Purbeck Beds

Purbeck Beds

      unit of sedimentary rocks exposed in southern England that spans the boundary between the Jurassic (Jurassic Period) and Cretaceous (Cretaceous Period) periods, approximately 145 million years ago. The highly varied Purbeck Beds, which overlie rocks of the Portland Beds, record a marked change in sedimentary facies, indicating major alterations in environmental conditions. Limestones (limestone), marls (marl), clays (clay mineral), and old soil horizons (soil) are present in thicknesses of up to 170 metres (560 feet).

      The type section is at Durlston Bay near Swanage, Dorset. Each of the Lower, Middle, and Upper Purbeck beds contains distinctive units. The Lower Purbeck is completely Jurassic in age, having been deposited during the Tithonian Age (Tithonian Stage), and the Upper Purbeck is entirely Cretaceous in age, having been deposited during the Berriasian Age (Berriasian Stage). The boundary between the two geologic time periods appears to occur in the Cinder Bed unit of the Middle Purbeck.

      The varied rock types of the Purbeck Beds were deposited in marine, marginal marine (such as brackish lagoons), and freshwater settings. Ancient land soils in the Lower Purbeck include the fossilized stumps of coniferous trees and primitive palmlike cycads (cycad). In addition, shales and clays occasionally contain fossil insects. The Middle and Upper Purbeck consist of freshwater limestones that are quarried for use as building stone. Marls and shales (shale) are interbedded with the limestones.

      The lowest unit of the Middle Purbeck, the Marly Freshwater Beds, has a Mammal Bed containing about 20 mammalian species. The Cinder Bed, located within the Middle Purbeck, is a marine unit containing varied fauna, including large quantities of oysters, trigonids (a type of Mesozoic clam), and fragments of echinoids (sea urchins). The Upper Building Stones unit of the Middle Purbeck contains fossils of turtles and fish that probably lived in brackish water. The Upper Purbeck contains freshwater fossils and is the source of “Purbeck Marble” building stones.

Carol Marie Tang
 

* * *


Universalium. 2010.

Look at other dictionaries:

  • Purbeck beds — Pur beck beds [So called from the Isle of Purbeck in England.] (Geol.) The strata of the Purbeck stone, or Purbeck limestone, belonging to the O[ o]litic group. See the Chart of {Geology}. [1913 Webster] …   The Collaborative International Dictionary of English

  • Purbeck-Marmor — Netzgewölbe aus Purbeck Marmor in der Kathedrale von Exeter …   Deutsch Wikipedia

  • 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… …   Wikipedia

  • Speeton Beds — Speeton Beds, in English geology, a series of clays well exposed at Speeton, near Filey on the Yorkshire coast.Peculiar interest attaches to these beds for they are the principal representatives in Britain of the marine phase of the Lower… …   Wikipedia

  • Isle of Purbeck — Fishermans ledge in Purbeck Gewässer Ärmelkanal …   Deutsch Wikipedia

  • Purbeckian — In geology, the Purbeckian refers to the highest and youngest member of the Jurassic system of rocks, also known as the Purbeck Beds. Either name is derived from the district known as the Isle of Purbeck in Dorset where the strata are splendidly… …   Wikipedia

  • Durdle Door — Durdle Door, Dorset. Durdle Door is a natural limestone arch on the Jurassic Coast near Lulworth in Dorset, England.[1] It is privately owned by the Welds, a family who ow …   Wikipedia

  • Iguanodon — Taxobox name = Iguanodon fossil range = Early Cretaceous image width = 250px image caption = Iguanodon bernissartensis skull regnum = Animalia phylum = Chordata classis = Sauropsida superordo = Dinosauria ordo = Ornithischia subordo = Cerapoda… …   Wikipedia

  • Cow Corner — Looking down to Worbarrow Bay and Cow Corner to the very left is Arish Mell Known as Cow Corner is the north western end of Worbarrow Bay, a small secluded bay on the south coast of the Isle of Purbeck, in Dorset England …   Wikipedia

  • Peter Bellinger Brodie — (1815 1897) was an English geologist and churchman, the son of P. B. Brodie, barrister, and nephew of Sir Benjamin C. Brodie. He was born in London in 1815. While residing with his father at Lincoln s Inn Fields, he gained some knowledge of… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”