Sedimentation and preservation of the Miocene Atacama Gravels in the Pedernales-Chañaral Area, Northern Chile: Climatic or Tectonic Control?

In recent years, longitudinal changes on the thin/thick-skinned tectonic styles of the Central Andes has been intensively discussedwhile other studies have considered the role ofmass transfers on the unloading of the orogen, and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the Peru–Chile trench. The search for paleo-climate records is therefore crucial for our understanding of the history of the Central Andes. In this paper, we focus on the Atacama Gravels, an extensive blanket ofMiocene continental deposits filling a Neogene paleo-valley systemalong the southern Atacama Desert in northern Chile. An east–west transect, between Pedernales and Chañaral (26°30′S), enabled us to carry out a sedimentological and tectonic study of the Atacama Gravels, based on logging and field observations along the Rio Salado canyon. New 39Ar–40Ar ages obtained on intercalated and overlying ignimbrites date the beginning of the Atacama Gravels sedimentation at around the Oligocene–Miocene boundary and cessation of sedimentation in the Late Miocene. Thirteen lithofacies, included within five facies associations (A1 to A5) were identified. Depositional environments vary from proximal alluvial fan (A1, A2) in the Precordillera through ephemeral fluvial (A3, A4) to distal playa lake (A5) in the Coastal Cordillera. No evidences of synsedimentary deformation have been found, showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes, and climate change appears to be the dominant controlling factor of sediment preservation. A progressive change from semiarid towards hyper-arid climatic conditions during theMiocene, led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleo-valley system formed during the Oligocene.