The Dent Group is a
group
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...
of Upper
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
sedimentary
Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...
and
volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic r ...
s in north-west
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
. It is the lowermost part of the
Windermere Supergroup
The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~, and exposed in northwest England, including the Pennines and correlates along its strike, in the Isle of Man and Ireland, and down-dip in the South ...
, which was deposited in the
foreland basin
A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
formed during the
collision
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...
between
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...
and
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of Wester ...
. It lies
unconformably
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
on the
Borrowdale Volcanic Group
The Borrowdale Volcanic Group is a group of igneous rock formations named after the Borrowdale area of the Lake District, in England. They are Caradocian (late Ordovician) in age (roughly 450 million years old). It is thought that they represent t ...
. This unit was previously known as the Coniston Limestone Group or Coniston Limestone Formation and should not be confused with the significantly younger (uppermost
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
)
Coniston Group
The Coniston Group is a Silurian lithostratigraphic group (a sequence of rock strata) in the southern Lake District and north-west Pennines of northern England. The name is derived from the small town of Coniston in Cumbria. The rocks of the ...
.
Distribution
The Dent Group is exposed in four areas, the southern Lake District as a narrow strip across the whole width of the outcrop, in the Cautley and
Dent
Dent may refer to:
People
* Dent (surname)
* Dent May (active 2007), American musician
* Dent Mowrey (1888–1960), American composer, musician and music teacher
* Dent Oliver (1918–1973), international speedway rider
Places France
* Dent d' ...
inliers, the
Cross Fell
Cross Fell is the highest mountain in the Pennines of Northern England and the highest point in England outside the Lake District. It is located in the North Pennines Area of Outstanding Natural Beauty. It lies within the county of Cumbria and ...
inlier and the
Craven inliers.
Lake District outcrop
In the main Lake District outcrop, the group consists of calcareous
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...
s and
mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s of the basal Kirkley Bank Formation,
micritic limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
s of the Broughton Moor Formation
and dark blue-gray
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
s of the Ashgill Formation.
Locally volcanic formations are developed, including the Yarlside Volcanic Formation at the base of the Kirkley Bank Formation and the High Haume Tuff and Appletreeworth Formations at the base of the Ashgill Formations.
Cautley and Dent inliers
In the Cautley and Dent inliers, rocks of the Dent Group occur in two fault bounded strips next to the
Dent Fault. In these inliers, calcareous mudstones of the Cautley Mudstone Formation
are overlain by the Ashgill Formation. The Cautley Volcanic Member is a unit of
volcaniclastic rocks, within the Cautley Mudstone Formation.
Cross Fell inlier
In the Cross Fell inlier, the basal Dufton Shale Formation
is followed by the Swindale Limestone Formation
and the Ashgill Formation. In the Keisley area, the uppermost part of the Dent Group is represented by the highly fossiliferous Keisley Limestone Formation, exposed in the core of an
anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
.
Craven inliers
The Craven inliers are found to the north of the
North Craven Fault. In these inliers, calcareous siltstones and muddy limestones of the Norber Formation
are overlain by siltstones and mudstones of the Sowerthwaite Formation.
The Dam House Bridge Tuff Member forms the basal part of the Sowerthwaite Formation.
Tectonic setting
The early
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
geology of northern England records the convergence and eventual collision of the Avalonia microcontinent with the continent of Laurentia. The underlying
Borrowdale Volcanic Group
The Borrowdale Volcanic Group is a group of igneous rock formations named after the Borrowdale area of the Lake District, in England. They are Caradocian (late Ordovician) in age (roughly 450 million years old). It is thought that they represent t ...
formed in an
island arc
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
, caused by the southerly
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
of the
crust of the now vanished
Iapetus Ocean
The Iapetus Ocean (; ) was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleoco ...
. At the end of active arc volcanism, the area began to subside, allowing deposition of the shallow marine Dent Group. The final closure of Iapetus occurred during the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
Acadian orogeny, with the resulting
continental collision
In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains prod ...
causing deformation of the Ordovician rocks, including
folding
Fold, folding or foldable may refer to:
Arts, entertainment, and media
* ''Fold'' (album), the debut release by Australian rock band Epicure
* Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot
*Abov ...
,
thrusting
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...
and
cleavage development.
Depositional environment
The Dent Group was deposited in a shallow water environment.
See also
*
List of fossiliferous stratigraphic units in England
See also
*Lists of fossiliferous stratigraphic units in Europe
* Lists of fossiliferous stratigraphic units in the United Kingdom
References
*
{{DEFAULTSORT:Fossiliferous stratigraphic units in England
England
England is a country ...
References
* {{cite web, title= Fossilworks: Gateway to the Paleobiology Database, author= ((Various Contributors to the Paleobiology Database)), url= http://www.fossilworks.org/cgi-bin/bridge.pl?action=home, access-date= 17 December 2021
Geological groups of the United Kingdom
Geologic formations of England
Ordovician System of Europe
Ordovician England
Geology of Cumbria
Geology of the Pennines