Blue Lias Formation | |
---|---|
Stratigraphic range: Rhaetian-Lower Sinemurian Planorbis-Semicostatum ~ | |
Type | Geological formation |
Unit of | Lias Group |
Sub-units | Wilmcote Limestone Member, Saltford Shale Member, Rugby Limestone Member |
Underlies | Charmouth Mudstone Formation |
Overlies | Lilstock Formation |
Thickness | up to 120 metres (390 ft) |
Lithology | |
Primary | Limestone |
Other | Mudstone |
Location | |
Region | Europe |
Country | United Kingdom |
Extent | South West England, Wales |
Type section | |
Location | Saltford railway cutting |
The Blue Lias is a geological formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassic times, between 195 and 200 million years ago. The Blue Lias is famous for its fossils, especially ammonites.
Its age corresponds to the Rhaetian to lower Sinemurian stages of the geological timescale, thus fully including the Hettangian stage. It is the lowest of the three divisions of the Lower Jurassic period and, as such, is also given the name Lower Lias . Stratigraphically it can be subdivided into three members: the Wilmcote Limestone, Saltford Shale and Rugby Limestone. [1]
The Blue Lias comprises decimetre scale alternations of argillaceous limestone and mudstone. These alternations are caused by short-term climatic variations during the Early Jurassic attributed to orbital forcing (Milankovitch cycles). These limestone-mudstone alternations pass up into a clay member formerly known as the Lower Lias Clay now the Charmouth Mudstone. This lithology consists of monotonous mudstones weathering to clay at the surface. Sparse thin limestone and nodule bands are seen where the rocks are exposed. The deposition of a clay-rich mudstone member normally indicates deposition in a deeper marine environment. The lowest beds of the formation are referred to as the "Pre-planorbis beds" in reference them being deposited before the first appearance of the ammonite Psiloceras planorbis.
In certain restricted parts of Britain, the lowermost member of the Blue Lias is the Wilmcote Limestone. It lies above the Cotham Member of the Lilstock Formation and beneath the Saltford Shale Member. [2] The Wilmcote Limestone of central England was formerly quarried close to Stratford-upon-Avon, for example at Wilmcote, Temple Grafton and Binton. It is roughly 200 million years old, dating back to the dawn of the Jurassic Period.
Much of the Wilmcote Limestone is very fine-grained, blue-grey when fresh, and very finely layered. Fossils are quite rare, except in the lowest beds. It was formerly used for a variety of purposes, including walling, building, paving, gravestones, cement-making and as a source of agricultural lime. It is no longer quarried, and most of the old quarries are either infilled or overgrown.
Geologists think that the Wilmcote Limestone originated as layers of fine-grained mud on the floor of a sheltered, shallow muddy sea or lagoon that covered parts of central England at the dawn of the Jurassic Period. Very little life could tolerate the stagnant conditions on the seabed. As a consequence the mud was seldom disturbed, which is why the fine, paper-like layering is preserved.
Above the sea bed, the shallower waters supported ammonites, fish, and marine reptiles (ichthyosaurs and plesiosaurs). Their remains were discovered in the Wilmcote Limestone quarries during the nineteenth century. The Warwickshire Museum [3] houses a collection of these fossils and some are on display at the Market Hall Museum in Warwick. [4] [5]
The Blue Lias is a prevalent feature of the cliffs around Lyme Regis and Charmouth, on the Jurassic Coast in Dorset, where it exists in layers of limestone interspersed with softer clay. It is also notable for its presence in Somerset, particularly around the Polden Hills, Keinton Mandeville and Glastonbury area, and it forms a broad plain across the East Midlands. It also appears near Whitby in Yorkshire and Southam in Warwickshire where a pub is named after it. There are outcrops along the coast of South Wales, notably that of the Vale of Glamorgan. [6] The type section of the Blue Lias is at Saltford near Bath.
Blue Lias is useful as a building stone, and as a source of lime for making lime mortar. Because it is argillaceous, the lime is hydraulic. Since the mid-nineteenth century, it has been used as a raw material for cement, in South Wales, Somerset, Warwickshire, and Leicestershire. The cement plant quarry at Rugby, Warwickshire is probably the best exposure of the formation: more than 100 layers can be seen.
In areas where Blue Lias is quarried it has been used in buildings and churches as well as tombstones in cemeteries. An example of a Blue Lias town is Street, near Glastonbury. Other examples of Blue Lias buildings can be found in the nearby towns of Somerton and Ilchester.
It remains popular in more modern-day surroundings where it is used in the construction of new housing developments and extensions for existing buildings in conservation areas. Blue Lias is mainly used in flooring, walling and paving slabs – both coursed and layered. It is also used in the making of flagstones and cobbles.
There are only four quarries in Somerset quarrying Blue Lias at present. AR Purnell at Ashen Cross Quarry in Somerton have been mining blue lias stone since 1996. Hadspen Quarry Ltd. Hadspen Quarry operate one in Keinton Mandeville. Ham & Doulting Stone Co Ltd. operate one of these, Tout Quarry near Somerton. [7]
The rock is rich in fossil remains from the Jurassic period. The blue-grey colour is provided by its iron content, enclosed to a large extent in pyrites. [8]
Theropods of the Blue Lias | ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Dracoraptor | D. hanigani | Lavernock Point | Pre- planorbis Beds, lowermost Hettangian | NMW 2015.5G.1–2015.5G.11 "a disarticulated, but associated partial skeleton" | A coelophysoid theropod | |
D. normani | Lower cliff face, Charmouth | Black Ven Marl Member? |
| An Averostran theropod, possible Ceratosaur. In 2024 was formally described, but referred to the Blue Lias. [10] A paper non quoted on the description relocated it on the Charmouth Mudstone Formation. [11] | ||
Sarcosaurus | S. woodi | Wilmcote | angulata zone, late Hettangian (NHMUK PV R3542) Rugby Limestone Member liasicus to semicosatum zones, lowermost Sinemurian (WARMS G667–690) | Paratype specimens: NHMUK PV R3542, complete right tibia WARMS G667–690, partial skeleton | Basal neotheropod, holotype specimen is known from the Scunthorpe Mudstone |
Pterosaurs of the Blue Lias | ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Dimorphodon | D. macronyx | Aust Cliff, Lyme Regis | NHMUK PV R 1034, NHMUK PV OR 41212, NHMUK PV R 1035 | A basal pterosaur |
Numerous fish species are known from the Blue Lias and overlying Charmouth Mudstone.
Fish of the Charmouth Mudstone Formation | ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Acrodus | A hybodont shark | |||||
Hybodus | A hybodont shark | |||||
Palidiplospinax | A synechodontiform shark | |||||
Squaloraja | S. tenuispina,S. polyspondyla | Closely related to modern chimaeras | ||||
Myriacanthus | M. paradoxus, M. granulatus | A myriacanthid closely related to modern chimaeras | ||||
Dorsetichthys | D. bechei | A stem-group teleost | ||||
"Coccolepis" | "C." liassicus | A coccolepidid fish, probably does not belong to the genus | ||||
Cosmolepis | C. ornatus | An enigmatic basal actinopterygian | ||||
Holophagus | H. gulo | A coelacanth | ||||
Chondrosteus | C. acipenseroides | A chondrosteid acipenseriform fish, related to sturgeon and paddlefish | ||||
Oxygnathus | O. ornatus | A palaeonisciform fish | ||||
Saurorhynchus | S. brevirostris,S. anningae | A member of Saurichthyiformes | ||||
Ptycholepis | P. gracilis, P. curtus | A palaeonisciform fish | ||||
Dapedium | Spp. | A dapediiform fish | ||||
Caturus | Spp. | An amiiform fish related to bowfins | ||||
Platysiagum | P. sclerocephalum | A platysiagid fish | ||||
Furo | F. orthostomus | A member of Ionoscopiformes within Halecomorphi |
Ichthyosaurs of the Blue Lias | ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Ichthyosaurus | I. larkini [12] | Lyme Regis & Charmouth in Dorset, Street and the north coast of Somerset, and other inland sites across England | Unknown, possibly Pre-planorbis beds | Holotype: BRSUG 25300, referred: AGC 11, CAMSM J5957, NHMUK PV OR5595 | ||
I. somersetensis [12] | Holotype: ANSP 15766 referred: BRSMG Cb4997, NHMUK PV OR2013AGC 16, ROM 26029 | |||||
I. communis [13] | Unknown | BMNH R1162 | ||||
Protoichthyosaurus | P. prostaxalis [14] | Somerset | Unknown, probably Pre-planorbis beds | Holotype: BRLSI M3553, "a partial skull, pectoral girdle and both forefins, preserved in ventral view" | ||
Wahlisaurus | W. massarae [15] | Sutton Hill (Stowey) Quarry, Bishop Sutton | Pre-planorbis beds | BRSMG Cg240, "a practically complete right coracoid" | ||
?Shastasauridae | Indeterminate | Penarth | Psiloceras planorbis Biozone | NMW95.61G.1, radius [16] | Estimated length of 12–15 metres | |
Temnodontosaurus [17] | T. platyodon | PV R 1158, consisting of a Skull, Lower Jaw and Cervical Vertebrae | ||||
Excalibosaurus | E. costini [18] [19] | North Somerset coastline | Arietites (Coroniceras) bucklandi Biozone | Holotype: BRSMG Cc881, "...a complete skull, with associated forefin, pectoral girdle, vertebrae and ribs..." referred: ROM 47697 |
Plesiosaurs of the Blue Lias | ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Atychodracon | A. megacephalus [20] | Street-on-the-Fosse | Lowermost Blue Lias | Holotype:BRSMG Cb 2335 | Rhomaleosaurid | |
