Rocks and soils of Timorasso's Colli Tortonesi DOC

I have previously treated the geology of Colli Tortonesi DOC but, while the treatment was comprehensive, the final product was unconsolidated. I address that shortcoming in this post.

The Villalvernia-Varzi Line is a key feature of the regional geology. It is an east-striking, slightly dipping fault zone which separates the sedimentary structures of the Tertiary Piedmont Basin to its south of from the Epiligurian Unit sediments in the wedge-top basin to its north. The role that this line has played is evidenced in the two charts following which show the comparative parent-rock geology. The Line is represented by the Antognola Formation in both of the charts.

In the charts below, we see evidence of at least four tectonic events south of the line and no equivalent activity in the north.

The chart immediately following shows a lack of alignment in sediment deposition within the two basins except for Sant'Agata Marl in the middle of the Late Miocene and no evidence of deposition in the Epiligurian Units post that phase. It is unclear as to whether that lack of deposits post the Sant'Agata phase was due to erosion in the Epiligurian column or an early uplift.

Screen shot from Festa, et al., 
Geological Map of the Villalvernia-Varzi Line

As shown in the chart below, the parent rocks have eroded down to primarily limestone, clays and marls. 

Descriptions of these soil types can be found on my soils page but I have also used the wein+ glossary to construct the table shown below.

Table 1. Soil types described in Colli Tortonesi (Descriptions curated from Wein+)

Soil Type	Description
Limestone	Rocks with a dominant proportion of calcium carbonate and smaller proportions of magnesium carbonate Widespread asa light grey or yellowish mineral and sedimentary rock Formed as sedimentary rock in the sea by deposition of calcareous shells and skeletons of small marine animals and as lake sediment Iron poorly available to plants in calcareous soils Usually yield wines with good acidity; particularly well-suited to white wine varieties Appreciated in cooler wine growing areas but require lime-tolerant rootstocks
Chalk	White, soft, fine-grained limestone that breaks down into a white powder Often contains flint Good water drainage due to its great porosity Soil easily penetrable by vine roots
Clay	A mixture of sand, silt, and clay Depending on the ratio, distinction made between sandy, silty, or clayey loam Colored yellow=brown by iron compounds Usually already decalcified Produces full=bodied, powerful wines
Marl	Grey or yellowish sedimentary rock of about half clay and half limestone Formed where clay particles were deposited on the fossil seabed with simultaneous lime precipitation Fertile, heavy soils with high pH values
Sand	Weathering products of hard rocks Grain size between 63 micrometers and 2 millimeters High proportions of quartz and feldspar Water-permeable, dry, and often infertile Produces fragrant, low-acid wines
Lime Tuff	Deposition of lime with air inclusions Porous structure caused by swelling mosses on whose plant  bodies lime precipitates Mosses eventually calcify and die, while new mosses are already growing above them

©Wine -- Mise en abyme

The Geology of Timorasso's Colli Tortonesi DOC

The most recent activity in European mountain-formation began with the 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.

At the "conclusion" of the Alpine orogeny, the area that is now Colli Tortonesi did not exist in its current form; rather, it was the eastern portion of a basin that rested beneath a remnant of the Tethys Sea -- the Tertiary Piedmont Basin (TPB) -- separated from another wedge-top basin -- the Epiligurian Succession -- by a tectonic fault called the Villalvernia-Varzi (VV) Line. Colli Tortonesi is roughly positioned on the sketch map below. I discuss the basement, the VV fault, and the successions in the material following. 

The Colli Tortonesi Basement

In the Oligocene - Miocene, Ligurian Units, comprised of fragments of the Ligurian-Piedmony Ocean basin and the transition zone to the Adria continental margin, was carted onto the more external domains corresponding to the continental margin of Adria (Tuscan-Umbrian domain). The resulting structure is illustrated in the chart below.

The figure shows the Villavernia-Vernia Line, a prominent feature throughout the entire basement profile. As shown in the figure, VVL separates the TPB from the Epiligurian Units.

Villalvernia-Varzi Line

The characteristics of the line are illustrated in the chart below.

Tertiary Piedmont Basin

As shown in the chart above, the TPB is located to the south of the VV Line.

Beginning in the late Eocene, the Tertiary Piedmont Basin (TPB) developed within the Epimesoalpine  Basin and "sealed the meso-Alpine deformations caused by various tectonic units ranging from the Ligurian Alps (Voltri Group, Brianconnais) to the Northern Apennines (Ligurian Units)." The TPB is divided into three sectors -- western (Langhe). central, and eastern (Borbera-Grue) -- by the Celle-Sandia and Sestria-Voltaggio fault lines. The sedimentary succession, and underlying basement rocks, are present in all three sectors, as shown in A below.

A - Deposits in the basin (Source: The Tertiary Piedmont
Basin, Mufti, et al.); B - Age ranges in the Stages of the Miocene
 (Source: Wikipedia); C - Composition and depths of Oligocene
and Miocene deposits (Source: Mufti, et al)

The TPB sedimentary succession began in the mid-Oligocene and continued through the Miocene. A (above) shows the population of sedimentations by geographic era while C shows the types and extent of sedimentation, exclusive of the Messinian period. The deposits through the Tortonian are predominantly terrigenous in nature -- that is, originating from land -- and are primarily sandstones and mudstones. At its deepest points the succession records a thickness of 6000 m (Mutti, et al.).

The Borbera Valley succession, the area of import for our purposes, is characterized by the sedimentary succession beginning with the Val Borbera conglomerates -- a 2500-m-thick layer of continental-to-marine conglomerates from the Lower Oligocene. In the Costa Merlassino area, Sandstone of Ranzano and Marls of Monte Piano are interposed between the Val Borbera Conglomerates and the Antola Unit. The Middle-Borbera-Valley succession is illustrated in the chart below.

The eastern (Borbera-Grue) sector:

• Exposes the lowermost part of the sedimentary succession of the TPB (upper Eocene)
• Experienced, during the Oligocene and Miocene, a major phase of subsidence, leading to the deposition of relatively deep-water turbidites
• After the Burdigalian, and concurrently with a phase of dramatic inversion of the entire basin, experienced a progressive uplift expressed by a general shallowing upward of the depositional profile.

External Ligurian Units and Epiligurian Unit

Epiligurian Units are found in the wedge-top basin to the north of the VV Line and describes the deposits into that basin between the Middle Eocene and Middle Miocene. The succession is illustrated graphically below.

Quaternary Succession

Quaternary deposits are 2.5 million years old and younger. All of the Quaternary deposits found in the region are located along the valley floors, indicating river-driven migration.

Quaternary soils in Colli Tortonesi
(Underlying map from Vietti. Used with permission)

Conclusion

The chart immediately below shows the distribution of rocks in the areas to the north and south of the Villalvernia - Varzi Line, inclusive. The chart indicates four distinct tectonic periods on the TPB-side of the line and "relative" calm in the area above. 

This chart summarizes the successions in the VVZ area and provides detailed descriptions of each rock formation encountered.

Source: https://www.unito.it/sites/default/files/temi_geologo_ii_2015_allegato1.pdf

©Wine -- Mise en abyme