| Purbeck Group | |
|---|---|
| Stratigraphic range:Tithonian–Berriasian | |
 Strata of the Purbeck Group in Durlston Bay, Dorset | |
| Type | Group |
| Sub-units | Lulworth Formation,Durlston Formation andHaddenham Formation |
| Underlies | Wealden Group |
| Overlies | Portland Group |
| Thickness | 45 to 120 m in South Dorset, 77 to 186 m in the Weald |
| Lithology | |
| Primary | Mudstone,Limestone |
| Other | evaporites,chert |
| Location | |
| Region | Europe |
| Country |  UK |
| Extent | SouthernEngland |
| Type section | |
| Named for | Isle of Purbeck |
| Location | Durlston Bay cliffs |
 Exposures of the Purbeck Group in Dorset shown in dark blue | |
ThePurbeck Group is anUpper Jurassic toLower Cretaceouslithostratigraphicgroup (a sequence ofrock strata) in south-eastEngland. The name is derived from the district known as theIsle of Purbeck inDorset where thestrata are exposed in the cliffs west ofSwanage.
The Purbeck Group is famous for its fossils ofreptiles and earlymammals. This sequence of rocks has gone by various names in the past including amongst others thePurbeck Beds,Purbeck Formation,Purbeck Limestone Formation andPurbeck Stone.[1]
Rocks of this age have in the past been called thePurbeckianstage by European geologists. The Purbeckian corresponds with theTithonian toBerriasian stages of the internationally usedgeologic timescale.
The stratigraphic organisation of the Purbeck Group has varied significantly over time since it was first labelled in 1816 for a section of limestone on the Isle of Purbeck. The labels Purbeck Limestone, Purbeck stone, Purbeck series and Purbeck strata were all used by Webster in 1816, with Purbeck Beds first used by SirWilliam Buckland in 1818. Works from 136 to 1884 provided some descriptions and labels of the locally-known beds, and the divisions of the Purbeck into upper, middle, and lower units. The label of Lulworth Beds was given to the strata below the Cinder Bed in 1963 by Casey, who called the remaining section the Durlston Beds. These were elevated to the status ofLulworth andDurlston Formations by Thompson in 1975, who introduced the Purbeck Group as their parent unit. Cope and colleagues considered the Purbeck as a formation in 1980, naming it the Purbeck Limestone Formation, while El Shahat and West in 1993 called it the Purbeck Formation. Ensom in 1985 and Clements in 1993 detailed the stratigraphy of the Worbarrow Tout and Durlston Bay sections respectively, with the former considering there to be 12 members in the Purbeck Limestone Formation, while the latter had an additional two members within the Lulworth and Durlston Formations of the Purbeck Limestone Group. The Members of Ensom and Clements were lowered to the status of Beds by Westhead and Mather in 1996 as they were of limited applicability outside of their originally-observed sections, with them only recognizing five members of the Lulworth and Durlston Formations, each encompassing multiple of the Beds of earlier studies.[2]
TheBritish Geological Survey recognises three formations and five members of the Purbeck Group, exposed differently across the span of southern England. In theWessex Basin (or Vectian Basin) area ofDorset where the Purbeck was first recognized, the younger Durlston Formation can be divided into the Peveril Point Member and Stair Hole Member, while the older Lulworth Formation is divided into the Worbarrow Tout Member, Ridgeway Member, and Mupe Member. Within theWeald Basin, the Lulworth and Durlston Formations can be recognized, but their subdivisions are undifferentiated, while outside the Dorset region the oldest and lowermost Purbeck Limestone below the Lulworth inWiltshire,Oxfordshire andBuckinghamshire has been given its own status as theHaddenham Formation.[1]
InEast Sussex the Purbeck Group is formally subdivided into the Blues and Greys Limestones members[3] The sequence was traditionally divided into three, though along different lines viz. Upper, Middle and Lower. The Upper Purbeck comprises 50 to 60 ft (15 to 18 m). of fresh-water clays and shales with limestones, thePurbeck marble andUnio-bed, in the lower part. The Middle division (50 ft (15 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 (13 cm), bed from which an interesting suite ofmammalian 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 famousPortland limestone) with several fossil soils known asdirt beds. This division is very extensively exposed on theIsle 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.
