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Niger Delta gravity-driven deformation above the relict Chain and Charcot oceanic fracture zones, Gulf of Guinea: Insights from analogue models
Abstract
The Niger Delta is a classic example of a passive margin delta that has gravitationally deformed above an overpressured shale decollement. The outboard Niger Delta clastic wedge, including the Akata Formation overpressured shale decollement, is differentially thickened across relict oceanic basement steps formed at the Chain and Charcot fracture zones. In this study, five analogue models were applied to investigate the effects of a differentially thickened overpressured shale decollement across relict stepped basement on Niger Delta gravity-driven deformation. Gravity-driven delta deformation was simulated by allowing a lobate, layered sandpack to deform by gravity above a ductile polymer. A first series of experiments had a featureless, horizontal basement whereas a second series had differentially thickened polymer above Niger Delta-like basement steps. Two syn-kinematic sedimentation patterns were also tested. Surface strains were analysed using digital image correlation and key models were reconstructed in 3D. All five model deltas spread radially outward and formed plan view arcuate delta top grabens and arcuate delta toe folds. The arcuate structures were segmented by dip-oriented radial grabens and delta toe oblique extensional tear faults, which were formed by along-strike extensional strains during spreading. Basement steps partitioned delta toe gravity spreading into dual, divergent directions. Similarities between the analogue model structures and the Niger Delta strongly suggest a history of outward radial gravity spreading at the Niger Delta. The Niger Delta western lobe has potentially spread downdip more rapidly due to a thicker or more highly overpressured underlying Akata Fm. shale detachment. Faster western lobe spreading may have produced the Niger Delta toe ‘dual lobe’ geometry, perturbed up dip Niger Delta top growth fault patterns, and implies that western lobe toe thrusts have been very active.
... In this paper, the style of deformation in the deep-offshore domain of the Niger Delta and the role of the shales of the Akata Formation have been extensively analyzed. According to J. E. Wu et al. (2015), deep-seated, transversal, inherited faults related to the segmentation of the passive margin offset the base of the Akata shales. These produced structural depressions controlling shale thickness variations (Cobbold et al., 2009), which in turn influence modes of deformation and structural style. ...
... The role of transversal, inherited faults in the Niger delta has been already illustrated at various scales (Benesh et al., 2014;J. E. Wu et al., 2015). Deep-seated structures like the Chain and Charcot fault zones ( Figure 14) are part of a series of transversal features that offset the passive margin during Aptian times (Briggs et al., 2009;Lehner & De Ruiter, 1977). These fault zones show an average spacing of about 150 km. Sharp variations of regional isopach maps across transversa ...
... Sharp variations of regional isopach maps across transversal structures indicate that they control the overlying units of the prograding Niger Delta (e.g., Cobbold et al., 2009;MacGregor et al., 2003). J. E. Wu et al. (2015) proposed that changes in Figure 13. (a) Sketch illustrating how the volume increase in the anticlines core, due to their amplification, can balance the volume of sediments accumulated within the subsiding synclines (assuming that the sedimentation rate is equal or higher than the deformation rate). ...
Based on a large 3D seismic data set in the deep‐water domain of the Niger Delta, this study challenges previous interpretations involving the occurrence of multiple detachments and extensive thrust flats, illustrating timing and mode of shales flow at the toe of the gravity system. Five units of syn‐kinematic sediments, reaching a maximum thickness of ∼800 m, accumulated in the tectonically subsiding synclines during fold amplification between ∼9.5 and ∼1.4 Ma. The volumes of syn‐kinematic units roughly balance those of the shales accumulated in the thickened cores of WNW trending anticlines. This feature is consistent with folding resulting from buckling controlled by the competence contrast between isopach Cenozoic units and underlying overpressured shales of the Akata Formation. A dense network of NE‐SW striking oblique extensional faults offsets a prominent anticline characterized by a NE‐SW trend (which is almost perpendicular to the regional fold trend). These faults form a narrow, continuous deformation zone extending for tens of kilometers along and beyond the length of the anticline. The faults, rooting within the shales of the Akata Formation, formed since ∼5 Ma and deform the seabed. Displacement distribution suggests mechanical interaction between isolated fault segments within the deformation zone. The latter is interpreted as the shallow expression of a deep‐seated fault zone inherited from the segmented passive margin and marked by gravity and magnetic data. Our results, providing a comprehensive picture of active deformation features and their relationships with deep‐seated faults, shed new light into the modes of interaction between gravity systems and underlying basement structures.
... Fazlikhani & Back, 2015b;Sapin et al., 2012), temporal and spatial variations in sediment accumulation rates (e.g. Chima et al., 2022;Rouby et al., 2011), the existence of fluid expulsion structures (Back & Morley, 2016), the variation of dips on base mobile-shale surfaces (Wu et al., 2015), the deep flow of mobile shale under either brittle or ductile conditions (e.g. Cohen & McClay, 1996;, and the growth history of supra-shale faults (e.g. ...
... Isochore maps show temporal changes in sediment thickness, which we infer record changes in accommodation driven by the migration of deformation (Figure 9a-f) (e.g. Erdi & Jackson, 2021;Wu et al., 2015). ...
... Figures 3 and 4). This interpretation is consistent, for example, with the study of Wu et al. (2015) and with the results from several physical models of shale-rich deltas (e.g. Mourgues et al., 2009), which suggest that landward-dipping listric fault systems are better developed when the dip of the mobile shales and their basal surface is relatively gentle. ...
Deformation on shale‐rich continental margins is commonly associated with thin‐skinned extension above mobile shales. Normal faulting and shale mobilization are widespread on such margins, being associated with and controlled by progradation and gravitational failure of deltaic sedimentary wedges. However, due to uncertainties in seismically imaging mobile shales, our understanding of problems like how base mobile‐shale controls deformation, and the shape, size, and distribution of shale structures remain poorly understood. We here use 3D seismic reflection data from the platform region of the Tarakan Basin, offshore eastern Indonesia to investigate the temporal and spatial evolution of thin‐skinned deformation of the Neogene sedimentary section. Our detailed seismic interpretation reveals up to 74 km long, concave‐ and convex‐into‐the‐basin normal faults, dipping both basinward (eastwards) and locally landward (westwards), which detach downwards on a basal mobile shale (Early‐Middle Miocene). The base of the mobile shale unit dips gently (< 17o) seaward, although older (Eocene‐Early Miocene), rift‐related normal faults originate local structural highs deforming the base of mobile shales. Our isochore (thickness map) analysis shows that supra‐shale normal faulting commenced in the Middle Miocene and was accompanied by the formation of hanging‐wall rollover folds and associated crestal grabens, with the subsequent along‐ and across strike migration of the deformation related to the nucleation, lateral linkage, and reactivation of individual fault systems. Updip growth normal faulting was also accompanied by the downslope flow of mobile shale, accompanied by parallel and perpendicular variations of the differential loading in the delta system, and local contraction and mobile shale‐upbuilding, resulting in the growth of large, margin‐parallel shale anticlines further downdip. The growth faults and anticlines are locally overlain by up to 5 km tall of mud pipes and volcanoes. We suggest that variations in the rate of sedimentary loading, mobile shale flow, fault growth, and gravitational failure of the delta system above a seaward‐dipping, but locally rugose base mobile‐shale surface, controlled Neogene deformation in the Tarakan Basin. We also demonstrate how variations in the trend and dip of the base mobile‐shale surface influences the position, timing of formation, and evolution of supra‐shale normal faults and their associated depocenters along shale‐rich, deltaic margins.
... Other than these two global reviews, most previous studies of PMFB have focused on specific examples of PMFB such as the Atwater-Mississippi fan foldbelt Peel et al., 1995), Perdido foldbelt Trudgill et al., 1999), Mexican Ridges foldbelt (Salomón-Mora et al., 2009), Campeche Salt Basin (Hasan and Mann, 2021), Foz do Amazonas (Reis et al., 2016), Pará-Maranhão and Barreirinhas Basin (Oliveira et al., 2013), Pelotas Basin (Strozyk et al., 2017), Niger Delta Wu et al., 2015;Zhang et al., 2021), Kwanza Basin (Evans and Jackson, 2020;Hudec and Jackson, 2004), Orange River Basin (de Vera et al., 2010), Lamu Basin (Cruciani et al., 2017;Cruciani and Barchi, 2016) and Rovuma Basin (Cai et al., 2020). Academic and industry studies have produced a considerable amount of high-quality and deep-penetration seismic reflection data from PMFB, and several of those seismic profiles and cross sections are used in this analysis. ...
... The depositional history of the Niger Delta began in the Eocene with deposition of a clastic wedge that is 12 km ( 7.5 mi.) thick. The Niger PMFB evolved above an overpressured shale detachment (Wu et al., 2015). ...
... The gravity system of the Niger Delta PMFB is characterized as a headward delta-top zone of extension, a midslope zone of translation and a delta-toe zone of contraction (Steele et al., 2009;Wu et al., 2015). The distance from the updip extensional zone to the downdip contraction zone is approximately 300 km ( 186 mi.). ...
Deepwater passive margin foldbelts (PMFB) are the present and future focus of hydrocarbon exploration and production due to the abundance of undrilled structural traps, the presence of natural oil seeps and advances in deepwater drilling technology. PMFB are characterized by linked updip extension and downdip contraction detached over a basinward dipping layer of salt or overpressured shale. PMFB are found on rifted- and tranform-passive margins around the world such as the Gulf of Mexico, offshore south Atlantic and East African margins. The linked kinematic system is driven by a combination of gravitational force resulting from thermal subsidence, onshore cratonic uplift, tectonic overstepping of the margin and deltaic depositional loading. Symmetric detachment folds typically dominate systems associated with salt detachment, whereas shale-detached PMFB are dominated by imbricate thrusts, fault-bend folds and fault propagation folds. The difference in structural style is caused by different rheology of the detachment layer.
... Other than these two global reviews, most previous studies of PMFB have focused on specific examples of PMFB such as the Atwater-Mississippi fan foldbelt Peel et al., 1995), Perdido foldbelt Trudgill et al., 1999), Mexican Ridges foldbelt (Salomón-Mora et al., 2009), Campeche Salt Basin (Hasan and Mann, 2021), Foz do Amazonas (Reis et al., 2016), Pará-Maranhão and Barreirinhas Basin (Oliveira et al., 2013), Pelotas Basin (Strozyk et al., 2017), Niger Delta Wu et al., 2015;Zhang et al., 2021), Kwanza Basin (Evans and Jackson, 2020;Hudec and Jackson, 2004), Orange River Basin (de Vera et al., 2010), Lamu Basin (Cruciani et al., 2017;Cruciani and Barchi, 2016) and Rovuma Basin (Cai et al., 2020). Academic and industry studies have produced a considerable amount of high-quality and deep-penetration seismic reflection data from PMFB, and several of those seismic profiles and cross sections are used in this analysis. ...
... The depositional history of the Niger Delta began in the Eocene with deposition of a clastic wedge that is 12 km ( 7.5 mi.) thick. The Niger PMFB evolved above an overpressured shale detachment (Wu et al., 2015). ...
... The gravity system of the Niger Delta PMFB is characterized as a headward delta-top zone of extension, a midslope zone of translation and a delta-toe zone of contraction (Steele et al., 2009;Wu et al., 2015). The distance from the updip extensional zone to the downdip contraction zone is approximately 300 km ( 186 mi.). ...
Deepwater Sedimentary Systems: Science, Discovery and Applications helps readers identify, understand and interpret deepwater sedimentary systems at various scales – both onshore and offshore. This book describes the best practices in the integration of geology, geophysics, engineering, technology and economics used to inform smart business decisions in these diverse environments. It draws on technical results...
... Cretaceous period marked the end of the rifting phase and the beginning of gravity tectonism as the main deformational process (Oladele and Ojo, 2013;Wu et al., 2015). Syn-sedimentary faults, folding and interplays between subsidence and sediment supply from Benue, Niger and Cross Rivers have controlled the progradation of the Niger Delta sequence, a distance of about 250 km (Merki, 1972;Evamy et al., 1978;Adesida et al., 1997). ...
... Syn-sedimentary faults, folding and interplays between subsidence and sediment supply from Benue, Niger and Cross Rivers have controlled the progradation of the Niger Delta sequence, a distance of about 250 km (Merki, 1972;Evamy et al., 1978;Adesida et al., 1997). The clastic wedge of the Niger Delta is about 12 km thick and in plan-view is seen as a concave-seaward lobate profile (Wu et al., 2015). The prograding depositional facies of the Niger Delta comprises of a continental shallow massive sand sequencethe Benin Formation; a coastal marine sequence of alternating sands and shalesthe Agbada Formation, and a basal marine shale unitthe Akata Formation which have been documented by Evamy et al. (1978), Tuttle et al. (1999) and Reijers (2011). ...
... This suggests that they may have structurally controlled these formations. Southwest of the study area, the Insert Figure 7: Low-pass filtered map of the residual fields for (a) magnetic and (b) gravity data generated from the power spectrum of their respective residual fields Insert Figure 8: Theta PGT maps of (a) magnetic and (b) gravity data generated from their respective low-pass filtered residual grids.NW-SE amplitude peak which extends from the south, cutting through the CFZ and CHFZ to Cotonou, coincide with the 2000 m sediment thickness boundary(Wu et al., 2015) within the Basin. This suggests that it may have structurally influenced sedimentation within the Gulf of Guinea. ...
This study mapped deep crustal network of the Equatorial Atlantic Fracture Zones in southern Nigeria using regional scale magnetic and gravity data in an attempt to link the Equatorial Atlantic Fracture Zones to structures of similar trend in the continental margin of southern Nigeria. The data were analysed by removing the regional trend of the data and isolating deep-seated anomalies of interest. Thereafter, the theta potential gradient tensor method was applied to delineate the network of the Equatorial Atlantic Fracture Zones and their linkage to other associated deep-seated lineaments in southern Nigeria. Within the limit of the study area, the results obtained revealed the network of the Romanche, Chain and Charcot Fractures Zones, and their extent, several hundreds of kilometers within the landmass of southern Nigeria. The build-up and transformation of the mapped lineaments further supported evidence of extensional crustal deformation within the Niger Delta Basin and Benue Trough axis which may have influenced rifting in the early Cretaceous.
... The Niger Delta basin is one of the classic shale tectonic province in the world where deepwater gravitational systems are widely developed and studied (e.g., Cohen & McClay, 1996;Corredor et al., 2005;Damuth, 1994;Jolly et al., 2017Jolly et al., , 2016Morley & Guerin, 1996;Pizzi et al., 2020;Rouby et al., 2011;Wu & Bally, 2000;Wu et al., 2015). Previous studies found that the structural development of deepwater gravitational systems is closely related to deformation within the ductile substrate, composed of undercompacted and overpressured mobile shales (e.g., Bonini, 2003;Cohen & McClay, 1996;Deville et al., 2006;Duerto & McClay, 2011;Fillon et al., 2013;Hansberry et al., 2014;Morley, 2003;Morley & Guerin, 1996;Morley et al., 2011Morley et al., , 2017Morley et al., , 1998Wu & Bally, 2000). ...
... 10.1029/2020TC006491 3 of 33 Figure 1. Regional structure map (a) and cross section (b) of the Niger Delta showing the distribution of gravity-driven structural domains across the basin, after Wu et al. (2015). The study area is located at a site between the translational and compressional domain, north of the study area from Maloney et al. (2010). ...
... a The shale thickness is calculated using a 2,600 m/s seismic velocity. tear faults (e.g., Benesh et al., 2014;Mueller & Talling, 1997), and they have been widely recognized in the deepwater Niger Delta (Figure 1a) as a result of differential radial spreading of the delta (Wu et al., 2015). In addition to the strike-slip displacement, this oblique fault also has a dip-slip component exhibiting high-angle thrust fault in cross section (Figure 10d). ...
This paper presents a three‐dimensional (3D) seismic‐based case study (∼1,200 km²) from the deepwater Niger Delta to examine the role of shale deformation in the structural development of a deepwater gravitational system. Tectono‐stratigraphic interpretation reveals that this system consists of two sets of major fold‐thrusts laterally separated by a central oblique detachment fold. A prominent shale thick beneath these structures is believed to have originated from tectonic deformation rather than a pretectonic thick, due to its complex internal structures. Seismic mapping of the growth units indicates synchronous initiation of the oblique detachment fold with the main thrusts and gradual growth in response to thickening shales. Downslope gravitational contraction is not considered the direct cause for the oblique shale‐detachment fold. Evidence from the 3D shale distribution and deformation styles within the shale unit reveals that shales that were squeezed out of adjacent shale‐thinning areas “flowed” laterally into the detachment‐fold core. Based on the spatial variation in structural deformation and growth strata distribution, this study proposes a model that considers differential contraction and differential loading from syntectonic sediments as two key factors leading to the 3D shale redistribution which ultimately determines the deformation styles and evolution history within the overburden. Additionally, seismic imaging within the shale unit recognizes various internal structures ranging from hundreds to thousands of meters in scale, and confirms what has been suggested in previous studies, that redistribution of shales occurred through a combination of multiscale brittle failures, ductile folding, and plastic flows.
... The basic elements of GDSs are extensional faults, folds, thrust faults (Rowan et al. 2004;de Vera et al. 2010) and associated mobile layer(s). Extension and contraction of GDSs are accommodated by normal faults in the proximal region and by folds and thrust faults in the distal margin, respectively (Corredor et al. 2005;Wu et al. 2015). Fold-and-thrust belts at the distal provinces of the gravitational collapse systems are increasingly becoming targets for hydrocarbon exploration in deepwater continental margins (Morley et al. 2011;Cruciani et al. 2017). ...
... The GDSs in the Niger Delta on the west margin of the Africa are well-studied. They are believed to be closely related to the progradation of the delta and to have been triggered by several factors, such as overpressured porefluids and mechanically weak layers (Corredor et al. 2005;Lei et al. 2010;Peter 2011;Wu et al. 2015). A similar GDS was observed near the study area, at the toe of the Baram Delta, NW Borneo Franke et al. 2008;Hesse et al. 2010;King et al. 2010;Cullen 2010). ...
... In general, the transitional domain has features of both the extensional and contractional domains, analogous to the results of Wu et al. (2015). It is a transition zone between the extensional and contractional domains. ...
Gravitational collapse structures are commonly observed in shelf-margin deltas underlain by mobile shales. However, these structures are rarely accompanied by mud diapirs. This paper presents an updated study of the gravity-driven system in the West Luconia Deltas, a shelf-margin delta system, in the Kangxi Depression, southern South China Sea. Compared to the classical shale-detachment model, the syn-collapse deformation in the contractional domain in this study is accommodated mainly by thrust faults combined with mud diapirs rather than simply imbricated thrusts. Based on seismic interpretation and structural analysis, this gravity-driven system is divided into three domains, the extensional domain, the contractional domain and the transitional domain. All of these domains are intruded by mud diapirs. The quantitative analysis of the amounts of extension and contraction suggests that these structures mainly resulted from gravitational collapse rather than the tectonic compression. Quantification of the relative contributions of gravity spreading and gliding indicates that the gravitational collapse was mainly driven by gravity spreading. Two episodes of collapse are suggested by the analysis of the progradation of the West Luconia Deltas and the features of the syn-collapse structures. The first episode was minor and not accompanied by diapirism, whereas the second episode was major and accompanied by diapirism. The entire evolution of the GDS is divided into five stages: (1) the first episode of the gravitational collapse, lasted from the earliest Middle Miocene to the earliest Pliocene; (2) the deposition of an interval between the syn-collapse strata and the mobile shale, occurred in the Early Pliocene.; (3) the initiation of the second episode of the gravitational collapse, lasted from the Early Pliocene to the Late Pliocene; (4) the attenuation and basinward migration of the gravity-driven deformation, lasted from the Late Pliocene to the Early Pleistocene; and (5) the ending of the gravitational collapse, lasted from the Early Pleistocene to the present. The last four stages were accompanied by intensive diapirs which pierced the overlying strata and became targets for the hydrocarbon exploration.
... Indeed, their deformation is achieved exclusively by gravity and they do not require additional shortening (Worrall and Snelson, 1989;Cobbold et al., 1995). Within a toe fold-and-thrust belt, three zones are differentiated downslope (e.g., Connors et al., 2009;Krueger and Grant, 2011;Mahanjane and Franke, 2014;Wu et al., 2015): ...
... In our model this zone is simulated by the inner domain, characterized by the extension that balances the transfer of material from the inner zone to the piedmont ( Figs. 2A and 3A). Overall, the normal faults of the inner domain draw a mirror arc with respect to the arc ( Figs. 2A and 3A) outlined by the toe fold-andthrust belt (Wu et al., 2015). In natural cases comparable to our model, passive rotation of structures and block rotation have been described with similar features to those observed during our model evolution [Okari oil field in the Niger Delta: e.g., Aminu and Olorunniwo (2011); Figs. ...
... 3A and 4A). In this zone, both arc-perpendicular and arc-parallel extension have been detected in toe fold-and-thrust belts (Wu et al., 2015); 3) a zone where the extension in the first two zones is balanced with shortening, which generates a fold-and-thrust belt. In our model, these compressive structures correspond to the curved topographic high (Fig. 4A). ...
We performed analogue experiments to investigate the genesis and the progressive formation of piedmonts. A silicone-sand parallelepiped was bounded by wood strips. The central part of the front strip was removed to permit the analogue material to flow through a gate, by gravity only in the first experiment, and also pushed from behind in the second one. In both cases, a piedmont arc formed in front of the gate, whose end points represented the arc tips of natural piedmonts. Deformation in the brittle layer (sand) was mostly accommodated by normal faults in the first experiment, versus oblique-slip faults in the second experiment. Extension separated blocks that rotated in opposite directions. In the piedmont, arc-parallel and arc-perpendicular stretching was observed. Our results can be applied to toe thrust belts or collapsed foreland fold-and-thrust belts, floored by evaporites and/or overpressured shale formations.
... Whiteman, 1982;Doust and Omatsola, 1990). Gravity deformation in the Niger Delta is controlled by the combination of (i) seaward-dipping slope at the shelf-slope break; (ii) deeply buried overpressured marine shales and (iii) high sedimentation/progradation of delta wedge (Whiteman, 1982;Doust and Omatsola, 1990;Damuth, 1994;Cohen and McClay, 1996; Morely and Guerin, 1996;Graue, 2000;Corredor et al., 2005;Cobbold et al., 2009;Wiener et al., 2010;Sapin et al., 2012;Wu et al., 2015;Fig. 1.5). ...
... Rapid sedimentation and progradation of the thick deltaic succession above the overpressured marine shales are the main triggering mechanism of shale mobility in the Niger Delta (Doust and Omatsola, 1990;Cobbold et al., 2009;Mourgues et al., 2009;Wiener et al., 2010;Sapin et al., 2012;Wu et al., 2015). The build-up of the Niger Delta up to 4,000 m below the sea-level, coupled with the greater bulk density of the overlying delta wedge (the Agbada Formation), often result to the vertical and lateral squeezing of the overpressured marine shales (the Akata Formation) into mobile shales (Burke, 1972). ...
The tilting of the western Niger Delta early during its evolution in the Cretaceous, resulted in the development of basin centre-dipping regional seismic surface, which facilitated gravity gliding from the Cretaceous to the Serravallian. An overall increase in sedimentation rates occurred in the western Niger Delta onwards from the Serravallian to the Pliocene and most of the Pleistocene, favouring gravity spreading. Mobile shales have been active in the western Niger Delta for at least the Burdigalian, and controlled submarine channels morphology, distribution and depositional patterns up to the Pliocene and the Pleistocene. The Pliocene and Pleistocene stratigraphy of the western Niger Delta intraslope basins were controlled by allocyclic forcings of 400 ka and 100 ka eccentricity, linked to glacio-eustatic sea-level changes over the Messinian-middle Pleistocene and the middle Pleistocene-present respectively, and basin tectonics forcing linked to shale tectonics. Despite the relatively lower sedimentation rates estimated on the western Niger Delta continental shelf and slope since the early Pliocene, the overall increase in gravity collapse, sediment thickness and distribution from the shelf to the deep basin, suggest an overall increase in sedimentation in this region. However, in the eastern Niger Delta, the actively subsiding counter regional normal faults and inferred delta lobe migration are thought to have contributed to the ongoing reduction in sedimentation since the Pleistocene. This PhD research not only improves our understanding of the tectonostratigraphic evolution of the western Niger Delta, highlighting the complex structural and stratigraphic trapping styles notably in the underexplored deepwater systems, but also shows the presence of syn-rift deposits that may host the Cretaceous-Paleogene petroleum play in the offshore western Niger Delta.
... A description of each attribute is shown in Table S1 (supplementary information can be accessed through the following link: S1). As with most subsurface studies, there are limited well penetrations to independently verify the analyzed faults within the study area; as such, "real" faults will be evaluated from our analyses by verifying cross sections for systematically offset reflectors (i.e., fault throws) and by assessing their map view likeness to known fault and fracture network patterns from geologic studies (Peacock et al., 2016) and structural analog models (Wu et al., 2015). These interpreted subtle faults and fractures should be calibrated to well data (e.g., image logs or cores) when it becomes available. ...
... T308 Interpretation / May 2021 algorithms are more quantitative and can perform repeated tasks with as many voxels as needed more quickly compared to humans. However, humans can recognize larger scale fault patterns (Wu et al., 2015;Peacock et al., 2016) and put them in a geologic context. For instance, a human interpreter can infer the presence of a fault by observing that the most-positive and most-negative curvature anomalies bracket a coherence anomaly. ...
