The Purbeck Group is famous for its fossils of insects, 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 Purbeckianstage 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 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 60ft (15 to 18m). of fresh-water clays and shales with limestones, the Purbeck marble and Unio-bed, in the lower part. The Middle division (50ft (15m)), mainly thin limestones with shaly partings, contains the principal building stones of the Swanage district; near the base of this subdivision there is a 5in (13cm), 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.
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.
The Jurassic-Cretaceous boundary is currently thought to be around the base of the "Cypris" Freestones Bed.
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 90ft (27m), but in Dorset it is typically between 45 and 120 metres (148 and 394ft) thick. In the Weald of East Sussex the Purbeck Group has a typical thickness of between 77 and 186 metres (253 and 610ft). 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.
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 and Paleobiota
Animals by the Purbeck lagoon
Palaeobiota
The Purbeck beds are well known for their diverse biota, containing terrestrial, freshwater and marine elements, representing one of the world's most diverse earliest Cretaceous faunas. These were deposited in a semi-arid environment in conditions ranging from freshwater lakes, brackish water, and tidal flats to lagoons. Several important horizons within the assemblage, such as the "Mammal Bed", have yielded a large number of remains.
Charophyte algae have been reported from the group.
Insects
Hundreds of insect species have been described from the Purbeck Group. A comprehensive account of Purbeck Group insects is given in Fossil Insects of the Purbeck Limestone Group of Southern England (2012).
Represented by footprints, probable eggshell fragments, a hadrosauriform femur, an ornithopod dorsal vertebral centrum, as well as a pedal phalanx, partial femur, and another indeterminate limb bone.
12Ensom, P.C. (2002). "The Purbeck Limestone Group of Dorset, southern England: a guide to lithostratigraphic terms". Special Papers in Palaeontology. 68: 7–12.
↑Codd, J.W. (2007) Analysis of the distribution and characteristics of landslips in the Weald of East Sussex. MSc dissertation, University of Brighton.
↑Coram, Robert A.; Jepson, James E. (2012). Fossil insects of the Purbeck Limestone group of Southern England: palaeoentomology from the dawn of the Cretaceous. Monograph series. Manchester: Siri scientific press. ISBN978-0-9567795-3-3.
↑Evans, Susan E.; McGowan, Gerard J. (2002). "Lissamphibian remains from the Purbeck Limestone Group, southern England". Special Papers in Palaeontology: 104–119.
↑Pérez-García, A. (May 2014). "Revision of the poorly known Dorsetochelys typocardium, a relatively abundant pleurosternid turtle (Paracryptodira) in the Early Cretaceous of Europe". Cretaceous Research. 49: 152–162. Bibcode:2014CrRes..49..152P. doi:10.1016/j.cretres.2014.02.015. ISSN0195-6671.
