Cataclastic Bands in Immature and Poorly Lithified Sandstone, Examples

TECTO-126315; No of Pages 12 Tectonophysics xxx (2014) xxx–xxx

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Tectonophysics

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Cataclastic bands in immature and poorly lithiﬁed sandstone, examples from Corsica, France

Anita Torabi

Centre for Integrated Petroleum Research (Uni CIPR), Uni Research, Allegt. 41, 5007 Bergen, Norway article info abstract

Article history: Cataclastic bands with intense cataclasis formed in phyllosilicate- and clay-rich poorly lithified sandstone have Received 21 January 2014 been identified and characterized for the first time. This study indicates that intense cataclasis could occur at Received in revised form 17 April 2014 shallow burial depth regardless of mineralogy of the rock at the time of deformation. The study was performed Accepted 9 May 2014 on a series of listric normal faults (displacement up to 10 m) in the Aghione Formation at Aleria Basin, Corsica, Available online xxxx France. In-situ measurements of permeability show a reduction in permeability up to two orders of magnitude

Keywords: in the damage zone (cataclastic bands), while permeability increases along the slip surfaces. The slip surfaces fl fl fi fi Cataclasis make conduits to uid ow, which is also con rmed by the eld observations. The permeability and porosity Phyllosilicate decrease within cataclastic bands is in agreement with an increase in P-wave velocity measured on the band Clay possibly due to compaction, introducing anisotropy to the rock. Permeability © 2014 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license Porosity (http://creativecommons.org/licenses/by-nc-nd/3.0/). P-wave velocity

1. Introduction bands, which are commonly called phyllosilicate band or framework (e.g. Heynekamp et al., 1999; Knipe et al., 1997). The phyllosilicate The process of deformation of high porosity sediments and poorly bands involve disaggregation and usually are classified under the disag- lithified rocks frequently involves formation of deformation bands gregation bands category. If the phyllosilicate and/or clay content of the (Aydin, 1978; Heynekamp et al., 1999; Sigda et al., 1999; Torabi, 2008 sediments or rocks increase to more than 40%, a phyllosilicate or clay and the references therein). Deformation bands are kinematically smear will occur at shallow depth (e.g. Clausen and Gabrielsen, 2002; classified as one of the three end members (e.g., Aydin et al., 2006 and Clausen et al., 2003; Fisher et al., 2000). Cataclasis or grain breakage in Fossen et al., 2007) namely dilation band (volume increase in the deformation bands is commonly expected at deeper burial depth or band), compaction band (volume decrease in the band), and shear higher confining pressure, but grain abrasion and low degree of band (no volume change in the band) or a combination of dilation and cataclasis in deformation bands formed in poorly lithified sandstone at shear or compaction and shear. However, deformation bands can be shallow burial depth have been previously reported in sediments and also classified based on the dominant deformation mechanism involved rocks with low content of clay and phyllosilicate, i.e. less than 15% in the formation of bands and therefore different types of bands may (e.g., Balsamo and Storti, 2010; Cashman and Cashman, 2000; Rawling form such as disaggregation band, cataclastic band and dissolution/ and Goodwin, 2003). Rawling and Goodwin (2003) report that the cementation bands (e.g. Torabi and Fossen, 2009). It is generally strength of grains (minerals) play a significant role in the deformation believed that at shallow burial depths and in poorly lithified rocks mechanism of poorly lithified sediments, in which a controlled particu- deformation bands are commonly formed by particulate or granular late flow can form in immature sediments with high content of rock flow mechanism (grain sliding or rolling) (Antonellini et al., 1994) fragments and feldspar. In a controlled particulate flow, deformation resulting in disaggregation bands when the phyllosilicate or clay of grains is selective, i.e. quartz grains are mainly deformed by abrasion, content is less than 15% (Fossen et at., 2007). Based on literature while feldspar grains break through transgranular fracturing and rock (e.g. Fisher et al., 2000; Fossen et al., 2007; Knipe et al., 1997; Torabi fragments by multi-cracking (Rawling and Goodwin, 2003). and Fossen, 2009), deformation of sediments or rocks with phyllosilicate The occurrence of intense cataclasis within a phyllosilicate- and/or and/or clay content between 15% and 40%, usually results in elongation clay-rich sand or sandstone is something that is not expected or has of phyllosilicate and/or clay mineral aggregates along localized not been previously addressed. In the present work, we identified cataclasitic bands with different degrees of cataclasis from low to intensive formed in a poorly lithified sandstone from the Aleria Basin Corsica, E-mail address: [email protected]. France. The studied sandstone contains more than 30% phyllosilicate

http://dx.doi.org/10.1016/j.tecto.2014.05.014 0040-1951/© 2014 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Please cite this article as: Torabi, A., Cataclastic bands in immature and poorly lithified sandstone, examples from Corsica, France, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.014 2 A. Torabi / Tectonophysics xxx (2014) xxx–xxx and clay particles. The influence of such deformation on petrophysical XRD (X-ray diffraction measured on powder of rock samples in (porosity and permeability) and acoustic properties (P-wave velocity) order to determine the average bulk composition of the rock) analysis of rocks has been investigated. was also performed on one of the sandstone samples in order to Previous studies have shown that phyllosilicate bands (because of quantify the mineralogical composition of the sandstone. An energy- the elongation of phyllosilicate grains along the band) and cataclastic dispersive X-ray spectroscopy detector (EDS) for elemental analysis bands (because of grain crushing and compaction) can impact the was used under electron microscopy for further identification of petrophysical properties of reservoir sandstones (e.g. Braathen et al., minerals that could not be captured by XRD analysis. 2009; Fisher and Knipe, 2001; Torabi and Fossen, 2009; Torabi et al., 2013). By studying the variation in petrophysical properties of different 3. Geology of Aleria Basin and the studied area types of deformation bands, Torabi and Fossen (2009) found that this variation depends on the degree of cataclasis in cataclastic bands and The island of Corsica is located between the Liguro-Provençal back on the phyllosilicate content in phyllosilicate bands. Recently, Torabi arc basin in the west and the Tyrrhenian Sea to the east. The island et al. (2013) showed the impact of cataclastic bands within fault dam- was part of the Iberian lithosphere which rifted off the main European age zone of faulted porous sandstones on fluid flow in petroleum reser- continent during the Oligocene-Miocene (Alvarez et al., 1974). voirs and on the capacity of reservoirs for CO2 storage underground. The geological framework of the island includes two distinct Torabi et al. (2013) indicate that a cluster of cataclastic bands (a zone structural terrains, Hercynian Corsica in the west and Alpine Corsica of deformation bands in decimetre width) can be locally as efficient as in the northeast of the island (Cavazza et al., 2007; Fig. 1). Hercynian a fault core in sealing fluid reservoirs (e.g. oil and CO2)resultingin Corsica is characterized by Palaeozoic granitoid rocks, volcanic withholding up to ~1 m column of these fluids. However, this study rocks of Hercynian orogeny, some Palaeozoic sedimentary rocks, and and most of the previous studies report the importance of cataclasis in Precambrian-Palaeozoic metamorphic rock units (Cavazza et al., clean sandstone, while the significance of grain crushing in immature 2007). The Alpine Corsica is part of the Alpine belt and is a nappe and phyllosilicate- and clay-rich sandstone has been undermined or stack that was thrusted on the Hercynian Corsica. less considered in the literature. There are two schools of thought with regard to the tectonic evolu- In this work, the deformation mechanism of immature and poorly tion of the Alpine Corsica during the Neogene to Quaternary: a) contin- lithified sandstone with high content of phyllosilicate and clay (more uous extension (Jolivet and Faccenna, 2000; Jolivet et al., 1990, 1991) than 30%) is investigated. The fieldwork for this study was performed episodal extension and inversion (Fellin et al., 2005). According to the on a series of listric normal faults developed in poorly consolidated, first group compression stopped in the late Oligocene and was followed high-porosity Aghione sandstone in the Aleria Basin, Corsica, France. by continuous extension. Continuous extension is suggested based on Field measurements of geological structures and in-situ measurements the regional significance of the extentional events which rule out the of permeability are presented. In addition, microstructural analyses of possibility of individual compressional structures (e.g. Jolivet et al., thin sections, porosity and permeability measured on backscatter 1991). images (BSE) of thin sections, and ultrasonic measurements of samples The extension and opening of the Provençal-Ligurian back-arc basin that were collected from the outcrop, are investigated in order to further led to the generation of different size sub-basins of different ages (Fellin characterize cataclastic bands observed in these immature sandstones. et al., 2005; Zarki-Jakni et al., 2004). Examples are the Saint Florent Basin Petrophysical properties (permeability and porosity) have been in the north and the Aleria and Marana Plains on the eastern coast of substantially decreased within these bands while acoustic properties Corsica (Fig. 1). The borders of these basins are extensional faults (P-wave velocity) are increased inside the band due to compaction which locally created accommodation space and post-sedimentary and porosity reduction, indicating anisotropy in the rock samples. deformation has been recorded within the rock units (Cavazza et al., 2007; Fellin et al., 2005). 2. Methodology The Aleria Plain corresponds to the onshore portion of the greater offshore Corsica Basin (Mauffret et al., 1999). The sedimentary rocks In this study a series of measurements have been carried out in of the Aleria Plain span from the Burdigalian to the Quaternary (Caron the field and laboratory. The field measurements include geometrical and Loye-Pilot, 1990). These rock units formed the youngest units of measurements of structural elements and in-situ permeability the Alpine Corsica and overlay the Alpine contacts (Felin et al., 2005). measurements. The laboratory work includes microstructural studies, We have studied a heavily faulted outcrop of Miocene rocks in the Aleria petrophysical properties measurements (using image processing), Plain northwest of Casaperta (Fig. 2). Fig. 2 shows a geological map of density, and ultrasonic measurements. part of the Aleria Basin around Casaperta. The outcrop is located on In order to carry out the field measurements, a 43 metre scan-line the eastern bank of the Corsigliese River before it joins the Tavignano was made along the outcrop almost perpendicular to the main orienta- River. The Miocene sedimentary rocks and Alpine Schist contact are tion of faults in the studied locality. The structural analysis of the out- faulted (Orszag-Sperber and Pilot, 1976). The regional fault system crop, measurements of faults orientations, deformation bands and has a relatively simple trace further south but it is split into branches fractures, and permeability measurements (using a Tiny-PermII) were in the studied area. There are several sub-parallel extensional faults performed along the scan-line (Appendix A). Samples of outcrop were (dashed lines on the map in Fig. 2) which have displaced both Alpine used for thin sections and ultrasonic measurements in the laboratory. units (e.g. Padoa et al., 2002) and the Miocene sedimentary rocks of The ultrasonic analysis was performed on samples in the laboratory the outcrop studied in this research. The oldest Miocene rock units in using a portable ultrasonic system by Geotron-Elektronik. In this device, the area are the conglomerates and sandy marls of the Saint Antoine the oven-dried sample is placed between two transducers. The length Formation that are exposed at the western bank of the Corsigliese and bulk density of the sample are given by the user as input parame- River (Caron and Loye-Pilot, 1990). The Aghione Formation (Langhian) ters, while the P-wave and S-wave velocities are measured by the which overlays conformably the Saint Antoine Formation has three instrument. Before performing the ultrasonic measurements, the bulk members (Fig. 2) including conglomerates with rhyolithic pebbles density of the samples was measured in the laboratory (Torabi and (m3-3), calcareous fossiliferous sandstones, (m3-2), and sand and Alikarami, 2012). sandy marls (m3-1). The m3-2 member is rich in micro-fauna and Polished thin sections of the samples were studied by both optical macro-fauna indicating a marine depositional environment. However, and electronic microscopes. Porosity and permeability were estimated the fossiliferous calcareous sandstone beds of m3-2 member are only by image-processing of high resolution backscatter images of thin developed further to the north (outside of our studied area) between sections (Torabi et al., 2008, Appendix B). the conglomerate (m3-3) and sandy marl units (m3-1) (Caron and

