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Slope to Basin-Floor Evolution of Channels to Lobes, Jurassic Los Molles Formation, Neuquén Basin, Argentina


Approximately 400m high clinoformal, shelf-slope-basin-floor deposits of Jurassic Los Molles Formation outcrops are evaluated for reservoir scale definition of facies and architectures in the La Jardinera field area, Neuquén Basin. Slope deposits represent the accretionary front of the prograding shelf margin, that was fed by a coarse grained shelf (Lajas Fm.). Mapping of a high-resolution satellite image draped on a digital elevation model resolved a sub-meter stratigraphic framework. An analysis of outcrops exposed along a 5km transect reveals the down-dip evolution of sand body architectures from shelf edge incisions filled with conglomerates, to confined channels in mid to upper slope reaches, to weakly confined channels on the lower slope and sheet-like lobes on the basin floor that also drape up onto the lower slope. Along the slope to basin floor profile the depositional architecture changes by overall decrease in bed thickness, grain size and degree of erosion. Confined slope channels are up to 25m deep, isolated within muddy slope deposits and have complex multistory fills marked by basal and internal erosion surfaces with mud-clast and/or pebble conglomerates. Channel axes contain amalgamated, medium to coarse sandstones that taper and fine towards channel margins over 200m. Down-dip, lower slope channels are up to 400m wide and less than 10 m deep. A downslope decrease in flow energy is indicated by marked reduction in mud clasts and conglomeratic material at basal erosional surfaces in weakly confined channels. However, distinct, meter scale erosion surfaces are recognizable where thin, ripple laminated sands are truncated on channel margins by amalgamated, structureless sands. Erosional surfaces are absent in laterally extensive (>10km), sheet-like lobes of basin-floor fans that are generally finer grained than lower or upper slope channel fills. However, there are lenticular debrites and thin micro-conglomerates associated with basin-floor fans. This relatively steep and narrow Neuquén Basin margin provides an excellent laboratory for monitoring down slope changes in sediment gravity flow bed thickness, grain size and facies, as well as channel to lobe transitions.