Petrología y Diagénesis de las Unidades Sedimentarias Precámbricas de Olavarría, Provincia de Buenos Aires

LUCÍA E. GÓMEZ PERAL

SEDICI UNIVERSIDAD NACIONAL DE LA PLATA

Lugar: La Plata (Nro de Tesis 0978 Museo de Ciencias Naturales); Año: 2011 p. 598

The study area comprises the outcrops of the nor-western part of the System of Tandilia, fundamentally the three hills near to the Olavarría locality. In this part of basin, the Neorpoterozoic successions of the Sierras Bayas Group are composed for the Villa Mónica, Cerro Largo, Olavarría y Loma Negra formations. On top of these successions the Cerro Negro Formation is discordantly overlaying the Sierras Bayas Group. One of the most important characteristics is that the complete succession is unmetamorphosed and almost undeformed.  During a first period of working they were surveyed the compilation and reading of the references referred to the Precambrian-Paleozoic sedimentary basin of the Tandilia System. In the same way, it was made a collection of the cartography, satellite images, and previously generated sedimentary profiles with the finality of make a digitalized and georreferenciated geological map from the study area indicating the best locations for fieldwork and sampling. By means of the treatment of the satellite images and using different software, combination of bands, and also georreferenciating the images, topographic maps and aerial photographs, it was possible to interpret the information integrated in a Geographic Information System from the Sierras Bayas-Olavarría area. Taking into account the previous works and based on the sedimentological attributes recognized in the outcrops there were identified, defined and redefined the sedimentary facies from the complete lithological successions (Sierras Bayas Group and Cerro Negro Formation), establishing a total of twenty seven sedimentary facies both carbonate and siliciclastics. Besides the macroscopic characterization, there was carried out a detailed petrographic study of the carbonate and siliciclastic facies. The recognizing of the different types of dolomite in the Villa Mónica Formation, and also by the identification of different stages of cementation made it possible the interpretation of the diagenetic history of this rocks, here associated with dolomitization process that begin with an earlier dolomitization of a micritic mudstone follow by burial dolomitization during mosodiagenesis. In addition, there were recognized post-dolomitization stages of cementation related to the incursion of hydrothermal fluids with begin with dissolution as well as generation of secondary porosity followed of later cementation. The proofs related to the uplift in a telogenetic regime associated with the ingress of meteoric water include the development of the karstic surface plus the generation of secondary porosity. In the Loma Negra Formation, the composition of the micritic to microsparitic base, the analysis of cements plus the interpretation of the other diagenetic process allowed to establish a diagenetic evolution model which begin with a marine diagenesis followed of early burial diagenesis and concludes with an uplift related to a telogenesis represented in the top of this carbonate sequence. The stylolites identified in both carbonate formations from the Sierras Bayas Group are referred to a lithostatic origin. In the dolostone and shale facies association of the Villa Mónica Formation the stylolites are related to a deep mesogenetic regime. On the other had, the pressure dissolution surfaces from the Loma Negra Formation were associated with eogenesis to early burial diagenesis. In the Cerro Negro Formation there were recognized the presence of dissolution seams correlated with an eogenetic regime. Petrographic analysis of the siliciclastic facies allowed to a detailed description of thirty microfacies with particular petrographic characteristics. From these microfacies twenty three belong to quartz arkosic sandstones facies association of the Villa Mónica Formation, seven are from Cerro Largo Formation and three from Olavarría Formation. The diagenetic processes recognized in the quartz arkosic sandstones facies association involve transformations occurred during eogenetic, mesogenetic (early and deep) and telogenetic regimes. The last stage is associated with an uplift of the succession. It is important that all the proofs lead to consider that Villa Mónica Formation were during the deep burial at depth over 3km. Interestingly, the diagenetic processes recognized in the Cerro Largo Formation are very different respect to the Villa Mónica Formation. In this case the transformations are related with eogenetic, early mesogenetic (with a lower depth of burial, maybe near 2 or 3km) and later telogenetic regimes. In the Olavarría Formation, the effects of diagenesis identified are in association with an early mesogenesis. In addition, most of the changes are related to eogenetic environment. Besides, other processes recognized were linked with meteoric diagenesis in a telogenetic event. Respect to all the telogenetic processes identified in the complete Precambrian-Paleozoic succession it was seen a relationship with an intense weathering and they are represented in a very different way regarding the area of analysis. Therefore, it was seen that in the Central and South hills this kind of diagenesis was much more intense and better represented compared with the same units in the North hills. Consequently, this could be referred to different tectonics mechanisms of uplift related to each part of the Sierras Bayas hills, with a concomitant different temporal interaction with the meteoric fluids. The incursion in the knowledge about geochemistry of carbonates and more specify in the analysis of C and O isotopes allowed to infer ages and diagenetic trends in the carbonate successions. In the same way, from the analysis of this data it was possible to correlate these units with other Neoproterozoic comparable successions in Uruguay and Brazil in the southwest margin of Gondwana. Finally, it is reported the presence of three phosphatic levels in the Olavarría-Sierras Bayas area. The two oldest levels, were identified in the Villa Mónica Formation, and the third (previously reported) is in the base of the Cerro Negro Formation. The first phosphate level is located in the top of quartz arkosic sandstones facies association. It is consider that was generated in situ by direct precipitation of the francolite related to the input of rich nutrient fluids brought by upwelling currents from the deep ocean. Later, the fosfogenesis would be able to originate, by recristalization of francolita, nodules and lenses of fluorapatite during the burial diagenesis.  Afterward, and by reworking of these phosphatic nodules and lenses as well as wakes and pelites associated, it was development a clastic phosphatic level from the base of the dolostones of the Villa Mónica Formation. Respect to the Cerro Negro Formation phosphatic level, there were identified phosphatic nodules related to the karstic surface. In this case of study, it was possible to confirm the association of this kind of phosphatic levels with sea level relatives changes like in other examples reported in different parts of world.