Salt Tectonics Research Articles

Secondary minibasins in orogens: Examples from the Sivas Basin (Turkey) and the sub-Alpine fold-and-thrust belt (France)

Inherited and syn-compressional salt-controlled structures in orogens are difficult to distinguish from compressional features because classical salt features (halokinetic growth strata, salt welds) can be misinterpreted as shortening-related structures (syn-kinematic growth strata, tectonic unconformities). In the Sivas basin in Turkey, syn-kinematic salt diapirism entirely shaped the architecture of the foreland and allowed the development of primary and secondary minibasins. One of the most impressive, the Tuzlagözü minibasin, formed by sinking into a salt feeder while being upturned due to compression. This minibasin has been nicknamed the “Turkish Vélodrome” because of its similarity with the well-known Vélodrome structure in the French Alps which shows growth strata and successive unconformities, commonly interpreted as foreland growth syncline features. Based on structural field descriptions and geometric analogies with the Tuzlagözü minibasin, we revisit the Vélodrome as a salt-related minibasin. We argue that the Vélodrome developed as a secondary minibasin above an Oligocene aerial salt glacier: sheet of Upper Triassic evaporites extruded during Alpine orogeny. This new interpretation enables to propose a revised evolution of the sub-Alpine fold-and-thrust belt. In line with other recent works, this study emphasises once again the important role of salt tectonics in the evolution of the sub-Alpine fold-and-thrust belt.

Tectonics characterization of the salt influenced structures: Insights from a seismic data interpretation in the eastern part of the Persian Gulf, Iran

Salt tectonics can influence the petroleum systems via the development and distribution of ductile migration pathways through the structural and stratigraphic traps. In this study, the effects of base-salt relief on the development of salt décollement horizon and the salt-influenced structures were investigated in the eastern part of the Persian Gulf, NE of the Afro-Arabian Plate. For this purpose, data from hydrocarbon exploration wells and seismic lines of the region have been used. To set the position in a time of the main stratigraphic and acoustic markers, and to compare well and seismic data, the processing of synthetic seismogram and the conversion of logs in the time domain has been performed for the selected well.Interpretation of seismic sections revealed the occurrence of a NE-SW trending horst structure and development of a natural model of the salt tectonics on its overburden of the Oligocene to Miocene sedimentary layers. The presence of Oligo-Miocene saliferous beds as a décollement has provided suitable conditions for overburden sediments gravitational sliding and development of thin-skinned extension which started during the Miocene time and it was evolved up to the present time. This condition controlled the geometry of salt-influenced structures in the eastern parts of the Persian Gulf and resulted in the formation of special structural features namely hereafter as Curly Bracket Structures in the overburden.

A two‐stage, fault‐controlled paleofluid system at the southern termination of the Gypsum Valley salt wall, Paradox Basin, Colorado, USA

AbstractThis study combines field structural analysis with thin‐section petrography, U‐Pb dating, and strontium, carbon and oxygen isotopic analysis of calcite fracture fills to constrain the evolution of the 2‐5 km scale paleofluid system around the faulted, plunging fold nose comprising the southern termination of the Gypsum Valley salt wall in the Paradox Basin, U.S.A. Brittle deformation in this region began with the formation of a down‐to‐the‐northeast, counter‐regional fault and then progressed into jointing and faulting in a radial pattern, followed by jointing in a concentric pattern. Coupled with increases in fracture abundance toward the faults, multiple stages of mineralization suggest that the faults served as efficient and long‐lived conduits for vertical fluid migration. Although fracture cement textures and calcite colour are variable throughout the area, the distribution of these characteristics does not correlate with fracture orientation, relative age, stratigraphic or structural position. Irrespective of the type of calcite comprising the fracture cements, δ13C values average near −7‰ (VPDB), whereas δ18O values cluster into groups whose averages are roughly 6‰ apart, with the more negative grouping stratigraphically restricted to fracture cements in Jurassic rocks. The stratigraphic segregation of δ18O values suggests the paleofluid system contained two distinct paleofluids, a more recent one comprised of meteoric waters and an older one comprising brine that originated in Pennsylvanian strata. 87Sr/86Sr ratios in fracture‐filling calcite cements indicate that the older fluid underwent fluid‐rock interaction with Permian strata and that this evolved fluid migrated upwards along the faults until the Triassic or Jurassic. Thereafter, fluid migrating along the faults was more meteoric and appears to have migrated downward along the faults, where it interacted with Permian strata. Consistent U‐Pb dates from carbonates precipitated from the older fluid suggest this stage of the paleofluid system was active around 240 Ma. Local burial history models and published temperatures for fracture cements elsewhere in the basin suggest the younger stage of the paleofluid system occurred during the Latest Cretaceous to Oligocene. This study highlights the spatial and temporal complexity of fluid systems in the vicinity of salt structures and emphasises the need to interpret them through careful integration of high resolution stratigraphic and structural data in the context of evolving salt tectonics.

