Stratigraphy, provenance, and timing of neogene sedimentation in the western valdés basin, patagonia. Accurate paleogeographic reconstructions as a key piece for andean-passive margin integration

CUITIÑO, JOSÉ I.; BILMES, ANDRÉS; BUONO, MÓNICA R.; BORDESE, SOFÍA; HERAZO, LORENA; SCASSO, ROBERTO A.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2023

0895-9811

With the aim to better understand the timing and geological evolution of the late Cenozoic stratigraphic record ofthe onshore, western Valdés Basin in northeast Chubut province (Patagonia, Argentina), we perform sedimentological,stratigraphic, and zircon U–Pb geochronologic analyses. The study includes the terrestrial Early MioceneTrelew Member of the Sarmiento Formation, the shallow marine Early Miocene Gaiman Formation, the shallowmarine to estuarine Late Miocene Puerto Madryn Formation, and the Plesitocene fluvial conglomerates of the RodadosPatagónicos. Based on our data set, together with the recognition of relevant stratigraphic surfaces, we dividethe succession into stratigraphically bounded units and define three cycles of sedimentation. The Lower Cyclerests on top of Surface 1, which comprise a long hiatus including the entire Oligocene Epoch. This cycle is inturn divided into two subunits separated by the transgressive, ravinement Surface 2. The Lower Cycle is separatedfrom the overlying Middle Cycle (=Puerto Madryn Formation) by the flat and regionally extensive Surface3. Finally, the latter is separated from the Upper Cycle (=Rodados Patagónicos) by Surface 4, a complex surfaceat the base of gravel deposits forming several terrace levels. Maximum depositional ages calculated from 10 zirconU–Pb samples indicate the succession between surface 1 and 4 extends from 21.5 to 2.7 Ma (Aquitanian-Piacenzian). The Lower Cycle is restricted to the 21.5–16 Ma interval (Aquitanian-Burdigalian); the Middle Cyclebetween 12 and 2.7 Ma (latest Serravalian-Piacenzian), with most of the new and previous ages clustering in theTortonian; whereas the Upper Cycle is restricted to the Pleistocene. Although most of the analyzed sedimentsshow fresh volcanic glass particles, older zircon ages are dominant, representing detrital components with a dominantpeak of Permian-aged grains, with subordinated Devonian, Jurassic, Cretaceous, and Paleogene peaks. Weinterpret the analyzed succession as a condensed section compared to the offshore Valdés Basin depocenter area.Sedimentation in the study area was controlled by periods of slow subsidence, probably related to thermal subsidenceand sediment compaction in the depocenter. Two main sources of sediments for the analyzed successionare envisaged: one represented by explosive volcanism located either in the Andes or in the North PatagonianMassif, the other through erosion of older rocks surrounding the North Patagonian Massif where late Paleozoicplutonic rocks are abundant. Particularly for the Middle and Upper cycles, an increase in Mesozoic contributionsuggests erosion from sediments of the Cañadón Asfalto Basin. The new ages and stratigraphic analyses here reportedare of major relevance for the integration of the onshore and offshore stratigraphies of the Valdés Basinwhich is key to connect the geologic evolution of the Andean and passive margin areas of southern South America.They are also of major relevance for the calibration of fossil communities, particularly for the abundant anddiverse marine vertebrates preserved in the Burdigalian Gaiman Formation.