Basement Highs Research Articles

Jurassic depocenters development, between Colorado basin, Chelforó sub-basin and Neuquén basin

Northern Patagonia, to the north of the Somún Cura Massif along the 39°S parallel, is characterized by the development of a series of Jurassic depocenters, initiating in the Andes to the west, across the Neuquén basin and the Pampean sector (Chelforó sub-basin) towards the Atlantic coast near the location of the Pedro Luro −1 well (onshore Colorado basin), and further east over the Argentine shelf, where multiple Jurassic depocenters form the Colorado, Rawson, and Valdés basins.Using marine and continental gravity data (Free air and Isostatic anomalies respectively), constrained by well and seismic data, an integrated interpretation workflow for the study area was carried out. The southern limit of the Neuquén Basin is controlled by the Huincul High, an E-W directed structure with a positive gravimetric anomaly, interpreted as a basement high limited by relative gravimetric lows to the north and south of the structure. The eastern sector includes a series of isolated depocenters, separated by basement highs (obliquely oriented) and limited to the south by the E-W anomaly of the Huincul High. These depocenters correspond to the Chelforó sub-basin, which was drilled in a recent exploration campaign obtaining Jurassic ages where seismic interpretation pointed out presumably Jurassic deposits. Between Valcheta and Sierra Grande, this positive anomaly turns NW and becomes the southern boundary of the Pedro Luro depocenter, partially drilled by the Pedro Luro- 1 well, with possibly Tithonian palynological records at total depth. Between the Pedro Luro depocenter and the Chelforó sub-basin some features consistent with basement highs with circular morphology have been identified on gravimetric data, located in the area limited between the Colorado and Río Negro rivers. Jurassic rift depocenters have also been interpreted further east, in the Colorado basin where depocenters are controlled by an E-W oriented basement high, and further east by NW trending structures near the position of the SDRs (seaward dipping reflectors on the continental-oceanic crust transition zone).The common evolution of these Jurassic depocenters is still poorly understood. They extend for more than 1500 km from the Andes to the Atlantic continental shelf and are controlled by structures located to the north of the Somún Cura Massif. The interpretation of seismic lines, and gravimetric data in areas without deep wells allow the identification of new exploration targets increasing the exploration potential of these basins.

Late Ediacaran carbonate production and REE+Y signatures tracing redox conditions in a Cadomian retroarc basin, Central Iberian Zone, Spain

The late Ediacaran carbonate production recorded in a Cadomian retroarc basin from the Iberian Peninsula provides an excellent opportunity to evaluate the tectonic and redox factors that controlled the infill of the foredeep troughs, located between uplifted orogenic wedges and exposed forebulge transects. In the Central Iberian Zone of the Iberian massif, the carbonates of the Villarta Formation follow two geographic belts, which represent short-term mosaics of uplifted and tilted fault-bounded basement highs. These recorded the nucleation of fringing reefal (rich in Cloudina and Sinotubulites microfossils embedded in thromboid textures) and shoal and back-barrier complexes, which episodically fed slope-related megabreccia lobes and channels. REE+Y datasets from the impure carbonates reflect contamination by detrital material, hydrothermal interaction and variable redox conditions, ranging from oxic to dysoxic, the latter emphasized by the deposition of kerogenous black shales in distal parts of the foredeep trough.

The Vienna Basin: Petroleum Systems, Storage and Geothermal Potential

The Vienna Basin is located in the transition zone between the Alps and the Carpathians. Its tectonostratigraphic structure is complex, and includes from base to top crystalline basement, autochthonous Mesozoic sediments, Cenozoic foreland basin deposits, the Alpine nappe system and a thick Neogene basin fill. The Vienna Basin area hosts one of the major hydrocarbon provinces of Central Europe, but also a significant geothermal potential. The petroleum system is mainly based on the Upper Jurassic Mikulov source rock in the authochthonous succession, which became mature during nappe stacking and Neogene basin subsidence. Migration along faults and via basement highs filled oil and gas reservoirs in all tectonostratigraphic units. Despite a 100-year-long exploration history, new discoveries are still being made. Two geothermal systems are known in the Vienna Basin. One system is based on hot fluids ascending along the major faults in the southern part of the basin and has been used for balenological purposes since Roman times. The other is based on deeply buried, highly permeable carbonate rocks in the Alpine wedge (e.g. Hauptdolomite Formation) and clastic reservoirs in the Neogene basin fill. Current projects aim to supply the city of Vienna with geothermal heat from Lower Miocene fluvial conglomerates.

The impact of the Abimael Ridge on the salt tectonics in the southern Santos Basin, Brazil: The Esmeralda exploration block case

The Merluza Graben and Abimael Ridge are first-order basement structures that influence not only the thickness, geometry, and shape of the salt layers, but also the distribution and geomorphology of post-salt sequences in the southern Santos Basin.The basement highs, associated with the implantation of the Abimael Ridge, interpose as a physical barrier to the salt flow, as the plastic, mobile evaporite sequences tend to occupy areas with less overburden pressure. Given that most pre-salt targets rely on evaporite layers as a sealing element, these basement structures can either favor or hamper the risk analysis of hydrocarbon prospects recognized in the area.This study focuses on the Esmeralda exploration block, offered by the Brazilian Agência Nacional de Petróleo (ANP) in 2022. It examines the impact of alternate geological interpretations on the economic evaluation of pre-salt exploratory plays in the southern Santos Basin.

Gravity and magnetic geophysical surveys for exploration of low sulphidation epithermal mining project. Marifil Complex, Chon Aike Igneous Province, Argentina

The Marifil Complex constitutes the early stage of the Chon Aike Igneous Province along eastern North Patagonia, Argentina. Geological, geochronological, and geochemical data indicate that this magmatism consists of two characteristic Lower Jurassic volcanic events (V0 and V1). The epithermal activity takes place in V1 as Au-Ag veins (Pavón Pivetta et al., 2024). Particularly, base metal veins are covered and masked by the important explosive volcanic activity related to the V1 volcanic stage. This makes geophysical exploration a highly recommended tool for interpreting subsurface geological features.With this in mind, we have carried out the acquisition and processing of gravity and magnetic data that is supported with drill holes and geochronological data. A series of gravity and magnetic highs and lows were recognized, together with major lineaments. Using Euler deconvolution, the depth of the sources that generate the anomalies was calculated. The analysis of these results allowed us to construct two perpendicular gravity 2D models for the calculation of thicknesses and depth of the rhyolitic coulée. The gravity lows were associated with the major thickness of rhyolitic coulée, reaching 290 m in the drill holes, and interpreted geophysically to have a deeper root. The gravity highs were interpreted in two possible ways: a basement depth decrease and/or the absence of the rhyolitic coulée. The minimum depth estimated for the basement highs is approximately 20 m. These interpretations are consistent with outcrop, drill hole and geochronologic data, and magnetic maps. The RMSE between the calculated and the observed anomalies of the models are about 0.316 and 0.519 mGal. Due to the shape of the anomalies obtained from the gravimetric maps, the structures that generate the anomalies were considered as generated by three-dimensional bodies. For this purpose, we created a 3D model that provides interesting data about the possible location of these non-outcropping gravity lows that are located in the first 200 m depth.These results demonstrate that the use of the gravity and magnetic method constitutes a useful, fast, economical, robust, and reliable tool for the identification of these volcanic structures.

