Heilbronner Weinberge

To grow wine, you need, apart from a sunny south-facing slope and a favourable climate, one thing above all: good soil. Which soil this is in the Heilbronn area and how it was created is what this earthcache is about.

There is no container hidden for this cache. To log it, you need to send us the answers to the three questions in the text below. You may then directly log a find. We will get in touch with you in case the answers are not correct.
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Fig.1 – View from the Wartberg

Rock in the Heilbronn Vineyards

Heilbronn is part of the Central European Basin (formerly the Germanic Basin). Its formation is traced back to the expansion of the Earth's crust in the Permian 299 to 252 million years ago. It then expanded to the south in the Triassic 252 to 200 million years ago. Characteristic of the Germanic Trias in southern Germany are the deposits of Buntsandstein ("Bunter sandstone"), then Muschelkalk ("mussel chalk") and then Keuper. While Keuper and Buntsandstein are mainland formed, Muschelkalk is based on a marine formation. The Triassic rocks, especially those of the Keuper period, are the most important witnesses of geological history in Heilbronn, along with ice age formations. The rock layers of the Middle Keuper surround the city east of the Neckar from the Wartberg in the north over the city forest at the Jägerhaus to the Staufenberg in the south.
The layers of the Keuper-mountains around Heilbronn are 235 to 200 million years old and up to 150 metres thick. The weathered marlstone on the slopes of the Wartberg, which mainly belongs to the Grabfeld Formation, with its blue-grey, green and red tones is ideal for viticulture. The Keuper marl arises from deposits of the rocks and is composed of clay and lime. It quickly breaks down in the air to soil, is air and water permeable, has a high nutrient content and is quickly warmed up by the sun - ideal conditions for successful viticulture.
A layer of up to 45 metres thick Stuttgart Formation, the Heilbronn sandstone, can be found atop of the Grabfeld Formation. It is not suitable for viticulture, but has been intensively mined in some places (see GC2Q3KJ). Its stones were built in a culturally and historically significant way at the town hall, Kilian's Church and at the main train station in Heilbronn, in Bad Wimpfen at the Red Tower, and sculptures at Heidelberg Castle and ornaments at Cologne Cathedral were made from it. This sandstone was created in the Middle Keuper 232 to 209 million years ago.
The soil at the foot of the Keuper-mountains, partly weathered to loess and rubble, dates from the last ice age around 30,000 years ago (see GC20EYN). Like the Keuper marl, it is a weathered soil that represents a wonderful basis for viticulture and produces the best plants.

1) Question:
At waypoint 1, one has a good view on the Wartberg and the other vineyards (Fig.1). Why do the vineyards in many places not extend to the top of the mountains, but have a forest atop of them? Use the geology of the mountains to justify your answer.

Outcrop at the "Rebstöckle" Viewpoint

At waypoint 2 you stand under the “Rebstöckle” viewpoint and you can easily see the outcrop there from the path (Fig.2). In its rock layers it shows the transition from the older Grabfeld Formation seen below to the younger Stuttgart Formation above.

Fig.2 – The outcrop

Grabfeld Formation

At the very bottom are older gray layers of estheria (Benk Formation) made up of thin-plate, blue-grey clay stones that also appear yellowish to ocher-coloured due to weathering. The "cell marls" at the base of the outcrop are striking. These mudstone layers are criss-crossed with bands of calcite.
The Grabfeld Formation (formerly Gipskeuper) in the Heilbronn area consists of a sequence of colourful clay and dolomite stones up to 150 metres thick with layers of gypsum leaching residues. Individual benches, such as the galena bench, malachite bench or anatine bench, are so striking due to their colour, fossil record and morphological characteristics that they are used as marker horizons to subdivide the monotonous series of rocks (Fig.3).
When the rocks of the Grabfeld Formation were deposited, the conditions were very changeable. In the period of the Grabfeld Formation, partly oversalted, shallow sea areas were transformed into dry areas that were only periodically flooded. The rock banks containing fossils always reveal the relatively short-term flooding with fresh sea water. You can find remains of mussels, small crustaceans, fish and occasional marine dinosaurs. Otherwise, however, the period of the Grabfeld Formation was a section of earth's history that was hostile to life.

Fig.3 – Marker horizon

2) Question:
In the lower area of the outcrop, in addition to the grey colour, there are also special structures that are somewhat reminiscent of a net. What are they? How many millimetres thick are the structures and at what distance to each other do they run?

Stuttgart Formation

The Stuttgart Formation (formerly Schilfsandstein) in the uppermost part of the wall clearly stands out in colour and bank from the finely layered clay stones of the Grabfeld Formation below. In the Heilbronn area, the Stuttgart Formation is a predominantly fine-grained, greenish-yellow to rust-brown coloured rock. Quartz, feldspar, mica and heavy minerals are the most important ingredients.
The Stuttgart Formation is mostly viewed as a river deposit. 225 million years ago, at the time of the Middle Keuper, there was a large mountainous mainland area in northern and northeastern Europe, the continent of Fennosarmatia. The rivers transported enormous amounts of weathering debris to the southwest and thus also into the Heilbronn area. Today's compact Stuttgart Formation strands in northeast-southwest direction are remnants of the sands deposited in the river arms. The clayey to sandy-clayey layers were deposited in the swampy still water lagoons between the river arms or with lower water currents.
Older layers were often washed out by the erosive power of the flowing water. As a result, strands of sandstone are now partially cut deep into the geologically older Grabfeld Formation below. Inclined, cross and flow ripple stratification prove that the Stuttgart Formation was deposited in running water. In the Stuttgart Formation you can find shells, skeletal parts of reptiles, such as labyrinthodont, and, above all, vegetable fossils. Remains of the up to six meters high tree-strong horsetail (Equisetites), the cycads and conifers, which were widespread at the time, are remarkable. This flora thrived near a shallow body of water or in the warm and humid lowlands of the mainland. The former name of the Stuttgart Formation was Schilfsandstein ("reed sandstone"). It can be traced back to the fossil horsetail, which used to be mistakenly interpreted as reed stalks.

3) Question:
If you look at the outcrop from bottom to top and ignore the few narrow banks, the colour initially changes from blue-grey to yellowish-ocher, then the structure changes from coarse to fine. But where exactly is the transition from the Grabfeld Formation to the Stuttgart Formation? At the change of colour or the change of structure?

References

[1] W. Ricken, T. Aigner und B. Jacobsen, "Levee-crevasse deposits from the German Schilfsandstein", Neues Jahrbuch für Geologie und Paläontologie 1998(2)77–94, DOI: 10.1127/njgpm/1998/1998/77
[2] Verkehrsverein Heilbronn e.V., http://www.weinpanorama.hn, 22. Apr 2021
[3] T. Aigner und G.H. Bachmann, "Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin", Sedimentary Geology 62(1)5–25, DOI: 10.1016/0037-0738(89)90098-5
[4] Landesamt für Geologie, Rohstoffe und Bergbau Freiburg, https://lgrbwissen.lgrb-bw.de/geotourismus/lehrpfade/geologische-lehrpfade-im-eigentlichen-sinne/keuperbergland/keuperweg-heilbronn, 22. Apr 2021