Bedrock Geology UK South: Jurassic
|This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK South and gives a brief explanation of their origins.
Author: A A Jackson (BGS); Contributor: P Stone (BGS)
200 to 145 million years ago
The Jurassic outcrop curves southwards from the coast of Yorkshire to Dorset with an extension running westward through the Bristol Channel into south Wales. An outlier occurs near Prees in the Cheshire Basin, with a significant occurrence at Mochras, concealed beneath younger sediments, on the west coast of Wales. The landscape is one of vales and escarpments. Many of the steeper slopes are wooded (Plate P667863), and villages with cottages of local stone in a variety of muted colours create the classic English landscape. Along the coast there are spectacular sea cliffs in Yorkshire, south Wales and Dorset.
The palaeogeographical landscape was inherited from the late Triassic so that the Worcester, Wessex and Cleveland basins continued as the main depositional areas with the East England Shelf as a long-lived stable area stretching east towards the deep North Sea graben (Figure P785824). The palaeolatitude is calculated at 30º to 40º north (Figure P785800f). There was a subtropical climate with a strong seasonal influence on rainfall and temperature, but some thin gypsum and red beds are recorded offshore in the Middle and Upper Jurassic of the Bristol Channel and indicate periods of severe aridity.
Besides the famous dinosaur bones, these rocks are richly fossiliferous with the remains of a variety of marine creatures that include reptiles, fish and some mammals, but it is perhaps the ammonites that are most characteristic of the Jurassic strata.
The transgression that had started during the late Triassic continued throughout the early Jurassic. The peneplained landscape was flooded as a result of a rise in sea level (these changes are well documented in the North Sea) combined with a steady regional subsidence that affected both basins and highs. A warm shallow sea covered most of southern Britain, lapping against the landmasses, with lagoonal and brackish water sediments marking the imprecise boundary between land and sea.
Jurassic strata generally rest conformably on the topmost Triassic (Rhaetian) in the basinal areas, but towards the London Platform, which probably serves as a model for the other landmasses, the Lower Jurassic Lias Group becomes progressively thinner; the lower beds are overlapped by younger ones and eventually the group dies out across the ancient landmass.
The Lias Group (J1) consists mainly of grey mudstone (Charmouth Mudstone and Whitby Mudstone formations) but thin alternations of limestone are common especially in the basal part (Blue Lias Formation; Plate P006953), and sandstone and siltstone occur higher in the group (Dyrham, Bridport Sand and Staithes Sandstone formations). Ferruginous limestone (Marlstone Rock and Cleveland Ironstone) was formerly an important source of iron for the blast furnaces of the Midlands and Yorkshire. The Lias Group is well exposed along the Dorset coast particularly at Lyme Regis, a focus for fossil hunters over the centuries including Mary Anning who collected the first Ichthyosaurus, Plesiosaurus and Pterodactylus in the early 1800s.
In the Cleveland Basin, a shallow-water inshore environment also prevailed, although the succession differs in detail. Unique to this area are jet deposits (Jet Rock Member of the Whitby Mudstone Formation, upper part of Lias Group) formed from tree trunks that were washed into the basin.
In south Wales and around Bristol, coarse bioclastic limestone accumulated around an archipelago of small islands (Mendip High, Glamorgan High) that stretched south-eastwards from the Welsh landmass. Evidence of the shoreline is exposed in places, for example at Bridgend in south Wales, where ‘islands’ of older strata were buried by Jurassic sediments. Only the earliest Jurassic strata are preserved in south Wales and it is not known how much of the Welsh landmass was eventually submerged but it is likely that much of the subsequent uplift occurred in the Cenozoic. At Mochras in west Wales a significant 1305m of Lias Group was proved in a borehole—this is three times the thickness of any Liassic strata found elsewhere in Britain. This borehole, drilled in the late 1960s, changed our understanding of Mesozoic geology and gave an insight into what might be found offshore.
A major change that may be attributed to thermal doming in the North Sea occurred during mid Jurassic times, and there is evidence of volcanic activity in offshore areas. A widespread hiatus is recorded in the sedimentation pattern (Mid Cimmerian unconformity), particularly at the basin margins, and Middle Jurassic strata rest unconformably on Lower Jurassic or older strata indicating considerable erosion.
