Border dolomite

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The name Grenzdolomit is on the one hand the name of a layer in the geological formation of the Triassic , on the other hand also a layer of the formation of the Permian . The name correspondence is obviously based on the lithological nature and seems to be local-historical.

Frontier dolomite (Triassic formation)

General geological classification and distribution

A carbonate sediment layer of fluctuating thickness (according to G. Richter in Thuringia 1 m to 5 m) within the lithostratigraphic unit of the Lower Keuper (synonymous: Unterkeuper , Lettenkohlenkeuper, Lettenkeuper, Lettenkohle) of the Germanic Triassic is called a border dolomite . In some cases, the sequence of layers was also called the border dolomite region, in which the border dolomite i. e. S. was endured. So z. B. 1894 in Franconia by KW Gümbel and similarly by H. Stille.

According to E. Kayser, the border dolomite is widespread throughout Germany with the exception of a peripheral area near Hanover. In a comprehensive lithostratigraphic analysis of the Keuper of the Central European Basin, M. Franz has compiled all recent geological data on the formation and genesis of the border dolomite in its northern distribution areas. This also includes a relevant overview map.

Both in the North German Basin, where the Lower Keuper occurs in its basin facies as an Erfurt Formation , as well as in southern Germany or other basin edge areas, the layer of the border dolomite forms a benchmark and marks the border to the overlying Middle Keuper.

Outcrops in the area

Examples of natural and artificial outcrops of the border dolomite in central and southern Germany:

  • Border dolomite cliffs in the Apfelstädt River near Wandersleben / Thuringia

  • Quarry near Vellberg- Eschenau: transition from Unter- / Mittelkeuper. The Unterkeuper ends with the yellow border dolomite (a little above the foot of the embankment) of the Erfurt formation , above it Mittelkeuper with plaster of paris from the grave field formation

Formation conditions and lithological nature

The facies of the time of the Lower Keuper in the Germanic Basin were predominantly terrestrial and brackish - estuarine (lettuce layers with coal seams), which was occasionally interrupted by marine ingresses . The strongest ingression led to the deposition of the border dolomite. The evidence for the marine character of the frontier dolomite genesis results from fossil finds in the frontier dolomite typical of the facies. In the border dolomite of Thuringia, specimens of the cephalopod class (cephalopods) Thuringionautilus (or Germanonautilus) jugatonodosus (ZIMMERM.) And Alloceratides schmidi (ZIMMERM.) Were found. The collections of the Natural History Museum Schloss Bertholdsburg Schleusingen have such a rare fossil find. This Germanonautilus stone core (leg. Gustav Compter ) from the border dolomite of the Erfurt formation near Apolda is of great national importance.

Border dolomite with myophoria (Costatoria) goldfussi in the Erfurt formation in Hohenlohe

The border dolomite is described petrographically by E. Kayser in 1922 as "sometimes dense, sometimes holey, sometimes oolithic, yellowish dolomitic limestone with the Myophoria Goldfussi mussel as an important key form". In a breakdown of the Lower Keuper of Thuringia, G. Richter describes the border dolomite as "yellow-gray to yellow dolomite, partly banky, partly flat, locally oolothic or sulphate ... alternating with gray carbonatic claystone layers". According to J. Dockter, 1974, the border dolomite in Thuringia consists of alternating layers of gray and yellow carbonate clay stones and dolomites. Carbonate banks reach a thickness of 50 cm and more. In this alternating position, calcium sulphate intercalations occur locally ; glauconite and pyrite are often found in the Dolomites . The Grenzdolomit almost always leads with two-shell schill , in places also with fish scales. The mussel Myophoria Costatoria goldfussi (v. ALB.), Which can appear in layers as rocks, is characteristic.

The cut stone border dolomite - technical rock characteristics

The border dolomite was already used as a local building block in the Middle Ages when there were favorable extraction options. One example is one of the most important fortified churches in Lower Franconia from the 12th century: Fortified church in Kleinlangheim near Würzburg. The locally extracted, hard, yellow border dolomite was built into the defensive walls.

In the Romanesque masonry of Gleichen Castle in Thuringia (Drei Gleichen area near Gotha / Erfurt), border dolomite was built into the curtain wall alongside other types of stone. The historical extraction sites were quarries in the nearby Apfelstädtaue and on the Stiedenberg east of Wandersleben.

