The BGS Lexicon of Named Rock Units — Result Details

Alness Spiculite Member

Computer Code: ALNS Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Oxfordian Age (JO) — Oxfordian Age (JO)
Lithological Description: The member comprises sandy spiculite grading to spiculitic sandstone. Ovoidal spicules dominate, being derived from the siliceous sponge Rhaxella. Spicules are very fine- to fine-grained sand grade. The sandstone is generally intensely bioturbated, but in the type section, wavy, horizontal laminae are preserved at the base. Bivalves, belemnites and ammonites occur in the type section. Two large scale upward coarsening cycles occurs from grey-brown, spiculitic siltstone into sandy spiculites. An upward fining unit grading from sandy spiculite to spiculitic siltstone is commonly present at the top of the member. A thin mudstone unit, or where this is missing, a higher gamma argillaceous horizon can be used to subdivide the member (Stephen et al., 1993).
Definition of Lower Boundary: The base is defined by a down-section transitional change from spiculites to mudstones. The boundary is marked by an inflection on the gamma-ray and sonic logs.
Definition of Upper Boundary: The top of the member is defined by an abrupt downward change from mudstones to spiculites in some sections, but a more transitional change in others. The boundary is marked by a decrease in gamma-ray values and a decrease in velocity.
Thickness: Up to 90 m.
Geographical Limits: Restricted to the Inner Moray Firth and thickly developed the central and northern part (Quadrants 11, 12 and 13 and the very northern part of Quadrant 19).
Parent Unit: Heather Formation (HTHE)
Previous Name(s): Alness Member (-5138)
Alness Spiculite Formation (-5139)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 11/25-1 between the depths 2751.5 and 2811 m below KB (Richards et al., 1993). 
Reference Section  North Sea well 12/22-3 between the depths 1281.5 and 1324.5 m (Richards et al., 1993). 
Reference(s):
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. Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham] 
Andrews, I J and Brown, S. 1987. Stratigraphic evolution of the Jurassic, Moray Firth. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 785-795. [Graham & Trotman, London] 
Stephen, K J, Underhill, J R, Partington, M A and Hedley, R J. 1993. The genetic sequence stratigraphy of the Hettangian to Oxfordian succession, Inner Moray Firth. In: Parker, J R (ed.) Petroleum geology of Northwest Europe: Proceedings of the 4th conference, 485-505. [Geological Society, London] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable