Composition and diagenesis of limestones from Salamanca Formation, Lower Tertiary, southeast of Patagonia, Argentina

MATHEOS, S. D., BREA, M., LOURENCO, F. BEILINSON, E.

Belem, Brasil

Congreso; 3rd Latinamerican Congress of Sedimentology; 2003

Universidade Federal de Pará

Geochemical and cathodoluminiscence studies have benn carried out in the limestones of Salamanca Formation, Coger Tertiary, cropping out in the southeast of Chubut Province, in order to determine their compositional aspects and to define their diagenetic stage. This sequence is about 15 m thick and lies discordantly above the Jurassic vulcanite of Chon Aike Formation (Andreis et al, 1975). The dominant texture is bioclastic packstone (wackstone) with porfids (5-6 cm in diameter) in the upper party, containing fossils (pelecypods, chinoderm spines, bryozoans, ostrácodos and foraminifera). In general, the fossils are cemented by radiaxial calcite (LMC); as an exception, the pelecypods are totally to partially silicified. To the top of the sequence, the size of the fossils increases and the faunistic diversification decreses. The stratofabric is of tabular type, with wedging beds in the upper part. Geochemical analysis have reveald a regular content of original calcite sediments (i.e. 277 ppm Sr, 700 ppm Mn and very low values of non-carbonate elements); this reflects a clear predominance of the carbonate fraction over the detritic one. At the same time, the Mg content (< 2%) and the staining of the samples show a LMC original composition. The X-ray diffraction analysis reflected a notorious abscenbce of clay minerals and a predominant clacitic composition with lithic traces. Two cement types were determined by cathodoluminiscence  analysis: a) prismatic rim cement, characteristic of marine environment, with euhedral-subhedral scalenoedric to prismatic calcite (LMC); these zones possess CL modelately bright to dull with < 450 ppm Mn+2 and between 1200 to 1550 ppm Fe+2; b) the second generation of blocky-type cement partially obliterating the residual porosity and composed of LMC, typical of a mixed water environment, has a higher content of activator elements, even inside the skeletal grains showing moderate and uniform bright orange luminescence, which revels values of 600 to 800 ppm Mn+2. In the lower part of the sequence, there has been identified a selective and differential dolomitization, represented by small nucleated rhombohedrons completely ferruginous (> 3000 ppm Fe+2), which give a nearly dull cathodoluminiscence. In resume, the resulting diagenetic trend indicates the precipitation of rim cement in early diagenetic stages followed by a later diagénesis represented by the formation of blocky cement and the later silicification of the valves. The selective dolomitization, restricted to a small area, and which only occurred in the lower part of the calcareous package, may evidence a mixture of pore and meteoric water.