Angoumies
The Angoumien is a geological group of the northern Aquitaine Basin that was deposited during the Turonian . It consists of two formations made up of fossil limestone .
etymology
The term Angoumian derives from the French Angoumois from - a historical region that Angoulême had as its center, Prefecture city of today's department Charente .
Geographical distribution
The group is open in the area around Angoulême, but extends much further south into the Dordogne department . Here it occurs particularly in the Périgord vert , but is also found in the Périgord blanc .
stratigraphy
The Angoumien is divided into two formations, a Lower Angoumien and an Upper Angoumien . Some authors also add the Ligérien and refer to it as Basales Angoumien . The Angoumian in the narrower sense superimposed on the concordant Ligérien and closes with a hard base ( English hard ground ) compared to the subsequent Coniacian .
Lower Angoumien
(Is shown on French geological maps as unit c3b )
The 15 to 20 meters thick Lower Angoumia, also known as the Angoulême Formation , begins with a 5 to 6 meter thick, thin-banked, microcrystalline lime of white or creamy color. It contains a lot of fine-grained debris and has a benthic microfauna consisting of miliolids and textulariids . In this situation, the first to appear rudists , represented by the taxa Sphaerulites patera and Biradiolites lumbricalis . Very rare cephalopods such as Romaniceras deveriari or Prionotropis fleuriausi were also found; this suggests relatively open marine deposit conditions.
The thin-banked layers are then overlaid by a massive, homogeneous, compact, mostly 8-10 meter thick limestone bar. This relatively soft lime of pure white color has chalk properties and consists mainly of rudists and their fragments. In the terrain, the bar often forms a geomorphological hardy (especially in the recrystallized state) with partly overhanging rock walls.
This generally relatively soft rock - it can already be cut with a hand saw - was once a very popular building block that was quarried in countless, mostly smaller, quarries. With him z. B. built a large part of the historical buildings in Angoulême and Périgueux . The dismantling has now largely come to a standstill.
The bar is sometimes subject to significant fluctuations in thickness (between 5 and 12 meters), it can even be up to 15 meters thick. The fluctuations are either tectonically caused or erosive in nature - for example, in the Mareuil anticline, the Upper Angoumia lies discordantly and there, in some cases, reaches into the Lower Angoumia with erosion channels. The chalky limestone consists mainly of rudist rubble with only a little cement. The rudist rubble consists mainly of the taxon Biradiolites lumbricalis , which is accompanied to a lesser extent by the taxa Durania cornupastoris , Praeradiolites peroni , Radiolites peroni , Radiolites ponsi , Radiolites beaumonti , Radiolites socialis and Hippurites requieni . Occasionally also find biostromes with undamaged rudists in living position (mainly the relatively small taxon Biradiolites ). On the hanging wall , the previously solid layer changes its character and thinner, rather hard, ocher-colored calcarenite layers now appear. These harder locations meant the upper limit for mining and were called the kettle (French chaudron ) by the miners . In addition to the usual fauna, there are also smaller biostromes of Biradiolites angulosus .
The normal facies described so far can change relatively quickly (ie over a distance of 500 meters) into a calcarenite facies. The calcarenite known as Pierre de Cheyroux was deposited in a littoral beach rock environment, it is coarse-grained, only weakly cemented, contains bioclasts and is white to yellow in color. The bioclasts are strongly rounded and consist of the remains of echinoderms , rudists, individual corals (polyps) and miliolids. The very massive calcarenites can be up to 12, sometimes up to 15 meters thick. Some of them are slanted . Their occurrence is limited to the areas southeast of the La Tour Blanche anticline and southeast of the Périgueux anticline . They point to an uplift of the two structures already during the Turonian.
The lower Angoumien ends with thin-banked, yellowish microcrystalline limestones that are only a few meters thick and are very rich in bioclasts - usually lamellibranchs and echinoderms. This final layer shows oblique stratification (seen in Brantôme ) and layer gaps .
Another facies change that can be observed in the west of the deposit area is significant. Here the usual reef facies merge into a very hard, fine-grained recrystallized limestone that no longer carries any bioclasts. This change of facies is likely to be linked to disturbances .
Upper Angoumien
(Shown as c3c )
The Upper Angoumien, also known as the Bourg-des-Maisons-Formation , can be identified relatively easily in the terrain by its typically arid dwarf flora with predominantly juniper vegetation . The total thickness varies between 20 and 40 meters. The type locality of the formation is at Bourg-des-Maisons .
The bottom layer element, which is up to 20 meters thick, consists of gray to beige, rudist-containing, cryptocrystalline limestone . According to the structural classification according to Dunham (1962), this is a Wackestone that can merge into Mudstone . Microcrystalline limestones, which are very rich in fine debris and bioclasts, are interposed in the Wackestone. The lowest layer member weathers in a bulbous to prismatic manner.
The fossil content is once again dominated by rudists that generate bio-currents. There are various taxa present, such as Durania cornupastoris , Praeradiolites ponsi , Radiolites praesauvagesi , Radiolites radiosus , Radiolites trigeri , Biradiolites quadratus , Biradiolites angulosus , Vaccinites praepetrocoriensis , Vaccinites petrocoriensonia and Hippurites requieni var. The benthic fossils consist of Lamellibranchiern, Acteonellen, Nerineen, Chaetetiden (calcareous sponges), Miliolids and others. They indicate relatively calm, very warm and only slightly deep deposit conditions. In the uppermost section, this layer segment can also switch to more restless conditions, as is the case in the west of the sedimentation area and near Périgueux. Relatively coarse (grain size> 2 millimeters), beige-colored limestone containing bioclasts was sedimented here. They have a very low degree of recrystallization and some of them have large oblique layers.
