Abstract : In sedimentary basins, source rocks may generate aqueous and hydrocarbon fluids during burial and
diagenesis. This fluid production in very-low permeability sediments is responsible for the development of
fluid overpressure, and in some cases leads to hydrofracturing processes. Such processes are evidenced by the
occurrence of mineralized fibrous veins distributed parallel to black shale layering in many mature basins,
known as “beef” or “bedded parallel fibrous calcite veins”. A link between the beef formation and the total
organic carbon (TOC) has been suggested since overpressure and hydrofracturing is most likely to be triggered
by a phase transition (solid-liquid) during chemical compaction and thermal maturation of kerogene.
In parallel, the organic matter preservation and TOC evolution in marine sediments are highly influenced
by eustatic and climatic fluctuations. At short to medium timescales (< 1.106 yr), these fluctuations are cyclic
and influenced by Milankovitch cycles, thus illustrating an orbital control on the stratigraphical evolution of
the organic matter content. Considering a presumed relationship between mechanisms of beef generation and
the TOC as well as a control of orbital parameters on the TOC evolution, we address the following question:
is there an orbital control on the beef distribution in source rock?
Hereby, we present the preliminary results of a field mission aiming to answer this question and
performed in the Neuquén Basin (Argentina) during March 2017. We conducted fieldwork in the Loconpué
area where the Late Jurassic Vaca Muerta Formation offers exceptional exposures composed of organic matterrich
black shales interbedded with numerous calcite beef of reliable lateral continuity (100’s meters) and
thickness (0.2 to 7 cm). We performed a detailed 100 m thick outcrop logging focusing on the beef
stratigraphical position, thickness and continuity. In parallel, we carried out a 10 cm interval shale sampling
in order to investigate magnetic susceptibility trends in the stratigraphical record. Knowing that the magnetic
susceptibility of sediments is a good indicator of the detrital fraction, we aim to decipher a possible climatic
and orbital signal in this Late Jurassic sedimentary record. Using these datasets, we later performed spectral
analyses on the beef stratigraphical distribution, the beef thickness and the magnetic susceptibility signal to
evidence a possible control of orbital parameters on beef distribution.