The BGS Lexicon of Named Rock Units — Result Details

Watchet Mudstone Formation

Computer Code: WATCH Preferred Map Code: notEntered
Status Code: Full
Age range: Rhaetian Age (TR) — Rhaetian Age (TR)
Lithological Description: Thin to thickly bedded pale and medium grey, calcareous mudstones with variable shell content, with lenticular or nodular beds of muddy limestone. Subordinate lithologies include porcellanous limestone, calcareous sandstone and siltstone.
Definition of Lower Boundary: The base of the formation is an erosion surface where calcareous mudstone infills an irregular surface at the top of off-white, porcellanous limestones of the White Lias Formation.
Definition of Upper Boundary: This is taken at the sharp upward change from pale grey, calcareous mudstone to laminated, fissile, brownish grey, organic-rich mudstones with common fish scales of the Blue Lias Formation (this is probably the Wilmecote Limestone Member).
Thickness: The formation ranges in thickness at outcrop from 0 to 0.5m in Somerset and is around 2m thick on the south Glamorgan coast. The thickest development is 4.8m in the St Fagan’s Borehole [ST1169 7813] (ST17NW/276).
Geographical Limits: North Somerset and south Glamorgan coasts, and the Bridgend area with possible extensions into mid-Somerset.
Parent Unit: Penarth Group (PNG)
Previous Name(s): Watchet Beds (-5077)
Watchet Member (-5078)
Alternative Name(s): Passes Laterally Into White Lias Formation
Stratotypes:
Reference Section  St Mary’s Well Bay, Glamorgan [ST 1745 6779] (Richardson, 1905; Gallois, 2009), exposing 2.2m of mudstone. It is overlain by the St Mary’s Well Bay Member of the Blue Lias Formation and underlain by the White Lias Formation. 
Type Section  Doniford Bay, Watchet, Somerset [ST 0830 4350], exposing a full thickness of c. 0.46m (Gallois, 2009). It is overlain by the Blue Lias Formation and underlain by the White Lias Formation. 
Reference(s):
Gallois, R. W. 2009. The stratigraphy of the Penarth group of the Severn Estuary area. Geoscience in south-west England, Vol. 12, 71-84 
Richardson, L. 1905. The Rhaetic and contiguous deposits of Glamorgan. Quarterly Journal of the Geological Society of London, Vol. 61, 385-422 
Richardson, L. 1911. The Rhaetic and contiguous deposits of west, mid, and part of east Somerset. Quarterly Journal of the Geological Society of London, Vol. 67, 1-74 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E262 E263 E278 E279