Lower Triassic lacustrine sediments in La Coipa area, Atacama, Chile

Lower Triassic lacustrine sediments in an area of at least 300 km² at the La Coipa Mine area (26°50′ S, 69°15′ W), in northern Chile, indicate a large, actively subsiding lake, which was at one time evaporitic. This lake was surrounded by highlands dominated by outcrops of Late Paleozoic volcanic rocks and granitoids. An open lacustrine environment includes black hemipelagic shales with intercalated thin and medium-grained turbidites probably representing a channel-levee complex. The coarse-grained, lake margin sediments are dominated by debris-flow deposits and turbidites with intercalated black shales. These represent either the subaqueous part of a fan delta (formed during low-stand) or subaqueous talus (formed in a high stand). The Lower Triassic age, given by palynomorphs, represents the first documentation of strata of this age in Chile and apparently in the Andes. It indicates that basins of probable extensional origin were forming prior to the previously accepted Middle-Late Triassic age.