DRAINAGE EVOLUTION AND CARBONATE DEPOSITION BASED ON URANIUM ISOTOPES IN GROUNDWATER AND TUFA FORMING WITHIN THE CALAMA BASIN OF NORTHERN CHILE

Extensive tufa deposits with exceptional preservation, the Santa Juana, occur in the Atacama Desert to the west of Calama, Chile. The tufas extend westward from the city limit across an elongate E-W orientated plateau between the parallel San Salvador and Loa Rivers. The San Salvador rises from springs close to the NW part of Calama and incises 250 m below the plateau surface. The Loa river, which rises in the Andes, flows in a channel with negligible incision through Calama, but within 9 km to the west, carves a canyon reaching a depth of 150 m. The Quaternary Santa Juana tufas were deposited both conformably on the underlying carbonate Opache Formation and unconformably within shallow to deep channels and canyons that incised the plateau surface. The distribution is interpreted to reflect a much higher groundwater table in the past.

Over 100 U-series disequilibria data are presented for tufa plus 7 samples of water. U-series dates span from the oldest the method can achieve, with some tufa reaching secular equilibrium, to samples formed within the last few hundred years. The oldest tufas are found on the plateau at edges of the Loa and San Salvador canyons. Following those, tufa between 300 and 20 kyr in age occur in dry gullies that feed into a third central, dry channel that incises the plateau by at most 30m between the two active rivers. Carbonate encrusted plant stems at the bottom of this channel and carbonate deposited in now dry cascades close to Calama indicate a fall in the water table over the last few thousand years, which human activity has likely contributed to over the last few hundred years. Tufas continue to form, but only in the active channels of the San Salvador and Loa rivers. Initial ²³⁴U/²³⁸U ratios have a distribution that is interpreted as showing carbonate dissolution of the underlying Opache limestone by groundwater charged with CO­₂ of different sources that reprecipitates as CO₂ exsolves. These data sets combine to portray an evolving drainage network during a time of active tectonics and climate drying, where competition between the San Salvador and eastward migration of a deep Loa canyon towards Calama has exacerbated water table decline and stopped the development of the third channel.