On the historically built border dolomite in the Gleichen castle complex, knowledge of the rock microstructure of the border dolomite was gained during the monument conservation work using modern petrographic sampling (light microscopy / scanning electron microscopy) (see the pore space representations in the report by H. Stück et al.).

According to K. Sterzl, historically built stone from the border dolomite in buildings in the Weimar area can always be recognized by their brown color. Darker brown appears on the side of the stone facing away from the weather, and ocher on the weather side (corroded surface). For border dolomites from the Erfurt / Apolda area, relevant rock-technical parameters are shown in the following table in comparison with other selected, local ashlar.

Rock technical parameters
Ashlar origin Bulk density [g / cm 3 ] Water absorption [mass%] Cube compressive strength [Mpa]
Chirotheriensandstein ( Upper Buntsandstein ) Bad Berka 2.08 7.7 25th
Grenzdolomit (Lower Keuper) Erfurt, Apolda to 2.36 to 12.5 10 to 40
Cycloidalkalk ( Upper Muschelkalk ) Weimar 2.87 0.51 90
Travertine (quaternary) Weimar 2.3 2.0 to 2.9 35 to 95
Thür. Bausandstein (middle red sandstone) Tonndorf 2.02 8.44 25th

Border dolomite (permformation)

Zechstein transgression in the edge area of ​​the Zechstein basin: carbonates ( mother seam , Zechstein limestone), here dark in color and containing a lot of C org , including copper slate (weathered brownish), are discordant on steeply sloping greywacke and clay slates of the Lower Carboniferous (large open-cast mine Kamsdorf near Saalfeld, Thuringia)

Within the lithological unit of the Zechstein , the carbonate mother seam in the layer of copper shale (Permian) during its formation in the threshold facies is sometimes referred to as border dolomite (synonymous also border limestone or productus limestone) .

Individual evidence

  1. ↑ a b G. Richter: In: W. Hoppe, G. Seidel: Geologie von Thüringen. 1974, p. 635.
  2. ^ KW Gümbel: Geology of Bavaria. 1894, p. 719.
  3. ^ H. Stille: On the stratigraphy of the German Lettenkohlenkeupergruppe. Jahrb. Preuss. Geol. Landesanstalt, 1908, I, p. 145.
  4. ↑ a b E. Kayser: Textbook of Geology. Volume 3 Geological Formation Science, 1922, p. 468.
  5. ↑ Matthias Franz: Lithographic and guide surface stratigraphy, chronostratigraphy, cyclo- and sequence stratigraphy of the Keuper in the eastern Central European basin (Germany, Poland) and the Danish basin (Denmark, Sweden). Dissertation, Martin Luther University Halle-Wittenberg, 2008 ( abstract & full text ).
  6. ↑ Matthias Franz: Lithographic and guide surface stratigraphy, chronostratigraphy, cyclo- and sequence stratigraphy of the Keuper in the eastern Central European basin (Germany, Poland) and the Danish basin (Denmark, Sweden). Dissertation, Martin Luther University Halle-Wittenberg, 2008, Fig. 5.1.1-3, p. 571 (PDF)
  7. ↑ K.-A. Tröger: Outline of the historical geology. 1980, p. 332.
  8. ↑ J. Dockter: In: W. Hoppe, G. Seidel: Geology of Thuringia. 1974, p. 672.
  9. ^ Germanonautilus - Museum Schloss Bertholdsburg Schleusingen
  10. ↑ J. Dockter: In: W. Hoppe, G. Seidel: Geology of Thuringia. 1974, p. 639.
  11. ^ Website Kleinlangheim
  12. ↑ Heidrun piece, Siegfried Siegesmund: Natural stone of the three equals - petrography and damage phenomena . In: The man-made, environmentally damaged castles 'Drei Gleichen' (Thuringia) as objects of renovation in cooperation with nature conservation and monument preservation . DBU funding project 26625-45, p. 23–39 ( PDF [accessed April 17, 2014]).
  13. ↑ Heidrun piece, Siegfried Siegesmund: Natural stone of the three equals - petrography and damage phenomena . In: The man-made, environmentally damaged castles 'Drei Gleichen' (Thuringia) as objects of renovation in cooperation with nature conservation and monument preservation . DBU funding project 26625-45, p. 23–39, here p. 27 ( PDF [accessed April 17, 2014]).
  14. ↑ a b K. Sterzl: Natural stones in Weimar - occurrence, weathering u. Use. Thesis . University of Bamberg, 2004, pp. 5 and 8.