The BGS Lexicon of Named Rock Units — Result Details

Balder Formation

Computer Code: BALD Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Eocene Epoch (GE) — Eocene Epoch (GE)
Lithological Description: The Balder Formation consists of light to dark grey, generally fisile, carbonaceous, pyritic mudstone with abundant tuff layers in the lowest part. The mudstone is well laminated in the lower part, with light to medium grey indurated siliceous mudstone alternating with medium to dark grey soft, fissile mudstone. In the upper part, the mudstone is fissile, but soft and poorly laminated. In parts of the Viking Graben, the tuff-rich mudstones include thin reddish grey layers, often with diffuse boundaries, and often associated with minor bioturbation. The tuffs mostly occur as thin layers up to a few centimetres in thickness, with sharp bases and commonly displaying normal size grading; these are interpreted as undisturbed air-fall tuffs. Several cored sections in the Viking Graben have revealed units several tens of centimeteres thick that are internally structureless or display dewatering structures; such units are interpreted as resulting from grvity-flow resedimentation of the primary air-fall tuff (Malm et al. 1984). Sporadic calcite-cemented concretions or concretionary beds are present in some sections, as indicated by high-velocity spikes (e.g. 9/18a-15). Sandstone units up to 200 m thick occur in the Balder Formation in central and northern parts of the Viking Graben; thinner sandstone units occur in southwestern parts of the Central Graben (principally Quadrant 29). The sandstones are typically well sorted, clean and poorly cemented, although tightly cemented sandstones occur locally. In the Viking Graben, some sandstones in the upper part of the formation are believed to have been emplaced by injection from lower levels. The sandstone units in 204/9-1 and 205/16-1 may be informally referred to as the 'Balder Sandstone'.
Definition of Lower Boundary: The base of the Balder Formation is taken at a sharp downward decrease in the thickness of tuff layers and in the proportion of tuff to laminated mudstone. It is marked on wireline logs by a sharp downward increase in gamma-ray values; a downward decrease in sonic velocity typically takes place a few metres below (e.g. 21/10-1). A blocky log response is displayed where sandstone is present in the lower part of the formation, as in parts of the South Viking Graben (e.g. 9/18a-15; 9/19-7Z). Where the Balder Formation rests directly on the Dornoch Formation (e.g. 14/25-1) it is represented by variably tuffaceous siltstone or mudstone, generally resting on low-gamma sandstone or lignite.
Definition of Upper Boundary: The top of the Balder Formation is normally defined by a downward change from grey-green or variegated mudstones of the Horda Formation, or from green-grey glauconitic siltstones of the basal Mousa Formation, to medium to dark grey, pyritic mudstone. In the south of Quadrants 21 and 22, the Balder Formation is locally overlain by sandstones of the Tay Formation. In basinal sections, the top of the Balder Formation is taken at the base of a sharp high-gamma peak (e.g. 21/10-1), which is believed to represent a phosphate-rich condensed section at the base of the Horda Formation. The upper part of the Balder Formation often displays low resistivity values, and a marked downward decrease in resistivity values commonly serves to identify the top of the formation in sections where the gamma peak is poorly developed or absent (e.g. 15/28-2).
Thickness: Typically 30 to 50 m thick; 100 m in Outer Moray Firth, 300 m+ in the South Viking. Typically 50-150 m on the UK NW Margin.
Geographical Limits: The Balder Formation is present throughout most of the Central and Northern North Sea. It is, however, absent from most sections in the Outer Moray Firth, where the Mousa Formation rests directly on the Dornoch Formation. It is also locally absent along the southwestern margin of the Central Graben (e.g. 21/18-1A, 21/25-6), as a result of erosion before or during deposition of the Tay Sandstone. Lott & Knox (1994) and Knox et al. (1997) extended the geographical range of the Balder Formation to include the Southern North Sea and West of Shetland areas, respectively.
Parent Unit: Moray Group (MORA)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  Norwegian Sector North Sea well N25/11-1 (Deegan & Scull 1977, p.34, fig.44; Isaksen & Tonstad 1989, p.46, fig.57): 1705-1780 m (5595-5840 ft) below KB. According to the definition proposed here (for the UK sector), the base of the formation would be placed at c. 1777 m. 
Reference Section  North Sea well 9/18a-15: 1589.5-1893 m (5215-6210 ft). 
Reference Section  North Sea well 15/28-2: 1734.5-1883.5 m (5690-6180 ft). 
Reference Section  North Sea well 21/10-1: 2068-2100 m (6784-6890 ft). 
Reference Section  North Sea well 22/5b-3: 2468.5-2497 m (8158-8192 ft). 
Knox, R W O'B, Holloway, S, Kirby, G A and Baily, H E. 1997. Stratigraphic nomenclature of the UK North West Margin. 2. Early Paleogene lithostratigraphy and sequence stratigraphy. British Geological Survey, Nottingham. 
Lott, G K and Knox, R W O'B. 1994. 7. Post-Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Knox, R W O'B and Holloway, S. 1992. 1. Paleogene 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. 
Isaksen, D and Tonstad, K. 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No.5 
Malm, O A, Christensen, O B, Furnes, H, Lovlie, R, Ruselatten, H and Ostby, K L. 1984. The Lower Tertiary Balder Formation: an organogenic and tuffaceaous deposit in the North Sea region. In: Spencer, A M et al. (eds) Petroleum geology of the North European margin, 149-170. Graham and Trotman, London. 
Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable