CONICET | Buscador de Institutos y Recursos Humanos

Deposits of La Manga Formation (middle Oxfordian) exposed in arroyo Los Blancos (southern Mendoza province) have been studied through a sedimentologic and microstratigraphic work. This section consists of mudstone-wackestones alternating with black shales and exhibits well developed centimeter-scale rhythmicity. Limestones are represented by laminated to massive calcareous mudstones, laminated radiolarian wackestones and laminated bioclastic wackestone to packstones. Radiolarian, bivalve filaments, ammonites, some sponge spicules, calcispheres and rare microcrinoids are the dominant biotic constituents. These sediments are interpreted to have been deposited from a combination of suspension fall-out and dilute turbidite flows in an anoxic to dysoxic deep environment. These deep facies would be the result of hemipelagic deposition. Thin graded beds of wackestones and bedding-plane concentrations of bioclasts interbedded with finergrained facies indicate that turbidity currents episodically interrupted background sedimentation. Evidence of poorly oxygenated bottom waters includes organic-rich shales, lamination, the presence of pyrite framboids and absence or scarcity of trace fossils and benthic fauna, considering that only isolated Chondrites and occasional Bositra have been recognized. The analysis of facies and the comparison with northern sections indicate that deposition at arroyo Los Blancos would be associated with a subsiding fault block during the middle Oxfordian. TOC and different types of organic matter are characterized using organic petrology, Rock-Eval pyrolisis and thermal madurity. The low Pyrolysis S yields coupled with the low hydrogen indices suggest poor kerogen convertibility. Vitrinite reflectance (Ro) ranges from 2.39 to 2.97 with an average of 2.70, while thermal alteration index (TAI) is 4+(5). Spectral analytical results of centimeter-scale rhythmic alterations of fine-grained limestones and black shales indicate the presence of orbital frequency related to eccentricity.dominant biotic constituents. These sediments are interpreted to have been deposited from a combination of suspension fall-out and dilute turbidite flows in an anoxic to dysoxic deep environment. These deep facies would be the result of hemipelagic deposition. Thin graded beds of wackestones and bedding-plane concentrations of bioclasts interbedded with finergrained facies indicate that turbidity currents episodically interrupted background sedimentation. Evidence of poorly oxygenated bottom waters includes organic-rich shales, lamination, the presence of pyrite framboids and absence or scarcity of trace fossils and benthic fauna, considering that only isolated Chondrites and occasional Bositra have been recognized. The analysis of facies and the comparison with northern sections indicate that deposition at arroyo Los Blancos would be associated with a subsiding fault block during the middle Oxfordian. TOC and different types of organic matter are characterized using organic petrology, Rock-Eval pyrolisis and thermal madurity. The low Pyrolysis S yields coupled with the low hydrogen indices suggest poor kerogen convertibility. Vitrinite reflectance (Ro) ranges from 2.39 to 2.97 with an average of 2.70, while thermal alteration index (TAI) is 4+(5). Spectral analytical results of centimeter-scale rhythmic alterations of fine-grained limestones and black shales indicate the presence of orbital frequency related to eccentricity.

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