Tectono-Sedimentary Evolution of the Distal Paleogene Andean Retroarc at 33° S: New Detrital Zircon U–Pb Geochronology From the Divisadero Largo Formation

Abstract

Sedimentary basins in the distal Cenozoic Andean retroarc yield an important geological archive that provides crucial insights into the tectonic and sedimentary processes associated with the different stages of mountain building. At 33° S, the tectonic and sedimentary processes that have operated during the Neogene and Quaternary periods of Andean orogenesis are well documented, whereas information on the Paleogene period remains fragmentary and partly enigmatic. The Paleogene sedimentation in the distal retroarc at this latitude is represented by the Divisadero Largo Formation, a 70-m-thick sedimentary unit that has been extensively studied for its fossil content, leading to the controversial definition of the late Eocene Divisaderan South American Land Mammal Age (SALMA). New zircon U–Pb geochronological data provide a valuable age constraint for Paleogene tectonic and sedimentary processes in the Southern Central Andes. Furthermore, we present the first detailed facies analysis of the Divisadero Largo Formation, combined with a sedimentary provenance study and a seismic subsurface characterisation of this unit. Our results indicate that the age of the Divisadero Largo Formation is Palaeocene to early Eocene (~65–41 Ma). Deposition of this unit occurred in a shallow, lacustrine depositional environment with variable water depths and was characterised by a low accumulation rate of 3 m/Myr. During this time, the sediment source was predominately located in the Andean magmatic arc; however, no conclusive evidence of significant Paleogene deformation exists. These characteristics (age, depositional environment, low accumulation rate and provenance) enable a regional correlation with Paleogene deposits farther south in the Neuquén Basin. In addition, based on U–Pb geochronology and sedimentary features, a 20 Myr hiatus could be defined between the Divisadero Largo Formation and overlying synorogenic deposits, as has been proposed farther south, reflecting the northernmost record of this hiatus. Taken together, these new observations help to refine a tectono-sedimentary model for the evolution of the Southern Central Andes retroarc basin at 33° S that comprises four stages preceding the well-documented Miocene contraction phase: (i) Late Jurassic–Early Cretaceous extension; (ii) Late Cretaceous contraction; (iii) Palaeocene–middle Eocene tectonic quiescence; and (iv) a renewed phase of late Eocene–Oligocene extension.