Basin Research - Volume 34, Issue 4, 2022

Quantitative assessment of terrigenous sediment flux to the deep basin has mainly been limited to the Cenozoic Era, given challenges with reconstruction of more ancient source to sink systems. Subsurface mapping in the northern Gulf of Mexico basin, where there is a robust database of wells and seismic data, allows estimation of sand‐size grain volumes within 12 chronostratigraphically defined Late Mesozoic supersequences. Five subsurface depocentres with thicknesses of >400 m are identified, revealing a history of sediment routing via major fluvial axes (paleo rivers) southward from a mid‐continent drainage divide. The interpreted paleo‐rivers are further confirmed by compilation and synthesis of published and novel detrital zircon provenance data showing combinations of age peak subsets that vary considerably between depocentres. This study quantifies sediment volumes within each depocentre by inverting porosity over defined gross rock volumes derived from subsurface well logs and 2D seismic mapping. Two temporal trends of progressively increasing grain volume can be related to climatic effects on fluvial discharge and sediment delivery to the depocentres: (1) Late Jurassic to Early Cretaceous (Oxfordian to early Aptian) and (2) Late Cretaceous (Late Aptian to Coniacian). The first temporal trend probably reflects the local climate becoming less arid with northward drift of the North American Plate into higher latitudes. The second trend mirrors increasing Cretaceous paleotemperatures and rising global sea levels. This second trend is also marked by increased shelf margin bypass of sand indicated by deep basin drilling. Partitioning of Late Mesozoic grain volume by fluvial axes shows that the Paleo‐Apalachicola and Paleo‐Mississippi rivers, sourced from Appalachian basement terranes, generated the largest and longest terrigenous sediment flux into the Gulf of Mexico basin in the Late Mesozoic. Overall, Late Mesozoic siliciclastic grain volumes are estimated to be an order of magnitude less than those previously documented for the Cenozoic Era.

Grain volumes of the Late Mesozoic supersequences partitioned by the paleo‐river interpreted as delivering sediment to the various Gulf Basin depocenters. The Paleo‐Apalachicola River, linked to the southern Appalachians through detrital zircon analysis, is the most temporally consistent and volumetrically important fluvial axis during the Mesozoic. Other paleo‐drainages show more episodic sediment delivery.

This paper presents the first comprehensive analysis of the present‐day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present‐day stress in this area that is located ca. 100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowledge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological features in the Malawi region. Considering the limited reliability of earthquake‐derived stress orientations near plate boundaries, poor coverage of low to moderate magnitude earthquakes in eastern Africa, and ambiguity about the latest activity of geological structure; other well‐established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1188 stress‐related borehole failures. The results indicate a mean regional trend of 045° ± 31° for the maximum horizontal stress (SHmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate‐scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500 and 100 km) order stress sources (i.e., large tectonic forces and lateral density variations). However, high‐resolution data used in this study reveal that third (between 100 and 1 km) and fourth (<1 km) order stress sources from stiffness contrasts, rock fabric and geological structures have a great impact on the stress perturbations at smaller and local scales.

Maximum horizontal stress orientation (SHmax) and the trend of fault lines (based on two different studies) in the Moatize Basin. There is a consistency between the trend of SHmax and fault lines in different parts of the study area that can explain the role of faults in localised perturbation of stress at basin scales.

The separation and characterisation of different deformation events in superimposed basins can be challenging due to the effects of overprinting and/or fault reactivation, combined with a lack of detailed geological or geophysical data. This paper shows how an onshore study can be enhanced using a targeted interpretation of contiguous structures offshore imaged by seismic reflection data. Two deformation events, including evidence of fault reactivation, are recognised and associated with the onshore part of the Lossiemouth Fault Zone (LFZ), southern‐central Inner Moray Firth Basin. The basin is thought to record a history of Permian to Cenozoic deformation, but it is difficult to conclusively define the age of faulting and fault reactivation. However, structures in onshore outcrops of Permo–Triassic strata show no evidence of fault growth, and new interpretation of seismic reflection profiles in the offshore area reveals that Permo–Triassic fills are widely characterised by subsidence and passive infill of post‐Variscan palaeotopography. We propose that sequences of reactivated faulting observed onshore and offshore can be correlated and can be shown in the latter domain to be Early Jurassic–Late Cretaceous, followed by localised Cenozoic reactivation. The workflow used here can be applied to characterise deformation events in other superimposed rift basins with contiguous onshore (surface)—offshore (subsurface) expressions.

