Revealing weathering, diagenetic and provenance evolution using petrography and geochemistry: A case of study from the Cretaceous to Cenozoic sedimentary record of the SE Chaco-Paraná basin in Uruguay. (January 2021) - Blanco, G.
Abstract: The Cretaceous rift to post-rift sedimentary sequences of the Paraná Basin and the Cenozoic sedimentary cover reflect the paleoclimate and tectonic influence on weathering and diagenesis, allowing to constrain provenance components. Detailed petrography enhanced by SEM and XRD, along with geochemistry of major, trace and rare earth elements provide a unique multi-technique approach. Weathering and diagenesis of the Arapey, Guichón, Fray Bentos and Salto formations are depicted. Provenance components of each of these units are deciphered. Implications regarding the poral system are evaluated in the context of the Salto Aquifer. The feldsphatic litharenites interlayered within the flood basalts of the Arapey Formation of Early Cretaceous age show a provenance from the NE mainly derived from the Precambrian felsic basement and basaltic rocks. Moderate Th/Sc ratio (1.3), low CIA (56) and low Th/U ratio (1.59) and strongly negative Eu/Eu* anomaly (0.65), among other geochemical proxies, indicate a wide compositional variation and low weathering of the source rocks under semiarid climatic conditions. Effective porosity is low because of an opaline and zeolitic cementation during early diagenesis. Up section, the lithic feldsarenites of the Guichón Formation (Late Cretaceous) are characterized by the dominance of volcanic and sedimentary lithoclasts. Low Th/Sc ratio (0.55) and CIA (56) values, moderate Th/U ratio (2.99) and Eu/Eu* negative anomaly (0.82) indicate the influence of local mafic source rock and low weathering under semiarid to wet climatic conditions. No effective porosity is preserved because of the smectitic matrix (11%) and of the calcitic cement. After a long period of erosion and no-deposition, the diamictites and wackes of the Fray Bentos Formation were deposited during the Oligocene. Angular fragments of basalts and arenites as lithoclasts, along with low Th/Sc ratio (0.52) and CIA values (58), moderate Th/U ratio (2.45) and Eu/Eu* negative anomaly (0.76) point to the influence of local sources probably related to tectonic uplift during deposition and scarce weathering. Well-developed calcretes indicate semiarid climatic conditions during burial, and porosity is only preserved after calcite dissolution. A braided fluvial environment was the depositional locus for the Pliocene Salto Formation. The arenites are dominated by rounded quartz, but the original clastic composition was modified due to strong dissolution processes that affected mainly the feldspar population. Low Th/Sc ratio (0.58), low to moderate CIA value (70), very low Th/U ratio (1.45) and moderately negative Eu/Eu* anomaly (0.82) confirm the influence of a mafic source. A wide range of variation of the Ce/Ce* anomaly (0.76–5.86), along with a high concentration of U, Cu, V, As are untypical geochemical characteristics for recycled quartz arenites indicating remobilization during early diagenesis rather than intense reworking. The porosity of the opal-free clastic rocks is extremely high, constituting an important local groundwater reservoir. Highlights: CIA and trace elements indicate low alteration and arid climatic condition. Post-sedimentary process affect the composition of the sedimentary rocks. Calcretes and silcretes are evaluated along the Meso-Cenozoic units. Basalts and Precambrian rocks are the main provenance source areas.