Blue Lias

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Blue Lias Formation
Ma
Lower Lias sequence exposed at Nash Point, Glamorgan, Wales
TypeGeological formation
Unit ofLias Group
Sub-unitsWilmcote Limestone Member, Saltford Shale Member, Rugby Limestone Member
UnderliesCharmouth Mudstone Formation
OverliesLilstock Formation
Thicknessup to 120 metres (390 ft)
Lithology
PrimaryLimestone
OtherMudstone
Location
RegionEurope
CountryUnited Kingdom
ExtentSouth West England,
Wales
Type section
LocationSaltford railway cutting

The Blue Lias is a

ammonites
.

Its age corresponds to the

Wilmcote Limestone, Saltford Shale and Rugby Limestone.[1]

Lithology and facies

The Blue Lias comprises decimetre scale alternations of

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
.

Wilmcote Limestone

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

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]

Occurrence

Blue Lias formation at Lyme Regis, Dorset

The Blue Lias is a prevalent feature of the cliffs around

type section of the Blue Lias is at Saltford near Bath
.

Use in construction

Ham stone
dressings around the windows

Blue Lias is useful as a building stone, and as a source of

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

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]

Paleofauna

The rock is rich in

pyrites.[8]

Dinosaurs

Dinosaurs 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
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

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

Fish

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
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

Ichthyosaurs of the Blue Lias
Genus Species Location Stratigraphic position Material Notes Images
Ichthyosaurus I. larkini[9] Somerset Unknown, possibly Pre-planorbis beds Holotype: BRSUG 25300, referred: AGC 11, CAMSM J5957, NHMUK PV OR5595
I. somersetensis[9] Holotype: ANSP 15766 referred: BRSMG Cb4997, NHMUK PV OR2013AGC 16, ROM 26029
I. communis[10] Somerset Unknown BMNH R1162
Protoichthyosaurus P. prostaxalis[11] Somerset Unknown, probably Pre-planorbis beds Holotype: BRLSI M3553, "a partial skull, pectoral girdle and both forefins, preserved in ventral view"
Wahlisaurus W. massarae[12] 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[13] Estimated length of 12–15 metres
Temnodontosaurus[14] T. platyodon PV R 1158, consisting of a Skull, Lower Jaw and Cervical Vertebrae

Plesiosaurs

Plesiosaurs of the Blue Lias
Genus Species Location Stratigraphic position Material Notes Images
Atychodracon A. megacephalus[15]
Street-on-the-Fosse
Lowermost Blue Lias Holotype:BRSMG Cb 2335 Rhomaleosaurid
Avalonnectes A. arturi[16] 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[16] Watchet, Somerset TTNCM 8348, postcranial skeleton Basal Plesiosauroid
Stratesaurus S. taylori[16] 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[17]

Insects

Insect compression fossils are known from the localities of Binton in Warwickshire and Copt Heath near Birmingham.[18]

See also

  • White Lias
  • List of dinosaur-bearing rock formations

References

  1. ^ Ambrose, K.; 2001: The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands Archived 7 March 2006 at the Wayback Machine, Proceedings of the Geologists' Association 112(2), pp. 97-110.
  2. ^ "Wilmcote Limestone Member". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 20 April 2014.
  3. ^ "Museum Service – Warwickshire Heritage and Culture". Archived from the original on 21 April 2014. Retrieved 20 April 2014.
  4. ^ Ambrose, K., 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian - Early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association vol. 112, 97-110.
  5. ^ Williams, B.J. & Whittaker, A., 1974. Geology of the Country around Stratford-upon-Avon and Evesham. Memoirs of the Geological Survey of Great Britain. HMSO, London.
  6. ^ Wilson et al., 1990 Geology of the South Wales Coalfield, Part VI, the country around Bridgend Mem Br Geol Surv sheet 261 & 262 (England and Wales)
  7. ^ "Strategic Stone Study: A Building Stone Atlas of Somerset and Exmoor" (PDF). English Heritage. p. 11. Retrieved 11 October 2011.
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  14. ^ "PV R 1158". NHM Data Portal. NHM. Retrieved 10 August 2022.
  15. ISSN 1094-8074
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External links