12 Portland West



140 to 144 million years ago (MYA).

The Purbeck Group consists mainly of fossiliferous thin-bedded limestone and mudstone. It has long been recognised as a distinctive unit, and it differs obviously from the massive Portland limestone beneath not only by the thin-bedding but also by the generally non-marine fauna, rarity of thick-shelled molluscs and absence of ammonites. The lagoonal and lacustrine fauna with vertebrates of continental origin contrasts with the marine faunas of the underlying rocks. The separation from the non-marine clastic Wealden Group above is lithological. Up to 246 numbered beds have been described in some detail, and five geological members can be mapped in the field. Many are named and the ostracod and gastropod content of each is known.


145 to 146 MYA

The Portland Sand and Portland Limestone formations represent the final shallowing on the last major sedimentary rhythm of the Dorset Jurassic (J5) which commenced with the deeper facies of the Oxford Clay; in the overlying Purbeck Group lacustrine and non-marine environments are finally developed. Well-exposed on the Isle of Portland and in Purbeck, the area was recognised as a key area in earliest days of investigation and in 1829 Brongniart coined the term Portlandian by which rocks of this age have been known internationally. The excellent Portland Stone has been quarried and exported nationally and internationally for over three centuries.
The Portland Sand comprises impure siltstones and with oyster beds grading up from the Kimmeridge Clay below; levels are full of rhaxellid sponges. It contains an important ammonite fauna, which provides the last link with Russian faunas before restriction of the Boreal seas. The overlying Portland Limestone has a lower unit full of cherty levels and a Basal Shell Bed, which has yielded rich bivalve and gastropod faunas. The Portland Freestone above is the famous building stone of shelly limestones and oolitic levels with local stromatoporoid occurrences. The giant ammonite faunas have been described at least since 1668; the biostratigraphy has been well described (Wimbledon and Cope, 1978).

147 to 157 MYA

The Kimmeridge Clay is represented in Dorset by a succession of mudrocks of variable organic carbon content, together with minor amounts of carbonate, siltstone, fine-grained sandstone and ironstone. The formation is remarkable in that it has some very high total organic carbon values averaging around 10 per cent but reaching up to about 60 percent. In contrast most mudrocks have a total organic carbon content of 0-2 percent. The individual mudrock beds of variable organic carbon content are generally tens of centimetres thick and reflect the changing chemical, physical and biological conditions in seawater at that time. The beds contain an abundant well-preserved fauna of mainly ammonites, bivalves, gastropods and vertebrates. It is the standard section for the Boreal Lower Tithonian (=Bolonian) ammonite zones and is probably the only section world wide with good exposure of the Rotunda and Fittoni Zones. Recent work on samples from the Kimmeridge Clay Formation in Dorset presents the only radiometric date for the marine Tithonian world-wide. The Eudoxus to Fittoni Zones are exposed uninterrupted between Brandy Bay and Chapman’s Pool. Further sections, particularly of the lowermost zones, occur at Ringstead Bay, Osmington Mills, Black Head and West Weare Cliffs on the Isle of Portland.