Sedimentology and architecture of late syn-rift to early post-rift transition in a deep-water half-graben, Neuquén Basin, Argentina

Abstract

The early post-rift period of many rift basins are characterized by deep-water conditions due to thermally-driven subsidence after rapid, large-magnitude, syn-rift subsidence. The transition from syn- to post-rift, which in subsurface data is defined by a post-rift (PRU) or breakup unconformity stage, can lead to major changes in sediment dispersal patterns and processes, conditioned by inherited rift basin topography (i.e. underfilled or overfilled rift basins). Generic tectono-stratigraphic models for deep-water rift basins do not capture this complexity, especially in early post-rift systems fed by multiple sources. To assess the impact of evolving post-rift physiography on stratigraphic architecture, a rare example of an exhumed post-rift deep-water system has been documented. The study area is the Chachil subbasin, a 17 km long and 17 km wide NW-SE trending half-graben in the SW Neuquén Basin, Argentina. The exposures permits the controlling factors on the paleogeographic organization from syn- to post-rift transition to be assessed. Ten measured sections were physically correlated along a >10 km long transect across the width of the southern border of the Chachil half-graben. The subbasin is filled by Triassic, volcano-sedimentary, syn-rift deposits, which thicken northeastward from 100s m to >2 km. The syn-rift deposits are overlain by an early Pliensbachian-late Toarcian syn- to post-rift marine succession that dips and thickens towards the hangingwall, and thins and onlaps towards a major granitic footwall bounding the southwestern margin of the subbasin. The syn- to post-rift succession comprises two major stratigraphic units. The first is a fining-upward succession (up to 50m thick) of slope-to-platform, mixed carbonate-calcareous clastic deposits, which are strongly influenced by rift-related relief (Chachil Formation) (Fig. 1-2). The second stratigraphic unit is a coarsening-upward succession (up to 2000m thick) of slope-to-basin floor, deep-water clastic deposits emplaced across gentler basin relief (Los Molles Formation) (Fig. 3). The gradual sedimentary infill indicates drowning of the carbonate system by a turbidite system in a subbasin dominated by subsidence. The turbidite system is composed of (i) a lower calcareous-rich mud-prone succession and (ii) an upper carbonaceous-rich lobe-dominated succession with abundant plant material and intrabasinal mudclasts (cm to dm). The lower mud-prone succession corresponds to massive mudstones with rare thin to medium beds of structureless siltstones and medium to fine-grained sandstones with shell fragments. The upper lobe-dominated succession corresponds to thick beds of poorly sorted, massive or normally graded very coarse to fine-grained sandstones, with planar or low angle-cross tangential stratification, intercalated into dark organic shales and pelagic mudstones, or thin bedded successions of fine to coarse-grained, structured or structureless sandstones and siltstones. The architecture and thickness patterns of the syn- to post-rift succession presents local variations of accommodation and readjustment of sedimentary systems across structures, expressed by abrupt facies changes. The evolution of syn- to post-rift sedimentation, from a shallow-water carbonate- to a deep-water clastic-dominated regime, record a transition from intra- to extra-basinal processes that occurred with a change of sediment yield, source provenance and climate. The development of predictive stratigraphic models that incorporate outcrop data is necessary to improve prediction of subseismic facies distribution in early post-rift sequences. This will allow us to better predict reservoir distribution and trap type for analogue systems developed under the influence of similar factors of control in underexplored post-rift basins.