baltica, and Avalonia and subduction of the crust of the Iapetus Ocean. The configuration of the world's landmasses prior to that orogeny is illustrated in the picture below.

Reconstruction of how the Iapetus Ocean and surrounding
continents might have been arranged during the late
Ediacaran Period. Source: wikipedia.com

The distribution of mountain ranges at the end of the Caledonian orogeny is illustrated below.

Present day coastlines are indicated in gray for reference.
Source: By Woudloper - Own work, CC BY-SA 1.0,
https://commons.wikimedia.org/w/index.php?curid=5038110

The granites formed during the Caledonian orogeny were later deformed during the Hercynian (Variscan) orogeny, a Late Paleozoic geologic process which deeply modified the continental crust of western Europe and the north and northwest of Africa. This orogeny resulted from the collision of Euramerica and Gondwana to form the supercontinent Pangea. The Variscan orogeny forms the basement of most of western and central Europe and is well preserved in Italy when the basement pops up (as it does in the Alps, Sardinia, Calabria, NE Sicily, and West Tuscany). The results of the Variscan orogeny are shown in the figure below. The only Italian section of the Variscan not superimposed by Alpine or Apennic thrusts are belts located in Sardinia.

After the Hercynian earth movement, the general area of Italy was covered by the sea for long periods. As such, limestones are typical for the Permian and Mesozoic eras. Towards the end of the Mesozoic, and during the Tertiary, gravels, sands, clay, limestones and marls were deposited.

Source: wikipedia.com

The most recent activity in the basement formation began with initial compression caused by subduction of the European plate under the African plate in the Jurassic period. Collision between the African and Eurasian plates resulted in increased deformation of Tethyian Sea deposits. The orogeny "produced intense metamorphism of preexisting rocks, crumpling of rock strata, and uplift accompanied by both normal and thrust faulting." Remnants of the Tethys Sea remain as the Mediterranean, Black, Caspian, and Aral Seas. The plate movement resulted in the Italian peninsula being driven northward and compressed into Europe.

Ranges resulting from the Alpine orogeny. Source: wikipedia.com

A schematic geological map of the Alps is shown below.

Schematic geological map of the Alps. Source: Public Domain,
https://commons.wikimedia.org/w/index.php?curid=4380219

The next phase in this story is the Tertiary Piedmont Basin and the formation of associated cover rocks.

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