MICROFOSSILS FROM THE SANTA MARTA FORMATION (UPPER SANTONIAN-LOWER CAMPANIAN) OF JAMES ROSS ISLAND, ANTARCTICA

DIEMMER KOCHHAN, K; DO MONTE GUERRA, R.; CONCHEYRO, A.; FAUTH, G.,; BAECKER, S.; VIVIERS, M; DE ARAÚJO CARVALHO, M.; RODRIGUEZ CABRAL RAMOS, R.

Gramado

Congreso; XXIII Congresso Brasileiro de Paleontologia; 2013

Micropalaeontological studies in the Antarctic Peninsula are relevant for a better understanding of the evolution of high-latitude marine basins during the Mesozoic. This study deals with calcareous nannofossils, benthic foraminifera, ostracods, and radiolarians of a sedimentary succession within the Lachman Crags Member - Santa Marta Formation (upper Santonian-lower Campanian), northwestern James Ross Island. Calcareous and siliceous microfossils were recovered using the traditional hydrogen peroxide method, and calcareous nannofossils were prepared using the decantation technique. Ninety-nine samples were analyzed, mainly composed of very fine to medium-grained tuffaceous sandstones, claystones, and siltstones. Seventy-three calcareous nannofossils species, benthic foraminiferal species, six ostracod species, and six radiolarian species were identified. Calcareous nannofossils occur mainly in the upper part of the section, the lower part contains a low diversity assemblage composed almost exclusively of dissolution-resistant taxa (e.g. Micula staurophora, Rhagodiscus splendens, Watznaueria barnesiae and Zeugrhabdotus diplogrammus), suggesting a higher degree of dissolution at this portion of the studied section. The upper assemblage is Campanian in age due to the co-occurrences of Gephyrobiscutum diabolum, Reinhardtites anthophorus and Eiffelithus eximius. Calcareous microfossils, such as ostracods and benthic foraminifera, are restricted to few stratigraphic levels described as tuffaceous sandstones cemented by calcium carbonate, while siliceous microfossils and agglutinated foraminifera also occur in different lithofacies. This suggests a preservational bias in the occurrences of microfossils within the studied section, possibly related to the presence/absence of calcium carbonate cement. The benthic foraminiferal fauna presents a low degree of endemism with the dominance of cosmopolitan taxa (e.g. Gyroidinoides globosus, Gavelinella sandidgei). Consistent occurrences of Gyroidinoides globosus suggest the dominance of deep-neritic to upper-bathyal paleodepths. The ostracod genera identified herein have already been reported for the Cretaceous Period of the Gondwana region and also suggest deposition on a shelf with warm water temperature and normal salinity. The studied radiolarian fauna, the first one described for the James Ross Island region, can be considered Late Cretaceous in age, also presenting the dominance of robust (dissolution resistant) cosmopolitan taxa.