The age of the base of the Gustav Group in the James Ross Basin, Antarctica
The Lagrelius Point Formation from its type area in north-west James Ross Island, Antarctica has yielded dinoflagellate cysts indicative of an earliest Aptian age. Reworked palynomorphs presumed to be from the Nordenskjöld Formation (Kimmeridgian-Berriasian) were also encountered. The Lagrelius Point Formation also contains Early Cretaceous spore-pollen floras of Austral aspect. The indigenous stratigraphically significant dinoflagellate cysts includeHerendeenia postprojecta,Muserongiaspp.,Odontochitinaspp. andOvoidinium cinctum. This assemblage indicates that the Lagrelius Point Formation, the oldest formation of the Gustav Group, is Aptian rather than Barremian in age. This in turn means that the base of the extensive Cretaceous marine succession in the James Ross Basin can be dated accurately for the first time. The Lagrelius Point Formation is correlated with the AustralianOdontochitina operculatadinoflagellate cyst Oppel Zone and theCyclosporites hughesiispore-pollen Interval Zone. The extensive record of Aptian marine sedimentation within the James Ross Basin can be correlated directly with that of other key localities in the Antarctic Peninsula and Scotia arc regions. There is still the possibility of a major stratigraphical hiatus in the preceding Hauterivian-Barremian stages.