Minibasin Subsidence Research Articles

Mass‐transport complexes (MTCs) document subsidence patterns in a northern Gulf of Mexico salt minibasin

AbstractMass‐transport complexes (MTCs) dominate the stratigraphic record of many salt‐influenced sedimentary basins. Commonly in such settings, halokinesis is invoked as a primary trigger for MTC emplacement, although the link between specific phases of salt movement, and related minibasin dynamics, remains unclear. Here, we use high‐quality 3D seismic reflection and well data to constrain the composition, geometry and distribution (in time and space) of six MTCs preserved in a salt‐confined, supra‐canopy minibasin in the northern Gulf of Mexico, and to assess how their emplacement relate to regional and local controls. We define three main tectono‐sedimentary phases in the development of the minibasin: (a) initial minibasin subsidence and passive diapirism, during which time deposition was dominated by relatively large‐volume MTCs (c. 25 km3) derived from the shelf‐edge or upper slope; (b) minibasin margin uplift and steepening, during which time small‐volume MTCs (c. 20 km3) derived from the shelf‐edge or upper slope were emplaced; and (c) active diapirism, during which time very small volume MTCs (c. 1 km3) were emplaced, locally derived from the diapir flanks or roofs. We present a generic model that emphasizes the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. Although based on a single data‐rich case study, our model may be applicable to other MTC‐rich, salt‐influenced sedimentary basins.

The competition for salt and kinematic interactions between minibasins during density-driven subsidence: observations from numerical models

Stratal geometries of salt-floored minibasins provide a record of the interplay between minibasin subsidence and sedimentation. Minibasin subsidence and resulting stratal geometries are frequently interpreted by considering the minibasins in isolation and implicitly assuming that internal geometries are the result of purely vertical halokinetic processes. However, minibasins rarely form in isolation and may record complex subsidence histories even in the absence of tectonic forces. In this study we use numerical models to investigate how minibasins subside in response to density-driven downbuilding. We show that minibasins subsiding in isolation result in simple symmetrical minibasins with relatively simple internal stratigraphic patterns. In contrast, where minibasins form in closely spaced arrays and subside at different rates, minibasins can kinematically interact due to complex patterns of flow in the encasing salt, even during simple density-driven subsidence. More specifically, we show that minibasins can: (1) prevent nearby minibasins from subsiding; (2) induce lateral translation of nearby minibasins; and (3) induce tilting and asymmetrical subsidence of nearby minibasins. We conclude that even in areas where no regional or dominant salt flow regime exists, minibasins can still be genetically related and the minibasin subsidence histories cannot be fully understood if considered in isolation. Thematic collection: This article is part of the Mechanics of salt systems: state of the field in numerical methods collection available at: https://www.lyellcollection.org/cc/mechanics-of-salt-systems

Minibasin depocentre migration during diachronous salt welding, offshore Angola

AbstractSalt tectonics is an important part of the geological evolution of many continental margins, yet the four‐dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high‐quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long‐term (>70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt‐tectonics and related minibasin evolution: (i) thin‐skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low‐displacement normal faults and related salt rollers. During this stage, local salt welding led to the along‐strike migration of fault‐bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning > 70 Myr of the salt‐tectonic history of this, and likely other salt‐rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono‐stratigraphic evolution of minibasins.

The stratigraphic record of minibasin subsidence, Precaspian Basin, Kazakhstan

AbstractMinibasins are fundamental components of many salt‐bearing sedimentary basins, where they may host large volumes of hydrocarbons. Although we understand the basic mechanics governing their subsidence, we know surprisingly little of how minibasins subside in three‐dimensions over geological timescales, or what controls such variability. Such knowledge would improve our ability to constrain initial salt volumes in sedimentary basins, the timing of salt welding and the distribution and likely charging histories of suprasalt hydrocarbon reservoirs. We use 3D seismic reflection data from the Precaspian Basin, onshore Kazakhstan to reveal the subsidence histories of 16, Upper Permian‐to‐Triassic, suprasalt minibasins. These minibasins subsided into a Lower‐to‐Middle Permian salt layer that contained numerous relatively strong, clastic‐dominated minibasins encased during an earlier, latest Permian phase of diapirism; because of this, the salt varied in thickness. Suprasalt minibasins contain a stratigraphic record of symmetric (bowl‐shaped units) and then asymmetric (wedge‐shaped units) subsidence, with this change in style seemingly occurring at different times in different minibasins, and most likely prior to welding. We complement our observations from natural minibasins in the Precaspian Basin with results arising from new physical sandbox models; this allows us to explore the potential controls on minibasin subsidence patterns, before assessing which of these might be applicable to our natural example. We conclude that due to uncertainties in the original spatial relationships between encased and suprasalt minibasins, and the timing of changes in style of subsidence between individual minibasins, it is unclear why such complex temporal and spatial variations in subsidence occur in the Precaspian Basin. Regardless of what controls the observed variability, we argue that vertical changes in minibasin stratigraphic architecture may not record the initial (depositional) thickness of underlying salt or the timing of salt welding; this latter point is critical when attempting to constrain the timing of potential hydraulic communication between sub‐salt source rocks and suprasalt reservoirs. Furthermore, temporal changes in minibasin subsidence style will likely control suprasalt reservoir distribution and trapping style.

