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The Angoumian is a
geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...
restricted to the northern
Aquitaine Basin The Aquitaine Basin is the second largest Mesozoic and Cenozoic sedimentary basin in France after the Paris Basin, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km2 onshore. It formed on Varisc ...
in France. The group consists of two fossiliferous
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...
formations deposited during the
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 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded ...
.


Etymology

The name Angoumian, in
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with France ...
''Angoumien'', is derived from the ''Angoumois'' region around the town of
Angoulême Angoulême (; Poitevin-Saintongeais: ''Engoulaeme''; oc, Engoleime) is a commune, the prefecture of the Charente department, in the Nouvelle-Aquitaine region of southwestern France. The inhabitants of the commune are known as ''Angoumoisins'' o ...
in the
Charente Charente (; Saintongese: ''Chérente''; oc, Charanta ) is a department in the administrative region of Nouvelle-Aquitaine, south western France. It is named after the river Charente, the most important and longest river in the department, an ...
.


Occurrence

The formation crops out in the surroundings of Angoulême, but also reaches farther south into the
Dordogne Dordogne ( , or ; ; oc, Dordonha ) is a large rural department in Southwestern France, with its prefecture in Périgueux. Located in the Nouvelle-Aquitaine region roughly half-way between the Loire Valley and the Pyrenees, it is named ...
(
Périgord Périgord ( , ; ; oc, Peiregòrd / ) is a natural region and former province of France, which corresponds roughly to the current Dordogne department, now forming the northern part of the administrative region of Nouvelle-Aquitaine. It is div ...
, i.e. Périgord Vert & Périgord Blanc).


Stratigraphy

The Angoumian is usually divided into two formations, a lower ''Angoumien inférieur'' and an upper ''Angoumien supérieur''. Some authors even attribute the Ligerian (in French ''Ligérien'') to the Angoumian and then call it ''basal Angoumian''. The Angoumian ''sensu stricto'' conformably overlies the Ligerian and closes with a
hardground Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone ...
towards the overlying
Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded b ...
.


Lower Angoumian

(Represented on French
geological map A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with str ...
s as unit ''c3b'') The 15–20 m thick lower Angoumian, also called ''Angoulême Formation'', starts with a 5–6 m thick, thinly bedded, ''microcrystalline limestone'' of white to creamy colour. This limestone is characterised by the incorporation of fine debris and by a
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
microfauna consisting of
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
such as
miliolida The Miliolida are an order of foraminifera with calcareous, porcelacous tests that are imperforate and commonly have a pseudochitinous lining. Tests are composed of randomly oriented calcite needles that have a high proportion of magnesium along ...
and
textulariida The Textulariida are an order of foraminifera that produce agglutinated shells or tests. An agglutinated test is one made of foreign particles glued together with an organic or calcareous cement to form an external shell on the outside of the ...
. The first
rudist Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms ...
s ('' Sphaerulites patera'' and '' Biradiolites lumbricalis'') start to appear in this level. Some rare
cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...
s have also been found (such as '' Romaniceras deverianum'' or '' Lecointriceras fleuriausianum'' for instance). These fairly thin beds then give way to a massive, homogeneous, normally 8–10 m thick,
chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Cha ...
y,
rudist Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms ...
-bearing limestone of pure white colour. The massive bed stands out
geomorphologically Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topography, topographic and bathymetry, bathymetric features created by physical, chemical or biological pro ...
, sometimes forming small cliffs in the countryside. Generally this rock is quite soft (it can be cut with a handsaw), it was therefore quarried extensively for building stone with hundreds of small quarries in operation; most of them have closed down by now. The massive member is affected by important thickness variations (its total thickness normally can vary from 5–12 m, sometimes even reaching 15 m), caused either by
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
disturbances or by post-depositional
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is di ...
(for instance, in the
Mareuil Anticline The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sediment, sedimentary sequence of the northeastern Aquitaine Basin. The northwest-southeast trending anticline was caused by tectonic movements probably s ...
, the upper Angoumian discordantly overlies the lower Angoumian, occasionally even cutting down into it). The ''chalky limestones'' are mainly composed of rudist debris with comparatively little
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement m ...
. The rudist debris consists almost exclusively of the species ''Biradiolites lumbricalis'' accompanied by minor '' Durania cornupastoris'', '' Praeradiolites peroni'', '' Radiolites peroni'', '' Radiolites ponsi'', '' Radiolites beaumonti'', '' Radiolites socialis'', and '' Hippurites requieni''. In some places, undamaged rudists (mainly the small-sized ''Biradiolites'') can be encountered in numerous biostromes. Towards the top, the massive bed changes its character and thinner beds of a very hard limestone or of an ocre calcarenite appear. This horizon is the upper limit of the quarrying activity and was called ''chaudron'' (kettle) by the miners. In addition to the usual fauna, it also comprises small biostromes of '' Biradiolites angulosus''. The common facies can change quite suddenly (within less than 500 m) into a coarse, weakly cemented, bioclastic facies (''Pierre de Cheyroux'') consisting of white to yellowish
calcarenite Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), carbonate grains. The grains consist of sand-size grains of either corals, shells, o ...
s — a
beachrock Beachrock is a friable to well-cemented sedimentary rock that consists of a variable mixture of gravel-, sand-, and silt-sized sediment that is cemented with carbonate minerals and has formed along a shoreline. Depending on location, the sedi ...
facies, indicating a
littoral The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...
position of the deposits. The bioclasts are very rounded and are composed of the debris of
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s, rudists, single
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...
s (polyps), and milioles. These rather massive ''calcarenites'' can reach a thickness of 12–15 m in places and sometimes are affected by
cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...
. They occur somewhat southeast of the ''La Tour Blanche anticline'' and the ''Périgueux anticline'', proving that these major structures were already beginning to rise during the Turonian. The lower formation finishes with a few metres of thinly bedded, yellowish, ''microcrystalline limestone'' rich in bioclasts (lamellibranchs and echinoderms). This member shows
cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...
and is affected by hiati as observed in Brantôme. The common rudist-bearing
reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic component, abiotic processes—deposition (geology), deposition of ...
al facies can completely change in the western depositional area, taking on a very hard, finely recrystallised aspect without any fossil debris. This facies change is probably due to faulting.


