Génis Unit
Encyclopedia
The Génis Unit is a Paleozoic
metasediment
ary succession of the southern Limousin
and belongs geologically to the Variscan
basement of the French
Massif Central
. The unit covers the age range Cambrian
/Ordovician
till Devonian
.
, the small town Génis
, situated in the northeastern Dordogne.
plateau that was peneplain
ed during the Paleogene
. The plateau's elevation oscillates between 300 and 400 meters. Geologically the unit's northern limit is the South Limousin Fault, a very important ductile, dextral wrench fault separating the Génis Unit from the Thiviers-Payzac Unit
to the north. To the south the unit is overlain by lias
sic sediments of the Aquitaine Basin
. In the east the group disappears beneath Permian
red beds of the Brive Basin. Its sedimentary succession can be best studied along the Auvézère
river.
Comparable units to the Génis Unit with an identical stratigraphic succession can be found in the Vendée
and in the Rouergue
.
magmatic rocks like gabbro
and basalt
ic pillow lava
, basic volcanoclastics and rare intercalations of chert
and pelite
. A lower Devonian
age can be attributed to the greenschist.
Incorporated at the base of the greenschist are limestone
lenses containing upper Silurian
conodont
s.
s quartz
, chlorite
and muscovite
(variety sericite). The sericite schist most likely is of Ordovician
and lower Silurian age (rare finds of Ordovician acritarch
s have been made).
. A close relationship with the Grès Armoricain in Brittany
is also considered. Therefore an Ordovician age of the arkose is most likely.
one encounters the Génis porphyroid. This rock represents alkaline, rhyolitic
ignimbrite
s (metaignimbrites) of lower Ordovician age (Tremadocian). Its mineralogy is composed of phenocryst
s of quartz, alkali feldspar
and plagioclase
(albite
) and a very fine-grained matrix (grain-size 5 μ) made of quartz, feldspars, sericite and rare chlorite. Original fiamme
are hard to discern, but welded glass
y layers are recognisable. The metaignimbrite is rich in potassium
and contains more than 70 % of SiO2.
Underlying the Génis porphyroid are the Donzenac schist and the Thiviers sandstone, the main formations of the Thiviers-Payzac Unit. In the Fugeyrollas anticline the Thiviers sandstone appears on the surface and thus also crops out within the Génis Unit. Both formations are supposed to cover the age range Neoproterozoic
to Cambrian
.
ed into tight, upright folds with a wavelength
of about 150 meters. The fold axes strike WNW-ESE (N 110) with a slight dip of about 10 ° to the east. The original sedimentary layering (S0) can often still be recognised dipping 75 to 80 ° to the north or to the south. Parallel to the folds' axial plane a distinct schistosity has developed (S1). The tight folds are overprinted by secondary folding and deformed into large anticlines and synclines with a wavelength of about 2 kilometers (Cubas syncline in the south followed by the Fougeyrollas anticline and the Génis syncline to the north). Imprinted on the layering are well developed stretching lineations also running more or less parallel to the fold axes. Newly formed metamorphic minerals are preferably arranged along this direction, although their lineations stray between N 110 and N 135. Furthermore the lineations are accompanied by microfolds, whose axes yet again follow the N 110-direction.
due to increasing overburden. The metamorphism followed a retrograde path (neoformation of chlorite) and recorded the epizonal conditions of the greenschist facies. This fact is very important for the geology of the Massif Central, because low-grade metamorphic successions are underrepresented and quite rare. Usually the metasediments in the Massif Central are strongly deformed and metamorphosed (under amphibolite facies conditions) and therefore only rough guesses can be made about their protoliths.
The retrograde metamorphism is known in other places of the Massif Central and is assigned a mid-Carboniferous
age .
Like the already mentioned South Limousin Fault the entire Génis Unit underwent ductile, dextral wrenching and can thus be conceived as a fairly wide, upright, WNW-ESE-striking shear zone
. Dextral shear criteria can be found in all of its formations. Asymmetrically deformed quartz pebbles in conglomeratic
layers of the Thiviers sandstone indicate dextral shearing. The same sense of movement is even more evident in the Génis porphyroid, where pressure shadows have formed around the quartz and alkali feldspar phenocrysts. Millimetre-sized shear bands within the Génis sericite schist also point at dextral shearing .
The timing of the shearing motions is based on comparisons with similar terrains in the Armorican Massif
(Chantonnay synform in the Vendée) and in the Rouergue. The dextral shearing in the Armorican Massif is dated as Namurian and Westphalian (Serpukhovian
till Moscovian
), i.e. 325 to 305 million years ago. It is therefore reasonable to assume a mid- to late Carboniferous age for the deformations in the Génis Unit of the southern Limousin.
