Sedimentological and provenance analysis of the Río Playas stratigraphic section: Implications for the evolution of the Alamor-Lancones Basin of southern Ecuador and northern Peru

Abstract

The Cretaceous Alamor-Lancones Basin is located in the forearc region of southern Ecuador and northern Peru. The eastern (Río Playas) domain includes the upper stratigraphic levels of the 1800 m thick basin-fill succession. Sedimentological, provenance and chronostratigraphic analyses of the Río Playas region help to elucidate the Late Cretaceous to Eocene evolution of the forearc region in southern Ecuador and neighboring regions.The oldest strata of the Río Playas region include Lower Cretaceous volcanic and volcaniclastic deposits of the Celica and Alamor formations, which were deposited in an arc-related extensional basin. These units are unconformably overlain by the Coniacian to Maastrichtian El Naranjo and Casanga formations. Lithofacies described for the El Naranjo and Casanga formations include sandy debrites, sandy turbidites, silty-muddy turbidites, sandy slumps, clast-supported debrites and rafted blocks. These lithofacies association suggests a deepwater submarine fan depositional environment.Provenance analyses for the Casanga and El Naranjo formations, including heavy mineral and petrographic data, show that these formations were derived from erosion of a volcanic arc and a minor granitic and metamorphic source terrane. Detrital zircon U-Pb ages for the Casanga Formation indicate contributions from Santonian age sources, which partially overlap stratigraphic ages, suggesting volcanism contemporaneous with deposition. In addition, paleocurrent analyses indicate sediment transport to the southeast, pointing to a sediment source along the South American margin.Volcanic and volcaniclastic rocks of the Paleocene Sacapalca Formation overlie the Casanga and Naranjo formations in angular unconformity. The Sacapalca Formation passes transitionally into fluvial sandstone, conglomerate and red beds of the Río Playas Formation. Overall, the succession recorded a transition from volcanic to alluvial fan deposition. Heavy mineral and sandstone QFL analyses show a significant input of metamorphic detritus in the Río Playas Formation, which indicates derivation from the Cordillera Real. This interpretation is also supported by detrital zircons of Proterozoic age. The change from submarine fan deposits of the El Naranjo and Casanga formations to the nonmarine Río Playas Formation suggests exhumation in response to an important tectonic event, likely deformation related to collision of the Caribbean Plateau to South America at 73–70 Ma.