The thick sequence of fossiliferous marine and non-marine sandstones and siltstones on Seymour I. represents the most complete well-exposed section of Upper Cretaceous to Lower Tertiary rocks known in the Southern Hemisphere. The succession has been divided into the Upper Campanian to possibly lowermost Tertiary Marambio Group comprised of the Lopez de Bertodano and Sobral formations and the Lower Tertiary Cross Valley and La Meseta formations of the Seymour Island Group.
The Lopez de Bertodano Formation consists of c. 1067 m of medium-grained to silty concretionary sandstones of Middle Campanian to possibly Maestrichtian age, and the Sobral Formation of c. 210 m of medium- to coarse-grained glauconitic concretionary sandstones. The precise age of the Sobral Formation is uncertain, but may be Maestrichtian to possibly Danian. The lower 110 m of the Seymour Island Group consists of non-marine gravelly to pebbly sandstones and tuffaceous plant-bearing siltstones of the Cross Valley Formation, overlying which are 450 m of fine-grained marine sandstones and siltstones with interbedded fossiliferous pebbly conglomerates. Recent studies of the diverse and abundant faunas and floras of the Seymour Island Group have shown that they are older than previously thought, the Cross Valley Formation being Palaeocene and the La Meseta Formation late Eocene to possibly earliest Oligocene.