Facies models for rocky shorelines and their application to transgressed basement highs in the North Sea

Rocky shorelines form where basement highs are eroded and flooded during marine transgressive events. Although the Mesozoic North Sea rift generated numerous platform margins and rotated fault blocks that acted as basement highs, rocky shoreline deposits have not previously been reported in this region. Rocky shoreline deposits are usually represented in the rock record by thin conglomerates overlying major unconformities. They are typically characterized by their ichnological aspects rather than by their depositional facies. This study used the sedimentological aspects of modern and Miocene rocky shorelines from Spain and Austria to create facies models, which were then applied to the recognition of rocky shorelines in the Mesozoic of the Central North Sea. Our results demonstrate that structureless, clast-supported, poorly to moderately sorted conglomerate–breccia deposits are associated with competent basement lithologies, which produce hard, resistant coastal cliffs around previously overlooked volcanic centres in the subsurface of the North Sea. The basement lithologies in most of the Central North Sea favoured the formation of softer coastal cliffs, with less resistant lithologies that did not generate or preserve gravel-sized particles. These softer coastal cliffs were mostly characterized by low-angle, unconformity-bounded sandstones and fine-grained deposits, precluding the preservation and recognition of Mesozoic rocky shores in much of the North Sea's stratigraphic record.

Evaluating the Physics of Outcrop‐To‐Outcrop Flow With Hydrothermal Flow Models

AbstractCold and diffuse hydrothermal circulation on mid‐ocean ridge flanks impacts heat and fluid fluxes between the seafloor and the ocean. One mode of this circulation is given by outcrop‐to‐outcrop flow, where seawater circulates through a crustal aquifer that connects two or more recharging and discharging seamounts or basement highs that outcrop through the less permeable sediment cover. The physical mechanism driving this flow is a lateral pressure gradient that is sustained by contrasting the hydrological properties of the recharging and discharging outcrops. To investigate the physical controls of this pressure gradient, we performed two‐dimensional numerical simulations of coupled heat transfer and fluid flow. We have modified aquifer permeability, outcrop permeability and width, outcrop distance, and sediment thickness to assess their mutual effects on the lateral pressure differences. We have also investigated how different flow patterns, resulting from changes in these parameters, manifest themselves in seafloor observables such as flow rates, aquifer temperatures, and heat flow. Our models show that outcrop‐to‐outcrop flow generally occurs for aquifer permeabilities ≥10−14 m2, depending on the basal heat input. High aquifer permeabilities correspond to fast flow rates and low fluid temperatures, whereas the maximum lateral pressure differences arise for lower permeabilities. The permeability and the geometric shape of the outcrops determine the flow direction, while the aquifer temperature is also affected by the distance between the outcrops. Thicker sediments increase the lateral pressure difference and the flow rate. Our models thus provide constraints for predicting subseafloor hydrothermal ridge flank flow behavior from regional field data.

Tectono‐sedimentary evolution of high‐displacement crustal‐scale normal faults and basement highs on rifted margins: Klakk Fault Complex and Frøya High, Mid‐Norwegian Margin

AbstractCrustal‐scale high‐displacement (>10 km) normal faults are not captured in existing tectono‐sedimentary models of rift basins. We used 2D and 3D seismic reflection and well data to perform a structural and source‐to‐sink analysis of the southern part of the Klakk Fault Complex and the western part of the Vingleia Fault Complex, Mid‐Norwegian rifted margin. The north–south trending Klakk Fault Complex has a zig‐zag to sinuous plan‐view geometry, forming a series of structural recesses and salients along strike. In cross‐section, the fault complex has a listric to convex‐up or low‐angle planar geometry with displacements above 20 km. This fault complex exhumed basement highs, the Frøya High and Sklinna Ridge, in its footwall and created a series of supradetachment basins, for example, the Rås Basin, in its hanging wall. In contrast, the northeast‐southwest trending Vingleia Fault Complex has a zig‐zag geometry in plan view and planar to listric geometry in cross‐section and displacement of up to 5 km. This fault has the Frøya High in its footwall and the southern Halten Terrace in its hanging wall. Restoration of selected structural cross‐sections shows a prominent fault‐parallel ridge, up to 15 km east of the Klakk Fault Complex interpreted as a palaeodrainage divide. This divide separates steep drainages developed along the west‐dipping footwall scarp to the Klakk Fault Complex, from broader, gentler east‐dipping drainages up to ca. 10 km long developed on a back‐tilted dip slopes along the eastern side of the Frøya High and Sklinna Ridge. Progressive headward erosion of active flank catchments was enhanced around topographically elevated structural salients to the point of capturing previous dip‐slope‐directed drainages during the earliest Cretaceous. A network of submarine canyons develop down‐dip of the drainage catchments along the Klakk Fault Complex scarp, whose geometries and length are controlled by their location with respect to the structural salients or recesses, and the presence of fault terraces. The middle Jurassic‐earliest Cretaceous synrift deposits form two seismic sequences that are filled with five distinctive seismic facies that record the evolution from a linked normal fault during rift climax to a high‐displacement stage. During the high displacement stage, exhumed local continental core complexes formed structural salients, separated along strike by structural recesses at the heads of supradetachment basins. Key elements of the high‐displacement fault stage include (i) the development of structural salients at sites of rift climax displacement maxima, (ii) development of supradetachment basins in rift climax displacement minima and (iii) migration of major depocentres away from the centre of rift climax fault segments. We synthesise these observations into a generic tectono‐sedimentary model for high‐displacement faults.

Exploration potential of Papua New Guinea – latest multiclient 3D provides new insight into the underexplored Papuan Basin over the Papuan and Eastern Plateaus

The Gulf of Papua, part of the regional Papuan Basin, is characterised by extensional to passive margin tectonics from the Triassic to Late Oligocene and collisional tectonics from the Late Oligocene until the present day. The basin has experienced rapid clastic deposition in the late Neogene which covers an underlying extensive Miocene carbonate shelf with local bioherms and associated carbonate facies. Underlying the carbonates, an array of deformed and complex rift basins are present, with potential petroleum plays in undifferentiated Mesozoic sediments which onlap basement highs. The Miocene carbonates are the primary exploration targets, along with Pliocene clastic turbidite sediments in the Fly River Delta, Western Gulf of Papua. It is postulated that these were charged by Mesozoic marine–deltaic/terrestrial source rocks that were deposited during the opening of the Coral Sea. The offshore Papuan Basin is underexplored and the Gulf of Papua play types seen in the onshore Aure Fold Belt and offshore Fly River Platform could potentially extend offshore into the Coral Sea area. New broadband Pre‐Stack Depth Migration seismic data covering a part of the Eastern and Papuan Plateaus reveals new insights into the structural development of the region, rift basin geometries, configuration, and association of rift clastics and post-rift carbonates. Internal carbonate features can be observed on the new seismic data set, and new plays in the yet undifferentiated rift sediments of Mesozoic age are proposed in this study.

Geophysical insights into basement faults influencing basin evolution: A case study from Tennant Creek Block, North Australian Craton

Geophysical interpretation and modelling of the poorly exposed Proterozoic basement rocks of the Tennant Creek Block in the North Australian Craton elucidated the distribution, location, connectivity, and overprinting relationships of regional faults. The prominent structural grain within the basement comprises west-northwest-striking faults that commonly bound geophysically defined domains of the Warramunga Formation and felsic plutons of the Tennant Creek Supersuite. Constrained by seismic reflection data, forward models of gravity and magnetic data identified several faults as major structures that link into a shallow south-dipping structure at approximately 20 km depth. The modelling also reveals a 20 km wide and 14 km southward deepening half-graben bounded by a northeast dipping normal fault. The half-graben forms a sub-basin filled with the Ooradidgee Group at the top of the basement stratigraphy. The west-northwest-trending structural grain of the basement is not reflected in the Tomkinson Creek Group in the overlying younger basin. The younger basin has dominant north-oriented faults and basement highs. This suggests basement structures had limited influence in controlling the evolution of younger basins in this region. Our approach has wider relevance, addressing similar challenges encountered in other cratonic areas globally, such as India and Africa, where sedimentary cover obscures critical aspects of basement architecture and tectonic evolution. This underscores the broader significance of our methodology in advancing the understanding of Precambrian terranes.

North Kara and Vize-Ushakov Composite Tectono-Sedimentary Elements, Kara Sea

Abstract North Kara Shelf and adjoining Severnaya Zemlya Archipelago represent a region of a complex tectonic interplay between two large Arctic fold-and-thrust belts: the Early Mesozoic Pay-Khoy-Novaya Zemlya–South Taimyr in the SW and the Late Paleozoic Taimyr–Severnaya Zemlya–in the S-SE and E, and an inferred Devonian/Carboniferous deformation zone in the N. Contractional deformations of different ages formed a series of basement highs and inverted sediment-cored swells that separate deeply subsided sedimentary basins. In the framework of the volume's terminology, this region consists of two composite tectono-sedimentary elements (CTSE), the Vize-Ushakov (VU CTSE) and North Kara (NK CTSE). The principal difference between these elements is in the age of underlying basement rocks, which, in turn, determines the stratigraphic span of the overlying sedimentary successions. In this chapter we characterize the geology of this remote Arctic region based on the available seismic data supported by the published results and our own studies.

Is the Yermak Plateau a continental fragment from North America? Constraints from Cretaceous and early Eocene magmatic events

The Yermak Plateau (YP) north of Svalbard is a prominent bathymetric feature in the Eurasia Basin of the Arctic Ocean, forming the northwesternmost margin of the Eurasian plate. Seismic data indicate that the YP comprises continental basement; however, little is known about its geology. New petrographic, geochemical, Sr–Nd isotopic, and Ar–Ar geochronological data were obtained on rock fragments, which were previously recovered from basement highs of the northeastern and southwestern YP and are dominantly of magmatic origin. These new data combined with available literature data, and comparisons with volcanic and sedimentary rocks from onshore and offshore areas adjacent to the YP indicate that the northeastern YP and the southwestern YP are different regarding their geological evolution. The southwestern YP comprises an alkaline basaltic suite for which an Ar–Ar biotite age of 51 Ma was previously reported. The suite was formed in a continental extensional regime offshore northern Svalbard. Associated sedimentary rocks (sandstone, several limestones) show petrographic similarity with rocks of the Devonian Old Red Sandstone on Svalbard. From the northeastern YP, in contrast, we recovered mildly alkaline basaltic rocks with mid-Cretaceous Ar–Ar ages (102 ± 3 and 98 ± 3 Ma). The rocks show certain geochemical characteristics (partial enrichments of P, Ba, and Eu), which overlap with similar-aged Cretaceous basaltic rocks from northern Ellesmere Island of Canada and North Greenland. We suggest that the northeastern YP is a continental fragment derived from the North American plate, which was separated from the conjugate Morris Jesup Rise and juxtaposed to the geologically distinct southwestern YP by the propagation of the Gakkel Ridge spreading center since the early Oligocene.Graphical

Geochemical and Isotopic (Nd, Sr) Tracing of the Origin of REE Enrichment in the Cambrian Georgina Basin Phosphorites

AbstractPhosphorites of the Georgina Basin (northern Australia) are an established economic source of phosphate and have recently been recognized to be a potential source of rare earth elements (REE). Previous bulk‐rock geochemistry work focused on the eastern margin of the basin revealed that phosphorites from the southern region have significantly higher (up to 0.5%) REE contents than equivalent prospects further to the north. In this study, we examine the origin of REE enrichment in the Georgina Basin phosphorites using an integrated geochemical, petrographic, mineralogical, and isotopic (Sm‐Nd and Sr) approach. The trace element geochemistry of primary phosphate minerals is consistent with a seawater origin for the REEs in phosphorites, with minimal input from the underlying basement rocks. Variations in total REE concentrations are controlled largely by the seawater composition, depositional environment, and the texture of the phosphorite (grainstone vs. mudstone phosphorite). Grainstone phosphorite yields on average higher concentrations of REEs compared to the mudstone phosphorite, likely due to the higher surface‐to‐mass ratio of the grainstone phosphorite resulting in the uptake of higher concentrations of REEs during coastal reworking. The formation of the low‐REE northern and central deposits took place during early transgression in oxidized, shallow supratidal to subtidal platformal environments. During the subsequent main transgressive event, fault‐controlled topography in the southern domain allowed phosphogenesis along basement highs to be facilitated along a redox boundary, where more saline and REE‐rich deep anoxic bottom waters, along with Fe‐cycling and bacterial mediation played a major role in the enrichment of REEs in the southern phosphorites.

Structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil)

We present a multi-scale conceptual model based on structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil). We assembled the model from a regional 2D seismic reflection line integrated with potential gravimetric field data and a local 3D seismic reflection volume integrated with well data (lithologies and in situ stress). (i) The geochemical isotope range of δ13CCO2 falls mostly within −7‰ and −5‰ and shows relatively high values for 3He/4He represented by an R/Ra rate of up to 5.60, indicating CO2 mantle generation and degassing. (ii) Seismic interpretation feasibly validated by potential gravimetric responses of the crustal structure (Moho discontinuity) show CO2 migration through deep-seated faults in a region of highly stretched continental crust with oceanward mantle uprising. (iii) Early Cretaceous basement highs generated in an obliquely syn-rift faulting system control CO2 accumulation in thermogenic travertines (hydrothermal carbonate reservoirs of continental lakes), and Aptian evaporites subsequently trap it.

Results of high-resolution technologies applied in the acquisition of seafloor information in the Colombian Caribbean Sea

We expound on a hydrographic and geophysical survey in the south of the Archipelago of San Andres, Providencia, and Santa Catalina (ASAPSC) in the Colombian Caribbean Sea. In 2017 and 2018, high-resolution data were acquired from the Caribbean Center for Oceanographic and Hydrographic Research (CIOH). The hydrographic data were processed and filtered per International Hydrographic Organization (IHO) standards and the geomagnetic data were processed, corrected, and filtered to improve the interpretation. We could then characterise the submarine relief, analyse the geomagnetic anomalies in the area, and identify different relief forms of volcanic origin. Regarding geomagnetic work, a spectral analysis of the anomalies revealed highly magnetic bodies in deep water and residual magnetic anomalies strongly related to the seabed morphology. Magnetic highs associated with the basement highs and volcanic cones were detected. Complementary spectral analysis showed that the shallowest magnetic sources were in the first 500 m, whereas the deepest magnetic sources were between the depths of 4 and 5 km. The geomagnetic data were also validated using Euler deconvolution analysis, which confirmed these magnetic anomalies. Thus, a direct relationship between the magnetic anomalies and bathymetry of the seafloor was confirmed, reinforcing the theory of the volcanic origin of these islands.

Differential Uplift Triggered Basin‐And‐Range System: Evidence From Low‐Temperature Thermochronology in Eastern NE Asia

AbstractSince the Mesozoic, eastern NE Asia has experienced multiple tectonic events, resulting in a complex structure and forming one of the world's largest Meso‐Cenozoic lacustrine basin systems. Presently, basin evolution models require further elucidation regarding the simultaneous generation of diverse rift basins and the potential impact stemming from the closure of the Mudanjiang Ocean, whose oceanic closure demarcated the boundary between the Songliao Basin and the eastern basins, raises questions about its influence on the development of the basin‐and‐range system. To address these questions, we augment new low‐temperature thermochronological data on basement highs separating the eastern NE Asia basins to investigate the shallow‐deep coupling process of tectonic evolution since the Mesozoic. The new cooling age pattern shows non‐overlapping and spatial differences among major basement highs. Inverse thermal modeling revealed five‐stage cooling episodes among the basement highs, but with different onset and cooling rates of each episode, indicating a significant differential uplift mode. A major reburial stage was identified throughout eastern NE Asia during the mid‐Cretaceous. Compiling cooling age patterns and inverse thermal modeling, we note the existence of a proto‐basin covering an area much larger than the previously contemplated “Pan‐Sanjiang” Basin. In general, our study indicates the final closure of the Mudanjiang Ocean occurred at ca. 150–140 Ma. Since the Early Cretaceous, with changes in the subduction direction, two‐stage flat slab subduction of the Paleo‐Pacific plate and the consequent subduction of the Pacific plate co‐dominated the basements' differential uplift and the formation of the eastern NE Asia basin‐and‐range framework.