Salt tectonics of the offshore Tarfaya Basin, Moroccan Atlantic margin

Salt tectonics play a critical role on passive margins evolution controlling aspects like structural style, subsidence patterns and thermal history, amongst others. The salt-bearing Atlantic passive margin of Morocco hosts one of the oldest stratigraphic records documenting the opening history of the Central Atlantic. However, the available seismic data is scarce and some offshore basins are still poorly studied, particularly in southern Morocco. Through the interpretation of an unpublished 2D/3D seismic dataset from the offshore Tarfaya Basin (SW Morocco), this study aims to highlight the key events that conditioned the evolution of this salt-bearing basin. From proximal to distal regions, the structural style of the basin is characterized by expulsion rollovers and salt-cored anticlines delimited by primary welded surfaces, evolving to buried salt sheets surrounded by thick minibasins and finally, diapirs actively deforming the modern seabed. From Late Triassic to Early Jurassic times, salt was deposited with a basinward thickening wedge-shaped geometry on a narrow trough developed over thinned continental crust. During the Jurassic, sedimentation and associated salt withdrawal triggered early salt deformation. Gravity gliding is a common process in salt-bearing passive margins that requires an originally continuous autochthonous salt layer with a minimum slope angle and longitude to thickness ratio of the overburden. However, in the Tarfaya Basin, the narrow geometry of the salt-bearing depocenter hampered this process. Early salt tectonics was probably triggered by slope progradation during the Early Jurassic. During the Early Cretaceous, the progradation of the Tan-Tan Delta promoted a continued basinward expulsion of salt, the development of a local salt-detached gravitational system and the proximal extrusion of salt sheets. Finally, from Late Cretaceous to the Present-day, shortening related to the convergence between Africa and Eurasia resulted in thick-skin inversion and the rejuvenation of precursor salt structures.

Evolution of rift-related cover-basement decoupling revealed by brecciation processes in the eastern Pyrenees

Breccias associated with tectonic, fluid and sedimentary evolution of rifted margins can provide information on a variety of processes reflecting the modes of extension. In this paper, we analyse the numerous breccias exposed in the Agly Massif that was part of the European side of the Cretaceous rift now inverted in the eastern Pyrenees. Using a combination of petrologic and sedimentologic analyses, field-based structural study, and multivariate analysis of clast shape and diversity, binding lithology and size, and breccia fabrics, we distinguish 5 types of breccias reflecting depositional, tectonic, and salt-related processes. The integration of these processes in the tectonic history of the eastern Pyrenees confirms the attribution of these breccias to the Cretaceous rifting. We emphasize the major role played by the evaporitic Triassic particularly during the first stages of rifting as a major decoupling level at the basement/cover interface. Salt tectonics and shearing assisted by the circulation of fluids are reflected by hydrofracturing at the base of the Mesozoic cover. As this weak mechanical layer is later extracted as extension increases, a brittle detachment system developed along the cover-basement interface to exhume of deep crust and mantle. The relationships between brecciation and Cretaceous extension in the Pyrenees argue for a mixed mode of rifting associated with ductile and brittle deformation during the formation of the hyper-extended rift domain.

Petroleum system analysis of the Paleozoic series in the Fars Platform of Iran

The Paleozoic petroleum system of the Fars Platform of Iran is analyzed using 1D petroleum system models of 17 deep wells drilled in the study area. The main aim was to investigate the hitherto undocumented influence of salt tectonics on the essential elements and processes of this petroleum system. For the first time, a better understanding about the geohistory evolution of this prolific petroleum system is provided over the Fars Platform. Modeling results indicate that hydrocarbon generation from the Silurian source rock was associated with salt withdrawal synclines. Salt tectonics further controlled the entrapment styles and the geometric distribution of carrier/reservoir rocks and their overlying evaporitic seals. Our findings provide new insights for identifying the remaining exploration potential in the Paleozoic petroleum system of the Fars Platform and suggest that the role of salt tectonics should be considered in the future basin and petroleum system modeling studies in this area.

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution.

Strike-Slip Salt Tectonic Structures Associated with Basement-Involved Faults: Analogies between the Frade Field and Surrounding Region in the Campos Basin, Offshore Brazil and the Zagros Fold-and-Thrust Belt, Iran

Strike-Slip Salt Tectonic Structures Associated with Basement-Involved Faults: Analogies between the Frade Field and Surrounding Region in the Campos Basin, Offshore Brazil and the Zagros Fold-and-Thrust Belt, Iran

Deformation of the Dachstein Limestone in the Dachstein thrust sheet (Eastern Alps, Austria)

Abstract Deformation affecting the Upper Triassic Dachstein Limestone has been analyzed in the Dachstein thrust sheet, the uppermost thrust unit of the central Northern Calcareous Alps (Eastern Alps). Different scales of deformation are discussed, from kilometer-scale thrusting down to folds in the order of tens of meters to meters. Observations are based on both conventional outcrop observations and on digital fieldwork performed on drone-captured virtual outcrops and on GoogleMaps 3D terrain renderizations. The structures observed were formed at different times and document the following events: 1) Late Triassic syn-depositional instability and slumping; 2) Late Triassic syn-depositional growth of the Hallstatt diapir; 3) Late Triassic syn-depositional, salt-driven, extensional faulting; 4) Jurassic-age re-activation of extensional faults; 5) (presumably) Early Cretaceous shortening in both east-west and north-south directions; and 6) (presumably) Late Cretaceous extensional re-activation of faults. The structures and their origin have a bearing on the interpretation of the tectonic evolution of the Dachstein thrust sheet, highlighting the potential relevance of salt tectonics in controlling its structure.

New Evidence of the Holokinetic Sequences around Suakin-1 and -2in the Sudanese Red Sea Area Using Integrated Geophysical Interpretation

Suakin-1 and Suakin-2 wells are in the Sudanese Red Sea segment where the hydrocarbon generation had been proved by previous studies, however, no reasonable reserve was evidenced due to the complexities of the surrounding salt structures. Six seismic lines were tied to Suakin-1 and -2 to delineate the controlling salt tectonics. The salt evacuation (Roho) and other salt bodies were recognized and matched with similar salt structures in analogous stratigraphic conditions as the Gulf of Mexico and Angola margin. While a previous inconsistent interpretation in the study area marked the high amplitude horizon of the Lower Zeit formation as the top of the Dungunab formation. Three seismic features indicated the presence of salt dome (autochthonous): velocity pull-up, dragging of the sedimentary layers forming mini basins around the third feature, which is the relative transparency of the seismic signal in two piercing like bodies. This interpretation similarly demarcated that the salt escaped east-wards, thus the mapped welded salt is believed to be formed after the salt evacuation. A 3D seismic with a far offset and wide range of azimuth is recommended for detailed imaging.

Distribution patterns of platinoids in halogen rock masses in the Southern Urals

The Early Permian halogen formations are widely spread in the territory of the Orenburg Region. The thickness of the rock-bearing strata grows southward and southeastward, and reaches maximum values in the Pre-Ural downfold area. This study focuses on distribution patterns of noble metals in halogen strata in the Southern Urals. Geochemical sampling of the test halogen strata in the Orenburg Region was composed of the Iletsk salt deposit samples and cores from the oil and gas reservoirs in the Sakmara–Ilek, Southern Buzuluk, Sol-Iletsk and Teplovsko–Derkul (north wall of Pre-Caspian Syncline) provinces, and was supported with the relevant geological documentation. The results obtained by the authors in detection of noble metals in rock salt point at their nonuniform distribution. The highest concentrations of platinum (3128 mg/t) and gold (2064 mg/t) occur at halite in the Peschanoe oil reservoir. The maximum silver content (3079 mg/t) is observed in rock salt in the Vershinovskoe oil reservoir. The highest palladium concentration (248 mg/t) is a feature of the Akobinsk gas reservoir. The lowest concentrations of platinum (43 mg/t) and palladium (3 mg/t) are found in the samples from the Iletsk salt deposit. The concentrations of gold, platinum and palladium vary the most of halite in the oil and gas reservoir. In the Iletsk rock salt samples, the ranges of the platinum and palladium concentrations are lower by an order of magnitudes. This may be reflective of the presence of a geochemical barrier at the salt–oil interface. It is found that all noble metals have higher content. The detection of noble metals in the salt-bearing strata in the Orenburg’s Urals have the applied relevance as the obtained results demonstrate interaction between the increased concentration of metals, halogenesis and salt tectonics phenomena.

Geomorphological imprint of opposing ocean bottom currents, a case study from the southeastern Brazilian Atlantic margin

The combination of oceanographic processes and a negligible fluvial terrestrial sediment contribution makes the Santos Basin margin (southwestern Brazil) a unique environment for studying thew morphological imprint of bottom currents on the seafloor morphology. We divide the seafloor into four domains: outer shelf, upper, middle, and lower slope, based on an analysis of sub-bottom Chirp profiles, multichannel seismic data, hydrographic data, numerical simulation outputs, and a seismic-derived bathymetry map. A clinoform at the outer shelf and upper slope, topped by erosional morphologies, developed below the SW-flowing Brazil Current (BC). At the bottom of the upper slope, a rugged and mounded surface around 600 mbsl coincides with the current flow reversal to the underlying NE-flowing Intermediate Western Boundary Current (IWBC). The smooth middle slope exhibits sheeted contourites interrupted by occasional channel-and-mound complexes.From the middle slope base, salt tectonics becomes a major player in setting the seafloor morphology. The slope-parallel Santos Channel develops above a subsurface diapir and the Cabo Frio Fault. The latter divides between the landward gap in subsurface Albian salt and the basinward rich diapir realm. Hence, the lower slope is reworked by the SW-flowing Deep Western Boundary Current (DWBC); however, salt tectonics dictates its morphology. Our study consists of a unique example for studying the morpho-sedimentary imprint of a counter-current flowing on the continental slope and also for exploring the interaction between bottom flows with a suite of endogenic processes, such as salt tectonics, fluid flows, and mass movements.

Mechanical relationship between strike-slip faulting and salt tectonics in the Northern Tunisian Atlas: The Bir-El-Afou salt structure

Wide variety of salt structures –from typical diapirs to large allochthonous salt sheets– are recognized in the Northern Tunisian Atlas. In this study, we present for the first time the presence of a diapiric structure, Bir-El-Afou Salt Structure (BEASS), which has formed in response to strike-slip tectonics. Structural mapping, mesoscale field observations, fault kinematic analysis, and interpretation of gravity data are used to interpret BEASS, which is currently associated with a restraining bend. The present-day NNE– trending restraining stepover is formed as a result of E-W dextral fault system. We assume that the restraining stepover is a product of the inversion of a pre-existing pull-apart basin that formed during the Cretaceous South Tethyan extension. Tectono-sedimentary relationship is used to argue for an early Cretaceous NW-SE regional extension and local transtension. During the opening of the pull-apart basin, salt movement took place in the southwestern corner of the basin which must have thinned due to its extension facilitating diapiric emplacement. Diapir growth during Aptian was followed by brief salt flow from the diapir crest towards the basin forming a salt sheet. A period of Tertiary contraction resulted in the inversion of the pull-apart basin and consequently, squeezing of BEASS.

Thin-skinned extensional salt tectonics, counterregional faults, and the Albian Gap of Brazil

