The jigsaw-puzzle fit of South America and Africa is an icon of plate tectonics and continental drift. Fieldwork in Angola since 2002 allows the correlation of onshore outcrops and offshore geophysical and wellcore data in the context of rift, sag, salt, and post-salt drift phases of the opening of the central South Atlantic. These outcrops, ranging in age from > 130 Ma to, < 71 Ma, record Early Cretaceous outpouring of the Etendeka-Parana Large Igneous Province (Bero Volcanic Complex) and rifting, followed by continental carbonate and siliciclastic deposition (Tumbalunda Formation) during the sagging of the nascent central South Atlantic basin. By the Aptian, evaporation of sea water resulted in thick salt deposits (Bambata Formation), terminated by seafloor spreading. The Equatorial Atlantic Gateway began opening by the early Late Cretaceous (100 Ma) and allowed flow of currents between the North and South Atlantic, creating environmental conditions that heralded the introduction of marine reptiles. These dramatic outcrops are a unique element of geoheritage because they arguably comprise the most complete terrestrially exposed geological record of the puzzle-like icon of continental drift.

Abstract Magmatic activity can severely alter the thermal structure of a sedimentary basin, with variable effects on the petroleum system. The Namibe Basin of Angola (Cretaceous South Atlantic rift) contains well-exposed magmatic and petroleum system elements and allows integrated assessment of how magmatic activity can modify the petroleum system. The basin was affected by syn-rift and post-rift magmatic events, and bitumen is observed within both the Pre- and Post-Salt stratigraphical sections. In the Pre-Salt, fluorescent bitumen has a lacustrine signature and is associated with calcite and quartz cements. Onshore Pre-Salt units are thermally immature, and therefore the source rock that generated the Pre-Salt bitumen is likely located offshore. Hydrocarbons migrated or re-migrated via magmatically driven fluids, reaching the present-day onshore. Closer to magmatic units, non-fluorescent pyrobitumen was instead observed, evidencing hydrocarbon cracking processes following emplacement. In the Post-Salt, bitumen is in situ and shows marine-like signatures compatible with an immediate Post-Salt source rock depositional environment. In the immediate Post-Salt, units with very high total organic carbon values (TOC; up to 13.8%) and excellent source rock properties (hydrogen index >600 mgHC g −1 TOC) have reached thermal maturation. Within the Namibe Basin these Post-Salt source units lie in proximity to major Turonian–Coniacian–Santonian volcanic centres and associated shallow intrusions, which are likely to have caused thermally forced maturation processes and generation of the Post-Salt hydrocarbons. This paper demonstrates the importance of an integrated field, petrographic and geochemical approach in unravelling the influence of magmatic activity on basin thermal structure and petroleum systems.

The Aptian Pre-Salt sedimentary succession cropping out in Cangulo palaeovalley onshore Namibe Basin (Angola) was studied by a combination of field and analytical techniques to constrain the sedimentary and diagenetic evolution of the uppermost sag sequence of the South Atlantic passive margin. Field observations allows definition of four transgressive-regressive cycles characterised by fluvial to tidal-influenced mixed clastic-carbonate and carbonate-dominated deposits, that locally show evidence of evaporite dissolution; highlighting that evaporite deposition started earlier than deposition of the regional South Atlantic Loeme-Bambata evaporite formations. Two separate pre-salt carbonate units have been differentiated within the Cangulo Fm; i) a lower transitional to marginal marine, and ii) a younger upper non-marine freshwater travertine system, that is documented for the first time in the west African margin. Transgressive-regressive cycles control the early diagenesis of the tidal carbonates that include dolomitization due to mixing fluids during transgressions, and karstification due to evaporite dissolution by meteoric water circulation during regressive events. Clastic supply appears to have been completely shut down during carbonate deposition, suggesting major climatic change associated with carbonate deposition. During the lowstand between the two carbonate units, fluid flow through Cangulo palaeovalley was re-established resulting in extensive karstification and formation of a large-scale erosional unconformity that is interpreted to be time equivalent to an intra Chela-Cuvo Fm. event. The top of the studied succession corresponds to the transgressive deposits of the Bambata evaporites that are not preserved in the Cangulo palaeovalley due to its erosion but are regionally developed. The results of this study can be directly linked to along strike age equivalent Pre-Salt successions cropping out in the Namibe, Benguela and Kwanza basins, and directly offshore Angola and Brazil using well and seismic data. These new data shed important new light and constraints on the depositional and diagenetic evolution of the complex Pre-Salt reservoir systems of the South Atlantic, and the depositional and bathymetric setting at the time of onset of the main south Atlantic evaporite deposition.