Upper Angoumian

(represented as unit ''c3c'') The upper Angoumian, also called ''Bourg-des-Maisons Formation'', can be fairly easily recognised in the countryside by its rather arid aspect with a typical flora of
juniper Junipers are coniferous trees and shrubs in the genus ''Juniperus'' () of the cypress family Cupressaceae. Depending on the taxonomy, between 50 and 67 species of junipers are widely distributed throughout the Northern Hemisphere, from the Arcti ...
s and dwarf bushes growing on it. The total thickness of the upper Angoumian varies from 20 to 40 m. The
type locality Type locality may refer to: * Type locality (biology) * Type locality (geology) See also * Local (disambiguation) * Locality (disambiguation) {{disambiguation ...
is at
Bourg-des-Maisons Bourg-des-Maisons (; oc, Borg de Maisons) is a commune in the Dordogne department in southwestern France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overs ...
. Its lowermost member of up to 20 m thickness is composed of grey to cream-coloured, rudist bearing, ''cryptocrystalline limestones''. Texturally they are
wackestone Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...
s grading into mudstones. They weather in a nodular and prismatic fashion and are interlayered with microcrystalline limestones rich in fine debris and bioclasts. The fossil content is dominated by rudist biostromes. The rudists are represented by ''Durania cornupastoris'', '' Praeradiolites ponsi'', '' Radiolites praesauvagesi'', '' Radiolites radiosus'', '' Radiolites trigeri'', '' Biradiolites quadratus'', '' Biradiolites angulosus'', '' Vaccinites praepetrocoriensis'', '' Vaccinites petrocoriensis'', and ''Hippurites requieni'' var. subpolygonia. The benthic fossils present indicate a calm and not very deep
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
; they comprise lamellibranchs,
acteonellidae Acteonellidae is an extinct family of gastropods in the informal group Lower Heterobranchia (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005). Taxonomy * Acteonellinae ** ''Acteonella'' d'Orbigny, 1843 - type genus ** ' ...
,
nerineoidea †Nerineoidea is a superfamily of extinct sea snails, fossil marine gastropod mollusks in the informal group Lower Heterobranchia. Families Families within the superfamily Nerineoidea include: * † Family Nerineidae The Nerineidae is an ...
, chaetetids, miliolids, and others. In some places, the top part undergoes a facies change to more agitated conditions, especially towards the West and near Périgueux (deposition of cream-coloured, coarse, bioclastic limestones, which are much less recrystallised; their
grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refe ...
is over 2 mm and they can show large cross-beds). The second member demonstrates pronounced thickness variations (from 8 to 12 m). In contrast to the first member it is composed either of micrites that are slightly debris-bearing, or of microcrystalline limestones rich in debris. Both rock types enclose the same fauna as the first member. The second member likewise can change its facies to calcarenites of the beachrock type. (These calcarenites persisted near Paussac and near Toulon (
Périgueux Périgueux (, ; oc, Peireguers or ) is a commune in the Dordogne department, in the administrative region of Nouvelle-Aquitaine, southwestern France. Périgueux is the prefecture of Dordogne, and the capital city of Périgord. It is al ...
) right from the lower Angoumian onwards). The upper Angoumian closes with about 5 m (to as much as 15 m in places) of grey to yellow, platy, ''marly limestones'' interlayered with yellow marls. Enclosed fossils are again rudists (''Praeradiolites praesinuatus'', '' Praeradiolites praecoquandi'', ''Vaccinites petrocoriensis'', and ''Hippurites requieni''), and chaetetids (
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...
s). The benthic microfauna is quite prominent and contains the foramnifera groups
rotaliida The Rotaliida are an order of Foraminifera, characterized by multilocular tests (shells) composed of bilamellar perforate hyaline lamellar calcite that may be optically radial or granular. In form, rotaliid tests are typically enrolled, but may ...
, discorbidae, and textulariida with the species '' Valvulammina picardi'' or '' Arenobulimina sp.'' as well as the
ostracode Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typical ...
species '' Bairdia'', '' Cythereis'', and '' Pterygocythereis''.