Summary of basin development using a representative 2D seismic profile.

Based on the integration of seismic and well log data, biostratigraphy, petrography, clumped isotope analyses, and basin modelling, a burial model is proposed for the Chalk Group that experienced hydrostatic and subsequently overpressured conditions, which resulted in the recognition of a palaeo‐lithification front.

This study proposes a unique workflow to unravel complex burial diagenetic histories of overpressured basins based on the integration of seismic and well log data, biostratigraphy, petrography, clumped isotope analyses and basin modelling. This approach is demonstrated with an example from the Chalk Group in the Danish Central Graben, where a seismic‐scale palaeo‐lithification front has been observed and studied in detail to elucidate the timing of the establishment of overpressured conditions and its relation to changing diagenetic activity. The palaeo‐lithification front separates high‐porosity chalks above, that dominantly underwent mechanical compaction and contact cementation, from low‐porosity chalk below, that dominantly underwent severe pressure dissolution and pore‐filling cementation. Analysis of chalk buried under hydrostatic conditions shows a strikingly similar lithification front between 1000 and 1200 m burial, much shallower than the lithification front in the Danish Central Graben at a current depth between 2100 and 2400 m below seafloor. The discrepancy of 1200 m is due to the establishment of overpressured conditions that limited the increase in effective stress as burial continued, finally halting burial compaction when formation fluids started to carry the lithostatic weight. Basin modelling data indicate that this occurred at the end of the Oligocene for large parts of the Danish Central Graben, which is much earlier than the Middle Miocene timing that is currently assumed. The results imply a regional occurrence of a relict lithification front in the North Sea Basin, its position guided by stratigraphy, but mainly dependent on the maximum effective stress experienced during its burial history. The study shows that the porosity bipartition is a remnant of the past and not from ongoing compaction as has previously been suggested. Since it was established before the thermal maturity of the main source rocks, chalk below the lithification front must have formed a sealing unit during hydrocarbon migration. The recognition of the lithification front is also of importance to velocity modelling and depth‐conversion since a non‐linear increase between velocity and depth is expected across this boundary. The methodology may be applied in other overpressured basins where the diagenetic state of reservoir rocks at the end of hydrostatic conditions must be constrained.

We document high densities of pockmarks across the Canterbury Margin. Using an interdisciplinary approach, we infer that pockmarks were formed by expulsion of methane, and saline and freshened groundwater. Sediment loading is the main driver of fluid flow across the slope, whereas methane generation and lower sea‐levels may account for fluid flow across the shelf.

Shallow seabed depressions attributed to focused fluid seepage, known as pockmarks, have been documented in all continental margins. In this study, we demonstrate how pockmark formation can be the result of a combination of multiple factors—fluid type, overpressures, seafloor sediment type, stratigraphy and bottom currents. We integrate multibeam echosounder and seismic reflection data, sediment cores and pore water samples, with numerical models of groundwater and gas hydrates, from the Canterbury Margin (off New Zealand). More than 6800 surface pockmarks, reaching densities of 100 per km², and an undefined number of buried pockmarks, are identified in the middle to outer shelf and lower continental slope. Fluid conduits across the shelf and slope include shallow to deep chimneys/pipes. Methane with a biogenic and/or thermogenic origin is the main fluid forming flow and escape features, although saline and freshened groundwaters may also be seeping across the slope. The main drivers of fluid flow and seepage are overpressure across the slope generated by sediment loading and thin sediment overburden above the overpressured interval in the outer shelf. Other processes (e.g. methane generation and flow, a reduction in hydrostatic pressure due to sea‐level lowering) may also account for fluid flow and seepage features, particularly across the shelf. Pockmark occurrence coincides with muddy sediments at the seafloor, whereas their planform is elongated by bottom currents.

