Provenance and geochronological insights into Late Cretaceous-Cenozoic foreland basin development in the Subandean Zone and Oriente Basin of Ecuador
The evolution of sediment source regions in the Andes of Ecuador can be linked to Late Cretaceous-Cenozoic development of a subduction-related magmatic arc, retroarc fold-thrust belt, and flexural foreland basin. An assessment of the chronostratigraphic, provenance, and depositional characteristics of 3.5 km thick clastic succession accompanied by seismic structural analysis of uplifted basement-rooted structuress in the Subandean and Oriente foreland basin provide insights into important Cretaceous and Cenozoic shifts in the paleogeographic framework and tectonic evolution of the Ecuadorian Andes. Detrital zircon U-Pb geochronological results for 17 sandstone samples (1845 individual ages) provide an integrated provenance framework for sedimentary fill of the Oriente Basin. The U-Pb results span six stratigraphic levels that range in age from the late Early Cretaceous (Hollín Formation) to Quaternary (Mesa and Mera Formations). Contributing source regions can be identified by diagnostic age populations representative of four principal tectonic provinces: (1) Western Cordillera magmatic arc (<100 Ma); (2) Eastern Cordillera basement (1300-250 Ma), Triassic intrusions (250-200 Ma) and Early and limited Late Cretaceous igneous rocks (145-66 Ma); (3) Subandean Zone fold-thrust belt (200-145 Ma ages accompanied by recycled signatures from Cretaceous strata); and (4) eastern cratonic regions of the Amazonian shield (>1300 Ma). U-Pb geochronological results for the uppermost Cretaceous Tena Formation, including a young population of 68.6 ± 3.5 Ma (n=3 grains), indicate a wholesale reversal from eastern cratonic sediment sources to early Andean sources that can be assigned to a Maastrichtian onset of shortening. The U-Pb results also reveal existence of previously undocumented andean basement sources of Pan-African age (900-650 Ma) and Late Cretaceous magmatic arc (89.8-68 Ma) signatures. Upsection shifts in representative U-Pb age spectra demonstrate the systematic introduction of new source regions as well as progressive recycling (cannibalization) of older basin fill. The recycled basin fill was incorporated into younger foreland deposits during eastward advance of the Andean fold-thrust belt. Such interactions were also recorded by thickness variations within the Paleocene-Eocene reflector package and reactivation of preexisting (inherited) normal faults that were identified using seismic structural analysis. This structural disruption was linked to uplift of basement-rooted blocks in the westernmost basin margin, causing the lack of westward thickening of the basin succession toward the Andean mountain belt.