Clay mineral assemblages and analcime formation in a Palaeogene fluvial- lacustrine sequence (Maíz Gordo Formation) from northwestern Argentina

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

SEDIMENTARY GEOLOGY

Elsevier

Año: 2007 vol. 201 p. 56 - 74

0037-0738

The Palaeogene Maíz Gordo Formation constitutes one of the main lacustrine events recorded in northwestern Argentina. It consists of sandstone, mudstone, and limestone beds 200 metres thick, deposited in a brackish-alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, with brief periods as an open one. We studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) seven sites, corresponding respectively to proximal, intermediate and transitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy and analcime formation. The basinward zonation of diagenetic minerals identified in the Maíz Gordo Lake was: mordenite® analcime® K-feldspar. Although this mineralogical zonation is not typical of saline-alkaline lakes, it indicates an increase in salinity and alkalinity towards the centre. In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards a warm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes in hydro geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pore. SEM images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets. In littoral and inner-lake settings the clay fraction is composed of muscovite, sometimes with subordinate smectite; analcime occurs in variable amounts in all sedimentary facies. Analcime appears in rock pores or filling veins, forming subhedral square to hexagonal, or anhedral rounded crystals, denoting that they coarsened at low to moderate degrees of supersaturation. Although the mordenite identified in a fluvial level would have been the precursor of analcime in the Maíz Gordo basin, no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found.. Hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase.