Tectonic signatures in arid continental basins: an example from the Upper Miocene-Pleistocene, Calama Basin, Andean forearc, northern Chile

The Calama Basin, a transtensional basin situated within the Andean forearc , and its extension to the Pampa del Tamarugal (Central Depression), is fil led with 700 m of sediments. Detailed sedimentary logging and Ar-40/Ar-39 d ates from interbedded volcanic deposits have established a lithostratigraph ic and chronostratigraphic framework for the upper 95 m thick Upper Miocene -Pleistocene succession. Five facies associations, fluvio-lacustrine diatom ite, palustrine carbonate, massive anhydrite, fluvial and alluvial fan, hav e been identified within this part of the basin-fill. The following events are recognised in the basin evolution: (1) deposition of diatomaceous lacus trine sediments in two separate depocentres in the Pampa del Tamarugal and the central Calama Basin, with alluvial fan accumulation along the basin ma rgins at 8 Ma; (2) localised deformation within the central Calama Basin be tween 7 and 6 Ma but continuous sedimentation elsewhere; (3) palustrine car bonate deposition across the entire area from 6 to 3 Ma except in the Pampa del Tamarugal where diatomaceous sedimentation continued until 5 Ma before being replaced by palustrine carbonate deposition; (4) widespread folding after 3 Ma and entrenchment as the drainage system cut down to reach a new base level caused by the Rio Loa breaching the Coastal Cordillera and reach ing the sea. Localised lacustrine deposition in topographic lows or tectoni cally created dams also occurred during this period. Comparison with simila r age facies elsewhere in northern Chile has shown changes such as these to be localised and diachronous, indicating that tectonic, controls on accomm odation space and drainage patterns are the primary influence on sedimentat ion in this tectonic setting. The lacustrine deposits do not appear to be a reliable recorder of low-frequency climatic change. This is exemplified by the change from lacustrine to palustrine deposition at 6-5 Ma which would appear to record a drying of the climate. However, a synchronous change fro m ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run-off. (C) 1999 Elsevier Science B .V. All rights reserved.