Mercia Mudstone Group

Mercia Mudstone Group Stratigraphic range: early Triassic to Rhaetian
Type	Group
Unit of	New Red Sandstone Supergroup
Sub-units	Blue Anchor Formation, Branscombe Mudstone Formation, Arden Sandstone Formation, Sidmouth Mudstone Formation, Tarporley Siltstone Formation
Underlies	Penarth Group
Overlies	Sherwood Sandstone Group
Thickness	over 1350m
Lithology
Primary	mudstone
Other	siltstone, sandstone, halite, anhydrite
Location
Country	England
Extent	widespread
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 – indeed 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.

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.

Clearly, older schemes will remain in maps and literature well into the future, 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.

The group outcrops widely across England, representing deposition within numerous Triassic basins, some of which are physically connected at depth. From the south there is an almost continuous outcrop from the Wessex Basin of east Devon, Somerset and Dorset, through the Bristol/South Wales area and the Worcester and Knowle Basins into the English Midlands and including the Needwood Basin of Staffordshire. Northwards the outcrop splits either side of the Pennines where deposition took place across the East Midlands Shelf of Nottinghamshire and through Yorkshire to the North Sea coast at Hartlepool. A western arm includes the Stafford and Cheshire basins, West Lancashire and the Carlisle Basin - the latter are connected at depth beneath the Irish Sea.

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