Clay mineral assemblages and analcime formation in a Palaeogene fluvial- lacustrine sequence (Maiz Gordo Formation) from Northwestern Argentina.

M. DO CAMPO; C. DEL PAPA.; J. JIMÉNEZ-MILLÁN; F. NIETO

SEDIMENTARY GEOLOGY

Elsevier

Año: 2007 vol. 201 p. 56 - 74

0037-0738

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]. It consists of sandstones, mudstones and limestones beds, 200 meters thick, deposited in a brackish-alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, although it turned into an open one during short periods. Its sedimentary facies and paleogeographic reconstruction pointed out to a lake with a ramp type margin in which periods of low and high energy alternated [1]. We studied the mineralogy of representative samples by X- ray diffraction (XRD) at bulk sample and in the < 2 µm sub-fraction with the aim to examine the changes in clay mineralogy between alluvial and lacustrine sediments. Scanning electron microscopy (SEM) was also carried out. Seven sites were considered in this study, which correspond respectively to proximal, medium and transitional positions of the fluvial environment -Tonco, Valle Encantado and Ayuzos-; marginal lake - Chuñapampa and Corralito -; and inner lake sub environment -Garabatal and Las Cañas. The XRD study of <2 µm sub-fractions revealed marked vertical changes in clay mineralogy in Tonco and Valle Encantado sites, and lateral changes from alluvial to lacustrine environments. In Tonco, smectite predominates at the base of the sequence, with subordinate illite and scarce kaolinite. Towards the top of Tonco site, a striking increase in kaolinite content (4 to 91%) is observed. On the contrary, in Valle Encantado site, kaolinite only prevails in one sample from the middle of the sequence, while in the other samples muscovite predominates with subordinate kaolinite (6-27 %), and scarce smectite. Meanwhile, in Ayuzos site, the clay fraction is dominated by muscovite with subordinate smectite, kaolinite is absent and its disappearance coincides with the first occurrence of analcime. The <2 µm sub-fraction of sediments from litt