Facies analysis of fine-grained deposits related to muddy underflows. Vaca Muerta Formation (Tithonian-Valanginian), central Neuquen Basin, Argentina

The conventional sedimentological model suggests that the accumulation of organic-rich fine-grained sedimentary rocks (<62.5 mu m) is uniquely associated with fallout processes in low energy depositional environments. This contribution analyzes organic-rich mudstones belonging to the lower section of the Vaca Muerta Formation at central areas of the Neuquen Basin (Arroyo Mulichinco, Tres Chorros and Rio Neuquen localities). The studied interval is characterized by the highest organic matter content of the Vaca Muerta Formation (up to 8% TOC). The associated mudstone deposits are usually highly compacted, thus obscuring the recognition of the original fabric and the analysis of mudstone depositional processes. Nevertheless, the common occurrence of carbonate concretions within these highly compacted intervals provides an exceptional preservation of mudstone primary fabric. Atter macroscopic study of polished slabs and thin sections of these cemented beds, a series of facies genetically linked to muddy underflows were recognized. The origin of these deposits is related to long-lived muddy hyperpycnal flows (quasi-steady mud flows) triggered by extreme river discharges during rainfall humid periods. During their travel basinward, hyperpycnal flows, originally composed of detrital mud, would be able to go through very low gradient reliefs, incorporating the available intrabasinal components (including organic matter) to their extrabasinal sedimentary load. The rapid basinward transfer of organic-rich mud would have provided a fast deposition and efficient burial of organic matter, avoiding its potential degradation at seafloor. Muddy underflows constitute a rational mechanism to explain the common occurrence of bituminous mudstones at central areas of the Neuquen Basin.