Thin-skinned extension detached on salt is mostly well understood. Faults may dip basinward or landward (counterregional) and intersect the top salt or equivalent weld at cusps/rollers. The polarity of faulting may switch over time, and with sufficient extension, the oldest suprasalt strata separate into rafts with intervening younger depocenters. Salt may break through extensionally thinned overburden to grow as passive diapirs, which can widen during ongoing extension but collapse if the salt supply cannot keep pace with extension. Counterregional normal faults are the most debated structures as they may form by: extension, with the footwall moving basinward relative to the hanging wall; or progressive basinward salt expulsion but only vertical movement of the overburden. Extensional counterregional structures are characterized by: (i) strata that predominantly thicken into the bounding structures, but with areas of basinward thinning due to fault/salt geometry and displacement transfer; (ii) highly elongate plan-view geometries; and (iii) the presence of coeval distal translation and contraction. The most controversial counterregional system is the Albian Gap of the Santos Basin, Brazil. We use the criteria above to argue that it formed by post-Albian thin-skinned extension, with little/no extension during the Albian. This contrasts with the Campos Basin along strike to the NE, where Albian and later extension were accommodated on basinward-dipping faults, and is explained by the rift architecture and consequent base-salt dip. The Aptian salt dipped basinward in the Campos Basin and extension was therefore triggered by Albian gravity gliding. However, the base salt dipped landward over a large area of the northern and central Santos Basin, so that Albian gravity gliding was minimal and Late Cretaceous progradational loading drove gravity spreading and the development of the counterregional system. Accommodation for sediment within the Albian Gap was provided in part by stretching and consequent thinning of the underlying salt.

Sub-areas in steep rock salt formations – what do we know about the geological composition of North German salt structures?

Abstract. Suitable host rocks for a repository for high-level radioactive waste (HLW) in Germany include not only clay and crystalline rocks but also rock salt formations in so-called flat and steep bedding (StandAG, 2017). Favourable repository relevant properties of rock salt are, e.g. the high heat conductivity, low porosity and permeability, and its viscoplastic deformation behaviour. Thicker salt deposits can be attributed to the formation of approx. 700 salt structures that have formed under various geological conditions in the North German Basin (NGB) over the last 250 million years. According to their shape and genesis, salt structures are classified as salt pillows (considered as flat bedding) or salt diapirs (steep bedding). Out of a total of 74 sub-areas in rock salt, 60 sub-areas in salt diapirs consisting of Permian evaporates were identified within the first phase of the German site selection procedure (BGE, 2020). At the current stage of the site selection process, a conservative approach has been adopted and the internal structure of the salt structures have not yet been taken into account for further classification of the identified sub-areas. However, the interior of the salt structures not only consists of rock salt but also of varying proportions of clay, carbonate and anhydrite rocks, as well as potassium salts formed by progressive evaporation of marine brines. Multi-phase salt tectonics has led to the folding of these differently composed layers and to complex internal structures. Therefore, detailed knowledge of the salt structure compositions is necessary to identify suitable rock salt areas for the designation of the containment providing rock zone. As a result of decades of research through extensive salt and potash mining, cavern storage and exploration for final waste disposal, Permian salt rocks represent a well-studied host rock in Germany. The use of different exploration methods and multidisciplinary data evaluation have led to a comprehensive understanding of the internal composition of some well-studied salt structures. Systematic studies have shown that several factors have influenced the formation of salt structures as well as their external shapes, sizes, and spatial distribution (e.g., Pollok et al., 2020). Furthermore, the amount and distribution of suitable host rocks varies greatly in different salt structures and is closely related to their internal structure. Since the interior of salt structures has not been considered in the site selection process so far, a classification of salt structures (or sub-areas) into certain types with varying internal composition and complexity is presented. By examining their lithofacial composition, genesis, and structural geological position within the basin, it is possible to narrow down these types to specific areas in the NGB. Without the acquisition of additional exploration data in this phase of the site selection process, this salt structure classification provides important data for the legally demanded assessment of the overall favourable geological situation.

Tectono-stratigraphic evolution of the Permo-Triassic succession in the fars platform of Iran: Implications for future exploration of the Paleozoic petroleum system