Deposition of the Lapataia Formation took place in the absence of coeval volcanism at the earliest stages of rifting of the Rocas Verdes basin. Thus, there is a significant time gap between the youngest detrital zircon age and the actual age of sedimentation.

Although being one of the most efficient tools to constrain the stratigraphy of sedimentary basins, detrital zircon geochronology may present limitations under certain conditions, and isotopic ages should be treated with caution before assigning a depositional age. This is the case of depositional environments that lack coeval volcanism, where there may exist a significant time gap between the age of the youngest detrital zircon available and the actual time of deposition of a sedimentary rock. In this paper, we put on a comprehensive analysis in which detrital geochronology is aided with other reliable geological constraints. As a case study, we evaluate the stratigraphic age and provenance of the Lapataia Formation from the Fuegian Andes of southernmost South America, a low‐grade metamorphic unit with a problematic stratigraphic definition in the context of the Rocas Verdes basin. Detrital ages show very wide age spectra, with a younger peak at ca. 267 Ma. However, based on detailed petrography and microstructural observations, as well as stratigraphic correlations at a regional scale, we conclude that the Lapataia Formation represents the earliest stages of Jurassic rifting during the opening of the Rocas Verdes basin. Erosion from older basement blocks in an environment lacking coeval volcanic activity explains the occurrence of Paleozoic and older grains, and the lack of younger zircons. This work offers a multifaceted approach, which may be essential to assist radiometric detrital geochronology at certain stages during basin evolution.

Sediment accumulation rates in foreland basins result from a complex interplay between surface and deep processes in both the exhumed relief domain and sedimentary basins. The growth and decay of a mountain belt during orogenic and post‐orogenic phases have been largely studied, thanks to thermochronological and structural studies. The sedimentary response of the orogenic phases in the preserved sediments of the surrounding basins is well known in terms of sedimentary filling patterns and architecture, but much less better quantified. Here, we performed a measurement of the siliciclastic sediment volumes of the Pyrenean retro‐foreland basin—the Aquitaine Basin and the Bay of Biscay during Cenozoic times—for a better understanding of the erosion and the sediment transfer and deposition during the convergence (syn‐orogenic) to post‐convergence (post‐orogenic) periods of the Pyrenees Mountain. The measurement of the compacted siliciclastic sediment is based on sediment thickness (isopach) maps of known lithologies, derived from the interpretation of 40,000 kms of seismic profiles. Thanks to the siliciclastic sediment volumes quantification and a well‐known retro‐foreland basin tectono‐sedimentary evolution, we bring quantitative results as:

1. The amount of preserved sediments is of 51,500 ± 16,800 km³ for the Cenozoic.
2. The siliciclastic sediment rate curve during Cenozoic shows two major increases around 26.0 and 2.5 Ma. The 26.0 Ma increase is clearly related to the erosion of the Pyrenees of tectonic origin. The major 2.5 Ma one would be mainly related to a climatic forcing.
3. The mass balance between the Aquitaine Platform and the deepest domains changes through time in favour of the deep domain. This might be explained by the ratio between subsidence that created accommodation space and the sediments feed by the mountain belt and stored in the Aquitaine Platform.

We performed a measurement of the siliciclastic sediment volumes of the Pyrenean retro‐foreland basin—the Aquitaine Basin and the Bay of Biscay during Cenozoic times—for a better understanding of the erosion and the sediment transfer and deposition during the convergence (syn‐orogenic) to post‐convergence (post‐orogenic) periods of the Pyrenees Mountain.

The Bohai Sea hosts a complex fluid flow system composed of various seafloor expressions and shallow fluid migration pathways (gas chimneys, mud diapirs, and dense Quaternary faults). Gas chimneys are classified into three types based on their distribution and seismic character, implying variability in formation processes.

Thorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e. pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type A formed by relatively rapid gas escape, Type B formed by episodic fluid expulsion and Type C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in the formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness <20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.

Schematic Reconstruction of the Santorini Mass‐Transport Cascade (SMTC). After a phase of volcanic quiescence (A), a rift pulse (B) triggered precursory mass‐wasting events (C), large‐scale sector collapses (D) and secondary sediment failures (E), which culminated in a change in the volcanic behaviour of the system (F).

Volcanic island sector collapses have the potential to trigger devastating tsunamis and volcanic eruptions that threaten coastal communities and infrastructure. Considered one of the most hazardous volcano‐tectonic regions in the world, the Christiana‐Santorini‐Kolumbo Volcanic Field (CSKVF) lies in the South Aegean Sea in an active rift zone. Previous studies identified an enigmatic voluminous mass‐transport deposit west and east of Santorini emplaced during the early evolution of the edifice. However, the distribution and volume as well as the nature and emplacement dynamics of this deposit remained unknown up to now. In this study, we use an extensive dataset of high‐resolution seismic profiles to unravel the distribution and internal architecture of this deposit. We show that it is located in all basins surrounding Santorini and has a bulk volume of up to 125 km³, thus representing the largest known volcanic island mass‐transport deposit in the entire Mediterranean Sea. We propose that the deposit is the result of a complex geohazard cascade that was initiated by an intensive rift pulse. This rifting event triggered a series of smaller precursory mass‐transport events before large‐scale sector collapses occurred on the northeastern flank of the extinct Christiana Volcano and on the southeastern flank of the nascent Santorini. This was followed by the emplacement of large‐scale secondary sediment failures on the slopes of Santorini, which transitioned into debris and turbidity flows that traveled far into the neighboring rift basins. Following this cascade, a distinct change in the volcanic behaviour of the CSKVF occurred, suggesting a close relationship between crustal extension, mass transport and volcanism. Cascading geohazards seem to be more common in the evolution of marine volcanic systems than previously appreciated. Wider awareness and a better understanding of cascading effects are crucial for more holistic hazard assessments.

This study presents a method for creating synthetic stratigraphy from overhead imagery paired with limited topographic data. Measurements of channel sand connectivity and overbank deposit geometry from synthetic stratigraphy of an experimental delta highlight that channel kinematics drive large‐scale stratigraphic architecture.

Characterization of groundwater aquifers and hydrocarbon reservoirs requires an understanding of the distribution and connectivity of subsurface sandbodies. In deltaic environments, distributary channel networks serve as the primary conduits for water and sediment. Once these networks are buried and translated into the subsurface, the coarse‐grained channel fills serve as primary conduits for subsurface fluids such as water, oil or gas. The temporal evolution of channels on the surface therefore plays a first‐order role in the 3D permeability and connectivity of subsurface networks. Land surface imagery is more broadly available than topographic or subsurface data, and time‐series imagery of river networks can hold useful information for constraining the shallow subsurface. However, these reconstructions require an understanding of the degree to which channel bathymetry and river kinematics affect connectivity of subsurface sandbodies. Here, we present a novel method for building synthetic cross sections using overhead images of an experimental delta. We use principal components analysis to extract river networks from surface imagery, then couple this with an inverse‐CDF method to estimate channel bathymetry, to generate a time‐series of synthetic delta topography. This synthetic topography is then transformed, accounting for deposition and subsidence, to produce synthetic stratigraphy that differentiates coarse‐grained channel fill from overbank and offshore deposition. We find that large‐scale subsurface architecture is relatively insensitive to details of channel bathymetry, but instead is primarily controlled by channel location and kinematics. We analyse the connectivity of sand bodies and the geometries of barriers to flow and find that periods of rapid sea‐level rise have more variability in sand body connectivity. We also find that barrier width decreases downstream during all sea‐level phases. Our method generates synthetic stratigraphy that closely resembles the large‐scale architecture and 2‐dimensional connectivity of the real stratigraphy built during the experiment it was based on. We anticipate that it will be broadly applicable to other experimental and field‐scale scenarios.