Alsace: Landscape creation

At a recent tasting of Trimbach wines held at Orlando's Wine Bar George, Jean Trimbach noted that the Alsace region was geologically unique and home to 800 different soil compositions. Wines of Alsace supports this contention: "Experts say if you walk 100 feet in any direction, you'll find a different soil composition, making Alsace a complex mosaic unlike any other wine region." I explore the origin of these soils in this post.

I have elsewhere described the Variscan Orogeny (380 - 280 mya) -- the mountain building resulting from collision of Gondwana and Euramerica and the formation of the supercontinent Pangea -- and note that at its conclusion, the current Vosges Mountain (France) and Black Forest (Germany) was a single north-south mountain range.

Nothing is ever static in nature though. Over millions of years, the range was eroded. Further, warming temperatures resulted in sea level rise and the submersion of the mountain range. The following chart shows depositional activity on the range over the ages.

Sedimentary deposits on the north-south mountain
and the succeeding Rhine Graben, Vosges Mountain,
and Black Forest (Source: moore.no)

During the Triassic period, sandstone, limestone, and marl layers accumulated over the mountain range. Eroded sand from the continents were transported to, and deposited in, the shallow seas (Bunsandstein). The remains of shelled organisms compressed the sand below -- while creating a layer of their own (Muschelkalk) -- leading to the creation of sandstone. This shelled layer eventually became a limestone layer. The Keuper (marl) layer is the youngest of the three and was formed by 'clay-sized particles of clay minerals, limestone, and organic material."

Another 570 - 790 m of sedimentation were laid down during the Jurassic area. Deposits were marls and limestone containing gypsum, iron, and fluorite.

The third of the significant European-landscape-creating Orogenies was the Alpine Orogeny which occurred about 50 mya and resulted from the collision of the European and African plates. This collision uplifted the north-south mountain range, cracking the Jurassic rock and causing a collapse of its central portion along two fault lines. The Jurassic and Triassic strata overlying the uplift were shed, with the shoulders of the graben bared to the basement granites. These shoulders became the Vosges of Alsace and the Black Forest of Germany.

Uplift was less intense in the north and, as a result, some of its Triassic sandstone was retained. Based on this uplift dichotomy, the southern Vosges is referred to as the High or Crystalline Vosges while the northern portion is referred to as the low or Sandstone Vosges.

"... the western side of the graben was not one clean break but a series of somewhat sinuous faults stepping down to the central gutter. The westernmost fracture is a major break along the base of the mountain known as the Vosges Fault. A second major break, sometimes as much as 3 km east of the Vosges Fault, is known as the Rhine Fault" (Wilson). The step faults -- "fault bundles" -- show minor displacement but are cut into criss-cross patterns by cross faults. Differential erosions of these fault blocks have produced low hills  -- the sub-Vosges Hills -- each with its own erosion-determined dominant strata, upon which the bulk of the Alsace vineyards lie.

The extent and a cross-section of the Rhine Graben are shown in the two charts following.

Cross-Section of Rhine Graben (Source:vinsalsace.com)

During the Oligocene era, the Jurassic and Triassic rocks in the lower parts of the valley were converted to limestone-rich marly conglomerates. At this time, the rock composition of the structural units were as follows:

• Vosges Mountains -- Granite and sandstone; sometimes shales
• Sub-Vosges Hills -- Incredible soil diversity
• Rhine alluvial plain -- Marl and alluvium.

During the Quaternary period the Vosges was subjected to glaciation down to the 1000 m altitude with a resultant scraping of the Triassic layers and revelation of the underlying granite. Below 1000 m the granite remains covered by sandstone.

The valley floor has been filled with water on numerous occasions with the associated carriage and deposition of fertile soil material.

In this post I have identified the origin and interplay of rock types in the Alsace region. In an upcoming post I will describe how these rocks relate to the soil types in place today.

©Wine -- Mise en abyme