Basin Research - Volume 36, Issue 2, 2024

In our study, we investigate the limitations of seismic imaging of mafic sills by developing a synthetic seismic dataset rooted in field outcrops. Our observations highlights the limitations of seismic imaging of mafic sills, often due to subseismic structures and high density contrast between igneous intrusions and sedimentary host rocks.

Sills play a leading role in the transport of magma in sedimentary basins. The contact between sills and host rocks reflects the acting emplacement processes during sill propagation and evolution. Recent studies have shown that the propagation of sills and dykes is strongly influenced by the lithology of the host rocks, but none have detailed documentation of marginal features in large‐scale intrusive complexes. Three‐dimensional seismic data is the primary method of mapping and investigating such complexes, but it is difficult to accurately image sills due to their low thickness compared to seismic resolution. By understanding the relationship between local lithology and marginal sill features, we can better understand the imaging of sills in seismic datasets and their resulting geometry. In this study, we present a seismic‐scale sill analogue through multiple high‐resolution three‐dimensional models, with corresponding logs and field observations from Cedar Mountains, San Rafael Swell, US. This model was further used to develop a synthetic seismic dataset, providing us with a strong control on which marginal sill features fall beneath seismic resolution. We found that lithology plays a critical control in sill geometry and morphology. In Cedar Mountains, sills emplaced within massive sandstones frequently exhibit strata‐discordant base contact with the host rock. Conversely, sills found within heterolithic intervals and mudstones typically display strata‐concordant base contact with the host rocks. Sills within heterolithic intervals also tend to exhibit a more complex segmentation with multiple broken bridges. Furthermore, our findings show that sills are more than 3.7 times more likely to intrude in mudstone compared to sandstone and heterolithic intervals. These results suggest how sill geometries can be adapted to interpret lithology in seismic datasets from sedimentary basins with little to no well control. We anticipate that our findings may provide better knowledge for interpreting sills in sedimentary basins and contribute to developing more sophisticated geomechanical emplacement models for igneous intrusions.

Geophysical and geological research in the Ulleung Basin has been ongoing since the 1970s, involving continuous seismic acquisition and multiple well‐drilling projects. This study utilized an integrated quantitative approach with conventional seismic interpretation based on recent trends to understand the history of the shelf‐margin development and individual controls that may have influenced each period in the Ulleung Basin. Quantitative analysis was conducted by measuring the progradation (Pse), aggradation (Ase) and sediment influx (Fc) of individual shelf margins in seven dip‐oriented seismic profiles, and calculating the shelf‐edge gradient (αse) and the P/A ratio. Based on these data and the ratio of accommodation to sediment supply (A/S ratio), which was interpreted from the stratal stacking pattern, five shelf‐edge trajectory types were defined and assigned to each shelf margin. By considering individual controls (eustatic fluctuations, sediment supply and tectonic events), we defined the three evolution intervals of the Ulleung Basin during the Middle Miocene to Late Miocene (15 to 6.5 Ma): (1) the upper Middle Miocene (15 to 11.63 Ma) characterized by a moderate sediment supply and high aggradation margin induced by rapid subsidence, (2) the lower Upper Miocene (10.8 to 10 Ma) as a high sediment supply and low aggradation margin associated with reworked sediments from the uplifted Dolgorae Thrust and (3) the uppermost Miocene (9.2 to 6.5 Ma) characterized by a low sediment supply and low aggradation margin experiencing sediment starvation. A comparison with worldwide continental margins indicated that the Ulleung Basin formed under a relatively low rate of progradation (low sediment supply; 4.79 km/Myr) and a high rate of aggradation (high shelf accommodation; 361.04 m/Myr) condition. The SW margin of the Ulleung Basin provides a unique example of understanding stratigraphic architecture variations under changing stress regimes of back‐arc setting (extensional to compressional) and understanding of individual controls that influence margin development.

Stratigraphic chart for the Ulleung Basin shows seismic units, eustatic changes, calculated sediment supply and measured shelf‐margin aggradation, with major tectonic events. Brief depositional models of three different seismic units imply the transitional changes from the opening to the closing in the ‘old’ back‐arc Ulleung Basin.

