OBSERVATIONS ON THE LITHOSTRATIGRAPHY AND AMMONITE SUCCESSION OF THE APTIAN (LOWER CRETACEOUS) LOWER GREENSAND OF CHALE BAY, ISLE-OF-WIGHT,UK

A revised lithostratigraphy is given for parts of the Atherfield Clay and Ferruginous Sands Formations (Lower-Upper Aptian) of the Lower Gre ensand Group of the standard section in Chale Bay, Isle of Wight, reta ining in essence the classic nomenclature of Fitton. Changes in zonal and subzonal boundaries are proposed on the basis of new collections o f ammonites obtained bz situ. The Upper Lobster Beds Member of the Ath erfield Clay is detached from the Subzone of Deshayesites callidiscus and placed in a separate Subzone of Deshayesites annelidus at the top of the Zone of D. forbesi. The influx of Megatyloceras, Cheloniceras, Pseudosaynella and Sinzovia at this level has palaeogeographic implica tions, pointing to the opening of a marine link with the Paris Basin a nd confirming the Upper Lobster Beds as a transgressive horizon. The u pper part of the Scaphites Beds Member of the Ferruginous Sands, previ ously included in the deshayesi Zone, is transferred to the Subzone of Dufrenoyia transitoria, at the base of the Zone of Tropaeum (T.) bowe rbanki, on the strength of the occurrence therein of species of Tropae um and Dufrenoyia. Tropaeum is not present below the bowerbanki Zone a s now defined; its primitive ancyloceratoid (hooked) forms such as T. (T.) hillsii have limit ed biochronological value, persisting alongsid e the criocone (spiral) species of the bowerbanki Zone and giving rise to a local horizon of T. (T.) pseudohillsi at the bottom of the Upper Crioceras Beds (basal Upper Aptian). A more precise delimitation of t he Walpen Clay & Sands from the Upper Crioceras Beds is made and the j unction of these two members of the Ferruginous Sands is reaffirmed as the Lower/Upper Aptian boundary, coinciding with the junction of the Zones of T. (T.) bowerbanki and Epicheloniceras martinioides. Bed-by-b ed collecting across the boundary shows that Epicheloniceras can no lo nger be taken as the prime biostratigraphical marker for the base of t he Upper Aptian; greater significance is given to the extinction of Du frenoyia. The sources of the genus Vectisites and some species of Ammo nitoceras and Caspianites, hitherto fixed only vaguely in the column, are pinpointed in the martinioides Zone. The horizon of Tropaeum (T.) drewi is confirmed as high in the bowerbanki Zone (meyendorffi Subzone ) and the validity of a 'drewi Zone' at the base of the German Upper A ptian (=base of Middle Aptian, Kemper, 1995) is questioned. In a revie w of the Lower/Upper Aptian boundary, it is suggested that the German stratum be called the Horizon of Cicatrites? tenuinodosus and placed t entatively at the top of the Lower Aptian. These and other changes in ammonite nomenclature are explained in the systematic notes. (C) 1998 Academic Press.