The BGS Lexicon of Named Rock Units — Result Details

Lark Formation

Computer Code: LARK Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Oligocene Epoch (GO) — Miocene Epoch (NM)
Lithological Description: The Lark Formation consists of a monotonous succession of brown-grey to grey-brown mudstone, silty in part, and locally grading upwards into sandstones of the Skade Formation. Thin sanstones of presumed shelf origin occur near the base of the Lark Formation in the South Viking Graben.
Definition of Lower Boundary: The base of the Lark Formation is marked by a downward change from relatively uniform grey-brown or brown-grey mudstones to a more variable association of green-grey mudstone, grey silty mudstone, or sandstone of the Horda Formation. The boundary is typically marked by a sharp downhole decrease in gamma-ray values.
Definition of Upper Boundary: In argillaceous sections, the top of the Lark Formation is marked by a downhole change fron green-grey, commonly glauconitic, silty mudstone (Nordland Group) to grey-brown or brown-grey mudstone. The boundary is marked by a downhole decrease in gamma values, with a distinct high-gamma peak often occurring in the basal sediments of the Nordland Group.
Thickness: The Lark Formation reaches a maximum thickness of c.1000 m within the Central Graben.
Geographical Limits: The Lark Formation is present over much of the Central and Northern North Sea, being absent only in western parts of the Outer Moray Firth and on the East Shetland Platform, where it passes into glauconitic sandstones and siltstones of shelf facies (Skade Formation). Lott & Knox (1994) extended the geographical extent of the Lark Formation to include the Southern North Sea area.
Parent Unit: Westray Group (WESY)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 21/10-4: 1217-1867 m (3993-6125 ft) below KB. 
Reference(s):
Lott, G K and Knox, R W O'B. 1994. 7. Post-Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham]. 
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