Enhancing the model of coarse-grained basin floor fans : characteristic trends within lobes and lobe complexes of the Jurassic Los Molles Fm., Neuquén Basin, Argentina

Basin floor fans can contain a wide range of grain sizes, though the most frequently described fans have tended to be the fine-grained sandy and muddier ones. A coarse-grained category of fan has been broadly described previously, but it lacks a depositional and facies model. The Mid-Jurassic deep-water marine deposits of Los Molles Fm. in Neuquén Basin, Argentina is an example of this type of fan and it is well exposed in outcrop. A detailed characterization of the basin-floor fans of La Jardinera is used to build a coarse-grained fan model. We made use of a high-resolution satellite image, drone imagery and 4000 m of logs with detailed measurements to build isopach and net/gross (NG) maps that with facies analysis allowed reconstruction of the fan and its lobe complexes (LC1-5). In addition, grain size, facies and bed thickness trends were used to refine the interpretation at a lobe scale within unit LC3. Lithofacies, NG ratios and sandstone body geometry helped define six facies associations; hemipelagic deposits, lobe fringe, off-axis lobes, on-axis lobes, distributary channels and debris flows. The facies associations build lobes (<10 m thick) and these are grouped into lobe complexes (~20-40 m thick). The studied five lobe complexes (LC1-5) are separated by fine-grained intervals (~ 4 m thick in average). The fan shows paleoflow trends towards the northwest at the bottom gradually changing to northeast at the top. The lobe complexes stack forward and backstep gently, with no major switches; they aggrade and shift laterally in an autocyclic manner, following topographic lows left by previous deposits. The maps at lobe complex scale show an overall elongated morphology and serrated geometries downdip, a normal response of focused sediment dispersal associated with channels and high-density turbidity currents. Detailed study of lobes 3 and 4 in LC3 show that 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 a coarse-grained basin floor fan system that differs from previous models; having a distinct elongated morphology, finger-like geometries and changes in facies associated with channelized features and variations on the type of flow from the axis to the fringes.