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The Atacama Planation Surface is an extensive west-dipping surface developed between 16[degrees]S and 27[degrees]S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic [sup.3]He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Abstract: Determining the response of fluvial systems to syn‐sedimentary halokinesis is important for reconstructing the palaeogeography of salt basins, determining the history of salt movement and predicting development and architecture of sandstone bodies for subsurface fluid extraction. To assess both the influence of salt movement on fluvial system development and the use of lithostratigraphic correlation schemes in salt basins we have analysed the Triassic Chinle Formation in the Paradox Basin, Utah. Results indicate that sandstone body development proximal to salt bodies should be considered at two scales: intra‐ (local) and inter‐ (regional) mini‐basin scale. At the intra‐mini basin or local scale, conformable packages of up to 12 m deep meandering fluvial channel deposits and associated overbank deposits are developed, which may thin, pinch‐out or become truncated towards salt highs. When traced down the axis of a mini‐basin, individual stories extend for a few hundred metres, and form part of amalgamated channel‐belt packages up to 60 m thick that can be traced for at least 25 km parallel to palaeoflow. Where salt movement outpaces sediment accumulation, progressive low angle unconformities are developed along the flanks of salt highs. Significantly, in mini‐basins with high sand supply, sandstone bodies are present across salt highs where they show increased amalgamation, decrease in thickness due to truncation and no change in internal sandstone body character. At inter mini‐basin or regional scale, spatial and temporal variations in accommodation space generated by differential salt movement strongly influence facies distributions and facies correlation lengths. Broad lithostratigraphic packages (5–50 m thick) can be correlated within mini‐basins, but correlation of these units between adjacent mini‐basins is problematic. Knowledge of fluvial system development at a regional scale is critical as, fluvial sediment distribution is focussed by topography generated by growing salt bodies, such that adjacent mini‐basins can have significant differences in sandstone body thickness, distribution and lateral extent. The observations from the Chinle Formation indicate that lithostratigraphic‐based correlation schemes can only be applied within mini‐basins and cannot be used to correlate between adjacent mini‐basins or across a salt mini‐basin province. The key to predicting sandstone body development is an understanding of the timing of salt movement and reconstructing fluvial drainage system development. [ABSTRACT FROM AUTHOR]

The Salar de Atacama forms one of a series of forearc basins developed along the western flank of the Central Andes. Exposed along the northwest margin of the basin, a salt-cored range, the Cordillera de la Sal, records the Mid-Miocene to recent sedimentological and structural development of this basin. Sediments of the Mid-Miocene Vilama Formation record the complex interaction between regional/local climate change, halokinesis and compressional deformation. This study reveals how these factors have controlled the facies development and distribution within the Salar de Atacama. Detailed sedimentary logging, cross-sections and present day geomorphology through the northern Cordillera de la Sal have been used to establish a lithostratigraphy, chronostratigraphy and the regional distribution of the Vilama Formation. The Vilama Formation documents an increase in aridity with a hiatus in sedimentation from Mid-Miocene to 9 Ma with initial uplift of the Cordillera de la Sal. From 9 Ma to 8.5 Ma deposition of a meandering fluvial system is recorded followed by a rapid decrease in sedimentation till 6 Ma. From 6 to 2 Ma, the deposition of extensive palustrine carbonates and distal alluvial-mudflat-lacustrine demonstrates the existence of an extensive lake within the Salar de Atacama. Post 2 Ma, the lake decreased in size and braided alluvial gravels associated with alluvial fans were widespread through the region suggesting a final shift to hyperarid conditions. By comparing the Vilama Formation with similar age facies throughout northern Chile and southern Peru, several shifts in climate are recognized. Climate signatures within northern Chile appear to be largely diachronous with the last regional event in the Mid-Miocene. Since that time, humid events have been restricted to either Precordillerian basins or the Central Atacama. Within the Central Atacama, the final switch to hyperarid conditions was not till the earliest Pleistocene, much later than previously estimated within the region. [ABSTRACT FROM AUTHOR]

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories. [ABSTRACT FROM AUTHOR]

Rates of fluvial incision reconstructed for the last 11 Ma from the major E-W fluvial systems of the western margin of the Andes (18°00'S to 20°15'S) highlight the existence of 3 distinct sectors. In Sector 1 (the northernmost part of Chile) the fluvial systems are exoheiric with a terminal base level in the Pacific Ocean and span the Coastal Cordillera, Longitudinal Valley, Precordillera and edge of the Western Cordillera, constraining the uplift of this region to a minimum of ~200 m/Ma (11-7 Ma), 150 m/Ma (7-3 Ma) and 120 m/Ma (3-0 Ma). In Sector 2, to the immediate south, the rivers are shorter and terminate in the Longitudinal Valley, spanning only the Precordillera and part of the Western Cordillera with incision rates of 100-50 m/Ma (11-7 Ma) to 50 m/Ma (7-3 Ma and 3-0 Ma). In southernmost Sector 3 the fluvial systems terminate in the Longitudinal Valley, spanning only the western Precordillera with incision rates of 50 m/Ma (11-7 Ma), 30m/ma (7-3 Ma) and 25 m/Ma (3-0 Ma). Comparison of incision rates between Sectors 1 and 2 constrains the uplift of the Coastal Cordillera to 60m/Ma in keeping with previous studies from the region. Differences between Sectors 2 and 3 (25-50 m/Ma) are attributable to drainage loss by tectonic beheading of catchments by the uplift of the Cordillera de Domeyko fault system. The data suggest that a substantial amount of surface uplift has occurred over the last 11 Ma along the western edge of the Andes which is in agreement with rapid but sustained uplift of the Andes in the Late Miocene. The study highlights the value of regional assessments of fluvial incision for quantitying the impacts and timing of uplift on a continental scale. [ABSTRACT FROM AUTHOR]