Angoumian
Encyclopedia
The Angoumian is a geological
group
restricted to the northern Aquitaine Basin. The group consists of two fossiliferous limestone
formation
s deposited during the Turonian
.
Angoumien, is derived from the Angoumois region around the town of Angoulême
in the Charente.
, i.e. Périgord Vert & Périgord Blanc).
(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 towards the overlying Coniacian
.
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
microfauna consisting of foraminifera
such as miliolida and textulariida
. The first rudists (Sphaerulites patera and Biradiolites lumbricalis) start to appear in this level. Some rare cephalopod
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
y, rudist-bearing limestone of pure white colour. The massive bed stands out geomorphologically
, 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
disturbances or by post-depositional erosion
(for instance, in the Mareuil Anticline
, 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
. 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
s — a beachrock
facies, indicating a littoral position of the deposits. The bioclasts are very rounded and are composed of the debris of echinoderm
s, rudists, single coral
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
. 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
and is affected by hiati as observed in Brantôme
.
The common rudist-bearing reef
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 fault
ing.
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
s and dwarf bushes growing on it. The total thickness of the upper Angoumian varies from 20 to 40 m. The type locality
is at Bourg-des-Maisons
.
Its lowermost member of up to 20 m thickness is composed of grey to cream-coloured, rudist bearing, cryptocrystalline limestones. Texturally they are wackestone
s grading into mudstone
s. 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; they comprise lamellibranchs
, acteonellidae
, nerineoidea
, 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 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
) 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 marl
s. Enclosed fossils are again rudists (Praeradiolites praesinuatus, Praeradiolites praecoquandi, Vaccinites petrocoriensis, and Hippurites requieni), and chaetetids (sponges). The benthic microfauna is quite prominent and contains the foramnifera groups rotaliida
, discorbidae
, and textulariida with the species Valvulammina picardi or Arenobulimina sp. as well as the ostracode species Bairdia, Cythereis, and Pterygocythereis.
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) – 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
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
. 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).
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 Champeau it dips 5° to the SW. Only near Beaussac
a greater dip (of about 30°) can be found caused by local fault
ing. Nearby in the Aucors quarry, a slump
overlies the rudist reef, followed by a grain flow. These synsedimentary structures point to a tectonically disturbed setting possibly related to the rising Mareuil Anticline
. Small strike-slip 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
), which indicate the direction of pressure solution
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.
.
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...
group
Group (stratigraphy)
A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are divided into formations and are sometimes themselves grouped into "supergroups"....
restricted to the northern Aquitaine Basin. The group consists of two fossiliferous 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....
formation
Formation
Formation may refer to:* Formation flying, aerobatics performed with several aircraft* Formation , a high-level military organization* Tactical formation, the physical deployment of military forces-Sports:...
s deposited during 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...
.
Etymology
The name Angoumian, 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...
Angoumien, is derived from the Angoumois region around the town of Angoulême
Angoulême
-Main sights:In place of its ancient fortifications, Angoulême is encircled by boulevards above the old city walls, known as the Remparts, from which fine views may be obtained in all directions. Within the town the streets are often narrow. Apart from the cathedral and the hôtel de ville, the...
in the Charente.
Occurrence
The formation crops out in the surroundings of Angoulême, but also reaches farther south into the Dordogne (PérigordPérigord
The Périgord is a former province of France, which corresponds roughly to the current Dordogne département, now forming the northern part of the Aquitaine région. It is divided into four regions, the Périgord Noir , the Périgord Blanc , the Périgord Vert and the Périgord Pourpre...
, 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 LigerianLigérian
The Ligérian is a regional geological substage of the Turonian. It designates sediments of the Lower Turonian in western France.- Etymology :The name Ligérian, in French Ligérien, is derived from the Latin Ligera, which stands for the Loire River....
(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 towards the overlying Coniacian
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 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...
.
Lower Angoumian
(Represented on French geological maps 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
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
microfauna consisting of foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
such as miliolida and textulariida
Textulariida
The Textulariida is a group of common foraminiferans that produce agglutinated shells, composed of foreign particles held in an organic or calcareous cement. Commonly the order is taken to include all such species, but genetic studies indicate that they are not all closely related, and several...
. The first rudists (Sphaerulites patera and Biradiolites lumbricalis) start to appear in this level. Some rare cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
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
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, rudist-bearing limestone of pure white colour. The massive bed stands out geomorphologically
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
, 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
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
disturbances or by post-depositional erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
(for instance, in the Mareuil Anticline
Mareuil Anticline
The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sedimentary sequence of the northeastern Aquitaine Basin...
, 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
Cementation (geology)
Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material within sediment pores; this is how "sediment" becomes "rock". The new pore-filling minerals form "bridges" between original sediment grains, thereby binding them together. So sand becomes...
. 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
thumb|250px|The [[Pietra di Bismantova]] in central [[Italy]] is an example of calcarenite formation.Calcarenite is a type of limestone that is composed predominately, more than 50 percent, of detrital sand-size , carbonate grains...
s — a beachrock
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...
facies, indicating a littoral position of the deposits. The bioclasts are very rounded and are composed of the debris of 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, rudists, single coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
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
Cross-bedding
In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...
. 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
Cross-bedding
In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...
and is affected by hiati as observed in Brantôme
Brantôme
Brantôme may refer to:*Brantôme, Dordogne, a commune in the Dordogne département in central France*Pierre de Bourdeille, seigneur de Brantôme , French soldier and historian*Brantôme , champion French racehorse...
.
The common rudist-bearing reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
al facies can completely change in the western depositional area, taking on a very hard, finely recrystallised
Recrystallization
Recrystallization may refer to:*Recrystallization *Recrystallization *Recrystallization...
aspect without any fossil debris. This facies change is probably due to fault
Fault
Fault may refer to:*Fault , planar rock fractures which show evidence of relative movement*Fault , in dog breeding, is an undesirable aspect of structure or appearance that indicates the dog should not be bred...
ing.
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
Juniper
Junipers are coniferous plants in the genus Juniperus of the cypress family Cupressaceae. Depending on taxonomic viewpoint, there are between 50-67 species of juniper, widely distributed throughout the northern hemisphere, from the Arctic, south to tropical Africa in the Old World, and to the...
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 (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....
is at Bourg-des-Maisons
Bourg-des-Maisons
Bourg-des-Maisons is a commune in the Dordogne department in southwestern France.-Population:-References:*...
.
Its lowermost member of up to 20 m thickness is composed of grey to cream-coloured, rudist bearing, cryptocrystalline limestones. Texturally they are wackestone
Wackestone
Wackestone is a matrix-supported carbonate rock that contains over 10% allochems in a carbonate mud matrix. This is part of the Dunham classification of carbonate rocks. In the other widely used classification due to Folk, an equivalent description would be, for example, an oopelmicrite, where the...
s grading into mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s. 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; they comprise lamellibranchs
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
, acteonellidae
Acteonellidae
Acteonellidae is an extinct family of gastropods in the informal group Lower Heterobranchia .- Taxonomy :* Acteonellinae** Acteonella d'Orbigny, 1843 - type genus...
, nerineoidea
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* † Family Ceritellidae...
, 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 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
Paussac-et-Saint-Vivien
Paussac-et-Saint-Vivien is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
and near Toulon (Périgueux
Périgueux
Périgueux is a commune in the Dordogne department in Aquitaine in southwestern France.Périgueux is the prefecture of the department and the capital of the region...
) 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 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...
s. Enclosed fossils are again rudists (Praeradiolites praesinuatus, Praeradiolites praecoquandi, Vaccinites petrocoriensis, and Hippurites requieni), and chaetetids (sponges). The benthic microfauna is quite prominent and contains the foramnifera groups rotaliida
Rotaliida
The Rotaliida are a large and abundant group of foraminiferans. They are primarily oceanic benthos, although some are common in shallower waters such as estuaries. They also include many important fossils, such as nummulites...
, discorbidae
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 textulariida with the species Valvulammina picardi or Arenobulimina sp. as well as the ostracode species Bairdia, Cythereis, and Pterygocythereis.
Depositional environment
After the generally transgressiveTransgression
Transgression may be:*a Biblical transgression, violation of God's ten commandments; sin *a legal transgression, a crime usually created by a social or economic boundary*a social transgression, violating a norm...
Ligerian (in comparison with its underlying Cenomanian
Cenomanian
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...
) dominated by open marine deposits, the Angoumian clearly shows a regressive
Marine regression
Marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land....
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) – 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
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
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
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...
. 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 synclineSyncline
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 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 Champeau it dips 5° to the SW. Only near Beaussac
Beaussac
Beaussac is a commune in the Dordogne department in Aquitaine region in southwestern France.The community is located in the Périgord Limousin Regional Nature Park.- History :...
a greater dip (of about 30°) can be found caused by local fault
Fault
Fault may refer to:*Fault , planar rock fractures which show evidence of relative movement*Fault , in dog breeding, is an undesirable aspect of structure or appearance that indicates the dog should not be bred...
ing. Nearby in the Aucors quarry, a slump
Slump
A Slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface...
overlies the rudist reef, followed by a grain flow. These synsedimentary structures point to a tectonically disturbed setting possibly related to the rising Mareuil Anticline
Mareuil Anticline
The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sedimentary sequence of the northeastern Aquitaine Basin...
. Small strike-slip 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
Stylolite
Stylolites are serrated surfaces at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock. Insoluble minerals like clays, pyrite, oxides remain within the stylolites and make them visible...
), which indicate the direction of pressure solution
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 either deposition in regions of relatively low stress...
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 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...
.