Alluvial fan and playa sedimentation in an Andean arid closed basin: the Pacencia Group, Antofagasta Province, Chile

Abstract

The mid-Tertiary Pacencia Group comprises over 2 km of continental sediments. It was deposited in a half-graben basin thought to have been active since early Tertiary times. Conglomerates along the western basin margin were deposited as longitudinal bars, rare transverse bars and channel fills with subsidiary debris flows, all in a distal fan pebbly braided stream environment. No proximal fan sediments are exposed. Petrographic and palaeocurrent analysis indicates these sediments were derived from Cretaceous and possibly Jurassic source areas to the northwest. To the east, within the basin, a succession of sands, silts, muds, thick evaporites and rare carbonates were deposited in playa sub-environments. Saliferous muds (central salt pan) are overlain by interbedded red mudstones and thin, high-energy ribbon and sheet sands (marginal dry mudflat and sandflat). Certain sand bodies carry stratiform Cui/Agi/Au mineralization. Lakes existed at various stages and were fed by permanent fluvial systems. The playa sediments were derived from crystalline source areas to the east. Proximal fan facies and sediment sources are now buried under the volcanic Andes.