Geochemical, petrographic, and geochronologic framework for the Salado block of eastern Ecuador: implications for Andean tectonics and Amazon basin evolution

The Cordillera Real of Ecuador includes at least five lithotectonic divisions interpreted as separate allochthonous blocks successively accreted to the Amazonian craton (Guyana Shield) of northern South America during Mesozoic subduction. The Salado block has been described as a Jurassic marginal basin structurally distinct from the Oriente retroarc foreland basin and eastern craton. This study presents the Middle Jurassic to Cretaceous geologic and palaeogeographic evolution of metasedimentary rocks of the Cuyuja and Upano units of the Salado block. Our results provide insights into the stratigraphic framework, depositional ages, and correlation with retroarc strata of the neighbouring Oriente Basin. Geochronological and geochemical data suggest that metamorphosed sedimentary rocks of the Upano unit are correlative with the nonmetamorphosed Misahualli and Chapiza formations located farther east in the adjacent Sub-Andean Zone and Oriente Basin. We propose that the Upano unit was deposited in an intraarc position within a Jurassic magmatic arc that extended along Colombia and Ecuador. Detrital zircon U-Pb data indicate that clastic sediments of the Cuyuja unit were derived from both, the Amazonian Craton to the east and Andean sources to the west. Stratigraphic, sedimentologic, and geochronological data from metamorphic rocks of the Cuyuja unit suggest that they are correlative with nonmetamorphosed sedimentary rocks of the Hollín Formation in the Sub-Andean Zone and Oriente Basin. The data presented in this study supports an autochthonous origin for the Salado block, in which metasedimentary rocks of the Upano and Cuyuja units in the west were contiguous with Jurassic to Cretaceous sedimentary rocks of the Oriente Basin in the east. The sharp contrast in metamorphic grade can be attributed to the structural incorporation of the western units into the growing Andean orogenic wedge during the Maastrichtian to Palaeocene initiation of shortening and inception of the adjacent foreland basin system in the northern Andes.