Eocene to Pleistocene lithostratigraphy, chronostratigraphy and tectono-sedimentary evolution of the Calama Basin, northern Chile

Abstract

New ⁴⁰Ar/³⁹Ar radiometric, sedimentological and structural data from post-Paleocene sedimentary strata in the Calama Basin, northern Chile suggest that the established lithostratigraphy of the basin-fill requires revision. A new lithostratigraphic scheme for the Eocene-Pleistocene stratigraphy of the Calama Basin is proposed here. The key features of this scheme are that the existing Calama Formation is retained although the age of the formation is redefined as (?Lower) Eocene to (?)Lower Miocene. The El Loa Formation is elevated to group status and redefined as Lower Miocene to Upper Pliocene in age. The El Loa Group includes four newly defined formations: the Jalquinche, Opache, Lasana and Chiquinaputo formations. The existing definition of the uppermost unit within the Calama Basin, the Upper Pliocene to Pleistocene Chiu-Chiu Formation, is retained. The tectono-sedimentary evolution of the Calama Basin-fill reveals three distinct unconformity-bounded phases of sedimentation: Eocene-Oligocene, early Miocene to late mid-Miocene and late Miocene to late Pliocene. Sedimentation commenced in the Eocene with deposition of alluvial braidplain deposits (Calama Formation). This style of sedimentation occurred across northern Chile throughout the Oligocene following the Incaic Orogeny. From 22 Ma to 10 Ma ephemeral fluvial sediments were developed along the Calama Basin flanks (Lasana Formation), playa sandflat and mudflat deposits (Jalquinche Formation) in the basin centre. Equivalently-aged sediments in both the Salar de Atacama and Pampa del Tamarugal basins also indicate deposition in endorheic basins. Late Miocene sedimentation occurred diachronously across the north Chilean forearc. The Pampa del Tamarugal and Calama Basin areas were linked around 6 Ma following tectonic activity. Regional palustrine carbonate sedimentation occurred in the Calama Basin centre (Opache Formation) with fluvial sedimentation along the eastern basin margin (Chiquinaputo Formation). The change in depositional style is attributed to increased rainfall in drainage basins. At about 3 Ma the Calama Basin was subject to gentle folding, followed by entrenchment of the Río Loa and Río San Salvador through Miocene to Pliocene strata to reach a new base-level caused by breaching of the Coastal Cordillera by the Río Loa to reach the Pacific. Localised diatomite and evaporite (anhydrite) deposition took place in depressions created by either folding of older strata or geomorphic features (Chiu-Chiu Formation). Up to 5 unconformities are present in the Calama Basin-fill and occur across either the whole of the basin or localised areas of the basin. They are not present in adjacent basins which suggests they were generated by movement on the Precordilleran/West Fissure Fault Systems. Facies analysis of the Eocene to Pleistocene succession suggests that sedimentation took place under an arid/semi-arid climate throughout this time period. Occasional fluctuations in climate suggesting increased runoff are indicated by the development of diatomites together with lacustrine and palustrine limestones. However, it is unlikely that the climate was never more humid than semi-arid. Similar facies developments in the Pampa del Tamarugal and Salar de Atacama basins during the Oligocene to Pleistocene suggest that climate changes affected the whole of the forearc and were not restricted to individual basins.