Model of continental sedimentation for the Cretaceous Central Argentinean rift: example from Las Quijadas hill, San Luis province

En este trabajo se estudian las sedimentitas continentales cretácicas (Grupo del Gigante) en la sierra de las Quijadas, ubicada en el noroeste de la provincia de San Luis. La información se obtuvo mediante el relevamiento de 15 perfiles sedimentológicos de detalle que permitieron definir 21 facies sedimentarias y 12 asociaciones de facies. Las facies permitieron interpretar los principales procesos de acumulación sedimentaria, mientras que a partir de las asociaciones de facies se propusieron los modelos paleoambientales. Los depósitos cretácicos se han acumulado en sistemas de abanico aluvial, lóbulos fluviales efímeros, planicies fangosas y barreales, dunas eólicas y ambientes lacustres hidrológicamente cerrados. El registro de la sierra de las Quijadas representa a los depósitos clásticos del margen activo de una cuenca extensional con geometría de hemigraben, desarrollada en un marco paleoclimático árido a semiárido. En él se reconocen dos secuencias deposicionales limitadas por sendas discordancias. Cada una de estas secuencias se compone de una fase de sinrift y otra de postrift. En las fases iniciales o de sinrift las asociaciones de facies reflejan pendientes fuertes, ascenso tectónico de las áreas de aporte ubicadas hacia el oeste (Alto del Desaguadero) y subsidencia mecánica en la cuenca. Las fases de postrift se caracterizan por depósitos efímeros de bajo gradiente y lacustres que responden a una alta tasa de acomodación y tendencia al ascenso del nivel de base geomorfológico que se vinculan con una etapa dominada por subsidencia termal generalizada.The Cretaceous continental deposits of the El Gigante Group in the northwest of San Luis Province (Sierra de las Quijadas), consist of proximal to distal alluvial fan - ephemeral fluvial, playa and lacustrine systems. The sedimentary record is composed of two sequences, each with different architecture and lithofacies compositions. The lower sequence is characterised by: (1) proximal and medial alluvial fan facies, constituted by clast-supported and matrix-supported conglomerates and subordinate sandstone and mudstone; (2) ephemeral sandstone lobes formed mainly of low-angle and trough cross-stratified sandstones, plane-bedded and cross-laminated sandstones and subordinate heterolitic and mudstone levels; (3) distal playa deposits comprising sheet-like heterolitic intervals composed of massive mudstones alternating with ripple cross-laminated fine-grained sandstones and siltstones; and (4) aeolian deposits represented by large-scale, cross stratified sandstones (dunes) associated with low-angle or horizontally laminated layers with dinosaur ichnites (dry interdunes or aeolian sand-sheet deposits). The facies associations of the upper sequence are: (1) terminal alluvial fan sheets characterized by pebbly sandstones and sandstones with a hole range of sedimentary structures (plane-bedding, planar a trough cross-bedding, ripple cross-lamination and massive layers); (2) proximal alluvial debris flow deposits associated with imbricated and massive clast-supported stream- and sheet-flood conglomerates; (3) amalgamated large-scale and wedge shaped cross-bedded sandstone sets interpreted as barjanoid ridges; and (4) massive and laminated lacustrine mudstones showing intercalations of nodular gypsum, laminated anhydrite and fine-grained lake margin sandstones. Alluvial and fluvial systems flowed east from the Alto del Desaguadero Uplift. During lower sequence time the climate was arid to semiarid. During the upper sequence the climate though still semiarid was more humid. The sequence stratigraphy provides a framework in which to asses the evolution of depositional style. The two fining upward sequences of Sierra de las Quijadas represent the sedimentary infill of an active margin of a half graben basin. Both sequences are characterized by an initial synrift stage and a final postrift stage. The initial stage was governed by regional tectonic subsidence. The sedimentary record comprises proximal alluvial fan facies which passed downstream into ephemeral fluvial facies and mudflat/lacustrine facies. A prominent change in depositional style from proximal to distal deposits occurred in the postrift stage. This suggests an increase in sediment accommodation and a general rise in base-level related to a change from mechanical to thermal basin subsidence.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica