ALBERT

Description: Seafloor topography affects the sediment gravity flows that interact with it. Understanding this interaction is critical for accurate predictions of sediment distribution and paleogeographic or structural reconstructions of deep-water basins. The effects of seafloor topography can be seen from the bed scale, through facies transitions toward intra-basinal slopes, to the basin scale, where onlap patterns reveal the spatial evolution of deep-water systems. Basin-margin onlap patterns are typically attributed to allogenic factors, such as sediment supply signals or subsidence rates, with few studies emphasizing the importance of predictable spatio-temporal autogenic flow evolution. This study aims to assess the autogenic controls on onlap by documenting onlap styles in the confined Eocene-to-Oligocene deep-marine Annot Basin of SE France. Measured sections, coupled with architectural observations, mapping, and paleogeographical interpretations, are used to categorize onlap styles and place them within a generic stratigraphic model. These observations are compared with a simple numerical model. The integrated stratigraphic model predicts that during progradation of a deep-water system into a confined basin successive onlap terminations will be partially controlled by the effect of increasing flow concentration. Initially thin-bedded low-density turbidites of the distal lobe fringe are deposited and drape basinal topography. As the system progrades these beds become overlain by hybrid beds and other deposits of higher-concentration flows developed in the proximal lobe fringe. This transition is therefore marked by intra-formational onlap against the underlying and lower-concentration lobe fringe that drapes the topography. Continued progradation results in deposition of lower-concentration deposits in the lobe off-axis, resulting in either further intra-formational onlap against the lobe fringe or onlap directly against the hemipelagic basin margin. Basinal relief is gradually reduced as axial and higher-volume flows become more prevalent during progradation, causing the basin to become a bypass zone for sediment routed down-dip. This study presents an autogenic mechanism for generating complex onlap trends without the need to invoke allogenic processes. This has implications for sequence-stratigraphic interpretations, basin subsidence history, and forward modeling of confined deep-water basins.

Description: Behavior of sediment gravity flows can be influenced by seafloor topography associated with salt structures; this can modify the depositional architecture of deep-water sedimentary systems. Typically, salt-influenced deep-water successions are poorly imaged in seismic reflection data, and exhumed systems are rare, hence the detailed sedimentology and stratigraphic architecture of these systems remains poorly understood. The exhumed Triassic (Keuper) Bakio and Guernica salt bodies in the Basque–Cantabrian Basin, Spain, were active during deep-water sedimentation. The salt diapirs grew reactively, then passively, during the Aptian–Albian, and are flanked by deep-water carbonate (Aptian–earliest Albian Urgonian Group) and siliciclastic (middle Albian–Cenomanian Black Flysch Group) successions. The study compares the depositional systems in two salt-influenced minibasins, confined (Sollube basin) and partially confined (Jata basin) by actively growing salt diapirs, comparable to salt-influenced minibasins in the subsurface. The presence of a well-exposed halokinetic sequence, with progressive rotation of bedding, beds that pinch out towards topography, soft-sediment deformation, variable paleocurrents, and intercalated debrites indicate that salt grew during deposition. Overall, the Black Flysch Group coarsens and thickens upwards in response to regional axial progradation, which is modulated by laterally derived debrites from halokinetic slopes. The variation in type and number of debrites in the Sollube and Jata basins indicates that the basins had different tectonostratigraphic histories despite their proximity. In the Sollube basin, the routing systems were confined between the two salt structures, eventually depositing amalgamated sandstones in the basin axis. Different facies and architectures are observed in the Jata basin due to partial confinement. Exposed minibasins are individualized, and facies vary both spatially and temporally in agreement with observations from subsurface salt-influenced basins. Salt-related, active topography and the degree of confinement are shown to be important modifiers of depositional systems, resulting in facies variability, remobilization of deposits, and channelization of flows. The findings are directly applicable to the exploration and development of subsurface energy reservoirs in salt basins globally, enabling better prediction of depositional architecture in areas where seismic imaging is challenging.

Description: Subsurface salt movement in the absence of external tectonic forces can affect contemporaneous sediment deposition, mask allocyclic signals, and deform older strata. We used a discrete element model (DEM) to better understand salt-related modification of a sedimentary sequence with an increasing sedimentation rate. This permitted quantification of thinning rates and analysis of the lateral extent of synkinematic layers. Results show realistic evolution of salt-related faults, defining two salt-withdrawal basins, beyond which strata are undeformed. Thinning of stratigraphy is four times greater between the salt flank and crest than between the undeformed zone and flank, confirming an intense zone of halokinetic modulation adjacent to the diapir. Early, slowly aggrading layers are isolated within the salt-withdrawal basin and strongly influenced by salt growth, whereas later, quickly aggrading layers are more laterally extensive, matching inferences made from subsurface and outcrop data. Halokinetic modulation reduces up the stratigraphic section, mirroring observations around the Pierce diapirs, in the North Sea, offshore UK. Our DEM provides quantitative insights into the dynamic interplay between halokinetic and allocyclic controls on salt-stratigraphic relationships.