218-11 Chemostratigraphy and U-Pb geochronology of the Late Cretaceous on James Ross Island, Antarctica

Abstract:

James Ross Island (Antarctic Peninsula) is an important high-latitude location for understanding climatic and ecologic changes through the Late Cretaceous as it has one of the most continuous sequences of Cretaceous strata in the Southern Hemisphere. Global temperatures declined in the Late Cretaceous, while angiosperms (flowering plants) evolved and diversified to become dominant components of terrestrial ecosystems. The Santa Marta (SMF) and Snow Hill Island (SIF) Formations contain abundant mixed terrestrial and marine fossils that document these changes in the Southern Hemisphere, an area traditionally lacking in data. NSF-sponsored paleontological expedition (G-074-E) recovered a wealth of fossils including many new species of plants, which will illuminate the diversity and ecology of these ancient polar forests. To put these findings into a global context, we aim to better constrain the strata from three field localities (Sitio Feliz, Ugly Creek, and 14S) using integrated U-Pb geochronology of carbonates and C-isotope chemostratigraphy. 

The three sections are currently assigned to the Campanian Stage based on bio- and magnetostratigraphy. Stratigraphically, the lower-most section is Sitio Feliz (Beta Member of the SMF), followed by Ugly Creek, and 14S (Gamma Member (SIF). To more precisely constrain each section, 198 samples were analyzed for d¹³C_org (‰ VPDB). Carbon isotope values range from -28.85‰ to -24.78‰, and average -26.10‰. These represent some of the first chemostratigraphic data from James Ross Island. A positive excursion in the Sitio Feliz profile is tentatively correlated to the Mid-Campanian Event, while a pronounced negative isotope excursion in the Ugly Creek section is preliminarily correlated to the Late Campanian Event. A minimum depositional age of 72.2 +/- 2.7Ma was obtained from U-Pb analyses of calcite infill (diagenetic) in vertebrate bone material from the Sitio Feliz section and is consistent with these interpretations. Additional U-Pb analysis of calcite cements from Ugly Creek and 14S along with zircon geochronology and Sr chemostratigraphy will test these interpretations.

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