Characterization of Oligo-Miocene evaporite-rich minibasins in the Sivas Basin, Turkey

Abstract The Sivas Basin in Turkey displays in its central part an Oligo-Miocene halokinetic province which acts as a major outcrop analogue to study salt-sediment interactions. Based on field geology observations, the present paper focuses on the geometry and sedimentology of several minibasins having the particularity of being mainly filled by gypsiferous deposits. Such type of evaporite-rich minibasins remain difficult to identify and are poorly studied in other halokinetic provinces. In the Sivas Basin, the evaporites were recycled from diapiric salts and precipitated in saline ponds emplaced above deflating diapiric stems. Diapir deflation resulted either from local transtensive strain, cessation of diapir feeding and/or subsurface dissolution of the diapiric salt. Minibasin subsidence was likely enhanced by the fast emplacement rate of the capping evaporites, together with the high density of the depositional sulfates compared to the diapiric halite. The evaporite-rich minibasins stand out from their surrounding siliciclastic counter-parts by their small dimension (lower than 1 km-wide), their encased teardrop shape, and their high internal deformations. The later include well-developed halokinetic sedimentary wedges, aerial mega-slumps or inverted flaps. Such structural features probably resulted from the ductile rheology of the evaporite infill and the complex pattern of downbuilding. Although secondary evaporitic minibasins have never been identified in other ancient halokinetic settings, our study highlights that they could developed in any evaporitic environments, coastal or continental, such as in the Precaspian Basin. The secondary minibasins described here can also act as field analogues of other primary evaporite-rich minibasins already suspected in salt giant basins (e.g. in the Santos Basin, Brazil).

Lateral mobility of minibasins during shortening: Insights from the SE Precaspian Basin, Kazakhstan

Minibasin provinces are widespread and can be found in all types of salt tectonic settings, many of which are prone to shortening. Previous studies of how minibasin provinces shorten assume that the salt between the minibasins is homogeneous and that the base of salt is flat or of low relief, such that minibasins are free to move laterally. Here we investigate how minibasin provinces respond to shortening when the lateral mobility of the minibasins is restricted by intra-salt sediment bodies, in order to gain a greater understanding of the controls on the structural styles and modes of tectono-stratigraphic evolution exhibited in minibasin provinces. We examine a borehole-constrained, 3D seismic reflection dataset from the SE Precaspian Basin (onshore western Kazakhstan). The study area is characterised by large, supra-salt minibasins and an array of smaller intra-salt sediment packages distributed between these larger minibasins. We first outline the evidence of episodic shortening between the Late Triassic and present, after the onset of supra-salt minibasin subsidence. Next, we document spatial variations in shortening style, showing how these relate to the concentration of intra-salt sediment packages. Finally, we develop synoptic models showing how intra-salt sediment packages influence both the lateral mobility of minibasins during shortening and the resultant structural style, and we compare and contrast our findings with existing models and other natural examples of shortened minibasin provinces. We conclude that minibasin provinces may have different degrees of lateral mobility depending on the presence, or absence, of intrasalt barriers, and that these variations provide a first-order control on basin-shortening style and tectono-stratigraphic evolution.