The pervasive shearing motions are also responsible for the fold structures in the Génis Unit, which can be interpreted as tear folds caused by the transpressive, ductile wrenching.
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
metasediment
Metasediment
In geology, metasediment is sediment or sedimentary rock that shows evidence of having been subjected to metamorphism. The overall composition of a metasediment can be used to identify the original sedimentary rock, even where they have been subject to high-grade metamorphism and intense...
ary succession of the southern Limousin
Limousin (région)
Limousin is one of the 27 regions of France. It is composed of three départements: Corrèze, Creuse and the Haute-Vienne.Situated largely in the Massif Central, as of January 1st 2008, the Limousin comprised 740,743 inhabitants on nearly 17 000 km2, making it the second least populated region of...
and belongs geologically to the Variscan
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
basement of the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
Massif Central
Massif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
. The unit covers the age range Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
/Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
till Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
.
Type locality
The Génis Unit was named after its type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
, the small town Génis
Génis
Génis is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-External links:* *...
, situated in the northeastern Dordogne.
Geographical Occurrence
The Génis Unit occurs in an elongated strip at the northeastern edge of the Dordogne department, following a WNW-ESE direction for about 26 kilometers. Its width across strike is not more than 5 kilometers. The unit forms part of the Bas Limousin, a basementBasement
__FORCETOC__A basement is one or more floors of a building that are either completely or partially below the ground floor. Basements are typically used as a utility space for a building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system...
plateau that was peneplain
Peneplain
A peneplain is a low-relief plain representing the final stage of fluvial erosion during times of extended tectonic stability. The existence of peneplains, and peneplanation as a geomorphological process, is not without controversy, due to a lack of contemporary examples and uncertainty in...
ed during the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
. The plateau's elevation oscillates between 300 and 400 meters. Geologically the unit's northern limit is the South Limousin Fault, a very important ductile, dextral wrench fault separating the Génis Unit from the Thiviers-Payzac Unit
Thiviers-Payzac Unit
The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France...
to the north. To the south the unit is overlain by lias
Lias Group
The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the low countries and the north of Germany...
sic sediments of the Aquitaine Basin
Aquitaine Basin (geology)
The Aquitaine Basin is after the Paris Basin the second largest Mesozoic and Cenozoic sedimentary basin in France, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km2 onshore. It formed on Variscan basement which was peneplained during the Permian...
. In the east the group disappears beneath Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
red beds of the Brive Basin. Its sedimentary succession can be best studied along the Auvézère
Auvézère
The Auvézère is a small river in the Dordogne department of France. It is a tributary of the Isle River, which is itself a tributary of the Dordogne River.-Geography:...
river.
Comparable units to the Génis Unit with an identical stratigraphic succession can be found in the Vendée
Vendée
The Vendée is a department in the Pays-de-la-Loire region in west central France, on the Atlantic Ocean. The name Vendée is taken from the Vendée river which runs through the south-eastern part of the department.-History:...
and in the Rouergue
Rouergue
Rouergue is a former province of France, bounded on the north by Auvergne, on the south and southwest by Languedoc, on the east by Gévaudan and on the west by Quercy...
.
Stratigraphy
The Génis Unit shows the following stratigraphic succession (from top to bottom):- Génis greenschist
- Génis sericite schist
- Puy-de-Cornut arkose
- Génis porphyroid
Génis greenschist
The Génis greenschist is the youngest formation of the Génis Unit. The greenschist derives from maficMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
magmatic rocks like gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
and basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
, basic volcanoclastics and rare intercalations of chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
and pelite
Pelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...
. A lower Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
age can be attributed to the greenschist.
Incorporated at the base of the greenschist are limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
lenses containing upper Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
s.
Génis sericite schist
The underlying Génis sericite schist is very rich in the mineralMineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
and muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
(variety sericite). The sericite schist most likely is of Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and lower Silurian age (rare finds of Ordovician acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s have been made).
Puy-de-Cornut arkose
Below the Génis sericite schist follows the Puy-de-Cornut arkose. The arkose is strongly silicified and forms erosionally resistant morphological ridges. It is supposed to be an equivalent of the Puy-des-Âges quartzite from the neighbouring Thiviers-Payzac UnitThiviers-Payzac Unit
The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France...
. A close relationship with the Grès Armoricain in Brittany
Brittany
Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...
is also considered. Therefore an Ordovician age of the arkose is most likely.
