# Angoumian

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The **Angoumian** is a [geological](/source/Geology) [group](/source/Group_(stratigraphy)) restricted to the northern [Aquitaine Basin](/source/Aquitaine_Basin) in France. The group consists of two [fossiliferous limestone](/source/Fossiliferous_limestone) [formations](/source/Formation_(stratigraphy)) deposited during the [Turonian](/source/Turonian).

## Etymology

The name Angoumian, in [French](/source/French_language) *Angoumien*, is derived from the *Angoumois* region around the town of [Angoulême](/source/Angoul%C3%AAme) in the [Charente](/source/Charente_(d%C3%A9partement)).

## Occurrence

The formation crops out in the surroundings of Angoulême, but also reaches farther south into the [Dordogne](/source/Dordogne_(d%C3%A9partement)) ([Périgord](/source/P%C3%A9rigord), 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](/source/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](/source/Hardground) towards the overlying [Coniacian](/source/Coniacian).

### Lower Angoumian

(Represented on French [geological maps](/source/Geological_map) 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](/source/Benthos) microfauna consisting of [foraminifera](/source/Foraminifera) such as [miliolida](/source/Miliolida) and [textulariida](/source/Textulariida). The first [rudists](/source/Rudist) (*[Sphaerulites patera](/source/Sphaerulites_patera)* and *[Biradiolites lumbricalis](/source/Biradiolites_lumbricalis)*) start to appear in this level. Some rare [cephalopods](/source/Cephalopod) have also been found (such as *[Romaniceras deverianum](/source/Romaniceras_deverianum)* or *[Lecointriceras fleuriausianum](/source/Lecointriceras_fleuriausianum)* for instance).

These fairly thin beds then give way to a massive, homogeneous, normally 8–10 m thick, [chalky](/source/Chalk), [rudist](/source/Rudist)-bearing limestone of pure white colour. The massive bed stands out [geomorphologically](/source/Geomorphology), 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](/source/Tectonics) disturbances or by post-depositional [erosion](/source/Erosion) (for instance, in the [Mareuil Anticline](/source/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](/source/Cementation_(geology)). The rudist debris consists almost exclusively of the species *Biradiolites lumbricalis* accompanied by minor *[Durania cornupastoris](/source/Durania_cornupastoris)*, *[Praeradiolites peroni](/source/Praeradiolites_peroni)*, *[Radiolites peroni](/source/Radiolites_peroni)*, *[Radiolites ponsi](/source/Radiolites_ponsi)*, *[Radiolites beaumonti](/source/Radiolites_beaumonti)*, *[Radiolites socialis](/source/Radiolites_socialis)*, and *[Hippurites requieni](/source/Hippurites_requieni)*. In some places, undamaged rudists (mainly the small-sized *Biradiolites*) can be encountered in numerous [biostromes](/source/Biostrome). 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](/source/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 [calcarenites](/source/Calcarenite) — a [beachrock](/source/Beachrock) facies, indicating a [littoral](/source/Littoral_zone) position of the deposits. The bioclasts are very rounded and are composed of the debris of [echinoderms](/source/Echinoderm), rudists, single [corals](/source/Coral) (polyps), and milioles. These rather massive *calcarenites* can reach a thickness of 12–15 m in places and sometimes are affected by [cross-bedding](/source/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](/source/Cross-bedding) and is affected by hiati as observed in [Brantôme](/source/Brant%C3%B4me,_Dordogne).

The common rudist-bearing [reefal](/source/Reef) facies can completely change in the western depositional area, taking on a very hard, finely [recrystallised](/source/Recrystallization_(geology)) aspect without any fossil debris. This facies change is probably due to [faulting](/source/Fault_(geology)).

### 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 [junipers](/source/Juniper) and dwarf bushes growing on it. The total thickness of the upper Angoumian varies from 20 to 40 m. The [type locality](/source/Type_locality_(geology)) is at [Bourg-des-Maisons](/source/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 [wackestones](/source/Wackestone) grading into [mudstones](/source/Mudstone). 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](/source/Praeradiolites_ponsi)*, *[Radiolites praesauvagesi](/source/Radiolites_praesauvagesi)*, *[Radiolites radiosus](/source/Radiolites_radiosus)*, *[Radiolites trigeri](/source/Radiolites_trigeri)*, *[Biradiolites quadratus](/source/Biradiolites_quadratus)*, *[Biradiolites angulosus](/source/Biradiolites_angulosus)*, *[Vaccinites praepetrocoriensis](/source/Vaccinites_praepetrocoriensis)*, *[Vaccinites petrocoriensis](/source/Vaccinites_petrocoriensis)*, and *Hippurites requieni* var. subpolygonia. The benthic fossils present indicate a calm and not very deep [depositional environment](/source/Depositional_environment); they comprise [lamellibranchs](/source/Bivalvia), [acteonellidae](/source/Acteonellidae), [nerineoidea](/source/Nerineoidea), [chaetetids](/source/Chaetetidae), 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](/source/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](/source/Paussac-et-Saint-Vivien) and near Toulon ([Périgueux](/source/P%C3%A9rigueux)) 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](/source/Marl). Enclosed fossils are again rudists (*Praeradiolites praesinuatus*, *[Praeradiolites praecoquandi](/source/Praeradiolites_praecoquandi)*, *Vaccinites petrocoriensis*, and *Hippurites requieni*), and chaetetids ([sponges](/source/Sponge)). The benthic microfauna is quite prominent and contains the foramnifera groups [Rotaliida](/source/Rotaliida), [Discorbidae](/source/Discorbidae), and Textulariida with the species *[Valvulammina picardi](/source/Valvulammina_picardi)* or *[Arenobulimina](/source/Arenobulimina)* sp. as well as the [ostracod](/source/Ostracod) genera *[Bairdia](/source/Bairdia)*, *[Cythereis](/source/Cythereis)*, and *[Pterygocythereis](/source/Pterygocythereis)*.

## Depositional environment

After the generally [transgressive](/source/Transgression_(geology)) Ligerian (in comparison with its underlying [Cenomanian](/source/Cenomanian)) dominated by open marine deposits, the Angoumian clearly shows a [regressive](/source/Marine_regression) 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) [1] – 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](/source/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](/source/Massif_Central_(geology)). 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 [synclines](/source/Syncline) 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](/source/Champeaux-et-la-Chapelle-Pommier) it dips 5° to the SW. Only near [Beaussac](/source/Beaussac) a greater dip (of about 30°) can be found caused by local faulting. Nearby in the Aucors quarry, a [slump](/source/Slump_(geology)) overlies the rudist reef, followed by a [grain flow](/source/Grain_flow). These synsedimentary structures point to a tectonically disturbed setting possibly related to the rising [Mareuil Anticline](/source/Mareuil_Anticline). Small [strike-slip](/source/Fault_(geology)#Strike-slip_faults) 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](/source/Slickolite) (a special kind of [stylolite](/source/Stylolite)), which indicate the direction of [pressure solution](/source/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.

## 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](/source/Before_present).

## References

1. Clarke LJ & Jenkins HC. (1999). New oxygen evidence for long-term Cretaceous climatic change in the Southern Hemisphere. *Geology*, **27**:699–702.

- 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. (in French)
- 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. (in French)
- Platel JP, et al. (1989). *Carte géologique de la France à 1/50 000*. Feuille Périgueux (Ouest). BRGM. (in French)

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Adapted from the Wikipedia article [Angoumian](https://en.wikipedia.org/wiki/Angoumian) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Angoumian?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.