Abstract

TOFALO, Ofelia R.; PAZOS, Pablo J.  and  SANCHEZ BETTUCCI, Leda. Compositional study of siliciclastics sandstones and paleosoils from the Mercedes Formation (Upper Cretaceous), Uruguay. Rev. Asoc. Geol. Argent. [online]. 2011, vol.68, n.4, pp.615-626. ISSN 0004-4822.

The composition and textures of epiclastic sedimentary rocks from the Mercedes Formation and the micromorphology of its paleosoils are described in order to study the provenance areas, the tectonic framework, the paleoclimatic fluctuations and the diagenetic processes affecting this unit during the Upper Cretaceous. The Mercedes Formation is located to the west and south of Uruguay, in the Parana Basin. The combined provenance analyses on conglomerates and sandstones show two different source areas with a transitional boundary between them. Towards the north, the volcanic clasts from the underlaying Arapey Formation predominate while there is a minor input from the sandstones of the Guichón Formation and from metamorphic and plutonic sources (Precambriam basement). Besides, in the central-south area the Precambrian clasts are dominant, and fragments from the Arapey Formation are scarce, while here, the Guichón Formation is not a source unit. The sandstone petrography shows interior to continental transitional cratonic provenance. Stable grains are more abundant than metastables and instables suggesting that the sediments where deposited under warm and humid climates. The increase of metaestables altered clasts in the central-south, suggests a near source area. The diagenetic processes were not uniform resulting in a varible porosity distribution, with a predominance of secondary porosity due to dissolution. The recognized processes are clay and iron oxides illuviation, clay neoformation and recristalization, carbonate and silica precipitation and disolution. The uppermost section of this unit has paleosoils, related to the complete infilling of the basin and tectonic quiescence, that suggest a change in the climate, the presence of palygorskyte and illuvial clays imply a semiarid seasonal climate.

Keywords : Upper Cretaceous; Continental sedimentary rock; Paleosoils; Tectonic framework; Paraná Basin.

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