Pre-rift salt controls structural style variability within rifts by decoupling sub- and supra-salt faults. However, the way in which this variability controls sediment erosion and dispersal, and facies distributions within the coeval syn-rift stratigraphic succession, remains poorly known. We here use 3D seismic reflection and borehole data to study the tectono-stratigraphic development of the Halten Terrace, offshore Mid-Norway, a salt-influenced rifted margin formed during Middle to Late Jurassic extension. On the eastern basin margin the rift structural style passes southwards from an unbreached extensional growth fold dissected by numerous horst and graben (Bremstein Fault Complex), into a single, through-going normal fault (Vingleia Fault Complex). This southwards change in structural style is likely related to the pinch-out of or a change in the dominant lithology (and thus rheology) within a pre-rift (Triassic) evaporite layer, which was thick and/or mobile enough in the north to decouple basement- and cover-involved faulting, and to permit extensional forced folding. As a result, the salt-influenced Bremstein Fault Complex underwent limited footwall uplift, with minor erosion of relatively small horsts supplying only limited volumes of sediment to the main downdip depocentre. In contrast, the Vingleia Fault Complex, which was directly coupled to basement, experienced significant uplift and extensive footwall erosion. The footwall of this structure also locally underwent salt-detached gravity gliding and collapse as the pre-rift detachment was tilted. Our results show that where through-going normal faults develop along the rift flanks, the presence of a pre-rift salt layer will suppress the topographic expression of the footwall. The pre-rift salt layer may however facilitate footwall collapse and limit the volume of sediment supplied to downdip basins. Our results also show that variable topography along the rift flanks facilitated the development of relatively small, localised, intra-rift flank accommodation that trapped flank-derived sediment, and which meant basins nearer the rift axis were starved of sediment.

The breakup of Africa from South America is associated with the emplacement of the Parana-Etendeka flood basalt province from around 134 Ma and the Tristan da Cunha plume. Yet many additional volcanic events occur that are younger than the main pulse of the Parana-Etendeka and straddle the rift to drift phases of the main breakup. This contribution reports on new geochronological constraints from the Angolan part of the African Margin. Three coastal and one inland section have been sampled stretching across some 400 Km, with Ar-39/Ar-40, U-Pb and Palaeontology used to provide age constraints. Ages from the new data range from similar to 100 to 81 Ma, with three main events (cr. 100, 91 and 82-81 Ma). Volcanic events are occurring within the Early to Late Cretaceous, along this part of the margin with a general younging towards Namibia. With the constraints of additional age information both onshore and offshore Angola, a clear younging trend at the early stages of rift to drift is recorded in the volcanic events that unzip from North to South. Similar age volcanic events are reported from the Brazilian side of the conjugate margin, and highlight the need to fully incorporate these relatively low volume volcanic pulses into the plate tectonic breakup models of the South Atlantic Margin.

Sedimentation in hangingwall dipslope settings is still a relatively underexplored topic in rift basin studies. A better understanding of the evolution of marine sedimentary environments in this kind of settings has to address the variations occurring both along the strike and down the dipslope. Previous work was mainly built on the analysis of subsurface data, relying on the visualization of coarse resolution (10s of m) seismic sections and sparsely located borehole logs (km apart). This study focuses on the sedimentology and stratal arrangement of excellent quality Miocene marine early syn‐rift and rift climax successions continuously exposed for more than 20 km along the strike of the hangingwall dipslope in the El Qaa Fault Block, Suez Rift, Egypt. The integration of traditional sedimentary field techniques and terrestrial LIDAR scanning allowed for a detailed analysis of dip and dip direction for the different depositional units. Three different phases of tilting were identified for the hangingwall dipslope, which controlled the overall evolution of the marine sedimentary environment in the area. The tilt of the hangingwall not only determined variations in facies, thickness and grain‐size of the deposits down the dipslope but also along its strike. The studied exposures of the El Qaa Fault Block dipslope constitute a unique outcrop analogue for marine sedimentation in hangingwall dipslopes.

Alpine Folded Belts and Extensional Basins (GRW events) , 15-16 march, 2018, Granada, Spain

Rift basin tectono‐stratigraphic models indicate that normal fault growth controls the sedimentology and stratigraphic architecture of syn‐rift deposits. However, such models have rarely been tested by observations from natural examples and thus remain largely conceptual. In this study we integrate 3D seismic reflection, and biostratigraphically constrained core and wireline log data from the Vingleia Fault Complex, Halten Terrace, offshore Mid‐Norway to test rift basin tectono‐stratigraphic models. The geometry of the basin‐bounding fault and its hangingwall, and the syn‐rift stratal architecture, vary along strike. The fault is planar along a much of its length, bounding a half‐graben containing a faultward‐thickening syn‐rift wedge. Locally, however, the fault has a ramp‐flat‐ramp geometry, with the hangingwall defined by a fault‐parallel anticline‐syncline pair. Here, an unusual bipartite syn‐rift architecture is observed, comprising a lower faultward‐expanding and an upper faultward‐thinning wedge. Fine‐grained basinfloor deposits dominate the syn‐rift succession, although isolated coarse clastics occur. The spatial and temporal distribution of these coarse clastics is complex due to syn‐depositional movement on the Vingleia Fault Complex. High rates of accommodation generation in the fault hangingwall led to aggradational stacking of fan deltas that rapidly (<5 km) pinch out basinward into offshore mudstone. In the south of the basin, rapid strain localization meant that relay ramps were short‐lived and did not represent major, long‐lived sediment entry points. In contrast, in the north, strain localization occurred later in the rift event, thus progradational shorefaces developed and persisted for a relatively long time in relay ramps developed between unlinked fault segments. The footwall of the Vingleia Fault Complex was characterized by relatively low rates of accommodation generation, with relatively thin, progradational hangingwall shorelines developed downdip of the fault block apex, sometime after the onset of sediment supply to the hangingwall. We show that rift basin tectono‐stratigraphic models need modifying to take into account along‐strike variability in fault structure and basin physiography, and the timing and style of syn‐rift sediment dispersal and facies, in both hangingwall and footwall locations.

Rifts III: Catching the Wave 22-24 March 2016 The Geological Society, Burlington House, London

Ephemeral evaporitic conditions developed within the uppermost part of the transgressive Late Sag sequence in the Namibe Basin (SW Angola), leading to the formation of extensive centimetre- to metre-thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals. The onshore pre-salt beds examined in this study are located up to 25 m underneath the major mid-Aptian evaporitic succession, which is typified at the outcrop by gypsiferous Bambata Formation and in the subsurface by the halite-dominated Loeme Formation.Carbonate-evaporite cycles mostly occur at the top of metre-thick regressive parasequences, which progressively onlap and overstep landward the former faulted (rift) topography, or fill major pre-salt palaeo-valleys. The sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules, dissolution-related gypsum breccia, and are cross-cut by thin satin-spar gypsum veins. Nodular and fine-grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates, whereas gypsum veins and breccia result from telogenetic processes.The carbonates display a broader diversity of facies, characterised by rapid lateral variations along strike. Thin dolomitic and calcitic bacterial-mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds. Dissolution-related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites. Sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra- to intertidal- sabkha and constitute high-frequency hypersaline precursor events, prior to the accumulation of the giant saline mid-Aptian Bambata and Loeme Formations.Petrographic and geochemical analyses reveal successive dissolution, recrystallisation and cementation phases that occurred during burial, uplift and exhumation, implying a complex diagenetic evolution of both gypsum and carbonates, influenced by pore fluids of diverse composition which distinctly varied from meso- to telogenetic domains. (C) 2016 Elsevier B.V. All rights reserved.

The Halten Terrace is underlain by a Triassic evaporitic package, resulting in vertically decoupled normal fault systems following subsequent extension. Four structural domains are identified along the eastern margin of the Halten Terrace, characterised by: i) thick-skinned normal faults affecting both sub-salt and supra-salt cover, ii) basement-restricted normal faults associated with fault-propagation folds, iii) thick-skinned, distributed normal faults, and iv) thick-skinned, localised normal faults. A fault domain boundary associated with an NE–SW striking basement fault corresponds to an abrupt change in style in the north of the Halten Terrace. Summed throw and estimated strain measurements show that throw and strain accommodated by the fault system increase southward, corresponding to a transition from distributed to localised faulting. The evaporite package is variable in thickness, but those variations do not correspond spatially to variations in structural style. Wells that penetrate the evaporite package, and volume attribute analysis of 3D seismic data, suggest variable evaporite facies. A change in seismic attributes from high-amplitude, low variance to low-amplitude, high variance corresponds to a change from decoupled to thick-skinned faulting. The sub-evaporite fault template, amount of strain accommodated across the fault system, and facies variations in the evaporite package are key influences on structural style.

