Análisis de terrazas fluviales en el río Guayllabamba, sector Perucho, provincia de Pichincha

Abstract

Lithostratigraphic mapping, petrographic characterization and provenance analyzes were performed to define the Cretaceous tectono-stratigraphic evolution of a narrow lithotectonic complex located to the northwest of Quito, among the villages of San José de Minas, Perucho, San Antonio and Calacalí.

Based on petrographic and lithostratigraphic analyzes have been defined at least two main Formations. The Pallatanga Formation, which represents an andesitic to basaltic oceanic terrane accreted to the continental margin during Upper Cretaceous times forming a basement block. The Natividad Formation, which in turn represents a volcanic to volcaniclastic sequence derived from the Río Cala arc, developed between the Campanian and Maastrichtian. A ~100 m wide plutonic body characterized as a Gabbro was interpreted to intrude the pre-existent lithologies of the study area.

Provenance analyzes were established using two widely recognized methods sandstone petrography (QFL diagrams) and meta-sediment geochemistry. The results of the provenance analyzes suggest a volcanic source of intermediate-mafic composition typical of an island arc for the Natividad Formation, deposited during the Late Cretaceous. Sedimentary facies analize indicate an underwater fan environment for the deposition of the Natividad Formation, in wich both turbiditic flow processes and debris flow dunamics dominated the Upper Cretaceous environments.

Geochemical analysis shows a marked tholeiitic affinity with P-MORB signatures of mantle plume for the basalts of the Pallatanga Formation. While for the gabbroic intrusion a calco-alkaline affinity together with semi-flat patterns on rare earth multi- element diagrams are indicative of magmas formed in a subduction context.

Finally, a transpressive tectonic regime into a ductile shear zone was deduced for the lithotectonic complex after a detailed structural analysis of foliation surfaces, fault plains, fold axis and fractures.