JURASSIC SHELF-MARGIN GROWTH IN NEUQUEN BACK-ARC BASIN: COARSE-GRAINED, RIVER-TIDE INTERACTION RESERVOIRS AT THE SHELF EDGE

Shelf-margin clinoforms (300m+) in Neuquen Basin can be seen both on subsurface and outcrop data. Significant aggradation of the shelf but slow forward growth of the margin prism (few km/My) suggest only moderate sediment supply. A river-delta system, with a dominating gravelly distributary channel system and more modest frontal mouth bars, brought sediment intermittently across the narrow (<40km) Lajas shelf out onto the shelf-edge area. There developed occasional slope hyperpycnal-flow sandbodies (up to 20 thick with multiple flows) that are slope elongate, non-cross stratified, upward coarsening and thickening and likely linked back to strong flooding episodes from the shelf-edge deltas. Persistent tidal currents running along and sub-parallel to the shelf break influenced the shelf-edge delta sandbodies in three ways: (1) Individual channelized river-flood events that typically brought meter-scale, poorly sorted alluvial gravel and sand to the shallow subtidal to intertidal zone of the delta front show an upward transition and accumulation of better sorted and laminated tidal sand deposits (<0.5 m) in the waning or inter-flood periods; (2) In off-axis areas less than a few hundred meters lateral to coeval, thick gravel channels, 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 when distributary channels and gravelly sediment in general had been pushed back across the shelf, thick (>10m) marine mudstone units accumulated on the shelf, as well as associated sharp-based, 5-10m thick, blocky or slightly upward-coarsening, well sorted white sandstones with stacked sets of cross strata. These are interpreted as transgressive shelf sand ridges oriented sub-parallel or oblique to the shelf edge.