Marbre de Guillestre
The Marbre de Guillestre is an Upper Triassic formation in the French and Italian western Alps .
etymology
The Marbre de Guillestre, often also known in the plural as Marbres de Guillestre , is named after its type locality Guillestre ( Saint Crépin quarry ) in the Hautes-Alpes department . Strictly speaking, in the petrographical sense, this is not a real marble ( French marbre ), but a lump of lime without recrystallization .
introduction
The formation of the Marbre de Guillestre forms part of the sedimentary shell of the Briançonnais - a tectonic outer zone of the Central Penninic . During the development of the European continental margin and the associated formation of the future Tethyan Alpine Ocean , the sedimentation history of the Briançonnais up to the central Malm is characterized by a stable high altitude, whereas the zone of the continental Dauphinois already experienced strong subsidence movements with simultaneous crust thinning (by listrian blocks) . Only from the Upper Jurassic onwards did the Briançonnais submerge itself and deposits of 10 to 30 meters thick tuberous limestone - the Marbre de Guillestre - occurred.
distribution
In addition to the type locality, the Marbre de Guillestre can be found throughout Briançonnais with its various ceilings and partial ceilings. In addition, it is found widespread in the Upper Jura of the Alpine internal zone, especially in Italy , where it is known as Ammonitico Rosso (or Rosso Ammonitico ) because of its red color and its wealth of ammonites and is traded under the name Veronese marble , for example . In addition to the Ammonitico Rosso, equivalent formations in the Southern Alps are the Rosso ad Aptici . Comparable facies occur even in the Upper Eastern Alps , such as in the Ruhpolding marble or in general in formations such as the aptych lime or the aptych layers . Also worth mentioning are the Triassic Adnet marble and the Scaglia Rossa from the Lower Cretaceous of the Southern Alps. Very similar facies are not exclusively tied to the Mesozoic , but can also be found in the Paleozoic , for example the Upper Devonian cephalopod limestone of the Rhenish Slate Mountains or the Griotte in France.
stratigraphy
At the type locality, the Marbre de Guillestre is discordant on the gray dolomitic triad of the Norium . Elsewhere, such as on external solid Argentera - Mercantour may also point Dogger (Upper Bathonian to Callovian down access), which is then in turn unconformably Upper Triassic.
The Marbre de Guillestre is in turn covered by light gray to white Calpionella limestone of the Tithonium, which is only a few meters thick . After a renewed discordance, this is followed by very thick planktonic calcareous schists of the Upper Cretaceous (with Globotruncana ).
The dolomotic Triassic was deposited in shallow shallow water. This can be seen from the benthic and bacterial traces of life in the sediment. Smaller faults pervade the dolomites and point to their tectonically induced expansion. In Lias and Dogger there was then emersion of Briançonnais, which now loomed up as an island-like high zone. From the upper bathonium onwards, the geanticline of the Briançonnais was slowly flooded 165 million years ago and gradually turned into a continental slope on which the pelagic sediments of the Marbre de Guillestre were finally sedimented at towering high points.
The following Calpionella limestones and limestone slates are also deep-water sediments like the Marbre de Guillestre. However, the discordance of the limestone schist does not indicate, as in the previous case, that it has fallen dry in the meantime, but rather that it is a sedimentation gap ( hiatus ), probably caused by eroding deep currents.
Sedimentology
The Marbre de Guillestre, sometimes also Marbre rose de Guillestre , is a fine-grained, more or less clay-rich limestone that is colored red or reddish due to trivalent iron oxide (Fe 3+ ). The tones, colored dark red by hematite, indicate oxidising conditions. With prolonged weathering, the lime takes on gray tones. The rock, which is quite hard and poorly layered, has a bulbous structure. The centimeter-sized, lighter colored tubers are more calcareous than the surrounding matrix and often stretched in the layering plane. Often there are also hardened layer horizons covered by an iron-manganese skin. They represent typical condensation horizons (hard ground, English hardgrounds ), which were formed by long interruptions or low speeds in the sedimentation. These tuber limestone facies are likely to have originated at a relatively great depth (probably deeper than 200 meters). Nevertheless, the sediment was well aerated by constant water exchange (cool currents) and also has a very diversified marine fauna. Usually the sedimentation environment is interpreted as a classic, pelagic high area. Changes in depth and flow velocity could definitely cause a sedimentation stop or renewed sediment dissolution.
