The BGS Lexicon of Named Rock Units — Result Details

Strathsteven Mudstone Formation

Computer Code: SSTV Preferred Map Code: notEntered
Status Code: Full
Age range: Callovian Age (JC) — Callovian Age (JC)
Lithological Description: Mudstone and siltstone, generally bituminous with significant subordinate units of silty sandstone, commonly muddy and glauconitic. At Brora, the overall thickness is about 55 m. The lowermost unit is the Brora Roof Bed - up to 2.3 m of bioturbated, shelly medium-grained sandstone with scattered quartzite pebbles and coal fragments. It is overlain by about 2.3 m of silty sand grading up into about 25 m of bituminous fissile mudstone with thin interbedded glauconitic sandstone beds. This is the Brora Shale Bed that, together with the Brora Roof Bed, makes up the Brora Shale Member (Arkell, 1933; Sykes, 1975). This is overlain by 10.1 m of coarsening-upwards glauconitic muddy and silty sandstones with a siltstone interbed - the Glauconitic Sandstone Member (Sykes, 1975), overlain by 15 m of bituminous sandy siltstone with mudstone intercalations; five fining-upwards cycles have been recognised - the Brora Brick Clay Member (Arkell, 1933; Sykes, 1975). The formation yields marine biota and marine microfossils throughout. The macrofauna is dominated by ammonites, belemnites, bivalves, brachiopods and gastropods (Cox and Sumbler, 2002, p.374) and dinoflagellate cysts are abundant and diverse (Riding, 2005). The sequence stratigraphy of the formation was interpreted by Stephen and Davies (1998).
Definition of Lower Boundary: The formation overlies the Brora Coal (Bed) of the Brora Coal Formation. The boundary is marked by a sharp planar lithological change where bioturbated medium-grained sandstone (the Brora Roof Bed) rests on the Brora Coal. The boundary appears to be erosional.
Definition of Upper Boundary: The upper boundary is defined by a change up from the bituminous sandy siltstone with mudstone intercalations of the Brora Brick Clay Member to the coarse siltstone to silty fine-grained sandstone of the Fascally Siltstone Member of the Clynekirkton Sandstone Formation. The boundary is conformable.
Thickness: At the type section at Brora, the thickness of the formation is about 55 m.
Geographical Limits: The Strathsteven Mudstone Formation is confined to the Moray Firth area, north-east Scotland. It is present in the coastal strip around Brora, up to the Brora-Helmsdale Fault, at outcrop and at depth. It correlates offshore in the Inner Moray Firth with the lower part of the Beatrice Formation of the Fladen Group (Richards et al., 1993).
Parent Unit: Sutherland Group (SUTH)
Previous Name(s): Brora Argillaceous Formation (BRARG)
Brora Argillaceous Series (-1382)
Alternative Name(s): none recorded or not applicable
Type Section  The type section is a composite of the foreshore south of the River Brora estuary and the cliffs of the River Brora. The type section of the Brora Shale Member is the foreshore [NC 904 031] south of the River Brora estuary, to the east of Inverbrora Farm [NC 893 034]. The stratotype of the Glauconitic Sandstone Member is in the cliffs in the north bank of the River Brora [NC 889 040]. The Type sections of the Brora Brick Clay Member are at the west end of cliffs of the River Brora [NC 887 038], and on the foreshore [NC 907 032] south of the mouth of the River Brora, (Sykes, 1975 as modified herein; British Geological Survey, 1998; Trewin and Hurst, 1993). 
Reference Section  Brick pit section [NC 897 040] near the village of Brora, exposes middle to upper part of the Brora Brick Clay Member, (Sykes, 1975). 
Reference Section  Shaft for coal [NC 8831 0185] at Strathsteven exposes Brora Roof Bed near the surface, overlying Brora Coal (Lee, 1925, p.79). 
Type Area  Brora area, Sutherland, Scotland. 
Barron, A J M, Lott, G K, and Riding, J B. 2012. Stratigraphical framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf. British Geological Survey Research Report RR/11/06. 
Sykes, R M. 1975. The stratigraphy of the Callovian and Oxfordian stages (Middle-Upper Jurassic) in northern Scotland. Scottish Journal of Geology, Vol.11, 51-78. 
Cox, B M and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series No.26. [Peterborough: Joint Nature Conservation Committee.] 
Lee, G W and Pringle, J. 1932. A synopsis of the Mesozoic rocks of Scotland. Transactions of the Geological Society of Glasgow, Vol.19, 158-224. 
Richards, P C, Lott, G K, Johnson, H, Knox, R W O'B. and Riding, J B. 1993. 3. Jurassic of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.). Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Trewin, N H, and Hurst, A R (editors). 1993. Excursion guide to the geology of east Sutherland and Caithness. (Edinburgh: Scottish Academic Press.) 
Riding, J B. 2005. Middle and Upper Jurassic (Callovian to Kimmeridgian) palynology of the onshore Moray Firth Basin, northeast Scotland. Palynology, Vol. 29, 87-142. 
Stephen, K J, and Davies, R J. 1998. Documentation of Jurassic sedimentary cycles from the Moray Firth Basin, United Kingdom North Sea. 481-506 in Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. SEPM Special Publication 60. Graciansky, P-C, Hardenbol, J, Jacquin, T, and Vail, P R (editors). 
Arkell W J, 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
British Geological Survey, 1998. Helmsdale. Scotland Sheet 103E. Solid and Drift. 1:50 000 Geology Series. British Geological Survey, Keyworth, Nottingham. 
Lee, G.W., 1925. Mesozoic rocks of East Sutherland and Ross, In: Read, H.H., Ross, G., Phemister, J. (Eds.), The geology of the country around Golspie, Sutherlandshire. Memoir of the Geological Survey, pp. 65-114. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable