The BGS Lexicon of Named Rock Units — Result Details

Tarn Moor Formation

Computer Code: TMF Preferred Map Code: TMF
Status Code: Full
Age range: Llanvirn Series (OE) — Llanvirn Series (OE)
Lithological Description: The lower part comprises laminated and thickly laminated mudstone with subordinate siltstone beds. The middle part is mudstone and locally, siltstone with up to 5% tuffaceous beds. The highest part comprises dark grey to black mudstones.
Definition of Lower Boundary: Not yet proved, probably unconformable on the Buttermere Formation.
Definition of Upper Boundary: Not seen as it is unconformably overlain by rocks of the Borrowdale Volcanic Group.
Thickness: 1000 to 1500m
Geographical Limits: Occurs within the southeast part of the Skiddaw inlier, bounded to the north by the Causey Pike Fault. It forms all of the Ullswater and Bampton inliers. It is unconformably overlain by the Borrowdale Volcanic Group, the youngest part of the Formation is in the Tarn Moor Tunnel.
Parent Unit: Skiddaw Group (SKG)
Previous Name(s): Tarn Moor Mudstone (-3083)
Tarn Moor Mudstone Formation (-3727)
Tarn Moor Mudstones (-1867)
Mudstones Of Tarn Moor (-4930)
Alternative Name(s): none recorded or not applicable
Type Area  Southeast part of the Skiddaw inlier, south of the Causey Pike Fault. The Formation equates with the Kirkland Formation of the Cross Fell Inlier (Burgess and Wadge, 1974). The base of the Formation has not yet been proved but its age and outcrop pattern indicate that it lies above the Buttermere Formation. It is likely that it lies unconformably on the highly disrupted Buttermere Formation. cooper et al, 1995. 
Type Area  The Ullswater and Bampton inliers. Mudstones of D. Murchisoni Biozone age were first described in the Tarn Moor Tunnel (Ullswater inlier) and named the Tarn Moor Mudstone (Wadge, Nutt and Skevington, 1972). The highest part of the Tarn Moor Formation (dark grey to black mudstones of the Murchisoni Biozone faulted against artus Biozone mudstones) is exposed in the Tarn Moor Tunnel where it is unconformably overlain by rocks of the Borrowdale Volcanic Group. Cooper et al, 1995. 
Cooper, A H, and Webb, B C. 1988. Slump Folds and Gravity Slide Structures in a Lower Palaeozoic Marginal Basin Sequence (The Skiddaw Group), north west England. Journal of Structural Geology, Vol.10, (5), p.463-472. 
Cooper, A H, Rushton, A W A, Molyneux, S G, Hughes, R A, Moore, R M and Webb, B C. 1995. The stratigraphy,correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District. Geological Magazine, Vol.132(2), 185-211. 
Wadge, A J, Nutt, M J C and Skevington, D, 1972. Geology of the Tarn Moor Tunnel in the Lake District. Bulletin of the Geological Survey of Great Britain, No. 41, 55 - 73. 
Burgess, I C and Wadge, A J. 1974. The geology of the Cross Fell area. Explanation of 1:25 000 Geological Special Sheet comprising parts of sheets NY 53, 62, 63, 64, 71, 72, 73. (London HMSO). 91pp. 
Moseley F. 1984. Lower Palaeozoic lithostratigraphical classification in the English Lake District. Geological Journal, Vol. 19, p. 239-247. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E030 E029