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19-509: The Wester Ross Supergroup consists of several groups that are geographically or structurally isolated from each other. The certainty of the correlation between the groups is variable, with the Torridon, Sleat and Morar groups considered as very likely to be lateral equivalents of each other, while the Iona and Tarskavaig groups and those on Shetland are likely but not proven. The Torridon Group

38-484: A formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of

57-563: Is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup

76-582: Is the main part of what used to be termed the "Torridonian", although that also included the Stoer Group, which is now known to be a completely separate and older sequence. The Sleat Group, which outcrops on the Sleat peninsula on Skye , underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is metamorphosed to greenschist facies and sits within

95-586: 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

114-866: 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

133-539: The "Torridonian" and formed the lower section of the Moine Supergroup . Comparisons between the Torridon Group with areas of lower strain in the Morar Group have found many similarities in lithology, thickness and interpreted depositional environment . This group is now thought to have been deposited as part of the same depositional basin, a foreland basin developed in front of the mountain belt formed by

152-819: The Grenville Orogeny. The Iona Group is exposed on the island of Iona and consists of two units separated by a zone of mylonite . The group has been assigned to the Wester Ross Supergroup based on its structural position beneath the Moine Thrust Zone and the ages of detrital zircons , which are similar to those found in the Sleat Group. Rocks of the Tarskavaig Group lie above the Tarskavaig Thrust on

171-557: The Kishorn Nappe, part of the Caledonian thrust belt , making its exact relationship to the other outcrops difficult to assess. The sequence consists of mainly coarse feldspathic sandstones deposited in a fluvial environment with some less common grey shales , probably deposited in a lacustrine environment. This sequence of variably metamorphosed and deformed sandstones (psammites) was originally considered as separate from

190-454: The Wester Ross Supergroup based on lithological similarities, the ages of the youngest detrital zircons and the timing of later metamorphism. The age of this sequence is constrained by a combination of detrital zircon geochronology and by the dating of later metamorphic events. The youngest detrital zircon and rutile ages are in the range 1070–1000 Ma, providing an upper bound. Metamorphic ages of 950–940 Ma have been calculated for garnets from

209-606: 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. Supergroup (geology) A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic , lithologic or paleontologic features ( facies ) that characterize it. Units must be mappable and distinct from one another, but

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228-479: The contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank,

247-437: The formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group

266-482: The lower part of the Morar Group, confirming that these rocks were affected by the Renlandian Orogeny (960–920 Ma) and providing a lower bound. Together these data give a depositional age range of 1000–960 Ma. The Sleat and Torridon groups have been interpreted as deposited in a rift setting. However the scale of the interpreted river systems that deposited the Torridon Group rocks and the consistency in

285-415: The main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for the unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to

304-473: The mountains formed by the Grenville Orogeny. Group (stratigraphy) 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 . Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying

323-427: The paleocurrent directions suggest that most of the sequence was deposited in a single basin. This is also true for the Morar Group, which has very similar thickness, sedimentary facies and interpreted paleocurrents to the Torridon Group. Combined with the detrital zircons that indicate a dominantly Grenvillian source, this has led to a reinterpretation that the supergroup was deposited within a foreland basin, in front of

342-511: The southwestern end of the Sleat peninsula. The group consists of a mixture of psammites, semi-pelites and pelites , which are deformed and no sedimentological analysis has been carried out, nor have the group been sampled for detrital zircons. From its structural position it has been tentatively correlated with the lower part of the supergroup. The Westing, Sand Voe and Yell Sound groups on Shetland are tentatively correlated with other groups within

361-468: 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)

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