Petrology and geochemistry of a limestone-marl rhythmite from the Southern Hemisphere: the Los Catutos Member (Vaca Muerta Formation), Middle Tithonian of Neuquén Basin

Abstract

A 25 m thick, rhythmic, limestone-marl alternation of Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), was studied bed by bed in several quarries near Zapala. Beds up to 1 m thick are of massive or laminated, lithographic, fossiliferous limestone, locally mottled or weakly bioturbated (?Chondrites), interbedded with thinner marl beds. Limestones and marls can be classified as pelbiomicrites and biopelmicrites, mainly composed of pellets, radiolaria, forams, ostracods, equinoids, spicles of sponges and gastropods, with minor amounts of peloids, feldspar, chert, muscovite, glauconite and phosphates. Values for δ¹⁸O and δ¹³C are similar to those of lithographic limestones from the northern hemisphere. δ¹³O values are most probably result of diagenetic modification of the primary signal but the δ¹³C agrees well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ¹³C point to productivity crisis in the sedimentary system with a periodicity of 400 ka, which can be a result of changes in the orbital eccentricity of the earth. Smectite and interlayers reflect pedogenic processes developed on volcanogenic source-rocks, whereas more abundant kaolinite in marls might reflect more humid conditions. With TOC up to 1.95% the rocks offer prospects as a regular to good source rock for hydrocarbons, although thermally immature. A low-energy, dysaerobic, open-marine environment is inferred for Los Catutos Member, with high sedimentation rates as a result of high productivity at the sea-surface and incoming plumes of suspended sediment from the inner shelf. Sediment accumulation in wide and gently dipping calcareous bulges in an outer ramp system is envisaged.