Evolution from syn-rift carbonates to early post-rift deep-marine intraslope lobes: The role of rift basin physiography on sedimentation patterns

The stratigraphic architecture of Early Jurassic strata exposed along a >10 km long transect in the Chachil Graben, an exhumed marine rift depocentre in the Neuquén Basin (Argentina), provides insights into the sedimentological and stratigraphic expression of the syn-rift to post-rift transition. A change from syn-rift intrabasinal carbonate to post-rift extrabasinal siliciclastic sedimentation is recorded, as well as variations in sediment supply and dispersal patterns across rift-related topography. The late syn-rift was marked by a transgression and development of a shallow-marine carbonate system, including carbonate platform deposits perched on fault-block highs and periplatform deposits accumulated in fault-block lows, which overlies continental volcano-sedimentary syn-rift deposits. Differential subsidence and basin deepening induced retrogradation of the carbonate system, which was progressively drowned and overlain by organic-rich calcareous mudstone that draped across rift structures at the onset of the early post-rift. The first extrabasinal siliciclastic influx led to progradation of an early post-rift intraslope lobe complex into the graben, which is associated with kilometre-scale clastic injectites. The depositional architecture, facies distribution and pinch-out style of intraslope lobes record the effects of an inherited compaction hinge, which acted as an oblique counterslope to sediment gravity flows. The occurrence of combined-flow bedforms, widespread erosion, and limited facies segregation across lobes bearing different hybrid event bed types, is in sharp contrast to sedimentological characteristics of existing intraslope lobe models. Documentation of the syn-rift to post-rift transition stratigraphy permitted identification of changes in thickness and facies resulting from the passive infill of inherited topography with early post-rift differential compaction. This architecture contrasts markedly with those developed during syn-rift normal faulting. Furthermore, the influence of local inherited topography on the development of early post-rift lobes is key to improve subsurface prediction of sandstone distribution and quality during assessment of hydrocarbon reservoirs and carbon storage sites.