• Salt tectonics has influenced the Paleozoic petroleum system of the Fars Platform. • Withdrawal synclines control the thickness/facies of the Permo-Triassic sequences. • Considerable potential exists for stratigraphic traps in this petroleum system. Using a new tectono-stratigraphic framework, this paper investigates the Paleozoic petroleum system of the Fars Platform (Southwest Iran). The essential elements and processes of this prolific petroleum system were strongly impacted during both syn -rift and post-rift phases. Extensive erosion during the syn -rift phase led to the local removal of the Latest Ordovician-Earliest Silurian Sarchahan source rock and deposition of continental clastic sediments which acted as important carrier rocks during hydrocarbon migration. During the post-rift phase, syn -depositional salt tectonics predominantly controlled the stratigraphy of the Permo-Triassic strata. The salt- withdrawal synclines not only controlled the facies and thickness of the reservoir-seal couplets, but also promoted hydrocarbon generation from the Silurian source rock in the resulting depocenters. These hydrocarbons could migrate toward structurally higher areas through areally-continuous carriers of the Lower Permian Faraghan Formation and from huge accumulations below efficient evaporitic seals. This model can better explain the present-day distribution of Paleozoic accumulations in the Fars Platform and sheds new light on the remaining potential (in stratigraphic traps) within the Paleozoic petroleum system.

The Merluza Graben: How a Failed Spreading Center Influenced Margin Structure, and Salt Deposition and Tectonics in the Santos Basin, Brazil

AbstractThe relative timing between crustal extension and salt deposition can vary spatially along passive margin salt basins as continents unzip, or as the locus of extension shifts toward the embryonic ocean spreading center. Determining the relative timing of salt deposition, rifting, and seafloor spreading is often problematic due to the diachronous nature of rifting, the ability of salt to fill pre‐existing topography, and the subsequent flow and deformation of that salt. We here use 2D PSDM seismic data and structural restorations to investigate the Merluza Graben, a large rift‐related depocentre located in the southern, most proximal part of the Santos Basin, Brazil, along‐strike of a failed spreading center, the Abimael Ridge. The graben is defined by up to 3.5 km of base‐salt relief along its basinward‐bounding fault and internal base‐salt horsts that are up to 1 km high. This compartmentalizes deformation, producing intra‐graben extensional and contraction salt structures, ramp‐syncline basins, and expulsion rollovers, resulting in a remarkably different salt‐tectonic structural style to that seen in the adjacent areas. We also conduct structural restorations to analyze the spatial and temporal evolution of salt‐tectonic structural styles and the relationship this has to potential prolonged crustal extension in the Merluza Graben. This approach further constrains local variations in the relative timing of rifting and salt deposition, and the impact this has on salt tectonics along the margin. The results of our study can be applied to better understand the tectono‐stratigraphic development of other salt‐bearing rifted margins.

Chemostratigraphy of Cumberland Group (Pennsylvanian) strata influenced by salt tectonics, Joggins Fossil Cliffs UNESCO World Heritage Site, eastern Canada

ABSTRACT The Pennsylvanian stratigraphy of the western Cumberland Basin has been influenced by salt tectonics, specifically the formation of the Minudie Anticline, a salt wall. South of the Minudie Anticline, along the shoreline of the Joggins Fossil Cliffs UNESCO World Heritage Site, the post–Boss Point Formation succession comprises an ∼ 3 km succession of strata assigned to the Little River, Joggins, Springhill Mines, and Ragged Reef formations. North of the Minudie anticline, the Grande Anse Formation lies in angular unconformity on the Boss Point and basal Little River formations. Biostratigraphic studies have not been able to discern whether the Grande Anse Formation is equivalent to all, or just one, of the Joggins to Ragged Reef units south of the salt wall (the Minudie Anticline). To further investigate the relationship of the Grande Anse Formation with the units along the Joggins shoreline, forty sandstone samples from the post–Boss Point Fm strata were selected for a chemostratigraphic study, using inductively coupled plasma mass spectrometry (ICP-MS) to determine major-element compositions. Transformed ICP-MS data, subjected to a Kruskal-Wallis test and post-hoc tests, show that there is no significant difference between Grande Anse and Ragged Reef formations in the mean values of almost all analyzed elements. In contrast, there are significant differences when comparing these two units and the older Little River, Joggins, and Springhill Mines formations in the case of elements usually encountered in detrital mineral phases (Si, Al, Ti, Na, and Fe). Sandstones of the Grande Anse and Ragged Reef formations show greater compositional maturity than the Little River, Joggins, and Springhill Mines formations. This trend is explained by a gradual overall change in paleoclimate from semiarid conditions during deposition of the Little River Formation to humid conditions during deposition of the Grande Anse and Ragged Reef formations, causing greater chemical weathering of the sediment. These findings indicate that > 2 km of sediment (Little River, Joggins, and Springhill Mines formations) accumulated south of the salt wall during the major episode of salt diapirism, followed by erosion of any topographic high associated with the salt wall, and accumulation of a further > 500 m of sediment (the laterally equivalent Ragged Reef and Grand Anse formations), all within a timespan of only ∼ 2 Myr.

