Ligérian
Encyclopedia
The Ligérian is a regional geological
substage of the Turonian
. It designates sediment
s of the Lower Turonian in western France
.
Ligérien, is derived from the Latin
Ligera, which stands for the Loire
River.
(southwestern part of the Paris Basin
) near Saumur
along the River Loire, type locality
of the Turonian. But it can equally be found in the northern Aquitaine Basin.
and is in turn overlain by the Angoumian
.
In the northern Aquitaine Basin the Ligérian, also known as the Villars Formation, shows the following stratigraphy
:
At its base the Ligérian consists of an alternating succession of nearly meter-sized limestone
beds and very thin intervening grey marl
y horizons. The limestone is white and very chalk
y.
At the contact with the underlying Cenomanian it is detritic and contains coarse quartz
grains, small-sized gravels and the fragments of ground-up lamellibranchs and echinoderm
s. Higher up in the section the marly layers gradually disappear and the rock takes on a massive, homogeneous and cryptocrystalline aspect (thickness 5 to 10 meters).
Stratification within the Ligérian, if recognisable, is nodular, wavy to platy. The rock disintegrates to short, prismatic columns. Because of its erodibility and incompetent nature the Ligérian forms slopes beneath the much more resistant Angoumian cliffs. Farmers grow cereals on its easily workable layers.
Compared to other Upper Cretaceous
rocks the Ligérian is rather poor in fossil content. Its fauna
consists mainly of lamellibranchs like arca noueliana, cardium productum, exogyra columba (oyster
) and inoceramus labiatus, of gasteropods (pleurotomaria galliennei), of hexacorals and of bryozoa
, towards the top annelid
s are appearing as well. Ammonites like collignoniceras peramplum, eutrophoceras sublaevigatus, lewesiceras peramplum, mammites revelieri and spathites reveliereanum as well as nautiloid
s like nautilus sublaevigatus are only rarely encountered. The microfauna is not very diversified either, present are ostracod
s (in the marls), textulariids and pelagic forams (hedbergella delrioensis, heterohelix, and globotruncana). Some discorbids
and some benthic forams like dorothia sp., marsonnella oxycona, eggerella sp. and tritaxia sp. do also occur.
Very common are burrow
s and pyrite
nodules within the chalky limestones.
The changeover to the overlying Angoumian is very progressive but can be defined by the first appearance of detritic layers, pellet
s of different sizes and quite common annelid tubes as well as by the disappearance of the pelagic forams. The lower 5 to 8 meters of the Angoumian cannot really be separated from the Ligérian in the field and are therefore most often attributed to the Ligérian on geological maps.
The thickness of the Ligérian in the northern Aquitaine Basin usually oscillates between 15 and 20 meters, but can exceptionally reach 42 meters (in syncline
s).
.
The Ligérian follows immediately after the oceanic anoxic event OAE-2, which closed the Cenomanian (and exterminated half of the ammonite species). Globally the sea level was rising and reached a highstand towards the end of the Ligérian. During the Angoumian the sea receded again (the depositional environment of the Angoumian is much closer to the shore). The ocean temperatures were very high – they reached their maximum later in the Upper Turonian.
; they are most probably due to differential overburden pressures during diagenesis.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
substage of the Turonian
Turonian
The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
. It designates sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s of the Lower Turonian in western France
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...
.
Etymology
The name Ligérian, in FrenchFrench language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
Ligérien, is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
Ligera, which stands for the Loire
Loire
Loire is an administrative department in the east-central part of France occupying the River Loire's upper reaches.-History:Loire was created in 1793 when after just 3½ years the young Rhône-et-Loire department was split into two. This was a response to counter-Revolutionary activities in Lyon...
River.
Occurrence
The Ligérian is encountered in the TouraineTouraine
The Touraine is one of the traditional provinces of France. Its capital was Tours. During the political reorganization of French territory in 1790, the Touraine was divided between the departments of Indre-et-Loire, :Loir-et-Cher and Indre.-Geography:...
(southwestern part of the Paris Basin
Paris Basin
Paris Basin may refer to:*As a hydrological basin, it is largely the basin of the River Seine* Paris Basin , the geological basin...
) near Saumur
Saumur
Saumur is a commune in the Maine-et-Loire department in western France.The historic town is located between the Loire and Thouet rivers, and is surrounded by the vineyards of Saumur itself, Chinon, Bourgueil, Coteaux du Layon, etc...
along the River Loire, type locality
Type 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....
of the Turonian. But it can equally be found in the northern Aquitaine Basin.
Stratigraphy
The Ligérian, abbreviated as c3a on French geological maps, is the lowest substage of the Turonian. It overlies the CenomanianCenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
and is in turn overlain by the Angoumian
Angoumian
The Angoumian is a geological group restricted to the northern Aquitaine Basin. The group consists of two fossiliferous limestone formations deposited during the Turonian.- Etymology :...