| Purbeck Group | Durlston Formation | Peveril Point Member | "Upper Purbeck" | Upper "Cypris" Clays and Shales Bed |
| Unio Bed | ||||
| Broken Shell Limestone Bed | ||||
| Stair Hole Member | "Middle Purbeck" | Chief Beef Bed | ||
| Corbula Bed | ||||
| Scallop Bed | ||||
| Intermarine Bed | ||||
| Cinder Bed | ||||
| Lulworth Formation | Worbarrow Tout Member | Cherty Freshwater Bed | ||
| Marly Freshwater Bed | ||||
| "Lower Purbeck" | Soft Cockle Bed | |||
| Hard Cockle Bed | ||||
| Ridgeway Member | "Cypris" Freestones Bed | |||
| Mupe Member | Broken Beds and basal beds |
In the past, many geologists have ranged the Purbeck Group with the overlying Lower CretaceousWealden 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 upperVolgian ofRussia. Contemporaneous rocks are also present in the neighbourhood ofBoulogne-sur-Mer, where they are characterized by thin limestones withCyrena andgypsiferous marls. These French outcrops occur, just like those in England, in the core of theWeald-Artois anticline. Purbeckian aged deposits occur even further south in theCharente. In north-westGermany three subdivisions are recognized in strata of the same age: in descending orderPurbeck Kalk,Serpulit andMünder Mergel.
The Purbeck Group outcrops follow the line of the Jurassic outcrop from Dorset, through the Vale of Wardour,Swindon,Garsington,Brill andAylesbury. In East Sussex, the Purbeck Group outcrops at three locations north and northwest west ofBattle, East Sussex and at Netherfield. They also occur at several other locations east ofHeathfield, East Sussex and at Beak's Wood nearBurwash. Deposits of evaporite minerals such asgypsum andanhydrite, within the Purbeck Group are mined and processed inMountfield, East Sussex.[4] InLincolnshire they are represented in part by theSpilsby Sands and inYorkshire by portions of theSpeeton Clay.
The rocks predominantly comprise calcareous mudstones though also includeclays,shales andmarls withmarly, tuffaceous and shellylimestones, occasionaloolitic and sandy strata, andevaporites.Nodules ofchert are present in some of the limestones.
The thickness of the formation in Wiltshire is 80 to 90 ft (27 m), but in Dorset it is typically between 45 and 120 metres (148 and 394 ft) thick. In theWeald ofEast Sussex the Purbeck Group has a typical thickness of between 77 and 186 metres (253 and 610 ft).[5] In most places the Purbeck Group rests conformably upon thePortland Group and it is conformably overlaid by theWealden Group; but there are in some districts distinct indications that the Portland Group was uplifted and worn away to some extent prior to the deposition of the Purbeck Group.
The building stones of the Purbeck Group have already been mentioned; the Purbeck or Paludina "marble", a grey or greenish limestone full of shells, was formerly extensively employed in cathedrals and churches. Stone tiles or slats 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.
No zonalfossil has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that ofPensphinctes transilorius of the European continent. The Purbeckian equivalents of Spilsby and Speeton are in the zone ofBelemnites lateralis. Other marine fossils areHemicidaris purbeckensis andOstrea distonta, the latter being abundant in the Cinder bed of the Middle Purbeck. The fresh-water mollusca includeViviparus (Paludina),Planorbis,Melanopsis,Unio,Cyrena. A large number of insect genera has been found in the Middle and Lower Purbeck Group.
Dinosaurs (Owenodon,Echinodon [known from "Isolated skull elements of at least [three] individuals."[6]]),crocodylians (Goniopholis,Petrosuchus),Cimoliosaurus, theplesiosaurs and thechelonians (Chelone,Pleurosternum) are representativereptiles. The mammals, mostly determined from lower jaws, found in the beds mentioned above includePlagiaulax,Amblotherium,Stylodon,Dorsetodon,Triconodon,Spalacotherium and several others. TheisopodcrustaceanArchaeoniscus brodiei is very common in the Purbeck of theVale of Wardour.
Reptile remains diagnostic to thegenus level (Megalosaurus sp (theropod indet),Opisthias) are among the fossils that have been recovered from the formation.[7]
Fossil eggs referable to theoogenusMycomorphoolithus are also known from the Purbeck Group.[8]
Thesilicified stumps and trunks ofcycads andconiferous trees, often surrounded by great masses of calcareousconcretions (burrs), are very noticeable in the dirt beds of Portland and nearLulworth.Chara is found in the fresh-water cherts of the Middle Purbeck.
This article incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Purbeckian".Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. pp. 657–658.