The base of the Albian Stage; comments on recent proposals

Three recent proposals for defining the base of the Albian Stage (Casey 1999; Kennedy et al. 2000; Hancock, 2001) are discussed on the basis of the ammonite succession. Since Breistroffer (1947), the stage boundary in terms of ammonites has been placed at the first occurrence of Leymeriella (Proleymeriella) schrammeni, the earliest of a succession of three Leymeriella faunas. In terms of geographical occurrence, this specific boundary can only be recognised clearly in the classic north German succession in the Hannover-Braunschweig area in which no permanent section is available. Casey (1999) favoured the boundary to be placed earlier, at the first appearance of the geographically widespread genusHypacanthoplites , a partial return to a boundary accepted in the earlier half of the 20th Century. Unfortunately, this proposal is not supported by an uncondensed permanent section in which Acanthoplites (Nolaniceras) occurs below the boundary and Hypacanthoplites above. Kennedy et al. (2000) raised the boundary to a position within the succession of Leymeriella faunas established in north Germany. This proposal is supported by a relatively uncondensed, more permanent section in the Vocontian Basin, France, in which multidisciplinary studies have been made. However, in terms of ammonites, the boundary point chosen is of local value only, set within a series of Leymeriella species chrons which are essentially confined to the European Province and immediately adjacent Tethyan and Boreal areas. The occurrence of the geographically widespread nannofossil Praediscosphaera columnata close to their ammonite-based boundary, lends support to the proposal that the first occurrence of this species should mark the base of the Albian Stage. However, there is globally a dearth of correlation data provided by other taxa, which would confirm that the first occurrence of P. columnata is isochronous. It is concluded that all previous proposals for the emplacement of the Aptian/Albian boundary are unsatisfactory. The radical proposal of Hancock (2001) that the base of the Albian Stage be placed at the geographically widespread and interprovincial first occurrence of the ammonite Lyelliceras lyelli, formerly regarded as the base of the Middle Albian Substage, is supported. Crown