The formation of small- to medium-sized Cenozoic basins drained by the Tagus river (Portugal and Spain). Example of sedimentary basins formed by crustal-scale folding

The Tagus River drains several small- to medium-sized Cenozoic sedimentary basins situated within a tectonic depression flanked by two basement highs. These basins were formed during the initial stages of the Cenozoic compressive event that affected the interior of the Iberian Peninsula, a consequence of the Africa-Eurasia convergence. These basins exhibit distinct characteristics typical of compressional sedimentary basins, being flanked by thrusts and reverse faults. The mechanism of their formation can be attributed to crustal-scale folds, showcasing a notable and possibly exceptional instance of compressional sedimentary basins created through this tectonic mechanism.

Facies analysis and sequence stratigraphy of shallow marine, coarse‐grained siliciclastic deposits in the southern Utsira High: The Late Jurassic intra‐Draupne Formation sandstones in the Johan Sverdrup Field (Norwegian North Sea)

AbstractThin, condensed coarse‐grained shallow marine successions can be difficult to describe and interpret, especially in the subsurface since the recognition of finer‐grained intervals, typically associated with sequence stratigraphic surfaces, is challenging. This lack of mudstones and siltstones means that they also typically make excellent reservoir intervals. The Oxfordian to Volgian intra‐Draupne Formation sandstones in the Johan Sverdrup Field, southern Utsira High, represent such a system. This study presents a new sequence stratigraphic model for the Johan Sverdrup Field that unravels the detailed depositional history of the succession and places its formation within a regional Late Jurassic tectonostratigraphic framework. The intra‐Draupne Formation sandstones comprise four parasequences deposited following a regional Kimmeridgian marine flooding event. Sediments were mainly supplied through West‐derived fan deltas from the Haugaland High and NW‐SE‐directed tidal currents reworking the Augvald Graben and the Avaldsnes High at the East. The oldest parasequence shows a distinctive suite of facies consisting of fine‐grained and mud‐rich bioturbated sandstones deposited in a semi‐restricted lagoon. Subsequent parasequences lack fine‐grained sediments and are dominated by bidirectional cross‐stratified, very coarse‐to coarse‐grained sandstones and gravels deposited in a tidal strait. A progressive reduction of fault‐related subsidence in the Middle Volgian along with Late Volgian‐Ryazanian sea‐level rise and inversion of pre‐existing structures promoted backstepping of the feeder systems, sediment starvation and the progressive deposition of the black and green‐red shales of the Draupne and Asgard formations. The results of this study account for features previously unidentified in the Johan Sverdrup Field and which have implications for understanding the deposition of coarse‐grained shallow marine successions around the Utsira High and other transgressed basement highs.

An early Cambrian post-rift basin within the Baltica–Iapetus passive margin (north-central Scandinavian Caledonides)

