Evolving fill‐and‐spill patterns across linked early post‐rift depocentres control lobe characteristics: Los Molles Formation, Argentina

Abstract

Inherited rift topography controls the sediment routeing, timing of sand supply, and sedimentary linkage of early post-rift depocentres. Exhumed examples of early post-rift turbidite systems are rare and previous studies have examined the evolution of individual depocentres; in contrast, the detailed evolution of early post-rift turbidite systems across multiple depocentres has never been documented. Current fill-and-spill models do not detail the stratigraphic architecture and evolution of sedimentological characteristics of multiple intraslope fans developed across topography, including bed type distributions. Here, the evolution of three intraslope fans that developed across two early post-rift depocentres is documented along an 18 km-long transect in the southern Neuquén Basin, Argentina. The relative chronology of sand supply in depocentres is constrained with new U-Pb ages, and sediment source areas with provenance analysis. The early post-rift intraslope fans record progradation of the system and progressive sedimentary linkage of post-rift depocentres, transverse to local syn-rift structures, with sediment routeing subparallel to the cratonic basin margin. The large-scale stratigraphic architecture of intraslope fans indicates an evolution as a fill-and-spill system, with initial confinement through flow stripping and overspill to spillover with erosion and bypass across a transverse topographic high separating the depocentres. Changes in early post-rift intraslope fan characteristics, including thickness, sandstone content, lobe complex stacking patterns, stratal termination patterns, and bed type distribution, record changing confinement through time within a depocentre, and spatially across depocentres. The strong spatial and vertical stratigraphic variability of transitional flow deposits and hybrid event beds reflects enhanced erosion, sediment bypass and flow transformation across transverse relief between the two depocentres during the spillover phase. These findings advance our understanding of early post-rift turbidite systems and refine fill-and-spill models, which will help the prediction of spatial and vertical changes in rock quality and connectivity in subsurface hydrocarbon reservoirs and CO2 storage sites.