Mercia Mudstone Group

Mercia Mudstone Group
Stratigraphic range: Early Triassic-Rhaetian
Type	Group
Unit of	New Red Sandstone Supergroup
Sub-units	Stratigraphy
Underlies	Penarth Group
Overlies	Sherwood Sandstone Group
Thickness	over 1,350 m (4,430 ft)
Lithology
Primary	Mudstone
Other	Siltstone, sandstone, halite, anhydrite
Location
Coordinates	52°18′19″N 1°41′47″W / 52.3052°N 1.6965°W / 52.3052; -1.6965
Region	Scotland, England, Wales
Country	United Kingdom
Type section
Named for	Mercia

The Mercia Mudstone Group is an early Triassic lithostratigraphic group (a sequence of rock strata) which is widespread in Britain, especially in the English Midlands—the name is derived from the ancient kingdom of Mercia which corresponds to that area. It is frequently encountered in older literature as the Keuper Marl or Keuper Marl Series.^[1]

The Mercia Mudstone Group is now divided into five formations recognised and mappable across its entire outcrop and subcrop. The formations are a mix of mudstones, siltstones, sandstones and halites. Historically this sequence of rocks has been subdivided in different ways with different names in each of the basinal areas in which it is found. Increasing knowledge of the sequences and the more recent development of seamless electronic mapping by the British Geological Survey (BGS) necessitated a reappraisal of these divisions. A report published by BGS in 2008 recommended the abandonment of previous divisions and naming schemes in favour of a simpler approach which, having now been adopted, is set out below.^[2]

• Blue Anchor Formation
• Branscombe Mudstone Formation
• Arden Sandstone Formation
• Sidmouth Mudstone Formation
• Tarporley Siltstone Formation

Older schemes will remain in maps and literature well into the future,^[3] providing a source of potential confusion. An example might be the Arden Sandstone Formation which previously enjoyed lower status as a member and also higher status as a group.