SICHAL FORMATION: AN EOCENE-EARLY OLIGOCENE BASIN SYNTECTONIC TO THE INCAIC OROGENIC EVENT AND CONTEMPORANEOUS TO EMPLACEMENT OF PORPHYRY COPPER RELATED PLUTONS, II REGION OF ANTOFAGASTA, CHILE

The Sichal basin, located in the Chilean Precordillera at 21°34'-22°00'S, formed as a syntectonic intramontane basin, between Paleozoic blocks uplift during the Incaic orogenic event. The basin initiated immediately after termination of Early-Middle Eocene volcanism (Icanche Fm.). Its first stage of evolution records proximal alluvial sedimentation that started in the Middle Eocene (c. 43 Ma), with provenance directions from the E, NNE, NNW and W. During this first stage, there occurred subordinate explosive volcanism (c. 37 Ma), characterized by dacitic pyroclastic flow deposits. At about this time, the progressive uplift of a basement block, situated in the center of the basin, deformed the deposits of the first stage and captured the sedimentation in two separate depocenters with contrasting thicknesses. A second cycle of activity, presumably from uplift of the western margin of the basin, permitted the progradation, from the W and NW, of an alluvial system of proximal and intermediate facies, across the entire basin, discordantly covering the deposits of the first cycle. The third and final stage of sedimentation consisted of alluvial playa facies characterized by a thick sequence of red siltstones and gypsum, probably deposited during the Early Oligocene, as they are unconformably overlain by gravels of the Altos de Pica Fm. that have a c. 26 Ma tuff interbedded. This last stage of Sichal sedimentation lacks evidence of local block uplift. The notable change of facies between the second and third stages could represent, in part, the cessation of local relief generation, and in part, the change to more arid conditions at the Eocene-Oligocene boundary that is registered both regionally and globally. Deformation that affects the Sichal Fm., prior to deposition of the Altos de Pica Fm., and evidence for progressive exhumation of porphyry copper deposits in the region, suggests compression continued into the Early Oligocene, though the main locus of surface deformation probably had migrated eastward. Porphyry copper-related plutons, in this part of the Chilean Precordillera (Quebrada Blanca, Collahuasi, El Abra, Fortuna and Chuquicamata), were emplaced at 38-31 Ma, during the latter half of the Incaic compressional phase (43-31 Ma) and contemporaneously with sedimentation in the Sichal basin.