Controls on sedimentation with transgression across a complex slope during the syn- to post-rift transition

PRIVAT, A.; HODGSON, D.M.; JACKSON, C.A-L.; SCHWARZ, E.; PEAKALL, J.

Congreso; 20th International Sedimentological Congress; 2018

IAS

The evolution of sedimentation in tectonically-active rift basins is widely overlooked during the transition from syn- to post-rift conditions. In particular, controls on the development of shallow- and deep-marine clastic systems across complex inherited rift basin physiography have been poorly demonstrated. For the first time, we examine sediment dispersal processes and resultant stratigraphic architecture recorded by an exhumed Early Jurassic shallow- to deep-marine succession during the syn- to post-rift transition, in the Neuquén Basin, Argentina. The succession was deposited across a complex slope, within partially connected volcanic rift sub-basins, which stratigraphic variability is constrained by a 27 km long, proximal-to-distal, slope-parallel transect, using 10 (250-450 m), high-resolution measured sections. In distal sub-basins, the early post-rift stage is marked by deposition of areally extensive, organic-rich mudstone (40-70 m thick), which records long-lived sand starvation. Through time, sandy flows reached the distal sub-basins, depositing thick (70-230 m), sandstone-rich packages texturally ?dirty? and immature at base and cleaning upward, and comprising lobes and minor channel-fills. These deposits are time-equivalent to thinner (maximum 160 m thick), clean, more mature, sandstone-rich successions including channels and minor lobes in proximal sub-basins. These relationships suggest trapping of cleaner sands in proximal shallow-marine sub-basins, bypass and accumulation of relatively ?dirty?, sand-rich material in distal sub-basins. Changes in sediment storage and the textural characteristics of sandstone deposits are associated with variations in the contributions from extrabasinal and intrabasinal sources, sediment supply rates, and the evolution of rift basin physiography. Interactions between sand-rich flows and inherited rift topography had a major impact on the location and reservoir quality of sand-rich accumulations, with significant implications for subsurface exploration of analogous slope systems in rift basins.