Understanding the Mercia Mudstone Group

The Mercia Mudstone Group represents a significant geological formation within the British Isles, primarily composed of mudstone and siltstone. Understanding its stratigraphy and lithological variations is crucial for various geological applications, from resource exploration to infrastructure development. The British Geological Survey (BGS) has undertaken extensive work to rationalise and unify the nomenclature applied to this group, acknowledging the complexities arising from historical regional variations in naming conventions. This article aims to provide a comprehensive overview of the Mercia Mudstone Group, detailing its constituent formations, their characteristics, and the rationale behind the BGS's revised lithostratigraphical framework.

What is the Mercia Mudstone Group?

The Mercia Mudstone Group, primarily found in England and Wales, is characterised by its predominantly red and, less commonly, green and grey mudstone and siltstone deposits. Significant halite (rock salt) deposits are found in thicker, basinal successions, alongside sulphate deposits like gypsum and anhydrite, and sandstone beds. The group spans the Mid Triassic (Anisian) to the latest Triassic (Rhaetian) periods. It typically overlies sandstones of the Sherwood Sandstone Group, though it can also rest directly on older rocks. The overlying unit is usually the Penarth Group, specifically the Westbury Formation.

The Challenge of Nomenclature

Historically, a plethora of local names have been applied to formations and members within the Mercia Mudstone Group. This arose due to several factors:

• Depositional restriction: Units were often confined to individual basins during their formation.
• Post-depositional isolation: Faulting and erosion subsequently isolated outcrops.
• Varying survey approaches: Differences in lithostratigraphical methods across different BGS mapping projects, especially over time, led to inconsistent naming.

This lack of a unified scheme presented challenges for creating seamless digital geological maps and 3D models, and for ensuring consistency in geological interpretation across the country. The BGS embarked on a review to address these issues.

The BGS Review and Revised Lithostratigraphy

The BGS review aimed to establish a logical, robust, and mappable lithostratigraphical scheme for the Mercia Mudstone Group. The primary objectives were to:

• Adequately describe the lithological and stratigraphical variability.
• Promote stability in nomenclature for clarity.
• Create a rationalised, mappable stratigraphy applicable across England and Wales.
• Support the compilation of a 'seamless' digital geological map and 3D model.

The review identified five key lithostratigraphical units (Units A to E) that were considered to have a high degree of continuity, either currently or historically. These units form the basis of the revised nomenclature.

The Five Formations of the Mercia Mudstone Group

Following a full rationalisation approach, the BGS has recommended the following five formations for the Mercia Mudstone Group:

1. Tarporley Siltstone Formation (Unit A)

This heterolithic unit is characterised by interlaminated and interbedded siltstone, mudstone, and sandstone. The sandstones are typically fine-grained and micaceous. Historically, it was known by various names such as 'Waterstones'.

Key Features:

• Lithology: Interbedded siltstone, mudstone, sandstone.
• Age: Mid Triassic (Anisian).
• Thickness: Varies, up to 220 m in the Cheshire Basin.
• Subdivisions: Malpas Sandstone Member, Woodthorpe Member, Retford Member, Colwick Member.

2. Sidmouth Mudstone Formation (Unit B)

Predominantly red and, less commonly, green and grey dolomitic mudstones and siltstones. This formation can contain substantial deposits of halite, which are of economic importance. It is characterised by the presence of 'skerries', resistant beds of sandstone.

Key Features:

• Lithology: Red and grey mudstones and siltstones, often with halite.
• Age: Mid Triassic (Anisian) to Late Triassic (Carnian).
• Thickness: Highly variable, up to 1600 m in the Cheshire Basin.
• Subdivisions: Numerous members including halite formations and mudstone formations, e.g., *Radcliffe Member, *Northwich Halite Member, *Wilkesley Halite Member.

