Belemnella occidentalis Zone

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Base: In Northern Ireland, the base of the zone is placed at the base of Bed C of the Ballycastle Chalk (sensu Fletcher, 1977). In East Anglia, the base of the zone is placed at an unspecified level in the upper part of the Trimmingham Sponge Beds Member (Wood, 1967; Johansen & Surlyk, 1990).

Top: This is the youngest zone in the Cretaceous of the UK, and so the top is undefined.

Range of index species: The index does not apparently range below the base of the nominate zone in Northern Ireland or East Anglia (Wilson & Manning, 1978).

Key fauna:

Brachiopoda: Chatwinothyris?
Cretirhynchia limbata s.s.
Bivalvia: Agerostrea lunata*
Pycnodonte vesiculare
Belemnoidea: Belemnella lanceolata
B. occidentalis
Echinoidea: Echinocorys belgica trans. E. ciplyensis
E. aff. limburgica
Galerites sp.

(*: occurs in the zone in East Anglia)

Faunal abundance & preservation: The zone is richly fossiliferous in East Anglia, but much less so in Northern Ireland where the extreme hardness of the chalk also makes collection of the fauna difficult (Peake & Hancock, 1970; Wood, 1967).

Bio-markers: In East Anglia, the Echinocorys near the base of the zone are transitional to E. ciplyensis, being flatter than typical E. belgica, and higher in the zone there is a profusion of the oyster Agerostrea lunata (Peake & Hancock, 1970; Johansen & Surlyk, 1990). Near the top of the zone in East Anglia there is a calcarenite characterised by huge, thick-shelled Pycnodonte vesiculare and large Galerites, and the Echinocorys are smaller and resemble E. limburgica (Peake & Hancock, 1970). In Northern Ireland, the zone has a limited fauna, being dominated by Belemnella, with Cretirhynchia, Chatwinothyris? and Echinocorys (Wood, 1967).

Age: Early Maastrichtian