The second layer segment is characterized by strong fluctuations in thickness (8 - 12 meters). In contrast to the lowest layer member, it has either micrite with only a little fine debris or microcrystalline limestone rich in biogenic debris. Their fossil content is the same as in the lowest layer member. The second layer segment can also change facies to calcarenites of the beach rock type. In the vicinity of Paussac and Le Toulon (suburb of Périgueux) the Kalkarenites had been around since the Lower Angoumia.
The uppermost layer of the upper member includes Angoumian 5 meters thick (and sometimes up to 15 meters growing), gray to yellow, platy marly limestone and interposed marls . It includes rudists such as Praeradiolites praesinuatus , Praeradiolites praecoquandi , Vaccinites petrocoriensis and Hippurites requieni, as well as Chaetetiden. The benthic microfauna is fairly well represented, it carries the ostracode genera Bairdia , Cythereis and Pterygocythereis as well as foraminifera of the Rotaliida (e.g. Discorbidae) and Textulariida (e.g. Valvulammina picardi or Arenobulimina sp. ).
At the end of the Upper Angoumien a hard ground is established.
Deposit conditions
After the Ligérien (lower Turonian), which was transgressive in comparison to the previous Cenomanium and which was clearly dominated by open marine conditions, the Angoumien, on the other hand, shows very clear regressive tendencies. This fact, in conjunction with the optimum climatic conditions during the Turonian period, caused the water temperatures on the northern Aquitaine Shelf to rise to around 10 ° C (compared to current values in the Atlantic) - ideal prerequisites for the expansion of the rudist reefs in the calm shelf area. Proof of the high temperatures are the miliolids, which need water temperatures of 24 to 30 ° C for their growth.
During the Lower Angoulême Formation (Angoulême Formation), a limestone mud lagoon with a low to moderate energy level had established itself on the shelf interior. The lagoon was framed or penetrated by numerous rudist reefs, the debris of which was mainly transported seaward. Starting from Périgueux, a larger reef moved to the northwest and thus divided the lagoon into an inner area northeast and east of Périgueux and an outer area towards the center of the Aquitaine Basin. On the more energetic side of the individual reefs, the coarser clastic layers were deposited, whereas in the calm lagoon cryptocrystalline and micritic limestone settled.
Towards the end of the Upper Angoumia (Bourg-des-Maisons-Formation) the regressive trend was clearly noticeable, as parts of the Aquitaine Shelf began to surface. An uplift movement in the Massif Central was primarily responsible for this . The regressive tendency can be clearly traced in the sediments of the outgoing Angoumiens, the development proceeded from clastically emphasized sediments over transformation phenomena up to breccification and finally to hard ground formation.
The total thickness of the Angoumien is around 35 to 60 meters, with the south-western half of the deposit area (towards the center of the basin) having a much greater thickness.
tectonics
The Angoumien forms part of two rather flat, NW-SE and WNW-ESE trending syncline , the syncline of Combiers-Saint-Crépin-de-Richemont in the northeast and the syncline of Gout-Rossignol-Léguillac in the southwest. The group is still mostly flat or dips with only 5 ° to the southwest (near Champeaux ). At Beaussac it shows a somewhat stronger dip (by 30 °), caused by local disturbances . In the immediate vicinity of the tilt of the Aucors quarry is located, in the above Rudistenriff a landslide (Engl. Slump ) with about following mass transport ( Grain Flow ) can be seen. These synsedimentary structures reveal local, restless sedimentation conditions that can possibly be related to the simultaneous movements at the Mareuil anticline . Tectonically induced displacements also occurred at the Mareuil anticline, so at the northern end of the latter, the strata were submerged at 35 ° to NNE. Smaller lateral shifts with a horizontal sense of movement (preferred is the NNW-SSE direction, subordinate the NW-SE direction) can be observed in Brantôme. It is also worth mentioning the presence of so-called slickolites - a special form of stylolite , which shows the direction of pressure release in the rock. The direction of pressure was generally vertical (corresponding to the load on the following formations), but it can also show significant deviations and reveal flat to horizontal stress fields.
Age
In terms of time, the Lower Angoumia is roughly equivalent to the upper two thirds of the Middle Turonium, the Upper Angoumia is identical to the Upper Turonium. The Angoumien therefore claims about the period 91 to 89 million years BP .
literature
- J. - P. Platel: Un modèle d'organization des biotopes à Rudistes: l'Angoumian de l'Aquitaine septentrionale . In: Bull. Soc. Linn. Bordeaux, IV, n ° 1 1974, p. 3-13 .
- J. - P. Platel: Les formations à Rudistes du Turonien moyen et supérieur de la plate-forme nord-aquitaine . In: Mém. Mus. nat. hist. nat., série c, tome XLIX . 1982, p. 91-96 .
- J. - P. Platel and others: Périgueux (Ouest) . In: Carte géologique de la France at 1/50 000 . BRGM, 1989.
- B. Bourgueuil, P. Moreau et al .: Angoulême XVII-32 . In: Carte géologique de la France at 1/50 000 . BRGM.