3. Arden Sandstone Formation (Unit C)

A heterolithic unit distinguished by grey and green mudstone interbedded with paler siltstone and varicoloured sandstone. It is often fossiliferous and characterised by an exotic clay mineral assemblage. It is known for forming cuesta-like features.

Key Features:

• Lithology: Grey/green mudstone, siltstone, varicoloured sandstone.
• Age: Late Triassic, late Carnian (Tuvalian).
• Thickness: Typically 7 to 24 m.
• Subdivisions: None.

4. Branscombe Mudstone Formation (Unit D)

This formation superficially resembles Unit B, but with less common sandstone-rich units. Structureless red-brown, dolomitic mudstones dominate, with abundant gypsum and anhydrite deposits. It has a consistent geophysical log profile.

Key Features:

• Lithology: Red-brown mudstones, gypsum, anhydrite.
• Age: Late Triassic, Norian to ?Rhaetian.
• Thickness: Up to 240 m in the Wessex Basin.
• Subdivisions: Several members including gypsum beds, e.g., Red Rock Gypsum Member, Seaton Mudstone Member.

5. Blue Anchor Formation (Unit E)

The uppermost formation of the Mercia Mudstone Group, characterised by pale greenish grey, dolomitic mudstones and siltstones, often referred to as the 'Tea Green Marl'. It is widespread across England and Wales.

Key Features:

• Lithology: Pale greenish grey, dolomitic mudstones and siltstones.
• Age: Late Triassic; late Norian to early Rhaetian.
• Thickness: Typically 5 to 20 m, up to 67 m.
• Subdivisions: Rydon Member, Williton Member.

Distribution of the Mercia Mudstone Group

The Mercia Mudstone Group crops out across various regions of England and Wales, including east Devon, Somerset, the Bristol region, south Wales, the Gloucester and Worcester regions, the central Midlands, Nottinghamshire, Yorkshire, Cheshire, Merseyside, and Cumbria. In many of these areas, the outcrop is masked by superficial deposits. The group also extends eastwards below younger Mesozoic rocks and pinches out around the London–Brabant Massif.

Contacting the British Geological Survey

For detailed geological information, publications, or to consult their extensive archives, you can contact the British Geological Survey through their official channels. Their website, www.bgs.ac.uk, provides comprehensive information on their services, research, and how to access their data and reports. Specific queries regarding publications can often be directed to their online shop or information offices.

Frequently Asked Questions

What is the primary lithology of the Mercia Mudstone Group?

The group is predominantly composed of red, and less commonly green and grey, mudstone and siltstone. It also contains significant deposits of halite (rock salt) and evaporites like gypsum and anhydrite in certain areas.

Why was a revision of the Mercia Mudstone Group nomenclature needed?

The historical use of numerous local names for formations and members within the group led to inconsistencies and difficulties in mapping and digital modelling. The BGS review aimed to create a unified, rationalised lithostratigraphical scheme.

What are the five proposed formations for the Mercia Mudstone Group?

The five formations are the Tarporley Siltstone Formation, Sidmouth Mudstone Formation, Arden Sandstone Formation, Branscombe Mudstone Formation, and Blue Anchor Formation.

What is the age range of the Mercia Mudstone Group?

The group ranges in age from Mid Triassic (Anisian) to the latest Triassic (Rhaetian).

Where can I find more detailed information about BGS publications?

More detailed information on BGS publications, including formal definitions of geological units, can be found on the BGS website, particularly in the BGS Lexicon of Named Rock Units, which is accessible online.

Conclusion

The British Geological Survey's efforts to rationalise the lithostratigraphy of the Mercia Mudstone Group have resulted in a more coherent and unified classification system. The adoption of five key formations – Tarporley Siltstone, Sidmouth Mudstone, Arden Sandstone, Branscombe Mudstone, and Blue Anchor – provides a consistent framework for geological mapping, research, and resource assessment across Great Britain. This revised nomenclature is essential for understanding the geological history and subsurface characteristics of this important Triassic sequence.