Subtle fault detection plays a vital role in reservoir development studies because faults may form baffles or conduits that significantly control how a petroleum reservoir is swept. Small-throw faults are often overlooked in interpreting seismic amplitude data. However, seismic attributes can aid in mapping small faults. Over the years, dozens of seismic attributes have been developed that offer additional features for interpreters with associated caveats. Using the Maui 3D seismic data acquired in the Offshore Taranaki Basin, New Zealand, we generate seismic attributes that are typically useful for fault detection. We find multi-attribute analysis provides greater geological information than would be obtained by the analysis of individual attribute volumes. We extract the geological content of multiple attributes in two ways: interactive co-rendering of different seismic attributes and the unsupervised machine learning algorithm self-organizing maps (SOM). Co-rendering seismic attributes that are mathematically independent but geologically interrelated provides a well-integrated structural image. We suggest eight combinations of sixteen various attributes useful for a human interpreter with interest in fault and fracture detection. Current interpretation display capabilities constrain co-rendering to only four attribute volumes. Therefore, we use principal component analysis (PCA) and SOM techniques to efficiently integrate the geological information contained within many attributes. This approach gathers the data into one classification volume based on interrelationships between seismic attributes. We show that our resulting SOM classification volume better highlights small faults that are difficult to image using conventional seismic interpretation techniques. We find SOM works best when a fault exhibits anomalous features for multiple attributes within the same voxel. On the other hand, human interpreters are more adept at recognizing spatial patterns within various attributes and can place them in an appropriate geologic context.
... Cowie et al., 2000, Soliva andSchultz, 2008]. The material was homogeneously deformed [McClay and Ellis, 1987, Vendeville and Cobbold, 1988, Wu et al., 2015 by a flexural bend in the upper part of the crust, which compensated for the massive subsidence that occurred at the southern margin of the basin. ...
... In the region, the post-rift succession, which is made of predominantly marine shales and silts (Lawrence et al. 2002;Brownfield and Charpentier 2006), has been tectonically affected by the frontal thrusts of the Niger Delta (Fig. 3b). These thrust faults are gravitydriven structures that formed in the Cenozoic as a result of the collapse of deltaic, overthickened shelf-slope strata (Damuth 1994;Corredor et al. 2005;Rouby et al. 2011;Wu et al. 2015;Pizzi et al. 2020). In the offshore region of the Niger Delta, the post-rift succession contains a number of sandy reservoirs units, mainly Eocene to Pliocene in age, which are commonly ascribed to deep-water channel systems and associated submarine fan systems (Tuttle et al. 1999;Liu et al. 2013;Zhang et al. 2015Zhang et al. , 2016. ...
This research seeks to test the hypothesis that rheology of submarine gravity-flows influences the planform geometry of associated fan lobe deposits. If this control exists, seismic geomorphology has the potential to predict the sedimentary characteristics of these deposits by analysing relatively simple seismic outputs such as time structure maps and amplitude maps. This predictive power could potentially aid identification of prospective reservoirs through automated processes of seismic interpretation.
In this work, Upper Cretaceous and Neogene deepwater reservoirs from East and West Africa with porosity in excess of 20% and multi-Darcy permeability were analysed. The overall external geometry of the reservoirs was described by integrating seismic amplitude extractions, structure and isochron maps derived from high quality 3D seismic data. The maps were generated from detailed interpretation of the top and base reservoir reflections. Core photos of the reservoirs were integrated into the seismic project and their examination allowed identification of dominant sedimentary facies, enabling a proposal for the classification of these deposits in terms of flow processes.
The research indicates confinement of flows exerts an obvious control on the shape of the deposits and resulted in sedimentary bodies with marked elongation at the seismic scale. However, the analysis of the reservoirs at the core scale indicated that these are dominated by massive, poorly sorted, medium- to coarse- grained sandstones, commonly with floating oversize clasts, as well as beds with disrupted laminae and sandy conglomerates. These characteristics may indicate ‘depositional freezing’ from high-density flows with yield strength, which may have also contributed to the limited lateral spreading of the deposits analysed.
The research also highlights the importance of the non-unique relationship between sedimentary processes and external geometries of resulting deposits at the seismic scale. This means that deposits with similar geometries may have markedly different sedimentary characteristics.
This work highlights the increasing importance of integrating detailed seismic geomorphology studies with accurate examination of core data to validate models of gravity flow processes and test their predictive value.
... The wide range of model materials applied to simulate different tectonic processes has been summarized and discussed in different review articles (Koyi, 1997;Ranalli, 2001;Reber et al. 2020), classifying them as materials used to model (1) the upper crust, (2) the middle crust, and (3) the lower crust and the mantle. These include sand (Davis & Dahlen, 1983;McClay & Ellis, 1987a;Krantz, 1991;McClay, 1996;Gutscher et al. 1998;McClay et al. 1998;Bonini et al. 2000;Schellart, 2000;Cobbold et al. 2001;Lohrmann et al. 2003;Panien et al. 2006;Dooley & Schreurs, 2012;Gomes, 2013;Wu et al. 2015;Dotare et al. 2016;Klinkmüller et al. 2016;Maestrelli et al. 2021;Santolaria et al. 2021), gelatine Brizzi et al. 2016), foam rubber Rosenau et al. , 2017, clays (Krantz, 1991;Henza et al. 2010;Gomes, 2013;Bonini et al. 2016;Bonanno et al. 2017;Hatem et al. 2017 (Panien et al. 2006), sugar (Schellart, 2000) and dry mortar (Gutscher et al. 1998). ...
In this study, we present a new granular rock-analogue material (GRAM) with a dynamic scaling suitable for the simulation of fault and fracture processes in analogue experiments. Dynamically scaled experiments allow the direct comparison of geometrical, kinematical and mechanical processes between model and nature. The geometrical scaling factor defines the model resolution, which depends on the density and cohesive strength ratios of model material and natural rocks. Granular materials such as quartz sands are ideal for the simulation of upper crustal deformation processes as a result of similar nonlinear deformation behaviour of granular flow and brittle rock deformation. We compared the geometrical scaling factor of common analogue materials applied in tectonic models, and identified a gap in model resolution corresponding to the outcrop and structural scale (1–100 m). The proposed GRAM is composed of quartz sand and hemihydrate powder and is suitable to form cohesive aggregates capable of deforming by tensile and shear failure under variable stress conditions. Based on dynamical shear tests, GRAM is characterized by a similar stress–strain curve as dry quartz sand, has a cohesive strength of 7.88 kPa and an average density of 1.36 g cm ⁻³ . The derived geometrical scaling factor is 1 cm in model = 10.65 m in nature. For a large-scale test, GRAM material was applied in strike-slip analogue experiments. Early results demonstrate the potential of GRAM to simulate fault and fracture processes, and their interaction in fault zones and damage zones during different stages of fault evolution in dynamically scaled analogue experiments.
... Gravitational collapse can be related to the presence beneath a sedimentary succession of ductile evaporites (e.g. Morley & Guerin, 1996), but on many margins it has been shown to be linked to the presence of overpressured shale levels, such as the Niger Delta (Morley & Guerin, 1996;Hooper et al., 2002;Morley et al., 2011;Wu et al., 2015), the northwestern Gulf of Mexico (Rowan et al., 2004), the Brunei margin (Berthelon et al., 2018) or the Amazon Fan (Cobbold et al., 2004;Oliveira, 2005;Morley et al., 2011;Perovano et al., 2011). ...
The Amazon River culminates in one of the world’s largest deep-sea fans, a shelf-slope wedge that has prograded seaward since the late Miocene while undergoing gravitational collapse above shale detachments. In order to examine the overpressure mechanisms acting in the Amazon Fan and affecting its gravity tectonics, I developed an innovative approach based on the integration of modeling methods commonly used in the oil &gas industry, applied here for the first time to a collapsing passive margin depocenter. Two regional seismic sections were interpreted, depth-converted, structurally restored and then used for basin and geomechanical modelling to investigate overpressure mechanisms and deformation along the detachments and associated extensional and compressional faultsduring the deposition of up to 6-10 km of sediment over the last 8 Ma. The modeling results provide information on the evolution of pore pressure and temperature and their implications for the operation of the southeast and northwest structural compartments of the gravity tectonic system. It is found that themain control on gravity tectonics was sediment supply, which differed in magnitude and style between the SE and NW compartments. In both compartments, progradation of the Amazon Fan drove the basinward migration of the deformation front in response to a seaward migration of overpressure along the detachment. In the SE compartment, fault activity was observed only during periods of higher sedimentation, whereas in the NW compartment, continuous fault activity reflected constant high sediment input over the last 8Ma. Disequilibrium compaction (undercompaction) is argued to be the primary mechanism of overpressure in the Amazon Fan, however the secondary role of inflationary overpressures cannot be excluded. The temperature-dependent smectite-illite transition window was present within the fan, mainly above the detachment in the SE compartment, but at the level of the detachment on the inner and outer part of the NW compartment. Thermogenic gas generation (by primary and secondary cracking) did not affect the gravity system in the SE compartment, where most gas was expelled prior to the growth of the fan and thereafter trapped in shale-rich layers beneath the detachment, whereas in the NW compartment it has contributed to higher pore pressure on the detachment and some faults. Thus, temperature-driven fluid mechanics played a different role in terms of inflationary overpressure in the two structural compartments. These differences are in part due to differing different crustal types beneath the two compartments, which syn- to post-rift basin modeling shows produceddistinct thermalhistories,modulated by thermal blanketing during the growth of the Amazon fan. In particular, this led to lower heat flow in the NW compartment over the last 8 Ma, accounting for the greater depth of the smectite-illite transition window relative to the detachment and the later expulsion of thermogenic gases. The findings of this thesis thus provide new insights into the evolution of pore pressure during the growth and collapse of the Amazon Fan, and distinguish particularities of each structural compartment linked to its long-term history. The results also show that the integration of basin modeling methodologies provides an extremely useful tool to investigate the tectonic and sedimentary dynamics of Late Cenozoic depocentres,even when there is limited data. As a perspective of future work, dedicated studies of the crustal and thermal history of the Amazon margin might be done.
... Gravitational collapse can be related to the presence beneath a sedimentary succession of ductile evaporites (e.g. Morley and Guerin, 1996), but on many margins it has been shown to be linked to the presence of overpressured shale levels, such as the Niger Delta (Morley and Guerin, 1996;Hooper et al., 2002;Morley et al., 2011;Wu et al., 2015), the northwestern Gulf of Mexico (Rowan et al., 2004), the Brunei margin (Berthelon et al., 2018) or the Amazon Fan (Cobbold et al., 2004;Oliveira, 2005;Morley et al., 2011;Perovano et al., 2011). ...
The Amazon Fan provides a natural laboratory to study the generation of overpressure, due to rapid late Cenozoic burial that has resulted in gravitational collapse above shale detachments. Here we examine collapse systems for the first time using the techniques of petroleum systems analysis. We propose an integrated methodology based on numerical modeling constrained by the structural restoration of a seismic profile across the southwestern fan. The results provide information on the evolution of pore pressure and temperature and their implications for the operation of the detachment and overlying extensional and compressional faults during the deposition of up to 6 km of sediment over the last 8 Ma. The modelled thermal history implies that fluid release by smectite-to-illite transformation has taken place within the thickening sedimentary succession, but has not significantly contributed to pore pressures along the detachment. Modeling of hydrocarbon generation and migration from source rocks beneath the fan indicates gas accumulated in successions at depths of 10²–10³ m beneath the detachment without influencing pore pressures along it. In contrast, model results indicate that overpressures have varied in response to disequilibrium compaction. Fault activity within the collapse system took place during phases of higher sedimentation rates, and ceased from 5.5 to 3.7 Ma when sediment supply to the SE fan decreased. From 2 Ma, renewed sediment flux and shelf-slope progradation led to a basinward migration both of overpressure along the detachment and of fault activity above it. We conclude that gravity tectonics in the Amazon Fan over the last 8 Ma have been mainly controlled by overpressures due to disequilibrium compaction, with secondary contributions from clay mineral transformation. Present-day pressure conditions show that the southeastern Amazon Fan is not at equilibrium and gravity driven deformation could occur at any time.
... Unlike orogenic belts, or accretionary prisms, the majority of fold belts developed on passive margins are controlled by gravitational instability rather than far-field plate convergence forces and Wu et al., 2015). The orange polygon is location of the 3D seismic data used in this work. ...
The temporal and spatial evolution of thrust fault arrays is currently poorly understood, and marine fold and thrust belts at the toe of passive margin gravitational systems, imaged by commercial 3D seismic reflection datasets, afford a unique opportunity to investigate this problem in three dimensions. Using an extensive 3D seismic data set and age data, the total cumulative strain (shortening) and interval strain rates have been calculated for 11 thrust-related folds mapped in the toe-thrust region of the southern lobe of the Niger Delta. For the first time, the sequence of thrust nucleation, propagation and linkage through time at a scale of 10 s km both along and across strike is documented. Short thrust segments had nucleated throughout the entire study area by 15 Ma. They then grew largely by lateral growth and linkage, increasing the fault trace length and generating asymmetric strain-distance plots, for the first 50% of their history. Thereafter, growth continued by shortening, with minimal along strike increase in fault length. Changes in shortening-distance data between adjacent structures across strike suggest that the change in growth mode occurred once the thrusts had linked in 3D through the common underlying detachment. Over the entire thrust array the strain rate varies through time, starting slowly (<200 m/Ma), then increasing between 9.5 and 3.7 Ma (200–400 m/Ma) before slowing down in the last ∼ 4 Ma (<150 m/Ma). The variation in strain rate is attributed to a change in boundary conditions of the gravitational system. An increase in sediment supply to the delta occurred in the late Miocene-Pliocene, driving higher shortening rates in the toe area. A subsequent reduction in sediment supply in the last ∼4 Ma led to a reduction in deformation rate and the cessation of activity on a number of the thrusts. Predictions of the critical taper wedge model are used to explain the near-synchronous growth of the entire thrust array over the last 15 Ma. Because sedimentation acts to lower the surface slope, the wedge can only continue to deform if shortening occurs over a wide area allowing the surface slope to build up. These new results suggest that models of piggyback fault propagation are not appropriate for deep-water fold and thrust belts.
... The Niger Delta lobes are linked laterally by a series of transfer faults, which also detach in the mobile shale, and extend across the slope from the shelf to the toe of slope, locally offsetting thrust-fold structures (Benesh, Plesch, & Shaw, 2014;Leduc, Davies, Densmore, & Imber, 2012;Morgan, 2004). Wu et al. (2015) propose that these large strike-slip faults partition deformation between the major sedimentary lobes following major oceanic fracture zones. Smaller examples, which have also been interpreted as tear faults, accommodate strain gradients between thrust sheets (Benesh et al., 2014). ...
The processes and deposits of deep‐water submarine channels are known to be influenced by a wide variety of controlling factors, both allocyclic and autocyclic. However, unlike their fluvial counterparts whose dynamics are well‐studied, the factors that control the long‐term behaviour of submarine channels, particularly on slopes undergoing active deformation, remain poorly understood. We combine seismic techniques with concepts from landscape dynamics to investigate quantitatively how the growth of gravitational‐collapse structures at or near the seabed in the Niger Delta have influenced the morphology of submarine channels along their length from the shelf edge to their deep‐water counterpart. From a three dimensional (3D), time‐migrated seismic‐reflection volume, which extends over 120 km from the shelf edge to the base of slope, we mapped the present‐day geomorphic expression of two submarine channels and active structures at the seabed, and created a Digital‐Elevation Model (DEM). A second geomorphic surface and DEM raster—interpreted to closer approximate the most recent active channel geometries—were created through removing the thickness of hemipelagic drape across the study area. The DEM rasters were used to extract the longitudinal profiles of channel systems with seabed expression, and we evaluate the evolution of channel widths, depths and slopes at fixed intervals downslope as the channels interact with growing structures. Results show the channel long profiles have a relatively linear form with localised steepening associated with seabed structures. We demonstrate that channel morphologies and their constituent architectural elements are sensitive to active seafloor deformation, and we use the geomorphic data to infer a likely distribution of bed shear stresses and flow velocities from the shelf edge to deep water. Our results give new insights into the erosional dynamics of submarine channels, allow us to quantify the extent to which submarine channels can keep pace with growing structures, and help us constrain the delivery and distribution of sediment to deep‐water settings.
... Such methodology permits to test the influence of some of the parameters that control the development of arcuate structural patterns in external zones. Some of these parameters are: (a) the variations in thickness of the deforming layers (e.g., Calassou et al., 1993;Mitra, 1997;Corrado et al., 1998;Soto et al., 2002;Storti et al., 2007); (b) the lateral variations in the rheology of the detachment and/or that of the deforming layers (Mitra, 1997;Macedo and Marshak, 1999;Cotton and Koyi, 2000;Schreurs et al., 2001;Bahroudi and Koyi, 2003;Luján et al., 2003Luján et al., , 2006bReiter et al., 2011); (c) the topography of the foreland (Marques and Cobbold, 2002); (d) the syn-tectonic sedimentation and/or erosion (Wu et al., 2015); (e) the presence of obstacles of different shapes and strength and/or previous structures Dominguez et al., 2000;Duarte et al., 2011;Ter Borgh et al., 2011); and (f) the shape, velocity, and motion direction of the indenter (Lu and Malavieille, 1994;Zweigel, 1998;Macedo and Marshak, 1999;Lickorish et al., 2002;Marshak, 2004;Crespo-Blanc and González-Sánchez, 2005;Crespo-Blanc, 2007, 2008Reiter et al., 2011;Crespo-Blanc et al., 2012Rauch, 2013). ...
The timing and kinematics of the different types of structures and the associated vertical-axis rotations that permit an arcuate external wedge to acquire progressively its curved shape throughout its deformation history—known as progressive arcs—are key questions in natural cases of arcuate fold-and-thrust belts that we want to address through analog modeling. We present laboratory models of fold-and-thrust belts formed with a backstop that deforms in map view to simulate progressive arcs in a thin-skinned tectonic regime. Our setup makes use of a deformable backstop rigid enough to push from behind the initial parallelepiped but deformable in map view. This innovative design permits us to increase the amplitude of the arc indenting in the model as its radius of curvature decreases, that is, it simulates a progressive arc. Taking the Gibraltar Arc external wedge situated in the western Mediterranean to scale our models in terms of rheology, velocities, and sizes, four types of experiments were made. We varied the type of substratum (sand or silicone), the silicone thickness, and the width and length of the initial analog pack in order to test the influence of each of these parameters on the resulting fold-and-thrust belts. All experiments led to the formation of arcuate wedges where strain was partitioned into: (a) arc-perpendicular shortening, accommodated by thrusts which main structural trend is broadly subparallel to the indenter shape and with divergent transport directions, and (b) arc-parallel stretching, accommodated by normal and conjugate strike-slip faults. The normal and strike-slip faults contributed to the fold-and-thrust belt segmentation and the formation of independent blocks that rotated clockwise and counterclockwise depending on their position within the progressive arc. Our experiments allow to simulate and understand the finite deformation mode of the external wedge of the Gibraltar Arc. Accordingly, they shed light on how an arcuate fold-and-thrust belt can develop progressively in terms of structural trend and transport directions, types and distribution of the structures accommodating strain partition, and timing of vertical-axis rotations.
... décollement (salt, overpressured mud or shale) (e.g. Rowan et al., 2004;Hamilton and De Vera, 2009;Butler and Paton, 2010;Butler and Turner, 2010;Morley et al., 2011;Wu et al., 2015;Scarselli et al., 2016;King and Morley, 2017) (Fig. 1b). Note that FTBs may have components of both plate convergence and gravitational deformation, creating hybrid systems, such as the Makran accretionary prism (Grando and McClay, 2007;Smith et al., 2012), the Cascadia (Washington) margin (McNeill et al., 1997), and the NW Borneo FTB (e.g. ...
... Our measurements of MR fold geometries are consistent with their previous observations (Fig. 13). Conceptual modeling in the Niger Delta suggests that an area of thicker underlying shale to a thinner overburden would result in a higher degree of deformation in the overlying strata and partitioned, faster gravitational spreading compared to the area with thinner shale (Wu et al., 2015). Variations in relative shale thicknesses have also suggested a link with over-pressure distribution, which is seen as a primary control on detachment in the Niger Delta (Wiener et al., 2010). ...
... Compared with traditional analysis of analogue experiments, the PIV technique improves the spatio-temporal resolution of deformation monitoring by at least one order of magnitude. Recent applications of this method in analogue experiments included strain analysis (e.g., Boutelier et al., 2014;Dooley et al., 2015), surface processes (e.g., Cruz et al., 2008;Reiter et al., 2011;Wu et al., 2015), seismic activity (e.g., Knappett et al., 2006;Rosenau et al., 2009), and stick-slip kinematics along faults (e.g., Yamada et al., 2006). By combining industrial CT and PIV techniques, we are able to measure velocity and strain fields, thus producing a high-resolution structural interpretation (e.g., see Fig. S1 in the electronic supplemental material). ...
The Nepal Himalaya fold-and-thrust belt is a classical two-décollement thrust–fold system. The belt has been extensively studied by analogue and numerical modeling, but the formation mechanism of the mid-crustal duplex structure beneath the northern part of the Lesser Himalaya remains debated. The formation of this thrust duplex has been attributed either to tectonic underplating along new ramps in response to the subducting Indian Plate, or to out-of-sequence thrusting along preexisting ramps. We conducted a series of experiments using analogue models, employing a model setup that was faithful to the geological setting of the Nepal Himalaya. We used industrial computed tomography (CT) and particle image velocimetry (PIV) to generate detailed deformation maps. The analogue models successfully reproduce the main structure of the Nepal Himalaya fold-and-thrust belt, showing that the two competing processes (i.e., underplating along new ramps and out-of-sequence thrusting along preexisting ramps) operate alternately to form the duplex structure. The models also show that underplating is associated with extensive foreland sedimentation, analogous to the foreland molassic sedimentation in the Sub-Himalaya, which leads to a systematic recession of the deformation front and a smaller horse. Finally, the recognition that out-of-sequence thrusting occurs in the interval between major underplating events provides important insights into the origin of the 2015 MW 7.8 Gorkha earthquake.
... Our study is the first stratigraphic and morphologic study of the principal body of the Yellow River subaqueous delta, and also marks the first time the discrete and spatially-confined lobate progradation, marked by laterally shifting locuses, have been mapped in detail over such a large spatial scale (tens to hundreds of kilometers). In previous studies (e.g., Olariu and Bhattacharya, 2006;Wu et al., 2015), laterallyshifting, deltaic lobate architectures were usually reported at much smaller scales, e.g., the shoreface near the existing river mouth, driven both by the avulsion of fluvial channels and seaward progradation. At small spatial scales, lobate architectures were also observed in many sediment-starved littoral/offshore environments, such as in estuarine tidal bars (Billy et al., 2012;Chaumillon et al., 2013), lobate sandbodies (Field and Roy, 1984) and linear barrier shorefaces (Liu and Goff, 2018). ...
Since 6 kyr BP, the subaerial deltas of the Yellow, the Yanshan and the Taihang river systems evolved along the Bohai Bay coast, forming several lobate progradational systems associated with frequent avulsions of these river systems. Their subaqueous parts consist of a multi-sourced subaqueous deltaic complex in Bohai Bay, but little is known about the complex's subsurface structure, morphology and evolutionary history. Here we investigate these characteristics using a dense, high-resolution seismic survey. Using stratigraphic superposition, seismic geometry, and the results of previous studies, we identified nine seismic units (SUs) in the post-Last Glacial Maximum stratigraphy, which are interpreted, from bottom to top, as: (1) fluvial/lacustrine sediments (SU0); (2) marine sands combined with estuarine/coastal deposits (SU1); and (3) a subaqueous fluviodeltaic complex (SUs2–8); these units correspond to the lowstand system tract, the transgressive and/or highstand system tracts, and the normal regressive system tract, respectively, and evolved as a function of eustatic fluctuations. The subaqueous fluviodeltaic complex is characterized by a discrete and spatially-confined lobate architecture, which is indicative of step-wise switching of subaqueous lobes. Two sets of progradational systems (PS), a northern one (SUs2–3) and a southern one (SUs4–8), were observed within the complex, tapering in opposite directions and bounded by an erosional unconformity (R3). These characteristics indicate that the two PS were sourced from different river systems and prograded at different times. R3 forms the base of the southern PS but cross cuts the northern PS, indicating that it is a diachronous surface marks the preexisting seafloor, which was further reworked/eroded by the progradation of the southern PS. In each PS, the internal subaqueous lobes correlated well with the onshore lobes/superlobes of the subaerial deltas, suggesting the lobate evolution of the complex is driven predominantly by alternating erosion and deposition caused by the variation in fluvial supply, which appears to be linked to the frequent avulsions of the major river systems, in addition to the multi-sourced supply. Based on the dipping geometries and the locations of the depocenters, we hypothesize that (1) the northern PS is the seaward extension of the ancient Haihe River's northern branch; (2) the bulk of southern PS in the middle and southern Bohai Bay is correlated with the principal body of the Yellow River subaqueous delta; (3) the majority of the volume of the northern PS in northern Bohai Bay consists of the modern Haihe River subaqueous delta.
... Lobate shoreface construction is common in deltaic settings, driven by both avulsion of fluvial channels and seaward progradation in different evolutionary timescales (e.g. Olariu and Bhattacharya, 2006;Wu et al., 2015). Our proposed mechanism (Fig. 11) is similar in suggesting a laterally shifting locus of deposition; however, rather than through avulsion, we propose lateral shifting opening, closing and migrating inlet channels within the barrier island itself. ...
Under rising sea level conditions, barrier islands are largely ephemeral features, eroded on the seaward side by the transgressing shoreline and reformed by overwash to a more landward position. Locally, however, and over shorter time scales, shorelines can either advance or retreat, even in an overall transgressive environment, and the stratigraphy and morphology of the shoreface can be significantly impacted by the evolution of shoreface-attached bedforms. Fire Island, New York, is a well-studied example of such variability, with a stable-to-accreting shoreline at the western end and a retreating shoreline on the eastern end. In this study, we seek to better understand these differences by investigating the lower-shoreface stratigraphy at both stable/accreting (Fire Island West, or FIW) and retreating (Fire Island East, or FIE) shorefaces, using ultra-high resolution chirp seismic reflection data. Within the barrier/marine sands (the seismic unit between seafloor and shoreface ravinement), we identify six seismic units (WSUs 1–6 from bottom to top) in the FIW survey and two units (ESU1 and ESU2 from bottom to top) in the FIE survey; these units constitute the modern lower shoreface wedge. The barrier shoreface in the FIW survey is dominated by discrete and spatially-confined lobes. Isopach maps indicate that the lobe shifting was an episodic process with westward-migrating depocenters. The prograding shoreface was constructed by this lobate deposition; we speculate that these are related to ebb deposition from ephemeral barrier breaches/inlets. In the FIE survey, ESU2 accounts for the majority accumulation of the barrier shoreface and it is more linear than the lobate structure observed within the FIW survey, possibly derived from eroded shoreface sediments. Portions of this unit are absent however, exposing lower Pleistocene units to the erosive forces.
... The mechanism required for faults within an evolving system to interact is generally discussed in terms of two fault models: the isolated model and the coherent model (Walsh et al. 2003a;Giba et al. 2012;Fossen & Rotevatn 2016). The isolated model suggests that a segmented fault array develops from random overlap and linkage of previously unrelated faults which initiate in a self-organized manner from natural heterogeneities and that strain is distributed homogenously (Cowie et al. 2000;Wu et al. 2015). According to the isolated fault model, faults develop during rifting from isolated heterogeneities within a rock volume by radial propagation and, as a result, individual segments of isolated structures expand laterally in three dimensions. ...
The initiation, growth and interaction of faults within an extensional rift is an inherently four-dimensional process where connectivity with time and depth are difficult to constrain. A 3D discrete element model is employed that represents the crust as a two-layered brittle-ductile system in which faults nucleate, propagate and interact in response to local heterogeneities and resulting stresses. Faults nucleate in conjugate sets throughout the model brittle crust; they grow through a combination of tip propagation and interaction of co-linear segments to form larger normal faults. Segment linkage occurs by merging of adjacent fault segments located along strike, downdip or oblique to one another. Finally, deformation localizes onto the largest faults. Displacement distribution on faults is highly variable with marked along-strike and temporal variations in displacement rates. Displacement maxima continuously migrate as smaller fault segments interact and link to form the final fault plane. As a result, displacement maxima associated with fault nucleation sites are not coincident with the location of the maximum finite displacement on a fault where segment linkage overprints the record. The observed style of fault growth is consistent with the isolated growth model in the earliest stages which then gives way to a coherent (constant-length) fault growth model at greater strains.
... décollement (salt, overpressured mud or shale) (e.g. Rowan et al., 2004;Hamilton and De Vera, 2009;Butler and Paton, 2010;Butler and Turner, 2010;Morley et al., 2011;Wu et al., 2015;Scarselli et al., 2016;King and Morley, 2017) (Fig. 1b). Note that FTBs may have components of both plate convergence and gravitational deformation, creating hybrid systems, such as the Makran accretionary prism (Grando and McClay, 2007;Smith et al., 2012), the Cascadia (Washington) margin (McNeill et al., 1997), and the NW Borneo FTB (e.g. ...
... The continental margin off the Niger Delta is undergoing outward radial gravity deformation caused by rapid seaward progradation (Damuth, 1994;Wu et al., 2015). This has created an upper extensional zone of listric faults, a translational zone of mud diapirs and ridges beneath the upper slope (Hovland et al., 1997), and a lower compressional zone of imbricated thrust structures beneath the lower slope and rise (Damuth, 1994;Morley and Guerin, 1996;Corredor et al., 2005) (Fig. 1). ...
Terraces on the modern seafloor are defined as topographically flat areas above the active submarine channel thalweg but within the confines of the channel-belt. They have been described from many modern submarine channels, but the controls on terrace distribution, evolution and stacking patterns are not well understood. In this study, we describe the architecture of the Mahin and Avon channel-belts and their associated terraces, located offshore Nigeria towards the northwest of the Niger Delta. The studied channel sections are < 10 km apart up-dip and converge downslope. They are on slopes with similar gradients, yet they have significantly different morphologies indicating that the interplay between sedimentary processes and channel evolution must be different.
... 4a). Experimentally this setting is produced in a sandbox experiment where sand is overlying a relatively homogeneously deforming material such as silica gel or basal rubber membrane (e.g., McClay and Ellis, 1987;Vendeville et al., 1987;Vendeville and Cobbold, 1988;Wu et al., 2015). As natural examples we could envisage a system comprising a A C C E P T E D M A N U S C R I P T ...
This paper presents a 3D seismic-based case study from the deep-water Niger Delta Basin to investigate sedimentary–tectonic interaction on growth sequence architecture within the thrust-related intraslope or piggyback basins. Gravitational contraction in the lower continental slope had yielded a series of thrust faults and associated folds in the study area, which formed several piggyback basins. These basins were filled by a suite of growth sequences with varying stratigraphic architecture. Analysis of the 3D seismic data recognized three primary seismic facies types respectively as: convergent, draping and chaotic, which contain seven subtypes. These facies types are combined to form different filling successions for convergent or chaotic growth sequences. The convergent growth sequences mainly occur in the deep section of basin fills during strong gravitational deformation, and always began with convergent-baselapping strata succeeded by convergent-thinning strata, representing pond-to-bypass transition in the ponded-basin accommodation space. The chaotic growth sequences mainly occur in the shallow section of basin fills in response to weak gravitational deformation, and usually began with debris-flow deposits succeeded by channel-levee complexes, reflecting dominant erosion-bypass processes in the slope accommodation space. A dynamic fill-and-spill model considering relationship between episodic sedimentation rate and structural growth rate is proposed to explain the formative mechanisms of growth strata units and associated successions. Interaction between glaciation or deglaciation and sea-level change and gravitational deformation history are suggested to be the factor which resulted in the complex stratal stacking patterns, including progradational or retrogradational stacking patterns within convergent growth sequences, and progradational stacking patterns within chaotic growth sequences.
In submarine settings, the growth of structurally‐influenced topography can play a decisive role in controlling the routing of sediments from shelf‐edge to deep water, and can determine depositional architectures and sediment characteristics. Here we use well‐constrained examples from the deep water Niger Delta, where gravity‐driven deformation has resulted in the development of a large fold and thrust belt, to illustrate how spatial and temporal variations in the rate of deformation have controlled the nature and locus of contrasting depositional styles. Published work in the study area using 3D seismic data has quantified the growth history of the thrust‐related folds at multiple locations using line‐length‐balancing, enabling cumulative strain for individual structures over time and along‐strike to be obtained. We integrate this information with seismic interpretation and facies analysis, focusing on the interval of maximum deformation (15 to 3.7 Ma), where maximum strain rates reached 7 %/Ma. Within this interval, we observe a vertical change in depositional architecture where: (1) leveed‐confined and linear channels pass upward in to (2) ponded lobes with erosionally‐confined channels and finally (3) channelized sheets. Our analysis demonstrate that this change is tectonically‐induced and diachronous across the fault array, and we characterise the extent to which structural growth controls both the distribution and the architecture of the turbidite deposits in such settings. In particular we show that leveed‐confined channels exist when they can exploit strain minima between growing faults or at their lateral tips. Conversely, as a result of fault linkage and increased strain rates submarine channels become erosional and may be forced to cross folds at their strain maxima (crests), where their pathways are influenced by across‐strike variations in shortening for individual structures. Our results enable us to propose new conceptual models of submarine channel deposition in structurally complex margins, and provide new insights into the magnitude of fault interaction needed to alter depositional style from leveed to erosionally confined channels, or to deflect seabed systems around growing structures.
Paleo-uplift is one of the main controlling factors that influence the style of deformation in salt-bearing fold-and-thrust belt (FTB). Whereas, how paleo-uplift amplitude influences salt-related deformations in FTB remains unclear. Here we designed four series of experimental models that contain a basal brittle décollement and a shallow ductile décollement. Our studies show that the presence of paleo-uplift in the ductile décollement has a profound impact on deformation propagation in the supra-salt overburden during the contraction, which localizes shortening strain in the proximal and central parts of the models and deters rapid deformation propagation towards the distal part. As the amplitude of the paleo-uplift increases, its influence on deformation propagation becomes stronger, resulting in out-of-sequence deformation towards the hinterland. The increase of paleo-uplift amplitude also controls the distribution and migration of rock salt during the contraction. Particularly, when the paleo-uplift height equals or exceeds the thickness of salt layer, the position of salt pinch-out or welding point localizes shortening strain and thereby initiates folding and thrusting deformation over the salt-base high. This process leads to salt fed into the fold and fault belt over the paleo-high and the uplift of the supra-salt layers. Our modeling results are highly comparable with natural examples of Kuqa FTB and provide insights to the salt-influenced deformations in the Qiulitage structural belt.
The interaction between sedimentary wedge dynamics and paleo-fracture zones is investigated offshore Western Niger Delta lobe (WNDL) to reconstruct the evolution of the delta from the Cretaceous to present. This was achieved through detailed regional seismic interpretation, calibrated with well data.
Our results suggest that high sedimentation rates in the WNDL since the Serravallian-Tortonian triggered the migration of the ‘Oligocene-Tortonian extensional zone’ and gravity spreading seawards (from a present-day onshore to a present-day offshore position), with extensional, translational and contractional deformation. An additional increase in sedimentation rate since the early Pliocene, further accelerated gravity spreading and the development of the present-day contractional front. A five-stage tectono-stratigraphic evolution of the offshore WNDL from the late Cretaceous-present is proposed.
Paleo-topographies formed by the Charcot and Chain Fracture Zones, exerted depositional control on the stratigraphic architecture of the offshore WNDL from the Cretaceous to Serravallian. Differential subsidence on both sides of the relict Charcot and Chain transform faults is responsible for the segmentation of gravity-driven deformation of the eastern and western Niger Delta lobes.
In addition, a comparison of the stratigraphic architecture of the Eastern Niger Delta Lobe (ENDL) and WNDL demonstrates a similar overall progradation and sediment bypass to the deep basin during the Pliocene. During the Pleistocene, the two lobes show a distinct evolution and architecture: the ENDL shows an overall retrogradation and sediment sequestration on the shelf whereas the WNDL displays an overall progradation and sediment bypass. This study documents long-term and large-scale control of delta dynamics and paleo-topography on gravity-driven deformation of the offshore eastern and western Niger Delta lobes, and similar analysis could be applied in the reconstruction of other passive margin basins.
Submarine channel complexes are often described as having a two-phase stratigraphic evolution where an initial phase of migration is followed by aggradation, generating a ‘hockey-stick shaped’ channel trajectory. However, the role of tectonic forcing in modifying time-integrated sedimentary architectures remains poorly understood. Here, we evaluate how tectonically driven changes in slope modify the evolution—both in terms of morphology and stratigraphic architecture—of submarine channels across a range of spatial scales from the fundamental architectural unit, a channel element, to the scale of a channel complex set, using examples from the Niger Delta system. From a 3D, time-migrated seismic reflection volume, we use amplitude extractions, frequency decomposition and RGB blending to determine channel stratigraphic architectures. These observations are used systematically to evaluate the development of cross-sectional and planform architectures as the channel systems interact with a range of active and pre-existing structural bathymetry. Our results indicate that while a channel complex’s stratigraphic architecture may be captured by a two-phase evolution on unstructured slopes, this model fails on structurally complex slopes. Unstructured slope channel complexes display a repeated arrangement of migration dominating the early stratigraphic record and subsequent aggradation. The late aggradational phase signals a decrease in the rate of growth in channel complex width and the rate of change in sinuosity relative to aggradation throughout the complex’s development. However, tectonically driven changes in sinuosity and the relative rates of channel migration and aggradation modify complex development significantly. We identify three end-member styles of channel-structure interaction, determined by the timing of bathymetry development and its associated style: (1) pre-channel structural bathymetry; (2) coeval positive relief, and (3) coeval negative relief. Where structural relief pre-dates channel inception, a principal adjustment is in the initial channel course with early channel elements being forced around positive relief of the structure, generating long-wavelength bends in the complex’s course. Where structure continues to modify slope creating positive and negative bathymetry during complex development, migration and bend development continue with complex width and channel element sinuosity increasing until abandonment. These observations demonstrate that submarine channel architecture and planform are highly sensitive to tectonic perturbation and we use these results to generate graphical models that show predicted architectural evolution of submarine channels on structurally complex slopes in general.
Many passive margins have deep-water, contractional fold belts that formed
above salt or shale. Margin failure, accommodated by proximal extension
and distal shortening, is caused by some combination of gravity gliding
above a basinward-dipping detachment and gravity spreading of a sedimentary wedge
with a seaward-dipping bathymetric surface. Gravitational failure is inherently selflimiting,
and sedimentation patterns provide fundamental control of deformation.
Continued shortening is driven primarily by shelf and upper-slope deposition, which
maintains the bathymetric slope and the gravity potential, and by increased basinward
tilting. Deformation is retarded or halted by distal thickening of the overburden
caused by the folding itself or by lower-slope and abyssal sedimentation. Net shortening
amounts and deformation rates are lower than in collisional/accretionary fold belts,
because the driving forces are weaker than those induced by lithospheric plate motions.
Structural styles vary but depend largely on the nature of the deˆcollement layer,
not the driving forces. Fold belts detached on shale typically comprise basinwardvergent
thrust imbricates and associated folds because of the relative strength and
frictional behavior of the plastic shale. Deformation does not occur until there is sufficient
overburden, and it is facilitated by high fluid pressures. In contrast, salt is a viscous
material with essentially no strength, which leads to symmetrical detachment folds and
early deformation beneath only a thin overburden. Moreover, the surface slope can be
reduced by proximal subsidence into salt and distal inflation of salt, and much of the
shortening can be accommodated by lateral squeezing of diapirs and salt massifs and by
extrusion of salt nappes.
