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This paper documents calcareous nannofossil assemblages and the bioevents recorded in the Austral basin of Tierra del Fuego, Argentina. Fifty samples from the type section of the Cerro Colorado Formation were analyzed. Nannoflora is moderately preserved and indicates a late Eocene to early Oligocene age for sedimentary rocks of this formation. Biostratigraphic results are based on distribution patterns of calcareous nannofossils assemblages, permitting comparison with the Southern Ocean zonations and some correlations with standard schemes. Our biostratigraphic interpretation shows an almost complete stratigraphic section spanning from the Priabonian to Rupelian Stages and calcareous nannofossil Chiasmolithus oamaruensis to Blackites spinosus zones, or the low latitude zones of Martini NP18 to NP21. The oldest fossiliferous rocks from the type locality of the Cerro Colorado Formation contain Chiasmolithus oamaruensis indicating nannofossil Zone NP18. The Eocene/Oligocene boundary was placed between the last occurrence (LO) of Reticulofenestra oamaruensis and the increasing in abundance (IB) of Reticulofenestra daviesii. Samples taken from the top of the section belong to the Estancia María Cristina Beds have Cyclicargolithus abisectus. During the late middle to late Eocene a hiatus is suggested between the Cerro Colorado and Leticia formations evident by a 39 Ma detrital zircon age dating, and the first occurrence (FO) of D ictyococcites bisectus. Furthermore, the presence of C. abisectus in the Estancia María Cristina Beds suggests an unconformity with the underlying Cerro Colorado Formation as previously inferred by stratigraphic relationships. • Calcareous nannofossils confirm a late Eocene to early Oligocene age. • The E/O boundary is placed in Tierra del Fuego, Austral basin. • The nanofossil bioevents are correlated with those of the Southern Ocean zonation. • The section spanning from the Chiasmolithus oamaruensis to B. spinosus Zones. [ABSTRACT FROM AUTHOR]

Marine sedimentary rocks of the lower Miocene Monte León Formation of southeastern Patagonia (Austral Basin), Argentina are important geological archives for better understanding regional tectonics, paleoenvironments, oceanography, and climate. In this paper, we describe assemblages of invertebrates, palynomorphs, foraminifera, ostracods and calcareous nannofossils in a stratigraphical and sedimentological framework, which suggest deposition during a transgressive-regressive cycle. From base to top, the lowermost outcropping deposits of the Punta Entrada Member represent an inner shelf environment, formed during a transgressive phase. The zone of maximum flooding is marked by a high diversity of protoperidiniaceans (heterotrophic) and offshore dinoflagellate cysts, an increase in the percentage and size of planktonic foraminifera, and a decrease in shallow water benthic foraminifera. Regressive deposits in the upper part of the Punta Entrada Member exhibit a progradational stratal stacking pattern and are characterized by an upward decrease in bioturbation and in the content of marine invertebrates; the dinoflagellate cyst assemblages point toward shallower and more restricted marine conditions than in underlying deposits. The cycle ends with the Monte Observación Member, which contains an impoverished and mostly reworked fauna of invertebrates. The presence of monospecific reefs of Crassostrea orbignyi , the decrease in dinoflagellate cysts and calcareous microfossils diversity, and the increase of continental palynomorphs suggest progressively shallower conditions and increasing influence of freshwater discharge. Although eustatic controls could have contributed to the sedimentary evolution of the Monte León Formation, the upward regressive trend is interpreted as the result of tectonism linked to the Andean orogeny, which led to the uplift, exhumation, and erosion of the highlands in the west. This is supported by the abundance of pyroclastic material, together with reworked specimens of Upper Cretaceous forams and Upper Cretaceous and middle Eocene dinoflagellate cysts in the upper part of the Punta Entrada Member and in the Monte Observación Member. • Multiproxy data indicate that study rocks represent a transgressive-regressive cycle. • A maximum flooding and/or higher organic flux zone was identified at the coastal cliffs. • The regressive trend was the result of tectonics, sediment supply dynamics, and volcanism. • Cretaceous/Eocene forams and dinocysts evidence exhumation and reworking from the Andes. [ABSTRACT FROM AUTHOR]