Identificación y Caracterización de Facies de la Formación Hollín en Centro Shaime: El Registro de una Transición Fluvio-Marina en la Región Sur Oriental del Ecuador

Abstract

This project was performed in order to identify and characterize the depositional environments, petrophysical characteristics (porosity and permeability) and sedimentary facies of outcrops analogous to the subsurface record of the Cretaceous Hollín Formation at Centro Shaime in the Zamora Chinchipe province, Cordillera del Condor, southeastern Ecuador. U-Pb detrital zircon dating of the basement rocks range between is 200 to 116 Ma, suggesting a pre-Hollín erosion of the Misahuallí and Alao Arcs associated whit Peltetec Event, and a 116 Ma as a maximum age for the Hollín Formation. Eight (8) lithofacies have been defined for the Main Member Hollín Formation, seven (7) sandy lithofacies and one (1) mud litofacie expressing a fluvial environment dominated by braided rivers (lag basal channel, fluvial bars and flood plain deposits), with high values of porosity and permeability; heavy minerals analysis indicates cratonic provenience whit volcanic influence. Seventeen (17) lithofacies are reported in the Upper Hollín interval. These have been distributed in two (2) supratidal lithofacies, seven (7) intertidal lithofacies, and eight (8) subtidal lithofacies, that are associated with a coastal environment with strong tidal influence and platform facies to the top. The best reservoir within this tide dominated environment correspond to the tidal channels, although these sands have a limited extension; heavy minerals analysis indicate cratonic provenience. The base of the Napo Formation corresponds to the continuation of the transgression recorded in the Upper Hollín Formation, and include platform shales and carbonates, with abundant Albian macrofossils. Quartzrich sandstones of the Basal Tena Member of the Maastrichtian Tena Formation overly in unconformity the Albian shales of the Napo Formation, suggesting a strong pre-Maastrichtian erosional event, characterized by the rapid exhumation of the upper cretaceous section of the Napo Formation, associated to the accretion between the Caribbean Large Igneous Province and the NW South American Plate in the Upper Cretaceous.