Due to the effects of sediment compaction, thermal subsidence and ‘post‐rift’ fault reactivation, the present‐day geometry of buried, ancient rift basins may not accurately reflect the geometry of the basin at any stage of its syn‐rift evolution. An understanding of the geometry of a rift basin through time is crucial for resolving the dynamics of continental rifting and in assessing the hydrocarbon prospectivity of such basins. In this study, we have restored the Late Jurassic–Early Cretaceous geometry of the southern Halten Terrace, offshore mid Norway, using a combination of well log‐ and core‐derived, sedimentological and stratigraphic data, seismic‐stratigraphic observations and reverse subsidence modelling. This integrated geological and geophysical approach has allowed the large number of input parameters involved in flexural backstripping and post‐rift thermal subsidence modelling to be constrained. We have thus been able to determine the regional structure of the basin at the end of the Late Jurassic–Early Cretaceous rift phase and the associated amount of crustal stretching. Our basin geometry reconstructions reveal that, during the latest syn‐rift period in the Late Jurassic–Early Cretaceous, the Halten Terrace was characterized by a series of isolated depocentres, located between footwall islands, which were not connected into a single depocentre until the Late Cretaceous (Coniacian). We show that two major unconformities, which are now vertically offset by ca . 2 km and located ca . 60 km apart, formed at similar subaerial elevations in the Late Jurassic–Early Cretaceous and were subsequently vertically offset by thermally induced tilting of the basin margin. Cretaceous sediments were deposited in a single, relatively unconfined basin in water depths of 1–1.5 km. The β profile that best restores palaeobathymetry to match our geological constraints is the same as that derived from summing visible post‐Late Triassic heave on faults plus 25–60% additional extension to account for sub‐seismic deformation. This indicates that, at least in the southern part of the Halten Terrace, the amount of upper‐crustal stretching during the Late Jurassic–Early Cretaceous rift phase is comparable to the total amount of lithospheric stretching, supporting a uniform pure‐shear stretching model.

The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre‐scale and patchy, dome‐like gypsum accumulations are well‐exposed within strongly incised present‐day river valleys along the eastern margin of the Namibe and Benguela basins (south‐west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution‐related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin‐spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the current at‐surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross‐cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics.

New, high-resolution lithofacies data from hanging-wall Miocene synrift (Rudeis Formation) exposures of the eastern Suez Rift margin, Egypt, reveal a submarine slope depositional system dominated by coarse-grained (pebble), heterogeneous, lenticular beds, formed by coalescing turbidity currents, slumps, and debris flows deposited on deforming submarine substrate. Flows include prerift clasts and contemporaneous shallow-marine fossil fragments originating from an uplifted eastern hinterland. Multiple terrestrial drainages debouched onto faulted offshore slopes or fed small fan deltas on narrow shelves (<0.5 km width). Steep, subaerial rift-flank topography also shed rock avalanche and landslide material offshore. The >360 m Rudeis Formation is divided into stratigraphic units R1 and R2, which exhibit upward-coarsening and unordered vertical motifs. Ongoing faulting influenced synrift deposition by controlling the locus of subsidence and gravity base level. Mesoscale faults became inactive during Rudeis Formation times, with strain localized on the large rift border fault system, leading to a wider basin with time. We compare 16 subaqueous rift-margin basin fills from various tectonic and geographic settings and show they generally represent proximal gravity-flow deposits dominated by nongraded beds. We find little commonality in vertical grain-size trends, highlighting the diversity of stratal architectures. Most basin fills show an inverse relationship between maximum clast size and shelf width. We propose a new model to capture the spectrum of sedimentary responses to rifting within rift-margin basins, varying as a function of shelf width, slope gradient, maximum grain size, and textural maturity. Rudeis Formation strata at north Wadi Baba represent a particularly coarse-grained end member, deposited on steep slopes, with a narrow shelf.

The Halten Terrace, offshore mid-Norway, is underlain by a Triassic evaporitic package that is rheologically weak, and led to decoupling of fault systems during Middle Jurassic to Early Cretaceous rifting. We use 2D and 3D reflection seismic data, constrained by wells, from the southern Bremstein Fault Complex of the Halten Terrace to map faults and key stratigraphic horizons, and analyse throw variations along faults, allowing us to constrain patterns of fault segmentation and linkage within the complex. The Bremstein Fault Complex has an overall tilted monoclinal geometry with localised fault systems at base salt level associated with overlying, highly distributed systems of normal faults. Vertical strain partitioning across the evaporite package means that sub-evaporite and supra-evaporite fault populations acted as semi-independent fault systems. Supra-evaporite faults are partly gravity-driven, and controlled by sub-evaporite faulting and consequent tilting of the evaporitic package. This behaviour leads to a wide variety of possible vertical linkage patterns of faults across the evaporite package. A greater variety of lateral segment linkage patterns occurs in evaporite-detached normal fault systems than in normal fault systems developed in the absence of evaporite units. Segment boundary styles can also be modified by migration of evaporite. Some segment boundaries are associated with a footwall anticline and hanging-wall syncline, in contrast to the footwall synclines and hanging-wall anticlines widely described in studies of normal fault systems.

Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault-related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity-driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid-Norway. This 22-km-long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first-order control on footwall erosion patterns is the overall fault-throw distribution.