Lake-level and salinity reconstruction from diatom analyses in Quillagua formation (late Neogene, Central Andean forearc, northern Chile)

A sedimentary and micropaleontological study of the Quillagua Formation pro vides a detailed paleohydrological reconstruction of the lacustrine system which occupied the present-day hyperarid Quillagua-Llamara fore-are Basin ( Northern Chile) from lattermost Miocene (5.8 +/- 0.4 Ma) to Early Pliocene times. Diatom and lithofacies analyses were carried out in two correlated s tratigraphic sections of the lacustrine system. The Quebrada Temblor sectio n is located at the southern margin of the ancient lake and is directly inf luenced by the freshwater inputs of a northward flowing fluvial system drai ning the Precordillera and Calama regions. The Cerro Mogote section occupie s a western marginal location in the northern zone sheltered from direct fl uvial inputs from the south but subjected to the activity of the alluvial f an systems of the Coastal Range. A mostly shallow oligosaline waterbody occ upied the basin during the interval studied, though with fluctuations in sa linity and the extent of the inner, open waters. The southern margin sector - represented by Quebrada Temblor - had, in general terms, a palustrine ol igosaline character with almost freshwater conditions during certain period s, which favoured the establishment of semi-permanent to permanent freshwat er plumes overlying a saline waterbody. Development of shoreline facies sub jected to desiccation events was also characteristic at the top of this sec tion. The innermost shallow lacustrine areas - represented by Ceno Mogote - maintained more homogeneous oligosaline characteristics and more persisten t open waters. They were also subjected to freshwater pulses mediated in th is case by the activity of the terminal alluvial fan zones. Paleohydrologic al evolution of the basin was strongly conditioned not only by shifts in th e climatic-tectonic system but by variations in the local hydrological para meters. Four hierarchical orders of variability in the lake level of the ba sin were distinguished both by stratigraphic analysis of lithofacies and fr om changes undergone by the diatom record. Strong intrasample mixing of dia toms of incompatible salinity spectra and the presence of fine laminated la custrine facies in some terms define the highest order short-term intra- or inter-annual pulses experienced by the lacustrine system (higher than 6th order). Diatom-based punctuated interruptions of the minor order bathymetri cal trends highlight the high frequency variability in the basin (probably 6th order, 0.001-0.01 Ma), while the deepening/shallowing facies defined by the arrangement of the decimetre thick lithological sequences revealed a l ower order of variability (5th order, 0.01-0.1 Ma). Combined lithofacies an d diatom analyses delineate the lowest order of variability (4th order, 0.1 -1 Ma) allowing the reconstruction of two well-established highstand and lo wstand situations which implied the existence of a regressive trend between two transgressive trends. This order of variability is related to the inte rplay of tectonic events in the Calama Basin, and the regional climatic evo lution during late Neogene to Pliocene times. (C) 1999 Elsevier Science B.V . All rights reserved.