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

Abstract

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 Neuqu en 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 volcanosedimentary 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 combinedflow 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.

Channel‐lobe transition zone development in tectonically active settings: Implications for hybrid bed development

“…The stratigraphic distribution of hybrid beds has been linked to the character of the supply slope and seafloor relief, where hybrid beds are invoked to develop during periods of disequilibrium in out‐of‐grade slopes (Haughton et al, 2003, 2009; Hodgson, 2009; Pierce et al, 2018; Spychala et al, 2017a, 2017b). Therefore, they are dominantly deposited during stages of fan initiation and growth (Kane & Pontén, 2012; Privat et al, 2021), or up‐dip erosion during spatial expansion at the CLTZ (Brooks et al, 2018a). The equilibrium state of the slope is important, as when a system is above‐grade (Prather, 2000, 2003) flows preferentially erode the slope to form a profile that is at equilibrium.…”

Substrate Entrainment, Depositional Relief, and Sediment Capture: Impact of a Submarine Landslide on Flow Process and Sediment Supply

“…The intercalation of such different facies suggests the juxtaposition of depositional environments of contrasting energy and/or different sediment sources. Either scenario could be possible given the complex sediment routing patterns and multiple transverse or axial sources available in the Neuquén Basin during the early post-rift setting (Vicente, 2005;Privat et al, 2021) and by analogy to other post-rift settings (e.g., Lien, 2005;Fugelli and Olsen, 2007;Hansen et al, 2021). The mass failure would trigger a new coarse-grained source due to slide scar position and geometry, promoting intermittent sand supply to an otherwise sand-starved environment (see "Origin and role of the Mass-wasting process as a trigger for turbidite systems development" in discussion).…”

“…One explanation is that the exposure exhumes the Unit 4B lobe obliquely, with the medium-to thick-bedded lobe axis deposits in the westernmost sector (CML-1 to CML-4) transitioning into an HEB-dominated fringe, being highly impacted by gentle seafloor topography (Soutter et al, 2019;Privat et al, 2021). Alternatively, the seafloor relief could have promoted the modification of flow pathways and deflection of flows, thus changing the downdip orientation (Figure 8).…”

Substrate Entrainment, Depositional Relief, and Sediment Capture: Impact of a Submarine Landslide on Flow Process and Sediment Supply

“…One explanation is that the exposure exhumes the Unit 4B lobe obliquely, with the medium-to thick-bedded lobe axis deposits in the westernmost sector (CML-1 to CML-4) transitioning into an HEB-dominated fringe, being highly impacted by gentle seafloor topography (Soutter et al, 2019;Privat et al, 2021). Alternatively, the seafloor relief could have promoted the modification of flow pathways and deflection of flows, thus changing the downdip orientation (Fig.…”