Avalonnectes | A. arturi [10] | Street, Somerset (referred specimen) | NHMUK 14550, "the posterior portion of the skull, and a partial postcranial skeleton" uncatalogued partial specimen | Rhomaleosaurid | ||
Eurycleidus | E. arcuatus | Street | BMNH 2030 (lectotype), 2027-2029, 2047, 2061, R1317-1319 (paralectotypes, probably belonging to the same individual) | Rhomaleosaurid | ||
Eoplesiosaurus | E. antiquior [10] | Watchet, Somerset | TTNCM 8348, postcranial skeleton | Basal Plesiosauroid | ||
Plesiosaurus | P. dolichodeirus [21] | Lyme Regis and Charmouth, Dorset | Sinemurian stage (upper Blue Lias, Shales-with-Beef member or lower Black Ven Marl | NHMUK PV OR 22656, complete skeleton | Plesiosaurid | |
Stratesaurus | S. taylori [10] | Street, Somerset | lowermost Hettangian | OUMNH J.10337, "a skull and partial postcranial skeleton including anterior cervical and pectoral vertebrae, a partial hindlimb and ilium" | Rhomaleosaurid | |
Thalassiodracon | T. hawkinsii | Street | Pre-planorbis Beds | BMNH 2018 "almost complete skeleton missing distal parts of limbs" CAMSM J.35181, partial skeleton | Pliosaurid affinities [22] |
Insect compression fossils are known from the localities of Binton in Warwickshire and Copt Heath near Birmingham. [23]
The Early Jurassic Epoch is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic–Jurassic extinction event, 201.3 Ma, and ends at the start of the Middle Jurassic 174.7 ±0.8 Ma.
Ichthyosaurus is a genus of ichthyosaurs from the Early Jurassic, with possible Late Triassic record, from Europe. It is among the best known ichthyosaur genera, as it is the type genus of the order Ichthyosauria.
The Polden Hills in Somerset, England are a long, low ridge, extending for 10 miles (16 km), and separated from the Mendip Hills, to which they are nearly parallel, by a marshy tract, known as the Somerset Levels. They are now bisected at their western end by the M5 motorway and a railway, the Bristol and Exeter Railway, part of the Great Western Main Line.
The Hettangian is the earliest age and lowest stage of the Jurassic Period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma. The Hettangian follows the Rhaetian and is followed by the Sinemurian.
Temnodontosaurus is an extinct genus of ichthyosaur from the Early Jurassic period. They lived between 200 and 175 million years ago (Hettangian-Toarcian) in what is now Western Europe and possibly Chile. It lived in the deeper areas of the open ocean. University of Bristol paleontologist Jeremy Martin described the genus Temnodontosaurus as "one of the most ecologically disparate genera of ichthyosaurs," although the number of valid Temnodontosaurus species has varied over the years.
Sarcosaurus is a genus of basal neotheropod dinosaur, roughly 3.5 metres (11 ft) long. It lived in what is now England and maybe Ireland and Scotland during the Hettangian-Sinemurian stages of the Early Jurassic, about 199-196 million years ago. Sarcosaurus is one of the earliest known Jurassic theropods, and one of only a handful of theropod genera from this time period. Along with Dracoraptor hanigani it is one of the two described neotheropods from the lowermost Jurassic of the United Kingdom.
Dapedium is an extinct genus of primitive marine neopterygian ray-finned fish. The first-described finding was an example of D. politum, found in the Lower Lias of Lyme Regis, on the Jurassic Coast of England. Dapedium lived in the Late Triassic to the Middle Jurassic period, from the late Norian to the early Aalenian.
Blue Anchor to Lilstock Coast SSSI is a 742.8 hectare geological Site of Special Scientific Interest between Blue Anchor and Lilstock in Somerset, notified in 1971.
The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, shales, marls and clays.
Coptoclavidae is an extinct family of aquatic beetles in the suborder Adephaga. The Coptoclavidae lived from the Late Triassic to the Early Cretaceous. Coptoclavidae is a member of the adephagan clade Dytiscoidea, which contains other aquatic beetles. Suggested reasons for their extinction to include the rise of teleost fish, or competition with Gyrinidae and Dytiscidae, which possess defensive secretions and sucking channels in the mandibles of larvae, which coptoclavids likely lacked. It has been suggested that the genus Timarchopsis and the subfamily Timarchopsinae are only distantly related to other coptoclavids based on cladistic analysis, with Timarchopsis being more closely related to geadephagans like carabids and trachypachids instead. Another study also suggested similarly for Coptoclavisca and possibly other coptoclaviscines.
