GEODYNAMIC EVOLUTION OF THE NEOGENE INTERMONTANE CHOTA BASIN, NORTHERN ANDES OF ECUADOR

The development of intermontane basins with a thick non-marine sedimen tary pile is one of the main characteristics of the Andean geodynamic evolution during the Neogene. The Chota basin is the northernmost Neog ene intermontane basin recognized in the Andes of Ecuador. This basin is located in the Interandean Depression and presents a sedimentary fi ll of continental deposits. The detailed stratigraphic and tectonic an alysis of this basin allows for a new subdivision of the sedimentary s equence in four main units with a total minimum thickness of 2400 m. S equence analysis suggests a cyclic evolution characterized by two mega sequences. The first one, represented by the Chota Unit, displays an e volution from proximal to more distal facies (braided alluvial to lacu strine facies) during the opening of the basin. The second one, repres ented by the Santa Rosa Unit to the west, and the Penas coloradas and Carpuela Units to the east, displays a thickening-coarsening evolution related with the filling of the basin and with the development of pro grading alluvial fan deposits.The structural analysis in the Chota bas in shows two main tectonic events. The first event is responsible for the opening of the basin, and is the result of a tensional tectonic pe riod with sigma 3 along a N130 degrees E direction. The second event c aused the closing of the basin, as a result of a regional compressiona l regime with a N120 degrees E trending, which then rotated to an E-W compressive trend. The Chota Basin is interpreted as a Neogene intermo ntane basin, with a sedimentary fill controlled by a continuous tecton ic deformation. The deposits of this basin recorded large scale geodyn amic events and are a unique example for the study of the Neogene evol ution of the Ecuadorian Northern Andes. Copyright (C) 1996 Elsevier Sc ience Ltd & Earth Sciences & Resources Institute