Scaled physical models illustrate the importance of progradation as a trigger for salt tectonics and formation of allochthonous sheets. Regional extension and contraction were excluded in the models. In our experiments, prograding wedges above a tabular, buoyant salt layer with a flat base expelled the salt basinward, forming the following structures proximally to distally: (1) sigmoidally distorted initially planar wedges, (2) relict salt pillows and salt welds, (3) basinward-dipping expulsion rollover and crestal graben, (4) rollover syncline, (5) landward-facing salt-cored monocline, and (6) distal inflated salt layer. This deformation zone amplified and advanced basinward during progradation; however, no diapiric salt structures formed. Over a buoyant salt layer whose basement had steps facing landward, progradation initially formed a broad anticline where salt flow was restricted across each basement step. Distal aggradation pinned the anticline and enhanced differential loading. The anticline actively pierced its crest, which had been thinned by faulting and erosion. Thereafter, the diapir grew passively, locally sourcing allochthonous salt sheets. This deformation cycle repeated over each basement step so that the age, amplitude, complexity, and maturity of salt-related structures decreased basinward. As each allochthonous salt sheet was buried and evacuated by sediment loading, arcuate peripheral normal faults formed along the sheet`s trailing edge, detached wrench faults formed along its lateral edges, and active piercement at its leading edge allowed the sheet to break out and climb stratigraphic levels. This process formed a multitiered complex of salt sheets that migrated basin-ward with time. Restorations of examples from various salt tectonic provinces support our model results.
Using conceptual reasoning and results from physical models, we describe the mechanical and kinematic characteristics of sediment wedges spreading seaward above a viscous evaporitic layer. Spreading can occur if the distal sediment overburden is thin and weak, or if it comprises preexisting salt bodies. Spreading is accommodated by proximal extension and an associated rise of diapiric ridges, by midslope seaward translation, and by distal shortening. Rapid sediment progradation can lead to the reactivation of older distal folds by later extension. Either retrogradation or renewed sedimentation following a long depositional hiatus can reactivate older grabens and diapirs in shortening. Copyright © 2005. The American Association of Petroleum Geologists. All rights reserved.
The deep-water Niger Delta includes two large fold and thrust belts, products of contraction caused by gravity-driven extension on the shelf that exhibit complex styles of thrusting. These contractional structures formed above multiple detachment levels in the over-pressured shales of the Akata Formation. Using the patterns of growth sedimentation, fold shapes, fault-plane seismic reflections, and combined conventional and shear fault-bend folding theories, we describe and model the structural styles and kinematics of the fault-related folds and imbricate thrust systems that compose these belts. Individual fault-related folds, involving both forethrusts and backthrusts, are characterized by long planar backlimbs that dip less than the associated fault ramps, with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation. Forelimbs are short compared to backlimbs, but growth strata show more consistent dips that suggest a component of folding by kink-band migration. Thus, we employ a combination of classic and shear fault-bend fold theories to describe these structures, including the influence of a weak basal detachment zone in the overpressured shales. We expand upon these theories to model the kinematics of imbricate thrust systems, which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta. Regional patterns of folded growth strata are used to define break-forward, break-backward, and coeval thrusting involving single and multiple detachment levels. We define two main types of imbricate thrust systems: type I system with a single basal detachment level and type II imbricate system with multiple basal detachment levels, which cause massive structural thickening of the Akata Formation and refolding of shallow thrust sheets. Through the sequential restoration of two regional cross sections across these systems, we resolve the structural styles, the timing and sequences of thrusting, as well as the regional amounts of shortening, all of which have important implications for hydrocarbon maturation and charge in the deep-water Niger Delta. Copyright © 2005. The American Association of Petroleum Geologists. All rights reserved.
In the Niger Delta, which has prograded onto the African continental shelf and neighbouring oceanic crust since the Eocene, abundant thin-skinned structures provide evidence for slope instability. The structures have detached on shale of the Akata Fm. At the toe-of-slope, the outer thrust belts contain compressional structures. The surface slopes and apical angles of the thrust wedges are small. Also, fore-thrusts alternate with back-thrusts. This is evidence for a small resistance to basal slip and it implies high values of fluid overpressure. Thrust faults are planar through the overburden, but strongly curved beneath it. By analogy with physical models, we take this as evidence for normal fluid pressures through the overburden, and for a pressure ramp, inducing seepage forces, beneath it. The seismic velocity of the Akata Fm. beneath the thrust belts is abnormally slow. This probably results from overpressure. Fluid vents are abundant within hanging-wall anticlines. A large one at a culmination communicates from a fore-thrust to the surface. The contents are seismically transparent, whereas the host rock is well stratified. We attribute the vent to fluidization. A nearby deep well encountered normal fluid pressures throughout the Agbada Fm., but a strong overpressure ramp beneath it. We argue that the most likely mechanism for explaining the observations is chemical compaction as a result of hydrocarbon generation, and it is suggested that the thrust belts have advanced seaward by a feedback mechanism.
Delta systems developed above ductile substrates such as overpressured shales and salt have been modelled using layered sand-packs above ductile silicone polymer layers. Gravity spreading of progradational sedimentary wedges produces delta-top and upper delta-slope grabens linked to delta-toe contractional fold-thrust and diapir zones. The delta-top grabens are bound by both regional and counter-regional listric growth faults. A basinward-stepping sequence of regional, counter-regional followed by regional faulting is commonly developed. Polymer pillows and ridges commonly develop in the footwalls of the major listric extensional faults and may evolve into reactive diapirs. Successive progradational loads generate new delta-top or upper delta-slope graben systems on top of older contractional belts where the ductile polymer layer has been thickened significantly. The analogue model results in cross-section show many similarities to examples of natural deltas and differential sedimentary load systems such as offshore Angola, the Niger and Nile Deltas, Kutai Basin, Kalimantan, the Baram delta, Brunei and the Orinoco delta, Columbus basin and offshore Trinidad.
In a preliminary series of experiments, using physical models mechanical processes of extensional tectonics have been investigated at various scales. By a suitable choice of model materials, experiments were performed at low cost in a natural gravity field. Upper layers of the lithosphere were modelled using sand; lower layers, using silicone putties of two different densities; the mantle asthenosphere was modelled using honey. The models deformed under their own weight or under absolute horizontal tension. Rates of extension were controlled using a stepper motor. Surface deformation and faulting were monitored using 35 mm time-lapse photography. Lower lithosphere topography was photographed through the transparent asthenosphere. Fault patterns in models with lithosphere only, were observed by serial sectioning. Otherwise, the brittle-ductile interface was observed after suctioning off the sand.
Simple experiments with uniformly extended sand layers only show that; (i) spacing of normal faults is a measure of the layer thickness; (ii) the length of fault trace increases with the amount of downthrow; and (iii) faults tend to form domino domains. Some experiments with a brittle layer on a ductile substrate show a mechanism of passive rifting where; (i) major faults occur in conjugate pairs, defining rift valleys; (ii) minor faults localize additional extension in rift-valley floors; and (iii) isostatic uplift of the viscous substrate causes uplift and tilting of rift rims. In freely floating continents, gravitational spreading leads to: (i) highly localized extension and thinning at continental margins and (ii) internal rifting.
ABSTRACT In the Nile deep-sea fan, thin-skinned deformation detaching on a layer of Messinian salt has generated an upslope to downslope progression from growth faults, to polygonal minibasins bounded by salt ridges, to buckle folds. Such progression is common in salt-bearing passive margins, where gravity spreading of the salt–sediment system causes proximal thin-skinned extension on the shelf and upper slope, and distal contraction along and in front of the lower slope. In the Eastern Nile deep-sea fan, this structural progression seems to be restricted to a corridor bounded by NW–SE-trending lineaments more than 200 km in length. These are associated with salt ridges and record strike–slip movements. In the absence of a large grid of deep-penetrating seismic data accurately imaging the basement, different likely hypotheses have been advanced about the origin of this corridor: (1) it may result from possible deep-seated tectonics related to the Rift of Suez, combined with salt-related deformation or, (2) by complex interaction between the overburden's gravity spreading and pre-existing pre-Messinian paleo-topographic features, particularly the possible buttressing effect of a seamount located North of the eastern Nile deep-sea fan. In order to understand how this corridor could have been generated, we used a series of physical experiments to test the effect on three-dimensional spreading of a sediment lobe of the following parameters: (1) active, crustal, oblique extension, (2) a dormant subsalt graben, (3) a passive buttress, such as a seamount and (4) complex paleo-topographic features along the Egyptian margin affecting initial salt distribution. These experiments show that the presence of a distal buttress, combined with a complex Messinian topography best explain the complex deformational pattern observed in the eastern Nile deep-sea fan.
Experimental models indicate that sedimentary lobes deposited above mobile evaporites deform gravitationally by spreading outward radially. Radial spreading forms both concentric and radial families of thin-skinned grabens that are then pierced diapirically by salt ridges. The overburden blocks located between the ridges become subsiding depocenters. Later, continued spreading after depletion of the salt layer forces salt ridges and diapirs to fall, forming rapidly subsiding bathymetric lows that channel or trap younger sediments. Spreading causes large horizontal overburden movements whose distribution and orientation change when the locus of regional deposition shifts. Copyright © 2005. The American Association of Petroleum Geologists. All rights reserved.
Many deltas exhibit gravitational deformation of their sedimentary cover. In these systems, the décollement layers do not always consist of rock salt but sometimes of overpressured shale. Unlike salt, the efficiency of detachment in shale depends on the magnitude of fluid overpressures and it varies through time and space, as rapid sedimentary burial progrades into deeper water. As a result, the gravity deformational domains are progressively translated seaward. Sandbox models involving high air pore pressures were used to simulate such gravity-driven shale tectonics in prograding deltas. Models were built with sand of various permeabilities and air was injected to simulate the mechanical effects of fluid overpressure. Our apparatus for the injection of air allowed us to control subsurface pressures in space and time during the experiments, and it was used to simulate the advance of the front of the overpressured domain during the sedimentary progradation. In our models, sand kept obeying a frictional behavior, for medium to high pore pressures, and the detachment appeared as very thin shear bands. Compressional belts that formed during the experiment were dominated by asymmetric basinward-verging fore-thrusts, as is often observed in deep-water, shale-detached foldbelts. Where the value of fluid pressures approached that of the lithostatic stress, sand was fluidized, resulting in ductile strains analogous to what occurs in highly overpressured mobile shale. During progradation, ancient buried thrustbelts were reactivated, thereby controlling later extension. During the experiments, sand volcanoes, analogous to mud volcanoes, formed in relation with tectonic structures. Some of them developed near normal faults but many of them formed directly above old buried thrusts.
Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50–80 km long while it increases dramatically at higher rates (100–1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.
We investigate the evolution of passive continental margin sedimentary basins that contain salt through two-dimensional (2D) analytical failure analysis and plane-strain finite-element modelling. We expand an earlier analytical failure analysis of a sedimentary basin/salt system at a passive continental margin to include the effects of submarine water loading and pore fluid pressure. Seaward thinning sediments above a weak salt layer produce a pressure gradient that induces Poiseuille flow in the viscous salt. We determine the circumstances under which failure at the head and toe of the frictional–plastic sediment wedge occurs, resulting in translation of the wedge, landward extension and seaward contraction, accompanied by Couette flow in the underlying salt. The effects of water: (i) increase solid and fluid pressures in the sediments; (ii) reduce the head to toe differential pressure in the salt and (iii) act as a buttress to oppose failure and translation of the sediment wedge. The magnitude of the translation velocity upon failure is reduced by the effects of water.
Regional integration of results from conventional exploration geochemistry, structural analysis, and gravity-magnetic data provide a comprehensive new understanding of Niger Delta petroleum systems. Nigeria is the 12th largest producer of crude oil in the world. Daily oil production from the Niger Delta is 2.1 million bbl, and recoverable reserves are estimated to be about 22.5 billion bbl. Historically, structural play types have dominated, although large stratigraphic traps have also been discovered. The basin has matured through one cycle of successful exploration, and future success depends on linking the geology of the shelf and onshore areas to deep-water areas and exploiting new play types in older producing areas. Three petroleum systems are present in the Niger Delta and delta frame: Lower Cretaceous (lacustrine), Upper Cretaceous-lower Paleocene (marine), and Tertiary (deltaic). One biodegraded seep oil from Nigerian tar sands along the northern flank of the Dahomey Embayment has been correlated to Neocomian source rocks in Ise-2 well. A source rock extract and pyrolyzate of the seep are similar to the Bucomazi petroleum system in the Lower Congo Basin. Oil recovered from Paleogene sandstones in Shango-1 well are inferred to be derived from Upper Cretaceous-lower Paleocene source rocks identified in Epiya-1 well, consisting of type II and II-III kerogens. The principal source for oil and gas in the Niger Delta is the Tertiary deltaic petroleum system, consisting of type II, II-III, and III kerogens. On the basis of oils and source rocks, source fades variation characteristic of this system has been regionally mapped in the northwestern part of the delta. Similar trends exist delta-wide and are responsible, along with burial, for controlling the complex distribution of gas and oil across the delta.