The Middle Jurassic of the Cleveland Basin and north-east England shows an intricate pattern of mudstone, sandstone, limestone, conglomerate, seatearth, coal and ironstone, deposited in fluviodeltaic conditions, in lagoons, brackish creeks and nearshore areas. Plant remains are common (including ginkgo and ferns), insects and reptilian footprints occur as well as a profusion of other hard-shelled fossils. Iron-rich sandstones and ironstones (Dogger Formation, Northampton Sand Formation) are widespread at the base of the Middle Jurassic, and they have been mined in many places. In the Cleveland Basin, the Dogger Formation underlies the Ravenscar Group (J2–3) which comprises fluviodeltaic sandstones and mudstones with some beds of coal and of limestone. Periodically marine transgressions established more uniform depositional conditions and some horizons may be traced for many kilometres.
In the East Midlands, a patchwork of diachronous facies has produced a Middle Jurassic sequence that varies both laterally and vertically. Sand and mud of the Inferior Oolite Group (J2) were deposited in coastal swamps and lakes (Grantham Formation), and bioclastic and ooidal carbonates (Lincolnshire Limestone Formation) in the shallow shelf sea that eventually flooded the region (J2). Several cycles of marine transgression followed by deltaic progradation and finally salt-marsh conditions can be determined in the Rutland Formation of the Great Oolite Group (J3). The Blisworth Limestone Formation records the re-establishment of marine conditions, but the overlying Blisworth Clay Formation accumulated in coastal lagoonal conditions with some fine-grained fluvial sediments (J3).
To the south, more open marine but generally shallow-water conditions prevailed, and ooidal and bioclastic limestones accumulated; these form the numerous named limestones of the Cotswolds, the Inferior Oolite Group (J2) and Great Oolite Group (J3). Mud deposition (Fuller’s Earth and Forest Marble formations) dominated to the south of the Mendip Hills, where the Frome Clay Formation replaces many of the Great Oolite limestones. At the top of the Great Oolite Group, the Cornbrash Formation is thin, but this fossiliferous limestone can be traced from Yorkshire through the Midlands to the south coast, which suggests that a marine transgression swept across a uniform landscape.
More uniform conditions existed after the Cornbrash transgression of the mid Jurassic. Sand was deposited (Kellaways Formation, Osgodby Formation J4; (Plate P006771) before mud spread over all but the most northerly parts. The Oxford Clay Formation (J4) was deposited in quiet marine conditions, but rhythmic changes are evident within it. The mudstones have been worked extensively in the past for the manufacture of bricks, and is partially self-firing owing to the relatively high carbon content. The mudstone is richly fossiliferous; the tougher fossils, belemnites and oysters such as Gryphaea, survive weathering and can often be seen on newly ploughed land.
The Corallian Group (J5) crops out in Yorkshire and can be traced from the south Midlands to Dorset. It is arenaceous in the lower part and calcareous above, and was deposited in shallow-marine conditions. Small coral reefs developed and patchworks of broken coral colonies are preserved locally as ‘rag’ rock. The reef environment sustained a rich variety of organisms whose remains were commonly reworked into bioclastic limestone. In the north and east Midlands, mudstone deposition dominated throughout this time (West Walton Formation).
The youngest Jurassic beds are the Portland and Purbeck groups (Plate P005775) that are well exposed in the cliffs of the Isle of Portland and eastwards at Swanage (Durlston Head). The Portland Group (J7) represents a return to warm shallow-water deposition; glauconitic sandstone and mudstone is overlain by chert and then by ooidal limestone that is famous for its gigantic ammonite fossils, and has been worked extensively to provide the distinctive pale-coloured dimension stone that has been used in many civic buildings around London. The Jurassic–Cretaceous boundary is placed near the base of the overlying Purbeck Group.
The Ampthill Clay and Kimmeridge Clay formations (Ancholme Group; J6) were deposited over a wide area of southern England and in the North Sea basins. Subsiding basins resulted in deeper water conditions and stratification of the water column, which provided ideal conditions for the preservation of organic matter. These organic-rich strata are the principal source-rock for hydrocarbons, both onshore and in the North Sea.