Middle Eocene dinoflagellate cysts of the Austral Basin: biostratigraphic and paleoceanographic implications

M. SOL GONZÁLEZ ESTEBENET; G. RAQUEL GUERSTEIN; MARTA ALPERIN; MARTÍN RODRIGUEZ RAISING

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

INTERNATIONAL PALAEONTOLOGICAL ASSOCIATION

Reconstruction of palaeoenvironmental and paleoceanographic evolution of the southwestern Atlantic Ocean during the Paleogene is prevented by the lack of bioestratigraphic tools to date and correlate the sedimentary units. During the middle Eocene a regional transgression covered the Austral Basin represented in the southwest of Santa Cruz province by the upper member of the Río Turbio Formation (UMRTF). These marine deposits contain well preserved organic-walled dinoflagellate cyst (dinocyst) assemblages dominated by Antarctic endemic assemblage species with important biostratigraphic and paleoenvironmental indicators. In this contribution we correlate the assemblages of the UMRTF from 2 outcrop sections and 3 cores drilled close to the Río Turbio locality using Statistical Compositional Analysis. The results show two well-defined clusters. One cluster comprises the samples of the lower part of UMRTF from all the localities and the other includes the samples of the upper part of the UMRTF. The comparison of our results with the high-resolution Southern Pacific Ocean dinoflagellate cyst zonation for middle-late Eocene allows us to date some of the dinoflagellate events recorded in UMRTF. The first abundant occurrence of Enneadocysta dictyostila (Menéndez) Fensome et al. 2006 and the presence of Hystrichosphaeridium truswelliae Wrenn and Hart and Arachnodinium antarcticum Wilson and Clowes in the lower part of the UMRTF endorse its correlation with the dinocysts?s zones SPDZ11 and SPDZ12 and indicate an age between 45.5 and 39 Ma. Upwards, the dominance of E. dictyostila is replaced in all the sections by Vozzhennikovia apertura Wilson, 1967a showing the transition from a distal setting in an inner-shelf environment with relatively warm waters to an intertidal deltaic plain with eutrophic shallow marine waters. A brief interval dominated by Imagidinium spp. coincides with the maximum flooding surface. In the upper part of the UMRTF the first common occurrence of Spinidinium macmurdoense (Wilson) Lentin and Williams is recorded at 37 Ma. and correlates with the Zone SPDZ13. Therefore the UMRTF ranges in age from late Lutetian to early Priabonian. The abundance of V. apertura and the increasing amount of non-marine palynomorphs towards the top of the sections indicate the transition from a tide-dominated deltaic to a continental environment. This paleoenvironmental reconstruction agrees with the proposed by the sequence stratigraphical analysis. The presence of the endemic dinocyst assemblages throughout the UMRTF suggests this unit was deposited during the early opening of the Drake Passage before the emplacement of an unrestricted Antarctic circumpolar flow.