Lily Jackson presents Andean River Sediments as a Window into the Tectonic History of Ecuador

Abstract:  The Andes of South America form the longest mountain range on Earth spanning over 7000 km. In contrast to much of the Andes, the tectonic history of the Ecuadorian segment involves one or many accretionary events with proposed timing ranging from the Late Cretaceous to the Eocene. Additionally, Ecuador’s two Cordilleras (Eastern Cordillera and Western Cordillera) may have been among the most recent segments of the Andes to attain high topography, though the age of the Cordilleras remains debated. Topographic growth causes erosional unroofing, producing detritus that is then deposited adjacent to the growing mountain belt and incorporated into the sedimentary record of the event. Modern river sediments faithfully represent the outcrop geology within their catchments. Pairing detrital zircon U-Pb geochronology of river sands with quantitative GIS models of surface geology allows for interpretation of sediment provenance within river catchments. In Ecuador, we find that various morphotectonic regions have diagnostic U-Pb age spectra that reveal erosion and sediment transport related to key tectonic events. Morphotectonic provinces of Ecuador include the: (1) Late Cretaceous-Cenozoic sub-Andean and Oriente retroarc foreland basin; (2) Eastern Cordillera; (3) Interandean Valley; (4) Western Cordillera; and (5) forearc lowlands and coastal plain. In the Oriente foreland basin, Cretaceous-Neogene sediments are the dominant surface exposures, with Jurassic arc-related rocks restricted to the sub-Andean Zone. Diagnostic U-Pb age spectra for these foreland zones contain Jurassic, Triassic, and Cretaceous age peaks. In contrast, Eastern Cordillera exposures are metamorphosed Paleozoic-Mesozoic rocks and non-metamorphosed Triassic-Jurassic igneous intrusions. Diagnostic U-Pb age spectra for the Eastern Cordillera contains large age populations of Proterozoic and Paleozoic ages representing recycled cratonic material. Western Cordillera surface geology involves Late Cretaceous basement rocks, Cretaceous-Paleogene sedimentary rocks and Cenozoic volcanic cover. Diagnostic U-Pb age spectra for the Western Cordillera contain Neogene-Quaternary age populations with minor Triassic and Cretaceous ages. River samples draining Paleogene units in the easternmost Western Cordillera show Proterozoic and Paleozoic ages indicative of exhumation, erosion, and westward transport of detritus from the Eastern Cordillera. Furthermore, the dominant Proterozoic and Paleozoic ages that characterize the Eastern Cordillera rivers provide support for multiple recycling events prior to the current tectonic configuration of the Ecuadorian Andes.

Bio:  Lily Jackson is a Ph.D. Candidate at the University of Texas at Austin in the Jackson School of Geosciences and a National Science Foundation Graduate Research Fellow. Lily is a native Arizonan from Prescott. She graduated magna cum laude with a Bachelor of Science from the University of Arizona in 2014. While in Tucson, Lily was the recipient of the Arizona Geological Society Scholarship Award in 2013, and served as a Student Councilor to the AGS executive committee in 2014. Lily was named the outstanding senior in Geosciences for her graduating class.

Lily’s research focuses on the Cenozoic tectonic history of northern Peru and Ecuador. Lily uses sedimentary basin analysis, detrital zircon U-Pb geochronology, stable isotope geochemistry, and field geology in her research.