Purbeckian
Encyclopedia
The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphic
group
(a sequence of rock strata) in south-east England
. The name is derived from the district known as the Isle of Purbeck
in Dorset
where the strata
are exposed in the cliffs west of Swanage
.
The Purbeck Group is famous for its fossils of reptiles and early mammals.
Rocks of this age have in the past been called the Purbeckian stage by European geologists. The Purbeckian corresponds with the Tithonian
to Berriasian
stages of the internationally used geologic timescale.
, Garsington
, Brill
and Aylesbury
. They also outcrop in a number of small inliers in West Sussex where they have been mined as a source of gypsum. In Lincolnshire
they are represented in part by the Spilsby Sands and in Yorkshire
by portions of the Speeton Clay
.
s, shale
s and marl
s with marly
, tufaceous and shelly limestone
s and occasional oolitic
and sandy strata. Nodules
of chert
are present in some of the limestones.
The thickness of the formation in Wiltshire is 80 to 90 ft (27.4 m), but in Dorset it reaches nearly 400 ft (121.9 m). In most places the Purbeck Group rests conformably upon the Portland Group and it is conformably overlaid by the Wealden Group
; but there are in some districts distinct indications that the Portland Group was uplifted and worn to some extent prior to the deposition of the Purbeck Group.
on account of the similarity of its fresh-water faunas; but the marine fossils, including the fishes, ally the Purbeck more closely with the Upper Jurassic rocks of other parts, and it may be regarded as the equivalent of the upper Volgian of Russia
. Contemporaneous rocks are also present in the neighborhood of Boulogne-sur-Mer
, where they are characterized by thin limestones with Cyrena and gypsiferous
marls. These French outcrops occur, just like those in England, in the core of the Weald-Artois anticline
. Purbeckian aged deposits occur even further south in the Charente
. In north-west Germany
three subdivisions are recognized in strata of the same age: in descending order Purbeck Kalk, Serpulit and Münder Mergel.
and Unjo-bed, in the lower part. The Middle division (50 ft (15.2 m)), mainly thin limestones with shaly partings, contains the principal building stones of the Swanage district; near the base of this subdivision there is a 5 in (12.7 cm), bed from which an interesting suite of mammal
ian remains has been obtained; in this portion of the Purbeck Group there are some marine bands. The Lower Purbeck consists of fresh-water and terrestrial deposits, marls, and limestones (the famous Portland limestone) with several fossil soils known as dirt beds. This division is very extensively exposed on the Isle of Portland
, where many of the individual beds are known by distinctive names. The chief building stones of Upwey belong to this part of the Purbeck Group.
has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that of Pensphinctes transilorius of the European continent. The Purbeckian equivalents of Spilsby and Speeton are in the zone of Belemnites lateralis. Other marine fossils are Hemicidaris purbeckensis
and Ostrea distonta
, the latter being abundant in the Cinder bed of the Middle Purbeck. The fresh-water mollusca include Viviparus
(Paludina), Planorbis
, Melanopsis
, Unio
, Cyrena. A large number of insect genera has been found in the Middle and Lower Purbeck Group.
Dinosaur
s (Owenodon
, Echinodon
[known from "Isolated skull elements of at least [three] individuals."]), crocodylians (Goniopholis
, Petrosuchus), Cimoliosaurus, the plesiosaur
s and the chelonian
s (Chelone
, Pleurosternum) are representative reptile
s. The mammals, mostly determined from lower jaws, found in the beds mentioned above include Plagiaulax
, Amblothenium, Stylodon, Dorsetodon
, Triconodon, Spalacothenium and several others. The isopod crustacean
Archeoniscus brodei is very common in the Purbeck of the Vale of Wardour.
The silicified stumps and trunks of cycads and coniferous trees, often surrounded by great masses of calcareous concretion
s (burrs), are very noticeable in the dirt beds of Portland and near Lulworth
. Chara is found in the fresh-water cherts of the Middle Purbeck.
, a grey or greenish limestone full of shells, was formerly extensively employed in cathedrals and churches. Stone tiles or slatts were once used locally for roofing from the Lower Purbeck of Portland, Swanage and Swindon. Gypsum was formerly worked from the Lower Purbeck at Swanage.
