Purbeck Group

The Purbeck Group (or Purbeck Beds) 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 and Berriasian 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, Swindon, Garsington, Brill and Aylesbury; they have been proved by borings to lie beneath younger rocks in Sussex; in Lincolnshire they are represented in part by the Spilsby Sands] and in Yorkshire by portions of the Speeton Clay.

Lithology and stratigraphy
The rocks include clays, shales and marls with marly, tufaceous and shelly limestones 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 m), but in Dorset it reaches nearly 400 ft (120 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.

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 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.

Subdivisions
The Purbeck Group in England is divisible into three subdivisions, viz. Upper, Middle and Lower. The Upper Purbeck comprises 50-60 ft (−3.0 m). of fresh-water clays and shales with limestones, the Purbeck marble and Unjo-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, 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 (95,160 ft (29,000 m)) 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.

Fossil content
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 (Iguanodon, Echinodon), 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, Amblothenium, Stylodon, Tniconodon, 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 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.