CONICET | Buscador de Institutos y Recursos Humanos

The Neoproterozoic sedimentary successions of the Tandilia Basin are exposed in Olavarr¨ªa and Barker areas and are composed of the Villa M¨®nica, Cerro Largo, Las Aguilas, Olavarr¨ªa and Loma Negra formations (Sierras Bayas Group) and the Cerro Negro Formation. On top of the Loma Negra Formation a regional unconformity, Barker Surface, was related to an important sea level fall which could be linked to Gaskiers glaciation, however no negative C isotopes excursions were observed. Carbon and oxygen isotopic data, combined with a detailed diagenetic study, obtained from undeformed and unmetamorphosed limestone, provide a new record of isotopic stratigraphic variations useful for regional and global correlations. The age Loma Negra Formation (~580-590Ma) was suggested from ¦Ä13C and 87Sr/86Sr trends and supported by Cloudina. In the Barker area this unit is composed almost exclusively of greenish (lower section) and black (upper) micritic limestones originated by suspension fall-out in open marine ramp and lagoonal environments. The d13C values range from +2.2 to +3.6¡ë and d18O(V-PDB) values vary from ¨C7.4 to ¨C13.5¡ë and all C- and O- isotope are very close with those reported for the Olavarr¨ªa area, and show similar excursions. Furthermore, such data are very consistent with Late Ediacarian carbonate successions from Uruguay and Brazil. The sequence is divided in three sections based on stratigraphic distribution of C- and O isotope values and petrography. Section 1: are 6m of greenish micritic limestones with d18O values from -13¡ë to -10. Section 2: 13 m of black micritic limestones with d18O values between -7.4 and -9.2¡ë. Section 3: the uppermost 4-5m of the black micritic limestones reflects a stronger influence of 18O depleted meteoric fluids with -13.7¡ë. A similar distribution was previously recognized in the Olavarr¨ªa area. For instance, in this work we remark that the d18O stratigraphic excursion shows a regional connotation in the Tandilia System. For Loma Negra Formation four different diagenetic regimens were identified: early marine diagenesis with micritic peloidal cement and authigenic pyrite; (I). Early meteoric diagenesis with dissolution-vein generation (1); recrystalization; sparitic LMC cementation (I) and opal and chert cementation. Early to intermediate burial diagenesis: neomorphism; chemical compaction and stylolitization; dissolution-vein generation (2); LMC cementation (II); transformation of opal to opal CT and chert to microquartz. Finally, the uplift of the succession drive in subaerial exposure and meteoric diagenesis (telodiagenesis): intense carbonate dissolution, precipitation of silica cements (chert and microquartz); replacement of calcite by chert; implantation of the karstic surface on top of the sequence. It is important to remark that neomorphism were not generalized and we observe preservation of the primary crystals and also algal lamination, furthermore the stylolitizacion is related to an early burial diagenesis. Preservation of organic matter was referred in previous work by ¦Ä13Corg and ¦¤¦Ä13C, and Mn/Sr, Fe/Sr, Ca/Sr and Rb/Sr supported a primary isotopic signature. Carbon isotope data for the Loma Negra Formation is considered as near-primary seawater signal and consistent with secular variation curve for Late Neoproterozoic. d18O excursions are associated with early metheoric diagenesis and most pervasive recrystalization for S1, lithification with mayor preservation of the primary texture in S2, and late metheoric diagenesis with decrease in pH and incursion of rich 16O fluids for S3, in this section the typical trend expected for an exposed surface with negative d13C was not observed.