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 .
17-522: The Durness Group is a geological group , a carbonate -dominated stratigraphic unit that forms a c. 170 km long narrow and discontinuous outcrop belt along the north-western coast of Scotland from the Isle of Skye and Loch Kishorn (on the mainland) in the south to Durness and Loch Eriboll in the north. It forms the youngest part of the foreland basin of the Moine Thrust Belt in
34-400: A bed, a flow, is a discrete extrusive volcanic stratum or body distinguishable by texture, composition, or other objective criteria. As in case of a bed, a flow should only be designated and named as a formal lithostratigraphic unit when it is distinctive, widespread, and useful for stratigraphic correlation. A band is a thin stratum that is distinguishable by a distinctive lithology or color and
51-666: A depositional sequence of major marine transgression and regression of the Laurentian craton at the Scottish Laurentian Margin Megasequence . The sedimentary rocks were deposited in a range of shallow marine environments. Evaporite facies deposition was restricted to the intertidal zone as is a rare constituent of successions . Fenestral carbonates, oolites , evaporate pseudomorphs , stromatolites and finely-laminated dolomites with desiccation cracks and teepees were deposited in
68-541: A stratum underlying another stratum. Typically, a stratum is generally one of a number of parallel layers that lie one upon another to form enormous thicknesses of strata. The bedding surfaces (bedding planes) that separate strata represent episodic breaks in deposition associated either with periodic erosion , cessation of deposition, or some combination of the two. Stacked together with other strata, individual stratum can form composite stratigraphic units that can extend over hundreds of thousands of square kilometers of
85-742: A supratidal to shallow subtidal setting. In more open marine conditions during maximum flooding events, the Sailmhor and Croisaphuill formations of the Durness Group saw burrowed carbonate mud dominated deposition. The thick dolostone and limestone beds succession of the Ghrudaidh Formation, the lowest part of the Durness Group, was formed in a range of supratidal, peritidal and shallow marine carbonate platform deposits. Quartz sand grains in its basal part disappear higher up. This has been attributed to an abrupt transgression causing
102-574: Is separated by visible surfaces known as either bedding surfaces or bedding planes . Prior to the publication of the International Stratigraphic Guide, older publications have defined a stratum as being either equivalent to a single bed or composed of a number of beds; as a layer greater than 1 cm in thickness and constituting a part of a bed; or a general term that includes both bed and lamina . Related terms are substrate and substratum (pl. substrata ),
119-499: The Earth 's surface. Individual stratum can cover similarly large areas. Strata are typically seen as bands of different colored or differently structured material exposed in cliffs , road cuts, quarries , and river banks. Individual bands may vary in thickness from a few millimeters to several meters or more. A band may represent a specific mode of deposition : river silt , beach sand , coal swamp , sand dune , lava bed, etc. In
136-586: 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
153-866: 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
170-559: The Scottish Northwest Highlands and is incorporated into this belt's lowermost thrust sheets , where it is often affected by thrust faulting . It overlies the Ardvreck Group . The formations of north-western Scotland were once part of a fragment of a then continuous south-eastern margin of Laurentia stretching from western Newfoundland to north-eastern Greenland , which at the time (and prior to
187-486: 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. Stratum In geology and related fields, a stratum ( pl. : strata ) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it
SECTION 10
#1732780430557204-710: The group has the youngest deposition in the Scottish sector of the Laurentian margin prior to the Grampian Orogeny. The age of the youngest strata is consistent with the dates for igneous intrusions associated with the Grampian orogeny (475–465 Ma) in Scotland whose peak metamorphism is dated at 473–465 Ma. Group (stratigraphy) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying
221-622: The group were formed by carbonate deposition on the southeast-facing continental margin that was subsiding passively. This deposition in Scotland formed part of the Great American Carbonate Bank, a continent-wide area of carbonate deposition created by Ordovician concurrent sedimentation across almost the whole of Laurentia. Deposition in the Ardvreck Group was predominantly marine sandstones . The Durness and Ardvreck groups Cambrian and Ordovician rocks constitute
238-653: The hinterland sediments to become far distant. The Ordovician units of the group are found only on Skye, at Stronchrubie in the Assynt area (the Early Ordovician Sailmhor Formation) and around Durness , which is the type area and has the most complete and most structurally intact Ordovician unit north of the Highland Boundary Fault . It has diagenetic overprint with extensive recrystallisation . The Durine Formation of
255-749: The opening of the Atlantic Ocean ) were contiguous to the west and to the north respectively. The Durness Group narrow outcrop also forms part of the foreland basin of the Grampian orogeny . Its age ranges from the Cambrian to the Middle Ordovician (mid- Dapingian ). The group is composed of pale grey, buff and dark grey dolostones with intervals of dolomitic limestone (an intermediate between dolomite rock and calcite limestone), limestone and subordinate chert . The Cambrian parts of
272-468: 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)
289-586: The study of rock and sediment strata, geologists have recognized a number of different types of strata, including bed , flow , band , and key bed . A bed is a single stratum that is lithologically distinguishable from other layers above and below it. In the classification hierarchy of sedimentary lithostratigraphic units, a bed is the smallest formal unit. However, only beds that are distinctive enough to be useful for stratigraphic correlation and geologic mapping are customarily given formal names and considered formal lithostratigraphic units. The volcanic equivalent of
#556443