Blue Lias Formation

The Blue Lias Formation is a sedimentary formation that occurs in the south, west and east of England, as well as in southern Wales , Northern Ireland and western Scotland . It was deposited in the period from the Upper Triassic to the Lower Jura .

etymology

Watercolor of the Lias Sea by Henry Thomas de la Bèche (1830)

The name Blue Lias is derived from the English. blue, alluding to the blue-gray color of the rock - in contrast to the underlying White Lias , which is white. Lias is an old dialect expression used by quarry workers for layers and refers to the characteristic layer structure.

history

As early as 1811, Mary & Joseph Anning discovered the first Ichthyosaurus in the Blue Lias formation near Lyme Regis . A first detailed description of the formation was made in 1826 by Henry Thomas de la Bèche . Thomas Wright created a stratigraphy of the formation in 1886, into which he incorporated his pioneering work on the Lias ammonites . A standard work is the Geological Survey Memoir from 1911 by HB Woodward and WAE Usher. In 1923 WD Lang began his microstratigraphic monumental work, in which he described the entire formation centimeter by centimeter.

geography

Nash Point, Glamorgan, Wales.

The Blue Lias Formation, often referred to simply as the Blue Lias , occurs in the following sedimentary basins in Great Britain :

• in the Wessex Basin on the south coast of England. There are beautiful outcrops on Dorset's Jurassic Coast , particularly on the cliffs of Charmouth and Lyme Regis . Upcoming is the Blue Lias Formation in the north of the Wessex Basin in Somerset , particularly around Glastonbury and in the Polden Hills .
• in the Bristol Channel Basin in south west England. The formation on the coast of South Wales and in the Vale of Glamorgan is open . The type locality of the Blue Lias Formation is at Saltford near Bath .
• in the Cardigan Basin on the west coast of Wales.
• in the Celtic Sea basin in south west Great Britain.
• in the Cleveland Basin in north west England. In the Eastern Midlands , the Blue Lias Formation forms a vast plain.
• in the Hebrides Basin on the north coast of Ireland ( Antrim ) and on the north-west coast of Scotland ( Skye ).

stratigraphy

The Blue Lias Formation west of Lyme Regis

The generally flat Blue Lias formation, the lowest formation of the Lias group , concordantly overlays the Langport member of the Lilstock formation of the Penarth group after a shift gap . In places it can reach down to the deeper Cotham member . The Blue Lias Formation is in turn concordantly overlaid by the Charmouth Mudstone Formation . Three members can be eliminated within the formation (from young to old):

• Rugby Limestone Member
• Saltford Shale Member
• Wilmcote Limestone Member

Lithologically , the blue lias formation, which is 30 to 40 meters thick, consists of a rhythmic alternation of limestone , marl , rarer siltstones and dark claystones in the decimeter range. The partially laminated, bulbous or massive and persistent limestone layers can reach a thickness of 10 to 30 centimeters. It is assumed that these alternating sequences are related to Milanković cycles . However, it is not clear which of these cycles is ultimately decisive for the Blue Lias formation ( precession with 26,000 or inclination of the earth's axis with 41,000 years).

An idealized change cycle that becomes between 20 and 100 centimeters thick is structured as follows (from hanging wall to lying):

• Final lime action
• Marl layer
• Bituminous mudstone layer
• Basal marl layer

The basal marl layer transgresses via the limestone layer of the previous cycle, which was pierced and digested on its surface by worm structures and other sediment dwellers. It can contain bivalves and ammonites . The bituminous, finely layered claystone layer, which can contain up to 10% organic matter and 5% calcium carbonate , lays concordantly on top . Its base is flat, but its surface is irregular, as there are buildings running through it. This layer, which is very likely in the deeper water, contains pyrite and is generally very poor in fossils, but can occasionally show ammonites, fish scales and small bivalves. It is assumed that there were anaerobic conditions on the seabed and that the organic content is due to explosively multiplying dinoflagellates in the photic range. The subsequent marl layer of 65% clay and 35% calcite is also heavily interspersed with worm structures at its upper limit. Inside the shell layer of couples find bivalves and sometimes even with oysters occupied tree trunk remains . The final lime layer is a calcilutite , which consists of 80% CaCO ₃ and 20% clay. It is quite rich in fossils (bivalves, gastropods , brachiopods , echinoderms and uncrushed ammonites) and its top is rich in worm structures of the Taxa Chondrites , Diplocraterion and Thalassinoides . The lime action was likely discontinued under regressive conditions .

The Blue Lias formation is built up from over 50 of these alternating sequences. After the lagunar Penarth group of the Rhaet, it is the first clearly marine formation in Great Britain in the Mesozoic. The water depth and thus the marine character of the formation increase towards the hanging wall and the marine fauna is diversifying.