Coevolution of Minibasin Subsidence and Sedimentation: Experiments

Abstract Subsidence patterns resulting from differential sediment loading on a mobile substrate (e.g., salt) are an important process for the development of accommodation and stratigraphic architectures in intraslope minibasins. Numerous studies of minibasin systems have focused on either the tectonic processes involved in salt deformation or the stratigraphic interpretation of the sedimentary fill of minibasins. This study focuses on the coevolution of depositional and tectonic processes to investigate the response of substrate movement to minibasin sedimentation. Using a silicone polymer to model a viscous mobile substrate, a series of 2D experiments were conducted to explore the effects of variation in 1) sediment supply rate, 2) depositional style (intermittent sediment supply), and 3) the thickness of the deformable substrate on subsidence patterns and minibasin stratigraphic development. Experimental results indicate that larger initial thickness of salt substrate as well as lower sedimentation rates result in greater amounts of subsidence for a given amount of deposit. Furthermore, in the experiments with intermittent sediment supply, increasing subsidence rate was observed as sedimentation continued, while decreasing subsidence rate occurred once sedimentation ceased. These accelerations and decelerations in subsidence were attenuated as the total thickness of the minibasin deposit increased and the thickness of the remaining salt decreased. Lower sediment supply rate led to a narrower but deeper minibasin formation. The increase in overall time allowed the salt substrate to have a greater response to the overburden. In contrast, the linked depositional and tectonic processes caused higher sediment supply rate to increase the planform size of the minibasin. Based on the experimental results, a new model of autostratigraphic minibasin evolution is suggested: 1) differential loading causes initial subsidence, 2) ponded (basin infilling) architecture occurs during a period of acceleration in subsidence, and 3) perched (spilling) architecture occurs over the duration of final subsidence deceleration.

The initiation of submarine slope failure and the emplacement of mass transport complexes in salt-related minibasins: A three-dimensional seismic-reflection case study from the Santos Basin, offshore Brazil

Three-dimensional seismic-reflection data are used to characterize the seismic expression and investigate the origin of two mass transport complexes (sensu [Moscardelli and Wood, 2007][1]) that are contained in two salt-related minibasins on the Sao Paulo Plateau, offshore SE Brazil. The mass transport complexes contain numerous slide blocks, which are expressed on seismic data as broadly tabular packages of weakly deformed reflections. Individual slide blocks are up to 50 m thick and up to 1 km2 in plan view. The slide blocks are flanked by laterally continuous reflections, which are interpreted as the seismic expression of either a thin mudstone-dominated debrite deposit or a thin interval of hemipelagic mudstones. The surface that bounds the base of the two mass transport complexes is planar, and this suggests that the associated gravity-flow event caused only limited erosion of the seafloor. The upper surfaces of the mass transport complexes are very rugose and onlapped by overlying strata, indicating that the slide blocks formed seabed relief of up to 20 m. The complexes occur at the base of a unit that thins toward the margins of the studied minibasins, and the slide blocks in the mass transport complexes decrease in size and density away from the salt-cored structural high that separates the two minibasins. Based on its seismic stratigraphic context and the spatial distribution of the slide blocks that they contain, the mass transport complexes are interpreted to have been derived from the failure of the margins of the salt-cored high in response to a period of relatively rapid minibasin subsidence. The results of this study indicate that large volumes of sedimentary material may be derived from the margins of salt-related minibasins in relatively distal submarine settings. From a geohazard perspective, this observation is important because the risk associated with submarine slope failure, and the transport and emplacement of large blocks, is typically thought to be low in relatively distal, deep-water environments. [1]: #ref-42

Salt tectonics in the North Sea Basin: a structural style template for seismic interpreters

Abstract The North Sea Basin contains a widespread Permian salt layer that reached a depositional thickness of c. 1 km in the basin centre. This layer profoundly affected structural style of the post-salt succession and the basin can be divided into structural domains on this basis. In combination with regional 3D seismic data and several thousand wells this makes the North Sea a natural laboratory for salt tectonics. Four principal structural domains are illustrated here. (1) Minibasin subsidence and salt wall growth on the West Central Shelf in the Late Permian to Triassic. This area was exhumed and differentially eroded prior to Jurassic rifting, creating palaeogeomorphology analogous to the present-day Paradox Basin, Utah. (2) Regional tilt during the Mesozoic and Cenozoic led to basin-scale gravity sliding with updip detached extensional faults and downdip compressional structures, similar to gravity sliding in the circum-Atlantic salt basins. (3) Jurassic rifting propagated across the salt basin, displaying spatial variation in extensional fault style, partly as a function of salt layer thickness. (4) North Sea salt thickness was not sufficient for salt canopy development but there are two suites of minor intrusions: cylindrical, passive diapirs with associated fault and fracture patterns in the central North Sea, and sills where Permian salt from reactive diapirs intruded along thin Triassic salt layers in the southern North Sea. Cretaceous to Palaeogene regional shortening affected all these domains, resulting in a variety of reactivation styles that do not fit within commonly used definitions of inversion tectonics. The North Sea salt tectonic domains form the basis of a matrix approach to salt structure initiating and driving mechanisms, and a mechanostratigraphic scheme for tectonic structure classification.

