Purbeck Group

Purbeck Group
Stratigraphic range:
Strata of the Purbeck Group in Durlston Bay, Dorset
Type	Group
Sub-units	Lulworth Formation, Durlston Formation and Haddenham 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	Southern England
Type section
Named for	Isle of Purbeck
Location	Durlston Bay cliffs
Exposures of the Purbeck Group in Dorset shown in dark blue

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. This sequence of rocks has gone by various names in the past including amongst others the Purbeck Beds, Purbeck Formation, Purbeck Limestone Formation and Purbeck Stone.

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.

History and stratigraphy

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 Sir William 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 of Lulworth and Durlston 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.

The British Geological Survey recognises three formations and five members of the Purbeck Group, exposed differently across the span of southern England. In the Wessex Basin (or Vectian Basin) area of Dorset 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 the Weald 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 in Wiltshire, Oxfordshire and Buckinghamshire has been given its own status as the Haddenham Formation.

In East Sussex the Purbeck Group is formally subdivided into the Blues and Greys Limestones members 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, the Purbeck marble and Unio-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 of mammalian 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.

Subdivisions of the Purbeck Group in Dorset

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

Correlations

In the past, many geologists have ranged the Purbeck Group with the overlying Lower Cretaceous Wealden 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 neighbourhood 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.

Lithology

Purbeck quarry in southern England

The Purbeck Group outcrops follow the line of the Jurassic outcrop from Dorset, through the Vale of Wardour, Swindon, Garsington, Brill and Aylesbury. In East Sussex, the Purbeck Group outcrops at three locations north and northwest west of Battle, East Sussex and at Netherfield. They also occur at several other locations east of Heathfield, East Sussex and at Beak's Wood near Burwash. Deposits of evaporite minerals such as gypsum and anhydrite, within the Purbeck Group are mined and processed in Mountfield, East Sussex. In Lincolnshire they are represented in part by the Spilsby Sands and in Yorkshire by portions of the Speeton Clay.

The rocks predominantly comprise calcareous mudstones though also include clays, shales and marls with marly, tuffaceous and shelly limestones, occasional oolitic and sandy strata, and evaporites. Nodules of chert 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 the Weald of East Sussex the Purbeck Group has a typical thickness of between 77 and 186 metres (253 and 610 ft). 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 away to some extent prior to the deposition of the Purbeck Group.

Use in construction

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.

Palaeoenvironment

Animals by the Purbeck lagoon

Palaeobiota

Thrombolites in the basal part of the Purbeck Group, Isle of Portland

No zonal fossil 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.

Dinosaurs (Owenodon, Echinodon [known from "Isolated skull elements of at least [three] individuals."]), crocodylians (Goniopholis, Petrosuchus), Cimoliosaurus, the plesiosaurs and the chelonians (Chelone, Pleurosternum) are representative reptiles. The mammals, mostly determined from lower jaws, found in the beds mentioned above include Plagiaulax, Amblotherium, Stylodon, Dorsetodon, Triconodon, Spalacotherium and several others. The isopod crustacean Archaeoniscus brodiei is very common in the Purbeck of the Vale of Wardour.

Reptile remains diagnostic to the genus level (Megalosaurus sp (theropod indet), Opisthias) are among the fossils that have been recovered from the formation.

Fossil eggs referable to the oogenus Mycomorphoolithus are also known from the Purbeck Group.

The silicified stumps and trunks of cycads and coniferous trees, often surrounded by great masses of calcareous concretions (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.

See also

• Geology of East Sussex
• List of dinosaur-bearing rock formations

References

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Purbeckian". Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. pp. 657–658.