Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources

Beatriz Coira,Patrocinio Flores,Claudia I Galli,Suzanne Mahlburg Kay,Daniel F Stockli,Emilio Eveling

doi:10.5027/andgeov49n2-3377

Abstract

Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene-Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations), and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to these tuff groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.

Highlights

In the Humahuaca and Casa Grande intermountain basins, Northwestern Argentina (Fig. 1), there are important records of ash-fall tuffs interspersed in their conspicuous Pliocene-Pleistocene sedimentary rock sequences
We present new petrographic and geochemical data and geochronological age constraints from selected ash-fall tuffs from Uquía and Mal Paso formations and from Pleistocene alluvial fans sheetflood deposits outcropping in those basins
The sedimentary rocks of the Uquía Formation exposed in the Quebrada de Humahuaca (Fig. 1), records numerous taxons of fossil mammals belonging to different lineages that have been used to document major events associated with the Great American Biotic Interchange (GABI) in South America during Pliocene and Early Pleistocene (Reguero et al, 2007; Reguero and Candela, 2008), and become a crucial unit in the calibration of the Uquía South America Land Mammal Age (SALMA) (Marshall et al, 1982)

Summary

Introduction

In the Humahuaca and Casa Grande intermountain basins, Northwestern Argentina (Fig. 1), there are important records of ash-fall tuffs interspersed in their conspicuous Pliocene-Pleistocene sedimentary rock sequences. The sedimentary rocks of the Uquía Formation exposed in the Quebrada de Humahuaca (Fig. 1), records numerous taxons of fossil mammals belonging to different lineages that have been used to document major events associated with the Great American Biotic Interchange (GABI) in South America during Pliocene and Early Pleistocene (Reguero et al, 2007; Reguero and Candela, 2008), and become a crucial unit in the calibration of the Uquía South America Land Mammal Age (SALMA) (Marshall et al, 1982) This formation (Upper Pliocene-Lower Pleistocene) is a carrier of well-preserved levels of tuffs widely distributed along the Quebrada de Humahuaca. A decrease in the pyroclastic contribution to the Humahuaca basin is recorded in the Pleistocene alluvial fan deposits distributed in it

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