(In part adapted from Weissmuller1996, Unger 1984 and Binsteiner 1990b)
The sources of chert presented here under the heading of "Ortenburger Jura" represent three separate small regions of which only the sites of Maierhof, Dötter and Obermühle lie in the Ortenburger area sensu stricto. All three sources have in common that they all belong to the same Upper Jurassic strata, the "Ortenburger Kieselnierenkalke", and represent the last vestiges of Jurassic limestones in the southeasternmost part of Bavaria around the valley of the Danube.
Originally the whole region must have been covered with Jurassic limestones, not unlike the area around Regensburg with which it shares a common geological history. During the Lower Cretaceous however, the limestones between Regensburg and Passau were exposed and subjected to heavy erosion which removed the major part of the Jurassic cover. The Alpine orogenisis did the rest, pushing down the mesozoic rocks and covering them with huge amounts of Molasse. Only very small patches survived which are now lying in the large tertiary basin between the crystalline mountains of the Bayrischer Wald in the North and the alpine foreland in the South.
If you are trying to identify the last islands of Jurassic sediments on a small-scale geological map like the 1:500 000 Geological map of Bavaria, you'll need a magnifying glass and still have to know where to look. The largest ones are those in the West near Münster, just north of Straubing and the well exposed area near Flintsbach near Winzer. The Patches in the Ortenburg area are difficult to spot even on the large scale 1:50 000 geological map L 7544 Griesbach i. Rottal. They lie mostly in the valley of the small Wolfach stream towards the North from the town of Ortenburg and more to the East, just North of Fürstenzell, only 10 kilometres from the German/Austrian border.
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Even if the westernmost source Münster is separated from the sampling point at Obermühle by more than 70 kilometres, the types of chert are very similar as all come from the Upper Jurassic (Upper Oxfordian, Malm β according to the regional stratigrafy) limestones locally known as the "Ortenburger Schichten" or "Ortenburger Kieselnierenkalke". The last name translates as "Siliceous kidney chalks", indicating that the presence of chert nodules is an important feature in these strata. In contrast to the Regensburg area where a lot of the cherts are present als tabular "Plattenhornsteine", the flint in the Ortenburg Strata is exclusively of the nodular "Knollenhornstein" variety.
One of the most typical features of this hornstone are the numerous inclusions, mostly ghost of fossil fragments
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|Last modified on:
November 16, 2002
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