Estratigrafía, sedimentología y geoquímica de las formaciones chapiza y hollín en la zona subandina: estratigrafía, sedimentología y geoquímica de la formación chapiza y los miembros alluvial valley fill y hollín inferior de la formación hollín, en el corte coca-codo sinclair, zona subandina, cuenca oriente.

Abstract

In this study, a macroscopic recognition of components in hand samples, the interpretation of mineral affinities determined in previous works, and X-ray fluorescence were carried out, with which 12 facies were recognized, sandy, conglomeratic, muddy-silty, volcanoclastic. and volcanic, distributed between the Chapiza Formation, and the Alluvial Valley Fill (AVF) and Lower Hollin members of the Hollin Formation, and which, correlated with their geochemical response, made it possible to determine the dominant fluvial and fluvio-alluvial depositional environment in the sedimentary sequence. from the Upper Jurassic to the Lower Cretaceous.

The Fe2O3/K2O and SiO2/Al2O3 ratios allowed the lithological distinction of the Fm. Chapiza from sandstones and arkoses from Lower Hollin and arkoses and shales from AVF. In general, the concentrations of Si, Th, Zr, W, Nb and Y are present in higher proportions in the Lower Hollin sandstones; while the Ca-related Sr increases its proportion in the fine-grained facies of the Fm. Chapiza. The high Al, Ti and Rb explains the presence of clay minerals in the fine-grained facies of the Fm. Chapiza and AVF, and the alteration of primary components in the Lower Hollin sandstones.

The variation of MgO-Fe2O3-SiO2/Al2O3 classifies the samples of the Fm. Chapiza, AVF and Lower Hollin as non-marine sediments, deposited under oxidizing conditions, reaffirmed with the indices Ni/Co<5 and V/Cr<2, and particularly with the depletion in U, V, Cu, Co, Mo, Zn and Ni in the Fm. Chapiza; conditions typically of a dry climate (Mg/Ca<2.5), contemporary with the volcanic activity of the Misahualli arc. The Zr/TiO2 vs. Nb/Y determines that the volcanic source that contributes sediments to the Chapiza Formation, the AVF and Lower Hollin members is of intermediate-basic to intermediate-acid composition. The weathering tendency increases in the Lower Hollin facies and decreases in the Fm. Chapiza and AVF (Th/U>4).