Limestone-dolostone rhythmites in the Caucete Group, sierra de Pie de Palo, Argentina

NAIPAUER MAXIMILIANO; GAUCHER CLAUDIO; CINGOLANI CARLOS ALBERTO

Mendoza, Argentina

Congreso; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

Academia Nacional de Ciencias

The Sierra de Pie de Palo belongs to the Western Sierras Pampeanas, and comprises a Grenvillian basement (Vujovich et al., 2004 and references therein) with a metasedimentary cover succession known as Difunta Correa Sedimentary Sequence (DCSS), thrusted onto the Caucete Group cropping out in the western flank of the range. Both units include thick carbonate deposits, which are characterized by different 87Sr/86Sr and d13C signatures. Whereas the DCSS yielded 87Sr/86Sr around 0.7073-0.7074 and d13C between +11.4 and –2.3 ‰ PDB (Galindo et al., 2004), the Caucete Group is characterized by higher 87Sr/86Sr ratios (0.7089-0.7091) and d13C around zero (-1.4 to +1.3 ‰ PDB) (Sial et al., 2001; Naipauer et al., in press). These data suggest a Neoproterozoic age for the DCSS and Cambrian age for the Caucete Group, in it were mentioned ichnofossils (Bordonaro et al., 1992). We carried out a petrographic survey in the southwestern of the Sierra de Pie de Palo (Quebrada La Lichona and San Ceferino area), where thick carbonate deposits occur, assigned to the Caucete Group. They are characterized by an alternation of millimetric dolostone and limestone laminae. Limestone laminae are composed by 85% calcite, 5-10% dolomite and 5-10% quartz grains. Dolostone laminae are made up of 83-96% dolomite and 4-17% calcite, and usually contain more abundant organic matter. Grain size of dolostones is finer than in the limestone laminae, although recrystallization of carbonates masks differences in grain size.Similar limestone-dolostone rhythmites were reported from Neoproterozoic successions of Uruguay, Brazil, Namibia and India (Gaucher, 2000 and references therein). In the Uruguayan occurrences (Arroyo del Soldado Group), as well as in the Pie de Palo rhythmites, dolostone laminae represents background sedimentation, and limestone laminae represent higher energy events. Higher terrigenous contents, coarser grain size and lower organic matter content in the limestone layers favour the previous interpretation.  In contrast, dolostone laminae contain almost no terrigenous grains and more organic matter. In proximal sections of the Arroyo del Soldado Group, limestone layers are up to 50 cm thick and bear all the characteristics of calcareous tempestites, including normal grading and hummocky cross-stratification. On the contrary, in distal sections of the same basin, limestone layers are 0.5-5 mm thick and do not develop cross bedding (Gaucher et al., 2004). This resembles limestone-dolostone rhythmites of the Sierra de Pie de Palo, supporting a deep-water setting for these carbonates. We envisage that genesis of dolomite is penecontemporaneous or primary, and related to bacterial sulphate-reduction (Burns et al., 2000), because of: (1) abundance of pyrite in the carbonates; (2) high content of organic matter, preferably in the dolostone layers; (3) occurrence of dolomitic intraclasts in the limestone laminae; and (4) very fine, rhythmic alternation of dolomitic and calcitic laminae. So far, the only known occurrences of limestone-dolostone rhythmites are Neoproterozoic in age. Thus, the occurrences in the Caucete Group would be the first Phanerozoic examples. This suggests that the special palaeoceanographic conditions leading to deposition of bacterially-mediated dolomite and limestone-dolostone rhythmites continued into the Cambrian.