CORRELATION OF NW EUROPEAN PURBECK WEALDEN (NONMARINE LOWER CRETACEOUS) AS SEEN FROM THE ENGLISH TYPE-AREAS

Correlation within and between the onshore basins is reviewed and revised. The English Purbeck and Wealden stratotypes (zoned by ostracods and palynomorphs) lie in separate sub-basins, overlap in time and are not fully correlated (especially the Wealden). The overall age range is Berriasian-earliest Aptian (∼ 141(146)-119(125) m.y. B.P.), with the top of the Purbeck Formation at the Berriasian-Valanginian boundary and the Wealden Group base about mid-Berriasian. North of the mid-European source-massifs the Purbeck-Wealden facies are almost wholly 'Purbeck' (Berriasian) in age, with only a little lower 'Wealden' (earliest Valanginian) above. Ostracodpalynomorph subdivision is good, and correlation with the English stratotypes also good as far east as north Germany except for the west and north Netherlands basins. The Purbeck records must be fairly complete except in Poland (Lower and lower Middle divisions only). Correlation of the Bornholm sequence is largely enigmatic. On the massifs, Wealden lithofacies in palaeokarstic pockets doubtfully equate with upper Wealden (Hauterivian-Barremian). Those in the Hautrage basin could include younger strata. South of the massifs, the Paris basin succession broadly resembles that in England and must also be substantially Berriasian to Barremian. Published evidence on the Purbeck is scant, but ostracods and palynomorphs allow correlation with parts at least of the three divisions of the English stratotype. Some direct correlation of the overlying Wealden has been achieved in the same way, aided indirectly by intercalations of marine Hauterivian and Barremian sediments. Purbeck-Wealden facies in the Boulonnais are curiously different from those in the Paris and English basins. Near the Channel coast the succession must be largely Portlandian, and exposures to the east reveal possible upper Wealden (Barremian). "Purbeckian" carbonates in the French and Swiss Jura, clearly Berriasian from their marine contexts, yield palynomorphs and ostracods showing that the Lower, Middle and Upper divisions of the Purbeck stratotype are represented in some degree. Event stratigraphy is used with caution owing to the risk of circular arguments. Thus the widespread occurrence of gypsum near the base of the sequences might not, as often assumed, have been generated by a regional climatic event. Likewise transgression-regression phases, marine or nonmarine, do not necessarily reflect eustatic changes of sea-level. Both dilemmas are encountered in the English basins. Only independent tests of synchroneity can establish the reality of such events. Future progress in correlation may be aided by the absolute dating of volcanogenic materials and washed-in contemporary glauconites. © 1991.