Field data from recent geological mapping over a major part of the north-central Scandinavian Caledonides combined with published information give a detailed view of early Cambrian basin successions, comprising the Gärdsjön formation (Gf; Jämtland supergroup) in the Lower Allochthon and autochthonous equivalents (Dividal Group). The Gf comprises ten units of sandstone and siltstone or mudstone (Gf I—X, > 300 m thick). Green siltstones with red layers (Gf VI, c. 521 to 519 Ma) and green–grey siltstones at the top (Gf X, c. 516.5 to 513.5 Ma) are regional key horizons. Gf V, VI, VII, IX, and X deposition may be related to eustatic events. Restoration of Caledonian shortening reveals a major “Hornavan-Vattudal basin” (HVB; > 330 km NW–SE, > 400 km NE-SW) between the Grong–Olden culmination in the S and the Akkajaure–Tysfjord culmination in the N. Published zircon ages imply the latter separated the HVB from those shed from the Timan orogen in the N. The eastern basin margin straddles the present Caledonian erosional margin. Basement highs identified here within the Nasafjället, Bångonåive, and Børgefjellet “basement” windows define the western margin. They separate the HVB from the outer shelf towards the Iapetus Ocean in the W. The onset of sedimentation is time-related with E–W extension at c. 544–534 Ma. NNE–SSW-directed extension occurs after c. 518 Ma, perhaps related with Timan late-orogenic extension. The HVB is distinctly younger (c. 535–513.5 Ma) than Rodinia break-up and Iapetus ocean formation (> 550 Ma), comparable with post-rift basins in inner parts of modern passive margins.Graphical

The cryptic stratigraphic record of the syn‐ to post‐rift transition in the offshore Campos Basin, SE Brazil

AbstractRift basins typically comprise three main tectono‐stratigraphic stages; pre‐, syn‐ and post‐rift. The syn‐rift stage is often characterised by the deposition of asymmetric wedges of growth strata that record differential subsidence caused by active normal faulting. The subsequent post‐rift stage is defined by long‐wavelength subsidence driven by lithospheric cooling and is typified by the deposition of broadly tabular stratal packages that drape any rift‐related relief. The stratigraphic contact between syn‐ and post‐rift rocks is often thought to be represented by an erosional unconformity. However, the late syn‐rift to early post‐rift stratigraphic record is commonly far more complex since (i) the associated tectonic transition is not instantaneous; (ii) net subsidence may be punctuated by transient periods of uplift; and (iii) strain often migrates oceanward during rifting until continental breakup is achieved with crustal rupture. Previous publications on the Eastern Brazilian marginal basins have not historically used the tripartite scheme outlined above, with the post–pre‐rift interval instead being subdivided into rift, sag and passive margin tectono‐stratigraphic stages. In addition, the sag stage has been previously described as late syn‐rift, early post‐rift or as a transition between the two, with the passive margin stage being equivalent to the classically defined post‐rift, drift stage. Two (rather than one) erosional unconformities are also identified within the rift‐to‐sag succession. In this work, we use 2D and 3D seismic reflection and borehole data to discuss the expression of and controls on the syn‐ to post‐rift transition in the shallow and deep water domains of the south‐central Campos Basin, south‐east Brazil. We identified three seismic–stratigraphic sequences bounded by unconformities, named lower and upper pre‐salt and salt. The lower pre‐salt interval is characterised by wedge‐shaped packages of reflections that thicken towards graben and half‐graben‐bounding normal faults. This stage ends with the development of an angular unconformity, inferred to form as a result of the onset of the oceanward migration of deformation. The upper pre‐salt is typically defined by packages of subparallel and relatively continuous reflections that are broadly lenticular and thin towards fault‐bound basement highs, but that locally contain packages that thicken against faults. The pre‐salt to salt contact is defined by an erosional unconformity that is largely restricted to basement highs, and which is inferred to have formed due to base‐level fall and uplift associated with local fault reactivation, resulting in the formation of channels of possible fluvial origin. Based on its geometries and seismic facies, we conclude that the lower pre‐salt interval is syn‐rifting and syn‐tectonic, deposited during active continental extension and upper crustal faulting affecting the entire evolving margin, whereas the overlying upper pre‐salt is syn‐rifting and post‐tectonic in the Campos Basin, deposited when extension and faulting had migrated seaward to the future location of the spreading centre. The results of our study support the arising notion that the syn‐rift sequence does not only display syn‐tectonic sedimentary packages, and thus the tripartite tectono‐stratigraphic model for rift development is too simplistic and cannot be applied when assessing rifts in the context of the regional development of continental margins.