The Marbre de Guillestre is a fossil limestone that emerged from the accumulation of countless calcareous shells of marine organisms (such as slightly deformed and loosened ammonites and their aptyches , belemnites , gastropods , globigerines, etc.) on the sea floor. The resulting lime sludge then gradually solidified into solid limestone in the course of the diagenesis under the loading pressure. Since there are no benthic fossils , it can be concluded that there is a relatively deep educational environment.
The origin of the tubers has not been fully clarified. This is probably due to a redistribution or compression effect, which is likely to have been triggered by differential compaction, ocean currents, sediment burrows or seismically caused landslides. As the sedimentation continued, the tubers that formed were enveloped by the surrounding matrix. Biological sedimentary phenomena are also conceivable, which contributed to an increased carbonate concentration around organism remains or bacterial accumulations. It is possible that diagenesis also increased the separation and geochemical separation of clay and limestone layers, so that in the end there were almost pure lime nodules in the midst of pure clay layers. The fact that there was no continuous layered clay-lime separation is explained by the lack of lime content in the clay. Lime migrates from clayey to calcareous areas, which was demonstrated by Brenneke (1977) through the enrichment of light oxygen isotopes in limestone banks. Concretions are even more extreme in this regard than tuber limestone .
Fossils
The existing fossils are difficult to determine, but the taxa Sowerbyceras , Lytoceras and PerÎsphinctes could still be determined in the ammonite fauna . The taxon Duvalia can be recognized among the belemnites .
Age
The pelagic fossils contained in the Marbre de Guillestre point to an Upper Jurassic age, more precisely to the Oxfordian and Lower Kimmeridgian , ie to the period 160 to 150 million years before today.
use
Due to its mechanical properties, the Marbre de Guillestre found a rich architectural use in the wider area of its type locality from the Middle Ages at the latest , for example in the construction of the fortress town of Mont-Dauphin , on the cathedral of Gap or on Notre-Dame-du-Réal in Embrun . Its massive appearance enables it to be prepared in large blocks and cuboids, which can, for example, be used for monolithic monuments, pillars, fountains, benches, tables or even steps. Its surface can be processed in many ways and can be polished. On polished surfaces, bulbs and fossils form very sophisticated decorative elements. When polished, the Marbre de Guillestre is used for funerary monuments, furniture and art objects. The bulbous character of the rock, however, is a certain shortcoming, especially with sculptures.
See also
literature
- Jacques Debelmas: Alpes du Dauphiné . In: Guides Géologiques Régionaux . Masson, 1983, ISBN 2-225-78276-8 .
- Pierre Thomas: Le "marbre griotte" jurassique supérieur du Briançonnais, dit "marbre de Guillestre", et ses ammonites . Ed .: Olivier Dequincey. 2017.
Individual evidence
- ^ ME Tucker: Sedimentology of Palaeozoic pelagic limestones: the Devonian Griotte (Southern France) and Cephalopodenkalk (Germany) . In: Internat. Assoc. Sedimentol. Spec. Publ. Volume 1 , 1974, p. 71-92 .
- ↑ C. Sturani: La couverture sédimentaire de l'Argentera-Mercantour dans le secteur comprised entre les Barricades et Vinadio (haute vallée de la Stura di Demonte, Italie) . In: Travaux du laboratoire de Geologie de la Facultè des Sciences de Grenoble . 1963, p. 83-124 .
- ^ D. Bernoulli and HC Jenkyns: Alpine, Mediterranean, and Central Atlantic Mesozoic facies in relation to the early evolution of the Tethys . In: Soc. Econ. Paleont. Mineral. Spec. Publ . tape 19 , 1974, p. 129-160 .
- ↑ JC Brenneke: A comparison of the stable oxygen and carbon isotope composition of Early Cretaceous and Late Jurassic carbonates from DSDP sites 105 and 367 . In: Initial Reports DSDP . tape 41 , 1977, pp. 937-956 .
- ↑ Hans Füchtbauer: Sediments and sedimentary rocks . E. Schweizerbart'sche Verlag Buchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 .