Angoumian

The Angoumien is a geological group of northern Aquitaine basin, which was deposited during the Turoniums. It consists of two formations which are made up of Fossilkalken.

Etymology

The term derives from the French Angoumien Angoumois - a historical region that Angoulême had as its center, the capital of present-day département of Charente.

Geographical Distribution

The Group is exposed in the vicinity of Angoulême, but still extends much further south into the Dordogne into it. Here it is especially prevalent in the Périgord Vert, but is also encountered in the Périgord Blanc.

Stratigraphy

The Angoumien is divided into two formations, in a lower and an upper Angoumien Angoumien. Some authors also expect even add the Ligérien and then refer to this as the basal Angoumien. The Angoumien in the strict sense superimposed concordant the Ligérien and closes with a hard base for the next downstream Coniacian.

Lower Angoumien

(Will be shown on French geological maps as a unit c3b )

The total of 15 to 20 meter thick Lower Angoumien, also known as Angoulême lineup, starting with a 5-6 meter thick, dünnbankigen, microcrystalline lime of white or cream in color. It contains a lot of fine-grained debris and has a benthic microfauna consisting of Milioliden and Textulariiden. In this situation, the first rudists, represented by the taxa sphaerulites patera and biradiolites lumbricalis appear. Also quite rare cephalopods such as romaniceras deveriari or prionotropis fleuriausi were encountered; this indicates relatively open marine depositional environment.

The dünnbankigen layers are then examined by a massive, homogeneous, compact, usually 8 - 10 meters thick superimposed Kalkbarre. This relatively soft limestone of pure white chalk ink has properties and is mainly composed of rudists and their fragments. The cash is in the terrain often geomorphic Härtling (especially in the recrystallized state ) with partly overhanging rock walls.

This generally but relatively soft rock - it may already be cut with a hand saw - was once a very desirable block that has been mined in countless, mostly smaller quarries. With it, for example, a large part of the historic buildings in Angoulême and Périgueux was built. The breakdown is but now come to a virtual standstill.

The cash is subject to some fairly significant thickness changes (between 5 and 12 meters ), it can be a maximum even up to 15 meters are powerful. The fluctuations are due to either tectonic or erosive nature - for example, lies in the Mareuil- anticline the Upper Angoumian unconformably and there engages mainly with erosion gullies in the Lower Angoumien. The Cretaceous limestones consist mainly of Rudistenschutt with little cement. The Rudistenschutt primarily consists of the taxon biradiolites lumbricalis together, which to a lesser extent of the taxa Durania cornupastoris, praeradiolites peroni, radio lites peroni, radio ponsi lites, radio beaumonti lites, radio lites socialis and Hippurites requieni is accompanied. Occasionally, also biostromes with undamaged rudists in living position (mainly the relatively small taxon biradiolites ). For the hanging wall of the previously continuous solid layer changes its character and there are now thinner, quite hard, ocher Kalkarenitlagen in appearance. This meant harder layers, the upper limit for the breakdown and were greeted by the miners as a boiler (French chaudron ) refers. In them occur in addition to the usual fauna in addition to even smaller biostromes of biradiolites angulosus.

The Normalfazies described so far can change relatively rapidly (ie, over a distance of 500 meters ) into a Kalkarenitfazies. The unit is called Pierre de Cheyroux calcarenite came in a littoral Beach rock environment for deposition, it is coarse-grained, poorly cemented, leads bioclasts and is white to yellowish color. The bioclasts are strongly rounded and consist of the remains of echinoderms, rudists, individual corals ( polyps) and Milioliden. The very massive calcarenites occur can indeed occasionally up to 15 meters are mighty to 12. They are partly obliquely layered. Their occurrence is limited to the areas south-east of La Tour- Blanche Anticline and southeast of Perigueux Anticline. They therefore indicate a lifting of the two structures already during the Turoniums.

The Lower Angoumien ends with only a few meters thick expectant, dünnbankigen, yellowish microcrystalline limestones, which are very rich in bioclasts - usually Lamellibranchia and echinoderms. This final situation shows cross-bedding (seen in Brantome ) and hiatuses.

Of importance is another facies, which can be observed in the west of the depositional environment. Here goes the usual Rifffazies into a very hard, fine-grained recrystallized limestone that no longer carries bioclasts. This facies is likely to be very likely linked to disturbances.

Upper Angoumien

( Shown as C3C )

The Upper Angoumien, also known as Bourg -des- Maisons- formation, can be relatively easily in the field recognized by its typical arid dwarf flora with vorwiegendem juniper vegetation. The total thickness varies between 20 and 40 meters. The type locality of the formation is at Bourg -des- Maisons.

The up to 20 m thick bottom layer member consists of gray to beige, rudistenhaltigen, crypto- crystalline limestones. According to the structural classification according to Dunham (1962 ), these are a Wackestone, which may pass into mudstone. In the Wackestone microcrystalline limestones are interposed, which are very rich in fine debris and bioclasts. The lowermost layer member weathered bulbous to prismatic.

