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Mid-Jurassic Shelf-Margin Growth in Neuquen Basin: Coarse-Grained, River-Tide Interaction at the Shelf Edge

Abstract

Research on the Jurassic shelf margin of southernmost Neuquen Basin, Argentina, is providing new outcrop examples of how coarse-grained river influx to the shelf-edge area interacted with shelf-margin tidal currents to produce an unusual assemblage of shelf-edge and upper slope sandbodies. Mid-Jurassic margin clinoforms (300m+) are well imaged in seismic-scale outcrops where occasional incisions funneled coarse-grained sediment from the Las Lajas shelf down onto Los Molles muddy deep-water slope. Significant aggradation of the shelf (stacked T-R sequences occur at all shelf sites) but slow forward growth of the margin prism (few km/My) suggest only moderate sediment supply. A periodically flooding river-delta system, with a dominating gravelly distributary channels and more modest sandy frontal mouth bars, brought flood sediment intermittently across the narrow (<40km) Lajas shelf. Exit of this sediment from the shelf break area onto the muddy slope occurred on widely spaced incisions, each some 2-3km wide and 10s of m deep, at times bringing coarse sediment to the basin floor. Persistent tidal currents running along and sub-parallel to the shelf break also influenced the shelf-edge deltas in: (1) channelized river-flood events of meter-scale, poorly sorted alluvial gravel and sand show an upward transition to dm-thick better sorted and laminated inter-flood, tidal sand deposits with double mud drapes and occasional tidal bundles; (2) within the incisions, but in off-axis areas, there are stacked sets of gravel-free, cross-bedded sands with frequent mud interbeds; stronger tidal signals including bi-directional cross strata suggest that these were tidally reworked delta-front areas (3) During periods of widespread transgressive flooding at the margin, thick (>10m) marine mudstone units draped the shelf edge and outer shelf, but contain sharp-based, 5-10m thick, blocky or slightly upward-coarsening, well sorted white sandstones with well-organized, stacked sets of cross strata. These are interpreted as transgressive shelf sand ridges oriented sub-parallel or oblique to the shelf edge, possibly attached to coastal headlands. A final type of sandbody, distal to all the others (possibly on the upper slope), without obvious tidal influence, is flat-laminated and up to 20 thick with multiple internal flows. They are slope elongate, upward coarsening and thickening and likely a distal product of strong flooding episodes from the shelf-edge deltas.