Fossil content

The pterosaur Dimorphodon

The Blue Lias Formation extends over five ammonite zones (from young to old):

• Arnioceras semicostatum
• Arietites bucklandi - Sinemurium
• Schlotheimia angulata - Hettangium
• Alsatites liasicus
• Psiloceras planorbis

Below is the preplanorbian zone , which represents the beginning of the hettangium.

In addition to the zone ammonites, there are other taxa such as B. Agassiceras , Arnioceras obliquicostatum , Arnioceras pseudokriodon , Caloceras intermedium , Charmasseiceras , Euagassiceras , Paracoroniceras lyra , Psiloceras plicatulum , also Nautiloidea such as Cenoceras striatum and Nautilus and belemnites .

In invertebrates , the blue lias formation contains bivalves ( Gryphaea , Inoceramus , Liogryphaea , Liostrea , Ostrea , Oxytoma inequivalvis , Pecten , Plagiostoma , Uniocardium and Volsella minima ), brachiopods ( Calcirhynchia and Spiriferina ), crinoids ( Pentacrinites ) and echinoderms.

Numerous vertebrate finds were also made in the Blue Lias formation:

• Fish , mainly Teleostomi ( Actinopterygii ). Most of the fish finds come from the Fish Bed and Fish Bed Shales in the uppermost section of the Blue Lias Formation, including the taxa:
  • Belonorhynchus
  • Caturus
  • Chondrosteus
  • Dapedium
  • Eugnathus
  • Heterolepidotus
  • Osteorachis
  • Pholidophorus
  • Ptycholepis

The shark-like cartilaginous fish Hybodus is worth mentioning .

• Dinosaurs , including the
  • Ceratosauria (undetermined remains from Warwickshire ), including
    • Sarcosaurus woodi (from Leicestershire ) and Sarcosaurus andrewsi (shin from Warwickshire)
    • Scelidosaurus harrisonii (two fairly complete finds from Dorset)
  • Theropoda (remains from Antrim and Dorset that cannot be identified, including " Megalosaurus lydekkeri ")
  • Thyreophora (undetermined remains from County Antrim )
• Ichthyosauria with the taxa
  • Eurypterygius communis , Eurypterygius intermedius and Ichthyosaurus platyodon
• Plesiosauria with
  • Rhomaleosaurus cromptoni
• Pterosauria
  • Dimorphodon macronyx (from Lyme Regis)

In addition to the tree trunks already mentioned, the Blue Lias formation also carries plant remains , including the Cycadales Cycadeoidea gracilis , Cycadeoidea pygmaea , Cycadites , Otozamites and Pagiophyllum as well as the ferns Ctenopteris and Thinfeldia . The lignite tree trunks indicate the presence of other conifers such as araucarias or cypresses .

Age

Absolute ages for the Blue Lias Formation are not known. Biostratigraphically , the hettangium begins about 4 meters above the basal, brown, lignite-containing claystone layer (layer H1). So the formation started in the uppermost Rhätium . The top layer (layer 53) ends the Arnioceras semicostatum - ammonite zone of the Sinemurium . The Blue Lias Formation thus spans roughly the period 200 to 194 million years BP .

literature

• Woodcock, N. & Strachan, R .: Geological History of Britain and Ireland . Blackwell Science, 2000, ISBN 0-632-03656-7 .

Individual evidence

1. ^ Henry Thomas De la Bèche: II. On the Lias of the Coast, in the Vicinity of Lyme Regis, Dorset. In: Transactions of the Geological Society of London . Series 2, volume 2 , no. 1 , January 1826, ISSN  2042-5295 , p. 21-30 , doi : 10.1144 / transgslb.2.1.21 ( biodiversitylibrary.org ).
2. ^ T. Wright: Monograph on the Lias Ammonites of the British Islands . In: Monograph of the Palaeontographical Society . London 1886 ( biodiversitylibrary.org - published between 1878 and 1886).
3. ↑ HB Woodward, WAE Ussher with contributions from AJ Jukes-Browne: The Geology of the Country near Sidmouth and Lyme Regis . In: Memoirs of the Geological Survey of England and Wales. Explanation of Sheets 326 and 340 . Her Majesty's Stationery Office, London 1911 ( archive.org ).
4. ^ WD Lang: The Blue Lias of the Devon and Dorset coasts . In: Proceedings of the Geologists' Association . tape 35 , 1924, pp. 169-185 , doi : 10.1016 / S0016-7878 (24) 80032-9 .

Web links

• Dr. Ian West with the Blue Lias formation west of Lyme Regis