Depositional environment

After the generally
transgressive Transgressive may mean: *Transgressive art, a name given to art forms that violate perceived boundaries *Transgressive fiction, a modern style in literature *Transgressive Records, a United Kingdom-based independent record label *Transgressive (l ...
Ligerian (in comparison with its underlying Cenomanian) dominated by open marine deposits, the Angoumian clearly shows a regressive trend. This fact combined with the generally warm climate during the Turonian allowed the temperature of the sea-water to reach unprecedented values on the inner shelf of the northern Aquitaine Basin (peaking at roughly 10 °C above today's ocean temperatures during the lower Angoumian) Clarke LJ & Jenkins HC. (1999). New oxygen evidence for long-term Cretaceous climatic change in the Southern Hemisphere. ''Geology'', 27:699–702. – together with the reigning calm conditions an ideal environment for the widespread expansion of rudist reefs. During the lower Angoumian (Angoulême Formation), a muddy, calcareous
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons' ...
had established itself on the inner shelf with a low to moderate energy level. The lagoon was fringed or interspersed with extensive rudist reefs shedding their debris mainly towards the open sea. A major reef extended from Périgueux northwestwards thus isolating an inner platform northeast and east of Périgueux from an outer shelf towards the centre of the Aquitaine Basin. On the high(er) energy side of the individual reefs, the more clastic dominated members formed, whereas in the calm inner lagoon, the cryptocrystalline and micritic members were deposited. By the end of the upper Angoumian (Bourg-des-Maisons Formation), the regression became noticeable as the northern part of the Aquitaine shelf started to fall dry due to uplift in the
Massif central The (; oc, Massís Central, ; literally ''"Central Massif"'') is a highland region in south-central France, consisting of mountains and plateaus. It covers about 15% of mainland France. Subject to volcanism that has subsided in the last 10,00 ...
. This is recorded in the sediments, which at first took on a clastic character and later underwent profound alterations, breccifications and hardground formation. The total thickness of the Angoumian varies between 35 and 60 m with a higher accumulation rate occurring on the southwestern side of the depositional area (towards the centre of the Aquitaine Basin).


Structural observations

The Angoumian forms part of two very open
syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...
s striking NW-SE (the ''Combiers-Saint-Crépin-de-Richemont syncline'' in the NE) and WNW-ESE (the ''Gout-Rossignol-Léguillac synline'' in the SW) respectively. The group is therefore mainly flat-lying, yet in some places (like near Champeaux it dips 5° to the SW. Only near
Beaussac Beaussac (; oc, Baussat) is a former commune in the Dordogne department in Nouvelle-Aquitaine region in southwestern France. On 1 January 2017, it was merged into the new commune Mareuil en Périgord. The community is located in the Périgord L ...
a greater dip (of about 30°) can be found caused by local faulting. Nearby in the Aucors quarry, a slump overlies the rudist reef, followed by a
grain flow A grain flow is a type of sediment-gravity flow in which the fluid can be either air or water, acts only as a lubricant, and grains within the flow remain in suspension due to grain-to-grain collisions that generate a dispersive pressure to prevent ...
. These synsedimentary structures point to a tectonically disturbed setting possibly related to the rising
Mareuil Anticline The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sediment, sedimentary sequence of the northeastern Aquitaine Basin. The northwest-southeast trending anticline was caused by tectonic movements probably s ...
. Small
strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecton ...
faults can be observed near Brantôme which mostly show a horizontal movement direction pointing to the SSE, rarely to the SE. There is also a fairly common occurrence of slickolites (a special kind of
stylolite Stylolites (Greek: ''stylos'', pillar; ''lithos'', stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a deformation process that decreases the total volume of rock. Minerals wh ...
), which indicate the direction of
pressure solution In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and ...
in the rock. This direction is mostly vertical, i.e. merely representing the overburden of the overlying strata. Yet significant aberrations from this direction can be observed as well, proving that the group was also under the influence of directional stresses.


Age

The lower Angoumian is coeval with the upper two-thirds of the middle Turonian, the upper Angoumian is equivalent to the upper Turonian. The Angoumian therefore represents approximately the time interval 91 to 89 million years BP.


References

* Platel JP. (1974). Un modèle d'organisation des biotopes à Rudistes: l'Angoumian de l'Aquitaine septentrionale. Bull. Soc. Linn. Bordeaux, IV, n°1, pp. 3–13. * Platel JP. (1982). Les formations à Rudistes du Turonien moyen et supérieur de la plate-forme nord-aquitaine. ''Mém. Mus. nat. hist. nat.'', série c, tome XLIX, pp. 91–96. * Platel JP, et al. (1989). ''Carte géologique de la France à 1/50 000''. Feuille Périgueux (Ouest). BRGM. {{in lang, fr Cretaceous geochronology Geology of France Sedimentology