Sedimentological and climatic control in the clay minerals distribution in the fluvial deposits of the Palo Pintado Formation (upper Miocene), Salta Province, Argentina

GALLI, C.I.; VIDES, M.E.; FLORES, P.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Associations of Sedimentologist

Succession of the Palo Pintado Formation, about 800 m thick, was deposited in Calchaquí river.Palaeoenvironmental interpretation of these deposits was obtained from sedimentological andpalaeontological data in the earlier studies which consist of an intermediate class between low andhigh sinuosity rivers forming a wandering fluvial system sand-gravel with semipermanentes smalllakes.The cycles are composed of conglomerates and sandstones, which are channeles (gravels bars and sandy bedforms) and overbank deposits (crevasse channels and small crevasse channel) and claystones in the floodplain deposits.The conglomerates on the base of Palo Pintado Formation, shows pebbles of slate andphyllite, and in less proportion granite and pegmatite that increase to the top (from 0% to 60%).The quartz is the principal component in the sandstones with monocrystalline grains(plutonic origin, Qm= 50-83%) and policrystalline. Undulatory extinction quartz (low grade metamorphic origen, Qme= 0-16%) is observed in less propotion. The policrystallinequartz (Qp= 45-3%) is constituted by two or more crystals with sutured contacts (metamorphicorigen) and straight contacts (plutonic origen).Other relevant detritics grains are the potash feldspars (K): orthoclase with high kaolinitization grade and microcline which appears rounded or subrounded, fresh grains. The plagioclase generally altered, can be identified by its polysynthetic, multiple twinning. The lithiccomponents are metamorphics: gneisses, schists, slate, phyllite, a few volcanics fragments andsandstones.Then, X-ray diffraction analysis were carried out on 27 samples of claystones. Clay minerals identified are illite, interstratified illitesmectite, smectite and kaolinite. The mineralogical composition (< 2mm) of clay includes non-clay mineral too: quartz, plagioclase and calcite. Illite and smectite associated with quartz and plagioclase appears more in the base of Palo Pintado Formation. The middle and the top levels are composed by smectite, illite and kaolinite with quartz, plagioclase, calcite, siderite and paligosrkite.Added to this, clay minerals incompletely altered such as illite and smectite are formed bybialitisation (hydrolysis) in warm-dry climates.The clays formed to these conditions are produced by preexistent minerals degradation in the soil,with sustraction of elements: Ca, Mg and in less propotion Si and Fe.The interaction between the mineralogy constituted by phyllosilicates and alkaline waters loaded in magnesium would be responsible for the smectite formation.To conclude, the upper Miocene deposits were derived from upper Neoproterozoic basement (Puncoviscana Formation and La Paya Formation) and Ordovician granites andpegmatites. The clay mineral assemblage of these source-area rocks is mostly smectite,interstratified illite-smectite, illite and kaolinite.The weathering of these materials under high temperature and rainfall favoured improved thecondition of the clay minerals hydrolysis and the kaolinite formation. Clay associations reflect boththe influence of the climate and to the lithology and composition of source areas.The clay minerals have been generated during a long period of time and under climaticstable conditions, under warm and humid state with low seasonal, more humid than othercontemporary regional formations.