Sedimentology and tectono-stratigraphic development of the syn- to post-rift transition in Southern Neuquen Basin (Argentina) and controls on early post-rift submarine lobes of the Los Molles Formation

Marine rift basin-fills record the interplay between sediment supply and accommodation, which controls the organization and evolution of depositional environments in time and space. This study documents the tectono-stratigraphic architecture of multiple exhumed Early Jurassic marine rift depocentres of the southwestern Neuquén Basin, Argentina. Petrographic analyses of sandstone and new U-Pb ages are used to revise the current chronostratigraphic framework framework and to reassess sediment sources. This provides new insights in the timing of depocentre linkages, contrasting syn- to post-rift transition signatures, and diachroneity of early post-rift lobes in adjacent rift basins. The development of intraslope lobes in the Los Molles Fm. is documented from the late Early Toarcian, while the true basin-floor of the Neuquén Basin remained sand-starved during the Early Jurassic.
At basin margin-scale, the late syn-rift is characterized by transverse systems, with mixed contribution of intrabasinal siliciclastic intrarift or volcanic arc-derived sediment supply and extrabasinal cratonic sediment supply from the southeastern basin margin. The early post-rift is characterized by the development of extrabasinal volcanic arc-derived intraslope fans, with axial sediment routeing across a low- to moderate-gradient ramp-type system flanking the volcanic island arc basin margin. At a regional-scale, inherited topography results in initial trapping of sands in depocentres proximal to the source, until healing of intrabasinal relief enables overspill, bypass and progradation of intraslope fans into more distal sand-starved depocentres. At the scale of individual depocentres, this study demonstrates the interactions of sediment gravity flows with subtle inherited rift and compaction-enhanced topography, and their impact on the characteristics of early post-rift lobes.
This study extends established tectono-stratigraphic models of marine rift basin-fills during the syn- to post-rift transition to backarc settings, and provides new outcrop-based models for different types of low aspect ratio early post-rift lobes.