In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into supergroups .
20-453: The Ribband Group is a geologic group in south-eastern Ireland . It is the most extensive stratigraphic unit in this part of Ireland. It underlies much of County Wexford . It overlies the Bray Group and Cahore Group. It is made up of medium to dark grey laminated greywacke siltstones and mudstones with occasional green beds . Turbidite structures are locally prominent and the unit
40-559: A Middle Triassic (240 to 230 million years) age and forms the middle part of the three-part Germanic Trias (that gives the Triassic its name) lying above the older Buntsandstein and below the younger Keuper . The Muschelkalk (" mussel - chalk ") consists of a sequence of limestone and dolomite beds . In the past, the time span in which the Muschelkalk was deposited could also be called "Muschelkalk". In modern stratigraphy , however,
60-473: A closer affinity with the Triassic Tethys realm . The Muschelkalk can be up to 100 meters thick; it is divisible into three subdivisions, of which the upper and lower are pale thin-bedded limestones with greenish-grey marls , the middle group being composed of gypsiferous and saliniferous marls with dolomite. Stylolites are common in all the Muschelkalk limestones. The lithostratigraphic status of
80-664: Is a classification by Brück & Molyneux (2011). The Ballymadder Shear Zone (just east of Hook Head on the coast of south County Wexford ), separates a to some extent different Cambrian succession immediately to the east to the one to its west. To its west the Clammers Point Unit (in the Bannow area) exposes a coastal section comprising Cahore Group and Ribband Group sediments. Group (stratigraphy) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying
100-742: Is conformably overlain by the Ribband Group; the Duncannon Group (a highly faulted, predominantly volcanic, platform sequence) which unconformably overlies the Ribband Group except in the west; the Kilcullen Group in the Comeragh Mountains , which is composed of sand-dominant turbidites (lower Ordovician to at least the Llandovery Epoch of the Silurian ) which are conformably underlain by
120-528: Is divided into six formations: Jena Formation , Rüdersdorf Formation , Udelfangen Formation , Freudenstadt Formation and Eschenbach Formation . The Lower Muschelkalk is sometimes called Wellenkalk , German : Welle the "wave" chalk, so called on account of the buckled wavy character the bedding has received. In the Saarland and Alsace and northern Eifel , the Lower Muschelkalk has more sandy beds,
140-604: Is dominantly a distal turbidite succession. Intercalated volcanic rocks are locally abundant. The ages of the rocks of the group range from the Cambrian to the Ordovician, Middle Cambrian - Llanvirn (Middle Ordovician). The Ribband group is one of the four Early Palaeozoic stratigraphic groups in SE Ireland. The others are: the Bray group (made up of laterally-derived flysch ) which
160-417: Is often composed entirely of the fragmentary stems of the crinoid Encrinus liliiformis ; higher up come beds with a series of ammonites , Ceratites compressus , Ceratites nodosus , and Ceratites semipartitus in ascending order. In Swabia and Franconia the highest beds are platy dolomites with Tringonodus sandergensis and the crustacean Bairdia . In addition to the fossils mentioned above,
180-513: The Muschelsandstein ., "mussel sandstone" The Middle Muschelkalk or Anbydnite Group consists mainly of evaporites (gypsum, anhydrite and halite ) and is divided into three formations: Karlstadt Formation , Heilbronn Formation and Diemel Formation . The sedimentary facies at the margins of the Germanic Basin is different and these deposits are classified as a separate formation,
200-571: The Glenwoody Formation , other strata (particularly in the lower part of the group) remain undivided into formations. Some well known groups of northwestern Europe have in the past also been used as units for chronostratigraphy and geochronology . These are the Rotliegend and Zechstein (both of Permian age); Buntsandstein , Muschelkalk , and Keuper ( Triassic in age); Lias , Dogger , and Malm ( Jurassic in age) groups. Because of
220-676: The Grafenwöhr Formation , which continues into the Upper Muschelkalk. In the Middle Muschelkalk, weathering can form characteristic cellular dolomite ( Zellendolomit ). The Upper Muschelkalk ( Hauptmuschelkalk ) is similar to the Lower Muschelkalk and consists of regular beds of shelly limestone, marl and dolomite. It is divided into six formations: Trochitenkalk , Meißner Formation , Irrel Formation , Gilsdorf Formation and Warburg Formation . The lower portion or Trochitenkalk
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#1732848640964240-771: The Low Countries , the North Sea and parts of Silesia , Poland and Denmark . Outcrops are found in Thuringia , the Harz , Franconia , Hesse , Swabia , and the Saarland and in Alsace . The Muschelkalk was deposited in a land-locked sea which, in the earlier part of its existence, had only imperfect communications with the more open waters of the Tethys Ocean to the south. The basin in which
260-820: The Wingate Sandstone , the Moenave Formation , the Kayenta Formation , and the Navajo Sandstone . Each of the formations can be distinguished from its neighbor by its lithology , but all were deposited in the same vast erg . Not all these formations are present in all areas where the Glen Canyon Group is present. Another example of a group is the Vadito Group of northern New Mexico . Although many of its strata have been divided into formations, such as
280-487: The Muschelkalk differs regionally. In Germany it is considered a group , in the Netherlands a formation . The German Muschelkalk Group is subdivided into three subgroups: Upper, Middle and Lower Muschelkalk. The Lower Muschelkalk consists mainly of limestone, calcareous marls and clayey marls. Some beds are composed of porous cellular limestone, the so-called Schaumkalk , there are also oolite beds. The Lower Muschelkalk
300-621: The Muschelkalk was deposited is called the Germanic Basin . Sometimes stratigraphic units with the same age from the Alps , southern Europe and even Asia are called Muschelkalk too. Of course these rocks have little history in common with the central European Muschelkalk except for similarities in fossil content. Closer at hand, the Alpine Muschelkalk differs in many respects from that of Central Europe, and in its characteristic fossil fauna has
320-689: The Ribband and Duncannon Groups. The sediments of the Ribband group pass upward into the Kilcullen Group turbidites. This reflects a continuous period of flysch sedimentation . Although most of the Group is unfossiliferous (without fossils), locally sparse graptolite faunas and acritarchs have been found. Their dating range from the Drumian stage of the Cambrian to the Early-Mid Ordovician. The following
340-491: The confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks, a group must not be defined by fossil taxonomy. Muschelkalk The Muschelkalk (German for "shell-bearing limestone"; French : calcaire coquillier ) is a sequence of sedimentary rock strata (a lithostratigraphic unit ) in the geology of central and western Europe. It has
360-526: The following are Muschelkalk forms: Coenothyris vulgaris, Mentzelia mentzeli and Spiriferina hirsuta, Myophoria vulgaris, Rhynchotites hirundo, Ceratites munsteri, Ptychites studeri, Balatonites balatonicus, Aspidura scutellata, Daonella lommeli , and in the Alpine region several rock-forming algae (for example, Baciryllium, Gyroporella, and Diplopora ). The salt beds are worked at Halle (Saale) , Bad Friedrichshall , Heilbronn , Szczecin and Erfurt . It
380-504: The name only applies to the layers of rock. The name Muschelkalk was first used by German geologist Georg Christian Füchsel (1722-1773). In 1834, Friedrich August von Alberti included it into the Triassic system . The name indicates a characteristic feature of the unit, namely the frequent occurrence of lenticular banks composed of fossil shells. The Muschelkalk is restricted to the subsurface in most of Germany and adjacent regions as
400-460: The stratigraphy of large regions. Geologists exploring a new area have sometimes defined groups when they believe the strata within the groups can be divided into formations during subsequent investigations of the area. It is possible for only some of the strata making up a group to be divided into formations. An example of a group is the Glen Canyon Group , which includes (in ascending order)
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