CONICET | Buscador de Institutos y Recursos Humanos

The Claromecó Basin (Carboniferous-Permian) is located in the south of Buenos Airesprovince, Argentina. The sedimentary infill of the basin has been recorded by the PANG0001and PANG0003 exploratory wells, among others, reaching the youngest lithological unit of thePillahuincó Group: Tunas Formation (Permian). The study of their synorogenic deposits is ofsignificant importance, as they represent a fundamental key to understand the evolution of theSW sector of the Gondwana margin (Tomezzoli et al. 2023). Furthermore, a speculativepetroleum system was defined, named Tunas-Tunas(?), with significant economic importancedue to the presence of coal beds and associated methane gas (Coal Bed Methane, CBM;Arzadún et al. 2016).Veins are mineral aggregates that precipitated from a fluid in dilational sites. Their analysisallows us to understand the interaction between mechanical and chemical processesassociated with the deformation history of rocks. The aim of this work is to survey and classifythe veins present in 729 meters of subsurface samples (cores) from the Tunas Formationbased on cement petrographic description, including microtextural characteristics, usingpolarization microscopy and cathodoluminescence (CL).Veins related to non-tectonic and tectonic fractures were recognized, depending on whetheror not tectonic stresses and/or geological structures such as faults and folds were involved intheir formation.Veins in non-tectonic fractures correspond to drainage fractures and bedding parallel veins(beef and cone-in-cone). Drainage fractures are present in laminated mudrocks, with highanglerespect bedding, meandering shape and apertures between 20 μm and 100 μm. In thisstudy case, the cement is composed mainly of subhedral pyrite and clay material, with nullluminescence under CL. Their origin is related to natural fluid pressure during early stages ofrock burial (Cunfei et al. 2016). The beef and cone-in-cone structures are lithologicallyrestricted to carbonaceous mudrocks. They are subparallel to the bedding with sinuous shapeand have a maximum aperture of 5000 μm. Cement is composed of calcite and quartz, bothwith fibrous habit and antitaxial growth morphology. The results of the CL tests indicate thatcalcite exhibited dull and orange moderate luminescence, whereas quartz is non-luminescent.Although beef origin is subject of several hypotheses, its presence can be explained byseepage forces result of fluid overpressure (Cobbold et al. 2013).Veins in tectonic fractures are present throughout the entire lithological record, includingextension and bedding parallel veins, are represented by shear and sigmoidal veins. Extensionveins display low and high angles respect to bedding, straight boundaries to the host rock andplanar geometry. The apertures of the vein ranges from 20 μm to 8000 μm and the cement ismainly composed of quartz, calcite and pyrite. Quartz has euhedral and subhedralmorphologies, showing locally crystal-size reduction related to dynamic recrystallization.Lattice-bladed calcite has euhedral crystals and deformation twins. Growth morphology isblocky syntaxial. The CL tests showed for calcite null/orange with moderate luminescence,whereas quartz is non-luminescent. The origin and angular relationship of tectonic fracturesrespect bedding (Mode I: extension fractures and Mode II: shear fractures) related to describedveins can be linked to folding (Nelson 2001).Shear veins and sigmoidal veins are parallel/subparallel to bedding, have a maximum apertureof up to 1 cm and cemented by quartz and calcite. Quartz has subhedral and anhedralmorphology, and are affected by dynamic recrystallization while euhedral calcite are in a latticebladed texture, with deformation twins. Under CL, quartz showed no response while calciteregistered dull/orange with low/moderate luminescence. These veins are common in shearzones, associated with the presence of slickenfibres or friction surfaces, registering flexuralslipmechanism during folding (Nelson 2001).Vein recognition and classification proposed in this work has led to the identification of twodistinct fracture scenarios in the Tunas Formation. The first scenario is associated with earlystages of rock burial, where fluid mobility and overpressure, reflecting organic matter thermalmaturation, would result in veins in non-tectonic fractures. The second scenario is associatedwith later folding, at an advanced diagenetic stage of the rock, which would result in theformation of veins in tectonic fractures. Both types of veins share characteristics in theircements, including mineralogy, infilling textures and CL response for calcite, suggesting acommon origin although dynamic recrystallization textures are only observed for veins intectonic fractures. Fluid inclusions microthermometry and fluorescence will be carried out inorder to advance in cement characteristics, to understand geochemical conditions in bothtypes of veins and link fluid mobility system during diagenesis and later tectonism.

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