147 to 157 million years ago (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.
158 to 163 MYA
Corallian. The deeper water facies of the Oxford Clay give way to shallower-water facies of sandstones, limestones and oolites in the Corallian Beds, the former name of this unit. Excellent examples of a variety of trace fossils can be found in many of the sandstone and limestone units. One of the sandstone units (the Bencliff Grit Member) shows an uncommonly preserved sedimentary structure termed swaley cross-stratification. This is one of the best exposures of such structures in western Europe. The Bencliff Grit Member contains mature hydrocarbons at the western end of the Bran Point exposure. This represents a dissected hydrocarbon reservoir: the hydrocarbons are most likely from the Lower Lias, similar to the nearby producing Wytch Farm and Kimmeridge Bay oil fields. In the middle of the Oxfordian succession there is an oolitic grainstone. The overlying Clavellata Member has a beautifully preserved ammonite/bivalve fauna. The Sandsfoot Grit Member and overlying Ringstead Waxy Clay probably represent a beach barrier complex and associated lagoonal facies. The top of the Corallian Group and base of the Kimmeridge Clay Formation contain a complex succession of very condensed beds with a rich, well-preserved fauna.
Oxford clay. The boundary between the Callovian and Oxfordian Stages is well exposed on the coast at Ham Cliff and is well-known for its ammonite faunas and contribution to Oxfordian biostratigraphy. Important individual features include the well-preserved aragonitic ammonites and nodule beds. In particular the Red Nodule Bed exposed at Furzy Cliff is associated with a beautifully preserved three dimensional ammonites and bivalves. The Dorset Oxfordian sections have contributed to significant publications and ongoing work in many fields. The Oxford Clay (Callovian) is hydrocarbon-rich and has produced fine faunas of marine invertebrates and reptiles. Callovian: upper Macrocephalus to Lamberti Zones.
The Kellaways Formation and Lower Oxford Clay are poorly exposed along the Fleet shore from East Fleet to near Tidmoor Point. They represent the initiation of the fourth major sedimentary rhythm (J4), which ends with the carbonates of the Corallian Beds (upper Oxfordian).
The Cornbrash is exposed at Shipmoor Point, Berry Knap and near East Fleet. This thin limestone comprises a shallow-water facies with abundant brachiopods, bivalves and other fauna. The faunas show a major change within the Cornbrash at which the Bathonian/Callovian boundary is drawn. Lower Cornbrash (Barry Member), Bajocian: Discus Zone: Upper Cornbrash (Fleet member), Callovian: Macrocephalus Zone.
Forest Marble.Discontinuously exposed just west of West Bay, between Burton Bradstock and Abbotsbury Castle, and along the shores of the Fleet. The overlying sequence from the mudstones of the Fuller’s Earth through mudstones and calcareous sands of the Forest Marble to the limestones of the Cornbrash represent the third major sedimentary cycle (J3) of the Jurassic and the Bathonian Stage.