Mineralogical and geochemical analysis of sodium bentonites in continental settings: The Uspallata Group (Triassic) of the Cuyana Basin, Mendoza province, Argentina

SALDUONDO, J.; ETCHEVERRY, R; CRAVERO, F; COMERIO M.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2020

0895-9811

The Triassic Uspallata Group of the Cuyana rift Basin is characterized by continental deposits including alluvial,fluvial and lacustrine systems, mainly composed of volcaniclastic (reworked) and pyroclastic sediments. Thepresent work focuses on the ash fall tuffs levels (bentonites) in order to understand the origin of their sodiumcomposition since classic worldwide natural sodium bentonites link with marine environments as a source ofsodium. The mineralogical attributes and relationship with diagenetic alterations have been explored by petrography,X-ray diffraction, and geochemical analysis. The tuff levels, recorded in the Cerro de las Cabras,Potrerillos and Cacheuta formations, provide the most reliable representation of the composition of the originalmaterial and diagenetic alterations due to these levels are devoid of detrital input. Accordingly, they representkey elements to explore the origin of huge bentonite deposits in the basin. In the study units, Na-montmorilloniteand analcime, found at concentrations of up to or more than 50 vol%, grew at the expense of vitric componentsand represent authigenic minerals developed under early diagenesis. Following sedimentological and paleogeographicevidence the alteration of volcanic glass occurred in hydrologically open lacustrine systems thatreached saline-alkaline conditions. The changes in the (Na + K)/H+ ratio influenced the formation of Namontmorilloniteand analcime through the succession. In addition, kaolinite is an important alteration mineralthat fills cavities, replaces the feldspar grains, and probably the early diagenetic minerals. The origin of kaoliniteis due to the combined effects of decomposition of organic matter and incursion of meteoric freshwater duringlate-stage telogenesis which generated the acidification of pore waters. Based on geochemical proxies, geochronologicaldata and the geodynamic context the input of high volumes of pyroclastic material may thus be theresult of explosive volcanism from an arc-subduction complex located to the west at these latitudes. The presentwork suggests that the composition of Na-rich pyroclastic ashes accumulated in hydrologically-open lake systemswhich conditioned the formation of sodium bentonites in continental settings.