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Scharnhausen Volcano revisited

Dr. Günter Schweigert is curator of Jurassic and Cretaceous invertebrates and micropalaeontology. Furthermore he is responsible for the scientific excavations of the Nusplingen Plattenkalk.

“ScharnhausenVolcanoe” – this term sounds much more spectacular than it is in reality. No lava, no rising ash clouds but only a poorly exposed tuff diatreme at the slope of a creek valley.

This diatreme is the northern most one of the “Swabian Volcano”, which had perforated the Swabian Alb in more than 350 places approximately 14 million years ago, in the Miocene. The special importance of the “Scharnhausen Volcano”: since 125 years it is proved that the Swabian Alb extended to the area of Stuttgart in the active time of volcanism, over 20 kilometres more distant than today, because of Upper Jurassic limestone xenoliths found in the filling of the diatreme.

My personal interest in the “Swabian Volcano” arouse at the beginning of my geological studies at Stuttgart University (1983-90). I had to give a seminary talk – my first one in life. The topic of this talk specified by the lecturer was the Tertiary volcanism in southwestern Germany. Since that time this subject did not get out of my focus.

The slope of the Körsch valley north of Filderstadt-Scharnhausen with the volcanic tuff diatreme. (G. Schweigert)


1892: Seemingly established – the Swabian Alb extends to Stuttgart

Some time ago I was visited by Dr. Wolfgang Roser, a retired teacher, who intended to write a booklet dealing with the “Swabian Volcano” and who wanted to avoid any erroneous statements. He showed me some rock samples of the Scharnhausen Volcano to get my expert opinion. As said above, this occurrence in the Körsch valley is well-known, because Upper Jurassic limestones in the diatreme filling prove that in mid-Miocene times, 14 million years ago, the Swabian Alb extended to the area of Stuttgart. Every student of geology in this country (including myself) learns these facts in lectures or on field trips. Many textbooks on regional geology and popular publications by the “Schwäbischer Albverein” yield this information as well. The source for this information are studies of the geologist Wilhelm Branco from Tübingen University published in 1892. In the printed version of a lecture presented in honour of the 45th birthday of King Wilhelm II of Württemberg, Branco reported in great detail on the just previously discovered Scharnhausen Volcano.

2018: False facts, false conclusions

I started to become suspicious because there were no Upper Jurassic limestones in the rock sample of Wolfgang Roser, and moreover, I noticed several strange inconsistencies in Brancoʼs publication.

No Upper Jurassic limestones: the bright pieces of rock in the diatreme filling represent marly limestones of the Lower Jurassic. (G. Schweigert)

As a scientist this situation let me not sleep any more. First I visited the outcrop of the diatreme and afterwards I went to Tübingen University to study Brancoʼs original material in the petrographic collection of the Mineralogical Institute. Fortunately Brancoʼs material had mostly survived – and I surprisingly realized that the purported Upper Jurassic limestones in Brancoʼs material are in fact of Early Jurassic age! Obviously, generations of geologists and geographers had parroted one from the other, and all of them took Brancoʼs statements at face value! In fact Branco had made rather plausible observations, but finally he followed the wrong track. Those of you who want to get more detailed information may read my publication cited below.

This dark grey limestone with oxidized pyrite nodules and whitish belemnites was interpreted by Branco as fritted Upper Jurassic limestone. However, it comes definitely from the Lower Jurassic. The preparation artefacts were created by Branco himself, who extracted a fragmentary belemnite. All three pieces belong to the same rock specimen. (G. Schweigert)

Landscape history newly interpreted

The unmasking of the supposed Upper Jurassic Limestones of the Scharnhausen Volcano sheds new light on the question how long it takes for the head erosion of the northern escarpment of the Swabian Alb (“Albtrauf”). At the times of volcanismn this escapment must have been located much further southwards than previously expected, and in the area of the present Filder plane there was a hilly landscape formed by Middle Jurassic rocks.


References

Schweigert, G. (2018): Der Scharnhäuser Vulkan – eine Bestandsaufnahme 125 Jahre nach Brancos Beschreibung. Jahreshefte der Gesellschaft für Naturkunde in Württemberg, 174:191-207.

One thought on “Scharnhausen Volcano revisited

  1. What can be stated for sure is the fact that it is Ostfldern-Scharnhausen,
    not Filderstadt-Scharnhausen, since Filderstadt is a different municipality.

    Still thank you very much for your interesting publication.

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