NEW EVIDENCE OF MARINE TRANSGRESSION IN THE INTERMONTANE BASINS OF ECUADOR DURING THE MIDDLE MIOCENE

The intermontane basins are geographically located to the south of Ecuador and represent continental, marine and transitional sequences, developed during periods of tectonic activity in the Tertiary. They were developed in two stages: i) coastal Pacific stage (15-11 Ma) and ii) intramountain stage (uplift of the Eastern Cordillera, 11-9 Ma). The Gonzanamá-Catamayo Basin is located in the province of Loja and represents the lower part of the sedimentary fill that was tectonically deposited on a hemigraben structure. This basin limits to the west by the Amotape-Tahuín terrane and the Celica Basin, and to the east by the Eastern Cordillera of Ecuador.

The basement of the Gonzanamá-Catamayo Basin consists of an Eocene-Oligocene volcanic/volcanoclastic stratigraphy of acidic rocks with ZTF ages between 40–25 Ma. The Gonzanamá Formation comprises Early Miocene sediments that represent the oldest lower part of the sedimentary fill in the basin and that constitute the subject of our study.

The lower member of the Gonzanamá Formation comprises Miocene sediments that overlie unconformably by intermediate volcanic and pyroclastic rocks of the Loma Blanca Formation. Sedimentological analysis for this member includes at the base, lithofacies of matrix-supported boulder conglomerate, matrix-supported cobble conglomerate. The upper part of this series consists of inorganic limestones intercalated with irregular to subspherical masses of chert, greenish siltstones, sandstones with phytodebris and coal. This lithofacies association suggests an initial deposition of alluvial fans followed by a deposition of sediments in shallow lakes in a coastal environment.

The middle member of the Gonzanamá Formation includes facies of sandy turbidites, tabular and amalgamed sandstones, black shales, laminated mudstones, fossiliferous limestones and sporadic acidic tuff beds. This facies association suggests deposition in a deep lake influenced by deltaic systems in a coastal environment. Different types of fossils found in this sedimentary sequence such as ostracods, sponge spicules and foraminifera suggest that this basin was located in a brackish water environment. Petrographic analysis of sandstones, heavy mineral data and some levels of tuff in the stratigraphic sequence show that the source area is derived from an active volcanic arc. Therefore, the analyzed stratigraphic section shows the coastal Pacific Stage during the Early Miocene of the Gonzanamá-Catamayo Basin.