Le substrat est constitué
de schistes briovériens affectés par
le métamorphisme à l’approche du massif
granitique de Locronan (développement d’une
foliation parallèle au litage).
En
pied de falaise, un platier ancien est fossilisé par un
poudingue ferrugineux englobant
des galets de lithologie variée, montrant quelques
éléments de taille importante,
comme les quartz, issus vraisemblablement de lentilles ou de veines de
ségrégation
de quartz et de filons présents dans les formations
briovériennes. Cette formation
correspondant à un milieu à forte
énergie est coiffée par une formation
gréseuse
admettant des galets bien émoussés, puis par des
grès roux correspondant à des sables
dont la granulométrie est voisine de celle des sables
actuels de la plage de Trezmalaouen.
Les
spécialistes qui se sont succédé sur
le site ont observé que la dalle ferrugineuse
présentait un réseau de fissures avec des
décalages s’apparentant à la
réactivation
de fractures du substratum schisteux. Des réajustements
tectoniques sont en effet
possibles en raison de la proximité de l’accident
Kerforne, qui débouche dans l’anse
du Ry, à proximité.
• Aplite
Une aplite est une roche filonienne leucocrate (riches en
minéraux dits blancs) de granulométrie
très fine. Les aplites sont associées, soit
à des roches magmatiques, soit à des roches
métasomatiques.
Les aplites sont caractérisées par la petite
taille de leurs cristaux (moins de 1 mm), avec une teneur
très basse en minéraux mafiques
(ferromagnésiens).
Les aplites sont constituées essentiellement de quartz, de
feldspath alcalin (orthoclase ou microperthite) et de plagioclase.
.
Questions
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.
Voir le chapitre "une
earthcache" pour les conditions de log.
Question
0 - Prenez une photo indivuelle de vous (pas de
photo collective), ou de votre objet
distinctif de géocacheur, ou de votre pseudo
écrit sur une feuille de papier ou dans votre
main... sur
la plage
et joignez-là à votre log ou à vos
réponses
Question
1
- Pour chacune des roches A B C sur la photo, décrivez-les
puis nommez-les.
Question
2- En fonction de leur
nature et des éléments de leur formation,
classez-le par
ordre chronologique d'apparition géologique, puis expliquez
ce
qu'il s'est passé ici.
An
Earthcache
This is not a
physical cache.
To log this cache, you must first
read its educational description of geology, then observe the site you
are on, and finally answer
the questions that will be asked. A higher earthcache difficulty rating
will require more advanced answers so your log can be validated.
You will then be able to log in "Found it" without waiting but you must
send me your
answers at the same time by contacting me either by email
in my profile, or via geocaching.com messaging (Message Center), and I
will contact you in case problem. Collective answers are accepted
provided that the photos remain individual and that the other members
of the team
are mentioned in the log.
'Found it' logs recorded without
answers or without
a photo or that are too rudimentary will be deleted; however, you can
log them in a 'write note' while you complete all of this later.
Geology
of the site
The substrate is made up of Brioverian schists
affected by metamorphism
approaching the granite massif of Locronan (development of foliation
parallel to the bedding).
At the foot of the cliff, an ancient reef flat (wave-cut plateform) is
fossilized by a
ferruginous pudding encompassing pebbles of varied lithology, showing
some large elements, such as quartz, probably coming from lenses or
veins of quartz segregation and veins present in the formations.
Brioverian. This formation corresponding to a high energy environment
is capped by a sandstone formation with very blunt pebbles, then by red
sandstones corresponding to sands whose grain size is close to that of
the current sands of the Trezmalaouen beach.
These formations are also present at the bottom of cavities and caves
indenting the rocky cliff. Ferruginous sandstones, clad on the rocky
cliff up to 5 to 8 m above high sea level and including a few angular
shards of shale, correspond to a fossil dune, also sandified by the
oxide-laden waters draining the slope littoral. This red dune, probably
contemporary with the beginning of a retreat of the sea, when vast
sandy foreshores found themselves subject to wind deflation, at the end
of the Eemian transgression, was quickly impregnated by limonite, which
preserved, when the gelifluxion flows subsequently planed the steep
coastline during the Weichselien.
The specialists who successively visited the site observed that the
ferruginous slab presented a network of cracks with offsets resembling
the reactivation of fractures in the shale bedrock. Tectonic
readjustments are in fact possible due to the proximity of the Kerforne
accident, which opens into the nearby Ry cove.
Source: SGMB
To
go further in geology
◊ Sedimentary rocks
Sedimentary rocks form at ambient temperature and pressure and come
from various particles (sand, gravel, dust, remains of living beings,
etc.), which are transported to the bottom of lakes, seas or oceans
then assemble before settling. consolidate.
These are rather soft rocks, scratchable by a metal blade.
They can be characterized by their formation process
• Formation by chemical precipitation, which gives chemical
sedimentary rock (no fossils, generally very fine grains).
Ex: certain limestones like chalk...
• Formation by chemical precipitation and remains of living
organisms in large quantities, which gives a biogenic sedimentary rock.
Ex: many forms of limestone: Oolithic, Echaillon, Urgonian,...
• Formation by erosion of a previously formed rock, the debris
of
which is transported and then deposited (sometimes with a few fossils
depending on the deposition environment), which gives a detrital
sedimentary rock.