The paradox of minibasin subsidence into salt: Clues to the evolution of crustal basins

Why do salt-floored minibasins subside? An almost universal explanation is that salt is forced from beneath the sinking basin by the weight of its sedimentary fill. This explanation is valid if the average density of the basin fill exceeds that of salt, which in the Gulf of Mexico needs at least 2300 m of siliciclastic fill to ensure enough compaction. However, most minibasins start sinking when they are much thinner than this. Some mechanism other than density inversion must explain the early history of these minibasins. Conventional understanding of minibasin subsidence is thus incomplete. Here, we identify five alternatives to density-driven subsidence of minibasins. (1) During diapir shortening, the squeezed diapirs inflate, leaving the intervening minibasins as bathymetric depressions. (2) In extensional diapir fall, stretching of a diapir causes it to sag, producing a minibasin above its subsiding crest. (3) During decay of salt topography, a dynamic salt bulge subsides as upward flow of salt slows, which lowers the salt surface below the regional sediment surface. (4) During sedimentary topographic loading, sediments accumulate as a bathymetric high above salt. (5) Finally, subsalt deformation affecting the base of salt may produce relief at the top of salt. Each mechanism (including density-driven subsidence) produces a different bathymetry, which interacts with sediment transport to produce different facies patterns in each type of minibasin. The particular mechanism for minibasin subsidence depends on the tectonic environment, regional bathymetry, and sedimentation rate. Their spatial variation on a continental margin creates provinces in which a given minibasin style is dominant. An appreciation of subsidence mechanisms should thus improve our understanding of minibasin fill patterns and allow genetic comparisons between minibasins. The mechanics of a minibasin sinking into fluid salt is in many ways analogous to a crustal basin sinking into a fluid asthenosphere. However, minibasins lack the complex rheologies, thermal histories, and compositional variations that make study of crustal basins so challenging. Minibasins are thus natural analogs and have the potential to elucidate fundamentals of subsidence mechanics.

Mesozoic dissolution tectonics on the West Central shelf, UK Central North Sea

3-D seismic mapping of the Upper Jurassic Kimmeridge Clay Formation on the West Central Shelf in the Central North Sea reveals a complex fault array which is constrained by seismic interpretation and well control to be of late Jurassic/early Cretaceous age. Fault shapes in plan-view range from linear to circular. Linear fault lengths are 200–300 m to 5 km, the strongly curved and circular faults range in diameter from 100–1000 m. Fault trends are apparently random and display no correlation in location or trend with basement (sub-Zechstein) structures. There is, however, a strong link between this fault pattern and the structure of the top Zechstein (top salt) surface. Linear faults occur at the edges of elongate salt walls and the circular faults lie directly above structures which have been interpreted here as tall, steep-sided salt chimneys. The salt chimneys are present only in the thick, elongate minibasins of Triassic sediment which lie between the salt walls. It is argued that salt dissolution controls the timing, location, orientation and shape of the late Jurassic/early Cretaceous faults. A model is provided to account for the development of both salt walls and chimneys. We suggest that early Triassic karstification of the Zechstein evaporites led to development of an array of circular collapse features. During the ensuing episode of Triassic halokinesis which led to minibasin subsidence and salt wall growth, salt passively ‘intruded’ the circular collapse features within the subsiding minibasins to form narrow salt chimneys. The resulting array of salt walls and chimneys was subject to dissolution during subsequent subaerial exposure and the late Jurassic marine transgression of the basin (creating the observed fault array), prior to sealing of the salt from circulating groundwater by compaction of the Upper Jurassic and Lower Cretaceous shales which blanket the area.

