Cenozoic stratigraphic development in the north Chilean forearc: implications for basin development and uplift history of the Central Andean margin

Analysis of the Cenozoicstratigraphicdevelopment of the forearc of northern Chile between 18°S and 23°30'S, allows constraints to be placed on the timing and nature of basin formation and the uplifthistory of the Central Andes. Chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc. Sections were taken from the Longitudinal Valley, Central Depression, Calama Basin, Salar de Atacama, Precordillera and the western flank of the WesternCordillera. Correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections.

Three chronostratigraphic units are defined based upon the presence of regional unconformities. Deposition of the Late Eocene to Early Miocene chronostratigraphic unit (38–19 Ma) commenced across an irregular unconformity surface between ~ 38 and 30 Ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites. Aggradation after 25 Ma resulted in development of a large broad basin over much of northern Chile that expanded eastwards through onlap onto basement. Deposition terminated around 19 Ma with the development of an angular unconformity over much, but not all of the study area. During deposition of the Early to Late Miocene chronostratigraphic unit (18–10 Ma) emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth affected sedimentation restricting accommodation space to small intra-thrust basins in the Precordillera and localised disruption and unconformity development in the Longitudinal Valley. The Late Miocene to present day chronostratigraphic unit (10–0 Ma) followed the development of a regional angular unconformity at 10 Ma. Sedimentation was restricted to a series of thrust-bounded endorheic basins in both the Central Depression and the Precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets. Alluvial fan, fluvial and lacustrine sedimentation dominated within the endorheic basins from ~ 8 to 3 Ma. After development of a regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the WesternCordillera. The scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the WesternCordillera by 30 Ma and that this has persisted to the present day. This area also shed material eastwards into the Altiplano. The presence of such a topographic feature by 30 Ma suggests that a significant proportion of Andean uplift had occurred prior to the Late Miocene. This important uplift phase should be incorporated into any model of Andean uplift. The evidence from the basin-fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 Ma and that a transition to a thrust-bounded foreland style basin took place after the development of the unconformity at 19 Ma.