The BGS Lexicon of Named Rock Units — Result Details

Dartmouth Group

Computer Code: DRTM Preferred Map Code: Drtm
Status Code: Full
Age range: Lochkovian Age (DO) — Pragian Age (DP)
Lithological Description: In the Plymouth region, the Dartmouth Group is divided into the Whitsand Bay Formation and the Bin Down Formation; this is an intercalation towards the top of the Whitsand Bay Formation. The Whitsand Bay Formation comprises reddish purple mudstone and silty mudstone (beds commonly up to 2m thick), green silty mudstone to muddy siltstone (with bedding units of between 5 to 10m thick), green to mauve siltstone (in massive beds up to 6m thick that are predominantly structureless but have local lenses and laminae), grey-green with some purple sandstone (beds up to 0.8m thick with local conglomerate above erosional bases) and off-white to pale green quartzite (isolated beds up to 0.5m thick, with some grading and low-angle cross lamination). The Bin Down Formation includes grey to dark grey cleaved mudstone, grey siltstone (laminated in place) and pale grey quartzitic sandstone. Interbedded with these sedimentary rocks are basaltic lavas and volcaniclastic beds including tuff and hyaloclastite.
Definition of Lower Boundary: This is a thrust-fault contact; this unit forms the hanging walls of two thrusts to the west of the Cawsand Fault. The thrusts are dextrally displaced by the Portnadler, Portwrinkle and Rame Faults, and place the Dartmouth Group on top of the Bovisand Formation of the Meadfoot Group. In the field, the lower boundary of the Dartmouth Group is distinguished by an abrupt transition from grey slaty mudstone with thin interbeds of grey-green siltstone and fine-grained sandstone of the Meadfoot Group at the fault plane, up into reddish purple mudstone and green silty mudstone to muddy siltstone of the Dartmouth Group.
Definition of Upper Boundary: This is a transitional boundary with the overlying Bovisand Formation of the Meadfoot Group. The contact is seen at Tregantle Cliffs [SX 388 526], where the uppermost strata of the Whitsand Bay Formation of the Dartmouth Group comprise interbedded red and green slaty mudstone with sporadic thin quartzose sandstone beds and grey slaty mudstone; this sequence is succeeded by rocks of the overlying Bovisand Formation, which comprise bioturbated grey mudstone with sandstone laminae, thin beds and load casts and black phosphatic nodules. Near Oldhouse Cove [SX 367 536] the contact is expressed as an upward passage into grey slaty mudstone with thin interbeds of grey-green siltstone and fine sandstone, with dark grey mudstone becoming predominant over an interval of 20m.
Thickness: To 3300m
Geographical Limits: Within the Variscan outcrop in southwest England, the Dartmouth Group forms an approximately east-west trending band, between Watergate Bay [SW 8335 6519] on the north Cornwall coast to Scabbacombe Head [SX 9224 5119] on the east coast of south Devon.
Parent Unit: Not Applicable (-)
Previous Name(s): The Wembury Siltstones (-4624)
Dartmouth Group [Obsolete Name And Code: Use DRTM] (DTB)
Dartmouth Beds (-2157)
Dartmouth Slates [Obsolete Name And Code] (DTMS)
The Scobbiscombe Sandstones (-2766)
The Yealm Formation (-2767)
The Warren Sandstones (-872)
Alternative Name(s): Whitsand Bay Formation
Bin Down Formation
Stratotypes:
Type Area  Along the coast of east Cornwall, between Freathy and Rame Head, exposing the Whitsand Bay Formation. Leveridge and others, 2002. 
Reference(s):
Smith, S A and Humphreys, B. 1989. Lakes and alluvial sandflat plays in the Dartmouth Group, south-west England. Proceedings of the Ussher Society, Vol.7, 118-124. 
Dineley, D L. 1966. The Dartmouth Beds of Bigbury Bay, south Devon. Quarterly Journal of the Geological Society, London. Vol.122, 187-217. 
Leveridge B E, Holder, M T, Goode, A J J, Scrivener R C, Jones N S and Merriman, R J. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales). 
Ussher, W A E. 1890. The Devonian rocks of south Devon. Quarterly journal of the Geological Society, London. Vol.46, 487-517. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E355 E356 E350 E348 E347 E346 E349