Monday, October 10, 2016

Umbria (Italy) and Orvieto landscapes and soils

Umbria, one of the smallest of the Italian regions, lies almost dead center on the peninsula and is the only region to border neither a body of water or a country. I recently visited the region and, as part of my reportage, will explore some of its landscape and soils characteristics in this post.

Source: olio2go.com
One hundred million years ago, much of Italy was an ocean floor. According to Menichetti and Coccione, the Umbria - Marche sedimentary basin formed in the late Triassic in a passive continental margin of the southern Tethys Ocean. In that basin, a 3000-m-thick stratigraphic succession "records the thermal and mechanical subsidence history from the Jurassic carbonate platforms to the pelagic realm of the Paleogene, while its upper part consists of Neogene terrigenous clastics that accumulated in a migratory foredeep system reflecting the encroachment of the Apennic deformation and sedimentation patterns into the Adriatic foreland" (Menichetti and Coccione).

Source: rocksinmyheadtoo.com
In describing the Apennines, a distinctive feature of the Italian Peninsula, and, as such, Umbria, Vezzani, et al., paint a picture of "lithotechnic assemblages that evolved through interaction between the African and European plates in the central Mediterranean with: (i) Mesozoic development of the Tethyan domain; (ii) Cretaceous-Eocene oceanic subduction; (iii) Oligocenic-Miocene and Pliocene convergence, continental collision and shortening; and (iv) late-Miocene - present extensional collapse of the contractional edifice." The Vezzani - Menichetti and Coccione arguments are tightly aligned.

According to Moti, geologic processes through the ages have resulted in the following soils distribution:
  • Alluvial sediments and debris along major river valleys
  • Gravels, sands, and clays deposited during the Pliocene and early Pleistocene
  • Marly deposits during the Oligocene - Miocene
  • Stratigraphic Umbria-Marche deposits from the Jurassic-Miocene
  • In the southwest, volcanic deposits from the eruptions of the Vulsino volcano.
This distribution is illustrated graphically in the figure below.
Schematic geologic map of Umbria.
Blue = Limestone; Gold = Sandstone rocks;
 Yellow = Inter-mountain basins; and
Purple = Volcanic complexes (Source:
Andrea Moti, researchgate.net)

Orvieto Landscape
Orvieto, one of the three major Umbrian wine regions, is centered around the town of Orvieto in southwest Umbria which, itself, sits on the northern edge of a broad volcanic plateau (alfina) which originated in the Quaternary period. Orvieto is shown in the map below.

Source: adventourus.com
The geological configuration of the Orvieto area is a direct result of neotectonic and volcanic events which took place in the Quaternary. The marine clays revealed by the departing sea experienced an extensional tectonic stage during the lower Pleistocene, resulting in a NW-SE fault. Magma flows and pyroclastics from the Vulsino volcano terminated against the raised block of this fault and backfilled to form the Alfena Plateau. Alfina Plateau formation dates from the middle Pleistocene.

Over time, the River Paglia and its tributaries cut the volcanic tuff of the Alfina Plateau into mesas (Bardano and Orvieto, for example) or buttes (Rocca Ripensa, for example). The distribution of soils in the Orvieto area is shown in the figure below. Note that the main difference between this distribution and the broader Umbrian distribution is the addition of a volcanic layer to the series.

1, talus (Oligocene); 2, recent and present alluvial sediments,
also terraced (Oligocene - Upper Pleistocene); 3, volcanic rocks
of the Alfina plateau (Middle Pleistocene); 4, gravels, sands and
clays (marine clastic sediments, Lower Pleistocene - Pliocene);
5, marls and sandstones (pre-Pliocenic bedrock); 6, River Paglia
and its main tributaries. Source: Moti.
The middle Pliocene clays form the base of the stratigraphic sequence in the Orvieto area and serves as reservoir for water flowing through from the uppermost layers. This is the old seabed present before the emergence of the Apennine range and these clays tend to be bluish in color -- tending towards grey -- and have high calcium carbonate content (marly clays). In some areas the CaCO₃ content can exceed 40% (argillaceous marls). These clays are also characterized by a good percentage of micaceous silt.

In mesas, there is a thin layer of volcanic origin from a fluvial-lacustrine environment that is called the Albornoz series. This soil type is probably incorporated into the topmost layers in non-mesa, non-butte structures.

The topmost stratigraphic layer in mesas, and a significant component of the recent alluvial sediments, is the effluvia of the Vulsino eruptions which occurred over a 300,000-year period. This lithoidal tuff with black scoriae includes "yellow-orange tuff of lithoidal texture with numerous inclusions of pumice and extremely friable rock of a grey color that incorrectly takes the name of pozzolano." Both the pozzolano and tuff have natural porosity.

Bibliography
Andrea Moti, An Example of Possible Application of Detailed Geological Maps. The 11 DOC/DOCG Destined to Wine Production in Umbria, researchgate.net
Corrado Cencetti, et al., The Rock of Orvieto (Umbria, Central Italy), researchgate.net
Mario Menichetti and Rodolfo Coccione, Umbria - Marche Apennine geological field trip, researchgate.net
Livio Vezzani, et al., Geology and Tectonic Evolution of the Central-Southern Apennines, Italy

©Wine -- Mise en abyme

No comments:

Post a Comment