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_Journal of Structural Geology 33 (2011) 154e168_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Pressure solution-fracturing interactions in weakly cohesive carbonate sediments and rocks: Example of the synsedimentary deformation of the Campanian chalk from the Mons Basin (Belgium) J. Richard*, J.P. Sizun U.M.R. C.N.R.S. 6249 Chrono-environnement, Département de Géosciences, U.F.R. Sciences et Techniques, Université de Franche-Comté, 16 route de Gray, 25030 Besançon cedex, France Article info Article history: Received 23 June 2010; Received in revised form 4 November 2010; Accepted 17 November 2010; Available online 27 November 2010 Keywords: Deformation Pressure solution Mass transfer Fracture Diagenesis Carbonate Abstract This paper documents the pressure solution-fracturing interactions in weakly cohesive carbonate sediments and rocks by studying the synsedimentary deformation of the Campanian chalk from the Mons Basin (Belgium). The present work shows that the development of a normal fault in a near-surface marine environment can promote significant mass transfers and volume changes in weakly cohesive micritic carbonate materials. The deformation corresponds to a mass redistribution from deformed zones adjacent to the fault plane towards outermost deformed zones. These mass transfers result from a faster return of interstitial fluid pressure to an initial state within outermost deformed zones. The deformation is rapidly controlled by the volume gains caused by diffused rupture of grain contacts inside outermost deformed zones and not by fracturing. Within deformed zones adjacent to the fault plane, mass losses and related chemical compaction lead to a decrease in reservoir qualities of the material and growth of a permeability barrier that rapidly restricts flow of interstitial fluids towards active fractures. Within outermost deformed zones, transport properties and reservoir qualities of the material are maintained or increased. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Pressure solution (fluid-enhanced deformation) is considered to be a major mechanism of rock deformation. In the upper crust, it often plays an important role within zones where it is spatially associated with brittle deformation (Gratier et al., 1999). Previous contributions (Mimran, 1975, 1977; Jones et al., 1984; Carrio-Schaffhauser and Gaviglio, 1990; Hellmann et al., 2002a, b; Angelier et al., 2006; Schroeder et al., 2006; Darquennes et al., 2007; Richard et al., 2002; Richard, 2008; Gaviglio et al., 1993, 1997, 1999, 2009) highlighted the interest to study the interactions between pressure solution and fracturing by examining experimental and natural deformation in carbonate rocks and particularly in chalk. A small number of these works focused on mass transfers and volume changes caused by the pressure solution-fracturing interactions and attempted to make clear at the grain scale, diagenetic processes at the origin of these mass transfers and volume changes. A first study (Richard, 2008) highlighted the interest of this approach. * Corresponding author. Tel.: +33 3 81666434; fax: +33 3 81666558. E-mail address: james.richard@univ-fcomte.fr (J. Richard). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.11.006 The present paper completes this first study by examining the synsedimentary deformation of the Campanian chalk from the Mons Basin in a near-surface marine environment. Several works (Angelier et al., 2006; Schroeder et al., 2006; Darquennes et al., 2007; Gaviglio et al., 1993, 1997, 1999, 2009) were devoted to the study of interactions between pressure solution and fracturing processes in chalk from the Mons Basin. These previous contributions have chiefly provided data about the petrophysical and petrographic impact of the pressure solution-fracturing interactions but did not examine mass and volume changes caused by these mechano-chemical interactions. Moreover, modifications of transport properties of chalk (essential for petroleum geologists) were poorly documented and no physicochemical model of the deformation mechanism was proposed. The purpose of the present paper is therefore (1) to qualitatively and quantitatively characterise the diagenetic signature (modifications of reservoir qualities and transport properties, elemental signature and nannofacies of chalk) of pressure solution-fracturing interactions caused by the development of a dip-slip normal fault in the "white chalk" from the Mons Basin, (2) to quantify mass transfers and volume changes, to determine their spatial distribution. J. Richard, J.P. Sizun Journal of Structural Geology 33 (2011) 154e168 155 (3) to make clear at the grain scale the diagenetic processes at the origin of these mass and volume changes and (3) to propose a physico-chemical model of the deformation mechanism. 2. Geological setting The Mons Basin (30 km long and 15 km wide) that constitutes a northern extension of the Paris Basin is located in southwestern Belgium (Fig. 1). It is filled by Cretaceous and Cenozoic deposits that unconformably overlap the Paleozoic basement (Fig. 1). Detailed tectonic investigations have been carried out in the Mons Basin (Vandycke et al., 1988, 1991; Vandycke and Bergerat, 1989; Vandycke, 2002). They highlighted that the Late Cretaceous tectonics was dominated by an extensional regime (NWeSE extension during the Late Campanian followed by a NEeSW extension during the Maastrichtian) interrupted by a strike-slip event during the Early Maastrichtian. The Late Campanian NWeSE extension led to the development of N040 x14E to N055 x14E normal faults, often associated with subvertical joints. Fig. 1. Geological map and geological section of the Mons Basin with location of the Hainaulte-Sambre quarry. 156 J. Richard, J.P. Sizun Journal of Structural Geology 33 (2011) 154e168 NWeSE extension results from a synsedimentary tectonics that probably played an important role in the basin subsidence (Vandycke et al., 1991). 3. Sampling, analytical techniques and methods 3.1. Sampling Core samples were collected in the Hainaulte-Sambre quarry located in the southern part of the Mons Basin near Harmignies (Fig. 1). The studied fault system results from the development of a dip-slip normal fault through a "white chalk" (insoluble residue below 3_) in the Obourg Chalk (Late Campanian lithostratigraphic unit). It shows a principal fault plane with subsidiary branches (Fig. 2). The strike of the principal fault plane F is N055 x14E, its dip is 67 x14S and the throw reaches 1.7 m (Fig. 2). The strike of the principal fault plane indicates that the studied fault system is related to the Late Campanian NWeSE extension and consequently to synsedimentary tectonics. Two cores (7 cm in diameter) were drilled at right angles with the strike of the principal fault plane in a horizontal position (at the core scale, the bedding plane can be considered as horizontal): one of 58.6 cm long through the hangingwall and one of 76.9 cm long through the footwall (Fig. 2). Both were drilled 1.2 m below a key bed with belemnites. They crosscut the principal fault plane (F), two of its subsidiary branches (f1 and f2) and joints. J. Richard, J.P. Sizun Journal of Structural Geology 33 (2011) 154e168 157 Fig. 2. Simplified sketch of the studied working face with location of samples. The studied fault system shows a principal fault plane (F) with subsidiary branches. The strike of the principal fault plane is N055 x14E, its dip is 67 x14S and the throw reaches 1.7 m. Two cores were drilled at right angles with the strike of the principal fault plane in a horizontal position: one of 58.6 cm long through the hangingwall and one of 76.9 cm long through the footwall. Both were drilled 1.2 m below a key bed with belemnites. They crosscut the principal fault plane (F), two of its subsidiary branches (f1 and f2) and joints. The samples showing a fault plane are in bold and italic whereas the samples showing a joint are in italic. J. Richard, J.P. 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