Late Neogene lacustrine record and palaeogeography in the Quillagua-Llamara basin, Central Andean fore-arc (northern Chile)

The Cenozoic Quillagua-Llamara basin (northern Chile, Central Andes) is an asymmetrical, intramassif fore-are basin with a relatively wide northern sector separated from a narrower southward extension by a basement threshold. The northern sector was characterised by a noticeable Oligocene?-late Neogene alluvial-fan and lacustrine dominated deposition which resulted in sequences up to 900 m thick, whereas the southern sector was often a bypass zone with thinner fluvial and lacustrine sediment accumulation. The basin infill includes two third-order alluvial-lacustrine unconformity-bounded units which include other higher-frequency (4th to 5th order) sequences. The evolution of the Late Miocene-Pliocene lacustrine episodes in the Quillagua-Llamara basin was not only controlled by the regional variations from arid to hyperarid palaeoclimate conditions, due to the combined influence of the Pacific high pressure cell, the rain shadow effect exerted by the rising Andes and the northward flowing cold oceanic currents, but also by: (a) the extensional tectonics and evolution and uplift of the fore-are region which defined the location and size of the depocentres; (b) the resulting basement palaeorelief which affected sediment thickness and facies distribution during the late basin-infill episodes; and (c) the tectonic modifications of watersheds, water divides and drainage networks in the Precordillera which caused considerable changes of water income in the lacustrine systems. Understanding of this regional tectonosedimentary evolution is a necessary first step before analysing of the low- to high-order lacustrine sequence changes in the region. Lacustrine water supply was very sensitive to tectonics; even gentle tectonic tilting and uplifting in critical water-divide zones could result in changes in water balance in the lacustrine basins and trigger variations in the depositional record. The very conspicuous, lacustrine regime changes recorded in the Quillagua-Llamara basin infill cannot be considered in themselves conclusive proof of an exclusive climatic forcing, since they took place close to either major regional drainage changes or to gentle but noticeable tectonic reactivation in the fore-are region. (C) 1999 Elsevier Science B.V. All rights reserved.