The fossil content is dominated again by biostrombildenden rudists. Available different taxa as Durania cornupastoris, Praeradiolites ponsi, Radio Lites praesauvagesi, Radio Lites radiosus, Radio Lites are trigeri, praepetrocoriensis Biradiolites quadratus, Biradiolites angulosus, Vaccinites, Vaccinites petrocoriensis and Hippurites requieni var subpolygonia. The benthic fossils consist of Lamellibranchia, Acteonellen, Nerineen, Chaetetiden ( calcareous sponges ), Milioliden and others. They express relatively quiet, very warm and very little deep deposition conditions. Section at the top of this layer can also switch to member via restless conditions, as is the case in the west of Sedimentationsraumes and near Perigueux. This relatively coarse (grain size> 2 mm) were beige, bioklastenhaltige limestones sediments. They have a very low degree of recrystallization and have some great cross-bedding body.

The second layer member is characterized by strong fluctuations of thickness ( 8-12 meters). In contrast to the lowest layer link leads either micrites with little fine debris or microcrystalline limestones rich in biogenic debris. Your fossil content is the same as in the lowest layer member. The second layer member may also undergo a facies towards Kalkareniten the Beach rock type. Near Paussac and Le Toulon ( suburb of Périgueux ), the calcarenites held since the Lower Angoumien.

The top layer member of the Upper Angoumien closes with 5 meters thick ( and sometimes growing up to 15 meters), gray to yellow, platy marly limestones and marls interposed. It contains rudists praesinuatus as Praeradiolites, Praeradiolites praecoquandi, Vaccinites petrocoriensis and Hippurites requieni and Chaetetiden. The benthic microfauna is fairly well represented, it performs the Ostrakodengattungen Bairdia, Cythereis and Pterygocythereis and foraminifera of Rotaliida (eg Discorbiden ) and Textulariida (eg Valvulammina picardi or Arenobulimina sp.).

At the end of the Upper Angoumien established a hard base.

Deposition conditions

After the transgressive compared to the previous Ligérien Cenomanian (lower Turonian ), which was clearly dominated by open marine conditions, the Angoumien shows, however, very significant regressive tendencies. This fact in combination with the climatic optimum during the Turoniums let the water temperatures on the northern Aquitanian shelf (compared with today's values ​​in the Atlantic ) to about 10 ° C higher values ​​rise - ideal conditions for the expansion of Rudistenriffe in the quiet shelf indoors. A testament to the high temperatures are the Milioliden, the water temperatures of 24 to 30 ° need for their growth.

During the Lower Angoumien ( Angoulême Formation) had established itself on the shelf Interior Kalkschlicklagune a low to moderate energy level. The lagoon was surrounded by numerous Rudistenriffen or enforce their detritus was mainly transported seaward. Starting from Périgueux drew a larger reef to the northwest and thus divided the lagoon into an inner region to the northeast and east of Périgueux and an outer region toward the center of the Aquitaine basin. On the higher energy side of the individual reefs gröberklastischen layer limbs were deposited, whereas cryptocrystalline in the tranquil lagoon and micritic limestones deposed.

Towards the end of the Upper Angoumien ( Bourg -des- Maisons Formation) of the regressive trend was clearly evident, as now began to appear part of the Aquitanian shelf. Responsible for this is likely to have been primarily a raising movement in the Massif Central. The regressive tendency can be clearly traced in the sediments of the late Angoumiens that development was proceeding from clastic sediments stressed about conversion phenomena towards Brekzifizierung and ultimately to hardground formation.

The total thickness of the Angoumien should be around 35 to 60 meters, with the West half of the depositional environment (in the direction Beck Center ) has a much higher rate of accumulation.

Tectonics

The Angoumien forms part of two rather flat, NW-SE and WNW- ESE trending Synklinalzüge, the synclinal of Combiers -Saint -Crépin -de -Richemont to the northeast and the synclinal from Gout - Rossignol - Léguillac in the southwest. The group still lies mostly flat or dive with only 5 ° to the southwest one ( at Champeaux ). In Beaussac it shows something stronger dip ( 30 ° ) caused by local disturbances. In the immediate vicinity of this tilt is the Aucors quarry in which is above the Rudistenriff a slump with about the following mass transport ( grain flow ) to see. These synsedimentary structures reveal localized, restless Sedimentationsverhältnisse that might be associated with the simultaneous movements of the anticline in Mareuil- connection. Tectonic displacements also occurred at the Mareuil- anticline, as can be observed to NNE at its north end a descent of the layers with 35 °. Smaller side shifts with horizontal sense of movement ( preferably the NNW-SSE direction, subordinate to the NW- SE direction ) can be observed in Brantome. Another noteworthy aspect is the presence of so-called Slickolites, a special form of stylolites, indicating the direction of pressure solution in the rock. The compression direction was generally vertically (corresponding to the applied load of the following formations ), but can also have significant deviations thereof and thus inclined to horizontal stress fields give shallow to detect.

Age

Since then, the Lower Angoumien is approximately the upper two-thirds of the Middle Turoniums equivalent to the Upper Angoumien is identical to the Upper Turonian. The Angoumien claimed thus roughly the period 91 bis 89 million years BP.