Petrología, geoquímica y diagénesis de sedimentitas y fósiles carbonáticos del Jurásico superior-Cretácico inferior de las cuencas Neuquina y Austral, Argentina

Science and Technology Production

Thesis

Authorship

Gómez Dacal, Alejandro Rubén

Date

06/04/2018

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This PhD work has three objectives. First, the description of sedimentary levels and carbonate fossil levels of upper Jurassic-lower Cretaceous from Neuquén basin (Vaca Muerta and Chachao Formations) and Austral basin (Springhill and Río Mayer Formations) through petrography, geochemistry and diagenesis. Second, the analysis of climatic features of the Tithonian-Valanginian interval through geochemistry data. Third, the correlation between the two basins through methodological too... This PhD work has three objectives. First, the description of sedimentary levels and carbonate fossil levels of upper Jurassic-lower Cretaceous from Neuquén basin (Vaca Muerta and Chachao Formations) and Austral basin (Springhill and Río Mayer Formations) through petrography, geochemistry and diagenesis. Second, the analysis of climatic features of the Tithonian-Valanginian interval through geochemistry data. Third, the correlation between the two basins through methodological tools like sequence stratigraphy, chemostatigraphy and fossils content. This work was done in six methodology steps: cartographic and bibliographic compilation, field activities, laboratory analysis (that is where the most extensive and innovative work has been done), two instances of office activities and intellectual processing of information. From the bibliographic and cartographic data, two areas of study were selected: Mendoza Shelf, near Malargüe city, in the south of the Mendoza Province, and the area between Argentino and San Martín Lakes, between El Calafate City and Tres Lagos City. The four studied formations (Vaca Muerta, Chachao, Springhill and Río Mayer) were assigned temporal intervals. As for Neuquén Basin, on the basis of ammonite biozones and for Austral Basin, taking into account the fossil associations. These temporal assignations allowed to outline the first correlation between both basins. During field activities, six detailed sedimentary logs were done (Río Salado, Puesto Loncoche and Cuesta del Chihuido in Neuquén Basin, and Subida del Chancho, Río Guanaco and Cerro Hobler in Austral Basin). Later, twenty-seven sedimentary facies were defined and after that, these facies were grouped according to vertical and lateral relation in ten facies association (five from Neuquén basin and five from Austral Basin). The facies association of basins, distal outer ramp, proximal outer ramp, distal middle ramp and proximal middle ramp were recognized in Neuquén Basin. A depositional environment of mixed/carbonate ramp was defined through these facies association. On the other hand, in Austral Basin, the facies associations of outer platform, inner platform, shoreface, foreshore and fluvial were defined. Two depositional environments were defined for these facies associations: siliciclastic/mixed platform and fluvial system. Five system tracks limited by stratigraphic boundaries were recognized in both basins as from sequence stratigraphy analysis. From these analyses, a second correlation in concordance with the first was done. The formations were described to know their elemental and mineralogical composition. For this characterization, petrographic analysis, staining of thin sections and X ray diffraction were done according to grain size and composition (siliciclastic, carbonate or mixed). In the Neuquén Basin, carbonates, silts and mixed facies were studied in detail. In the pelitic fraction, quartz and calcite are the most abundant minerals; between them they reach 90% of the sample. The increase in quartz content always coincides with a decrease in calcite and vice versa. In the clay fraction: illite, smectite and interstratified illite/smectite are the most abundant type of clay. In addition, there are smaller amounts of kaolinite and chlorite. The carbonates facies are composed by skeletal elements. Regarding the mineralogy, they are mainly composed of low-magnesium calcite. In respect of the taphonomic preservation, the fragments of the same fossil genera were recognized from totally fragmented to articulated and in life position. The mixed rocks are scarce, and differ from the carbonates by the presence of small percentages of terrigenous elements. In the Austral Basin, siliciclastics, silts and mixed facies were studied in detail. In the pelitic fraction, quartz is the most abundant mineral; in addition, there are smaller amounts of calcite and clays. In the clay fraction: illite, kaolinite and chlorite are the most abundant clays, and smectite and interstratified illite/smectite (I/S) are less abundant. Regarding the sandstones and conglomerates, the lithic composition is the most abundant. Mixed rocks differ from siliciclastic ones due to the presence of not-oysters bivalves, dasycladacean algae and belemnites. Provenance areas of the detrital elements were defined from the analysis of compositional data. In the Vaca Muerta Formation, the detrital input would come from the magmatic arc, Triassic- early Jurassic volcanic lands and of the underlying Tordillo Formation. The Chachao Formation is integrally composed of skeletal elements, which denote its basinal characteristics. The source of the terrigenous material that makes up the Springhill and Río Mayer formations would be the El Quemado volcano-sedimentary complex. The study of the diagenesis was divided in four parts taking into account the compositional and grain size characteristics: pelitic fraction (Neuquén and Austral Basins), carbonate facies (Neuquén Basin), siliciclastic facies (Austral Basin) and fossils (Neuquén and Austral Basins). In the pelitic fraction, the detrital or authigenic origin of the illite and interstratified I/S were determined. In addition, the degree of diagenetic alteration was quantified according to the percentage of expansive layers. In the Vaca Muerta Formation (Neuquén Basin), a part of the illite and all the interstratified I/S were considered authigenic. According to the percentage of expansive layers, the deposits would have reached early mesodiagenesis. In the Springhill and Río Mayer formations (Austral Basin), the interstratified I/S and most of the illite are authigenic. From the study of the percentage of expansive layers, it can be concluded that the Rio Mayer Formation would have reached a stage of late mesodiagenesis. In the Springhill Formation, on the other hand, it was not possible to quantify the degree of diagenetic alteration with the use of clay minerals. In the carbonate levels of the Vaca Muerta and Chachao Formations, diagenetic features corresponding to the cementation, neomorphism, piritization, phosphatization, dissolution and compaction processes were