Integrated stratigraphy and paleontology of the lower Miocene Monte León Formation, southeastern Patagonia, Argentina: Unraveling paleonvironmental changes and factors controlling sedimentation

PARRAS, A; GUERSTEIN G. R.; PÉREZ PANERA J.P; GRIFFIN, M; NAÑEZ, C; CUSMINSKY, G; QUIROGA, A

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2020

0031-0182

UNCORRECTEDMarine 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 depositionduring a transgressive-regressive cycle. From base to top, the lowermost outcropping deposits of the PuntaEntrada Member represent an inner shelf environment, formed during a transgressive phase. The zone of maximumflooding is marked by a high diversity of protoperidinaceans (heterotrophic) and offshore dinoflagellatecysts, an increase in the percentage and size of planktonic foraminifera, and a decrease in shallow water benthicforaminifera. Regressive deposits in higher parts of the Punta Entrada Member exhibit a progradational stratalstacking 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 thanin underlying deposits. The cycle ends with the Monte Observación Member, which contains an impoverished andmostly reworked fauna of invertebrates. The presence of monospecific reefs of Crassostrea orbignyi, the decrease indinoflagellate cysts and calcareous microfossils diversity, and the increase of continental palynomorphs suggestprogressively shallower conditions and increasing influence of freshwater discharge. Although eustatic controlscould have contributed to the sedimentary evolution of the Monte León Formation, the upward regressive trendis interpreted as the result of tectonism linked to the Andean orogeny, which led to the uplift, exhumation, anderosion of the highlands in the west. This is supported by the abundance of pyroclastic material, together withreworked specimens of Upper Cretaceous forams and Upper Cretaceous and middle Eocene dinoflagellate cysts inthe upper part of the Punta Entrada Member and in the Monte Observación Member.