Life at the continental–marine interface: palaeoenvironments and biota of the Alcobaça Formation (Late Jurassic, Central Portugal), with a formal definition of the unit appended

The Kimmeridgian Alcobaça Formation of the Lusitanian Basin forms a mixed carbonate–siliciclastic unit between basinal deposits of the Abadia Formation, and fluvial–terrestrial strata of the Lourinhã Formation. This study presents >2.5 km of detailed logs of nine outcrop sections of the Alcobaça Formation in its type region. Eight of these sections encircle the Caldas da Rainha Diapir, which was a prominent, emergent, passive salt diapir during the time of deposition. Palaeoenvironments of the unit form a complex mosaic of low- to high-energy, carbonate- or siliciclastic-dominated shallow shelf settings; coastal embayments and lagoons; and coastal plains with rivers, lakes and playas. In the strata, abundant microfauna is often joined by a rich macrofauna, usually dominated by bivalves. Locally, corals, calcareous sponges or oysters form meadows or patch reefs. These autochthonous to parautochthonous remnants of former communities are assigned to 35 benthic macrofaunal associations. The integration of palaeoecological analysis of these associations with microfaunal and sedimentological data provides constraint on their salinity ranges, which range from slightly hypersaline to freshwater. Frequent temporal and spatial salinity fluctuations are attributed to variations in relative sea-level, salt tectonics or climate. The NNE-trending Caldas da Rainha Diapir induced pronounced facies differentiation. Predominantly, non-marine siliciclastic facies in the northwest and carbonate to siliciclastic, marine to brackish facies in the southwest are contrasted by shallow-marine carbonate facies east of the diapir. Comprehensive exposure and well-preserved fossils make the Alcobaça Formation an excellent showcase to demonstrate how biofacies analysis can help to disentangle the interplay of climate changes, sea-level fluctuations and salt tectonics. Based on the improved characterisation of the unit, the Alcobaça Formation is formally defined, and seven members are established.

Interplay between base‐salt relief, progradational sediment loading and salt tectonics in the Nordkapp Basin, Barents Sea – Part II

AbstractReprocessed, regional, 2D seismic reflection profiles, 3D seismic volumes and well data (exploration and shallow boreholes), combined with 2D structural restorations and 1D backstripping were used to study the post‐salt evolution of the Nordkapp Basin in Barents Sea. The post‐salt evolution took place above a pre‐salt basement and basin configuration affected by multiple rift events that influenced the depositional facies and thickness of Pennsylvanian‐lower Permian‐layered evaporite sequence. Initially, regional mid‐late Permian extension reactivated pre‐salt Carboniferous faults, caused minor normal faulting in the Permian strata and triggered slight salt mobilization towards structural highs. The main phase of salt mobilization occurred during earliest Triassic when thick and rapidly prograding sediments entered from the east into the Nordkapp Basin. In the early‐mid‐Triassic, the change in the direction of progradation and sediment entry‐points shifted to the NW led to rotation of the earlier‐formed mini‐basins and shift of dominant salt evacuation direction to the south. The prograding sediment influx direction, sediment transport velocity and thickness influenced the dynamics of the early to late passive diapirism, salt expulsion and depletion along the strike of the basin. The basin topography resulting from salt highs and mini‐basin lows strongly affected the Triassic progradational fairways and determined the distinct sediment routing patterns. Minor rejuvenation of the salt structures and rotation of the mini‐basins took place at the Triassic–Jurassic transition, due to far‐field stresses caused by the evolving Novaya Zemlya fold‐and‐thrust belt to the east. This rejuvenation influenced the sediment dispersal routings and caused formation of dwarf secondary mini‐basins. The second and main rejuvenation phase took place during likely early‐mid‐Eocene when propagated far‐field stresses from the transpressional Eurekan/Spitsbergen orogeny to the NW inverted pre‐salt normal faults, reactivated the structural highs and rejuvenated the salt structures. The studied processes and study outcomes can be applicable to other evaporite‐dominated basins worldwide.