Sedimentological and paleoenvironmental study of the Peñas Coloradas Formation of the Chota basin, Ecuador

Abstract:

The Andes of Ecuador are divided in two north-south sub-parallel chains, the Cordillera Occidental (CO) to the west, and the Cordillera Real (CR) to the east. In between these is the Interandean Depression, a valley that spans the northern half of Ecuador with average elevations over 2000 m. Several exhumed basins there such as the Chota, Guayllabamba y Latacunga-Ambato have alluvial fan, fluviatile, lacustrine, and volcanoclastic deposits that preserve a record of the sedimentological and structural settings that existed in the past. These intermontane basins also reveal changes in climate through paleoenvironmental records. The Chota Basin is the northernmost intermontane basin of the Ecuadorian Andes. Its basin fill is defined by four units; Chota, Santa Rosa, Peñas Coloradas, and Carpuela Formations. In this study, we will carry out a detailed study of the sedimentology of the Peñas Coloradas Formation (PCF) of the Chota basin, which is exposed in the eastern part of the basin with a thickness of ~600 m. The regional geology allows us to carefully trace source areas using provenance deposits because CR -derived sediments are sourced in Paleozoic and Jurassic plutonic and metamorphic rocks, while CO-derived sediments are sourced in a Cretaceous oceanic terrane. Previous studies done using heavy mineral analyses have suggested that the CR is the main source for the PCF, but preliminary results show abundant volcanic material in the deposits more consistent with a western source. We plan to carry out further heavy mineral analyses and clast counts in conglomeratic strata to better constrain this aspect. Furthermore, we will carry out a detailed facies analysis to determine changes in depositional environments over time, and XRD analyses of clay to determine clay assemblages and infer paleoclimate conditions of the Chota basin. This will allow us to constrain tectonic and climatic signals that were prevalent during the deposition of the PCF.