Sedimentation and preservation of the Miocene Atacama Gravels in the Pedernales–Chañaral Area, Northern Chile: Climatic or tectonic control?

Abstract

In recent years, longitudinal changes on the thin/thick-skinned tectonic styles of the Central Andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading 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 of Miocene continental deposits filling a Neogene paleo-valley system along the southern Atacama Desert in northern Chile. An east-west transect, between Pedernales and Chañaral (26º30'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 39 Ar-40 Ar 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 syn-sedimentary 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 semi-arid towards hyper-arid climatic conditions during the Miocene, led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the Oligocene.

Geomorphologic evidence for the late Pliocene onset of hyperaridity in the Atacama Desert

“…1), to the south of our primary study area, suggests that the infi lling of the Central Depression began with the burial of an Oligocene erosional landscape formed during a more pluvial period, and continued during a long period of declining precipitation. Nalpas et al (2008) concluded that deposition ceased with the onset of hyperarid conditions, between 1 The recent change in the Pliocene-Pleistocene boundary (Gibbard et al, 2009) from 1.8 to 2.6 Ma may ultimately change of some the epoch assignments of the geological mapping units in our fi eld area. In our landscape reconstructions, we use the previously assigned relative geological ages, where the Pleistocene was assigned a beginning at 1.8 Ma.…”

“…The relative thickness of the gravels, approximately tens of meters to ~100 m depending on location (Riquelme et al, 2007), combined with ages of intercalated ashes and ignimbrites, indicates that deposition of these gravels occurred over an extended period of several million years (e.g., Clark et al, 1967;Riquelme et al, 2007). Recent work by Riquelme et al (2007) and Nalpas et al (2008) along the Rio Salado (near Chanaral, Fig. 1), to the south of our primary study area, suggests that the infi lling of the Central Depression began with the burial of an Oligocene erosional landscape formed during a more pluvial period, and continued during a long period of declining precipitation.…”

Role of climate and tectonics in the geomorphologic evolution of the Semiarid Chilean Andes between 27-32°S.

“…En la Región de Atacama, las gravas presentan una gradación vertical que va desde facies sedimentarias fluviales en la base, a facies sedimentarias aluviales en el techo. Esta gradación resulta de la progresiva desecación climática del Mioceno Medio, que origina la actual condición hiperárida del Desierto de Atacama, y del progresivo alzamiento y basculamiento de la región de antearco (Riquelme et al, 2003(Riquelme et al, , 2007Nalpas et al, 2008). La colmatación del relieve por las Gravas de Atacama da paso a la formación del extenso Pediplano de Atacama (Hollingworth, 1964;Sillitoe et al, 1968;Mortimer, 1973).…”

“…1c). A lo largo del Desierto de Atacama, los pedimentos han sido clásicamente asociados a condiciones de relativa quiescencia tectónica y ambientes climáticos áridos (e.g., Sillitoe et al, 1968;Paskoff, 1970;Mortimer, 1973;Alpers y Brimhall, 1988;Montgomery et al, 2001;Riquelme et al, 2003Riquelme et al, , 2007Nishiizumi et al, 2005;Schlunegger et al, 2006;Kober et al, 2007;Farías et al, 2008;Nalpas et al, 2008;Placzek et al, 2009). Los pedimentos corresponden a superficies casi planas, escasamente degradadas por la incisión de los sistemas fluviales, y que resultan de un balance entre alteración del sustrato y transporte de sedimentos (para una discusión reciente ver Strudley y Murray, 2007).…”

Sedimentology-based reconstructions of paleoclimate changes in the Central Andes in response to the uplift of the Andes, Arica region between 19 and 21°S latitude, northern Chile

“…At 10-8 Ma, the formation of[3-m-thick salic gypsisols have been used to suggest a paleoclimate shift to hyperaridity (\20 mm/year precipitation; Rech et al 2006). Similarly, there are a number of reasons to infer a shift to a hyperarid climate: (1) contemporaneous cessation of supergene oxidation in the Atacama Desert (Sillitoe and McKee 1996;Arancibia et al 2006), (2) termination of channel incision by rivers with sources in the Coastal Cordillera , (3) cessation of Atacama Gravel accumulation by braided rivers with sources in the Western Escarpment (Nalpas et al 2008), and (4) termination of supergene enrichment and mineralization in the Late Miocene, or even later (Fig. 8).…”