-
la diagénèse, qui modifie le sédiment pour le transformer en roche sédimentaire (ici une roche calcaire).
Le vecteur principal d'origine des sédiments étant ici l'eau, la nature des débris à l'origine de la roche dépend fortement de la vitesse du courant, comme le montre le diagramme de Hjuström :
► The geological series of Cularo
Grenoble (from its former Gallic name Cularo) is a metropolis located at the gates of the Alps, between the massifs of the Vercors, the Chartreuse, the Taillefer and the chain of Belledonne.
Ancient Roman city, then capital of the Dauphiné, Grenoble very soon used for its monuments, its houses, dikes, ramparts or bridges the local materials closest.
This geological richness is evident when you walk around the city. It is enough to stroll through the streets and gardens to see in a simple and pleasant way various natures of rocks, geological or hydrological phenomena.
This series entitled Géol'Cularo will make it possible to discover some of these geological secrets. The stopping points of this walk are largely inspired by the geological circuit proposed by the site "Circuit géologique dans les rues de Grenoble: de la Porte de France au Jardin des Plantes".
► The observation site: The Crypt of St. Lawrence
The old church of Saint Laurent, the only district on the right bank of the Isère in Grenoble, is itself built on an oratory dedicated to Saint Oyand.
This oratory dates from the 6th century and became underground as a result of new edifications and was known under the improper name of St. Lawrence Crypt by extension of the patronage of the superior church.
This crypt has a colonnade set up to support it, and among this colonnade (20 columns), 6 are extracted from a particular rock, the marble of Vimines, a conglomerate of pudding type with pebbles aureoled with red.
► Geographical location
The marble colonnades of Vimines come from quarries in the hamlet of Terre Rouge, formerly exploited near Chambéry.
The BRGM map gives the following indications: g2B. - breaches and pudding, often with red cement ("marble de Vimines").
These oligocene deposits are in the form of lenticular outcrops (several tens of meters thick), very discontinuous.
► Conglomerates
In general, the conglomerates are clastic sedimentary rocks formed by a minimum of 50% rock debris of larger than 2 mm (class rudites) and bound by a cement.
This is this large size of debris which differentiates conglomerate compared to limestone, which it consists of sediment with a size up to 2mm.
There are two types of conglomerates :
- The breaches, when the majority of elements are angular.
- The pudding, if the elements are rounded, rolled (= pebble).
► The Vimines conglomerate
More specifically, the conglomerate of Vimines (or Vimines marble) is composed of fine pebbles trapped in a dark pink gangue (calcareous concretions).
Flowing and polished, it takes on the appearance of a pink marble.
This conglomerate formed during the erosion phases succeeding the Alpine orogeny. These figured elements are globally rounded indicating a fluvial transport, mainly of torrential regime. Some are nonetheless relatively angular with low transport.
The aureole and the rather red ocher cement testifies to their richness in iron oxide.
This aureole was established following the action of algae, in a process analogous to the formation of stromatolites.
This action of photosynthesis of the algae consumes the carbon dioxide present in the water and displaces the equilibrium of the carbonates towards the precipitation: calcium carbonate rich in iron oxides also dissolved in the water giving rise to these Limestone around the pebbles.
► The principles of sedimentation applied to conglomerates
The sedimentary cycle that led to the sandstone conglomerate includes four phases :
- Weathering / erosion of a rock, here the limestone enhanced by the alpine orogeny.
- Transportation by a natural element (water, wind or ice). In our case, the shape and size of the rollers indicates a transport by water.
- Sedimentation or deposit.
- Diagenesis, which modifies the sediment and turn it into sedimentary rock (sandstone rock here).
The main vector leading to sedimentation being water, the nature of the debris forming the rock depends strongly on the flow velocity, as shown in the diagram Hjuström :
► Sources bibliographiques / Bibliographical sources
Les Questions / The Questions
!!! ATTENTION !!! Cette EarthCache est soumise à horaires (photo Horaires). Entrer dans le musée archéologique de Grenoble-Saint Laurent (Entrée gratuite, accès au TrealHead 1) et descendre dans la crypte en suivant la signalétique.
!!! WARNING !!! This EarthCache is subject to schedule (see photo Horaires). Enter in the Archaeological Museum of Grenoble-Saint Laurent ( Free entrance , access to TrealHead 1) and go down into the crypt following the signs.
La lecture attentive du descriptif de la cache, ainsi qu'une observation des éléments de terrain et un peu de déduction sont normalement suffisants pour répondre aux questions de cette EarthCache.
A careful reading of the description of the cache, as well as observation of terrain features and some deduction is usually sufficient to answer questions of this EarthCache.
Questions pour valider :"Les conglomérats de la Crypte"
Questions to validate: "Conglomerates of the Crypt"
Vous voici dans la crypte, nous allons étudier une des colonnes en particulier (photo WP1_large)
Here you are in the crypt, we will study one of the columns in particular (photo WP1_large)
- Question 1 : Décrire la roche de la colonne sous la zone rouge (taille et forme des éléments qui la composent, couleur des éléments et du ciment). De quel type de conglomérat s'agit-il ? Justifier votre réponse.
- Question 1 : Describe the rock of the column under the red zone (size and shape of the elements that compose it, color of elements and cement). What type of conglomerate is it? Justify your answer.
- Question 2 : Quel est la longueur du bloc de la photo WP1_zoom. Décrivez le (dureté, couleur, granularité). Pensez-vous qu'il s'agisse de calcaire ? Justifier votre réponse
- Question 2 : What is the length of the block of the photo WP1_zoom. Describe the (hardness, color, granularity). Do you think it's limestone? Justify your answer
- Question 3 : Quelle est la taille des plus gros galets ; déduisez-en la vitesse du courant ayant déposer ses éléments. Comment pouvez-vous qualifier le débit de ce cours d'eau ?
- Question 3 : What is the size of the largest pebbles; Deduce the speed of the current having deposited its elements. How do you qualify the flow of this stream?
Vous pouvez vous loguer sans attendre notre confirmation,
mais vous devez nous envoyer les réponses en même temps soit par mail via notre profil (
fafahakkai), soit via la messagerie geocaching.com (Message Center).
S'il y a des problèmes avec vos réponses nous vous en ferons part.
Les logs enregistrés sans réponses seront supprimés.Une photo de vous ou de votre GPS à l'entrée du musée archéologique sera la bienvenue, mais n'est pas obligatoire.
You can log this cache without waiting for our confirmation, but you must send us the answers at the same time, by e-mail via our profile (fafahakkai) or by the system of Message Center of geocaching.com.
If there is a problem with your answers we will notify you. The logs recorded without answers will be deleted.
A photo of you or your GPS at the entrance of the archaeological museum will be welcome, but not mandatory.
Rappel concernant les « Earthcaches »: Il n'y a pas de conteneur à rechercher ni de logbook à renseigner. Il suffit de se rendre sur les lieux, de répondre aux questions ci-dessus et de nous renvoyer les réponses.
Reminder concerning "Earthcaches": there is neither a container to look for nor a logbook to sign. One need only go to the location, answer to the differents questions and send us the answers.