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...
group
Group (stratigraphy)
A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are divided into formations and are sometimes themselves grouped into "supergroups"....
(a sequence of rock strata) in south-east England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. The name is derived from the district known as the Isle of Purbeck
Isle of Purbeck
The Isle of Purbeck, not a true island but a peninsula, is in the county of Dorset, England. It is bordered by the English Channel to the south and east, where steep cliffs fall to the sea; and by the marshy lands of the River Frome and Poole Harbour to the north. Its western boundary is less well...
in Dorset
Dorset
Dorset , is a county in South West England on the English Channel coast. The county town is Dorchester which is situated in the south. The Hampshire towns of Bournemouth and Christchurch joined the county with the reorganisation of local government in 1974...
where the strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
are exposed in the cliffs west of Swanage
Swanage
Swanage is a coastal town and civil parish in the south east of Dorset, England. It is situated at the eastern end of the Isle of Purbeck, approximately 10 km south of Poole and 40 km east of Dorchester. The parish has a population of 10,124 . Nearby are Ballard Down and Old Harry Rocks,...
.
The Purbeck Group is famous for its fossils of reptiles and early mammals.
Rocks of this age have in the past been called the Purbeckian stage by European geologists. The Purbeckian corresponds with the Tithonian
Tithonian
In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
to Berriasian
Berriasian
In the geological timescale, the Berriasian is an age or stage of the Early or Lower Creteceous. It is the oldest or lowest subdivision in the entire Cretaceous. It spanned between 145.5 ± 4.0 Ma and 140.2 ± 3.0 Ma...
stages of the internationally used geologic timescale.
Outcrops
The Purbeck Groups outcrops follow the line of the Jurassic outcrop from Dorset, through the Vale of Wardour, SwindonSwindon
Swindon is a large town within the borough of Swindon and ceremonial county of Wiltshire, in South West England. It is midway between Bristol, west and Reading, east. London is east...
, Garsington
Garsington
Garsington is a village and civil parish about southeast of Oxford in Oxfordshire.-Notable Garsington buildings:The earliest part of the Church of England parish church of Saint Mary is the Norman tower, built towards the end of the 12th century. The Gothic Revival architect Joseph Clarke restored...
, Brill
Brill
Brill is a village and civil parish in Aylesbury Vale district in Buckinghamshire, England, close to the boundary with Oxfordshire. It is about north-west of Long Crendon and south-east of Bicester...
and Aylesbury
Aylesbury
Aylesbury is the county town of Buckinghamshire in South East England. However the town also falls into a geographical region known as the South Midlands an area that ecompasses the north of the South East, and the southern extremities of the East Midlands...
. They also outcrop in a number of small inliers in West Sussex where they have been mined as a source of gypsum. In Lincolnshire
Lincolnshire
Lincolnshire is a county in the east of England. It borders Norfolk to the south east, Cambridgeshire to the south, Rutland to the south west, Leicestershire and Nottinghamshire to the west, South Yorkshire to the north west, and the East Riding of Yorkshire to the north. It also borders...
they are represented in part by the Spilsby Sands and in Yorkshire
Yorkshire
Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...
by portions of the Speeton Clay
Speeton Clay
The Speeton Clay is a Mesozoic geologic formation. Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus.-See also:* List of dinosaur-bearing rock formations...
.
Lithology and stratigraphy
The rocks include clayClay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s and marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
s with marly
Marly
-France:* Marly, in the Moselle département* Marly, in the Nord département* Marly-Gomont, in the Aisne département* Marly-la-Ville, in the Val-d'Oise département...
, tufaceous and shelly limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s and occasional oolitic
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
and sandy strata. Nodules
Nodule (geology)
A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock...
of chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
are present in some of the limestones.
The thickness of the formation in Wiltshire is 80 to 90 ft (27.4 m), but in Dorset it reaches nearly 400 ft (121.9 m). In most places the Purbeck Group rests conformably upon the Portland Group and it is conformably overlaid by the Wealden Group
Wealden Group
The Wealden Group is a group in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental facies sedimentary rocks of Valanginian to Barremian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays...