Génis porphyroid
Below a pronounced angular unconformityUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
one encounters the Génis porphyroid. This rock represents alkaline, rhyolitic
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
s (metaignimbrites) of lower Ordovician age (Tremadocian). Its mineralogy is composed of phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s of quartz, alkali feldspar
Alkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
(albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
) and a very fine-grained matrix (grain-size 5 μ) made of quartz, feldspars, sericite and rare chlorite. Original fiamme
Fiamme
Fiamme are lens-shapes, usually mm to cm in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density currents. The name fiamme comes from the Italian word for flames,...
are hard to discern, but welded glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
y layers are recognisable. The metaignimbrite is rich in potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and contains more than 70 % of SiO2.
Underlying the Génis porphyroid are the Donzenac schist and the Thiviers sandstone, the main formations of the Thiviers-Payzac Unit. In the Fugeyrollas anticline the Thiviers sandstone appears on the surface and thus also crops out within the Génis Unit. Both formations are supposed to cover the age range Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
to Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
.
Structural organisation
The Génis Unit is thoroughly foldFold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
ed into tight, upright folds with a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of about 150 meters. The fold axes strike WNW-ESE (N 110) with a slight dip of about 10 ° to the east. The original sedimentary layering (S0) can often still be recognised dipping 75 to 80 ° to the north or to the south. Parallel to the folds' axial plane a distinct schistosity has developed (S1). The tight folds are overprinted by secondary folding and deformed into large anticlines and synclines with a wavelength of about 2 kilometers (Cubas syncline in the south followed by the Fougeyrollas anticline and the Génis syncline to the north). Imprinted on the layering are well developed stretching lineations also running more or less parallel to the fold axes. Newly formed metamorphic minerals are preferably arranged along this direction, although their lineations stray between N 110 and N 135. Furthermore the lineations are accompanied by microfolds, whose axes yet again follow the N 110-direction.
Metamorphism and structural evolution
During the Variscan orogeny the original sedimentary succession of the Génis Unit subsided and was metamorphosedMetamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
due to increasing overburden. The metamorphism followed a retrograde path (neoformation of chlorite) and recorded the epizonal conditions of the greenschist facies. This fact is very important for the geology of the Massif Central, because low-grade metamorphic successions are underrepresented and quite rare. Usually the metasediments in the Massif Central are strongly deformed and metamorphosed (under amphibolite facies conditions) and therefore only rough guesses can be made about their protoliths.
The retrograde metamorphism is known in other places of the Massif Central and is assigned a mid-Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
age .
Like the already mentioned South Limousin Fault the entire Génis Unit underwent ductile, dextral wrenching and can thus be conceived as a fairly wide, upright, WNW-ESE-striking shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
. Dextral shear criteria can be found in all of its formations. Asymmetrically deformed quartz pebbles in conglomeratic
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
layers of the Thiviers sandstone indicate dextral shearing. The same sense of movement is even more evident in the Génis porphyroid, where pressure shadows have formed around the quartz and alkali feldspar phenocrysts. Millimetre-sized shear bands within the Génis sericite schist also point at dextral shearing .
The timing of the shearing motions is based on comparisons with similar terrains in the Armorican Massif
Armorican Massif
The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers...
(Chantonnay synform in the Vendée) and in the Rouergue. The dextral shearing in the Armorican Massif is dated as Namurian and Westphalian (Serpukhovian
Serpukhovian
The Serpukhovian is in the ICS geologic timescale the uppermost stage or youngest age of the Mississippian, the lower subsystem of the Carboniferous. The Serpukhovian age lasted from 328.3 Ma tot 318.1 Ma...
till Moscovian
Moscovian (Carboniferous)
The Moscovian is in the ICS geologic timescale a stage or age in the Pennsylvanian, the youngest subsystem of the Carboniferous. The Moscovian age lasted from 311.7 ± 1.1 to 306.5 ± 1.0 Ma, is preceded by the Bashkirian and is followed by the Kasimovian...
), i.e. 325 to 305 million years ago. It is therefore reasonable to assume a mid- to late Carboniferous age for the deformations in the Génis Unit of the southern Limousin.
The pervasive shearing motions are also responsible for the fold structures in the Génis Unit, which can be interpreted as tear folds caused by the transpressive, ductile wrenching.
Sources
- BRGM. Feuille Juillac. Carte géologique de la France à 1/50 000
- Peterlongo, J.M. (1978). Massif Central. Guides Géologiques Régionaux. Masson. ISBN=2-225-49753-2
- Roig, J.-Y., Faure, M. & Ledru, P. (1996). Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids. Geologische Rundschau, 85, pp. 138–153