The BGS Lexicon of Named Rock Units — Result Details

Lower Nordland Unit

Computer Code: NORDL Preferred Map Code: notEntered
Status Code: Full
Age range: Miocene Epoch (NM) — Miocene Epoch (NM)
Lithological Description: On the shelf, lithological information is derived from BGS boreholes and commercial wells. These sampled the outer shelf sheet deposit, which is a sandstone-dominated facies, the Muckle Ossa Sandstone, characteristically green, glauconitic, and bioclastic (Stoker et al., 1993; Evans et al., 1997). The sandstone facies dominates the entire cored section in most of the sampled intervals, although well 202/8-1 records interbedded lignites and thin mudstones, and a basal mudstone facies underlies the sandstone. The regional extent of this mudstone remains unknown. Sampling of the deeper-water succession is restricted to a few commercial wells sited on the upper slope, primarily in blocks 208 and 209. Well 208/15-1A penetrated the acoustically-structureless upper slope section north of 61°N, and documented a sandstone-dominated facies, composed of yellow-brown, medium- to coarse-grained, poorly sorted, bioclastic and glauconitic sand, with sporadic concretions of unspecified composition. Subordinate interbeds of dark grey, soft, sandy mudstone were also logged. Similar sandstones were recorded in nearby wells 209/6-1 and 209/12-1, although the upper part of the section in 209/6-1 appears to be dominated by pale brown-grey mudstones with thin interbeds of medium- to coarse-grained, shelly sands. In contrast, well 209/9-1 proved an interbedded sequence of grey to dark grey, soft to firm, calcareous mudstones and olive grey to brown-black, well sorted, glauconitic sandstones with occasional shell fragments, lignitic and shale clasts. Farther south, well 204/28-1 may have recovered traces of glauconitic sandstone from the discrete, mounded, mass-flow package developed in this area. Deeper-water sediments, in particular the sediment drifts, have not been sampled. On the basis of regional evidence, it is likely that they comprise relatively fine-grained lithologies (Stoker et al., 1993).
Definition of Lower Boundary: On the shelf, the lower boundary appears to be a composite erosion surface which truncates Palaeogene and older strata, and was, at least in part, formed in association with the latest Oligocene/earliest Miocene unconformity. The latter is a distinctive, commonly irregular, sequence boundary which forms the base of this unit, and the Nordland Group in general, on the slope and basin floor. The easily recognised, seismic expression of the lower boundary is one of the main distinguishing criteria in the recognition of the Nordland Group, west of Shetland.
Definition of Upper Boundary: The upper boundary is everywhere marked by an angular unconformity; this is the intra-Neogene unconformity (INU) over most of the area, but where this is absent it is marked by the glacial unconformity. On the shelf and upper slope, the nature of the boundary is characteristically erosional, varying from planar, where associated with the INU, to irregular, where glacial erosion has occurred. In deeper water, the boundary is more commonly a depositional surface with the overlying Middle Nordland strata displaying onlapping and downlapping relations.
Thickness: On the West Shetland Shelf, the Lower Nordland unit is preserved as an eroded, fairly uniform sheet deposit, locally up to 150 m thick. This contrasts with the slope and basinal areas where the unit is characterised by a variable geometry dependant upon its component packages. South of 61°N, prograding wedges with associated mounded slump deposits, up to 180 m thick, occur in the middle to upper slope region, whilst eroded remnants of mounded, upslope-accreting sediment drifts, up to 120 m thick, are present on the lower slope. In the area of block 204, these deposits are overlain by a discrete, thick, mounded, mass-flow deposit up to 140 m thick. The basin floor is characterised by a sheet drape up to 50 m thick, with sporadic, channel infills which may exceed 150 m in thickness. North of 61°N, the slope and basinal deposits are dominated by sheeted-to-mounded drift deposits, which locally exceed 200 m thick, and commonly migrate up the slope. Some overlap with slump bodies occurs on the upper slope.
Geographical Limits: The Lower Nordland unit extends from the outer part of the West Shetland Shelf into the Faroe-Shetland Channel. On the outermost shelf and upper slope, it has been mostly eroded away, although a shelf-to-basinal connection is preserved in block 205. The landward limit is marked by a well-defined erosional pinch-out, due mainly to glacial erosion. The basinal deposits are locally absent at the south-west end of the Faroe-Shetland Channel, and in the south-western part of the North Sea Fan.
Parent Unit: Nordland Group (NORD)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  North Sea well 202/08- 1: 221.0 - 365.0 m below sea bed (Stoker, 1999). 
Reference Section  North Sea well 208/15- 1A: 367.0 - 447.0 m below sea bed (Stoker, 1999). 
Reference(s):
Stoker, M S. 1999. Stratigraphic nomenclature of the UK North West Margin. 3. Mid- to Late Cenozoic Stratigraphy. BGS, Nottingham. 
Stoker, M S, Hitchen, K, and Graham, C C. 1993. United Kingdom offshore regional report: the geology of the Hebrides and West Shetland shelves and adjacent deep-water areas. (London: HMSO for the British Geological Survey.) 
Evans, D, Morton, A C, Wilson, S, Jolley, D, and Barreiro, B A. 1997. Palaeo-environmental significance of marine and terrestrial Tertiary sediments on the NW Scottish shelf in BGS borehole 77/7. Scottish Journal of Geology, 33, 31-42. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable