Syn-sedimentary shear zones 1
I. Moretti1, S. Calassou2, P. Victor1,3, M. Molinaro4 and L. Maerten1
Institut Français du Pétrole, Div Géologie-Géochimie, Av de Bois Préau, 92852 Rueil Malmaison, France (e-mail:
[email protected]) 2 TotalFinaElf, Centre Scientifique et Technique JF, 64000 Pau, France 3 Present address: University of Hannover, Institut für Geologie und Paläontologie, Callinstr. 30, D-30167 Hannover, Germany 4 University of Orsay, Laboratoire Tectonique et Bassins, Université Paris Sud, UMR 8616, 91405 Orsay, France ABSTRACT: In deltaic and turbiditic deposits along passive margins, such as the
Lower Congo basin on the West African margin, the main deformation is not induced by a regional tectonic stress field but by the increase of the sedimentary load leading to gravitational instabilities. The local stress field in such an environment is drastically influenced by the lithological discontinuities, which can reorientate the principal stresses induced by sedimentary loading. In this paper we document the localization of particular faults, called sedimentary shear zones (SSZ); these are formed along the borders of a sandy channel embedded in more fine-grained sediments, as a function of the lithological contrast. Furthermore we describe the reorientation of the stress field, perpendicular to the channel borders in the channel interior. Examples from 3D seismic lines and field studies in SW Ireland and Tunisia are compared with results from analogue and geomechanical models, leading to an integrated interpretation for the formation of the SSZ. KEYWORDS: channel, turbidite, gravity gliding, stress orientation
INTRODUCTION Exploration in the deep offshore and the improved quality of 3D seismic data in sedimentary basins along passive margins has highlighted new features in turbiditic systems. On the seismic data from the West African margin, explorationists observe structural discontinuities along the channel boundaries that cannot be explained by sedimentary processes alone. This is especially true in channel-levee complexes, structures which are defined by lateral or vertical stacking of sandy elementary channels and finer-grained overbank deposits, as described by Kolla et al. (2001) from offshore Angola or Clark et al. (1992). In this paper we describe and analyse the observed structures by a multidisciplinary approach, using seismic 3D survey interpretation and field observation, as well as results from analogue and geomechanical modelling. Gravity spreading context The stress field, and hence the driving force of the deformation, along passive margins after the cessation of the rifting phase cannot be characterized by a regional stress field, either extensional or compressive. Deformation is rather controlled by gravitational instabilities induced by loading of the sedimentary pile due to continuous influx (Calassou & Moretti 2003). In addition, along some margins, for example the West African, regional tilt occurs due to an uplift of the continent. Sediments can spread in various directions because the offshore borders are unconfined and the displacement can be both perpendicular and parallel to the margin (Panien et al. 2001; Calassou & Moretti 2003; Victor & Moretti 2003). Multidirectional gravitational spreading is also enhanced by Petroleum Geoscience, Vol. 9 2003, pp. 221–232
basal décollement horizons like salt or overpressured shales. The importance of the material behaviour of the sediments, especially in localizing compressive deformation, is well documented in the Gulf of Mexico (Diegel et al. 1995; Rowan et al. 1999) and in the West African margin (Duval et al. 1992). At the channel-levee complex scale, the material contrast between the sandy channel and the finer-grained levee is expected to influence the local stress field and, therefore, to control the deformation pattern. In this paper, we will describe that this leads to the formation of structural discontinuities. SEISMIC DATA The TotalFinaElf seismic data presented here all come from the West African margin. The seismic data used for channel description are 3D high-resolution data. Inter-trace: 6.5 m; Inter-sample: 4 ms. The picking of stratigraphic time lines, sedimentary facies and faults were made by a TotalFinaElf team. Attribute maps – such as coherency, amplitude, dip-azimuth – were generated. After the Atlantic break up, the West African margin was the site of an evaporitic event (Albian), then the post-rift subsidence and gravity-glided deformation started. Deposition of carbonates took place during the Turonian to Eocene. Contemporaneously, salt tectonics were active, characterized by raft and growth faults in the proximal domain (extensional) and by diapers, as well as compressive structures such as thrusts units, in the distal domain. From Oligocene times until now a more regional sedimentary infilling took place in the Zaire/Congo deltas and only few new faults formed in the extensional domain. This regional sedimentary influx was 1354-0793/03/$15.00 2003 EAGE/Geological Society of London
222
I. Moretti et al. + 20 km from the axis to the pinch-out of the levees.
Fig. 1. Seismic time line across a levee channel on the West Africa margin. (a) Non-interpreted section from high-resolution 3D seismic data. This section is perpendicular to the channel complex axis. (b) Interpreted section. Vertical stack of channel complexes. Time lines are faulted by very flat and convergent faults rooted at the base of the channel complex. This detachment is located on a sedimentary surface (erosion/basal-lag). Main faults are located at the edge between the channel axis and the levees and not close to the erosive part of each elementary channel.
associated with a major uplift of the African margin. The sedimentary pile above the salt is more than 6 km thick, with locally more than 4 km of Oligo-Miocene to Quaternary turbiditic deposits (Duval et al. 1992; Lundin 1992). Regional cross-sections can be found in Rouby et al. (2002) and Calassou & Moretti (2003). We will focus our study on the characteristics of the sandy channel and fine-grained levees in these turbidites, located in the Tertiary translated domain. Turbidites of the deep-offshore Miocene deposits of the southern part of the lower Congo basin show particular geometries of vertical stacking of channel-levee complexes (CC). The channel-levee complex is defined in cross-section by + an axis of high density sedimentary transit and two symmetrical levees built by overspill low density currents; + grain density decrease from the axial/proximal position to the distal position;
The depositional system shown on Figure 1 comprises a stack of three channel-levee complexes A, B and C. In this case, the time elapsed from the deposition of CC-A to the abandonment of CC-C is 2 Ma. The axis of the lower one, CC-A, is characterized by an erosional surface, rather flat. There is a sandy basal-lag and a heterogeneous distribution along the channel axis of dominantly sandy infill. A sandy overspill caps the CC-A infill. A hemipelagic event is preserved at the top of the spill, predating CC-B. It constitutes a maximum relative flooding surface. This hemipelagic event is a very important time line that can be correlated, using biostratigraphic analysis, from the channel axis to the distal levees of the channel and to the continuous edges of the pinch-out of the levees. The correlation is possible only by considering the boundaries of CC-B and CC-C as faults and not as erosional surfaces. Considering the sedimentary dynamics, the observed amount of erosion is low during the formation of CC-B and CC-C. The main volume of the previous deposition is preserved during each channel event. Our interpretation proposes the development of particular faults – sedimentary shear zones (SSZ) – which accommodate an important horizontal displacement. Figure 2 shows a restoration perpendicular to the channel axis. The amount of displacement as shown in Figure 2 is 600 m (10% of extension). These SSZ are not rare and are often visible on the 3D seismic data, which cover the prospective part of this area. They do not accommodate slumps or rotated blocks, which fall into a depression. Because this deformation is progressive and continues after channel deposition, growth faults develop and the shapes are due to the progressive rotation of the levee material (Fig. 1). These syn-sedimentary faults define a 6 km long depression along the CC axis. Possible origin of this deformation The channel-levee complex deposits are located on the continental slope, where sedimentary loading and gravity gliding are the main parameters controlling deformation. We can observe interactions between sedimentation on the levees and SSZ movements. These interactions are due to continuous (through time) anomalies of topography constrained by the fault displacement. This means that between two events of
Fig. 2. Tentative restoration of the channel deformation on a cross-section perpendicular to the channel axis. This restoration shows 600 m of extension (10%). This extension took place during the channel complex deposit. It is probably responsible for the vertical capture of the channel complex. This mechanism is a hypothesis to explain the vertical stack of the channel complex.
Syn-sedimentary shear zones catastrophic turbidites, the system has enough time to readjust the disequilibrium induced by the previous deposition at this scale of analysis. Nevertheless, questions remain about the relationships between ‘instantaneous’ deformation and the deformation of all the sedimentary succession. Dewatering and the initiation of compaction may be responsible for a part of the displacement but the 600 m of extension perpendicular to the axis appears too large to be fully explained by these processes. Figure 3 proposes an evolution of this kind of channellevee complex. Of course, the sedimentary mechanisms of channel-levees may differ, but we can propose two extreme behaviours. If the velocity of sediment deposition is high, the compensation mechanism is dominant (spatial homogeneous distribution). If the velocity of deformation is high (this velocity could be increased by the dewatering processes), SSZ develop. These SSZ faults constrain variations in sedimentation. In this case, a vertical channel-complex stack, i.e. a heterogeneous distribution of turbiditic sediment, can develop. Of course, natural examples are a complex interaction between these two cases. Another type of structure also characterizes the internal deformation of the sedimentary pile. Panien et al. (2001) have shown how channel boudinage separates sandy channels into individual boudins. The underlying and overlying shales are deformed in a ductile manner and do not show brittle fracturing, as in the sandy channel. As in the case shown in Panien et al. (2001, fig. 2), the shale–sand interface acts as a decollement level. The authors interpreted the data in terms of flattening of the shaly sequence without localized faulting as long as the sediments react in a ductile manner and are unconfined. Panien et al. (2001) set up some analogue models to test this hypothesis of flattening due to gravitational instabilities. In this paper, we will focus on the influence of this deformation on the sedimentation. Obviously the dewatering processes will not be modelled but the influence of the rheological contrast between the channel and the shale during the gliding can be approximated. In the case of channel boudinage, the initial sand–shale interface is a bed, but in the case of the SSZ it is a lateral facies change; however, the behaviour remains the same. FIELD DATA Field work has been carried out to observe these synsedimentary shear zones and boudinaged channels in outcrop. The field data described in this paper are derived from two different basins: the Numidian basin outcropping in northern Tunisia (Wezel 1970) and the Upper Carboniferous of the Namurian Clare Basin in southwest Ireland. The Clare Basin example shows exceptional exposures of channels and associated SSZ, sometimes in 3D outcrop situations. In Tunisia, the size of the outcrops does not permit a 3D view of the structure, nevertheless spectacular boudinage of a sandy bed surrounded by shales and silts can be observed, such as that shown in Figure 4. Clare Basin (Ireland) The Namurian deposits of the West Clare Basin outcrop on cliff exposures along the northern Atlantic coast of the Clare peninsula in SW Ireland (Fig. 5a). These deposits rest upon Visean Carbonates and are formed by a thick (400–1900 m) detritic sequence evolving from shale and turbidites at the base (Shannon Group) to a deltaic complex at the top (Central Clare Group) (Fig. 5b).
223
The entire succession is strongly affected by a wide variety of syn-sedimentary deformation structures (Gill 1979; Martinsen 1989; Martinsen & Bakken 1990; Strachan 2002). In the deltaic sequences this deformation is manifested essentially as normal growth faulting (Rider 1978; Crans et al. 1980). Postdepositional Variscan deformation led to large wavelength folds and quartz-filled fractures (Coller 1984) and can be easily distinguished from syn-sedimentary structures considered in this paper. The examples of SSZ illustrated here are situated in the deltaic part of the succession. These deposits represent the distal sedimentation of a fluvial delta and, in this context, sandbodies are represented by small braided or meandering channel systems embedded in the clay matrix of the upper delta plain. Each channel body has an average width of 10 m to 20 m at most and a depth of a few metres. Castle Point. The lithology at this site (location in Figure 5a) is formed essentially by fine-grained sandstone organized in large lenticular bodies. Two stratigraphic units can be distinguished in this outcrop (Fig. 6a) which are separated by a discontinuity. A channel filled with fine-grained sandstone is embedded in a succession of parallel layers of laminated claystone and siltstone. The eastern channel margin can be identified as an erosive surface whereas the western channel margin seems to be more complex. It originated as an erosive surface similar to the eastern margin. This erosive surface continues as a discontinuity into the overlying sediments that form an asymmetric wedge thickening towards the discontinuity. The geometry of the wedge can be interpreted as a growth fault, which implies that the discontinuity on the western channel margin was reactivated as a normal fault. This interpretation is furthermore supported by the observation that the channel fill is rotated towards the west caused by displacement along the discontinuity. Further evidence that the discontinuity along the channel margin has been reactivated as a fault plane comes from the surface corresponding to the base of the channel, which is characterized by closely spaced ripple marks and sand-injected fractures deformed towards the south (Fig. 6b). From all these observations we infer that the lithological boundary between channel fill and surrounding fine-grained material was reactivated as a fault plane and thus can be described as a SSZ. It can be observed for 15–20 m towards the north, following the western channel margin. This implies that the channel margin, which is a rheologic discontinuity in the sedimentary pile, has controlled the initial localization of the fault. In a second stage, the fault controlled the deposition of further channel fill material and, therefore, the local accumulation of sediments, as shown by the syn-kinematic wedge. Green Island. The site of Green Island (Fig. 5a), situated in deltaic sediments, offers a second example of a channel margin reactivated as a syn-sedimentary normal fault. The outcrop (Fig. 7a) shows a normal fault plane that is turning from N65 to N100, along with a channel margin in a meander bend. The fault is dipping 30 to the north. Locally, some shale levels are sheared in a ductile manner within the fault zone, indicating that the fault was active when the sediment was still unlithified. The footwall of this fault is formed essentially of folded and disrupted shale mixed with lumps of sandstone, while the hanging wall consists of channelled sandstone. This sandstone is not a continuous body but appears to be formed by a succession of stacked channel bodies. The sandstone–shale interface is interpreted as a normal fault localized along the
224
I. Moretti et al.
Fig. 3. Sedimentary flattening and possible schematic evolution of a stack of channel-levee complexes (depositional system). This model is based on the seismic observations made on channel-levee complexes in offshore Gulf of Guinea. dz, deformation zone.
channel margin due to the rheological contrast between the two lithological units (channelled sandstone and fine-grained levee material).
These observations suggest that the presence and the geometry of the fault are a direct consequence of the influence of a meandering sandy channel surrounded by its fine-grained
Syn-sedimentary shear zones
Fig. 4. Boudinage of a sandstone layer (about 1 m thick) in shale-dominated series of the Numidian flysh in Tabarka (northwest Tunisia). The outcrop is located in the city of Tabarka, just above the castle. The current tilt, 40 east is due to the Miocene compression.
levee. This is illustrated by the conceptual model in Figure 7b, in which we propose that the fault is localized along the inner bend of a channel meander. ANALOGUE MODELLING Analogue modelling was performed to understand better the process of evolution of the SSZ. Systematic experiments were conducted to study the influence of the basal slope angle and the presence of a brittle sandy channel in a ductile environment on the fault development in the sedimentary pile subjected to multidirectional extension. Set-up Experiments were carried out in a 40.536 cm sandbox. Each model is composed of a 3 cm thick basal silicone layer representing a possible décollement horizon in the mobile substratum, such as salt or overpressured shales. The initial trapezoid form of the silicone block was chosen to initiate divergent gliding and spreading of the model, as observed in lobate deltas or deep sea fans with free convex downslope borders. The silicone putty (PDMS, Dow Corning SG36) was used to simulate the overpressured shales or salt of the substratum, which is assumed to have a Newtonian viscous rheology. Dry quartz sand and corundum powder are used to simulate the brittle behaviour of the channel fill and the
225
Fig. 5. The Namurian West Clare Basin, Ireland: (a) geological map, with location of sites (after Wignall & Best 2000); (b) stratigraphy (modified after Rider 1978).
sedimentary overburden. We use two different brittle analogues (quartz sand and corundum powder) with the same mechanical properties to achieve an attenuation contrast for X-ray tomography, which is employed to visualize internal deformation. Models deform under their own weight for various basal slope angles ranging from 0–5. Syn-deformational alternating layers of sand and corundum powder are sieved on top of the model with a constant sedimentation rate of 0.4 cm h1, simulating syn-deformational sedimentation. During the experiment, computerized X-ray tomography – as described by Colletta et al. (1991) – enables analysis of the kinematic evolution, as well as the three-dimensional geometry of the model. Different channel geometries (straight or meandering) were created in the basal silicone layer, which display different directions towards the maximum slope angle to test the different parameters controlling possible SSZ localization. Further systematic investigations have been conducted and are described in full detail in Victor & Moretti (2003). In this paper we restrict description to the SSZ-related experiments. Results All our models demonstrate that the evolution of a polygonal fault pattern due to multidirectional extension (Victor &
226
I. Moretti et al.
Fig. 6. Example of a faulted channel margin (SSZ) at Castle Point. (a) Outcrop in deltaic deposits showing two distinct stratigraphic units: the pre-kinematic channel infill (in pink) and the syn-kinematic wedge (in yellow) associated with the movement of fault A. The N–S fracture on the right of the image is post-basin and related to Variscan deformation. (b) Detail on deformed ripple marks and sand-injected fractures crossing the channel base. Sense of deformation suggests displacement of sediment towards the south.
Moretti 2001, 2003) is strongly influenced by the presence of a sandy channel. Figure 8 shows that faults are localized in the channel interior, orientated perpendicular to the channel boundaries during the early deformation stages (6% extension). These faults are termed channel-cutting faults and are attributed to the competency contrast between the brittle sandy channel and ductile silicone putty. During continuation of the experiments we soon observed the localization of normal faults following the strike of the channel boundaries (Fig. 9). These faults are initiated after about 15% extension at the sandy channel–silicone interface depicted on X-ray tomographic sections perpendicular to the channel (Fig. 10). The activity of this normal fault leads to the rotation of the sandy channel compared to the surrounding strata. The same rotation geometry was also observed in the field as described previously. Once the fault is initiated along the strike of the channel border it propagates into the syn-kinematic overburden and strongly influences the development of the polygonal fault pattern (Fig. 8). Fault strike orientation displayed in equal-area stereo projections strongly deviates from the fault pattern developed in experiments without a channel. Localization of the SSZ observed in experiments does not seem to be influenced by the amount of slope angle. Neverthe-
less, we observed that the most likely localization of a SSZ is always on the downslope side of a channel, with a strike orientation of the channel boundary not exceeding 45 towards the maximum slope angle (Figs 8, 9). In the experiment with a channel exactly parallel to the slope no SSZ is developed (Fig. 8). These observations suggest that the localization of the SSZ is controlled by the direction of the channel towards the maximum slope direction. The models demonstrate that the formation of the SSZ is localized along the strike of channel boundaries. They nucleate at the channel/silicone interface and propagate upward into the syn-kinematic sediment, forming major growth faults. Nucleation of these faults seems to be a result of the rheological anisotropy across the silicone–sand interface. This plane of instability is reactivated to localize the extensional strain of the model. GEOMECHANICAL APPROACH We investigated the perturbed stress field, as caused by a sandy channel embedded in a more shale-prone matrix, using a 2D finite element code. We used 2D elastic models because they provide a first-order understanding of how the contrasting
Syn-sedimentary shear zones
227
Fig. 7. Example of SSZ at Green Island. (a) Outcrop in deltaic sediments showing a fault plane turning from N65 to N100 (from right to left of image). The hanging wall is composed of channelled sandstone, the footwall is mainly composed of sheared claystone. (b) Proposed model for outcrop in (a). Fault geometry is controlled by the geometry of a meandering channel.
rheology between the sandy channel and the surrounding shale in a unidirectional extension setting might redistribute deformation and influence stress trajectories. Model configuration The basic geometry of the model (Fig. 11) consists of a quarter-circle block containing a finite Z-shaped channel at its centre. Triangular elements have been used to mesh the different parts with an adjustment of the element shape to fit the edge of the channel. This particular geometry has been used to reproduce the analogue model configuration. In order to simulate a homogeneous radial extension of 1% (e.g. infinitesimal strain), which might correspond to the very first stage of the deformation by gravitationally driven deformation, we have adopted the following boundary conditions. The upper left corner is fixed in the x and y directions. The two straight borders of the model are fixed in the direction normal to their orientation but free in the tangential direction in
order to accommodate border parallel deformation. The quarter-circular border has been displaced by L0.01 (i.e. 1% extension) along the normal direction (xlocal) of the border and is free in the tangential direction ( ylocal). This boundary condition is used to accommodate border parallel deformation and the undesirable boundary effects. Two elastic constants, Poisson’s ratio () and Young’s modulus (E), were used to characterize the behaviour of the linear elastic solids. We used =0.25 and E=30 GPa for the sandstone of the channel and =0.15 and E=25 GPa for the surrounding shale. These are typical values, as described by Clark (1966). We used a plane stress model so that material is allowed to move in and out of the plane. Computed stress perturbations Figure 11b shows how stresses have been perturbed within the model. The two principal stresses 1 and 2 have been normalized to the stress that would occur in a model without a
228
I. Moretti et al.
Fig. 8. Comparison of straight channel experiments with the formation of a polygonal fault pattern in the no-channel experiment shows how fault localization is strongly influenced by the presence of a brittle channel in a ductile fine-grained sedimentary pile. After about 6% of extension normal faults are localized in the channel interior with fault strike orientation perpendicular to the channel borders. Note the direction of maximum slope that does not have an influence on fault orientation. After 120 minutes (15% extension) we observe particular fault patterns influenced by channel orientation. Lower hemisphere stereo projections show an almost uniform fault strike orientation for experiment 10/3 (channel at 0). A bimodal fault strike orientation is observed for experiment 10/6 (channel at 45) and an isotropic distribution for fault strike orientation is observed for experiment 10/4 (channel at 90). During progressive deformation fault localization along channel borders is manifested and leads to the development of major normal faults especially in experiments 10/6 and 10/4.
channel. Such a model, without rheological contrast, produces a homogeneous deformation where 1=2, as seen on Figure 12 (third row). We see on Figure 11b that stresses are perturbed within and around the channel. The least tensile principal stress (1) is increased within the channel and reduced outside the channel, whereas the most tensile principal stress (2) is increased within and outside the channel. Note that the stresses are all tensile. Far from the perturbing channel, the normalized stresses are equal to 1 (white colour in Fig. 11b), meaning that there is no stress perturbation. Figure 11c shows that there is a complete reorganization of the principal stress directions. The least tensile principal stress (1) is perpendicular and parallel to the edge of the channel, inside and outside the channel respectively. On the other hand, the most tensile principal stress (2) is parallel and perpendicular to the edge of the channel, inside and outside the channel, respectively. These results show that fracturing would occur first inside and/or adjacent to the channel as long as the stresses reach the tensile or shear strength of the rocks. This implies, if we assume
that the most compressive principal stress is vertical for a 3D model – which is the case for gravity gliding, that fractures (e.g. faults or joints) would form perpendicular to the edge of the channel inside the channel and parallel to the edge of the channel outside and adjacent to the channel. Effect of contrasting mechanical properties The effect of rheology contrast on the stress direction pattern has been analysed with the same model configuration; only the material properties (e.g. and E) of the channel and the surrounding medium have been modified (see Fig. 12). Figure 12 (first and second rows) shows that a small contrast in the Poisson’s ratio (e.g. down to 0.7%) or the Young’s moduli (e.g. down to 0.4%) is sufficient to redistribute the principal stress directions. When there is no rheology contrast, the stress distribution is homogeneous with 1=2 and no preferential stress direction, as shown on Figure 12 (third row). As soon as the channel is less competent than the surrounding medium (e.g. 1>2 or E1>E2 in Fig. 12, lowest row) the
229
Syn-sedimentary shear zones
Fig. 9. The experiment modelling evolution of a meandering channel depicts the formation of sedimentary shear zones (SSZ) along the channel boundaries of a sandy channel in a ductile silicone matrix. Note that sedimentary shear zones are mainly developed along downdip channel borders that have an angle >45 with the maximum slope direction.
Fig. 10. X-ray tomographic sections taken perpendicular to channel strike show the development of channel-bounding growth faults that can be identified as SSZ. Interpreted sections correspond to frames in tomographic sections and depict the initiation of the channel-bounding fault and the syn-kinematic rotation of the channel fill induced by fault activity.
principal stresses are switched such that, inside the channel, 1 becomes parallel to the edge of the channel and 2 becomes perpendicular to the edge of the channel. Given the limitation of the mechanical modelling, we none the less show that simple elastic models capture some of the principal effects of rheology contrast in a unidirectional extension setting and illustrate the accompanying field observations and analogue model results.
DISCUSSION AND CONCLUSIONS The analysis of natural examples, as well as analogue and geomechanical modelling, has shown the existence of the SSZ as an important structure affecting a sedimentary basin during deposition. With the aid of modelling we can also propose a viable mechanism for the formation of the SSZ.
230
I. Moretti et al.
Fig. 11. Geomechanical model and results. (a) Model configuration (see text for detail). (b) Contours of the principal stresses 1 and 2. The stresses have been normalized to the stress that would occur in a model without channel. Contour interval is 0.025. (c) Principal stress trajectories.
The structures localized along the channel borders observed on seismic sections from Block 17 offshore Angola are clearly identified as tectonic structures: the seismic arguments make it improbable that they have a sedimentary origin. This idea of formation of normal faults along channel margins is very well supported by field data collected in the Clare Basin of SW Ireland. A detailed study of channels showed that many of them develop faults along their margins, which could in some cases be identified as faults developed during sedimentation (Castle Point) or before lithification (Green Island). The analogue and geomechanical models were set up to test the conditions for the formation of the SSZ. The analogue models showed the evolution of SSZ during syn-kinematic sedimentation and the geometry of SSZ were mapped in three dimensions. We observed a first stage of faulting localized in the channel interior. Orientation of these normal faults was
always perpendicular to the channel border margin and led to subsequent boudinage of the channel. Geomechanical modelling showed that normal faulting inside the channel is controlled by the stress perturbation as a consequence of contrasting material properties inside and outside the channel. According to the stress reorientation we expect fractures to open perpendicular to the channel border margin inside the channel and parallel to the channel along its border margin and adjacent external regions. These results are consistent with the results of the analogue experiments. Ongoing deformation of the analogue models showed the development of normal faults that are localized along the channel borders, evolving to major growth faults propagating into the sedimentary overburden. Such a structure is well documented by field data at Castle Point (Fig. 6). Localization of these faults is also predicted by the stress perturbation pattern resulting from geomechanical modelling. Along channel
Syn-sedimentary shear zones
231
Fig. 12. Effect of contrasting mechanical properties: (a) Poisson’s ratio variation; (b) Young’s modulus variation.
borders and outside the channel the most tensile stress (2) is orientated perpendicular to the channel margin. If the tensile strength of the material is exceeded we would, therefore, expect the evolution of joints or normal faults parallel to the channel border, which again is consistent with analogue experiments. This development of faults parallel to the channel border is the mechanism responsible for the evolution of the SSZ. According to the modelling approaches we propose that the mechanical material contrast between the sandy channel fill and its surrounding fine-grained levee is the controlling parameter for the localization of SSZ. Geomechanical modelling allowed us to test the influence of differing material properties. Results show that already a slight difference in rheological properties across the border of the channel is sufficient to reorientate the stress field. This observation supports the formation of SSZ in the deltaic sequences in the Clare Basin where we observe only a
slight material contrast between channel fill and surrounding sediments. This kind of approach to analyse the influence of material contrast cannot be approached with sandbox experiments since the variability of properties for analogue materials is limited. A number of conclusions can be drawn from this study. 1. The SSZ is a tectonic structure localized along channel margins. There is no sedimentary process that could explain formation of the observed structures without tectonic deformation. 2. The structures are formed during sedimentation, as indicated by field observations and analogue experiments. 3. The controlling factor for fault localization along channel margins is the competency contrast across the channel border caused by lateral facies changes.
232
I. Moretti et al.
4. Channel boudinage is a second phenomenon of synsedimentary deformation in the interior of the sedimentary pile and is also controlled by contrasting material properties of the brittle channel and the ductile fine-grained levee material. This study has been carried out under the auspices of the FSHproject CINEMOD between IFP, TFE and GDF. Jean Gerard from TFE, Laurent Escaré from GDF, Pierre Vergeli and Antonio Benedetto from the Orsay-Paris Sud University participated in the fieldwork in Ireland. Michel Rosener, Jean Marie Mengus and Remy Eschard were in the field in Tunisia. We thank Jean Marie Mengus, Camille Schlitter and Corinne Fichen for their help with the X-ray tomography acquisition and processing.
REFERENCES Calassou, S. & Moretti, I. 2003. Sedimentary flattening and multi-extensional deformation along the west African margin. Marine and Petroleum Geology, 20, 71–82. Clark, S.P. Jr (ed.) 1966. Handbook of Physical Constants (revised edn). Geological Society of America, Memoir, 97. Clark, J.D., Kenyon, N.H. & Pickering, K.T. 1992. Quantitative anaylsis of the geometry of the submarine channels: implication for the classification of submarine fans. Geology, 20, 633–636. Coller, D.W. 1984. Variscan Structures in the upper Palaeozoic rocks of West Central Ireland. In: Hutton, D.H.W. & Sanderson, D.J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publications, 14, 185–194. Colletta, B., Letouzey, J., Pinedo, R., Ballard, J.F. & Balé, P. 1991. Computerised X-ray tomography analysis of sandbox models: examples of thin-skinned thrust systems. Geology, 19, 1063–1067. Crans, W., Mandl, G. & Haremboure, J. 1980. On the theory of growth faulting: a geomechanical model based on gravity sliding. Journal of Petroleum Geology, 2(3), 265–307. Diegel, F.A., Karlo, J.F., Schuster, D.C., Shoup, R.C. & Travers, P.M. 1995. Cenozoic structural evolution and tectono stratigraphic framework of the northern Gulf Coast continental margin. In: Jackson, M.P., Roberts, D. &
Snelson, S. (eds) Salt Tectonics, a global perspective. AAPG Memoir, 65, 109–151. Duval, B., Cramez, C. & Jackson, M.P. 1992. Raft tectonics in Kwanza Basin, Angola. Marine and Petroleum Geology, 9, 389–404. Gill, W.D. 1979. Syndepositional sliding and slumping in the West Clare Namurian Basin, Ireland. Special Papers of the Geological Survey of Ireland, 4. Kolla, V., Bourges, Ph., Urruty, J-M. & Safa, P. 2001. Evolution of deep-water Tertiary sinuous channels offshore Angola (west Africa) and implications for reservoir architecture. AAPG Bulletin, 85(8), 1373–1405. Lundin, E. 1992. Thin-skinned extensional tectonics on a salt detachment, northern Kwanza basin, Angola. Marine and Petroleum Geology, 9, 405–411. Martinsen, O.J. 1989. Styles of soft-sediment deformation on a Namurian (Carboniferous) delta slope, Western Irish Namurian Basin, Ireland. In: Whateley, M.K.G. & Pickering, K.T. (eds) Deltas: Sites and Traps for Fossil Fuels. Geological Society, London, Special Publications, 41, 167–177. Martinsen, O.J. & Bakken, B. 1990. Extensional and compressional zones in slumps and slides in the Namurian of County Clare, Ireland. Journal of the Geological Society, London, 147, 153–164. Panien, M., Moretti, I. & Calassou, S. 2001. Analogical model of the deformation of sandy submarine channels in shaly pelagic sediments. Oil and Gas Science and Technology, 56, 319–325. Rider, M.H. 1978. Growth faults in Carboniferous of Western Ireland. AAPG Bulletin, 62, 2191–2213. Rouby, D., Raillard, S., Guillocheau, F., Bouroullec, R. & Nalpas, T. 2002. Kinematics of a growth fault/raft system of the West African margin using 3-D Restoration. Journal of Structural Geology, 24, 783–796. Rowan, M.G., Jackson, M. & Trudgill, B.D. 1999. Salt-related fault families and fault welds in the northern Gulf of Mexico. AAPG Bulletin, 83, 1454–1484. Strachan, L. 2002. Slump-initited ands controlled syndepositional sandstone remobilization: an example from the Namurian of County of Clare, Ireland. Sedimentology, 49, 25–41. Victor, P. & Moretti, I. 2001. 3D Geometry of polygonal patterns formed by multidirectional gravitational gliding in analogue experiments. EAPG, Extended abstract. Victor, P. & Moretti, I. 2003. 3D Geometry of polygonal patterns formed by multidirectional gravitational gliding in analogue experiments. Marine and Petroleum Geology, in press. Wezel, F.C. 1970. Numidian flysch: an Oligocene–early Miocene continental rise deposit off the African platform. Nature, London, 228, 275–276. Wignall, P.B. & Best, J.L. 2000. The Western Irish Namurian Basin reassessed. Basin Research, 12, 59–78.
Received 8 July 2002; revised typescript accepted 7 April 2003.