Ex: Sandstone
◊ Magmatic rocks
These rocks come from deep within the earth, within the mantle.
These are very hard rocks (cannot be scratched with a knife) and
without schistosity (no presence of "beds" in the rock).
There are two families of igneous rocks, which will depend on how and
where they crystallized.
• Volcanic rocks
They come from magma which enters the earth's crust then makes its way
to the surface and gives rise to lava flows. Due to this rise,
crystallization is very rapid, which produces very small crystals.
The resulting rock will be a rock with fine crystals that are generally
not distinguishable with the naked eye (micro-grained).
Ex: Basalt
• Plutonic rocks
They come from magma which has remained stuck in the earth's crust and
will crystallize there.
When this magma crystallizes inside the earth's crust, the lowering of
its temperature is slow and the slower the crystallization, the larger
and more visible the crystals will be. The rock is said to be grainy.
Ex: Granite
◊ Metamorphic rocks
These rocks are called secondary and come from the transformation of a
primitive rock under the effect of modifications in temperature (T) and
pressure (P) conditions. These transformations lead to a reorganization
of the elements in the rock and a recrystallization of the minerals in
the solid state.
Ex: shale
◊ Insertion of veins
A vein is a sheet of rock, a few centimeters to a few meters thick,
intersecting the surrounding area (which distinguishes it from the sill
or lopolite) and most often corresponding to the filling of a
stratification joint or a fracture (fault , diaclase) by magmatic rocks
(pegmatites, aplites, basalt, etc.), sedimentary rocks or by
hydrothermal deposits (quartz, micas, ores, etc.). The rocks present
are of magmatic origin (pegmatite whose shrinkage faults due to its
cooling are filled by late magma), or metamorphic after reworking of
the surrounding area itself.
• Pegmatite
Pegmatite (from the Greek pegma, "which is assembled, coagulated") is a
magmatic rock with large crystals (of size greater than 1 cm, and which
can reach several meters), frequently automorphic - that is to say in
the form of a perfect crystal or, at least, limited by flat crystal
faces. We speak of a phenocryst (from the Greek phainein, to shine and
crystal) for a crystal of a size such that it is visible to the naked
eye, with a diameter ranging from one millimeter to ten centimeters.
Most rocks with a pegmatitic texture have a composition close to
granite, with quartz, feldspars and micas (especially muscovite) as
essential minerals, but are depleted in mafic minerals (silicates rich
in magnesium and iron) and enriched in incompatible elements (
concentration in residual silicate liquids of elements normally rare on
the planet).
Pegmatites form veins (pockets or veins), on the edge or in the
immediate vicinity of the granite plutons with which they are
genetically associated. They generally correspond to the residual
liquid, rich in water, at the end of crystallization of a granite
magma. The abundance of water facilitates the diffusion of chemical
elements and allows the growth of large crystals.
• Diorite
Diorite is a plutonic magmatic rock with a grainy texture, generally
light with darker areas. It is made up of large, very dark green
crystals of amphibole, other milky white ones being feldspar
(plagioclase) and other translucent ones being quartz, and mica. This
mineralogical assemblage is linked to a slow cooling characteristic of
a magma chemically intermediate between a granitic magma and a basaltic
magma.
• Gabbro
A gabbro is a rock with a grainy structure predominantly green to black
in color. It is composed of pyroxene, amphibole and olivine, giving it
this dark color, and plagioclase. The structure of a gabbro is
generally coarse-grained, with crystals 1 mm or larger in size. A
gabbro with finer grains is called a dolerite, although the term
"microgabbro" is often used when additional descriptor is desired.
• Dolerite
Dolerite is a microgabbro, that is to say a dense, hard and massive,
fine-grained eruptive rock, corresponding to a basalt which solidified
(relatively) slowly in a vein, then underwent light metamorphism. It is
made up of grains visible under a magnifying glass. Greenish to bluish
in color, it is composed of rectangular rods of crisscrossed
plagioclase feldspar, between which are dark minerals (pyroxenes,
olivine, etc.). This compact rock comes from the conduits connecting
the magma chamber to the volcanic apparatus on the surface. It is an
intermediate rock between gabbros (grainy) and basalts (lavas with a
fine paste).
The term dolerite is also used, in the broad sense, to designate
microdiorites and microgabbros.
• Aplite
An aplite is a leucocratic vein rock (rich in so-called white minerals)
with a very fine grain size. Aplites are associated either with
magmatic rocks or with metasomatic rocks.
Aplites are characterized by the small size of their crystals (less
than 1 mm), with a very low content of mafic (ferromagnesian) minerals.
Aplites are essentially made up of quartz, alkali feldspar (orthoclase
or microperthite) and plagioclase.
.
Questions
Careful reading of the description
of the cache, as
well as an observation of the terrain features and a little deduction
are normally sufficient to answer the questions of this
EarthCache.
See
chapter "an
earthcache" for log conditions.
Question
0 - Take an individual photo of yourself (no
collective photo), or your distinctive geocacher object, or your
nickname written on a sheet of paper or in your hand... on the beach,
and
attach it to your log or your answers
Question
1
- For each of the rocks A B C in the photo, describe them then name
them.
Question
2- Depending on their
nature and the elements of their formation, classify them in
chronological order of geological appearance, then explain what
happened here.