Estratigrafía del Grupo Mina Verdún, Proterozoico de Minas, Uruguay

Abstract

The aim of this contribution is to introduce the stratigraphic framework of the Mina Verdún Quarry, located in the eastern side of Verdún Hill, Minas, Uruguay (Fig. 1), and to characterise their sedimentary units. The paleoenvironments involved are outlined as well as the sedimentological significance of the very well developed stromatolites. The stratigraphic position and composition of this sedimentary succession used to be uncertain and the limestone from the Mina Verdún Quarry was alternatively considered part of diverse litostratigraphic units (Mac Millan, 1933; Caorsi and Goñi, 1958; Bossi et al., 1965; Midot, 1984; Preciozzi et al., 1985; Bossi and Navarro, 2000; Sánchez Betucci, 1998; Sánchez Betucci et al., 2001). From the base to the top (Fig. 2), the sedimentary succession consists of the Mina Verdún Group, Las Palmas Formation and the quartzites of the Verdún Hill (Fig. 3). Mina Verdún Group (Poiré et al., 2003b) is conformed by: Don Mario Formation (Poiré et al., 2003b): 40 m thick, base not exposed; massive and laminated, black shales (Fig 4a), which are rich in organic matter (Fig. 5a). XRD analysis shows manly quartz, with scarce calcite, clay (illite, chlorite, and IS and CS interlayer) and feldspars (Fig. 6). Applying the Bhatia and Crook (1986), Taylor and McLennan (1985) and McLennan et al. (1990, 1993) plots (Fig. 7) their geochemistry suggests a high degree of weathering and reworking in the source area compose of a differentiated upper continental crust. La Toma Formation (Poiré et al., 2003b): 15 m thick; fine-grained, dark green marls (Fig. 4b and c), bearing up to 45 % of silica and 35 % of CaCO3.