a) b) Cap Corse

N Saint Florent Bastia

45 N Alps Marana

Pyrenees Apennines Ligurian-Provençal Basin C

Tyrrhenian Sea Iberia S Corte Valencia trough

Aléria Ajacio Africa 200 km 35 N 10 W 0 10 E 20 E

Neogene-Quaternary

Alpine Corsica

Hercynian Corsica

Studied area Bonifacio 20 km

Fig. 1. a) Location of Corsica island with respect to main regional structures; b) Map of Corsica presenting tectonic classiﬁcation of different regions in Corsica including Hercynian and Alpine Corsica. Modiﬁed and redrawn after Cavazza et al., 2007.

Loye-Pilot, 1990). Within the m3-1 unit sandy channels have also been smaller displacement, which could also be called shear fractures or sim- reported (Caron and Loye-Pilot, 1990; Orszag-Sperber and Pilot, 1976). ply slip surfaces based on Fossen (2010) classification) cross the last Towards the top, the Aghione Formation becomes sandier (m3-1). The synthetic fault, where it becomes horizontal along the section Aghione Formation is overlaid by the Alitzone Formation (sands and (Fig. 4a). These minor faults, which are mostly synthetic to the main gravels). The contact between the Aghion and Alitzone Formations is faults have been also mapped through the scan-line. Displacement gradual (Caron and Loye-Pilot, 1990). varies along the minor faults with the maximum about 0.5 m. Most of the faults present open slip surfaces (Figs. 4a&5a). Fault 4. Results cores include crushed rocks and sometimes lenses of deformed rocks (Figs. 4a&5a). In the damage zone of these faults, there are deformation 4.1. Outcrop description bands crossing each other with different trends (Fig. 5b). In addition to deformation bands, fractures are present in the damage zone of the The outcrop of the studied locality (Casaperta) consists of mainly studied faults. poorly consolidated sandstones with thin coal beds (corresponding to Geometrical measurements of faults, fractures, and deformation m3-1 member) of the Aghione Formation and partly conglomerates bands were plotted on lower hemisphere stereonet diagrams (Fig. 4). (corresponding to the m3-3 member), Fig. 2. Fig. 3 illustrates a mosaic The main faults trend NW–SE with dips varying between 40° and 60° photo from the studied outcrop with a structural interpretation. Five (Fig. 4b). The fractures dominant trends are similar to faults (Fig. 4c) samples from the damage zone close to the faults and two from the and the deformation bands strike 100°, 142°, and 160° (Fig. 4d). In fault cores were carefully taken along the scan-line for the purpose of general, faults, fractures, and deformation bands show parallel trends further analysis. Apart from one fault core sample from a conglomeratic (Fig. 4). unit, the rest of samples are from poorly lithified sandstone (Fig. 3). Based on field and microscopic observations the exposed sandstone The well-exposed part of the outcrop is 43 m long and comprises a has heterogeneous grain size. XRD and microscopic analyses of the sam- set of synthetic listric NW–SE trending normal faults (faults F1 to 5 in ples show that the poorly consolidated sandstone contain abundant Fig. 3). These synthetic faults join each other at the westernmost part phyllosilicate and clay minerals (more than 30%, see Table 1)andis of the outcrop towards a horizontal detachment with displacement up quite immature and very poorly sorted. In addition to the minerals iden- to 10 m (Figs. 3 and 4a). Fault F2 (Fig. 3) is traceable to the river floor tified by XRD analysis, coal, iron oxide and iron sulphide were observed providing a nice 3D view of the fault. The displacements of the synthetic in the BSE images examined by EDS during electron-microscopy (Figs. 6, faults vary between few centimetres to ten metres. 7). Coal was also observed as thin layers between sandstone layers that The synthetic faults are crossed by a set of antithetic faults (faults A1 have been displaced by small faults (Fig. 7e). Previous work (Caron and to A4 in Fig. 3). The 5 synthetic and 4 antithetic faults mapped along the Loye-Pilot, 1990; Orszag-Sperber and Pilot, 1976) has suggested shal- scan-line are considered here as main faults in this locality because of low marine to fluvial depositional environment for these sediments, their larger displacements (up to 10 m). Several minor faults (with which has been confirmed by the present analyses and observations

Fig. 2. Geological map and stratigraphic column for part of the Aleria Basin; modified and redrawn from Geological map of Pietra-Di-Verde 1/50,000, after Caron et al., 1990. Note the location of main regional faults as black dashed lines both on the map. of some microfossils (e.g. foraminifera) in this study. The outcrop has deformed or broken phyllosilicate and clay particles, which is a new been shallowly buried (probably around 100 m) at the time of faulting observation in such poorly consolidated sandstone with more than (Torabi, 2012; Torabi et al., 2007). The iron-oxide and iron-sulphide 30% phyllosilicate and clay (Fig. 8). cementation (Fig. 6) appears to be a syn-deformation feature since we The cataclastic bands with low grain crushing are mineralogically di- can find these cements in small pieces trapped inside broken and de- vided into two sub-types: coal-dominated bands and iron-dominated formed phyllosilicate grains, where these grains are kinking. Further bands (Fig. 8). Thin coal-dominated bands involve smearing of coal analysis is needed in order to prove this idea, which would be outside along the bands and a weak cataclasis (Fig. 8b & d), while iron- of the scope of this study. dominated bands are slightly thicker than the coal-dominated bands and can be observed both as individual bands as well as in the margin 4.2. Microstructural study of deformation bands of the cataclastic bands with high degree of cataclasis (Fig. 8b & e). The iron-dominated bands show cataclasis to some extent and most of Two types of cataclastic bands based on the degree of cataclasis the pores have been filled with iron oxide and iron sulphide (Figs. 6, 8). were identified through microscopic studies of thin sections of the Aghione sandstone. The cataclastic bands with high degree of cataclasis 4.3. Permeability and porosity measurements (Fig. 8a & c) are thicker (more than 1 mm, measured on BSE images), while the cataclastic bands with low degree of cataclasis and less Permeability was measured both in the field parallel to deformation grain crushing (Fig. 8b & d & e) are thinner (around 0.5 mm, measured structures using a Tiny-PermII (Fig. 9a) and on BSE images of thin sec- on BSE images), Fig. 8. On BSE images, all of the bands contain tions using image processing (Torabi et al., 2008, Fig. 9b). The results

Fig. 3. A mosaic picture of the studied outcrop with the main synthetic (red dash-lines) and antithetic (black dash-lines) faults. The orange dashed line shows the boundary between the lower conglomerate and sandstone. Some of the displaced layers (solid blue and yellow lines) and sample locations have been marked on the picture. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 4. a) A close-up picture of the horizontal fault (red dashed line) in the outcrop (location of the picture is shown on the cross section of outcrop in Fig. 3) which is cut by small synthetic faults (black dashed lines); b, c, and d) Stereograms (lower hemisphere, equal area) of the studied faults, fractures and deformation bands, respectively. N refers to the number of data measured in the field for each category. Grey great circles are the synthetic faults, while the black great circles are the antithetic faults. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) of permeability measurements from the two approaches show an (these samples include deformation bands) or from the fault core agreement in reduction of permeability from host rock to deformation (five sandstone samples from damage zone and one from fault core bands (for all kind of bands described in Section 4.2) and also to the plus an additional fault core sample from conglomeratic unit). Due to fault cores. the irregular shape of the samples and the resulted uncertainty in Vs The permeability reduction within the bands can be up to two orders measurements because of the sample size, only Vp velocities are of magnitude relative to the host rock, which is comparable to the per- presented in this work. A plot of Vp versus bulk density of the samples meability reduction in the fault core (Fig. 9). However, Tiny-PermII data is presented in Fig. 11. In general, the P-wave velocities are low for show an increase of permeability around slip surfaces, which is con- most of the samples (~1100 m/s for the conglomerate sample and firmed with the field observation of remnants of fluid flow (reaction ~1800 m/s for sandstone samples). The velocities measured in this front) on open fault slip surfaces that have promoted fluid flow in the study are lower than those previously reported for unconsolidated faults (Fig. 5). The variation in the permeability values of different sand and sandstones (e.g. previous study by Wang, 2000); this could types of bands (different symbols in Fig. 9b) can be attributed to the be related to the mineralogy and deformation of these samples and degree of cataclasis within these bands. In particular, thick cataclastic will be further discussed in the next section. bands with high degree of grain crushing show higher permeability The highest velocity measurements belong to a deformation band. reduction relative to the host rock (solid symbols) in comparison This is the only sandstone sample from damage zone with a shape to the two other bands in this study (thin coal-dominated and that allowed velocity measurements in two different dimensions on iron-dominated bands). When making a comparison between the band (1810 m/s and 2402 m/s) and also perpendicular to the band coal-dominated and iron-dominated bands, it appears that the iron- (1742 m/s), see the solid squares on Fig. 11a and directions in dominated bands show slightly higher permeability reduction than Fig. 11b. The results show higher velocities on the band than outside the coal-dominated bands. This could be due to the fact that many the band (perpendicular to the band, Fig. 11a & b). grains have been coated with iron oxide besides the pore-filling iron sulphide (Fig. 6), which is an early diagenetic effect, in addition to the 5. Discussion and conclusions slight cataclasis within these bands. Porosity is very high in the host rock with an average around 40% and is reduced within the bands to Based on previous studies (Cavazza et al., 2007; Fellin et al., 2005), an average around 25% (Fig. 10). faulting in the area has happened after sedimentation but when the sediments were still very porous (the porosity of the host rock is still 4.4. Ultrasonic measurements around 40%). The main regional faults that have been previously mapped (dashed lines in Fig. 2)areNE–SW trending. These faults are ei- Ultrasonic measurements were performed on the deformed samples ther located at the boundary of the Alpine Corsica and Aleria Basin that were gathered either from the damage zone close to the faults (Faults 1 and 3 in Fig. 2) or displace rock units in the Aleria Basin

locality, but the presence of intensive cataclasis within phyllosilicate- and clay-rich shallowly buried sandstone is a new observation that has been made through this study. Based on literature (e.g. Fisher et al., 2000 and Fossen et al., 2007) these kinds of rocks would deform in a ductile manner rather than a brittle manner and cause smearing of phyllosilicate and clay minerals along deformation bands. The present observations, however, contradict this concept. This new observation could bring new insight into the understanding of deformation behaviour of these kinds of rocks and could probably challenge the previous ideas about deformation mechanism of similar materials. The brittle deformation of phyllosilicate and clay minerals at low stress level is not well-known and probably needs further experimental investiga- tion at different stress levels and drainage conditions. The presence of two types of cataclastic bands with different degrees of cataclasis could be attributed to the local variation in mineralogy, poor sorting, and immaturity of the studied sandstone. The porosity and permeability measurements on the bands are comparable to those previously reported for cataclastic bands measured parallel to the bands (e.g. Lothe et al., 2002; Torabi et al., 2013), and the degree of cataclasis in the present study seems to be more important than impurities such as coal and iron-oxide or iron-sulphide in reducing permeability of the bands (Fig. 9). Therefore, the cataclasis is considered to play a more significant role on fluid flow behaviour of these rocks when it comes to the movement of fluid or storage capacity of such a reservoir. On the other hand, evidence of fluid flow concentrated around slip surfaces is a good sign for the conductivity of the reservoir but needs to be taken with care if a risk assessment of possible leakage scenarios is

the case when evaluating such reservoirs for the purpose of CO2 storage.

5.1. P-wave velocity and anisotropy in the samples

P-wave velocity changes inside the cataclastic bands according to measuring direction on the band in three dimensions (Fig. 11). This Fig. 5. a) Fault cores and open slip surfaces with reaction fronts around slip surfaces in the could indicate anisotropy of the elastic properties in the measured di- studied outcrop. b) Two sets of deformation bands that cross each other (100° and 142°) mensions in the sandstone sample. Torabi and Alikarami (2012) and re- are observed in the outcrop. The one with 100° strike displaces the other set (~1 cm ﬁ displacement). cently Torabi and Zari (2014) report different values of M-modulus (the compressional P-wave modulus), Young's modulus and Poisson's ratio for cataclastic bands and their corresponding host rocks. This is expected from cataclastic bands since they usually show variation in their (Fault 2 in Fig. 2). The northern tip of the NNE–SSW trending Fault 3 microtexture and physical properties along them, see e.g. Figs. 2 and 7 in (Fig. 2) extends to the studied area. The strike of the fault cluster Torabi et al. (2008), where they have examined a cataclastic band in mapped in our study is NW–SE which crosses the main structural three dimensions by studying two perpendicular thin sections. They re- trend (NNE–SSW). There is almost a gap in the previously mapped port that permeability varies up to two orders of magnitude when mea- faults (Caron and Loye-Pilot, 1990) around the studied outcrop. sured in two perpendicular orientations on a single cataclastic band The traces of Faults 2 and 3 (Fig. 2) might be linked together by the (Torabi et al., 2008). The ultrasonic measurements show higher P- NW–SE trending fault cluster that has been mapped in this study but wave velocities within the band and are in agreement with the results to support this hypothesis, more ﬁeld work is needed in the future. from image processing of thin sections that present lower porosity The shallow deformation has resulted in deformation bands in the values in the bands. This could be an evidence of compaction and damage zone of the studied faults but what makes these bands spectac- grain locking process which follows the grain crushing (cataclasis) ular is the grain crushing and cataclasis in spite of the high content of within the bands. However, from the lower values of Vp and density phyllosilicate and clay minerals in the host rock. Cataclastic bands for this study in comparison to previous published data in Fig. 11, formed in quartz-rich sandstone at shallow burial depth have been re- one could predict that the relationship between density and velocity ported by many authors before (e.g. recent studies by Balsamo and measurements does not correspond to those previously reported Storti, 2010; Balsamo et al., 2010; Balsamo and Storti, 2011 and Ballas for unconsolidated sand, and sandstones with more than 15% clay et al., 2013). In addition, Rolseth (2008) reports on cataclastic bands (e.g. Wang, 2000), although our samples include more than 30% clay formed in quartz-rich sandstone in the Aleria Basin close to the studied and phyllosilicates (Fig. 11). The reason for this discrepancy could be

Table 1 Result from XRD analysis of one of the sandstone samples.

Illite/smectite lllite + Mica Kaolinite Chlorite Quartz K feldspar Plagioclase Calcite Dolomite Siderite Pyrite Hematite Total

0 20.8 2.8 6.9 37.8 14.4 14.9 TR 2.5 0 0 0 100

Dolomite is Ferroan and Minor barite was observed. TR means trace.

a) b)

20 µ 20 µ

Iron Sulfide

Iron Oxide

Fig. 6. BSE images and EDS analyses of thin sections show of iron sulphide (a and c) and iron oxide in the samples (b and d).

20 µm 100 µm

Fig. 7. BSE images and EDS analyses of thin sections show evidences of coal (a–d). e) Field observation of coal layers displaced by small faults.

Fig. 8. a) Photomicrograph taken with optical microscope showing a cataclastic band with high degree of cataclasis; b) Photomicrograph taken with optical microscope showing iron- dominated and coal dominated bands. c) BSE image of a thick cataclastic band (band in a) with intense cataclasis. d) and e) BSE images of thin coal-dominated and iron-dominated bands with low degree of cataclasis.

1000 100 Host rock Host rock Slip Surface Fault core Fault 100 core Fault 10 Deformation band Deformation Deformation band Deformation Permeability (md) 10 1

1 0.1

Fig. 9. a) Permeability values for different fault related rocks using Tiny-PermII in the field, there is a reduction of permeability from host rock to deformation bands and fault core, while permeability increases in the slip surfaces. b) Permeability values measured on the BSE image of thin sections. Solid symbols present values of thick cataclastic bands; open symbols those of iron-dominated bands, and crosses and stars values of coal-dominated bands. Different symbols present different samples. One of the fault core samples is from conglomerate (grey square). related to two factors: (i) the presence of coal and abundant factor for deformation mechanism of porous sandstone at shallow phyllosilicate and clay in the current samples and (ii) deformation in burial depth. the samples, the presence of deformed and crushed grains; both of • Permeability is reduced up to two orders of magnitude within the these factors would increase heterogeneity and bring anisotropy to studied cataclastic bands. the behaviour of the material when subjected to wave propagation. • Slip surfaces in this poorly lithified sandstone can form conduits Low velocity and low density in coal-dominated sediments have been to fluid flow around the faults, which might either enhance the reported before (Yao and Han, 2008). In addition, phyllosilicates be- conductivity of the reservoir in a desirable way or make leakage cause of their platy shape can induce anisotropy to the sediments. The pathways in worst cases. position and distribution of clay minerals influences the seismic velocity • Several factors have caused the discrepancy between the present and density of the rock (Wang, 2000). Pore-filling clay does not change velocity–density data and those previously published; these factors the wave velocity as such but increases the bulk density, on the other include the abundance of clay minerals and phyllosilicates and hand, the abundance of clay in the matrix of sandstone, which is the thepresenceofcoalaswellasdeformedgrainsinthestudied case in the current samples, will reduce seismic velocity but does not af- samples. fect the density of the rock (Wang, 2000). The following conclusions can be made from this study: Acknowledgement

• Cataclastic bands can form in phyllosilicate- and clay-rich poorly This research was performed at the Centre for Integrated Petroleum lithified sandstone at shallow burial depth. Research at Uni Research in Norway and was partly funded through • This study indicates that cataclasis could occur at shallow burial Consortium R&D Project 207806 (IMPACT Project, http://org.uib.no/ depth regardless of mineralogy of the rock at the time of deforma- cipr/Project/IMPACT) sponsored by the CLIMIT programme at the tion. This suggests that mineralogy might not be an important Research Council of Norway and Statoil. The financial support by the sponsors is appreciated. Alvar Braathen and Jan Tveranger are acknowl- 1000 edged for introducing me to the field localities in Corsica. I am very grateful to Behzad Alaei for his field assistance and contributions to this research. Jeremy Rohmer from BRGM is thanked for translation of the geological map legend from French into English. Donald T. Seifried is appreciated for reviewing and copy-editing of the manuscript. The chief editor of Tectonophysics, Prof. Laurent Jolivet and the two anonym 100 reviewers are acknowledged for their constructive comments.

Appendix A. Modiﬁed after Tiny-PermII User's Manual, NER

In-situ permeability measurements were performed by a Tiny- PermII manufactured by New England Research (NER). In order to

Permeability (md) 10 perform the measurements, a rubber nozzle is pressured against the sample and the device is vacuumed by pulling a plunger. When the device is ready, the air is sent to the sample with a single stroke of a Host rock syringe. The plunger is held until the measurement is completed Deformation bands and the result displays on the monitor. The result (TP)isrelatedto permeability (K) using Eq. (A.1): 1 20 25 30 35 40 45 50 ¼ : ðÞþ : ð : Þ Porosity (%) TP 0 8206 log K 12 8737 A 1

Fig. 10. Porosity–permeability relationship for host rock and deformation band samples where TP is the Tiny-PermII reading. When performing the tests in the estimated through image processing of BSE image of thin sections. ﬁeld, the weathered surface was cleaned using a geological hammer.

4000

sandstone, >15% clay

3000 Sandstone, <15% clay

Vp m/s Shale 2000 Deformed samples unconsolidated sandthis study &

1000 1000 1400 1800 2200 2600 3000 Density kgm-3 b) z Vp=1810 m/s

x Vp=2402 m/s

y Vp=1742 m/s

Fig. 11. a) P-wave velocity measurements versus estimated density for the corresponding samples; square and diamond symbols are from this study. Solid diamonds are sandstone samples from damage zone, open diamonds are fault core samples, the greyish open symbol is from conglomerate, while the solid squares are the deformation band samples. The data from the current study are compared with the reported velocity–density data by Wang (2000). Both density and P-wave velocity values are lower for the deformed samples in this work, even smaller than those of unconsolidated sand previously reported by Wang, 2000. b) P-wave velocity measurements were performed on 3 dimensions on a sample with a deformation band, two measurements on the deformation band (1810 m/s and 2402 m/s) and one perpendicular to deformation band (1742 m/s).

Measurements on cracks and open fractures were avoided. For each pores, and (i, j) indicates the position of pixels in the image. The pore sample point, 3 measurements were made and an average value of one-point correlation function, Eq. (B.2), S1, gives information about them was used as the representative value for that point. the volume fraction of the pores in an image consisting of M by N pixels (Garboczi et al., 1999), which is the porosity (φ). Appendix B. Image processing 1 X S ¼ φ ¼ bf N ¼ f ðB:2Þ 1 ðÞi; j MxN ðÞi; j In the backscatter images, the sandstone grain and pore phases occu- ij py different spatial positions. In such a two-phase (grain–pore) system i ¼ 1; 2; 3; …M and j ¼ 1; 2; 3; …N: of volume v each phase occupies a subvolume, v1 (for grains) and subvolume v2 (for pores), and a characteristic or indicator function can be defined as follows: For permeability estimation through the image processing approach (Torabi et al., 2008), a pore–pore two point correlation function was ∈ ðÞ¼ 0 r v1 ð : Þ used on binary images of thin sections using Eq. (B.3). f r ∈ : B 1 1 r v2 1 X S ðÞx; y ¼ bf f N ¼ f f : ðB:3Þ 2 ðÞi; j ðÞiþx; jþy MxN ðÞi; j ðÞiþx; jþy Each image includes a matrix of M by N pixels. In order to be able to ij use the indicator function to characterize the microgeometry of the Using Eq. (B.4), the specific surface area of the pores is calculated and samples, the colour intensity range should be limited to two pixel values finally the permeability is estimated utilizing a modified version of the representing the two phases of the medium (binary image). For a binary Kozney–Carman relation (Eq. (B.5), Walsh and Brace, 1984; Blair et al., image we define a function f(i,j) that is zero for grains and one for 1996), where F the formation factor has an exponential relation with

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