The caption of this graphical abstract is as follows: Schematic representation of the reconstructed diagenetic evolution model showing the stepwise mechanisms resulting in the higher secondary porosity in the central part of a sandstone unit. The middle interval B has a better grain sorting, resulting in a greater depositional porosity. Although the interval B experienced a stronger mechanical compaction, it had significantly less early carbonate cements. These resulted in a greater preserved porosity in the interval B prior to the dissolution phase. During the dissolution phase, LMWOA produced in the adjacent top and bottom shales were limited, resulting in minor secondary porosity in marginal sandstones. However, the higher preserved porosity in interval B facilitated higher advection flux of LMWOA‐rich water and ultimately resulted in higher secondary porosity. COPL, compactional porosity loss; LMWOA, low‐molecular‐weight organic acids.

Higher secondary porosity was observed in the centre of a sandstone unit in the Eocene Shahejie Formation fan delta front sandstones from the Bozhong Depression, Bohai Bay Basin. This differs from past studies showing secondary porosity mainly in the marginal parts of sandstones adjacent to shales. This study utilized reactive transport models involving low‐molecular‐weight organic acids (LMWOA) to discuss potential processes resulting in the contrary distribution of secondary porosity. An interface model simulating LMWOA diffusion from adjacent shales to the sandstone resulted in secondary porosity in sandstones adjacent to shales. In contrast, an advection model simulating advective transport of LMWOA parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to the dissolution through LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA‐rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates and ultimately higher secondary porosity. This study indicates that grain sorting during sediment deposition, early carbonate cementation, LMWOA production in adjacent shales, and advection processes collectively control the diagenetic reactions and the distribution of secondary porosity in sandstones.

The Zagros foreland basin is an important sedimentary archive for the tectonic and paleoclimatic evolution of the Zagros Mountains and the entire Neotethyan Arabia–Iran collision zone. By combining new geochemical high‐resolution whole rock XRF data with clay mineralogy and soluble salt geochemistry we propose an evolution of the sedimentary environment in the Lurestan arc from the Serravallian to the early Pleistocene, closing a gap in understanding the complex exhumation history of the central Zagros mountain belt. An increase in ultramafic sedimentary input indicates a shift from provenance 1 to provenance 2 by ophiolite exhumation at ca.10 Ma in the Imbricated Zagros north of the Lurestan arc. Our data further indicates that the sedimentary environment of parts of the Lahbari Mb within the Lurestan arc represents a piedmont deposition of fine‐grained alluvial fans and siltstones with aeolian contribution deposited under hyper‐arid climate conditions. These represent provenance shift 3 and were likely sourced from evaporites of the underlying Gachsaran Fm and fluvial deposits of the Lower Aghajari Mb (provenance 1 and provenance 2), uplifted by the Mountain Front Flexure at around 5.6 Ma. Combining XRF whole rock data with clay mineral data refines formation conditions of the clay minerals in the foreland basin such as palygorskite, which is revealed to be authigenic in origin in the Lower Aghajari Mb. as a function of varying Mg‐content due to variations of erosion of the ultramafic and mafic rocks in the Imbricate Zagros belt. Palygorskite in the Lahbari Member is likely both inherited from the Neogene Gachsaran evaporites as well as of authigenic origin.

Y/Ni versus Cr/V diagram (Hiscott, 1984) as an indicator of ultramafic input for samples from the Gachsaran Fm, the Lower Aghajari Mb and the Lahbari Mb UM‐ultramafic, GR‐granitic, ME‐metamorphic endmembers and mixing hyperboles modified from Dinelli et al. (1999).

Sequences and facies of the Upper Jurassic–Lower Cretaceous succession in the wells of the Eastern Barents Sea. Flattened on the Barremian unconformity (K1).