Since 1952, the Niger Delta has been explored primarily for oil. At the end of 2000, at least 5200 wells had been drilled on the delta. Initial reserves from this drilling total about 66,100 MMBOE. This has resulted in 240 producing fields. Current production from these fields amounts to more than 2 million BOPD. Exploration of the offshore deep-water areas started in 1995 and has resulted in the discovery of four giant fields and other minor discoveries. Most publications on the Niger Delta refer to the area as an oil basin, but there exists a very large gas-reserve base. Gas statistics have been included here because of the increased importance of gas to Nigeria and its recognition of this as an important source of energy for the area. All of the data contained in this paper includes information gathered and updated to the end of the year 2000.
This paper describes one of the world's major hydrocarbon provinces, the Niger delta. An introduction describes the regional setting of the delta, the deltaic framework and morphology, and the present morphology. Major sections within the paper consider the stratigraphy, basin architecture and tectonics, as well as oil and gas systems and a review of exploration activity. There are a number of interpreted regional seismic profiles. The paper is concluded with a reference list. -D.J.Sanderson
Basin development of the Atlantic margin of Africa, according to seismic evidence, began with rifting, in an alluvial and lacustrine environment, followed by regional subsidence and later marine conditions. Evaporites commonly were deposited during the transition from rifting to subsidence. Regional subsidence probably began after separation of continental blocks. Oceanic fracture zones and related tectonic trends onshore allow for a subdivision of the African Atlantic marginal belt into four segments: Northwest Africa, Equatorial, Cameroon-Gabon-Angola, and Walvis Ridge-Cape. Each segment has its own basin history.
The Tertiary Niger delta covers an area of about 75,000 sq km and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft (9,000 to 12,000 m). The development of the delta has been dependent on the balance between the rate of sedimentation and the rate of subsidence. The source rocks of the Niger delta yield a light waxy paraffinic oil, which is transformed bacterially to a heavier nonwaxy crude at temperatures below 150 to 180 degree F. The observed uneven distribution of oil and gas in the delta cannot be explained in terms of the passage of the source rocks through the oil-generating zone into the gas-generating zone, with early structures receiving mainly oil and late traps receiving mainly gas. Rather, the hydrocarbon distribution probably is the result of original heterogeneity of the source rock and of segregation during migration and remigration.
The Sanga Sanga Block contains four large to giant hydrocarbon fields in mid- to upper Miocene deltaic sandstones of the Mahakam Delta, eastern Kalimantan (Indonesia). These fields occur in the northeast-trending Mahakam fold belt, which is characterized by long, tight, fault-bounded anticlines and broad synclines and cored by overpressured shales. Onshore sections of the fold belt are strongly deformed, uplifted, and eroded, whereas the eastern offshore sections are little deformed and buried by the progradational delta wedge. Section balancing of depth-converted seismic lines, together with scaled analog modeling, was used to develop a new tectonic model of inverted delta growth faults for the evolution of the Mahakam fold belt. Section balancing shows that the fault-bounded anticlines of the Sanga Sanga Block are formed by contractional reactivation of early delta-top extensional growth faults. The change from gravity-driven extension to regional contraction occurred at around 14 Ma. Anticlinal folds controlled local sedimentation patterns and influenced the distribution of the reservoir channel sands in the main hydrocarbon fields. Scaled analog models of progradational loading above a ductile substrate produced delta-top extensional growth faults and 'depobelts,' together with delta-toe fold-thrust. Contraction inverted the extensional growth faults and depobelts, producing tight, fault-bounded anticlines. The results support the model of delta inversion and, thus, the most viable explanation for the geometric, kinematic, and mechanical evolution of the structures in the Sanga Sanga Block. The inverted delta model has applications to other hydrocarbon-bearing deltas around Borneo and in other contracted delta systems.
The margin between Côte d’Ivoire and the Niger Delta is a region with a common structural history, this being reflected in similarities in the stratigraphic response and play fairways identified across the region. There has been significant exploration on the narrow shelf characterizing the margin, resulting in a series of modest oil and gas discoveries. It is shown in this paper that many of the aspects of the plays in the unexplored deep-water regions of the margin are considerably more favourable to the development of giant fields than those on the shelf. This play-fairway review is based on the integration of existing publications with focused studies of multiclient 3-D seismic data over a number of areas.
Play fairways are classified by seismic sequence and trap type, with an analysis of each undertaken. The most attractive deep-water play types are: (1) anticlinal traps involving late syn-transform (Apto-Albian) and early post-transform (Late Cretaceous) reservoirs, (2) combination traps involving ponded turbidites on the shoreward flanks of these highs, and (3) stratigraphic traps associated with large Late Cretaceous submarine fan complexes. The anticlinal play is associated with the terminations of the St Paul and Romanche fracture zones, with the more recent structuring generally associated with the latter. 3-D imaging and amplitude mapping is critical to prospect delineation, particularly for the combination and stratigraphic plays. Active kitchens are evidenced involving Early and Late Cretaceous source rocks in the Côte d’Ivoire and western Ghana to Nigeria segments of the region, which are consequently upgraded. Considerable volumetric potential is indicated that promises to make the region one of significant new exploration activity in coming years.
The interpretation of 2-D and 3-D seismic data acquired over the ultradeep water (1500–4000 m), lower delta-slope region of the Niger Delta indicates the continuation of the principal stratigraphic subdivisions out onto the continental rise. The Niger Delta sediment apron is still over 4 km in thickness in 4000 m of water. The continuation of a thick sediment pile into ultradeep water, coupled with the existence of a proven petroleum system on the higher parts of the slope, extends the potential region for mature Palaeogene and possibly older, oilsource rocks, up to and beyond the limit of current drilling technology. Contrary to previous models, the Miocene-Holocene Agbada Formation does not thin out oceanward relative to the underlying Akata Formation. The continuation of this seismic-event-based division of the Tertiary succession into and throughout the ultradeep-water province indicates an abrupt change in depositional style with the arrival of major submarine fans.
The collapse of the Niger Delta sediment cone above overpressured muds of the Akata Formation is manifested in the ultradeep-water region as thrusting. The style of deformation varies across the area, depending on the level of detachment within the Akata Formation. The thrusting is divided into compartments by transfer zones against which toe-thrusts terminate or are offset. The toe-thrusts can be seen to have antecedents in the framework of rifts that predated breakup. These rift elements are imaged beneath the western delta region and reflect possible extent of extended continental crust in this area.
The lower slope is traversed by large turbidite channels that extend from the upper parts of the slope to beyond the outermost toe-thrusts. These channels provide a route for sand to be brought into these water depths from the shelf and from mud-diapir-bounded mini-basins higher on the slope.
The existence of underlying, presumed Late Cretaceous rift elements beneath parts of the ultradeep-water areas raise the possibility of additional source rocks within the ultradeep waters of the Niger Delta.
[1] A model is developed which couples fully saturated porous compaction to the viscous-plastic deformation of the skeleton matrix. The Darcy fluid flow during compaction is described by an advection-diffusion equation for the excess pressure with two source/sink terms that depend on the mechanical compressibility and viscous compaction of the pore space, the latter representing the effect of pressure solution. The incompressible deformation of the composite medium is described by a force balance equation and its rheology can be viscous, plastic, or viscoplastic (Bingham material). For the plastic and viscoplastic cases, the coupling between the compacting and plastically deforming parts of the system is through the Drucker-Prager frictional-plastic yield criterion modified by Terzaghi's principle, so that the yield strength depends on the effective dynamical pressure. The coupled system is solved using a two-dimensional (2-D) finite element method. Two problems are solved to demonstrate the behavior of our theory. The first considers compaction of a uniform sediment layer. The numerical results agree with the predictions of the nondimensional control parameters and previously published results. The second problem concerns 2-D kinematic progradation of deltaic sediments. Substratum and delta sediments have the same compaction properties and a Bingham rheology during deviatoric deformation, such that the delta undergoes linear postyield viscous flow. For certain depositional regimes, overpressure is generated. When pore pressures approach critical values, yielding occurs and the delta front fails and becomes unstable, spreading gravitationally under its own weight. The flow velocity is limited to geological rates by the Bingham viscosity. For the range of parameter values considered, pressure solution is the most effective mechanism for generating near-lithostatic fluid pressures that lead to initial failure, and it appears that mechanical compaction hardly contributes to the fluid overpressure at this stage.
The occurrence, in large shelf edge deltas, of gravity driven deformation related to overpressured shales is now well documented in many natural examples. In this paper, we explored the implications of the migration of the delta front on the kinematics of gravity driven deformation, in a case where the stratigraphic framework is detailed enough to discuss deformation kinematics and sedimentary supply at high temporal resolution (×0.1Myr).We determined the deformation for the last 4Myr at the scale of the whole Eastern Niger Delta. The upslope extensional domain showed EW asymmetric grabens. The transitional domain displayed (i) deformation restricted to folding to the east and (ii) a thicker sedimentary wedge and a larger deformation accommodated by thrust related folds to the west. The compressional front was a fold and thrust belt.Extensional deformation decreased in amount and rate over the Plio-Pleistocene. It also evolved significantly from distributed over the whole area to localized either to the western or eastern domains over that period. The compressional deformation followed the same evolution (i.e. the greater the extension, the greater the compression in either time or space), demonstrating the strong coupling between the extensional and compressional domains.Gravity deformation during the Plio-Pleistocene was driven by the continental slope spanning the coastal plain to distal deposits over the oceanic crust (about 2800m of gravity potential). The superimposition of the delta front migration area and the extensional domain show that the associated pressure gradient localized the extensional domain. Deformation was closely linked to migrations of the delta front and the sedimentary supply reaching the delta: (i) the decrease in extension rate over the Plio-Pleistocene is related to a decrease in sedimentary supply over the Plio-Pleistocene, and, (ii) the spatial variations in extension intensity and rate are related to spatial variations of sedimentary supply.
Two- and three-dimensional (3-D) seismic reflection data acquired over oceanic crust in the deepwater west Niger Delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading. Using the 3-D seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the Moho that are interpreted as thrusts. Thrust propagation results in the development of associated hanging-wall anticlines and footwall synclines. These structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them. The Charcot Ridge is located 140 km south of these thrusts and is a significantly larger structure. It is a triangular-shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the NE–SW-oriented Charcot Fracture Zone. Two interpretations are possible for the role of the fracture zone in the development of the Charcot Ridge: (1) A thin-skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward, with detachment occurring close to the level of the Moho. The ridge forms as a result of translation and folding above a crustal-scale ramp-flat thrust geometry. (2) A thick-skinned model where there is no detachment close to the Moho, with the thrust fault being much steeper, penetrating the crust and probably the mantle lithosphere. In this interpretation the structure formed owing to the compressional reactivation of the fracture zone. Approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 Ma ago. The Charcot Ridge represents one of the largest thrust structures to be identified in a passive margin setting. Many other compressional folds with the same orientation formed to the northeast in the Benue Trough, probably during the Santonian, as a result of a change in the spreading direction during South Atlantic rifting. We speculate that the same causal mechanism applies for the formation of the Charcot Ridge.
Scaled analogue models of fault structures are powerful tools for investigating the progressive development of extensional deformation. The results of a wide ranging programme of extensional modelling are briefly reviewed and are analysed in terms of the deformation mechanics and rheological behaviour of the modelling materials. Extensional models have been constructed using homogeneous dry quartz sand with isotropic behaviour; sand and dry china clay mixtures; with behaviour controlled by competency; and sand with dry vermiculite mica flakes to simulate anisotropic systems. Rheological tests show that these materials deform by Navier-Coulomb failure with friction angles between 30° and 35°. Extensional faults developed within the models are dilatant granular shear zones, whose widths are grain-size dependent, being widest in the sand models and narrowest in faults cutting the fine-grained clay layers. Initial fault-bedding cutoff angles are high, 60°-70°, but during progressive deformation shear strains may change this, depending upon the boundary conditions of the model. Measurements of angular shear strains within domino fault arrays show significant zones of shear either parallel to the basal detachment or parallel to bounding faults. Small bulk dilations are found in most models as a result of grain packing rearrangements during extension. The primary extensional architecture of the models is controlled by the underlying detachment configuration. Despite limitations to the models, it is believed that the rheologies, the modelling materials and the deformation mechanics within the models realistically simulate brittle deformation of sedimentary rocks in the upper crust.
We investigate the evolution of rifted continental margin shale tectonics using 2D finite-element models that couple sediment deformation to compaction-driven Darcy fluid flow via the effective stress. Fluid overpressures are generated in the models by a combination of mechanical compaction (grain rearrangement) and viscous compaction (grain dissolution and local reprecipitation), and lead to failure and flow of viscoplastic Bingham shale. Model results indicate that pore fluid pressures must be 90-95% of the lithostatic pressure to cause shale failure and delta destabilization. Mechanical compaction alone is insufficient to generate fluid overpressures required for failure in the models and viscous compaction is the primary source. The numerical models include delta progradation, lateral lithology variation, and flexural isostatic compensation. Seaward shale flow and associated overburden deformation results in the formation of landward regional and counter-regional fault-bounded extensional basins, a transitional domain of thickened and folded shale beneath the continental slope, and a seaward fold and thrust belt at the delta toe. The structural styles generated by the preliminary numerical models are compatible with features observed in unstable Cenozoic deltas (e.g. the Niger Delta) and provide additional insight into the fundamental relationships between deltaic sedimentation, fluid pressure generation, and margin-scale gravity spreading.
