Stratigraphy and paleoenvironments of Los Lanos Formation (La Rioja): A condensed sequence in the broken foreland of the Central Andes

Abstract

In the central region of the Sierras Pampeanas, within the Cenozoic cover, sparse outcrops of a composite fluvio-eolian unit with abundant calcretes have been included within the Los Llanos Formation. This succession of ~40 m is well exposed in the Sierra Brava, Sierra de Los Llanos and Sierra de Ulapes-Las Minas. Since the Los Llanos Formation was defined (also known as Estratos de Los Llanos) little detailed stratigraphic-sedimentologic work has been carried out as to properly understand its depositional environments and stratigraphic context. Mapping and facies analysis led to separate two members within this unit and defined their paleoenvironments. Eight main facies were defined: 1) clast supported conglomerates, 2) microbreccias, 3) megacross-bedded sandstones, 4) strongly cemented calcareous sandstones, 5) sandstones and shales with columnar structure, 6) conglomeratic sandstones with siliceous cement, 7) red clayey-silt, 8) pebbly mudstones. This facies are grouped into (I) fluvio-eolian and (II) paleosoil-rich alluvial plain facies associations, partly coincident with the two recognized members, characterizing an alluvial setting with frequent intervals of condensation. Frequent condensation in synorogenic alluvial settings allows prediction of relatively distal positions within the foreland, probably next to the forebulge depozone. Because of the scarcity of well-constrained paleontological data and lack of isotopic dating, our process- response paleoclimatic interpretation allows a better correlation of the Los Llanos Formation with the Middle Miocene- Pliocene climatic events, widely recognized within the Andean foreland.