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Paleogeographic Reconstruction and Characteristic Trends of a Basin Floor Fan in Los Molles Fm, Neuquen Basin, Argentina


The Jurassic deep-water marine deposits of Los Molles Fm in Neuquén Basin, Argentina (La Jardinera area) have received much attention through the years; however, a detailed characterization of the basin floor fans that outcrop in the area was missing. We made use of a high-resolution satellite image, drone imagery and 3500 m of logs with detailed measurements to build isopach and net/gross (NG) maps that with facies analysis helped reconstruct the evolution of the system. In addition, grain size, facies and bed thickness trends were useful to refine the interpretation at a lobe scale in unit LC3. Lithofacies, NG ratios and sandstone bodies’ geometries help defined six facies association; hemipelagic deposits, fringe lobes, off axis lobes, on axis lobes, distributary channels and debris flows. The facies associations define lobe elements that are grouped into lobes (<10 m thick) and these into lobe complexes (~20-40 m thick). The studied basin floor fan comprises five lobe complexes (LC1-5) separated by fine-grained intervals. Here, we evaluated only LC1-4, since LC5 is poorly exposed. LC1 shows paleoflows towards the north and another one towards the east; it is composed of two lobes with high proportion of unconfined deposits. LC2 shows northeastward paleoflows, it is composed of three lobes that onlap and fill relative low areas left by LC1. LC3 exhibits paleoflows to the east and it is composed of six lobes, five of which show channels on their axis, evidencing the most proximal setting on the system. The lobes of LC3 aggrade and migrate laterally towards the NW. LC4 develops on top, with its axis shifting to the south and backstepping, exhibiting only unconfined lobe elements. Detailed study of lobes 2 and 3 in LC3 show interesting trends in axial sections. Proximal to lobe axis beds are thicker (>40 cm), grain size is greater (medium sand to granules) and main facies are conglomerates and structureless sandstones. Off axis, beds are thinner (<40 cm), grain size ranges from fine to medium sand and there is an increase on normally graded and laminated sands. These trends are associated with the confinement and density of the flow. From lobe axis to off-axis, channelized elements disappear and the facies vary from high density to low density turbidites. The present work shows that Los Molles Fm has significant sandstone units that likely form good reservoirs when present in the subsurface and a good analog for coarse-grained fan deposits.