Sunday, April 28, 2013

Contextualizing Kimmeridgian soils

Writing on winegeeks.com (The World's Top 10 Wine Soils), Sunny Brown undertook the description of the top 10 wine soils ranked based on "... the unique qualities, historical importance and market appeal of the wines." The top rated soil was "A basin of limestone marl that starts in England and runs all the way down through the vineyards of Champagne, the Loire Valley and ultimately Burgundy. It is on this ridge, known as the Kimmeridgian, that the best vineyards lie." While the soils underlying Burgundy, Champagne, and the Loire may be the best in the world -- and while Kimmeridgian soil can be found in each of these regions -- Kimmeridgian soil is given broader scope in the Brown post than is the case. I present my arguments below.

At varying periods of the earth's history, portions of present-day Europe were covered by shallow seas. Such was the condition during the Jurassic period in the time span we now call the Kimmeridgian Age, identified by ammonite and oyster fossils found in associated strata.

Modification of IUGS Chronostratigraphic
Chart (stratigraphy.org)
Ammonite -- Kimmeridgian marker
Exogyra virgula -- Kimmeridgian
marker (Source: zeably.com)

According to James Wilson (Terroir: The Role of Geology, Climate and Culture in the Making of French Wines, University of California, 1998), French geologist Alcide d'Obigny, while working in the south of England in the middle of the 18th century, named the Jurassic limestone in Dorset as Portlandian. In his work east of that site, in the vicinity of the village of Kimmeridge, he encountered a dark marl in the layer below the Portlandian and named it Kimmeridgian. The Kimmeridgian soil found in France differs from that found in Britain in that it is a "relatively uniform chalky marl and thin marly limestone containing many lenses or banks of seashells" (Wilson).

Strata from periods post the Jurassic continued to be deposited into the shallow seas and many of these layers were forced to the surface when the area that is known today as the Paris Basin began a slow sag during the late Tertiary and Quaternary periods. This slow tilting of the Basin allowed the Seine, Aube, Yonne, and Loire rivers to "downcut through the rising ridges, thus cutting the Kimmeridgian-Portlandian outcrop band into an archipelago of wine areas" (Wilson).


Source: timkeen.com

Now that we know the origin of the Kimmeridgian, lets examine its extent. In the 10-best-soils article, Brown indicated that the vineyards of Champagne, Burgundy, and the Loire Valley reside upon this hallowed soil. But the images above would seem to indicate otherwise. The figures show that upper Jurassic deposits (which would include Portlandian and Kimmeridigian deposits) came to the surface in a band that falls below the Champagne  and Loire bands and above the Burgundy band.

And Wilson supports this observation. He, in fact, refers to the Kimmeridgian outcrops as the Kimmeridgian Chain in that they are distinct and separate from their associated wine regions. The primary Kimmeridgean vineyard sites in France are: (i) the Aube sub-region of Champagne; (ii) the Chablis, Tonnerre, and Auxerrois areas of Burgundy; and (iii) the Pouilly, Sancerre, and Menetou-Salon areas of the Loire Valley.


Wilson sees the key to Kimmeridgian soil as the way it works with its Portlandian partner. The marly soil of the Kimmeridgian develop good structure and water-retention characteristics and is easy to cultivate. The hard limestone of the Portlandian contains many fossils and fragments and is also cracked by frost. This enables aeration of the slopes as well as aiding in drainage.

Chablis is the "big island" in the Kimmeridgian chain and is home to some of the finest Chardonnay known. The defined region was recognized in 1923 by the Wine Tribunals as being grown on a sub-soil of Kimmeridgian limestone while wine grown anywhere else in Chablis would be classed Petit Chablis. The mid-slope in Chablis maps almost perfectly to the Kimmeridgian outcrop with the soft, carbonate-rich mud rock being capped by Portlandian Barrios limestone and supported by Calcares à Astarte, itself a limestone (Jennifer Huggett, Geology and Wine: A review, Proceedings of the Geologists Association, 2006). This south-facing Kimmeridgian slope has significant sun exposure and is home to the Chablis Grand Cru vineyards.

Source: justburgundy.com

Wilson notes that geologic conditions identical to those experienced by the Grand Cru slope extend both northeast and southwest but that the vineyards on those sites are classed as Premiers Crus. That is an indication that Kimmeridgian soil is not the key ingredient in the making of a Grand Cru Chablis. As a matter of fact, the reference to Kimmeridgian limestone in the definition of Chablis was discontinued in 1976, a tacit admission, according to Huggett, that "slope and orientation are of greater importance to wine quality in Chablis."

In the vicinity of Sancerre and Pouilly-sur-Loire (Central Vineyards sub-region of the Loire Valley), faulting has caused the east bank of the river (Pouilly side) to be lowered, thus causing the typical Kimmeridgian slope to lie flat and the retention of elements of the Tertiary (sands, clays, and freshwater limestones) and Quaternary (high-river-terrace sands, gravel, and clay) period. The town of Sancerre sits atop a fault ridge the eastern side of which is Kimmeridgian topped by Cretaceous soils while the west side is brush-covered gravel slopes. Further west, the best vineyards sit on the classic Portlandian-Kimmeridgian combination. The Portlandian here is thinner but, according to Wilson, is reinforced by ferruginous sandstones of the basal Cretaceous and weathering of the sandstone infuses the rock with iron. Wilson talks about three "lithologic Kimmeridgian zones" in Sancerre:
  1. St. Doulchard marl -- this soil weathers into "terres blanches" (white earth). Also contains glauconite, a granular mica that is rich in iron
  2. Calcaire à Astartes -- lies below the St. Dulchard and identified by the presence of the clam Astartes
  3. Calcaire de Tonnerre -- soft and porous limestone in its Sancerre incarnation. Hard around its home base of Tonnerre.
The Calcaires weather in combination to form a pebbly soil called caillottes.

Quincy and Reuilly are not considered part of the Kimmeridgian chain as they have been recipients of the Tertiary outwash from the Massif Central, a result of the breakdown of the "Portlandian barrier." The Kimmeridgian strata disappears under the Lower Cretaceous west of Reuilly.

To summarize then, Kimmeridgian soil is the result of the Paris-Basin-sag-related upthrusting of sedimentary layers that were laid down during the upper Jurassic age. Kimmeridgian formations usually exhibit as slopes capped by Portlandian limestone and generally exist as islands remote from their associated wine regions; regions which have soil profiles that are dependent on the era and types of their formations.


©Wine -- Mise en abyme

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