Echinocorys depressula Subzone

From Earthwise
Jump to navigation Jump to search

The subzone is generally developed in regions where the parent O. pilula Zone can be recognised (see above).

Base: In southern England, the base of the subzone is placed at the highest of the three Friar's Bay Marls (Young & Lake, 1988), and in Northern Ireland is coincident with the base of the Creggan Chalk (Wilson & Manning, 1978).

Top: In southern England, the top of the subzone is placed at a thin marl c. 1 m beneath the Old Nore Marl (Young & Lake, 1988). In Northern Ireland, the top of the Creggan Chalk is within the E. depressula Subzone, but because of a probable non-sequence with the immediately overlying Boheeshane Chalk (Wilson & Manning, 1978), the top of the Creggan Chalk is probably not coincident with the top of the subzone.

Range of index species: In southern England, data on the range of the index are variable. Mortimore (1986) recorded that it ranged from the upper M. testudinarius Zone, through the U. anglicus Zone, to the top of the nominate subzone. However, Young & Lake (1988) restricted the range of the index to the nominate subzone and basal part of the Subzone of abundant O. pilula. Data on the range of the index in Northern Ireland are incomplete.

Key fauna:

Brachiopoda: Cretirhynchia exsculpta
Bivalvia: Oxytoma (Hypoxytoma) tenuicostatum*
Pseudoperna boucheroni
Belemnoidea: Actinocamax versus
Echinoidea: Echinocorys depressula
E. tectiformis

(*:occurs in the subzone in Northern Ireland)

Faunal abundance & preservation: In southern England the subzone is rather poorly fossiliferous, and many of the echinoids which dominate the fauna are crushed. In Northern Ireland, the subzone seems to be relatively more fossiliferous than in southern England (Wilson & Manning, 1978), particularly in the basal part.

Bio-horizons: Within the subzone, Pseudoperna boucheroni is common in the basal part in both Northern Ireland and southern England, and in the latter region, an acme of Actinocamax versus occurs towards the middle of the subzone (Mortimore, 1986; Young & Lake, 1988; Wilson & Manning, 1978).

Age: Campanian