BGS Zone 20 (=UKB16 of Hart et al., 1989)
Base: Inception of Gavelinella usakensis.
Top: Inception of Gavelinella monterelensis.
- Eouvigerina gracilis and Reussella kelleri extinction at the base of the foraminiferal zone, immediately below the Arundel Sponge Bed and lateral equivalents.
- 'Bolivinoides culverensis becomes common with the inception of Gavelinella usakensis immediately below the Arundel Sponge Bed.
- Gavelinella cristata becomes extinct immediately above the Arundel Sponge Bed.
- Rare Gavelinella cf. voltziana appears for the first time above the Lancing Flint. Gavelinella trochus is also present, although the exact stratigraphical position of its inception is not fully known due to rarity of the species in the earliest part of its range.
- Gavelinella lorneiana generally disappears from the record, temporarily, immediately above the Lancing Flint and lateral equivalents (Lazarus Effect). It reappears in the middle part of the foraminiferal zone (middle G. quadrata macrofaunal zone) at the Cotes Bottom Flint.
- Rugoglobigerina pilula, Stensioeina exsculpta gracilis and common Gavelinella stelligera disappear from the record at the top of the zone (at the Farlington Marls and lateral equivalents).
20i. (un-named subzone): From the base of the zone to the inception of Eouvigerina galeata.
20ii. Eouvigerina galeata Subzone: Index species has its inception between Pepper Box Marls and Lancing Flint
20iii. Pullenia quaternaria Subzone: Marked by the inception of the index species at Whiteclff Marl (upper G. quadrata Zone) Gavelinella clementiana and Spiroplectammina baudouiniana have their inception at the base of the subzone, but are generally rare.
20iv. Bolivinoides decoratus Subzone: Defined by the extinction of common B. culverensis and the inception of common B. decoratus at Scratchell’s Marl 1 in the uppermost G. quadrata macrofaunal zone.
Macrofaunal correlation: G. quadrata Zone (excluding the lowermost part).