ICHNOLOGY AND SEDIMENTOLOGY OF THE JURASSIC-CRETACEOUS VACA MUERTA FORMATION BLACK SHALES OF ARGENTINA: DELINEATING ENVIRONMENTAL FACTORS CONTROLLING BENTHIC COLONIZATION

The Jurassic-Cretaceous Vaca Muerta Formation is a transgressive-regressive shallow-marine succession mainly composed by mudstone, marl and limestone with high organic matter content, commonly referred to as black shales. Sedimentological and ichnological analysis was performed in the Picún Leufú area, in Neuquén Province, Argentina. At the base, the transgressive cycle shows transgressive shoreface, bay margin and distal bay deposits. The transgressive shoreface is represented by unbioturbated, cross-bedded sandstone erosionally overlying the Tordillo Formation eolian dune deposits. The bay margin deposits are composed by wave-rippled sandstone and siltstone capped by microbial mats and containing Palaeophycus, Thalassinoides and Teichichnus. The distal bay deposits are characterized by mudstone, marl and wackestone showing alternation of intervals with burrow mottling overprinted by a firmground Thalassinoides suite, and intervals with softground Planolites and Teichichnus.

Above the transgressive interval, depositional lobe and mixed siliciclastic-carbonate ramp deposits occur, representing the regressive cycle. The depositional lobe facies is composed by lobe and axial lobe channel deposits. This facies is interbedded with the mixed ramp deposits, and it is interpreted as produced by concentrated density flows and turbidity flows. The mixed ramp deposits are subdivided into inner, mid and outer ramp. Depositional lobe and mid and outer ramp facies are represented by unbioturbated mudstone and sandstone with abundant posidoniid bivalves and ammonites. Inner ramp deposits contain alternating bioturbated interbar facies with Palaeophycus and Thalassinoides, and unburrowed bar facies.

The transgressive deposits show the establishment of a suspension feeder infauna and alternation of softground and firmground suites, indicating high energy conditions and continuous erosion and exhumation of consolidated substrate. In the regressive cycle, bioturbation is mostly precluded due to oxygen-depleted conditions. Posidoniid bivalves reflect short-term benthic colonization as a result of fluctuating oxygen levels. The bioturbated inner ramp deposits suggest the onset of more oxygenated waters, with bar migration being a major control on benthic colonization.