Impact of salt on the structure of the Central North Sea hydrocarbon fairways

The Central North Sea lies within the North Permian evaporite Basin, which was largely flexural but with locally significant basement faults. While the main evaporite basin fill is halite, seismically reflective basin margin facies delineate the salt basin shape. The marginal Zechstein strata record Permian salt tectonics including listric faulting, slumping of kilometre-scale carbonate rafts and halite diapirism, all related to localized pre-Triassic gravity-driven collapse of the Zechstein basin margin. Triassic thickness in the Central North Sea (CNS) was directly influenced by salt, the most significant phase of salt tectonics in the history of the CNS in terms of volume of salt redistributed and dissolved. Lower–Middle Triassic ‘minibasins’ subsided into the salt over much of the CNS. Different initiating mechanisms operated around the basin–differential loading was important near sediment input points, and thin-skinned extension on the platforms balanced a pulse of basement extension in the Central Graben. Minibasin subsidence was accommodated by salt wall growth; salt was eroded during periods of base-level fall and redeposited locally and in the Southern North Sea. The minibasins touched down on basement progressively from the basin margins towards the Central Graben, the timing being a function of salt thickness. The diachronous halting of accommodation space generation combined with the effects of erosion led to a variety of field relationships between the Middle–Upper Triassic Skagerrak sands and the underlying salt–minibasin system. Regionally, the Skagerrak sands are thickest near input points and in the Central Graben, where accommodation space was supplemented by basement faulting. The platforms were subaerially exposed and eroded for much of the period between the late Triassic and late Jurassic. On these platforms, the distribution of the relatively thin Late Jurassic section was controlled by a combination of regional tilt and pre-existing topography created by differential erosion (including dissolution) of the salt walls versus grounded minibasins. The late Jurassic Fulmar shallow marine sands are thickest on the differentially eroded salt highs, which constituted several hundred foot-deep palaeovalleys. Rifting occurred in the Central Graben where tilts of detachments on top of major basement fault blocks caused thin-skinned extension in the overlying strata, either for the first time, or reactivating Triassic minibasin systems. The Fulmar sands are thickest where basement-controlled accommodation space was accentuated by detached fault systems (e.g. fallen diapirs) near sediment input points in the Central Graben. The Early Cretaceous was a time of tectonic stabilization following Late Jurassic basement rifting, although the perched detachment systems in the major basement graben continued to evolve. Accommodation space for Upper Jurassic and Lower Cretaceous turbidites was influenced by up-dip salt high collapse and down-dip ‘rim syncline’ growth. By the end of the Early Cretaceous these systems had stabilized and the surrounding basement highs were sufficiently blanketed to shut off salt dissolution. Chalk isopach variations pick out subtle regional tilts and local basement fault reactivations. Major flexural subsidence in the Tertiary tilted the basin again and precipitated local cover slip, e.g. extensional faults balanced by down-dip folds on the West Central Shelf and diapir rejuvenation on the graben margin. A variety of inversion structures are present in the CNS and are dominantly Cretaceous and Tertiary in age. Several styles of inversion structure occur including thin-skinned and thick-skinned compression, diapir rejuvenation, compaction and dissolution-related features. Salt governed the geometry of many of these structures and in turn the geometry of the Tertiary sand fairways.

Oil Migration in a Major Growth Fault: Structural Analysis of the Pathfinder Core, South Eugene Island Block 330, Offshore Louisiana

The Pathfinder core, collected in the South Eugene Island Block 330 field, offshore Louisiana, provides an outstanding sample of structures associated with a major growth fault that abuts a giant oil field and that is thought to have acted as a conduit for hydrocarbon migration into the producing reservoirs. Where cored, the growth-fault zone cuts semiconsolidated Pliocene-Pleistocene mudstone and is over 100 m wide. The fault zone in the core consists of three structural domains, each characterized by a distinct rock type, distribution of fault dips and dip azimuths, and distribution of spacing between adjacent faults and fractures. Although all of the domains contain oil-bearing sands, only faults and fractures in the deepest domain contain oil, even though the oil-barren fault domains contain numerous faults and fractures that are parallel to those containing oil in the deepest domain. The deepest domain is also distinguished from the other two domains by a greater degree of structural complexity and by a well-defined power-law distribution of fault and fracture spacings. Sediments in this domain behaved as competent rock with respect to fault and fracture spacing, whereas the departure from power-law distribution of fault and fracture spacing in the other two domains may reflect deformation of unconsolidated sediment. This departure from a power-law spacing distribution in the upper two domains, combined with stable isotope data that indicate low-temperature water-rock interaction within a gouge zone that separates these two fault domains, indicates that the faults in those domains may have been active only early in the history of the growth fault zone, when the sampled sediments were at shallow burial depths. Thus, these faults may predate oil migration. In contrast, the faults in the oil-bearing domain appear to have been active later in the fault zone’s history, when the sediments faulted as competent rock and when geologic and organic geochemical investigations indicate oil migrated into the Block 330 reservoirs. Even though oil is present in sands throughout the core, its restriction to faults and fractures in the youngest sampled portion of the fault zone implies that oil migrated only through that part of the fault that was active during the time when oil had access to it. The absence of oil in fractures or faults in the other, probably older, fault domains indicates that the oil was never sufficiently pressured to flow up the fault zone on its own, either by hydraulic fracture or by increased permeability as a result of decreased effective stress. Instead, fluid migration along faults and fractures in the Pathfinder core was enhanced by permeability created in response to relatively far-field stresses related to minibasin subsidence.