| Computer Code: |
LOSD |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Triassic Period (T)
— Triassic Period (T) |
| Lithological Description: |
The Lossiehead Formation in Quadrant 12 is composed of reddish brown, white and pale grey sandstones and reddish brown, olive-grey, grey and purple mudstones and siltstones. In the south of the area, the sandstones fine upwards from mainly fine to medium grained, but occasionally coarse and pebbly at the base, to very fine or fine grained. Further north, the sandstones are mainly very fine or fine grained throughout. Sorting is poor or moderate, and the sandstones are variably argillaceous and variably calcareous.
Sandstone predominates over mudstone in central Quadrant 13, where the Lossiehead Formation is thickest. The sandstones display variable grain-size trends, the coarsest sandstones occurring at the base of the formation in well 13/18-1 but at the top of the formation in nearby well 13/13-1. Otherwise, the sediments are very similar to those in Quadrant 12, except that they include a few thin beds of limestone that may be calcretes. |
| Definition of Lower Boundary: |
Sandstones of the Lossiehead Formation rest on relatively clean sandstones of the Hopeman Sandstone Formation in the western Inner Moray Firth. The boundary is generally sharp, and is marked by a downward decrease in gamma-ray values (e.g. 12/28-1). In southeastern Quadrant 12 and southwestern Quadrant 13, the Lossiehead Formation rests on silty Triassic mudstones (Smith Bank Formation), the boundary being marked by a sharp downward increase in gamma-ray values (e.g. 12/30-1). Further east, the Lossiehead Formation oversteps the basal Triassic formations to rest on Upper Permian clastic sediments (Bosies Bank Formation). Mudstone forms a more significant component of the Upper Permian section, and the boundary is defined by a downward increase in gamma-ray values, velocity and resistivity (e.g. 13/19-1). |
| Definition of Upper Boundary: |
The Lossiehead Formation is conformably overlain by the Stotfield Calcrete Formation in the western Inner Moray Firth. The boundary is sharp in some sections, transitional in others (e.g. 12/28-1). It is generally marked by a downward increase in gamma-ray values and corresponding decrease in velocity and resistivity. The boundary between equivalent sediments onshore at Stotfield is transitional, with highly silicified sandstone grading up into an increasingly carbonate-rich rock containing pockets of unaltered sandstone (Naylor et al., 1989). In the remaining eastern areas of the Inner Moray Firth, the Lossiehead Formation is directly overlain by Middle Jurassic or younger strata. |
| Thickness: |
The Lossiehead Formation thickens northeastwards across Quadrants 11 and 12; it is about 160 m thick in well 11/30-6 and about 410 m thick in well 12/24-2. Equivalent sediments are about 75 m thick in coastal outcrops around Lossiemouth (Peacock et al., 1968). Middle Jurassic uplift has caused partial erosion of the Triassic from southwestern Quadrant 13, where the Lossiehead Formation is less than 115 m thick. The formation comprises the whole of the Triassic north of the Halibut Horst and, despite partial erosion, it is also at its thickest in that area, up to 770 m of sediments having been drilled in central Quadrant 13. Roberts et al. (1990) estimated that up to 1200 m of Triassic deposits are present adjacent to the Wick Fault. They suggested that local thickness variations in the Triassic were caused by synsedimentary faulting that partitioned the Inner Moray Firth Basin into a number of small sub-basins. |
| Geographical Limits: |
The Lossiehead Formation is restricted to the Inner Moray Firth. Bounded to the north by the Wick Fault and to the west by the Helmsdale and Great Glen faults, the formation has been eroded over, and to the south of, the Halibut Horst. The formation extends southwards through Quadrants 17 and 18 to the Moray Firth coast. |
| Parent Unit: |
Heron Group (HERO)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 12/28- 1: 2303.5-2672.5 m (7557-8768 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 12/30- 1: 1925.5-2100 m (6317-6890 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 13/19- 1: 1203-1481.5 m (3947-41860 ft) (Cameron, 1993). |
| Reference(s): |
| Andrews, I J, Long, D, Richards, P C, Thomson, A R, Brown, S, Chesher, J A, and McCormac, M. 1990. The geology of the Moray Firth. (London: British Geological Survey.) |
| Frostick, L, Reid, I, Jarvis, J and Eardley, H. 1988. Triassic sediments of the Inner Moray Firth, Scotland: early rift deposits. Journal of the Geological Society (London), Vol. 145, 235-248. |
| Naylor, H, Turner, P, Vaughan, D J and Fallick, A E. 1989. The Cherty Rock, Elgin: A petrographic and isotopic study of a Permo-Triassic calcrete. Geological journal, Vol. 24, p. 205 - 221. |
| Peacock, J D, Berridge, N G, Harris, A L, May, F. 1968. The geology of the Elgin District. Memoir of the Geological Survey of Scotland. Sheets 86 and 96 (Scotland). (Edinburgh: HMSO). |
| Roberts, A M, Badley, M E, Price, J D, and Huck, I W. 1990. The structural history of a transtensional basin: Inner Moray Firth, NE Scotland. Journal of the Geological Society, London 147, 87-103. |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