Scaled physical models of progradational sand wedges above a ductile polymer substrate have successfully simulated the formation of delta-top graben systems synchronous with the development of delta-toe fold-thrust belts. delta-top graben systems are characterized by paired regional and counter-regional listric growth faults. The latter form without total removal of polymer from beneath the graben system. The delta-toe fold-thrust belts are formed by polymer bulges and fault-related detachment folds that have thickening of the ductile polymer in their cores. Multiload wedge models show the formation of younger delta-top extensional graben systems above older fold-thrust belts. The analog model results show structures similar to those found in many delta systems and provide kinematic models for their evolution.
We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Cera and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present-day positions with respect to South America and Africa.
Recently acquired 2D seismic data shot over the western Niger Delta have enabled a pre-delta rift framework to be delineated inshore of a transform fault dominated continental margin which lies beneath the later, delta sediment apron. The delta apron has been deformed by toe-of-slope thrusting where faults have climbed from a detachment surface at or near the top of the over-pressured Akata Formation mudstones. The overlying mixed clastic succession of the Agbada Formation has been faulted by a broadly oceanward stepping series of NW-SE trending thrusts climbing from this detachment level. The principal thrusts have been offset by NE-SW trending transfer zones, the positions of which have been inherited from trends within a pre-delta rift framework that underlies part of the western delta slope.
3D seismic data partly covering the 2D grid show turbidite channel complexes at numerous stratigraphic levels within the Agbada Formation and clustered in particular areas of the slope. Commonly, submarine channels can be seen to have cut through the relief caused by folding at the positions of intersection with transfer fault zones. These data show the relationship between structure and channel formation and highlight the importance of transfer fault zones in localizing channel systems on the lower slope. Nevertheless, the 2D seismic data has provided an explanation for the location of the transfer zones within the toe-thrust belt in the form of an underlying structural framework, and both data types have contributed to the understanding of controls on reservoir distribution in an area where the principal sand delivery systems are perpendicular to the main structural trend.
We have done a series of simple tests on sandpacks, involving upward flow of compressed air through the pores and its effect on the yield strength. The ultimate objective is to model deformation coupled with fluid flow in sedimentary basins. For all tests, we used a single batch of Fontainebleau sand, sieved to a grain size between 0.200 and 0.315 mm, poured into a cylindrical container and then fluidized. The density of this sand was 1.585 g/cm3, irrespective of sand thickness. The lithostatic pressure was proportional to the thickness of the sandpack. A yield envelope was obtained by shearing the sandpack horizontally. Compressed air entered the base of the sandpack and flowed upwards through the pore spaces. For 69 measurements on air flow without shearing, a plot of discharge velocity versus gradient of fluid pressure is close to a straight line, verifying Darcy's law and yielding an intrinsic permeability of about 1.7 darcy for the sand. For 72 tests on simultaneous shearing and fluid flow, the estimated effective stress (lithostatic stress minus estimated pore fluid pressure) ranged from 0 to 1600 Pa and the pore fluid ratio (between air pressure and lithostatic pressure) from 0.0 to 1.0. A plot of shear stress versus effective stress at failure is almost linear, verifying Terzaghi's principle of effective stress. The best-fit slope (coefficient of internal friction) is about 0.55 and the intercept on the shear stress axis (cohesion) is less than 85 Pa. The tests show that it is feasible to use compressed air within sandpacks, as a means of modelling deformation coupled with fluid flow. The next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures.
The initiation of analogue studies of rock flow is stimulated by improving our knowledge of suitable model materials. Bouncing Putties and “Plasticines” are the most frequently used model materials in analogue studies of flow instabilities in deforming rocks. Polydimethyl-siloxane (PDMS) and polyborondimethylsiloxane (PBDMS), both substrates of Bouncing Putty, are introduced as convenient geological model materials. The chemistry of PDMS, PBDMS, Bouncing Putties and “Plasticines” is reviewed. A comprehensive set of instructions and graphs is provided for the manipulation of these model materials.
The Florence rise is located southwest of the island of Cyprus and belongs to the western part of the Cyprus arc. The Florence rise is an accretionary prism, with some amount of strike slip, where the thick layer of evaporitic Messinian decouples deformation in the post-Messinian cover from that in the pre-Messinian “basement.” The basement structural highs, whether presently active or inactive, influence the deformation and displacement patterns of the salt and its overburden.Our first experiment focused on the presence, in nature, of normal faults located above basement thrusts. Deformation of the salt layer and its overburden was influenced by several processes. On one hand, the entire model was subjected to regional shortening, and basement thrusts formed. On the other hand, the local vertical rise associated with basement thrusts created local slopes down which the salt and overburden glided gravitationally, in some places leading to the formation of normal faults in an otherwise compressional regional setting.Our second experiment was designed to model the buttressing effect of the Florence rise and the Eratosthenes Seamount on thin-skinned displacement patterns during regional gravity spreading of the Nile cone. Results indicate that in the Northeastern distal region, buttressing by these two basement highs forced the salt and its overburden to flow northeastward, as a lateral escape toward the free boundary of the region.
Gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment. In examples from the Gulf of Mexico and the Brazilian margin, gliding has produced three main structural domains: an uppermost domain of downdip extension; an intermediate domain of rigid gliding; and a lowermost domain of downdip contraction. Domain boundaries are established by changes in slope.In this paper, we examine three kinds of gravitational gliding, depending on the paths followed by material particles. In ideal parallel gliding, particle paths are parallel straight lines, trending downslope. This should occur where the margin is perfectly straight. In ideal radial gliding, particle paths are radii of a circle and the margin is shaped like a circular cone. Natural margins will not have ideal shapes; but divergent gliding will tend to occur off coastal salients; convergent gliding, off coastal re-entrants.A simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding. Changes in radius during divergent gliding produce strike-parallel extension; during convergent gliding, they produce strike-parallel contraction. Vertical strains also differ. Divergent gliding produces an uppermost domain of strong vertical thinning, balanced by extensions in all horizontal directions. Similarly, convergent gliding produces a lowermost domain of strong vertical thickening, balanced by contractions in all horizontal directions. These deformed states cannot be restored by simple techniques based on section balancing.We have done three experiments using analogue materials: sand, to model the brittle behaviour of sediments; silicone putty, to model the ductile behaviour of basal layers of evaporite. The experiments were properly scaled to account for gravitational forces. Experiment I reproduced convergent gliding above a basement with a conical upper surface. Strike-parallel contraction was responsible for radial folds on the lower slopes. The lowermost domain had a complex fold pattern, indicating contraction in all horizontal directions. Experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface. Strike-parallel extension produced radial rifts on the upper slopes. The uppermost domain had a complex polygonal pattern of normal faults, indicating extension in all horizontal directions. Experiment 3 also reproduced divergent gliding, but due to non-uniform sediment thickness.We have examined the many seismic profiles, both downdip and alongstrike, available for the Brazilian margin. Alongstrike profiles show structures which we attribute to strike-parallel strains. Around the Santos re-entrant, syn-sedimentary growthfolds indicate strike-parallel contraction, increasing downslope. This argues for a mechanism of convergent gliding. In contrast, around the Campos salient, normal growth faults and associated salt rollers indicate strike-parallel extension, decreasing downslope. This argues for a mechanism of divergent gliding.The pattern of oil fields in the Campos area, including the giant Marlim field, seems to be closely controlled by divergent gravitational gliding and associated processes.Radial gliding should occur on other passive margins as well, especially where the coastline is sinuous. Structures around the western Gulf of of Mexico seem to indicate convergent gliding at the scale of the entire Gulf. Structures at a smaller scale around the Mississippi delta seem to indicate divergent gliding.
Scaled sandbox models were used to investigate the 4D evolution of pull-apart basins formed above underlapping releasing stepovers in both pure strike-slip and transtensional basement fault systems. Serial sectioning and 3D volume reconstruction permitted analysis of the full 3D fault geometries. Results show that very different pull-apart basins are developed in transtension compared to pure strike-slip. Both types of models produced elongate, sigmoidal to rhomboidal pull-apart systems, but the transtensional pull-apart basins were significantly wider and uniquely developed a basin margin of en-echelon oblique-extensional faults. Dual, opposing depocentres formed in the transtensional model whereas a single, central depocentre formed in pure strike-slip. In transtension, a distinct narrow graben system formed above the principal displacement zones (PDZs). Cross-basin fault systems that linked the offset PDZs formed earlier in the transtensional models.Sequential model runs to higher PDZ displacements allowed the progressive evolution of the fault systems to be evaluated. In cross-section, transtensional pull-aparts initiated as asymmetric grabens bounded by planar oblique-extensional faults. With increasing displacement on the PDZs, basin subsidence caused these faults to become concave-upwards and lower in dip angle due to fault block collapse towards the interior of the basin. In addition, strain partitioning caused fault slip to become either predominantly extensional or strike-slip. The models compare closely with the geometries of natural pull-apart basins including the southern Dead Sea fault system and the Vienna Basin, Austria.
The western margin of the Gulf of Mexico (Veracruz State, Mexico) displays an extensive Neogene gravitational system, whereby the Neogene siliciclastic sediments are detached from underlying Mesozoic carbonates along decollement surface in Oligocene underpressured clays. Rapidly subsiding half-grabens develop above the footwall associated with major listric faults, whereas mini-basins develop in the deepest parts of the slope, in conjunction with the growth of west-verging compressional features. Between the high-angle normal faults in the west and the thrust anticlines in the east, a wide roll-over structure has grown progressively, resulting in a major topographic break in the morphology of the slope profile.Coupled forward kinematic modelling (Thrustpack) and stratigraphic modelling (Dionisos), calibrated against seismic profiles and two key nearshore wells, have been applied to representative regional transects across the margin, in order to quantify the vertical (subsidence) and horizontal (gravitational) deformations, and to discuss the impact of various key parameters on the distribution of debris flow depositions. These simulations are compared with data from distant ODP wells and discussed in the scope of current exploration strategies in both the US and Mexican portions of the Gulf of Mexico basin.
Brittle faults reflect a complex strain history that emerges from contrasting modes of distributed and localised deformation, and their interaction on various spatial and temporal scales. To better understand this process, we monitor the displacement field in scaled tectonic model experiments using high-resolution optical image correlation techniques (particle imaging velocimetry, PIV).2D and 3D surface displacement data of extensional and contractional sandbox experiments show that the mode, pattern, and temporal variation of strain accumulation are strongly dependent on the non-linear strain-dependent frictional strength of granular model materials similar to natural deformation processes in brittle rocks. Strain hardening and softening control the shear zone formation.The pattern of localised deformation is established much earlier than is visible from visual inspection of the experiment. Evolution of distributed strain in the surrounding material of the shear zones and discontinuous shear re-localisation control later stages irrespective of the kinematic boundary conditions.High-resolution optical strain monitoring quantifies the spatial and temporal patterns of strain accumulation in our model experiments with unprecedented detail. Together with detailed characterisation of the deformation behaviour of the model materials, our experiments will help to re-evaluate important scaling issues, and allow accurate comparison of analogue experiments with numerical simulations.
We describe the first three-dimensional imaging of the termination of a continent-ocean fracture zone (COFZ), the Chain Fracture Zone, located offshore of the Niger Delta. The COFZ marks the abrupt transition between extended continental crust, comprising multiple half-graben, and oceanic crust that has a pervasive seafloor-spreading fabric. It preserves a history of continent-continent shearing followed by oceanic crust accretion and continent-ocean shearing during the inception of Atlantic rifting. The termination is marked by steeply dipping faults with sigmoidal planform and thrusts that probably formed as a result of continent-continent or continent-ocean shearing. These are crosscut by the seafloor-spreading fabric that formed during the subsequent phase of oceanic crust accretion. The accreted oceanic crust is cut by listric and planar faults that curve in the direction of the COFZ, where they terminate. The transition from continental to oceanic crust across the COFZ is sharp and resolvable to 100–200 m. Complexes of lava flows emanate from volcanoes along the COFZ, bifurcating and trifurcating down the volcano flanks. The volcanoes are 2–5.5 km wide and 1.4 km in height relative to adjacent oceanic crust and were injected at the COFZ, probably as the spreading center migrated along it.
3D volume reconstruction and visualisation of delta models e applications to seismic interpretation
- E Frankowicz
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- U Ngala
- K Mcclay
Frankowicz, E., Wu, J.E., de Vera, J., Ngala, U., McClay, K., 2008. 3D volume reconstruction and visualisation of delta models e applications to seismic interpretation. In: 2008 AAPG Annual Convention Conference Abstract.
The growth history of toe thrusts of the Niger Delta and the role of pore pressure
- S W Krueger
- N T Grant
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and the role of pore pressure. In: McClay, K.R., Shaw, J.H., Suppe, J. (Eds.), AAPG
Memoir 94, Thrust Fault-related Folding, pp. 357e390.
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during continental drift offshore Niger Delta, equatorial Africa. Tectonics 28 (1).
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Conceptual model of structures formed by radial spreading across basement steps at the Niger Delta, based on insights from the analogue models in this study
- Fig
Fig. 18. Conceptual model of structures formed by radial spreading across basement steps at the Niger Delta, based on insights from the analogue models in this study.