The Upper Jurassic–Lower Cretaceous sedimentary rocks in the Eastern Barents Sea Basin are up to 2 km thick and represent one of the least studied Arctic intervals. Here, for the first time, we present a detailed analysis of 43,000 km of 2D seismic profiles, as well as well‐log and core data from 24 offshore wells with the aim to create a comprehensive sequence stratigraphic framework that can be integrated with the rest of the basin. Results show that (1) seven third‐order sequences and five types of clinoforms can be identified based on integrated seismic and well data. The age of each sequence was established based on published biostratigraphic investigations along with new dinocyst interpretations included in this study; (2) the deep marine basin was gradually filled with sediments coming from north, east and south as a response to HALIP, Canada Basin opening and Cimmerian uplift of Novaya Zemlya, and was preserved only in the south‐western part of the Barents Sea Basin at the end of Early Cretaceous and (3) both Eastern Barents Sea and West Siberia Basin share similarities in sedimentary environments and tectonic setting, though the spatial distribution of clastic reservoirs in Upper Jurassic and Lower Cretaceous mega‐sequence heavily depends on the source areas that require more provenance focused research. The results presented here can be used in further regional exploration in the area and to better understand the geodynamic evolution of the Greater Barents Sea Basin.

Continental margins develop long submarine slopes, linking the shallow shelves along the continental landmasses to the deep abyssal plain. They are the results of a complex interaction between destructive and constructive processes, although by and large they are sites of deposition. There is a great amount of variation between the length, height, smoothness, gradient and variation thereof between the slope profiles; however, there is also recurring similarity in their shape. The similitude has suggested systematic relationships between the shape and the processes forming them, and led to studies on geomorphological categorisation based on curvatures. The potential for prediction of along‐strike variations and connection between morphology and sedimentary process is herein approached through broadening the mathematical functions used, detailed measurement, observation and curve‐fitting of over 150 passive continental margins. Previously, three functions have been used to categorise submarine slopes. The present study finds that four mathematical functions closely match the slopes: Linear, Gaussian, exponential and quadratic (positive and negative/inverse), and reveals that the fourth slope, the quadratic, is by far the most common. While exponential and quadratic slopes are similar there is a crucial difference in the way in which the angle of the slope changes. This study suggests that quadratic slopes represent systematically decreasing sediment deposition with distance, previously attributed to exponential slopes. Exponential slopes meanwhile, represent slope readjustment profiles with upper sediment bypass and lower slope aggradation. Linear slopes, which form the longest low‐angle slopes, form in response to high sediment input. Abrupt shelf‐edges form in shallower water and develop longer slope aprons, suggesting formation from erosional processes. This implies that the quintessential sigmoidal (s‐shaped, Gaussian function) slope, with a smooth rollover, represents the fundamental depositional slope profile.

A figure showing two examples of the normalised expression of each type of curvature, along with specific examples of the different types of curvature (locations on map).

In the late Permian and Triassic, the continental Mitu Group formed in extensional basins along the length of the Cordillera Oriental and Altiplano of present‐day Peru. Given the presence of coeval arc systems only in northern Chile and southern Ecuador but not in Peru the tectonic setting of the Mitu basin has been interpreted variably as orthogonal continental rift, sinistral transtensional rift, aulacogen and back‐arc basin. The Mitu Group comprises continental mass flow and alluvial fan, fluvial, aeolian and minor lacustrian facies and hosts thick piles of subalkaline and alkaline intermediate and felsic ignimbrites and mafic lavas. The age of the Mitu Group had originally been established as ranging from the Late Permian to the late Triassic on the basis of structural considerations and scarce biostratigraphic data. Recently, U–Pb zircon ages from ignimbrites and sedimentary rocks have been taken to constrain the Mitu Group to the Middle and Late Triassic. We performed a sedimentological, heavy mineral, and zircon geochronological and Lu‐Hf isotope study of the Mitu Group in 14 sections mainly in southern and central Peru, and one section in northern Peru. Ten new U–Pb concordia ages on ignimbrites intercalated in the Mitu Group successions offer a new robust stratigraphic framework and constrain the stratigraphy of the Mitu Group between 260 and 205 Ma. In combination with maximum likelihood ages of deposition derived from detrital zircon, U–Pb geochronology places the deposition of the Mitu Group between ca. 270 and 194 Ma (lower Guadalupian into the Sinemurian). Detrital zircon U–Pb age distributions and heavy mineral assemblages reflect a strongly recycled Precambrian Amazonian and Palaeozoic proto‐Andean provenance. The Palaeozoic detrital age patterns are highly variable, and temporally and spatially random. A local provenance can generally not be identified. εHf(t) values in zircon obtained from ignimbrites and sedimentary rocks indicate variable degrees of crustal recycling. In the course of the Palaeozoic, εHf(t) values become on average progressively less negative, with a large proportion particularly of Mitu age zircons' εHf(t) values encompassing less evolved and moderately juvenile compositions. Along strike of the basin stratigraphic thicknesses, and rates and times of accumulation vary strongly with larger thicknesses and rates being registered in southern Peru. This suggests that the Mitu basin had been divided into a number of subbasins with individual histories of subsidence, accumulation, and volcanism. Absent a magmatic arc, late Permian–Triassic Peru evolved in a sinistral plate tectonic and regional framework expressed particularly in the sinistral Late Gondwanide orogeny predating the Mitu Group. We interpret that a sinistral transform fault linked the subduction zones in southern Ecuador and northern Chile and that the Mitu Group basin has formed by sinistral transtensional and transpressional movements along a related transcurrent fault inboard of the transform margin. In the broader framework of the accretionary Terra Australis orogen along the western Gondwana margin, this interpretation fits its evolution in the Andean segment in an internally consistent way.

Spatial and temporal correlation of Late Permian‐Triassic Mitu Group continental and volcanic facies showing two main pulses of volcanism.

The conglomerates preserved in alluvial fans in the Swiss Molasse basin provide crucial insights into the sedimentary dynamics of these routing systems. In particular, the architectural trends and grain sizes of such deposits reveal information on the sediment fluxes and record variations in the intermittency — a proxy for the fan's activity — that indicate on the relative importance of tectonic or climatic controls on sediment production and transport. Here, we calculated intermittencies from sediment transport dynamics using the ratio between the long‐term average and the short‐term instantaneous unit sediment fluxes during bankfull discharge conditions. For this, we collected grain size data from three palaeo‐fan systems that were active through Oligo–Miocene times and that reveal preserved proximal–distal relationships. The three fan systems, which we term the western, central and eastern fans, show significant differences in their long‐term sediment budget but equivalent magnitudes of the sediment transport dynamics expressed through the intermittency factor. The eastern fan records a low long‐term sediment flux (5.7 km² Myr⁻¹), which needed the fan to be active during ca. 8 h yr⁻¹ (intermittency factor of 0.89 × 10⁻³). The western fan reveals a higher long‐term sediment flux (16.2 km² Myr⁻¹), which could have accumulated during ca. 16 h yr⁻¹ (intermittency factor of 1.83 × 10⁻³), thereby reflecting a more active system. The central fan records the largest long‐term sediment flux (40.3 km² Myr⁻¹), where ca. 57 h yr⁻¹ of sediment transport would be required to deposit the supplied material (intermittency factor of 6.53 × 10⁻³), thus representing the most active system. By relating these characteristics to the regional exhumation history, we consider that the central fan mainly recorded the transient response of the Alpine surface to the break‐off of the European mantle lithosphere slab. Contrarily, the western and eastern fans were formed during the Alpine evolution when steady‐state conditions between uplift and erosion were reached and when sediment fluxes to the basin were lower. Despite differences in the tecto‐geomorphic and climatic boundary conditions, our data suggest that these Oligo–Miocene megafans could accomplish their mean annual sediment transport work within a few hours or days per year.

This study investigates how three palaeo‐depositional systems situated in the Swiss Molasse basin (North to the Central European Alps; details in Fig. 1) record the tecto‐geomorphic and climatic boundary conditions of the source area through Oligo‐Miocene times. The granulometric properties of the conglomerates at these palaeo‐fan systems, paired with information on the spatial distribution of accommodation space, are used to infer the activity – or intermittency – of these dispersal systems and provide information on the long‐term sediment budgets.