identified. These processes were carried out during the stages of eogenesis and early mesogenesis. Diagenesis affects differentially the deep and shallow facies. In the deep facies, the diagenesis was characterized by the low development of the processes of marine cementation and neomorphism. On the other hand, the shallow facies were characterized by the abundance of dissolution and neomorphism and precipitation of drusiform cement (these characteristics are typical of the action of meteoric diagenetic processes). In the siliciclastics and mixed sandstones levels of the Austral Basin, diagenetic features corresponding to the glauconitization, marine carbonate cementation, quartz cementation, clay authigenesis, burial carbonate cementation, dissolution and compaction processes were identified. These processes were carried out during the stages of eogenesis, early mesogenesis and late mesogenesis. For the Springhill Formation, three diagenetic trains corresponding to the fluvial, mixed marine and siliciclastic marine facies were established, on the basis of the active diagenetic processes. The study of fossil diagenesis was especially focused on selecting the best preserved sectors for geochemical and chemostratigraphic studies. It was carried out on oyster (Neuquén Basin) and belemnites (Austral Basin) specimens selected as from their taphonomic attributes, size, equidistance, abundance and distribution. From studies of cathodoluminescence and scanning electron microscopy, it can be concluded that the diagenesis affects differently the diverse micro textures that compose the shell of oysters and belemnites. The oysters of the genus Aetostreon sp. reflect a very good preservation in their layers with foliated growth, while those of "chalky" growth are altered. In the case of Belemnites of the genus Belemnopsis sp. show a very good preservation of the rostra, but they have high luminescence in sectors such as the apical line and the alveolar groove. From the geochemical study of the fossiliferous fragments, the potential of isotopic preservation was defined in both species, and the sea in which these organisms lived was characterized water (temperature, salinity, depth and oxygenation). Major and minor elements were studied by X-ray fluorescence, ICP-AES and ICP-MS. Most of the analyzed samples are within the stipulated ranges for the taxon and the temporal interval values, although there is a smaller amount that presents diagenetic enrichment or impoverishment. Nonetheless, there are no correlations between elements or elemental relationships and isotopic data that can be linked to diagenesis. In this way, it is concluded that these alterations did not condition the isotopic values. From the isotopic results, paleotemperatures and paleosalinities were calculated, characterizing the seas of the two basins as warm (with temperatures between 24 and 25 ºC) and euhaline (with salinity between 31 and 35 g/l). Through the study of rare earth elements and yttrium, important observations were made on diagenetic alterations, levels of oxygenation and depth of water. From the calculation of enrichments in the MREY, values were characterized as primary (without diagenetic alteration). The waters where the fossil species had their life cycle were defined as oxic given that they present negative anomalies of Ce recorded in all samples of the two basins. Through the study of REY spiders and elemental anomalies, it was corroborated that the organisms of the Austral Basin lived and precipitated their shells in deeper environments than those of the Neuquén Basin. Chemostatigraphic curves were made with the data of ?13C, ?18O y ?13CORG. The analysis of chemostratigraphic curves provides important information about isotopic anomalies and facilitated the correlation between basins. From the analysis of the chemoestratigraphic curves of ?13C, the positive isotopic excursion known as "Weissert anomaly" was recognized in the outcrops of both basins. In the Neuquén Basin it was observed with maximum values of ~2-3‰VPDB, in the levels of the Chachao Formation. In the Austral Basin, on the other hand, the anomaly was identified towards the top of the Lower Rio Mayer Formation and it was recognized with a maximum value of only -0.56‰VPDB. In this case, it was defined as a "ghost anomaly" due to the negative value (given the precipitation of the shells in deep water) and because the event was recognized thanks to the very good correlation between ?13C curves. The Weissert event was restricted in both basins from the end of the early Valanginian to the beginning of the late Valanginian through its fossil content. This correlation made it possible to compare the sections studied with other contemporary basins around the world. From the analysis of the chemoestratigraphic curves of ?13CORG, it was possible to conclude that in the sections Río Salado and Puesto Loncoche, there is a good correlation with the results obtained with the fossiliferous material, especially towards the section of the curve corresponding to the Tithonian. On the other hand, on the central portion of the Vaca Muerta Formation, in the last levels corresponding to the Tithonian, we defined an anomaly (with less intensity than the Weissert) that can be observed both in the chemostatigraphic curves of ?13CORG, as in those of ?13C. This event was called "Tithonian anomaly". Conclusions about the paleoclimate were reached through the integrated analysis of the compositional, geochemical and chemostratigraphic data. During the TithonianValanginian interval there would have been a greenhouse that would have raised the global temperature of the waters. This event would have extended the equatorial climate beyond the tropics, restricting cold conditions to a few polar regions and balancing the temperature of the waters in these two latitudinally distant basins. The climate during the interval studied would have been warm and arid, with short periods of dry conditions called "episodes of environmental change". The most important variations of the chemoestratigraphic curves of ?13C were reflected in these episodes. In contrast, the compositions of the deposits do not reflect these changes and it would have been controlled by the relative changes in the level of the sea. The third correlation tool among basins was provided by the chemostratigraphic analysis since there is a marked parallel between the ?13C chemostratigraphic curves of the Puesto Loncoche (Neuquén Basin) and Río Guanaco (Austral Basin) sedimentary profiles. In conclusion, it is possible to assert that there existed in both basins different depositional and diagenetic histories that shared the same climatic context. The analysis of the chemostatigraphic curves, the fossil content and the stratigraphic surfaces constitute an efficient tool that allowed establishing the comparative temporal evolution of the TithonianValanginian interval in the Neuquén and Austral basins.