SANTONIAN-MAASTRICHTIAN ANTARCTIC AMMONITES: DIVERSITY CHANGES, SEDIMENTARY CYCLES, AND PALEOCEANOGRAPHIC EVENTS

The 3-km-thick Santonian-Maastrichtian succession of the James Ross Basin, Antarctica includes three sequences that record fourteen ammonite associations: the N (Santonian-early Campanian); the NG (late Campanian-early Maastrichtian); and the MG (early Maastrichtian-Danian) Sequences. The ammonite generic diversification pattern, particularly kossmaticeratids, is analyzed. The N Sequence has the highest generic richness (S: 24), kossmaticeratids, mainly Natalites, show an increment by the late early Campanian. Diversity is minimum at the Santonian, then it increases gradually to an early Campanian maximum, and decreases at the latest early Campanian. Diversity in the NG Sequence is very peculiar and lower than in the N Sequence. The kossmaticeratids Neograhamites and Gunnarites are dominant, representing more than 50% of the total abundance. The NG Sequence has the highest kossmaticeratid richness, which is approximately constant with S close to 10. The MG Sequence diversity curve is similar to that of the previous one but has a lowered richness (S: 5-8) and an absolute dominance of the kossmaticeratids Maorites and Grossouvrites, the abundance of which is over 80% for most of the sequence. The diversity pattern of the N Sequence is typical for transgressive-regressive cycles, where maximum diversity is controlled by enlargement of the shelf during peak transgression. Conversely, the diversity pattern of the NG and MG Sequences differs markedly and do not reflect the expansion of the shelf during transgression. This unusual pattern is accompanied by dominance of the Kossmaticeratidae concomitant with a known temperature decline of the sea water, supporting the idea that kossmaticeratids were a stenothermal group. The Cretaceous cooling peak in Antarctica is in the Maastrichtian, apparently coincident with the minimum kossmaticeratid diversity in Antarctica and the maximun Maastrichtian kossmaticeratid diversity reached at lower latitudes, e.g. in Australia, suggesting the displacement of the group towards its preferred sea-water temperature.

Financial support by PICTO 36315 FONCYT, DNA, Argentina.