.
In the northern Aquitaine Basin the Ligérian, also known as the Villars Formation, shows the following stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
:
At its base the Ligérian consists of an alternating succession of nearly meter-sized 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....
beds and very thin intervening grey marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
y horizons. The limestone is white and very chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
y.
At the contact with the underlying Cenomanian it is detritic and contains coarse 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,...
grains, small-sized gravels and the fragments of ground-up lamellibranchs and echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. Higher up in the section the marly layers gradually disappear and the rock takes on a massive, homogeneous and cryptocrystalline aspect (thickness 5 to 10 meters).
Stratification within the Ligérian, if recognisable, is nodular, wavy to platy. The rock disintegrates to short, prismatic columns. Because of its erodibility and incompetent nature the Ligérian forms slopes beneath the much more resistant Angoumian cliffs. Farmers grow cereals on its easily workable layers.
Compared to other Upper Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
rocks the Ligérian is rather poor in fossil content. Its fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
consists mainly of lamellibranchs like arca noueliana, cardium productum, exogyra columba (oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
) and inoceramus labiatus, of gasteropods (pleurotomaria galliennei), of hexacorals and of bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
, towards the top annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s are appearing as well. Ammonites like collignoniceras peramplum, eutrophoceras sublaevigatus, lewesiceras peramplum, mammites revelieri and spathites reveliereanum as well as nautiloid
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
s like nautilus sublaevigatus are only rarely encountered. The microfauna is not very diversified either, present are ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
s (in the marls), textulariids and pelagic forams (hedbergella delrioensis, heterohelix, and globotruncana). Some discorbids
Discorbacea
Discorbacea is a superfamily of rotaliid foraminifera, with a range extending from the Middle Triassic to the present, characterized by chambers arranged in a low trochspiral; an umbilical or interiomarginal aperture, with or without supplementary apertures; and a wall structure that is optically...
and some benthic forams like dorothia sp., marsonnella oxycona, eggerella sp. and tritaxia sp. do also occur.
Very common are burrow
Burrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
s and pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
nodules within the chalky limestones.
The changeover to the overlying Angoumian is very progressive but can be defined by the first appearance of detritic layers, pellet
Pellet
Pellet are small particles typically created by compressing an original material. Specific items often termed 'pellet' include:* Pelletizing is the industrial process used to create pellets, using a [pellet mill] or equipment for extrusion and * Spheroids is the process for the manufacture of...
s of different sizes and quite common annelid tubes as well as by the disappearance of the pelagic forams. The lower 5 to 8 meters of the Angoumian cannot really be separated from the Ligérian in the field and are therefore most often attributed to the Ligérian on geological maps.
The thickness of the Ligérian in the northern Aquitaine Basin usually oscillates between 15 and 20 meters, but can exceptionally reach 42 meters (in syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
s).
Depositional environment
The finely ground bioclastic remnants, the pelagic forams and the occasional ammonites all indicate a depositional environment bordering onto open marine conditions.Age
The Ligérian can be correlated with the Lower Turonian substage, therefore representing the time period 93.5 to 91 million years BPBefore Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
.
The Ligérian follows immediately after the oceanic anoxic event OAE-2, which closed the Cenomanian (and exterminated half of the ammonite species). Globally the sea level was rising and reached a highstand towards the end of the Ligérian. During the Angoumian the sea receded again (the depositional environment of the Angoumian is much closer to the shore). The ocean temperatures were very high – they reached their maximum later in the Upper Turonian.
Tectonic features
Besides common fractures and occasional faults the Ligérian shows in places remarkable pressure cones that can reach a decimeter in size. They resemble elephant's feet and are lined with near vertical slickolitesSlickenside
thumb|Slickensides on a sample of sandstone of the [[Juniata Formation]], from an outcrop on [[U.S. Route 322|Rt 322]] northeast of State College, Pennsylvania250px|thumb|left|How slickenfibre steps form and show sense of movement on a fault...
; they are most probably due to differential overburden pressures during diagenesis.
Literature
- Cassoudebat, M., Platel, J.-P. (1973). Sédimentologie et paléogéographie du Turonien de la bordure septentrionale du bassin aquitain. Bulletin BRGM, (2), section I, n° 2, p. 85-102.
- Gradstein, F., Ogg, J. & Smith, A. (2004). A Geologic Time Scale 2004. Cambridge University Press. ISBN 0-521-78673-8.
- Platel, J.-P. et al. (1989). Feuille Périgueux (Ouest). Carte géologique de la France à 1/50 000. BRGM.