The stratigraphic evolution of ancient salt giants is controversial, mainly due to the absence of modern analogues that are of comparable scale and thickness and that occur in similar tectonic and hydrological settings. Fur thermore, investigating the original stratigraphy of salt giants is often made difficult by postdepositional flow and dissolution. Layered evaporites of the Ariri Formation in the Santos Basin (offshore Brazil), deposited during open ing of the South Atlantic Ocean, form part of one such salt giant. Despite being well imaged in seismic data and being penetrated by more than 50 boreholes, little work has explored the stratigraphic architecture of this unit and what this may tell us about the syndepositional tectonics, basin physi og raphy, and variations in climate and sea level. Here we integrate threedimen sional seismic and borehole data from the São Paulo Plateau, deepwater Santos Basin, to document the intrasalt stratigraphy of the Ariri Formation. Our analysis suggests a combination of an arid paleoclimate, lowamplitude local sealevel variations, and basin physiography controlled the deposition of this thick (2.5 km) salt sequence during a short time span (<530 k.y.). The Ariri Formation records at least 12 cycles of basin desiccation and filling, re sulting in the deposition of four key units (A1-A4) that have a distinct com position and therefore seismic expression; i.e., lowfrequency, transparent, chaotic seismic facies represent relatively haliterich (>85%) units (A1 and A3), whereas highfrequency, highly reflective seismic facies represent still relatively haliterich (65%-85% halite) units, but contain relatively high pro portions (15%-35%) of anhydrite and bittern salts (i.e., K and Mgrich salts; A2 and A4 units). Our findings suggest that during salt deposition the Santos Basin was characterized by a series of subbasins of varying water depth; as a result the thickness and composition of these units vary laterally and are spa tially related to structural domains. Overall, thinner salt (~1.8 km) and higher anhydrite net thickness (~350 m) occur toward the structurally high Sugar Loaf domain, compared to flanking, structurally lower domains where the mean salt thickness is >2.2 km and anhydrite net thickness are less (~180 m). In addition, stratigraphic variations in the basin suggest that seawater incur sions came from the south, through the São Paulo and Walvis Ridges; conse quently, more anhydrite was deposited closer to the ridge, whereas more bit tern salts were deposited in more distal and restricted locations. The results of our study, although based on an analysis of Aptian salts preserved offshore Brazil, offer valuable insights into the sedimentology and stratigraphic archi tecture and evolution of other ancient salt giants.
Faulting and earthquakes occur extensively along the flanks of the East African Rift System, including an offshore branch in the western Indian Ocean, resulting in remobilization of sediment in the form of landslides. To date, constraints on the occurrence of submarine landslides at margin scale are lacking, leaving unanswered a link between rifting and slope instability. Here, we show the first overview of landslide deposits in the post-Eocene stratigraphy of the Tanzania margin and we present the discovery of one of the biggest landslides on Earth: the Mafia mega-slide. The emplacement of multiple landslides, including the Mafia mega-slide, during the early-mid Miocene is coeval with cratonic rifting in Tanzania, indicating that plateau uplift and rifting in East Africa triggered large and potentially tsunamigenic landslides likely through earthquake activity and enhanced sediment supply. This study is a first step to evaluate the risk associated with submarine landslides in the region.
Salt dissolution occurs when evaporite-dominated rocks come into contact with NaCl-undersaturated fluids. Salt dissolution can positivity and negatively impact hydrocarbon and mineral exploration, seismic imaging, drilling, and structural restorations in salt-bearing sedimentary basins. However, due to typically poor seismic imaging and a lack of borehole data, few studies have analysed the detailed morphology of salt dissolution-related features (i.e., salt karst) and how this relates to intrasalt stratigraphic heterogeneity, and associated deformation within post-salt overburden. Here we integrate high-quality 3D seismic reflection data, a regional 2D seismic reflection line, and borehole data from the Santos Basin, offshore Brazil to characterise salt dissolution-related features at the crest of buried salt diapirs. We recognise: (i) flat (<10◦), halite-dominated crests mainly characterised by up to 100 m tall, sub-circular mounds, likely comprising insoluble evaporite; (ii) rugose, evaporite-interbedded crests mainly characterised by up to 100 m deep, oval-to-circular sinkholes above more soluble evaporite units; and (iii) up to 60 m tall breccia pipes, capped by collapse-related sinkholes within the overburden above the flat and rugose crests. Reverse-basin modelling suggests salt dissolution occurred in a fully submarine environment in water depths of 1900 m (± 100 m), which, in combination with seismic-stratigraphic-relationships in post-salt strata, suggests dissolution occurred due to (i) superjacent NaCl-undersaturated seawater which penetrated exposed, thin (up to 60 m) roofs; and (ii) lateral updip migration of formation fluids from flanking submarine channels and lobes. We are therefore able to demonstrate a direct link between the intrasalt stratigraphic heterogeneity, and the style of salt karst, and related deformation in post-salt sedimentary overburden, providing evidence for widespread dissolution of salt in a fully submarine environment.
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