Basin-fill development from marine to intermontane settings: Lithostratigraphy, Sedimentology and Paleogeography of the Malacatos-Vilcabamba basin in the southern Ecuadorian Andes

Abstract

A detailed lithostratigraphic and sedimentologic study was performed in the southern Andes of Ecuador along the Malacatos-Vilcabamba intermontane basin, a thrust-controlled inverted basin developed during the middle Miocene. We defined twenty-three sedimentological facies grouped into seven facies associations and three different depositional systems (deltaic, tidal flat and alluvial fan) for the entire basin fill. The 500–1100 m thick Malacatos-Vilcabamba basin fill currently lies at about 1470 m above current sea level. It is unconformably underlain by a metamorphic and volcanic basement and usually exhibits some evidence of synsedimentary deformation and synchronous volcanism. Long-term patterns of basin sedimentation suggest a basal fining-upward sequence of marginal-marine lithofacies that gradually passes upward into a coarsening-upward sequence (conglomeratic) typical of a continental setting. The history of basin-filling was initiated in the middle to late Miocene (15-10 Ma) as a lowland basin. Proximal facies within a deltaic system under tidal influence were preserved as the San José and San Francisco Formations, while a flat tidal system alternating between arid and humid climatic episodes was preserved as the Santo Domingo Formation. During the subsequent continental sedimentation, which occurred in the late Miocene (9-5 Ma), the prior basin-fill was partially deformed and uplifted. This episode of basin inversion is primarily evidenced by the arrival of thick alluvial successions (conglomerates) composing the Cerro Mandango Formation. Compressive tectonics continues throughout the Pliocene with alluvial sedimentation (Suro Unit) and the gradual inversion of the basin-bounding normal faults into thrusting systems that currently predominates at basin borders. An insight into the connection via marine incursions between the Miocene basins and either the Pacific Ocean or the eastern Oriente basin, is provided herein as a discussion based on the data collected and a local paleogeographic model.