Lias Group

The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) 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.

In the past, the name Lias was often not only used for the sequence of rock layers, but also for the timespan during which they were formed. It was thus an alternative name for the Early Jurassic epoch of the geologic timescale. It is now known that the Lias is Rhaetian to Toarcian in age (over a period of ca. 20 million years between 200 to 180 million years ago) and thus also includes a part of the Triassic. The use of the name "Lias" for a unit of time is therefore slowly disappearing.

Subdivisions

In southern England, the Lias Group is often divided into Lower, Middle and Upper subgroups. In Somerset the Lias is divided into the following formations (from top to base):

• Helwell Marl
• Doniford Shale
• Quantock Beds
• Kilve Shale
• Blue Lias
• St. Audries Shale
• Aldergrove Beds

It is underlain by the Late Triassic Penarth Group,^[2] and overlain, after a stratigraphic hiatus, by the Early Cretaceous Upper Greensand Formation.

In Dutch lithostratigraphy, the name Lias has no official status, however, it is often used for the lower part of the Altena Group in the subsurface of the Netherlands and the southern North Sea.^[3]

In northern Germany, the Lias Group consists of nine formations (from top to base):^[4]

• Opalinuston
• Dörnten-Formation
• Posidonia Shale
• Amaltheenton
• Capricornu-Formation
• Raricostaten-Formation
• Arieten Sandstone
• Liassicus Sandstone
• Psilonoten Sandstone

See also

• White lias

References

Notes

Literature

• Wong, Th.E., (2007): Jurassic, in: Wong, Th.E.; Batjes, D.A.J. & Jager, J. de (eds.): Geology of the Netherlands, Royal Netherlands Academy of Arts and Sciences, pp. 107–125.