| Computer Code: |
HTHE |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Bathonian Age (JN)
— Oxfordian Age (JO) |
| Lithological Description: |
The formation comprises medium to dark grey to brown mudstone and siltstone, predominantly, with sporadic thin sandstone stringers or concretions of pale brown limestone or dolomitic limestone. The mudstone is usually poorly fissile, soft to firm and moderately calcareous (due to the common calcareous microfossils). The mudstone is locally carbonaceous and pyritic. The limestone is generally microcrystalline and hard producing a low gamma and high velocity spikes. The sandstone stringers are usually friable, fine-grained and quartzose; locally with glauconite and mica. Thicker sandstone bodies, which are given member status (Bruce, Freshney and Ling sandstone members), are mainly very fine and fine-grained, sometimes micaceous and occasionally calcite cemented. A spiculitic sandstone occurs in the Inner Moray Firth (Alness Spiculite Member). |
| Definition of Lower Boundary: |
The nature of the lower boundary varies as the formation overlies a number of different units. It is usually recognised by a downward transition from mudstone and siltstone (Heather Formation) to sandstone or interbedded sandstone, siltstone and coal (Hugin, Tarbert or, in the Inner Moray Firth, Beatrice formations). The lower boundary is marked by a down-section decrease in gamma and velocity values. In the Outer Moray Firth and Central Graben the downward transition from mudstone to thinly interbedded sandstone, mudstone and coal (Stroma Member of the Pentland Formation) or red and grey-brown Triassic mudstone (Smith Bank Formation). |
| Definition of Upper Boundary: |
Mudstone of the Kimmeridge Clay overlies mudstone of the Heather Formation, so that the boundary is difficult to locate lithologically. It is usually taken at a sharp down-section decrease in gamma values, sometime associated with a decrease in resistivity and increase in velocity. However, in basinal sections there are a number of geophysical excursions that could be confused with the boundary. Often there is a down section lightening of the colour of the mudstones at the boundary, a decease in organic content and an increasein the proportion of thin limestone beds. Locally, the Brae, Fulmar or Piper formations overlie the Heather Formation and in this case the boundary can be recognised by the downward change from sandstone to mudstone. |
| Thickness: |
East Shetland Basin: up to 350 m. South Viking Graben: up to 600 m. Central North Sea: up to 700 m. |
| Geographical Limits: |
East Shetland Basin, Viking Graben and Central North Sea. North Sea Quadrants 1-3, 7-9, 11-22, 28-31. The Heather Formation is also defined west of Shetland. |
| Parent Unit: |
Humber Group (HMBG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 211/21- 1A at a depth of 2769-2840 m below KB (Deegan and Scull, 1977; Richards et al., 1993). |
| Reference Section |
North Sea well 3/01- 2 at 3535-3717 m depth (Richards et al., 1993). |
| Reference Section |
North Sea well 9/10c- 2 at 4201-4533 m depth (Richards et al., 1993). |
| Reference Section |
North Sea well 11/30- 2 at 1638.5-1929 m depth (Richards et al., 1993). |
| Reference Section |
North Sea well 30/08- 1 at 3990-4542.5 m depth (Richards et al., 1993). |
| Reference(s): |
| 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. |
| Andrews, I J, and Brown, S. 1987. Stratigraphic evolution of the Jurassic, Moray Firth. 785-795 in Petroleum Geology of North West Europe. Brooks, J, and Glennie, K W (editors). (London: Graham and Trotman.) |
| Boldy, S A R, and Brearly, S. 1990. Timing, nature and sedimentary result of Jurassic tectonism in the Outer Moray Firth. 259-279 in Tectonic events responsible for Britain's oil and gas reserves. Hardman, R F P, and Brooks, J (editors). Special Publication of the Geological Society, London, No.55. |
| Harker, S D, Mantel, K A, Morton, D J and Riley, L A. 1993. The stratigraphy of Oxfordian-Kimmeridgian (Late Jurassic) reservoir sandstones in the Witch Ground Graben, UK North Sea. American Association of Petroleum Geologists Bulletin, 77, 1693-1709. |
| 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 (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 |
