Clay mineral transformations from alluvial to lacustrine environment in the Paleogene Maiz Gordo Formation (NW Argentina).

CAMPO,; CECILIA EUGENIA DEL PAPA; MILLÁN, J, J.; F, N.

Paris

Congreso; Bridging Clay; 2006

The Maíz Gordo Formation constitutes, by its thickness, extension and time span, one of the main Paleogene lacustrine events registered in north-western Argentina [1]. This unit consists of mudstones and limestones beds, 200 meters thick, interpreted as deposited in a brackish-alkaline lake. The basin evolved mainly as a closed system, but during short periods it turned into an open one. Its sedimentary facies and paleogeographic reconstruction pointed out to a lake with a ramp type margin in which alternated periods of low and high energy [1]. We study the mineralogy of representative samples by X- ray diffraction (XRD) at bulk sample and also in the < 2 µm sub-fraction with the aim to examine the changes in clay mineralogy between alluvial and lacustrine sediments. Besides, selected samples were study with scanning electron microscopy (SEM). From the seven sites considered in this study three – Tonco, Valle Encantado and Ayuzos – respectively represent proximal, medium and transitional positions of the fluvial environment. Two sites – Chuñapampa and Corralito – are representative of marginal lake sub environment and two sites – Garabatal and Las Cañas – of the inner lake sub environment. Studied sites are approximately located in a west-east section from the source area to inner basin and each locality laterally correlates with the next; allowing the detection of mineralogical changes and diminishing the effect of different source-area provenance. The mineralogical study revealed that distinct clay mineral associations prevail in the <2 µm sub-fraction of fine-grained sediment of the different fluvial sites. In Tonco site kaolinite is the most abundant mineral in almost all samples, muscovite abundances range from 12 to 46 %, and smectite is very scarce or is absent. Progressing basinward, in Valle Encantado site, muscovite is the predominant clay mineral with subordinate kaolinite (5-26%) and scarce smectite. Meanwhile, in Ayuzos site, transitional to lake settings, the clay fraction is dominated by muscovite with subordinate smectite, kaolinite is absent and it disappearance coincides with the first occurrence of analcime, which is a widespread mineral in all the lithologies from lake-littoral to inner lake settings. The <2 µm sub-fraction of sediments from littoral and inner lake settings are composed by detrital muscovite and subordinate smectite or entirely by muscovite, in all cases kaolinite is absent. SEM micrograph indicates that illitization of kaolinite takes place in an arenite from Valle Encantado. Likewise, textural evidences of smectite authigenesis were recognized during the SEM study of a feldesphatic wacke from Ayuzos and a carbonate mudstone from inner lake settings. In this sample SEM micrographs also reveal albite overgrowths over detrital plagioclase. We interpret the observed changes in clay mineral assemblages as the result of the development of a hydrogeochemical gradient in basinward direction from acidic to neutral fluvial waters and alkaline waters of inner lake settings. [1]del Papa, C.E., Jour South Am Earth Sci 12 (1999), 389.