; but there are in some districts distinct indications that the Portland Group was uplifted and worn to some extent prior to the deposition of the Purbeck Group.
Correlations
In the past, many geologists have ranged the Purbeck Group with the overlying Lower Cretaceous Wealden GroupWealden Group
The Wealden Group is a group in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental facies sedimentary rocks of Valanginian to Barremian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays...
on account of the similarity of its fresh-water faunas; but the marine fossils, including the fishes, ally the Purbeck more closely with the Upper Jurassic rocks of other parts, and it may be regarded as the equivalent of the upper Volgian of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. Contemporaneous rocks are also present in the neighborhood of Boulogne-sur-Mer
Boulogne-sur-Mer
-Road:* Metropolitan bus services are operated by the TCRB* Coach services to Calais and Dunkerque* A16 motorway-Rail:* The main railway station is Gare de Boulogne-Ville and located in the south of the city....
, where they are characterized by thin limestones with Cyrena and gypsiferous
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
marls. These French outcrops occur, just like those in England, in the core of the Weald-Artois anticline
Weald-Artois Anticline
The Weald–Artois anticline is a large anticline, a geological structure running between the regions of the Weald in southern England and the Artois in northeastern France. The fold formed during the Alpine orogeny, from the late Oligocene to middle Miocene as an uplifted form of the Weald basin...
. Purbeckian aged deposits occur even further south in the Charente
Charente
Charente is a department in southwestern France, in the Poitou-Charentes region, named after the Charente River, the most important river in the department, and also the river beside which the department's two largest towns, Angoulême and Cognac, are sited.-History:Charente is one of the original...
. In north-west Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
three subdivisions are recognized in strata of the same age: in descending order Purbeck Kalk, Serpulit and Münder Mergel.
Subdivisions
The Purbeck Group in England is divisible into three subdivisions, viz. Upper, Middle and Lower. The Upper Purbeck comprises 50–60 ft (15.2–18.3 m). of fresh-water clays and shales with limestones, the Purbeck marblePurbeck Marble
Purbeck Marble is a fossiliferous limestone quarried in the Isle of Purbeck, a peninsula in south-east Dorset, England.It is one of many kinds of Purbeck Limestone, deposited in the late Jurassic or early Cretaceous periods....
and Unjo-bed, in the lower part. The Middle division (50 ft (15.2 m)), mainly thin limestones with shaly partings, contains the principal building stones of the Swanage district; near the base of this subdivision there is a 5 in (12.7 cm), bed from which an interesting suite of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian remains has been obtained; in this portion of the Purbeck Group there are some marine bands. The Lower Purbeck consists of fresh-water and terrestrial deposits, marls, and limestones (the famous Portland limestone) with several fossil soils known as dirt beds. This division is very extensively exposed on the Isle of Portland
Isle of Portland
The Isle of Portland is a limestone tied island, long by wide, in the English Channel. Portland is south of the resort of Weymouth, forming the southernmost point of the county of Dorset, England. A tombolo over which runs the A354 road connects it to Chesil Beach and the mainland. Portland and...
, where many of the individual beds are known by distinctive names. The chief building stones of Upwey belong to this part of the Purbeck Group.
Fossil content
No zonal fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that of Pensphinctes transilorius of the European continent. The Purbeckian equivalents of Spilsby and Speeton are in the zone of Belemnites lateralis. Other marine fossils are Hemicidaris purbeckensis
Hemicidaris
Hemicidaris is an extinct genus of echinoid that lived from the Middle Jurassic to the Early Cretaceous. Its remains have been found in Africa, Asia, and Europe.-External links:* in the Paleobiology Database...
and Ostrea distonta
Ostrea
Ostrea is a genus of oyster in the family Ostreidae. Evidence of a number of species of this genus in the geological fossil record demonstrates the ancient nature of this genus, and also gives testimony to the species that co-existed with members of the Ostrea genus...
, the latter being abundant in the Cinder bed of the Middle Purbeck. The fresh-water mollusca include Viviparus
Viviparus
Viviparus, common name river snails, is a genus of large, freshwater snails with an operculum, aquatic gastropod mollusks. They are primitive members of the superorder Caenogastropoda.-Distribution:This genus is palaearctic in distribution....
(Paludina), Planorbis
Planorbis
Planorbis is a genus of freshwater air-breathing snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails, or planorbids. All species in this genus have sinistral or left-coiling shells.- Description :...
, Melanopsis
Melanopsis
Melanopsis is a genus of freshwater snails with a gill and an operculum, aquatic gastropod mollusks in the family Melanopsidae. The genus first appeared in the Cretaceous.Melanopsis is the type genus of the family Melanopsidae.-Distribution:...
, Unio
Unio
Unio can refer to:* Unio, a genus in the family Unionidae * UNIO Satu Mare, machine building company from Romania* Utdanningsgruppenes Hovedorganisasjon , a national trade union center in Norway....
, Cyrena. A large number of insect genera has been found in the Middle and Lower Purbeck Group.
Dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s (Owenodon
Owenodon
Owenodon is a genus of iguanodontian dinosaur known from a partial lower jaw discovered in Early Cretaceous-age rocks of Durlston Bay, Dorset, United Kingdom. The specimen, NHM R2998, comes from the Purbeck Limestone, dating to the middle Berriasian stage...
, Echinodon
Echinodon
Echinodon meaning "hedgehog tooth" in reference to the spines on its teeth , occasionally known as Saurechinodon, is a genus of small European dinosaur of the early Cretaceous Period , 140 million years ago.-Discovery and species:The type specimen was discovered by Samuel Beckles in the Purbeck...
[known from "Isolated skull elements of at least [three] individuals."]), crocodylians (Goniopholis
Goniopholis
Goniopholis is an extinct genus of crocodyliform that lived in North America, Europe and Asia during the Late Jurassic and Early Cretaceous. Being semi-aquatic it is very similar to modern crocodiles...
, Petrosuchus), Cimoliosaurus, the plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
s and the chelonian
Chelonian
Chelonian has multiple, interrelated meanings:*Green turtle*Chelonia, the superorder uniting turtles, tortoises and terrapins with the "proto-turtle" Australochelys*Chelonians, monsters from Doctor Who...
s (Chelone
Chelone
Chelone may refer to:* Chelone , commonly known as turtleheads* Chelone...
, Pleurosternum) are representative reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s. The mammals, mostly determined from lower jaws, found in the beds mentioned above include Plagiaulax
Plagiaulax
Plagiaulax is a genus of mammal from the Lower Cretaceous of Europe. It was a member of the also extinct order Multituberculata, and shared the world with dinosaurs. It is of the suborder "Plagiaulacida" and family Plagiaulacidae. The genus was named by Hugh Falconer in 1857.Fossil remains of the...
, Amblothenium, Stylodon, Dorsetodon
Dorsetodon
Dorsetodon is an extinct genus of mammal from the Early Cretaceous Purbeck Group of Britain. It is represented by isolated lower molars.-See also:* Prehistoric mammal** List of prehistoric mammals...
, Triconodon, Spalacothenium and several others. The isopod crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
Archeoniscus brodei is very common in the Purbeck of the Vale of Wardour.
The silicified stumps and trunks of cycads and coniferous trees, often surrounded by great masses of calcareous concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
s (burrs), are very noticeable in the dirt beds of Portland and near Lulworth
Lulworth
Lulworth is the popular name for an area on the coast of Dorset, South West England notable for its castle and cove. However there is no actual place or feature called simply "Lulworth", the villages are East and West Lulworth and the coastal feature is Lulworth Cove.See:*East Lulworth *Lulworth...
. Chara is found in the fresh-water cherts of the Middle Purbeck.
Use in construction
The building stones of the Purbeck Group have already been mentioned; the Purbeck or Paludina marbleMarble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...
, a grey or greenish limestone full of shells, was formerly extensively employed in cathedrals and churches. Stone tiles or slatts were once used locally for roofing from the Lower Purbeck of Portland, Swanage and Swindon. Gypsum was formerly worked from the Lower Purbeck at Swanage.