Stratigraphic and tectonic characterization of the basal breccias of intermontane Andean basin: the Peñas Coloradas Formation of the Chota Basin of northern Ecuador.

Abstract:

The Chota Basin is located in northern Ecuador, in the province of Imbabura; it represents the northernmost basin of the Interandean Depression. The stratigraphy of this intramontane basin preserves a record of the processes that created the surrounding mountain range, and its subsequent deformation informs us of the tectonic processes there as well. This basin is thought to have formed at about 6 Ma, and thus is one of the most relevant for the study of the Neogene evolution of the Andes in the northern part of the country. The focus of our work is the breccia deposits of the Peñas Coloradas Formation (PCF). Although there are only two previous studies carried out in this basin, Barragán et al. (1996) and Winkler et al. (2005), the interpretations of different aspects of the basin are incongruent between them. Among the main differences are the age of the basin, the processes that lead to the origin of the basin (i.e. inverted extensional basin vs foreland setting), and the stratigraphic sequence of the basin (the PCF is placed either at the base or the middle of the basin stratigraphy). In order to help resolve the discrepancies and in an effort to better understand the PCF and the tectono-stratigraphic context of its formation we carry out fieldwork with the purpose of creating refined geologic maps, stratigraphic columns, and cross-sections. Our preliminary results show that even though the PCF has been previously described as a unit with uniform lithologies throughout, there is varied internal stratigraphy that shows episodes of lacustrine, channelized and sheet flow deposits. The strata also show lateral variations in thickness, which may be related to the origin of the basin. The structural style is also quite different between the unit that possibly underlies the PCF (the Chota Formation), which has large overturned sections and abundant recumbent folding, and the PCF breccias, which is characterized by a train of upright folds. This allow us to place new constraints on the evolution of the Chota basin.