Offaster pilula Zone
This zone is only recognised as a separate interval in southern England and Northern Ireland. In northern England, the zone probably equates with the greater part of the Sphenoceramus (Inoceramus) lingua Zone, and it equates with the lower part of the 'Gonioteuthis Zone' in East Anglia.
Base: In southern England, the base of the zone 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 zone is placed at Telscombe Marl 4 (Mortimore, 1986; Young & Lake, 1988), and in Northern Ireland is coincident with the top of the Creggan Chalk (but the zone here is incompletely developed; Wilson & Manning, 1978).
Range of index: In southern England, the index ranges from a short distance above the base of the zone to the upper G. quadrata Zone (Mortimore, 1986). In Northern Ireland, the index appears to be absent from the lower part of the zone, but related forms appear to range into the overlying G. quadrata and B. mucronata zones (Wilson & Manning, 1978).
|Inoceramus (comminuted shell extremely abundant in Northern Ireland)|
|O. pilula planata|
Faunal abundance & preservation: In southern England, the lower part of the zone is typically poorly fossiliferous, and the echinoids that dominate the fauna are usually crushed. Localised fossil-rich bands occur higher up, but the fauna still tends to be rather sporadic. In Northern Ireland, the basal part of the zone is moderately fossiliferous, but the higher beds less so (Wilson & Manning, 1978). The extreme hardness of the Chalk Group in Northern Ireland means that fossils are difficult to extract.
Bio-markers: The zone is characterised by a succession of bio-markers that can be used to differentiate particular horizons. Thus, the lower part of the zone is indicated by locally common Echinocorys depressula and E. tectiformis with sparse records of Offaster pilula (Young & Lake, 1988). Offaster pilula becomes more common in the higher part of the zone, where it is separated into two belts of abundance; the lower associated with an acme of Echinocorys truncata; an acme of Echinocorys cincta intervening between the two belts of O. pilula, and an acme of Offaster pilula planata coinciding with the top of the upper belt, at the top of the zone (Young & Lake, 1988). In Northern Ireland, the only bio-markers are Echinocorys depressula and E. tectiformis, suggesting that the higher part of the pilula Zone is absent here (Wilson & Manning, 1978).
|Subzones:||Subzone of abundant Offaster pilula|
|Echinocorys depressula Subzone|