Omma is a genus of beetles in the family Ommatidae. Omma is an example of a living fossil. The oldest species known, O. liassicum, lived during the final stage of the Triassic (Rhaetian), over 200 million years ago, though the placement of this species in Omma has been questioned. Numerous other fossil species are known from the Jurassic and Cretaceous of Europe and Asia. The only living species is Omma stanleyi, which is endemic to Australia. Three other extant species endemic to Australia that were formerly part of this genus were moved to the separate genus Beutelius in 2020.Omma stanleyi is strongly associated with wood, being found under Eucalyptus bark and exhibiting thanatosis when disturbed. Its larval stage and many other life details are unknown due to its rarity. Males are typically 14–20 mm in length, while females are 14.4-27.5 mm. Omma stanleyi occurs throughout eastern Australia from Victoria to Central Queensland.
The Charmouth Mudstone Formation is a geological formation in England, dating to the Early Jurassic (Sinemurian–Pliensbachian). It forms part of the lower Lias Group. It is most prominently exposed at its type locality in cliff section between Lyme Regis and Charmouth but onshore it extends northwards to Market Weighton, Yorkshire, and in the subsurface of the East Midlands Shelf and Wessex Basin. The formation is notable for its fossils, including those of ammonites and marine reptiles and rare dinosaur remains. The formation played a prominent role in the history of early paleontology, with its Lyme Regis-Charmouth exposure being frequented by fossil collectors including Mary Anning.
The Lilstock Formation is a geologic formation in England. It preserves bivalve, insect and other invertebrate fossils, as well as fossil fish of Agkistracanthus mitgelensis and the basal theropod dinosaur Newtonsaurus cambrensis dating back to the Rhaetian of the Triassic period.
The Scunthorpe Mudstone is a geologic formation in England. It preserves plesiosaur fossils dating back to the Late Triassic (Rhaetian) to Early Jurassic (Hettangian) period. It predominantly consists of grey mudstone with thin beds of argillaceous limestone and calcareous siltstone. The Ichthyosaur Wahlisaurus is known from the formation. As is the holotype specimen of the dinosaur Sarcosaurus.
This timeline of ichthyosaur research is a chronological listing of events in the history of paleontology focused on the ichthyosauromorphs, a group of secondarily aquatic marine reptiles whose later members superficially resembled dolphins, sharks, or swordfish. Scientists have documented ichthyosaur fossils at least as far back as the late 17th century. At that time, a scholar named Edward Lhuyd published a book on British fossils that misattributed some ichthyosaur vertebrae to actual fishes; their true nature was not recognized until the 19th century. In 1811, a boy named Joseph Anning discovered the first ichthyosaur fossils that would come to be scientifically recognized as such. His sister Mary would later find the rest of its skeleton and would go on to become a respected fossil collector and paleontologist in her own right.
Fossil Beach, at Sedbury, Gloucestershire, England, is beneath the Sedbury Cliffs by the River Severn and is known as a rich source of easily discovered fossils. It is within the Severn Estuary Site of Special Scientific Interest (SSSI).
The Beacon Limestone Formation, historically known as the Junction Bed, is a formation of the early Jurassic age (Pliensbachian–Toarcian). It lies above the Dyrham Formation and below the Bridport Sand Formation. It forms part of the Lias Group. It is found within the Wessex Basin and parts of Somerset, in England. It is well known for the Strawberry Bank Lagerstätte, which contains the 3-dimensionally preserved remains of vertebrates, including marine crocodyliformes, ichthyosaurs and fish, as well as insect compression fossils.
Dermapteridae is an extinct family of earwigs known from the Late Triassic to Mid Cretaceous, it is part of the extinct suborder Archidermaptera, alongside Protodiplatyidae and Turanovia. It was first named as a subfamily by Vishniakova in 1980, and elevated to family status by Engel in 2003 without discussion.
Protorthophlebia is an extinct genus of scorpionflies, known from the Triassic and Jurassic periods of Eurasia. It was originally considered a member of the family Orthophlebiidae, but was later placed as the only genus within the family Protorthophlebiidae within the superfamily Panorpoidea.
Ichthyotitan is an extinct genus of giant ichthyosaur from the Late Triassic (Rhaetian), known from the Westbury Mudstone Formation in Somerset, England. It is believed to be a shastasaurid, extending the family's range by 13 million years up to the latest Triassic. The discovery of Ichthyotitan has been considered evidence that shastasaurids were still thriving until their disappearance in the Triassic–Jurassic extinction event.