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

摘要

A sedimentary and micropaleontological study of the Quillagua Formation provides a detailed paleohydrological reconstruction of the lacustrine system which occupied the present-day hyperarid Quillagua–Llamara fore-arc 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 stratigraphic sections of the lacustrine system. The Quebrada Temblor section is located at the southern margin of the ancient lake and is directly influenced by the freshwater inputs of a northward flowing fluvial system draining the Precordillera and Calama regions. The Cerro Mogote section occupies a western marginal location in the northern zone sheltered from direct fluvial inputs from the south but subjected to the activity of the alluvial fan systems of the Coastal Range. A mostly shallow oligosaline waterbody occupied the basin during the interval studied, though with fluctuations in salinity and the extent of the inner, open waters. The southern margin sector — represented by Quebrada Temblor — had, in general terms, a palustrine oligosaline character with almost freshwater conditions during certain periods, which favoured the establishment of semi-permanent to permanent freshwater plumes overlying a saline waterbody. Development of shoreline facies subjected to desiccation events was also characteristic at the top of this section. The innermost shallow lacustrine areas — represented by Cerro Mogote — maintained more homogeneous oligosaline characteristics and more persistent open waters. They were also subjected to freshwater pulses mediated in this case by the activity of the terminal alluvial fan zones. Paleohydrological evolution of the basin was strongly conditioned not only by shifts in the climatic–tectonic system but by variations in the local hydrological parameters. Four hierarchical orders of variability in the lake level of the basin were distinguished both by stratigraphic analysis of lithofacies and from changes undergone by the diatom record. Strong intrasample mixing of diatoms of incompatible salinity spectra and the presence of fine laminated lacustrine 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 bathymetrical 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 lower order of variability (5th order, 0.01–0.1 Ma). Combined lithofacies and diatom analyses delineate the lowest order of variability (4th order, 0.1–1 Ma) allowing the reconstruction of two well-established highstand and lowstand situations which implied the existence of a regressive trend between two transgressive trends. This order of variability is related to the interplay of tectonic events in the Calama Basin, and the regional climatic evolution during late Neogene to Pliocene times.