Geodynamic evolution of the Neogene intermontane Chota basin, Northern Andes of Ecuador

Abstract

The development of intermontane basins with a thick non-marine sedimentary pile is one of the main characteristics of the Andean geodynamic evolution during the Neogene. The Chota basin is the northernmost Neogene intermontane basin recognized in the Andes of Ecuador. This basin is located in the Interandean Depression and presents a sedimentary fill of continental deposits. The detailed stratigraphic and tectonic analysis of this basin allows for a new subdivision of the sedimentary sequence in four main units with a total minimum thickness of 2400 m. Sequence analysis suggests a cyclic evolution characterized by two megasequences. The first one, represented by the Chota Unit, displays an evolution from proximal to more distal facies (braided alluvial to lacustrine facies) during the opening of the basin. The second one, represented by the Santa Rosa Unit to the west, and the Peñas coloradas and Carpuela Units to the east, displays a thickening-coarsening evolution related with the filling of the basin and with the development of prograding alluvial fan deposits. The structural analysis in the Chota basin shows two main tectonic events. The first event is responsible for the opening of the basin, and is the result of a tensional tectonic period with σ3 along a N130 °E direction. The second event caused the closing of the basin, as a result of a regional compressional regime with a N120 °E trending, which then rotated to an E-W compressive trend. The Chota Basin is interpreted as a Neogene intermontane basin, with a sedimentary fill controlled by a continuous tectonic deformation. The deposits of this basin recorded large scale geodynamic events and are a unique example for the study of the Neogene evolution of the Ecuadorian Northern Andes.