Jurassic to Early Paleogene sedimentation in the Amazon region of Ecuador: Implications for the paleogeographic evolution of northwestern South America

The retroarc Amazon region of Ecuador recorded distinct events that are preserved in the Mesozoic-Cenozoic sedimentary record of the Subandean Zone and Oriente Basin. In this contribution, we review the Jurassic to Paleocene geologic and paleogeographic evolution of the Ecuadorian foreland using stratigraphic, sedimentologic, provenance, and geochronological data. Our results provide new insights into the sedimentary systems, stratigraphic framework, depositional ages, and sediment dispersal patterns of retroarc basin fill. During the Early Jurassic, marine deposits of the Santiago Formation formed a carbonate platform in eastern Ecuador. At ~180 Ma, an eastward-prograding deltaic system transported volcaniclastic material from a western volcanic arc. Nonmarine conditions prevailed during most of the Middle to Late Jurassic, characterized by deposition of red beds and intercalated volcanic and volcaniclastic deposits of the Chapiza Formation. Clastic and volcanic deposition dated from ~160 to 130 Ma was coeval with regional extension, as suggested by sedimentologic and seismic data from the Oriente Basin. Provenance results show that a ~ 120 Ma fluvial system at the base of the Hollín Formation was initially sourced from the west and progressively evolved to a mixed source with contributions from the Amazonian Craton and incipient Andean topography. Deposition of the Hollín Formation occurred in a northwest-flowing fluvial system that evolved into an estuary system confined by the Andes to the west and the Amazonian Craton to the east. Coastal to shallow marine deposits of the Napo Formation accumulated during regional sea-level variations throughout the Late Cretaceous. The geometry of coastal deposits and supporting provenance data suggest a drainage system characterized by west-flowing rivers derived chiefly from the Amazonian Craton. The latest Cretaceous (~70 Ma) onset of shortening in the Northern Andes coincided with collision of the Caribbean Plateau. This tectonic episode induced a continental-scale drainage reorganization documented in provenance data showing an Andean origin of the east-directed fluvial deposits of the Maastrichtian–Paleocene Tena Formation.