The BGS Lexicon of Named Rock Units — Result Details

Horda Formation

Computer Code: HORD Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Eocene Epoch (GE) — Oligocene Epoch (GO)
Lithological Description: The Horda Formation consists predominantly of mudstone with subordinate sandstone. The mudstone is typically green-grey to olive-grey, but grades to brown-grey in the siltier, proximal sections and near the top of the formation in the Outer Moray Firth and South Viking Graben. Grey-red to purple mudstones, grading to waxy claystones, are prominent in the lower parts of many distal, condensed sections, especially in the Central Graben. They are associated with sporadic thin tephra layers, often preserved as green bentonitic clay. Minor amounts of variegated mudstone also occur in these condensed sections, with colours including cream, pale grey, pale green, and dark grey. Isolated sand bodies occur within the lower and middle parts of the formation, with more continuous sands occurring at the top of the formation in the South Viking Graben and Outer Moray Firth.
Definition of Lower Boundary: Over most of its area, the Horda Formation rests on the Balder Formation, the boundary being marked by a downward change from variegated mudstone and claystone to dark grey, carbonaceous claystone. Horda mudstones rest on sandstones of the Mousa Formation in western sections of the Northern North Sea (e.g. 9/7-1, 9/12b-6); elsewhere, they rest on mudstones of the Balder Formation or, in parts of the Outer Moray Firth, on sandstones or lignites of the Beauly Member. The base of the Horda Formation is unconformable over salt-induced highs in the south of Quadrant 21, with the Tay Sandstone Member resting variously on the Balder Formation, Sele Formation (e.g. 21/25-6), or Lista Formation (e.g. 21/22-1).
Definition of Upper Boundary: In mudstone-dominated sections, the upper boundary is characterized by a downward change from brown mudstone of the Lark Formation to green-grey mudstone (e.g. 22/1-1A, 21/22-1). On wireline logs, the boundary is marked by a downward decrease in gamma-ray values. In the South Viking Graben and Outer Moray Firth, the high-gamma mudstone at the base of the Lark Formation rests directly on glauconitic sandstone (e.g. 9/4-1) or on laterally equivalent brown-grey, silty mudstone (e.g. 16/18-1).
Thickness: The Horda Formation can reach over 800 m in thickness in the northern North Sea.
Geographical Limits: The Horda Formation is present over much of the Central and Northern North Sea, but passes laterally into the Mousa Formation along the eastern margin of the East Shetland Platform and in the west of the Outer Moray Firth.
Parent Unit: Stronsay Group (STRY)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 22/1-1 A: 1992-2379.5 m (6535-7806 ft) below KB. 
Reference Section  North Sea well 9/4-1: 1026.5-1835 m (3368-602l ft). 
Reference Section  North Sea well 16/18-1: 1852.5-2356.5 m (6078-7732 ft). 
Reference Section  North Sea well 21/22-1: 1076-1234.5 m (3530-4050 ft). 
Reference(s):
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable