Mullion

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Mullion rocks on the L 106 at the southwest exit of Dedenborn. View of the underside of a Grauwackenbank dismantled into mullions.

In geology, a mullion or mullion structure is a regularly divided, hard rock bank that has convex bulges on its top and bottom. This gives it the appearance of a bundle of Gothic pillars, from which the name is derived. Mullions occur in sedimentary rock sequences with a marked contrast between hard and soft layers. The term was introduced in 1891 by the British geologist Joseph Nolan.

morphology

Mullions in a former quarry near Rouette in the Ardennes .

A single mullion body has an approximately rectangular cross-section, the top and bottom (based on the layering ) show the shape of a segment of a circle. The height / width ratio of a mullion cross-section is always greater than 1; values ​​between 1.5 and 2.5 were measured most frequently. The sides are usually slightly concave. Between the individual mullion bodies there are thin spindles made of light-colored quartz . Rock banks that have been broken up in mullions are always embedded in softer clay slate .

In rare cases, with graded stratification, half-mullions have been observed that are convex only on the underside of the layer.

distribution

In Germany, mullions are widespread in the Eifel between Einruhr and Monschau , the best known is the natural monument at the southwestern exit of Dedenborn . The Mullion deposits continue on Belgian territory as far as Bastogne . Mullion occurrences outside the Rhenish Slate Mountains have been described from the Neoproterozoic Moine supergroup in Scotland .

Emergence

The well-studied deposits in the Rhenish Slate Mountains originated in sandstones and greywacke , which are located in the lower part of an 8 to 10 km thick sequence of sediments from the Devonian . After the solidification of the rocks, at the end of the Upper Devonian, either due to the expansion of the earth's crust or due to overpressure of the fluids trapped in the sandstones , the banks disintegrated and the vertical quartz veins were separated from the circulating pore waters. When the layers were compressed during the Variscan orogeny in the Upper Carboniferous, the vertical quartz veins initially acted as mechanical abutments, creating the characteristic convex bulges of the sandstone and greywacken banks. Only in the later course of the mountain formation were the sediment layers folded. Between the formation of the mullions and the application of the folds there was a slight change in the main stress direction, which means that the longitudinal axes of the mullions are always at an angle of about 10 ° to a maximum of 40 ° to the direction of the fold axes .

literature

Web links

Individual evidence

  1. ^ Hans Murawski: Dictionary of Tectonics. 8th edition, p. 150, Enke, Stuttgart 1983.
  2. ^ G. Wilson: Mullion and rodding structures in the Moine Series of Scotland. Proceedings Geol. Assoc., Vol. 64, pp. 118-151. London 1953.