CHARACTERIZATION OF THE STRATIGRAPHY OF AN ANDEAN INTRAMONTANE BASIN: A STUDY OF THE SILANTE FM. IN NORTHERN ECUADOR

Ecuador, located in the northwestern part of South America, is traversed by the Andes in an approximate north-south direction, and is segmented into the following tectonic provinces from east to west: the Oriente Basin, Subandean Zone, Cordillera Real, Inter-Andean Valley, Western Cordillera (WC), and Coastal Forearc Basin. The WC represents an accreted oceanic terrane thought to have collided with South America in the Upper Cretaceous, and is formed by an oceanic crust basement covered by deep-marine deposits. The Silante Formation (SF) is a continental sequence deposited in an intramontane basin within the WC. This unit has been well described in the central part of the WC, where it is composed of breccias, matrix-supported conglomerates, red mudstones, shales, siltstones, and violaceous tuffs. However, the age and stratigraphic context of the SF are still controversial, as different authors have proposed different scenarios. For example, Egüez (1986) proposed that the SF was deposited in the Late Cretaceous between marine deposits, while Vallejo (2007) proposed that it was deposited in the Paleocene in discordance over marine deposits. More recently, Almagor (2019) proposed that the SF was deposited in Oligocene-Miocene times and overlies Oligocene volcanic deposits.

In this study we carry out a geologic survey of the SF along the Salinas-Lita road, which cuts the WC in an east-west direction in northern Ecuador. This transect provides a series of road and river cuts across the WC, where the SF and underlying Cretaceous bedrock are well exposed. We use geological mapping and measurements of stratigraphic sections and structural features to characterize the depositional setting and subsequent deformation of the SF. We show that the formation is composed of two distinctive sub-units and is underlain by a time-transgressive unconformity, and that the present basin margins are fault contacts. We compare our results with those from the central WC, and propose a model for basin formation that involves the SF filling pre-existing topography in the WC, and interpret its subsequent deformation in the context of the tectonic evolution of the WC. This study adds to the collective understanding of the evolution of accretionary orogens in subduction settings.