| Computer Code: |
NORDM |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Pliocene Epoch (NP)
— Mid Pleistocene (QPM) |
| Lithological Description: |
On the shelf, lithological information is derived from BGS boreholes and commercial wells. Most of these sampled the prograding wedge on the outer shelf, which was proved to consist of an interbedded sequence of sands and muds (e.g. 82/10, 84/04). The sands are compact and firm, display a range of colours, including dark grey, grey brown, greenish grey and olive grey, are generally poorly sorted, very fine- to very coarse-grained, and slightly muddy, with common scattered pebbles, shell debris, and occasional glauconite. By way of variation, sands in the lower part of 84/04 are very fine- to fine-grained and well-sorted. The top of this sand unit may coincide with a shift in log response, although the signal is muted due to the start of the 'thicker' drill collars. The interbedded muds vary from very dark grey to yellowish brown, are commonly firm to hard, and also contain scattered pebbles and shell fragments. The sand facies is predominant in the recovered cores, consistent with the natural gamma-log signatures. There are also subordinate interbeds of laminated to thin-bedded sands and muds, and rare gravel bands (e.g. 84/04), sporadic lignites (e.g. 202/8-1), and thin limestones and siltstones (e.g. 206/8-2).
The deeper-water slope succession, in front of the prograding wedge, was tested by a few commercial wells with variable recovery. On the upper to middle slope, well 208/15-1A proved an interbedded sequence of sands and muds comparable to the outer shelf, with the sand facies similarly dominant. In contrast to the outer shelf, the muds were generally softer. Thin limestones were reported from well 208/27-1. A significant change in the nature of the slope-apron occurs to the north of Shetland where wells 219/20-1 and 219/28-1 document a mud-dominated succession on the North Sea Fan.
The basinal succession has not been sampled, although the predominance of sediment-drift deposits implies relatively fine-grained lithologies. A high terrigenous component is likely due to the proximity of the West Shetland and Faroe shelves, although information from the Rockall Trough (cf. Stoker et al., 1993) suggests that interbeds of hemipelagic oozes cannot be discounted. |
| Definition of Lower Boundary: |
The lower boundary is the intra-Neogene unconformity (INU). This is an angular unconformity on the shelf and slope, being erosional on the shelf, but more probably a surface of non-deposition on the slope. It is downlapped by clinoforms of the prograding wedge, and onlapped by upslope-accreting basinal strata. In the basin, the boundary appears sub-parallel with the underlying strata of the Lower Nordland unit, and may be locally transitional. In the North Sea Fan, the INU may have re-excavated the latest Oligocene/earliest Miocene unconformity. |
| Definition of Upper Boundary: |
The upper boundary is the glacial unconformity. On the shelf and upper slope, this is characteristically planar to irregular, and erosive. Lower on the slope, the boundary is more planar in form and commonly a downlap surface for the overlying Upper Nordland sediments. In the basin, the boundary appears more conformable, and may locally be transitional with the Upper Nordland unit. |
| Thickness: |
West of Shetland, the Middle Nordland unit characteristically forms a shelf-margin, prograding wedge of sediment, which is up to 350 m in thickness on the outer shelf and upper slope but thins into the basin where it is generally <100 m thick. Farther north, the slope and deep-water section appear to be generally thicker where (1) the basin floor widens and deepens, and (2) the North Sea Fan is developed. The thicker basinal accumulations have been locally moulded by bottom currents into sediment-drift and -wave forms. In blocks 208 and 209, the slope sediments have locally failed in the area of the Miller Slide. |
| Geographical Limits: |
The Middle Nordland unit occurs widely on the West Shetland Margin, extending from the outer West Shetland Shelf into the Faroe-Shetland Channel. The landward limit of the unit is marked by a well-defined erosional pinch-out on the outer shelf, being everywhere truncated by the glacial unconformity. The unit is locally absent at the south-west end of the Faroe-Shetland Channel. |
| Parent Unit: |
Nordland Group (NORD)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
BGS borehole 82/10: 46.9 - 103.5 m below sea bed (Stoker, 1999). |
| Reference Section |
BGS borehole 84/04: 62.5 - 154.0 m below sea bed (Stoker, 1999). |
| Reference Section |
North Sea well 208/15- 1A: 75.0-367.0 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.) |
| Cockcroft, D N. 1987. The Quaternary sediments of the Shetland Platform and adjacent continental shelf margin. Unpublished Ph.D. thesis, University of Keele. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
